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Versions: (draft-stewart-natsupp-tsvwg) 00 01 02 03 04 05 06 07 08 09 10 11 12

Network Working Group                                         R. Stewart
Internet-Draft                                             Netflix, Inc.
Intended status: Standards Track                               M. Tuexen
Expires: January 3, 2019                                    I. Ruengeler
                                        Muenster Univ. of Appl. Sciences
                                                            July 2, 2018


Stream Control Transmission Protocol (SCTP) Network Address Translation
                                Support
                    draft-ietf-tsvwg-natsupp-12.txt

Abstract

   The Stream Control Transmission Protocol (SCTP) provides a reliable
   communications channel between two end-hosts in many ways similar to
   the Transmission Control Protocol (TCP).  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).

   This document describes the protocol extensions required for the SCTP
   endpoints and the mechanisms for NATs necessary to 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 https://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 January 3, 2019.






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Copyright Notice

   Copyright (c) 2018 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
   (https://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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  SCTP NAT Traversal Scenarios  . . . . . . . . . . . . . .   6
       4.1.1.  Single Point Traversal  . . . . . . . . . . . . . . .   6
       4.1.2.  Multi Point Traversal . . . . . . . . . . . . . . . .   7
     4.2.  Limitations of Classical NAPT for SCTP  . . . . . . . . .   8
     4.3.  The SCTP Specific Variant of NAT  . . . . . . . . . . . .   8
   5.  Data Formats  . . . . . . . . . . . . . . . . . . . . . . . .  12
     5.1.  Modified Chunks . . . . . . . . . . . . . . . . . . . . .  12
       5.1.1.  Extended ABORT Chunk  . . . . . . . . . . . . . . . .  12
       5.1.2.  Extended ERROR Chunk  . . . . . . . . . . . . . . . .  13
     5.2.  New Error Causes  . . . . . . . . . . . . . . . . . . . .  13
       5.2.1.  VTag and Port Number Collision Error Cause  . . . . .  13
       5.2.2.  Missing State Error Cause . . . . . . . . . . . . . .  14
       5.2.3.  Port Number Collision Error Cause . . . . . . . . . .  15
     5.3.  New Parameters  . . . . . . . . . . . . . . . . . . . . .  15
       5.3.1.  Disable Restart Parameter . . . . . . . . . . . . . .  16
       5.3.2.  VTags Parameter . . . . . . . . . . . . . . . . . . .  16
   6.  Procedures for SCTP End Points and NATs . . . . . . . . . . .  17
     6.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  17
     6.2.  Association Setup Considerations  . . . . . . . . . . . .  18
     6.3.  Handling of Internal Port Number and Verification Tag
           Collisions  . . . . . . . . . . . . . . . . . . . . . . .  18
     6.4.  Handling of Internal Port Number Collisions . . . . . . .  19
     6.5.  Handling of Missing State . . . . . . . . . . . . . . . .  20
     6.6.  Handling of Fragmented SCTP Packets . . . . . . . . . . .  22
     6.7.  Multi-Point Traversal Considerations  . . . . . . . . . .  22
   7.  Various Examples of NAT Traversals  . . . . . . . . . . . . .  23
     7.1.  Single-homed Client to Single-homed Server  . . . . . . .  23



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     7.2.  Single-homed Client to Multi-homed Server . . . . . . . .  25
     7.3.  Multihomed Client and Server  . . . . . . . . . . . . . .  28
     7.4.  NAT Loses Its State . . . . . . . . . . . . . . . . . . .  32
     7.5.  Peer-to-Peer Communication  . . . . . . . . . . . . . . .  34
   8.  Socket API Considerations . . . . . . . . . . . . . . . . . .  39
     8.1.  Get or Set the NAT Friendliness
           (SCTP_NAT_FRIENDLY) . . . . . . . . . . . . . . . . . . .  40
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  40
     9.1.  New Chunk Flags for Two Existing Chunk Types  . . . . . .  40
     9.2.  Three New Error Causes  . . . . . . . . . . . . . . . . .  41
     9.3.  Two New Chunk Parameter Types . . . . . . . . . . . . . .  42
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  42
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  42
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  43
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  43
     12.2.  Informative References . . . . . . . . . . . . . . . . .  43
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  44

1.  Introduction

   Stream Control Transmission Protocol [RFC4960] provides a reliable
   communications channel between two end-hosts in many ways similar to
   TCP [RFC0793].  With the widespread deployment of Network Address
   Translators (NAT), specialized code has been added to NAT for TCP
   that allows multiple hosts to reside behind a NAT using private
   addresses (see [RFC6890]) and yet use only a single globally unique
   IPv4 address, even when two hosts (behind a NAT) choose the same port
   numbers for their connection.  This additional code is sometimes
   classified as Network Address and Port Translation (NAPT).  Please
   note that this document focuses on the case where the NAT maps
   multiple private addresses to a single public address.  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 and this host can only be
   single-homed.  The only alternative for supporting legacy NATs is to
   use UDP encapsulation as specified in [RFC6951].

   This document describes an SCTP specific variant NAT and specific
   packets and procedures to help NATs provide similar features of NAPT
   in the single-point and multi-point 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.

   It is possible and desirable to make these changes for a number of
   reasons:





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   o  It is desirable for SCTP internal end-hosts on multiple platforms
      to be able to share a NAT's public IP address in the same way that
      a TCP session can use a NAT.

   o  If a NAT does not need to change any data within an SCTP packet it
      will reduce the processing burden of NAT'ing SCTP by NOT needing
      to execute the CRC32c checksum required by SCTP.

   o  Not having to touch the IP payload makes the processing of ICMP
      messages in NATs easier.

   An SCTP-aware NAT will need to follow these procedures for generating
   appropriate SCTP packet formats.

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

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

                   Table 1: Communication possibilities

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








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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.  Familiarity with the terminology used in [RFC4960] and
   [RFC5061] is assumed.

   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 SCTP Verification Tag (VTag) 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.  Motivation

4.1.  SCTP NAT Traversal Scenarios

   This section defines the notion of single and multi-point NAT
   traversal.

4.1.1.  Single Point Traversal

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


     Internal Network     |       External Network
                          |
   +---------+            |               /--\/--\           +---------+
   |  SCTP   |         +-----+           /        \          |  SCTP   |
   |end point|=========| NAT |========= | Internet | ========|end point|
   |    A    |         +-----+           \        /          |    B    |
   +---------+            |               \--/\--/           +---------+
                          |

                            Single NAT scenario

   A variation of this case is shown below, i.e., multiple NATs in a
   single path:












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          Internal | External : Internal | External
                   |          :          |
   +---------+     |          :          |       /--\/--\    +---------+
   |  SCTP   |  +-----+       :       +-----+   /        \   |  SCTP   |
   |end point|==| NAT |=======:=======| NAT |==| Internet |==|end point|
   |    A    |  +-----+       :       +-----+   \        /   |    B    |
   +---------+     |          :          |       \--/\--/    +---------+
                   |          :          |

                           Serial NATs scenario

   In this single point traversal scenario, we must acknowledge that
   while one of the main benefits of SCTP multi-homing is redundant
   paths, the NAT function represents a single point of failure in the
   path of the SCTP multi-home association.  However, the rest of the
   path may still benefit from path diversity provided by SCTP multi-
   homing.

   The two SCTP endpoints in this case can be either single-homed or
   multi-homed.  However, the important thing is that the NAT (or NATs)
   in this case sees all the packets of the SCTP association.

4.1.2.  Multi Point Traversal

   This case involves multiple NATs and each NAT only sees some of the
   packets in the SCTP association.  An example is shown below:


            Internal       |      External
                        +------+             /---\/---\
   +---------+  /=======|NAT A |=========\  /          \     +---------+
   |  SCTP   | /        +------+          \/            \    |  SCTP   |
   |end point|/       ...                 |   Internet   |===|end point|
   |    A    |\                            \            /    |    B    |
   +---------+ \        +------+          / \          /     +---------+
                \=======|NAT B |=========/   \---\/---/
                        +------+
                           |

                          Parallel NATs scenario

   This case does NOT apply to a single-homed SCTP association (i.e.,
   BOTH endpoints in the association use only one IP address).  The
   advantage here is that the existence of multiple NAT traversal points
   can preserve the path diversity of a multi-homed association for the
   entire path.  This in turn can improve the robustness of the
   communication.




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4.2.  Limitations of Classical NAPT for SCTP

   Using classical NAPT may result in changing one of the SCTP port
   numbers during the processing which requires the recomputation of the
   transport layer checksum.  Whereas for UDP and TCP this can be done
   very efficiently, for SCTP the checksum (CRC32c) over the entire
   packet needs to be recomputed.  This would considerably add to the
   NAT computational burden, however hardware support may mitigate this
   in some implementations.

   An SCTP endpoint may have multiple addresses but only has a single
   port number.  To make multipoint traversal work, all the NATs
   involved must recognize the packets they see as belonging to the same
   SCTP association and perform port number translation in a consistent
   way.  One possible way of doing this is to use pre-defined table of
   ports and addresses configured within each NAT.  Other mechanisms
   could make use of NAT to NAT communication.  Such mechanisms are not
   to be deployable on a wide scale base and thus not a recommended
   solution.  Therefore the SCTP variant of NAT has been developed.

4.3.  The SCTP Specific Variant of NAT

   In this section we assume that we have multiple SCTP capable hosts
   behind a NAT which has one Public-Address.  Furthermore we are
   focusing in this section on the single point traversal scenario.

   The modification of SCTP packets sent to the public Internet is easy.
   The source address of the packet has to be replaced with the Public-
   Address.  It may also be necessary to establish some state in the NAT
   box to handle incoming packets, which is discussed later.

   For SCTP packets coming from the public Internet the destination
   address of the packets has to be replaced with the Private-Address of
   the host the packet has to be delivered to.  The lookup of the
   Private-Address is based on the External-VTag, External-Port,
   Internal-VTag and the Internal-Port.

   For the SCTP NAT processing the NAT box has to maintain a table of
   Internal-VTag, Internal-Port, External-VTag, External-Port, Private-
   Address, and whether the restart procedure is disabled or not.  An
   entry in that table is called a NAT state control block.  The
   function Create() obtains the just mentioned parameters and returns a
   NAT-State control block.

   The entries in this table fulfill some uniqueness conditions.  There
   must not be more than one entry with the same pair of Internal-Port
   and External-Port.  This rule can be relaxed, if all entries with the
   same Internal-Port and External-Port have the support for the restart



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   procedure enabled.  In this case there must be no more than one entry
   with the same Internal-Port, External-Port and Ext-VTag and no more
   than one entry with the same Internal-Port, External-Port and Int-
   VTag.

   The processing of outgoing SCTP packets containing an INIT-chunk is
   described in the following figure.  The scenario shown is valid for
   all message flows in this section.


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


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

                Create(Initiate-Tag, Int-Port, 0, Ext-Port, Priv-Addr,
                       RestartSupported)
                Returns(NAT-State control block)

              Translate To:

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


   Normally a NAT control block will be created.  However, it is
   possible that there is already a NAT control block with the same
   External-Address, External-Port, Internal-Port, and Internal-VTag but
   different Private-Address.  In this case the INIT MUST be dropped by
   the NAT and an ABORT MUST be sent back to the SCTP host with the
   M-Bit set and an appropriate error cause (see Section 5.1.1 for the
   format).  The source address of the packet containing the ABORT chunk
   MUST be the destination address of the packet containing the INIT
   chunk.

   It is also possible that a connection to External-Address and
   External-Port exists without an Internal-VTag conflict but the
   External-Address does not support the DISABLE_RESTART feature (noted
   in the NAT control block when the prior connection was established).
   In such a case the INIT SHOULD be dropped by the NAT and an ABORT




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   SHOULD be sent back to the SCTP host with the M-Bit set and an
   appropriate error cause (see Section 5.1.1 for the format).

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


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

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


                             Translate To:

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


   The processing of incoming SCTP packets containing INIT-ACK chunks is
   described in the following figure.  The Lookup() function getting as
   input the Internal-VTag, Internal-Port, External-VTag, and External-
   Port, returns the corresponding entry of the NAT table and updates
   the External-VTag by substituting it with the value of the Initiate-
   Tag of the INIT-ACK chunk.  The wildcard character signifies that the
   parameter's value is not considered in the Lookup() function or
   changed in the Update() function, respectively.




















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

                                            INIT-ACK[Initiate-Tag]
                               Pub-Addr:Int-Port <---- Ext-Addr:Ext-Port
                                                Int-VTag

            Lookup(Int-VTag, Int-Port, *, Ext-Port)
            Update(*, *, Initiate-Tag, *)

            Returns(NAT-State control block containing Priv-Addr)

                  INIT-ACK[Initiate-Tag]
   Priv-Addr:Int-Port <------ Ext-Addr:Ext-Port
                      Int-VTag


   In the case Lookup fails, the SCTP packet is dropped.  The Update
   routine inserts the External-VTag (the Initiate-Tag of the INIT-ACK
   chunk) in the NAT state control block.

   The processing of incoming SCTP packets containing an ABORT or
   SHUTDOWN-COMPLETE chunk with the T-Bit set is described in the
   following figure.


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

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

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

            Returns(NAT-State control block containing Priv-Addr)

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


   The processing of other incoming SCTP packets is described in the
   following figure.



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

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

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

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

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


   For an incoming packet containing an INIT-chunk a table lookup is
   made only based on the addresses and port numbers.  If an entry with
   an External-VTag of zero is found, it is considered a match and the
   External-VTag is updated.

   This allows the handling of INIT-collision through NAT.

5.  Data Formats

   This section defines the formats used to support NAT traversal.
   Section 5.1 and Section 5.2 describe chunks and error causes sent by
   NATs and received by SCTP end points.  Section 5.3 describes
   parameters sent by SCTP end points and used by NATs and SCTP end
   points.

5.1.  Modified Chunks

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

5.1.1.  Extended ABORT Chunk

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



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   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.

   [NOTE:

      ASSIGNMENT OF M-BIT TO BE CONFIRMED BY IANA.

   ]

5.1.2.  Extended ERROR Chunk

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

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

   [NOTE:

      ASSIGNMENT OF M-BIT TO BE CONFIRMED BY IANA.

   ]

5.2.  New Error Causes

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

5.2.1.  VTag and Port Number Collision Error Cause

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




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   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.

   [NOTE:

      ASSIGNMENT OF CAUSE-CODE TO BE CONFIRMED BY IANA.

   ]

5.2.2.  Missing State Error Cause

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

   Cause Code: 2 bytes (unsigned integer)
      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.



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   [NOTE:

      ASSIGNMENT OF CAUSE-CODE TO BE CONFIRMED BY IANA.

   ]

5.2.3.  Port Number Collision Error Cause

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

   Cause Code: 2 bytes (unsigned integer)
      This 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.

   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.

   [NOTE:

      ASSIGNMENT OF CAUSE-CODE TO BE CONFIRMED BY IANA.

   ]

5.3.  New Parameters

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








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5.3.1.  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.

   [NOTE:

      ASSIGNMENT OF PARAMETER TYPE TO BE CONFIRMED BY IANA.

   ]

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

5.3.2.  VTags Parameter

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

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




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   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.

   [NOTE:

      ASSIGNMENT OF PARAMETER TYPE TO BE CONFIRMED BY IANA.

   ]

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

6.  Procedures for SCTP End Points and NATs

6.1.  Overview

   When an SCTP endpoint is behind an SCTP-aware NAT a number of
   problems may arise as it tries to communicate with its peer:

   o  IP addresses can not not be included in the SCTP packet.  This is
      discussed in Section 6.2.




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   o  More than one host 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 6.3.

   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 6.4.

   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 6.5.

   o  NAT boxes need to deal with SCTP packets being fragmented at the
      IP layer.  This is discussed in Section 6.6.

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

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

6.2.  Association Setup Considerations

   The association setup procedure defined in [RFC4960] allows multi-
   homed SCTP end points to exchange its IP-addresses by using IPv4 or
   IPv6 address parameters in the INIT and INIT-ACK chunks.  However,
   this can't be used when NATs are present.

   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 6.7 MUST be used.

   The INIT and INIT-ACK chunk SHOULD contain the Disable Restart
   parameter defined in Section 5.3.1.

6.3.  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 service provided by some
   hosts in the Internet.  This means that the External-Port is the
   same.  If they both choose the same Internal-Port and Internal-VTag,



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   the NAT box cannot distinguish between incoming packets anymore.  But
   this is very unlikely.  The Internal-VTags are chosen at random and
   if the Internal-Ports are also chosen from the ephemeral port range
   at random 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 therefore avoid collisions deterministically.

   The same can happen with the External-VTag 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 5.1.1).  If 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
   and the appropriate error cause code for colliding NAT table state is
   included, MUST reinitiate the association setup procedure after
   choosing a new initiate tag, if the association is in COOKIE-WAIT
   state.  In any other state, the SCTP endpoint MUST 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 5.2.1).

6.4.  Handling of Internal Port Number Collisions

   When two SCTP hosts are behind an SCTP-aware NAT 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.

   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 address and port numbers.  For
   the server it looks like the association is being restarted.  To



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   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 5.2.3) 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.

6.5.  Handling of Missing State

   If the NAT box receives a packet from the internal network for which
   the lookup procedure does not find an entry in the 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



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   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 5.2.2) MUST be included and the new M-bit of the ERROR
   chunk MUST be set (see Section 5.1.2).

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

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

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

   o  Generate a new ASCONF chunk containing the VTags parameter (see
      Section 5.3.2) 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 5.2.1).

   If an SCTP endpoint receives an ERROR with 'Internal Port Number and
   Verification Tag collision' as the error cause and the packet in the



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

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

6.6.  Handling of Fragmented SCTP Packets

   A NAT box MUST support IP reassembly of received fragmented SCTP
   packets.  The fragments may arrive in any order.

   When an SCTP packet has to be fragmented by the NAT box and the IP
   header forbids fragmentation a corresponding ICMP packet SHOULD be
   sent.

6.7.  Multi-Point Traversal Considerations

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







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7.  Various Examples of NAT Traversals

   Please note that this section is informational only.

   The addresses being used in the following examples are IPv4 addresses
   for private-use networks and for documentation as specified in
   [RFC6890].  However, the method described here is not limited to this
   NAT44 case.

7.1.  Single-homed Client to Single-homed Server

   The internal client starts the association with the external server
   via a four-way-handshake.  Host A starts by sending an INIT chunk.


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

      INIT[Initiate-Tag = 1234]
   10.0.0.1:1 ------> 203.0.113.1:2
           Ext-VTtag = 0


   A NAT entry is created, the source address is substituted and the
   packet is sent on:


          NAT creates entry:
          +---------+--------+----------+--------+-----------+
   NAT    |  Int    |  Int   |   Ext    |   Ext  |    Priv   |
          |  VTag   |  Port  |   VTag   |   Port |    Addr   |
          +---------+--------+----------+--------+-----------+
          |  1234   |    1   |     0    |    2   |  10.0.0.1 |
          +---------+--------+----------+--------+-----------+

                                   INIT[Initiate-Tag = 1234]
                     192.0.2.1:1 ------------------------> 203.0.113.1:2
                                           Ext-VTtag = 0






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   Host B receives the INIT and sends an INIT-ACK with the NAT's
   external address as destination address.


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

                                    INIT-ACK[Initiate-Tag = 5678]
                      192.0.2.1:1 <----------------------- 203.0.113.1:2
                                            Int-VTag = 1234

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


   INIT-ACK[Initiate-Tag = 5678]
   10.0.0.1:1 <------ 203.0.113.1:2
             Int-VTag = 1234


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





















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

            COOKIE-ECHO
   10.0.0.1:1 ------> 203.0.113.1:2
          Ext-VTag = 5678

                                         COOKIE-ECHO
                      192.0.2.1:1 -----------------------> 203.0.113.1:2
                                       Ext-VTag = 5678


                                          COOKIE-ACK
                      192.0.2.1:1 <----------------------- 203.0.113.1:2
                                       Int-VTag = 1234

               COOKIE-ACK
   10.0.0.1:1 <------ 203.0.113.1:2
              Int-VTag = 1234


7.2.  Single-homed Client to Multi-homed Server

   The internal client is single-homed whereas the external server is
   multi-homed.  The client (Host A) sends an INIT like in the single-
   homed case.






















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                                                  +--------+
                                  /--\/--\      /-|Router 1| \
   +------+         +-----+      /        \    /  +--------+  \ +------+
   | Host | <-----> | NAT | <-> | Internet | ==                =| Host |
   |   A  |         +-----+      \        /    \  +--------+  / |   B  |
   +------+                       \--/\--/      \-|Router 2|-/  +------+
                                                  +--------+

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


    INIT[Initiate-Tag = 1234]
   10.0.0.1:1 ---> 203.0.113.1:2
          Ext-VTag = 0


   NAT creates entry:


          +---------+--------+----------+--------+-----------+
   NAT    |  Int    |  Int   |   Ext    |   Ext  |    Priv   |
          |  VTag   |  Port  |   VTag   |   Port |    Addr   |
          +---------+--------+----------+--------+-----------+
          |  1234   |    1   |     0    |    2   |  10.0.0.1 |
          +---------+--------+----------+--------+-----------+

                                INIT[Initiate-Tag = 1234]
                   192.0.2.1:1 --------------------------> 203.0.113.1:2
                                     Ext-VTag = 0


   The server (Host B) includes its two addresses in the INIT-ACK chunk,
   which results in two NAT entries.















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                                                  +--------+
                                  /--\/--\      /-|Router 1| \
   +------+         +-----+      /        \    /  +--------+  \ +------+
   | Host | <-----> | NAT | <-> | Internet | ==                =| Host |
   |   A  |         +-----+      \        /    \  +--------+  / |   B  |
   +------+                       \--/\--/      \-|Router 2|-/  +------+
                                                  +--------+

                  INIT-ACK[Initiate-tag = 5678, IP-Addr = 203.0.113.129]
                   192.0.2.1:1 <-------------------------- 203.0.113.1:2
                                    Int-VTag = 1234


   NAT does need to change the table for second address:


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


   INIT-ACK[Initiate-Tag = 5678]
   10.0.0.1:1 <--- 203.0.113.1:2
            Int-VTag = 1234


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




















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                                                  +--------+
                                  /--\/--\      /-|Router 1| \
   +------+         +-----+      /        \    /  +--------+  \ +------+
   | Host | <-----> | NAT | <-> | Internet | ==                =| Host |
   |   A  |         +-----+      \        /    \  +--------+  / |   B  |
   +------+                       \--/\--/      \-|Router 2|-/  +------+
                                                  +--------+

          COOKIE-ECHO
   10.0.0.1:1 ---> 203.0.113.1:2
          ExtVTag = 5678

                                      COOKIE-ECHO
                   192.0.2.1:1 --------------------------> 203.0.113.1:2
                                     Ext-VTag = 5678


                                        COOKIE-ACK
                   192.0.2.1:1 <-------------------------- 203.0.113.1:2
                                      Int-VTag = 1234

             COOKIE-ACK
   10.0.0.1:1 <--- 203.0.113.1:2
            Int-VTag = 1234


7.3.  Multihomed Client and Server

   The client (Host A) sends an INIT to the server (Host B), but does
   not include the second address.





















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                  +-------+
               /--| NAT 1 |--\       /--\/--\
   +------+   /   +-------+   \     /        \     +--------+
   | Host |===                 ====| Internet |====| Host B |
   |   A  |   \   +-------+   /     \        /     +--------+
   +------+    \--| NAT 2 |--/       \--/\--/
                  +-------+

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

    INIT[Initiate-Tag = 1234]
   10.0.0.1:1 --------> 203.0.113.1:2
            Ext-VTag = 0


   NAT 1 creates entry:


          +---------+--------+----------+--------+-----------+
   NAT 1  |  Int    |  Int   |   Ext    |   Ext  |    Priv   |
          |  VTag   |  Port  |   VTag   |   Port |    Addr   |
          +---------+--------+----------+--------+-----------+
          |  1234   |    1   |     0    |    2   |  10.0.0.1 |
          +---------+--------+----------+--------+-----------+


                                   INIT[Initiate-Tag = 1234]
                        192.0.2.1:1 ---------------------> 203.0.113.1:2
                                           ExtVTag = 0


   Host B includes its second address in the INIT-ACK, which results in
   two NAT entries in NAT 1.















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                        +-------+
               /--------| NAT 1 |--------\       /--\/--\
   +------+   /         +-------+         \     /        \    +--------+
   | Host |===                             ====| Internet |===| Host B |
   |   A  |   \         +-------+         /     \        /    +--------+
   +------+    \--------| NAT 2 |--------/       \--/\--/
                        +-------+

                  INIT-ACK[Initiate-Tag = 5678, IP-Addr = 203.0.113.129]
                      192.0.2.1:1 <----------------------- 203.0.113.1:2
                                      Int-VTag = 1234


   NAT 1 does not need to update the table for second address:


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


     INIT-ACK[Initiate-Tag = 5678]
   10.0.0.1:1 <-------- 203.0.113.1:2
               Int-VTag = 1234


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




















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                        +-------+
               /--------| NAT 1 |--------\       /--\/--\
   +------+   /         +-------+         \     /        \    +--------+
   | Host |===                             ====| Internet |===| Host B |
   |   A  |   \         +-------+         /     \        /    +--------+
   +------+    \--------| NAT 2 |--------/       \--/\--/
                        +-------+

             COOKIE-ECHO
   10.0.0.1:1 --------> 203.0.113.1:2
             Ext-VTag = 5678

                                           COOKIE-ECHO
                           192.0.2.1:1 ------------------> 203.0.113.1:2
                                         Ext-VTag = 5678


                                           COOKIE-ACK
                           192.0.2.1:1 <------------------ 203.0.113.1:2
                                         Int-VTag = 1234

               COOKIE-ACK
   10.0.0.1:1 <------- 203.0.113.1:2
              Int-VTag = 1234


   Host A announces its second address in an ASCONF chunk.  The address
   parameter contains an undefined address (0) to indicate that the
   source address should be added.  The lookup address parameter within
   the ASCONF chunk will also contain the pair of VTags (external and
   internal) so that the NAT may populate its table completely with this
   single packet.


                        +-------+
               /--------| NAT 1 |--------\       /--\/--\
   +------+   /         +-------+         \     /        \    +--------+
   | Host |===                             ====| Internet |===| Host B |
   |   A  |   \         +-------+         /     \        /    +--------+
   +------+    \--------| NAT 2 |--------/       \--/\--/
                        +-------+

   ASCONF [ADD-IP=0.0.0.0, INT-VTag=1234, Ext-VTag = 5678]
   10.1.0.1:1 --------> 203.0.113.129:2
            Ext-VTag = 5678


   NAT 2 creates complete entry:



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


                   ASCONF [ADD-IP,Int-VTag=1234, Ext-VTag = 5678]
                    192.0.2.129:1 ---------------------> 203.0.113.129:2
                                         Ext-VTag = 5678

                                         ASCONF-ACK
                    192.0.2.129:1 <--------------------- 203.0.113.129:2
                                      Int-VTag = 1234

           ASCONF-ACK
 10.1.0.1:1 <----- 203.0.113.129:2
          Int-VTag = 1234


7.4.  NAT Loses Its State

   Association is already established between Host A and Host B, when
   the NAT loses its state and obtains a new public address.  Host A
   sends a DATA chunk to Host B.


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

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

                  DATA
   10.0.0.1:1 ----------> 203.0.113.1:2
               Ext-VTag = 5678


   The NAT box cannot find entry for the association.  It sends ERROR
   message with the M-Bit set and the cause "NAT state missing".



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

     ERROR [M-Bit, NAT state missing]
   10.0.0.1:1 <---------- 203.0.113.1:2
             Ext-VTag = 5678


   On reception of the ERROR message, Host A sends an ASCONF chunk
   indicating that the former information has to be deleted and the
   source address of the actual packet added.


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

 ASCONF [ADD-IP,DELETE-IP,Int-VTag=1234, Ext-VTag = 5678]
 10.0.0.1:1 ----------> 203.0.113.129:2
           Ext-VTag = 5678

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

         ASCONF [ADD-IP,DELETE-IP,Int-VTag=1234, Ext-VTag = 5678]
                        192.0.2.2:1 -------------------> 203.0.113.129:2
                                        Ext-VTag = 5678


   Host B adds the new source address and deletes all former entries.












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

                                          ASCONF-ACK
                        192.0.2.2:1 <------------------- 203.0.113.129:2
                                         Int-VTag = 1234

                ASCONF-ACK
 10.1.0.1:1 <---------- 203.0.113.129:2
           Int-VTag = 1234

                DATA
 10.0.0.1:1 ----------> 203.0.113.1:2
          Ext-VTag = 5678
                                           DATA
                        192.0.2.2:1 -------------------> 203.0.113.129:2
                                      Ext-VTag = 5678


7.5.  Peer-to-Peer Communication

   If two hosts are behind NATs, they have to get knowledge of the
   peer's public address.  This can be achieved with a so-called
   rendezvous server.  Afterwards the destination addresses are public,
   and the association is set up with the help of the INIT collision.
   The NAT boxes create their entries according to their internal peer's
   point of view.  Therefore, NAT A's Internal-VTag and Internal-Port
   are NAT B's External-VTag and External-Port, respectively.  The
   naming of the verification tag in the packet flow is done from the
   sending peer's point of view.


















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


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

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

   INIT[Initiate-Tag = 1234]
   10.0.0.1:1 --> 203.0.113.1:2
           Ext-VTag = 0


   NAT A creates entry:


          +---------+--------+----------+--------+-----------+
   NAT A  |  Int    |  Int   |   Ext    |   Ext  |    Priv   |
          |  VTag   |  Port  |   VTag   |   Port |    Addr   |
          +---------+--------+----------+--------+-----------+
          |  1234   |    1   |     0    |    2   |  10.0.0.1 |
          +---------+--------+----------+--------+-----------+

                           INIT[Initiate-Tag = 1234]
                  192.0.2.1:1 ----------------> 203.0.113.1:2
                               Ext-VTag = 0


   NAT B processes INIT, but cannot find an entry.  The SCTP packet is
   silently discarded and leaves the NAT table of NAT B unchanged.

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




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   Now Host B sends INIT, which is processed by NAT B.  Its parameters
   are used to create an entry.


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

                                               INIT[Initiate-Tag = 5678]
                                              192.0.2.1:1 <-- 10.1.0.1:2
                                                            Ext-VTag = 0

          +---------+--------+-----------+----------+--------+
   NAT B  |  Int    |  Int   |    Priv   |   Ext    |   Ext  |
          |  VTag   |  Port  |    Addr   |   VTag   |   Port |
          +---------+--------+-----------+----------+--------+
          |  5678   |    2   |  10.1.0.1 |     0    |    1   |
          +---------+--------+-----------+----------+--------+

                             INIT[Initiate-Tag = 5678]
                  192.0.2.1:1  <--------------- 203.0.113.1:2
                                     Ext-VTag = 0


   NAT A processes INIT.  As the outgoing INIT of Host A has already
   created an entry, the entry is found and updated:





















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

                  VTag != Int-VTag, but Ext-VTag == 0, find entry.
          +---------+--------+----------+--------+-----------+
   NAT A  |  Int    |  Int   |   Ext    |   Ext  |    Priv   |
          |  VTag   |  Port  |   VTag   |   Port |    Addr   |
          +---------+--------+----------+--------+-----------+
          |   1234  |   1    |   5678   |    2   |  10.0.0.1 |
          +---------+--------+----------+--------+-----------+

    INIT[Initiate-tag = 5678]
   10.0.0.1:1 <-- 203.0.113.1:2
             Ext-VTag = 0


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





























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

   INIT-ACK[Initiate-Tag = 1234]
   10.0.0.1:1 --> 203.0.113.1:2
         Ext-VTag = 5678


                       INIT-ACK[Initiate-Tag = 1234]
                  192.0.2.1:1 ----------------> 203.0.113.1:2
                                Ext-VTag = 5678

                                                NAT B updates entry:

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

                                           INIT-ACK[Initiate-Tag = 1234]
                                              192.0.2.1:1 --> 10.1.0.1:2
                                                         Ext-VTag = 5678


   The lookup for COOKIE-ECHO and COOKIE-ACK is successful.



















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

                                                     COOKIE-ECHO
                                              192.0.2.1:1 <-- 10.1.0.1:2
                                                   Ext-VTag = 1234

                                 COOKIE-ECHO
                  192.0.2.1:1 <------------- 203.0.113.1:2
                                 Ext-VTag = 1234

          COOKIE-ECHO
   10.0.0.1:1 <-- 203.0.113.1:2
          Ext-VTag = 1234

          COOKIE-ACK
   10.0.0.1:1 --> 203.0.113.1:2
          Ext-VTag = 5678

                                 COOKIE-ACK
                  192.0.2.1:1 ----------------> 203.0.113.1:2
                                 Ext-VTag = 5678

                                                       COOKIE-ACK
                                              192.0.2.1:1 --> 10.1.0.1:2
                                                    Ext-VTag = 5678


8.  Socket API Considerations

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

   Please note that this section is informational only.

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








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8.1.  Get or Set the NAT Friendliness (SCTP_NAT_FRIENDLY)

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

   struct sctp_assoc_value {
     sctp_assoc_t assoc_id;
     uint32_t assoc_value;
   };

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

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

9.  IANA Considerations

   [NOTE to RFC-Editor:

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

   ]

   [NOTE to RFC-Editor:

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

   ]

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

9.1.  New Chunk Flags for Two Existing Chunk Types

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

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





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

9.2.  Three New Error Causes

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

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









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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] |
          +-------+--------------------------------+-----------+

9.3.  Two New Chunk Parameter Types

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

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

                           Chunk Parameter Types

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

10.  Security Considerations

   State maintenance within a NAT is always a subject of possible Denial
   Of Service attacks.  This document recommends that at a minimum a NAT
   runs a timer on any SCTP state so that old association state can be
   cleaned up.

   For SCTP end points, this document does not add any additional
   security considerations to the ones given in [RFC4960], [RFC4895],
   and [RFC5061].  In particular, SCTP is protected by the verification
   tags and the usage of [RFC4895] against off-path attackers.

11.  Acknowledgments

   The authors wish to thank Gorry Fairhurst, Bryan Ford, David Hayes,
   Alfred Hines, Karen E.  E.  Nielsen, Henning Peters, Timo Voelker,
   Dan Wing, and Qiaobing Xie for their invaluable comments.





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   In addition, the authors wish to thank David Hayes, Jason But, and
   Grenville Armitage, the authors of [DOI_10.1145_1496091.1496095], for
   their suggestions.

12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4895]  Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
              "Authenticated Chunks for the Stream Control Transmission
              Protocol (SCTP)", RFC 4895, DOI 10.17487/RFC4895, August
              2007, <https://www.rfc-editor.org/info/rfc4895>.

   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, DOI 10.17487/RFC4960, September 2007,
              <https://www.rfc-editor.org/info/rfc4960>.

   [RFC5061]  Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
              Kozuka, "Stream Control Transmission Protocol (SCTP)
              Dynamic Address Reconfiguration", RFC 5061,
              DOI 10.17487/RFC5061, September 2007,
              <https://www.rfc-editor.org/info/rfc5061>.

   [RFC6096]  Tuexen, M. and R. Stewart, "Stream Control Transmission
              Protocol (SCTP) Chunk Flags Registration", RFC 6096,
              DOI 10.17487/RFC6096, January 2011,
              <https://www.rfc-editor.org/info/rfc6096>.

12.2.  Informative References

   [DOI_10.1145_1496091.1496095]
              Hayes, D., But, J., and G. Armitage, "Issues with network
              address translation for SCTP", ACM SIGCOMM Computer
              Communication Review Vol. 39, pp. 23,
              DOI 10.1145/1496091.1496095, December 2008.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.







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   [RFC6458]  Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
              Yasevich, "Sockets API Extensions for the Stream Control
              Transmission Protocol (SCTP)", RFC 6458,
              DOI 10.17487/RFC6458, December 2011,
              <https://www.rfc-editor.org/info/rfc6458>.

   [RFC6890]  Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman,
              "Special-Purpose IP Address Registries", BCP 153,
              RFC 6890, DOI 10.17487/RFC6890, April 2013,
              <https://www.rfc-editor.org/info/rfc6890>.

   [RFC6951]  Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream
              Control Transmission Protocol (SCTP) Packets for End-Host
              to End-Host Communication", RFC 6951,
              DOI 10.17487/RFC6951, May 2013,
              <https://www.rfc-editor.org/info/rfc6951>.

Authors' Addresses

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

   Email: randall@lakerest.net


   Michael Tuexen
   Muenster University of Applied Sciences
   Stegerwaldstrasse 39
   48565 Steinfurt
   DE

   Email: tuexen@fh-muenster.de


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

   Email: i.ruengeler@fh-muenster.de








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