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Versions: (draft-deconinck-multipath-quic) 00 01 02 03

QUIC Working Group                                         Q. De Coninck
Internet-Draft                                            O. Bonaventure
Intended status: Standards Track                               UCLouvain
Expires: March 1, 2020                                   August 29, 2019


                Multipath Extensions for QUIC (MP-QUIC)
                   draft-deconinck-quic-multipath-03

Abstract

   This document specifies extensions to the QUIC protocol to enable,
   other than connection migration, simultaneous usage of multiple paths
   for a single connection.  Those extensions remain compliant with the
   current single-path QUIC design.  They allow devices to benefit from
   multiple network paths while preserving the privacy features of QUIC.

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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   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."

   This Internet-Draft will expire on March 1, 2020.

Copyright Notice

   Copyright (c) 2019 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
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   4
     2.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   4
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  What is a Path? . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Beyond Connection Migration . . . . . . . . . . . . . . .   5
     3.3.  Establishment of a Multipath QUIC Connection  . . . . . .   7
     3.4.  Architecture of Multipath QUIC  . . . . . . . . . . . . .   7
     3.5.  Path Establishment  . . . . . . . . . . . . . . . . . . .   8
     3.6.  Exchanging Data over Multiple Paths . . . . . . . . . . .   9
     3.7.  Exchanging Addresses  . . . . . . . . . . . . . . . . . .  10
     3.8.  Coping with Address Removals  . . . . . . . . . . . . . .  11
     3.9.  Path Migration  . . . . . . . . . . . . . . . . . . . . .  12
     3.10. Sharing Path Policies . . . . . . . . . . . . . . . . . .  13
     3.11. Congestion Control  . . . . . . . . . . . . . . . . . . .  13
   4.  Mapping Path ID to Connection IDs . . . . . . . . . . . . . .  13
   5.  Using Multiple Paths  . . . . . . . . . . . . . . . . . . . .  14
     5.1.  Multipath Negotiation . . . . . . . . . . . . . . . . . .  14
       5.1.1.  Transport Parameter Definition  . . . . . . . . . . .  14
     5.2.  Coping with Additional Remote Addresses . . . . . . . . .  15
     5.3.  Receive Path State  . . . . . . . . . . . . . . . . . . .  15
     5.4.  Sending Path State  . . . . . . . . . . . . . . . . . . .  17
     5.5.  Dynamic Management of Paths . . . . . . . . . . . . . . .  19
     5.6.  Losing Addresses  . . . . . . . . . . . . . . . . . . . .  19
     5.7.  Scheduling Strategies . . . . . . . . . . . . . . . . . .  19
   6.  Modifications to QUIC frames  . . . . . . . . . . . . . . . .  20
     6.1.  NEW_CONNECTION_ID Frame . . . . . . . . . . . . . . . . .  20
     6.2.  RETIRE_CONNECTION_ID Frame  . . . . . . . . . . . . . . .  22
     6.3.  ACK Frame . . . . . . . . . . . . . . . . . . . . . . . .  22
   7.  New Frames  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     7.1.  PATH_UPDATE Frame . . . . . . . . . . . . . . . . . . . .  23
     7.2.  ADD_ADDRESS Frame . . . . . . . . . . . . . . . . . . . .  24
     7.3.  REMOVE_ADDRESS Frame  . . . . . . . . . . . . . . . . . .  25
     7.4.  PATHS Frame . . . . . . . . . . . . . . . . . . . . . . .  25
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  27
     8.1.  Nonce Computation . . . . . . . . . . . . . . . . . . . .  27
     8.2.  Validation of Exchanged Addresses . . . . . . . . . . . .  28
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  28
     9.1.  QUIC Transport Parameter Registry . . . . . . . . . . . .  28
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  28
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  29
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  29
     11.2.  Informative References . . . . . . . . . . . . . . . . .  29
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  31
     A.1.  Since draft-deconinck-quic-multipath-02 . . . . . . . . .  31
     A.2.  Since draft-deconinck-quic-multipath-01 . . . . . . . . .  31



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     A.3.  Since draft-deconinck-quic-multipath-00 . . . . . . . . .  31
     A.4.  Since draft-deconinck-multipath-quic-00 . . . . . . . . .  31
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  32

1.  Introduction

   Endhosts have evolved.  Today's endhosts are equipped with several
   network interfaces and users expect to be able to seamlessly switch
   from one to another or use them simultaneously to aggregate or grab
   bandwidth whenever needed.  During the last years, several multipath
   extensions to transport protocols have been proposed
   [RFC6824],[MPRTP], [SCTPCMT].  Multipath TCP [RFC6824] is the most
   mature one.  It is already deployed on popular smartphones, but also
   for other use cases [RFC8041].

   With regular TCP and UDP, all the packets that belong to a given flow
   share the same 5-tuple that acts as an identifier for this flow.
   Such characterization prevents these flows from using multiple paths.
   QUIC [I-D.ietf-quic-transport] does not use the 5-tuple as an
   implicit connection identifier.  A QUIC flow is identified by two
   Connection IDs (Source and Destination).  This enables QUIC flows to
   cope with events affecting the 5-tuple, such as NAT rebinding or IP
   address changes.  The QUIC connection migration feature, described in
   more details in [I-D.ietf-quic-transport], is key to migrate a flow
   from one 5-tuple to another one.  This migration feature offers the
   opportunity for QUIC to sustain a connection over multiple paths, but
   still there is a void to specify simultaneous usage of available
   paths for a single connection.  Use cases such as bandwidth
   aggregation or seamless network handovers would be applicable to
   QUIC, as they are now with Multipath TCP.  An early performance
   evaluation of such use cases and a comparison between Multipath QUIC
   and Multipath TCP may be found in [MPQUIC].

   In this document, we leverage many of the lessons learned from the
   design of Multipath TCP and the comments received on the first
   versions of this draft to propose extensions to the current QUIC
   design to enable it to simultaneously use several paths.  This
   document focuses mainly on paths that are locally visible to an
   endpoint.  This document is organized as follows.  It first provides
   in Section 3 an overview of the operation of Multipath QUIC.  It then
   states changes required in the current QUIC design
   [I-D.ietf-quic-transport] and specifies in Section 5 the usage of
   multiple paths.  Finally, it discusses some security considerations.








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2.  Conventions and Definitions

   The words "MUST", "MUST NOT", "SHOULD", and "MAY" are used in this
   document.  It's not shouting; when they are capitalized, they have
   the special meaning defined in [RFC2119].

   We assume that the reader is familiar with the terminology used in
   [I-D.ietf-quic-transport].  In addition, we define the following
   terms:

   o  Path: A logical association seen by a QUIC host to reach its peer,
      on which packets can be sent.  It is typically characterized by
      (Source IP Address, Source Port Number, Destination IP Address,
      Destination Port Number).  A path is unidirectional, similar to
      the Connection IDs.  A path is internally identified by using a
      Path ID and uses a potentially changing Connection ID identifying
      the parent connection in packets exchaged on that path.

   o  Initial Paths: The paths used by peers for the establishment of
      the QUIC connection.  The cryptographic handshake is done on these
      paths.  These are identified by Path ID 0.

2.1.  Notational Conventions

   Packet and frame diagrams use the format described in Section 2.1 of
   [I-D.ietf-quic-transport].

3.  Overview

   The current design of QUIC [I-D.ietf-quic-transport] provides
   reliable transport with multiplexing and security.  A wide range of
   devices on today's Internet are multihomed.  Examples include
   smartphones equipped with both WLAN and cellular interfaces, but also
   regular dual-stack hosts that use both IPv4 and IPv6.  Experience
   with Multipath TCP has shown that the ability to combine different
   paths during the lifetime of a connection provides various benefits
   including bandwidth aggregation or seamless handovers
   [RFC8041],[IETFJ].

   The current design of QUIC does not enable multihomed devices to
   efficiently use different paths simultaneously.  We first explain why
   a multipath extension would be beneficial to QUIC and then describe
   it at a high level.








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3.1.  What is a Path?

   Before going into details, let us first define what is called a
   "path".  Since the early days of computer networks, a path was
   denoted by a 4-tuple (Source IP Address, Source Port Number,
   Destination IP Address, Destination Port Number).  It is namely a UDP
   road from the local host to the remote one.  Considering a smartphone
   interacting with a single-homed server, the smartphone might want to
   use one path over the WLAN network and another over the cellular one.
   Those paths are not necessarily disjoint.  For example, when
   interacting with a dual-stack server, a smartphone may create two
   paths over the Wi-Fi network, one over IPv4 and the other over IPv6.
   However, networking experiences showed that packets following a
   direction do not always share the same road as the packets in the
   opposite direction.  Therefore, QUIC paths are host-specific, i.e.,
   the path from A to B is different from the one from B to A.  This
   potentially enables the use of unidirectional networks such as
   sattelites, unapplicable when using TCP.

3.2.  Beyond Connection Migration

   Unlike TCP [RFC0793], QUIC is not bound to a particular 4-tuple
   during the lifetime of a connection.  A QUIC connection is identified
   by a Connection ID, placed in the public header of each QUIC packet.
   This enables hosts to continue the connection even if the 4-tuple
   changes due to, e.g., NAT rebinding.  This ability to shift a
   connection from one 4-tuple to another is called Connection
   Migration.  One of its use cases is fail-over when the IP address in
   use fails but another one is available.  A device losing the WLAN
   connectivity can then continue the connection over its cellular
   interface, for instance.

   A QUIC peer can thus start on a given path, denoted as the initial
   path, and end on another one.  However, the current QUIC design
   [I-D.ietf-quic-transport] assumes that only one symmetric path is in
   use for a given connection.  The specification does not support means
   to distinguish path migration from simultaneous usage of available
   asymmetric paths for a given connection.

   This document fills that void.  Concretely, instead of switching the
   4-tuple for the whole connection, this draft first proposes
   mechanisms to communicate endhost addresses to the peer.  It then
   leverages the address validation process with the PATH_CHALLENGE and
   PATH_RESPONSE frames proposed in [I-D.ietf-quic-transport] to verify
   if additional addresses advertised by the communicating host are
   available and actually belong to it.  In this case, those addresses
   can be used to create new paths to spread packets over several




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   networks following a traffic distribution policy that is out of scope
   of this document.

   When multiple paths are available, different delays may be
   experienced as a function of the initial path selected for the
   establishment of the QUIC connection.  The first version of the
   specification does not discuss considerations related to the
   selection of the initial path to place the connection.

   The example of Figure 1 shows a possible data exchange between a
   dual-homed client performing a request fitting in two packets and a
   single-homed server.  Notice that the Path ID used by an host over a
   specific network is not necessarily the same as the one used by the
   remote peer.  In the presented example, phone sends packets over WLAN
   on path 1 and over LTE on path 2, while the packets sent by the
   server over WLAN are on path 2 and over LTE are on path 1.

 Server                           Phone                           Server
 via WLAN                                                        via LTE
 -------                         -------                           -----
   | Pkt(DCID=B,PN=1,frames=[       |                                |
   |  STREAM("Request (1/2)")])     | Pkt(DCID=D,PN=1,frames=[       |
   |<-------------------------------|  STREAM("Request (2/2)")])     |
   | Pkt(DCID=A,PN=1,frames=[       |--------                        |
   |  ACK(PID=1,LargestAcked=1)])   |       |----------              |
   |------------------------------->|                 |----------    |
   | Pkt(DCID=A,PN=2,frames=[       |                           |--->|
   |  STREAM("Response 1")])        | Pkt(DCID=C,PN=1,frames=[       |
   |------------------------------->|  ACK(PID=2,LargestAcked=1),    |
   |                                |  STREAM("Response 2")])   -----|
   | Pkt(DCID=B,PN=2,frames=[       |                 ----------|    |
   |  ACK(PID=2, LargestAcked=2),   |       ----------|              |
   |  ACK(PID=1, LargestAcked=1)])  |<------|                        |
   |<-------------------------------|                                |
   | Pkt(DCID=A,PN=3,frames=[       | Pkt(DCID=D,PN=2,frames=[       |
   |  STREAM("Response 3")])        |  STREAM("Response 4")])        |
   |------------------------------->|                            ----|
   |                                |                   ---------|   |
   |            ...                 |    ...  <---------|            |

                  Figure 1: Dataflow with Multipath QUIC

   The remaining of this section focuses on providing a high-level
   overview of the multipath operations in QUIC.







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3.3.  Establishment of a Multipath QUIC Connection

   A Multipath QUIC connection starts like a regular QUIC connection.  A
   cryptographic handshake takes place with CRYPTO frames and follows
   the classical process [I-D.ietf-quic-transport] [I-D.ietf-quic-tls].
   It is during that process that the multipath capability is negotiated
   between hosts.  This is performed using the max_sending_paths
   transport parameter, where both hosts advertise their support for the
   multipath extension.  Any value different from 0 indicates that the
   host wants to support multipath over the connection.  If one of the
   hosts does not advertise the max_sending_paths transport parameter,
   the negotiated value is 0, meaning that the QUIC connection will not
   use the multipath extensions presented in this document.

   The handshake is performed on a given path.  This path is called the
   Initial path and is identified by Path ID 0.

3.4.  Architecture of Multipath QUIC

   Once established, a Multipath QUIC connection is composed of two or
   more asymmetric paths.  Each asymmetric path is associated with a
   different four-tuple and identified by a Path ID, as shown in
   Figure 2.

             +-----------------------------------------------+
             |           Connection (MSCID, MDCID)           |
             |   +---------+ +---------+ ... +------------+  |
             |   | Sending | | Sending | ... |  Sending   |  |
             |   | Path 0  | | Path 1  |     | Path N - 1 |  |
             |   | Tuple   | | Tuple'  | ... |   Tuple"   |  |
             |   |  PDCID  | |  PDCID' | ... |    PDCID"  |  |
             |   |  PN     | |  PN'    |     |    PN"     |  |
             |   +---------+ +---------+ ... +------------+  |
             |   +---------+ +---------+ ... +------------+  |
             |   | Receive | | Receive | ... |  Receive   |  |
             |   | Path 0  | | Path 1  |     | Path M - 1 |  |
             |   | Tuple   | | Tuple'  | ... |   Tuple"   |  |
             |   |  PSCID  | |  PSCID' |     |    PSCID"  |  |
             |   |  PN     | |  PN'    |     |    PN"     |  |
             |   +---------+ +---------+ ... +------------+  |
             +-----------------------------------------------+

              Figure 2: Architectural view of Multipath QUIC

   As described before, a Multipath QUIC connection starts on the
   Initial Path, identified by Path ID 0.  For Multipath QUIC, this
   document proposes two levels of asymmetric Connection IDs.  The first
   ones are the Main (or Primary) Connection IDs (MCIDs).  Both the Main



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   Source Connection ID (MSCID) and the Main Destination Connection ID
   (MDCID) uniquely identify the connection, as with the current QUIC
   design.  The second ones are the Path Connection IDs (PCIDs), written
   in the Connection ID field of the public header.  The PCIDs act as
   the path identifiers for packets.  Depending on the direction of the
   path (either sending path or receiving one), the host keeps either
   the Path Source Connection ID (PSCID, for the receive paths) or the
   Path Destination Connection ID (PDCID, for the sending paths).
   Notice that the PDCID of a sending path of an host is the same as the
   PSCID of the corresponding receive path of the remote.  Preventing
   the linkability of different paths is an important requirement for
   the multipath extension [I-D.huitema-quic-mpath-req].  Using PCIDs as
   implicit path identifier makes this linkability harder than having
   explicit signaling as in the early version of this draft and does not
   require public header change to keep invariants
   [I-D.ietf-quic-invariants].  The MCIDs of a connection will be the
   PCIDs of the Initial Path.  In the example of Figure 1, if the
   connection started using WLAN, then the Destination Connection ID A
   is both the PDCID of the WLAN sending path and the MDCID of the
   connection.

   In addition to the PCIDs, some additional information is kept for
   each asymmetric path.  The Path ID identifies the assymetric path at
   the frame level and ensures uniqueness of the nonce (see Section 8.1
   for details).  A congestion window is maintained for each sending
   path.  Hosts can also collect network measurements on a per-path
   basis, such as one-way delay measurements and lost packets.

3.5.  Path Establishment

   The max_sending_paths transport parameter exchanged during the
   cryptographic handshake determines if multiple paths can be used.
   Then, hosts provide to their peer the Path Connection ID to use on
   asymmetric paths.  Unlike Multipath TCP [RFC6824], both hosts
   dynamically control how many sending paths can currently be in use by
   the peer, i.e., the number of different Path IDs proposed to the
   peer.  Notice that the peers might advertise a different number of
   sending Path IDs to their peer, setting different limits to the
   sending and receive paths of each host.  The max_sending_paths
   transport parameter indicates the maximum number of sending paths the
   host would support to receive from the peer.

   Hosts propose new receive paths with an extended version of the
   NEW_CONNECTION_ID frame (see Section 6.1).  This frame provides to
   the peer the PDCID for a given sending path.  Upon reception of the
   frame, the peer can start using the proposed sending path Path ID
   with the provided PDCID.  Therefore, once an host sent such
   NEW_CONNECTION_ID frame, it MUST be ready to receive packets from



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   that Path ID with the proposed PCID.  As frames are encrypted, adding
   new paths does not leak cleartext identifiers
   [I-D.huitema-quic-mpath-req].

   A server might provide several Path Connection IDs for the same
   asymmetric Path IDs with multiple NEW_CONNECTION_ID frames.  This can
   be useful to cope with migration cases, as described in Section 3.9.
   Multipath QUIC is asymmetrical.  Any host can start using new sending
   paths once their corresponding PDCIDs have been provided by the
   remote peer.

   Hosts are not able to create new paths as long as the peer does not
   send NEW_CONNECTION_ID frames.  To limit the latency of the path
   handshake, hosts should send those frames as soon as possible, i.e.,
   just after the 0-RTT handshake packet.

   Sending useful data on a fresh new sending path might lead to poor
   performance as the network path used by the QUIC path is not usable
   yet.  A typical case is when a server wants to initiate a new path to
   a client behind a NAT.  The client would possibly never receive this
   packet, leading to connectivity issues on that path.  To avoid such
   issues, a remote address MUST have been validated as described in
   [I-D.ietf-quic-transport] before sending packets on a sending path
   using it.  A host MUST be prepared to receive packets on paths it
   advertised.

   Because attaching to new networks may be volatile and an endpoint
   does not have full visibility on multiple paths that may be available
   (e.g., hosts connected to a CPE), an MP-QUIC capable endhost SHOULD
   advertise a max_sending_paths value of at least 4 and SHOULD propose
   at least 4 receive paths to its peer.

3.6.  Exchanging Data over Multiple Paths

   A QUIC packet acts as a container for one of more frames.  Multipath
   QUIC uses the same STREAM frames as QUIC to carry data.  A byte
   offset is associated to the data payload.  One of the key design
   decision of (Multipath) QUIC is that frames are independent of the
   packets carrying them.  This implies that a frame transmitted over
   one path could be retransmitted later on another path without any
   change.

   The path on which data is sent is a packet-level information.  This
   means a frame can be sent regardless of the path of the packet
   carrying it.  Furthermore, because the data offset is a frame-level
   information, there is no need to define additional sequence numbers
   to cope with reordering across paths, unlike Multipath TCP [RFC6824]
   that uses a Data Sequence Number at the MPTCP level.  Other flow



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   control considerations like the stream receive window advertised by
   the MAX_STREAM_DATA frame remain unchanged when there are multiple
   paths.

   However, Multipath QUIC might face reordering at packet-level when
   using paths having different latencies.  The presence of different
   Path Connection IDs ensures that the packets sent over a given path
   will contain monotonically increasing packet numbers.  To ensure more
   flexibility and potentially to reduce the ACK block section of the
   ACK frame when aggregating bandwidth of paths exhibiting different
   network characteristics, each path keeps its own monotonically
   increasing Packet Number space.  This potentially allows sending up
   to 2 * 2^62 * 2^62 packets on a QUIC connection since each path has
   its own packet number space.

   The ACK frame is also modified to allow per-path packet
   acknowledgments.  This remains compliant with the independence
   between packets and frames while providing more flexibility to hosts
   to decide on which sending path they want to send receive path
   acknowledgments.  Looking again at Figure 1, packets that were sent
   over a given sending path (e.g., the "Response 2" packet on server
   sending path 1 with DCID C) can be acknowledged on another sending
   path (here, client sending path 1 with DCID B) that does not
   correspond to the same underlying network to limit the latency due to
   ACK transmissions on high-latency paths and to enable the usage of
   unidirectional networks.  Such scheduling decision would not have
   been possible in Multipath TCP [RFC6824] which must acknowledge data
   on the (symmetric) path it was received on.

3.7.  Exchanging Addresses

   When a multi-homed mobile device connects to a dual-stacked server
   using its IPv4 address, it is aware of its local addresses (e.g., the
   Wi-Fi and the cellular ones) and the IPv4 remote address used to
   establish the QUIC connection.  If the client wants to create new
   paths over IPv6, it needs to learn the other addresses of the remote
   peer.

   This is possible with the ADD_ADDRESS frames that are sent by a
   Multipath QUIC host to advertise its current addresses.  Each
   advertised address is identified by an Address ID.  The addresses
   attached to a host can vary during the lifetime of a Multipath QUIC
   connection.  A new ADD_ADDRESS frame is transmitted when a host has a
   new address.  This ADD_ADDRESS frame is protected as other QUIC
   control frames, which implies that it cannot be spoofed by attackers.
   The communicated address is first validated by the receiving host
   before it starts using it.  This ensures that the address actually
   belongs to the peer and that the peer can send and receive packets on



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   that address.  It also prevents hosts from launching amplification
   attacks to a victim address.

   If the client is behind a NAT, it could announce a private address in
   an ADD_ADDRESS frame.  In such situations, the server would not be
   able to validate the communicated address.  The client might still
   use its NATed addresses to start a new sending path.  To enable the
   server to make the link between the private and the public addresses,
   Multipath QUIC provides the PATHS frame that lists current active
   Path IDs.  Notice that an host might also discover the public
   addresses of its peer by observing its remote IP addresses associated
   to the connection.

   Likewise, the client may be located behind a NAT64.  As such it may
   announce an IPv6 address in an ADD_ADDRESS frame, that will be
   received over IPv4 by an IPv4-only server.  The server should not
   discard that address, even if it is not IPv6-capable.

   An IPv6-only client may also receive from the server an ADD_ADDRESS
   frame which may contain an IPv4 address.  The client should rely on
   means, such as [RFC7050] or [RFC7225], to learn the IPv6 prefix to
   build an IPv4-converted IPv6 address.

   A receive path is active as soon as the host has sent the
   NEW_CONNECTION_ID frames proposing the corresponding Path Connection
   IDs to its peer.  A sending path is active when its has received its
   Path Connection IDs and its is bound to a validated 4-tuple.  The
   PATHS frame indicates the local and remote Address IDs that the path
   uses.  With this information, the server can then validate the public
   address and associate the advertised with the perceived addresses.

   Hosts that are connected behind an address sharing mechanism may
   collect the external IP address and port numbers assigned to the
   hosts and then use there addresses in the ADD_ADDRESS.  Means to
   gather such information include, but not limited to, UPnP IGD, PCP,
   or STUN.

3.8.  Coping with Address Removals

   During the lifetime of a QUIC connection, a host might lose some of
   its addresses.  A concrete example is a smartphone going out of
   reachability of a Wi-Fi network or shutting off one of its network
   interfaces.  Such address removals are advertised by using
   REMOVE_ADDRESS frames.  The REMOVE_ADDRESS frame contains the Address
   ID of the lost address previously communicated through ADD_ADDRESS.






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3.9.  Path Migration

   At a given time, a Multipath QUIC endpoint gathers a set of sending
   and receive paths, each associated to a 4-tuple.  To address privacy
   issues due to the linkability of addresses with Connection IDs, hosts
   should avoid changing the 4-tuple used by a sending path.  There
   remain situations where this change is unavoidable.  These can be
   categorized into two groups: host-aware changes (e.g., network
   handover from Wi-Fi to cellular) and host-unaware changes (e.g., NAT
   rebinding).

   For the host-aware case, let us consider the case of a Multipath QUIC
   connection where the client is a smartphone with both Wi-Fi and
   cellular.  It advertised both addresses and the server currently
   enables only one client-sending path, the initial one.  The Initial
   Path uses the Wi-Fi address.  Then, for some reason, the Wi-Fi
   address becomes unusable.  To preserve connectivity, the client might
   then decide to use the cellular address for the Initial Path.  It
   thus sends a REMOVE_ADDRESS announcing the loss of the Wi-Fi address
   and a PATHS frame to inform that the Initial Path is now using the
   cellular address.  If the cellular address validation succeeds (which
   could have been done as soon as the cellular address was advertised),
   the server can continue exchanging data through the cellular address.

   However, both server and client might want to change their path used
   on the cellular address for privacy concerns.  If the server provides
   an additional path (e.g., Path ID 42) through NEW_CONNECTION_ID frame
   at the beginning of the connection, the client can perform the path
   change directly and avoid using the Initial Path Connection ID on the
   cellular network.  This can be done using the PATH_UPDATE frame.  It
   can indicate that the host stopped to use the Initial Path to use
   Path ID 42 instead.  This frame is placed in the first packet sent to
   the new sending path with its corresponding PCID.  The client can
   then send the REMOVE_ADDRESS and PATHS frames on this new path.
   Compared to the previous case, it is harder to link the paths with
   the IP addresses to observe that they belong to the same Multipath
   QUIC connection.

   For the host-unaware case, the situation is similar.  In case of NAT
   rebinding, the server will observe a change in the 2-tuple (source
   IP, source port) of the receive path of the packet.  The server first
   validates that the 2-tuple actually belongs to the client
   [I-D.ietf-quic-transport].  If it is the case, the server can send a
   PATH_UPDATE frame on a previously communicated but unused Path ID.
   The client might have sent some packets with a given PCID on a
   different 4-tuple, but the server did not use the given PCID on that
   4-tuple.  Because some on-path devices may rewrite the source IP
   address to forward packets via the available network attachments



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   (e.g., an host located behind a multi-homed CPE), the server may
   inadvertently conclude that a path is not anymore valid leading thus
   to frequently sending PATH_UPDATE frames as a function of the traffic
   distribution scheme enforced by the on-path device.  To prevent such
   behavior, the server SHOULD wait for at least X seconds to ensure
   this is about a connection migration and not a side effect of an on-
   path multi-interfaced device.

3.10.  Sharing Path Policies

   Some access networks are subject to a volume quota.  To prevent a
   peer from aggressively using a given path while available resources
   can be freely grabbed using existing paths, it is desirable to
   support a signal to indicate to a remote peer how it must place data
   into available paths.  An approach may consist in indicating in an
   ADD_ADDRESS the type of the interface (e.g., cellular, WLAN, fixed)
   through the interface-type field.  The remote peer may rely on
   interface-type to select the path to be used for sending data.  For
   example, fixed interfaces will be preferred over WLAN and cellular
   interfaces, and WLAN interface will be preferred over cellular
   interface.

   This information might also be used to avoid draining battery for
   some devices.

3.11.  Congestion Control

   The QUIC congestion control scheme is defined in
   [I-D.ietf-quic-recovery].  This congestion control scheme is not
   suitable when several sending paths are active.  Using the congestion
   control scheme defined in [I-D.ietf-quic-recovery] with Multipath
   QUIC would result in unfairness.  Each sending path of a Multipath
   QUIC connection MUST have its own congestion window.  The windows of
   the different paths MUST be coupled together.  Multipath TCP uses the
   LIA congestion control scheme specified in [RFC6356].  This scheme
   can immediately be adapted to Multipath QUIC.  Other coupled
   congestion control schemes have been proposed for Multipath TCP such
   as [OLIA].

4.  Mapping Path ID to Connection IDs

   As described in the overview section, hosts need to identify on which
   sending path packets are sent.  The Path ID must then be inferred
   from the public header.  This is done by using Path Connection IDs in
   addition to Main Connection IDs.

   The Master Connection IDs are determined during the cryptographic
   handshake and actually correspond to both Connection IDs in the



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   current QUIC design [I-D.ietf-quic-transport].  The Path Connection
   IDs of the Initial Path (with Path ID 0) are equal to the Main
   Connection IDs.  The Path Connection IDs of the other paths are
   determined when the NEW_CONNECTION_ID frames are exchanged.

   The server MUST ensure that all advertised Path Connection IDs are
   available for the whole connection lifetime.  Once it sends a
   NEW_CONNECTION_ID frame containing a PCID, both hosts can start
   receiving packets with the advertised Connection ID as belonging to
   the corresponding receive path.  Hosts MUST wait until the reception
   of a NEW_CONNECTION_ID frame before sending packets on the
   corresponding sending path.

   Hosts MUST ensure that they can receive packets coming from their
   peer using the PCID they proposed in the NEW_CONNECTION_ID frame they
   sent and associate it with the corresponding receive Path ID.  Upon
   reception of the NEW_CONNECTION_ID frame, hosts MUST acknowledge it
   and MUST store the advertised Destination Path Connection ID and the
   Path ID of the proposed path.

5.  Using Multiple Paths

   This section describes in details the multipath QUIC operations.

5.1.  Multipath Negotiation

   The Multipath Negotiation takes place during the cryptographic
   handshake with the max_sending_paths transport parameter.  A QUIC
   connection is initially single-path in QUIC, and all packets prior to
   handshake completion MUST be exchanged over the Initial Path.  During
   this process, hosts advertise their support for multipath operations.
   Any value different from 0 indicates that the host supports multipath
   operations over the connection, i.e., the extensions defined in this
   document (not to be mixed with the availability of local multiple
   paths).  If a host does not send the max_sending_paths transport
   parameter during the cryptographic handshake, the remote MUST assume
   a value of 0, leading to a single-path connection over the Initial
   Path.  If both hosts advertise their support of the multipath
   extensions, NEW_CONNECTION_ID frames can later propose Path IDs that
   can then be used for the connection.

5.1.1.  Transport Parameter Definition

   An endhost MAY use the following transport parameter:

   max_sending_paths (0x0020):  Indicate the support of the multipath
      extension presented in this document, encoded as an unsigned 8-bit
      integer.  Any value different from 0 indicates support of the



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      extension.  The actual value indicates the number of sending paths
      the host advertising the value would like to receive from its
      peer.  If absent, the value for this parameter is 0, meaning that
      a host omitting this transport parameter does not agree to use the
      multipath extension over the connection.

5.2.  Coping with Additional Remote Addresses

   The usage of multiple networks paths is often done using multiple IP
   addresses.  For instance, a smartphone willing to use both Wi-Fi and
   LTE will use the corresponding addresses assigned by these networks.
   It can then safely send packets to a previously-used IP address of a
   server.  The server can receive packets sourced with different IP
   addresses, but it MUST first validate the new remote IP addresses
   before starting sending packets to those addresses.

   Similarly, additional addresses could be communicated using
   ADD_ADDRESS frames.  Such addresses MUST be validated before starting
   to send packets to them.  This requirement is explained in
   Section 8.2.

   The validation of an address could be performed with PATH_CHALLENGE
   and PATH_RESPONSE frames as described in [I-D.ietf-quic-transport].
   A validated address MAY be cached for a given host for a limited
   amount of time.

5.3.  Receive Path State

   When proposing asymmetric paths to the peer, hosts maintain some
   state for their receive paths.  The possible receive path states are
   depicted in Figure 3.

         o
         | send first NEW_CONNECTION_ID with the associated Path ID
         v
    +--------+
    | ACTIVE |
    +--------+
         |
         | receive PATH_UPDATE
         |
         v
    +--------+
    | CLOSED |
    +--------+

              Figure 3: Finite-State Machine of receive paths




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   Upon the sending of the NEW_CONNECTION_ID proposing the corresponding
   Path ID, the receive path is in ACTIVE state.  In this state, hosts
   MUST ensure that they can associate a packet with the PCID and its
   corresponding receive path at any time, as they must be ready to
   figure out the path used by the remote host.  ACTIVE is the state
   where the receive path can be used to receive packets.

   Eventually, a receive path may be closed.  This is signaled by the
   reception of a PATH_UPDATE frame.  In that case, the path is in
   CLOSED state.  In that state, packets MUST NOT be received over it.
   A host MUST keep some minimal receive path state to avoid ambiguities
   and CLOSED path reuse.

   In any receive path state (ACTIVE or CLOSED), hosts MUST keep the
   following receive path information:

   o  Path ID: encoded as a 8-byte integer.  It uniquely identifies the
      receive path in the connection.  This value is immutable.

   o  Main Connection IDs: they make the link between the path and the
      QUIC
      connection it belongs to.  These values are immutable.

   o  Source Path Connection IDs: it makes the link between the packet's
      Connection ID field and the receive path.  This value can be
      updated by sending subsequent NEW_CONNECTION_ID frames.

   o  Receive Path State: the current state of the receive path, one of
      the values presented in Figure 3.

   In the ACTIVE state, the following elements MUST be tracked:

   o  Packet Number Space: each receive path is associated with its own
      monotonically increasing packet number space, dedicated for the
      packet receptions.  Packet number considerations described in
      [I-D.ietf-quic-transport] apply within a given receive path.

   o  Current 4-tuple: the tuple (sIP, dIP, sport, dport) observed to
      receive
      packets over this path.  This value is mutable, because it might
      receive a packet with a different validated remote address and/or
      port than the one currently recorded.  If an host observes a
      change in the 4-tuple of the receive path, it SHOULD send a
      NEW_CONNECTION_ID with an increased Retire Prior To field to make
      the peer change the Path Connection ID.

   o  Current (local Address ID, remote Address ID) tuple: those
      identifiers come from the ADD_ADDRESS sent (local) and received



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      (remote).  The addresses on which the connection was established
      have Address ID 0.  The reception of PATHS frames helps hosts to
      associate the remote Address ID used by the path.

   o  Performance metrics: basic statistics such as number of packets
      received can be collected on a per-receive path basis.

5.4.  Sending Path State

   During the Multipath QUIC connection, hosts maintain some state for
   sending paths.  Information about the sending path that hosts are
   required to store depends on its state.  The possible sending path
   states are depicted in Figure 4.

         o
         | receive a first NEW_CONNECTION_ID with the associated Path ID
         v
   +----------+
   |  READY   |
   +----------+
         |
         | path usage or PATH_UPDATE
         |
         v      reception or sending of REMOVE_ADDRESS
    +--------+ -------------------------------------> +----------+
    | ACTIVE |            4-tuple validated           | DISABLED |
    +--------+ <------------------------------------- +----------+
         |                                                  |
         +---------------------------------------------------
         |
         | sending PATH_UPDATE
         v
    +--------+
    | CLOSED |
    +--------+

              Figure 4: Finite-State Machine of sending paths

   Once a sending path has been proposed by the peer in a received
   NEW_CONNECTION_ID frame, it is in the READY state.  In this
   situation, hosts MUST keep the following sending path information:

   o  Path ID: encoded as a 4-byte integer.  It uniquely identifies the
      sending path in the connection.  This value is immutable.

   o  Main Connection IDs: they make the link between the path and the
      QUIC
      connection it belongs to.  These values are immutable.



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   o  Path Connection IDs: they make the link between the packet's
      Connection ID field and the sending path.  This value can be
      updated with subsequent NEW_CONNECTION_ID frames.

   o  Sending Path State: the current state of the sending path, one of
      the values presented in Figure 4.

   When the host wants to start using the path over a validated address,
   the path goes to the ACTIVE state.  This is the state where a sending
   path can be used to send packets.  Having a path in ACTIVE state only
   guarantees that it can be used, but the host is not forced to.  In
   addition to the fields required in the READY state, the following
   elements MUST be tracked:

   o  Packet Number Space: each sending path is associated with its own
      monotonically increasing packet number space, dedicated for the
      packet sendings.  Packet number considerations described in
      [I-D.ietf-quic-transport] apply within a given sending path.

   o  Current 4-tuple: the tuple (sIP, dIP, sport, dport) used to send
      packets over this path.  This value is mutable, as the host might
      decide to change its local address and/or port.  A host that
      changes the 4-tuple of a sending path SHOULD migrate it.

   o  Current (local Address ID, remote Address ID) tuple: those
      identifiers come from the ADD_ADDRESS sent (local) and received
      (remote).  This enables a host to temporarily stop using a sending
      path when, e.g., the remote Address ID is declared as lost in a
      REMOVE_ADDRESS.  The addresses on which the connection was
      established have Address ID 0.  The reception of PATHS frames
      helps hosts to associate the remote Address ID used by the path.

   o  Performance metrics: basic statistics such as one-way delay or
      number of packets sent can be collected on a per-path basis.  This
      information can be useful when a host needs to perform packet
      scheduling decisions and flow control management.

   It might happen that a sending path is temporarily unavailable,
   because one of the endpoint's addresses is no more available or
   because the server decided to decrease the number of active paths.
   In such cases, the path goes to the DISABLED state.  In that state,
   the host MUST NOT send packets on it.  At this state, the host might
   want to restart using the path over a validated 4-tuple, switching
   the path state back to ACTIVE state.  However, its congestion window
   MUST be restarted and its performance metrics SHOULD be reset.

   Eventually, a sending path may be closed.  This is signaled by the
   PATH_UPDATE frame.  In that case, the sending path is in CLOSED



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   state.  In that state, packets MUST NOT be sent over it.  A host MUST
   keep the same path state field as in the READY state, to avoid
   ambiguities and CLOSED path reuse.

5.5.  Dynamic Management of Paths

   Both hosts determine how many sending paths their peer can use over a
   Multipath QUIC connection.  It is their responsibility to propose
   asymmetric Path IDs with corresponding PCIDs that could be used by
   the peer.  In addition, they dynamically control the number of active
   paths that can be used for the connection, as they provide and
   potentially retire the Connection IDs usable by the peer.  A host can
   propose several different Path Connection IDs with NEW_CONNECTION_ID
   frames for a given Path ID.  This can be useful in migration
   scenarios.

5.6.  Losing Addresses

   During the lifetime of a connection, a host might lose addresses,
   e.g., a network interface that was shut down.  All the ACTIVE sending
   paths that were using that local address MUST stop sending packets
   from that address.  To advertise the address loss to the peer, the
   host MUST send a REMOVE_ADDRESS frame indicating which Address IDs
   has been lost.  The host MUST also send a PATHS frame indicating the
   status of the remaining ACTIVE paths.

   Upon reception of the REMOVE_ADDRESS, the receiving host MUST stop
   using the ACTIVE paths affected by the address removal.

   Hosts MAY reuse one of these sending paths by changing the assigned
   4-tuple.  In this case, it MUST send a PATHS frame describing that
   change.

5.7.  Scheduling Strategies

   The current QUIC design [I-D.ietf-quic-transport] offers a two-
   dimensional scheduling space, i.e., which frames will be packed
   inside a given packet.  With the use of multiple paths, a third
   dimension is added, i.e., the sending path on which the packet will
   be sent.  This dimension can have a non negligible impact on the
   operations of Multipath QUIC, especially if the available sending
   paths exhibit very different network characteristics.

   The progression of the data flow depends on the reception of the
   MAX_DATA and MAX_STREAM_DATA frames.  Those frames SHOULD be
   duplicated on several or all paths in use.  This would limit the
   head-of-line blocking issue due to the transmission of the frames
   over a slow path.



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   The sending path on which ACK frames are sent impacts the peer.  The
   ACK frame is notably used to determine the latency of a combination
   of asymmetric path.  In particular, the peer can compute the round-
   trip-time of the combination of its sending path with its receive
   one.  The peer would compute the latency as the sum of the forward
   delay of the acknowledged path and the return delay of the path used
   to send the ACK frame.  Choosing between acknowledging packets
   symmetrically (on path B to A if packet was sent on A to B) or not is
   up to the implementation.  However, hosts SHOULD keep a consistent
   acknowledgement strategy.  Selecting a random path to acknowledge
   packets will possibly increase the variability of the latency
   estimation, especially if paths exhibit very different network
   characteristics.  Notice that with a receive timestamp field in the
   ACK frame, this would enable host to estimate each asymmetric path
   one-way delay.  Unlike MAX_DATA and MAX_STREAM_DATA, ACK frames
   SHOULD NOT be systematically duplicated on several paths as they can
   induce a large network overhead.

6.  Modifications to QUIC frames

   The multipath extension allows hosts to send packets over multiple
   paths.  Since nearly all QUIC frames are independent of packets, no
   change is required for most of them.  The only exceptions are the
   NEW_CONNECTION_ID and the ACK frames.  The NEW_CONNECTION_ID and
   RETIRE_CONNECTION_ID are modified to provide Destination Path
   Connection ID negotiation for each asymmetric path.  The ACK frame
   contains packet-level information with the Largest Acknowledged
   field.  Since the Packet Numbers are now associated to asymmetric
   paths, the ACK frame must contain the Path ID it acknowledges.

6.1.  NEW_CONNECTION_ID Frame

   The NEW_CONNECTION_ID frame (type=0x0b) as defined by
   [I-D.ietf-quic-transport] keeps its ability to provide the client
   with alternative connection IDs that can be used to break linkability
   when migrating connections.  It also allows the host to indicate
   which destination connection IDs the peer must use to take advantage
   of multiple sending paths.

   The NEW_CONNECTION_ID is as follows:











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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Path ID  (i)                       ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Sequence Number (i)                    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Retire Prior To (i)                    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Length (8)  |                                               |
   +-+-+-+-+-+-+-+-+         Connection ID                         +
   |                                                             ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                   Stateless Reset Token (128)                 +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 5: NEW_CONNECTION_ID frame adapted to Multipath QUIC

   Compared to the frame specified in [I-D.ietf-quic-transport], a Path
   ID field of variable size is prefixed to associate the Path ID with
   the Connection ID.  If the multipath extension was not negotiated
   during the connection establishment, the NEW_CONNECTION_ID frame is
   the same as the one presented in [I-D.ietf-quic-transport].  This
   frame can be sent by both hosts.  Upon reception of the frame with a
   specified path ID, the peer can update the related sending path state
   either to READY or ACTIVE and store the communicated Connection ID as
   the Destination Connection ID of the sending path.

   A host MUST NOT start using a path as long as the peer have not
   proposed a Destination Connection ID with a NEW_CONNECTION_ID frame.
   To limit the delay of the multipath usage upon handshake completion,
   hosts SHOULD send NEW_CONNECTION_ID frames for receive paths they
   allow using as soon the connection establishment completes.

   To cope with privacy issues, it should be hard to make the link
   between two different connections or two different paths of a same
   connection by just looking at the Connection ID contained in packets.
   Therefore, Path Connection IDs chosen by hosts MUST be random.







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6.2.  RETIRE_CONNECTION_ID Frame

   The RETIRE_CONNECTION_ID frame (type=0x19) as defined by
   [I-D.ietf-quic-transport] still allows an endpoint to indicate that
   it will no longer use a connection ID that was issued by its peer.
   The multipath extensions link Connection IDs to paths.  Therefore,
   this frame should contains the Path ID on which it applies.

   The format of the adapted RETIRE_CONNECTION_ID is shown below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Path ID  (i)                       ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Sequence Number (i)                    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Figure 6: RETIRE_CONNECTION_ID frame adapted to Multipath QUIC

   The frame is handled as described in [I-D.ietf-quic-transport] on a
   path basis.

6.3.  ACK Frame

   The format of the modified ACK frame is shown below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Path ID (i)                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Largest Acknowledged (i)                 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         ACK Delay (i)                       ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      ACK Range Count (i)                    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      First ACK Range (i)                    ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         ACK Ranges (*)                      ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         [ECN Counts]                        ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 7: ACK frame adapted to Multipath





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   Compared to the ACK frame in the current QUIC design
   [I-D.ietf-quic-transport], the ACK frame is prefixed by a variable
   size Path ID field indicating to which path the acknowledged PSNs
   relate to.  Notice that if the multipath extension was not negotiated
   during the connection handshake, the ACK frame is the same as the one
   presented in [I-D.ietf-quic-transport].

   Since frames are independent of packets, and the path notion relates
   to the packets, the ACK frames can be sent on any path, unlike
   Multipath TCP [RFC6824] which is constrained to send ACKs on the same
   path.

   Notice that using timestamps would help peers to estimate the one-way
   delay of the paths.  These would also be beneficial for single-path
   cases.

7.  New Frames

   To support the multipath operations, new frames have been defined to
   coordinate hosts.  This draft uses a type field containing 0x20 to
   indicate that the frame is related to multipath operations.

7.1.  PATH_UPDATE Frame

   The PATH_UPDATE frame is used by a host either to migrate a path or
   to close it.  This indicates to the remote that the closed path MUST
   NOT be used anymore and it can use the proposed one instead, if any.
   The proposed type for the PATH_UPDATE is 0x21.  A PATH_UPDATE frame
   is shown below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Closed Path ID (i)                   ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Proposed Path ID (i)                  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 8: PATH_UPDATE Frame

   The PATH_UPDATE frame contains the following fields:

   o  Closed Path ID: A variable-length integer corresponding to the
      Path ID of the path that is closed.

   o  Proposed Path ID: A variable-length integer corresponding to the
      Path ID of the path that substitutes the closed path.  If the
      value is 0, no path is proposed.



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   Upon the transmission or the reception of the PATH_UPDATE frame, the
   path with the Path ID referenced in Closed Path ID MUST be in the
   CLOSED state.  If the proposed Path ID is different of 0, the path
   MUST be ACTIVE.

7.2.  ADD_ADDRESS Frame

   The ADD_ADDRESS frame is used by a host to advertise its currently
   reachable addresses.  The proposed type for the ADD_ADDRESS frame is
   0x22.  An ADD_ADDRESS frame is shown below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|0|0|P|IPVers.|Address ID (8) |Interface T.(8)|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       IP Address (32/128)                   ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          [Port (16)]          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 9: ADD_ADDRESS Frame

   The ADD_ADDRESS frame contains the following fields:

   o  Reserved bits: the three most-significant bits of the first byte
      are set to 0, and are reserved for future use.

   o  P bit: the fourth most-significant bit of the first byte
      indicates, if set, the presence of the Port field.

   o  IPVers.: the remaining four least-significant bits of the first
      byte contain the version of the IP address contained in the IP
      Address field.

   o  Address ID: an unique identifier for the advertised address for
      tracking and removal purposes.  This is needed when, e.g., a NAT
      changes the IP address such that both hosts see different IP
      addresses for a same path endpoint.

   o  Interface Type: used to provide an indication about the interface
      type to which this address is bound.  The following values are
      defined: o 0: fixed.  Used as default value.  o 1: WLAN o 2:
      cellular

   o  IP Address: the advertised IP address, in network order.





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   o  Port: this optional field indicates the port number related to the
      advertised IP address.  When this field is present, it indicates
      that a path can use the 2-tuple (IP addr, port).

   Upon reception of an ADD_ADDRESS frame, the receiver SHOULD store the
   communicated address for future use.  The receiver MUST NOT send
   packets others than validation ones to the communicated address
   without having validated it.  ADD_ADDRESS frames SHOULD contain
   globally reachable addresses.  Link-local and possibly private
   addresses SHOULD NOT be exchanged.

7.3.  REMOVE_ADDRESS Frame

   The REMOVE_ADDRESS frame is used by a host to signal that a
   previously announced address was lost.  The proposed type for the
   REMOVE_ADDRESS frame is 0x23.  A REMOVE_ADDRESS frame is shown below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+
   |Address ID (8) |
   +-+-+-+-+-+-+-+-+

                      Figure 10: REMOVE_ADDRESS Frame

   The frame contains only one field, Address ID, being the identifier
   of the address to remove.  A host SHOULD stop using paths using the
   removed address and set them in the UNUSABLE state.  If the
   REMOVE_ADDRESS contains an Address ID that was not previously
   announced, the receiver MUST silently ignore the frame.

7.4.  PATHS Frame

   The PATHS frame communicates the paths state of the sending host to
   the peer.  It allows the sender to communicate its active paths to
   the peer in order to detect potential connectivity issue over paths.
   Its proposed type is 0x24.  The format of the PATHS frame is shown
   below.













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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Sequence (i)                       ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    ActiveReceivePaths (i)                   ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    ActiveSendingPaths (i)                   ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Receive Path Info Section (*)               ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Sending Path Info Section (*)               ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 11: PATHS Frame

   The PATHS frame contains the following fields:

   o  Sequence: A variable-length integer.  This value starts at 0 and
      increases by 1 for each PATHS frame sent by the host.  It allows
      identifying the most recent PATHS frame.

   o  ActiveReceivePaths: the current number of additional receive paths
      considered as being active from the sender point of view, i.e.,
      (the number of active receive paths - 1).

   o  ActiveSendingPaths: the current number of additional sending paths
      considered as being active from the sender point of view, i.e.,
      (the number of active sending paths - 1).

   o  Receive Path Info Section: contains information about all the
      active receive paths (i.e., there are ActivePaths + 1 entries).

   o  Sending Path Info Section: contains information about all the
      active sending paths (i.e., there are ActivePaths + 1 entries).

   Both Receive Path Info and Sending Path Info Sections share the same
   format, which is shown below.













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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Path ID 0 (i)                       ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |LocAddrID 0 (8)|RemAddrID 0 (8)|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Path ID N (i)                       ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |LocAddrID N (8)|RemAddrID N (8)|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 12: Path Info Section

   The fields in the Path Info Section are:

   o  Path ID: the Path ID of the active path the sending host provides
      information about.

   o  LocAddrID: the local Address ID of the address currently used by
      the path.

   o  RemAddrID: the remote Address ID of the address currently used by
      the path.

   The Path Info section only contains the Local and Remote Address IDs
   so far, but this section can be extended later with other potentially
   useful information.

8.  Security Considerations

8.1.  Nonce Computation

   With Multipath QUIC, each path has its own packet number space.  With
   the current nonce computation [I-D.ietf-quic-tls], using twice the
   same packet number over two different paths leads to the same
   cryptographic nonce.  Depending on the size of the Initial Value (and
   hence the nonce), there are two ways to mitigate this issue.

   If the Initial Value has a length of 8 bytes, then a packet number
   used on a given path MUST NOT be reused on another path of the
   connection, and therefore at most 2^64 packets can be sent on a QUIC
   connection.  This means there will be packet number skipping at path
   level, but the packet number will remain monotonically increasing on
   each path.




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   If the Initial Value has a length of 9 or more, then the
   cryptographic nonce computation is now performed as follows.  The
   nonce, N, is formed by combining the packet protection IV (either
   client_pp_iv_n or server_pp_iv_n) with the Path ID and the packet
   number.  The 64 bits of the reconstructed QUIC packet number in
   network byte order is left-padded with zeros to the size of the IV.
   The Path ID encoded in its variable-length format is right-padded
   with zeros to the size of the IV.  The Path IV is computed as the
   exclusive OR of the padded Path ID and the IV.  The exclusive OR of
   the padded packet number and the Path IV forms the AEAD nonce.

8.2.  Validation of Exchanged Addresses

   To use addresses communicated by the peer through ADD_ADDRESS frames,
   hosts are required to validate them before using paths to these
   addresses.  The main reason for this validation is that the remote
   host might have sent, purposely or not, a packet with a source IP
   that does not correspond to the IP of the remote interface.  This
   could lead to amplification attacks where the client start using a
   new path with a source IP corresponding to the victim's one.  Without
   validation, the server might then flood the victim.  Similarly for
   ADD_ADDRESS frames, a malicious server might advertise the IP address
   of a victim, hoping that the client will use it without validating it
   before.

9.  IANA Considerations

9.1.  QUIC Transport Parameter Registry

   This document defines a new transport parameter for the negotiation
   of multiple paths.  The following entry in Table 1 should be added to
   the "QUIC Transport Parameters" registry under the "QUIC Protocol"
   heading.

                +--------+----------------+---------------+
                | Value  | Parameter Name | Specification |
                +--------+----------------+---------------+
                | 0x0020 | max_paths      | Section 5.1.1 |
                +--------+----------------+---------------+

          Table 1: Addition to QUIC Transport Parameters Entries

10.  Acknowledgements

   We would like to thanks Masahiro Kozuka and Kazuho Oku for their
   numerous comments on the first version of this draft.  We also thanks
   Philipp Tiesel for his early comments that led to the current design,
   and Ian Swett for later feedbacks.  We also want to thank Christian



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   Huitema for his draft about multipath requirements to identify
   critical elements about the multipath feature.  Mohamed Boucadair
   provided lot of useful inputs on the second version of this document.
   Maxime Piraux helped us to improve the third version of this draft.

11.  References

11.1.  Normative References

   [I-D.ietf-quic-invariants]
              Thomson, M., "Version-Independent Properties of QUIC",
              draft-ietf-quic-invariants-06 (work in progress), July
              2019.

   [I-D.ietf-quic-recovery]
              Iyengar, J. and I. Swett, "QUIC Loss Detection and
              Congestion Control", draft-ietf-quic-recovery-22 (work in
              progress), July 2019.

   [I-D.ietf-quic-tls]
              Thomson, M. and S. Turner, "Using TLS to Secure QUIC",
              draft-ietf-quic-tls-22 (work in progress), July 2019.

   [I-D.ietf-quic-transport]
              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-22 (work
              in progress), July 2019.

   [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>.

11.2.  Informative References

   [Cellnet]  Paasch, C., Detal, G., Duchene, F., Raiciu, C., and O.
              Bonaventure, "Exploring Mobile/Wi-Fi Handover with
              Multipath TCP", ACM SIGCOMM workshop on Cellular Networks
              (Cellnet'12) , 2012.

   [I-D.huitema-quic-mpath-req]
              Huitema, C., "QUIC Multipath Requirements", draft-huitema-
              quic-mpath-req-01 (work in progress), January 2018.

   [IETFJ]    Bonaventure, O. and S. Seo, "Multipath TCP Deployments",
              IETF Journal , November 2016.





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   [MPQUIC]   De Coninck, Q. and O. Bonaventure, "Multipath QUIC: Design
              and Evaluation", 13th International Conference on emerging
              Networking EXperiments and Technologies (CoNEXT 2017).
              http://multipath-quic.org , December 2017.

   [MPRTP]    Singh, V., Ahsan, S., and J. Ott, "MPRTP: Multipath
              considerations for real-time media", Proceedings of the
              4th ACM Multimedia Systems Conference , 2013.

   [OLIA]     Khalili, R., Gast, N., Popovic, M., Upadhyay, U., and J.
              Le Boudec, "MPTCP is not pareto-optimal: performance
              issues and a possible solution", Proceedings of the 8th
              international conference on Emerging networking
              experiments and technologies, ACM , 2012.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.

   [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>.

   [RFC6824]  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/info/rfc6824>.

   [RFC7050]  Savolainen, T., Korhonen, J., and D. Wing, "Discovery of
              the IPv6 Prefix Used for IPv6 Address Synthesis",
              RFC 7050, DOI 10.17487/RFC7050, November 2013,
              <https://www.rfc-editor.org/info/rfc7050>.

   [RFC7225]  Boucadair, M., "Discovering NAT64 IPv6 Prefixes Using the
              Port Control Protocol (PCP)", RFC 7225,
              DOI 10.17487/RFC7225, May 2014,
              <https://www.rfc-editor.org/info/rfc7225>.

   [RFC8041]  Bonaventure, O., Paasch, C., and G. Detal, "Use Cases and
              Operational Experience with Multipath TCP", RFC 8041,
              DOI 10.17487/RFC8041, January 2017,
              <https://www.rfc-editor.org/info/rfc8041>.

   [SCTPCMT]  Iyengar, J., Amer, P., and R. Stewart, "Concurrent
              multipath transfer using SCTP multihoming over independent
              end-to-end paths", IEEE/ACM Transactions on networking,
              Vol. 14, no 5 , 2006.



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Appendix A.  Change Log

A.1.  Since draft-deconinck-quic-multipath-02

   o  Consider asymmetric paths

A.2.  Since draft-deconinck-quic-multipath-01

   o  Include path policies considerations

   o  Add practical considerations thanks to Mohamed Boucadair inputs

   o  Adapt the RETIRE_CONNECTION_ID frame

   o  Updated text to match draft-ietf-quic-transport-18

A.3.  Since draft-deconinck-quic-multipath-00

   o  Comply with asymmetric Connection IDs

   o  Add max_paths transport parameter to negotiate initial number of
      active paths

   o  Path ID as a regular varint

   o  Remove max_path_id transport parameter

   o  Updated text to match draft-ietf-quic-transport-14

A.4.  Since draft-deconinck-multipath-quic-00

   o  Added PATH_UPDATE frame

   o  Added MAX_PATHS frame

   o  No more packet header change

   o  Implicit Path ID notification using Connection ID and
      NEW_CONNECTION_ID
      frames

   o  Variable-length encoding for Path ID

   o  Updated text to match draft-ietf-quic-transport-10

   o  Fixed various typos





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Authors' Addresses

   Quentin De Coninck
   UCLouvain

   Email: quentin.deconinck@uclouvain.be


   Olivier Bonaventure
   UCLouvain

   Email: Olivier.Bonaventure@uclouvain.be







































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