An Alternate Path Model for Multipath QUIC
draft-gage-quicmp-pathmodel-00
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Author | Bill Gage | ||
| Last updated | 2023-11-11 | ||
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draft-gage-quicmp-pathmodel-00
QUIC Working Group W. Gage
Internet-Draft Unaffiliated
Intended status: Standards Track 11 November 2023
Expires: 14 May 2024
An Alternate Path Model for Multipath QUIC
draft-gage-quicmp-pathmodel-00
Abstract
The path model used in the current MPQUIC draft binds a connection
identifier to a path. In fact, the sequence number of a connection
identifier is used as an implicit path identifier. This has a number
of consequences that may cause MPQUIC to diverge from the principles
of RFC9000. One of these consequences, for example, is to associate
each connection with a different application data packet number space
rather than maintaining a single application data packet number space
across all connections as defined in RFC9000.
This document proposes a different path model for Multipath QUIC
using explicit path identifiers, enabling a multipath management
framework that retains the principles and operations of RFC9000.
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
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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."
This Internet-Draft will expire on 14 May 2024.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Multipath Management . . . . . . . . . . . . . . . . . . . . 4
3. Path Identification . . . . . . . . . . . . . . . . . . . . . 5
3.1. (Sidebar) Comparison to MPQUIC-DRAFT . . . . . . . . . . 5
3.2. (Sidebar) Use of multiple application data packet number
spaces . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. High-level Overview . . . . . . . . . . . . . . . . . . . . . 6
5. Path Activation and Removal . . . . . . . . . . . . . . . . . 7
5.1. Path Activation . . . . . . . . . . . . . . . . . . . . . 7
5.2. Path Removal . . . . . . . . . . . . . . . . . . . . . . 8
6. Path Maintenance . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Path Transmission Status . . . . . . . . . . . . . . . . 8
6.2. Path Congestion Control . . . . . . . . . . . . . . . . . 9
6.3. Path RTT Measurements . . . . . . . . . . . . . . . . . . 9
7. Packet Scheduling . . . . . . . . . . . . . . . . . . . . . . 9
8. Packet Loss Detection and Recovery . . . . . . . . . . . . . 10
9. Multipath Frame Types . . . . . . . . . . . . . . . . . . . . 10
9.1. MULTIPATH_CHALLENGE . . . . . . . . . . . . . . . . . . . 11
9.2. MULTIPATH_RESPONSE . . . . . . . . . . . . . . . . . . . 11
9.3. MULTIPATH_STATUS . . . . . . . . . . . . . . . . . . . . 11
9.4. MULTIPATH_STATUS_ACK . . . . . . . . . . . . . . . . . . 11
9.5. MULTIPATH_ABANDON . . . . . . . . . . . . . . . . . . . . 11
9.6. MULTIPATH_ABANDON_ACK . . . . . . . . . . . . . . . . . . 12
9.7. MULTIPATH_BINDING . . . . . . . . . . . . . . . . . . . . 12
9.8. MULTIPATH_ECN . . . . . . . . . . . . . . . . . . . . . . 12
9.9. MULTIPATH_MAX_DATA . . . . . . . . . . . . . . . . . . . 12
10. Security Considerations . . . . . . . . . . . . . . . . . . . 13
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
11.1. New QUIC transport parameters . . . . . . . . . . . . . 13
11.2. New QUIC frame types . . . . . . . . . . . . . . . . . . 13
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
12.1. Normative References . . . . . . . . . . . . . . . . . . 13
12.2. Informative References . . . . . . . . . . . . . . . . . 14
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Introduction
Architecturally, one may consider two models for multi-path
operations: model (A) is a collection of uni-path QUIC constructs
while model (B) is a uni-path QUIC construct operating over a
collection of paths.
Model (A) is like multipath TCP [MPTCP] and appears to be the model
of the current MPQUIC design [MPQUIC-DRAFT]. Model (B) is like a TCP
connection operating over a layer 2 link aggregation group [LAG]. In
(B), a TCP segment can be transmitted in an IP datagram over any of
the links in the LAG.
If we apply the principles of (B) to Multipath QUIC, then path
management is distinct from connection management. Conceptually the
Multipath QUIC stack is an [RFC9000] entity sitting on top of a path
management entity with a shim entity between them to direct a QUIC
packet over one of the available paths.
This document proposes a QUIC multipath model similar to (B).
In the proposed model, a QUIC packet can be sent over any of the
available (and unrestricted) paths. Since connection identifiers are
independent of path, a QUIC packet received over any path is
processed in the same way as a packet received over the single path
construct of [RFC9000] - i.e. there is a single application data
packet number space and an ACK received over any path contains
unambiguous packet numbers. While congestion control must clearly be
path-specific, connection management, key management and packet loss
recovery are not path-specific.
1.1. Conventions
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.
1.2. Terminology
This document uses the following terminology:
session: a collection of QUIC connections and paths used to exchange
QUIC packets between two endpoints.
TBC: to be completed
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TBD: to be determined
2. Multipath Management
Connection migration to a new path is already supported in [RFC9000].
A goal of multipath QUIC is to provide multiple paths that can be
simultaneously active and to explicitly manage the use of paths.
Similar to [MPQUIC-DRAFT], this proposal is based on several basic
design points:
* Re-use the mechanisms of [RFC9000] as much as possible. In
particular, this proposal uses path validation based on [RFC9000]
and re-uses all of the connection management, key management and
loss recovery procedures of [RFC9000].
* Use the same packet header formats as [RFC9000] to avoid
differences between multipath and non-multipath traffic over a
particular path.
* Do not modify frame formats defined in [RFC9000]; if necessary,
define new frame types for multipath-specific operations.
Multipath QUIC requires changes to the path management mechanisms
specified in Section 9 of [RFC9000]:
* 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 or lost.
In addition, multipath QUIC may require changes to several QUIC path-
specific procedures described in [RFC9002]:
* congestion control;
* RTT measurements;
* PMTU discovery.
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3. Path Identification
A path may be associated with the 4-tuple of an IP/UDP datagram
(source IP address, destination IP address, source UDP port, and
destination UDP port). However, this document explicitly assigns an
identifier to each path to decouple path management from the 4-tuple
of the IP/UDP datagram used to transport a QUIC packet.
A path identifier is an integer that is unique across all paths
associated with a session. The initial (default) path (i.e. the path
used for the exchange of QUIC initial and handshake packets) is
implicitly assigned the path identifier (PathID) zero (0). New path
identifiers MUST increase monotonically. A path identifier assigned
to one path MUST NOT be reused as the identifier for a different path
within the session.
If the 4-tuple associated with a QUIC connection changes without the
use of multipath validation (Section 5.1), this is considered a
passive migration event (e.g. due to a NAT rebinding) and is outside
the scope of this document - i.e. it is already covered by [RFC9000].
QUIC connections exist and are managed independently of paths. An
outgoing QUIC packet may be transmitted over any of the available and
active paths, subject to any constraints that may have been placed on
path usage by either of the QUIC endpoints (Section 6.1). Similarly,
an incoming QUIC packet received over any path will be processed
according to [RFC9000], as though it had been received over a uni-
path transport between the QUIC endpoints.
3.1. (Sidebar) Comparison to [MPQUIC-DRAFT]
[MPQUIC-DRAFT] does not use explicit path identifiers; instead, a
connection identifier is used as an implicit path identifier with
several consequences:
* a connection is bound to a specific path;
* there can be be only one connection per path.
In addition, QUIC packet loss and recovery in [MPQUIC-DRAFT] is
performed separately for each path. To simplify implementation,
[MPQUIC-DRAFT] introduces the concept of multiple application data
packet number spaces with a different number space for each
connection (path). This is in contrast to [RFC9000] where there is a
single application data packet number space.
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3.2. (Sidebar) Use of multiple application data packet number spaces
One consequence of the use of multiple packet number spaces is that
it may be difficult to use [MPQUIC-DRAFT] in combination with a QUIC-
aware UDP proxy [QUIC-PROXY].
If the proxy is operating in forwarding mode, an uplink QUIC short
header packet received over a (virtual) CID on the network segment
between the proxy and a client is mapped to a destination CID on the
network segment between the proxy and a server. Nothing else in the
QUIC packet is changed and parts of the QUIC packet header -
including the packet number - are protected by a header protection
key known only to the client and server.
If multipath QUIC is used between the proxy and client, and uni-path
QUIC [RFC9000] is used between the proxy and server, then a change in
the path between proxy and client cannot affect the packet numbering.
In other words, multipath QUIC would need to preserve the packet
numbering spaces defined by [RFC9000] and not introduce a new set of
packet number spaces that would prevent interop with [RFC9000]
compliant servers.
4. High-level Overview
The multipath extensions to QUIC proposed in this document enable the
simultaneous use of different paths to exchange non-probing QUIC
frames. This differs from [RFC9000] where the connection migration
procedure selects only one path to exchange non-probing frames.
A multipath QUIC session between peers starts with a standard QUIC
handshake over an initial (default) path. As indicated by [RFC9000],
an endpoint MUST NOT attempt to activate a new path before the
handshake is confirmed. The peers use a new enable_multipath
transport parameter during the handshake to negotiate the use of
multipath capabilities. A new max_active_paths transport parameter
limits the maximum number of active paths that can be used between
the peers.
To add a new path to an existing multipath QUIC session, a client
starts a path validation on the chosen path. A new path can only be
used to transport non-probing frames once the path has been validated
using mechanisms similar to those described in Section 8 of
[RFC9000]. New MULTIPATH_CHALLENGE and MULTIPATH_RESPONSE frames are
used to validate the path and to assign an identifier to the path. A
new MULTIPATH_STATUS frame may be used to control use of a path and a
new MULTIPATH_BINDING frame may be used to bind a QUIC connection
identifier to a specific set of paths.
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A new MULTIPATH_ABANDON frame may be used to abandon a path between
peers, preventing further use of that path to exchange QUIC packets.
A MULTIPATH_ABANDON frame includes the identifier assigned to the
path to be abandoned, allowing the frame to be forwarded over any of
the (allowable) paths active at the time of transmission.
The multipath operations described in this document do not change the
basic operations described in [RFC9000]. In particular, none of the
following procedures described in [RFC9000] are affected by the use
of multiple paths:
* connection management (e.g. the use of NEW_CONNECTION_ID frames
and subsequent rotation of connection identifiers);
* key management (e.g. use of key phase bit) and derivation of AEAD
parameters;
* packet loss detection and loss recovery (e.g. using type 0x02 ACK
frames without ECN counts).
However, changes to [RFC9002] procedures are required to deal with
path-dependent characteristics such as path MTU size, RTT and
congestion. For example, a new MULTIPATH_ECN frame may be used to
provide path-specific ECN information.
5. Path Activation and Removal
TBC.
5.1. Path Activation
operation overview:
* to initiate communications over a new path, an endpoint sends a
MULTIPATH_CHALLENGE frame (Section 9.1) containing a new path
identifier (PathID) and an unpredictable payload.
* the frame is encapsulated (in a QUIC packet) in an IP/UDP datagram
where the 4-tuple of the datagram corresponds to the new path.
* discovery of the peer endpoint IP address and UDP port is outside
the scope of this document.
* the peer confirms used of the new path by responding with a
MULTIPATH_RESPONSE frame (Section 9.2) that echoes the received
PathID and payload.
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* if the initiating endpoint does not receive a confirming
MULTIPATH_RESPONSE frame, it may transmit a new
MULTIPATH_CHALLENGE frame using the same (or a different) IP/UDP
4-tuple but with a new PathID and a different payload.
TBC.
5.2. Path Removal
operation overview:
* to terminate communications over an established path, an endpoint
sends a MULTIPATH_ABANDON frame (Section 9.5) containing the
PathID of the path to be abandoned.
* if the endpoint does not receive a MULTIPATH_ABANDON_ACK frame
(Section 9.6) in response, the MULTIPATH_ABANDON frame may be
retransmitted over the same or a different path.
* the MULTIPATH_ABANDON and MULTIPATH_ABANDON_ACK frames may be
transmitted over any path that is active (and allowable) at the
time of transmission.
TBC.
6. Path Maintenance
TBC.
6.1. Path Transmission Status
operation overview:
* an initiating endpoint may send an initiator MULTIPATH_STATUS
frame (Section 9.3) to inform its peer of the desired status of a
path.
* if the peer agrees with the status change, the peer should respond
with a MULTIPATH_STATUS_ACK frame (Section 9.4) that echoes the
sequence number and status of the corresponding MULTIPATH_STATUS
frame.
* if the peer does not agree with the status change, the peer should
respond with a new responder MULTIPATH_STATUS frame (Section 9.3)
to inform the initiator of its desired status of the path. The
path status sequence number in the new MULTIPATH_STATUS frame MUST
be greater than the path status sequence number in the initiator
MULTIPATH_STATUS frame.
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* an endpoint may also send a MULTIPATH_BINDING frame (Section 9.7)
to bind a QUIC connection identifier to a specific path or set of
paths.
* the MULTIPATH_STATUS, MULTIPATH_STATUS_ACK and MULTIPATH_BINDING
frames may be transmitted over any path that is active (and
allowable) at the time of transmission.
TBC.
6.2. Path Congestion Control
operation overview:
* congestion control is applied per path, as described in Section 7
of [RFC9002].
* QUIC packets sent on one path do not affect the congestion state
of another path.
* an endpoint may send a MULTIPATH_ECN frame (Section 9.8) to its
peer to report ECN information received over a path.
* an endpoint may send a MULTIPATH_MAX_DATA frame (Section 9.9) to
limit the number of bytes-in-flight allowed on a path.
* the MULTIPATH_ECN and MULTIPATH_MAX_DATA frames may be transmitted
over any path that is active (and allowable) at the time of
transmission.
TBC.
6.3. Path RTT Measurements
TBC.
7. Packet Scheduling
operation overview:
* a QUIC packet may be scheduled for transmission over a given path
only if:
- the path state is not "paused" or "closed";
- the destination connection identifier has not been bound to
another path (Section 9.7);
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- transmission of the packet does not increase the number of
bytes-in-flight beyond the congestion window of the path
(Section 6.2)
* if more than one path is eligible for transmission of a packet,
the algorithm used to select the path is beyond the scope of this
document. An implementation may, for example, use the precedence
value provided in a MULTIPATH_CHALLENGE, MULTIPATH_RESPONSE or
MULTIPATH_STATUS frame.
* a precedence value is an integer that may be used to differentiate
between paths when scheduling the transmission of a QUIC packet:
- in general, a path with a higher precedence value is preferred
over a path with a lower precedence value;
- multiple paths may be assigned the same precedence value;
- congestion control may override precedence to allow
transmission over a less congested path;
- a precedence value of zero (0) may be used to indicate that the
path is in a "standby" mode and should not be selected unless
no other path is available.
TBC.
8. Packet Loss Detection and Recovery
operation overview:
* QUIC senders use acknowledgements to detect lost packets and a PTO
to ensure acknowledgements are received.
* loss detection through acknowledgements is the same as described
in Section 6.1 of [RFC9002].
* loss detection through PTO requires derivation of a PTO period
that accommodates the different RTT that may be experienced over
different paths (Section 6.3).
TBC.
9. Multipath Frame Types
TBC.
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9.1. MULTIPATH_CHALLENGE
MULTIPATH_CHALLENGE Frame {
Type (i) = Type_mpChallenge,
PathID (i),
Initiator precedence (i),
Data (64),
}
TBC.
9.2. MULTIPATH_RESPONSE
MULTIPATH_RESPONSE Frame {
Type (i) = Type_mpResponse,
PathID (i),
Responder precedence (i),
Data (64),
}
TBC.
9.3. MULTIPATH_STATUS
MULTIPATH_STATUS Frame {
Type (i) = Type_mpStatus,
PathID (i),
Path Status sequence number (i),
Path Status (i),
[Initiator precedence (i)]
}
TBC.
9.4. MULTIPATH_STATUS_ACK
MULTIPATH_STATUS_ACK Frame {
Type (i) = Type_mpStatusAck,
PathID (i),
Path Status sequence number (i),
Path Status (i),
[Responder precedence (i)]
}
TBC.
9.5. MULTIPATH_ABANDON
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MULTIPATH_ABANDON Frame {
Type (i) = Type_mpAbandon,
PathID (i),
Reason Code (i)
}
TBC.
9.6. MULTIPATH_ABANDON_ACK
MULTIPATH_ABANDON_ACK Frame {
Type (i) = Type_mpAbandonAck,
PathID (i)
}
TBC.
9.7. MULTIPATH_BINDING
MULTIPATH_BINDING Frame {
Type (i) = Type_mpBinding,
PathID (i),
CID sequence number (i)
}
TBC.
9.8. MULTIPATH_ECN
MULTIPATH_ECN Frame {
Type (i) = Type_mpECN,
PathID (i),
ECN counts (..)
}
TBC.
9.9. MULTIPATH_MAX_DATA
MULTIPATH_MAX_DATA Frame {
Type (i) = Type_mpMaxData,
PathID (i),
Maximum data (i)
}
TBC.
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10. Security Considerations
TBD.
11. IANA Considerations
11.1. New QUIC transport parameters
* enable_multipath
* max_active_paths
TBC.
11.2. New QUIC frame types
* Type_mpChallenge
* Type_mpResponse
* Type_mpStatus
* Type_mpStatusAck
* Type_mpBinding
* Type_mpAbandon
* Type_mpAbandonAck
* Type_mpECN
* Type_mpMaxData
TBC.
12. References
12.1. Normative References
[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>.
[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>.
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[RFC9000] 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>.
[RFC9002] 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>.
12.2. Informative References
[LAG] Wikipedia, "Link aggregation", 18 October 2023,
<https://en.wikipedia.org/wiki/Link_aggregation>.
[MPQUIC-DRAFT]
Liu, Y., Ma, Y., De Coninck, Q., Bonaventure, O., Huitema,
C., and M. Kühlewind, "Multipath Extension for QUIC", Work
in Progress, Internet-Draft, draft-ietf-quic-multipath-06,
23 October 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-quic-multipath-06>.
[MPTCP] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
"TCP Extensions for Multipath Operation with Multiple
Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,
<https://www.rfc-editor.org/rfc/rfc6824>.
[QUIC-PROXY]
Pauly, T., Rosenberg, E., and D. Schinazi, "QUIC-Aware
Proxying Using HTTP", Work in Progress, Internet-Draft,
draft-ietf-masque-quic-proxy-00, 17 August 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-masque-
quic-proxy-00>.
Acknowledgements
The authors would like to thank (manyfolks) for their input and
contributions.
Author's Address
Bill Gage
Unaffiliated
Ottawa
Canada
Email: billgage.ietf@gmail.com
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