Internet DRAFT - draft-xie-tsvwg-sctp-nat

draft-xie-tsvwg-sctp-nat



Network Working Group                                             Q. Xie
Internet-Draft                                                  Motorola
Expires: January 7, 2005                                      R. Stewart
                                                     Cisco Systems, Inc.
                                                             M. Holdrege
                                                           Strix Systems
                                                            July 9, 2004



                   SCTP NAT Transverse Considerations
                    draft-xie-tsvwg-sctp-nat-00.txt


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   Copyright (C) The Internet Society (2004).  All Rights Reserved.


Abstract


   This document provides guidelines and solutions for dealing with SCTP
   association transversing NAT and similar middleboxes.








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Table of Contents


   1.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  SCTP NAT Transverse Scenarios  . . . . . . . . . . . . . . . .  3
     3.1   Single Point Transvers . . . . . . . . . . . . . . . . . .  3
     3.2   Multi-point Traverse . . . . . . . . . . . . . . . . . . .  4
   4.  Considerations for SCTP NAT transverse . . . . . . . . . . . .  4
     4.1   Simple NAT enhancement to support singly-homed SCTP
           associations . . . . . . . . . . . . . . . . . . . . . . .  4
     4.2   NAT enhancement to support multi-homed SCTP
           associations through a single traverse point . . . . . . .  5
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  5
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   6.1   Normative References . . . . . . . . . . . . . . . . . . . .  6
   6.2   Informative References . . . . . . . . . . . . . . . . . . .  6
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . .  6
       Intellectual Property and Copyright Statements . . . . . . . .  8


































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1.  Conventions


   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when
   they appear in this document, are to be interpreted as described in
   [2].


2.  Introduction


   It is the job of Network Address Translators (NAT) [3] and
   middleboxes [6] that utilize a NAT function to manipulate address and
   port information in the IP header.  This poses a challenge for hosts
   that attempt to use certain end-to-end protocols [5].  This issue has
   drawn increasingly wide attention from the IP development and service
   community and much work has been done to ameliorate the situation for
   UDP, TCP and other protocols.


   The same issue not only exists for SCTP [4], but also may become a
   more difficult issue when SCTP associations are multi-homed.  This
   document provides guidelines and solutions for dealing with SCTP and
   NAT traversal.  In the following discussion, we will simply refer to
   NAT as a function, but note that many types of middleboxes employ NAT
   functions.


3.  SCTP NAT Transverse Scenarios


3.1  Single Point Transvers


   In this case, all packets in the SCTP association go through a single
   NAT, as shown below:


      +------+          +-----+          +------+
      |SCTP  |==========| NAT |==========|SCTP  |
      |end A |          +-----+          |end B |
      +------+                           +------+


   A variation of this case is shown below, i.e., multiple NATs in a
   single transverse path:


      +------+      +-----+     +-----+       +------+
      |SCTP  |======| NAT |=::==| NAT |=======|SCTP  |
      |end A |      +-----+     +-----+       |end B |
      +------+                                +------+


   The two SCTP endpoints in this case can be either singly-homed or
   multi-homed.  However, the important thing is that the NAT (or NATs)
   in this case sees ALL the packets of the SCTP association.





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   In this single traverse point scenario, we must acknowledge that
   while one of the main benefits of SCTP multi-homing is redundant
   paths, the NAT function represents a single point of failure in the
   path of the SCTP multi-home association.  However, the rest of the
   path may still benefit from path diversity provided by SCTP
   multi-homing.


3.2  Multi-point Traverse


   This case involves multiple NATs and each NAT only sees some of the
   packets in the SCTP association.  An example is shown below:


                     +------+
                /====|NAT A |====\
      +------+ /     +------+     \ +------+
      |SCTP  |/       ...          \|SCTP  |
      |end A |\                    /|end B |
      +------+ \     +------+     / +------+
                \====|NAT B |====/
                     +------+


   This case does NOT apply to a singly-homed SCTP association (i.e.,
   BOTH endpoints in the association use only one IP address).  The
   advantage here is that the existance of multiple NAT traverse points
   can preserve the path diversity of a multi-homed association for the
   entire path.  This in turn can improve the robustness of the
   communication.


   To make this work, however, all the NATs involved must recognize the
   packets they see as belonging to the same SCTP association and
   perform address translation in a consistent way.  This can only be
   possible if there exists some inter-NAT protocol which can provide
   coordination amongst the NATs when the association is set up.
   Further study is needed for this scenario.  In the rest of this
   document, we will only discuss the single traverse point scenario.


4.  Considerations for SCTP NAT transverse


   In any type of traverse, the NAT must understand the SCTP protocol.
   Since SCTP is relatively new (compared to UDP or TCP), some older
   existing NATs that are capable of handling UDP or TCP traverse will
   need to be enhanced for SCTP.  In this section we discuss what
   considerations should be made for that NAT enhancement.


4.1  Simple NAT enhancement to support singly-homed SCTP associations


   In a singly-homed SCTP association, each endpoint uses only one IP
   address and the association will always go through a single NAT




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   traverse point.  In such cases, the enhancement needed for a NAT to
   support SCTP is relatively simple: 1) recognize the protocol number
   for SCTP, and 2) locate and change the transport ports (if needed).
   The location of the transport port numbers are the same as both TCP
   and UDP so these should be minor changes for NAT software.


   It is important that the endpoints not to list the address again
   within the INIT or INIT-ACK when setting up the association.


   Note, the use of a single address by the endpoint may be the result
   of either the endpoint only has a single address assigned to the host
   or due to user of the SCTP stack only specifies to bind one address
   to the endpoint.


4.2  NAT enhancement to support multi-homed SCTP associations through a
    single traverse point


   At the association setup, the NAT needs to recognize and intercept
   the INIT, or INIT-ACK chunk.  The NAT will look into the chunk and
   find out all the listed IP addresses (including the source IP address
   in the packet's IP header).  The NAT will map this address list to a
   _list_ of external addresses.  Then, the NAT will reconstruct the
   INIT or INIT-ACK chunk, fill it with external addresses, re-calculate
   the checksum, and finally forward the packet on to the peer.  This
   approach is transparent to the endpoints and can support
   multi-homing.


   The NAT function is capable of variety in translating one to many or
   many to one addresses in the chunk.  Depending on network
   architectural requirements, NAT administrators may choose to map many
   internal addresses to a single external address.  It should be noted
   that this will reduce path diversity in a portion of the network.
   Alternatively, by mapping 1 internal address to many external
   addresses can increase the path diversity in a portion of the
   network.


   Editor's Note: Add discussion about ASCONF handling by the NAT.


   Editor's Note: Add some scenario examples.


5.  Security Considerations


   See RFC XXXX on SCTP security considerations.  See RFC XXXX on NAT
   security considerations.  See RFC XXXX for IAB comments on NAT.


6.  References






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6.1  Normative References


   [1]  Bradner, S., "The Internet Standards Process -- Revision 3", BCP
        9, RFC 2026, October 1996.


   [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.


6.2  Informative References


   [3]  Srisuresh, P. and M. Holdrege, "IP Network Address Translator
        (NAT) Terminology and Considerations", RFC 2663, August 1999.


   [4]  Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer,
        H., Taylor, T., Rytina, I., Kalla, M., Zhang, L. and V. Paxson,
        "Stream Control Transmission Protocol", RFC 2960, October 2000.


   [5]  Holdrege, M. and P. Srisuresh, "Protocol Complications with the
        IP Network Address  Translator", RFC 3027, January 2001.


   [6]  Swale, R., Mart, P., Sijben, P., Brim, S. and M. Shore,
        "Middlebox Communications (midcom) Protocol Requirements", RFC
        3304, August 2002.



Authors' Addresses


   Qiaobing Xie
   Motorola, Inc.
   1501 W. Shure Drive, 2-F9
   Arlington Heights, IL  60004
   US


   Phone: +1-847-632-3028
   EMail: qxie1@email.mot.com



   Randall R. Stewart
   Cisco Systems, Inc.
   8725 West Higgins Road
   Suite 300
   Chicago, IL  60631
   USA


   Phone: +1-815-477-2127
   EMail: rrs@cisco.com






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   Matt Holdrege
   Strix Systems
   Suite 110, 26610 Agoura Road
   Calabasas, CA  91302
   USA


   EMail: matt@strixsystems.com













































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Acknowledgment


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   Internet Society.





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