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Versions: 00 draft-ietf-mptcp-rfc6824bis

MPTCP Working Group                                       O. Bonaventure
Internet-Draft                                                B. Hesmans
Updates: 6824 (if approved)                                    UCLouvain
Intended status: Experimental                               M. Boucadair
Expires: December 18, 2015                                        Orange
                                                           June 16, 2015


                   Experimental Multipath TCP option
                 draft-bonaventure-mptcp-exp-option-00

Abstract

   This document proposes a generic format for an experimental Multipath
   TCP option.  This experimental option is expected to replace the
   option type that was reserved for private use in [RFC6824].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on December 18, 2015.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  The Experimental Multipath TCP option . . . . . . . . . . . .   3
   3.  Security considerations . . . . . . . . . . . . . . . . . . .   6
   4.  IANA considerations . . . . . . . . . . . . . . . . . . . . .   6
   5.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Multipath TCP is an extension to TCP [RFC0793] that was specified in
   [RFC6824].  Like other TCP extensions, Multipath TCP relies on TCP
   options to exchange information.  To mitigate some forms of middlebox
   interference, Multipath TCP uses only one TCP option identified by
   Kind=30 [RFC6824].  TCP Option Kind=30 has a variable length and
   includes a subtype field that can support up to 16 different
   Multipath TCP options.  As of this writing, 8 different Multipath TCP
   options are defined [RFC6824] :

   o  subtype=0x0 : MP_CAPABLE

   o  subtype=0x1 : MP_JOIN

   o  subtype=0x2 : DSS

   o  subtype=0x3 : ADD_ADDR

   o  subtype=0x4 : REMOVE_ADDR

   o  subtype=0x5 : MP_PRIO

   o  subtype=0x6 : MP_FAIL

   o  subtype=0x7 : MP_FASTCLOSE

   In addition, subtype=0x8 is already reserved for a new version of the
   ADD_ADDR option [I-D.ietf-mptcp-rfc6824bis] and subtype=0xf is
   reserved for use in private testbeds [RFC6824].  There are thus less
   than six different Multipath TCP options that are available to extend
   the protocol.

   The Multipath TCP options defined above cover the basic
   functionalities for Multipath TCP but previous experience with TCP



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   indicates that it can be expected that Multipath TCP implementors
   will propose new extensions that require a new option subtype.  Some
   authors have already proposed such extensions
   [I-D.boucadair-mptcp-symmetric] [I-D.paasch-mptcp-syncookies]
   [I-D.xu-mptcp-prmp].  Given the small number of available Multipath
   TCP options, it will be difficult to officially reserve them for all
   proposed utilisations even if experiments with these new Multipath
   TCP options would probably be useful.

   This document proposes a shared use of the Multipath TCP option
   subtype=0xf following a similar approach as described in [RFC6994]
   for TCP.  This approach is efficient as it encourages conducting new
   experiments while not consuming the limited option codepoint space.

2.  The Experimental Multipath TCP option

   The extensibility of TCP depends largely on the definition of new TCP
   options.  As of this writing, 30 different TCP options have been
   assigned by the IANA among the 256 possible values.  Two of these
   option types (253 and 254) have been reserved as experimental options
   in [RFC4727].  However, this document did not specify how these
   experimental options should be used.  [RFC6994] clarified the
   situation by proposing two encodings for these experimental options
   to support a shared use of the same option kind by different
   experimental options.  Basically, [RFC6994] proposes to use a 16 bits
   experiment identifier in the first two bytes of the option as shown
   in the figure below.

      0          1          2          3
      01234567 89012345 67890123 45678901
      +--------+--------+--------+--------+
      |  Kind  | Length |       ExID      |
      +--------+--------+--------+--------+
      |     option contents...
      +--------+--------+---...


   [RFC6994] extends this format to support 32 bits identifiers where
   the second 16 bits word is used to encode a magic number.

      0          1          2          3
      01234567 89012345 67890123 45678901
      +--------+--------+--------+--------+
      |  Kind  | Length |       ExID      |
      +--------+--------+--------+--------+
      |   ExID (con't)  |  option contents...
      +--------+--------+--------+---




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   The experimental TCP options defined in [RFC6994] could in theory be
   used over any TCP connection, including the subflows used by
   Multipath TCP.  However, using them as such to support experimental
   features on a Multipath TCP connection would cause two practical
   problems.  The first problem is that the utilisation of such an
   option should be negotiated during the three-way handshake by adding
   the experimental option in the SYN and SYN+ACK segments.  Given the
   limited space for the options in these segments when Multipath TCP is
   used and other options are already used, this is not a valid
   approach.  Second, middleboxes could accept the Multipath TCP option
   and drop or modify an experimental TCP option that is used to support
   an experimental Multipath TCP feature.  This would increase the
   complexity of the middlebox handling algorithms inside a Multipath
   TCP implementation.

   For the above two reasons, we propose the utilisation of an
   Experimental Multipath TCP option that is encoded as shown in the
   figure below.

                           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
      +---------------+---------------+-------+-----------------------+
      |     Kind      |    Length     |Subtype| Flags |  Experiment   |
      +---------------+---------------+-------+-------+---------------+
      | Id. (16 bits) | Subtype-specific data  (variable length)  ...
      +----------------------------------------------------------- ...


   Flags is a set of 4 flags:

      +-+-+-+-+
      |S|U|r|r|
      +-+-+-+-+

   where "rr" are for future assignment as additional flag bits.  r bits
   MUST each be sent as zero and MUST be ignored on receipt.

   The Experimental Multipath TCP option has a variable length.  It is
   always of kind=30 and Subtype=0xf.  The Experiment identifier is a 16
   bits integer that shall be assigned by using the same procedure as
   defined in [RFC6994].  [RFC6994] also supports the utilisation of 32
   bits Experiment Identifiers.  These 32 bits identifiers are composed
   of a 16 bits identifier assigned on a FCFS basis and a 16 bits magic
   number.  At this stage, we propose that the Experimental Multipath
   TCP option uses only the 16 bits Experiment Identifier.

   The two high order flags that are included in the Experimental
   Multipath TCP option have the following semantics :



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   o  "S" flag (highest order bit) : This is the synchronising bit.
      When set to 1, it indicates that the host sending this option
      expects a reply from the remote host with an option having the
      same experiment identifier, but possibly containing other data.

   o  "U" flag (second highest order bit) : When set to 1, this flag
      indicates that the experimental option was received by the sending
      host but it was unable to parse it.

   The two low order flags are currently reserved for further use.  They
   MUST be set to zero when sending and ignored upon reception.

   To use the Experimental Multipath TCP option with a given experiment
   identifier over a Multipath TCP connection, the sending host must
   first verify the ability of the remote host to support this
   particular Experimental option.  For this, it first sends in any
   valid TCP segment, including a duplicate acknowledgement, an
   Experimental Multipath TCP option with the "S" flag set.  Upon
   reception of this option, the receiving host will verify whether it
   supports it.  If yes, it shall return a TCP segment that contains the
   experimental option with the same identifier and the "S" and the "U"
   flags reset.  This option may contain additional data depending on
   its semantics.  If the receiving host does not recognise the
   experimental option that it has received, it shall return a TCP
   segment that contains the received experimental option with the "S"
   flag set to 0 and the "U" flag set to 1.  Upon reception of this
   option, the sending host knows that the experimental option cannot be
   used over this Multipath TCP connection.

   Once the utilisation of a specific Experimental Multipath TCP option
   has been negotiated over a Multipath TCP connection, both hosts can
   use this experimental Multipath TCP option.  If a host receives an
   Experimental Multipath TCP option with the "U" flag set to 0 that it
   does not support or contains information that it cannot parse, it
   shall return the exact option that it received with the "U" flag set
   to indicate the error to the remote host.  If an invalid option is
   received with the "U" flag set to 0, it must be silently discarded.

   Future documents specifying new experimental MPTCP options should
   specify the extract semantic of the Subtype-specific data and whether
   additional validation operations are to be followed at both sides.
   Note, data can be included in an experimental data concurrently with
   the capability check (S/U).

   As a side effect of the use of a shared codepoint, an overhead of the
   2 octets will be consumed in the option space for each experimental
   MPTCP option.  This overhead should be taken into account when
   drawing conclusions related to the co-existence of multiple MPTCP



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   experimental options or, even worse, between experimental MPTCP
   options and experimental TCP options.

3.  Security considerations

   This document does not modify the security considerations for
   Multipath TCP.

4.  IANA considerations

   This document proposes to modify the Multipath TCP option subtype 0xf
   that was reserved for utilisation in private testbed to support the
   Experimental Multipath TCP option.

   The same considerations as for [RFC6994] apply.  IANA should create a
   "Multipath TCP Experimental Option Identifiers (MPTCP ExIDs)"
   registry.  This registry contains the 16 bits ExIDs and a reference
   (description, document pointer, or assignee name and e-mail contact)
   for each entry.  MPTCP ExIDs are assigned on a First Come, First
   Served (FCFS) [RFC5226].

   IANA will advise applicants of duplicate entries to select an
   alternate value, as per typical FCFS processing.

   IANA will record known duplicate uses to assist the community in both
   debugging assigned uses as well as correcting unauthorized duplicate
   uses.

   IANA should impose no requirement on making a registration other than
   indicating the desired codepoint and providing a point of contact.  A
   short description or acronym for the use is desired but should not be
   required.

5.  Conclusion

   This document has proposed a new experimental Multipath TCP option
   that is intended to replace the already defined, but apparently
   currently unused, Multipath TCP option reserved for private testbed.
   This new Experimental Multipath TCP option will enable implementors
   to perform experiments with Multipath TCP options without using the
   limited Multipath TCP option subtypes.

6.  Acknowledgements

   This work was partially supported by the FP7-Trilogy2 project.  This
   document was heavily inspired by [RFC6994].





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7.  References

7.1.  Normative References

   [RFC6824]  Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
              "TCP Extensions for Multipath Operation with Multiple
              Addresses", RFC 6824, January 2013.

7.2.  Informative References

   [I-D.boucadair-mptcp-symmetric]
              Boucadair, M. and C. Jacquenet, "An Extension to MPTCP for
              Symmetrical Sub-Flow Management", draft-boucadair-mptcp-
              symmetric-02 (work in progress), March 2015.

   [I-D.ietf-mptcp-rfc6824bis]
              Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
              "TCP Extensions for Multipath Operation with Multiple
              Addresses", draft-ietf-mptcp-rfc6824bis-04 (work in
              progress), March 2015.

   [I-D.paasch-mptcp-syncookies]
              Paasch, C., Biswas, A., and D. Haas, "Making Multipath TCP
              robust for stateless webservers", draft-paasch-mptcp-
              syncookies-00 (work in progress), April 2015.

   [I-D.xu-mptcp-prmp]
              Xu, C., Huang, H., Zhang, H., Xiong, C., and L. Zhu,
              "Multipath Transmission Control Protocol (MPTCP) Partial
              Reliability Extension", draft-xu-mptcp-prmp-00 (work in
              progress), April 2015.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, September 1981.

   [RFC4727]  Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4,
              ICMPv6, UDP, and TCP Headers", RFC 4727, November 2006.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC6994]  Touch, J., "Shared Use of Experimental TCP Options", RFC
              6994, August 2013.







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Authors' Addresses

   Olivier Bonaventure
   UCLouvain

   Email: Olivier.Bonaventure@uclouvain.be


   Benjamin Hesmans
   UCLouvain

   Email: Benjamin.Hesmans@uclouvain.be


   Mohamed Boucadair
   Orange

   Email: mohamed.boucadair@orange.com

































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