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Versions: (draft-stewart-natsupp-tsvwg) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15

Network Working Group                                         R. Stewart
Internet-Draft                                             Netflix, Inc.
Intended status: Standards Track                               M. Tuexen
Expires: May 7, 2020                                        I. Ruengeler
                                        Muenster Univ. of Appl. Sciences
                                                        November 4, 2019


Stream Control Transmission Protocol (SCTP) Network Address Translation
                                Support
                      draft-ietf-tsvwg-natsupp-14

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 NAT devices 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 May 7, 2020.






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Copyright Notice

   Copyright (c) 2019 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 Endpoints and NAT Devices . . . . . . . .  17
     6.1.  Association Setup Considerations for Endpoints  . . . . .  18
     6.2.  Handling of Internal Port Number and Verification Tag
           Collisions  . . . . . . . . . . . . . . . . . . . . . . .  18
       6.2.1.  NAT Device Considerations . . . . . . . . . . . . . .  19
       6.2.2.  Endpoint Considerations . . . . . . . . . . . . . . .  19
     6.3.  Handling of Internal Port Number Collisions . . . . . . .  19
       6.3.1.  NAT Device Considerations . . . . . . . . . . . . . .  20
       6.3.2.  Endpoint Considerations . . . . . . . . . . . . . . .  20
     6.4.  Handling of Missing State . . . . . . . . . . . . . . . .  21
       6.4.1.  NAT Device Considerations . . . . . . . . . . . . . .  21



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       6.4.2.  Endpoint Considerations . . . . . . . . . . . . . . .  21
     6.5.  Handling of Fragmented SCTP Packets by NAT Devices  . . .  23
     6.6.  Multi-Point Traversal Considerations for Endpoints  . . .  23
   7.  Various Examples of NAT Traversals  . . . . . . . . . . . . .  23
     7.1.  Single-homed Client to Single-homed Server  . . . . . . .  23
     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 NAT devices
   so that only a translation of IP addresses is supported.  The end
   result of this is that only one SCTP-capable host can successfully
   operate behind such a NAT and this host can only be single-homed.
   The only alternative for supporting legacy NAT devices is to use UDP
   encapsulation as specified in [RFC6951].

   This document specifies procedures allowing a NAT to support SCTP by
   providing similar features to those provided by a NAPT for TCP and
   other supported protocols.  The document also specifies a set of data
   formats for SCTP packets and a set of SCTP endpoint procedures to
   support NAT traversal.  An SCTP implementation supporting these



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   procedures can assure that in both single-homed and multi-homed cases
   a NAT will maintain the appropriate state without the NAT needing to
   change port numbers.

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

   o  It is desirable for SCTP internal end-hosts on multiple platforms
      to be able to share a NAT's public IP address 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 NAT devices 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 Device | 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 it can be seen that when a NAT device does not support
   the extension no communication can occur.  This assumes that the NAT
   device does not handle SCTP packets at all and all SCTP packets sent
   externally from behind a NAT device are discarded by the NAT.  In
   some cases, where the NAT device supports the feature but one of the
   two hosts does not support the feature, communication may occur but



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   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", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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) (see
      Section 3.1 of [RFC4960]) 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
      device that 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  |
   |endpoint|=========| NAT |=======| Internet |==========|endpoint|
   |    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  |
   |endpoint|=========| NAT |========= | Internet | ========|endpoint|
   |    A   |         +-----+           \        /          |    B   |
   +--------+            |               \--/\--/           +--------+
                         |

                            Single NAT scenario

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












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

                        Serial NAT Devices scenario

   Although one of the main benefits of SCTP multi-homing is redundant
   paths, In this single point traversal scenario 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 device (or
   NAT devices) in this case sees all the packets of the SCTP
   association.

4.1.2.  Multi Point Traversal

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


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

                       Parallel NAT devices 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 by the NAPT device.  Whereas for UDP and TCP
   this can be done very efficiently, for SCTP the checksum (CRC32c)
   over the entire packet needs to be recomputed.  See Appendix B of
   [RFC4960] for details of the CRC32c computation.  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 NAT devices
   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 a pre-defined table of
   ports and addresses configured within each NAT.  Other mechanisms
   could make use of NAT to NAT communication.  Such mechanisms have not
   been deployabled on a wide scale base and thus are not a recommended
   solution.  Therefore the SCTP variant of NAT has been developed.

4.3.  The SCTP-Specific Variant of NAT

   In this section it is allowed that there are multiple SCTP capable
   hosts behind a NAT that has one Public-Address.  Furthermore this
   section focuses on the single point traversal scenario.

   The modification of SCTP packets sent to the public Internet is
   simple: 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 device to later handle incoming packets.

   For the SCTP NAT processing the NAT device has to maintain a NAT
   binding 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 NAT binding table is called a NAT
   state control block.  The function Create() obtains the just
   mentioned parameters and returns a NAT-State control block.

   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.

   The entries in the NAT binding table need to fulfill some uniqueness
   conditions.  There must not be more than one entry NAT binding table
   with the same pair of Internal-Port and External-Port.  This rule can



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   be relaxed, if all NAT binding table entries with the same Internal-
   Port and External-Port have the support for the restart 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
   NAT binding table 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 binding table entry will be created.

   However, it is possible that there is already a NAT binding table
   entry 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



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   in the NAT binding table entry when the prior connection was
   established).  In such a case the INIT SHOULD be dropped by the NAT
   and an ABORT SHOULD be sent back to the SCTP host with the M-Bit set
   and an appropriate error cause (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 binding 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
   NAT devices and received by SCTP endpoints.  Section 5.3 describes
   parameters sent by SCTP endpoints and used by NAT devices and SCTP
   endpoints.

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 to RFC-Editor:

      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 to RFC-Editor:

      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.  IANA is requested to assign
      the value 0x00B0 for this cause code.

   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 to RFC-Editor:

      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.  IANA is requested to assign the value 0x00B1
      for this cause code.

   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 to RFC-Editor:

      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.  IANA is requested to assign the value
      0x00B2 for this cause code.

   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 to RFC-Editor:

      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.  IANA is requested to assign the value 0xC007
      for this parameter type.

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

   [NOTE to RFC-Editor:

      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.  IANA is requested to assign the value 0xC008 for this
      parameter type.

   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 to RFC-Editor:

      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 Endpoints and NAT Devices

   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 6.1.





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   o  More than one host behind a NAT device could select 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 6.2.

   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 6.3.

   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 6.4.

   o  NAT devices need to deal with SCTP packets being fragmented at the
      IP layer.  This is discussed in Section 6.5.

   o  An SCTP endpoint may be behind two NAT devices providing
      redundancy.  The method to set up this scenario is discussed in
      Section 6.6.

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

6.1.  Association Setup Considerations for Endpoints

   The association setup procedure defined in [RFC4960] allows multi-
   homed SCTP endpoints to exchange its IP-addresses by using IPv4 or
   IPv6 address parameters in the INIT and INIT-ACK chunks.  However,
   this doesn't work when NAT devices 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 6.6 MUST be used.

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

6.2.  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



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   same.  If they both choose the same Internal-Port and Internal-VTag,
   the NAT device 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 that has to match.
   A NAPT device 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.

6.2.1.  NAT Device Considerations

   If the NAT device detects a collision of internal port numbers and
   verification tags, it MUST send an ABORT chunk with the M-bit set if
   the collision is triggered by an INIT or INIT-ACK chunk.  If such a
   collision is triggered by an ASCONF chunk, it MUST send an ERROR
   chunk with the M-bit.  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 ERROR or ABORT chunk MUST include the error cause
   with cause code 'VTag and Port Number Collision' (see Section 5.2.1).

6.2.2.  Endpoint Considerations

   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 binding 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.

6.3.  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.



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6.3.1.  NAT Device Considerations

   The NAT, when processing the INIT-ACK, should note in its NAT binding
   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 device does the following when processing
   an INIT:

   o  If the INIT is destined to an external address and port for which
      the NAT device has no outbound connection, it MUST allow the INIT
      creating an NAT binding table entry.

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

   o  If the external server does not support the Disable Restart
      extension the NAT device 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 sent in response to the INIT 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 in response to the ASCONF chunk.

6.3.2.  Endpoint Considerations

   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
   overcome this limitation the client sends a Disable Restart parameter
   in the INIT-chunk.

   When the server receives this parameter it does the following:

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

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






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   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.

6.4.  Handling of Missing State

6.4.1.  NAT Device Considerations

   If the NAT device receives a packet from the internal network for
   which the lookup procedure does not find an entry in the NAT binding
   table, a packet containing an ERROR chunk is sent back with the M-bit
   set.  The source address of the packet containing the ERROR chunk
   MUST be the destination address of the incoming SCTP packet.  The
   verification tag is reflected and the T-bit is set.  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 error cause 'Missing State' (see
   Section 5.2.2) MUST be included and the M-bit of the ERROR chunk MUST
   be set (see Section 5.1.2).

   If the NAT device receives a packet for which it has no NAT binding
   table entry and the packet contains an ASCONF chunk with the VTags
   parameter, the NAT device MUST update its NAT binding table according
   to the verification tags in the VTags parameter and the optional
   Disable Restart parameter.

6.4.2.  Endpoint Considerations

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

   o  It SHOULD validate that the verification tag is reflected by
      looking at the VTag that would have been included in the outgoing
      packet.  If the validation fails, discard the incoming ERROR
      chunk.

   o  It SHOULD validate that the peer of the SCTP association supports
      the dynamic address extension.  If the validation fails, discard
      the incoming ERROR chunk.

   o  It SHOULD 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 device can recover the appropriate




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      state.  The procedures for generating an ASCONF chunk can be found
      in [RFC5061].

   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 and 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
   Error Chunk contains an ASCONF with the VTags parameter, careful
   examination of the association is required.  The endpoint does the
   following:

   o  It MUST validate that the verification tag is reflected by looking
      at the VTag that would have been included in the outgoing packet.
      If the validation fails, it MUST discard the packet.

   o  It MUST validate that the peer of the SCTP association supports
      the dynamic address extension.  If the peer does not support it,
      the NAT Device MUST discard the incoming ERROR chunk.

   o  If the association is attempting to add an address (i.e. following
      the procedures in Section 6.6) then the endpoint MUST NOT consider
      the address part of the association and SHOULD make no further
      attempt to add the address (i.e. cancel any ASCONF timers and
      remove any record of the path), since the NAT devie 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 device, then the endpoint MUST
      abort the association.  This would occur only if the local NAT
      device 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 device looses its
      state).





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6.5.  Handling of Fragmented SCTP Packets by NAT Devices

   A NAT device 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 device and the IP
   header forbids fragmentation a corresponding ICMP packet SHOULD be
   sent.

6.6.  Multi-Point Traversal Considerations for Endpoints

   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 NAT
   devices.  The address to add is the wildcard address and the lookup
   address SHOULD also contain the VTags parameter and optionally the
   Disable Restart parameter as illustrated above.

7.  Various Examples of NAT Traversals

   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




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   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


   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 NAT binding table for the 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 NAT binding table for the 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 NAT binding table entry
   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 a 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







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   The NAT device cannot find an entry in the NAT binding table for the
   association.  It sends ERROR an message with the M-Bit set and the
   cause "NAT state missing".


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

     ERROR [M-Bit, NAT state missing]
   10.0.0.1:1 <---------- 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 to this association and deletes
   all other addresses from this association.






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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 NAT devices and want to communicate with each
   other, 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 devices 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 (internal/external) of the
   verification tag in the packet flow is done from the sending host's
   point of view.

















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


   NAT Binding 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 binding 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 sends 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 requested 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 requested 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 requested 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 requested 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 requested 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
   requested 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 endpoints, 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>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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>.



Stewart, et al.            Expires May 7, 2020                 [Page 43]


Internet-Draft              SCTP NAT Support               November 2019


   [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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