Network Working Group                                         R. Stewart
Internet-Draft                                            Adara Networks
Intended status: Standards Track                               M. Tuexen
Expires: December 3, 2011 September 13, 2012                                 I. Ruengeler
                                        Muenster Univ. of Applied Appl. Sciences
                                                            June 1, 2011
                                                          March 12, 2012

Stream Control Transmission Protocol (SCTP) Network Address Translation
                                Support
                    draft-ietf-tsvwg-natsupp-01.txt
                    draft-ietf-tsvwg-natsupp-02.txt

Abstract

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

   This document describes an the protocol extensions required for the SCTP specific chunks and procedures
   endpoints to help NAT's provide similar features of NAPT in the single point
   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
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 3, 2011. September 13, 2012.

Copyright Notice

   Copyright (c) 2011 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Problem space overview Space Overview . . . . . . . . . . . . . . . . . . . .  5
   5.  Association Setup Considerations . . . . . . . . . . . . . . .  5
   6.  Handling of internal port number Internal Port Number and verification tag
       collisions Verification Tag
       Collisions . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   6.  6
   7.  Handling of missing state Internal Port Number Collisions  . . . . . . . . .  7
   8.  Handling of Missing State  . . . . . . . . .  7
     6.1. . . . . . . . . .  9
   9.  Multi Point Traversal considerations Considerations . . . . . . . . . . .  8
     6.2.  Handling of internal port number collisions . . 11
   10. Socket API Considerations  . . . . .  8
   7. . . . . . . . . . . . . . 11
     10.1.  Get or Set the NAT Friendliness (SCTP_NAT_FRIENDLY) . . . 11
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   8. 12
     11.1.  New Chunk Flags for Two Chunk Types . . . . . . . . . . . 12
     11.2.  Three New Error Causes  . . . . . . . . . . . . . . . . . 13
     11.3.  Two New Chunk Parameter Types . . . . . . . . . . . . . . 13
   12. Security considerations Considerations  . . . . . . . . . . . . . . . . . . . 10
   9. 14
   13. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 10
   10. 14
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     10.1. 14
     14.1.  Normative References  . . . . . . . . . . . . . . . . . . . 10
     10.2. 14
     14.2.  Informative References  . . . . . . . . . . . . . . . . . . 11 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11 15

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

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

   A NAT will need to follow these proceedures procedures for generating appropriate
   SCTP packet formats.  NAT's should refer to
   xxxxbehavedraftxxx [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 |
      +---------------+------------+---------------+---------------+
      | Supports    Support    | Supports   Support  | Supports    Support    |      Yes      |
      |    Support    |   Support  |   No Support  |    Limited    |
      |    Support    | No Support |    Support    |      None     |
      |    Support    | No Support |   No Support  | Limited      None     |
      |   No Support  | No   Support  |    Support    | None    Limited    |
      |   No Support  | No   Support  |   No Support  | None    Limited    |
      |   No Support  | No Support |    Support    | Limited      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 for the most part the
   current situation i.e.  SCTP packets sent externally from behind a
   NAT are discarded by the NAT.  In some cases, where the NAT supports
   the feature but one of the two external hosts does NOT not support the
   feature communication may occur but in a limited way.  For example
   only one host may be able to have a connection when a collision case
   occurs.

2.  Conventions

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

3.  Terminology

   For this discussion we will use several

   This document uses the following terms, which we will define
   and point out are depicted in a figure.

   o
   Figure 1.

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

   o

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

   o

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

   o

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

   o

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

   o

   External-VTag (Ext-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 32-bit tag that must accompany any incoming SCTP
      packet for this association to the External-Address.

   o

   Public-Address (Pub-Addr) - (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|==========| point|=========| NAT |======= | |=======| Internet | ========== |end |==========|end point|
    |    A    |         +-----+        \        /           |    B    |
    +---------+ Internal   |            \--/\--/     External +---------+    External+---------+
     Internal      Port    |                            Port   External
       VTag                |                                       VTag

                       Figure 1: Basic network setup

4.  Problem space overview Space Overview

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

   o  More than one server behind a NAT may pick the same V-Tag 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 6.

   o  When an SCTP endpoint is a server and talking with multiple peers
      and the peers are behind the same NAT, to the server the two
      endpoints cannot be distinguished.  This case is discussed in
      Section 6.2. 7.

   o  A NAT could at one point during a conversation restart causing all
      of its state to be lost.  This problem and its solution is
      discussed in Section 6. 8.

   o  An SCTP endpoint may be behind two NAT's giving it redundancy.
      The method to set up this scenario is discussed in Section 6.1. 9.

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

5.  Handling of internal port number and verification tag collisions

   Consider the case where two hosts in the Private-Address space  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 9 MUST be used.

   The INIT and INIT-ACK chunk SHOULD contain the Disable Restart
   parameter defined in Section 7.

6.  Handling of Internal Port Number and Verification Tag Collisions

   Consider the case where two hosts in the Private-Address space want
   to set up an SCTP association with the same server running on the
   same host in the Internet.  This means that the External-Port and the
   External-Address are the same.  If they both choose the same
   Internal-Port and Internal-VTag, the NAT box cannot distinguish
   incoming packets anymore.  But this is very unlikely.  The Internal-
   VTags are chosen at random and if the Internal-Ports are also chosen
   from the ephemeral port range at random this gives a 46 bit 46-bit random
   number which has to match.  In the TCP like NAPT case the NAT box can
   control the 16 bit 16-bit Natted Port.

   The same can happen when the INIT-ACK is processed by the NAT.

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

   The sender of the packet containing the INIT chunk, upon reception of
   an ABORT with M-bit set SHOULD reinitiate the association setup
   procedure after choosing a new initiate tag.  These proceedures 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 a INIT-ACK).  If the endpoint is in any
   other state an SCTP endpoint SHOULD NOT respond.

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

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Type = 6    | Reserved  |M|T|           Length              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \                                                               \
     /                   zero or more Error Causes                   /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Extended ABORT chunk

   The following error cause with cause code 0x00B0 (Colliding NAT table
   entry) (V-tag and Port
   Number Collision) MUST be included in the ABORT chunk:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Cause Code=0x00B0 Code = 0x00B0        |     Cause Length=Variable Length = Variable   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \                          INIT chunk                          /
     /                                                              \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

6.

                V-tag and Port Number Collision error cause

   FIXME: What to do when this is collision happens when processing an
   ASCONF chunk?

7.  Handling of missing state

   If the NAT box receives Internal Port Number Collisions

   When two SCTP hosts are behind a packet for which NAT and using the lookup procedure does
   not find an entry recommendations in
   [I-D.ietf-behave-sctpnat] it is possible that two SCTP hosts in the NAT table, a packet containing
   Private-Address space will want to set up an ERROR
   packet is sent back SCTP association with
   the M-bit set.  The source address of same server running on the
   packet containing same host in the ERROR chunk MUST Internet.  For the
   NAT appropriate tracking may be performed by assuring that the destination address of VTags
   are unique between the incoming SCTP packet.  The verification tag is reflected.

   The ERROR chunk two hosts as defined in [RFC4960] is therefore extended by using
   [I-D.ietf-behave-sctpnat].  But for the following format:

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Type = 9    | Reserved  |M|T|           Length              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                   zero or more Error Causes                   /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Cause Code=0x00B1         |      Cause Length=Variable    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                       Incoming Packet                        /
   /                                                              \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Upon reception by an external SCTP end-point with this ERROR chunk server on the
   receiver SHOULD take
   internet this means that the following actions:

   o  Validate External-Port and the verifcation tag is reflected by looking at External-Address
   are the V-tag
      that would same.  If they both have been included in the outgoing packet.

   o  Validate that chosen the peer of same Internal-Port the SCTP assocation supports
   server cannot distinguish both associations based on the dynamic address extension, if it does not discard and
   port numbers.  For the incoming ERROR
      chunk.

   o  Generate a new ASCONF chunk as defined below including both sets
      of V-tags so that server it looks like the NAT may recover association is being
   restarted.  To overcome this limitation the appropriate state.  The
      procedures for generating an ASCONF can be found client sends a Disable
   Restart parameter in [RFC5061] the INIT-chunk which is defined as follows:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Parameter         Type = 0xC008 0xC007         |     Parameter         Length = 16     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 ASCONF-Request Correlation ID                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Internal Verification Tag                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   External Verification Tag 4            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Disable Restart parameter

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

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

   o  Disable the restart 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 following error cause with cause code 0x00B2 (Port Number
   Collision) MUST be included in the ABORT chunk:

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

                     Port Number Collision error cause

8.  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
   the destination address of the incoming SCTP packet.  The
   verification tag is reflected and the T-bit is set.  Please note that
   such an packet containing an ERROR chunk SHOULD NOT be sent if the
   received packet contains an ABORT, SHUTDOWN-COMPLETE or INIT-ACK
   chunk.

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

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Type = 9    | Reserved  |M|T|           Length              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \                                                               \
     /                   zero or more Error Causes                   /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Extended ERROR chunk

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

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Cause Code = 0x00B1        |     Cause Length = Variable   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \                       Incoming Packet                        /
     /                                                              \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Missing State error cause

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

   o  Validate 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 V-tags parameter as
      defined in Figure 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].

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

                        Figure 2: V-tags parameter

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

6.1.

9.  Multi Point Traversal considerations Considerations

   If a multi-homed SCTP end-point behind a NAT connects to a peer, it
   SHOULD first set up the association single-homed with only one
   address causing the first NAT to populate its state.  Then it SHOULD
   adds
   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 vtag parameter pair illustrated
   above.

6.2.  Handling of internal port number collisions

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

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Type = 0xC007         |          Length=4             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

   o  Include in the INIT-ACK a DISABLE_RESTART parameter to inform lookup
   address SHOULD also contain the
      client that it will support V-tags parameter and optionally the feature.

   o
   Disable Restart parameter as illustrated above.

10.  Socket API Considerations

   This section describes how the restart procdures socket API defined in [RFC4960] for this
      association.

   Servers that support this feature will need to be capable of
   maintaining multiple connections to what appears [RFC6458] is
   extended to be the same peer
   (behind the NAT) differentiated only by the vtags.

   The NAT, when processing provide a way for the INIT-ACK, should application to control NAT
   friendliness.

   Please note in its internal
   table that this section is informational only.

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

10.1.  Get or Set the external server supports the DISABLE_RESTART
   extension. NAT Friendliness (SCTP_NAT_FRIENDLY)

   This note is socket option can be used when establishing to set the NAT friendliness for future
   associations
   (i.e. when processing and and retrieve the value for future and current 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 INIT from
      association identifier or SCTP_FUTURE_ASSOC for this query.  It is
      an internal host) error to use SCTP_{CURRENT|ALL}_ASSOC in assoc_id.

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

11.  IANA Considerations

   [NOTE to RFC-Editor:

      "RFCXXXX" is to decide if the
   connection should be allowed. replaced by the RFC number you assign this
      document.

   ]

   [NOTE to RFC-Editor:

      The NAT MUST do suggested values for the following when
   processing an INIT:

   o  If chunk type and the INIT chunk parameter
      types are tentative and to be confirmed by IANA.

   ]

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

11.1.  New Chunk Flags for Two Chunk Types

   As defined in [RFC6096] two chunk flags have to an external address and port be assigned by IANA
   for which
      the NAT has no outbound connection, allow the INIT creating an
      internal mapping table.

   o  If the INIT matches ERROR chunk.  The suggested value for the external address T bit is 0x01 and port of an already
      existing connection, validate that the external server supports
   for the DISABLE_RESTART feature.  If it does allow M bit is 0x02.

   This requires an update of the INIT "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
      forwarded.

   o  If the external server does NOT support assigned by IANA for
   the DISABLE_RESTART
      extension ABORT chunk.  The suggested value of the NAT MUST send M bit is 0x02.

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

                 ABORT Chunk Flags

                  Chunk Flag Value Chunk Flag Name Reference
                  0x01             T bit set.

   The following           [RFC4960]
                  0x02             M Bit           [RFCXXXX]
                  0x04             Unassigned
                  0x08             Unassigned
                  0x10             Unassigned
                  0x20             Unassigned
                  0x40             Unassigned
                  0x80             Unassigned

11.2.  Three New Error Causes

   Three error cause with cause code 0x00B2 (Duplicate Local
   Port with DISABLE_RESTART not Supported) MUST causes have to be included assigned by IANA.  It is suggested to
   use the values given below.

   This requires three additional lines in the
   ABORT chunk:

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | "Error Cause Code=0x00B2         | Codes"
   registry for SCTP:

    Chunk Parameter Types

       Value  Cause Length=Variable    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                          INIT Code                                       Reference
    --------  ------------------------------------------------ ---------
    176       V-tag and Port Number Collision                  [RFCXXXX]
    177       Missing State                                    [RFCXXXX]
    178       Port Number Collision                            [RFCXXXX]

11.3.  Two New Chunk Parameter Types

   Two chunk                          /
   /                                                              \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

7. parameter types have to be assigned by IANA.  It is
   suggested to use the values given below.  IANA Considerations

   TBD

8. 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     V-tags (0xC008)                                  [RFCXXXX]

12.  Security Considerations

   The document does not add any additional security considerations

   TBD

9. to
   the ones given in [RFC4960], [RFC4895], and [RFC5061].

13.  Acknowledgments

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

10.

14.  References

10.1.

14.1.  Normative References

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

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

   [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.

10.2.

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

14.2.  Informative References

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G.,

   [RFC5735]  Cotton, M. and
              E. Lear, "Address Allocation for Private Internets", L. Vegoda, "Special Use IPv4 Addresses",
              BCP 5, 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 1918, February 1996. 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-06 (work in progress),
              March 2012.

Authors' Addresses

   Randall R. Stewart
   Adara Networks
   Chapin, SC  29036
   USA

   Phone:
   US

   Email: randall@lakerest.net

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

   Email: tuexen@fh-muenster.de

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

   Email: i.ruengeler@fh-muenster.de