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Versions: (draft-stewart-behave-sctpnat) 00 01 02 03 04 05 06 07 08 09

Network Working Group                                         R. Stewart
Internet-Draft                                                    Huawei
Intended status: BCP                                           M. Tuexen
Expires: July 3, 2011                                       I. Ruengeler
                                      Muenster Univ. of Applied Sciences
                                                       December 30, 2010


Stream Control Transmission Protocol (SCTP) Network Address Translation
                    draft-ietf-behave-sctpnat-04.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 variant of NAT which
   provides 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 July 3, 2011.

Copyright Notice



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   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  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  SCTP NAT Traversal Scenarios . . . . . . . . . . . . . . . . .  4
     4.1.  Single Point Traversal . . . . . . . . . . . . . . . . . .  4
     4.2.  Multi Point Traversal  . . . . . . . . . . . . . . . . . .  5
   5.  Limitations of classical NAPT for SCTP . . . . . . . . . . . .  6
   6.  The SCTP specific variant of NAT . . . . . . . . . . . . . . .  6
   7.  Nat to SCTP  . . . . . . . . . . . . . . . . . . . . . . . . . 10
   8.  Handling of fragmented SCTP packets  . . . . . . . . . . . . . 10
   9.  Simplification for small NATs  . . . . . . . . . . . . . . . . 10
   10. Various examples of NAT traversals . . . . . . . . . . . . . . 10
     10.1. Single-homed client to single-homed server . . . . . . . . 10
     10.2. Single-homed client to multi-homed server  . . . . . . . . 13
     10.3. Multihomed client and server . . . . . . . . . . . . . . . 15
     10.4. NAT loses its state  . . . . . . . . . . . . . . . . . . . 18
     10.5. Peer-to-Peer Communication . . . . . . . . . . . . . . . . 20
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 24
   12. Security considerations  . . . . . . . . . . . . . . . . . . . 24
   13. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 25
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 25
     14.2. Informative References . . . . . . . . . . . . . . . . . . 25
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25











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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 proposes an SCTP specific variant NAT that provides the
   NAPT functionality without changing SCTP port numbers.  The authors
   feel it is possible and desirable to make these changes for a number
   of reasons.

   o  It is desirable for SCTP internal end-hosts on multiple platforms
      to be able to share a NAT's public IP address, much as TCP does
      today.

   o  If a NAT does not need to change any data within an SCTP packet it
      will reduce the processing burden of NAT'ing SCTP by NOT needing
      to execute the CRC32c checksum required by SCTP.

   o  Not having to touch the IP payload makes the processing of ICMP
      messages in NATs easier.


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.





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   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.


   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.  SCTP NAT Traversal Scenarios

   This section defines the notion of single and multi-point NAT
   traversal.

4.1.  Single Point Traversal

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





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   Internal Network      |       External Network
                         |
 +---------+             |               /--\/--\            +---------+
 |  SCTP   |          +-----+           /        \           |  SCTP   |
 |end point|==========| NAT |========= | Internet | =========|end point|
 |    A    |          +-----+           \        /           |    B    |
 +---------+             |               \--/\--/            +---------+
                         |

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

         Internal | External :: Internal | External
                  |          ::          |
 +---------+      |          ::          |       /--\/--\    +---------+
 |  SCTP   |   +-----+       ::       +-----+   /        \   |  SCTP   |
 |end point|===| NAT |=======::=======| NAT |==| Internet |==|end point|
 |    A    |   +-----+       ::       +-----+   \        /   |    B    |
 +---------+      |          ::          |       \--/\--/    +---------+
                  |          ::          |

   In this single point traversal 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.

   The two SCTP endpoints in this case can be either single-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.

4.2.  Multi Point Traversal

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


          Internal       |      External
                      +------+             /---\/---\
 +---------+  /=======|NAT A |=========\  /          \       +---------+
 |  SCTP   | /        +------+          \/            \      |  SCTP   |
 |end point|/       ...                 |   Internet   |=====|end point|
 |    A    |\                            \            /      |    B    |
 +---------+ \        +------+          / \          /       +---------+
              \=======|NAT B |=========/   \---\/---/
                      +------+
                         |



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   This case does NOT apply to a single-homed SCTP association (i.e.,
   BOTH endpoints in the association use only one IP address).  The
   advantage here is that the existence of multiple NAT traversal 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.


5.  Limitations of classical NAPT for SCTP

   Using classical NAPT may result in changing one of the SCTP port
   numbers during the processing which requires the recomputation of the
   transport layer checksum.  Whereas for UDP and TCP this can be done
   very efficently, for SCTP the checksum (CRC32c) over the entire
   packet needs to be recomputed.  This would add considerable to the
   NAT computational burden, however hardware support may mitigate this
   in some implementations.

   An SCTP endpoint may have multiple addresses but only has a single
   port number.  To make multipoint travesal work, all the NATs involved
   must recognize the packets they see as belonging to the same SCTP
   association and perform port number translation in a consistent way.
   One possible way of doing this is to use pre-defined table of ports
   and addresses configured within each NAT.  Other mechanisms could
   make use of NAT to NAT communcation.  Such mechanisms are considered
   by the authors to be undeployable on a wide scale base and thus not a
   recommended solution.  Therefore the SCTP variant of NAT has been
   developed.


6.  The SCTP specific variant of NAT

   In this section we assume that we have multiple SCTP capable hosts
   behind a NAT which has one Public-Address.  Furthermore we are
   focusing in this section on the single point traversal scenario.

   The modification of SCTP packets sent to the public Internet is easy.
   The source address of the packet has to be replaced with the Public-
   Address.  It may also be necessary to establish some state in the NAT
   box to handle incoming packets, which is discussed later.

   For SCTP packets coming from the public Internet the destination
   address of the packets has to be replaced with the Private-Address of
   the host the packet has to be delivered to.  The lookup of the
   Private-Address is based on the External-VTag, External-Port,
   External-Address, Internal-VTag and the Internal-Port.

   For the SCTP NAT processing the NAT box has to maintain a table of



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   Internal-VTag, Internal-Port, Private-Address, External-VTag,
   External-Port and External-Address.  An entry in that table is called
   a NAT state control block.  The function Create() obtains the just
   mentioned parameters and returns a NAT-State control block.

   The processing of outgoing SCTP packets containing an INIT-chunk is
   described in the following figure.  The scenario shown is valid for
   all message flows in this section.

                                         /--\/--\
 +--------+           +-----+           /        \            +--------+
 | Host A | <-------> | NAT | <------> | Internet | <-------> | Host B |
 +--------+           +-----+           \         /           +--------+
                                         \--/\---/


              INIT[Initiate-Tag]
 Priv-Addr:Int-Port ------> Ext-Addr:Ext-Port
                   Ext-VTag=0

                    Create(Initiate-Tag,Internal-Port,Private-Address,
                           0,External-Port,External-Address)
                    Returns(NAT-State control block)

            Translate To:

                         INIT[Initiate-Tag]
            Pub-Addr:Int-Port ------> Ext-Addr:Ext-Port
                             Ext-VTag=0

   It should be noted that normally a NAT control block will be created.
   However, it is possible that there is already a NAT control block
   with the same External-Address, External-Port, Internal-Port, and
   Internal-VTag but different Private-Address.  In this case the INIT
   SHOULD be dropped by the NAT and an ABORT SHOULD be sent back to the
   SCTP host with the M-Bit set and an appropriate error cause [please
   see xxx-tsvwg-document-xx for the format].

   It is also possible that a connection to External-Address and
   External-Port exists without anInternal-VTag conflict but the
   External-Address does not support the DISABLE_RESTART feature (noted
   in the NAT control block when the prior connection was established).
   In such a case the INIT SHOULD be dropped by the NAT and an ABORT
   SHOULD be sent back to the SCTP host with the M-Bit set and an
   appropriate error cause [please see xxx-tsvwg-document-xx for the
   format].

   The processing of outgoing SCTP packets containing no INIT-chunk is



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   described in the following figure.

                                         /--\/--\
 +--------+           +-----+           /        \            +--------+
 | Host A | <-------> | NAT | <------> | Internet | <-------> | Host B |
 +--------+           +-----+           \         /           +--------+
                                         \--/\---/

 Priv-Addr:Int-Port ------> Ext-Addr:Ext-Port
                   Ext-VTag


                             Translate To:

                             Pub-Addr:Int-Port ------> Ext-Addr:Ext-Port
                                               Ext-VTag

   The processing of incoming SCTP packets containing INIT-ACK chunks is
   described in the following figure.  The Lookup() function getting as
   input the Internal-VTag, Internal-Port, External-VTag (=0), External-
   Port, and External-Address, returns the corresponding entry of the
   NAT table and updates the External-VTag by substituting it with the
   value of the Initiate-Tag of the INIT-ACK chunk.  The wildcard
   character signifies that the parameter's value is not considered in
   the Lookup() function or changed in the Update() function,
   respectively.

                                         /--\/--\
 +--------+           +-----+           /        \            +--------+
 | Host A | <-------> | NAT | <------> | Internet | <-------> | Host B |
 +--------+           +-----+           \         /           +--------+
                                         \--/\---/

                                          INIT-ACK[Initiate-Tag]
                             Pub-Addr:Int-Port <------ Ext-Addr:Ext-Port
                                               Int-VTag

            Lookup(Internal-VTag,Internal-Port,*,
                   0,External-Port,External-Address)
            Update(*, *, *, Initiate-Tag, *, *)

            Returns(NAT-State control block containing Private-Address)

                INIT-ACK[Initiate-Tag]
 Priv-Addr:Int-Port <------ Ext-Addr:Ext-Port
                    Int-VTag

   In the case Lookup fails, the SCTP packet is dropped.  The Update



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   routine inserts the External-VTag (the Initiate-Tag of the INIT-ACK
   chunk) in the NAT state control block.

   The processing of incoming SCTP packets containing an ABORT or
   SHUTDOWN-COMPLETE chunk with the T-Bit set is described in the
   following figure.

                                         /--\/--\
 +--------+           +-----+           /        \            +--------+
 | Host A | <-------> | NAT | <------> | Internet | <-------> | Host B |
 +--------+           +-----+           \         /           +--------+
                                         \--/\---/

                             Pub-Addr:Int-Port <------ Ext-Addr:Ext-Port
                                               Ext-VTag

             Lookup(0, Internal-Port, *,External-VTag,
                    External-Port, External-Address)

             Returns(NAT-State control block containing Private-Address)

 Priv-Addr:Int-Port <------ Ext-Addr:Ext-Port
                    Ext-VTag

   The processing of other incoming SCTP packets is described in the
   following figure.

                                         /--\/--\
 +--------+           +-----+           /        \            +--------+
 | Host A | <-------> | NAT | <------> | Internet | <-------> | Host B |
 +--------+           +-----+           \         /           +--------+
                                         \--/\---/

                             Pub-Addr:Int-Port <------ Ext-Addr:Ext-Port
                                               Int-VTag

               Lookup(Internal-VTag, Internal-Port, *,
                        *, External-Port, External-Address)

               Returns(NAT-State control block containing Local-Address)

 Priv-Addr:Int-Port <------ Ext-Addr:Ext-Port
                    Int-VTag

   For an incoming packet containing an INIT-chunk a table lookup is
   made only based on the addresses and port numbers.  If an entry with
   an External-VTag of zero is found, it is considered a match and the
   External-VTag is updated.



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   This allows the handling of INIT-collision through NAT.


7.  Nat to SCTP

   This document at various places discusses the sending of specialized
   SCTP chunks (e.g. an ABORT with M bit set).  These chunks and
   proceedures are not defined in this document, but instead are defined
   in [xxxxx].  The NAT implementor should refer to [xxxx] for detailed
   descriptions of packet format and procedures.


8.  Handling of fragmented SCTP packets

   A NAT box MUST support IP reassembly of received fragmented SCTP
   packets.  The fragments may arrive in any order.

   When an SCTP packet has to be fragmented by the NAT box and the IP
   header forbids fragmentation a corresponding ICMP packet SHOULD be
   sent.


9.  Simplification for small NATs

   Small NAT boxes, i.e.  NAT boxes which only have to support a small
   number of concurrent SCTP associations, MAY not take the external
   address into account when processing packets.  Therefore the
   External-Address could also be removed from the NAT table.

   This simplification may make implementing a NAT box easier, however,
   the collision probability is higher than using a mapping which takes
   the external address into account.


10.  Various examples of NAT traversals

10.1.  Single-homed client to single-homed server

   The internal client starts the association with the external server
   via a four-way-handshake.  Host A starts by sending an INIT chunk.











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                                         /--\/--\
 +--------+           +-----+           /        \            +--------+
 | Host A | <-------> | NAT | <------> | Internet | <-------> | Host B |
 +--------+           +-----+           \         /           +--------+
                                         \--/\---/
        +---------+--------+-----------+----------+--------+-----------+
 NAT    |  Int    |  Int   |    Priv   |   Ext    |   Ext  |    Ext    |
        |  VTag   |  Port  |    Addr   |   VTag   |   Port |    Addr   |
        +---------+--------+--- -------+----------+--------+-----------+

    INIT[Initiate-Tag = 1234]
 10.0.0.1:1 ------> 100.0.0.1:2
         Ext-VTtag = 0

   A NAT entry is created, the source address is substituted and the
   packet is sent on:

        NAT creates entry:
        +---------+--------+-----------+----------+--------+-----------+
 NAT    |  Int    |  Int   |    Priv   |   Ext    |   Ext  |    Ext    |
        |  VTag   |  Port  |    Addr   |   VTag   |   Port |    Addr   |
        +---------+--------+-----------+----------+--------+-----------+
        |  1234   |    1   |  10.0.0.1 |     0    |    2   | 100.0.0.1 |
        +---------+--------+-----------+----------+--------+-----------+

                                  INIT[Initiate-Tag = 1234]
                    101.0.0.1:1 ---------------------------> 100.0.0.1:2
                                         Ext-VTtag = 0

   Host B receives the INIT and sends an INIT-ACK with the NAT's
   external address as destination address.




















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                                         /--\/--\
 +--------+           +-----+           /        \            +--------+
 | Host A | <-------> | NAT | <------> | Internet | <-------> | Host B |
 +--------+           +-----+           \         /           +--------+
                                         \--/\---/

                                  INIT-ACK[Initiate-Tag = 5678]
                    101.0.0.1:1 <--------------------------- 100.0.0.1:2
                                          Int-VTag = 1234

 NAT updates entry:
        +---------+--------+-----------+----------+--------+-----------+
 NAT    |  Int    |  Int   |    Priv   |   Ext    |   Ext  |    Ext    |
        |  VTag   |  Port  |    Addr   |   VTag   |   Port |    Addr   |
        +---------+--------+-----------+----------+--------+-----------+
        |  1234   |    1   |  10.0.0.1 |    5678  |    2   | 100.0.0.1 |
        +---------+--------+-----------+----------+--------+-----------+


 INIT-ACK[Initiate-Tag = 5678]
 10.0.0.1:1 <------ 100.0.0.1:2
           Int-VTag = 1234


   The handshake finishes with a COOKIE-ECHO acknowledged by a COOKIE-
   ACK.

                                         /--\/--\
 +--------+           +-----+           /        \            +--------+
 | Host A | <-------> | NAT | <------> | Internet | <-------> | Host B |
 +--------+           +-----+           \         /           +--------+
                                         \--/\---/

          COOKIE-ECHO
 10.0.0.1:1 ------> 100.0.0.1:2
        Ext-VTag = 5678

                                          COOKIE-ECHO
                    101.0.0.1:1 ---------------------------> 100.0.0.1:2
                                        Ext-VTag = 5678


                                          COOKIE-ACK
                    101.0.0.1:1 <--------------------------- 100.0.0.1:2
                                          Int-VTag = 1234

             COOKIE-ACK
 10.0.0.1:1 <------ 100.0.0.1:2



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            Int-VTag = 1234

10.2.  Single-homed client to multi-homed server

   The internal client is single-homed whereas the external server is
   multi-homed.  The client (Host A) sends an INIT like in the single-
   homed case.

                                                 +--------+
                                 /--\/--\      /-|Router 1| \
 +------+          +-----+      /        \    /  +--------+  \  +------+
 | Host | <------> | NAT | <-> | Internet | ==                ==| Host |
 |   A  |          +-----+      \        /    \  +--------+  /  |   B  |
 +------+                        \--/\--/      \-|Router 2|-/   +------+
                                                 +--------+

        +---------+--------+-----------+----------+--------+-----------+
 NAT    |  Int    |  Int   |    Priv   |   Ext    |   Ext  |    Ext    |
        |  VTag   |  Port  |    Addr   |   VTag   |   Port |    Addr   |
        +---------+--------+--- -------+----------+--------+-----------+


  INIT[Initiate-Tag = 1234]
 10.0.0.1:1 ---> 100.0.0.1:2
        Ext-VTag = 0

   NAT creates entry:

        +---------+--------+-----------+----------+--------+-----------+
 NAT    |  Int    |  Int   |    Priv   |   Ext    |   Ext  |    Ext    |
        |  VTag   |  Port  |    Addr   |   VTag   |   Port |    Addr   |
        +---------+--------+-----------+----------+--------+-----------+
        |  1234   |    1   |  10.0.0.1 |     0    |    2   | 100.0.0.1 |
        +---------+--------+-----------+----------+--------+-----------+

                                INIT[Initiate-Tag = 1234]
                 101.0.0.1:1 ------------------------------> 100.0.0.1:2
                                       Ext-VTag = 0


   The server (Host B) includes its two addresses in the INIT-ACK chunk,
   which results in two NAT entries.









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                                                 +--------+
                                 /--\/--\      /-|Router 1| \
 +------+          +-----+      /        \    /  +--------+  \  +------+
 | Host | <------> | NAT | <-> | Internet | ==                ==| Host |
 |   A  |          +-----+      \        /    \  +--------+  /  |   B  |
 +------+                        \--/\--/      \-|Router 2|-/   +------+
                                                 +--------+

                     INIT-ACK[Initiate-tag = 5678, IP-Addr = 100.1.0.1]
                 101.0.0.1:1 <------------------------------ 100.0.0.1:2
                                        Int-VTag = 1234

   NAT updates first entry and creates entry for second address:

        +---------+--------+-----------+----------+--------+-----------+
 NAT    |  Int    |  Int   |    Priv   |   Ext    |   Ext  |    Ext    |
        |  VTag   |  Port  |    Addr   |   VTag   |   Port |    Addr   |
        +---------+--------+-----------+----------+--------+-----------+
        |  1234   |    1   |  10.0.0.1 |    5678  |    2   | 100.0.0.1 |
        |  1234   |    1   |  10.0.0.1 |    5678  |    2   | 100.1.0.1 |
        +---------+--------+-----------+----------+--------+-----------+


 INIT-ACK[Initiate-Tag = 5678]
 10.0.0.1:1 <--- 100.0.0.1:2
          Int-VTag = 1234

   The handshake finishes with a COOKIE-ECHO acknowledged by a COOKIE-
   ACK.






















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                                                 +--------+
                                 /--\/--\      /-|Router 1| \
 +------+          +-----+      /        \    /  +--------+  \  +------+
 | Host | <------> | NAT | <-> | Internet | ==                ==| Host |
 |   A  |          +-----+      \        /    \  +--------+  /  |   B  |
 +------+                        \--/\--/      \-|Router 2|-/   +------+
                                                 +--------+

        COOKIE-ECHO
 10.0.0.1:1 ---> 100.0.0.1:2
        ExtVTag = 5678

                                      COOKIE-ECHO
                 101.0.0.1:1 ------------------------------> 100.0.0.1:2
                                      Ext-VTag = 5678


                                      COOKIE-ACK
                 101.0.0.1:1 <------------------------------ 100.0.0.1:2
                                      Int-VTag = 1234

           COOKIE-ACK
 10.0.0.1:1 <--- 100.0.0.1:2
          Int-VTag = 1234

10.3.  Multihomed client and server

   The client (Host A) sends an INIT to the server (Host B), but does
   not include the second address.

                       +-------+
              /--------| NAT 1 |--------\       /--\/--\
 +------+    /         +-------+         \     /        \     +--------+
 | Host |====                             ====| Internet |====| Host B |
 |   A  |    \         +-------+         /     \        /     +--------+
 +------+     \--------| NAT 2 |--------/       \--/\--/
                       +-------+

        +---------+--------+-----------+----------+--------+-----------+
 NAT 1  |  Int    |  Int   |    Priv   |   Ext    |   Ext  |    Ext    |
        |  VTag   |  Port  |    Addr   |   VTag   |   Port |    Addr   |
        +---------+--------+--- -------+----------+--------+-----------+

  INIT[Initiate-Tag = 1234]
 10.0.0.1:1 --------> 100.0.0.1:2
          Ext-VTag = 0

   NAT 1 creates entry:



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        +---------+--------+-----------+----------+--------+-----------+
 NAT 1  |  Int    |  Int   |    Priv   |   Ext    |   Ext  |    Ext    |
        |  VTag   |  Port  |    Addr   |   VTag   |   Port |    Addr   |
        +---------+--------+-----------+----------+--------+-----------+
        |  1234   |    1   |  10.0.0.1 |     0    |    2   | 100.0.0.1 |
        +---------+--------+-----------+----------+--------+-----------+


                                  INIT[Initiate-Tag = 1234]
                      101.0.0.1:1 -------------------------> 100.0.0.1:2
                                          ExtVTag = 0

   Host B includes its second address in the INIT-ACK, which results in
   two NAT entries in NAT 1.

                       +-------+
              /--------| NAT 1 |--------\       /--\/--\
 +------+    /         +-------+         \     /        \     +--------+
 | Host |====                             ====| Internet |====| Host B |
 |   A  |    \         +-------+         /     \        /     +--------+
 +------+     \--------| NAT 2 |--------/       \--/\--/
                       +-------+

                      INIT-ACK[Initiate-Tag = 5678, IP-Addr = 100.1.0.1]
                      101.0.0.1:1 <------------------------- 100.0.0.1:2
                                           Int-VTag = 1234

   NAT 1 updates first entry and creates complete entry for second
   address:

        +---------+--------+-----------+----------+--------+-----------+
 NAT 1  |  Int    |  Int   |    Priv   |   Ext    |   Ext  |    Ext    |
        |  VTag   |  Port  |    Addr   |   VTag   |   Port |    Addr   |
        +---------+--------+-----------+----------+--------+-----------+
        |  1234   |    1   |  10.0.0.1 |    5678  |    2   | 100.0.0.1 |
        |  1234   |    1   |  10.0.0.1 |    5678  |    2   | 100.1.0.1 |
        +---------+--------+-----------+----------+--------+-----------+


   INIT-ACK[Initiate-Tag = 5678]
 10.0.0.1:1 <---------100.0.0.1:2
             Int-VTag = 1234

   The handshake finishes with a COOKIE-ECHO acknowledged by a COOKIE-
   ACK.






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                       +-------+
              /--------| NAT 1 |--------\       /--\/--\
 +------+    /         +-------+         \     /        \     +--------+
 | Host |====                             ====| Internet |====| Host B |
 |   A  |    \         +-------+         /     \        /     +--------+
 +------+     \--------| NAT 2 |--------/       \--/\--/
                       +-------+

           COOKIE-ECHO
 10.0.0.1:1 --------> 100.0.0.1:2
           Ext-VTag = 5678

                                          COOKIE-ECHO
                         101.0.0.1:1 ----------------------> 100.0.0.1:2
                                          Ext-VTag = 5678


                                           COOKIE-ACK
                         101.0.0.1:1 <---------------------- 100.0.0.1:2
                                          Int-VTag = 1234

             COOKIE-ACK
 10.0.0.1:1 <------- 100.0.0.1:2
            Int-VTag = 1234

   Host A announces its second address in an ASCONF.  The address
   parameter contains an undefined address (0) to indicate that the
   source address should be added.  The lookup address pararmeter within
   the ASCONF chunk will also contain the pair of Vtags (external and
   internal) so that the NAT may populate its table completely with this
   single packet.

                       +-------+
              /--------| NAT 1 |--------\       /--\/--\
 +------+    /         +-------+         \     /        \     +--------+
 | Host |====                             ====| Internet |====| Host B |
 |   A  |    \         +-------+         /     \        /     +--------+
 +------+     \--------| NAT 2 |--------/       \--/\--/
                       +-------+

 ASCONF [ADD-IP=0.0.0.0, INT-VTag=1234, Ext-VTag = 5678]
 10.1.0.1:1 --------> 100.1.0.1:2
          Ext-VTag = 5678

   NAT 2 creates complete entry:






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        +---------+--------+-----------+----------+--------+-----------+
 NAT 2  |  Int    |  Int   |    Priv   |   Ext    |   Ext  |    Ext    |
        |  VTag   |  Port  |    Addr   |   VTag   |   Port |    Addr   |
        +---------+--------+-----------+----------+--------+-----------+
        |  1234   |    1   |  10.1.0.1 |    5678  |    2   | 100.1.0.1 |
        +---------+--------+-----------+----------+--------+-----------+


                   ASCONF [ADD-IP,Int-VTag=1234, Ext-VTag = 5678]
                      101.1.0.1:1 -------------------------> 100.1.0.1:2
                                         Ext-VTag = 5678

                                             ASCONF-ACK
                      101.1.0.1:1 <------------------------- 100.1.0.1:2
                                            Int-VTag = 1234

           ASCONF-ACK
 10.1.0.1:1 <----- 100.1.0.1:2
          Int-VTag = 1234

10.4.  NAT loses its state

   Assocation is already established between Host A and Host B, when the
   NAT loses its state and obtains a new public address.  Host A sends a
   DATA chunk to Host B.

                                          /--\/--\
 +--------+              +-----+         /        \         +--------+
 | Host A | <----------> | NAT | <----> | Internet | <----> | Host B |
 +--------+              +-----+         \        /         +--------+
                                          \--/\--/

        +---------+--------+-----------+----------+--------+-----------+
 NAT A  |  Int    |  Int   |    Priv   |   Ext    |   Ext  |    Ext    |
        |  VTag   |  Port  |    Addr   |   VTag   |   Port |    Addr   |
        +---------+--------+-----------+----------+--------+-----------+
        |  1234   |    1   |  10.0.0.1 |    5678  |    2   | 100.0.0.1 |
        +---------+--------+-----------+----------+--------+-----------+

                DATA
 10.0.0.1:1 ----------> 100.0.0.1:2
             Ext-VTag = 5678

   The NAT box cannot find entry for the association.  It sends ERROR
   message with the M-Bit set and the cause "NAT state missing".






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                                            /--\/--\
   +--------+              +-----+         /        \         +--------+
   | Host A | <----------> | NAT | <----> | Internet | <----> | Host B |
   +--------+              +-----+         \        /         +--------+
                                            \--/\--/

     ERROR [M-Bit, NAT state missing]
   10.0.0.1:1 <---------- 100.0.0.1:2
             Ext-VTag = 5678

   On reception of the ERROR message, HOST A sends an ASCONF indicating
   that the former information has to be deleted and the source address
   of the actual packet added.

                                          /--\/--\
 +--------+              +-----+         /        \         +--------+
 | Host A | <----------> | NAT | <----> | Internet | <----> | Host B |
 +--------+              +-----+         \        /         +--------+
                                          \--/\--/

 ASCONF [ADD-IP,DELETE-IP,Int-VTag=1234, Ext-VTag = 5678]
 10.0.0.1:1 ----------> 100.1.0.1:2
           Ext-VTag = 5678

        +---------+--------+-----------+----------+--------+-----------+
 NAT A  |  Int    |  Int   |    Priv   |   Ext    |   Ext  |    Ext    |
        |  VTag   |  Port  |    Addr   |   VTag   |   Port |    Addr   |
        +---------+--------+-----------+----------+--------+-----------+
        |  1234   |    1   |  10.0.0.1 |    5678  |    2   | 100.0.0.1 |
        +---------+--------+-----------+----------+--------+-----------+

         ASCONF [ADD-IP,DELETE-IP,Int-VTag=1234, Ext-VTag = 5678]
                        102.1.0.1:1 -----------------------> 100.1.0.1:2
                                        Ext-VTag = 5678

   Host B adds the new source address and deletes all former entries.















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                                          /--\/--\
 +--------+              +-----+         /        \         +--------+
 | Host A | <----------> | NAT | <----> | Internet | <----> | Host B |
 +--------+              +-----+         \        /         +--------+
                                          \--/\--/

                                              ASCONF-ACK
                        102.1.0.1:1 <----------------------- 100.1.0.1:2
                                             Int-VTag = 1234

                ASCONF-ACK
 10.1.0.1:1 <---------- 100.1.0.1:2
 Int-VTag = 1234                     1

                DATA
 10.0.0.1:1 ----------> 100.0.0.1:2
          Ext-VTag = 5678
                                                DATA
                        102.1.0.1:1 -----------------------> 100.1.0.1:2
                                        Ext-VTag = 5678

10.5.  Peer-to-Peer Communication

   If two hosts are behind NATs, they have to get knowledge of the
   peer's public address.  This can be achieved with a so-called
   rendezvous server.  Afterwards the destination addresses are public,
   and the association is set up with the help of the INIT collision.
   The NAT boxes create their entries according to their internal peer's
   point of view.  Therefore, NAT A's Internal-VTag and Internal-Port
   are NAT B's External-VTag and External-Port, respectively.  The
   naming of the verification tag in the packet flow is done from the
   sending peer's point of view.



















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            Internal | External           External | Internal
                     |                             |
                     |          /--\/---\          |
 +--------+      +-------+     /         \     +-------+      +--------+
 | Host A |<---->| NAT A |<-->| Internet  |<-->| NAT B |<---->| Host B |
 +--------+      +-------+     \         /     +-------+      +--------+
                     |          \--/\---/          |


 NAT-Tables
        +---------+--------+-----------+----------+--------+-----------+
 NAT A  |  Int    |  Int   |    Priv   |   Ext    |   Ext  |    Ext    |
        |  VTag   |  Port  |    Addr   |   VTag   |   Port |    Addr   |
        +---------+--------+--- -------+----------+--------+-----------+

        +---------+--------+-----------+----------+--------+-----------+
 NAT B  |  Int    |  Int   |    Priv   |   Ext    |   Ext  |    Ext    |
        |  v-tag  |  port  |    addr   |   v-tag  |   port |   addr    |
        +---------+--------+--- -------+----------+--------+-----------+

 INIT[Initiate-Tag = 1234]
 10.0.0.1:1 --> 100.0.0.1:2
         Ext-VTag = 0

   NAT A creates entry:

        +---------+--------+-----------+----------+--------+-----------+
 NAT A  |  Int    |  Int   |    Priv   |   Ext    |   Ext  |    Ext    |
        |  VTag   |  Port  |    Addr   |   VTag   |   Port |    Addr   |
        +---------+--------+-----------+----------+--------+-----------+
        |  1234   |    1   |  10.0.0.1 |     0    |    2   | 100.0.0.1 |
        +---------+--------+-----------+----------+--------+-----------+

                         INIT[Initiate-Tag = 1234]
                101.0.0.1:1 ----------------> 100.0.0.1:2
                                 Ext-VTag = 0


   NAT B processes INIT, but cannot find an entry.  The SCTP packet is
   silently discarded and leaves the NAT table of NAT B unchanged.


        +---------+--------+-----------+----------+--------+-----------+
 NAT B  |  Int    |  Int   |    Priv   |   Ext    |   Ext  |    Ext    |
        |  VTag   |  Port  |    Addr   |   VTag   |   Port |    Addr   |
        +---------+--------+-----------+----------+--------+-----------+

   Now Host B sends INIT, which is processed by NAT B. Its parameters



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   are used to create an entry.

            Internal | External           External | Internal
                     |                             |
                     |          /--\/---\          |
 +--------+      +-------+     /         \     +-------+      +--------+
 | Host A |<---->| NAT A |<-->| Internet  |<-->| NAT B |<---->| Host B |
 +--------+      +-------+     \         /     +-------+      +--------+
                     |          \--/\---/          |

                                               INIT[Initiate-Tag = 5678]
                                              101.0.0.1:1 <-- 10.1.0.1:2
                                                          Ext-VTag = 0

        +---------+--------+-----------+----------+--------+-----------+
 NAT B  |  Int    |  Int   |    Priv   |   Ext    |   Ext  |    Ext    |
        |  VTag   |  Port  |    Addr   |   VTag   |   Port |    Addr   |
        +---------+--------+-----------+----------+--------+-----------+
        |  5678   |    2   |  10.1.0.1 |     0    |    1   | 101.0.0.1 |
        +---------+--------+-----------+----------+--------+-----------+

                           INIT[Initiate-Tag = 5678]
                101.0.0.1:1  <--------------- 100.0.0.1:2
                                   Ext-VTag = 0

   NAT A processes INIT.  As the outgoing INIT of Host A has already
   created an entry, the entry is found and updated:

            Internal | External           External | Internal
                     |                             |
                     |          /--\/---\          |
 +--------+      +-------+     /         \     +-------+      +--------+
 | Host A |<---->| NAT A |<-->| Internet  |<-->| NAT B |<---->| Host B |
 +--------+      +-------+     \         /     +-------+      +--------+
                     |          \--/\---/          |

                VTag != Int-VTag, but Ext-VTag == 0, find entry.
        +---------+--------+-----------+----------+--------+-----------+
 NAT A  |  Int    |  Int   |    Priv   |   Ext    |   Ext  |    Ext    |
        |  VTag   |  Port  |    Addr   |   VTag   |   Port |    Addr   |
        +---------+--------+-----------+----------+--------+-----------+
        |   1234  |   1    |  10.0.0.1 |   5678   |    2   | 100.0.0.1 |
        +---------+--------+-----------+----------+--------+-----------+

  INIT[Initiate-tag = 5678]
 10.0.0.1:1 <-- 100.0.0.1:2
           Ext-VTag = 0




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   Host A send INIT-ACK, which can pass through NAT B:

            Internal | External           External | Internal
                     |                             |
                     |          /--\/---\          |
 +--------+      +-------+     /         \     +-------+      +--------+
 | Host A |<---->| NAT A |<-->| Internet  |<-->| NAT B |<---->| Host B |
 +--------+      +-------+     \         /     +-------+      +--------+
                     |          \--/\---/          |

 INIT-ACK[Initiate-Tag = 1234]
 10.0.0.1:1 --> 100.0.0.1:2
       Ext-VTag = 5678


                     INIT-ACK[Initiate-Tag = 1234]
                101.0.0.1:1 ----------------> 100.0.0.1:2
                              Ext-VTag = 5678

                                              NAT B updates entry:

        +---------+--------+-----------+----------+--------+-----------+
 NAT B  |  Int    |  Int   |    Priv   |   Ext    |   Ext  |    Ext    |
        |  VTag   |  Port  |    Addr   |   VTag   |   Port |    Addr   |
        +---------+--------+-----------+----------+--------+-----------+
        |  5678   |    2   |  10.1.0.1 |   1234   |   1    | 101.0.0.1 |
        +---------+--------+-----------+----------+--------+-----------+

                                           INIT-ACK[Initiate-Tag = 1234]
                                              101.0.0.1:1 --> 10.1.0.1:2
                                                     Ext-VTag = 5678

   The lookup for COOKIE-ECHO and COOKIE-ACK is successful.


















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            Internal | External           External | Internal
                     |                             |
                     |          /--\/---\          |
 +--------+      +-------+     /         \     +-------+      +--------+
 | Host A |<---->| NAT A |<-->| Internet  |<-->| NAT B |<---->| Host B |
 +--------+      +-------+     \         /     +-------+      +--------+
                     |          \--/\---/          |

                                                      COOKIE-ECHO
                                              101.0.0.1:1 <-- 10.1.0.1:2
                                                      Ext-VTag = 1234

                               COOKIE-ECHO
                101.0.0.1:1 <------------- 100.0.0.1:2
                               Ext-VTag = 1234

        COOKIE-ECHO
 10.0.0.1:1 <-- 100.0.0.1:2
        Ext-VTag = 1234

        COOKIE-ACK
 10.0.0.1:1 --> 100.0.0.1:2
        Ext-VTag = 5678

                               COOKIE-ACK
                101.0.0.1:1 ----------------> 100.0.0.1:2
                               Ext-VTag = 5678

                                                     COOKIE-ACK
                                              101.0.0.1:1 --> 10.1.0.1:2
                                                    Ext-VTag = 5678


11.  IANA Considerations

   TBD


12.  Security considerations

   State maintenance within a NAT is always a subject of possible Denial
   Of Service attacks.  This document recommends that at a minimum a NAT
   runs a timer on any SCTP state so that old association state can be
   cleaned up.







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13.  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.


14.  References

14.1.  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.

14.2.  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: randall@lakerest.net


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

   Email: tuexen@fh-muenster.de






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

   Email: i.ruengeler@fh-muenster.de












































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