Network Working Group                                         C. Huitema
Internet-Draft                                      Private Octopus Inc.
Intended status: Informational                           January 7, 2018
Expires: July 11, 2018

                      QUIC Multipath Requirements


   This document describes the requirement and plausible architecture of
   QUIC multipath extensions.  While the first version of QUIC is not
   scheduled to include multipath extensions, there are risks that
   decisions made in this first version might preclude some options that
   we may later find attractive.  An early review of multipath extension
   requirements and issues should minimize that risk.

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   the Trust Legal Provisions and are provided without warranty as
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  QUIC Multipath Scenarios  . . . . . . . . . . . . . . . . . .   3
   3.  Basic Architecture And Discussion Points  . . . . . . . . . .   4
   4.  QUIC Multipath Requirements . . . . . . . . . . . . . . . . .   5
     4.1.  Path Initiation . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Privacy . . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.3.  Security  . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.4.  Non Detectability . . . . . . . . . . . . . . . . . . . .   6
     4.5.  Error Detection . . . . . . . . . . . . . . . . . . . . .   7
     4.6.  Congestion Control  . . . . . . . . . . . . . . . . . . .   8
     4.7.  Flow Control  . . . . . . . . . . . . . . . . . . . . . .   9
     4.8.  Packet Encryption . . . . . . . . . . . . . . . . . . . .   9
     4.9.  Sequence Number Encryption  . . . . . . . . . . . . . . .  10
     4.10. Key Phases  . . . . . . . . . . . . . . . . . . . . . . .  11
     4.11. Key and sequence numbers per path . . . . . . . . . . . .  11
   5.  Next Steps  . . . . . . . . . . . . . . . . . . . . . . . . .  11
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   7.  Informative References  . . . . . . . . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   QUIC is a multiplexed and secure transport protocol that runs on top
   of UDP.  QUIC aims to provide a flexible set of features that allow
   it to be a general-purpose transport for multiple applications.  The
   QUIC working group is chartered to deliver a first version of the
   protocol in 2018.  The current version is defined in a set of drafts,
   describing the base transport [I-D.ietf-quic-transport], congestion
   and recovery [I-D.ietf-quic-recovery], use of TLS
   [I-D.ietf-quic-tls], and mapping of HTTP over QUIC

   According to its charter, the QUIC Working Group will first focus on
   delivering a first version of the protocol in 2018.  This first
   version will focus on the basic scenario: client connecting to server
   over a single path, for a single application, HTTP.  Multipath
   extensions will only be addressed later, once this first version is
   ready.  Focusing on a simple version and delivering it first is
   obviously a pretty reasonable way to manage a complex project like
   the development of QUIC.  However, there is a risk that decisions
   made during the development of the first version might preclude some
   options that we may later find attractive when developing the
   multipath extension.

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   It is almost impossible to discuss Multipath QUIC in an IETF context
   without making references to the Multipath extensions for TCP (MTCP),
   specified in [RFC6824].  The specification explains how to provide
   the same type of service to applications as TCP (i.e., reliable
   bytestream), while using multiple TCP flows across potentially
   disjoint paths.  The details of the MTCP design are very specific to
   TCP, but many high level principles and experiences can inform the
   design of multipath QUIC.

   The purpose of this document is to explore the multipath requirements
   and plausible solutions, in order to inform the development of QUIC
   V1 and hopefully facilitate a smooth introduction of multipath
   extensions after V1 is standardized.

2.  QUIC Multipath Scenarios

   The classic multipath scenarios involve splitting a connection's
   traffic over several paths, aiming for either better reliability by
   using several redundant paths, or better performance by balancing the
   traffic load over several path.  For example, we see load balancing
   between equal cost multipaths using MTCP documented in [RFC8041].
   Redundancy and load balancing define broad scenarios which will
   certainly be applicable to QUIC, but a couple of more specific
   scenarios have been discussed during the early development of QUIC.
   These scenarios include NAT re-binding, client migration, and anycast

   The classical example of client migration is the transition from one
   type of connectivity, e.g.  Wi-Fi, to another, e.g.  LTE.  Client
   migration is often handled through a "make before break" strategy, in
   which a new "path" is established and tested before switching to it
   and decommisioning the old path.  Such strategies involve maintaining
   the old and the new path active during the testing time, and thus
   require a simple form of multipath management.  This is one of the
   scenarios studied in the context of MTCP, and documented in

   Many services are deployed using anycast addresses, which allow a
   client to get automatically connected to the closest available
   server.  However, the connectivity between client and server can be
   affected by changes in routing conditions.  For better reliability,
   it may be desirable to transition the connection to use an "unicast"
   address specific to that server instance.  The transition from
   anycast to unicast requires a connection migration similar to the
   client migration described in the previous paragraph.  That, too,
   requires a simple form of multipath management.

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   An even simpler form of migration happens when a client is located
   behind a NAT.  The NAT will sometimes changed the mapping between the
   private address of the client and the public address visible to the
   server.  From the server point of view, this may have some
   commonality with the "client migration" scenario described above,
   since the client appears to move to a new address.  There are however
   some marked differences, such as the absence of the overlap period
   seen in the make before break scenario, or the fact that the client
   is generally unaware of the change in NAT mappings.  For
   simplification, we will not consider NAT rebinding as a multipath

3.  Basic Architecture And Discussion Points

   QUIC is effectively a layered protocol, in which a stream of packets
   carries traffic multiplexed as a number of streams.

          | Application     Stream oriented API


          | Stream layer | Stream oriented frames, e.g. STREAM

          |              | Control of stream, scheduling of frames

          |              | Retransmission of STREAM and other frames


          | Packet layer | Packet headers, encryption

          |              | Error detection via ACK frames


          | network        UDP packets

   Most experts will recognize that the partition between the stream
   layer and the packet layer provides a natural opportunity for
   implementing multipath.  For example, when describing a pre-standard
   experiment with QUIC multipath [I-D.deconinck-multipath-quic], De
   Coninck and Bonaventure state that "Since nearly all QUIC frames are
   independent of packets, no change is required (for supporting
   multipath)".  This implies a broad agreement for an architecture
   where stream frames, at the stream layer, can be exchanged over
   several paths.

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   We can thus establish a set of consensual principles.  A connection
   could be composed of many paths, each defined by a specific pair of
   client and server IP addresses and UDP ports.  The set of paths
   associated to a connection could vary over time.  Nodes could use
   various strategies to send stream oriented frames over the available
   paths.  Losses of packets will be detected at the packet layer, and
   the corresponding stream-oriented frames may be retransmitted over
   any of the available paths.  But to go from these easily drawn
   principles to a more precise architecture, we need to consider a set
   of requirements.

4.  QUIC Multipath Requirements

4.1.  Path Initiation

   In a multipath scenario, one of the peers starts a new path after
   becoming aware that different addresses are available.  In some
   scenarios, clients can start new paths because they gain additional
   connectivity, e.g. on Wi-Fi in addition to LTE.  In these cases, the
   server will learn the availability of the new path by observing the
   IP addresses and ports on which the new packet is received.

   In other scenarios, servers may ask clients to initiate new paths,
   for example in order to transition away from an anycast address.
   This requires communicating the additional addresses to the other
   peer.  This may require defining an "ADD_ADDRESS" frame, as suggested
   in [I-D.deconinck-multipath-quic], if we want the additonal addresses
   to be passed withing QUIC.  An alternative might be to let
   applications exchange addresses as part of their application
   protocol, and to use an API to "plumb" these addresses in the QUIC

   Paths are normally initiated by having one peer send an encrypted
   packet to the other peer.  The encrypted packet carries a connection
   identifiers that was previously associated with the connection,
   typically through a "NEW_CONNECTION_ID" frame.  Privacy issues
   related to the use of connection identifiers are discussed in
   Section 4.2.  In addition to privacy issues, we need to also consider
   the security issues, and specifically the risk of enabling denial of
   service attacks, as discussed in Section 4.3.

4.2.  Privacy

   When traffic is carried over multiple paths, it become observable at
   many points, and this has privacy implications.  For example, if
   packets belonging to a given connection carry some unique
   identifiers, observers could use these identifiers to track client

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   migrations through several paths, and thus potentially expose the
   successive locations of a particular user.

   The generally agreed privacy requirement is to prevent "linkability"
   between multiple paths.  In QUIC, linkability could derive from
   observing components in the packet headers, such as connection
   identifiers or sequence numbers, an possibly also clear text
   parameters of the initial packet and the TLS "Client Hello" message
   that it carries.

   In order to prevent linkability, we should adopt a set of clear
   guidelines, such as never reusing the same connection identifier over
   several paths, or breaking potential correlation betwen the sequence
   numbers used on multiple paths.  We should also require that all
   messages used to set up additional paths be encrypted, in order to
   avoid leaking clear text identifiers.  (The experiment described in
   [I-D.deconinck-multipath-quic] does not meet this privacy

   Note that these requirements are necessary, but may not be
   sufficient.  Connections can potentially be correlated by observing
   packet timings and other characteristics.

4.3.  Security

   Path creation mechanisms can potentially be abused to enable denial
   of service against third parties.  A rogue client could for example
   send path creation packets in which the source address is spoofed,
   and points to a third party.  If the server accepts the path, it
   could then direct a stream of packets to that third party,
   contributing to a denial of service attack.

   Similar attacks could be mounted by "man-in-the-middle" or "man-on-
   the-side" attackers, would could capture a legitimate client packet
   and replay it with a different source address.

   These attacks are discussed in the transport draft
   [I-D.ietf-quic-transport] in the context of connection migration.
   The proposed solution is to have an explicit handshake in which each
   peer demonstrates that it can send and receive packets on the
   proposed path.

4.4.  Non Detectability

   Lots of efforts are being made in the design of QUIC to prevent
   "ossification", which can happen if middleboxes somehow start parsing
   QUIC packets and dropping them if some parameter does not have an
   expected value.  Ideally, we would like to encrypt the entire packet,

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   including its header, to prevent such mistreatments, but some fields
   like connection ID or sequence numbers still have to be exposed.  If
   a multipath extension exposed new packet header fields, it would run
   the risk of failing in the presence of already ossified middleboxes,
   or conversely of creating an expectation in middleboxes that would
   increase the risk of ossification.

   To prevent such risks, the multipath extensions should use pretty
   much the same packet header formats as QUIC V1.  (The experiment
   described in [I-D.deconinck-multipath-quic] was not attempting to
   meet this requirement.)

4.5.  Error Detection

   In QUIC, errors are detected by observing holes in the received
   acknowledgements.  A packet will be declared lost if it is not
   acknowledged by the receiver, while later transmitted packets are.
   We could design multipath extensions to operate error detection
   either separately on each path, or globally on the entire set of

   Global processing require that the sequence numbers be coordinated
   between the different paths, e.g. sending packets 1, 2, 3 on Path 1,
   then packets 4, 5, 6 on Path 2, etc.  In order to meet the
   requirements asserted in Section 4.2, the packet sequence numbers
   will have to be encrypted, as explained in Section 4.9.

   If a global sequence number is used, implementations can manage a
   single packet retransmission queue for packets sent to all paths.
   The ACK format currently defined in QUIC can be retained.  However,
   we may expect that different paths have different latency and
   different queuing delays, which will manifest itself as a large
   amount of out of sequence deliveries and ACK frames carrying many
   ranges.  This may make error detection less efficient, but
   implementations can regain efficiencies by keeping track of which
   packet was sent on what path, and then doing smarter treatment.  For
   example, instead of marking a packet lost after a higher sequenced
   packet was acknowledged on any path, implementations could do that
   only if the higher sequenced packet was acknowledged on the same

   Independent processing assumes that there will be independent packet
   sequence numbers on each space.  For example, there may be packets 1,
   2, 3 on Path 1, and a different set of packets 1, 2, 3 on Path 2.
   Each sequence number would of course start at a different random
   offset to break correlation.  Independent numbering solves the
   privacy issues caused by packet number linkability, but causes
   potential problems with AEAD encryption, as explained in Section 4.8.

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   If independent sequence numbers are used for each path,
   implementations will have to maintain separate retransmission queues
   for each path.  ACK frames will need to be specific to a given path,
   either because they are sent on that path, or because they carry an
   identifier of the path (the solution adopted in
   [I-D.deconinck-multipath-quic]).  The "identifier" variant allows
   acknowledgements for one path to be sent on another path, which may
   prove very useful when dealing with some error conditions.

4.6.  Congestion Control

   We assume that different "QUIC paths" will be routed over different
   network paths.  Each path will be composed of links of different
   latency and capacity, and will go through routers experiencing
   different levels of load.  There are potential exceptions, such as
   one path using IPv6 addresses and another using IPv4 addresses when
   these addresses in fact correspond to the same network interfaces and
   will be routed similarly -- as pointed in
   [I-D.deconinck-multipath-quic].  But even with this IPv4/IPv6
   example, chances are that some network elements will perform
   differently, for example by performing Network Address Translation
   for IPv4.

   Since different paths experience different network conditions, it
   follows that congestion control should be executed separately for
   each path.  We should note that the experimental congestion control
   for MTCP [RFC6356] tries to meet three separate goals:

   o  Goal 1 (Improve Throughput) A multipath flow should perform at
      least as well as a single path flow would on the best of the paths
      available to it.

   o  Goal 2 (Do no harm) A multipath flow should not take up more
      capacity from any of the resources shared by its different paths
      than if it were a single flow using only one of these paths.  This
      guarantees it will not unduly harm other flows.

   o  Goal 3 (Balance congestion) A multipath flow should move as much
      traffic as possible off its most congested paths, subject to
      meeting the first two goals.

   Having separate instances of congestion control on each path will
   meet the second goal, and will also meet the first goal if combined
   with proper scheduling of frames on different paths.  However, purely
   independent control of each path is not sufficient to meet the third
   goal, which is why "coupled congestion control" is introduced in
   [RFC6356].  This is largely a research issue.  It is likely that
   initial deployments of multipath QUIC will start with a coupled

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   congestion control similar to [RFC6356], before exploring

   Congestion control algorithms require measuring acknowledgements,
   losses and delays on each specific path.  This happens naturally if
   independent sequence numbers are used for each path.  It will also
   happen if the solution uses a global sequence number space, as long
   as the congestion control algorithm is aware of the path associated
   with the acknowledgement or loss of individual packet, or with delay

4.7.  Flow Control

   Congestion control is a voluntary algorithm performed independently
   by each peer.  It does not protect against misbehaving peers that
   would send too much data and overwhelm the receiver.  In the first
   version of QUIC, this protection is ensured by connection level flow
   control, using MAX DATA frames, and stream level flow control, using
   MAX STREAM DATA frames.

   At some theoretical level, one may worry that the connection level
   MAX DATA frame may not provide sufficient protection against
   excessive use of individual paths.  For example, in a migration
   scenario, a low capacity path may inherit large credits from MAX DATA
   frames received on a high capacity path.  If this turned out to be a
   real problem, peers would need to manage an independent flow control
   limit on each path, perhaps using a multipath specific MAX PATH DATA

   On the other hand, we can observe that MTCP uses the same flow
   control window for each separate path.  This was done after
   considering using different windows for different paths, as explained
   in [NSDI12].  If the same window is used for all paths, we have the
   guarantee that should one path fail, all packets sent on that path
   can be retransmitted on any of the other paths, without having to
   wait for flow control updates.

   Based on the MTCP experience, we should assume that in the first
   versions of multipath QUIC, flow control will remain a global "stream
   level" function.

4.8.  Packet Encryption

   QUIC packets are protected by AEAD encryption.  The security of the
   AEAD algorithm depends on never using the same nonce for a given key.
   If the same nonce was used twice, the encryption algorithm would
   repeat the same "stream cipher" twice, and simple XOR could provide
   cues on the packet's content.  In QUIC V1, the nonce uniqueness

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   requirement is met by mixing the packet sequence number with an
   otherwise preset IV.  Since each packet carries a different sequence
   number, the nonce never repeats.

   If different paths use uncoordinated sequence numbers, there is a
   risk that at some points the numbers on one path repeat numbers
   previously used on another path.  If all paths used the same
   encryption key, this would lead to AEAD failure.  To avoid that
   failure, different paths will have to use either different keys or
   otherwise ensure the uniqueness of nonces.

   Different encryption keys per path could be obtained in various ways,
   such as mixing the path specific connection ID with a master secret,
   or considering that different paths use different iterations of the
   secret expansion algorithm.  However, it can be argued that this
   management of multiple keys significantly increases the complexity of
   the protocol, especially if we consider the management of key

   Instead of just mixing the packet sequence number with the preset
   AEAD IV, implementations could create nonces by mixing the IV with
   both the path specific connection ID and the packet sequence number.
   This would provide for nonce uniqueness across multiple paths,
   without imposing significant management burden.  It may require
   changing the API to the AEAD encryption in some stacks, but the
   change is trivial.

4.9.  Sequence Number Encryption

   Privacy issues arise if the same sequence number space is used across
   multiple paths, as adversaries can observe the numbers and use them
   to link otherwise independent paths.  This privacy issue is mitigated
   if different paths use different sequence number spaces, but it can
   also be mitigated if the packet sequence numbers are encrypted.

   Packet sequence number encryption is not commonly practiced, but it
   is easy to imagine.  For example, implementation could first encrypt
   the packet's payload using AEAD, and then derive a unique masking
   number by computing the keyed hash or the encryption of the last 16
   bytes in the encrypted payload, and then XOR the sequence number
   field in the packet header with that masking number.

   On the other hand, on-path diagnostic tools currently observe the
   packet numbers to detect transmission errors and thus potential
   network level issues.  Encrypting the packet numbers would impede
   these tools.

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4.10.  Key Phases

   It is generally considered unsafe to encrypt "too much" data with a
   single encryption key.  QUIC supports a key update mechanism.  Short
   packet headers include a KEY_PHASE bit that "allows for refreshes of
   keying material by either peer".

   If the same encryption key is used for all paths, the key updates
   will happen at the same time for all different paths.  An adversary
   might be able to observe the simultaneous changes in the KEY_PHASE
   bit and link otherwise uncorrelated paths.

   This KEY_PHASE bit linkability is a privacy issue, but whether it is
   important is debatable.  For client or server migration scenarios,
   the issue can be easily mitigated by not engaging in key updates
   during a migration, but for load balancing scenarios this simple
   mitigation will not suffice.  Other mitigations could include
   delaying the key transition by random values on different paths, or
   performing double transitions on one path while the other paths
   remain silent.

   The KEY_PHASE bit linkability does not occur if different paths use
   independently derived key materials, which is one of the option
   suggested in Section 4.8.

4.11.  Key and sequence numbers per path

   Our initial analysis pointed to two possible designs, a global
   sequence packet sequence number common to all paths, or separate
   packet sequence numbers for each path.  After reviewing the
   consequences, it is clear that the simplest design is to use
   different keys and separate sequence numbers for each path.  This is
   consistent with the use of a separate CONNECTION-ID for each path.
   It does not require encrypting the sequence number, and thus does not
   impede develoment of network quality monitoring tools.  It can also
   provide an alternative to the KEY_PHASE mechanism: if too much data
   was sent on a path and new keying material is needed, just open a new

5.  Next Steps

   The first version of QUIC is progressing steadily, and engaging on
   multipath discussion might detract from that progress.  However, it
   seems that parity with GQUIC requires handling at least the client
   migration scenario, in addition to NAT rebinding.  As soon as we
   start working on that scenario, we will have to tackle a number of
   multipath related issues.

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   The early design of connection migration mechanisms privilege the
   simplest multipath options, and in particular the use of a single
   sequence number space for the old and new streams.  This is fine, but
   we should probably start considering several of the issues listed in
   Section 4.  In particular:

   o  the standard should mandate the use of different connection ID for
      different paths, as explained in Section 4.2

   o  implementations should consider the use of "path tags" for packets
      in the retransmission queue, as explained in Section 4.5.

   o  implementations should manage separate congestion control contexts
      for each path, as explained in Section 4.6.

   o  the standard should consider simple ways to associate different
      encryption contexts with each path and each connection-ID.

6.  Acknowledgements

   Many of the issues in this draft were already debated in the QUIC
   working group.  Thanks to Yoshifumi Nishida and Martin Thomson for
   their constructive comments.  Thanks also to Olivier Bonaventure and
   Quentin De Coninck for detailed feedback on the first version of this
   draft, and for many references to the experience gained with MTCP.

7.  Informative References

              Coninck, Q. and O. Bonaventure, "Multipath Extension for
              QUIC", draft-deconinck-multipath-quic-00 (work in
              progress), October 2017.

              Bishop, M., "Hypertext Transfer Protocol (HTTP) over
              QUIC", draft-ietf-quic-http-08 (work in progress),
              December 2017.

              Iyengar, J. and I. Swett, "QUIC Loss Detection and
              Congestion Control", draft-ietf-quic-recovery-08 (work in
              progress), December 2017.

              Thomson, M. and S. Turner, "Using Transport Layer Security
              (TLS) to Secure QUIC", draft-ietf-quic-tls-08 (work in
              progress), December 2017.

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              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-08 (work
              in progress), December 2017.

   [NSDI12]   Raicius, C., Paasch, C., Barre, S., Ford, A., Honda, M.,
              Duchene, F., Bonaventure, O., and M. Handley, "How hard
              can it be? designing and implementing a deployable
              multipath TCP", NSDI'12 Proceedings of the 9th USENIX
              conference on Networked Systems Design and Implementation,
              April 2012,

   [RFC6356]  Raiciu, C., Handley, M., and D. Wischik, "Coupled
              Congestion Control for Multipath Transport Protocols",
              RFC 6356, DOI 10.17487/RFC6356, October 2011,

   [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,

   [RFC8041]  Bonaventure, O., Paasch, C., and G. Detal, "Use Cases and
              Operational Experience with Multipath TCP", RFC 8041,
              DOI 10.17487/RFC8041, January 2017,

Author's Address

   Christian Huitema
   Private Octopus Inc.
   Friday Harbor  WA  98250


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