Stream Control Transmission Protocol (SCTP) Network Address Translation for Endpoints
draft-ietf-tsvwg-natsupp-03

Network Working Group                                         R. Stewart
Internet-Draft                                            Adara Networks
Intended status: Standards Track                               M. Tuexen
Expires: April 12, 2013                                     I. Ruengeler
                                        Muenster Univ. of Appl. Sciences
                                                         October 9, 2012

Stream Control Transmission Protocol (SCTP) Network Address Translation
                             for Endpoints
                    draft-ietf-tsvwg-natsupp-03.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 NAT's 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 April 12, 2013.

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Copyright Notice

   Copyright (c) 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
   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
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Problem Space Overview . . . . . . . . . . . . . . . . . . . .  5
   5.  Handling of Internal Port Number and Verification Tag
       Collisions . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   6.  Handling of Missing State  . . . . . . . . . . . . . . . . . .  7
   7.  Multi Point Traversal Considerations . . . . . . . . . . . . .  8
   8.  Handling of Internal Port Number Collisions  . . . . . . . . .  8
   9.  SCTP Socket API Considerations . . . . . . . . . . . . . . . . 10
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 10
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     13.1.  Normative References  . . . . . . . . . . . . . . . . . . 10
     13.2.  Informative References  . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11

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1.  Introduction

   Stream Control Transmission Protocol [RFC4960] provides a reliable
   communications channel between two end-hosts in many ways similar to
   TCP [RFC0793].  With the widespread deployment of Network Address
   Translators (NAT), specialized code has been added to NAT for TCP
   that allows multiple hosts to reside behind a NAT and yet use only a
   single globally unique IPv4 address, even when two hosts (behind a
   NAT) choose the same port numbers for their connection.  This
   additional code is sometimes classified as Network Address and Port
   Translation (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 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 maintain the proper
   state without needing to change port numbers.

   A NAT will need to follow these procedures for generating appropriate
   SCTP packet formats.  NAT's 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 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 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.  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.

   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 8.

   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.

   o  An SCTP endpoint may be behind two NAT's giving it redundancy.
      The method to set up this scenario is discussed in Section 7.

   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 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

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   Internal-Port and Internal-VTag, the NAT box cannot distinguish
   incoming packets anymore.  But this is very unlikely.  The Internal-
   VTags are chosen at random and if the Internal-Ports are also chosen
   from the ephemeral port range at random this gives a 46-bit random
   number which has to match.  In the TCP like NAPT case the NAT box can
   control the 16-bit Natted Port.

   However, in this unlikely event the NAT box MUST respond to the INIT
   chunk by sending an ABORT chunk with the M-bit set.  The 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 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                   /
     \                                                               \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The following error cause with cause code 0x00B0 (Colliding NAT table
   entry) 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        |     Cause Length = Variable   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \                          INIT chunk                          /
     /                                                              \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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6.  Handling of Missing State

   If the NAT box receives a packet for which the lookup procedure does
   not find an entry in the NAT table, a packet containing an ERROR
   packet is sent back with the M-bit set.  The source address of the
   packet containing the ERROR chunk MUST be the destination address of
   the incoming SCTP packet.  The verification tag is reflected.

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

      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 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 as defined below including both sets
      of VTags so that the NAT may recover the appropriate state.  The
      procedures for generating an ASCONF chunk can be found in
      [RFC5061].

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

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

   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).

7.  Multi Point Traversal 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
   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.

8.  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

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   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 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 external server 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 (Duplicate Local
   Port with DISABLE_RESTART not Supported) MUST be included in the
   ABORT chunk:

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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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Cause Code = 0x00B2        |     Cause Length = Variable   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \                          INIT chunk                          /
     /                                                              \
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

9.  SCTP Socket API Considerations

   TBD

10.  IANA Considerations

   M-Bit for ABORT and ERROR chunk (0x02).

   Error cause Colliding NAT table entry, (0x00B1).

   Error cause Duplicate Local Port with DISABLE_RESTART not Supported,
   (0x00B2).

   Disable restart parameter (0xC007).

   ASCONF Parameter (0xC008).

11.  Security Considerations

   TBD

12.  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.

13.  References

13.1.  Normative References

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

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   [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.

   [I-D.ietf-tsvwg-sctp-udp-encaps]
              Tuexen, M. and R. Stewart, "UDP Encapsulation of SCTP
              Packets", draft-ietf-tsvwg-sctp-udp-encaps-04 (work in
              progress), July 2012.

13.2.  Informative References

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

   [RFC6083]  Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram
              Transport Layer Security (DTLS) for Stream Control
              Transmission Protocol (SCTP)", RFC 6083, January 2011.

   [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-06 (work in progress),
              March 2012.

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Authors' Addresses

   Randall R. Stewart
   Adara Networks
   Chapin, SC  29036
   USA

   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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