QUIC: A UDP-Based Multiplexed and Secure Transport

[x]

Indicates that x is optional

{x}

Indicates that x is encrypted

x (A)

Indicates that x is A bits long

x (A/B/C) …

Indicates that x is one of A, B, or C bits long

x (*) …

Indicates that x is variable-length

QUIC relies on a combined cryptographic and transport handshake for setting up a secure transport connection. QUIC connections are expected to commonly use 0-RTT handshakes, meaning that for most QUIC connections, data can be sent immediately following the client handshake packet, without waiting for a reply from the server. QUIC provides a dedicated stream (Stream ID 0) to be used for performing the cryptographic handshake and QUIC options negotiation. The format of the QUIC options and parameters used during negotiation are described in this document, but the handshake protocol that runs on Stream ID 0 is described in the accompanying cryptographic handshake draft [QUIC-TLS].¶

When application messages are transported over TCP, independent application messages can suffer from head-of-line blocking. When an application multiplexes many streams atop TCP’s single-bytestream abstraction, a loss of a TCP segment results in blocking of all subsequent segments until a retransmission arrives, irrespective of the application streams that are encapsulated in subsequent segments. QUIC ensures that lost packets carrying data for an individual stream only impact that specific stream. Data received on other streams can continue to be reassembled and delivered to the application.¶

QUIC’s packet framing and acknowledgments carry rich information that help both congestion control and loss recovery in fundamental ways. Each QUIC packet carries a new packet number, including those carrying retransmitted data. This obviates the need for a separate mechanism to distinguish acknowledgments for retransmissions from those for original transmissions, avoiding TCP’s retransmission ambiguity problem. QUIC acknowledgments also explicitly encode the delay between the receipt of a packet and its acknowledgment being sent, and together with the monotonically-increasing packet numbers, this allows for precise network roundtrip-time (RTT) calculation. QUIC’s ACK frames support up to 256 ACK blocks, so QUIC is more resilient to reordering than TCP with SACK support, as well as able to keep more bytes on the wire when there is reordering or loss.¶

QUIC implements stream- and connection-level flow control. At a high level, a QUIC receiver advertises the maximum amount of data that it is willing to receive on each stream. As data is sent, received, and delivered on a particular stream, the receiver sends MAX_STREAM_DATA frames that increase the advertised limit for that stream, allowing the peer to send more data on that stream.¶

In addition to this stream-level flow control, QUIC implements connection-level flow control to limit the aggregate buffer that a QUIC receiver is willing to allocate to all streams on a connection. Connection-level flow control works in the same way as stream-level flow control, but the bytes delivered and the limits are aggregated across all streams.¶

TCP headers appear in plaintext on the wire and are not authenticated, causing a plethora of injection and header manipulation issues for TCP, such as receive-window manipulation and sequence-number overwriting. While some of these are mechanisms used by middleboxes to improve TCP performance, others are active attacks. Even “performance-enhancing” middleboxes that routinely interpose on the transport state machine end up limiting the evolvability of the transport protocol, as has been observed in the design of MPTCP [RFC6824] and in its subsequent deployability issues.¶

Generally, QUIC packets are always authenticated and the payload is typically fully encrypted. The parts of the packet header which are not encrypted are still authenticated by the receiver, so as to thwart any packet injection or manipulation by third parties. Some early handshake packets, such as the Version Negotiation packet, are not encrypted, but information sent in these unencrypted handshake packets is later verified as part of cryptographic processing.¶

QUIC connections are identified by a Connection ID, a 64-bit unsigned number randomly generated by the server. QUIC’s consistent connection ID allows connections to survive changes to the client’s IP and port, such as those caused by NAT rebindings or by the client changing network connectivity to a new address. QUIC provides automatic cryptographic verification of a rebound lient, since the client continues to use the same session key for encrypting and decrypting packets. The consistent connection ID can be used to allow migration of the connection to a new server IP address as well, since the Connection ID remains consistent across changes in the client’s and the server’s network addresses.¶

QUIC version negotiation allows for multiple versions of the protocol to be deployed and used concurrently. Version negotiation is described in Section 7.1.¶

Header Form:

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

Long Packet Type:

The remaining seven bits of octet 0 contain the packet type. This field can indicate one of 128 packet types. The types specified for this version are listed in Table 1.

Connection ID:

Octets 1 through 8 contain the connection ID. Section 5.6 describes the use of this field in more detail.

Packet Number:

Octets 9 to 12 contain the packet number. Section 5.7 describes the use of packet numbers.

Version:

Octets 13 to 16 contain the selected protocol version. This field indicates which version of QUIC is in use and determines how the rest of the protocol fields are interpreted.

Payload:

Octets from 17 onwards (the rest of QUIC packet) are the payload of the packet.

The header form, packet type, connection ID, packet number and version fields of a long header packet are version-independent. The types of packets defined in Table 1 are version-specific. See Section 5.8 for details on how packets from different versions of QUIC are interpreted.¶

(TODO: Should the list of packet types be version-independent?)¶

The interpretation of the fields and the payload are specific to a version and packet type. Type-specific semantics for this version are described in the following sections.¶

Header Form:

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

Connection ID Flag:

The second bit (0x40) of octet 0 indicates whether the Connection ID field is present. If set to 1, then the Connection ID field is present; if set to 0, the Connection ID field is omitted. The Connection ID field can only be omitted if the omit_connection_id transport parameter (Section 7.3.1) is specified by the intended recipient of the packet.

Key Phase Bit:

The third bit (0x20) of octet 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.

Short Packet Type:

The remaining 5 bits of octet 0 include one of 32 packet types. Table 2 lists the types that are defined for short packets.

Connection ID:

If the Connection ID Flag is set, a connection ID occupies octets 1 through 8 of the packet. See Section 5.6 for more details.

Packet Number:

The length of the packet number field depends on the packet type. This field can be 1, 2 or 4 octets long depending on the short packet type.

Protected Payload:

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

The packet type in a short header currently determines only the size of the packet number field. Additional types can be used to signal the presence of other fields.¶

The header form, connection ID flag and connection ID of a short header packet are version-independent. The remaining fields are specific to the selected QUIC version. See Section 5.8 for details on how packets from different versions of QUIC are interpreted.¶

A Version Negotiation packet has long headers with a type value of 0x01 and is sent only by servers. The Version Negotiation packet is a response to a client packet that contains a version that is not supported by the server.¶

The packet number, connection ID and version fields echo corresponding values from the triggering client packet. This allows clients some assurance that the server received the packet and that the Version Negotiation packet was not carried in a packet with a spoofed source address.¶

A Version Negotiation packet is never explicitly acknowledged in an ACK frame by a client. Receiving another Client Initial packet implicitly acknowledges a Version Negotiation packet.¶

The payload of the Version Negotiation packet is a list of 32-bit versions which the server supports, as shown below.¶

See Section 7.1 for a description of the version negotiation process.¶

Cleartext packets are sent during the handshake prior to key negotiation.¶

All cleartext packets contain the current QUIC version in the version field.¶

The payload of cleartext packets also includes an integrity check, which is described in [QUIC-TLS].¶

Packets that are protected with 0-RTT keys are sent with long headers. Packets that are protected with 1-RTT keys MAY be sent with long headers. The different packet types explicitly indicate the encryption level and therefore the keys that are used to remove packet protection.¶

Packets protected with 0-RTT keys use a type value of 0x06. The connection ID field for a 0-RTT packet is selected by the client.¶

The client can send 0-RTT packets after having received a packet from the server if that packet does not complete the handshake. Even if the client receives a different connection ID from the server, it MUST NOT update the connection ID it uses for 0-RTT packets. This enables consistent routing for all 0-RTT packets.¶

Packets protected with 1-RTT keys that use long headers use a type value of 0x07 for key phase 0 and 0x08 for key phase 1; see [QUIC-TLS] for more details on the use of key phases. The connection ID field for these packet types MUST contain the value selected by the server, see Section 5.6.¶

The version field for protected packets is the current QUIC version.¶

The packet number field contains a packet number, which increases with each packet sent, see Section 5.7 for details.¶

The payload is protected using authenticated encryption. [QUIC-TLS] describes packet protection in detail. After decryption, the plaintext consists of a sequence of frames, as described in Section 6.¶

QUIC connections are identified by their 64-bit Connection ID. All long headers contain a Connection ID. Short headers indicate the presence of a Connection ID using the CONNECTION_ID flag. When present, the Connection ID is in the same location in all packet headers, making it straightforward for middleboxes, such as load balancers, to locate and use it.¶

The client MUST choose a random connection ID and use it in Client Initial packets (Section 5.4.1) and 0-RTT packets (Section 5.5). If the client has received any packet from the server, it uses the connection ID it received from the server for all packets other than 0-RTT packets.¶

When the server receives a Client Initial packet and decides to proceed with the handshake, it chooses a new value for the connection ID and sends that in a Server Cleartext packet. The server MAY choose to use the value that the client initially selects.¶

Once the client receives the connection ID that the server has chosen, it uses this for all subsequent packets that it sends, except for any 0-RTT packets, which all have the same connection ID.¶

The packet number is a 64-bit unsigned number and is used as part of a cryptographic nonce for packet encryption. Each endpoint maintains a separate packet number for sending and receiving. The packet number for sending MUST increase by at least one after sending any packet, unless otherwise specified (see Section 5.7.1).¶

A QUIC endpoint MUST NOT reuse a packet number within the same connection (that is, under the same cryptographic keys). If the packet number for sending reaches 2^64 - 1, the sender MUST close the connection without sending a CONNECTION_CLOSE frame or any further packets; a server MAY send a Stateless Reset (Section 7.7.5) in response to further packets that it receives.¶

To reduce the number of bits required to represent the packet number over the wire, only the least significant bits of the packet number are transmitted. The actual packet number for each packet is reconstructed at the receiver based on the largest packet number received on a successfully authenticated packet.¶

A 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 0xaa82f30e, then a packet containing a 16-bit value of 0x1f94 will be decoded as 0xaa831f94.¶

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 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 0x6afa2f, sending a packet with a number of 0x6b4264 requires a 16-bit or larger packet number encoding; whereas a 32-bit packet number is needed to send a packet with a number of 0x6bc107.¶

Version Negotiation (Section 5.3) and Server Stateless Retry (Section 5.4.2) packets have special rules for populating the packet number field.¶

• the location of the header form flag,
• the location of the Connection ID flag in short headers,
• the location and size of the Connection ID field in both header forms,
• the location and size of the Version field in long headers, and
• the location and size of the Packet Number field in long headers.

Implementations MUST assume that an unsupported version uses an unknown packet format. All other fields MUST be ignored when processing a packet that contains an unsupported version.¶

QUIC’s connection establishment begins with version negotiation, since all communication between the endpoints, including packet and frame formats, relies on the two endpoints agreeing on a version.¶

A QUIC connection begins with a client sending a handshake packet. The details of the handshake mechanisms are described in Section 7.2, but all of the initial packets sent from the client to the server MUST use the long header format and MUST specify the version of the protocol being used.¶

When the server receives a packet from a client with the long header format, it compares the client’s version to the versions it supports.¶

If the version selected by the client is not acceptable to the server, the server discards the incoming packet and responds with a Version Negotiation packet (Section 5.3). This includes a list of versions that the server will accept.¶

To avoid packet amplification attacks a server MUST NOT send a Version Negotiation packet that is larger than the packet it responds to. It is anticipated that this is ample space for all QUIC versions that a single server might need to advertise.¶

A server sends a Version Negotiation packet for every packet that it receives with an unacceptable version. This allows a server to process packets with unsupported versions without retaining state. Though either the initial client 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.¶

If the packet contains a version that is acceptable to the server, the server proceeds with the handshake (Section 7.2). This commits the server to the version that the client selected.¶

When the client receives a Version Negotiation packet from the server, it should select an acceptable protocol version. If the server lists an acceptable version, the client selects that version and reattempts to create a connection using that version. Though the contents of a packet might not change in response to version negotiation, a client MUST increase the packet number it uses on every packet it sends. Packets MUST continue to use long headers and MUST include the new negotiated protocol version.¶

The client MUST use the long header format and include its selected version on all packets until it has 1-RTT keys and it has received a packet from the server which is not a Version Negotiation packet.¶

A client MUST NOT change the version it uses unless it is in response to a Version Negotiation packet from the server. Once a client receives a packet from the server which is not a Version Negotiation packet, it MUST ignore other Version Negotiation packets on the same connection. Similarly, a client MUST ignore a Version Negotiation packet if it has already received and acted on a Version Negotiation packet.¶

A client MUST ignore a Version Negotiation packet that lists the client’s chosen version.¶

Version negotiation uses unprotected data. The result of the negotiation MUST be revalidated as part of the cryptographic handshake (see Section 7.3.4).¶

QUIC relies on a combined cryptographic and transport handshake to minimize connection establishment latency. QUIC allocates stream 0 for the cryptographic handshake. Version 0x00000001 of QUIC uses TLS 1.3 as described in [QUIC-TLS]; a different QUIC version number could indicate that a different cryptographic handshake protocol is in use.¶

• authenticated key exchange, where
  • a server is always authenticated,
  • a client is optionally authenticated,
  • every connection produces distinct and unrelated keys,
  • keying material is usable for packet protection for both 0-RTT and 1-RTT packets, and
  • 1-RTT keys have forward secrecy
• authenticated values for the transport parameters of the peer (see Section 7.3)
• authenticated confirmation of version negotiation (see Section 7.3.4)
• authenticated negotiation of an application protocol (TLS uses ALPN [RFC7301] for this purpose)
• for the server, the ability to carry data that provides assurance that the client can receive packets that are addressed with the transport address that is claimed by the client (see Section 7.5)

The initial cryptographic handshake message MUST be sent in a single packet. Any second attempt that is triggered by address validation MUST also be sent within a single packet. This avoids having to reassemble a message from multiple packets. Reassembling messages requires that a server maintain state prior to establishing a connection, exposing the server to a denial of service risk.¶

The first client packet of the cryptographic handshake protocol MUST fit within a 1232 octet QUIC packet payload. This includes overheads that reduce the space available to the cryptographic handshake protocol.¶

Details of how TLS is integrated with QUIC is provided in more detail in [QUIC-TLS].¶

During connection establishment, both endpoints make authenticated declarations of their transport parameters. These declarations are made unilaterally by each endpoint. Endpoints are required to comply with the restrictions implied by these parameters; the description of each parameter includes rules for its handling.¶

The format of the transport parameters is the TransportParameters struct from Figure 6. This is described using the presentation language from Section 3 of [I-D.ietf-tls-tls13].¶

The extension_data field of the quic_transport_parameters extension defined in [QUIC-TLS] contains a TransportParameters value. TLS encoding rules are therefore used to encode the transport parameters.¶

QUIC encodes transport parameters into a sequence of octets, which are then included in the cryptographic handshake. Once the handshake completes, the transport parameters declared by the peer are available. Each endpoint validates the value provided by its peer. In particular, version negotiation MUST be validated (see Section 7.3.4) before the connection establishment is considered properly complete.¶

Definitions for each of the defined transport parameters are included in Section 7.3.1.¶

A server can process an initial cryptographic handshake messages from a client without committing any state. This allows a server to perform address validation (Section 7.5, or to defer connection establishment costs.¶

A server that generates a response to an initial packet without retaining connection state MUST use the Server Stateless Retry packet (Section 5.4.2). This packet causes a client to reset its transport state and to continue the connection attempt with new connection state while maintaining the state of the cryptographic handshake.¶

A server MUST NOT send multiple Server Stateless Retry packets in response to a client handshake packet. Thus, any cryptographic handshake message that is sent MUST fit within a single packet.¶

In TLS, the Server Stateless Retry packet type is used to carry the HelloRetryRequest message.¶

Transport protocols commonly spend a round trip checking that a client owns the transport address (IP and port) that it claims. Verifying that a client can receive packets sent to its claimed transport address protects against spoofing of this information by malicious clients.¶

This technique is used primarily to avoid QUIC from being used for traffic amplification attack. In such an attack, a packet is sent to a server with spoofed source address information that identifies a victim. If a server generates more or larger packets in response to that packet, the attacker can use the server to send more data toward the victim than it would be able to send on its own.¶

Several methods are used in QUIC to mitigate this attack. Firstly, the initial handshake packet is padded to at least 1280 octets. This allows a server to send a similar amount of data without risking causing an amplification attack toward an unproven remote address.¶

A server eventually confirms that a client has received its messages when the cryptographic handshake successfully completes. This might be insufficient, either because the server wishes to avoid the computational cost of completing the handshake, or it might be that the size of the packets that are sent during the handshake is too large. This is especially important for 0-RTT, where the server might wish to provide application data traffic - such as a response to a request - in response to the data carried in the early data from the client.¶

To send additional data prior to completing the cryptographic handshake, the server then needs to validate that the client owns the address that it claims.¶

Source address validation is therefore performed during the establishment of a connection. TLS provides the tools that support the feature, but basic validation is performed by the core transport protocol.¶

QUIC connections are identified by their 64-bit Connection ID. QUIC’s consistent connection ID allows connections to survive changes to the client’s IP and/or port, such as those caused by client or server migrating to a new network. Connection migration allows a client to retain any shared state with a connection when they move networks. This includes state that can be hard to recover such as outstanding requests, which might otherwise be lost with no easy way to retry them.¶

• application close (Section 7.7.2)
• idle timeout (Section 7.7.3)
• immediate close (Section 7.7.4)
• stateless reset (Section 7.7.5)

The 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.¶

A PADDING frame has no content. That is, a PADDING frame consists of the single octet that identifies the frame as a PADDING frame.¶

An endpoint may use a RST_STREAM frame (type=0x01) to abruptly terminate a stream.¶

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

Stream ID:

The 32-bit Stream ID of the stream being terminated.

Error code:

A 32-bit error code which indicates why the stream is being closed.

Final offset:

A 64-bit unsigned integer indicating the absolute byte offset of the end of data written on this stream by the RST_STREAM sender.

Error Code:

A 32-bit error code which indicates the reason for closing this connection.

Reason Phrase Length:

A 16-bit unsigned number specifying the length of the reason phrase in bytes. Note that a CONNECTION_CLOSE frame cannot be split between packets, so in practice 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 to not give details beyond the Error Code. This SHOULD be a UTF-8 encoded string [RFC3629].

The MAX_DATA frame (type=0x04) 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.¶

Maximum Data:

A 64-bit unsigned integer indicating the maximum amount of data that can be sent on the entire connection, in units of 1024 octets. That is, the updated connection-level data limit is determined by multiplying the encoded value by 1024.

All data sent in STREAM frames counts toward this limit, with the exception of data on stream 0. The sum of the largest received offsets on all streams - including closed streams, but excluding stream 0 - MUST NOT exceed the value advertised by a receiver. An endpoint MUST terminate a connection with a QUIC_FLOW_CONTROL_RECEIVED_TOO_MUCH_DATA error if it receives more data than the maximum data value that it has sent, unless this is a result of a change in the initial limits (see Section 7.3.2).¶

The MAX_STREAM_DATA frame (type=0x05) is used in flow control to inform a peer of the maximum amount of data that can be sent on a stream.¶

Stream ID:

The stream ID of the stream that is affected.

Maximum Stream Data:

A 64-bit unsigned integer indicating the maximum amount of data that can be sent on the identified stream, in units of octets.

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, unless this is a result of a change in the initial limits (see Section 7.3.2).¶

The MAX_STREAM_ID frame (type=0x06) informs the peer of the maximum stream ID that they are permitted to open.¶

Maximum Stream ID:

ID of the maximum peer-initiated stream ID for the connection.

Loss or reordering can mean that a MAX_STREAM_ID frame can be received which states a lower stream limit than the client has previously received. MAX_STREAM_ID frames which do not increase the maximum stream ID MUST be ignored.¶

A peer MUST NOT initiate a stream with a higher stream ID than the greatest maximum stream ID it has received. An endpoint MUST terminate a connection with a STREAM_ID_ERROR error if a peer initiates a stream with a higher stream ID than it has sent, unless this is a result of a change in the initial limits (see Section 7.3.2).¶

Endpoints can use PING frames (type=0x07) to verify that their peers are still alive or to check reachability to the peer. The PING frame contains no additional fields. The receiver of a PING frame simply needs to acknowledge the packet containing this frame.¶

A PING frame has no additional fields.¶

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. An application protocol SHOULD provide guidance about the conditions under which generating a PING is recommended. This guidance SHOULD indicate whether it is the client or the server that is expected to send the PING. Having both endpoints send PING frames without coordination can produce an excessive number of packets and poor performance.¶

A connection will time out if no packets are sent or received for a period longer than the time specified in the idle_timeout transport parameter (see Section 7.7). However, state in middleboxes might time out earlier than that. Though REQ-5 in [RFC4787] recommends a 2 minute timeout interval, experience shows that sending packets every 15 to 30 seconds is necessary to prevent the majority of middleboxes from losing state for UDP flows.¶

A sender sends a BLOCKED frame (type=0x08) when it wishes to send data, but is unable to due to connection-level flow control (see Section 11.2.1). BLOCKED frames can be used as input to tuning of flow control algorithms (see Section 11.1.2).¶

The BLOCKED frame does not contain a payload.¶

A sender sends a STREAM_BLOCKED frame (type=0x09) when it wishes to send data, but is unable to due to stream-level flow control. This frame is analogous to BLOCKED (Section 8.8).¶

Stream ID:

A 32-bit unsigned number indicating the stream which is flow control blocked.

A sender MAY send a STREAM_ID_BLOCKED frame (type=0x0a) when it wishes to open a stream, but is unable to due to the maximum stream ID limit set by its peer (see Section 8.6). This does not open the stream, but informs the peer that a new stream was needed, but the stream limit prevented the creation of the stream.¶

The STREAM_ID_BLOCKED frame does not contain a payload.¶

A server sends a NEW_CONNECTION_ID frame (type=0x0b) to provide the client with alternative connection IDs that can be used to break linkability when migrating connections (see Section 7.6.1).¶

Sequence:

A 16-bit sequence number. This value starts at 0 and increases by 1 for each connection ID that is provided by the server. The sequence value can wrap; the value 65535 is followed by 0. When wrapping the sequence field, the server MUST ensure that a value with the same sequence has been received and acknowledged by the client. The connection ID that is assigned during the handshake is assumed to have a sequence of 65535.

Connection ID:

A 64-bit connection ID.

Stateless Reset Token:

A 128-bit value that will be used to for a stateless reset when the associated connection ID is used (see Section 7.7.5).

Stream ID:

The 32-bit Stream ID of the stream being ignored.

Error Code:

The application-specified reason the sender is ignoring the stream.

Receivers send ACK frames to inform senders which packets they have received and processed, as well as which packets are considered missing. The ACK frame contains between 1 and 256 ACK blocks. ACK blocks are ranges of acknowledged packets. Implementations SHOULD NOT generate ACK packets in response to packets which only contain ACKs. However, they SHOULD ACK those packets when sending ACKs for other packets.¶

To limit ACK blocks to those that have not yet been received by the sender, the receiver SHOULD track which ACK frames have been acknowledged by its peer. Once an ACK frame has been acknowledged, the packets it acknowledges SHOULD not be acknowledged again.¶

A receiver that is only sending ACK frames will not receive acknowledgments for its packets. Sending an occasional MAX_DATA or MAX_STREAM_DATA frame as data is received will ensure that acknowledgements are generated by a peer. Otherwise, an endpoint MAY send a PING frame once per RTT to solicit an acknowledgment.¶

To limit receiver state or the size of ACK frames, a receiver MAY limit the number of ACK blocks it sends. A receiver can do this even without receiving acknowledgment of its ACK frames, with the knowledge this could cause the sender to unnecessarily retransmit some data. When this is necessary, the receiver SHOULD acknowledge newly received packets and stop acknowledging packets received in the past.¶

Unlike TCP SACKs, QUIC ACK blocks are irrevocable. Once a packet has been acknowledged, even if it does not appear in a future ACK frame, it remains acknowledged.¶

A client MUST NOT acknowledge Version Negotiation or Server Stateless Retry packets. These packet types contain packet numbers selected by the client, not the server.¶

QUIC ACK frames contain a timestamp section with up to 255 timestamps. Timestamps enable better congestion control, but are not required for correct loss recovery, and old timestamps are less valuable, so it is not guaranteed every timestamp will be received by the sender. A receiver SHOULD send a timestamp exactly once for each received packet containing retransmittable frames. A receiver MAY send timestamps for non-retransmittable packets. A receiver MUST not send timestamps in unprotected packets.¶

A sender MAY intentionally skip packet numbers to introduce entropy into the connection, to avoid opportunistic acknowledgement attacks. The sender SHOULD close the connection if an unsent packet number is acknowledged. The format of the ACK frame is efficient at expressing blocks of missing packets; skipping packet numbers between 1 and 255 effectively provides up to 8 bits of efficient entropy on demand, which should be adequate protection against most opportunistic acknowledgement attacks.¶

• The first three bits must be set to 101 indicating that this is an ACK frame.
• The N bit indicates whether the frame contains a Num Blocks field.
• The two LL bits encode the length of the Largest Acknowledged field. The values 00, 01, 02, and 03 indicate lengths of 8, 16, 32, and 64 bits respectively.
• The two MM bits encode the length of the ACK Block Length fields. The values 00, 01, 02, and 03 indicate lengths of 8, 16, 32, and 64 bits respectively.

An ACK frame is shown below.¶

Num Blocks (opt):

An optional 8-bit unsigned value specifying the number of additional ACK blocks (besides the required First ACK Block) in this ACK frame. Only present if the ‘N’ flag bit is 1.

Num Timestamps:

An unsigned 8-bit number specifying the total number of <packet number, timestamp> pairs in the Timestamp Section.

Largest Acknowledged:

A variable-sized unsigned value representing the largest packet number the peer is acknowledging in this packet (typically the largest that the peer has seen thus far.)

ACK Delay:

The time from when the largest acknowledged packet, as indicated in the Largest Acknowledged field, was received by this peer to when this ACK was sent.

ACK Block Section:

Contains one or more blocks of packet numbers which have been successfully received, see Section 8.13.1.

Timestamp Section:

Contains zero or more timestamps reporting transit delay of received packets. See Section 8.13.2.

• The first two bits must be set to 11, indicating that this is a STREAM frame.
• F is the FIN bit, which is used for stream termination.
• The SS bits encode the length of the Stream ID header field. The values 00, 01, 02, and 03 indicate lengths of 8, 16, 24, and 32 bits long respectively.
• The OO bits encode the length of the Offset header field. The values 00, 01, 02, and 03 indicate lengths of 0, 16, 32, and 64 bits long respectively.
• The D bit indicates whether a Data Length field is present in the STREAM header. When set to 0, this field indicates that the Stream Data field extends to the end of the packet. When set to 1, this field indicates that Data Length field contains the length (in bytes) of the Stream Data field. The option to omit the length should only be used when the packet is a “full-sized” packet, to avoid the risk of corruption via padding.

A STREAM frame is shown below.¶

Stream ID:

The stream ID of the stream (see Section 10.1).

Offset:

A variable-sized unsigned number specifying the byte offset in the stream for the data in this STREAM frame. When the offset length is 0, the offset is 0. The first byte in the stream has an offset of 0. The largest offset delivered on a stream - the sum of the re-constructed offset and data length - MUST be less than 2^64.

Data Length:

An optional 16-bit unsigned number specifying the length of the Stream Data field in this STREAM frame. This field is present when the D bit is set to 1.

Stream Data:

The bytes from the designated stream to be delivered.

A stream frame’s Stream Data MUST NOT be empty, unless the FIN bit is set. When the FIN flag is sent on an empty STREAM frame, the offset in the STREAM frame is the offset of the next byte that would be sent.¶

Stream multiplexing is achieved by interleaving STREAM frames from multiple streams into one or more QUIC packets. A single QUIC packet can include multiple STREAM frames from one or more streams.¶

Implementation note: One of the benefits of QUIC is avoidance of head-of-line blocking across multiple streams. When a packet loss occurs, only streams with data in that packet are blocked waiting for a retransmission to be received, while other streams can continue making progress. Note that when data from multiple streams is bundled into a single QUIC packet, loss of that packet blocks all those streams from making progress. An implementation is therefore advised to bundle as few streams as necessary in outgoing packets without losing transmission efficiency to underfilled packets.¶

Traditional ICMP-based path MTU discovery in IPv4 [RFC1191] is potentially vulnerable to off-path attacks that successfully guess the IP/port 4-tuple and reduce the MTU to a bandwidth-inefficient value. TCP connections mitigate this risk by using the (at minimum) 8 bytes of transport header echoed in the ICMP message to validate the TCP sequence number as valid for the current connection. However, as QUIC operates over UDP, in IPv4 the echoed information could consist only of the IP and UDP headers, which usually has insufficient entropy to mitigate off-path attacks.¶

• Set the IPv4 Don’t Fragment (DF) bit on a small proportion of packets, so that most invalid ICMP messages arrive when there are no DF packets outstanding, and can therefore be identified as spurious.
• Store additional information from the IP or UDP headers from DF packets (for example, the IP ID or UDP checksum) to further authenticate incoming Datagram Too Big messages.
• Any reduction in PMTU due to a report contained in an ICMP packet is provisional until QUIC’s loss detection algorithm determines that the packet is actually lost.

Streams are identified by an unsigned 32-bit integer, referred to as the Stream ID. To avoid Stream ID collision, clients initiate streams using odd-numbered Stream IDs; streams initiated by the server use even-numbered Stream IDs.¶

Stream ID 0 (0x0) is reserved for the cryptographic handshake. Stream 0 MUST NOT be used for application data, and is the first client-initiated stream.¶

A QUIC endpoint cannot reuse a Stream ID. Streams MUST be created in sequential order. Open streams can be used in any order. Streams that are used out of order result in lower-numbered streams in the same direction being counted as open.¶

Stream IDs are usually encoded as a 32-bit integer, though the STREAM frame (Section 8.14) permits a shorter encoding when the leading bits of the stream ID are zero.¶

The semantics of QUIC streams is based on HTTP/2 streams, and the lifecycle of a QUIC stream therefore closely follows that of an HTTP/2 stream [RFC7540], with some differences to accommodate the possibility of out-of-order delivery due to the use of multiple streams in QUIC. The lifecycle of a QUIC stream is shown in the following figure and described below.¶

Note that this diagram shows stream state transitions and the frames and flags that affect those transitions only. It is possible for a single frame to cause two transitions: receiving a RST_STREAM frame, or a STREAM frame with the FIN flag cause the stream state to move from “idle” to “open” and then immediately to one of the “half-closed” states.¶

The recipient of a frame that changes stream state will have a delayed view of the state of a stream while the frame is in transit. Endpoints do not coordinate the creation of streams; they are created unilaterally by either endpoint. Endpoints can use acknowledgments to understand the peer’s subjective view of stream state at any given time.¶

In the absence of more specific guidance elsewhere in this document, implementations SHOULD treat the receipt of a frame that is not expressly permitted in the description of a state as a connection error (see Section 12).¶

If an endpoint is no longer interested in the data being received, it MAY send a STOP_SENDING frame on a stream in the “open” or “half-closed (local)” state to prompt closure of the stream in the opposite direction. This typically indicates that the receiving application is no longer reading from the stream, but is not a guarantee that incoming data will be ignored.¶

STREAM frames received after sending STOP_SENDING are still counted toward the connection and stream flow-control windows, even though these frames will be discarded upon receipt. This avoids potential ambiguity about which STREAM frames count toward flow control.¶

Upon receipt of a STOP_SENDING frame on a stream in the “open” or “half-closed (remote)” states, an endpoint MUST send a RST_STREAM with an error code of QUIC_RECEIVED_RST. If the STOP_SENDING frame is received on a stream that is already in the “half-closed (local)” or “closed” states, a RST_STREAM frame MAY still be sent in order to cancel retransmission of previously-sent STREAM frames.¶

While STOP_SENDING frames are retransmittable, an implementation MAY choose not to retransmit a lost STOP_SENDING frame if the stream has already been closed in the appropriate direction since the frame was first generated. See Section 9.¶

An endpoint limits the number of concurrently active incoming streams by adjusting the maximum stream ID. An initial value is set in the transport parameters (see Section 7.3.1) and is subsequently increased by MAX_STREAM_ID frames (see Section 8.6).¶

The maximum stream ID is specific to each endpoint and applies only to the peer that receives the setting. That is, clients specify the maximum stream ID the server can initiate, and servers specify the maximum stream ID the client can initiate. Each endpoint may respond on streams initiated by the other peer, regardless of whether it is permitted to initiated new streams.¶

Endpoints MUST NOT exceed the limit set by their peer. An endpoint that receives a STREAM frame with an ID greater than the limit it has sent MUST treat this as a stream error of type STREAM_ID_ERROR (Section 12), unless this is a result of a change in the initial offsets (see Section 7.3.2).¶

A receiver MUST NOT renege on an advertisement; that is, once a receiver advertises a stream ID via a MAX_STREAM_ID frame, it MUST NOT subsequently advertise a smaller maximum ID. A sender may receive MAX_STREAM_ID frames out of order; a sender MUST therefore ignore any MAX_STREAM_ID that does not increase the maximum.¶

Once a stream is created, endpoints may use the stream to send and receive data. Each endpoint may send a series of STREAM frames encapsulating data on a stream until the stream is terminated in that direction. Streams are an ordered byte-stream abstraction, and they have no other structure within them. STREAM frame boundaries are not expected to be preserved in retransmissions from the sender or during delivery to the application at the receiver.¶

When new data is to be sent on a stream, a sender MUST set the encapsulating STREAM frame’s offset field to the stream offset of the first byte of this new data. The first byte of data that is sent on a stream has the stream offset 0. The largest offset delivered on a stream MUST be less than 2^64. A receiver MUST ensure that received stream data is delivered to the application as an ordered byte-stream. Data received out of order MUST be buffered for later delivery, as long as it is not in violation of the receiver’s flow control limits.¶

An endpoint MUST NOT send data on any stream without ensuring that it is within the data limits set by its peer. The cryptographic handshake stream, Stream 0, is exempt from the connection-level data limits established by MAX_DATA. Stream 0 is still subject to stream-level data limits and MAX_STREAM_DATA.¶

Flow control is described in detail in Section 11, and congestion control is described in the companion document [QUIC-RECOVERY].¶

Stream multiplexing has a significant effect on application performance if resources allocated to streams are correctly prioritized. Experience with other multiplexed protocols, such as HTTP/2 [RFC7540], shows that effective prioritization strategies have a significant positive impact on performance.¶

QUIC does not provide frames for exchanging prioritization information. Instead it relies on receiving priority information from the application that uses QUIC. Protocols that use QUIC are able to define any prioritization scheme that suits their application semantics. A protocol might define explicit messages for signaling priority, such as those defined in HTTP/2; it could define rules that allow an endpoint to determine priority based on context; or it could leave the determination to the application.¶

A QUIC implementation SHOULD provide ways in which an application can indicate the relative priority of streams. When deciding which streams to dedicate resources to, QUIC SHOULD use the information provided by the application. Failure to account for priority of streams can result in suboptimal performance.¶

Stream priority is most relevant when deciding which stream data will be transmitted. Often, there will be limits on what can be transmitted as a result of connection flow control or the current congestion controller state.¶

Giving preference to the transmission of its own management frames ensures that the protocol functions efficiently. That is, prioritizing frames other than STREAM frames ensures that loss recovery, congestion control, and flow control operate effectively.¶

Stream 0 MUST be prioritized over other streams prior to the completion of the cryptographic handshake. This includes the retransmission of the second flight of client handshake messages, that is, the TLS Finished and any client authentication messages.¶

STREAM frames that are determined to be lost SHOULD be retransmitted before sending new data, unless application priorities indicate otherwise. Retransmitting lost STREAM frames can fill in gaps, which allows the peer to consume already received data and free up flow control window.¶

There are some edge cases which must be considered when dealing with stream and connection level flow control. Given enough time, both endpoints must agree on flow control state. If one end believes it can send more than the other end is willing to receive, the connection will be torn down when too much data arrives.¶

Conversely if a sender believes it is blocked, while endpoint B expects more data can be received, then the connection can be in a deadlock, with the sender waiting for a MAX_DATA or MAX_STREAM_DATA frame which will never come.¶

On receipt of a RST_STREAM frame, an endpoint will tear down state for the matching stream and ignore further data arriving on that stream. This could result in the endpoints getting out of sync, since the RST_STREAM frame may have arrived out of order and there may be further bytes in flight. The data sender would have counted the data against its connection level flow control budget, but a receiver that has not received these bytes would not know to include them as well. The receiver must learn the number of bytes that were sent on the stream to make the same adjustment in its connection flow controller.¶

To avoid this de-synchronization, a RST_STREAM sender MUST include the final byte offset sent on the stream in the RST_STREAM frame. On receiving a RST_STREAM frame, a receiver definitively knows how many bytes were sent on that stream before the RST_STREAM frame, and the receiver MUST use the final offset to account for all bytes sent on the stream in its connection level flow controller.¶

As with flow control, this document leaves when and how many streams to make available to a peer via MAX_STREAM_ID to implementations, but offers a few considerations. MAX_STREAM_ID frames constitute minimal overhead, while withholding MAX_STREAM_ID frames can prevent the peer from using the available parallelism.¶

Implementations will likely want to increase the maximum stream ID as peer-initiated streams close. A receiver MAY also advance the maximum stream ID based on current activity, system conditions, and other environmental factors.¶

The final offset is the count of the number of octets that are transmitted on a stream. For a stream that is reset, the final offset is carried explicitly in the RST_STREAM frame. Otherwise, the final offset is the offset of the end of the data carried in STREAM frame marked with a FIN flag.¶

An endpoint will know the final offset for a stream when the stream enters the “half-closed (remote)” state. However, if there is reordering or loss, an endpoint might learn the final offset prior to entering this state if it is carried on a STREAM frame.¶

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

Once a final offset for a stream is known, it cannot change. If a RST_STREAM or STREAM frame causes the final offset to change for a stream, an endpoint SHOULD respond with a FINAL_OFFSET_ERROR error (see Section 12). A receiver SHOULD treat receipt of data at or beyond the final offset as a FINAL_OFFSET_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 offset state for closed streams, which could mean a significant state commitment.¶

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 8.3). An endpoint MAY close the connection in this manner, even if the error only affects a single stream.¶

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 CONNECTION_CLOSE 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 7.7.5).¶

An endpoint that receives an invalid CONNECTION_CLOSE frame MUST NOT signal the existence of the error to its peer.¶

If the error affects a single stream, but otherwise leaves the connection in a recoverable state, the endpoint can send a RST_STREAM frame (Section 8.2) with an appropriate error code to terminate just the affected stream.¶

Stream 0 is critical to the functioning of the entire connection. If stream 0 is closed with either a RST_STREAM or STREAM frame bearing the FIN flag, an endpoint MUST generate a connection error of type PROTOCOL_VIOLATION.¶

Some application protocols make other streams critical to that protocol. An application protocol does not need to inform the transport that a stream is critical; it can instead generate appropriate errors in response to being notified that the critical stream is closed.¶

An endpoint MAY send a RST_STREAM frame in the same packet as a CONNECTION_CLOSE frame.¶

0x00000000-0x3FFFFFFF:

Application-specific error codes. Defined by each application-layer protocol.

0x40000000-0x7FFFFFFF:

Reserved for host-local error codes. These codes MUST NOT be sent to a peer, but MAY be used in API return codes and logs.

0x80000000-0xBFFFFFFF:

QUIC transport error codes, including packet protection errors. Applicable to all uses of QUIC.

0xC0000000-0xFFFFFFFF:

Cryptographic error codes. Defined by the cryptographic handshake protocol in use.

This section lists the defined QUIC transport error codes that may be used in a CONNECTION_CLOSE or RST_STREAM frame. Error codes share a common code space. Some error codes apply only to either streams or the entire connection and have no defined semantics in the other context.¶

NO_ERROR (0x80000000):

An endpoint uses this with CONNECTION_CLOSE to signal that the connection is being closed abruptly in the absence of any error. An endpoint uses this with RST_STREAM to signal that the stream is no longer wanted or in response to the receipt of a RST_STREAM for that stream.

INTERNAL_ERROR (0x80000001):

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

CANCELLED (0x80000002):

An endpoint sends this with RST_STREAM to indicate that the stream is not wanted and that no application action was taken for the stream. This error code is not valid for use with CONNECTION_CLOSE.

FLOW_CONTROL_ERROR (0x80000003):

An endpoint received more data than it permitted in its advertised data limits (see Section 11).

STREAM_ID_ERROR (0x80000004):

An endpoint received a frame for a stream identifier that exceeded its advertised maximum stream ID.

STREAM_STATE_ERROR (0x80000005):

An endpoint received a frame for a stream that was not in a state that permitted that frame (see Section 10.2).

FINAL_OFFSET_ERROR (0x80000006):

An endpoint received a STREAM frame containing data that exceeded the previously established final offset. Or an endpoint received a RST_STREAM frame containing a final offset that was lower than the maximum offset of data that was already received. Or an endpoint received a RST_STREAM frame containing a different final offset to the one already established.

FRAME_FORMAT_ERROR (0x80000007):

An endpoint received a frame that was badly formatted. For instance, an empty STREAM frame that omitted the FIN flag, or an ACK frame that has more acknowledgment ranges than the remainder of the packet could carry. This is a generic error code; an endpoint SHOULD use the more specific frame format error codes (0x800001XX) if possible.

TRANSPORT_PARAMETER_ERROR (0x80000008):

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.

VERSION_NEGOTIATION_ERROR (0x80000009):

An endpoint received transport parameters that contained version negotiation parameters that disagreed with the version negotiation that it performed. This error code indicates a potential version downgrade attack.

PROTOCOL_VIOLATION (0x8000000A):

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

QUIC_RECEIVED_RST (0x80000035):

Terminating stream because peer sent a RST_STREAM or STOP_SENDING.

FRAME_ERROR (0x800001XX):

An endpoint detected an error in a specific frame type. The frame type is included as the last octet of the error code. For example, an error in a MAX_STREAM_ID frame would be indicated with the code (0x80000106).

An attacker receives an STK from the server and then releases the IP address on which it received the STK. The attacker may, in the future, spoof this same address (which now presumably addresses a different endpoint), and initiate a 0-RTT connection with a server on the victim’s behalf. The attacker then spoofs ACK frames to the server which cause the server to potentially drown the victim in data.¶

There are two possible mitigations to this attack. The simplest one is that a server can unilaterally create a gap in packet-number space. In the non-attack scenario, the client will send an ACK frame with the larger value for largest acknowledged. In the attack scenario, the attacker could acknowledge a packet in the gap. If the server sees an acknowledgment for a packet that was never sent, the connection can be aborted.¶

The second mitigation is that the server can require that acknowledgments for sent packets match the encryption level of the sent packet. This mitigation is useful if the connection has an ephemeral forward-secure key that is generated and used for every new connection. If a packet sent is protected with a forward-secure key, then any acknowledgments that are received for them MUST also be forward-secure protected. Since the attacker will not have the forward secure key, the attacker will not be able to generate forward-secure protected packets with ACK frames.¶

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 endpoint might intentionally fragment the data on stream buffers in order to cause disproportionate memory commitment. An adversarial endpoint could open a stream and send some STREAM frames containing arbitrary fragments of the stream content.¶

The attack 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 the stream fragmentation attack. Mitigations could consist of avoiding over-committing memory, delaying reassembly of STREAM frames, implementing heuristics based on the age and duration of reassembly holes, or some combination.¶

An adversarial endpoint can open lots 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 10.1. However, when several streams are initiated at short intervals, transmission error may cause STREAM DATA frames opening streams to be received out of sequence. A receiver is obligated to open intervening streams if a higher-numbered stream ID is received. Thus, on a new connection, opening stream 2000001 opens 1 million streams, as required by the specification.¶

The number of active streams is limited by the concurrent stream limit transport parameter, as explained in Section 10.4. If chosen judisciously, this limit mitigates 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.¶

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

The “QUIC Transport Parameters” registry governs a 16-bit space. This space is split into two spaces that are governed by different policies. Values with the first byte in the range 0x00 to 0xfe (in hexadecimal) are assigned via the Specification Required policy [RFC5226]. Values with the first byte 0xff are reserved for Private Use [RFC5226].¶

Value:

The numeric value of the assignment (registrations will be between 0x0000 and 0xfeff).

Parameter Name:

A short mnemonic for the parameter.

Specification:

A reference to a publicly available specification for the value.

The nominated expert(s) verify that a specification exists and is readily accessible. The expert(s) are encouraged to be biased towards approving registrations unless they are abusive, frivolous, or actively harmful (not merely aesthetically displeasing, or architecturally dubious).¶

The initial contents of this registry are shown in Table 4.¶

• 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, RST_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 RST_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 RST_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)
• RST_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 RST_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