Multipath Extension for QUIC

Introduction This document specifies an extension to QUIC version 1 to enable the simultaneous usage of multiple paths for a single connection. This contrasts with the base QUIC protocol that includes a connection migration mechanism that selects only one path at a time to exchange such packets. The path management specified in fulfills multiple goals: it directs a peer to switch sending through a new preferred path, and it allows the peer to release resources associated with the old path. The multipath extension specified in this document requires several changes to that mechanism:

Simultaneous transmission of non-probing packets on multiple paths.
Continuous use of an existing path even if non-probing packets have been received on another path.
Introduction of an path identifier to manage connection IDs and packet number spaces per path.
Removal of paths that have been abandoned.

As such, this extension specifies a departure from the specification of path management in and therefore requires negotiation between the two endpoints using a new transport parameter, as specified in . Further, as different packet number spaces are used for each path, this specification requires the use of non-zero connection IDs in order to identify the path and respective packet number space. To add a new path to an existing QUIC connection with multipath support, a client starts a path validation on the chosen path, as further described in . A new path can only be used once the associated 4-tuple has been validated by ensuring that the peer is able to receive packets at that address (see ). In this version of the document, a QUIC server does not initiate the creation of a path, but it can validate a new path created by a client. Each endpoint may use several IP addresses for the connection. In particular, the multipath extension supports the following scenarios.

The client uses multiple IP addresses and the server listens on only one.
The client uses only one IP address and the server listens on several ones.
The client uses multiple IP addresses and the server listens on several ones.
The client uses only one IP address and the server listens on only one.

Note that in the last scenario, it still remains possible to have multiple paths over the connection, given that a path is not only defined by the IP addresses being used, but also the port numbers. In particular, the client can use one or several ports per IP address and the server can listen on one or several ports per IP address. In addition to these core features, an application using the multipath extension will typically need additional algorithms to handle the number of active paths and how they are used to send packets. As these differ depending on the application's requirements, this proposal only specifies a simple basic packet scheduling algorithm (see Section ), in order to provide some basic implementation guidance. However, more advanced algorithms as well as potential extensions to enhance signaling of the current path status are expected as future work. Further, this proposal does also not cover address discovery and management. Addresses and the actual decision process to setup or tear down paths are assumed to be handled by the application that is using the QUIC multipath extension. This is sufficient to address the first aforementioned scenario. However, this document does not prevent future extensions from defining mechanisms to address the remaining scenarios.

Basic Design Points This proposal is based on several basic design points:

Re-use as much as possible mechanisms of QUIC version 1. In particular, this proposal uses path validation as specified for QUIC version 1 and aims to re-use as much as possible of QUIC's connection migration.
Use the same packet header formats as QUIC version 1 to minimize the difference between multipath and non-multipath traffic being exposed on wire.
Congestion Control must be per-path (following ) which usually also requires per-path RTT measurements
PMTU discovery should be performed per-path
The use of this multipath extension requires the use of non-zero length connection IDs in both directions.
Connection IDs are associated with a path ID. The path initiation associates that path ID with a 4-tuple of source and destination IP address as well as source and destination port.
Migration is detected without ambiguity when a packet arrives with a connection ID pointing to an existing path ID, but the connection ID and/or the 4-tuple are different from the value associated with that path (see ).
Paths can be closed at any time, as specified in .
It is possible to create multiple paths sharing the same 4-tuple. Each of these paths can be closed at any time, like any other path.

Further the design of this extension introduces an explicit path identifier and use of multiple packet number spaces as further explained in the next sections.

Introduction of an Explicit Path Identifier This extension specifies a new path identifier (Path ID), which is an integer between 0 and 2^32-1 (inclusive). Path identifies are generated monotonically increasing and cannot be reused. The connection ID of a packet binds the packet to a path identifier, and therefore to a packet number space. The same Path ID is used in both directions to address a path in the new multipath control frames, such as PATH_ABANDON , PATH_STANDBY }, PATH_AVAILABLE as well as MP_ACK . Further, connection IDs are issued per Path ID. That means each connection ID is associated with exactly one path identifier but multiple connection IDs are usually issued for each path identifier. The Path ID of the initial path is 0. Connection IDs which are issued by a NEW_CONNECTION_ID frame respectively are associated with Path ID 0. Also, the Path ID for the connection ID specified in the "preferred address" transport parameter is 0. Use of the "preferred address" is considered as a migration event that does not change the Path ID.

Use of Multiple Packet Number Spaces This extension uses multiple packet number spaces, one for each active path. As such, each path maintains distinct packet number states for sending and receiving packets, as in . Using multiple packet number spaces enables direct use of the loss recovery and congestion control mechanisms defined in on a per-path basis. Each Path ID-specific packet number space starts at packet number 0. When following the packet number encoding algorithm described in , the largest packet number (largest_acked) that has been acknowledged by the peer in the Path ID-specfic packet number space is initially set to "None". Using multiple packet number spaces requires changes in the way AEAD is applied for packet protection, as explained in . More concretely, the Path ID is used to construct the packet protection nonce in addition to the packet number in order to enable use of the same packet number on different paths. Respectively, the Path ID is limited to 32 bits to ensure a unique nonce. Additional consideration on key updates are explained in .

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 when, and only when, they appear in all capitals, as shown here. We assume that the reader is familiar with the terminology used in . When this document uses the term "path", it refers to the notion of "network path" used in .

Handshake Negotiation and Transport Parameter This extension defines a new transport parameter, used to negotiate the use of the multipath extension during the connection handshake, as specified in . The new transport parameter is defined as follows:

initial_max_path_id (current version uses 0x0f739bbc1b666d09): a variable-length integer specifying the maximum path identifier an endpoint is willing to maintain at connection initiation. This value MUST NOT exceed 2^32-1, the maximum allowed value for the Path ID due to restrictions on the nonce calculation (see ).

For example, if initial_max_path_id is set to 1, only connection IDs associated with Path IDs 0 and 1 should be issued by the peer. If an endpoint receives an initial_max_path_id transport parameter with value 0, the peer aims to enable the multipath extension without allowing extra paths immediately. Setting initial_max_path_id parameter is equivalent to sending a MAX_PATH_ID frame () with the same value. As such to allow for the use of more paths later, endpoints can send the MAX_PATH_ID frame to increase the maximum allowed path identifier. If either of the endpoints does not advertise the initial_max_path_id transport parameter, then the endpoints MUST NOT use any frame or mechanism defined in this document. When advertising the initial_max_path_id transport parameter, the endpoint MUST use non-zero length Source and Destination Connection IDs. If an initial_max_path_id transport parameter is received and the carrying packet contains a zero-length connection ID, the receiver MUST treat this as a connection error of type MP_PROTOCOL_VIOLATION and close the connection. The initial_max_path_id parameter MUST NOT be remembered (). New paths can only be used after handshake completion. This extension does not change the definition of any transport parameter defined in . The initial_max_path_id transport parameter limits the initial maximum number of active paths that can be used during a connection. The active_connection_id_limit transport parameter limits the maximum number of active connection IDs per path when the initial_max_path_id parameter is negotiated successfully. Endpoints might prefer to retain spare connection IDs so that they can respond to unintentional migration events (). Cipher suites with a nonce shorter than 12 bytes cannot be used together with the multipath extension. If such a cipher suite is selected and the use of the multipath extension is negotiated, endpoints MUST abort the handshake with a TRANSPORT_PARAMETER error. The MP_ACK frame, as specified in , is used to acknowledge 1-RTT packets. Compared to the ACK frame as specified in , the MP_ACK 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 MP_ACK frames instead of ACK frames, also for acknowledging so far unacknowledged 0-RTT packets, using Path ID 0.

Path Management After completing the handshake, endpoints have agreed to enable multipath support. They can also start using multiple paths, unless both server preferred addresses and a disable_active_migration transport parameter were provided by the server, in which case a client is forbidden to establish new paths until "after a client has acted on a preferred_address transport parameter" (). This document does not specify how an endpoint that is reachable via several addresses announces these addresses to the other endpoint. In particular, if the server uses the preferred_address transport parameter, clients cannot assume that the initial server address and the addresses contained in this parameter can be simultaneously used for multipath (). Furthermore, this document does not discuss when a client decides to initiate a new path. We delegate such discussion to separate documents. To let the peer open a new path, an endpoint needs to provide its peer with connection IDs for at least one unused path identifier. Endpoints SHOULD use the MP_NEW_CONNECTION_ID frame to provide new connection IDs and, respectively, the MP_RETIRE_CONNECTION_ID frame to retire connection IDs after a successful handshake indicating multipath support by both endpoints. To open a new path, an endpoint MUST use a connection ID associated with a new, unused Path ID. Still, the receiver may observe a connection ID associated with a used Path ID on different 4-tuples due to, e.g., NAT rebinding. In such a case, the receiver reacts as specified in by initiating path validation but MUST use a new connection ID for the same Path ID. This proposal adds four multipath control frames for path management:

PATH_ABANDON frame for the receiver side to abandon the path (see ),
PATH_STANDBY and PATH_AVAILABLE frames to express a preference in path usage (see and ), and
MAX_PATH_ID frame for increasing the limit of active paths.

All new frames are sent in 1-RTT packets .

Path Initiation Opening a new path requires the use of a new connection ID (see ). Instead of NEW_CONNECTION_ID frame as specified in , 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 ), 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 , 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 () unless it has previously validated that address. MP_ACK frames (defined in ) can be returned on any path. If the MP_ACK is preferred to be sent on the same path as the acknowledged packet (see for further guidance), it can be beneficial to bundle an MP_ACK frame with the PATH_RESPONSE frame during path validation. If the server receives a PATH_CHALLENGE before receiving a MP_NEW_CONNECTION_ID for the specific path, it SHOULD ignore the PATH_CHALLENGE. Note that the MP_NEW_CONNECTION_ID might be sent in the same packet and in this case the PATH_CHALLENGE SHOULD be processed. 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 initiation attempt. However, as the used Path ID is anyway consumed, the endpoint MUST explicitly close the path as specified in . 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 () and the congestion control limits for this path are respected. Further, in contrast with the specification in , 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 with a path ID pointing to an existing path but with a different connection ID or from a different 4-tuple than the one previously associated with that path ID should be considered as a path migration as further discussed in . As specified in , 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 .

Path Status 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. That means that peers may have different usage preferences for the same path. Depending on the data 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 all active paths are marked as "standby", no guidance is provided about which path should be used. If an endpoints starts using a path that was marked as "standby" by its peer because it has detected issues on the paths marked as "available", it is RECOMMENDED to update its own path state signaling such that the peer avoids using the broken path. An enpoints that detects a path breakage can also explicitly close the path by sending a PATH_ABANDON frame (see section ) in order to avoid that its peer keeps using it and enable faster switch over to a standby path. If the endpoints do not want to close the path immediately, as connectivity could be re-established, PING frames can potentially be used to quickly detect connectivity changes and switch back in a timely way. 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.

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. Note that other explicit closing mechanisms of still apply on the whole connection. In particular, the reception of either a CONNECTION_CLOSE () or a Stateless Reset () closes the connection. An endpoint that wants to close a path MUST explicitly terminate the path by sending a PATH_ABANDON frame. 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 further ). This is true whether the decision to close the path results from implicit signals such as idle time or packet losses (see {(idle-time-close}}) or for any other reason, such as management of local resources. It is also possible to abandon a path for which no packet has been sent (see ). When an endpoint receives a PATH_ABANDON frame, it MUST send a corresponding PATH_ABANDON frame if it has not already done so. It MUST stop sending any new packet on the abandoned path, and it MUST treat all connection identifiers received from the peer for that path as immediately retired. However, knowledge of the connection identifiers received from the peer and of the state of the number space associated to the path SHOULD be retained while packets from the peer might still be in transit, i.e., for a delay of 3 PTO after the PATH_ABANDON frame has been received from the peer, both to avoid generating spurious stateless packets as specified in and to be able to acknowledge the last packets received from the peer as specified in ). After receiving or sending a PATH_ABANDON frame, the endpoints SHOULD promptly send MP_ACK frames to acknowledge all packets received on the path and not yet acknowledged, as specified in ). When an endpoint finally deletes all resource associated with the path, the packets sent over the path and not yet acknowledged MUST be considered lost. After a path is abandoned, the Path ID MUST NOT be reused for new paths, as the Path ID is part of the nonce calculation . PATH_ABANDON frames can be sent on any path, not only the path that is intended to be closed. Thus, a path can be abandoned even if connectivity on that path is already broken. Respectively, if there is still an active path, it is RECOMMENDED to send the PATH_ABANDON frames on another path. If a PATH_ABANDON frame is received for the only active path of a QUIC connection, the receiving peer SHOULD send a CONNECTION_CLOSE frame and enter the closing state. If the client received a PATH_ABANDON frame for the last open path, it MAY instead try to open a new path, if available, and only initiate connection closure if path validation fails or a CONNECTION_CLOSE frame is received from the server. Similarly the server MAY wait for a short, limited time such as one PTO if a path probing packet is received on a new path before sending the CONNECTION_CLOSE frame.

Avoiding Spurious Stateless Resets The peers that send a PATH_ABANDON frame MUST treat all connection identifiers received from the peer for the Path ID as immediately retired. The Stateless Reset Tokens associated with these connection identifiers MUST NOT be used to identify Stateless Reset packets per . Due to packet losses and network delays, packets sent on the path may well arrive after the PATH_ABANDON frames have been sent or received. If these packets arrive after the connection identifiers sent to the peer have been retired, they will not be recognized as bound for the local connection and could trigger the peer to send a Stateless Reset packet. The rule to "retain knowledge of connection ID for 3 PTO after receiving a PATH_ABANDON" is intended to reduce the risk of sending such spurious stateless packets, but it cannot completely avoid that risk. The immediate retirement of connection identifiers received for the path guarantees that spurious stateless reset packets sent by the peer will not cause the closure of the QUIC connection.

Handling MP_ACK for abandoned paths When an endpoint decides to send a PATH_ABANDON frame, there may still be some unacknowledged packets. Some other packets may well be in transit, and could be received shortly after sending the PATH_ABANDON frame. As specified above, the endpoints SHOULD send MP_ACK frames promptly, to avoid unnecessary data retransmission after the peer deletes path resources. These MP_ACK frames MUST be sent on a different path than the path being abandoned. MP_ACK frames received after the endpoint has entirely deleted a path MUST be silently discarded.

Idle Timeout allows for closing of connections if they stay idle for too long. The connection idle timeout when using the multipath extension is defined as "no packet received on any path for the duration of the idle timeout". When only one path is available, servers MUST follow the specifications in . When more than one path is available, hosts shall monitor the arrival of non-probing packets and the acknowledgements for the packets sent over each path. Hosts SHOULD stop sending traffic on a path if for at least the period of the idle timeout as specified in (a) no non-probing packet was received or (b) no non-probing packet sent over this path was acknowledged, but MAY ignore that rule if it would disqualify all available paths. To avoid idle timeout of a path, endpoints can send ack-eliciting packets such as packets containing PING frames () on that path to keep it alive. Sending periodic PING frames also helps prevent middlebox timeout, as discussed in . Server implementations need to select the sub-path idle timeout as a trade-off between keeping resources, such as connection IDs, in use for an excessive time or having to promptly re-establish a path after a spurious estimate of path abandonment by the client. Endpoints that desire to close a path because of the idle timer rule MUST do so explicitly by sending a PATH_ABANDON frame on another active path, as defined in .

Early Abandon The are scenarios in which an endpoint will receive a PATH_ABANDON frame before receiving or sending any traffic on a path. For example, if the client tries to initiate a path and the path cannot be established, it will send a PATH_ABANDON frame (see ). An endpoint may also decide to abandon a path for any reason, for example, removing a hole from the sequence of Path IDs in use. This is not an error. The endpoint that receive such a PATH_ABANDON frame must treat it as specified in .

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 , 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.

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 . 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 . Usually, it is desired to provide at least one additional connection ID for all used paths, to allow for migration. As further specified in connection IDs cannot be issued more than once on the same connection and therefore are unique for the scope of the connection, regardless of the associated Path ID. Over a given path, both endpoints use connection IDs associated to a given Path ID. To initiate a path, each endpoint needs to advertise at least one connection ID for a given Path ID to its peer. Endpoints SHOULD NOT introduce discontinuity in the issuing of Path IDs through their connection ID advertisements as path initiation requires available connection IDs for the same Path ID on both sides. For instance, if the maximum Path ID limit is 2 and the endpoint wants to provide connection IDs for only one Path ID inside range [1, 2], it should select Path ID 1 (and not Path ID 2). Similarly, endpoints SHOULD consume Path IDs in a continuous way, i.e., when creating paths. However, endpoints cannot expect to receive new connection IDs or path initiation attempts with in order use of Path IDs due to out-of-order delivery or path validation failure. 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. Endpoints MUST NOT issue new connection IDs with Path IDs greater than the Maximum Path Identifier field in MAX_PATH_ID frames (see Section ) or the value of initial_max_path_id transport parameter if no MAX_PATH_ID frame was received yet. Receipt of a frame with a greater Path ID is a connection error as specified in Section . When an endpoint finds it has not enough available unused path identifiers, it SHOULD send a MAX_PATH_ID frame to inform the peer that it could use new active path identifiers. If the client has consumed all the allocated connection IDs for a path, it is supposed to retire those that are not actively used anymore, and the server is supposed to provide replacements for that path, see . Sending a MP_RETIRE_CONNECTION_ID frame indicates that the connection ID will not be used anymore. In response, if the path is still active, the peer SHOULD provide new connection IDs using MP_NEW_CONNECTION_ID frames. Retirement of connection IDs will not retire the Path ID that corresponds to the connection ID or any other path ressources as the packet number space is associated with a path. The peer that sends the MP_RETIRE_CONNECTION_ID frame can keep sending data on the path that the retired connection ID was used on but has to use a different connection ID for the same Path ID when doing so.

Packet Protection Packet protection for QUIC version 1 is specified in . 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.

Nonce Calculation 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. Therefore, 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. In order to guarantee the uniqueness of the nonce, the Path ID is limited to a max value of 2^32-1. 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.

Key Update The Key Phase bit update process is specified in . The general principles of key update are not changed in this specification. Following , 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 (). 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 which used three times the PTO of the only one active path. Following , the Key Phase bit is protected, so sending multiple packets with Key Phase bit flipping at the same time should not cause linkability issue.

Examples

Path Establishment illustrates an example of new path establishment using multiple packet number spaces. In this example it is assumed that both endpoint have indicated an initial_max_path_id value of at least 2, which means both endpoints can use Path IDs 0, 1, and 2. Note that Path ID 0 is already used for the initial path.

Example of new path establishment <-- 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], MP_ACK[PathID=1, PN=0] Checks AEAD using nonce(Path ID 1, PN 0) 1-RTT[1]: DCID=S1, PATH_RESPONSE[Y], MP_ACK[PathID=1, PN=0], ... --> ]]> In , 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. In this example the Path ID 1 is used which is the smallest unused Path ID available as recommended in . Respectively, the client chooses the connection ID S1 as the Destination Connection ID of the new path.

Path Closure illustrates an example of path closure. In this example, the client wants to close the path with Path ID 1. It sends the PATH_ABANDON frame to terminate the path. After receiving the PATH_ABANDON frame with Path ID 1, the server also send a PATH_ABANDON frame with Path ID 1.

Example of closing a path. (server tells client to abandon a path) <-1-RTT[Y]: DCID=C1 PATH_ABANDON[Path ID=1], MP_ACK[PATH ID=1, PN=X] 1-RTT[U]: DCID=S2 MP_ACK[Path ID=1, PN=Y] -> ]]> Note that the last acknowledgement needs to be send on a different path. This examples assumes another path which uses connection ID S2 exists.

Implementation Considerations

Number Spaces As stated in , when the multipath extension is negotiated, each path uses a separate packet number space. This is a major difference from , 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.

Congestion Control When the QUIC multipath extension is used, senders manage per-path congestion status as required in . However, in 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 linked increased algorithm (LIA) congestion control scheme specified in 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 .

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 . Example of ACK delays using multiple paths

ACK Path \ Data path	Terrestrial	Satellite
Terrestrial	100ms	350ms
Satellite	350ms	600ms

The MP_ACK 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 MP_ACK frames. In the example above, we may assume that the MP_ACK 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 MP_ACK came over the satellite channel, but it is still the right value for computing for example the PTO timeout: if an MP_ACK is not received after more than 350ms, either the data packet or its MP_ACK were probably lost. The simplest implementation is to compute smoothedRTT and RTTvar per regardless of the path through which MP_ACK frames are received. This algorithm will provide good results, except if the set of paths changes and the MP_ACK 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 MP_ACK that produced the minimum RTT was received, and to restart the minimum RTT computation if that path is abandoned.

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

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. As noted in , STREAM frame boundaries are not expected to be preserved when data is retransmitted. Especially when STREAM frames have to be retransmitted over a different path with a smaller MTU limit, new smaller STREAM frames might need to be sent instead.

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.

Keep Alive The QUIC specification defines an optional keep alive process, see . 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.

Connection ID Changes and NAT Rebindings 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 ). If path validation process succeeds, the endpoints set the path's congestion controller and round-trip time estimator according to . 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.

New Frames All frames defined in this document MUST only be sent in 1-RTT packets. If an endpoint receives a multipath-specific frame in a different packet type, it MUST close the connection with an error of type FRAME_ENCODING_ERROR. Receipt of multipath-specific frames that use a Path ID that is greater than the announced Maximum Paths value in the MAX_PATH_ID frame or in the initial_max_path_id transport parameter, if no MAX_PATH_ID frame was received yet, MUST be treated as a connection error of type MP_PROTOCOL_VIOLATION. If an endpoint receives a multipath-specific frame with a path identifier that it cannot process anymore (e.g., because the path might have been abandoned), it MUST silently ignore the frame.

MP_ACK Frame The MP_ACK frame (types TBD-00 and TBD-01) is an extension of the ACK frame specified in . 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, MP_ACK frames also contain the sum of QUIC packets with associated ECN marks received on the acknowledged packet number space up to this point. MP_ACK frame is formatted as shown in .

MP_ACK Frame Format Compared to the ACK frame specified in , 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 MP_ACK frame.

PATH_ABANDON Frame The PATH_ABANDON frame informs the peer to abandon a path. PATH_ABANDON frames are formatted as shown in .

PATH_ABANDON Frame Format 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 , 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.

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 .

PATH_STANDBY Frame Format 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.

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 .

PATH_AVAILABLE Frame Format 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.

MP_NEW_CONNECTION_ID frames The MP_NEW_CONNECTION_ID frame (type=0x15228c09) is an extension of the NEW_CONNECTION_ID frame specified in . 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 . MP_NEW_CONNECTION_ID frames are formatted as shown in .

MP_NEW_CONNECTION_ID Frame Format Compared to the NEW_CONNECTION_ID frame specified in , 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 , the sequence number applies on a per-path context. This means different connection IDs on different paths may have the same sequence number value. The Retire Prior To field indicates which connection IDs should be retired among those that share the Path ID in the Path Identifier field. Connection IDs associated with different path IDs are not affected. 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. The last paragraph of specifies how to verify the Retire Prior To field of an incoming NEW_CONNECTION_ID frame. The same rule applies for MP_RETIRE_CONNECTION_ID frames, but it applies per path. After the multipath extension is negotiated successfully, the rule for RETIRE_CONNECTION_ID frame is only applied for Path ID 0.

MP_RETIRE_CONNECTION_ID frames The MP_RETIRE_CONNECTION_ID frame (type=0x15228c0a) is an extension of the RETIRE_CONNECTION_ID frame specified in . 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 , 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 ). MP_RETIRE_CONNECTION_ID frames are formatted as shown in .

MP_RETIRE_CONNECTION_ID Frame Format Compared to the RETIRE_CONNECTION_ID frame specified in , 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 is also per path. The MP_RETIRE_CONNECTION_ID frame retires the Connection ID with the specified Path ID and sequence number. The processing of an incoming RETIRE_CONNECTION_ID frame is described in . The same processing applies for MP_RETIRE_CONNECTION_ID frames per path, while the processing of an RETIRE_CONNECTION_ID frame is only applied for Path ID 0.

MAX_PATH_ID frames A MAX_PATH_ID frame (type=0x15228c0c) informs the peer of the maximum path identifier it is permitted to use. MAX_PATH_ID frames are formatted as shown in .

MAX_PATH_ID Frame Format MAX_PATH_ID frames contain the following field:

Maximum Path Identifier:: The maximum path identifier that the sending endpoint is willing to accept. This value MUST NOT exceed 2^32-1, the maximum allowed value for the Path ID due to restrictions on the nonce calculation (see ). The Maximum Path Identifier value MUST NOT be lower than the value advertised in the initial_max_path_id transport parameter.

Receipt of an invalid Maximum Path Identifier value MUST be treated as a connection error of type MP_PROTOCOL_VIOLATION. Loss or reordering can cause an endpoint to receive a MAX_PATH_ID frame with a smaller Maximum Path Identifier value than was previously received. MAX_PATH_ID frames that do not increase the path limit MUST be ignored.

Error Codes Multipath QUIC transport error codes are 62-bit unsigned integers following . This section lists the defined multipath QUIC transport error codes that can be used in a CONNECTION_CLOSE frame with a type of 0x1c. These errors apply to the entire connection. MP_PROTOCOL_VIOLATION (experiments use 0x1001d76d3ded42f3): An endpoint detected an error with protocol compliance that was not covered by more specific error codes.

IANA Considerations This document defines a new transport parameter for the negotiation of enable multiple paths for QUIC, and three new frame types. The draft defines provisional values for experiments, but we expect IANA to allocate short values if the draft is approved. The following entry in should be added to the "QUIC Transport Parameters" registry under the "QUIC Protocol" heading. Addition to QUIC Transport Parameters Entries

Value	Parameter Name.	Specification
TBD (current version uses 0x0f739bbc1b666d09)	initial_max_path_id

The following frame types defined in should be added to the "QUIC Frame Types" registry under the "QUIC Protocol" heading. Addition to QUIC Frame Types Entries

Value	Frame Name	Specification
TBD-00 - TBD-01 (experiments use 0x15228c00-0x15228c01)	MP_ACK
TBD-02 (experiments use 0x15228c05)	PATH_ABANDON
TBD-03 (experiments use 0x15228c07)	PATH_STANDBY
TBD-04 (experiments use 0x15228c08)	PATH_AVAILABLE
TBD-05 (experiments use 0x15228c09)	MP_NEW_CONNECTION_ID
TBD-06 (experiments use 0x15228c0a)	MP_RETIRE_CONNECTION_ID
TBD-06 (experiments use 0x15228c0c)	MAX_PATH_ID

The following transport error code defined in should be added to the "QUIC Transport Error Codes" registry under the "QUIC Protocol" heading. Error Code for Multipath QUIC

Value	Code	Description	Specification
TBD (experiments use 0x1001d76d3ded42f3)	MP_PROTOCOL_VIOLATION	Multipath protocol violation

Security Considerations The multipath extension retains all the security features of and but requires some additional consideration regarding the following amendments:

the need of potential additional resources as connection IDs are now maintained per-path;
the provisioning of multiple concurrent path contexts and the associated resources;
the possibility to create and use multiple simultaneous paths and the corresponding increased amplification risk for request forgery attacks;
the changes on encryption requirements due to the use of multiple packet number spaces.

Memory Allocation for Per-Path Resources The initial_max_path_id transport parameter and the Max Path ID field in the MAX_PATH_ID frame limit the number of paths an endpoint is willing to maintain and accordingly limit the associated path resources. Furthermore, as connection IDs have to be issued by both endpoint for the same path ID before an endpoint can open a path, each endpoint can further control the per-path resource usage (beyond the connection IDs) by limiting the number of Path ID that it issues connection IDs for. Therefore, to avoid unnecessarily resource usage, that potentially could be exploited in a resource exhaustion attack, endpoints should allocate those additional path resource, such as e.g. for packet number handling, only after path validation has successfully completed.

Request Forgery with Spoofed Address The path validation mechanism as specified in for migration is used unchanged for initiation of new paths in this extension. Respectively the security considerations on source address spoofing as outlined in equally apply. Similarly, the anti-amplification limits as specified in need to be followed to limit the amplification risk. However, while only allows the use of one path simultaneously and therefore only one path migration at the time should be validated, this extension allows for multiple open paths, that could in theory be migrated all at the same time, and it allows for multiple active paths that could be initialised simultaneously. Therefore, each path could be used to further amplify an attack. Respectively endpoints needs limit the number of maximum paths and might consider additional measures to limit the number of concurrent path validation processes e.g. by pacing them out or limiting the number of path initiation attempts over a certain time period.

Use of Transport Layer Security and the AEAD Encryption Nonce The multipath extension as specified in this document is only enabled after a successful handshake when both endpoints indicate support for this extension. Respectively, all new frames defined in this extension are only used in 1-RTT packets. As the handshake is not changed by this extension, the transport security mechanisms as specified in , such as encryption key exchange and peer authentication, remain unchanged as well and the respective security considerations in applied unaltered. Note that with the use of this extension, multiple nonces can be in use simultaneously for the same AEAD key. Further note, that the limits as discussed on Appendix B of apply to the total number of packets sent on all paths. This specification changes the AEAD calculation by using the path identifier as part of AEAD encryption nonce (see ). To ensure a unique nonce, path identifiers are limited to 32 bits and cannot be reused for another path in the same connection.

Contributors This document is a collaboration of authors that combines work from three proposals. Further contributors that were also involved one of the original proposals are:

Qing An
Zhenyu Li

Acknowledgments Thanks to Marten Seemann, Kazuho Oku, and Martin Thomson for their thorough reviews and valuable contributions!