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


Stream Control Transmission Protocol (SCTP) Network Address Translation
                                Support
                    draft-ietf-tsvwg-natsupp-05.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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 29, 2013.




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Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.





































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Data Formats . . . . . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Modified Chunks  . . . . . . . . . . . . . . . . . . . . .  6
       4.1.1.  Extended ABORT Chunk . . . . . . . . . . . . . . . . .  6
       4.1.2.  Extended ERROR Chunk . . . . . . . . . . . . . . . . .  7
     4.2.  New Error Causes . . . . . . . . . . . . . . . . . . . . .  7
       4.2.1.  VTag and Port Number Collision Error Cause . . . . . .  7
       4.2.2.  Missing State Error Cause  . . . . . . . . . . . . . .  8
       4.2.3.  Port Number Collision Error Cause  . . . . . . . . . .  8
     4.3.  New Parameters . . . . . . . . . . . . . . . . . . . . . .  9
       4.3.1.  Disable Restart Parameter  . . . . . . . . . . . . . .  9
       4.3.2.  VTags Parameter  . . . . . . . . . . . . . . . . . . .  9
   5.  Problem Space and Procedures . . . . . . . . . . . . . . . . . 10
     5.1.  Problem Space Overview . . . . . . . . . . . . . . . . . . 11
     5.2.  Association Setup Considerations . . . . . . . . . . . . . 11
     5.3.  Handling of Internal Port Number and Verification Tag
           Collisions . . . . . . . . . . . . . . . . . . . . . . . . 11
     5.4.  Handling of Internal Port Number Collisions  . . . . . . . 12
     5.5.  Handling of Missing State  . . . . . . . . . . . . . . . . 13
     5.6.  Multi-Point Traversal Considerations . . . . . . . . . . . 15
   6.  Socket API Considerations  . . . . . . . . . . . . . . . . . . 15
     6.1.  Get or Set the NAT Friendliness (SCTP_NAT_FRIENDLY)  . . . 16
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
     7.1.  New Chunk Flags for Two Existing Chunk Types . . . . . . . 16
     7.2.  Three New Error Causes . . . . . . . . . . . . . . . . . . 17
     7.3.  Two New Chunk Parameter Types  . . . . . . . . . . . . . . 18
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 18
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 19
     10.2. Informative References . . . . . . . . . . . . . . . . . . 19
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19















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1.  Introduction

   Stream Control Transmission Protocol [RFC4960] provides a reliable
   communications channel between two end-hosts in many ways similar to
   TCP [RFC0793].  With the widespread deployment of Network Address
   Translators (NAT), specialized code has been added to NAT for TCP
   that allows multiple hosts to reside behind a NAT using private
   addresses (see [RFC5735]) 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 SCTP specific packets and procedures to help
   NATs provide similar features of NAPT in the single-point and multi-
   point traversal scenario.  An SCTP implementation supporting this
   extension will follow these procedures to assure that in both single-
   homed and multi-homed cases a NAT will maintain the proper state
   without needing to change port numbers.

   A 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



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   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.





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

                       Figure 1: Basic network setup


4.  Data Formats

4.1.  Modified Chunks

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

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

   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.














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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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   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.

4.2.  New Error Causes

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

4.2.1.  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 cause code for the VTag and Port
      Number Collision Error Cause.  The suggested value of this field
      for IANA is 0x00B0.

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

   Chunk: variable length
      The Cause-Specific Information is filled with the chunk that
      caused this error.  This can be an INIT, INIT-ACK, or ASCONF
      chunk.  Note that if the entire chunk will not fit in the ERROR
      chunk or ABORT chunk being sent then the bytes that do not fit are
      truncated.




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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 the Missing State
      Error Cause.  The suggested value of this field for IANA is
      0x00B1.

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

   Incoming Packet: variable length
      The Cause-Specific Information is filled with the IPv4 or IPv6
      packet that caused this error.  The IPv4 or IPv6 header MUST be
      included.  Note that if the packet will not fit 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Cause Code = 0x00B2        |     Cause Length = Variable   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \                             chunk                            /
     /                                                              \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Cause Code: 2 bytes (unsigned integer)
      This field holds the IANA defined cause code for the Port Number
      Collision Error Cause.  The suggested value of this field for IANA
      is 0x00B2.

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





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   Chunk: variable length
      The Cause-Specific Information is filled with the chunk that
      caused this error.  This can be an INIT, INIT-ACK, or ASCONF
      chunk.  Note that if the entire chunk will not fit in the ERROR
      chunk or ABORT chunk being sent then 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 the RESTART procedure is
   requested to be disabled.  Both endpoints of an association MUST
   include this parameter in the INIT chunk and INIT-ACK chunk when
   establishing an association and MUST include it in the ASCONF chunk
   when adding an address to successfully disable 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Type = 0xC007         |         Length = 4            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

   Parameter Length: 2 bytes (unsigned integer)
      This field holds the length in bytes of the parameter.  The value
      MUST be 4.

   This parameter MAY appear in INIT, INIT-ACK and ASCONF chunks and
   MUST NOT appear in any other chunk.

4.3.2.  VTags Parameter

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










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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Parameter Type = 0xC008   |     Parameter Length = 16     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 ASCONF-Request Correlation ID                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   Internal Verification Tag                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   External Verification Tag                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

   Parameter Length: 2 bytes (unsigned integer)
      This field holds the length in bytes of the parameter.  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 of
      the ASCONF-ACK response parameter.  The sender of the ASCONF can
      use this same value in the ASCONF-ACK to find which request the
      response is for.  Note that the receiver MUST NOT change this 32-
      bit value.

   Internal Verification Tag: 4 bytes (unsigned integer)
      The Verification Tag that the internal host has chosen for its
      communication.  The Verification Tag is a unique 32-bit tag that
      must accompany any incoming SCTP packet for this association to
      the 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 chunk.


5.  Problem Space and Procedures





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5.1.  Problem Space Overview

   When an SCTP endpoint is behind a NAT which supports
   [I-D.ietf-behave-sctpnat] a number 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 the same peer server.  This creates a
      situation where the NAT will not be able to tell the two
      associations apart.  This situation is discussed in Section 5.3.

   o  When an SCTP endpoint is a server communicating with multiple
      peers and the peers are behind the same NAT, then the two
      endpoints cannot be distinguished by the server.  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.  The
      method to set up this scenario is discussed in Section 5.6.

   Each 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 MUST
   NOT be any IPv4 Address parameter, IPv6 Address parameter, or
   Supported Address Types parameter in the INIT-chunk.  The INIT-ACK
   chunk MUST NOT contain any IPv4 Address parameter or IPv6 Address
   parameter.

   If the association should finally be multi-homed, the procedure in
   Section 5.6 MUST be used.

   The INIT and INIT-ACK chunk SHOULD contain the Disable Restart
   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 the Private-Address space want
   to set up an SCTP association with the same server running on the
   same host in the Internet.  This means that the External-Port and the
   External-Address are the same.  If they both choose the same
   Internal-Port and Internal-VTag, the NAT box cannot distinguish
   between incoming packets anymore.  But this is very unlikely.  The



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   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
   with the M-bit set if the collision is triggered by an INIT or INIT-
   ACK chunk or send an ERROR chunk with the M-bit set if the collision
   is triggered by an ASCONF chunk.  The M-bit is a new bit defined by
   this document to express to SCTP that the source of this packet is a
   "middle" box, not the peer SCTP endpoint (see Section 4.1.1).  In a
   packet containing an INIT-ACK chunk triggers the collision, the
   corresponding packet containing the ABORT chunk MUST contain the same
   source and destination address and port numbers as the packet
   containing the INIT-ACK chunk.  In the other two cases, the source
   and destination address and port numbers MUST be swapped.

   The sender of the packet containing the INIT chunk or the 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 code for colliding NAT table state is
   included AND the 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 include the error cause
   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 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
   the same server running on the same host in the Internet.  For the
   NAT appropriate tracking may be performed by assuring that the VTags
   are unique between the two hosts as defined in
   [I-D.ietf-behave-sctpnat].  But for the external SCTP server on the
   internet this means that the External-Port and the External-Address
   are the same.  If they both have chosen the same Internal-Port the
   server cannot distinguish between both associations based on the



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   address and port numbers.  For the server it looks like the
   association is being restarted.  To overcome this limitation the
   client sends a Disable Restart parameter in the INIT-chunk.

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

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

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

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

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

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

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

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

   The 'Port Number Collision' error cause (see Section 4.2.3) MUST be
   included in the ABORT chunk.

   If the collision is triggered by an ASCONF chunk, a packet containing
   an ERROR chunk with the 'Port Number Collision' error cause MUST be
   sent back.

5.5.  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 NAT table, a
   packet containing an ERROR chunk is sent back with the M-bit set.
   The source address of the packet containing the ERROR chunk MUST be



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   the destination address of the incoming SCTP packet.  The
   verification tag is reflected and 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 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 4.2.2) MUST be included and the new M-bit of the ERROR chunk
   MUST be set (see Section 4.1.2).

   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  Generate a new ASCONF chunk containing the VTags parameter (see
      Section 4.3.2) and the Disable Restart parameter if the
      association is using the disabled restart feature.  By processing
      this packet the NAT can recover the appropriate state.  The
      procedures for generating an ASCONF chunk can be found in
      [RFC5061].

   If the NAT box receives a packet for which it has no NAT table entry
   and the packet contains an ASCONF chunk with the VTags parameter, the
   NAT box MUST 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 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 will realize that it has an
   'Internal Port Number and Verification Tag collision'.  In such a
   case the NAT MUST send an ERROR chunk with the error cause code set
   to 'VTag and Port Number Collision' (see Section 4.2.1).

   If an SCTP endpoint receives an ERROR with 'Internal Port Number and



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   Verification Tag collision' as the 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 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 the procedures in Section 5.6) 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 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.


6.  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.



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6.1.  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.  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.  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:




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                             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.  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:









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                             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.  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.  Security Considerations

   The document does not add any additional security considerations to
   the ones given in [RFC4960], [RFC4895], and [RFC5061].


9.  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.  References







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10.1.  Normative References

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

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

   [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.  Informative References

   [RFC5735]  Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses",
              BCP 153, RFC 5735, January 2010.

   [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.

   [I-D.ietf-behave-sctpnat]
              Stewart, R., Tuexen, M., and I. Ruengeler, "Stream Control
              Transmission Protocol (SCTP) Network Address Translation",
              draft-ietf-behave-sctpnat-07 (work in progress),
              October 2012.













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Authors' Addresses

   Randall R. Stewart
   Adara Networks
   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

























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