QUIC: A UDP-Based Multiplexed and Secure Transport

QUIC combines version negotiation with the rest of connection establishment to avoid unnecessary roundtrip delays. A QUIC client proposes a version to use for the connection, and encodes the rest of the handshake using the proposed version. If the server does not speak the client-chosen version, it forces version negotiation by sending back a Version Negotiation packet to the client, causing a roundtrip of delay before connection establishment.¶

This mechanism eliminates roundtrip latency when the client’s optimistically-chosen version is spoken by the server, and incentivizes servers to not lag behind clients in deployment of newer versions. Additionally, an application may negotiate QUIC versions out-of-band to increase chances of success in the first roundtrip and to obviate the additional roundtrip in the case of version mismatch.¶

QUIC relies on a combined crypto 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 1) to be used for performing the crypto 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 1 is described in the accompanying crypto 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 acks 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, closely following HTTP/2’s flow control mechanisms. At a high level, a QUIC receiver advertises the absolute byte offset within each stream up to which the receiver is willing to receive data. As data is sent, received, and delivered on a particular stream, the receiver sends WINDOW_UPDATE frames that increase the advertised offset 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 highest received offset are all aggregates 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 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 under crypto cover.¶

PUBLIC_RESET packets that reset a connection are currently not authenticated.¶

QUIC connections are identified by a 64-bit Connection ID, randomly generated by the client. 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 client, 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.¶

All QUIC packets begin with a QUIC Common header, as shown below.¶

• Flags:
  • 0x01 = VERSION. The semantics of this flag depends on whether the packet is sent by the server or the client. A client MAY set this flag and include exactly one proposed version. A server may set this flag when the client-proposed version was unsupported, and may then provide a list (0 or more) of acceptable versions as a part of version negotiation (described in Section XXX.)
  • 0x02 = PUBLIC_RESET. Set to indicate that the packet is a Public Reset packet.
  • 0x04 = DIVERSIFICATION_NONCE. Set to indicate the presence of a 32-byte diversification nonce in the header. (DISCUSS_AND_MODIFY: This flag should be removed along with the Diversification Nonce bits, as discussed further below.)
  • 0x08 = CONNECTION_ID. Indicates the Connection ID is present in the packet. This must be set in all packets until negotiated to a different value for a given direction. For instance, if a client indicates that the 5-tuple fully identifies the connection at the client, the connection ID is optional in the server-to-client direction.
  • 0x30 = PACKET_NUMBER_SIZE. These two bits indicate the number of low-order-bytes of the packet number that are present in each packet.
    • 11 indicates that 6 bytes of the packet number are present
    • 10 indicates that 4 bytes of the packet number are present
    • 01 indicates that 2 bytes of the packet number are present
    • 00 indicates that 1 byte of the packet number is present
  • 0x40 = MULTIPATH. This bit is reserved for multipath use.
  • 0x80 is currently unused, and must be set to 0.
• Connection ID: An unsigned 64-bit random number chosen by the client, used as the identifier of the connection. Connection ID is tied to a QUIC connection, and remains consistent across client and/or server IP and port changes.

• QUIC Version: A 32-bit opaque tag that represents the version of the QUIC protocol. Only present in the client-to-server direction, and if the VERSION flag is set. Version Negotiation is described in Section XXX.
• DISCUSS_AND_REPLACE: Diversification Nonce: A 32-byte nonce generated by the server and used only in the Server->Client direction to ensure that the server is able to generate unique keys per connection. Specifically, when using QUIC’s 0-RTT crypto handshake, a repeated CHLO with the exact same connection ID and CHLO can lead to the same (intermediate) initial-encryption keys being derived for the connection. A server-generated nonce disallows a client from causing the same keys to be derived for two distinct connections. Once the connection is forward-secure, this nonce is no longer present in packets. This nonce can be removed from the packet header if a requirement can be added for the crypto handshake to ensure key uniqueness. The expectation is that TLS1.3 meets this requirement. Upon working group adoption of this document, this requirement should be added to the crypto handshake requirements, and the nonce should be removed from the packet format.
• Packet Number: The lower 8, 16, 32, or 48 bits of the packet number, based on the PACKET_NUMBER_SIZE flag. Each Regular packet is assigned a packet number by the sender. The first packet sent by an endpoint MUST have a packet number of 1.
• AEAD Data: A Regular packet’s header, which includes the Common Header, and the Version, Diversification Nonce, and Packet Number fields, is authenticated but not encrypted. The rest of a Regular packet, starting with the first frame, is both authenticated and encrypted. Immediately following the header, Regular packets contain AEAD (Authenticated Encryption with Associated Data) data. This data must be decrypted in order for the contents to be interpreted. After decryption, the plaintext consists of a sequence of frames, as shown (frames are described in Section XXX).

A Version Negotiation packet is only sent by the server, MUST have the VERSION flag set, and MUST include the full 64-bit Connection ID. The rest of the Version Negotiation packet is a list of 4-byte versions which the server supports, as shown below.¶

A Public Reset packet MUST have the PUBLIC_RESET flag set, and MUST include the full 64-bit connection ID. The rest of the Public Reset packet is encoded as if it were a crypto handshake message of the tag PRST, as shown below.¶

• RNON (public reset nonce proof) - a 64-bit unsigned integer.
• RSEQ (rejected packet number) - a 64-bit packet number.
• CADR (client address) - the observed client IP address and port number. This is currently for debugging purposes only and hence is optional.

DISCUSS_AND_REPLACE: The crypto handshake message format is described in the QUIC crypto document, and should be replaced with something simpler when this document is adopted. The purpose of the tag-value map following the PRST tag is to enable the receiver of the Public Reset packet to reasonably authenticate the packet. This map is an extensible map format that allows specification of various tags, which should again be replaced by something simpler.¶

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 XX, but all of the initial packets sent from the client to the server MUST have the VERSION flag set, and MUST specify the version of the protocol being used.¶

When the server receives a packet from a client with the VERSION flag set for a connection that has not yet been established, it compares the client’s version to the versions it supports.¶

• If the client’s version is acceptable to the server, the server MUST use this protocol version for the lifetime of the connection. All subsequent packets sent by the server MUST have the version flag off.
• If the client’s version is not acceptable to the server, the server MUST send a Version Negotiation packet to the client. This packet will have the VERSION flag set and will include the server’s set of supported versions. On subsequently received packets for the same connection ID with the unacceptable version, the server MUST continue responding with a Version Negotiation packet.

When the client receives a Version Negotiation packet from the server, it should select an acceptable protocol version. If such a version is found, the client MUST resend all packets using the new version, and the resent packets MUST use new packet numbers. These packets MUST continue to have the VERSION flag set and MUST include the new negotiated protocol version.¶

The client MUST send its version on all packets until it receives a packet from the server with the VERSION flag off. If version negotiation is successful, the client should receive a packet from the server with the VERSION flag off indicating the end of version negotiation. All subsequent packets the client sends MUST have the version flag off.¶

Once the server receives a packet from the client with the VERSION flag off, it MUST ignore the VERSION flag in subsequently received packets.¶

The Version Negotiation packet is unencrypted and exchanged without authentication. To avoid a downgrade attack, the client needs to verify its record of the server’s version list in the Version Negotiation packet and the server needs to verify its record of the client’s originally proposed version. Therefore, the client and server MUST include this information later in their corresponding crypto handshake data.¶

QUIC relies on a combined crypto and transport handshake to minimize connection establishment latency. QUIC provides a dedicated stream (Stream ID 1) to be used for performing a combined connection and security handshake (streams are described in detail in Section XXX). The crypto handshake protocol encapsulates and delivers QUIC’s transport handshake to the peer on the crypto stream. The first QUIC packet from the client to the server MUST carry handshake information as data on Stream ID 1.¶

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. QUIC also provides automatic cryptographic verification of a rebound client, since the client continues to use the same session key for encrypting and decrypting packets.¶

DISCUSS: Simultaneous migration. Is this reasonable?¶

TODO: Perhaps move mitigation techniques from Security Considerations here.¶

1. Explicit Shutdown: An endpoint sends a CONNECTION_CLOSE frame to the peer initiating a connection termination. An endpoint may send a GOAWAY frame to the peer prior to a CONNECTION_CLOSE to indicate that the connection will soon be terminated. A GOAWAY frame signals to the peer that any active streams will continue to be processed, but the sender of the GOAWAY will not initiate any additional streams and will not accept any new incoming streams. On termination of the active streams, a CONNECTION_CLOSE may be sent. If an endpoint sends a CONNECTION_CLOSE frame while unterminated streams are active (no FIN bit or RST_STREAM frames have been sent or received for one or more streams), then the peer must assume that the streams were incomplete and were abnormally terminated.
2. Implicit Shutdown: The default idle timeout for a QUIC connection is 30 seconds, and is a required parameter (ICSL) in connection negotiation. The maximum is 10 minutes. If there is no network activity for the duration of the idle timeout, the connection is closed. By default a CONNECTION_CLOSE frame will be sent. A silent close option can be enabled when it is expensive to send an explicit close, such as mobile networks that must wake up the radio.
3. Abrupt Shutdown: An endpoint may send a Public Reset packet at any time during the connection to abruptly terminate an active connection. A Public Reset packet SHOULD only be used as a final recourse. Commonly, a public reset is expected to be sent when a packet on an established connection is received by an endpoint that is unable decrypt the packet. For instance, if a server reboots mid-connection and loses any cryptographic state associated with open connections, and then receives a packet on an open connection, it should send a Public Reset packet in return. (TODO: articulate rules around when a public reset should be sent.)

TODO: Connections that are terminated are added to a TIME_WAIT list at the server, so as to absorb any straggler packets in the network. Discuss TIME_WAIT list.¶

STREAM frames implicitly create a stream and carry stream data. A STREAM frame is shown below.¶

• Frame Type: The Frame Type byte is an 8-bit value containing various flags, and is formatted as the following 8 bits: 1FDOOOSS.
  • The leftmost bit must be set to 1 indicating that this is a STREAM frame.
  • ‘F’ is the FIN bit, which is used for stream termination.
  • 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.
  • The ‘OOO’ bits encode the length of the Offset header field as 0, 16, 24, 32, 40, 48, 56, or 64 bits long.
  • The ‘SS’ bits encode the length of the Stream ID header field as 8, 16, 24, or 32 bits. (DISCUSS: Consider making this 8, 16, 32, 64.)
• Stream ID: A variable-sized unsigned ID unique to this stream.
• Offset: A variable-sized unsigned number specifying the byte offset in the stream for the data in this STREAM frame. The first byte in the stream has an offset of 0.
• Data Length: An optional 16-bit unsigned number specifying the length of the Stream Data field in this STREAM frame.

A STREAM frame MUST have either non-zero data length or the FIN bit set.¶

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

Receivers send ACK frames to inform senders which packets they have received, 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.¶

To limit the ACK blocks to the ones that haven’t yet been received by the sender, the sender periodically sends STOP_WAITING frames that signal the receiver to stop acking packets below a specified sequence number, raising the “least unacked” packet number at the receiver. A sender of an ACK frame thus reports only those ACK blocks between the received least unacked and the reported largest observed packet numbers. It is recommended for the sender to send the most recent largest acked packet it has received in an ack as the STOP_WAITING frame’s least unacked value.¶

Unlike TCP SACKs, QUIC ACK blocks are irrevocable. Once a packet is acked, even if it does not appear in a future ack frame, it is assumed to be acked.¶

A sender MAY intentionally skip packet numbers to introduce entropy into the connection, to avoid opportunistic ack attacks. The sender MUST close the connection if an unsent packet number is acked. 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 ack attacks.¶

• Frame Type: The Frame Type byte is an 8-bit value containing various flags. This byte is formatted as the following 8 bits: 01NULLMM.
  • The first two bits must be set to 01 indicating that this is an ACK frame.
  • The ‘N’ bit indicates whether the frame has more than 1 ack range.
  • The ‘U’ bit is unused.
  • The two ‘LL’ bits encode the length of the Largest Acked field as 1, 2, 4, or 6 bytes long.
  • The two ‘MM’ bits encode the length of the Ack Block Length fields as 1, 2, 4, or 6 bytes long.
• Largest Acked: A variable-sized unsigned value representing the largest packet number the peer is acking in this packet (typically the largest that the peer has seen thus far.)
• Ack Delay: Time from when the largest acked, as indicated in the Largest Acked field, was received by this peer to when this ack was sent.
• Ack Block Section:
  • 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.
  • First Ack Block Length: An unsigned packet number delta that indicates the number of contiguous additional packets being acked starting at the Largest Acked.
  • Gap To Next Block (opt, repeated): An unsigned number specifying the number of contiguous missing packets from the end of the previous ack block to the start of the next.
  • Ack Block Length (opt, repeated): An unsigned packet number delta that indicates the number of contiguous packets being acked starting after the end of the previous gap. Along with the previous field, this field is repeated “Num Blocks” times.
• Timestamp Section:
  • Num Timestamps: An unsigned 8-bit number specifying the total number of <packet number, timestamp> pairs following, including the First Timestamp.
  • Delta Largest Acked (opt): An optional 8-bit unsigned packet number delta specifying the delta between the largest acked and the first packet whose timestamp is being reported. In other words, this first packet number may be computed as (Largest Acked - Delta Largest Acked.)
  • First Timestamp (opt): An optional 32-bit unsigned value specifying the time delta in microseconds, from the beginning of the connection to the arrival of this packet.
  • Delta Largest Observed (opt, repeated): (Same as above.)
  • Time Since Previous Timestamp (opt, repeated): An optional 16-bit unsigned value specifying time delta from the previous reported timestamp. It is encoded in the same format as the Ack Delay. Along with the previous field, this field is repeated “Num Timestamps” times.

• Frame Type: The Frame Type byte is an 8-bit value that must be set to 0x06 indicating that this is a STOP_WAITING frame.
• Least Unacked Delta: A variable-length packet number delta with the same length as the packet header’s packet number. Subtract it from the complete packet number of the enclosing packet to determine the least unacked packet number. The resulting least unacked packet number is the earliest packet for which the sender is still awaiting an ack. If the receiver is missing any packets earlier than this packet, the receiver SHOULD consider those packets to be irrecoverably lost and MUST NOT report those packets as missing in subsequent acks.

• Frame Type: The Frame Type byte is an 8-bit value that must be set to 0x04 indicating that this is a WINDOW_UPDATE frame.
• Stream ID: ID of the stream whose flow control windows is being updated, or 0 to specify the connection-level flow control window.
• Byte offset: A 64-bit unsigned integer indicating the absolute byte offset of data which can be sent on the given stream. In the case of connection level flow control, the cumulative number of bytes which can be sent on all currently open streams.

• Frame Type: The Frame Type byte is an 8-bit value that must be set to 0x05 indicating that this is a BLOCKED frame.
• Stream ID: A 32-bit unsigned number indicating the stream which is flow control blocked. A non-zero Stream ID field specifies the stream that is flow control blocked. When zero, the Stream ID field indicates that the connection is flow control blocked.

• Frame type: The Frame Type is an 8-bit value that must be set to 0x01 specifying that this is a RST_STREAM frame.
• Stream ID: The 32-bit Stream ID of the stream being terminated.
• Byte 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 why the stream is being closed.

• Frame Type: An 8-bit value that must be set to 0x02 specifying that this is a CONNECTION_CLOSE frame.
• 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. This may be zero if the sender chooses to not give details beyond the QuicErrorCode.
• Reason Phrase: An optional human-readable explanation for why the connection was closed.

• Frame type: An 8-bit value that must be set to 0x03 specifying that this is a GOAWAY frame.
• Error Code: A 32-bit field error code which indicates the reason for closing this connection.
• Last Good Stream ID: The last Stream ID which was accepted by the sender of the GOAWAY message. If no streams were replied to, this value must be set to 0.
• Reason Phrase Length: A 16-bit unsigned number specifying the length of the reason phrase. This may be zero if the sender chooses to not give details beyond the error code.
• Reason Phrase: An optional human-readable explanation for why the connection was closed.

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. For the purpose of state transitions, the FIN flag is processed as a separate event to the frame that bears it; a STREAM frame with the FIN flag set can cause two state transitions. When the FIN bit is sent on an empty STREAM frame, the offset in the STREAM frame MUST be one greater than the last data byte sent on this stream.¶

Both endpoints have a subjective view of the state of a stream that could be different when frames are in transit. Endpoints do not coordinate the creation of streams; they are created unilaterally by either endpoint. The negative consequences of a mismatch in states are limited to the “closed” state after sending RST_STREAM, where frames might be received for some time after closing.¶

Streams are identified by an unsigned 32-bit integer, referred to as the StreamID. To avoid StreamID collision, clients MUST initiate streams usinge odd-numbered StreamIDs; streams initiated by the server MUST use even-numbered StreamIDs.¶

A StreamID of zero (0x0) is reserved and used for connection-level flow control frames (Section XX); the StreamID of zero cannot be used to establish a new stream.¶

StreamID 1 (0x1) is reserved for the crypto handshake. StreamID 1 MUST NOT be used for application data, and MUST be the first client- initiated stream.¶

Streams MUST be created or reserved in sequential order, but MAY be used in arbitrary order. A QUIC endpoint MUST NOT reuse a StreamID on a given connection.¶

An endpoint can limit the number of concurrently active incoming streams by setting the MSPC parameter (see Section XX) in the transport parameters. The maximum concurrent streams setting is specific to each endpoint and applies only to the peer that receives the setting. That is, clients specify the maximum number of concurrent streams the server can initiate, and servers specify the maximum number of concurrent streams the client can initiate.¶

Streams that are in the “open” state or in either of the “half- closed” states count toward the maximum number of streams that an endpoint is permitted to open. Streams in any of these three states count toward the limit advertised in the MSPC setting.¶

Endpoints MUST NOT exceed the limit set by their peer. An endpoint that receives a STREAM frame that causes its advertised concurrent stream limit to be exceeded MUST treat this as a stream error of type QUIC_TOO_MANY_OPEN_STREAMS (Section XX).¶

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. 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 any stream data without consulting the congestion controller and the flow controller, with the following two exceptions.¶

• The crypto handshake stream, Stream 1, MUST NOT be subject to congestion control or connection-level flow control, but MUST be subject to stream-level flow control.
• An application MAY exclude specific stream IDs from connection- level flow control. If so, these streams MUST NOT be subject to connection-level flow control.

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

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 WINDOW_UPDATE which will never come.¶

An attacker receives an STK from the server and then releases the IP address on which it received the STK. The attacked may in the future, spoof this same address (which now presumably addresses a different endpoint), and initiates a 0-RTT connection with a server on the victim’s behalf. The attacker then spoofs ack packets 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 with a larger largest acked. In the attack scenario, the attacker may ack a packet in the gap. If the server sees an ack for a packet that was never sent, the connection can be aborted.¶

The second mitigation is that the server can require that acks 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 encrypted with a forward-secure key, then any acks that are received for them must also be forward-secure encrypted. Since the attacker will not have the forward secure key, the attacker will not be able to generate forward-secure encrypted ack packets.¶