Network Working Group                                         R. Stewart
Internet-Draft                                            Adara Networks                                             Netflix, Inc.
Intended status: Standards Track                               M. Tuexen
Expires: March 13, 2014 August 29, 2015                                    I. Ruengeler
                                        Muenster Univ. of Appl. Sciences
                                                      September 09, 2013
                                                       February 25, 2015

Stream Control Transmission Protocol (SCTP) Network Address Translation
                                Support
                    draft-ietf-tsvwg-natsupp-06.txt
                    draft-ietf-tsvwg-natsupp-07.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 (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 the protocol extensions required for the SCTP
   endpoints to help NATs provide similar features of NAPT in the
   single-point and multi-point traversal scenario.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on March 13, 2014. August 29, 2015.

Copyright Notice

   Copyright (c) 2013 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4   5
   4.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  SCTP NAT Traversal Scenarios  . . . . . . . . . . . . . .   6
       4.1.1.  Single Point Traversal  . . . . . . . . . . . . . . .   6
       4.1.2.  Multi Point Traversal . . . . . . . . . . . . . . . .   7
     4.2.  Limitations of Classical NAPT for SCTP  . . . . . . . . .   7
     4.3.  The SCTP Specific Variant of NAT  . . . . . . . . . . . .   8
   5.  Data Formats  . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  12
     5.1.  Modified Chunks . . . . . . . . . . . . . . . . . . . . .   5
       4.1.1.  12
       5.1.1.  Extended ABORT Chunk  . . . . . . . . . . . . . . . .   5
       4.1.2.  12
       5.1.2.  Extended ERROR Chunk  . . . . . . . . . . . . . . . .   5
     4.2.  12
     5.2.  New Error Causes  . . . . . . . . . . . . . . . . . . . .   6
       4.2.1.  13
       5.2.1.  VTag and Port Number Collision Error Cause  . . . . .   6
       4.2.2.  13
       5.2.2.  Missing State Error Cause . . . . . . . . . . . . . .   7
       4.2.3.  13
       5.2.3.  Port Number Collision Error Cause . . . . . . . . . .   7
     4.3.  14
     5.3.  New Parameters  . . . . . . . . . . . . . . . . . . . . .   8
       4.3.1.  14
       5.3.1.  Disable Restart Parameter . . . . . . . . . . . . . .   8
       4.3.2.  14
       5.3.2.  VTags Parameter . . . . . . . . . . . . . . . . . . .   8
   5.  Problem Space and  15
   6.  Procedures  . . . . . . . . . . . . . . . .   9
     5.1.  Problem Space . . . . . . . . .  16
     6.1.  Overview  . . . . . . . . . . . . . . . . .   9
     5.2. . . . . . . .  16
     6.2.  Association Setup Considerations  . . . . . . . . . . . .  10
     5.3.  17
     6.3.  Handling of Internal Port Number and Verification Tag
           Collisions  . . . . . . . . . . . . . . . . . . . . . . .  10
     5.4.  17
     6.4.  Handling of Internal Port Number Collisions . . . . . . .  11
     5.5.  18
     6.5.  Handling of Missing State . . . . . . . . . . . . . . . .  12
     5.6.  Multi-Point Traversal Considerations  . . . . . . .  19
     6.6.  Handling of Fragmented SCTP Packets . . .  14
   6.  Socket API Considerations . . . . . . . .  21
     6.7.  Multi-Point Traversal Considerations  . . . . . . . . . .  14
     6.1.  Get or Set the  21
   7.  Various Examples of NAT Friendliness
           (SCTP_NAT_FRIENDLY) . Traversals  . . . . . . . . . . . . .  21
     7.1.  Single-homed Client to Single-homed Server  . . . . .  14
   7.  IANA Considerations . .  21
     7.2.  Single-homed Client to Multi-homed Server . . . . . . . .  23
     7.3.  Multihomed Client and Server  . . . . . . . . . . .  15
     7.1.  New Chunk Flags for Two Existing Chunk Types . . .  26
     7.4.  NAT Loses Its State . . .  15
     7.2.  Three New Error Causes . . . . . . . . . . . . . . . .  30
     7.5.  Peer-to-Peer Communication  .  16
     7.3.  Two New Chunk Parameter Types . . . . . . . . . . . . . .  17  32
   8.  Security  Socket API Considerations . . . . . . . . . . . . . . . . . . .  17  37
     8.1.  Get or Set the NAT Friendliness
           (SCTP_NAT_FRIENDLY) . . . . . . . . . . . . . . . . . . .  38
   9.  Acknowledgments  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  38
     9.1.  New Chunk Flags for Two Existing Chunk Types  . . . . .  17 .  38
     9.2.  Three New Error Causes  . . . . . . . . . . . . . . . . .  39
     9.3.  Two New Chunk Parameter Types . . . . . . . . . . . . . .  40
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  40
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  40
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     10.1.  41
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     10.2.  41
     12.2.  Informative References . . . . . . . . . . . . . . . . .  18  41
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18  41

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 using private
   addresses (see [RFC6890]) 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 (NAPT).  To date,
   specialized code for SCTP has not yet been added to most NATs so that
   only true NAT is available.  The end result of this is that only one
   SCTP capable host can be behind a NAT.

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

   A

   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.

   An SCTP-aware NAT will need to follow these procedures for generating
   appropriate SCTP packet formats.  NATs should refer to [I-D.ietf-behave-sctpnat]
   for the BCP in using these formats.

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

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

                   Table 1: Communication possibilities

   From the table we can see that when a NAT does not support the
   extension no communication can occur.  This is because for the most
   part of the current situation i. e.  SCTP packets sent externally
   from behind a NAT are discarded by the NAT.  In some cases, where the
   NAT supports the feature but one of the two external hosts does not
   support the feature, communication may occur but in a limited way.
   For example only one host may be able to have a connection when a
   collision case occurs.

2.  Conventions

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

3.  Terminology

   This document uses the following terms, which are depicted in
   Figure 1.

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

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

   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.

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

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

   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.

   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

                       Figure 1: Basic network setup

4.  Data Formats  Motivation

4.1.  Modified Chunks  SCTP NAT Traversal Scenarios

   This section presents existing chunks defined in [RFC4960] that are
   modified by this document. defines the notion of single and multi-point NAT
   traversal.

4.1.1.  Extended 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  Single Point Traversal

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

     Internal Network     |   Type = 6       External Network
                          | Reserved  |M|T|           Length
   +---------+            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               /--\/--\           +---------+
   |  SCTP   |         +-----+           /        \          |  SCTP   |
   |end point|=========| NAT |========= | Internet | ========|end point|
   |    A    |         +-----+           \        /                   zero or more Error Causes          |    B    |
   +---------+            |               \--/\--/           +---------+
                          |

                            Single NAT scenario

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

   The ABORT chunk        /   |    B    |
   +---------+     |          :          |       \--/\--/    +---------+
                   |          :          |

                           Serial NATs scenario

   In this single point traversal scenario, we must acknowledge that
   while one of the main benefits of SCTP multi-homing is extended to add redundant
   paths, the new 'M-bit'.  The M-bit
   indicates to NAT function represents a single point of failure in the receiver
   path of the ABORT chunk that SCTP multi-home association.  However, the chunk was not
   generated 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 peer SCTP endpoint, but instead by a middle box. association.

4.1.2.  Extended 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  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       |   Type = 9      External
                        +------+             /---\/---\
   +---------+  /=======|NAT A |=========\  /          \     +---------+
   | Reserved  |M|T|           Length  SCTP   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ /        +------+          \/            \    |  SCTP   |
   |end point|/       ...                 |   Internet   |===|end point|
   |    A    |\                            \            /                   zero or more Error Causes                   /    |    B    |
   +---------+ \        +------+          / \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The ERROR chunk defined in [RFC4960] is extended          /     +---------+
                \=======|NAT B |=========/   \---\/---/
                        +------+
                           |

                          Parallel NATs scenario

   This case does NOT apply to add a single-homed SCTP association (i.e.,
   BOTH endpoints in the new
   'M-bit'. association use only one IP address).  The M-bit indicates to
   advantage here is that the receiver existence of multiple NAT traversal points
   can preserve the ERROR chunk that path diversity of a multi-homed association for the chunk was not generated by
   entire path.  This in turn can improve the peer robustness of the
   communication.

4.2.  Limitations of Classical NAPT for SCTP endpoint,

   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 efficiently, 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 instead by only has a middle box.

4.2.  New Error Causes

   This section defines single
   port number.  To make multipoint traversal work, all the new error causes added by 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 document.

4.2.1.  VTag is to use pre-defined table of
   ports and Port Number Collision Error Cause

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

   Cause Code: 2 bytes (unsigned integer)
      This field holds the IANA defined cause code for addresses configured within each NAT.  Other mechanisms
   could make use of NAT to NAT communication.  Such mechanisms are
   considered by the VTag authors not to be deployable on a wide scale base
   and Port
      Number Collision Error Cause. thus not a recommended solution.  Therefore the SCTP variant of
   NAT has been developed.

4.3.  The suggested value SCTP Specific Variant of NAT

   In this field
      for IANA 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 0x00B0.

   Cause Length: 2 bytes (unsigned integer)
      This field holds easy.
   The source address of the length packet has to be replaced with the Public-
   Address.  It may also be necessary to establish some state in bytes the NAT
   box to handle incoming packets, which is discussed later.

   For SCTP packets coming from the public Internet the destination
   address of the error cause.  The
      value MUST packets has to be replaced with the length Private-Address of
   the Cause-Specific Information plus 4.

   Chunk: variable length host the packet has to be delivered to.  The Cause-Specific Information lookup of the
   Private-Address is filled with based on the chunk that
      caused this error.  This can be an INIT, INIT-ACK, or ASCONF
      chunk.  Note that if External-VTag, External-Port,
   External-Address, Internal-VTag and the entire chunk will not fit in Internal-Port.

   For the ERROR
      chunk or ABORT chunk being sent then the bytes that do not fit are
      truncated.

4.2.2.  Missing State Error Cause

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

   Cause Code: 2 bytes (unsigned integer)
      This field holds the IANA defined cause code for SCTP NAT processing the Missing State
      Error Cause.  The suggested value NAT box has to maintain a table of this field for IANA is
      0x00B1.

   Cause Length: 2 bytes (unsigned integer)
      This field holds
   Internal-VTag, Internal-Port, Private-Address, External-VTag,
   External-Port and whether the length restart procedure is disabled or not.
   An entry in bytes of that table is called a NAT state control block.  The
   function Create() obtains the error cause. just mentioned parameters and returns a
   NAT-State control block.

   The
      value MUST entries in this table fulfill some uniqueness conditions.  There
   must not be more than one entry with the length same pair of the Cause-Specific Information plus 4.

   Incoming Packet: variable length
      The Cause-Specific Information is filled Internal-Port
   and External-Port.  This rule can be relaxed, if all entries with the IPv4 or IPv6
      packet that caused
   same Internal-Port and External-Port have the support for the restart
   procedure enabled.  In this error.  The IPv4 or IPv6 header MUST case there must be
      included.  Note that if no more than one entry
   with the packet will not fit same Internal-Port, External-Port and Ext-VTag and no more
   than one entry with the same Internal-Port, External-Port and Int-
   VTag.

   The processing of outgoing SCTP packets containing an INIT-chunk is
   described in the ERROR chunk
      or ABORT chunk being sent then the bytes that do not fit are
      truncated.

4.2.3.  Port Number Collision Error Cause

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ following figure.  The scenario shown is valid for
   all message flows in this section.

                                          /--\/--\
   +--------+          +-----+           /        \           +--------+
   |    Cause Code = 0x00B2 Host A |     Cause Length = Variable <------> |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ NAT | <------> | Internet | <------> | Host B |
   +--------+          +-----+           \                             chunk         /
     /                                                              \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Cause Code: 2 bytes (unsigned integer)
      This field holds          +--------+
                                          \--/\---/

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

                      Create(Initiate-Tag, Int-Port, Priv-Addr, 0)
                      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 IANA defined 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 code (see
   Section 5.1.1 for the Port Number
      Collision Error Cause. format).  The suggested value source address of this field for IANA
      is 0x00B2.

   Cause Length: 2 bytes (unsigned integer)
      This field holds the length in bytes of packet
   containing the error cause.  The
      value ABORT chunk MUST be the length destination address of the Cause-Specific Information plus 4.

   Chunk: variable length
      The Cause-Specific Information is filled with
   packet containing the chunk that
      caused this error.  This can be an INIT, INIT-ACK, or ASCONF INIT chunk.  Note

   It is also possible that if a connection to External-Address and
   External-Port exists without an Internal-VTag conflict but the entire chunk will
   External-Address does not fit support the DISABLE_RESTART feature (noted
   in the ERROR
      chunk or ABORT chunk being sent then NAT control block when the bytes that do not fit are
      truncated.

4.3.  New Parameters

   This section defines new parameters and their valid appearance
   defined by this document.

4.3.1.  Disable Restart Parameter

   This parameter is used to indicate that prior connection was established).
   In such a case the RESTART procedure is
   requested to INIT SHOULD be disabled.  Both endpoints of an association MUST
   include this parameter in dropped by the INIT chunk NAT and INIT-ACK chunk when
   establishing an association and MUST include it in ABORT
   SHOULD be sent back to the ASCONF chunk
   when adding SCTP host with the M-Bit set and an address to successfully disable
   appropriate error cause (see Section 5.1.1 for the restart procedure.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ format).

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

                                          /--\/--\
   +--------+          +-----+           /        \           +--------+
   |         Type = 0xC007 Host A |         Length = 4 <------> |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Parameter Type: 2 bytes (unsigned integer)
      This field holds the IANA defined parameter type for 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 Disable
      Restart Parameter. following figure.  The suggested 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 this field for IANA is
      0xC007.

   Parameter Length: 2 bytes (unsigned integer)
      This field holds the length in bytes Initiate-Tag of the parameter. INIT-ACK chunk.  The wildcard
   character signifies that the parameter's value
      MUST be 4.

   This parameter MAY appear is not considered in INIT,
   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(Int-VTag, Int-Port, *, 0, Ext-Port)
            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 and ASCONF chunks and
   MUST NOT appear
   chunk) in any 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, Int-Port, *, Ext-VTag, Ext-Port)

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

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

   The processing of other chunk.

4.3.2.  VTags Parameter

   This parameter incoming SCTP packets is used to help a described in the
   following figure.

                                          /--\/--\
   +--------+          +-----+           /        \           +--------+
   | Host A | <------> | NAT recover from state loss. | <------> | Internet | <------> | Host B |
   +--------+          +-----+           \         /          +--------+
                                          \--/\---/

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

            Lookup(Int-VTag, Int-Port, *, *, Ext-Port)

            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.

   This allows the handling of INIT-collision through NAT.

5.  Data Formats

5.1.  Modified Chunks

   This section presents existing chunks defined in [RFC4960] that are
   modified by this document.

5.1.1.  Extended 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Parameter   Type = 0xC008 6    |     Parameter Reserved  |M|T|           Length = 16     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 ASCONF-Request Correlation ID                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   Internal Verification Tag              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   External Verification Tag                   |
   \                                                               \
   /                   zero or more Error Causes                   /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Parameter Type: 2 bytes (unsigned integer)
      This field holds

   The ABORT chunk is extended to add the new 'M-bit'.  The M-bit
   indicates to the receiver of the ABORT chunk that the chunk was not
   generated by the peer SCTP endpoint, but instead by a middle box.

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

   The ERROR chunk defined in [RFC4960] is extended to add the new
   'M-bit'.  The M-bit indicates to the receiver of the ERROR chunk that
   the chunk was not generated by the peer SCTP endpoint, but instead by
   a middle box.

5.2.  New Error Causes

   This section defines the new error causes added by this document.

5.2.1.  VTag and Port Number Collision Error Cause

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

   Cause Code: 2 bytes (unsigned integer)
      This field holds the IANA defined parameter type cause code for the VTags
      Parameter. VTag and Port
      Number Collision Error Cause.  The suggested value of this field
      for IANA is 0xC008.

   Parameter 0x00B0.

   Cause Length: 2 bytes (unsigned integer)
      This field holds the length in bytes of the parameter. error cause.  The
      value MUST be 16.

   ASCONF-Request Correlation ID: 4 bytes (unsigned integer)
      This is an opaque integer assigned by the sender to identify each
      request parameter.  The receiver of the ASCONF Chunk will copy
      this 32-bit value into the ASCONF Response Correlation ID field length of the ASCONF-ACK response parameter. Cause-Specific Information plus 4.

   Chunk: variable length
      The sender of Cause-Specific Information is filled with the ASCONF can
      use chunk that
      caused this same value error.  This can be an INIT, INIT-ACK, or ASCONF
      chunk.  Note that if the entire chunk will not fit in the ASCONF-ACK to find which request ERROR
      chunk or ABORT chunk being sent then the
      response is for.  Note bytes that the receiver MUST NOT change this
      32-bit value.

   Internal Verification Tag: do not fit are
      truncated.

5.2.2.  Missing State Error Cause

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

   Cause Code: 2 bytes (unsigned integer)
      The Verification Tag that
      This field holds the internal host has chosen IANA defined cause code for its
      communication. the Missing State
      Error Cause.  The Verification Tag is a unique 32-bit tag that
      must accompany any incoming SCTP packet for suggested value of this association to
      the Private-Address.

   External Verification Tag: 4 field for IANA is
      0x00B1.

   Cause Length: 2 bytes (unsigned integer)  The
      Verification Tag that
      This field holds the host holding length in bytes of the External-Address has
      chosen for its communication. error cause.  The VTag is a unique 32-bit tag
      that must accompany any incoming SCTP packet for this association
      to the External-Address.

   This parameter MAY appear in ASCONF chunks and
      value MUST NOT appear in any
   other chunk.

5.  Problem Space and Procedures

5.1.  Problem Space Overview

   When an SCTP endpoint is behind a NAT which supports
   [I-D.ietf-behave-sctpnat] a number be the length of problems may arise as it tries
   to communicate with its peer:

   o  More than one server behind a NAT may pick the same VTag and
      source port when talking to Cause-Specific Information plus 4.

   Incoming Packet: variable length
      The Cause-Specific Information is filled with the same peer server.  This creates a
      situation where IPv4 or IPv6
      packet that caused this error.  The IPv4 or IPv6 header MUST be
      included.  Note that if the NAT packet will not be able to tell the two
      associations apart.  This situation is discussed fit in Section 5.3.

   o  When an SCTP endpoint is a server communicating with multiple
      peers and the peers are behind the same NAT, ERROR chunk
      or ABORT chunk being sent then the two
      endpoints cannot be distinguished by the server. bytes that do not fit are
      truncated.

5.2.3.  Port Number Collision Error Cause

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Cause Code = 0x00B2        |     Cause Length = Variable   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                             chunk                            /
   /                                                              \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Cause Code: 2 bytes (unsigned integer)
      This case is
      discussed in Section 5.4.

   o  A restart of a NAT during a conversation could cause a loss of its
      state.  This problem and its solution is discussed in Section 5.5.

   o  An SCTP endpoint may be behind two NATs providing redundancy. field holds the IANA defined cause code for the Port Number
      Collision Error Cause.  The
      method to set up suggested value of this scenario field for IANA
      is discussed 0x00B2.

   Cause Length: 2 bytes (unsigned integer)
      This field holds the length in Section 5.6.

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

5.2.  Association Setup Considerations

   Every association MUST initially be set up single-homed.  There the error cause.  The
      value MUST
   NOT be any IPv4 Address parameter, IPv6 Address parameter, or
   Supported Address Types parameter in the INIT-chunk. length of the Cause-Specific Information plus 4.

   Chunk: variable length
      The INIT-ACK
   chunk MUST NOT contain any IPv4 Address parameter or IPv6 Address
   parameter.

   If Cause-Specific Information is filled with the association should finally chunk that
      caused this error.  This can be multi-homed, an INIT, INIT-ACK, or ASCONF
      chunk.  Note that if the procedure entire chunk will not fit in
   Section 5.6 MUST be used.

   The INIT and INIT-ACK the ERROR
      chunk SHOULD contain or ABORT chunk being sent then the bytes that do not fit are
      truncated.

5.3.  New Parameters

   This section defines new parameters and their valid appearance
   defined by this document.

5.3.1.  Disable Restart Parameter

   This parameter defined in Section 4.3.1.

5.3.  Handling of Internal Port Number and Verification Tag Collisions

   Consider the case where two hosts in is used to indicate that the Private-Address space want RESTART procedure is
   requested to set up be disabled.  Both endpoints of 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
   between 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 MUST
   include this gives a
   46-bit random number which has to match.  In the TCP like NAPT case
   the NAT box can control parameter in the 16-bit Natted Port INIT chunk and therefor avoid
   collisions deterministically.

   The same can happen when an INIT-ACK chunk or when
   establishing an ASCONF chunk is
   processed by the NAT.

   However, in this unlikely event the NAT box association and MUST send an ABORT chunk
   with the M-bit set if include it in the collision is triggered by an INIT or INIT-
   ACK ASCONF chunk or send
   when adding an ERROR chunk with address to successfully disable the M-bit set if restart procedure.

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

   Parameter Type: 2 bytes (unsigned integer)
      This field holds the collision
   is triggered by an ASCONF chunk.  The M-bit is a new bit IANA defined by
   this document to express to SCTP that parameter type for the source Disable
      Restart Parameter.  The suggested value of this packet field for IANA is a
   "middle" box, not the peer SCTP endpoint (see Section 4.1.1).  In a
   packet containing an INIT-ACK chunk triggers the collision,
      0xC007.

   Parameter Length: 2 bytes (unsigned integer)
      This field holds the
   corresponding packet containing length in bytes of the ABORT chunk parameter.  The value
      MUST contain the same
   source be 4.

   This parameter MAY appear in INIT, INIT-ACK and destination address ASCONF chunks and port numbers as
   MUST NOT appear in any other chunk.

5.3.2.  VTags Parameter

   This parameter is used to help a NAT recover from state loss.

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

   Parameter Type: 2 bytes (unsigned integer)
      This field holds the packet
   containing IANA defined parameter type for the INIT-ACK chunk.  In VTags
      Parameter.  The suggested value of this field for IANA is 0xC008.

   Parameter Length: 2 bytes (unsigned integer)
      This field holds the other two cases, length in bytes of the source
   and destination address and port numbers parameter.  The value
      MUST be swapped.

   The 16.

   ASCONF-Request Correlation ID: 4 bytes (unsigned integer)
      This is an opaque integer assigned by the sender to identify each
      request parameter.  The receiver of the packet containing ASCONF Chunk will copy
      this 32-bit value into the INIT chunk or ASCONF Response Correlation ID field of
      the receiver ASCONF-ACK response parameter.  The sender of the INIT-ACK chunk, upon reception of an ABORT chunk with M-bit set,
   SHOULD reinitiate ASCONF can
      use this same value in the association setup procedure after choosing a
   new initiate tag.  These procedures SHOULD be followed only if ASCONF-ACK to find which request the
   appropriate error cause code for colliding NAT table state
      response is
   included AND for.  Note that the association is in receiver MUST NOT change this
      32-bit value.

   Internal Verification Tag: 4 bytes (unsigned integer)
      The Verification Tag that the COOKIE-WAIT state (i. e. it internal host has chosen for its
      communication.  The Verification Tag is
   awaiting an INIT-ACK).  If a unique 32-bit tag that
      must accompany any incoming SCTP packet for this association to
      the endpoint Private-Address.

   External Verification Tag: 4 bytes (unsigned integer)  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.

   This parameter MAY appear in ASCONF chunks and MUST NOT appear in any
   other state chunk.

6.  Procedures

6.1.  Overview

   When an SCTP endpoint SHOULD NOT respond.

   The sender of the ASCONF chunk, upon reception of is behind an ERROR chunk with
   M-bit set, MUST stop adding the path to the association.

   The sender SCTP-aware NAT a number of the ERROR or ABORT chunk MUST include the error cause
   problems may arise as it tries to communicate with cause code 'VTag and Port Number Collision' (see Section 4.2.1).

5.4.  Handling of Internal Port Number Collisions

   When two SCTP hosts are its peer:

   o  More than one host behind a NAT and using the recommendations in
   [I-D.ietf-behave-sctpnat] it is possible that two SCTP hosts in the
   Private-Address space will want to set up an SCTP association with may pick the same server running on VTag and source
      port when talking to the same host in the Internet.  For peer server.  This creates a
      situation where the NAT appropriate tracking may will not be performed by assuring that the VTags
   are unique between able to tell the two hosts as defined
      associations apart.  This situation is discussed in
   [I-D.ietf-behave-sctpnat].  But for the external Section 6.3.

   o  When an SCTP endpoint is a server on the
   internet this means that the External-Port communicating with multiple
      peers and the External-Address peers are the same.  If they both have chosen behind the same Internal-Port NAT, then the
   server two
      endpoints cannot distinguish between both associations based on the
   address and port numbers.  For the server it looks like be distinguished by the
   association server.  This case is being restarted.  To overcome this limitation the
   client sends a Disable Restart parameter
      discussed in the INIT-chunk.

   When the server receives this parameter it MUST do the following: Section 6.4.

   o  Include  A restart of a Disable Restart parameter NAT during a conversation could cause a loss of its
      state.  This problem and its solution is discussed in the INIT-ACK to inform the
      client that it will support the feature. Section 6.5.

   o  Disable the restart procedures  An SCTP endpoint may be behind two NATs providing redundancy.  The
      method to set up this scenario is discussed in Section 6.7.

   Each of these mechanisms requires additional chunks and parameters,
   defined in [RFC4960] for this
      association.

   Servers that support this feature will need to document, and possibly modified handling procedures
   from those specified in [RFC4960] fdafdfafdafdafdafdafdasf.

6.2.  Association Setup Considerations

   Every association MUST initially be capable of
   maintaining multiple connections to what appears to set up single-homed.  There MUST
   NOT be any IPv4 Address parameter, IPv6 Address parameter, or
   Supported Address Types parameter in the same peer
   (behind the NAT) differentiated only by the VTags. INIT-chunk.  The NAT, when processing INIT-ACK
   chunk MUST NOT contain any IPv4 Address parameter or IPv6 Address
   parameter.

   If the INIT-ACK, association should note in its internal
   table that finally be multi-homed, the association supports procedure in
   Section 6.7 MUST be used.

   The INIT and INIT-ACK chunk SHOULD contain the Disable Restart extension.
   This note is used when establishing future associations (i. e. when
   processing an INIT from an internal host) to decide if the connection
   should be allowed.  The NAT MUST do
   parameter defined in Section 5.3.1.

6.3.  Handling of Internal Port Number and Verification Tag Collisions

   Consider the following when processing an
   INIT:

   o  If case where two hosts in the INIT is destined Private-Address space want
   to set up an external address and port for which
      the NAT has no outbound connection, allow SCTP association with the INIT creating an
      internal mapping table.

   o  If same server running on the INIT matches
   same host in the external address and port of an already
      existing connection, validate Internet.  This means that the external server supports External-Port and the Disable Restart feature, if it does allow
   External-Address are the INIT to be
      forwarded.

   o same.  If they both choose the external server does not support the Disable Restart
      extension same
   Internal-Port and Internal-VTag, the NAT MUST send an ABORT with the M-bit set. box cannot distinguish
   between incoming packets anymore.  But this is very unlikely.  The 'Port Number Collision' error cause (see Section 4.2.3) MUST be
   included
   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 and therefor avoid
   collisions deterministically.

   The same can happen when an INIT-ACK chunk or an ASCONF chunk is
   processed by the NAT.

   However, in this unlikely event the NAT box MUST send an ABORT chunk.

   If chunk
   with the M-bit set if the collision is triggered by an ASCONF chunk, a packet containing INIT or INIT-
   ACK chunk or send an ERROR chunk with the 'Port Number Collision' error cause MUST be
   sent back.

5.5.  Handling of Missing State

   If M-bit set if the NAT box receives collision
   is triggered by an ASCONF chunk.  The M-bit is a packet from the internal network for which new bit defined by
   this document to express to SCTP that the lookup procedure does source of this packet is a
   "middle" box, not find an entry in the NAT table, peer SCTP endpoint (see Section 5.1.1).  If a
   packet containing an ERROR INIT-ACK chunk is sent back with triggers the M-bit set.
   The source address of collision, the
   corresponding packet containing the ERROR ABORT chunk MUST be contain the same
   source and destination address of the incoming SCTP packet.  The
   verification tag is reflected and port numbers as the T-bit is set.  Please note that
   such a packet
   containing an ERROR chunk SHOULD NOT be sent if the
   received packet contains an ABORT, SHUTDOWN-COMPLETE or INIT-ACK chunk.  An ERROR chunk  In the other two cases, the source
   and destination address and port numbers MUST NOT be sent if swapped.

   The sender of the received packet
   contains an ERROR chunk with containing the M-bit set.

   When sending INIT chunk or the ERROR receiver of
   the INIT-ACK chunk, upon reception of an ABORT chunk with M-bit set,
   SHOULD reinitiate the association setup procedure after choosing a
   new initiate tag.  These procedures SHOULD be followed only if the
   appropriate error cause Missing code for colliding NAT table state (see
   Section 4.2.2) MUST be is
   included and AND the new association is in the COOKIE-WAIT state (i. e. it is
   awaiting an INIT-ACK).  If the endpoint is in any other state an SCTP
   endpoint SHOULD NOT respond.

   The sender of the ASCONF chunk, upon reception of an ERROR chunk with
   M-bit set, MUST stop adding the path to the association.

   The sender of the ERROR or ABORT chunk MUST be set include the error cause
   with cause code 'VTag and Port Number Collision' (see Section 4.1.2).

   Upon reception 5.2.1).

6.4.  Handling of this ERROR chunk by an Internal Port Number Collisions

   When two SCTP endpoint the receiver
   SHOULD take the following actions:

   o  Validate that the verification tag hosts are behind an SCTP-aware NAT it is reflected by looking at the
      VTag possible that would have been included
   two SCTP hosts in the outgoing packet.

   o  Validate that the peer of the Private-Address space will want to set up an
   SCTP association supports with the
      dynamic address extension, if it does not discard same server running on the incoming
      ERROR chunk.

   o  Generate a new ASCONF chunk containing same host in the
   Internet.  For the NAT appropriate tracking may be performed by
   assuring that the VTags parameter (see
      Section 4.3.2) and are unique between the Disable Restart parameter if two hosts.  But for
   the
      association is using external SCTP server on the disabled restart feature.  By processing internet this packet means that the NAT can recover
   External-Port and the appropriate state.  The
      procedures for generating an ASCONF chunk can be found in
      [RFC5061]. External-Address are the same.  If they both
   have chosen the NAT box receives a packet for which it has no NAT table entry
   and same Internal-Port the packet contains an ASCONF chunk with server cannot distinguish
   between both associations based on the VTags parameter, address and port numbers.  For
   the
   NAT box MUST update its NAT table according to server it looks like the verification tags association is being restarted.  To
   overcome this limitation the client sends a Disable Restart parameter
   in the VTags INIT-chunk.

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

   o  Include a Disable Restart parameter.

   The peer SCTP endpoint receiving such an ASCONF chunk SHOULD either
   add the address and respond with an acknowledgment, if parameter in the address is
   new INIT-ACK to inform the association (following all
      client that it will support the feature.

   o  Disable the restart procedures defined in
   [RFC5061]).  Or, if the address is already part [RFC4960] for this
      association.

   Servers that support this feature will need to be capable of
   maintaining multiple connections to what appears to be the association, same peer
   (behind the SCTP endpoint MUST NOT respond with an error, but instead should
   respond with an ASCONF-ACK chunk acknowledging NAT) differentiated only by the address but take
   no action (since VTags.

   The NAT, when processing the address is already INIT-ACK, should note in the association).

   Note that it is possible its internal
   table that upon receiving an ASCONF chunk
   containing the VTags parameter association supports the NAT will realize that it has an
   'Internal Port Number and Verification Tag collision'.  In such a
   case Disable Restart extension.
   This note is used when establishing future associations (i. e. when
   processing an INIT from an internal host) to decide if the connection
   should be allowed.  The NAT MUST send do the following when processing an ERROR chunk with
   INIT:

   o  If the error cause code set INIT is destined to 'VTag an external address and Port port for which
      the NAT has no outbound connection, allow the INIT creating an
      internal mapping table.

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

   o  If the external server does not support the Disable Restart
      extension the NAT MUST send an ABORT with the M-bit set.

   The 'Port Number Collision' error cause (see Section 4.2.1). 5.2.3) MUST be
   included in the ABORT chunk.

   If the collision is triggered by an SCTP endpoint receives ASCONF chunk, a packet containing
   an ERROR chunk with 'Internal Port Number and
   Verification Tag collision' as the 'Port Number Collision' error cause and MUST be
   sent back.

6.5.  Handling of Missing State

   If the NAT box receives a packet from the internal network for which
   the lookup procedure does not find an entry in the
   Error Chunk contains NAT table, a
   packet containing an ASCONF ERROR chunk is sent back with the VTags parameter, careful
   examination M-bit set.
   The source address of the association is required.  The endpoint packet containing the ERROR chunk MUST do be
   the
   following:

   o  Validate that destination address of the incoming SCTP packet.  The
   verification tag is reflected by looking at and the
      VTag T-bit is set.  Please note that would have been included
   such a packet containing an ERROR chunk SHOULD NOT be sent if the
   received packet contains an ABORT, SHUTDOWN-COMPLETE or INIT-ACK
   chunk.  An ERROR chunk MUST NOT be sent if the received packet
   contains an ERROR chunk with the M-bit set.

   When sending the ERROR chunk, the new error cause Missing state (see
   Section 5.2.2) MUST be included and the new M-bit of the ERROR chunk
   MUST be set (see Section 5.1.2).

   Upon reception of this ERROR chunk by an SCTP endpoint the receiver
   SHOULD take the following actions:

   o  Validate that the verification tag is reflected by looking at the
      VTag that would have been included in the outgoing packet.

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

   o  If the association is attempting to add an address (i. e.
      following  Generate a new ASCONF chunk containing the procedures in VTags parameter (see
      Section 5.6) then the endpoint MUST-
      NOT consider 5.3.2) and the address part of Disable Restart parameter if the
      association and SHOULD make
      no further attempt to add the address (i. e. cancel any ASCONF
      timers and remove any record of is using the path), since disabled restart feature.  By processing
      this packet the NAT has a
      VTag collision and the association cannot easily create a new VTag
      (as it would if can recover the error occurred when sending appropriate state.  The
      procedures for generating an INIT).

   o ASCONF chunk can be found in
      [RFC5061].

   If the endpoint NAT box receives a packet for which it has no other path, i.  e. NAT table entry
   and the procedure was
      executed due to missing a state in packet contains an ASCONF chunk with the NAT, then VTags parameter, the endpoint
   NAT box MUST
      abort the association.  This would occur only update its NAT table according to the verification tags
   in the VTags parameter and the optional Disable Restart parameter.

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

   Note that it is possible that upon receiving an ASCONF chunk
   containing the VTags parameter the NAT
      restarted will realize that it has an
   'Internal Port Number and accepted Verification Tag collision'.  In such a new association before attempting
   case the NAT MUST send an ERROR chunk with the error cause code set
   to
      repair 'VTag and Port Number Collision' (see Section 5.2.1).

   If an SCTP endpoint receives an ERROR with 'Internal Port Number and
   Verification Tag collision' as the missing state (Note error cause and the packet in the
   Error Chunk contains an ASCONF with the VTags parameter, careful
   examination of the association is required.  The endpoint MUST do the
   following:

   o  Validate that this the verification tag is no different than what
      happens reflected by looking at the
      VTag that would have been included in the outgoing packet.

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

   o  If the association is attempting to add an address (i. e.
      following the procedures in Section 6.7) then the endpoint MUST-
      NOT consider the address part of the association and SHOULD make
      no further attempt to add the address (i. e. cancel any ASCONF
      timers and remove any record of the path), since the NAT has a
      VTag collision and the association cannot easily create a new VTag
      (as it would if the error occurred when sending an INIT).

   o  If the endpoint has no other path, i.  e. the procedure was
      executed due to missing a state in the NAT, then the endpoint MUST
      abort the association.  This would occur only if the local NAT
      restarted and accepted a new association before attempting to
      repair the missing state (Note that this is no different than what
      happens to all TCP connections when a NAT looses its state).

5.6.  Multi-Point Traversal Considerations

   If a multi-homed SCTP endpoint behind a looses its state).

6.6.  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.

6.7.  Multi-Point Traversal Considerations

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

7.  Various Examples of NAT Traversals

7.1.  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.

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

      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  |
          |  VTag   |  Port  |    Addr   |   VTag   |   Port |
          +---------+--------+-----------+----------+--------+
          |  1234   |    1   |  10.0.0.1 |     0    |    2   |
          +---------+--------+-----------+----------+--------+

                                   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.

                                          /--\/--\
   +--------+          +-----+           /        \           +--------+
   | 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  |
          |  VTag   |  Port  |    Addr   |   VTag   |   Port |
          +---------+--------+-----------+----------+--------+
          |  1234   |    1   |  10.0.0.1 |    5678  |    2   |
          +---------+--------+-----------+----------+--------+

   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
              Int-VTag = 1234

7.2.  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  |
          |  VTag   |  Port  |    Addr   |   VTag   |   Port |
          +---------+--------+-----------+----------+--------+

    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  |
          |  VTag   |  Port  |    Addr   |   VTag   |   Port |
          +---------+--------+-----------+----------+--------+
          |  1234   |    1   |  10.0.0.1 |     0    |    2   |
          +---------+--------+-----------+----------+--------+

                                  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.

                                                  +--------+
                                  /--\/--\      /-|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 does need to change the table for second address:

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

   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.

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

7.3.  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  |
          |  VTag   |  Port  |    Addr   |   VTag   |   Port |
          +---------+--------+--- -------+----------+--------+

    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  |
          |  VTag   |  Port  |    Addr   |   VTag   |   Port |
          +---------+--------+-----------+----------+--------+
          |  1234   |    1   |  10.0.0.1 |     0    |    2   |
          +---------+--------+-----------+----------+--------+

                                   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 does not need to update the table for second address:

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

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

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

                        +-------+
               /--------| 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 chunk.  The address
   parameter contains an undefined address (0) to indicate that the
   source address should be added.  The lookup address parameter 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:

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

                     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

7.4.  NAT Loses Its State

   Association 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    |  Int    |  Int   |    Priv   |   Ext    |   Ext  |
          |  VTag   |  Port  |    Addr   |   VTag   |   Port |
          +---------+--------+-----------+----------+--------+
          |  1234   |    1   |  10.0.0.1 |    5678  |    2   |
          +---------+--------+-----------+----------+--------+

                  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".

                                            /--\/--\
   +--------+              +-----+         /        \         +--------+
   | 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 chunk
   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    |  Int    |  Int   |    Priv   |   Ext    |   Ext  |
          |  VTag   |  Port  |    Addr   |   VTag   |   Port |
          +---------+--------+-----------+----------+--------+
          |  1234   |    1   |  10.0.0.1 |    5678  |    2   |
          +---------+--------+-----------+----------+--------+

           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.

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

                  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

7.5.  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.

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

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

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

   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  |
          |  VTag   |  Port  |    Addr   |   VTag   |   Port |
          +---------+--------+-----------+----------+--------+
          |  1234   |    1   |  10.0.0.1 |     0    |    2   |
          +---------+--------+-----------+----------+--------+

                           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  |
          |  VTag   |  Port  |    Addr   |   VTag   |   Port |
          +---------+--------+-----------+----------+--------+

   Now Host B sends INIT, which is processed by NAT B.  Its parameters
   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  |
          |  VTag   |  Port  |    Addr   |   VTag   |   Port |
          +---------+--------+-----------+----------+--------+
          |  5678   |    2   |  10.1.0.1 |     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  |
          |  VTag   |  Port  |    Addr   |   VTag   |   Port |
          +---------+--------+-----------+----------+--------+
          |   1234  |   1    |  10.0.0.1 |   5678   |    2   |
          +---------+--------+-----------+----------+--------+

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

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

             Internal | External           External | Internal
                      |                             |
                      |          /--\/---\          |
   +--------+     +-------+     /         \     +-------+     +--------+
   | Host A |<--->| NAT connects to a peer, it
   SHOULD first set up the association single-homed with only one
   address causing the first A |<-->| Internet  |<-->| NAT to populate its state.  Then it SHOULD
   add each IP address using ASCONF chunks sent via their respective
   NATs. 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  |
          |  VTag   |  Port  |    Addr   |   VTag   |   Port |
          +---------+--------+-----------+----------+--------+
          |  5678   |    2   |  10.1.0.1 |   1234   |   1    |
          +---------+--------+-----------+----------+--------+

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

   The address to add is the wildcard address and the lookup
   address SHOULD also contain the VTags parameter for COOKIE-ECHO and optionally the
   Disable Restart parameter as illustrated above.

6. COOKIE-ACK is successful.

             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

8.  Socket API Considerations

   This section describes how the socket API defined in [RFC6458] is
   extended to provide a way for the application to control NAT
   friendliness.

   Please note that this section is informational only.

   A socket API implementation based on [RFC6458] is extended by
   supporting one new read/write socket option.

6.1.

8.1.  Get or Set the NAT Friendliness (SCTP_NAT_FRIENDLY)

   This socket option uses the option_level IPPROTO_SCTP and the
   option_name SCTP_NAT_FRIENDLY.  It can be used to enable/disable the
   NAT friendliness for future associations and retrieve the value for
   future and specific ones.

   struct sctp_assoc_value {
     sctp_assoc_t assoc_id;
     uint32_t assoc_value;
   };

   assoc_id:  This parameter is ignored for one-to-one style sockets.
      For one-to-many style sockets the application may fill in an
      association identifier or SCTP_FUTURE_ASSOC for this query.  It is
      an error to use SCTP_{CURRENT|ALL}_ASSOC in assoc_id.

   assoc_value:  A non-zero value indicates a NAT-friendly mode.

7.

9.  IANA Considerations

   [NOTE to RFC-Editor:

      "RFCXXXX" is to be replaced by the RFC number you assign this
      document.

   ]

   [NOTE to RFC-Editor:

      The suggested values for the chunk type and the chunk parameter
      types are tentative and to be confirmed by IANA.

   ]

   This document (RFCXXXX) is the reference for all registrations
   described in this section.  The suggested changes are described
   below.

7.1.

9.1.  New Chunk Flags for Two Existing Chunk Types

   As defined in [RFC6096] two chunk flags have to be assigned by IANA
   for the ERROR chunk.  The suggested value for the T bit is 0x01 and
   for the M bit is 0x02.

   This requires an update of the "ERROR Chunk Flags" registry for SCTP:

                             ERROR Chunk Flags

            +------------------+-----------------+-----------+
            | Chunk Flag Value | Chunk Flag Name | Reference |
            +------------------+-----------------+-----------+
            | 0x01             | T bit           | [RFCXXXX] |
            | 0x02             | M bit           | [RFCXXXX] |
            | 0x04             | Unassigned      |           |
            | 0x08             | Unassigned      |           |
            | 0x10             | Unassigned      |           |
            | 0x20             | Unassigned      |           |
            | 0x40             | Unassigned      |           |
            | 0x80             | Unassigned      |           |
            +------------------+-----------------+-----------+

   As defined in [RFC6096] one chunk flag has to be assigned by IANA for
   the ABORT chunk.  The suggested value of the M bit is 0x02.

   This requires an update of the "ABORT Chunk Flags" registry for SCTP:

                             ABORT Chunk Flags

            +------------------+-----------------+-----------+
            | Chunk Flag Value | Chunk Flag Name | Reference |
            +------------------+-----------------+-----------+
            | 0x01             | T bit           | [RFC4960] |
            | 0x02             | M bit           | [RFCXXXX] |
            | 0x04             | Unassigned      |           |
            | 0x08             | Unassigned      |           |
            | 0x10             | Unassigned      |           |
            | 0x20             | Unassigned      |           |
            | 0x40             | Unassigned      |           |
            | 0x80             | Unassigned      |           |
            +------------------+-----------------+-----------+

7.2.

9.2.  Three New Error Causes

   Three error causes have to be assigned by IANA.  It is suggested to
   use the values given below.

   This requires three additional lines in the "Error Cause Codes"
   registry for SCTP:

                             Error Cause Codes

          +-------+--------------------------------+-----------+
          | Value | Cause Code                     | Reference |
          +-------+--------------------------------+-----------+
          | 176   | VTag and Port Number Collision | [RFCXXXX] |
          | 177   | Missing State                  | [RFCXXXX] |
          | 178   | Port Number Collision          | [RFCXXXX] |
          +-------+--------------------------------+-----------+

7.3.

9.3.  Two New Chunk Parameter Types

   Two chunk parameter types have to be assigned by IANA.  It is
   suggested to use the values given below.  IANA should assign these
   values from the pool of parameters with the upper two bits set to
   '11'.

   This requires two additional lines in the "Chunk Parameter Types"
   registry for SCTP:

                           Chunk Parameter Types

            +----------+--------------------------+-----------+
            | ID Value | Chunk Parameter Type     | Reference |
            +----------+--------------------------+-----------+
            | 49159    | Disable Restart (0xC007) | [RFCXXXX] |
            | 49160    | VTags (0xC008)           | [RFCXXXX] |
            +----------+--------------------------+-----------+

8.

10.  Security Considerations

   The

   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.

   For SCTP end-points, this document does not add any additional
   security considerations to the ones given in [RFC4960], [RFC4895],
   and [RFC5061].

9.

11.  Acknowledgments

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

10.

12.  References

10.1.

12.1.  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.

   [RFC4895]  Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
              "Authenticated Chunks for the Stream Control Transmission
              Protocol (SCTP)", RFC 4895, August 2007.

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

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

   [RFC6096]  Tuexen, M. and R. Stewart, "Stream Control Transmission
              Protocol (SCTP) Chunk Flags Registration", RFC 6096,
              January 2011.

10.2.

12.2.  Informative References

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

   [RFC6458]  Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
              Yasevich, "Sockets API Extensions for the Stream Control
              Transmission Protocol (SCTP)", RFC 6458, December 2011.

   [RFC6890]  Cotton, M., Vegoda, L., Bonica, R., and B. Haberman,
              "Special-Purpose IP Address Registries", BCP 153, RFC
              6890, April 2013.

   [I-D.ietf-behave-sctpnat]
              Stewart, R., Tuexen, M., and I. Ruengeler, "Stream Control
              Transmission Protocol (SCTP) Network Address Translation",
              draft-ietf-behave-sctpnat-08 (work in progress), February
              2013.

Authors' Addresses

   Randall R. Stewart
   Adara Networks
   Netflix, Inc.
   Chapin, SC  29036
   US

   Email: randall@lakerest.net
   Michael Tuexen
   Muenster University of Applied Sciences
   Stegerwaldstrasse 39
   48565 Steinfurt
   DE

   Email: tuexen@fh-muenster.de

   Irene Ruengeler
   Muenster University of Applied Sciences
   Stegerwaldstrasse 39
   48565 Steinfurt
   DE

   Email: i.ruengeler@fh-muenster.de