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Versions: 00                                                            
Network Working Group                                         M. Thomson
Internet-Draft                                                   Mozilla
Updates: 7983 (if approved)                            December 03, 2017
Intended status: Informational
Expires: June 6, 2018

           Principles for Multiplexing of UDP-based Protocols


   The existence of protocols that rely on multiplexing of different
   protocols could be used to justify the imposition of constraints on
   the operation of other protocols.  The existence of a multiplexed
   protocol does not inherently justify constraints on protocols that
   participate or might participate in the protocol.

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
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   This Internet-Draft will expire on June 6, 2018.

Copyright Notice

   Copyright (c) 2017 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
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   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

1.  Introduction

   The use of multiple protocols that share a 5-tuple is possible in
   unordered transport like UDP.  Multiplexing in this fashion creates a
   meta-protocol that is comprised of the set of protocols that are

   A specific example of such a meta-protocol is documented in RFC 7983
   [RT-MUX].  This protocol comprises up to 5 different protocols: STUN
   [STUN], ZRTP [ZRTP], DTLS [DTLS], TURN channels [TURN], and RTP
   [RTP].  The meta-protocol that is composed from this set of protocols
   is commonly deployed for use in real-time communications.  Of
   particular note is the STUN protocol [STUN], which is specifically
   designed to be multiplexed with another protocol.

   The existence of a multiplexed meta-protocol can be used to justify
   constraints on the definition of new protocols, or the addition of
   changes to protocols that participate.  These constraints should be
   considered in the context of the protocol in use.  A protocol design
   for use outside of a multiplexed context should not be unduly
   constrained by the limitations of a chosen multiplexing scheme.

2.  Multiplexing is Almost Always Possible

   Any protocol that includes integrity checks can be multiplexed with
   any other protocol.  A sufficiently strong checksum or cryptographic
   authentication tag allows arriving datagrams to be rejected by any
   protocol that the datagram was not intended for with high

   New protocols often require confidentiality and integrity and so use
   a solution like TLS [TLS].  Other protocols benefit greatly from
   being robust against errors in the relatively weak checksum
   [CHECKSUM] provided by internet protocols [CHECKSUM-FAIL].  Thus, the
   inclusion of strong integrity checks is beneficial for any internet
   protocol, which makes the protocol highly likely to be classified by
   a recipient as intended.

   Note:  This does not mean that protocols are distinguishable to
      intermediaries.  Protocols that include use keyed integrity checks
      will not be identifiable to entities that do not have access to

   Relying on an integrity check also probabilistic, meaning that
   shorter integrity checks increase the chance that a datagram could be

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   incorrectly classified by a recipient.  While the probability of
   collision is negligible for a protocol that uses an integrity check
   with 128 bits or more of redundancy, a shorter integrity check could
   be vulnerable to collisions and mis-classification.

   As long as no more than one participating protocol lacks an integrity
   check of sufficient strength, protocols can be demultiplexed with
   high reliability.  However, this form of demultiplexing can be
   grossly inefficient, as every datagram needs to be validated once for
   each potential protocol that is in use.

3.  Multiplexing Considerations

   Practical multiplexing schemes rely on simpler and more direct
   differences between protocols.  For instance, RFC 7983 [RT-MUX]
   separates protocols based on the value of the first octet.  While
   this scheme is cheap and effective, it has the drawback of using a
   limited space of potential values.  Of the 256 possible values for
   the first octet, RFC 7983 consumes 132 values.  Adding more protocols
   to the set that can be multiplexed would reduce the available space

   The design in RFC 7983 was initially opportunistic in nature.  The
   original set of protocols that were selected included only DTLS,
   SRTP, and STUN.  Since then, compatibility with that scheme has
   constrained the design of other protocols so that they fit within
   this scheme.  This is not an indefinitely sustainable posture.

   The need to multiplex can create unwanted constraints on the designs
   of protocols, particularly as protocols evolve.  For a protocol that
   is used exclusively or predominantly in a multiplexed meta-protocol,
   this does not present a significant burden, but protocols that have
   uses in other contexts are different.

   For instance, DTLS 1.3 [DTLS13] defines a record layout that uses the
   first octet in a very different fashion to earlier versions; this
   design is constrained by the need to remain compatible with the
   scheme in RFC 7983 [RT-MUX].

   Multiplexing should not be a primary consideration in design of new
   protocols that might be multiplexed.  Similary, the design of
   extensions or revisions to protocols should not be constrained by the
   potential for multiplexing.  Multiplexing considerations might be
   paid greater attention depending on how likely it is that the
   protocol will be used together with other protocols.

   For example, a new version of RTP [RTP] might consider setting the
   version field to 3.  While this space in the multiplexing scheme in

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   [RT-MUX] is currently unallocated, changing the RTP version greatly
   reduces the space available for other protocols.  Because RTP is
   frequently used in a multiplexed context, changing the RTP header is
   best avoided.

3.1.  Alternative Multiplexing Designs

   Protocol multiplexing works most efficiently when the protocol in use
   for any given packet can be identified using only the contents of the
   packet.  Stateful multiplexing - where multiplexing rules change
   based on protocol state - is possible, but is best avoided.

   A protocol that intends to multiplex several protocols can avoid this
   sort of pressure by adding an explicit multiplexing protocol layer.
   The simplest multiplexing protocol is a shim of a small number of
   octets that is added to the start or end of every datagram.  The
   value of these octets is then used to identify the protocol.

   For a scheme like that in [RT-MUX], it is possible to use a shim for
   only some protocols, whether other protocols are multiplexed based on
   their inherent observable properties.

   Expanding the size of datagrams can have implications for protocol
   operation.  Trivially, it reduces the space in the path maximum
   transfer unit (PMTU) [PMTUv4], [PMTUv6] available to the multiplexed

   More substantially, the addition of octets to a protocol datagram -
   especially at the start - can also mean that the apparent format of
   datagrams changes.  If a multiplexed meta-protocol adds octets to the
   start of payloads, this reduces the degree to which the multiplexed
   meta-protocol can be identified correctly.  Some middleboxes have
   been shown to observe and discriminate based on the content of flows,
   even when large parts of a flow is encrypted and therefore
   inaccessible to the middlebox.  Octets added to the start of a
   datagram could cause middleboxes to apply different treatment to a
   protocol when it is multiplexed than when it is not.

4.  No Expectation of Successful Multiplexing

   A multiplexing scheme that relies on certain properties of a protocol
   cannot expect those properties to remain fixed as that protocol

   It's possible that the designers of a protocol will explicitly
   guarantee that certain properties won't change over time, such as the
   invariants defined in [QUIC-INVARIANTS], in which case a multiplexing
   scheme could be designed based on those guarantees.

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   Choosing a multiplexing scheme that relies on properties of a
   protocol remaining constant can either impose unwanted constraints on
   the design of a protocol that is selected for multiplexing, or it can
   cause some or all features of that protocol to be unusable in the
   multiplexed context when that protocol changes.

5.  Security Considerations

   The ability to use new protocol features can have a material impact
   on security if access to updated or added security-related features
   is prevented by an incompatibility with a chosen multiplexing scheme.

6.  IANA Considerations

   This document has no IANA actions.

7.  Informative References

              Braden, R., Borman, D., and C. Partridge, "Computing the
              Internet checksum", RFC 1071, DOI 10.17487/RFC1071,
              September 1988, <https://www.rfc-editor.org/info/rfc1071>.

              Stone, J. and C. Partridge, "When the CRC and TCP checksum
              disagree", Proceedings of the conference on Applications,
              Technologies, Architectures, and Protocols for Computer
              Communication - SIGCOMM '00, DOI 10.1145/347059.347561,

   [DTLS]     Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [DTLS13]   Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", draft-ietf-tls-dtls13-02 (work in progress), October

   [PMTUv4]   Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              DOI 10.17487/RFC1191, November 1990,

   [PMTUv6]   McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
              "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
              DOI 10.17487/RFC8201, July 2017,

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              Thomson, M., "Version-Independent Properties of QUIC",
              draft-thomson-quic-invariants-00 (work in progress),
              November 2017.

   [RT-MUX]   Petit-Huguenin, M. and G. Salgueiro, "Multiplexing Scheme
              Updates for Secure Real-time Transport Protocol (SRTP)
              Extension for Datagram Transport Layer Security (DTLS)",
              RFC 7983, DOI 10.17487/RFC7983, September 2016,

   [RTP]      Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <https://www.rfc-editor.org/info/rfc3550>.

   [STUN]     Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              DOI 10.17487/RFC5389, October 2008,

   [TLS]      Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [TURN]     Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
              Relays around NAT (TURN): Relay Extensions to Session
              Traversal Utilities for NAT (STUN)", RFC 5766,
              DOI 10.17487/RFC5766, April 2010,

   [ZRTP]     Zimmermann, P., Johnston, A., Ed., and J. Callas, "ZRTP:
              Media Path Key Agreement for Unicast Secure RTP",
              RFC 6189, DOI 10.17487/RFC6189, April 2011,

Author's Address

   Martin Thomson

   Email: martin.thomson@gmail.com

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