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Versions: (draft-deconinck-multipath-quic) 00

QUIC Working Group                                         Q. De Coninck
Internet-Draft                                            O. Bonaventure
Intended status: Standards Track                               UCLouvain
Expires: September 6, 2018                                 March 5, 2018


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

Abstract

   This document proposes extensions to the QUIC protocol to enable the
   usage of multiple paths over a single connection.  Those changes
   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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://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 September 6, 2018.

Copyright Notice

   Copyright (c) 2018 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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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 . . . . . . . . . . . . . . . . .   3
     2.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   4
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  What is a Path? . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Beyond Connection Migration . . . . . . . . . . . . . . .   5
     3.3.  Establishment of a Multipath QUIC Connection  . . . . . .   6
     3.4.  Architecture of Multipath QUIC  . . . . . . . . . . . . .   7
     3.5.  Path Establishment  . . . . . . . . . . . . . . . . . . .   7
     3.6.  Exchanging Data over Multiple Paths . . . . . . . . . . .   8
     3.7.  Exchanging Addresses  . . . . . . . . . . . . . . . . . .   9
     3.8.  Coping with Address Removals  . . . . . . . . . . . . . .  10
     3.9.  Path Migration  . . . . . . . . . . . . . . . . . . . . .  10
     3.10. Congestion Control  . . . . . . . . . . . . . . . . . . .  11
   4.  Mapping Path ID to Connection ID  . . . . . . . . . . . . . .  11
   5.  Using Multiple Paths  . . . . . . . . . . . . . . . . . . . .  12
     5.1.  Multipath Negotiation . . . . . . . . . . . . . . . . . .  12
       5.1.1.  Transport Parameter Definition  . . . . . . . . . . .  13
     5.2.  Variable-Length Encoding of the Path ID . . . . . . . . .  13
     5.3.  Coping with Additional Remote Addresses . . . . . . . . .  13
     5.4.  Path State  . . . . . . . . . . . . . . . . . . . . . . .  14
     5.5.  Dynamic Management of Paths . . . . . . . . . . . . . . .  16
     5.6.  Loosing Addresses . . . . . . . . . . . . . . . . . . . .  17
     5.7.  Scheduling Strategies . . . . . . . . . . . . . . . . . .  17
   6.  Modifications to QUIC frames  . . . . . . . . . . . . . . . .  18
     6.1.  NEW_CONNECTION_ID Frame . . . . . . . . . . . . . . . . .  18
     6.2.  ACK Frame . . . . . . . . . . . . . . . . . . . . . . . .  20
   7.  New Frames  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     7.1.  MAX_PATHS Frame . . . . . . . . . . . . . . . . . . . . .  20
     7.2.  PATH_UPDATE Frame . . . . . . . . . . . . . . . . . . . .  21
     7.3.  ADD_ADDRESS Frame . . . . . . . . . . . . . . . . . . . .  22
     7.4.  REMOVE_ADDRESS Frame  . . . . . . . . . . . . . . . . . .  23
     7.5.  PATHS Frame . . . . . . . . . . . . . . . . . . . . . . .  23
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  25
     8.1.  Nonce Computation . . . . . . . . . . . . . . . . . . . .  25
     8.2.  Validation of Exchanged Addresses . . . . . . . . . . . .  25
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26
     9.1.  QUIC Transport Parameter Registry . . . . . . . . . . . .  26
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  26
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  26
     11.2.  Informative References . . . . . . . . . . . . . . . . .  27
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  28
     A.1.  Since draft-deconinck-multipath-quic-00 . . . . . . . . .  28
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28




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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
   bandwidth.  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 and for other use cases [RFC8041].

   With regular TCP and UDP, all the packets that belong to a given flow
   contain the same 5-tuple that acts as an identifier for this flow.
   This prevents such 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 its Connection
   ID.  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 use multiple paths.  Use cases such as bandwidth aggregation
   or seamless network handovers would be applicable to QUIC, as they
   are now with Multipath TCP.  A first performance evaluation of such
   use cases and a comparison between Multipath QUIC and Multipath TCP
   may be found in [MPQUIC].

   In this draft, we leverage many of the lessons learned from the
   design of Multipath TCP and comments received on the first version of
   this draft to propose extensions to the current QUIC design to enable
   it to simultaneously use several paths.  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.

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
   the QUIC documents [I-D.ietf-quic-transport].  In addition, we
   define:

   o  Path: A logical association between two hosts over which packets
      can be sent.  A path is internally identified by a Path ID and



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      uses a potentially changing Connection ID in packets exchanged on
      that path.

   o  Initial Path: The path used for the establishment of the QUIC
      connection.  The cryptographic handshake is done on this path.  It
      is 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], with the following additional convention:

   x (j) ... Indicates that x uses the variable-length encoding scheme
   described in Section 5.2.

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 WiFi 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.  We first explain why a multipath
   extension would be beneficial to QUIC and then describe it at a high
   level.

3.1.  What is a Path?

   Before going into details, let us first define what is called a
   "path".  A path is a UDP flow between two hosts denoted by a 4-tuple
   (source IP address, destination IP address, source port, destination
   port).  On a smartphone interacting with a single-homed server, the
   mobile device might want to use one path over the WiFi 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 WiFi network, one over IPv4
   and the other over IPv6.








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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.  Another of its use cases is fail-over when the address in
   use fails but another one is available.  A mobile device loosing the
   WiFi connectivity can then continue the connection over its cellular
   interface.

   A QUIC connection can thus start on a given path and end on another
   one.  However, the current QUIC design [I-D.ietf-quic-transport]
   assumes that only one path is in use for a given connection.  Instead
   of switching the 4-tuple for the whole connection, this draft first
   proposes mechanisms to communicate endhost addresses to the peer.  It
   then leverage 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 networks.  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.

























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

3.3.  Establishment of a Multipath QUIC Connection

   A Multipath QUIC connection starts like a regular QUIC connection.  A
   cryptographic handshake takes place over Stream ID 0 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_path_id
   transportparameter, where both hosts advertise the maximum number of
   paths, in addition to the first one, that they are able to use over
   the connection.  The negotiated value will determine how many bytes
   will be used to encode the Path ID in frames.  If one of the hosts
   does not advertise the max_path_id transport parameter, the
   negotiated value is 0, meaning that the QUIC connection will remain
   single-path.

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






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3.4.  Architecture of Multipath QUIC

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

             +---------------------------------------------+
             |              Connection (MCID)              |
             |   +--------+ +--------+ ... +------------+  |
             |   | Path 0 | | Path 1 |     | Path N - 1 |  |
             |   | Tuple  | | Tuple' | ... |   Tuple"   |  |
             |   |  PCID  | |  PCID' |     |    PCID"   |  |
             |   |  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
   draft proposes two levels of Connection IDs.  The first one is the
   Master Connection ID (MCID).  It uniquely identifies the connection,
   as with the current QUIC design.  The second one is the Path
   Connection ID (PCID).  Packets belonging to a given path share the
   same PCID written in the Connection ID field of the public header.
   The PCID acts as an implicit path identifier for packets.  Preventing
   the linkability of different paths is an important requirement for
   the multipath extension [I-D.huitema-quic-mpath-req].  Using PCID as
   implicit path identifier makes this linkability harder than having
   explicit signaling as in the previous version of this draft and does
   not require public header change to keep invariants
   [I-D.ietf-quic-invariants].  The MCID of a connection will be the
   PCID of the Initial Path.  In the example of Figure 1, if the
   connection started on the WiFi network, then Connection ID A is both
   the PCID of the WiFi path and the MCID of the connection.

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

3.5.  Path Establishment

   The max_path_id transport parameter exchanged during the
   cryptographic handshake determines an upper bound on the number of
   paths that can be created over the connection, in addition to the



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   Initial one.  Then, hosts must agree on which paths can be used, and
   with which Path Connection IDs.  Unlike Multipath TCP [RFC6824], the
   server dynamically controls how many paths can currently be in use.
   This can be done using a new MAX_PATHS frame indicating how many
   additional paths can be simultaneously in use.

   The server proposes new paths with an extended version of the
   NEW_CONNECTION_ID frame (see Section 6.1).  That frame proposes a
   PCID for a given Path ID.  Once the client receives and acknowledges
   the frame, it can start using the new path by placing the PCID in the
   Connection ID field of the packets.  The server can start using a
   path once the corresponding NEW_CONNECTION_ID has been acknowledged.
   Both hosts store the NEW_CONNECTION_ID information in order to cope
   with the remote trying to use a new path.  As frames are encrypted,
   the establishment of new paths does not leak cleartext identifiers
   [I-D.huitema-quic-mpath-req].

   A server might provide more Path IDs with NEW_CONNECTION_ID frames
   than the value advertised in the MAX_PATHS frame.  This can be useful
   to cope with migration cases, as described in Section 3.9.  In such
   cases, hosts can only use a subset of the proposed Path IDs.
   Multipath QUIC is fully symmetrical.  Both the client and the server
   can start using new paths once their corresponding PCID have been
   negotiated.  It might happen that both hosts try to create paths with
   different Path IDs, such that there are more paths than the
   advertised number in MAX_PATHS frame.  In that case, the paths with
   the lowest Path IDs will be used while the others will stop.

   The client is not able to create new paths as long as the server does
   not send NEW_CONNECTION_ID and MAX_PATHS frames.  To limit the
   latency of the path handshake, the server can send those frames just
   after the 0-RTT handshake packet.

   Although a path can be first used by any host, it might not be
   practical for one of the peers to send an initial packet on new
   paths.  A possible cause 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
   path using it.

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



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   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
   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 the Path ID
   in the public header 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 2^32 * 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 path they want to send path acknowledgments.
   Looking again at Figure 1, packets that were sent over a given path
   (e.g., the response2 packet on path 2 with CID B) can be acknowledged
   on another path (here, path 1 with CID A) to limit the latency due to
   ACK transmissions on high-latency paths.  Such scheduling decision
   would not have been possible in Multipath TCP [RFC6824] which must
   acknowledge data on the path is was received on.

3.7.  Exchanging Addresses

   When a multi-homed mobile device connects to a dual-stacked server on
   its IPv4 address, it is aware of its local addresses (e.g., the WiFi
   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



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   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
   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 address to start a new 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 of
   the sending host.  A path is called active when it was created over a
   validated 4-tuple and is still in use.  The frame indicates the local
   Address ID that the path uses.  With this information, the server can
   then validate the public address and associate the advertised with
   the perceived addresses.

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 WiFi 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.

3.9.  Path Migration

   At a given time, a Multipath QUIC connection gathers a set of paths,
   each using 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 path.  There remain situations where
   this change is unavoidable.  These can be categorized into two
   groups: host-aware changes (e.g., network handover from WiFi 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 WiFi and
   cellular.  It advertised both addresses and the server currently
   enables only one path, the Initial one.  The Initial Path uses the
   WiFi address.  Then, for some reason, the WiFi 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 WiFi address and a PATHS



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   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 the 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 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 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 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.

3.10.  Congestion Control

   The QUIC congestion control scheme is defined in
   [I-D.ietf-quic-recovery].  This congestion control scheme is not
   suitable when several paths are active.  Using the congestion control
   scheme defined in [I-D.ietf-quic-recovery] with Multipath QUIC would
   result in unfairness.  Each 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 ID

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




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   The Master Connection ID is determined during the cryptographic
   handshake and actually corresponds to the Connection ID in the
   current QUIC design [I-D.ietf-quic-transport].  The Path Connection
   ID of the Initial path (with Path ID 0) is equal to the Master
   Connection ID.  The Path Connection ID 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 the PCID, the server can start
   receiving packets with the advertised Connection ID as belonging to
   the corresponding path.  The server MUST wait until the reception of
   the frame acknowledgment before starting to send packets on that
   path.

   Upon reception of the NEW_CONNECTION_ID frame, the client MUST
   acknowledge it and MUST store the advertised Path Connection ID and
   the Path ID of the proposed path.  It MUST ensure that it can receive
   packets coming from the server using the PCID and associate it with
   the corresponding Path ID.

5.  Using Multiple Paths

   This section describes in details the multipath operations with the
   QUIC protocol.

5.1.  Multipath Negotiation

   The Multipath Negotiation takes place during the cryptographic
   handshake with the max_path_id 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 the maximum path ID they are willing to
   use.  The maximum path ID that can be used over a connection is the
   minimum of both advertised values.  If one host does not send the
   max_path_id transport parameter during the cryptographic handshake,
   the remote MUST assume a value of 0, leading to a single-path
   connection over the Initial Path.  NEW_CONNECTION_ID frames can later
   propose Path IDs in the range of max_path_id inclusive.  If a client
   receives a NEW_CONNECTION_ID proposing a Path ID greater than the
   negotiated maximum value, it MUST trigger a CONNECTION_CLOSE frame
   with an error code set to 0x10b with a reason phrase describing this
   issue.








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5.1.1.  Transport Parameter Definition

   An endhost MAY use the following transport parameter:

   max_path_id (0x0020):  The maximum path ID transport parameter
      indicates the number of paths in addition to the Initial Path that
      the host agrees to support over the connection, encoded as an
      unsigned 32-bit integer.  This sets a upper bound on the Path IDs
      that can be used.  The default value for this parameter is 0,
      meaning that a host omitting this transport parameter does not
      agree to use multiple paths over the connection.

5.2.  Variable-Length Encoding of the Path ID

   From the negotiated maximum Path ID, hosts can infer the amount of
   bits required to encode the Path ID.  If the maximum Path ID is 255,
   only one byte suffices.  Similarly, if one of the hosts wants a
   single-path connection, either by advertising a max_path_id of 0 or
   by omitting it, the Path ID field should not be present.

      +------------------------------+------------------------------+
      | Negotiated Max Path ID Range | Path ID Field Length (Bytes) |
      +------------------------------+------------------------------+
      | 0                            | 0                            |
      |                              |                              |
      | 1-255                        | 1                            |
      |                              |                              |
      | 256-65535                    | 2                            |
      |                              |                              |
      | 65536-4294967295             | 4                            |
      +------------------------------+------------------------------+

                   Table 1: Summary of Path ID Encodings

   The proposed encoding is summarized in Table 1.

   Because of security considerations described in Section 8.1, a host
   reusing packet numbers across paths must limit the number of
   advertised paths depending on the size of the Initial Value.  In
   particular, a host MUST NOT advertise a max_path_id whose field
   length exceeds (Initial Value Length) - 8.

5.3.  Coping with Additional Remote Addresses

   The usage of multiple networks paths if often done using multiple IP
   addresses.  For instance, a smartphone willing to use both WiFi and
   LTE will use the corresponding addresses provided by the networks.
   It can then safely send packets to a previously-used IP address of a



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   server.  The server can receive packets with different IPs, 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.

5.4.  Path State

   During the Multipath QUIC connection, hosts maintain some state for
   paths.  Information about the path that hosts are required to store
   depends on its state.  The possible path states are depicted in
   Figure 3.

   +----------+
   | PROPOSED |
   +----------+
         |
         | path usage
         |
         |       restricted by MAX_PATHS
         v    or affected by REMOVE_ADDRESS
    +--------+ --------------------------> +----------+
    | ACTIVE |       reusing path          | UNUSABLE |
    +--------+ <-------------------------- +----------+
         |                                       |
         | PATH_UPDATE                           |
         +---------------------------------------+
         |
         v
    +--------+
    | CLOSED |
    +--------+

                  Figure 3: Finite-State Machine of paths

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





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

   o  Master Connection ID: encoded as a 8-byte number.  It makes the
      link between the path and the QUIC connection it belongs to.  This
      value is immutable.

   o  Path Connection ID: encoded as a 8-byte integer.  It makes the
      link between the packet's Connection ID field and the path.  This
      value can be updated with subsequent NEW_CONNECTION_ID frames.

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

   Hosts MUST ensure that they can associate a packet with the PCID and
   its corresponding path at any time, as they must be ready to figure
   out the path used by the remote host.

   Either the host received a packet with the corresponding PCID coming
   from a validated address or it wants to start using the path to a
   validated address, the path goes to the ACTIVE state.  This is the
   state where a path can be used to send and receive packets.  In
   addition to the fields required in the PROPOSED state, the following
   elements MUST be tracked:

   o  Packet Number Space: each path is associated with its own
      monotonically increasing packet number space.  Each endpoint
      maintains a separate packet number for sending and receiving
      packets.  Packet number considerations described in
      [I-D.ietf-quic-transport] apply within a given path.

   o  Current 4-tuple: the tuple (sIP, dIP, sport, dport) used to send
      or receive
      packets over this path.  This value is mutable, either because the
      host decides to change its local address and/or port, or because
      it receives a packet with a different validated remote address
      and/or port than the one currently recorded.  A host that changes
      the 4-tuple of a 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 mark a path as UNUSABLE 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.





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   o  Performance metrics: basic statistics such as round-trip-time or
      number of packets sent and received 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 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 is in UNUSABLE state and the host MUST NOT send
   packets on it.  The host keeps the same information as in the ACTIVE
   state for the path.  It might happen that packets are received on
   that path.  If the path is in UNUSABLE state because of a decrease of
   the active paths, packets MUST be discarded silently.  If it is
   because a REMOVE_ADDRESS was received for the remote Address ID of
   the path, the host SHOULD validate the remote address first before
   reusing it.  If this validation suceeds, or the number of paths
   increased again, a path in the UNUSABLE state can go into ACTIVE
   state, but its congestion window MUST be restarted and its
   performance metrics SHOULD be reset.

   Eventually, a path may either be migrated to another one or closed.
   This is signalled by the PATH_UPDATE frame.  In that case, the path
   is in CLOSED state.  In that state, packets MUST NOT be sent or
   received over it.  A host MUST keep the same path state field as in
   the PROPOSED state, to avoid ambiguities and CLOSED path reuse.

5.5.  Dynamic Management of Paths

   The server is the main protagonist in determining how many paths can
   be used over a Multipath QUIC connection.  It is its reponsibility to
   propose Path IDs with corresponding PCIDs that could be used by the
   client.  In addition, it dynamically controls the number of active
   paths that can be used for the connection.  This is performed by
   sending the MAX_PATHS frame that sets an upper limit on the number of
   additional active paths.  A host can propose more Path IDs with
   NEW_CONNECTION_ID frames than the number of additional paths it
   agrees to use simultaneously.  This can be useful in migration
   scenarios, where the server wants to share a pool of Path IDs that
   can be directly used to migrate paths.

   As long as no MAX_PATHS frame was received from the server, the
   client MUST assume that only one path can be used.  At the beginning
   of the QUIC connection, this means that only the Initial Path can be
   used.  In this case however, during the connection, another path can
   be used, but the Initial Path MUST have been migrated to it.  The
   server can then increase the value advertised in the MAX_PATHS frame
   to allow hosts to use new paths among the proposed Path ID pool.




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   By default, the client does not set any limit to the number of paths
   which can be used.  If a client wants to limit the number of
   (additional) paths the server can use, it can also send a MAX_PATHS
   frame.  In this case, the number of additional paths that can be used
   over the connection is the minimum between last advertised value from
   both sides.

   A host can also decrease the number of paths that can be used over a
   connection.  If this value decreases below the current number of
   active paths, hosts MUST put in UNUSABLE state the paths in the
   ACTIVE state with the highest Path IDs.  Server and client MUST stop
   sending packets over UNUSABLE paths once the MAX_PATHS has been sent
   or received.  The host restricting the number of paths SHOULD allow
   receiving packets on newly UNUSABLE paths for one round-trip-time.
   After this delay, hosts SHOULD silently discard received packets.

   When the server proposes more Path IDs than the negotiated maximum
   number of ACTIVE paths, it might happen that both hosts decide at the
   same time to create paths with different Path IDs, such as there are
   more created paths than the negotiated maximum value.  In this case,
   the hosts MUST only keep as ACTIVE the paths with the lowest Path
   IDs.  Paths with the highest Path IDs are in the UNUSABLE state and
   packets SHOULD be silently discarded.

5.6.  Loosing Addresses

   During the lifetime of a connection, a host might lose addresses,
   e.g., a network interface that was shut down.  All the ACTIVE paths
   that were using that local address MUST be set in the UNUSABLE state.
   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 set the
   ACTIVE paths affected by the address removal into the UNUSABLE state.

   The host that locally lost the address MAY reuse one of these 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 path on which the packet will be sent.
   This dimension can have a non negligible impact on the operations of



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   Multipath QUIC, especially if the available 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.

   The path on which ACK frames are sent impacts the peer.  The ACK
   frame is notably used to determine the latency of a path.  If the ACK
   frame is sent on the path it acknowledges, then the peer can compute
   the round-trip-time of that path.  Otherwise, the peer would compute
   the latency as the sum of the foward delay of the acknowledged path
   and the return delay of the path used to send the ACK frame.
   Choosing between acknowledging packets on the same path or on a
   specific path 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.  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 is
   modified to provide Path Connection ID negotiation for each path.
   The ACK frame contains packet-level information with the Largest
   Acknowledged field.  Since the Packet Numbers are now associated to
   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 server to indicate
   which connection IDs the client must use to take advantage of
   multiple 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  (j)                       ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Sequence (i)                       ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                        Connection ID (64)                     +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                   Stateless Reset Token (128)                 +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 4: NEW_CONNECTION_ID 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 MUST only be sent by the server.  Upon reception of the frame
   with a specified path ID, the client can start using the new path
   with the matching Connection ID.

   A host MUST NOT start using a path if no related NEW_CONNECTION_ID
   frame has been received (at client side) or acknowledged (at server
   side).  To limit the delay of the multipath usage upon handshake
   completion, the server SHOULD send NEW_CONNECTION_ID frames for paths
   it allows 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 the server MUST be random.

   It might happen that a server proposes a Connection ID in a
   NEW_CONNECTION_ID frame that is already in use by the client for
   another connection with another server.  In such cases, the client
   MUST warn the server that this Connection ID is not available and ask
   for another Connection ID.  This request for asking another




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   Connection ID for a particular path will follow the same mechanism as
   described in [I-D.ietf-quic-transport].

6.2.  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 (j)                       ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Largest Acknowledged (i)                 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          ACK Delay (i)                      ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       ACK Block Count (i)                   ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          ACK Blocks (*)                     ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 5: ACK Frame adapted to Multipath

   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.

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.  MAX_PATHS Frame

   The MAX_PATHS frame is used by hosts to control the number of paths
   that can be simultaneously in the ACTIVE state on a given connection.
   The proposed type for the MAX_PATHS frame is 0x20.  A MAX_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)                       ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Num Additional Active Paths (j)             ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 6: MAX_PATHS Frame

   The MAX_PATHS frame contains the following fields:

   o  Sequence: A variable-length integer.  This value starts at 0 and
      increases by 1 for each change of number of active paths that is
      provided by the host.

   o  Num Additional Active Paths: A variable-length integer indicating
      how many additional paths can be used over the connection.  The
      number of paths that can be in ACTIVE state is thus (Num
      Additional Active Paths + 1).

   Once the connection is established, hosts MUST assume that the server
   sent a MAX_PATHS frame with sequence -1 and number of additional
   active paths equal to 0 and the client sent a MAX_PATHS frame with
   sequence -1 and number of additional active paths equal to the
   negotiated max_path_id value.

7.2.  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 (j)                   ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Proposed Path ID (j)                  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 7: MIGRATE_PATH Frame

   The MIGRATE_PATH frame contains the following fields:





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   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.

   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 have been either in the PROPOSED or in the UNUSABLE state and
   MUST now be considered as ACTIVE.

7.3.  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) |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       IP Address (32/128)                   ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          [Port (16)]          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 8: 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.




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

   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.

7.4.  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 9: 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.

7.5.  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)                       ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        ActivePaths (j)                      ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Path Info Section (*)                   ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 10: 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  ActivePaths: the current number of additional paths considered as
      being active from the sender point of view, i.e., (the number of
      active paths - 1).  ActivePaths MUST be lower or equal to the last
      value advertised by MAX_PATHS frame.

   o  Path Info Section: contains information about all the active paths
      (i.e., there are ActivePaths + 1 entries).  The format of this
      section 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 0 (j)                       ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |LocAddrID 0 (8)|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                  ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Path ID N (j)                       ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |LocAddrID N (8)|
   +-+-+-+-+-+-+-+-+

                       Figure 11: 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.




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

   The Path Info section only contains the Local Address ID 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.

   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 described in
   Section 5.2 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




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   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 2 should be added to
   the "QUIC Transport Parameters" registry under the "QUIC Protocol"
   heading.

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

          Table 2: 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.
   We also want to thanks Christian Huitema for his draft about
   multipath requirements to identify critical elements about the
   multipath feature.

11.  References

11.1.  Normative References

   [I-D.ietf-quic-invariants]
              Thomson, M., "Version-Independent Properties of QUIC",
              draft-ietf-quic-invariants-00 (work in progress), February
              2018.

   [I-D.ietf-quic-recovery]
              Iyengar, J. and I. Swett, "QUIC Loss Detection and
              Congestion Control", draft-ietf-quic-recovery-09 (work in
              progress), January 2018.

   [I-D.ietf-quic-tls]
              Thomson, M. and S. Turner, "Using Transport Layer Security
              (TLS) to Secure QUIC", draft-ietf-quic-tls-10 (work in
              progress), March 2018.




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   [I-D.ietf-quic-transport]
              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-10 (work
              in progress), March 2018.

   [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/WiFi 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.

   [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>.




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   [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>.

   [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.

Appendix A.  Change Log

A.1.  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

Authors' Addresses

   Quentin De Coninck
   UCLouvain

   Email: quentin.deconinck@uclouvain.be


   Olivier Bonaventure
   UCLouvain

   Email: Olivier.Bonaventure@uclouvain.be




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