Multipath Extension for QUIC

1.1. Conventions 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.¶

We assume that the reader is familiar with the terminology used in [QUIC-TRANSPORT]. When this document uses the term "path", it refers to the notion of "network path" used in [QUIC-TRANSPORT].¶

4.1. Path Initiation

Opening a new path requires the use of a new connection ID (see Section 9.5 of [QUIC-TRANSPORT]). Instead of NEW_CONNECTION_ID frame as specified in Section 19.15 of [QUIC-TRANSPORT], each endpoint uses the MP_NEW_CONNECTION_ID frame as specified in this extension to issue Path ID-specific connections IDs. The same Path ID is used in both directions. As such to open a new path, both sides need at least one connection ID (see Section 5.1.1 of [QUIC-TRANSPORT]), which is associated with the same, unused Path ID. When the peer receives the PATH_CHALLENGE, it MUST pick a Connection ID with the same Path ID for sending the PATH_RESPONSE.¶

When the multipath extension is negotiated, a client that wants to use an additional path MUST first initiate the Address Validation procedure with PATH_CHALLENGE and PATH_RESPONSE frames as described in Section 8.2 of [QUIC-TRANSPORT], unless it has previously validated that address. After receiving packets from the client on a new path, if the server decides to use the new path, the server MUST perform path validation (Section 8.2 of [QUIC-TRANSPORT]) unless it has previously validated that address.¶

If validation succeeds, the client can continue to use the path. If validation fails, the client MUST NOT use the path and can remove any status associated to the path initation attempt. However, as the used Path ID is anyway consumed, and the endpoint MUST abandon the path by sending a PATH_ABANDON frame and retire all corresponding connection IDs by sending MP_RETIRE_CONNECTION_ID frames on another path to inform the peer that the Path ID cannot be used anymore.¶

Section 9.1 of [QUIC-TRANSPORT] introduces the concept of "probing" and "non-probing" frames. A packet that contains at least one "non-probing" frame is a "non-probing" packet. When the multipath extension is negotiated, the reception of a "non-probing" packet on a new path needs to be considered as a path initiation attempt that does not impact the path status of any existing path. Therefore, any frame can be sent on a new path at any time as long as the anti-amplification limits (Section 21.1.1.1 of [QUIC-TRANSPORT]) and the congestion control limits for this path are respected.¶

Further, in contrast with the specification in Section 9 of [QUIC-TRANSPORT], the server MUST NOT assume that receiving non-probing packets on a new path with a new connection ID indicates an attempt to migrate to that path. Instead, servers SHOULD consider new paths over which non-probing packets have been received as available for transmission. Reception of QUIC packets containing a connection ID that is already in use but has a different 4-tuple than previously observed with this connection ID should be considered as a path migration as further discussed in Section 7.8.¶

As specified in Section 9.3 of [QUIC-TRANSPORT], the server is expected to send a new address validation token to a client following the successful validation of a new client address. In situations where multiple paths are established, the client may have received several tokens, each tied to a different address. When considering using a token for subsequent connections, the client ought to carefully select the token to use, due to the inherent ambiguity associated with determining the exact address to which a token is bound. To alleviate such a token ambiguity issue, a server may issue a token that is capable of validating any of the previously validated addresses. Further guidance on token usage can be found in Section 8.1.3 of [QUIC-TRANSPORT].¶

4.2. Path State Management

An endpoint uses the PATH_STANDBY and PATH_AVAILABLE frames to inform the peer that it should send packets with the preference expressed by these frames. Note that an endpoint might not follow the peer’s advertisements, but these frames are still a clear signal of the peer's preference of path usage. Each peer indicates its preference of path usage independently of the other peer. It means that peers may have different usage preferences for the same path. Depending on the sender's decisions, this may lead to usage of paths that have been indicated as "standby" by the peer or non-usage of some locally available paths.¶

PATH_AVAILABLE indicates that a path is "available", i.e., it suggests to the peer to use its own logic to split traffic among available paths. PATH_STANDBY marks a path as "standby", i.e., it suggests that no traffic should be sent on that path if another path is available. If no frame indicating a path usage preference was received for a certain path, the preference of the peer is unknown and the sender needs to decide based on it own local logic if the path should be used.¶

Endpoints use the Path ID in these frames to identify which path state is going to be changed. Note that both frames can be sent via a different path and therefore might arrive in different orders. The PATH_AVAILABLE and PATH_STANDBY frames share a common sequence number space to detect and ignore outdated information.¶

If all active paths are marked as "standby", no guidance is provided about which path should be used.¶

4.3. Path Close

Each endpoint manages the set of paths that are available for transmission. At any time in the connection, each endpoint can decide to abandon one of these paths, for example following changes in local connectivity or local preferences. After an endpoint abandons a path, the peer can expect to not receive any more non-probing packets on that path.¶

An endpoint that wants to close a path SHOULD explicitly terminate the path by sending a PATH_ABANDON frame (see Section 4.3.1). Note that while abandoning a path will cause connection ID retirement, the inverse is not true: retiring the associated connection IDs does not indicate path abandonment (see Section 4.5.1). Implicit signals such as idle time or packet losses might be the only way for an endpoint to detect path closure (see {(idle-time-close}}) if connectivity is broken on that path.¶

Note that other explicit closing mechanisms of [QUIC-TRANSPORT] still apply on the whole connection. In particular, the reception of either a CONNECTION_CLOSE (Section 10.2 of [QUIC-TRANSPORT]) or a Stateless Reset (Section 10.3 of [QUIC-TRANSPORT]) closes the connection.¶

4.4. Refusing a New Path

An endpoint may deny the establishment of a new path initiated by its peer during the address validation procedure. According to [QUIC-TRANSPORT], the standard way to deny the establishment of a path is to not send a PATH_RESPONSE in response to the peer's PATH_CHALLENGE.¶

4.5. Allocating, Consuming, and Retiring Connection IDs

With the multipath extension, each connection ID is associated with one path that is identified by the Path ID that is specified in the Path Identifier field of the MP_NEW_CONNECTION_ID frame Section 8.5. The Path ID 0 indicates the initial path of the connection. Respectively, the connection IDs used during the handshake belong to the initial path with Path ID 0. The MP_NEW_CONNECTION_ID frame is used to issue new connection IDs for all paths. In order to let the peer open new paths, it is RECOMMENDED to proactively issue a Connection ID for at least one unused path ID, as long as it remains compatible with the peer's Maximum Path ID limit.¶

Each endpoint maintains the set of connection IDs received from its peer for each path, any of which it can use when sending packets on that path; see also Section 5.1 of [QUIC-TRANSPORT]. Usually, it is desired to provide at least one additional connection ID for all used paths, to allow for migration.¶

Section 5.1.2. of [QUIC-TRANSPORT] specifies the retirement of connection IDs. In order to identify a connection ID correctly when the multipath extension is used, endpoints have to use the MP_RETIRE_CONNECTION_ID frame instead of the RETIRE_CONNECTION_ID frame to indicate the respective Path ID together with the connection ID sequence number, at least for all paths with a Path ID other than 0. Endpoints can also use MP_NEW_CONNECTION_ID and MP_RETIRE_CONNECTION_ID for the initial path with Path ID 0, however, the use of NEW_CONNECTION_ID and RETIRE_CONNECTION_ID is still valid as well and endpoints need to process these frames accordingly as corresponding to Path ID 0.¶

If the client has used all the allocated connection IDs for a path, it is supposed to retire those that are not used anymore, and the server is supposed to provide replacements for that path, see Section 5.1.2. of [QUIC-TRANSPORT]. Sending a MP_RETIRE_CONNECTION_ID frame indicates that the connection ID will not be used anymore. If the path is still active, the peer SHOULD replace it with a new connection ID using a MP_NEW_CONNECTION_ID frame.¶

Endpoints MUST NOT issue new connection IDs with Path IDs greater than the Maximum Path Identifier field in MAX_PATHS frames (see Section Section 8.7). When an endpoint finds it has not enough available unused path identifiers, it SHOULD send a MAX_PATHS frame to inform the peer that it could use new active path identifiers.¶

4.6. Path States

Figure 1 shows the states that an endpoint's path can have.¶

       o
       | PATH_CHALLENGE sent/received on new path
       v
 +------------+    Path validation failed
 | Validating |----------------------------------+
 +------------+                                  |
       |                                         |
       | PATH_RESPONSE received                  |
       |                                         |
       v                                         |
 +------------+        Idle timeout              |
 |   Active   |----------------------------------+
 +------------+                                  |
       |                                         |
       | PATH_ABANDON sent/received              |
       v                                         |
 +------------+                                  |
 |   Closing  |                                  |
 +------------+                                  |
       |                                         |
       | MP_RETIRE_CONNECTION_ID sent && received|
       | or                                      |
       | Path's draining timeout                 |
       | (at least 3 PTO)                        |
       v                                         |
 +------------+                                  |
 |   Closed   |<---------------------------------+
 +------------+

In all but the "Closed" states, hosts have to track the following information.¶

• Associated 4-tuple: The tuple (source IP, source port, destination IP, destination port) used by the endpoint to send packets over the path.¶

• Associated Path Identifier: The Path ID used to address the path. The endpoint relies on the path identifier to send path control information and specifically acknowledge packets belonging to that path-specific packet number space.¶

• Associated Destination Connection IDs: The connection IDs used to send packets over the path.¶

In Active state, hosts also tracks the following information:¶

• Associated Source Connection IDs: The connection IDs used to receive packets over the path.¶

A path in the "Validating" state performs path validation as described in Section Section 4.1.¶

The endpoint can use all paths in the "Active" state, provided that the congestion control and flow control currently allow sending of new data on a path.¶

"Closing" state is entered after an PATH_ABANDON frame was sent or received. In this state, the endpoint waits for three PTOs before sending MP_RETIRE_CONNECTION_ID frames. This allows for graceful tear down and processing of in-flight packets.¶

When a path reaches the "Closed" state, the endpoint releases all the path's associated resources, including the associated connection IDs and the path identifier.¶

5.1. Sending Acknowledgements

The ACK_MP frame, as specified in Section 8.1, is used to acknowledge 1-RTT packets. Compared to the ACK frame as specified in Section 19.3 of [QUIC-TRANSPORT], the ACK_MP frame additionally contains the receiver's Path ID to identify the path-specific packet number space.¶

As multipath support is unknown during the handshake, acknowledgements of Initial and Handshake packets are sent using ACK frames. If the multipath extension has been successfully negotiated, ACK frames in 1-RTT packets acknowledge packets for the path with Path ID 0.¶

After the handshake concluded if negotiation of multipath support succeeded, endpoints SHOULD use ACK_MP frames instead of ACK frames, also for acknowledging so far unacknowledged 0-RTT packets, using Path ID 0.¶

ACK_MP frames (defined in Section 8.1) can be returned on any path. If the ACK_MP is preferred to be sent on the same path as the acknowledged packet (see Section 7.3 for further guidance), it can be beneficial to bundle an ACK_MP frame with the PATH_RESPONSE frame during path validation.¶

5.2. Packet Protection

Packet protection for QUIC version 1 is specified in Section 5 of [QUIC-TLS]. The general principles of packet protection are not changed for the multipath extension specified in this document. No changes are needed for setting packet protection keys, initial secrets, header protection, use of 0-RTT keys, receiving out-of-order protected packets, receiving protected packets, or retry packet integrity. However, the use of multiple number spaces for 1-RTT packets requires changes in AEAD usage.¶

Section 5.3 of [QUIC-TLS] specifies AEAD usage, and in particular the use of a nonce, N, formed by combining the packet protection IV with the packet number. When multiple packet number spaces are used, the packet number alone would not guarantee the uniqueness of the nonce.¶

In order to guarantee the uniqueness of the nonce, the nonce N is calculated by combining the packet protection IV with the packet number and with the least significant 32 bits of the Path ID.¶

To calculate the nonce, a 96-bit path-and-packet-number is composed of the least significant 32 bits of the Path ID in network byte order, two zero bits, and the 62 bits of the reconstructed QUIC packet number in network byte order. If the IV is larger than 96 bits, the path-and-packet-number is left-padded with zeros to the size of the IV. The exclusive OR of the padded packet number and the IV forms the AEAD nonce.¶

For example, assuming the IV value is 6b26114b9cba2b63a9e8dd4f, the Path ID is 3, and the packet number is aead, the nonce will be set to 6b2611489cba2b63a9e873e2.¶

5.3. Key Update

The Key Phase bit update process is specified in Section 6 of [QUIC-TLS]. The general principles of key update are not changed in this specification. Following [QUIC-TLS], the Key Phase bit is used to indicate which packet protection keys are used to protect the packet. The Key Phase bit is toggled to signal each subsequent key update.¶

Because of network delays, packets protected with the older key might arrive later than the packets protected with the new key, however receivers can solely rely on the Key Phase bit to determine the corresponding packet protection key, assuming that there is sufficient interval between two consecutive key updates (Section 6.5 of [QUIC-TLS]).¶

When this specification is used, endpoints SHOULD wait for at least three times the largest PTO among all the paths before initiating a new key update after receiving an acknowledgement that confirms the receipt of the previous key update. This interval is different from that in [QUIC-TLS] which used three times the PTO of the only one active path.¶

Following Section 5.4 of [QUIC-TLS], the Key Phase bit is protected, so sending multiple packets with Key Phase bit flipping at the same time should not cause linkability issue.¶

6.1. Path Establishment

Figure 2 illustrates an example of new path establishment using multiple packet number spaces.¶

   Client                                                  Server

   (Exchanges start on default path)
   1-RTT[]: MP_NEW_CONNECTION_ID[C1, Seq=0, PathID=1] -->
             <-- 1-RTT[]: MP_NEW_CONNECTION_ID[S1, Seq=0, PathID=1]
             <-- 1-RTT[]: MP_NEW_CONNECTION_ID[S2, Seq=0, PathID=2]
   ...
   (starts new path)
   1-RTT[0]: DCID=S1, PATH_CHALLENGE[X] -->
                           Checks AEAD using nonce(Path ID 1, PN 0)
        <-- 1-RTT[0]: DCID=C1, PATH_RESPONSE[X], PATH_CHALLENGE[Y],
                                                 ACK_MP[PID=1,PN=0]
   Checks AEAD using nonce(Path ID 1, PN 0)
   1-RTT[1]: DCID=S1, PATH_RESPONSE[Y],
             ACK_MP[PID=1, PN=0], ... -->

In Figure 2, the endpoints first exchange new available connection IDs with the NEW_CONNECTION_ID frame. In this example, the client provides one connection ID (C1 with Path ID 1), and server provides two connection IDs (S1 with Path ID 1, and S2 with Path ID 2).¶

Before the client opens a new path by sending a packet on that path with a PATH_CHALLENGE frame, it has to check whether there is an unused connection IDs for the same unused Path ID available for each side. Respectively, the client chooses the connection ID S1 as the Destination Connection ID of the new path.¶

6.2. Path Closure

In this example, the client detects a network environment change (client's 4G/Wi-Fi is turned off, Wi-Fi signal is fading below a threshold, or the quality of RTT or loss rate is becoming worse) and wants to close an existing path.¶

Figure 3 illustrates an example of path closing. For the first path, the server's 1-RTT packets use DCID C1 and the client's 1-RTT packets use DCID S1 for the path with Path ID 1. For the second path, the server's 1-RTT packets use DCID C2 and the client's 1-RTT packets use DCID S2 for the path with Path ID 2.¶

Note that the paths use different packet number spaces. In this case, the client is going to close the first path with Path ID 1 but it sends the ABANBON frame over the second path using the DCID S2. In this example, the server confirms the path closure by sending an PATH_ABANDON frame by for the same Path ID to the client also using the other path with DCID C2. Both the client and the server can close the path after receiving the MP_RETIRE_CONNECTION_ID frame for that path.¶

Client                                                      Server

(client tells server to abandon a path)
1-RTT[X]: DCID=S2 PATH_ABANDON[Path ID=1]->
                           (server tells client to abandon a path)
               <-1-RTT[Y]: DCID=C2 PATH_ABANDON[Path ID=1],
                                               ACK_MP[PATH ID=1, PN=X]
(client retires the corresponding CID)
1-RTT[U]: DCID=S2 MP_RETIRE_CONNECTION_ID[Path ID=1, Seq=0],
ACK_MP[Path ID=1, PN=Y] ->
                            (server retires the corresponding CID)
           <- 1-RTT[V]: DCID=C2 RETIRE_CONNECTION_ID[Path ID1, Seq=0],
                                                ACK_MP[Path ID=1, PN=U]

7.1. Number Spaces

As stated in Section 1, when the multipath extension is negotiated, each path uses a separate packet number space. This is a major difference from [QUIC-TRANSPORT], which only defines three number spaces (Initial, Handshake and Application packets).¶

The relation between packet number spaces and paths is fixed. Connection IDs are separately allocated for each Path ID. Rotating the connection ID on a path does not change the Path ID. NAT rebinding, though it changes the 4-tuple of the path, also does not change the path identifier. The packet number space does not change when connection ID rotation happens within a given Path ID.¶

Data associated with the transmission and reception such RTT measurements, congestion control state, or loss recovery are maintained per packet number space and as such per Path ID.¶

7.2. Congestion Control

When the QUIC multipath extension is used, senders manage per-path congestion status as required in Section 9.4 of [QUIC-TRANSPORT]. However, in [QUIC-TRANSPORT] only one active path is assumed and as such the requirement is to reset the congestion control status on path migration. With the multipath extension, multiple paths can be used simultaneously, therefore separate congestion control state is maintained for each path. This means a sender is not allowed to send more data on a given path than congestion control for that path indicates.¶

When a Multipath QUIC connection uses two or more paths, there is no guarantee that these paths are fully disjoint. When two (or more paths) share the same bottleneck, using a standard congestion control scheme could result in an unfair distribution of the bandwidth with the multipath connection getting more bandwidth than competing single paths connections. Multipath TCP uses the LIA congestion control scheme specified in [RFC6356] to solve this problem. This scheme can immediately be adapted to Multipath QUIC. Other coupled congestion control schemes have been proposed for Multipath TCP such as [OLIA].¶

7.3. Computing Path RTT

Acknowledgement delays are the sum of two one-way delays, the delay on the packet sending path and the delay on the return path chosen for the acknowledgements. When different paths have different characteristics, this can cause acknowledgement delays to vary widely. Consider for example a multipath transmission using both a terrestrial path, with a latency of 50ms in each direction, and a geostationary satellite path, with a latency of 300ms in both directions. The acknowledgement delay will depend on the combination of paths used for the packet transmission and the ACK transmission, as shown in Table 1.¶

The ACK_MP frames describe packets that were sent on the specified path, but they may be received through any available path. There is an understandable concern that if successive acknowledgements are received on different paths, the measured RTT samples will fluctuate widely, and that might result in poor performance. While this may be a concern, the actual behavior is complex.¶

The computed values reflect both the state of the network path and the scheduling decisions by the sender of the ACK_MP frames. In the example above, we may assume that the ACK_MP will be sent over the terrestrial link, because that provides the best response time. In that case, the computed RTT value for the satellite path will be about 350ms. This lower than the 600ms that would be measured if the ACK_MP came over the satellite channel, but it is still the right value for computing for example the PTO timeout: if an ACK_MP is not received after more than 350ms, either the data packet or its ACK_MP were probably lost.¶

The simplest implementation is to compute smoothedRTT and RTTvar per Section 5.3 of [QUIC-RECOVERY] regardless of the path through which MP_ACKs are received. This algorithm will provide good results, except if the set of paths changes and the ACK_MP sender revisits its sending preferences. This is not very different from what happens on a single path if the routing changes. The RTT, RTT variance and PTO estimates will rapidly converge to reflect the new conditions. There is however an exception: some congestion control functions rely on estimates of the minimum RTT. It might be prudent for nodes to remember the path over which the ACK_MP that produced the minimum RTT was received, and to restart the minimum RTT computation if that path is abandoned.¶

7.4. Packet Scheduling

The transmission of QUIC packets on a regular QUIC connection is regulated by the arrival of data from the application and the congestion control scheme. QUIC packets that increase the number of bytes in flight can only be sent when the congestion window allows it. Multipath QUIC implementations also need to include a packet scheduler that decides, among the paths whose congestion window is open, the path over which the next QUIC packet will be sent. Most frames, including control frames (PATH_CHALLENGE and PATH_RESPONSE being the notable exceptions), can be sent and received on any active path. The scheduling is a local decision, based on the preferences of the application and the implementation.¶

Note that this implies that an endpoint may send and receive ACK_MP frames on a path different from the one that carried the acknowledged packets. As noted in Section 7.3 the values computed using the standard algorithm reflect both the characteristics of the path and the scheduling algorithm of ACK_MP frames. The estimates will converge faster if the scheduling strategy is stable, but besides that implementations can choose between multiple strategies such as sending ACK_MP frames on the path they acknowledge packets, or sending ACK_MP frames on the shortest path, which results in shorter control loops and thus better performance.¶

7.5. Retransmissions

Simultaneous use of multiple paths enables different retransmission strategies to cope with losses such as: a) retransmitting lost frames over the same path, b) retransmitting lost frames on a different or dedicated path, and c) duplicate lost frames on several paths (not recommended for general purpose use due to the network overhead). While this document does not preclude a specific strategy, more detailed specification is out of scope.¶

7.6. Handling different PMTU sizes

An implementation should take care to handle different PMTU sizes across multiple paths. One simple option if the PMTUs are relatively similar is to apply the minimum PMTU of all paths to each path. The benefit of such an approach is to simplify retransmission processing as the content of lost packets initially sent on one path can be sent on another path without further frame scheduling adaptations.¶

7.7. Keep Alive

The QUIC specification defines an optional keep alive process, see Section 5.3 of [QUIC-TRANSPORT]. Implementations of the multipath extension should map this keep alive process to a number of paths. Some applications may wish to ensure that one path remains active, while others could prefer to have two or more active paths during the connection lifetime. Different applications will likely require different strategies. Once the implementation has decided which paths to keep alive, it can do so by sending Ping frames on each of these paths before the idle timeout expires.¶

7.8. Connection ID Changes and NAT Rebindings

Section 5.1.2 of [QUIC-TRANSPORT] indicates that an endpoint can change the connection ID it uses to another available one at any time during the connection. As such a sole change of the Connection ID without any change in the address does not indicate a path change and the endpoint can keep the same congestion control and RTT measurement state.¶

While endpoints assign a connection ID to a specific sending 4-tuple, networks events such as NAT rebinding may make the packet's receiver observe a different 4-tuple. Servers observing a 4-tuple change will perform path validation (see Section 9 of [QUIC-TRANSPORT]). If path validation process succeeds, the endpoints set the path's congestion controller and round-trip time estimator according to Section 9.4 of [QUIC-TRANSPORT].¶

Section 9.3 of [QUIC-TRANSPORT] allows an endpoint to skip validation of a peer address if that address has been seen recently. However, when the multipath extension is used and an endpoint has multiple addresses that could lead to switching between different paths, it should rather maintain multiple open paths instead.¶

8.1. ACK_MP Frame

The ACK_MP frame (types TBD-00 and TBD-01) is an extension of the ACK frame specified in Section 19.3 of [QUIC-TRANSPORT]. It is used to acknowledge packets that were sent on different paths, as each path as its own packet number space. If the frame type is TBD-01, ACK_MP frames also contain the sum of QUIC packets with associated ECN marks received on the acknowledged packet number space up to this point.¶

ACK_MP frame is formatted as shown in Figure 4.¶

  ACK_MP Frame {
    Type (i) = TBD-00..TBD-01
         (experiments use  0x15228c00-0x15228c01),
    Path Identifier (i),
    Largest Acknowledged (i),
    ACK Delay (i),
    ACK Range Count (i),
    First ACK Range (i),
    ACK Range (..) ...,
    [ECN Counts (..)],
  }

Compared to the ACK frame specified in [QUIC-TRANSPORT], the following field is added.¶

Path Identifier:

The Path ID associated with the packet number space of the 0-RTT and 1-RTT packets which are acknowledged by the ACK_MP frame.¶

8.2. PATH_ABANDON Frame

The PATH_ABANDON frame informs the peer to abandon a path.¶

PATH_ABANDON frames are formatted as shown in Figure 5.¶

  PATH_ABANDON Frame {
    Type (i) = TBD-02 (experiments use 0x15228c05),
    Path Identifier (i),
    Error Code (i),
    Reason Phrase Length (i),
    Reason Phrase (..),
  }

PATH_ABANDON frames contain the following fields:¶

Path Identifier:

The Path ID to abandon.¶

Error Code:

A variable-length integer that indicates the reason for abandoning this path.¶

Reason Phrase Length:

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

Reason Phrase:

Additional diagnostic information for the closure. This can be zero length if the sender chooses not to give details beyond the Error Code value. This SHOULD be a UTF-8 encoded string [RFC3629], though the frame does not carry information, such as language tags, that would aid comprehension by any entity other than the one that created the text.¶

PATH_ABANDON frames are ack-eliciting. If a packet containing a PATH_ABANDON frame is considered lost, the peer SHOULD repeat it.¶

After sending the PATH_ABANDON frame, the endpoint MUST NOT send frames that use the Path ID anymore, even on other network paths.¶

8.3. PATH_STANDBY frame

PATH_STANDBY Frames are used by endpoints to inform the peer about its preference to not use the indicated path for sending. PATH_STANDBY frames are formatted as shown in Figure 6.¶

  PATH_STANDBY Frame {
    Type (i) = TBD-03 (experiments use 0x15228c07)
    Path Identifier (i),
    Path Status Sequence Number (i),
  }

PATH_STANDBY Frames contain the following fields:¶

Path Identifier:

The Path ID the status update corresponds to. All Path IDs that have been issued MAY be specified, even if they are not yet in use over a path.¶

Path Status Sequence Number:

A variable-length integer specifying the sequence number assigned for this PATH_STANDBY frame. The sequence number space is shared with the PATH_AVAILABLE frame and the sequence number MUST be monotonically increasing generated by the sender of the PATH_STANDBY frame in the same connection. The receiver of the PATH_STANDBY frame needs to use and compare the sequence numbers separately for each Path ID.¶

Frames may be received out of order. A peer MUST ignore an incoming PATH_STANDBY frame if it previously received another PATH_STANDBY frame or PATH_AVAILABLE for the same Path ID with a Path Status sequence number equal to or higher than the Path Status sequence number of the incoming frame.¶

PATH_STANDBY frames are ack-eliciting. If a packet containing a PATH_STANDBY frame is considered lost, the peer SHOULD resend the frame only if it contains the last status sent for that path -- as indicated by the sequence number.¶

A PATH_STANDBY frame MAY be bundled with a MP_NEW_CONNECTION_ID frame or a PATH_RESPONSE frame in order to indicate the preferred path usage before or during path initiation.¶

8.4. PATH_AVAILABLE frame

PATH_AVAILABLE frames are used by endpoints to inform the peer that the indicated path is available for sending. PATH_AVAILABLE frames are formatted as shown in Figure 7.¶

  PATH_AVAILABLE Frame {
    Type (i) = TBD-03 (experiments use 0x15228c08),
    Path Identifier (i),
    Path Status Sequence Number (i),
  }

PATH_AVAILABLE frames contain the following fields:¶

Path Identifier:

The Path ID the status update corresponds to.¶

Path Status Sequence Number:

A variable-length integer specifying the sequence number assigned for this PATH_AVAILABLE frame. The sequence number space is shared with the PATH_STANDBY frame and the sequence number MUST be monotonically increasing generated by the sender of the PATH_AVAILABLE frame in the same connection. The receiver of the PATH_AVAILABLE frame needs to use and compare the sequence numbers separately for each Path ID.¶

Frames may be received out of order. A peer MUST ignore an incoming PATH_AVAILABLE frame if it previously received another PATH_AVAILABLE frame or PATH_STANDBY frame for the same Path ID with a Path Status sequence number equal to or higher than the Path Status sequence number of the incoming frame.¶

PATH_AVAILABLE frames are ack-eliciting. If a packet containing a PATH_AVAILABLE frame is considered lost, the peer SHOULD resend the frame only if it contains the last status sent for that path -- as indicated by the sequence number.¶

A PATH_AVAILABLE frame MAY be bundled with a MP_NEW_CONNECTION_ID frame or a PATH_RESPONSE frame in order to indicate the preferred path usage before or during path initiation.¶

8.5. MP_NEW_CONNECTION_ID frames

The MP_NEW_CONNECTION_ID frame (type=0x15228c09) is an extension of the NEW_CONNECTION_ID frame specified in Section 19.15 of [QUIC-TRANSPORT]. It is used to provide its peer with alternative connection IDs for 1-RTT packets for a specific path. The peer can then use a different connection ID on the same path to break linkability when migrating on that path; see also Section 9.5 of [QUIC-TRANSPORT].¶

MP_NEW_CONNECTION_ID frames are formatted as shown in Figure 8.¶

MP_NEW_CONNECTION_ID Frame {
  Type (i) = 0x15228c09,
  Path Identifier (i),
  Sequence Number (i),
  Retire Prior To (i),
  Length (8),
  Connection ID (8..160),
  Stateless Reset Token (128),
}

Compared to the NEW_CONNECTION_ID frame specified in Section 19.15 of [QUIC-TRANSPORT], the following field is added:¶

Path Identifier:

The Path ID associated with the connection ID. This means the provided connection ID can only be used on the corresponding path.¶

Note that, other than for the NEW_CONNECTION_ID frame of Section 19.15 of [QUIC-TRANSPORT], the sequence number applies on a per-path context. This means different connection IDs on different paths may have the same sequence number value. Respectively, the Retire Prior To field indicates which connection IDs should be retired for the path with the Path ID in the Path Identifier field.¶

Note that the NEW_CONNECTION_ID frame can only be used to issue or retire connection IDs for the initial path with Path ID 0.¶

8.6. MP_RETIRE_CONNECTION_ID frames

The MP_RETIRE_CONNECTION_ID frame (type=0x15228c0a) is an extension of the RETIRE_CONNECTION_ID frame specified in Section 19.16 of [QUIC-TRANSPORT]. It is used to indicate that it will no longer use a connection ID for a specific path that was issued by its peer. To retire the connection ID used during the handshake on the initial path, Path ID 0 is used. Sending a MP_RETIRE_CONNECTION_ID frame also serves as a request to the peer to send additional connection IDs for this path (see also Section 5.1 of [QUIC-TRANSPORT], unless the path specified by the Path ID has been abandoned. New path-specific connection IDs can be delivered to a peer using the MP_NEW_CONNECTION_ID frame (see Section Section 8.5).¶

MP_RETIRE_CONNECTION_ID frames are formatted as shown in Figure 9.¶

MP_RETIRE_CONNECTION_ID Frame {
  Type (i) = 0x15228c0a,
  Path Identifier (i),
  Sequence Number (i),
}

Compared to the RETIRE_CONNECTION_ID frame specified in Section 19.16 of [QUIC-TRANSPORT], the following field is added:¶

Path Identifier:

The Path ID associated with the connection ID to retire.¶

Note that the RETIRE_CONNECTION_ID frame can only be used to retire connection IDs for the initial path with Path ID 0.¶

As the MP_NEW_CONNECTION_ID frames applies the sequence number per path, the sequence number in the MP_RETIRE_CONNECTION_ID frame also needs to be considered in the context of the Path Identifier field.¶

8.7. MAX_PATHS frames

A MAX_PATHS frame (type=0x15228c0b) informs the peer of the cumulative number of paths it is permitted to open.¶

When there are not enough unused path identifiers, endpoints SHOULD send MAX_PATHS frame to inform the peer that new path identifiers are available.¶

MAX_PATHS frames are formatted as shown in Figure 10.¶

MAX_PATHS Frame {
  Type (i) = 0x15228c0b,
  Maximum Paths (i),
}

MAX_PATHS frames contain the following field:¶

Maximum Path Identifier:

A count of the cumulative number of paths that can be opened over the lifetime of the connection. This value MUST NOT exceed 2^32-1, as Path IDs are defined with a maximum value 2^32-1 as the 32 bits of the Path ID are used to calculate the nonce (see Section Section 5.2). The Maximum Paths value MUST NOT be lower than the value advertised in the initial_max_paths transport parameter. Receipt of an invalid Maximum Paths value MUST be treated as a connection error of type MP_PROTOCOL_VIOLATION.¶

Loss or reordering can cause an endpoint to receive a MAX_PATHS frame with a smaller Maximum Paths value than was previously received. MAX_PATHS frames that do not increase the path limit MUST be ignored.¶