1.1. Document Structure

This document describes the core QUIC protocol and is structured as follows:¶

• Streams are the basic service abstraction that QUIC provides.¶

  • Section 2 describes core concepts related to streams,¶
  • Section 3 provides a reference model for stream states, and¶
  • Section 4 outlines the operation of flow control.¶

• Connections are the context in which QUIC endpoints communicate.¶

  • Section 5 describes core concepts related to connections,¶
  • Section 6 describes version negotiation,¶
  • Section 7 details the process for establishing connections,¶
  • Section 8 describes address validation and critical denial-of-service mitigations,¶
  • Section 9 describes how endpoints migrate a connection to a new network path,¶
  • Section 10 lists the options for terminating an open connection, and¶
  • Section 11 provides guidance for stream and connection error handling.¶

• Packets and frames are the basic unit used by QUIC to communicate.¶

  • Section 12 describes concepts related to packets and frames,¶
  • Section 13 defines models for the transmission, retransmission, and acknowledgment of data, and¶
  • Section 14 specifies rules for managing the size of datagrams carrying QUIC packets.¶

• Finally, encoding details of QUIC protocol elements are described in:¶

  • Section 15 (versions),¶
  • Section 16 (integer encoding),¶
  • Section 17 (packet headers),¶
  • Section 18 (transport parameters),¶
  • Section 19 (frames), and¶
  • Section 20 (errors).¶

Accompanying documents describe QUIC's loss detection and congestion control [QUIC-RECOVERY], and the use of TLS and other cryptographic mechanisms [QUIC-TLS].¶

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

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

1.2. Terms and Definitions

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

Commonly used terms in this document are described below.¶

QUIC:

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

Endpoint:

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

Client:

The endpoint that initiates a QUIC connection.¶

Server:

The endpoint that accepts a QUIC connection.¶

QUIC packet:

A complete processable unit of QUIC that can be encapsulated in a UDP datagram. One or more QUIC packets can be encapsulated in a single UDP datagram.¶

Ack-eliciting packet:

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

Frame:

A unit of structured protocol information. There are multiple frame types, each of which carries different information. Frames are contained in QUIC packets.¶

Address:

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

Connection ID:

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

Stream:

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

Application:

An entity that uses QUIC to send and receive data.¶

This document uses the terms "QUIC packets", "UDP datagrams", and "IP packets" to refer to the units of the respective protocols. That is, one or more QUIC packets can be encapsulated in a UDP datagram, which is in turn encapsulated in an IP packet.¶

1.3. Notational Conventions

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

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

Individual fields include length information, plus indications about fixed value, optionality, or repetitions. Individual fields use the following notational conventions, with all lengths in bits:¶

x (A):

Indicates that x is A bits long¶

x (i):

Indicates that x holds an integer value using the variable-length encoding described in Section 16¶

x (A..B):

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

x (L) = C:

Indicates that x has a fixed value of C; the length of x is described by L, which can use any of the length forms above¶

x (L) = C..D:

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

[x (L)]:

Indicates that x is optional and has a length of L¶

x (L) ...:

Indicates that x is repeated zero or more times and that each instance has a length of L¶

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

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

Figure 1 provides an example:¶

Example Structure {
  One-bit Field (1),
  7-bit Field with Fixed Value (7) = 61,
  Field with Variable-Length Integer (i),
  Arbitrary-Length Field (..),
  Variable-Length Field (8..24),
  Field With Minimum Length (16..),
  Field With Maximum Length (..128),
  [Optional Field (64)],
  Repeated Field (8) ...,
}

When a single-bit field is referenced in prose, the position of that field can be clarified by using the value of the byte that carries the field with the field's value set. For example, the value 0x80 could be used to refer to the single-bit field in the most significant bit of the byte, such as One-bit Field in Figure 1.¶

2.1. Stream Types and Identifiers

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

Streams are identified within a connection by a numeric value, referred to as the stream ID. A stream ID is a 62-bit integer (0 to 2⁶²-1) that is unique for all streams on a connection. Stream IDs are encoded as variable-length integers; see Section 16. A QUIC endpoint MUST NOT reuse a stream ID within a connection.¶

The least significant bit (0x01) of the stream ID identifies the initiator of the stream. Client-initiated streams have even-numbered stream IDs (with the bit set to 0), and server-initiated streams have odd-numbered stream IDs (with the bit set to 1).¶

The second least significant bit (0x02) of the stream ID distinguishes between bidirectional streams (with the bit set to 0) and unidirectional streams (with the bit set to 1).¶

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

The stream space for each type begins at the minimum value (0x00 through 0x03, respectively); successive streams of each type are created with numerically increasing stream IDs. A stream ID that is used out of order results in all streams of that type with lower-numbered stream IDs also being opened.¶

2.2. Sending and Receiving Data

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

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

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

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

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

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

2.3. Stream Prioritization

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

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

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

2.4. Operations on Streams

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

On the sending part of a stream, an application protocol can:¶

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

On the receiving part of a stream, an application protocol can:¶

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

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

3.1. Sending Stream States

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

       o
       | Create Stream (Sending)
       | Peer Creates Bidirectional Stream
       v
   +-------+
   | Ready | Send RESET_STREAM
   |       |-----------------------.
   +-------+                       |
       |                           |
       | Send STREAM /             |
       |      STREAM_DATA_BLOCKED  |
       v                           |
   +-------+                       |
   | Send  | Send RESET_STREAM     |
   |       |---------------------->|
   +-------+                       |
       |                           |
       | Send STREAM + FIN         |
       v                           v
   +-------+                   +-------+
   | Data  | Send RESET_STREAM | Reset |
   | Sent  |------------------>| Sent  |
   +-------+                   +-------+
       |                           |
       | Recv All ACKs             | Recv ACK
       v                           v
   +-------+                   +-------+
   | Data  |                   | Reset |
   | Recvd |                   | Recvd |
   +-------+                   +-------+

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

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

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

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

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

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

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

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

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

3.2. Receiving Stream States

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

       o
       | Recv STREAM / STREAM_DATA_BLOCKED / RESET_STREAM
       | Create Bidirectional Stream (Sending)
       | Recv MAX_STREAM_DATA / STOP_SENDING (Bidirectional)
       | Create Higher-Numbered Stream
       v
   +-------+
   | Recv  | Recv RESET_STREAM
   |       |-----------------------.
   +-------+                       |
       |                           |
       | Recv STREAM + FIN         |
       v                           |
   +-------+                       |
   | Size  | Recv RESET_STREAM     |
   | Known |---------------------->|
   +-------+                       |
       |                           |
       | Recv All Data             |
       v                           v
   +-------+ Recv RESET_STREAM +-------+
   | Data  |--- (optional) --->| Reset |
   | Recvd |  Recv All Data    | Recvd |
   +-------+<-- (optional) ----+-------+
       |                           |
       | App Read All Data         | App Read Reset
       v                           v
   +-------+                   +-------+
   | Data  |                   | Reset |
   | Read  |                   | Read  |
   +-------+                   +-------+

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

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

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

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

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

When a STREAM frame with a FIN bit is received, the final size of the stream is known; see Section 4.5. The receiving part of the stream then enters the "Size Known" state. In this state, the endpoint no longer needs to send MAX_STREAM_DATA frames; it only receives any retransmissions of stream data.¶

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

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

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

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

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

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

3.3. Permitted Frame Types

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

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

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

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

3.4. Bidirectional Stream States

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

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

3.5. Solicited State Transitions

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

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

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

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

An endpoint SHOULD copy the error code from the STOP_SENDING frame to the RESET_STREAM frame it sends, but it can use any application error code. An endpoint that sends a STOP_SENDING frame MAY ignore the error code in any RESET_STREAM frames subsequently received for that stream.¶

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

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

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

4.1. Data Flow Control

QUIC employs a limit-based flow control scheme where a receiver advertises the limit of total bytes it is prepared to receive on a given stream or for the entire connection. This leads to two levels of data flow control in QUIC:¶

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

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

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

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

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

Once a receiver advertises a limit for the connection or a stream, it is not an error to advertise a smaller limit, but the smaller limit has no effect.¶

A receiver MUST close the connection with an error of type FLOW_CONTROL_ERROR if the sender violates the advertised connection or stream data limits; see Section 11 for details on error handling.¶

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

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

4.2. Increasing Flow Control Limits

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

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

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

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

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

When a sender receives credit after being blocked, it might be able to send a large amount of data in response, resulting in short-term congestion; see Section 7.7 of [QUIC-RECOVERY] for a discussion of how a sender can avoid this congestion.¶

4.3. Flow Control Performance

If an endpoint cannot ensure that its peer always has available flow control credit that is greater than the peer's bandwidth-delay product on this connection, its receive throughput will be limited by flow control.¶

Packet loss can cause gaps in the receive buffer, preventing the application from consuming data and freeing up receive buffer space.¶

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

4.4. Handling Stream Cancellation

Endpoints need to eventually agree on the amount of flow control credit that has been consumed on every stream, to be able to account for all bytes for connection-level flow control.¶

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

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

4.5. Stream Final Size

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

A sender always communicates the final size of a stream to the receiver reliably, no matter how the stream is terminated. The final size is the sum of the Offset and Length fields of a STREAM frame with a FIN flag, noting that these fields might be implicit. Alternatively, the Final Size field of a RESET_STREAM frame carries this value. This guarantees that both endpoints agree on how much flow control credit was consumed by the sender on that stream.¶

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

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

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

4.6. Controlling Concurrency

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

If a max_streams transport parameter or a MAX_STREAMS frame is received with a value greater than 2⁶⁰, this would allow a maximum stream ID that cannot be expressed as a variable-length integer; see Section 16. If either is received, the connection MUST be closed immediately with a connection error of type TRANSPORT_PARAMETER_ERROR if the offending value was received in a transport parameter or of type FRAME_ENCODING_ERROR if it was received in a frame; see Section 10.2.¶

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

Once a receiver advertises a stream limit using the MAX_STREAMS frame, advertising a smaller limit has no effect. MAX_STREAMS frames that do not increase the stream limit MUST be ignored.¶

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

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

5.1. Connection ID

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

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

Multiple connection IDs are used so that endpoints can send packets that cannot be identified by an observer as being for the same connection without cooperation from an endpoint; see Section 9.5.¶

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

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

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

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

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

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

5.2. Matching Packets to Connections

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

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

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

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

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

Invalid packets that lack strong integrity protection, such as Initial, Retry, or Version Negotiation, MAY be discarded. An endpoint MUST generate a connection error if processing the contents of these packets prior to discovering an error, or fully revert any changes made during that processing.¶

5.3. Operations on Connections

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

When implementing the client role, an application protocol can:¶

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

When implementing the server role, an application protocol can:¶

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

In either role, an application protocol can:¶

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

6.1. Sending Version Negotiation Packets

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

This system allows a server to process packets with unsupported versions without retaining state. Though either the Initial packet or the Version Negotiation packet that is sent in response could be lost, the client will send new packets until it successfully receives a response or it abandons the connection attempt.¶

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

6.2. Handling Version Negotiation Packets

Version Negotiation packets are designed to allow for functionality to be defined in the future that allows QUIC to negotiate the version of QUIC to use for a connection. Future Standards Track specifications might change how implementations that support multiple versions of QUIC react to Version Negotiation packets received in response to an attempt to establish a connection using this version.¶

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

How to perform version negotiation is left as future work defined by future Standards Track specifications. In particular, that future work will ensure robustness against version downgrade attacks; see Section 21.12.¶

6.3. Using Reserved Versions

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

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

7.1. Example Handshake Flows

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

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

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

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

Client                                                  Server
Initial[0]: CRYPTO[CH] ->
                                 Initial[0]: CRYPTO[SH] ACK[0]
                       Handshake[0]: CRYPTO[EE, CERT, CV, FIN]
                                 <- 1-RTT[0]: STREAM[1, "..."]
Initial[1]: ACK[0]
Handshake[0]: CRYPTO[FIN], ACK[0]
1-RTT[0]: STREAM[0, "..."], ACK[0] ->
                                          Handshake[1]: ACK[0]
         <- 1-RTT[1]: HANDSHAKE_DONE, STREAM[3, "..."], ACK[0]

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

Client                                                  Server
Initial[0]: CRYPTO[CH]
0-RTT[0]: STREAM[0, "..."] ->
                                 Initial[0]: CRYPTO[SH] ACK[0]
                                  Handshake[0] CRYPTO[EE, FIN]
                          <- 1-RTT[0]: STREAM[1, "..."] ACK[0]
Initial[1]: ACK[0]
Handshake[0]: CRYPTO[FIN], ACK[0]
1-RTT[1]: STREAM[0, "..."] ACK[0] ->
                                          Handshake[1]: ACK[0]
         <- 1-RTT[1]: HANDSHAKE_DONE, STREAM[3, "..."], ACK[1]

7.2. Negotiating Connection IDs

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

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

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

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

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

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

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

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

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

7.3. Authenticating Connection IDs

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

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

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

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

An endpoint MUST treat the following as a connection error of type TRANSPORT_PARAMETER_ERROR or PROTOCOL_VIOLATION:¶

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

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

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

Client                                                  Server
Initial: DCID=S1, SCID=C1 ->
                                  <- Initial: DCID=C1, SCID=S3
                             ...
1-RTT: DCID=S3 ->
                                             <- 1-RTT: DCID=C1

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

Client                                                  Server
Initial: DCID=S1, SCID=C1 ->
                                    <- Retry: DCID=C1, SCID=S2
Initial: DCID=S2, SCID=C1 ->
                                  <- Initial: DCID=C1, SCID=S3
                             ...
1-RTT: DCID=S3 ->
                                             <- 1-RTT: DCID=C1

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

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

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

7.4. Transport Parameters

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

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

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

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

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

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

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

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

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

7.5. Cryptographic Message Buffering

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

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

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

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

8.1. Address Validation during Connection Establishment

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

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

For the client, the value of the Destination Connection ID field in its first Initial packet allows it to validate the server address as a part of successfully processing any packet. Initial packets from the server are protected with keys that are derived from this value (see Section 5.2 of [QUIC-TLS]). Alternatively, the value is echoed by the server in Version Negotiation packets (Section 6) or included in the Integrity Tag in Retry packets (Section 5.8 of [QUIC-TLS]).¶

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

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

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

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

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

Client                                                  Server
Initial[0]: CRYPTO[CH] ->
                                                <- Retry+Token
Initial+Token[1]: CRYPTO[CH] ->
                                 Initial[0]: CRYPTO[SH] ACK[1]
                       Handshake[0]: CRYPTO[EE, CERT, CV, FIN]
                                 <- 1-RTT[0]: STREAM[1, "..."]

8.2. Path Validation

Path validation is used by both peers during connection migration (see Section 9) to verify reachability after a change of address. In path validation, endpoints test reachability between a specific local address and a specific peer address, where an address is the 2-tuple of IP address and port.¶

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

Path validation does not validate that a peer can send in the return direction. Acknowledgments cannot be used for return path validation because they contain insufficient entropy and might be spoofed. Endpoints independently determine reachability on each direction of a path, and therefore return reachability can only be established by the peer.¶

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

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

An endpoint MAY include other frames with the PATH_CHALLENGE and PATH_RESPONSE frames used for path validation. In particular, an endpoint can include PADDING frames with a PATH_CHALLENGE frame for Path Maximum Transmission Unit Discovery (PMTUD); see Section 14.2.1. An endpoint can also include its own PATH_CHALLENGE frame when sending a PATH_RESPONSE frame.¶

An endpoint uses a new connection ID for probes sent from a new local address; see Section 9.5. When probing a new path, an endpoint can ensure that its peer has an unused connection ID available for responses. Sending NEW_CONNECTION_ID and PATH_CHALLENGE frames in the same packet, if the peer's active_connection_id_limit permits, ensures that an unused connection ID will be available to the peer when sending a response.¶

An endpoint can choose to simultaneously probe multiple paths. The number of simultaneous paths used for probes is limited by the number of extra connection IDs its peer has previously supplied, since each new local address used for a probe requires a previously unused connection ID.¶

9.1. Probing a New Path

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

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

9.2. Initiating Connection Migration

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

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

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

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

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

9.3. Responding to Connection Migration

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

If the recipient permits the migration, it MUST send subsequent packets to the new peer address and MUST initiate path validation (Section 8.2) to verify the peer's ownership of the address if validation is not already underway. If the recipient has no unused connection IDs from the peer, it will not be able to send anything on the new path until the peer provides one; see Section 9.5.¶

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

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

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

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

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

9.4. Loss Detection and Congestion Control

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

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

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

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

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

9.5. Privacy Implications of Connection Migration

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

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

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

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

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

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

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

A client might wish to reduce linkability by switching to a new connection ID, source UDP port, or IP address (see [RFC8981]) when sending traffic after a period of inactivity. Changing the address from which it sends packets at the same time might cause the server to detect a connection migration. This ensures that the mechanisms that support migration are exercised even for clients that do not experience NAT rebindings or genuine migrations. Changing address can cause a peer to reset its congestion control state (see Section 9.4), so addresses SHOULD only be changed infrequently.¶

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

9.6. Server's Preferred Address

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

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

9.7. Use of IPv6 Flow Label and Migration

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

The flow label generation MUST be designed to minimize the chances of linkability with a previously used flow label, as a stable flow label would enable correlating activity on multiple paths; see Section 9.5.¶

[RFC6437] suggests deriving values using a pseudorandom function to generate flow labels. Including the Destination Connection ID field in addition to source and destination addresses when generating flow labels ensures that changes are synchronized with changes in other observable identifiers. A cryptographic hash function that combines these inputs with a local secret is one way this might be implemented.¶

10.1. Idle Timeout

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

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

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

To avoid excessively small idle timeout periods, endpoints MUST increase the idle timeout period to be at least three times the current Probe Timeout (PTO). This allows for multiple PTOs to expire, and therefore multiple probes to be sent and lost, prior to idle timeout.¶

10.3. Stateless Reset

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

A stateless reset is not appropriate for indicating errors in active connections. An endpoint that wishes to communicate a fatal connection error MUST use a CONNECTION_CLOSE frame if it is able.¶

To support this process, an endpoint issues a stateless reset token, which is a 16-byte value that is hard to guess. If the peer subsequently receives a Stateless Reset, which is a UDP datagram that ends in that stateless reset token, the peer will immediately end the connection.¶

A stateless reset token is specific to a connection ID. An endpoint issues a stateless reset token by including the value in the Stateless Reset Token field of a NEW_CONNECTION_ID frame. Servers can also issue a stateless_reset_token transport parameter during the handshake that applies to the connection ID that it selected during the handshake. These exchanges are protected by encryption, so only client and server know their value. Note that clients cannot use the stateless_reset_token transport parameter because their transport parameters do not have confidentiality protection.¶

Tokens are invalidated when their associated connection ID is retired via a RETIRE_CONNECTION_ID frame (Section 19.16).¶

An endpoint that receives packets that it cannot process sends a packet in the following layout (see Section 1.3):¶

Stateless Reset {
  Fixed Bits (2) = 1,
  Unpredictable Bits (38..),
  Stateless Reset Token (128),
}

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

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

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

The resulting minimum size of 21 bytes does not guarantee that a Stateless Reset is difficult to distinguish from other packets if the recipient requires the use of a connection ID. To achieve that end, the endpoint SHOULD ensure that all packets it sends are at least 22 bytes longer than the minimum connection ID length that it requests the peer to include in its packets, adding PADDING frames as necessary. This ensures that any Stateless Reset sent by the peer is indistinguishable from a valid packet sent to the endpoint. An endpoint that sends a Stateless Reset in response to a packet that is 43 bytes or shorter SHOULD send a Stateless Reset that is one byte shorter than the packet it responds to.¶

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

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

Endpoints MUST discard packets that are too small to be valid QUIC packets. To give an example, with the set of AEAD functions defined in [QUIC-TLS], short header packets that are smaller than 21 bytes are never valid.¶

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

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

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

Using a randomized connection ID results in two problems:¶

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

This stateless reset design is specific to QUIC version 1. An endpoint that supports multiple versions of QUIC needs to generate a Stateless Reset that will be accepted by peers that support any version that the endpoint might support (or might have supported prior to losing state). Designers of new versions of QUIC need to be aware of this and either (1) reuse this design or (2) use a portion of the packet other than the last 16 bytes for carrying data.¶

11.1. Connection Errors

Errors that result in the connection being unusable, such as an obvious violation of protocol semantics or corruption of state that affects an entire connection, MUST be signaled using a CONNECTION_CLOSE frame (Section 19.19).¶

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

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

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

As the AEAD for Initial packets does not provide strong authentication, an endpoint MAY discard an invalid Initial packet. Discarding an Initial packet is permitted even where this specification otherwise mandates a connection error. An endpoint can only discard a packet if it does not process the frames in the packet or reverts the effects of any processing. Discarding invalid Initial packets might be used to reduce exposure to denial of service; see Section 21.2.¶

11.2. Stream Errors

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

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

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

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

12.1. Protected Packets

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

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

Retry packets use an AEAD function [AEAD] to protect against accidental modification.¶

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

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

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

12.2. Coalescing Packets

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

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

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

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

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

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

12.3. Packet Numbers

The packet number is an integer in the range 0 to 2⁶²-1. This number is used in determining the cryptographic nonce for packet protection. Each endpoint maintains a separate packet number for sending and receiving.¶

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

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

Packet numbers are divided into three spaces in QUIC:¶

Initial space:

All Initial packets (Section 17.2.2) are in this space.¶

Handshake space:

All Handshake packets (Section 17.2.4) are in this space.¶

Application data space:

All 0-RTT (Section 17.2.3) and 1-RTT (Section 17.3.1) packets are in this space.¶

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

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

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

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

A QUIC endpoint MUST NOT reuse a packet number within the same packet number space in one connection. If the packet number for sending reaches 2⁶²-1, the sender MUST close the connection without sending a CONNECTION_CLOSE frame or any further packets; an endpoint MAY send a Stateless Reset (Section 10.3) in response to further packets that it receives.¶

A receiver MUST discard a newly unprotected packet unless it is certain that it has not processed another packet with the same packet number from the same packet number space. Duplicate suppression MUST happen after removing packet protection for the reasons described in Section 9.5 of [QUIC-TLS].¶

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

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

12.4. Frames and Frame Types

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

Packet Payload {
  Frame (8..) ...,
}

The payload of a packet that contains frames MUST contain at least one frame, and MAY contain multiple frames and multiple frame types. An endpoint MUST treat receipt of a packet containing no frames as a connection error of type PROTOCOL_VIOLATION. Frames always fit within a single QUIC packet and cannot span multiple packets.¶

Each frame begins with a Frame Type, indicating its type, followed by additional type-dependent fields:¶

Frame {
  Frame Type (i),
  Type-Dependent Fields (..),
}

Table 3 lists and summarizes information about each frame type that is defined in this specification. A description of this summary is included after the table.¶

The format and semantics of each frame type are explained in more detail in Section 19. The remainder of this section provides a summary of important and general information.¶

The Frame Type in ACK, STREAM, MAX_STREAMS, STREAMS_BLOCKED, and CONNECTION_CLOSE frames is used to carry other frame-specific flags. For all other frames, the Frame Type field simply identifies the frame.¶

The "Pkts" column in Table 3 lists the types of packets that each frame type could appear in, indicated by the following characters:¶

I:

Initial (Section 17.2.2)¶

H:

Handshake (Section 17.2.4)¶

0:

0-RTT (Section 17.2.3)¶

1:

1-RTT (Section 17.3.1)¶

ih:

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

For more details about these restrictions, see Section 12.5. Note that all frames can appear in 1-RTT packets. An endpoint MUST treat receipt of a frame in a packet type that is not permitted as a connection error of type PROTOCOL_VIOLATION.¶

The "Spec" column in Table 3 summarizes any special rules governing the processing or generation of the frame type, as indicated by the following characters:¶

N:

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

C:

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

P:

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

F:

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

The "Pkts" and "Spec" columns in Table 3 do not form part of the IANA registry; see Section 22.4.¶

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

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

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

12.5. Frames and Number Spaces

Some frames are prohibited in different packet number spaces. The rules here generalize those of TLS, in that frames associated with establishing the connection can usually appear in packets in any packet number space, whereas those associated with transferring data can only appear in the application data packet number space:¶

• PADDING, PING, and CRYPTO frames MAY appear in any packet number space.¶
• CONNECTION_CLOSE frames signaling errors at the QUIC layer (type 0x1c) MAY appear in any packet number space. CONNECTION_CLOSE frames signaling application errors (type 0x1d) MUST only appear in the application data packet number space.¶
• ACK frames MAY appear in any packet number space but can only acknowledge packets that appeared in that packet number space. However, as noted below, 0-RTT packets cannot contain ACK frames.¶
• All other frame types MUST only be sent in the application data packet number space.¶

Note that it is not possible to send the following frames in 0-RTT packets for various reasons: ACK, CRYPTO, HANDSHAKE_DONE, NEW_TOKEN, PATH_RESPONSE, and RETIRE_CONNECTION_ID. A server MAY treat receipt of these frames in 0-RTT packets as a connection error of type PROTOCOL_VIOLATION.¶

13.1. Packet Processing

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

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

An endpoint SHOULD treat receipt of an acknowledgment for a packet it did not send as a connection error of type PROTOCOL_VIOLATION, if it is able to detect the condition. For further discussion of how this might be achieved, see Section 21.4.¶

13.2. Generating Acknowledgments

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

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

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

13.3. Retransmission of Information

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

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

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

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

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

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

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

13.4. Explicit Congestion Notification

QUIC endpoints can use ECN [RFC3168] to detect and respond to network congestion. ECN allows an endpoint to set an ECN-Capable Transport (ECT) codepoint in the ECN field of an IP packet. A network node can then indicate congestion by setting the ECN-CE codepoint in the ECN field instead of dropping the packet [RFC8087]. Endpoints react to reported congestion by reducing their sending rate in response, as described in [QUIC-RECOVERY].¶

To enable ECN, a sending QUIC endpoint first determines whether a path supports ECN marking and whether the peer reports the ECN values in received IP headers; see Section 13.4.2.¶