QUIC: A UDP-Based Multiplexed and Secure Transport

• Streams are the basic service abstraction that QUIC provides.
  • Section 2 describes core concepts related to streams,
  • Section 3 provides a reference model for stream states, and
  • Section 4 outlines the operation of flow control.
• Connections are the context in which QUIC endpoints communicate.
  • Section 5 describes core concepts related to connections,
  • Section 6 describes version negotiation,
  • Section 7 details the process for establishing connections,
  • Section 8 specifies critical denial of service mitigation mechanisms,
  • Section 9 describes how endpoints migrate a connection to a new network path,
  • Section 10 lists the options for terminating an open connection, and
  • Section 11 provides general guidance for error handling.
• Packets and frames are the basic unit used by QUIC to communicate.
  • Section 12 describes concepts related to packets and frames,
  • Section 13 defines models for the transmission, retransmission, and acknowledgement of data, and
  • Section 14 specifies rules for managing the size of packets.
• Finally, encoding details of QUIC protocol elements are described in:
  • Section 15 (Versions),
  • Section 16 (Integer Encoding),
  • Section 17 (Packet Headers),
  • Section 18 (Transport Parameters),
  • Section 19 (Frames), and
  • Section 20 (Errors).

Accompanying documents describe QUIC’s loss detection and congestion control [QUIC-RECOVERY], and the use of TLS for key negotiation [QUIC-TLS].¶

This document defines QUIC version 1, which conforms to the protocol invariants in [QUIC-INVARIANTS].¶

To refer to QUIC version 1, cite this document. References to the limited set of version-independent properties of QUIC can cite [QUIC-INVARIANTS].¶

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

Commonly used terms in the document are described below.¶

QUIC:

The transport protocol described by this document. QUIC is a name, not an acronym.

QUIC packet:

A complete processable unit of QUIC that can be encapsulated in a UDP datagram. Multiple QUIC packets can be encapsulated in a single UDP datagram.

Ack-eliciting Packet:

A QUIC packet that contains frames other than ACK, PADDING, and CONNECTION_CLOSE. These cause a recipient to send an acknowledgment; see Section 13.2.1.

Endpoint:

An entity that can participate in a QUIC connection by generating, receiving, and processing QUIC packets. There are only two types of endpoint in QUIC: client and server.

Client:

The endpoint that initiates a QUIC connection.

Server:

The endpoint that accepts a QUIC connection.

Address:

When used without qualification, the tuple of IP version, IP address, and UDP port number that represents one end of a network path.

Connection ID:

An identifier that is used to identify a QUIC connection at an endpoint. Each endpoint selects one or more Connection IDs for its peer to include in packets sent towards the endpoint. This value is opaque to the peer.

Stream:

A unidirectional or bidirectional channel of ordered bytes within a QUIC connection. A QUIC connection can carry multiple simultaneous streams.

Application:

An entity that uses QUIC to send and receive data.

Packet and frame diagrams in this document use a custom format. The purpose of this format is to summarize, not define, protocol elements. Prose defines the complete semantics and details of structures.¶

Complex fields are named and then followed by a list of fields surrounded by a pair of matching braces. Each field in this list is separated by commas.¶

x (A):

Indicates that x is A bits long

x (i):

Indicates that x uses the variable-length encoding in Section 16

x (A..B):

Indicates that x can be any length from A to B; A can be omitted to indicate a minimum of zero bits and B can be omitted to indicate no set upper limit; values in this format always end on an octet boundary

x (?) = C:

Indicates that x has a fixed value of C with the length described by ?, as above

x (?) = C..D:

Indicates that x has a value in the range from C to D, inclusive, with the length described by ?, as above

[x (E)]:

Indicates that x is optional (and has length of E)

x (E) …:

Indicates that x is repeated zero or more times (and that each instance is length E)

This document uses network byte order (that is, big endian) values. Fields are placed starting from the high-order bits of each byte.¶

By convention, individual fields reference a complex field by using the name of the complex field.¶

Streams can be unidirectional or bidirectional. Unidirectional streams carry data in one direction: from the initiator of the stream to its peer. Bidirectional streams allow for data to be sent in both directions.¶

Streams are identified within a connection by a numeric value, referred to as the stream ID. A stream ID is a 62-bit integer (0 to 2^62-1) that is unique for all streams on a connection. Stream IDs are encoded as variable-length integers; see Section 16. A QUIC endpoint MUST NOT reuse a stream ID within a connection.¶

The least significant bit (0x1) of the stream ID identifies the initiator of the stream. Client-initiated streams have even-numbered stream IDs (with the bit set to 0), and server-initiated streams have odd-numbered stream IDs (with the bit set to 1).¶

The second least significant bit (0x2) of the stream ID distinguishes between bidirectional streams (with the bit set to 0) and unidirectional streams (with the bit set to 1).¶

The two least significant bits from a stream ID therefore identify a stream as one of four types, as summarized in Table 1.¶

The stream space for each type begins at the minimum value (0x0 through 0x3 respectively); successive streams of each type are created with numerically increasing stream IDs. A stream ID that is used out of order results in all streams of that type with lower-numbered stream IDs also being opened.¶

STREAM frames (Section 19.8) encapsulate data sent by an application. An endpoint uses the Stream ID and Offset fields in STREAM frames to place data in order.¶

Endpoints MUST be able to deliver stream data to an application as an ordered byte-stream. Delivering an ordered byte-stream requires that an endpoint buffer any data that is received out of order, up to the advertised flow control limit.¶

QUIC makes no specific allowances for delivery of stream data out of order. However, implementations MAY choose to offer the ability to deliver data out of order to a receiving application.¶

An endpoint could receive data for a stream at the same stream offset multiple times. Data that has already been received can be discarded. The data at a given offset MUST NOT change if it is sent multiple times; an endpoint MAY treat receipt of different data at the same offset within a stream as a connection error of type PROTOCOL_VIOLATION.¶

Streams are an ordered byte-stream abstraction with no other structure visible to QUIC. STREAM frame boundaries are not expected to be preserved when data is transmitted, retransmitted after packet loss, or delivered to the application at a receiver.¶

An endpoint MUST NOT send data on any stream without ensuring that it is within the flow control limits set by its peer. Flow control is described in detail in Section 4.¶

Stream multiplexing can have a significant effect on application performance if resources allocated to streams are correctly prioritized.¶

QUIC does not provide a mechanism for exchanging prioritization information. Instead, it relies on receiving priority information from the application that uses QUIC.¶

A QUIC implementation SHOULD provide ways in which an application can indicate the relative priority of streams. An implementation uses information provided by the application to determine how to allocate resources to active streams.¶

This document does not define an API for QUIC, but instead defines a set of functions on streams that application protocols can rely upon. An application protocol can assume that an implementation of QUIC provides an interface that includes the operations described in this section. An implementation designed for use with a specific application protocol might provide only those operations that are used by that protocol.¶

• write data, understanding when stream flow control credit (Section 4.1) has successfully been reserved to send the written data;
• end the stream (clean termination), resulting in a STREAM frame (Section 19.8) with the FIN bit set; and
• reset the stream (abrupt termination), resulting in a RESET_STREAM frame (Section 19.4) if the stream was not already in a terminal state.

• read data; and
• abort reading of the stream and request closure, possibly resulting in a STOP_SENDING frame (Section 19.5).

An application protocol can also request to be informed of state changes on streams, including when the peer has opened or reset a stream, when a peer aborts reading on a stream, when new data is available, and when data can or cannot be written to the stream due to flow control.¶

Figure 2 shows the states for the part of a stream that sends data to a peer.¶

The sending part of a stream that the endpoint initiates (types 0 and 2 for clients, 1 and 3 for servers) is opened by the application. The “Ready” state represents a newly created stream that is able to accept data from the application. Stream data might be buffered in this state in preparation for sending.¶

Sending the first STREAM or STREAM_DATA_BLOCKED frame causes a sending part of a stream to enter the “Send” state. An implementation might choose to defer allocating a stream ID to a stream until it sends the first STREAM frame and enters this state, which can allow for better stream prioritization.¶

The sending part of a bidirectional stream initiated by a peer (type 0 for a server, type 1 for a client) starts in the “Ready” state when the receiving part is created.¶

In the “Send” state, an endpoint transmits - and retransmits as necessary - stream data in STREAM frames. The endpoint respects the flow control limits set by its peer, and continues to accept and process MAX_STREAM_DATA frames. An endpoint in the “Send” state generates STREAM_DATA_BLOCKED frames if it is blocked from sending by stream or connection flow control limits Section 4.1.¶

After the application indicates that all stream data has been sent and a STREAM frame containing the FIN bit is sent, the sending part of the stream enters the “Data Sent” state. From this state, the endpoint only retransmits stream data as necessary. The endpoint does not need to check flow control limits or send STREAM_DATA_BLOCKED frames for a stream in this state. MAX_STREAM_DATA frames might be received until the peer receives the final stream offset. The endpoint can safely ignore any MAX_STREAM_DATA frames it receives from its peer for a stream in this state.¶

Once all stream data has been successfully acknowledged, the sending part of the stream enters the “Data Recvd” state, which is a terminal state.¶

From any of the “Ready”, “Send”, or “Data Sent” states, an application can signal that it wishes to abandon transmission of stream data. Alternatively, an endpoint might receive a STOP_SENDING frame from its peer. In either case, the endpoint sends a RESET_STREAM frame, which causes the stream to enter the “Reset Sent” state.¶

An endpoint MAY send a RESET_STREAM as the first frame that mentions a stream; this causes the sending part of that stream to open and then immediately transition to the “Reset Sent” state.¶

Once a packet containing a RESET_STREAM has been acknowledged, the sending part of the stream enters the “Reset Recvd” state, which is a terminal state.¶

Figure 3 shows the states for the part of a stream that receives data from a peer. The states for a receiving part of a stream mirror only some of the states of the sending part of the stream at the peer. The receiving part of a stream does not track states on the sending part that cannot be observed, such as the “Ready” state. Instead, the receiving part of a stream tracks the delivery of data to the application, some of which cannot be observed by the sender.¶

The receiving part of a stream initiated by a peer (types 1 and 3 for a client, or 0 and 2 for a server) is created when the first STREAM, STREAM_DATA_BLOCKED, or RESET_STREAM frame is received for that stream. For bidirectional streams initiated by a peer, receipt of a MAX_STREAM_DATA or STOP_SENDING frame for the sending part of the stream also creates the receiving part. The initial state for the receiving part of a stream is “Recv”.¶

The receiving part of a stream enters the “Recv” state when the sending part of a bidirectional stream initiated by the endpoint (type 0 for a client, type 1 for a server) enters the “Ready” state.¶

An endpoint opens a bidirectional stream when a MAX_STREAM_DATA or STOP_SENDING frame is received from the peer for that stream. Receiving a MAX_STREAM_DATA frame for an unopened stream indicates that the remote peer has opened the stream and is providing flow control credit. Receiving a STOP_SENDING frame for an unopened stream indicates that the remote peer no longer wishes to receive data on this stream. Either frame might arrive before a STREAM or STREAM_DATA_BLOCKED frame if packets are lost or reordered.¶

Before a stream is created, all streams of the same type with lower-numbered stream IDs MUST be created. This ensures that the creation order for streams is consistent on both endpoints.¶

In the “Recv” state, the endpoint receives STREAM and STREAM_DATA_BLOCKED frames. Incoming data is buffered and can be reassembled into the correct order for delivery to the application. As data is consumed by the application and buffer space becomes available, the endpoint sends MAX_STREAM_DATA frames to allow the peer to send more data.¶

When a STREAM frame with a FIN bit is received, the final size of the stream is known; see Section 4.4. The receiving part of the stream then enters the “Size Known” state. In this state, the endpoint no longer needs to send MAX_STREAM_DATA frames, it only receives any retransmissions of stream data.¶

Once all data for the stream has been received, the receiving part enters the “Data Recvd” state. This might happen as a result of receiving the same STREAM frame that causes the transition to “Size Known”. After all data has been received, any STREAM or STREAM_DATA_BLOCKED frames for the stream can be discarded.¶

The “Data Recvd” state persists until stream data has been delivered to the application. Once stream data has been delivered, the stream enters the “Data Read” state, which is a terminal state.¶

Receiving a RESET_STREAM frame in the “Recv” or “Size Known” states causes the stream to enter the “Reset Recvd” state. This might cause the delivery of stream data to the application to be interrupted.¶

It is possible that all stream data has already been received when a RESET_STREAM is received (that is, in the “Data Recvd” state). Similarly, it is possible for remaining stream data to arrive after receiving a RESET_STREAM frame (the “Reset Recvd” state). An implementation is free to manage this situation as it chooses.¶

Sending RESET_STREAM means that an endpoint cannot guarantee delivery of stream data; however there is no requirement that stream data not be delivered if a RESET_STREAM is received. An implementation MAY interrupt delivery of stream data, discard any data that was not consumed, and signal the receipt of the RESET_STREAM. A RESET_STREAM signal might be suppressed or withheld if stream data is completely received and is buffered to be read by the application. If the RESET_STREAM is suppressed, the receiving part of the stream remains in “Data Recvd”.¶

Once the application receives the signal indicating that the stream was reset, the receiving part of the stream transitions to the “Reset Read” state, which is a terminal state.¶

The sender of a stream sends just three frame types that affect the state of a stream at either sender or receiver: STREAM (Section 19.8), STREAM_DATA_BLOCKED (Section 19.13), and RESET_STREAM (Section 19.4).¶

A sender MUST NOT send any of these frames from a terminal state (“Data Recvd” or “Reset Recvd”). A sender MUST NOT send a STREAM or STREAM_DATA_BLOCKED frame for a stream in the “Reset Sent” state or any terminal state, that is, after sending a RESET_STREAM frame. A receiver could receive any of these three frames in any state, due to the possibility of delayed delivery of packets carrying them.¶

The receiver of a stream sends MAX_STREAM_DATA (Section 19.10) and STOP_SENDING frames (Section 19.5).¶

The receiver only sends MAX_STREAM_DATA in the “Recv” state. A receiver MAY send STOP_SENDING in any state where it has not received a RESET_STREAM frame; that is states other than “Reset Recvd” or “Reset Read”. However there is little value in sending a STOP_SENDING frame in the “Data Recvd” state, since all stream data has been received. A sender could receive either of these two frames in any state as a result of delayed delivery of packets.¶

A bidirectional stream is composed of sending and receiving parts. Implementations may represent states of the bidirectional stream as composites of sending and receiving stream states. The simplest model presents the stream as “open” when either sending or receiving parts are in a non-terminal state and “closed” when both sending and receiving streams are in terminal states.¶

Table 2 shows a more complex mapping of bidirectional stream states that loosely correspond to the stream states in HTTP/2 [HTTP2]. This shows that multiple states on sending or receiving parts of streams are mapped to the same composite state. Note that this is just one possibility for such a mapping; this mapping requires that data is acknowledged before the transition to a “closed” or “half-closed” state.¶

Note (*1):

A stream is considered “idle” if it has not yet been created, or if the receiving part of the stream is in the “Recv” state without yet having received any frames.

If an application is no longer interested in the data it is receiving on a stream, it can abort reading the stream and specify an application error code.¶

If the stream is in the “Recv” or “Size Known” states, the transport SHOULD signal this by sending a STOP_SENDING frame to prompt closure of the stream in the opposite direction. This typically indicates that the receiving application is no longer reading data it receives from the stream, but it is not a guarantee that incoming data will be ignored.¶

STREAM frames received after sending a STOP_SENDING frame are still counted toward connection and stream flow control, even though these frames can be discarded upon receipt.¶

A STOP_SENDING frame requests that the receiving endpoint send a RESET_STREAM frame. An endpoint that receives a STOP_SENDING frame MUST send a RESET_STREAM frame if the stream is in the Ready or Send state. If the stream is in the “Data Sent” state, an endpoint MAY defer sending the RESET_STREAM frame until the packets containing outstanding data are acknowledged or declared lost. If any outstanding data is declared lost, the endpoint SHOULD send a RESET_STREAM frame instead of retransmitting the data.¶

An endpoint SHOULD copy the error code from the STOP_SENDING frame to the RESET_STREAM frame it sends, but MAY use any application error code. The endpoint that sends a STOP_SENDING frame MAY ignore the error code carried in any RESET_STREAM frame it receives.¶

STOP_SENDING SHOULD only be sent for a stream that has not been reset by the peer. STOP_SENDING is most useful for streams in the “Recv” or “Size Known” states.¶

An endpoint is expected to send another STOP_SENDING frame if a packet containing a previous STOP_SENDING is lost. However, once either all stream data or a RESET_STREAM frame has been received for the stream - that is, the stream is in any state other than “Recv” or “Size Known” - sending a STOP_SENDING frame is unnecessary.¶

An endpoint that wishes to terminate both directions of a bidirectional stream can terminate one direction by sending a RESET_STREAM frame, and it can encourage prompt termination in the opposite direction by sending a STOP_SENDING frame.¶

• Stream flow control, which prevents a single stream from consuming the entire receive buffer for a connection by limiting the amount of data that can be sent on any stream.
• Connection flow control, which prevents senders from exceeding a receiver’s buffer capacity for the connection, by limiting the total bytes of stream data sent in STREAM frames on all streams.

Senders MUST NOT send data in excess of either limit.¶

A receiver sets initial limits for all streams by sending transport parameters during the handshake (Section 7.4). A receiver sends MAX_STREAM_DATA (Section 19.10) or MAX_DATA (Section 19.9) frames to the sender to advertise larger limits.¶

A receiver can advertise a larger limit for a stream by sending a MAX_STREAM_DATA frame with the corresponding stream ID. A MAX_STREAM_DATA frame indicates the maximum absolute byte offset of a stream. A receiver could use the current offset of data consumed to determine the flow control offset to be advertised.¶

A receiver can advertise a larger limit for a connection by sending a MAX_DATA frame, which indicates the maximum of the sum of the absolute byte offsets of all streams. A receiver maintains a cumulative sum of bytes received on all streams, which is used to check for violations of the advertised connection or stream data limits. A receiver might use a sum of bytes consumed on all streams to determine the maximum data limit to be advertised.¶

Once a receiver advertises a limit for the connection or a stream, it MAY advertise a smaller limit, but this has no effect.¶

A receiver MUST close the connection with a FLOW_CONTROL_ERROR error (Section 11) if the sender violates the advertised connection or stream data limits.¶

A sender MUST ignore any MAX_STREAM_DATA or MAX_DATA frames that do not increase flow control limits.¶

If a sender has sent data up to the limit, it will be unable to send new data and is considered blocked. A sender SHOULD send a STREAM_DATA_BLOCKED or DATA_BLOCKED frame to indicate it has data to write but is blocked by flow control limits. If a sender is blocked for a period longer than the idle timeout (Section 10.1), the receiver might close the connection even when the sender has data that is available for transmission. To keep the connection from closing, a sender that is flow control limited SHOULD periodically send a STREAM_DATA_BLOCKED or DATA_BLOCKED frame when it has no ack-eliciting packets in flight.¶

Implementations decide when and how much credit to advertise in MAX_STREAM_DATA and MAX_DATA frames, but this section offers a few considerations.¶

To avoid blocking a sender, a receiver MAY send a MAX_STREAM_DATA or MAX_DATA frame multiple times within a round trip or send it early enough to allow for recovery from loss of the frame.¶

Control frames contribute to connection overhead. Therefore, frequently sending MAX_STREAM_DATA and MAX_DATA frames with small changes is undesirable. On the other hand, if updates are less frequent, larger increments to limits are necessary to avoid blocking a sender, requiring larger resource commitments at the receiver. There is a trade-off between resource commitment and overhead when determining how large a limit is advertised.¶

A receiver can use an autotuning mechanism to tune the frequency and amount of advertised additional credit based on a round-trip time estimate and the rate at which the receiving application consumes data, similar to common TCP implementations. As an optimization, an endpoint could send frames related to flow control only when there are other frames to send or when a peer is blocked, ensuring that flow control does not cause extra packets to be sent.¶

A blocked sender is not required to send STREAM_DATA_BLOCKED or DATA_BLOCKED frames. Therefore, a receiver MUST NOT wait for a STREAM_DATA_BLOCKED or DATA_BLOCKED frame before sending a MAX_STREAM_DATA or MAX_DATA frame; doing so could result in the sender being blocked for the rest of the connection. Even if the sender sends these frames, waiting for them will result in the sender being blocked for at least an entire round trip.¶

When a sender receives credit after being blocked, it might be able to send a large amount of data in response, resulting in short-term congestion; see Section 6.9 in [QUIC-RECOVERY] for a discussion of how a sender can avoid this congestion.¶

Endpoints need to eventually agree on the amount of flow control credit that has been consumed, to avoid either exceeding flow control limits or deadlocking.¶

On receipt of a RESET_STREAM frame, an endpoint will tear down state for the matching stream and ignore further data arriving on that stream.¶

RESET_STREAM terminates one direction of a stream abruptly. For a bidirectional stream, RESET_STREAM has no effect on data flow in the opposite direction. Both endpoints MUST maintain flow control state for the stream in the unterminated direction until that direction enters a terminal state, or until one of the endpoints sends CONNECTION_CLOSE.¶

The final size is the amount of flow control credit that is consumed by a stream. Assuming that every contiguous byte on the stream was sent once, the final size is the number of bytes sent. More generally, this is one higher than the offset of the byte with the largest offset sent on the stream, or zero if no bytes were sent.¶

The final size of a stream is always signaled to the recipient. The final size is the sum of the Offset and Length fields of a STREAM frame with a FIN flag, noting that these fields might be implicit. Alternatively, the Final Size field of a RESET_STREAM frame carries this value. This ensures that all ways that a stream can be closed result in the number of bytes on the stream being reliably transmitted. This guarantees that both endpoints agree on how much flow control credit was consumed by the stream.¶

An endpoint will know the final size for a stream when the receiving part of the stream enters the “Size Known” or “Reset Recvd” state (Section 3). The receiver MUST use the final size of the stream to account for all bytes sent on the stream in its connection level flow controller.¶

An endpoint MUST NOT send data on a stream at or beyond the final size.¶

Once a final size for a stream is known, it cannot change. If a RESET_STREAM or STREAM frame is received indicating a change in the final size for the stream, an endpoint SHOULD respond with a FINAL_SIZE_ERROR error; see Section 11. A receiver SHOULD treat receipt of data at or beyond the final size as a FINAL_SIZE_ERROR error, even after a stream is closed. Generating these errors is not mandatory, but only because requiring that an endpoint generate these errors also means that the endpoint needs to maintain the final size state for closed streams, which could mean a significant state commitment.¶

An endpoint limits the cumulative number of incoming streams a peer can open. Only streams with a stream ID less than (max_stream * 4 + initial_stream_id_for_type) can be opened; see Table 1. Initial limits are set in the transport parameters; see Section 18.2. Subsequent limits are advertised using MAX_STREAMS frames; see Section 19.11. Separate limits apply to unidirectional and bidirectional streams.¶

If a max_streams transport parameter or MAX_STREAMS frame is received with a value greater than 2^60, this would allow a maximum stream ID that cannot be expressed as a variable-length integer; see Section 16. If either is received, the connection MUST be closed immediately with a connection error of type FRAME_ENCODING_ERROR; see Section 10.2.¶

Endpoints MUST NOT exceed the limit set by their peer. An endpoint that receives a frame with a stream ID exceeding the limit it has sent MUST treat this as a connection error of type STREAM_LIMIT_ERROR (Section 11).¶

Once a receiver advertises a stream limit using the MAX_STREAMS frame, advertising a smaller limit has no effect. A receiver MUST ignore any MAX_STREAMS frame that does not increase the stream limit.¶

As with stream and connection flow control, this document leaves when and how many streams to advertise to a peer via MAX_STREAMS to implementations. Implementations might choose to increase limits as streams close to keep the number of streams available to peers roughly consistent.¶

An endpoint that is unable to open a new stream due to the peer’s limits SHOULD send a STREAMS_BLOCKED frame (Section 19.14). This signal is considered useful for debugging. An endpoint MUST NOT wait to receive this signal before advertising additional credit, since doing so will mean that the peer will be blocked for at least an entire round trip, and potentially for longer if the peer chooses not to send STREAMS_BLOCKED frames.¶

An endpoint that is unable to ensure that a peer has flow control credit on the order of the current BDP will have receive throughput limited by flow control. Lost packets can cause gaps in the receive buffer, delaying the application from consuming data and freeing up flow control window.¶

Sending timely updates of flow control limits can improve performance. Sending packets only to provide flow control updates can increase network load and adversely affect performance. Sending flow control updates along with other frames, such as ACK frames, reduces the cost of those updates.¶

Each connection possesses a set of connection identifiers, or connection IDs, each of which can identify the connection. Connection IDs are independently selected by endpoints; each endpoint selects the connection IDs that its peer uses.¶

The primary function of a connection ID is to ensure that changes in addressing at lower protocol layers (UDP, IP) do not cause packets for a QUIC connection to be delivered to the wrong endpoint. Each endpoint selects connection IDs using an implementation-specific (and perhaps deployment-specific) method that will allow packets with that connection ID to be routed back to the endpoint and to be identified by the endpoint upon receipt.¶

Connection IDs MUST NOT contain any information that can be used by an external observer (that is, one that does not cooperate with the issuer) to correlate them with other connection IDs for the same connection. As a trivial example, this means the same connection ID MUST NOT be issued more than once on the same connection.¶

Packets with long headers include Source Connection ID and Destination Connection ID fields. These fields are used to set the connection IDs for new connections; see Section 7.2 for details.¶

Packets with short headers (Section 17.3) only include the Destination Connection ID and omit the explicit length. The length of the Destination Connection ID field is expected to be known to endpoints. Endpoints using a load balancer that routes based on connection ID could agree with the load balancer on a fixed length for connection IDs, or agree on an encoding scheme. A fixed portion could encode an explicit length, which allows the entire connection ID to vary in length and still be used by the load balancer.¶

A Version Negotiation (Section 17.2.1) packet echoes the connection IDs selected by the client, both to ensure correct routing toward the client and to demonstrate that the packet is in response to a packet sent by the client.¶

A zero-length connection ID can be used when a connection ID is not needed to route to the correct endpoint. However, multiplexing connections on the same local IP address and port while using zero-length connection IDs will cause failures in the presence of peer connection migration, NAT rebinding, and client port reuse. An endpoint MUST NOT use the same IP address and port for multiple connections with zero-length connection IDs, unless it is certain that those protocol features are not in use.¶

When an endpoint uses a non-zero-length connection ID, it needs to ensure that the peer has a supply of connection IDs from which to choose for packets sent to the endpoint. These connection IDs are supplied by the endpoint using the NEW_CONNECTION_ID frame (Section 19.15).¶

Incoming packets are classified on receipt. Packets can either be associated with an existing connection, or - for servers - potentially create a new connection.¶

Endpoints try to associate a packet with an existing connection. If the packet has a non-zero-length Destination Connection ID corresponding to an existing connection, QUIC processes that packet accordingly. Note that more than one connection ID can be associated with a connection; see Section 5.1.¶

If the Destination Connection ID is zero length and the addressing information in the packet matches the addressing information the endpoint uses to identify a connection with a zero-length connection ID, QUIC processes the packet as part of that connection. An endpoint can use just destination IP and port or both source and destination addresses for identification, though this makes connections fragile as described in Section 5.1.¶

Endpoints can send a Stateless Reset (Section 10.3) for any packets that cannot be attributed to an existing connection. A stateless reset allows a peer to more quickly identify when a connection becomes unusable.¶

Packets that are matched to an existing connection are discarded if the packets are inconsistent with the state of that connection. For example, packets are discarded if they indicate a different protocol version than that of the connection, or if the removal of packet protection is unsuccessful once the expected keys are available.¶

Invalid packets that lack strong integrity protection, such as Initial, Retry, or Version Negotiation, MAY be discarded. An endpoint MUST generate a connection error if processing the contents of these packets prior to discovering an error resulted in changes to connection state that cannot be reverted.¶

This document does not define an API for QUIC, but instead defines a set of functions for QUIC connections that application protocols can rely upon. An application protocol can assume that an implementation of QUIC provides an interface that includes the operations described in this section. An implementation designed for use with a specific application protocol might provide only those operations that are used by that protocol.¶

• open a connection, which begins the exchange described in Section 7;
• enable Early Data when available; and
• be informed when Early Data has been accepted or rejected by a server.

• listen for incoming connections, which prepares for the exchange described in Section 7;
• if Early Data is supported, embed application-controlled data in the TLS resumption ticket sent to the client; and
• if Early Data is supported, retrieve application-controlled data from the client’s resumption ticket and enable rejecting Early Data based on that information.

• configure minimum values for the initial number of permitted streams of each type, as communicated in the transport parameters (Section 7.4);
• control resource allocation of various types, including flow control and the number of permitted streams of each type;
• identify whether the handshake has completed successfully or is still ongoing;
• keep a connection from silently closing, either by generating PING frames (Section 19.2) or by requesting that the transport send additional frames before the idle timeout expires (Section 10.1); and
• immediately close (Section 10.2) the connection.

If the version selected by the client is not acceptable to the server, the server responds with a Version Negotiation packet; see Section 17.2.1. This includes a list of versions that the server will accept. An endpoint MUST NOT send a Version Negotiation packet in response to receiving a Version Negotiation packet.¶

This system allows a server to process packets with unsupported versions without retaining state. Though either the Initial packet or the Version Negotiation packet that is sent in response could be lost, the client will send new packets until it successfully receives a response or it abandons the connection attempt. As a result, the client discards all state for the connection and does not send any more packets on the connection.¶

A server MAY limit the number of Version Negotiation packets it sends. For instance, a server that is able to recognize packets as 0-RTT might choose not to send Version Negotiation packets in response to 0-RTT packets with the expectation that it will eventually receive an Initial packet.¶

Version Negotiation packets are designed to allow future versions of QUIC to negotiate the version in use between endpoints. Future versions of QUIC might change how implementations that support multiple versions of QUIC react to Version Negotiation packets when attempting to establish a connection using this version.¶

A client that supports only this version of QUIC MUST abandon the current connection attempt if it receives a Version Negotiation packet, with the following two exceptions. A client MUST discard any Version Negotiation packet if it has received and successfully processed any other packet, including an earlier Version Negotiation packet. A client MUST discard a Version Negotiation packet that lists the QUIC version selected by the client.¶

How to perform version negotiation is left as future work defined by future versions of QUIC. In particular, that future work will ensure robustness against version downgrade attacks; see Section 21.11.¶

For a server to use a new version in the future, clients need to correctly handle unsupported versions. Some version numbers (0x?a?a?a?a as defined in Section 15) are reserved for inclusion in fields that contain version numbers.¶

Endpoints MAY add reserved versions to any field where unknown or unsupported versions are ignored to test that a peer correctly ignores the value. For instance, an endpoint could include a reserved version in a Version Negotiation packet; see Section 17.2.1. Endpoints MAY send packets with a reserved version to test that a peer correctly discards the packet.¶

Details of how TLS is integrated with QUIC are provided in [QUIC-TLS], but some examples are provided here. An extension of this exchange to support client address validation is shown in Section 8.1.2.¶

Once any address validation exchanges are complete, the cryptographic handshake is used to agree on cryptographic keys. The cryptographic handshake is carried in Initial (Section 17.2.2) and Handshake (Section 17.2.4) packets.¶

Figure 5 provides an overview of the 1-RTT handshake. Each line shows a QUIC packet with the packet type and packet number shown first, followed by the frames that are typically contained in those packets. So, for instance the first packet is of type Initial, with packet number 0, and contains a CRYPTO frame carrying the ClientHello.¶

Multiple QUIC packets – even of different packet types – can be coalesced into a single UDP datagram; see Section 12.2. As a result, this handshake may consist of as few as 4 UDP datagrams, or any number more (subject to limits inherent to the protocol, such as congestion control or anti-amplification). For instance, the server’s first flight contains Initial packets, Handshake packets, and “0.5-RTT data” in 1-RTT packets with a short header.¶

Figure 6 shows an example of a connection with a 0-RTT handshake and a single packet of 0-RTT data. Note that as described in Section 12.3, the server acknowledges 0-RTT data in 1-RTT packets, and the client sends 1-RTT packets in the same packet number space.¶

A connection ID is used to ensure consistent routing of packets, as described in Section 5.1. The long header contains two connection IDs: the Destination Connection ID is chosen by the recipient of the packet and is used to provide consistent routing; the Source Connection ID is used to set the Destination Connection ID used by the peer.¶

During the handshake, packets with the long header (Section 17.2) are used to establish the connection IDs used by both endpoints. Each endpoint uses the Source Connection ID field to specify the connection ID that is used in the Destination Connection ID field of packets being sent to them. After processing the first Initial packet, each endpoint sets the Destination Connection ID field in subsequent packets it sends to the value of the Source Connection ID field that it received.¶

When an Initial packet is sent by a client that has not previously received an Initial or Retry packet from the server, the client populates the Destination Connection ID field with an unpredictable value. This Destination Connection ID MUST be at least 8 bytes in length. Until a packet is received from the server, the client MUST use the same Destination Connection ID value on all packets in this connection.¶

The Destination Connection ID field from the first Initial packet sent by a client is used to determine packet protection keys for Initial packets. These keys change after receiving a Retry packet; see Section 5.2 of [QUIC-TLS].¶

The client populates the Source Connection ID field with a value of its choosing and sets the Source Connection ID Length field to indicate the length.¶

The first flight of 0-RTT packets use the same Destination Connection ID and Source Connection ID values as the client’s first Initial packet.¶

Upon first receiving an Initial or Retry packet from the server, the client uses the Source Connection ID supplied by the server as the Destination Connection ID for subsequent packets, including any 0-RTT packets. This means that a client might have to change the connection ID it sets in the Destination Connection ID field twice during connection establishment: once in response to a Retry, and once in response to an Initial packet from the server. Once a client has received a valid Initial packet from the server, it MUST discard any subsequent packet it receives with a different Source Connection ID.¶

A client MUST change the Destination Connection ID it uses for sending packets in response to only the first received Initial or Retry packet. A server MUST set the Destination Connection ID it uses for sending packets based on the first received Initial packet. Any further changes to the Destination Connection ID are only permitted if the values are taken from NEW_CONNECTION_ID frames; if subsequent Initial packets include a different Source Connection ID, they MUST be discarded. This avoids unpredictable outcomes that might otherwise result from stateless processing of multiple Initial packets with different Source Connection IDs.¶

The Destination Connection ID that an endpoint sends can change over the lifetime of a connection, especially in response to connection migration (Section 9); see Section 5.1.1 for details.¶

The choice each endpoint makes about connection IDs during the handshake is authenticated by including all values in transport parameters; see Section 7.4. This ensures that all connection IDs used for the handshake are also authenticated by the cryptographic handshake.¶

Each endpoint includes the value of the Source Connection ID field from the first Initial packet it sent in the initial_source_connection_id transport parameter; see Section 18.2. A server includes the Destination Connection ID field from the first Initial packet it received from the client in the original_destination_connection_id transport parameter; if the server sent a Retry packet, this refers to the first Initial packet received before sending the Retry packet. If it sends a Retry packet, a server also includes the Source Connection ID field from the Retry packet in the retry_source_connection_id transport parameter.¶

The values provided by a peer for these transport parameters MUST match the values that an endpoint used in the Destination and Source Connection ID fields of Initial packets that it sent. Including connection ID values in transport parameters and verifying them ensures that that an attacker cannot influence the choice of connection ID for a successful connection by injecting packets carrying attacker-chosen connection IDs during the handshake.¶

An endpoint MUST treat absence of the initial_source_connection_id transport parameter from either endpoint or absence of the original_destination_connection_id transport parameter from the server as a connection error of type TRANSPORT_PARAMETER_ERROR.¶

• absence of the retry_source_connection_id transport parameter from the server after receiving a Retry packet,
• presence of the retry_source_connection_id transport parameter when no Retry packet was received, or
• a mismatch between values received from a peer in these transport parameters and the value sent in the corresponding Destination or Source Connection ID fields of Initial packets.

If a zero-length connection ID is selected, the corresponding transport parameter is included with a zero-length value.¶

Figure 7 shows the connection IDs (with DCID=Destination Connection ID, SCID=Source Connection ID) that are used in a complete handshake. The exchange of Initial packets is shown, plus the later exchange of 1-RTT packets that includes the connection ID established during the handshake.¶

Figure 8 shows a similar handshake that includes a Retry packet.¶

In both cases (Figure 7 and Figure 8), the client sets the value of the initial_source_connection_id transport parameter to C1.¶

When the handshake does not include a Retry (Figure 7), the server sets original_destination_connection_id to S1 and initial_source_connection_id to S3. In this case, the server does not include a retry_source_connection_id transport parameter.¶

When the handshake includes a Retry (Figure 8), the server sets original_destination_connection_id to S1, retry_source_connection_id to S2, and initial_source_connection_id to S3.¶

Each endpoint validates transport parameters set by the peer. The client confirms that the retry_source_connection_id transport parameter is absent if it did not process a Retry packet.¶

During connection establishment, both endpoints make authenticated declarations of their transport parameters. Endpoints are required to comply with the restrictions that each parameter defines; the description of each parameter includes rules for its handling.¶

Transport parameters are declarations that are made unilaterally by each endpoint. Each endpoint can choose values for transport parameters independent of the values chosen by its peer.¶

The encoding of the transport parameters is detailed in Section 18.¶

QUIC includes the encoded transport parameters in the cryptographic handshake. Once the handshake completes, the transport parameters declared by the peer are available. Each endpoint validates the values provided by its peer.¶

Definitions for each of the defined transport parameters are included in Section 18.2.¶

An endpoint MUST treat receipt of a transport parameter with an invalid value as a connection error of type TRANSPORT_PARAMETER_ERROR.¶

An endpoint MUST NOT send a parameter more than once in a given transport parameters extension. An endpoint SHOULD treat receipt of duplicate transport parameters as a connection error of type TRANSPORT_PARAMETER_ERROR.¶

Endpoints use transport parameters to authenticate the negotiation of connection IDs during the handshake; see Section 7.3.¶

Application Layer Protocol Negotiation (ALPN; see [ALPN]) allows clients to offer multiple application protocols during connection establishment. The transport parameters that a client includes during the handshake apply to all application protocols that the client offers. Application protocols can recommend values for transport parameters, such as the initial flow control limits. However, application protocols that set constraints on values for transport parameters could make it impossible for a client to offer multiple application protocols if these constraints conflict.¶

Implementations need to maintain a buffer of CRYPTO data received out of order. Because there is no flow control of CRYPTO frames, an endpoint could potentially force its peer to buffer an unbounded amount of data.¶

Implementations MUST support buffering at least 4096 bytes of data received in out-of-order CRYPTO frames. Endpoints MAY choose to allow more data to be buffered during the handshake. A larger limit during the handshake could allow for larger keys or credentials to be exchanged. An endpoint’s buffer size does not need to remain constant during the life of the connection.¶

Being unable to buffer CRYPTO frames during the handshake can lead to a connection failure. If an endpoint’s buffer is exceeded during the handshake, it can expand its buffer temporarily to complete the handshake. If an endpoint does not expand its buffer, it MUST close the connection with a CRYPTO_BUFFER_EXCEEDED error code.¶

Once the handshake completes, if an endpoint is unable to buffer all data in a CRYPTO frame, it MAY discard that CRYPTO frame and all CRYPTO frames received in the future, or it MAY close the connection with a CRYPTO_BUFFER_EXCEEDED error code. Packets containing discarded CRYPTO frames MUST be acknowledged because the packet has been received and processed by the transport even though the CRYPTO frame was discarded.¶

Connection establishment implicitly provides address validation for both endpoints. In particular, receipt of a packet protected with Handshake keys confirms that the client received the Initial packet from the server. Once the server has successfully processed a Handshake packet from the client, it can consider the client address to have been validated.¶

Additionally, a server MAY consider the client address validated if the client uses a connection ID chosen by the server and the connection ID contains at least 64 bits of entropy.¶

Prior to validating the client address, servers MUST NOT send more than three times as many bytes as the number of bytes they have received. This limits the magnitude of any amplification attack that can be mounted using spoofed source addresses. For the purposes of avoiding amplification prior to address validation, servers MUST count all of the payload bytes received in datagrams that are uniquely attributed to a single connection. This includes datagrams that contain packets that are successfully processed and datagrams that contain packets that are all discarded.¶

Clients MUST ensure that UDP datagrams containing Initial packets have UDP payloads of at least 1200 bytes, adding padding to packets in the datagram as necessary. A client that sends padded datagrams allows the server to send more data prior to completing address validation.¶

Loss of an Initial or Handshake packet from the server can cause a deadlock if the client does not send additional Initial or Handshake packets. A deadlock could occur when the server reaches its anti-amplification limit and the client has received acknowledgements for all the data it has sent. In this case, when the client has no reason to send additional packets, the server will be unable to send more data because it has not validated the client’s address. To prevent this deadlock, clients MUST send a packet on a probe timeout (PTO, see Section 6.2 of [QUIC-RECOVERY]). Specifically, the client MUST send an Initial packet in a UDP datagram that contains at least 1200 bytes if it does not have Handshake keys, and otherwise send a Handshake packet.¶

A server might wish to validate the client address before starting the cryptographic handshake. QUIC uses a token in the Initial packet to provide address validation prior to completing the handshake. This token is delivered to the client during connection establishment with a Retry packet (see Section 8.1.2) or in a previous connection using the NEW_TOKEN frame (see Section 8.1.3).¶

In addition to sending limits imposed prior to address validation, servers are also constrained in what they can send by the limits set by the congestion controller. Clients are only constrained by the congestion controller.¶

Path validation is used during connection migration (see Section 9) by the migrating endpoint to verify reachability of a peer from a new local address. In path validation, endpoints test reachability between a specific local address and a specific peer address, where an address is the two-tuple of IP address and port.¶

Path validation tests that packets sent on a path to a peer are received by that peer. Path validation is used to ensure that packets received from a migrating peer do not carry a spoofed source address.¶

Path validation does not validate that a peer can send in the return direction. The peer performs independent validation of the return path.¶

Path validation can be used at any time by either endpoint. For instance, an endpoint might check that a peer is still in possession of its address after a period of quiescence.¶

Path validation is not designed as a NAT traversal mechanism. Though the mechanism described here might be effective for the creation of NAT bindings that support NAT traversal, the expectation is that one or other peer is able to receive packets without first having sent a packet on that path. Effective NAT traversal needs additional synchronization mechanisms that are not provided here.¶

An endpoint MAY include PATH_CHALLENGE and PATH_RESPONSE frames that are used for path validation with other frames. In particular, an endpoint can pad a packet carrying a PATH_CHALLENGE for Path Maximum Transfer Unit (PMTU) discovery (see Section 14.2.1), or an endpoint can include a PATH_RESPONSE with its own PATH_CHALLENGE.¶

When probing a new path, an endpoint might want to ensure that its peer has an unused connection ID available for responses. The endpoint can send NEW_CONNECTION_ID and PATH_CHALLENGE frames in the same packet. This ensures that an unused connection ID will be available to the peer when sending a response.¶

To initiate path validation, an endpoint sends a PATH_CHALLENGE frame containing an unpredictable payload on the path to be validated.¶

An endpoint MAY send multiple PATH_CHALLENGE frames to guard against packet loss. However, an endpoint SHOULD NOT send multiple PATH_CHALLENGE frames in a single packet. An endpoint SHOULD NOT send a PATH_CHALLENGE more frequently than it would an Initial packet, ensuring that connection migration is no more load on a new path than establishing a new connection.¶

The endpoint MUST use unpredictable data in every PATH_CHALLENGE frame so that it can associate the peer’s response with the corresponding PATH_CHALLENGE.¶

On receiving a PATH_CHALLENGE frame, an endpoint MUST respond by echoing the data contained in the PATH_CHALLENGE frame in a PATH_RESPONSE frame. A PATH_RESPONSE frame does not need to be sent on the network path where the PATH_CHALLENGE was received; a PATH_RESPONSE can be sent on any network path. An endpoint MUST NOT delay transmission of a packet containing a PATH_RESPONSE frame unless constrained by congestion control.¶

An endpoint MUST NOT send more than one PATH_RESPONSE frame in response to one PATH_CHALLENGE frame; see Section 13.3. The peer is expected to send more PATH_CHALLENGE frames as necessary to evoke additional PATH_RESPONSE frames.¶

Path validation succeeds when a PATH_RESPONSE frame is received that contains the data that was sent in a previous PATH_CHALLENGE frame. This validates the path on which the PATH_CHALLENGE was sent.¶

Receipt of an acknowledgment for a packet containing a PATH_CHALLENGE frame is not adequate validation, since the acknowledgment can be spoofed by a malicious peer.¶

Path validation only fails when the endpoint attempting to validate the path abandons its attempt to validate the path.¶

This timeout allows for multiple PTOs to expire prior to failing path validation, so that loss of a single PATH_CHALLENGE or PATH_RESPONSE frame does not cause path validation failure.¶

Note that the endpoint might receive packets containing other frames on the new path, but a PATH_RESPONSE frame with appropriate data is required for path validation to succeed.¶

When an endpoint abandons path validation, it determines that the path is unusable. This does not necessarily imply a failure of the connection - endpoints can continue sending packets over other paths as appropriate. If no paths are available, an endpoint can wait for a new path to become available or close the connection.¶

A path validation might be abandoned for other reasons besides failure. Primarily, this happens if a connection migration to a new path is initiated while a path validation on the old path is in progress.¶

An endpoint MAY probe for peer reachability from a new local address using path validation (Section 8.2) prior to migrating the connection to the new local address. Failure of path validation simply means that the new path is not usable for this connection. Failure to validate a path does not cause the connection to end unless there are no valid alternative paths available.¶

An endpoint uses a new connection ID for probes sent from a new local address; see Section 9.5 for further discussion. An endpoint that uses a new local address needs to ensure that at least one new connection ID is available at the peer. That can be achieved by including a NEW_CONNECTION_ID frame in the probe.¶

Receiving a PATH_CHALLENGE frame from a peer indicates that the peer is probing for reachability on a path. An endpoint sends a PATH_RESPONSE frame in response, as per Section 8.2.¶

PATH_CHALLENGE, PATH_RESPONSE, NEW_CONNECTION_ID, and PADDING frames are “probing frames”, and all other frames are “non-probing frames”. A packet containing only probing frames is a “probing packet”, and a packet containing any other frame is a “non-probing packet”.¶

An endpoint can migrate a connection to a new local address by sending packets containing non-probing frames from that address.¶

Each endpoint validates its peer’s address during connection establishment. Therefore, a migrating endpoint can send to its peer knowing that the peer is willing to receive at the peer’s current address. Thus an endpoint can migrate to a new local address without first validating the peer’s address.¶

When migrating, the new path might not support the endpoint’s current sending rate. Therefore, the endpoint resets its congestion controller and RTT estimate, as described in Section 9.4.¶

The new path might not have the same ECN capability. Therefore, the endpoint verifies ECN capability as described in Section 13.4.¶

To establish reachability on the new path, an endpoint initiates path validation (Section 8.2) on the new path. An endpoint MAY defer path validation until after a peer sends the next non-probing frame to its new address.¶

Path validation is necessary to verify reachability of a peer on a new network path. Acknowledgments cannot be used for path validation as they contain insufficient entropy and might be spoofed. No method is provided to establish return reachability, as endpoints independently determine reachability on each direction of a path.¶

Receiving a packet from a new peer address containing a non-probing frame indicates that the peer has migrated to that address.¶

In response to such a packet, an endpoint MUST start sending subsequent packets to the new peer address and MUST initiate path validation (Section 8.2) to verify the peer’s ownership of the unvalidated address.¶

An endpoint MAY send data to an unvalidated peer address, but it MUST protect against potential attacks as described in Section 9.3.1 and Section 9.3.2. An endpoint MAY skip validation of a peer address if that address has been seen recently. In particular, if an endpoint returns to a previously-validated path after detecting some form of spurious migration, skipping address validation and restoring loss detection and congestion state can reduce the performance impact of the attack.¶

An endpoint only changes the address that it sends packets to in response to the highest-numbered non-probing packet. This ensures that an endpoint does not send packets to an old peer address in the case that it receives reordered packets.¶

After changing the address to which it sends non-probing packets, an endpoint could abandon any path validation for other addresses.¶

Receiving a packet from a new peer address might be the result of a NAT rebinding at the peer.¶

After verifying a new client address, the server SHOULD send new address validation tokens (Section 8) to the client.¶

The capacity available on the new path might not be the same as the old path. Packets sent on the old path MUST NOT contribute to congestion control or RTT estimation for the new path.¶

On confirming a peer’s ownership of its new address, an endpoint MUST immediately reset the congestion controller and round-trip time estimator for the new path to initial values (see Appendices A.3 and B.3 in [QUIC-RECOVERY]) unless the only change in the peer’s address is its port number. Because port-only changes are commonly the result of NAT rebinding or other middlebox activity, the endpoint MAY instead retain its congestion control state and round-trip estimate in those cases instead of reverting to initial values. In cases where congestion control state retained from an old path is used on a new path with substantially different characteristics, a sender may transmit too aggressively until the congestion controller and the RTT estimator have adapted. Generally, implementations are advised to be cautious when using previous values on a new path.¶

There may be apparent reordering at the receiver when an endpoint sends data and probes from/to multiple addresses during the migration period, since the two resulting paths may have different round-trip times. A receiver of packets on multiple paths will still send ACK frames covering all received packets.¶

While multiple paths might be used during connection migration, a single congestion control context and a single loss recovery context (as described in [QUIC-RECOVERY]) may be adequate. For instance, an endpoint might delay switching to a new congestion control context until it is confirmed that an old path is no longer needed (such as the case in Section 9.3.3).¶

A sender can make exceptions for probe packets so that their loss detection is independent and does not unduly cause the congestion controller to reduce its sending rate. An endpoint might set a separate timer when a PATH_CHALLENGE is sent, which is cancelled if the corresponding PATH_RESPONSE is received. If the timer fires before the PATH_RESPONSE is received, the endpoint might send a new PATH_CHALLENGE, and restart the timer for a longer period of time. This timer SHOULD be set as described in Section 6.2.1 of [QUIC-RECOVERY] and MUST NOT be more aggressive.¶

Using a stable connection ID on multiple network paths would allow a passive observer to correlate activity between those paths. An endpoint that moves between networks might not wish to have their activity correlated by any entity other than their peer, so different connection IDs are used when sending from different local addresses, as discussed in Section 5.1. For this to be effective, endpoints need to ensure that connection IDs they provide cannot be linked by any other entity.¶

At any time, endpoints MAY change the Destination Connection ID they transmit with to a value that has not been used on another path.¶

An endpoint MUST NOT reuse a connection ID when sending from more than one local address, for example when initiating connection migration as described in Section 9.2 or when probing a new network path as described in Section 9.1.¶

Similarly, an endpoint MUST NOT reuse a connection ID when sending to more than one destination address. Due to network changes outside the control of its peer, an endpoint might receive packets from a new source address with the same destination connection ID, in which case it MAY continue to use the current connection ID with the new remote address while still sending from the same local address.¶

These requirements regarding connection ID reuse apply only to the sending of packets, as unintentional changes in path without a change in connection ID are possible. For example, after a period of network inactivity, NAT rebinding might cause packets to be sent on a new path when the client resumes sending. An endpoint responds to such an event as described in Section 9.3.¶

Using different connection IDs for packets sent in both directions on each new network path eliminates the use of the connection ID for linking packets from the same connection across different network paths. Header protection ensures that packet numbers cannot be used to correlate activity. This does not prevent other properties of packets, such as timing and size, from being used to correlate activity.¶

An endpoint SHOULD NOT initiate migration with a peer that has requested a zero-length connection ID, because traffic over the new path might be trivially linkable to traffic over the old one. If the server is able to associate packets with a zero-length connection ID to the right connection, it means that the server is using other information to demultiplex packets. For example, a server might provide a unique address to every client, for instance using HTTP alternative services [ALTSVC]. Information that might allow correct routing of packets across multiple network paths will also allow activity on those paths to be linked by entities other than the peer.¶

A client might wish to reduce linkability by employing a new connection ID and source UDP port when sending traffic after a period of inactivity. Changing the UDP port from which it sends packets at the same time might cause the packet to appear as a connection migration. This ensures that the mechanisms that support migration are exercised even for clients that do not experience NAT rebindings or genuine migrations. Changing port number can cause a peer to reset its congestion state (see Section 9.4), so the port SHOULD only be changed infrequently.¶

An endpoint that exhausts available connection IDs cannot probe new paths or initiate migration, nor can it respond to probes or attempts by its peer to migrate. To ensure that migration is possible and packets sent on different paths cannot be correlated, endpoints SHOULD provide new connection IDs before peers migrate; see Section 5.1.1. If a peer might have exhausted available connection IDs, a migrating endpoint could include a NEW_CONNECTION_ID frame in all packets sent on a new network path.¶

QUIC allows servers to accept connections on one IP address and attempt to transfer these connections to a more preferred address shortly after the handshake. This is particularly useful when clients initially connect to an address shared by multiple servers but would prefer to use a unicast address to ensure connection stability. This section describes the protocol for migrating a connection to a preferred server address.¶

Migrating a connection to a new server address mid-connection is not supported by the version of QUIC specified in this document. If a client receives packets from a new server address when the client has not initiated a migration to that address, the client SHOULD discard these packets.¶

Endpoints that send data using IPv6 SHOULD apply an IPv6 flow label in compliance with [RFC6437], unless the local API does not allow setting IPv6 flow labels.¶

The IPv6 flow label SHOULD be a pseudo-random function of the source and destination addresses, source and destination UDP ports, and the Destination Connection ID field. The flow label generation MUST be designed to minimize the chances of linkability with a previously used flow label, as this would enable correlating activity on multiple paths; see Section 9.5.¶

A possible implementation is to compute the flow label as a cryptographic hash function of the source and destination addresses, source and destination UDP ports, Destination Connection ID field, and a local secret.¶

If a max_idle_timeout is specified by either peer in its transport parameters (Section 18.2), the connection is silently closed and its state is discarded when it remains idle for longer than the minimum of both peers max_idle_timeout values.¶

Each endpoint advertises a max_idle_timeout, but the effective value at an endpoint is computed as the minimum of the two advertised values. By announcing a max_idle_timeout, an endpoint commits to initiating an immediate close (Section 10.2) if it abandons the connection prior to the effective value.¶

An endpoint restarts its idle timer when a packet from its peer is received and processed successfully. An endpoint also restarts its idle timer when sending an ack-eliciting packet if no other ack-eliciting packets have been sent since last receiving and processing a packet. Restarting this timer when sending a packet ensures that connections are not closed after new activity is initiated.¶

To avoid excessively small idle timeout periods, endpoints MUST increase the idle timeout period to be at least three times the current Probe Timeout (PTO). This allows for multiple PTOs to expire prior to idle timeout, ensuring the idle timeout does not expire as a result of a single packet loss.¶

An endpoint sends a CONNECTION_CLOSE frame (Section 19.19) to terminate the connection immediately. A CONNECTION_CLOSE frame causes all streams to immediately become closed; open streams can be assumed to be implicitly reset.¶

After sending a CONNECTION_CLOSE frame, an endpoint immediately enters the closing state; see Section 10.2.1. After receiving a CONNECTION_CLOSE frame, endpoints enter the draining state; see Section 10.2.2.¶

An immediate close can be used after an application protocol has arranged to close a connection. This might be after the application protocol negotiates a graceful shutdown. The application protocol can exchange messages that are needed for both application endpoints to agree that the connection can be closed, after which the application requests that QUIC close the connection. When QUIC consequently closes the connection, a CONNECTION_CLOSE frame with an application-supplied error code will be used to signal closure to the peer.¶

The closing and draining connection states exist to ensure that connections close cleanly and that delayed or reordered packets are properly discarded. These states SHOULD persist for at least three times the current Probe Timeout (PTO) interval as defined in [QUIC-RECOVERY].¶

Disposing of connection state prior to exiting the closing or draining state could cause could result in an endpoint generating a stateless reset unnecessarily when it receives a late-arriving packet. Endpoints that have some alternative means to ensure that late-arriving packets do not induce a response, such as those that are able to close the UDP socket, MAY end these states earlier to allow for faster resource recovery. Servers that retain an open socket for accepting new connections SHOULD NOT end the closing or draining states early.¶

Once its closing or draining state ends, an endpoint SHOULD discard all connection state. The endpoint MAY send a stateless reset in response to any further incoming packets belonging to this connection.¶

A stateless reset is provided as an option of last resort for an endpoint that does not have access to the state of a connection. A crash or outage might result in peers continuing to send data to an endpoint that is unable to properly continue the connection. An endpoint MAY send a stateless reset in response to receiving a packet that it cannot associate with an active connection.¶

A stateless reset is not appropriate for signaling error conditions. An endpoint that wishes to communicate a fatal connection error MUST use a CONNECTION_CLOSE frame if it has sufficient state to do so.¶

To support this process, a token is sent by endpoints. The token is carried in the Stateless Reset Token field of a NEW_CONNECTION_ID frame. Servers can also specify a stateless_reset_token transport parameter during the handshake that applies to the connection ID that it selected during the handshake; clients cannot use this transport parameter because their transport parameters do not have confidentiality protection. These tokens are protected by encryption, so only client and server know their value. Tokens are invalidated when their associated connection ID is retired via a RETIRE_CONNECTION_ID frame (Section 19.16).¶

This design ensures that a stateless reset packet is - to the extent possible - indistinguishable from a regular packet with a short header.¶

A stateless reset uses an entire UDP datagram, starting with the first two bits of the packet header. The remainder of the first byte and an arbitrary number of bytes following it are set to values that SHOULD be indistinguishable from random. The last 16 bytes of the datagram contain a Stateless Reset Token.¶

To entities other than its intended recipient, a stateless reset will appear to be a packet with a short header. For the stateless reset to appear as a valid QUIC packet, the Unpredictable Bits field needs to include at least 38 bits of data (or 5 bytes, less the two fixed bits).¶

The resulting minimum size of 21 bytes does not guarantee that a stateless reset is difficult to distinguish from other packets if the recipient requires the use of a connection ID. To achieve that end, the endpoint SHOULD pad all packets it sends to at least 22 bytes longer than the minimum connection ID that it might request the peer to include in packets that the peer sends. This ensures that any stateless reset sent by the peer is indistinguishable from a valid packet sent to the endpoint. An endpoint that sends a stateless reset in response to a packet that is 43 bytes or shorter SHOULD send a stateless reset that is one byte shorter than the packet it responds to.¶

These values assume that the Stateless Reset Token is the same length as the minimum expansion of the packet protection AEAD. Additional unpredictable bytes are necessary if the endpoint could have negotiated a packet protection scheme with a larger minimum expansion.¶

An endpoint MUST NOT send a stateless reset that is three times or more larger than the packet it receives to avoid being used for amplification. Section 10.3.3 describes additional limits on stateless reset size.¶

Endpoints MUST discard packets that are too small to be valid QUIC packets. With the set of AEAD functions defined in [QUIC-TLS], packets that are smaller than 21 bytes are never valid.¶

Endpoints MUST send stateless reset packets formatted as a packet with a short header. However, endpoints MUST treat any packet ending in a valid stateless reset token as a stateless reset, as other QUIC versions might allow the use of a long header.¶

An endpoint MAY send a stateless reset in response to a packet with a long header. Sending a stateless reset is not effective prior to the stateless reset token being available to a peer. In this QUIC version, packets with a long header are only used during connection establishment. Because the stateless reset token is not available until connection establishment is complete or near completion, ignoring an unknown packet with a long header might be as effective as sending a stateless reset.¶

An endpoint cannot determine the Source Connection ID from a packet with a short header, therefore it cannot set the Destination Connection ID in the stateless reset packet. The Destination Connection ID will therefore differ from the value used in previous packets. A random Destination Connection ID makes the connection ID appear to be the result of moving to a new connection ID that was provided using a NEW_CONNECTION_ID frame (Section 19.15).¶

• The packet might not reach the peer. If the Destination Connection ID is critical for routing toward the peer, then this packet could be incorrectly routed. This might also trigger another Stateless Reset in response; see Section 10.3.3. A Stateless Reset that is not correctly routed is an ineffective error detection and recovery mechanism. In this case, endpoints will need to rely on other methods - such as timers - to detect that the connection has failed.
• The randomly generated connection ID can be used by entities other than the peer to identify this as a potential stateless reset. An endpoint that occasionally uses different connection IDs might introduce some uncertainty about this.

This stateless reset design is specific to QUIC version 1. An endpoint that supports multiple versions of QUIC needs to generate a stateless reset that will be accepted by peers that support any version that the endpoint might support (or might have supported prior to losing state). Designers of new versions of QUIC need to be aware of this and either reuse this design, or use a portion of the packet other than the last 16 bytes for carrying data.¶

Errors that result in the connection being unusable, such as an obvious violation of protocol semantics or corruption of state that affects an entire connection, MUST be signaled using a CONNECTION_CLOSE frame (Section 19.19). An endpoint MAY close the connection in this manner even if the error only affects a single stream.¶

Application-specific protocol errors are signaled using the CONNECTION_CLOSE frame with a frame type of 0x1d. Errors that are specific to the transport, including all those described in this document, are carried in the CONNECTION_CLOSE frame with a frame type of 0x1c.¶

A CONNECTION_CLOSE frame could be sent in a packet that is lost. An endpoint SHOULD be prepared to retransmit a packet containing a CONNECTION_CLOSE frame if it receives more packets on a terminated connection. Limiting the number of retransmissions and the time over which this final packet is sent limits the effort expended on terminated connections.¶

An endpoint that chooses not to retransmit packets containing a CONNECTION_CLOSE frame risks a peer missing the first such packet. The only mechanism available to an endpoint that continues to receive data for a terminated connection is to use the stateless reset process (Section 10.3).¶

If an application-level error affects a single stream, but otherwise leaves the connection in a recoverable state, the endpoint can send a RESET_STREAM frame (Section 19.4) with an appropriate error code to terminate just the affected stream.¶

Resetting a stream without the involvement of the application protocol could cause the application protocol to enter an unrecoverable state. RESET_STREAM MUST only be instigated by the application protocol that uses QUIC.¶

The semantics of the application error code carried in RESET_STREAM are defined by the application protocol. Only the application protocol is able to cause a stream to be terminated. A local instance of the application protocol uses a direct API call and a remote instance uses the STOP_SENDING frame, which triggers an automatic RESET_STREAM.¶

Application protocols SHOULD define rules for handling streams that are prematurely cancelled by either endpoint.¶

QUIC packets have different levels of cryptographic protection based on the type of packet. Details of packet protection are found in [QUIC-TLS]; this section includes an overview of the protections that are provided.¶

Version Negotiation packets have no cryptographic protection; see [QUIC-INVARIANTS].¶

Retry packets use an authenticated encryption with associated data function (AEAD; [AEAD]) to protect against accidental modification.¶

Initial packets use an AEAD, the keys for which are derived using a value that is visible on the wire. Initial packets therefore do not have effective confidentiality protection. Initial protection exists to ensure that the sender of the packet is on the network path. Any entity that receives an Initial packet from a client can recover the keys that will allow them to both read the contents of the packet and generate Initial packets that will be successfully authenticated at either endpoint.¶

All other packets are protected with keys derived from the cryptographic handshake. The cryptographic handshake ensures that only the communicating endpoints receive the corresponding keys for Handshake, 0-RTT, and 1-RTT packets. Packets protected with 0-RTT and 1-RTT keys have strong confidentiality and integrity protection.¶

The Packet Number field that appears in some packet types has alternative confidentiality protection that is applied as part of header protection; see Section 5.4 of [QUIC-TLS] for details. The underlying packet number increases with each packet sent in a given packet number space; see Section 12.3 for details.¶

Initial (Section 17.2.2), 0-RTT (Section 17.2.3), and Handshake (Section 17.2.4) packets contain a Length field that determines the end of the packet. The length includes both the Packet Number and Payload fields, both of which are confidentiality protected and initially of unknown length. The length of the Payload field is learned once header protection is removed.¶

Using the Length field, a sender can coalesce multiple QUIC packets into one UDP datagram. This can reduce the number of UDP datagrams needed to complete the cryptographic handshake and start sending data. This can also be used to construct PMTU probes; see Section 14.4.1. Receivers MUST be able to process coalesced packets.¶

Coalescing packets in order of increasing encryption levels (Initial, 0-RTT, Handshake, 1-RTT; see Section 4.1.4 of [QUIC-TLS]) makes it more likely the receiver will be able to process all the packets in a single pass. A packet with a short header does not include a length, so it can only be the last packet included in a UDP datagram. An endpoint SHOULD include multiple frames in a single packet if they are to be sent at the same encryption level, instead of coalescing multiple packets at the same encryption level.¶

Receivers MAY route based on the information in the first packet contained in a UDP datagram. Senders MUST NOT coalesce QUIC packets with different connection IDs into a single UDP datagram. Receivers SHOULD ignore any subsequent packets with a different Destination Connection ID than the first packet in the datagram.¶

Every QUIC packet that is coalesced into a single UDP datagram is separate and complete. The receiver of coalesced QUIC packets MUST individually process each QUIC packet and separately acknowledge them, as if they were received as the payload of different UDP datagrams. For example, if decryption fails (because the keys are not available or any other reason), the receiver MAY either discard or buffer the packet for later processing and MUST attempt to process the remaining packets.¶

Retry packets (Section 17.2.5), Version Negotiation packets (Section 17.2.1), and packets with a short header (Section 17.3) do not contain a Length field and so cannot be followed by other packets in the same UDP datagram. Note also that there is no situation where a Retry or Version Negotiation packet is coalesced with another packet.¶

The packet number is an integer in the range 0 to 2^62-1. This number is used in determining the cryptographic nonce for packet protection. Each endpoint maintains a separate packet number for sending and receiving.¶

Packet numbers are limited to this range because they need to be representable in whole in the Largest Acknowledged field of an ACK frame (Section 19.3). When present in a long or short header however, packet numbers are reduced and encoded in 1 to 4 bytes; see Section 17.1.¶

Version Negotiation (Section 17.2.1) and Retry (Section 17.2.5) packets do not include a packet number.¶

• Initial space: All Initial packets (Section 17.2.2) are in this space.
• Handshake space: All Handshake packets (Section 17.2.4) are in this space.
• Application data space: All 0-RTT (Section 17.2.3) and 1-RTT (Section 17.3) encrypted packets are in this space.

As described in [QUIC-TLS], each packet type uses different protection keys.¶

Conceptually, a packet number space is the context in which a packet can be processed and acknowledged. Initial packets can only be sent with Initial packet protection keys and acknowledged in packets that are also Initial packets. Similarly, Handshake packets are sent at the Handshake encryption level and can only be acknowledged in Handshake packets.¶

This enforces cryptographic separation between the data sent in the different packet number spaces. Packet numbers in each space start at packet number 0. Subsequent packets sent in the same packet number space MUST increase the packet number by at least one.¶

0-RTT and 1-RTT data exist in the same packet number space to make loss recovery algorithms easier to implement between the two packet types.¶

A QUIC endpoint MUST NOT reuse a packet number within the same packet number space in one connection. If the packet number for sending reaches 2^62 - 1, the sender MUST close the connection without sending a CONNECTION_CLOSE frame or any further packets; an endpoint MAY send a Stateless Reset (Section 10.3) in response to further packets that it receives.¶

A receiver MUST discard a newly unprotected packet unless it is certain that it has not processed another packet with the same packet number from the same packet number space. Duplicate suppression MUST happen after removing packet protection for the reasons described in Section 9.3 of [QUIC-TLS].¶

Endpoints that track all individual packets for the purposes of detecting duplicates are at risk of accumulating excessive state. The data required for detecting duplicates can be limited by maintaining a minimum packet number below which all packets are immediately dropped. Any minimum needs to account for large variations in round trip time, which includes the possibility that a peer might probe network paths with much larger round trip times; see Section 9.¶

Packet number encoding at a sender and decoding at a receiver are described in Section 17.1.¶

The payload of QUIC packets, after removing packet protection, consists of a sequence of complete frames, as shown in Figure 11. Version Negotiation, Stateless Reset, and Retry packets do not contain frames.¶

The payload of a packet that contains frames MUST contain at least one frame, and MAY contain multiple frames and multiple frame types. Frames always fit within a single QUIC packet and cannot span multiple packets.¶

The frame types defined in this specification are listed in Table 3. The Frame Type in ACK, STREAM, MAX_STREAMS, STREAMS_BLOCKED, and CONNECTION_CLOSE frames is used to carry other frame-specific flags. For all other frames, the Frame Type field simply identifies the frame. These frames are explained in more detail in Section 19.¶

I:

Initial (Section 17.2.2)

H:

Handshake (Section 17.2.4)

0:

0-RTT (Section 17.2.3)

1:

1-RTT (Section 17.3)

ih:

Only a CONNECTION_CLOSE frame of type 0x1c can appear in Initial or Handshake packets.

Section 4 of [QUIC-TLS] provides more detail about these restrictions. Note that all frames can appear in 1-RTT packets. An endpoint MUST treat receipt of a frame in a packet type that is not permitted as a connection error of type PROTOCOL_VIOLATION.¶

N:

Packets containing only frames with this marking are not ack-eliciting; see Section 13.2.

C:

Packets containing only frames with this marking do not count toward bytes in flight for congestion control purposes; see [QUIC-RECOVERY].

P:

Packets containing only frames with this marking can be used to probe new network paths during connection migration; see Section 9.1.

F:

The content of frames with this marking are flow controlled; see Section 4.

The “Pkts” and “Spec” columns in Table 3 do not form part of the IANA registry; see Section 22.3.¶

An endpoint MUST treat the receipt of a frame of unknown type as a connection error of type FRAME_ENCODING_ERROR.¶

All QUIC frames are idempotent in this version of QUIC. That is, a valid frame does not cause undesirable side effects or errors when received more than once.¶

The Frame Type field uses a variable-length integer encoding (see Section 16) with one exception. To ensure simple and efficient implementations of frame parsing, a frame type MUST use the shortest possible encoding. For frame types defined in this document, this means a single-byte encoding, even though it is possible to encode these values as a two-, four- or eight-byte variable-length integer. For instance, though 0x4001 is a legitimate two-byte encoding for a variable-length integer with a value of 1, PING frames are always encoded as a single byte with the value 0x01. This rule applies to all current and future QUIC frame types. An endpoint MAY treat the receipt of a frame type that uses a longer encoding than necessary as a connection error of type PROTOCOL_VIOLATION.¶

A packet MUST NOT be acknowledged until packet protection has been successfully removed and all frames contained in the packet have been processed. For STREAM frames, this means the data has been enqueued in preparation to be received by the application protocol, but it does not require that data is delivered and consumed.¶

Once the packet has been fully processed, a receiver acknowledges receipt by sending one or more ACK frames containing the packet number of the received packet.¶

An endpoint SHOULD treat receipt of an acknowledgment for a packet it did not send as a connection error of type PROTOCOL_VIOLATION, if it is able to detect the condition.¶

Endpoints acknowledge all packets they receive and process. However, only ack-eliciting packets cause an ACK frame to be sent within the maximum ack delay. Packets that are not ack-eliciting are only acknowledged when an ACK frame is sent for other reasons.¶

When sending a packet for any reason, an endpoint SHOULD attempt to include an ACK frame if one has not been sent recently. Doing so helps with timely loss detection at the peer.¶

In general, frequent feedback from a receiver improves loss and congestion response, but this has to be balanced against excessive load generated by a receiver that sends an ACK frame in response to every ack-eliciting packet. The guidance offered below seeks to strike this balance.¶

QUIC packets that are determined to be lost are not retransmitted whole. The same applies to the frames that are contained within lost packets. Instead, the information that might be carried in frames is sent again in new frames as needed.¶

New frames and packets are used to carry information that is determined to have been lost. In general, information is sent again when a packet containing that information is determined to be lost and sending ceases when a packet containing that information is acknowledged.¶

• Data sent in CRYPTO frames is retransmitted according to the rules in [QUIC-RECOVERY], until all data has been acknowledged. Data in CRYPTO frames for Initial and Handshake packets is discarded when keys for the corresponding packet number space are discarded.
• Application data sent in STREAM frames is retransmitted in new STREAM frames unless the endpoint has sent a RESET_STREAM for that stream. Once an endpoint sends a RESET_STREAM frame, no further STREAM frames are needed.
• ACK frames carry the most recent set of acknowledgements and the acknowledgement delay from the largest acknowledged packet, as described in Section 13.2.1. Delaying the transmission of packets containing ACK frames or resending old ACK frames can cause the peer to generate an inflated RTT sample or unnecessarily disable ECN.
• Cancellation of stream transmission, as carried in a RESET_STREAM frame, is sent until acknowledged or until all stream data is acknowledged by the peer (that is, either the “Reset Recvd” or “Data Recvd” state is reached on the sending part of the stream). The content of a RESET_STREAM frame MUST NOT change when it is sent again.
• Similarly, a request to cancel stream transmission, as encoded in a STOP_SENDING frame, is sent until the receiving part of the stream enters either a “Data Recvd” or “Reset Recvd” state; see Section 3.5.
• Connection close signals, including packets that contain CONNECTION_CLOSE frames, are not sent again when packet loss is detected, but as described in Section 10.
• The current connection maximum data is sent in MAX_DATA frames. An updated value is sent in a MAX_DATA frame if the packet containing the most recently sent MAX_DATA frame is declared lost, or when the endpoint decides to update the limit. Care is necessary to avoid sending this frame too often as the limit can increase frequently and cause an unnecessarily large number of MAX_DATA frames to be sent; see Section 4.2.
• The current maximum stream data offset is sent in MAX_STREAM_DATA frames. Like MAX_DATA, an updated value is sent when the packet containing the most recent MAX_STREAM_DATA frame for a stream is lost or when the limit is updated, with care taken to prevent the frame from being sent too often. An endpoint SHOULD stop sending MAX_STREAM_DATA frames when the receiving part of the stream enters a “Size Known” state.
• The limit on streams of a given type is sent in MAX_STREAMS frames. Like MAX_DATA, an updated value is sent when a packet containing the most recent MAX_STREAMS for a stream type frame is declared lost or when the limit is updated, with care taken to prevent the frame from being sent too often.
• Blocked signals are carried in DATA_BLOCKED, STREAM_DATA_BLOCKED, and STREAMS_BLOCKED frames. DATA_BLOCKED frames have connection scope, STREAM_DATA_BLOCKED frames have stream scope, and STREAMS_BLOCKED frames are scoped to a specific stream type. New frames are sent if packets containing the most recent frame for a scope is lost, but only while the endpoint is blocked on the corresponding limit. These frames always include the limit that is causing blocking at the time that they are transmitted.
• A liveness or path validation check using PATH_CHALLENGE frames is sent periodically until a matching PATH_RESPONSE frame is received or until there is no remaining need for liveness or path validation checking. PATH_CHALLENGE frames include a different payload each time they are sent.
• Responses to path validation using PATH_RESPONSE frames are sent just once. The peer is expected to send more PATH_CHALLENGE frames as necessary to evoke additional PATH_RESPONSE frames.
• New connection IDs are sent in NEW_CONNECTION_ID frames and retransmitted if the packet containing them is lost. Retransmissions of this frame carry the same sequence number value. Likewise, retired connection IDs are sent in RETIRE_CONNECTION_ID frames and retransmitted if the packet containing them is lost.
• NEW_TOKEN frames are retransmitted if the packet containing them is lost. No special support is made for detecting reordered and duplicated NEW_TOKEN frames other than a direct comparison of the frame contents.
• PING and PADDING frames contain no information, so lost PING or PADDING frames do not require repair.
• The HANDSHAKE_DONE frame MUST be retransmitted until it is acknowledged.

Endpoints SHOULD prioritize retransmission of data over sending new data, unless priorities specified by the application indicate otherwise; see Section 2.3.¶

Even though a sender is encouraged to assemble frames containing up-to-date information every time it sends a packet, it is not forbidden to retransmit copies of frames from lost packets. A sender that retransmits copies of frames needs to handle decreases in available payload size due to change in packet number length, connection ID length, and path MTU. A receiver MUST accept packets containing an outdated frame, such as a MAX_DATA frame carrying a smaller maximum data than one found in an older packet.¶

A sender SHOULD avoid retransmitting information from packets once they are acknowledged. This includes packets that are acknowledged after being declared lost, which can happen in the presence of network reordering. Doing so requires senders to retain information about packets after they are declared lost. A sender can discard this information after a period of time elapses that adequately allows for reordering, such as a PTO (Section 6.2 of [QUIC-RECOVERY]), or on other events, such as reaching a memory limit.¶

Upon detecting losses, a sender MUST take appropriate congestion control action. The details of loss detection and congestion control are described in [QUIC-RECOVERY].¶

QUIC endpoints can use Explicit Congestion Notification (ECN) [RFC3168] to detect and respond to network congestion. ECN allows a network node to indicate congestion in the network by setting a codepoint in the IP header of a packet instead of dropping it. Endpoints react to congestion by reducing their sending rate in response, as described in [QUIC-RECOVERY].¶

Note that supporting ECN requires being able to set and read the ECN codepoints in the IP headers of datagrams carrying QUIC packets. On platforms where this is not possible, QUIC cannot support ECN.¶

To use ECN, QUIC endpoints first determine whether a path supports ECN marking and the peer is able to access the ECN codepoint in the IP header. A network path does not support ECN if ECN marked packets get dropped or ECN markings are rewritten on the path. An endpoint validates the use of ECN on the path, both during connection establishment and when migrating to a new path (Section 9).¶

A client MUST expand the payload of all UDP datagrams carrying Initial packets to at least the smallest allowed maximum packet size (1200 bytes) by adding PADDING frames to the Initial packet or by coalescing the Initial packet; see Section 12.2. Sending a UDP datagram of this size ensures that the network path from the client to the server supports a reasonable Path Maximum Transmission Unit (PMTU). This also helps reduce the amplitude of amplification attacks caused by server responses toward an unverified client address; see Section 8.¶

Datagrams containing Initial packets MAY exceed 1200 bytes if the client believes that the network path and peer both support the size that it chooses.¶

A server MUST discard an Initial packet that is carried in a UDP datagram with a payload that is less than the smallest allowed maximum packet size of 1200 bytes. A server MAY also immediately close the connection by sending a CONNECTION_CLOSE frame with an error code of PROTOCOL_VIOLATION; see Section 10.2.3.¶

The server MUST also limit the number of bytes it sends before validating the address of the client; see Section 8.¶

The Path Maximum Transmission Unit (PMTU) is the maximum size of the entire IP packet including the IP header, UDP header, and UDP payload. The UDP payload includes the QUIC packet header, protected payload, and any authentication fields. The PMTU can depend on path characteristics, and can therefore change over time. The largest UDP payload an endpoint sends at any given time is referred to as the endpoint’s maximum packet size.¶

An endpoint SHOULD use DPLPMTUD (Section 14.3) or PMTUD (Section 14.2.1) to determine whether the path to a destination will support a desired maximum packet size without fragmentation. In the absence of these mechanisms, QUIC endpoints SHOULD NOT send IP packets larger than the smallest allowed maximum packet size.¶

Both DPLPMTUD and PMTUD send IP packets that are larger than the current maximum packet size, referred to as PMTU probes. All QUIC packets that are not sent in a PMTU probe SHOULD be sized to fit within the maximum packet size to avoid the packet being fragmented or dropped ([RFC8085]).¶

If a QUIC endpoint determines that the PMTU between any pair of local and remote IP addresses has fallen below the smallest allowed maximum packet size of 1200 bytes, it MUST immediately cease sending QUIC packets, except for those in PMTU probes or those containing CONNECTION_CLOSE frames, on the affected path. An endpoint MAY terminate the connection if an alternative path cannot be found.¶

Each pair of local and remote addresses could have a different PMTU. QUIC implementations that implement any kind of PMTU discovery therefore SHOULD maintain a maximum packet size for each combination of local and remote IP addresses.¶

A QUIC implementation MAY be more conservative in computing the maximum packet size to allow for unknown tunnel overheads or IP header options/extensions.¶

Datagram Packetization Layer PMTU Discovery (DPLPMTUD; [DPLPMTUD]) relies on tracking loss or acknowledgment of QUIC packets that are carried in PMTU probes. PMTU probes for DPLPMTUD that use the PADDING frame implement “Probing using padding data”, as defined in Section 4.1 of [DPLPMTUD].¶

Endpoints SHOULD set the initial value of BASE_PMTU (see Section 5.1 of [DPLPMTUD]) to be consistent with the minimum QUIC packet size. The MIN_PLPMTU is the same as the BASE_PMTU.¶

QUIC endpoints implementing DPLPMTUD maintain a maximum packet size (DPLPMTUD MPS) for each combination of local and remote IP addresses.¶

PMTU probes are ack-eliciting packets.¶

Endpoints could limit the content of PMTU probes to PING and PADDING frames as packets that are larger than the current maximum packet size are more likely to be dropped by the network. Loss of a QUIC packet that is carried in a PMTU probe is therefore not a reliable indication of congestion and SHOULD NOT trigger a congestion control reaction; see Section 3, Bullet 7 of [DPLPMTUD]. However, PMTU probes consume congestion window, which could delay subsequent transmission by an application.¶

Packet numbers are integers in the range 0 to 2^62-1 (Section 12.3). When present in long or short packet headers, they are encoded in 1 to 4 bytes. The number of bits required to represent the packet number is reduced by including only the least significant bits of the packet number.¶

The encoded packet number is protected as described in Section 5.4 of [QUIC-TLS].¶

Prior to receiving an acknowledgement for a packet number space, the full packet number MUST be included.¶

After an acknowledgement is received for a packet number space, the sender MUST use a packet number size able to represent more than twice as large a range than the difference between the largest acknowledged packet and packet number being sent. A peer receiving the packet will then correctly decode the packet number, unless the packet is delayed in transit such that it arrives after many higher-numbered packets have been received. An endpoint SHOULD use a large enough packet number encoding to allow the packet number to be recovered even if the packet arrives after packets that are sent afterwards.¶

As a result, the size of the packet number encoding is at least one bit more than the base-2 logarithm of the number of contiguous unacknowledged packet numbers, including the new packet.¶

For example, if an endpoint has received an acknowledgment for packet 0xabe8bc, sending a packet with a number of 0xac5c02 requires a packet number encoding with 16 bits or more; whereas the 24-bit packet number encoding is needed to send a packet with a number of 0xace8fe.¶

At a receiver, protection of the packet number is removed prior to recovering the full packet number. The full packet number is then reconstructed based on the number of significant bits present, the value of those bits, and the largest packet number received on a successfully authenticated packet. Recovering the full packet number is necessary to successfully remove packet protection.¶

Once header protection is removed, the packet number is decoded by finding the packet number value that is closest to the next expected packet. The next expected packet is the highest received packet number plus one. For example, if the highest successfully authenticated packet had a packet number of 0xa82f30ea, then a packet containing a 16-bit value of 0x9b32 will be decoded as 0xa82f9b32. Example pseudo-code for packet number decoding can be found in Appendix A.¶

Header Form:

The most significant bit (0x80) of byte 0 (the first byte) is set to 1 for long headers.

Fixed Bit:

The next bit (0x40) of byte 0 is set to 1. Packets containing a zero value for this bit are not valid packets in this version and MUST be discarded.

Long Packet Type:

The next two bits (those with a mask of 0x30) of byte 0 contain a packet type. Packet types are listed in Table 5.

Type-Specific Bits:

The lower four bits (those with a mask of 0x0f) of byte 0 are type-specific.

Version:

The QUIC Version is a 32-bit field that follows the first byte. This field indicates the version of QUIC that is in use and determines how the rest of the protocol fields are interpreted.

Destination Connection ID Length:

The byte following the version contains the length in bytes of the Destination Connection ID field that follows it. This length is encoded as an 8-bit unsigned integer. In QUIC version 1, this value MUST NOT exceed 20. Endpoints that receive a version 1 long header with a value larger than 20 MUST drop the packet. In order to properly form a Version Negotiation packet, servers SHOULD be able to read longer connection IDs from other QUIC versions.

Destination Connection ID:

The Destination Connection ID field follows the Destination Connection ID Length field, which indicates the length of this field. Section 7.2 describes the use of this field in more detail.

Source Connection ID Length:

The byte following the Destination Connection ID contains the length in bytes of the Source Connection ID field that follows it. This length is encoded as a 8-bit unsigned integer. In QUIC version 1, this value MUST NOT exceed 20 bytes. Endpoints that receive a version 1 long header with a value larger than 20 MUST drop the packet. In order to properly form a Version Negotiation packet, servers SHOULD be able to read longer connection IDs from other QUIC versions.

Source Connection ID:

The Source Connection ID field follows the Source Connection ID Length field, which indicates the length of this field. Section 7.2 describes the use of this field in more detail.

The header form bit, Destination and Source Connection ID lengths, Destination and Source Connection ID fields, and Version fields of a long header packet are version-independent. The other fields in the first byte are version-specific. See [QUIC-INVARIANTS] for details on how packets from different versions of QUIC are interpreted.¶

Reserved Bits:

Two bits (those with a mask of 0x0c) of byte 0 are reserved across multiple packet types. These bits are protected using header protection; see Section 5.4 of [QUIC-TLS]. The value included prior to protection MUST be set to 0. An endpoint MUST treat receipt of a packet that has a non-zero value for these bits after removing both packet and header protection as a connection error of type PROTOCOL_VIOLATION. Discarding such a packet after only removing header protection can expose the endpoint to attacks; see Section 9.3 of [QUIC-TLS].

Packet Number Length:

In packet types that contain a Packet Number field, the least significant two bits (those with a mask of 0x03) of byte 0 contain the length of the packet number, encoded as an unsigned, two-bit integer that is one less than the length of the packet number field in bytes. That is, the length of the packet number field is the value of this field, plus one. These bits are protected using header protection; see Section 5.4 of [QUIC-TLS].

Length:

The length of the remainder of the packet (that is, the Packet Number and Payload fields) in bytes, encoded as a variable-length integer (Section 16).

Packet Number:

The packet number field is 1 to 4 bytes long. The packet number is protected using header protection; see Section 5.4 of [QUIC-TLS]. The length of the packet number field is encoded in the Packet Number Length bits of byte 0; see above.

This version of QUIC defines a single packet type that uses the short packet header.¶

Header Form:

The most significant bit (0x80) of byte 0 is set to 0 for the short header.

Fixed Bit:

The next bit (0x40) of byte 0 is set to 1. Packets containing a zero value for this bit are not valid packets in this version and MUST be discarded.

Spin Bit:

The third most significant bit (0x20) of byte 0 is the latency spin bit, set as described in Section 17.3.1.

Reserved Bits:

The next two bits (those with a mask of 0x18) of byte 0 are reserved. These bits are protected using header protection; see Section 5.4 of [QUIC-TLS]. The value included prior to protection MUST be set to 0. An endpoint MUST treat receipt of a packet that has a non-zero value for these bits, after removing both packet and header protection, as a connection error of type PROTOCOL_VIOLATION. Discarding such a packet after only removing header protection can expose the endpoint to attacks; see Section 9.3 of [QUIC-TLS].

Key Phase:

The next bit (0x04) of byte 0 indicates the key phase, which allows a recipient of a packet to identify the packet protection keys that are used to protect the packet. See [QUIC-TLS] for details. This bit is protected using header protection; see Section 5.4 of [QUIC-TLS].

Packet Number Length:

The least significant two bits (those with a mask of 0x03) of byte 0 contain the length of the packet number, encoded as an unsigned, two-bit integer that is one less than the length of the packet number field in bytes. That is, the length of the packet number field is the value of this field, plus one. These bits are protected using header protection; see Section 5.4 of [QUIC-TLS].

Destination Connection ID:

The Destination Connection ID is a connection ID that is chosen by the intended recipient of the packet. See Section 5.1 for more details.

Packet Number:

The packet number field is 1 to 4 bytes long. The packet number has confidentiality protection separate from packet protection, as described in Section 5.4 of [QUIC-TLS]. The length of the packet number field is encoded in Packet Number Length field. See Section 17.1 for details.

Packet Payload:

Packets with a short header always include a 1-RTT protected payload.

The header form bit and the connection ID field of a short header packet are version-independent. The remaining fields are specific to the selected QUIC version. See [QUIC-INVARIANTS] for details on how packets from different versions of QUIC are interpreted.¶

Transport parameters with an identifier of the form 31 * N + 27 for integer values of N are reserved to exercise the requirement that unknown transport parameters be ignored. These transport parameters have no semantics, and may carry arbitrary values.¶

This section details the transport parameters defined in this document.¶

Many transport parameters listed here have integer values. Those transport parameters that are identified as integers use a variable-length integer encoding; see Section 16. Transport parameters have a default value of 0 if the transport parameter is absent unless otherwise stated.¶

original_destination_connection_id (0x00):

The value of the Destination Connection ID field from the first Initial packet sent by the client; see Section 7.3. This transport parameter is only sent by a server.

max_idle_timeout (0x01):

The max idle timeout is a value in milliseconds that is encoded as an integer; see (Section 10.1). Idle timeout is disabled when both endpoints omit this transport parameter or specify a value of 0.

stateless_reset_token (0x02):

A stateless reset token is used in verifying a stateless reset; see Section 10.3. This parameter is a sequence of 16 bytes. This transport parameter MUST NOT be sent by a client, but MAY be sent by a server. A server that does not send this transport parameter cannot use stateless reset (Section 10.3) for the connection ID negotiated during the handshake.

max_udp_payload_size (0x03):

The maximum UDP payload size parameter is an integer value that limits the size of UDP payloads that the endpoint is willing to receive. UDP datagrams with payloads larger than this limit are not likely to be processed by the receiver.

The default for this parameter is the maximum permitted UDP payload of 65527. Values below 1200 are invalid.

This limit does act as an additional constraint on datagram size in the same way as the path MTU, but it is a property of the endpoint and not the path; see Section 14. It is expected that this is the space an endpoint dedicates to holding incoming packets.

initial_max_data (0x04):

The initial maximum data parameter is an integer value that contains the initial value for the maximum amount of data that can be sent on the connection. This is equivalent to sending a MAX_DATA (Section 19.9) for the connection immediately after completing the handshake.

initial_max_stream_data_bidi_local (0x05):

This parameter is an integer value specifying the initial flow control limit for locally-initiated bidirectional streams. This limit applies to newly created bidirectional streams opened by the endpoint that sends the transport parameter. In client transport parameters, this applies to streams with an identifier with the least significant two bits set to 0x0; in server transport parameters, this applies to streams with the least significant two bits set to 0x1.

initial_max_stream_data_bidi_remote (0x06):

This parameter is an integer value specifying the initial flow control limit for peer-initiated bidirectional streams. This limit applies to newly created bidirectional streams opened by the endpoint that receives the transport parameter. In client transport parameters, this applies to streams with an identifier with the least significant two bits set to 0x1; in server transport parameters, this applies to streams with the least significant two bits set to 0x0.

initial_max_stream_data_uni (0x07):

This parameter is an integer value specifying the initial flow control limit for unidirectional streams. This limit applies to newly created unidirectional streams opened by the endpoint that receives the transport parameter. In client transport parameters, this applies to streams with an identifier with the least significant two bits set to 0x3; in server transport parameters, this applies to streams with the least significant two bits set to 0x2.

initial_max_streams_bidi (0x08):

The initial maximum bidirectional streams parameter is an integer value that contains the initial maximum number of bidirectional streams the peer may initiate. If this parameter is absent or zero, the peer cannot open bidirectional streams until a MAX_STREAMS frame is sent. Setting this parameter is equivalent to sending a MAX_STREAMS (Section 19.11) of the corresponding type with the same value.

initial_max_streams_uni (0x09):

The initial maximum unidirectional streams parameter is an integer value that contains the initial maximum number of unidirectional streams the peer may initiate. If this parameter is absent or zero, the peer cannot open unidirectional streams until a MAX_STREAMS frame is sent. Setting this parameter is equivalent to sending a MAX_STREAMS (Section 19.11) of the corresponding type with the same value.

ack_delay_exponent (0x0a):

The acknowledgement delay exponent is an integer value indicating an exponent used to decode the ACK Delay field in the ACK frame (Section 19.3). If this value is absent, a default value of 3 is assumed (indicating a multiplier of 8). Values above 20 are invalid.

max_ack_delay (0x0b):

The maximum acknowledgement delay is an integer value indicating the maximum amount of time in milliseconds by which the endpoint will delay sending acknowledgments. This value SHOULD include the receiver’s expected delays in alarms firing. For example, if a receiver sets a timer for 5ms and alarms commonly fire up to 1ms late, then it should send a max_ack_delay of 6ms. If this value is absent, a default of 25 milliseconds is assumed. Values of 2^14 or greater are invalid.

disable_active_migration (0x0c):

The disable active migration transport parameter is included if the endpoint does not support active connection migration (Section 9) on the address being used during the handshake. When a peer sets this transport parameter, an endpoint MUST NOT use a new local address when sending to the address that the peer used during the handshake. This transport parameter does not prohibit connection migration after a client has acted on a preferred_address transport parameter. This parameter is a zero-length value.

preferred_address (0x0d):

The server’s preferred address is used to effect a change in server address at the end of the handshake, as described in Section 9.6. This transport parameter is only sent by a server. Servers MAY choose to only send a preferred address of one address family by sending an all-zero address and port (0.0.0.0:0 or ::.0) for the other family. IP addresses are encoded in network byte order.

The preferred_address transport parameter contains an address and port for both IP version 4 and 6. The four-byte IPv4 Address field is followed by the associated two-byte IPv4 Port field. This is followed by a 16-byte IPv6 Address field and two-byte IPv6 Port field. After address and port pairs, a Connection ID Length field describes the length of the following Connection ID field. Finally, a 16-byte Stateless Reset Token field includes the stateless reset token associated with the connection ID. The format of this transport parameter is shown in Figure 22.

The Connection ID field and the Stateless Reset Token field contain an alternative connection ID that has a sequence number of 1; see Section 5.1.1. Having these values sent alongside the preferred address ensures that there will be at least one unused active connection ID when the client initiates migration to the preferred address.

The Connection ID and Stateless Reset Token fields of a preferred address are identical in syntax and semantics to the corresponding fields of a NEW_CONNECTION_ID frame (Section 19.15). A server that chooses a zero-length connection ID MUST NOT provide a preferred address. Similarly, a server MUST NOT include a zero-length connection ID in this transport parameter. A client MUST treat violation of these requirements as a connection error of type TRANSPORT_PARAMETER_ERROR.

active_connection_id_limit (0x0e):

The active connection ID limit is an integer value specifying the maximum number of connection IDs from the peer that an endpoint is willing to store. This value includes the connection ID received during the handshake, that received in the preferred_address transport parameter, and those received in NEW_CONNECTION_ID frames. The value of the active_connection_id_limit parameter MUST be at least 2. An endpoint that receives a value less than 2 MUST close the connection with an error of type TRANSPORT_PARAMETER_ERROR. If this transport parameter is absent, a default of 2 is assumed. If an endpoint issues a zero-length connection ID, it will never send a NEW_CONNECTION_ID frame and therefore ignores the active_connection_id_limit value received from its peer.

initial_source_connection_id (0x0f):

The value that the endpoint included in the Source Connection ID field of the first Initial packet it sends for the connection; see Section 7.3.

retry_source_connection_id (0x10):

The value that the server included in the Source Connection ID field of a Retry packet; see Section 7.3. This transport parameter is only sent by a server.

If present, transport parameters that set initial flow control limits (initial_max_stream_data_bidi_local, initial_max_stream_data_bidi_remote, and initial_max_stream_data_uni) are equivalent to sending a MAX_STREAM_DATA frame (Section 19.10) on every stream of the corresponding type immediately after opening. If the transport parameter is absent, streams of that type start with a flow control limit of 0.¶

A client MUST NOT include any server-only transport parameter: original_destination_connection_id, preferred_address, retry_source_connection_id, or stateless_reset_token. A server MUST treat receipt of any of these transport parameters as a connection error of type TRANSPORT_PARAMETER_ERROR.¶

A PADDING frame (type=0x00) has no semantic value. PADDING frames can be used to increase the size of a packet. Padding can be used to increase an initial client packet to the minimum required size, or to provide protection against traffic analysis for protected packets.¶

PADDING frames are formatted as shown in Figure 23, which shows that PADDING frames have no content. That is, a PADDING frame consists of the single byte that identifies the frame as a PADDING frame.¶

Endpoints can use PING frames (type=0x01) to verify that their peers are still alive or to check reachability to the peer.¶

PING frames are formatted as shown in Figure 24, which shows that PING frames have no content.¶

The receiver of a PING frame simply needs to acknowledge the packet containing this frame.¶

The PING frame can be used to keep a connection alive when an application or application protocol wishes to prevent the connection from timing out; see Section 10.1.2.¶

Receivers send ACK frames (types 0x02 and 0x03) to inform senders of packets they have received and processed. The ACK frame contains one or more ACK Ranges. ACK Ranges identify acknowledged packets. If the frame type is 0x03, ACK frames also contain the sum of QUIC packets with associated ECN marks received on the connection up until this point. QUIC implementations MUST properly handle both types and, if they have enabled ECN for packets they send, they SHOULD use the information in the ECN section to manage their congestion state.¶

QUIC acknowledgements are irrevocable. Once acknowledged, a packet remains acknowledged, even if it does not appear in a future ACK frame. This is unlike reneging for TCP SACKs ([RFC2018]).¶

Packets from different packet number spaces can be identified using the same numeric value. An acknowledgment for a packet needs to indicate both a packet number and a packet number space. This is accomplished by having each ACK frame only acknowledge packet numbers in the same space as the packet in which the ACK frame is contained.¶

Version Negotiation and Retry packets cannot be acknowledged because they do not contain a packet number. Rather than relying on ACK frames, these packets are implicitly acknowledged by the next Initial packet sent by the client.¶

ACK frames are formatted as shown in Figure 25.¶

Largest Acknowledged:

A variable-length integer representing the largest packet number the peer is acknowledging; this is usually the largest packet number that the peer has received prior to generating the ACK frame. Unlike the packet number in the QUIC long or short header, the value in an ACK frame is not truncated.

ACK Delay:

A variable-length integer encoding the acknowledgement delay in microseconds; see Section 13.2.5. It is decoded by multiplying the value in the field by 2 to the power of the ack_delay_exponent transport parameter sent by the sender of the ACK frame; see Section 18.2. Compared to simply expressing the delay as an integer, this encoding allows for a larger range of values within the same number of bytes, at the cost of lower resolution.

ACK Range Count:

A variable-length integer specifying the number of Gap and ACK Range fields in the frame.

First ACK Range:

A variable-length integer indicating the number of contiguous packets preceding the Largest Acknowledged that are being acknowledged. The First ACK Range is encoded as an ACK Range; see Section 19.3.1 starting from the Largest Acknowledged. That is, the smallest packet acknowledged in the range is determined by subtracting the First ACK Range value from the Largest Acknowledged.

ACK Ranges:

Contains additional ranges of packets that are alternately not acknowledged (Gap) and acknowledged (ACK Range); see Section 19.3.1.

ECN Counts:

The three ECN Counts; see Section 19.3.2.

An endpoint uses a RESET_STREAM frame (type=0x04) to abruptly terminate the sending part of a stream.¶

After sending a RESET_STREAM, an endpoint ceases transmission and retransmission of STREAM frames on the identified stream. A receiver of RESET_STREAM can discard any data that it already received on that stream.¶

An endpoint that receives a RESET_STREAM frame for a send-only stream MUST terminate the connection with error STREAM_STATE_ERROR.¶

RESET_STREAM frames are formatted as shown in Figure 28.¶

Stream ID:

A variable-length integer encoding of the Stream ID of the stream being terminated.

Application Protocol Error Code:

A variable-length integer containing the application protocol error code (see Section 20.2) that indicates why the stream is being closed.

Final Size:

A variable-length integer indicating the final size of the stream by the RESET_STREAM sender, in unit of bytes; see Section 4.4.

An endpoint uses a STOP_SENDING frame (type=0x05) to communicate that incoming data is being discarded on receipt at application request. STOP_SENDING requests that a peer cease transmission on a stream.¶

A STOP_SENDING frame can be sent for streams in the Recv or Size Known states; see Section 3.1. Receiving a STOP_SENDING frame for a locally-initiated stream that has not yet been created MUST be treated as a connection error of type STREAM_STATE_ERROR. An endpoint that receives a STOP_SENDING frame for a receive-only stream MUST terminate the connection with error STREAM_STATE_ERROR.¶

STOP_SENDING frames are formatted as shown in Figure 29.¶

Stream ID:

A variable-length integer carrying the Stream ID of the stream being ignored.

Application Protocol Error Code:

A variable-length integer containing the application-specified reason the sender is ignoring the stream; see Section 20.2.

A CRYPTO frame (type=0x06) is used to transmit cryptographic handshake messages. It can be sent in all packet types except 0-RTT. The CRYPTO frame offers the cryptographic protocol an in-order stream of bytes. CRYPTO frames are functionally identical to STREAM frames, except that they do not bear a stream identifier; they are not flow controlled; and they do not carry markers for optional offset, optional length, and the end of the stream.¶

CRYPTO frames are formatted as shown in Figure 30.¶

Offset:

A variable-length integer specifying the byte offset in the stream for the data in this CRYPTO frame.

Length:

A variable-length integer specifying the length of the Crypto Data field in this CRYPTO frame.

Crypto Data:

The cryptographic message data.

There is a separate flow of cryptographic handshake data in each encryption level, each of which starts at an offset of 0. This implies that each encryption level is treated as a separate CRYPTO stream of data.¶

The largest offset delivered on a stream - the sum of the offset and data length - cannot exceed 2^62-1. Receipt of a frame that exceeds this limit MUST be treated as a connection error of type FRAME_ENCODING_ERROR or CRYPTO_BUFFER_EXCEEDED.¶

Unlike STREAM frames, which include a Stream ID indicating to which stream the data belongs, the CRYPTO frame carries data for a single stream per encryption level. The stream does not have an explicit end, so CRYPTO frames do not have a FIN bit.¶

A server sends a NEW_TOKEN frame (type=0x07) to provide the client with a token to send in the header of an Initial packet for a future connection.¶

NEW_TOKEN frames are formatted as shown in Figure 31.¶

Token Length:

A variable-length integer specifying the length of the token in bytes.

Token:

An opaque blob that the client may use with a future Initial packet. The token MUST NOT be empty. An endpoint MUST treat receipt of a NEW_TOKEN frame with an empty Token field as a connection error of type FRAME_ENCODING_ERROR.

An endpoint might receive multiple NEW_TOKEN frames that contain the same token value if packets containing the frame are incorrectly determined to be lost. Endpoints are responsible for discarding duplicate values, which might be used to link connection attempts; see Section 8.1.3.¶

Clients MUST NOT send NEW_TOKEN frames. Servers MUST treat receipt of a NEW_TOKEN frame as a connection error of type PROTOCOL_VIOLATION.¶

• The OFF bit (0x04) in the frame type is set to indicate that there is an Offset field present. When set to 1, the Offset field is present. When set to 0, the Offset field is absent and the Stream Data starts at an offset of 0 (that is, the frame contains the first bytes of the stream, or the end of a stream that includes no data).
• The LEN bit (0x02) in the frame type is set to indicate that there is a Length field present. If this bit is set to 0, the Length field is absent and the Stream Data field extends to the end of the packet. If this bit is set to 1, the Length field is present.
• The FIN bit (0x01) of the frame type is set only on frames that contain the final size of the stream. Setting this bit indicates that the frame marks the end of the stream.

An endpoint MUST terminate the connection with error STREAM_STATE_ERROR if it receives a STREAM frame for a locally-initiated stream that has not yet been created, or for a send-only stream.¶

STREAM frames are formatted as shown in Figure 32.¶

Stream ID:

A variable-length integer indicating the stream ID of the stream; see Section 2.1.

Offset:

A variable-length integer specifying the byte offset in the stream for the data in this STREAM frame. This field is present when the OFF bit is set to 1. When the Offset field is absent, the offset is 0.

Length:

A variable-length integer specifying the length of the Stream Data field in this STREAM frame. This field is present when the LEN bit is set to 1. When the LEN bit is set to 0, the Stream Data field consumes all the remaining bytes in the packet.

Stream Data:

The bytes from the designated stream to be delivered.

When a Stream Data field has a length of 0, the offset in the STREAM frame is the offset of the next byte that would be sent.¶

The first byte in the stream has an offset of 0. The largest offset delivered on a stream - the sum of the offset and data length - cannot exceed 2^62-1, as it is not possible to provide flow control credit for that data. Receipt of a frame that exceeds this limit MUST be treated as a connection error of type FRAME_ENCODING_ERROR or FLOW_CONTROL_ERROR.¶

A MAX_DATA frame (type=0x10) is used in flow control to inform the peer of the maximum amount of data that can be sent on the connection as a whole.¶

MAX_DATA frames are formatted as shown in Figure 33.¶

Maximum Data:

A variable-length integer indicating the maximum amount of data that can be sent on the entire connection, in units of bytes.

All data sent in STREAM frames counts toward this limit. The sum of the largest received offsets on all streams - including streams in terminal states - MUST NOT exceed the value advertised by a receiver. An endpoint MUST terminate a connection with a FLOW_CONTROL_ERROR error if it receives more data than the maximum data value that it has sent. This includes violations of remembered limits in Early Data; see Section 7.4.1.¶

A MAX_STREAM_DATA frame (type=0x11) is used in flow control to inform a peer of the maximum amount of data that can be sent on a stream.¶

A MAX_STREAM_DATA frame can be sent for streams in the Recv state; see Section 3.1. Receiving a MAX_STREAM_DATA frame for a locally-initiated stream that has not yet been created MUST be treated as a connection error of type STREAM_STATE_ERROR. An endpoint that receives a MAX_STREAM_DATA frame for a receive-only stream MUST terminate the connection with error STREAM_STATE_ERROR.¶

MAX_STREAM_DATA frames are formatted as shown in Figure 34.¶

Stream ID:

The stream ID of the stream that is affected encoded as a variable-length integer.

Maximum Stream Data:

A variable-length integer indicating the maximum amount of data that can be sent on the identified stream, in units of bytes.

When counting data toward this limit, an endpoint accounts for the largest received offset of data that is sent or received on the stream. Loss or reordering can mean that the largest received offset on a stream can be greater than the total size of data received on that stream. Receiving STREAM frames might not increase the largest received offset.¶

The data sent on a stream MUST NOT exceed the largest maximum stream data value advertised by the receiver. An endpoint MUST terminate a connection with a FLOW_CONTROL_ERROR error if it receives more data than the largest maximum stream data that it has sent for the affected stream. This includes violations of remembered limits in Early Data; see Section 7.4.1.¶

A MAX_STREAMS frame (type=0x12 or 0x13) inform the peer of the cumulative number of streams of a given type it is permitted to open. A MAX_STREAMS frame with a type of 0x12 applies to bidirectional streams, and a MAX_STREAMS frame with a type of 0x13 applies to unidirectional streams.¶

MAX_STREAMS frames are formatted as shown in Figure 35;¶

Maximum Streams:

A count of the cumulative number of streams of the corresponding type that can be opened over the lifetime of the connection. This value cannot exceed 2^60, as it is not possible to encode stream IDs larger than 2^62-1. Receipt of a frame that permits opening of a stream larger than this limit MUST be treated as a FRAME_ENCODING_ERROR.

Loss or reordering can cause a MAX_STREAMS frame to be received that state a lower stream limit than an endpoint has previously received. MAX_STREAMS frames that do not increase the stream limit MUST be ignored.¶

An endpoint MUST NOT open more streams than permitted by the current stream limit set by its peer. For instance, a server that receives a unidirectional stream limit of 3 is permitted to open stream 3, 7, and 11, but not stream 15. An endpoint MUST terminate a connection with a STREAM_LIMIT_ERROR error if a peer opens more streams than was permitted. This includes violations of remembered limits in Early Data; see Section 7.4.1.¶

Note that these frames (and the corresponding transport parameters) do not describe the number of streams that can be opened concurrently. The limit includes streams that have been closed as well as those that are open.¶

A sender SHOULD send a DATA_BLOCKED frame (type=0x14) when it wishes to send data, but is unable to do so due to connection-level flow control; see Section 4. DATA_BLOCKED frames can be used as input to tuning of flow control algorithms; see Section 4.2.¶

DATA_BLOCKED frames are formatted as shown in Figure 36.¶

Maximum Data:

A variable-length integer indicating the connection-level limit at which blocking occurred.

A sender SHOULD send a STREAM_DATA_BLOCKED frame (type=0x15) when it wishes to send data, but is unable to do so due to stream-level flow control. This frame is analogous to DATA_BLOCKED (Section 19.12).¶

An endpoint that receives a STREAM_DATA_BLOCKED frame for a send-only stream MUST terminate the connection with error STREAM_STATE_ERROR.¶

STREAM_DATA_BLOCKED frames are formatted as shown in Figure 37.¶

Stream ID:

A variable-length integer indicating the stream that is blocked due to flow control.

Maximum Stream Data:

A variable-length integer indicating the offset of the stream at which the blocking occurred.

A sender SHOULD send a STREAMS_BLOCKED frame (type=0x16 or 0x17) when it wishes to open a stream, but is unable to due to the maximum stream limit set by its peer; see Section 19.11. A STREAMS_BLOCKED frame of type 0x16 is used to indicate reaching the bidirectional stream limit, and a STREAMS_BLOCKED frame of type 0x17 is used to indicate reaching the unidirectional stream limit.¶

A STREAMS_BLOCKED frame does not open the stream, but informs the peer that a new stream was needed and the stream limit prevented the creation of the stream.¶

STREAMS_BLOCKED frames are formatted as shown in Figure 38.¶

Maximum Streams:

A variable-length integer indicating the maximum number of streams allowed at the time the frame was sent. This value cannot exceed 2^60, as it is not possible to encode stream IDs larger than 2^62-1. Receipt of a frame that encodes a larger stream ID MUST be treated as a STREAM_LIMIT_ERROR or a FRAME_ENCODING_ERROR.

An endpoint sends a NEW_CONNECTION_ID frame (type=0x18) to provide its peer with alternative connection IDs that can be used to break linkability when migrating connections; see Section 9.5.¶

NEW_CONNECTION_ID frames are formatted as shown in Figure 39.¶

Sequence Number:

The sequence number assigned to the connection ID by the sender, encoded as a variable-length integer; see Section 5.1.1.

Retire Prior To:

A variable-length integer indicating which connection IDs should be retired; see Section 5.1.2.

Length:

An 8-bit unsigned integer containing the length of the connection ID. Values less than 1 and greater than 20 are invalid and MUST be treated as a connection error of type FRAME_ENCODING_ERROR.

Connection ID:

A connection ID of the specified length.

Stateless Reset Token:

A 128-bit value that will be used for a stateless reset when the associated connection ID is used; see Section 10.3.

An endpoint MUST NOT send this frame if it currently requires that its peer send packets with a zero-length Destination Connection ID. Changing the length of a connection ID to or from zero-length makes it difficult to identify when the value of the connection ID changed. An endpoint that is sending packets with a zero-length Destination Connection ID MUST treat receipt of a NEW_CONNECTION_ID frame as a connection error of type PROTOCOL_VIOLATION.¶

Transmission errors, timeouts and retransmissions might cause the same NEW_CONNECTION_ID frame to be received multiple times. Receipt of the same frame multiple times MUST NOT be treated as a connection error. A receiver can use the sequence number supplied in the NEW_CONNECTION_ID frame to handle receiving the same NEW_CONNECTION_ID frame multiple times.¶

If an endpoint receives a NEW_CONNECTION_ID frame that repeats a previously issued connection ID with a different Stateless Reset Token or a different sequence number, or if a sequence number is used for different connection IDs, the endpoint MAY treat that receipt as a connection error of type PROTOCOL_VIOLATION.¶

The Retire Prior To field counts connection IDs established during connection setup and the preferred_address transport parameter; see Section 5.1.2. The Retire Prior To field MUST be less than or equal to the Sequence Number field. Receiving a value greater than the Sequence Number MUST be treated as a connection error of type FRAME_ENCODING_ERROR.¶

Once a sender indicates a Retire Prior To value, smaller values sent in subsequent NEW_CONNECTION_ID frames have no effect. A receiver MUST ignore any Retire Prior To fields that do not increase the largest received Retire Prior To value.¶

An endpoint that receives a NEW_CONNECTION_ID frame with a sequence number smaller than the Retire Prior To field of a previously received NEW_CONNECTION_ID frame MUST send a corresponding RETIRE_CONNECTION_ID frame that retires the newly received connection ID, unless it has already done so for that sequence number.¶

An endpoint sends a RETIRE_CONNECTION_ID frame (type=0x19) to indicate that it will no longer use a connection ID that was issued by its peer. This may include the connection ID provided during the handshake. Sending a RETIRE_CONNECTION_ID frame also serves as a request to the peer to send additional connection IDs for future use; see Section 5.1. New connection IDs can be delivered to a peer using the NEW_CONNECTION_ID frame (Section 19.15).¶

Retiring a connection ID invalidates the stateless reset token associated with that connection ID.¶

RETIRE_CONNECTION_ID frames are formatted as shown in Figure 40.¶

Sequence Number:

The sequence number of the connection ID being retired; see Section 5.1.2.

Receipt of a RETIRE_CONNECTION_ID frame containing a sequence number greater than any previously sent to the peer MUST be treated as a connection error of type PROTOCOL_VIOLATION.¶

The sequence number specified in a RETIRE_CONNECTION_ID frame MUST NOT refer to the Destination Connection ID field of the packet in which the frame is contained. The peer MAY treat this as a connection error of type PROTOCOL_VIOLATION.¶

An endpoint cannot send this frame if it was provided with a zero-length connection ID by its peer. An endpoint that provides a zero-length connection ID MUST treat receipt of a RETIRE_CONNECTION_ID frame as a connection error of type PROTOCOL_VIOLATION.¶

Endpoints can use PATH_CHALLENGE frames (type=0x1a) to check reachability to the peer and for path validation during connection migration.¶

PATH_CHALLENGE frames are formatted as shown in Figure 41.¶

Data:

This 8-byte field contains arbitrary data.

Including 64 bits of entropy in a PATH_CHALLENGE frame ensures that it is easier to receive the packet than it is to guess the value correctly.¶

The recipient of this frame MUST generate a PATH_RESPONSE frame (Section 19.18) containing the same Data.¶

A PATH_RESPONSE frame (type=0x1b) is sent in response to a PATH_CHALLENGE frame.¶

PATH_RESPONSE frames are formatted as shown in Figure 42, which is identical to the PATH_CHALLENGE frame (Section 19.17).¶

If the content of a PATH_RESPONSE frame does not match the content of a PATH_CHALLENGE frame previously sent by the endpoint, the endpoint MAY generate a connection error of type PROTOCOL_VIOLATION.¶

An endpoint sends a CONNECTION_CLOSE frame (type=0x1c or 0x1d) to notify its peer that the connection is being closed. The CONNECTION_CLOSE with a frame type of 0x1c is used to signal errors at only the QUIC layer, or the absence of errors (with the NO_ERROR code). The CONNECTION_CLOSE frame with a type of 0x1d is used to signal an error with the application that uses QUIC.¶

If there are open streams that have not been explicitly closed, they are implicitly closed when the connection is closed.¶

CONNECTION_CLOSE frames are formatted as shown in Figure 43.¶

Error Code:

A variable-length integer error code that indicates the reason for closing this connection. A CONNECTION_CLOSE frame of type 0x1c uses codes from the space defined in Section 20.1. A CONNECTION_CLOSE frame of type 0x1d uses codes from the application protocol error code space; see Section 20.2.

Frame Type:

A variable-length integer encoding the type of frame that triggered the error. A value of 0 (equivalent to the mention of the PADDING frame) is used when the frame type is unknown. The application-specific variant of CONNECTION_CLOSE (type 0x1d) does not include this field.

Reason Phrase Length:

A variable-length integer specifying the length of the reason phrase in bytes. Because a CONNECTION_CLOSE frame cannot be split between packets, any limits on packet size will also limit the space available for a reason phrase.

Reason Phrase:

A human-readable explanation for why the connection was closed. This can be zero length if the sender chooses not to give details beyond the Error Code. This SHOULD be a UTF-8 encoded string [RFC3629].

The application-specific variant of CONNECTION_CLOSE (type 0x1d) can only be sent using 0-RTT or 1-RTT packets; see Section 4 of [QUIC-TLS]. When an application wishes to abandon a connection during the handshake, an endpoint can send a CONNECTION_CLOSE frame (type 0x1c) with an error code of APPLICATION_ERROR in an Initial or a Handshake packet.¶

The server uses a HANDSHAKE_DONE frame (type=0x1e) to signal confirmation of the handshake to the client.¶

HANDSHAKE_DONE frames are formatted as shown in Figure 44, which shows that HANDSHAKE_DONE frames have no content.¶

A HANDSHAKE_DONE frame can only be sent by the server. Servers MUST NOT send a HANDSHAKE_DONE frame before completing the handshake. A server MUST treat receipt of a HANDSHAKE_DONE frame as a connection error of type PROTOCOL_VIOLATION.¶

QUIC frames do not use a self-describing encoding. An endpoint therefore needs to understand the syntax of all frames before it can successfully process a packet. This allows for efficient encoding of frames, but it means that an endpoint cannot send a frame of a type that is unknown to its peer.¶

An extension to QUIC that wishes to use a new type of frame MUST first ensure that a peer is able to understand the frame. An endpoint can use a transport parameter to signal its willingness to receive extension frame types. One transport parameter can indicate support for one or more extension frame types.¶

Extensions that modify or replace core protocol functionality (including frame types) will be difficult to combine with other extensions that modify or replace the same functionality unless the behavior of the combination is explicitly defined. Such extensions SHOULD define their interaction with previously-defined extensions modifying the same protocol components.¶

Extension frames MUST be congestion controlled and MUST cause an ACK frame to be sent. The exception is extension frames that replace or supplement the ACK frame. Extension frames are not included in flow control unless specified in the extension.¶

An IANA registry is used to manage the assignment of frame types; see Section 22.3.¶

This section lists the defined QUIC transport error codes that may be used in a CONNECTION_CLOSE frame with a type of 0x1c. These errors apply to the entire connection.¶

NO_ERROR (0x0):

An endpoint uses this with CONNECTION_CLOSE to signal that the connection is being closed abruptly in the absence of any error.

INTERNAL_ERROR (0x1):

The endpoint encountered an internal error and cannot continue with the connection.

CONNECTION_REFUSED (0x2):

The server refused to accept a new connection.

FLOW_CONTROL_ERROR (0x3):

An endpoint received more data than it permitted in its advertised data limits; see Section 4.

STREAM_LIMIT_ERROR (0x4):

An endpoint received a frame for a stream identifier that exceeded its advertised stream limit for the corresponding stream type.

STREAM_STATE_ERROR (0x5):

An endpoint received a frame for a stream that was not in a state that permitted that frame; see Section 3.

FINAL_SIZE_ERROR (0x6):

An endpoint received a STREAM frame containing data that exceeded the previously established final size. Or an endpoint received a STREAM frame or a RESET_STREAM frame containing a final size that was lower than the size of stream data that was already received. Or an endpoint received a STREAM frame or a RESET_STREAM frame containing a different final size to the one already established.

FRAME_ENCODING_ERROR (0x7):

An endpoint received a frame that was badly formatted. For instance, a frame of an unknown type, or an ACK frame that has more acknowledgment ranges than the remainder of the packet could carry.

TRANSPORT_PARAMETER_ERROR (0x8):

An endpoint received transport parameters that were badly formatted, included an invalid value, was absent even though it is mandatory, was present though it is forbidden, or is otherwise in error.

CONNECTION_ID_LIMIT_ERROR (0x9):

The number of connection IDs provided by the peer exceeds the advertised active_connection_id_limit.

PROTOCOL_VIOLATION (0xa):

An endpoint detected an error with protocol compliance that was not covered by more specific error codes.

INVALID_TOKEN (0xb):

A server received a client Initial that contained an invalid Token field.

APPLICATION_ERROR (0xc):

The application or application protocol caused the connection to be closed.

CRYPTO_BUFFER_EXCEEDED (0xd):

An endpoint has received more data in CRYPTO frames than it can buffer.

AEAD_LIMIT_REACHED (0xe):

An endpoint has reached the confidentiality or integrity limit for the AEAD algorithm used by the given connection.

CRYPTO_ERROR (0x1XX):

The cryptographic handshake failed. A range of 256 values is reserved for carrying error codes specific to the cryptographic handshake that is used. Codes for errors occurring when TLS is used for the crypto handshake are described in Section 4.8 of [QUIC-TLS].

See Section 22.4 for details of registering new error codes.¶

In defining these error codes, several principles are applied. Error conditions that might require specific action on the part of a recipient are given unique codes. Errors that represent common conditions are given specific codes. Absent either of these conditions, error codes are used to identify a general function of the stack, like flow control or transport parameter handling. Finally, generic errors are provided for conditions where implementations are unable or unwilling to use more specific codes.¶

The management of application error codes is left to application protocols. Application protocol error codes are used for the RESET_STREAM frame (Section 19.4), the STOP_SENDING frame (Section 19.5), and the CONNECTION_CLOSE frame with a type of 0x1d (Section 19.19).¶

As an encrypted and authenticated transport QUIC provides a range of protections against denial of service. Once the cryptographic handshake is complete, QUIC endpoints discard most packets that are not authenticated, greatly limiting the ability of an attacker to interfere with existing connections.¶

Once a connection is established QUIC endpoints might accept some unauthenticated ICMP packets (see Section 14.2.1), but the use of these packets is extremely limited. The only other type of packet that an endpoint might accept is a stateless reset (Section 10.3), which relies on the token being kept secret until it is used.¶

During the creation of a connection, QUIC only provides protection against attack from off the network path. All QUIC packets contain proof that the recipient saw a preceding packet from its peer.¶

Addresses cannot change during the handshake, so endpoints can discard packets that are received on a different network path.¶

The Source and Destination Connection ID fields are the primary means of protection against off-path attack during the handshake. These are required to match those set by a peer. Except for an Initial and stateless reset packets, an endpoint only accepts packets that include a Destination Connection ID field that matches a value the endpoint previously chose. This is the only protection offered for Version Negotiation packets.¶

The Destination Connection ID field in an Initial packet is selected by a client to be unpredictable, which serves an additional purpose. The packets that carry the cryptographic handshake are protected with a key that is derived from this connection ID and salt specific to the QUIC version. This allows endpoints to use the same process for authenticating packets that they receive as they use after the cryptographic handshake completes. Packets that cannot be authenticated are discarded. Protecting packets in this fashion provides a strong assurance that the sender of the packet saw the Initial packet and understood it.¶

These protections are not intended to be effective against an attacker that is able to receive QUIC packets prior to the connection being established. Such an attacker can potentially send packets that will be accepted by QUIC endpoints. This version of QUIC attempts to detect this sort of attack, but it expects that endpoints will fail to establish a connection rather than recovering. For the most part, the cryptographic handshake protocol [QUIC-TLS] is responsible for detecting tampering during the handshake.¶

Endpoints are permitted to use other methods to detect and attempt to recover from interference with the handshake. Invalid packets may be identified and discarded using other methods, but no specific method is mandated in this document.¶

An attacker might be able to receive an address validation token (Section 8) from a server and then release the IP address it used to acquire that token. At a later time, the attacker may initiate a 0-RTT connection with a server by spoofing this same address, which might now address a different (victim) endpoint. The attacker can thus potentially cause the server to send an initial congestion window’s worth of data towards the victim.¶

Servers SHOULD provide mitigations for this attack by limiting the usage and lifetime of address validation tokens; see Section 8.1.3.¶

An endpoint that acknowledges packets it has not received might cause a congestion controller to permit sending at rates beyond what the network supports. An endpoint MAY skip packet numbers when sending packets to detect this behavior. An endpoint can then immediately close the connection with a connection error of type PROTOCOL_VIOLATION; see Section 10.2.¶

A request forgery attack occurs where an endpoint causes its peer to issue a request towards a victim, with the request controlled by the endpoint. Request forgery attacks aim to provide an attacker with access to capabilities of its peer that might otherwise be unavailable to the attacker. For a networking protocol, a request forgery attack is often used to exploit any implicit authorization conferred on the peer by the victim due to the peer’s location in the network.¶

For request forgery to be effective, an attacker needs to be able to influence what packets the peer sends and where these packets are sent. If an attacker can target a vulnerable service with a controlled payload, that service might perform actions that are attributed to the attacker’s peer, but decided by the attacker.¶

For example, cross-site request forgery [CSRF] exploits on the Web cause a client to issue requests that include authorization cookies [COOKIE], allowing one site access to information and actions that are intended to be restricted to a different site.¶

As QUIC runs over UDP, the primary attack modality of concern is one where an attacker can select the address to which its peer sends UDP datagrams and can control some of the unprotected content of those packets. As much of the data sent by QUIC endpoints is protected, this includes control over ciphertext. An attack is successful if an attacker can cause a peer to send a UDP datagram to a host that will perform some action based on content in the datagram.¶

This section discusses ways in which QUIC might be used for request forgery attacks.¶

This section also describes limited countermeasures that can be implemented by QUIC endpoints. These mitigations can be employed unilaterally by a QUIC implementation or deployment, without potential targets for request forgery attacks taking action. However these countermeasures could be insufficient if UDP-based services do not properly authorize requests.¶

The attacks commonly known as Slowloris ([SLOWLORIS]) try to keep many connections to the target endpoint open and hold them open as long as possible. These attacks can be executed against a QUIC endpoint by generating the minimum amount of activity necessary to avoid being closed for inactivity. This might involve sending small amounts of data, gradually opening flow control windows in order to control the sender rate, or manufacturing ACK frames that simulate a high loss rate.¶

QUIC deployments SHOULD provide mitigations for the Slowloris attacks, such as increasing the maximum number of clients the server will allow, limiting the number of connections a single IP address is allowed to make, imposing restrictions on the minimum transfer speed a connection is allowed to have, and restricting the length of time an endpoint is allowed to stay connected.¶

An adversarial sender might intentionally send fragments of stream data in an attempt to cause disproportionate receive buffer memory commitment and/or creation of a large and inefficient data structure.¶

An adversarial receiver might intentionally not acknowledge packets containing stream data in an attempt to force the sender to store the unacknowledged stream data for retransmission.¶

The attack on receivers is mitigated if flow control windows correspond to available memory. However, some receivers will over-commit memory and advertise flow control offsets in the aggregate that exceed actual available memory. The over-commitment strategy can lead to better performance when endpoints are well behaved, but renders endpoints vulnerable to the stream fragmentation attack.¶

QUIC deployments SHOULD provide mitigations against stream fragmentation attacks. Mitigations could consist of avoiding over-committing memory, limiting the size of tracking data structures, delaying reassembly of STREAM frames, implementing heuristics based on the age and duration of reassembly holes, or some combination.¶

An adversarial endpoint can open a large number of streams, exhausting state on an endpoint. The adversarial endpoint could repeat the process on a large number of connections, in a manner similar to SYN flooding attacks in TCP.¶

Normally, clients will open streams sequentially, as explained in Section 2.1. However, when several streams are initiated at short intervals, loss or reordering may cause STREAM frames that open streams to be received out of sequence. On receiving a higher-numbered stream ID, a receiver is required to open all intervening streams of the same type; see Section 3.2. Thus, on a new connection, opening stream 4000000 opens 1 million and 1 client-initiated bidirectional streams.¶

The number of active streams is limited by the initial_max_streams_bidi and initial_max_streams_uni transport parameters, as explained in Section 4.5. If chosen judiciously, these limits mitigate the effect of the stream commitment attack. However, setting the limit too low could affect performance when applications expect to open large number of streams.¶

QUIC and TLS both contain frames or messages that have legitimate uses in some contexts, but that can be abused to cause a peer to expend processing resources without having any observable impact on the state of the connection.¶

Messages can also be used to change and revert state in small or inconsequential ways, such as by sending small increments to flow control limits.¶

If processing costs are disproportionately large in comparison to bandwidth consumption or effect on state, then this could allow a malicious peer to exhaust processing capacity.¶

While there are legitimate uses for all messages, implementations SHOULD track cost of processing relative to progress and treat excessive quantities of any non-productive packets as indicative of an attack. Endpoints MAY respond to this condition with a connection error, or by dropping packets.¶

An on-path attacker could manipulate the value of ECN codepoints in the IP header to influence the sender’s rate. [RFC3168] discusses manipulations and their effects in more detail.¶

An on-the-side attacker can duplicate and send packets with modified ECN codepoints to affect the sender’s rate. If duplicate packets are discarded by a receiver, an off-path attacker will need to race the duplicate packet against the original to be successful in this attack. Therefore, QUIC endpoints ignore the ECN codepoint field on an IP packet unless at least one QUIC packet in that IP packet is successfully processed; see Section 13.4.¶

Stateless resets create a possible denial of service attack analogous to a TCP reset injection. This attack is possible if an attacker is able to cause a stateless reset token to be generated for a connection with a selected connection ID. An attacker that can cause this token to be generated can reset an active connection with the same connection ID.¶

If a packet can be routed to different instances that share a static key, for example by changing an IP address or port, then an attacker can cause the server to send a stateless reset. To defend against this style of denial of service, endpoints that share a static key for stateless reset (see Section 10.3.2) MUST be arranged so that packets with a given connection ID always arrive at an instance that has connection state, unless that connection is no longer active.¶

More generally, servers MUST NOT generate a stateless reset if a connection with the corresponding connection ID could be active on any endpoint using the same static key.¶

In the case of a cluster that uses dynamic load balancing, it is possible that a change in load balancer configuration could occur while an active instance retains connection state. Even if an instance retains connection state, the change in routing and resulting stateless reset will result in the connection being terminated. If there is no chance of the packet being routed to the correct instance, it is better to send a stateless reset than wait for the connection to time out. However, this is acceptable only if the routing cannot be influenced by an attacker.¶

This document defines QUIC Version Negotiation packets in Section 6 that can be used to negotiate the QUIC version used between two endpoints. However, this document does not specify how this negotiation will be performed between this version and subsequent future versions. In particular, Version Negotiation packets do not contain any mechanism to prevent version downgrade attacks. Future versions of QUIC that use Version Negotiation packets MUST define a mechanism that is robust against version downgrade attacks.¶

Deployments should limit the ability of an attacker to target a new connection to a particular server instance. This means that client-controlled fields, such as the initial Destination Connection ID used on Initial and 0-RTT packets SHOULD NOT be used by themselves to make routing decisions. Ideally, routing decisions are made independently of client-selected values; a Source Connection ID can be selected to route later packets to the same server.¶

A complete security analysis of QUIC is outside the scope of this document. This section provides an informal description of the desired security properties as an aid to implementors and to help guide protocol analysis.¶

QUIC assumes the threat model described in [SEC-CONS] and provides protections against many of the attacks that arise from that model.¶

For this purpose, attacks are divided into passive and active attacks. Passive attackers have the capability to read packets from the network, while active attackers also have the capability to write packets into the network. However, a passive attack may involve an attacker with the ability to cause a routing change or other modification in the path taken by packets that comprise a connection.¶

Attackers are additionally categorized as either on-path attackers or off-path attackers; see Section 3.5 of [SEC-CONS]. An on-path attacker can read, modify, or remove any packet it observes such that it no longer reaches its destination, while an off-path attacker observes the packets, but cannot prevent the original packet from reaching its intended destination. Both types of attackers can also transmit arbitrary packets.¶

Properties of the handshake, protected packets, and connection migration are considered separately.¶

All QUIC registries allow for both provisional and permanent registration of codepoints. This section documents policies that are common to these registries.¶

IANA [SHALL add/has added] a registry for “QUIC Transport Parameters” under a “QUIC” heading.¶

The “QUIC Transport Parameters” registry governs a 62-bit space. This registry follows the registration policy from Section 22.1. Permanent registrations in this registry are assigned using the Specification Required policy ([RFC8126]).¶

Parameter Name:

A short mnemonic for the parameter.

The initial contents of this registry are shown in Table 6.¶

Additionally, each value of the format 31 * N + 27 for integer values of N (that is, 27, 58, 89, …) are reserved and MUST NOT be assigned by IANA.¶

IANA [SHALL add/has added] a registry for “QUIC Frame Types” under a “QUIC” heading.¶

The “QUIC Frame Types” registry governs a 62-bit space. This registry follows the registration policy from Section 22.1. Permanent registrations in this registry are assigned using the Specification Required policy ([RFC8126]), except for values between 0x00 and 0x3f (in hexadecimal; inclusive), which are assigned using Standards Action or IESG Approval as defined in Section 4.9 and 4.10 of [RFC8126].¶

Frame Name:

A short mnemonic for the frame type.

In addition to the advice in Section 22.1, specifications for new permanent registrations SHOULD describe the means by which an endpoint might determine that it can send the identified type of frame. An accompanying transport parameter registration is expected for most registrations; see Section 22.2. Specifications for permanent registrations also need to describe the format and assigned semantics of any fields in the frame.¶

The initial contents of this registry are tabulated in Table 3. Note that the registry does not include the “Pkts” and “Spec” columns from Table 3.¶

IANA [SHALL add/has added] a registry for “QUIC Transport Error Codes” under a “QUIC” heading.¶

The “QUIC Transport Error Codes” registry governs a 62-bit space. This space is split into three regions that are governed by different policies. Permanent registrations in this registry are assigned using the Specification Required policy ([RFC8126]), except for values between 0x00 and 0x3f (in hexadecimal; inclusive), which are assigned using Standards Action or IESG Approval as defined in Section 4.9 and 4.10 of [RFC8126].¶

Code:

A short mnemonic for the parameter.

Description:

A brief description of the error code semantics, which MAY be a summary if a specification reference is provided.

The initial contents of this registry are shown in Table 7.¶

• Require the same connection ID on coalesced packets (#3800, #3930)
• Allow caching of packets that can’t be decrypted, by allowing the reported acknowledgment delay to exceed max_ack_delay prior to confirming the handshake (#3821, #3980, #4035, #3874)
• Allow connection ID to be used for address validation (#3834, #3924)
• Required protocol operations are no longer directed at implementations, but are features provided to application protocols (#3838, #3935)
• Narrow requirements for reset of congestion state on path change (#3842, #3945)
• Add a three times amplification limit for sending of CONNECTION_CLOSE with reduced state (#3845, #3864)
• Change error code for invalid RETIRE_CONNECTION_ID frames (#3860, #3861)
• Recommend retention of state for lost packets to allow for late arrival and avoid unnecessary retransmission (#3956, #3957)
• Allow a server to reject connections if a client reuses packet numbers after Retry (#3989, #3990)
• Limit recommendation for immediate acknowledgment to when ack-eliciting packets are reordered (#4001, #4000)

• Made SERVER_BUSY error (0x2) more generic, now CONNECTION_REFUSED (#3709, #3690, #3694)
• Allow TRANSPORT_PARAMETER_ERROR when validating connection IDs (#3703, #3691)
• Integrate QUIC-specific language from draft-ietf-tsvwg-datagram-plpmtud (#3695, #3702)
• disable_active_migration does not apply to the addresses offered in server_preferred_address (#3608, #3670)

• Allowed CONNECTION_CLOSE in any packet number space, with a requirement to use a new transport-level error for application-specific errors in Initial and Handshake packets (#3430, #3435, #3440)
• Clearer requirements for address validation (#2125, #3327)
• Security analysis of handshake and migration (#2143, #2387, #2925)
• The entire payload of a datagram is used when counting bytes for mitigating amplification attacks (#3333, #3470)
• Connection IDs can be used at any time, including in the handshake (#3348, #3560, #3438, #3565)
• Only one ACK should be sent for each instance of reordering (#3357, #3361)
• Remove text allowing a server to proceed with a bad Retry token (#3396, #3398)
• Ignore active_connection_id_limit with a zero-length connection ID (#3427, #3426)
• Require active_connection_id_limit be remembered for 0-RTT (#3423, #3425)
• Require ack_delay not be remembered for 0-RTT (#3433, #3545)
• Redefined max_packet_size to max_udp_datagram_size (#3471, #3473)
• Guidance on limiting outstanding attempts to retire connection IDs (#3489, #3509, #3557, #3547)
• Restored text on dropping bogus Version Negotiation packets (#3532, #3533)
• Clarified that largest acknowledged needs to be saved, but not necessarily signaled in all cases (#3541, #3581)
• Addressed linkability risk with the use of preferred_address (#3559, #3563)
• Added authentication of handshake connection IDs (#3439, #3499)
• Opening a stream in the wrong direction is an error (#3527)

• Change format of transport parameters to use varints (#3294, #3169)

• Define the use of CONNECTION_CLOSE prior to establishing connection state (#3269, #3297, #3292)
• Allow use of address validation tokens after client address changes (#3307, #3308)
• Define the timer for address validation (#2910, #3339)

• Added HANDSHAKE_DONE to signal handshake confirmation (#2863, #3142, #3145)
• Add integrity check to Retry packets (#3014, #3274, #3120)
• Specify handling of reordered NEW_CONNECTION_ID frames (#3194, #3202)
• Require checking of sequence numbers in RETIRE_CONNECTION_ID (#3037, #3036)
• active_connection_id_limit is enforced (#3193, #3197, #3200, #3201)
• Correct overflow in packet number decode algorithm (#3187, #3188)
• Allow use of CRYPTO_BUFFER_EXCEEDED for CRYPTO frame errors (#3258, #3186)
• Define applicability and scope of NEW_TOKEN (#3150, #3152, #3155, #3156)
• Tokens from Retry and NEW_TOKEN must be differentiated (#3127, #3128)
• Allow CONNECTION_CLOSE in response to invalid token (#3168, #3107)
• Treat an invalid CONNECTION_CLOSE as an invalid frame (#2475, #3230, #3231)
• Throttle when sending CONNECTION_CLOSE after discarding state (#3095, #3157)
• Application-variant of CONNECTION_CLOSE can only be sent in 0-RTT or 1-RTT packets (#3158, #3164)
• Advise sending while blocked to avoid idle timeout (#2744, #3266)
• Define error codes for invalid frames (#3027, #3042)
• Idle timeout is symmetric (#2602, #3099)
• Prohibit IP fragmentation (#3243, #3280)
• Define the use of provisional registration for all registries (#3109, #3020, #3102, #3170)
• Packets on one path must not adjust values for a different path (#2909, #3139)

• Allow ClientHello to span multiple packets (#2928, #3045)
• Client Initial size constraints apply to UDP datagram payload (#3053, #3051)
• Stateless reset changes (#2152, #2993)
  • tokens need to be compared in constant time
  • detection uses UDP datagrams, not packets
  • tokens cannot be reused (#2785, #2968)
• Clearer rules for sharing of UDP ports and use of connection IDs when doing so (#2844, #2851)
• A new connection ID is necessary when responding to migration (#2778, #2969)
• Stronger requirements for connection ID retirement (#3046, #3096)
• NEW_TOKEN cannot be empty (#2978, #2977)
• PING can be sent at any encryption level (#3034, #3035)
• CONNECTION_CLOSE is not ack-eliciting (#3097, #3098)
• Frame encoding error conditions updated (#3027, #3042)
• Non-ack-eliciting packets cannot be sent in response to non-ack-eliciting packets (#3100, #3104)
• Servers have to change connection IDs in Retry (#2837, #3147)

• Rules for preventing correlation by connection ID tightened (#2084, #2929)
• Clarified use of CONNECTION_CLOSE in Handshake packets (#2151, #2541, #2688)
• Discourage regressions of largest acknowledged in ACK (#2205, #2752)
• Improved robustness of validation process for ECN counts (#2534, #2752)
• Require endpoints to ignore spurious migration attempts (#2342, #2893)
• Transport parameter for disabling migration clarified to allow NAT rebinding (#2389, #2893)
• Document principles for defining new error codes (#2388, #2880)
• Reserve transport parameters for greasing (#2550, #2873)
• A maximum ACK delay of 0 is used for handshake packet number spaces (#2646, #2638)
• Improved rules for use of congestion control state on new paths (#2685, #2918)
• Removed recommendation to coordinate spin for multiple connections that share a path (#2763, #2882)
• Allow smaller stateless resets and recommend a smaller minimum on packets that might trigger a stateless reset (#2770, #2869, #2927, #3007).
• Provide guidance around the interface to QUIC as used by application protocols (#2805, #2857)
• Frames other than STREAM can cause STREAM_LIMIT_ERROR (#2825, #2826)
• Tighter rules about processing of rejected 0-RTT packets (#2829, #2840, #2841)
• Explanation of the effect of Retry on 0-RTT packets (#2842, #2852)
• Cryptographic handshake needs to provide server transport parameter encryption (#2920, #2921)
• Moved ACK generation guidance from recovery draft to transport draft (#1860, #2916).

• Connection ID lengths are now one octet, but limited in version 1 to 20 octets of length (#2736, #2749)

• Error codes are encoded as variable-length integers (#2672, #2680)
• NEW_CONNECTION_ID includes a request to retire old connection IDs (#2645, #2769)
• Tighter rules for generating and explicitly eliciting ACK frames (#2546, #2794)
• Recommend having only one packet per encryption level in a datagram (#2308, #2747)
• More normative language about use of stateless reset (#2471, #2574)
• Allow reuse of stateless reset tokens (#2732, #2733)
• Allow, but not require, enforcing non-duplicate transport parameters (#2689, #2691)
• Added an active_connection_id_limit transport parameter (#1994, #1998)
• max_ack_delay transport parameter defaults to 0 (#2638, #2646)
• When sending 0-RTT, only remembered transport parameters apply (#2458, #2360, #2466, #2461)
• Define handshake completion and confirmation; define clearer rules when it encryption keys should be discarded (#2214, #2267, #2673)
• Prohibit path migration prior to handshake confirmation (#2309, #2370)
• PATH_RESPONSE no longer needs to be received on the validated path (#2582, #2580, #2579, #2637)
• PATH_RESPONSE frames are not stored and retransmitted (#2724, #2729)
• Document hack for enabling routing of ICMP when doing PMTU probing (#1243, #2402)

• Refine discussion of 0-RTT transport parameters (#2467, #2464)
• Fewer transport parameters need to be remembered for 0-RTT (#2624, #2467)
• Spin bit text incorporated (#2564)
• Close the connection when maximum stream ID in MAX_STREAMS exceeds 2^62 - 1 (#2499, #2487)
• New connection ID required for intentional migration (#2414, #2413)
• Connection ID issuance can be rate-limited (#2436, #2428)
• The “QUIC bit” is ignored in Version Negotiation (#2400, #2561)
• Initial packets from clients need to be padded to 1200 unless a Handshake packet is sent as well (#2522, #2523)
• CRYPTO frames can be discarded if too much data is buffered (#1834, #2524)
• Stateless reset uses a short header packet (#2599, #2600)

• Removed version negotiation; version negotiation, including authentication of the result, will be addressed in the next version of QUIC (#1773, #2313)
• Added discussion of the use of IPv6 flow labels (#2348, #2399)
• A connection ID can’t be retired in a packet that uses that connection ID (#2101, #2420)
• Idle timeout transport parameter is in milliseconds (from seconds) (#2453, #2454)
• Endpoints are required to use new connection IDs when they use new network paths (#2413, #2414)
• Increased the set of permissible frames in 0-RTT (#2344, #2355)

• Stream-related errors now use STREAM_STATE_ERROR (#2305)
• Endpoints discard initial keys as soon as handshake keys are available (#1951, #2045)
• Expanded conditions for ignoring ICMP packet too big messages (#2108, #2161)
• Remove rate control from PATH_CHALLENGE/PATH_RESPONSE (#2129, #2241)
• Endpoints are permitted to discard malformed initial packets (#2141)
• Clarified ECN implementation and usage requirements (#2156, #2201)
• Disable ECN count verification for packets that arrive out of order (#2198, #2215)
• Use Probe Timeout (PTO) instead of RTO (#2206, #2238)
• Loosen constraints on retransmission of ACK ranges (#2199, #2245)
• Limit Retry and Version Negotiation to once per datagram (#2259, #2303)
• Set a maximum value for max_ack_delay transport parameter (#2282, #2301)
• Allow server preferred address for both IPv4 and IPv6 (#2122, #2296)
• Corrected requirements for migration to a preferred address (#2146, #2349)
• ACK of non-existent packet is illegal (#2298, #2302)

• Stream limits are defined as counts, not maximums (#1850, #1906)
• Require amplification attack defense after closing (#1905, #1911)
• Remove reservation of application error code 0 for STOPPING (#1804, #1922)
• Renumbered frames (#1945)
• Renumbered transport parameters (#1946)
• Numeric transport parameters are expressed as varints (#1608, #1947, #1955)
• Reorder the NEW_CONNECTION_ID frame (#1952, #1963)
• Rework the first byte (#2006)
  • Fix the 0x40 bit
  • Change type values for long header
  • Add spin bit to short header (#631, #1988)
  • Encrypt the remainder of the first byte (#1322)
  • Move packet number length to first byte
  • Move ODCIL to first byte of retry packets
  • Simplify packet number protection (#1575)
• Allow STOP_SENDING to open a remote bidirectional stream (#1797, #2013)
• Added mitigation for off-path migration attacks (#1278, #1749, #2033)
• Don’t let the PMTU to drop below 1280 (#2063, #2069)
• Require peers to replace retired connection IDs (#2085)
• Servers are required to ignore Version Negotiation packets (#2088)
• Tokens are repeated in all Initial packets (#2089)
• Clarified how PING frames are sent after loss (#2094)
• Initial keys are discarded once Handshake are available (#1951, #2045)
• ICMP PTB validation clarifications (#2161, #2109, #2108)

Substantial editorial reorganization; no technical changes.¶

• Merge ACK and ACK_ECN (#1778, #1801)
• Explicitly communicate max_ack_delay (#981, #1781)
• Validate original connection ID after Retry packets (#1710, #1486, #1793)
• Idle timeout is optional and has no specified maximum (#1765)
• Update connection ID handling; add RETIRE_CONNECTION_ID type (#1464, #1468, #1483, #1484, #1486, #1495, #1729, #1742, #1799, #1821)
• Include a Token in all Initial packets (#1649, #1794)
• Prevent handshake deadlock (#1764, #1824)

• Streams open when higher-numbered streams of the same type open (#1342, #1549)
• Split initial stream flow control limit into 3 transport parameters (#1016, #1542)
• All flow control transport parameters are optional (#1610)
• Removed UNSOLICITED_PATH_RESPONSE error code (#1265, #1539)
• Permit stateless reset in response to any packet (#1348, #1553)
• Recommended defense against stateless reset spoofing (#1386, #1554)
• Prevent infinite stateless reset exchanges (#1443, #1627)
• Forbid processing of the same packet number twice (#1405, #1624)
• Added a packet number decoding example (#1493)
• More precisely define idle timeout (#1429, #1614, #1652)
• Corrected format of Retry packet and prevented looping (#1492, #1451, #1448, #1498)
• Permit 0-RTT after receiving Version Negotiation or Retry (#1507, #1514, #1621)
• Permit Retry in response to 0-RTT (#1547, #1552)
• Looser verification of ECN counters to account for ACK loss (#1555, #1481, #1565)
• Remove frame type field from APPLICATION_CLOSE (#1508, #1528)

• Changes to integration of the TLS handshake (#829, #1018, #1094, #1165, #1190, #1233, #1242, #1252, #1450, #1458)
  • The cryptographic handshake uses CRYPTO frames, not stream 0
  • QUIC packet protection is used in place of TLS record protection
  • Separate QUIC packet number spaces are used for the handshake
  • Changed Retry to be independent of the cryptographic handshake
  • Added NEW_TOKEN frame and Token fields to Initial packet
  • Limit the use of HelloRetryRequest to address TLS needs (like key shares)
• Enable server to transition connections to a preferred address (#560, #1251, #1373)
• Added ECN feedback mechanisms and handling; new ACK_ECN frame (#804, #805, #1372)
• Changed rules and recommendations for use of new connection IDs (#1258, #1264, #1276, #1280, #1419, #1452, #1453, #1465)
• Added a transport parameter to disable intentional connection migration (#1271, #1447)
• Packets from different connection ID can’t be coalesced (#1287, #1423)
• Fixed sampling method for packet number encryption; the length field in long headers includes the packet number field in addition to the packet payload (#1387, #1389)
• Stateless Reset is now symmetric and subject to size constraints (#466, #1346)
• Added frame type extension mechanism (#58, #1473)

• Enable server to transition connections to a preferred address (#560, #1251)
• Packet numbers are encrypted (#1174, #1043, #1048, #1034, #850, #990, #734, #1317, #1267, #1079)
• Packet numbers use a variable-length encoding (#989, #1334)
• STREAM frames can now be empty (#1350)

• Swap payload length and packed number fields in long header (#1294)
• Clarified that CONNECTION_CLOSE is allowed in Handshake packet (#1274)
• Spin bit reserved (#1283)
• Coalescing multiple QUIC packets in a UDP datagram (#1262, #1285)
• A more complete connection migration (#1249)
• Refine opportunistic ACK defense text (#305, #1030, #1185)
• A Stateless Reset Token isn’t mandatory (#818, #1191)
• Removed implicit stream opening (#896, #1193)
• An empty STREAM frame can be used to open a stream without sending data (#901, #1194)
• Define stream counts in transport parameters rather than a maximum stream ID (#1023, #1065)
• STOP_SENDING is now prohibited before streams are used (#1050)
• Recommend including ACK in Retry packets and allow PADDING (#1067, #882)
• Endpoints now become closing after an idle timeout (#1178, #1179)
• Remove implication that Version Negotiation is sent when a packet of the wrong version is received (#1197)

• Added PATH_CHALLENGE and PATH_RESPONSE frames to replace PING with Data and PONG frame. Changed ACK frame type from 0x0e to 0x0d. (#1091, #725, #1086)
• A server can now only send 3 packets without validating the client address (#38, #1090)
• Delivery order of stream data is no longer strongly specified (#252, #1070)
• Rework of packet handling and version negotiation (#1038)
• Stream 0 is now exempt from flow control until the handshake completes (#1074, #725, #825, #1082)
• Improved retransmission rules for all frame types: information is retransmitted, not packets or frames (#463, #765, #1095, #1053)
• Added an error code for server busy signals (#1137)
• Endpoints now set the connection ID that their peer uses. Connection IDs are variable length. Removed the omit_connection_id transport parameter and the corresponding short header flag. (#1089, #1052, #1146, #821, #745, #821, #1166, #1151)

• Clarified requirements for BLOCKED usage (#65, #924)
• BLOCKED frame now includes reason for blocking (#452, #924, #927, #928)
• GAP limitation in ACK Frame (#613)
• Improved PMTUD description (#614, #1036)
• Clarified stream state machine (#634, #662, #743, #894)
• Reserved versions don’t need to be generated deterministically (#831, #931)
• You don’t always need the draining period (#871)
• Stateless reset clarified as version-specific (#930, #986)
• initial_max_stream_id_x transport parameters are optional (#970, #971)
• ACK delay assumes a default value during the handshake (#1007, #1009)
• Removed transport parameters from NewSessionTicket (#1015)

• The long header now has version before packet number (#926, #939)
• Rename and consolidate packet types (#846, #822, #847)
• Packet types are assigned new codepoints and the Connection ID Flag is inverted (#426, #956)
• Removed type for Version Negotiation and use Version 0 (#963, #968)
• Streams are split into unidirectional and bidirectional (#643, #656, #720, #872, #175, #885)
  • Stream limits now have separate uni- and bi-directional transport parameters (#909, #958)
  • Stream limit transport parameters are now optional and default to 0 (#970, #971)
• The stream state machine has been split into read and write (#634, #894)
• Employ variable-length integer encodings throughout (#595)
• Improvements to connection close
  • Added distinct closing and draining states (#899, #871)
  • Draining period can terminate early (#869, #870)
  • Clarifications about stateless reset (#889, #890)
• Address validation for connection migration (#161, #732, #878)
• Clearly defined retransmission rules for BLOCKED (#452, #65, #924)
• negotiated_version is sent in server transport parameters (#710, #959)
• Increased the range over which packet numbers are randomized (#864, #850, #964)

• Replaced FNV-1a with AES-GCM for all “Cleartext” packets (#554)
• Split error code space between application and transport (#485)
• Stateless reset token moved to end (#820)
• 1-RTT-protected long header types removed (#848)
• No acknowledgments during draining period (#852)
• Remove “application close” as a separate close type (#854)
• Remove timestamps from the ACK frame (#841)
• Require transport parameters to only appear once (#792)

• Stateless token is server-only (#726)
• Refactor section on connection termination (#733, #748, #328, #177)
• Limit size of Version Negotiation packet (#585)
• Clarify when and what to ack (#736)
• Renamed STREAM_ID_NEEDED to STREAM_ID_BLOCKED
• Clarify Keep-alive requirements (#729)

• Introduce STOP_SENDING frame, RESET_STREAM only resets in one direction (#165)
• Removed GOAWAY; application protocols are responsible for graceful shutdown (#696)
• Reduced the number of error codes (#96, #177, #184, #211)
• Version validation fields can’t move or change (#121)
• Removed versions from the transport parameters in a NewSessionTicket message (#547)
• Clarify the meaning of “bytes in flight” (#550)
• Public reset is now stateless reset and not visible to the path (#215)
• Reordered bits and fields in STREAM frame (#620)
• Clarifications to the stream state machine (#572, #571)
• Increased the maximum length of the Largest Acknowledged field in ACK frames to 64 bits (#629)
• truncate_connection_id is renamed to omit_connection_id (#659)
• CONNECTION_CLOSE terminates the connection like TCP RST (#330, #328)
• Update labels used in HKDF-Expand-Label to match TLS 1.3 (#642)

• Change STREAM and RESET_STREAM layout
• Add MAX_STREAM_ID settings

• The size of the initial packet payload has a fixed minimum (#267, #472)
• Define when Version Negotiation packets are ignored (#284, #294, #241, #143, #474)
• The 64-bit FNV-1a algorithm is used for integrity protection of unprotected packets (#167, #480, #481, #517)
• Rework initial packet types to change how the connection ID is chosen (#482, #442, #493)
• No timestamps are forbidden in unprotected packets (#542, #429)
• Cryptographic handshake is now on stream 0 (#456)
• Remove congestion control exemption for cryptographic handshake (#248, #476)
• Version 1 of QUIC uses TLS; a new version is needed to use a different handshake protocol (#516)
• STREAM frames have a reduced number of offset lengths (#543, #430)
• Split some frames into separate connection- and stream- level frames (#443)
  • WINDOW_UPDATE split into MAX_DATA and MAX_STREAM_DATA (#450)
  • BLOCKED split to match WINDOW_UPDATE split (#454)
  • Define STREAM_ID_NEEDED frame (#455)
• A NEW_CONNECTION_ID frame supports connection migration without linkability (#232, #491, #496)
• Transport parameters for 0-RTT are retained from a previous connection (#405, #513, #512)
  • A client in 0-RTT no longer required to reset excess streams (#425, #479)
• Expanded security considerations (#440, #444, #445, #448)

• Defined short and long packet headers (#40, #148, #361)
• Defined a versioning scheme and stable fields (#51, #361)
• Define reserved version values for “greasing” negotiation (#112, #278)
• The initial packet number is randomized (#35, #283)
• Narrow the packet number encoding range requirement (#67, #286, #299, #323, #356)
• Defined client address validation (#52, #118, #120, #275)
• Define transport parameters as a TLS extension (#49, #122)
• SCUP and COPT parameters are no longer valid (#116, #117)
• Transport parameters for 0-RTT are either remembered from before, or assume default values (#126)
• The server chooses connection IDs in its final flight (#119, #349, #361)
• The server echoes the Connection ID and packet number fields when sending a Version Negotiation packet (#133, #295, #244)
• Defined a minimum packet size for the initial handshake packet from the client (#69, #136, #139, #164)
• Path MTU Discovery (#64, #106)
• The initial handshake packet from the client needs to fit in a single packet (#338)
• Forbid acknowledgment of packets containing only ACK and PADDING (#291)
• Require that frames are processed when packets are acknowledged (#381, #341)
• Removed the STOP_WAITING frame (#66)
• Don’t require retransmission of old timestamps for lost ACK frames (#308)
• Clarified that frames are not retransmitted, but the information in them can be (#157, #298)
• Error handling definitions (#335)
• Split error codes into four sections (#74)
• Forbid the use of Public Reset where CONNECTION_CLOSE is possible (#289)
• Define packet protection rules (#336)
• Require that stream be entirely delivered or reset, including acknowledgment of all STREAM frames or the RESET_STREAM, before it closes (#381)
• Remove stream reservation from state machine (#174, #280)
• Only stream 1 does not contribute to connection-level flow control (#204)
• Stream 1 counts towards the maximum concurrent stream limit (#201, #282)
• Remove connection-level flow control exclusion for some streams (except 1) (#246)
• RESET_STREAM affects connection-level flow control (#162, #163)
• Flow control accounting uses the maximum data offset on each stream, rather than bytes received (#378)
• Moved length-determining fields to the start of STREAM and ACK (#168, #277)
• Added the ability to pad between frames (#158, #276)
• Remove error code and reason phrase from GOAWAY (#352, #355)
• GOAWAY includes a final stream number for both directions (#347)
• Error codes for RESET_STREAM and CONNECTION_CLOSE are now at a consistent offset (#249)
• Defined priority as the responsibility of the application protocol (#104, #303)

• Replaced DIVERSIFICATION_NONCE flag with KEY_PHASE flag
• Defined versioning
• Reworked description of packet and frame layout
• Error code space is divided into regions for each component
• Use big endian for all numeric values

• Adopted as base for draft-ietf-quic-tls
• Updated authors/editors list
• Added IANA Considerations section
• Moved Contributors and Acknowledgments to appendices