Multipath Extension for QUIC
draft-ietf-quic-multipath-03
The information below is for an old version of the document.
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|---|---|---|---|
| Authors | Yanmei Liu , Yunfei Ma , Quentin De Coninck , Olivier Bonaventure , Christian Huitema , Mirja Kühlewind | ||
| Last updated | 2022-10-24 (Latest revision 2022-07-11) | ||
| Replaces | draft-lmbdhk-quic-multipath | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-quic-multipath-03
QUIC Working Group Y. Liu, Ed.
Internet-Draft Y. Ma
Intended status: Standards Track Alibaba Inc.
Expires: 27 April 2023 Q. De Coninck, Ed.
UCLouvain
O. Bonaventure
UCLouvain and Tessares
C. Huitema
Private Octopus Inc.
M. Kuehlewind, Ed.
Ericsson
24 October 2022
Multipath Extension for QUIC
draft-ietf-quic-multipath-03
Abstract
This document specifies a multipath extension for the QUIC protocol
to enable the simultaneous usage of multiple paths for a single
connection.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the QUIC Working Group
mailing list (quic@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/quic/.
Source for this draft and an issue tracker can be found at
https://github.com/mirjak/draft-lmbdhk-quic-multipath.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on 27 April 2023.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions and Definitions . . . . . . . . . . . . . . . 5
2. High-level overview . . . . . . . . . . . . . . . . . . . . . 5
3. Handshake Negotiation and Transport Parameter . . . . . . . . 6
4. Path Setup and Removal . . . . . . . . . . . . . . . . . . . 7
4.1. Path Initiation . . . . . . . . . . . . . . . . . . . . . 8
4.2. Path State Management . . . . . . . . . . . . . . . . . . 8
4.3. Path Close . . . . . . . . . . . . . . . . . . . . . . . 8
4.3.1. Use PATH_ABANDON Frame to Close a Path . . . . . . . 9
4.3.2. Refusing a New Path . . . . . . . . . . . . . . . . . 10
4.3.3. Effect of RETIRE_CONNECTION_ID Frame . . . . . . . . 10
4.3.4. Idle Timeout . . . . . . . . . . . . . . . . . . . . 11
4.4. Path States . . . . . . . . . . . . . . . . . . . . . . . 11
5. Congestion Control . . . . . . . . . . . . . . . . . . . . . 13
6. Computing Path RTT . . . . . . . . . . . . . . . . . . . . . 14
7. Packet Scheduling . . . . . . . . . . . . . . . . . . . . . . 15
8. Recovery . . . . . . . . . . . . . . . . . . . . . . . . . . 15
9. Packet Number Space and Use of Connection ID . . . . . . . . 16
9.1. Using Zero-Length connection ID . . . . . . . . . . . . . 16
9.1.1. Sending Acknowledgements and Handling Ranges . . . . 17
9.1.2. Loss and Congestion Handling With Zero-Length CID . . 17
9.1.3. RTT Estimation Considerations when SPNS is Used . . . 18
9.1.4. ECN and Zero-Length CID Considerations . . . . . . . 19
9.1.5. Restricted Sending to Zero-Length CID Peer . . . . . 19
9.2. Using non-zero length CID and Multiple Packet Number
Spaces . . . . . . . . . . . . . . . . . . . . . . . . . 20
9.2.1. Packet Protection for QUIC Multipath . . . . . . . . 21
9.2.2. Key Update for QUIC Multipath . . . . . . . . . . . . 21
10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 22
10.1. Path Establishment . . . . . . . . . . . . . . . . . . . 22
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10.2. Path Closure . . . . . . . . . . . . . . . . . . . . . . 23
11. Implementation Considerations . . . . . . . . . . . . . . . . 25
11.1. Handling different PMTU sizes . . . . . . . . . . . . . 25
11.2. Keep Alive . . . . . . . . . . . . . . . . . . . . . . . 26
12. New Frames . . . . . . . . . . . . . . . . . . . . . . . . . 26
12.1. PATH_ABANDON Frame . . . . . . . . . . . . . . . . . . . 26
12.2. PATH_STATUS frame . . . . . . . . . . . . . . . . . . . 28
12.3. ACK_MP Frame . . . . . . . . . . . . . . . . . . . . . . 29
13. Error Codes . . . . . . . . . . . . . . . . . . . . . . . . . 30
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
15. Security Considerations . . . . . . . . . . . . . . . . . . . 31
16. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 31
17. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 32
18. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
18.1. Normative References . . . . . . . . . . . . . . . . . . 32
18.2. Informative References . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33
1. Introduction
This document specifies an extension to QUIC version 1
[QUIC-TRANSPORT] to enable the simultaneous usage of multiple paths
for a single connection.
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 avoid the
risk of packets being dropped by middleboxes (which may only
support QUIC version 1)
* Congestion Control must be per-path (following [QUIC-TRANSPORT])
which usually also requires per-path RTT measurements
* PMTU discovery should be performed per-path
* A path is determined by the 4-tuple of source and destination IP
address as well as source and destination port. Therefore, there
can be at most one active paths/connection ID per 4-tuple.
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The path management specified in Section 9 of [QUIC-TRANSPORT]
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. Multipath requires several changes to
that mechanism:
* Allow simultaneous transmission of non-probing frames on multiple
paths.
* Continue using an existing path even if non-probing frames have
been received on another path.
* Manage the removal of paths that have been abandoned.
As such, this extension specifies a departure from the specification
of path management in Section 9 of [QUIC-TRANSPORT] and therefore
requires negotiation between the two endpoints using a new transport
parameter, as specified in Section 3.
This extension uses multiple packet number spaces. When multipath is
negotiated, each destination connection ID is linked to a separate
packet number space. Using multiple packet number spaces enables
direct use of the loss recovery and congestion control mechanisms
defined in [QUIC-RECOVERY].
Some deployments of QUIC use zero-length connection IDs. When a node
selects to use zero-length connection IDs, it is not possible to use
different connection IDs for distinguishing packets sent to that node
over different paths. This extension also specifies a way to use
zero-length CID by using the same packet number space on all paths.
However, when using the same packet number space on multiple paths,
out of order delivery is likely. This causes inflation of the number
of acknowledgement ranges and therefore of the the size of ACK
frames. Senders that accept to use a single number space on multiple
paths when sending to a node using zero-length CID need to take
special care to minimize the impact of multipath delivery on loss
detection, congestion control, and ECN handling. This proposal
specifies algorithms for controlling the size of acknowledgement
packets and ECN handling in Section Section 9.1 and Section 9.1.4.
This proposal does 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. Further, this proposal only specifies a simple
basic packet scheduling algorithm, in order to provide some basic
implementation guidance. However, more advanced algorithms as well
as potential extensions to enhance signaling of the current path
state are expected as future work.
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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]. In addition, we define the following terms:
* Path: refers to the 4-tuple {source IP address, source port
number, destination IP address, destination port number}. A path
refers to "network path" used in [QUIC-TRANSPORT].
* Path Identifier (Path ID): An identifier that is used to identify
a path in a QUIC connection at an endpoint. Path Identifier is
used in multipath control frames (etc. PATH_ABANDON frame) to
identify a path. By default, it is defined as the sequence number
of the destination Connection ID used for sending packets on that
particular path, but alternative definitions can be used if the
length of that connection ID is zero.
* Packet Number Space Identifier (PN Space ID): An identifier that
is used to distinguish packet number spaces for different paths.
It is used in 1-RTT packets and ACK_MP frames. Each node
maintains a list of "Received Packets" for each of the CID that it
provided to the peer, which is used for acknowledging packets
received with that CID.
The difference between Path Identifier and Packet Number Space
Identifier, is that the Path Identifier is used in multipath control
frames to identify a path, and the Packet Number Space Identifier is
used in 1-RTT packets and ACK_MP frames to distinguish packet number
spaces for different paths. Both identifiers have the same value,
which is the sequence number of the connection ID, if a non-zero
connection ID is used. If the connection ID is zero length, the
Packet Number Space Identifier is 0, while the Path Identifier is
selected on path establishment.
2. High-level overview
The multipath extensions to QUIC proposed in this document enable the
simultaneous utilization of different paths to exchange non-probing
QUIC frames for a single connection. This contrasts with the base
QUIC protocol [QUIC-TRANSPORT] that includes a connection migration
mechanism that selects only one path to exchange such frames.
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A multipath QUIC connection starts with a QUIC handshake as a regular
QUIC connection. See further Section 3. The peers use the
enable_multipath transport parameter during the handshake to
negotiate the utilization of the multipath capabilities. The
active_connection_id_limit transport parameter limits the maximum
number of active paths that can be used during a connection. A
multipath QUIC connection is thus an established QUIC connection
where the enable_multipath transport parameter has been successfully
negotiated.
To add a new path to an existing multipath QUIC connection, a client
starts a path validation on the chosen path, as further described in
Section 4. 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. A new path can only be used once it has been
validated. Each endpoint associates a Path identifier to each path.
This identifier is notably used when a peer sends a PATH_ABANDON
frame to indicate that it has closed the path whose identifier is
contained in the PATH_ABANDON frame.
In addition to these core features, an application using Multipath
QUIC 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, their
specification is out of scope of this document.
3. 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 [QUIC-TRANSPORT]. The new transport parameter is
defined as follows:
* name: enable_multipath (TBD - experiments use 0xbabf)
* value: 0 (default) for disabled.
The valid options for the value field are listed in Table 1:
+========+================================================+
| Option | Definition |
+========+================================================+
| 0x0 | don't support multipath |
+--------+------------------------------------------------+
| 0x1 | supports multipath as defined in this document |
+--------+------------------------------------------------+
Table 1: Available value for enable_multipath
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If for any one of the endpoints, the parameter is absent or set to 0,
the endpoints MUST fallback to [QUIC-TRANSPORT] with single active
path and MUST NOT use any frame or mechanism defined in this
document.
If endpoint receives an unexpected value for the transport parameter
"enable_multipath", it MUST treat this as a connection error of type
TRANSPORT_PARAMETER_ERROR (specified in Section 20.1 of
[QUIC-TRANSPORT]) and close the connection.
This extension does not change the definition of any transport
parameter defined in Section 18.2. of [QUIC-TRANSPORT].
Inline with the definition in [QUIC-TRANSPORT]
disable_active_migration also disables multipath support, except
"after a client has acted on a preferred_address transport parameter"
(Section 18.2. of [QUIC-TRANSPORT]).
The transport parameter "active_connection_id_limit" [QUIC-TRANSPORT]
limits the number of usable Connection IDs, and also limits the
number of concurrent paths. For the QUIC multipath extension this
limit even applies when no connection ID is exposed in the QUIC
header.
4. Path Setup and Removal
After completing the handshake, endpoints have agreed to enable
multipath feature and can start using multiple paths. 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 SHOULD NOT 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
in separate documents.
This proposal adds one multipath control frame for path management:
* PATH_ABANDON frame for the receiver side to abandon the path (see
Section 12.1)
All the new frames are sent in 1-RTT packets [QUIC-TRANSPORT].
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4.1. Path Initiation
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 Path Identifier field in PATH_STATUS frame to identify
which path's 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.
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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.
4.3.1. Use PATH_ABANDON Frame to Close a Path
Both endpoints, namely the client and the server, can close a path,
by sending PATH_ABANDON frame (see Section 12.1) which abandons the
path with a corresponding Path Identifier. Once a path is marked as
"abandoned", it means that the resources related to the path, such as
the used connection IDs, can be released. However, information
related to data delivered over that path SHOULD not be released
immediately as acknowledgments can still be received or other frames
that also may trigger retransmission of data on another path.
The endpoint sending the PATH_ABANDON frame SHOULD consider a path as
abandoned when the packet that contained the PATH_ABANDON frame is
acknowledged. When releasing resources of a path, the endpoint
SHOULD send a RETIRE_CONNECTION_ID frame for the connection IDs used
on the path, if any.
The receiver of a PATH_ABANDON frame SHOULD NOT release its resources
immediately, but SHOULD wait for the reception of the
RETIRE_CONNECTION_ID frame for the used connection IDs or 3 RTOs.
Usually, it is expected that the PATH_ABANDON frame is used by the
client to indicate to the server that path conditions have changed
such that the path is or will be not usable anymore, e.g. in case of
a mobility event. The PATH_ABANDON frame therefore indicates to the
receiving peer that the sender does not intend to send any packets on
that path anymore but also recommends to the receiver that no packets
should be sent in either direction. The receiver of an PATH_ABANDON
frame MAY also send an PATH_ABANDON frame to signal its own
willingness to not send any packet on this path anymore.
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If connection IDs are used, 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.
If no connection IDs are used and the PATH_ABANDON frame has to send
on the path that is intended to be closed, it is possible that the
packet containing the PATH_ABANDON frame or the packet containing the
ACK for the PATH_ABANDON frame cannot be received anymore and the
endpoint might need to rely on an idle time out to close the path, as
described in Section 4.3.4.
Retransmittable frames, that have previously been sent on the
abandoned path and are considered lost, SHOULD be retransmitted on a
different 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 enters 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 RTO if a path probing packet is received on a new path
before sending the CONNECTION_CLOSE frame.
4.3.2. 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. An endpoint that has negotiated the usage of the
multipath extension MAY use an explicit method by sending on another
active path a PATH_ABANDON frame containing the Path Identifier of
the refused path, but only if the PATH_CHALLENGE arrives in a packet
using a non-zero length Connection ID.
4.3.3. Effect of RETIRE_CONNECTION_ID Frame
Receiving a RETIRE_CONNECTION_ID frame causes the endpoint to discard
the resources associated with that connection ID. If the connection
ID was used by the peer to identify a path from the peer to this
endpoint, the resources include the list of received packets used to
send acknowledgements. The peer MAY decide to keep sending data
using the same IP addresses and UDP ports previously associated with
the connection ID, but MUST use a different connection ID when doing
so.
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Note that if the sender retires a Connection ID that is still used by
in-flight packets, it may receive ACK_MP frames referencing the
retired Connection ID. If the sender stops tracking sent packets
with retired Connection ID, these would be spuriously marked as lost.
To avoid such performance issue without keeping retired Connection ID
state, an endpoint should first stop sending packets with the to-be-
retired Connection ID, then wait for all in-flight packets to be
either acknowledged or marked as lost, and finally retire the
Connection ID.
4.3.4. Idle Timeout
[QUIC-TRANSPORT] allows for closing of connections if they stay idle
for too long. The connection idle timeout in multipath QUIC 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 [QUIC-TRANSPORT].
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 Section 10.1.
of [QUIC-TRANSPORT] (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 (Section 19.2 of
[QUIC-TRANSPORT]) on that path to keep it alive. Sending periodic
PING frames also helps prevent middlebox timeout, as discussed in
Section 10.1.2 of [QUIC-TRANSPORT].
Server MAY release the resource associated with paths for which no
non-probing packet was received for a sufficiently long path-idle
delay, but SHOULD only release resource for the last available path
if no traffic is received for the duration of the idle timeout, as
specified in Section 10.1 of [QUIC-TRANSPORT]. This means if all
paths remain idle for the idle timeout, the connection is implicitly
closed.
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 reestablish a path after
a spurious estimate of path abandonment by the client.
4.4. Path States
Figure 1 shows the states that an endpoint's path can have.
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o
| PATH_CHALLENGE sent/received on new path
v
+------------+ Path validation abandoned
| Validating |----------------------------------+
+------------+ |
| |
| PATH_RESPONSE received |
| |
v |
+------------+ Path blackhole detected |
| Active |----------------------------------+
+------------+ |
| |
| PATH_ABANDONED sent/received |
v |
+------------+ |
| Closing | |
+------------+ |
| |
| Path's draining timeout |
| (at least 3 PTO) |
v |
+------------+ |
| Closed |<---------------------------------+
+------------+
Figure 1: States of a path
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.
If multiple packet number spaces are used over the connection, hosts
MUST also track the following information.
* Path Packet Number Space: The endpoint maintains a separate packet
number for sending and receiving packets over this path. Packet
number considerations described in [QUIC-TRANSPORT] apply within
the given path.
In the "Active" state, hosts MUST also track the following
information.
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* Associated Source Connection ID: The Connection ID used to receive
packets over the path.
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_ABANDONED 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_ABANDONED 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. Congestion Control
Senders MUST manage per-path congestion status, and MUST NOT send
more data on a given path than congestion control on that path
allows. This is already a requirement of [QUIC-TRANSPORT].
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].
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6. 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.
+======================+=============+===========+
| ACK Path \ Data path | Terrestrial | Satellite |
+======================+=============+===========+
| Terrestrial | 100ms | 350ms |
+----------------------+-------------+-----------+
| Satellite | 350ms | 600ms |
+----------------------+-------------+-----------+
Table 2: Example of ACK delays using multiple
paths
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.
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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. 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 can only be sent when the
congestion window of at least one path is open.
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. Many
factors can influence the definition of these algorithms and their
precise definition is outside the scope of this document. Various
packet schedulers have been proposed and implemented, notably for
Multipath TCP. A companion draft [I-D.bonaventure-iccrg-schedulers]
provides several general-purpose packet schedulers depending on the
application goals.
Note that the receiver could use a different scheduling strategy to
send ACK(_MP) frames. The recommended default behaviour consists in
sending ACK(_MP) frames on the path they acknowledge packets. Other
scheduling strategies, such as sending ACK(_MP) frames on the lowest
latency path, might be considered, but they could impact the sender
with side effects on, e.g., the RTT estimation or the congestion
control scheme. When adopting such asymetrical acknowledgment
scheduling, the receiver should at least ensure that the sender
negotiated one-way delay calculation mechanism (e.g.,
[QUIC-Timestamp]).
8. Recovery
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.
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9. Packet Number Space and Use of Connection ID
If the connection ID is present (non-zero length) in the packet
header, the connection ID is used to identify the path. If no
connection ID is present, the 4 tuple identifies the path. The
initial path that is used during the handshake (and multipath
negotiation) has the path ID 0 and therefore all 0-RTT packets are
also tracked and processed with the path ID 0. For 1-RTT packets,
the path ID is the sequence number of the Destination Connection ID
present in the packet header, as defined in Section 5.1.1 of
[QUIC-TRANSPORT], or also 0 if the Connection ID is zero-length.
If non-zero-length Connection IDs are used, an endpoint MUST use
different Connection IDs on different paths. Still, the receiver may
observe the same Connection ID used on different 4-tuples due to,
e.g., NAT rebinding. In such case, the receiver reacts as specified
in Section 9.3 of [QUIC-TRANSPORT].
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 path identifiers. If
for any reason ACK frames are received in 1-RTT packets while the
state of multipath negotiation is ambiguous, they MUST be interpreted
as acknowledging packets sent on path 0.
9.1. Using Zero-Length connection ID
If a zero-length connection ID is used, one packet number space for
all paths. That means the packet sequence numbers are allocated from
the common number space, so that, for example, packet number N could
be sent on one path and packet number N+1 on another.
In this case, ACK frames report the numbers of packets that have been
received so far, regardless of the path on which they have been
received. That means the sender needs to maintain an association
between sent packet numbers and the path over which these packets
were sent. This is necessary to implement per path congestion
control, as explained in Section 9.1.2.
Further, the receiver of packets with zero-length connection IDs
should implement handling of acknowledgements as defined in
Section 9.1.1.
ECN handing is specified in Section 9.1.4, and mitigation of the RTT
measurement is further explained in Section 9.1.3.
If a node does not want to implement this logic, it MAY instead limit
its use of multiple paths as explained in Section 9.1.5.
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9.1.1. Sending Acknowledgements and Handling Ranges
If zero-length CID and therefore also a single packet number space is
used by the sender, the receiver MAY send ACK frames instead of
ACK_MP frames to reduce overhead as the additional path ID field will
anyway always carry the same value.
If senders decide to send packets on paths with different
transmission delays, some packets will very likely be received out of
order. This will cause the ACK frames to carry multiple ranges of
received packets. The large number of range increases the size of
ACK frames, causing transmission and processing overhead.
The size and overhead of the ACK frames can be controlled by the
combination of one or several of the following:
* Not transmitting again ACK ranges that were present in an ACK
frame acknowledged by the peer.
* Delay acknowledgements to allow for arrival of "hole filling"
packets.
* Limit the total number of ranges sent in an ACK frame.
* Limiting the number of transmissions of a specific ACK range, on
the assumption that a sufficient number of transmissions almost
certainly ensures reception by the peer.
* Send multiple messages for a given path in a single socket
operation, so that a series of packets sent from a single path
uses a series of consecutive sequence numbers without creating
holes.
9.1.2. Loss and Congestion Handling With Zero-Length CID
When sending to a zero-length CID receiver, senders may receive
acknowledgements that combine packet numbers received over multiple
paths. However, even if one packet number space is used on multiple
path the sender MUST maintain separate congestion control state for
each path. Therefore, senders MUST be able to infer the sending path
from the acknowledged packet numbers, for example by remembering
which packet was sent on what path. The senders MUST use that
information to perform congestion control on the relevant paths, and
to correctly estimate the transmission delays on each path. (See
Section 9.1.3 for specific considerations about using the ACK Delay
field of ACK frames, and Section 9.1.4 for issues on using ECN
marks.)
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Loss detection as specified in [QUIC-RECOVERY] uses algorithms based
on timers and on sequence numbers. When packets are sent over
multiple paths, loss detection must be adapted to allow for different
RTTs on different paths. When sending to zero-length CID receivers,
packets sent on different paths may be received out of order.
Therefore, senders cannot directly use the packet sequence numbers to
compute the Packet Thresholds defined in Section 6.1.1 of
[QUIC-RECOVERY]. Relying only on Time Thresholds produces correct
results, but is somewhat suboptimal. Some implementations have been
getting good results by not just remembering the path over which a
packet was sent, but also maintaining an order list of packets sent
on each path. That ordered list can then be used to compute
acknowledgement gaps per path in Packet Threshold tests.
9.1.3. RTT Estimation Considerations when SPNS is Used
When SPNS is in use, accurate RTT estimation requires more careful
considerations. According to [QUIC-RECOVERY], an endpoint generates
an RTT sample on receiving an ACK frame that meets the following two
conditions: (1) the largest acknowledged packet number is newly
acknowledged, and (2) at least one of the newly acknowledged packets
was ack-eliciting. The RTT sample, latest_rtt is calculated as the
time elapsed since the largest acknowledged packet was sent.
However, when applying the above algorithm with SPNS, one may
encounter the following issues: (1) RTT of some paths are not updated
timely if ACKs are mostly returned from other paths, and (2) ACK
frames depend on the largest received packet of the connection, not
the path, and the resulted RTT sample may be the sum of the one-way
delays of two different paths. One solution for accurate RTT
measurements is to employ time-stamps as described in
[QUIC-Timestamp]. If one chooses not to use time-stamps but wants to
get reasonable estimation of RTTs on multiple paths with single
packet number space, the following practices can be used:
For packet receiver (ACK sender):
* Maintain an ACK threshold and an ACK timer for each path. A path
should send an ACK when it receives ack-eliciting-threshold number
of ack-eliciting packets (e.g., two) on this path, and an ack-
eliciting packet must be acknowledged within MAX_ACK_DELAY.
* Write ACK frame based on the largest received packet of the path.
Start the ACK ranges with the largest received packet number of
that path, which means that the "Largest Acknowledged" field is
the path's largest packet number and the "ACK Delay" field is the
delay time of the path's largest received packet.
For packet sender (ACK receiver):
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* Maintain an unacked list for each path to retrieve the packets
that has been sent when an ACK is received. It can coexist with
the unacked list of the connection layer or packet number space
layer.
* Generate RTT sample for a path when the following conditions are
met: (1) the largest acknowledged packet number is newly
acknowledged by the ACK received from this path, and (2) at least
one of the newly acknowledged packets was ack-eliciting.
9.1.4. ECN and Zero-Length CID Considerations
ECN feedback in QUIC is provided based on counters in the ACK frame
(see Section 19.3.2. of [QUIC-TRANSPORT]). That means if an ACK
frame acknowledges multiple packets, the ECN feedback cannot be
accounted to a specific packet.
There are separate counters for each packet number space. However,
sending to zero-length CID receivers, the same number space is used
for multiple paths. Respectively, if an ACK frames acknowledges
multiple packets from different paths, the ECN feedback cannot
unambiguously be assigned to a path.
If the sender marks its packets with the ECN capable flags, the
network will expect standard reactions to ECN marks, such as slowing
down transmission on the sending path. If zero-length CID is used,
the sending path is however ambiguous. Therefore, the sender MUST
treat a CE marking as a congestion signal on all sending paths that
have been by a packet that was acknowledged in the ACK frame
signaling the CE counter increase.
A host that is sending over multiple paths to a zero-length CID
receiver MAY disable ECN marking and send all subsequent packets as
Not-ECN capable.
9.1.5. Restricted Sending to Zero-Length CID Peer
Hosts that are designed to support multipath using multiple number
spaces MAY adopt a conservative posture after negotiating multipath
support with a peer using zero-length CID. The simplest posture is
to only send data on one path at a time, while accepting packets on
all acceptable paths. In that case:
* the attribution of packets to path discussed in Section 9.1.2 are
easy to solve because packets are sent on a single path,
* the ACK Delays are correct,
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* the vast majority of ECN marks relate to the current sending path.
Of course, the hosts will only take limited advantage from the
multipath capability in these scenarios. Support for "make before
break" migrations will improve, but load sharing between multiple
paths will not work.
9.2. Using non-zero length CID and Multiple Packet Number Spaces
If packets contain a non-zero CID, each path has its own packet
number space for transmitting 1-RTT packets and a new ACK frame
format is used as specified in Section 12.3. Compared to the QUIC
version 1 ACK frame, the ACK_MP frames additionally contains a Packet
Number Space Identifier (PN Space ID). The PN Space ID used to
distinguish packet number spaces for different paths and is simply
derived from the sequence number of Destination Connection ID.
Therefore, the packet number space for 1-RTT packets can be
identified based on the Destination Connection ID in each packet.
As soon as the negotiation of multipath support is completed,
endpoints SHOULD use ACK_MP frames instead of ACK frames for
acknowledgements of 1-RTT packets on path 0, as well as for 0-RTT
packets that are acknowledged after the handshake concluded.
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.
ACK_MP frame (defined in Section 12.3) can be returned via either a
different path, or the same path identified by the Path Identifier,
based on different strategies of sending ACK_MP frames.
Using multiple packet number spaces requires changes in the way AEAD
is applied for packet protection, as explained in Section 9.2.1, and
tighter constraints for key updates, as explained in Section 9.2.2.
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9.2.1. Packet Protection for QUIC Multipath
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. If 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 path identifier.
The path ID for 1-RTT packets is the sequence number of
[QUIC-TRANSPORT], or zero if the Connection ID is zero-length.
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.
9.2.2. Key Update for QUIC Multipath
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
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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.
10. Examples
10.1. Path Establishment
Figure 2 illustrates an example of new path establishment using
multiple packet number spaces.
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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[Seq=2,PN=0]
Checks AEAD using nonce(CID sequence 1, PN 0)
1-RTT[1]: DCID=S2, PATH_RESPONSE[Y],
ACK_MP[Seq=1, PN=0], ... -->
Figure 2: Example of new path establishment
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.
10.2. 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 when both the client
and the server use non-zero-length CIDs. 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,
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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 received packet's DCID over that path (path identifier type
0x00), hence using the path_id 1. Optionally, the server confirms
the path closure by sending an PATH_ABANDON frame using the sequence
number of the received packet's DCID over that path (path identifier
type 0x00) as path identifier, which corresponds to the path_id 2.
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[path_id_type=0, path_id=1]->
(server tells client to abandon a path)
<-1-RTT[Y]: DCID=C1 PATH_ABANDON[path_id_type=0, path_id=2],
ACK_MP[Seq=2, PN=X]
(client retires the corresponding CID)
1-RTT[U]: DCID=S3 RETIRE_CONNECTION_ID[2], ACK_MP[Seq=1, PN=Y] ->
(server retires the corresponding CID)
<- 1-RTT[V]: DCID=C2 RETIRE_CONNECTION_ID[1], ACK_MP[Seq=3, PN=U]
Figure 3: Example of closing a path when both the client and the
server choose to receive non-zero-length CIDs.
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Figure 4 illustrates an example of path closing when the client
chooses to receive zero-length CIDs while the server chooses to
receive non-zero-length CIDs. Because there is a zero-length CID in
one direction, single packet number spaces are used. For the first
path, the client's 1-RTT packets use DCID S2, which has a sequence
number of 2. For the second path, the client's 1-RTT packets use
DCID S3, which has a sequence number of 3. Again, in this case, the
client is going to close the first path. Because the client now
receives zero-length CID packets, it needs to use path identifier
type 0x01, which identifies a path by the DCID sequence number of the
packets it sends over that path, and hence, it uses a path_id 2 in
its PATH_ABANDON frame. The server SHOULD stop sending new data on
the path indicated by the PATH_ABANDON frame after receiving it.
However, The client may want to repeat the PATH_ABANDON frame if it
sees the server continuing to send data. When the client's
PATH_ABANDON frame is acknowledged, it sends out a
RETIRE_CONNECTION_ID frame for the CID used on the first path. The
server can readily close the first path when it receives the
RETIRE_CONNECTION_ID frame from the client. However, since the
client will not receive a RETIRE_CONNECTION_ID frame, after sending
out the RETIRE_CONNECTION_ID frame, the client waits for 3 RTO before
closing the path.
Client Server
(client tells server to abandon a path)
1-RTT[X]: DCID=S2 PATH_ABANDON[path_id_type=1, path_id=2]->
(server stops sending on that path after
receiving PATH_ABANDON)
(client retires the corresponding CID
after PATH_ABANDON is acknowledged)
1-RTT[X+1]: DCID=S3 RETIRE_CONNECTION_ID[2]->
Figure 4: Example of closing a path when the client chooses to
receive zero- length CIDs while the server chooses to receive
non-zero-length CIDs
11. Implementation Considerations
11.1. 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.
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11.2. 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.
12. New Frames
All the new frames MUST only be sent in 1-RTT packet, and MUST NOT
use other encryption levels.
If an endpoint receives multipath-specific frames from packets of
other encryption levels, it MUST return MP_PROTOCOL_VIOLATION as a
connection error and close the connection.
12.1. PATH_ABANDON Frame
The PATH_ABANDON frame informs the peer to abandon a path. More
complex path management can be made possible with additional
extensions (e.g., PATH_STATUS frame in [I-D.liu-multipath-quic] ).
PATH_ABANDON frames are formatted as shown in Figure 5.
PATH_ABANDON Frame {
Type (i) = TBD-02 (experiments use 0xbaba05),
Path Identifier (..),
Error Code (i),
Reason Phrase Length (i),
Reason Phrase (..),
}
Figure 5: PATH_ABANDON Frame Format
PATH_ABANDON frames contain the following fields:
Path Identifier: An identifier of the path, which is formatted as
shown in Figure 6.
* Identifier Type: Identifier Type field is set to indicate the type
of path identifier.
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- Type 0: Refer to the connection identifier issued by the sender
of the control frame. Note that this is the connection
identifier used by the peer when sending packets on the to-be-
closed path. This method SHOULD be used if this connection
identifier is non-zero length. This method MUST NOT be used if
this connection identifier is zero-length.
- Type 1: Refer to the connection identifier issued by the
receiver of the control frame. Note that this is the
connection identifier used by the sender when sending packets
on the to-be-closed path. This method MUST NOT be used if this
connection identifier is zero-length.
- Type 2: Refer to the path over which the control frame is sent
or received.
* Path Identifier Content: A variable-length integer specifying the
path identifier. If Identifier Type is 2, the Path Identifier
Content MUST be empty.
Path Identifier {
Identifier Type (i) = 0x00..0x02,
[Path Identifier Content (i)],
}
Figure 6: Path Identifier Format
Note: If the receiver of the PATH_ABANDON frame is using non-zero
length Connection ID on that path, endpoint SHOULD use type 0x00 for
path identifier in the control frame. If the receiver of the
PATH_ABANDON frame is using zero-length Connection ID, but the peer
is using non-zero length Connection ID on that path, endpoints SHOULD
use type 0x01 for path identifier. If both endpoints are using
0-length Connection IDs on that path, endpoints SHOULD only use type
0x02 for path identifier.
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.
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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.
If the Identifier Type is 0x00 or 0x01, PATH_ABANDON frames MAY be
sent on any path, not only the path identified by the Path Identifier
Content field. If the Identifier Type if 0x02, the PATH_ABANDON
frame MUST only be sent on the path that is intended to be abandoned.
12.2. 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),
Path Identifier (..),
Path Status sequence number (i),
Path Status (i),
}
Figure 7: PATH_STATUS Frame Format
PATH_STATUS Frames contain the following fields:
Path Identifier: An identifier of the path, which is formatted as
shown in Figure 6. Exactly the same as the definition of Path
Identifier in {#path-abandon-frame}.
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 Path Identifier.
Available values of Path Status field are:
* 1: Standby
* 2: Available
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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 receive 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 Path Identifier with a sequence number equal to or
higher than the 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.
12.3. 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 when 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 8.
ACK_MP Frame {
Type (i) = TBD-00..TBD-01 (experiments use 0xbaba00..0xbaba01),
Packet Number Space Identifier (i),
Largest Acknowledged (i),
ACK Delay (i),
ACK Range Count (i),
First ACK Range (i),
ACK Range (..) ...,
[ECN Counts (..)],
}
Figure 8: ACK_MP Frame Format
Compared to the ACK frame specified in [QUIC-TRANSPORT], the
following field is added.
Packet Number Space Identifier: An identifier of the path packet
number space, which is the sequence number of Destination Connection
ID of the 1-RTT packets which are acknowledged by the ACK_MP frame.
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If the endpoint receives 1-RTT packets with zero-length Connection
ID, it SHOULD use Packet Number Space Identifier 0 in ACK_MP frames.
If an endpoint receives an ACK_MP frame with a packet number space ID
which was never issued by endpoints (i.e., with a sequence number
larger than the largest one advertised), it MUST treat this as a
connection error of type MP_PROTOCOL_VIOLATION and close the
connection. If an endpoint receives an ACK_MP frame with a packet
number space ID which is no more active (e.g., retired by a
RETIRE_CONNECTION_ID frame or belonging to closed paths), it MUST
ignore the ACK_MP frame without causing a connection error.
When using a single packet number space, endhosts MUST NOT send
ACK_MP frames. If an endhost receives an ACK_MP frame while a single
packet number space was negotiated, it MUST treat this as a
connection error of type MP_PROTOCOL_VIOLATION and close the
connection.
13. Error Codes
Multipath QUIC transport error codes are 62-bit unsigned integers
following [QUIC-TRANSPORT].
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 0xba01): An endpoint detected
an error with protocol compliance that was not covered by more
specific error codes.
14. IANA Considerations
This document defines a new transport parameter for the negotiation
of enable multiple paths for QUIC, and two 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 Table 3 should be added to the "QUIC Transport
Parameters" registry under the "QUIC Protocol" heading.
+==============================+==================+===============+
| Value | Parameter Name. | Specification |
+==============================+==================+===============+
| TBD (experiments use 0xbabf) | enable_multipath | Section 3 |
+------------------------------+------------------+---------------+
Table 3: Addition to QUIC Transport Parameters Entries
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The following frame types defined in Table 4 should be added to the
"QUIC Frame Types" registry under the "QUIC Protocol" heading.
+==============================+==============+===============+
| Value | Frame Name | Specification |
+==============================+==============+===============+
| TBD-00 - TBD-01 (experiments | ACK_MP | Section 12.3 |
| use 0xbaba00-0xbaba01) | | |
+------------------------------+--------------+---------------+
| TBD-02 (experiments use | PATH_ABANDON | Section 12.1 |
| 0xbaba05) | | |
+------------------------------+--------------+---------------+
| TBD-03 (experiments use | PATH_STATUS | Section 12.2 |
| 0xbaba06) | | |
+------------------------------+--------------+---------------+
Table 4: Addition to QUIC Frame Types Entries
The following transport error code defined in Table 5 should be added
to the "QUIC Transport Error Codes" registry under the "QUIC
Protocol" heading.
+==============+=======================+============+===============+
| Value | Code |Description | Specification |
+==============+=======================+============+===============+
| TBD | MP_PROTOCOL_VIOLATION |Multipath | Section 13 |
| (experiments | |protocol | |
| use 0xba01) | |violation | |
+--------------+-----------------------+------------+---------------+
Table 5: Error Code for Multipath QUIC
15. Security Considerations
TBD
16. 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
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17. Acknowledgments
TBD
18. References
18.1. Normative References
[QUIC-TLS] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
<https://www.rfc-editor.org/rfc/rfc9001>.
[QUIC-TRANSPORT]
Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/rfc/rfc9000>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/rfc/rfc3629>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
18.2. Informative References
[I-D.bonaventure-iccrg-schedulers]
Bonaventure, O., Piraux, M., De Coninck, Q., Baerts, M.,
Paasch, C., and M. Amend, "Multipath schedulers", Work in
Progress, Internet-Draft, draft-bonaventure-iccrg-
schedulers-02, 25 October 2021,
<https://datatracker.ietf.org/doc/html/draft-bonaventure-
iccrg-schedulers-02>.
[I-D.liu-multipath-quic]
Liu, Y., Ma, Y., Huitema, C., An, Q., and Z. Li,
"Multipath Extension for QUIC", Work in Progress,
Internet-Draft, draft-liu-multipath-quic-04, 5 September
2021, <https://datatracker.ietf.org/doc/html/draft-liu-
multipath-quic-04>.
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[OLIA] Khalili, R., Gast, N., Popovic, M., Upadhyay, U., and J.
Le Boudec, "MPTCP is not pareto-optimal: performance
issues and a possible solution", Proceedings of the 8th
international conference on Emerging networking
experiments and technologies, ACM , 2012.
[QUIC-Invariants]
Thomson, M., "Version-Independent Properties of QUIC",
RFC 8999, DOI 10.17487/RFC8999, May 2021,
<https://www.rfc-editor.org/rfc/rfc8999>.
[QUIC-RECOVERY]
Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
and Congestion Control", RFC 9002, DOI 10.17487/RFC9002,
May 2021, <https://www.rfc-editor.org/rfc/rfc9002>.
[QUIC-Timestamp]
Huitema, C., "Quic Timestamps For Measuring One-Way
Delays", Work in Progress, Internet-Draft, draft-huitema-
quic-ts-08, 28 August 2022,
<https://datatracker.ietf.org/doc/html/draft-huitema-quic-
ts-08>.
[RFC6356] Raiciu, C., Handley, M., and D. Wischik, "Coupled
Congestion Control for Multipath Transport Protocols",
RFC 6356, DOI 10.17487/RFC6356, October 2011,
<https://www.rfc-editor.org/rfc/rfc6356>.
Authors' Addresses
Yanmei Liu (editor)
Alibaba Inc.
Email: miaoji.lym@alibaba-inc.com
Yunfei Ma
Alibaba Inc.
Email: yunfei.ma@alibaba-inc.com
Quentin De Coninck (editor)
UCLouvain
Email: quentin.deconinck@uclouvain.be
Olivier Bonaventure
UCLouvain and Tessares
Email: olivier.bonaventure@uclouvain.be
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Christian Huitema
Private Octopus Inc.
Email: huitema@huitema.net
Mirja Kuehlewind (editor)
Ericsson
Email: mirja.kuehlewind@ericsson.com
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