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

Network Working Group                                         R. Stewart
Internet-Draft                                                Researcher
Intended status: BCP                                           M. Tuexen
Expires: August 20, 2009                                    I. Ruengeler
                                      Muenster Univ. of Applied Sciences
                                                       February 16, 2009


Stream Control Transmission Protocol (SCTP) Network Address Translation
                    draft-ietf-behave-sctpnat-01.txt

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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   This Internet-Draft will expire on August 20, 2009.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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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.

































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  SCTP NAT Traversal Scenarios . . . . . . . . . . . . . . . . .  5
     4.1.  Single Point Traversal . . . . . . . . . . . . . . . . . .  5
     4.2.  Multi Point Traversal  . . . . . . . . . . . . . . . . . .  6
   5.  The SCTP specific variant of NAT . . . . . . . . . . . . . . .  7
   6.  Handling of internal port number collisions  . . . . . . . . . 10
   7.  Handling of internal port number and verification tag
       collisions . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   8.  Handling of missing state  . . . . . . . . . . . . . . . . . . 11
   9.  Multi Point Traversal considerations . . . . . . . . . . . . . 12
   10. Handling of fragmented SCTP packets  . . . . . . . . . . . . . 13
   11. Simplification for small NATs  . . . . . . . . . . . . . . . . 13
   12. Various examples of NAT traversals . . . . . . . . . . . . . . 13
     12.1. Single-homed client to single-homed server . . . . . . . . 13
     12.2. Single-homed client to multi-homed server  . . . . . . . . 15
     12.3. Multihomed client and server . . . . . . . . . . . . . . . 16
     12.4. NAT loses its state  . . . . . . . . . . . . . . . . . . . 18
     12.5. Peer-to-Peer Communication . . . . . . . . . . . . . . . . 19
   13. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
   14. Security considerations  . . . . . . . . . . . . . . . . . . . 22
   15. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 23
   16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     16.1. Normative References . . . . . . . . . . . . . . . . . . . 23
     16.2. Informative References . . . . . . . . . . . . . . . . . . 23
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23






















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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.  Normally this is the port that
      will be translated by the NAPT to a different port number.

   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

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    |
 +---------+      |          ::          |       \--/\--/    +---------+
                  |          ::          |

   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.

   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.

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.

   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 may require
   that a pre-defined table of ports and addresses were shared between
   the NATs.  Other external management schemes that help multiple NATs
   coordinate a multi-homed SCTP association could be investigated.


5.  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
   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 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.











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                                         /--\/--\
 +--------+           +-----+           /        \            +--------+
 | 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, External-VTag,
   Internal-VTag but different Private-Address.  In this case the INIT
   SHOULD be dropped and an ABORT MAY be sent back.

   The processing of outgoing SCTP packets containing no INIT-chunk is
   described in the following figure.

 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.










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                                          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
   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.

                             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.

                             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



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   made only based on the addresses and port numbers.  If an entry with
   an Internal-VTag of zero is found, it is considered a match and the
   Internal-VTag is updated.

   This allows the handling of INIT-collision through NAT.


6.  Handling of internal port number collisions

   There is one drawback of the SCTP specific variant of NAT compared to
   a NAPT solution like the ones available for TCP.  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 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 limitation the client sends a
   NAT_SUPPORTED 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 will also use the
   verification tag to look up the association.  However, this will make
   it impossible to restart such associations.


7.  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-
   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 source
   address of the packet containing the ABORT chunk MUST be the



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   destination address of the SCTP packet containing the INIT chunk.
   The sender of the packet containing the INIT chunk MAY start the
   association setup procedure after choosing a new initiate tag.

   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) SHOULD 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                          /
   /                                                              \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


8.  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:











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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   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                        /
   /                                                              \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   If an end-point receives a packet with this ERROR chunk it MAY send
   an SCTP packet with an ASCONF chunk containing an Add IP Address
   parameter followed by a vtag parameter:

   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.


9.  Multi Point Traversal considerations

   If a multi-homed SCTP end-point behind a NAT connects to a peer, it
   first sets up the association single-homed.  Then it adds each IP



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   address using ASCONF chunks.  The address to add is the wildcard
   address and the lookup address also.  The ASCONF chunks SHOULD also
   contain a vtag parameter.


10.  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.


11.  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.


12.  Various examples of NAT traversals

12.1.  Single-homed client to single-homed server

   The internal client starts the association with the external server
   via a four-way-handshake.

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

 Host A sends INIT:

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



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         Ext-VTtag = 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-VTtag = 0


                                  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

          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



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12.2.  Single-homed client to multi-homed server

   The internal client is single-homed whereas the external server is
   multi-homed.  The server includes its addresses in the INIT-ACK
   chunk, which results in two NAT entries.

                                                 +--------+
                                 /--\/--\      /-|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


                     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 |



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        +---------+--------+-----------+----------+--------+-----------+


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

        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

12.3.  Multihomed client and server


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



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       +---------+--------+-----------+----------+--------+-----------+
       |  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


                     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

          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 the second address

ASCONF [ADD-IP,INT-VTag=1234, Ext-VTag = 5678]



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10.1.0.1:1 --------> 100.1.0.1:2
         Ext-VTag = 5678

      NAT 2 creates complete entry:
       +---------+--------+-----------+----------+--------+-----------+
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

12.4.  NAT loses its state

   Assocation is already established between Host A and 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 |
        +---------+--------+-----------+----------+--------+-----------+

 The NAT loses its state and obtaines a new public address.


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




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                        NAT cannot find entry: sends ERROR message
   ERROR [M-Bit, NAT state missing]
 10.0.0.1:1 <---------- 100.0.0.1:2
           Ext-VTag = 5678


 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 new source address and deletes all former entries.

                                              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

12.5.  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



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   sending peer's point of view.

            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    |
        +---------+--------+--- -------+----------+--------+-----------+

 Host A sends INIT:

 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

                                          SCTP packet silently discarded

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



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                                                    Host B sends INIT:

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

                                     NAT B processes INIT:

        +---------+--------+-----------+----------+--------+-----------+
 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 |
        +---------+--------+-----------+----------+--------+-----------+

                                     NAT B forwards INIT:

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

                NAT A processes INIT and updates entry:

                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 |
        +---------+--------+-----------+----------+--------+-----------+

                NAT A forwards INIT:

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

 Host A send INIT-ACK:

 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




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                                              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

                                                      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


13.  IANA Considerations

   TBD


14.  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



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   cleaned up.


15.  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.


16.  References

16.1.  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.

16.2.  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
   Researcher
   Chapin, SC  29036
   USA

   Phone:
   Email: randall@lakerest.net












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   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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