Internet DRAFT - draft-lmbdhk-quic-multipath
draft-lmbdhk-quic-multipath
QUIC Working Group Y. Liu
Internet-Draft Y. Ma
Intended status: Standards Track Alibaba Inc.
Expires: 28 April 2022 Q. De Coninck
O. Bonaventure
UCLouvain
C. Huitema
Private Octopus Inc.
M. Kuehlewind, Ed.
Ericsson
25 October 2021
Multipath Extension for QUIC
draft-lmbdhk-quic-multipath-00
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 28 April 2022.
Copyright Notice
Copyright (c) 2021 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
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions and Definitions . . . . . . . . . . . . . . . 4
2. Handshake Negotiation and Transport Parameter . . . . . . . . 5
3. Path Setup and Removal . . . . . . . . . . . . . . . . . . . 6
3.1. Path Initiation . . . . . . . . . . . . . . . . . . . . . 7
3.2. Path Close . . . . . . . . . . . . . . . . . . . . . . . 7
3.2.1. Use PATH_ABANDON Frame to Close a Path . . . . . . . 7
3.2.2. Effect of RETIRE_CONNECTION_ID Frame . . . . . . . . 8
3.2.3. Idle Timeout . . . . . . . . . . . . . . . . . . . . 9
3.3. Path States . . . . . . . . . . . . . . . . . . . . . . . 9
4. Congestion Control . . . . . . . . . . . . . . . . . . . . . 11
5. Computing Path RTT . . . . . . . . . . . . . . . . . . . . . 11
6. Packet Scheduling . . . . . . . . . . . . . . . . . . . . . . 12
7. Packet Number Space and Use of Connection ID . . . . . . . . 13
7.1. Using One Packet Number Space . . . . . . . . . . . . . . 13
7.1.1. Sending Acknowledgements and Handling Ranges . . . . 14
7.2. Using Multiple Packet Number Spaces . . . . . . . . . . . 15
7.2.1. Packet Protection for QUIC Multipath . . . . . . . . 15
7.2.2. Key Update for QUIC Multipath . . . . . . . . . . . . 16
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.1. Path Establishment . . . . . . . . . . . . . . . . . . . 17
8.2. Path Closure . . . . . . . . . . . . . . . . . . . . . . 18
9. Implementation Considerations . . . . . . . . . . . . . . . . 19
10. New Frames . . . . . . . . . . . . . . . . . . . . . . . . . 19
10.1. PATH_ABANDON Frame . . . . . . . . . . . . . . . . . . . 19
10.2. ACK_MP Frame . . . . . . . . . . . . . . . . . . . . . . 21
11. Error Codes . . . . . . . . . . . . . . . . . . . . . . . . . 22
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
13. Security Considerations . . . . . . . . . . . . . . . . . . . 23
14. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 23
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15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
16.1. Normative References . . . . . . . . . . . . . . . . . . 23
16.2. Informative References . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction
This document specifies an extension to QUIC v1 [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-v1. In particular
this proposal uses path validation as specified for QUIC v1 and
aims to re-use as much as possible of QUIC's connection migration.
* Use the same packet header formats as QUIC v1 to avoid the risk of
packets being dropped by middleboxes (which may only support QUIC
v1)
* Congestion Control, RTT measurements and PMTU discovery should be
per-path (following [QUIC-TRANSPORT])
* 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.
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 2.
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This proposal supports the negotiation of either the use of one
packet number space for all paths or the use of separate packet
number spaces per path. While separate packet number spaces allow
for more efficient ACK encoding, especially when paths have highly
different latencies, this approach requires the use of a connection
ID. Therefore use of a single number space can be beneficial in
highly constrained networks that do not benefit from exposing the
connection ID in the header. While both approaches are supported by
the specification in this version of the document, the intention for
the final publication of a multipath extension for QUIC is to choose
one option in order to avoid incompatibility. More evaluation and
implementation experience is needed to select one approach before
final publication. Some discussion about pros and cons can be found
here: https://github.com/mirjak/draft-lmbdhk-quic-
multipath/blob/master/presentations/PacketNumberSpace_s.pdf
As currently defined in this version of the draft the use of multiple
packet number spaces requires the use of connection IDs is both
directions. Today's deployments often only use destination
connection ID when sending packets from the client to the server as
this addresses the most important use cases for migration, like NAT
rebinding or mobility events. Further discussion and work is
required to evaluate if the use of multiple packet number spaces
could be supported as well when the connection ID is only present in
one direction.
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.
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:
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* 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 multi-path 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. 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 follow:
* name: enable_multipath (TBD - experiments use 0xbabf)
* value: 0 (default) for disabled. Endpoints use 2-bits in the
value field for negotiating one or more PN spaces, available
option value for client and server are listed in Table 1 :
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+===============+===========================+================+
| Client Option | Definition | Allowed server |
| | | responses |
+===============+===========================+================+
| 0x0 | don't support multi-path | 0x0 |
+---------------+---------------------------+----------------+
| 0x1 | only support one PN space | 0x0 or 0x1 |
| | for multi-path | |
+---------------+---------------------------+----------------+
| 0x2 | only support multiple PN | 0x0 or 0x2 |
| | spaces for multi-path | |
+---------------+---------------------------+----------------+
| 0x3 | support both one PN space | 0x0, 0x1 or |
| | and multiple PN space | 0x2 |
+---------------+---------------------------+----------------+
Table 1: Available value for enable_multipath
If the peer does not carry the enable_multipath transport parameter,
which means the peer does not support multipath, endpoint MUST
fallback to [QUIC-TRANSPORT] with single path and MUST NOT use any
frame or mechanism defined in this document. If endpoint receives
unexpected value for the transport parameter "enable_multipath", it
MUST treat this as a connection error of type MP_CONNECTION_ERROR and
close the connection.
Note that 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.
3. 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 discuss when a client decides to initiate a new path. We
delegate such discussion in separate documents.
This proposal adds one multi-path control frame for path management:
* PATH_ABANDON frame for the receiver side to abandon the path
Section 10.1
All the new frames are sent in 1-RTT packets [QUIC-TRANSPORT].
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3.1. 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
of [QUIC-TRANSPORT]. After receiving packets from the client on the
new paths, the servers MAY in turn attempt to validate these paths
using the same mechanisms.
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.
3.2. 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.
An endpoint that wants to close a path SHOULD NOT rely on implicit
signals like idle time or packet losses, but instead SHOULD use
explicit request to terminate path by sending the PATH_ABANDON frame
(see Section 10.1).
3.2.1. 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 10.1) 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.
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The receiver of a PATH_ABANDON frame SHOULD NOT release its resources
immediately but SHOULD wait for the receive 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
an 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.
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
connection 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 sent 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 Section 3.2.3.
Retransmittable frames, that have previously been send 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.
3.2.2. 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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3.2.3. 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, servers shall monitor the
arrival of non-probing packets on the available paths. Servers
SHOULD stop sending traffic on paths through where no non-probing
packet was received in the last 3 path RTTs, but MAY ignore that rule
if it would disqualify all available paths. 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.
3.3. 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 Associated CID have been retired |
+------------+ Path's idle timeout |
| Active |----------------------------------+
+------------+ |
| |
| PATH_ABANDONED sent/received |
v |
+------------+ |
| Closing | |
+------------+ |
| |
| Associated CID have been retired |
| Path's idle timeout |
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.
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. Consequently, the endhost is not able
to send nor receive packets on this path anymore.
4. 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].
5. 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 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
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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.
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 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.
6. 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.
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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.
7. 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.
7.1. Using One Packet Number Space
If the multipath option is negotiated to use one packet number space
for all paths, 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.
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 senders 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.
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When a packet is acknowledged, the state of the congestion control
MUST be updated for the path where the acknowledged packet was
originally sent. The RTT is calculated based on the delay between
the transmission of that packet and its first acknowledgement (see
Section 5) and is used to update the RTT statistics for the sending
path.
Also loss detection MUST be adapted to allow for different RTTs on
different paths. For example, timer computations should take into
account the RTT of the path on which a packet was sent. Detections
based on packet numbers shall compare a given packet number to the
highest packet number received for that path.
7.1.1. Sending Acknowledgements and Handling Ranges
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.
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7.2. Using Multiple Packet Number Spaces
If the multipath option is enabled with a value of 2, 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 10.2. Compared to
the QUIC v1 ACK frame, the MP_ACK 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 packets.
As soon as the negotiation of multipath support with value 2 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 Section 10.2 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 7.2.1, and
tighter constraints for key updates, as explained in Section 7.2.2.
7.2.1. Packet Protection for QUIC Multipath
Packet protection for QUIC v1 is specified is 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.
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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 None, 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 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.
7.2.2. Key Update for QUIC Multipath
The Key Phase bit update process for QUIC v1 is specified in
Section 6 of [QUIC-TLS]. The general principles of key update are
not changed in this specification. Following QUIC v1, the Key Phase
bit is used to indicate which packet protection keys are used to
protect the packet. The Key Phase bit is toggled to signal each
subsequent key update. Because of network delays, packets protected
with the older key might arrive later than the packets protected with
the new key. Therefore, the endpoint needs to retain old packet keys
to allow these delayed packets to be processed and it must
distinguish between the new key and the old key. In QUIC V1, 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
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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.
8. Examples
8.1. Path Establishment
Figure 2 illustrates an example of new path establishment using
multiple packet number spaces.
Client Server
(Exchanges start on default path)
1-RTT[]: NEW_CONNECTION_ID[C1, Seq=1] -->
<-- 1-RTT[]: NEW_CONNECTION_ID[S1, Seq=1]
<-- 1-RTT[]: NEW_CONNECTION_ID[S2, Seq=2]
...
(starts new path)
1-RTT[0]: DCID=S2, PATH_CHALLENGE[X] -->
Checks AEAD using nonce(CID sequence 2, PN 0)
<-- 1-RTT[0]: DCID=C1, PATH_RESPONSE[X], PATH_CHALLENGE[Y],
ACK_MP[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
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In Figure 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 an 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 used, to
allow for new paths or migration.
8.2. 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.
In Figure Figure 3 the server's 1-RTT packets use DCID C1, which has
a sequence number of 1, 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 server's 1-RTT packets use DCID C2, which has a
sequence number of 2; the client's 1-RTT packets use CID S3, which
has a sequence number of 3. Note that two paths use different packet
number space.
Thee client initiates the path closure for the path with ID 1 by
sending a packet with an PATH_ABANDON frame. When the server
received the PATH_ABANDON frame, it also sends an PATH_ABANDON frame
in the next packet. Afterwards the connection IDs in both directions
can be retired using the RETIRE_CONNECTION_ID frame.
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Client Server
(client tells server to abandon a path)
1-RTT[X]: DCID=S2 PATH_ABANDON[path_id=1]->
(server tells client to abandon a path)
<-1-RTT[Y]: DCID=C1 PATH_ABANDON[path_id=2], ACK_MP[Seq=2, PN=X]
(client abandons the path that it is using)
1-RTT[U]: DCID=S3 RETIRE_CONNECTION_ID[2], ACK_MP[Seq=1, PN=Y] ->
(server abandons the path that it is using)
<- 1-RTT[V]: DCID=C2 RETIRE_CONNECTION_ID[1], ACK_MP[Seq=3, PN=U]
Figure 3: Example of closing a path (path id type=0x00)
9. Implementation Considerations
TDB
10. 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.
10.1. 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 4.
PATH_ABANDON Frame {
Type (i) = TBD-03 (experiments use 0xbaba05),
Path Identifier (..),
Error Code (i),
Reason Phrase Length (i),
Reason Phrase (..),
}
Figure 4: 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 5.
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* Identifier Type: Identifier Type field is set to indicate the type
of path identifier.
- Type 0: Refer to the connection identifier used by the sender
of the control frame when sending data over the specified 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 used by the receiver
of the control frame when sending data over the specified 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 5: 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.
10.2. 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 6.
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 6: ACK_MP Frame Format
Compared to the ACK frame specified in [QUIC-TRANSPORT], the
following field is added.
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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.
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 a ACK_MP frame with a non-existing packet
number space ID, it MUST treat this as a connection error of type
MP_PROTOCOL_VIOLATION and close the connection.
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.
11. Error Codes
Multi-path 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.
12. 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 2 |
+------------------------------+------------------+---------------+
Table 3: Addition to QUIC Transport Parameters Entries
The following frame types defined in Table 4 should be added to the
"QUIC Frame Types" registry under the "QUIC Protocol" heading.
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+==============================+==============+===============+
| Value | Frame Name | Specification |
+==============================+==============+===============+
| TBD-00 - TBD-01 (experiments | ACK_MP | Section 10.2 |
| use 0xbaba00-0xbaba01) | | |
+------------------------------+--------------+---------------+
| TBD-02 (experiments use | PATH_ABANDON | Section 10.1 |
| 0xbaba05) | | |
+------------------------------+--------------+---------------+
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 |Multi-path | Section 11 |
| (experiments | |protocol | |
| use 0xba01) | |violation | |
+--------------+-----------------------+------------+---------------+
Table 5: Error Code for Multi-path QUIC
13. Security Considerations
TBD
14. 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
15. Acknowledgments
TBD
16. References
16.1. Normative References
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[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/info/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/info/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/info/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/info/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/info/rfc8174>.
16.2. Informative References
[I-D.bonaventure-iccrg-schedulers]
Bonaventure, O., Piraux, M., Coninck, Q. D., Baerts, M.,
Paasch, C., and M. Amend, "Multipath schedulers", Work in
Progress, Internet-Draft, draft-bonaventure-iccrg-
schedulers-02, 25 October 2021,
<https://www.ietf.org/archive/id/draft-bonaventure-iccrg-
schedulers-02.txt>.
[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://www.ietf.org/archive/id/draft-liu-
multipath-quic-04.txt>.
[OLIA] Khalili, R., Gast, N., Popovic, M., Upadhyay, U., and J.-
Y. 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.
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[QUIC-Invariants]
Thomson, M., "Version-Independent Properties of QUIC",
RFC 8999, DOI 10.17487/RFC8999, May 2021,
<https://www.rfc-editor.org/info/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/info/rfc9002>.
[QUIC-Timestamp]
Huitema, C., "Quic Timestamps For Measuring One-Way
Delays", Work in Progress, Internet-Draft, draft-huitema-
quic-ts-06, 12 September 2021,
<https://www.ietf.org/archive/id/draft-huitema-quic-ts-
06.txt>.
[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/info/rfc6356>.
Authors' Addresses
Yanmei Liu
Alibaba Inc.
Email: miaoji.lym@alibaba-inc.com
Yunfei Ma
Alibaba Inc.
Email: yunfei.ma@alibaba-inc.com
Quentin De Coninck
UCLouvain
Email: quentin.deconinck@uclouvain.be
Olivier Bonaventure
UCLouvain
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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