Multipath Extension for QUIC
draft-lmbdhk-quic-multipath-00

Document Type Active Internet-Draft (individual)
Authors Yanmei Liu  , Yunfei Ma  , Quentin De Coninck  , Olivier Bonaventure  , Christian Huitema  , Mirja K├╝hlewind 
Last updated 2021-10-25
Replaces draft-liu-multipath-quic, draft-deconinck-quic-multipath, draft-huitema-quic-mpath-option
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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
   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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   extracted from this document must include Simplified BSD License text
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   provided without warranty as described in the Simplified BSD License.

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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