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

Following [QUIC-TRANSPORT], each endpoint uses NEW_CONNECTION_ID frames to issue usable connections IDs to reach it. Before an endpoint adds a new path by initiating path validation, it MUST check whether at least one unused Connection ID is available for each side.¶

If the transport parameter "active_connection_id_limit" is negotiated as N, the server provided N Connection IDs, and the client is already actively using N paths, the limit is reached. If the client wants to start a new path, it has to retire one of the established paths.¶

When the multipath option is negotiated, clients that want to use an additional path MUST first initiate the Address Validation procedure with PATH_CHALLENGE and PATH_RESPONSE frames described in Section 8.2 of [QUIC-TRANSPORT]. 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 succeed, the client can send non-probing, 1-RTT packets on the new paths. 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 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.¶

4.2. Path State Management

An endpoint uses PATH_STATUS frames to inform that the peer should send packets in the preference expressed by these frames. Notice that the endpoint might not follow the peer’s advertisements, but the PATH_STATUS frame is still a clear signal of suggestion for the preference of path usage by the peer.¶

PATH_STATUS frame describes 2 kinds of path states:¶

• Mark a path as "available", i.e., allow the peer to use its own logic to split traffic among available paths.¶
• Mark a path as "standby", i.e., suggest that no traffic should be sent on that path if another path is available.¶

Endpoints use Destination Connection ID Sequence Number field in PATH_STATUS frame to identify which path state is going to be changed. Notice that PATH_STATUS frame can be sent via a different path. An Endpoint MAY ignore the PATH_STATUS frame if it would make all the paths unavailable in a single connection.¶

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, following for example changes in local connectivity or changes in local preferences. After an endpoint abandons a path, the peer will not receive any more non-probing packets on that path. Non-probing packets are defined in Section 9.1 of [QUIC-TRANSPORT].¶

An endpoint that wants to close a path SHOULD use explicit request to terminate the path by sending the PATH_ABANDON frame (see Section 4.3.1). Note that while abandoning a path will cause Connection ID retirement, only retiring the associated Connection ID does not necessarily advertise path abandon (see Section 4.3.3). However, implicit signals such as idle time or packet losses might be the only way for an endhost to detect path closure (see Section 4.3.4).¶

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. 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 abandoned
 | Validating |----------------------------------+
 +------------+                                  |
       |                                         |
       | PATH_RESPONSE received                  |
       |                                         |
       v                                         |
 +------------+     Path blackhole detected      |
 |   Active   |----------------------------------+
 +------------+                                  |
       |                                         |
       | PATH_ABANDON sent/received              |
       v                                         |
 +------------+                                  |
 |   Closing  |                                  |
 +------------+                                  |
       |                                         |
       | Path's draining timeout                 |
       | (at least 3 PTO)                        |
       v                                         |
 +------------+                                  |
 |   Closed   |<---------------------------------+
 +------------+

In non-final states, hosts have to track the following information.¶

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

In Active state, hosts MUST also track the following information:¶

• Associated Source Connection ID: The Connection ID used to receive packets over the path. The endpoint relies on its sequence number to send path control information and specifically acknowledge packets belonging to that Connection ID-specific packet number space.¶

A path in the "Validating" state performs path validation as described in Section 8.2 of [QUIC-TRANSPORT]. An endhost should not send non-probing frames on a path in "Validating" state, as it has no guarantee that packets will actually reach the peer.¶

The endhost can use all the paths in the "Active" state, provided that the congestion control and flow control currently allow sending of new data on a path. Note that if a path became idle due to a timeout, endpoints SHOULD send PATH_ABANDON frame before closing the path.¶

In the "Closing" state, the endhost SHOULD NOT send packets on this path anymore, as there is no guarantee that the peer can still map the packets to the connection. The endhost SHOULD wait for the acknowledgment of the PATH_ABANDON frame before moving the path to the "Closed" state to ensure a graceful termination of the path.¶

When a path reaches the "Closed" state, the endhost releases all the path's associated resources, including the associated Connection IDs. Endpoints SHOULD send RETIRE_CONNECTION_ID frames for releasing the associated Connection IDs following [QUIC-TRANSPORT]. Considering endpoints are not expected to send packets on the current path in the "Closed" state, endpoints can send RETIRE_CONNECTION_ID frames on other available paths. Consequently, the endhost is not able to send nor receive packets on this path anymore.¶

5.1. Sending Acknowledgements

The ACK_MP frame, as specified in Section 8.1, is used to acknowledge 1-RTT packets. Compared to the QUIC version 1 ACK frame, the ACK_MP frame additionally contains the receiver's Destination Connection ID Sequence Number field to distinguish the Connection ID-specific packet number space.¶

Acknowledgements of Initial and Handshake packets MUST be carried using ACK frames, as specified in [QUIC-TRANSPORT]. The ACK frames, as defined in [QUIC-TRANSPORT], do not carry the Destination Connection ID Sequence Number field to identify the packet number space. If the multipath extension has been successfully negotiated, ACK frames in 1-RTT packets acknowledge packets sent with the Connection ID having sequence number 0.¶

As soon as the negotiation of multipath support is completed, endpoints SHOULD use ACK_MP frames instead of ACK frames to acknowledge application data packets, including 0-RTT packets, using the initial Connection ID with sequence number 0 after the handshake concluded.¶

ACK_MP frame (defined in Section 8.1) SHOULD be sent on the path it received packet with the Connection ID of the packet number space it acknowledges. However, an ACK_MP frame can be returned via a different path, based on different strategies of sending ACK_MP frames.¶

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 QUIC Multipath. 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 Destination Connection ID sequence number.¶

Section 19 of [QUIC-TRANSPORT] encodes the Connection ID Sequence Number as a variable-length integer, allowing values up to 2^62-1; in this specification, a range of less than 2^32-1 values MUST be used before updating the packet protection key.¶

To calculate the nonce, a 96 bit path-and-packet-number is composed of the 32 bit Connection ID Sequence Number in 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 Connection ID Sequence Number 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 for QUIC version 1 is specified in Section 6 of [QUIC-TLS]. The general principles of key update are not changed in this specification. Following QUIC version 1, 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. Therefore, the endpoint needs to retain old packet keys to allow these delayed packets to be processed and it must distinguish between the new key and the old key. In QUIC version 1, this is done using packet numbers so that the rule is made simple: Use the older key if packet number is lower than any packet number frame the current key phase.¶

When using multiple packet number spaces on different paths, some care is needed when initiating the Key Update process, as different paths use different packet number spaces but share a single key. When a key update is initiated on one path, packets sent to another path needs to know when the transition is complete. Otherwise, it is possible that the other paths send packets with the old keys, but skip sending any packets in the current key phase and directly jump to sending packet in the next key phase. When that happens, as the endpoint can only retain two sets of packet protection keys with the 1-bit Key Phase bit, the other paths cannot distinguish which key should be used to decode received packets, which results in a key rotation synchronization problem.¶

To address such a synchronization issue, if key update is initialized on one path, the sender SHOULD send at least one packet with the new key on all active paths. Further, an endpoint MUST NOT initiate a subsequent key update until a packet with the current key has been acknowledged on each 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[]: NEW_CONNECTION_ID[C1, Seq=1] -->
                       <-- 1-RTT[]: NEW_CONNECTION_ID[S1, Seq=1]
                       <-- 1-RTT[]: NEW_CONNECTION_ID[S2, Seq=2]
   ...
   (starts new path)
   1-RTT[0]: DCID=S2, PATH_CHALLENGE[X] -->
                   Checks AEAD using nonce(CID sequence 2, PN 0)
     <-- 1-RTT[0]: DCID=C1, PATH_RESPONSE[X], PATH_CHALLENGE[Y],
                                              ACK_MP[PID=2,PN=0]
   Checks AEAD using nonce(CID sequence 1, PN 0)
   1-RTT[1]: DCID=S2, 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 sequence number 1), and server provides two Connection IDs (S1 with sequence number 1, and S2 with sequence number 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 available for each side. In this example, the client chooses the Connection ID S2 as the Destination Connection ID in the new path.¶

If the client has used all the allocated CID, it is supposed to retire those that are not used anymore, and the server is supposed to provide replacements, as specified in [QUIC-TRANSPORT]. Usually, it is desired to provide one more Connection ID as currently in use, to allow for new paths or migration.¶

6.2. Path Closure

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

Figure 3 illustrates an example of path closing. For the first path, the server's 1-RTT packets use DCID C1, which has a sequence number of 1; the client's 1-RTT packets use DCID S2, which has a sequence number of 2. For the second path, the server's 1-RTT packets use DCID C2, which has a sequence number of 2; the client's 1-RTT packets use DCID S3, which has a sequence number of 3. Note that the paths use different packet number spaces. In this case, the client is going to close the first path. It identifies the path by the sequence number of the DCID its peer uses for sending packets over that path, hence using the DCID sequence number 1 (which relates to C1). Optionally, the server confirms the path closure by sending an PATH_ABANDON frame by indicating the sequence number of the DCID the client uses to send over that path, which corresponds to the sequence number 2 (of S2). Both the client and the server can close the path after receiving the RETIRE_CONNECTION_ID frame for that path.¶

Client                                                      Server

(client tells server to abandon a path)
1-RTT[X]: DCID=S2 PATH_ABANDON[dcid_seq_num=1]->
                           (server tells client to abandon a path)
               <-1-RTT[Y]: DCID=C1 PATH_ABANDON[dcid_seq_num=2],
                                               ACK_MP[PID=2, PN=X]
(client retires the corresponding CID)
1-RTT[U]: DCID=S3 RETIRE_CONNECTION_ID[2], ACK_MP[PID=1, PN=Y] ->
                            (server retires the corresponding CID)
 <- 1-RTT[V]: DCID=C2 RETIRE_CONNECTION_ID[1], ACK_MP[PID=3, PN=U]

7.1. Number Spaces

As stated in Section 1, when multipath is negotiated, each Destination Connection ID is linked to a separate packet number space. This a big difference between implementations of QUIC as specified in [QUIC-TRANSPORT], which only have to manage three number spaces for Initial, Handshake and Application packets.¶

Implementation of multipath capable QUIC will need to carefully model the relations between paths and number spaces, as shown in Figure 4.¶

   +-------------------------+
   | CID received from peer: |
   | Previous Sender Number  |
   | Space                   |-- - - - - - +
   +-------------------------+
   +-------------------------+             |
   | CID received from peer: |
   | Sender Number Space     |             |
   +-------------------------+             v
                      ^             +----------------+
                      |             | Path (4 tuple) |
                      +-------------| - RTT          |
                +------------------>| - Congestion   |
                |                   |   state        |
                v                   +----------------+
   +-------------------------+             ^
   | CID provided to peer:   |             |
   | Receiver Number Space   |
   +-------------------------+             |
   +-------------------------+
   | CID previously used by  |-- - - - - - +
   | Peer: old Receiver      |
   | Number Space            |
   +-------------------------+

The path is defined by the 4-tuple through which packets are received and sent. Packets sent on the path will include the Destination Connection ID currently used for that path, selected from the list of CID provided by the peer. Packets received on the path carry a Destination CID selected by the peer from the list provided to that peer.¶

The relation between CIDs and paths is not fixed. A node may decide to rotate the Destination CID it uses, a NAT may decide to change the 4-tuple over which packets from that path will be received. Implementation will have to manage these evolving relations.¶

Data associated with the transmission and reception on a given path can be associated to either the "path state", or to the state of either the sender or receiver number spaces. For example:¶

• RTT measurements and congestion state are logically associated with the 4-tuple. They will remain unchanged if data starts being received or sent through the same 4-tuple using new CIDs.¶
• Implementations of loss recovery typically maintain lists of packets sent and not yet acknowledged. Such information, along with the value of the next PN to use for sending, is logically associated with the "Sender Number Space", and with the peer-provided CID used for sending on the path.¶
• Sending of acknowledgement requires keeping track of the PN of received packets and of acknowledgements previously sent. Such information is logically associated with the "Receiver Number Space", and with the CID used by the peer for sending on the path.¶

When the link between paths and CID changes, the information tied to the now unused CID remains useful for some time. For example, the list of packet numbers to acknowledge maintained in the old receiver number space could still be used to send ACK_MP frames for that number space. Similarly, the list of packets sent but not yet acknowledged with an old sender number space can be used when processing incoming ACK_MP frames for that number space. Such data should not be discarded immediately after a CID change, but only later, for example when the CID is retired.¶

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 2.¶

Using the default algorithm specified in [QUIC-RECOVERY] would result in suboptimal performance, computing average RTT and standard deviation from series of different delay measurements of different combined paths. At the same time, early tests showed that it is desirable to send ACKs through the shortest path because a shorter ACK delay results in a tighter control loop and better performances. The tests also showed that it is desirable to send copies of the ACKs on multiple paths, for robustness if a path experiences sudden losses.¶

An early implementation mitigated the delay variation issue by using time stamps, as specified in [QUIC-Timestamp]. When the timestamps are present, the implementation can estimate the transmission delay on each one-way path, and can then use these one way delays for more efficient implementations of recovery and congestion control algorithms.¶

If timestamps are not available, implementations could estimate one way delays using statistical techniques. For example, in the example shown in Table 1, implementations can use "same path" measurements to estimate the one way delay of the terrestrial path to about 50ms in each direction, and that of the satellite path to about 300ms. Further measurements can then be used to maintain estimates of one way delay variations, using logical similar to Kalman filters. But statistical processing is error-prone, and using time stamps provides more robust measurements.¶

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. A reasonable default consists in sending ACK_MP frames on the path they acknowledge packets, but the receiver must not assume its peer will do so.¶

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 for 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 performs 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.¶

If an endpoint uses a new Connection ID after an idle period and a NAT rebinding leads to a 4-tuple changes on the same packet, the receiving endpoint may not be able to associate the packet to an existing path and will therefore consider this as a new path. This leads to an inconsistent view of open paths at both peers, however, as the "old" path will not work anymore, it will be silently closed after the idle timeout expires.¶

8.1. ACK_MP Frame

The ACK_MP frame (types TBD-00 and TBD-01; experiments use 0xbaba00..0xbaba01) is an extension of the ACK frame defined by [QUIC-TRANSPORT]. It is used to acknowledge packets that were sent on different paths using multiple packet number spaces. If the frame type is TBD-01, ACK_MP frames also contain the sum of QUIC packets with associated ECN marks received on the connection up to this point.¶

ACK_MP frame is formatted as shown in Figure 5.¶

  ACK_MP Frame {
    Type (i) = TBD-00..TBD-01 (experiments use 0xbaba00..0xbaba01),
    Destination Connection ID Sequence Number (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.¶

Destination Connection ID Sequence Number:

The sequence number of the Connection ID identifying the packet number space of the 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 6.¶

  PATH_ABANDON Frame {
    Type (i) = TBD-02 (experiments use 0xbaba05),
    Destination Connection ID Sequence Number (i),
    Error Code (i),
    Reason Phrase Length (i),
    Reason Phrase (..),
  }

PATH_ABANDON frames contain the following fields:¶

Destination Connection ID Sequence Number:

The sequence number of the Destination Connection ID used by the receiver of the frame to send packets over the path 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 SHOULD be acknowledged. If a packet containing a PATH_ABANDON frame is considered lost, the peer SHOULD repeat it.¶

8.3. PATH_STATUS frame

PATH_STATUS Frame are used by endpoints to inform the peer of the current status of one path, and the peer should send packets according to the preference expressed in these frames. PATH_STATUS frames are formatted as shown in Figure 7.¶

  PATH_STATUS Frame {
    Type (i) = TBD-03 (experiments use 0xbaba06),
    Destination Connection ID Sequence Number (i),
    Path Status sequence number (i),
    Path Status (i),
  }

PATH_STATUS Frames contain the following fields:¶

Destination Connection ID Sequence Number:

The sequence number of the Destination Connection ID used by the receiver of this frame to send packets over the path the status update corresponds to.¶

Path Status sequence number:

A variable-length integer specifying the sequence number assigned for this PATH_STATUS frame. The sequence number MUST be monotonically increasing generated by the sender of the PATH_STATUS frame in the same connection. The receiver of the PATH_STATUS frame needs to use and compare the sequence numbers separately for each Destination Connection ID Sequence Number.¶

Available values of Path Status field are:¶

• 1: Standby¶
• 2: Available¶

Endpoints use PATH_STATUS frame to inform the peer whether it prefer to use this path or not. If an endpoint receives a PATH_STATUS frame containing 1-Standby status, it SHOULD stop sending non-probing packets on the corresponding path, until it receives a new PATH_STATUS frame containing 2-Available status with a higher sequence number referring to the same path.¶

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

PATH_STATUS frames SHOULD be acknowledged. If a packet containing a PATH_STATUS frame is considered lost, the peer should only repeat it if it was the last status sent for that path -- as indicated by the sequence number.¶