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Versions: 00 01 draft-ietf-tsvwg-natsupp

Network Working Group                                         R. Stewart
Internet-Draft                                                    Huawei
Intended status: Standards Track                               M. Tuexen
Expires: December 31, 2010                                  I. Ruengeler
                                      Muenster Univ. of Applied Sciences
                                                           June 29, 2010


Stream Control Transmission Protocol (SCTP) Network Address Translation
                                Support
                   draft-stewart-natsupp-tsvwg-01.txt

Abstract

   Stream Control Transmission Protocol [RFC4960] provides a reliable
   communications channel between two end-hosts in many ways similar to
   TCP [RFC0793].  With the widespread deployment of Network Address
   Translators (NAT), specialized code has been added to NAT for TCP
   that allows multiple hosts to reside behind a NAT and yet use only a
   single globally unique IPv4 address, even when two hosts (behind a
   NAT) choose the same port numbers for their connection.  This
   additional code is sometimes classified as Network Address and Port
   Translation or NAPT.  To date, specialized code for SCTP has NOT yet
   been added to most NATs so that only pure NAT is available.  The end
   result of this is that only one SCTP capable host can be behind a
   NAT.

   This document describes an SCTP specific chunks and procedures to
   help NAT's provide similar features of NAPT in the single point and
   multi-point traversal scenario.

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 December 31, 2010.




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Copyright Notice

   Copyright (c) 2010 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
   to this document.  Code Components extracted from this document must
   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.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Problem space overview . . . . . . . . . . . . . . . . . . . .  5
   5.  Handling of internal port number and verification tag
       collisions . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   6.  Handling of missing state  . . . . . . . . . . . . . . . . . .  7
     6.1.  Multi Point Traversal considerations . . . . . . . . . . .  8
     6.2.  Handling of internal port number collisions  . . . . . . .  8
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   8.  Security considerations  . . . . . . . . . . . . . . . . . . . 10
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 10
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 10
     10.2. Informative References . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11

















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1.  Introduction

   Stream Control Transmission Protocol [RFC4960] provides a reliable
   communications channel between two end-hosts in many ways similar to
   TCP [RFC0793].  With the widespread deployment of Network Address
   Translators (NAT), specialized code has been added to NAT for TCP
   that allows multiple hosts to reside behind a NAT and yet use only a
   single globally unique IPv4 address, even when two hosts (behind a
   NAT) choose the same port numbers for their connection.  This
   additional code is sometimes classified as Network Address and Port
   Translation or NAPT.  To date, specialized code for SCTP has NOT yet
   been added to most NATs so that only true NAT is available.  The end
   result of this is that only one SCTP capable host can be behind a
   NAT.

   This document describes an SCTP specific chunks and procedures to
   help NAT's provide similar features of NAPT in the single point and
   multi-point traversal scenario.  An SCTP implementation supporting
   this extension will follow these procedures to assure that in both
   single homed and multi-homed cases a NAT will maintian the proper
   state without needing to change port numbers.

   A NAT will need to follow these proceedures for generating
   appropriate SCTP packet formats.  NAT's should refer to
   xxxxbehavedraftxxx for the BCP in using these formats.

   When considering this feature it is possible to have multiple levels
   of support.  At each level, the Internal Host, External Host and NAT
   may or may not support the features described in this document.  The
   following table illustrates the results of the various combinations
   of support and if communications can occur between two endpoints.

      +---------------+------------+---------------+---------------+
      | Internal Host | NAT        | External Host | Communication |
      +---------------+------------+---------------+---------------+
      | Supports      | Supports   | Supports      | Yes           |
      | Support       | No Support | No Support    | None          |
      | Support       | Support    | No Support    | Limited       |
      | No Support    | No Support | Support       | None          |
      | Support       | No Support | Support       | None          |
      | No Support    | Support    | Support       | Limited       |
      | No Support    | No Support | No Support    | None          |
      +---------------+------------+---------------+---------------+

   From the table we can see that when a NAT does not support the
   extension no communication can occur.  This is for the most part the
   current situation i.e.  SCTP packets sent externally from behind a
   NAT are discarded by the NAT.  In some cases, where the NAT supports



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   the feature but one of the two external hosts does NOT support the
   feature communication may occur but in a limited way.  For example
   only one host may be able to have a connection when a collision case
   occurs.


2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].


3.  Terminology

   For this discussion we will use several terms, which we will define
   and point out in a figure.

   o  Private-Address (Priv-Addr) - The private address that is known to
      the internal host.

   o  Internal-Port (Int-Port) - The port number that is in use by the
      host holding the Private-Address.

   o  Internal-VTag (Int-VTag) - The Verification Tag that the internal
      host has chosen for its communication.  The VTag is a unique 32
      bit tag that must accompany any incoming SCTP packet for this
      association to the Private-Address.

   o  External-Address (Ext-Addr) - The address that an internal host is
      attempting to contact.

   o  External-Port (Ext-Port) - The port number of the peer process at
      the External-Address.

   o  External-VTag (Ext-VTag) - The Verification Tag that the host
      holding the External-Address has chosen for its communication.
      The VTag is a unique 32 bit tag that must accompany any incoming
      SCTP packet for this association to the External-Address.

   o  Public-Address (Pub-Addr) - The public address assigned to the NAT
      box which it uses as a source address when sending packets towards
      the External-Address.








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   Internal Network      |         External Network
                         |
             Private     | Public                    External
 +---------+ Address     | Address     /--\/--\      Address +---------+
 |  SCTP   |          +-----+         /        \             |  SCTP   |
 |end point|==========| NAT |======= | Internet | ========== |end point|
 |    A    |          +-----+         \        /             |    B    |
 +---------+ Internal    |             \--/\--/     External +---------+
  Internal      Port     |                             Port    External
    VTag                 |                                       VTag



4.  Problem space overview

   When an SCTP endpoint is behind a NAT which supports xxxnatdraftxxx a
   number of problems may arise as it trys to communicate with its peer.

   o  More than one server behind a NAT may pick the same V-Tag and
      source port when talking to the same peer server.  This creates a
      situation where the NAT will not be able to tell the two
      associations apart.  This situation is discussed in Section 5

   o  When an SCTP endpoint is a server and talking with multiple peers
      and the peers are behind the same NAT, to the server the two
      endpoints cannot be distinguished.  This case is discussed in
      Section 6.2.

   o  A NAT could at one point during a conversation restart causing all
      of its state to be lost.  This problem and its solution is
      discussed in Section 6.

   o  An SCTP endpoint may be behind two NAT's giving it redundancy.
      The method to set up this scenario is discussed in Section 6.1.

   Each of these solutions requires additional chunks and parameters,
   defined in this document, and possibly modified handling procedures
   from those specified in [RFC4960].


5.  Handling of internal port number and verification tag collisions

   Consider the case where two hosts in the Private-Address space want
   to set up an SCTP association with the same server running on the
   same host in the Internet.  This means that the External-Port and the
   External-Address are the same.  If they both choose the same
   Internal-Port and Internal-VTag, the NAT box cannot distinguish
   incoming packets anymore.  But this is very unlikely.  The Internal-



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   VTags are chosen at random and if the Internal-Ports are also chosen
   from the ephemeral port range at random this gives a 46 bit random
   number which has to match.  In the TCP like NAPT case the NAT box can
   control the 16 bit Natted Port.

   However, in this unlikely event the NAT box MUST respond to the INIT
   chunk by sending an ABORT chunk with the M-bit set.  The M-bit is a
   new bit defined by this document to express to SCTP that the source
   of this packet is a "middle" box, not the peer SCTP endpoint.  The
   source address of the packet containing the ABORT chunk MUST be the
   destination address of the SCTP packet containing the INIT chunk.

   The sender of the packet containing the INIT chunk, upon reception of
   an ABORT with M-bit set SHOULD reinitiate the association setup
   procedure after choosing a new initiate tag.  These proceedures
   SHOULD be followed only if the appropriate error cause code for
   colliding NAT table state is included AND the association is in the
   COOKIE-WAIT state (i.e. it is awaiting a INIT-ACK).  If the endpoint
   is in any other state an SCTP endpoint SHOULD NOT respond.

   The ABORT chunk defined in [RFC4960] is therefore extended by using
   the following format:

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Type = 6    | Reserved  |M|T|           Length              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                   zero or more Error Causes                   /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The following error cause with cause code 0x00B0 (Colliding NAT table
   entry) MUST be included in the ABORT chunk:

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Cause Code=0x00B0         |      Cause Length=Variable    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                          INIT chunk                          /
   /                                                              \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+







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6.  Handling of missing state

   If the NAT box receives a packet for which the lookup procedure does
   not find an entry in the NAT table, a packet containing an ERROR
   packet is sent back with the M-bit set.  The source address of the
   packet containing the ERROR chunk MUST be the destination address of
   the incoming SCTP packet.  The verification tag is reflected.

   The ERROR chunk defined in [RFC4960] is therefore extended by using
   the following format:

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Type = 9    | Reserved  |M|T|           Length              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                   zero or more Error Causes                   /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The following error cause with cause code 0x00B1 (Missing NAT table
   entry) SHOULD be included in the ERROR chunk:

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Cause Code=0x00B1         |      Cause Length=Variable    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                       Incoming Packet                        /
   /                                                              \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Upon reception by an SCTP end-point with this ERROR chunk the
   receiver SHOULD take the following actions:

   o  Validate the verifcation tag is reflected by looking at the V-tag
      that would have been included in the outgoing packet.

   o  Validate that the peer of the SCTP assocation supports the dynamic
      address extension, if it does not discard the incoming ERROR
      chunk.

   o  Generate a new ASCONF chunk as defined below including both sets
      of V-tags so that the NAT may recover the appropriate state.  The
      procedures for generating an ASCONF can be found in [RFC5061]





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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Parameter Type = 0xC008   |     Parameter Length = 16     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 ASCONF-Request Correlation ID                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Internal Verification Tag                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   External Verification Tag                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   If the NAT box receives a packet for which it has no NAT table entry
   and the packet contains an ASCONF chunk with a vtag parameter, the
   NAT box MUST update its NAT table according to the verification tags
   in the vtag parameter.

   The peer SCTP endpoint receiving such an ASCONF chunk SHOULD either
   add the address and respond with an acknowledgment, if the address is
   new to the assocation (following all procedures defined in
   [RFC5061]).  Or, if the address is already part of the association,
   the SCTP endpoint MUST NOT respond with an error, but instead should
   respond with an ASCONF-ACK acknowledging the address but take no
   action (since the address is already in the association).

6.1.  Multi Point Traversal considerations

   If a multi-homed SCTP end-point behind a NAT connects to a peer, it
   SHOULD first set up the association single-homed with only one
   address causing the first NAT to populate its state.  Then it SHOULD
   adds each IP address using ASCONF chunks sent via their respective
   NATs.  The address to add is the wildcard address and the lookup
   address SHOULD also contain the vtag parameter pair illustrated
   above.

6.2.  Handling of internal port number collisions

   When two SCTP hosts are behind a NAT and using the recommendations in
   xxxxbehavexxx it is possible that two SCTP hosts in the Private-
   Address space will want to set up an SCTP association with the same
   server running on the same host in the Internet.  For the NAT
   appropriate tracking may be performed by assuring that the vtags are
   unique between the two hosts as defined in xxxxbehavexxx.  But for
   the external SCTP server on the internet this means that the
   External-Port and the External-Address are the same.  If they both
   have chosen the same Internal-Port the server cannot distinguish both
   associations based on the address and port numbers.  For the server
   it looks like the association is being restarted.  To overcome this



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   limitation the client sends a DISABLE_RESTART parameter in the INIT-
   chunk which is defined as follows:

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Type = 0xC007         |          Length=4             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   When the server receives this parameter it MUST do the following:

   o  Include in the INIT-ACK a DISABLE_RESTART parameter to inform the
      client that it will support the feature.

   o  Disable the restart procdures defined in [RFC4960] for this
      association.

   Servers that support this feature will need to be capable of
   maintaining multiple connections to what appears to be the same peer
   (behind the NAT) differentiated only by the vtags.

   The NAT, when processing the INIT-ACK, should note in its internal
   table that the external server supports the DISABLE_RESTART
   extension.  This note is used when establishing future associations
   (i.e. when processing an INIT from an internal host) to decide if the
   connection should be allowed.  The NAT MUST do the following when
   processing an INIT:

   o  If the INIT is destined to an external address and port for which
      the NAT has no outbound connection, allow the INIT creating an
      internal mapping table.

   o  If the INIT matches the external address and port of an already
      existing connection, validate that the external server supports
      the DISABLE_RESTART feature.  If it does allow the INIT to be
      forwarded.

   o  If the external server does NOT support the DISABLE_RESTART
      extension the NAT MUST send an ABORT with the 'M' bit set.

   The following error cause with cause code 0x00B2 (Duplicate Local
   Port with DISABLE_RESTART not Supported) MUST be included in the
   ABORT chunk:








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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Cause Code=0x00B2         |      Cause Length=Variable    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                          INIT chunk                          /
   /                                                              \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


7.  IANA Considerations

   TBD


8.  Security considerations

   TBD


9.  Acknowledgments

   The authors wish to thank Qiaobing Xie, Henning Peters, Bryan Ford,
   David Hayes, Alfred Hines, Dan Wing, and Jason But for their
   invaluable comments.


10.  References

10.1.  Normative References

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

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol",
              RFC 4960, September 2007.

   [RFC5061]  Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
              Kozuka, "Stream Control Transmission Protocol (SCTP)
              Dynamic Address Reconfiguration", RFC 5061,
              September 2007.







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10.2.  Informative References

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.


Authors' Addresses

   Randall R. Stewart
   Huawei
   Chapin, SC  29036
   USA

   Phone:
   Email: rstewart@huawei.com


   Michael Tuexen
   Muenster Univ. of Applied Sciences
   Stegerwaldstr. 39
   48565 Steinfurt
   Germany

   Email: tuexen@fh-muenster.de


   Irene Ruengeler
   Muenster Univ. of Applied Sciences
   Stegerwaldstr. 39
   48565 Steinfurt
   Germany

   Email: i.ruengeler@fh-muenster.de

















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