Network Working Group                                         C. Porfiri
Internet-Draft                                               Ericsson AB
Intended status: Standards Track                           29 April 2022
Expires: 31 October 2022


Stream Control Transmission Protocol (SCTP) Network Address Translation
                                Support
                  draft-porfiri-tsvwg-sctp-natsupp-03

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 functions for TCP that allows multiple hosts to
   reside behind a NAT function and yet share a single IPv4 address,
   even when two hosts (behind a NAT function) 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 needed for the SCTP
   endpoints and the mechanisms for NAT functions necessary to provide
   similar features of NAPT in the single point and multipoint 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 31 October 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.



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   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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Motivation and Overview . . . . . . . . . . . . . . . . . . .   6
     4.1.  SCTP NAT Traversal Scenarios  . . . . . . . . . . . . . .   6
       4.1.1.  Single Point Traversal  . . . . . . . . . . . . . . .   6
       4.1.2.  Multipoint Traversal  . . . . . . . . . . . . . . . .   8
     4.2.  Limitations of Classical NAPT for SCTP  . . . . . . . . .   9
     4.3.  The SCTP-Specific Variant of NAT  . . . . . . . . . . . .   9
     4.4.  Compatibility and increamental deployment . . . . . . . .  14
     4.5.  Differences with Current NAT Support Draft  . . . . . . .  15
   5.  Data Formats  . . . . . . . . . . . . . . . . . . . . . . . .  15
     5.1.  Modified Chunks . . . . . . . . . . . . . . . . . . . . .  16
       5.1.1.  Extended ABORT Chunk  . . . . . . . . . . . . . . . .  16
       5.1.2.  Extended ERROR Chunk  . . . . . . . . . . . . . . . .  16
       5.1.3.  Extended INIT-ACK Chunk . . . . . . . . . . . . . . .  16
     5.2.  New Error Causes  . . . . . . . . . . . . . . . . . . . .  17
       5.2.1.  Port Number Collision Error Cause . . . . . . . . . .  17
     5.3.  New Parameters  . . . . . . . . . . . . . . . . . . . . .  18
       5.3.1.  Repetita Juvant Parameter . . . . . . . . . . . . . .  18
   6.  Procedures for SCTP Endpoints and NAT Functions . . . . . . .  18
     6.1.  Association Setup Considerations for Endpoints  . . . . .  19
     6.2.  Association Setup Considerations for NAT  . . . . . . . .  19
     6.3.  Handling of Internal Port Number Collisions . . . . . . .  19
       6.3.1.  NAT Function Considerations . . . . . . . . . . . . .  20
       6.3.2.  Endpoint Considerations . . . . . . . . . . . . . . .  21
     6.4.  Handling of Missing State . . . . . . . . . . . . . . . .  21
       6.4.1.  NAT Function Considerations . . . . . . . . . . . . .  21
       6.4.2.  Endpoint Considerations . . . . . . . . . . . . . . .  22
     6.5.  Handling of Fragmented SCTP Packets by NAT Functions  . .  22
     6.6.  Multipoint Traversal Considerations for Endpoints . . . .  22
       6.6.1.  NAT Function Considerations . . . . . . . . . . . . .  23
       6.6.2.  Endpoint Considerations . . . . . . . . . . . . . . .  23
     6.7.  Path Probing considerations . . . . . . . . . . . . . . .  24
   7.  Examples of Operation . . . . . . . . . . . . . . . . . . . .  24
     7.1.  Single Homed Association Setup  . . . . . . . . . . . . .  25
     7.2.  Single Homed Association Setup with Collision . . . . . .  25



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     7.3.  Multi Homed Association Setup . . . . . . . . . . . . . .  25
     7.4.  Multi Homed Association Setup . . . . . . . . . . . . . .  26
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
     8.1.  New Chunk Flags for Two Existing Chunk Types  . . . . . .  28
     8.2.  Four New Error Causes . . . . . . . . . . . . . . . . . .  29
     8.3.  Two New Chunk Parameter Types . . . . . . . . . . . . . .  30
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  30
   10. Normative References  . . . . . . . . . . . . . . . . . . . .  31
   11. Informative References  . . . . . . . . . . . . . . . . . . .  31
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  33
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  33

1.  Introduction

   Stream Control Transmission Protocol (SCTP) [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
   functions for TCP that allows multiple hosts to reside behind a NAT
   function using private-use addresses (see [RFC6890] ) and yet share a
   single IPv4 address, even when two hosts (behind a NAT function)
   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 function maps a single or multiple internal addresses to a single
   external address and vice versa.

   To date, specialized code for SCTP has not yet been added to most NAT
   functions 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 function and this host can
   only be single-homed.  The only alternative for supporting legacy NAT
   functions is to use UDP encapsulation as specified in [RFC6951] .

   The NAT function in the document refers to NAPT functions described
   in Section 2.2 of [RFC3022] , NAT64 [RFC6146] , or DS-Lite AFTR
   [RFC6333] .

   This document specifies procedures allowing a NAT function to support
   SCTP by providing similar features to those provided by a NAPT for
   TCP (see [RFC5382] and [RFC7857] ), UDP (see [RFC4787] and [RFC7857]
   ), and ICMP (see [RFC5508] and [RFC7857] ).  This 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 procedures can assure that in both single-homed and
   multi-homed cases a NAT function will maintain the appropriate state
   without the NAT function needing to change port numbers.




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

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

   *  If a NAT function 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 used by SCTP.

   *  Not having to touch the IP payload makes the processing of ICMP
      messages by NAT functions easier.

   An SCTP-aware NAT function will need to follow these procedures for
   generating appropriate SCTP packet formats, this is needed under
   circumstances detailed in this document and only triggered by the
   detection of an SCTP packet containing an INIT chunk.

   When considering SCTP-aware NAT it is possible to have multiple
   levels of support.  At each level, the Internal Host, Remote Host,
   and NAT function does or does not support the procedures described in
   this document.

   The reference configuration for NAT support is depicted in the
   following figure:


       Internal Network    |         External Network
                           |
                Internal   |   External              Remote
                Address    |   Address  /--\/--\    Address
     +--------+         +-----+        /        \           +--------+
     | Host A |=========| NAT |=======| Network  |==========| Host B |
     +--------+         +-----+        \        /           +--------+
                Internal   |            \--/\--/     Remote
      Internal    Port     |                           Port   Remote
        VTag               |                                   VTag


                       Figure 1: Basic Network Setup

   In the above Figure 1 the NAT hides Host A whereas Host B is directly
   connected to the public internet.  Host A has a private IP address,
   NAT and Host B have public IP addresses.






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   The following table illustrates the results of the various
   combinations of support and if communications can occur between two
   endpoints with reference to Figure 1, the NAT adaptation is the one
   described in the current document.

      +===============+==============+=============+===============+
      | Internal Host | NAT Function | Remote Host | Communication |
      +===============+==============+=============+===============+
      |    Support    |   Support    |   Support   |      Yes      |
      +---------------+--------------+-------------+---------------+
      |    Support    |   Support    |  No Support |      Yes      |
      +---------------+--------------+-------------+---------------+
      |    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 no communication can occur when a
   NAT function does not support SCTP-aware NAT.  This assumes that the
   NAT function does not handle SCTP packets at all and all SCTP packets
   sent from behind a NAT function are discarded by the NAT function.

   In some cases, where the NAT function supports SCTP-aware NAT but the
   local host does not support the feature, communication can possibly
   occur in a limited way.  For example, only one host can have a
   connection when a collision case occurs.

   When a SCTP host is deployed behind a NAT and both support SCTP-aware
   NAT, the communication will suceed independently from the remote
   peer.

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.



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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.

   Internal-Address (Int-Addr)
      An internal address that is known to the internal host.

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

   Internal-VTag (Int-VTag)
      The SCTP Verification Tag (VTag) (see Section 3.1 of [RFC4960] )
      that the internal host has chosen for an association.  The VTag is
      a unique 32-bit tag that accompanies any incoming SCTP packet for
      this association to the Internal-Address.

   Remote-Address (Rem-Addr)
      The address that an internal host is attempting to contact.

   Remote-Port (Rem-Port)
      The port number used by the host holding the Remote-Address.

   Remote-VTag (Rem-VTag)
      The Verification Tag (VTag) (see Section 3.1 of [RFC4960] ) that
      the host holding the Remote-Address has chosen for an association.
      The VTag is a unique 32-bit tag that accompanies any outgoing SCTP
      packet for this association to the Remote-Address.

   External-Address (Ext-Addr)
      An external address assigned to the NAT function, that it uses as
      a source address when sending packets towards a Remote-Address.

4.  Motivation and Overview

4.1.  SCTP NAT Traversal Scenarios

   This section defines the notion of single and multipoint NAT
   traversal.

4.1.1.  Single Point Traversal

   In this case, all packets in the SCTP association go through a single
   NAT function, as shown in Figure 2 .





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      Internal Network    |       External Network
                          |
                          |               /--\/--\
    +--------+         +-----+           /        \          +--------+
    | Host A |=========| NAT |========= | Network  | ========| Host B |
    +--------+         +-----+           \        /          +--------+
                          |               \--/\--/
                          |

                   Figure 2: Single NAT Function Scenario

   A variation of this case is shown in Figure 3 , i.e., multiple NAT
   functions in the forwarding path between two endpoints.



          Internal | External : Internal | External
                   |          :          |
                   |          :          |       /--\/--\
    +--------+  +-----+       :       +-----+   /        \   +--------+
    | Host A |==| NAT |=======:=======| NAT |==| Network  |==| Host B |
    +--------+  +-----+       :       +-----+   \        /   +--------+
                   |          :          |       \--/\--/
                   |          :          |

                  Figure 3: Serial NAT Functions Scenario

   Another case where the Endpoint is ditributed among SCTP Hosts is
   shown in Figure 4 where multiple Hosts behave as Server and share the
   same Internal Port.  A Load Balancer node supports NAT when a new
   Association request comes.  The description of the Load Balancer
   function and its interwork with NAT function is out of the scope of
   this document.


















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      Internal Network    |       External Network
                          |
                          |               /--\/--\
    +--------+         +-----+           /        \          +--------+
    | Host A |====+====| NAT |========= | Network  | ========| Host B |
    +--------+    |    +-----+           \        /          +--------+
                  |       | \             \--/\--/
    +--------+    |       |  \
    | Host B |====+       |   \
    +--------+    |       |    \
                  |       |     +----------+
    +--------+    |       |     | Load     |
    | Host C |====+       |     | Balancer |
    +--------+            |     +----------+

                  Figure 4: Distributed Endpoint Scenario

   Although one of the main benefits of SCTP multi-homing is redundant
   paths, in the single point traversal scenario the NAT function
   represents a single point of failure in the path of the SCTP multi-
   homed association.  However, the rest of the path can 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 function
   in this case sees all the packets of the SCTP association.

4.1.2.  Multipoint Traversal

   This case involves multiple NAT functions and each NAT function only
   sees some of the packets in the SCTP association.  An example is
   shown in Figure 5 .



             Internal      |      External
                        +------+             /---\/---\
                /=======|NAT A |=========\  /          \
    +--------+ /        +------+          \/            \    +--------+
    | Host A |/            |              |    Network   |===| Host B |
    +--------+\            |              /\            /    +--------+
               \        +------+         /  \          /
                \=======|NAT B |========/    \---\/---/
                        +------+
                           |

                 Figure 5: Parallel NAT Functions Scenario




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   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.

4.2.  Limitations of Classical NAPT for SCTP

   Using classical NAPT possibly results in changing one of the SCTP
   port numbers during the processing, which requires the recomputation
   of the transport layer checksum by the NAPT function.  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, even though
   hardware support can mitigate this in some implementations.

   An SCTP endpoint can have multiple addresses but only has a single
   port number to use.  To make multipoint traversal work, all the NAT
   functions involved need to 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 port numbers and addresses configured
   within each NAT function.  Other mechanisms could make use of NAT to
   NAT communication.  Such mechanisms have not been deployed on a wide
   scale base and thus are not a preferred solution.  Therefore an SCTP
   variant of NAT function has been developed and is described in draft-
   ietf-tsvwg-natsupp-23 that is the version at the current time.  This
   document describes an alternative to that function exploiting most of
   the same principles.  Rather than being radically different, it can
   be seen as a subset with some limitations but less complex and
   requiring minor computational effort at the SCTP Endpoints and at the
   NAT functions (see Section 4.3 ).

4.3.  The SCTP-Specific Variant of NAT

   In this section it is allowed that there are multiple SCTP capable
   hosts behind a NAT function that share one External-Address.  This
   section focuses on the single point traversal scenario (see
   Section 4.1.1 ) as well as on the multipoint trasversal NAT (see
   Section 4.1.2 ).

   The modification of outgoing SCTP packets sent from an internal host
   is simple: the source address of the packets has to be replaced with
   the External-Address.  It might also be necessary to establish some
   state in the NAT function to later handle incoming packets.




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   Typically, the NAT function has to maintain a NAT binding table of
   Internal-Port, Remote-Port, Internal-Address, Remote-Address.  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.  Create() instantiates a
   supervision timer on the NAT-State control block that has duration
   greather than 2 * HB.interval and lower than 4 * HB.interval (see
   section 15 of [RFC4960] ).  A NAT function MAY allow creating NAT-
   State control blocks via a management interface.

   For SCTP packets coming from the external realm of the NAT function
   the destination address of the packets has to be replaced with the
   Internal-Address of the host to which the packet has to be delivered,
   if a NAT state entry is found.  The lookup of the Internal-Address is
   based on the Remote-Address, Remote-Port and the Internal-Port.  The
   lookup function retarts the Nat-State control block supervision
   timer.

   The entries in the NAT binding table need to fulfill some uniqueness
   conditions.  There can not be more than one entry NAT binding table
   with the same 4-tuple of Internal-Address, Remote-Address, Internal-
   Port and Remote-Port.

   NAT is able understanding that the SCTP packet transports an INIT
   chunk because the SCTP common header will have VTAG=0 (see section
   3.1 of [RFC4960] )

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





















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


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

         if lookup(Int-Port, Rem-Port, Rem-Addr) == true
            if lookup(Int-Addr, Int-Port, Rem-Port, Rem-Addr) == false
                sendAbort(Rem-Addr, Rem-Port, Int-Addr, Int-Port, M-bit)
            else
                Returns(control block)
                forwardPkt(Ext-Addr, Int-Port, Rem-Addr, Rem-Port)
         else
                Create(Int-Port, Rem-Port, Int-Addr, Rem-Addr)
                Returns(control block)
                forwardPkt(Ext-Addr, Int-Port, Rem-Addr, Rem-Port)


           Translates To:

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



   In the normal case a NAT binding table entry will be created.

   However, it is possible that there is already a NAT binding table
   entry with the same Remote-Address, Internal-Port and Remote-Port but
   different Internal-Address.  In this case the packet containing the
   INIT chunk MUST be dropped by the NAT and a packet containing an
   ABORT chunk SHOULD be sent to the SCTP host that originated the
   packet with the M bit set and 'Port Number Collision' 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.

   In case that there's already a a NAT binding table entry with the
   same Remote-Address, Internal-Port, Remote-Port and the same
   Internal-Address, meaning that the INIT chunk is a new attempt for
   the same Association, the NAT entry is reused.




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   The processing of outgoing SCTP packets containing chunks other than
   INIT is described in the following figure.


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

   Int-Addr:Int-Port ------> Rem-Addr:Rem-Port
                     Rem-VTag

                 if lookup(Int-Port, Rem-Port, Rem-Addr) == false
                        Create(Int-Port, Rem-Port, Int-Addr, Rem-Addr)
                        Returns(control block)
                 forwardPkt(Ext-Addr, Int-Port, Rem-Addr, Rem-Port)

                 Translates To:

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


   The processing of incoming SCTP packets containing an INIT chunk is
   illustrated in the following figure.  This scenario is valid for all
   message flows in this section.
























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


                                           INIT [Initiate-Tag]
                              Ext-Addr:Int-Port <---- Rem-Addr:Rem-Port
                                               Int-VTag=0


              if lookup(Int-Port, Rem-Port, Rem-Addr) == true
                      Returns(control block)
                      forwardPkt(Rem-Addr, Rem-Port, Int-Addr, Int-Port)
              else
                  if INIT contains RJ option
                      send INIT-ACK to the INIT source
                  else
                      Create(Int-Port, Rem-Port, Int-Addr, Rem-Addr)
                      Returns(control block)
                      forwardPkt(Rem-Addr, Rem-Port, Int-Addr, Int-Port)


             Translates To:

                          INIT[Initiate-Tag]
             Int-Addr:Int-Port <------ Rem-Addr:Rem-Port
                              Int-VTag=0



   When INIT chunk contains the RJ option set, it's a duplicate of the
   INIT used for establishing the association.  In such case the reason
   for RJ option is to be recognized by the NAT function that will reply
   to the sender instead of the SCTP Host.  This allows the SCTP
   Endpoint to be distributed among hosts, and since the NAT function
   cannot arbitraly choose among hosts, it takes the role of the unknown
   host in answering to the INIT issuer so that it can proceed with the
   ASCONF handshake and extend the association.  The final step of
   setting the path between the NAT function and the unknown host will
   be completed by the host receiving ASCONF and sending an INIT with RJ
   option towards the remote peer.








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   The processing of incoming SCTP packets containing chunk different
   than INIT is illustrated in the following figure.  The Lookup()
   function has as input the Remote-Address, Remote-Port and the
   Internal-Port.  It returns the corresponding entry of the NAT binding
   table.


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

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

                 if lookup(Int-Port, Rem-Port, Rem-Addr) == true
                    Returns(NAT-State control block containing Int-Addr)
                    forwardPkt(Ext-Addr, Int-Port, Rem-Addr, Rem-Port)

                   Translates To:

    Int-Addr:Int-Port <------ Rem-Addr:Rem-Port
                      Int-VTag


   In the case where the Lookup function fails because it does not find
   an entry, the SCTP packet is dropped.

4.4.  Compatibility and increamental deployment

   The current proposal for adding SCTP-capable NAT function is meant to
   provide backwards compatibility in both involved functionality and
   being compatible with legacy SCTP remote terminations that doesn't
   implement it.

   The compatibility at NAT tracking mechanism allows the NAT functionto
   be able hiding also SCTP stack that doesn't implement the current
   specfication, at the same time an SCTP stack implementing the current
   specification canbe deployed in a NAT scenario where the NAT doesn't
   implement it.  In either cases the SCTP termination will be
   accomplished with limitations as described earlier.

   The compatibility at network level is proposed in a way that makes it
   possible deploying a cluster of SCTP termination behind a NAT
   function still with full compatibility towards legacy networking.  As
   an example, the scenario described in Figure 2 shows Host A being
   hidden by NAT and Host B being directly connected to the internet.



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   In such case only Host A and NAT need to implement the current
   specification whilst Host B can neglect it.  The same applies to more
   complex scenarios such as the ones shown in Figure 4 or in Figure 5.

4.5.  Differences with Current NAT Support Draft

   This section describes the differences with the existing draft-ietf-
   tsvwg-natsupp.

   From a functional perspective, the major difference between is in the
   compatibility towards legacy SCTP hosts.  The NAT-adaptation
   specified in this document allows interoperability between SCTP hosts
   even when the remote peer hasn't implemented it.  Not even is
   mandatory that all the NAT devices in the path do implement it as
   long as they allow SCTP packets to pass through transparently.  On
   the existing draft-ietf-tsvwg-natsupp, the specification needs to be
   implemented on all SCTP Hosts and all NAT devices in the network in
   order to work.

   The main technical difference is that the NAT function is simpler and
   doesn't require explicit handling of NAT missing states.  Actually in
   this proposal NAT doesn't need to parse all the SCTP payloads.  NAT
   only parses INIT chunks, filtering of SCTP packets containing INIT
   chunks is based on checking the SCTP Common Header and discriminate
   the behavior based on Verification Tag = 0, that indicates the SCTP
   packet contains an INIT chunk.  The NAT supervises the association by
   means of a timer, if no SCTP packets are seen within a certain time,
   NAT assumes that the association is closed and will remove the
   related NAT-entry.

   The other difference is in the role of the SCTP User.  In the current
   proposal it's up to the SCTP User to change the originating Endpoint
   (i.e. choose a different port number) if collision is detected.  The
   current proposal guarantees that at each node being in a path
   belonging to an association, there will be only one 4-uple describing
   that association, that means the NAT doesn't need to take care of
   VTAG.

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 functions and received by SCTP endpoints.  Section 5.3 describes
   parameters sent by SCTP endpoints and used by NAT functions and SCTP
   endpoints.






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5.1.  Modified Chunks

   This section presents existing chunks defined in [RFC4960] for which
   additional flags are specified 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                   /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   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
   (e.g., NAT).

   [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.]

5.1.3.  Extended INIT-ACK Chunk







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

   The INIT ACK chunk defined in [RFC4960] is extended to add the new 'M
   bit'.  The M bit indicates to the receiver of the INIT-ACK 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.  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.




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   [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.

5.3.1.  Repetita Juvant Parameter

   Repetita Juvant is a latin phase standing for "repeating does good".
   It's sually said as a jocular remark to defend the speaker's (or
   writer's) choice to repeat some important piece of information to
   ensure reception by the audience.

   The RJ Parameter is used as Optional Parameter in the INIT chunk.
   The RJ parameter is used to indicate that INIT chunk is the
   repetition of an already sent one even if it comes from a different
   source address.  It's used from either peers before sending ASCONF in
   order to setup the NATs in the path.

    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 = 0xXXXX         |         Length = 8            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

6.  Procedures for SCTP Endpoints and NAT Functions

   If an SCTP endpoint is behind an SCTP-aware NAT, a number of problems
   can arise as it tries to communicate with its peers:

   *  IP addresses can not be included in the SCTP packet.  This is
      discussed in Section 6.1 .

   *  More than one host behind a NAT function could select the same
      source port number when initiating an association with the same
      peer server.  This creates a situation where the NAT function will
      not be able to forward the INIT chunk.  This situation is
      discussed in Section 6.3 .

   *  A restart of a NAT function during a conversation could cause a
      loss of its state.  This problem and its solution is discussed in
      Section 6.4 .

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




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   *  An SCTP endpoint can be behind two NAT functions in parallel
      providing redundancy.  The method to set up this scenario is
      discussed in Section 6.6 .

   The mechanisms to solve these problems require additional chunks and
   parameters, defined in this document, and modified handling
   procedures from those specified in [RFC4960] as described below.

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 does not work when NAT functions are present.

   Every association setup from a host behind a NAT function MUST NOT
   use multiple internal addresses.  The INIT chunk MUST NOT contain an
   IPv4 Address parameter, IPv6 Address parameter, or Supported Address
   Types parameter.  The INIT ACK chunk MUST NOT contain any IPv4
   Address parameter or IPv6 Address parameter using non-global
   addresses.  The INIT chunk and the INIT ACK chunk MUST NOT contain
   any Host Name parameters.

   If the association is intended to be finally multi-homed, the
   procedure in Section 6.6 MUST be used.

6.2.  Association Setup Considerations for NAT

   When Endpoint is Distributed, NAT needs the cooperation of a Load
   Balancer function for handling incoming and outgoing Association
   Requests.  It's up to the Load Balancer internal design the strategy
   for permitting a Distributed Endpoint to handle the traffic.
   Functionally, it's important that Load Balancer provides NAT a way
   for assigning Associations to multiple SCTP Hosts.

6.3.  Handling of Internal Port Number Collisions

   Consider the case where two hosts in the Internal-Address space want
   to set up an SCTP association with the same service provided by some
   remote host.  This means that the Remote-Port is the same.  If they
   both choose the same Internal-Port the NAT function will experience
   collision when receiving the INIT and trying to create an Entry in
   the NAT Tables.  In such case NAT will send an ABORT chunk with M-bit
   set to the SCTP Client.  Since it's up to the SCTP User Application
   to choose the Internal Port, it may be that an Association chooses
   the Internal Port from the ephemeral port range at random (see
   [RFC6056] ), this would make the probability for Port Number
   Collision low.



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   At the Association initialization, the Client will experience one out
   of three alternative answers from the network:

   *  INIT-ACK from the peer, this means a viable path exists between
      peers, all the involved NATs have NAT tables properly configured
      and the Association can be established.

   *  ABORT with M-bit set from one of the NATs within the path, this
      means that the Association cannot be established.  The SCTP User
      application SHOULD decide whether to retry with a different
      Internal Port or to give up.  The way SCTP and the SCTP User
      interact in this case is implementation dependent.

   *  ABORT from the remote peer.

   The way SCTP and SCTP User Application interact can be either:

   *  An application can request a specific local port number (in the
      socket API, using bind() with a non-zero port number) and in case
      of a local port number collision, the connection setup has to
      fail.  It is up to the application to close() the socket and
      restart from the beginning.

   *  An application leaves the local port number selection up to the
      SCTP stack (in the socket socket API by either calling bind() with
      a zero port number or not calling bind() at all before calling
      connect() or sendto()).  However, once the port number is chosen,
      it can not be changed.  So in case of a local port number
      collision, the association setup has to fail.  It is up to the
      application to close() the socket and restart from the beginning.

   *  An application leaves the local port number selection up to the
      SCTP stack (in the socket socket API by either calling bind() with
      a zero port number or not calling bind() at all before calling
      connect() or sendto()).  In addition, it indicates that the SCTP
      can change the local port number over time (in the socket API this
      would be calling an IPPROTO_SCTP level new socket option).  In
      this case, the SCTP stack can automatically retry a connection
      setup in case of an local port number collision.

6.3.1.  NAT Function Considerations

   NAT function checks for collision only on packets containing INIT
   chunk.  If the NAT function detects a collision of internal port
   numbers, it SHOULD send a packet containing an ABORT chunk with the M
   bit set.  The M bit is a new bit defined by this document to express
   to SCTP that the source of this packet is a "middle" box, not the
   peer SCTP endpoint (see Section 5.1.1 ).  the source and destination



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   address and port numbers MUST be swapped.

   The sender of the packet containing an ERROR or ABORT chunk MUST
   include the error cause with cause code 'Port Number Collision' (see
   Section 5.2.1 ).

   If the INIT chunk contains the RJ option the NAT function MUST NOT
   forward the INIT chunk to the SCTP Host but it MUST reply to the
   remote peer with INIT-ACK chunk with the M bit set.  The M bit is a
   new bit defined by this document to express to SCTP that the source
   of this packet is a "middle" box (see Section 5.1.3 ).  The
   information contained in INIT-ACK chunk SHOULD be copied from the
   INIT chunk.  The value for Initiate Tag and Initial TSN MAY be chosen
   random.

6.3.2.  Endpoint Considerations

   The sender of the packet containing the INIT chunk upon reception of
   a packet containing an ABORT chunk with M bit set and the appropriate
   error cause code for colliding NAT binding table state is included,
   SHOULD evaluate the reason for ABORT.  If the reason is "Port Number
   Collision" it SHOULD reinitiate the association setup procedure after
   choosing a new Internal Port.

6.4.  Handling of Missing State

6.4.1.  NAT Function Considerations

   When experiencing a restart, the NAT function will start handling
   SCTP packets with time difference between the ones containing INIT
   chunks and all the other ones.  Handling of SCTP packets containing
   INIT chunks will start at least 4 * HB.interval after handling other
   SCTP packets (see section 15 of [RFC4960] ).  This avoids race
   condition between the recreation of existing Entries in the NAT
   Table and the creation of new ones from new Association requests.

   If the NAT function receives a packet not containing an INIT chunk
   from the internal network for which the lookup procedure does not
   find an entry in the NAT binding table, it must create an Entry for
   that packet and forward it.  If the NAT function receives a packet
   not containing an INIT chunk from the external network for which the
   lookup procedure does not find an entry in the NAT binding table, it
   must silently drop it.








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6.4.2.  Endpoint Considerations

   Upon restart of a NAT function, the endpoint will experience
   connectivity interruption, depending on the Association state it will
   keep on retrying sending SCTP packets containint DATA chunks or HB
   chunks.  Since the longest interval between SCTP packets is
   HB.interval, it will be able restoring the connectivity at most 2 *
   HB.interval after NAT function is back at work.

   If the Endpoint is trying to establish an Association, it will
   experience a longer connectivity unavalilability of more than 4 *
   HB.interval as NAT needs to rebuild the NAT Table with the existing
   Associations first.

6.5.  Handling of Fragmented SCTP Packets by NAT Functions

   SCTP minimizes the use of IP-level fragmentation.  However, it can
   happen that using IP-level fragmentation is needed to continue an
   SCTP association.  For example, if the path MTU is reduced and there
   are still some DATA chunk in flight, which require packets larger
   than the new path MTU.  If IP-level fragmentation can not be used,
   the SCTP association will be terminated in a non-graceful way.  See
   [RFC8900] for more information about IP fragmentation.

   Therefore, a NAT function MUST be able to handle IP-level fragmented
   SCTP packets.  The fragments MAY arrive in any order.

   When an SCTP packet can not be forwarded by the NAT function due to
   MTU issues and the IP header forbids fragmentation, the NAT MUST send
   back a "Fragmentation needed and DF set" ICMPv4 or PTB ICMPv6 message
   to the internal host.  This allows for a faster recovery from this
   packet drop.

6.6.  Multipoint Traversal Considerations for Endpoints

   If a multi-homed SCTP endpoint behind a NAT function connects to a
   peer, it MUST first set up the association single-homed with only one
   destination address causing the first NAT function to populate its
   state.

   Once an Association has been created, it's possible to add further
   external IP addresses for the peer to use, but before adding each IP
   address it must be created the needed set of Entries in all NAT
   functions towards all the peer's IP addresses.  An INIT chunk
   containing a RJ option (see Section 5.3.1 ) SHOULD be sent towards
   all peers IP addresses using a path selector that is expected to
   result in another external addres than association creation.  The
   reason why an INIT chunk with RJ option set is to be used is for



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   permitting the remote to be able discriminating between a request for
   a new Association in case of Distributed Endpoint.  The result from
   that INIT is according to the given rules for Association setup (see
   Section 6.1 ) and can cause collision.  The reception of INIT ACK
   confirms that the path from the new IP address and the remote one is
   available and that all the NATs involved are properly configured.  In
   case INIT ACK has M-bit set, the remote Endpoint is distributed.

   After succefull confirmation, the Endpoint SHOULD add each IP address
   using packets containing ASCONF chunks sent via their respective NAT
   functions.  The address used in the Add IP address parameter is the
   wildcard address (0.0.0.0 or ::0) and the address parameter in the
   ASCONF chunk SHOULD also contain the VTags parameter.

   When an Endpoint gets a new Remote IP Address added to an
   Association, it SHOULD send INIT chunks with RJ option towards from
   all its own IP Addresses towards that address in order to properly
   set all the NATs in the path.

6.6.1.  NAT Function Considerations

   NAT function differentiates the behavior towards INIT chunk depending
   on the RJ option.  If the RJ option exists and the packet contains an
   incoming INIT chunk, the NAT function SHOULD NOT forward the INIT
   chunk towards the SCTP Host, it shall reply instead with an INIT ACK
   chunk with the M-bit set.  Section 5.1.3 ).  NAT function SHOULD
   create INIT ACK data by using the parameters from the received INIT
   chunk.

6.6.2.  Endpoint Considerations

   When the Endpoint receives an INIT chunk with RJ option set, it will
   ignore the RJ option and handle INIT as in the legacy case.

   The Endpoint originating INIT chunk with RJ option set can receive
   different answers:

   *  When receiving INIT ACK, it will assume the NATs on the path are
      properly set and the Endpoint can continue with the ASCONF
      procedure.

   *  When receiving as ABORT with M-bit set, it shall assume that a
      path is not possible to be established.  The Endpoint SHOULD retry
      after a time greather than 4 * HB.interval.







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   *  When receiving an ABORT without M-bit set, it shall assume that
      some temporary NAT configuration has led the INIT towards the
      wrong SCTP Host.  The Endpoint SHOULD retry after a time greather
      than 4 * HB.interval.

6.7.  Path Probing considerations

   The SCTP protocol relies on continous path probing by means of data
   sending or using the Heartbeat mechanism as specified in section 5.4
   of [RFC4960] The adoption of the NAT mechanisms as described in this
   document introduces a criticality in the Path Probing mechanism of
   SCTP.

   The problem happens when, due to network problem, one or more
   secondary paths belonging to an Association will experience timeout
   in Path probing so than in some of the NAT functions used in the path
   there's no SCTP traffic for the given Association, causing the NAT
   entry to be canceled because of supervision timeout.

   It is recommended that before sending HEARTBEAT to an UNCONFIRMED
   address, an INIT chunk with RJ paramter set is sent so that NAT
   functions in the path can setup entries in the NAT tables properly.

7.  Examples of Operation

   This section describes examples of Association Establishements using
   the reference scenario depicted in Figure 6 .  Hosts A1 and A2
   implement a distributed client towards the same remote Host.  Hosts
   B1 and B2 implement a distributed Endpoint 'B' acting as Server.  The
   Load Balancer functionality is not shown as it doesn't affect SCTP
   protocol.

      Internal     |           External          |      Internal
                +------+                      +------+
             +==|NAT A |==\    /--\/--\    /==|NAT C |==+
+--------+   |  +------+   \  /        \  /   +------+  |     +--------+
|Host A1 +---+     |        \/          \/       |      +-----|Host B1 |
|        +-+ |     |        |  Network  |        |      |  +--+        |
+--------+ | |     |        /\          /\       |      |  |  +--------+
           | |  +------+   /  \        /  \   +------+  |  |
           +====|NAT B |==/    \--\/--/    \==|NAT D |=====+
           | |  +------+                      +------+  |  |
+--------+ | |     |                             |      |  |  +--------+
|Host A2 +-|-+     |                             |      +--|--+Host B2 |
|        +-+       |                             |         +--+        |
+--------+         |                             |            +--------+

      Figure 6: Parallel NAT with distributed endpoints Scenario



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7.1.  Single Homed Association Setup

   This section describes a successfull Association Establishment from
   A1 towards the distributed endpoint B.  The sequence chart is shown
   in Figure 7 .

A1      A2      NAT A       NAT B       NAT C       NAT D       B1    B2
|       |       |           |           |           |           |     |
+--------------}| INIT      |           |           |           |     |
|       |       +----------------------}|           |           |     |
|       |       |           |           +----------------------}|     |
|       |       |           |           |           |           |     |
|       |       |           |           |{----------------------+     |
|       |       |{----------------------+           |           |     |
|{--------------+ INIT ACK  |           |           |           |     |
|       |       |           |           |           |           |     |

         Figure 7: Single Homed successfull Association Setup

7.2.  Single Homed Association Setup with Collision

   This section describes a successfull Association Establishment from
   A2 towards the distributed endpoint B.  The collision happens at NAT
   A.  The sequence chart is shown in Figure 8 .

A1      A2      NAT A       NAT B       NAT C       NAT D       B1    B2
|       |       |           |           |           |           |     |
|       +------}| INIT      |           |           |           |     |
|       |{------+ ABORT     |           |           |           |     |
|       |       |           |           |           |           |     |
|       +------}| INIT      |           |           |           |     |
|       |       +----------------------}|           |           |     |
|       |       |           |           +----------------------------}|
|       |       |           |           |           |           |     |
|       |       |           |           |{----------------------------+
|       |       |{----------------------+           |           |     |
|       {-------+ INIT ACK  |           |           |           |     |
|       |       |           |           |           |           |     |

Figure 8: Single Homed successfull Association Setup after congestion

7.3.  Multi Homed Association Setup

   This section describes how the single homed established at
   Section 7.1 becomes multihomed.  Note that the decision for what peer
   has to handle the INIT message requires support of Load Balancer.
   It's assumed that a Load Balancer exists and provides NAT with the
   right information.  Success happens at all steps.  Figure 9 .



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A1      A2      NAT A       NAT B       NAT C       NAT D       B1    B2
|       |       |           |           |           |           |     |
+--------------------------}| INIT RJ   |           |           |     |
|       |       |           +----------}|           |           |     |
|       |       |           |{----------+           |           |     |
|{--------------------------+ INIT ACK  |           |           |     |
|       |       |           |           |           |           |     |
+--------------------------}| ASCONF    |           |           |     |
|       |       |           +----------}|           |           |     |
|       |       |           |           +----------------------}|     |
|       |       |           |           |{----------------------+     |
|       |       |           |{----------+           |           |     |
|{--------------------------+ ASCONF ACK|           |           |     |
|       |       |           |           |           |           |     |
|       |       |           | INIT RJ   |{----------------------+     |
|       |       |           |{----------+           |           |     |
|       |       |           +----------}|           |           |     |
|       |       |           | INIT ACK  +----------------------}|     |
|       |       |           |           |           |           |     |


         Figure 9: Multi Homed successfull Association Setup

7.4.  Multi Homed Association Setup

   This section describes how the multihome homed established at
   Section 7.3 becomes multihomed from the other peer.  Success happens
   at all steps.  Figure 10 .























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A1      A2      NAT A       NAT B       NAT C       NAT D       B1    B2
|       |       |           |           |           |           |     |
|       |       |           | INIT RJ   |           |{----------+     |
|       |       |{----------------------------------+           |     |
|       |       +----------------------------------}|           |     |
|       |       |           | INIT ACK  |           +----------}|     |
|       |       |           | INIT RJ   |           |{----------+     |
|       |       |           |{----------------------+           |     |
|       |       |           +----------------------}|           |     |
|       |       |           | INIT ACK  |           +----------}|     |
|       |       |           |           |           |           |     |
|       |       |           | ASCONF    |           |{----------+     |
|       |       |{----------------------------------+           |     |
|{--------------+           |           |           |           |     |
+--------------}|           | ASCONF ACK|           |           |     |
|       |       +----------------------------------}|           |     |
|       |       |           |           |           +----------}|     |
|       |       |           |           |           |           |     |
+--------------}|           | INIT RJ   |           |           |     |
|       |       +----------------------------------}|           |     |
|       |       |{----------------------------------+           |     |
|{--------------+           | INIT ACK  |           |           |     |
|       |       |           |           |           |           |     |
+--------------------------}| INIT RJ   |           |           |     |
|       |       |           +----------------------}|           |     |
|       |       |           |{----------------------+           |     |
|{--------------------------+ INIT ACK  |           |           |     |
|       |       |           |           |           |           |     |


         Figure 10: Multi Homed successfull Association Setup

8.  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.








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8.1.  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:

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

                                 Table 2

   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













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            +==================+=================+===========+
            | 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      |           |
            +------------------+-----------------+-----------+

                                 Table 3

8.2.  Four New Error Causes

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

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

   Error Cause Codes

          +=======+================================+===========+
          | Value | Cause Code                     | Reference |
          +=======+================================+===========+
          | 176   | VTag and Port Number Collision | [RFCXXXX] |
          +-------+--------------------------------+-----------+
          | 177   | Missing State                  | [RFCXXXX] |
          +-------+--------------------------------+-----------+
          | 178   | Port Number Collision          | [RFCXXXX] |
          +-------+--------------------------------+-----------+
          | 179   | VTag Not Found                 | [RFCXXXX] |
          +-------+--------------------------------+-----------+

                                 Table 4






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8.3.  Two New Chunk Parameter Types

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

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

                                  Table 5

9.  Security Considerations

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

   Generic issues related to address sharing are discussed in [RFC6269]
   and apply to SCTP as well.

   For SCTP endpoints not disabling the restart procedure, this document
   does not add any additional security considerations to the ones given
   in [RFC4960] , [RFC4895] , and [RFC5061] .

   SCTP endpoints disabling the restart procedure, need to monitor the
   status of all associations to mitigate resource exhaustion attacks by
   establishing a lot of associations sharing the same IP addresses and
   port numbers.

   In any case, SCTP is protected by the verification tags and the usage
   of [RFC4895] against off-path attackers.

   For IP-level fragmentation and reassembly related issues see
   [RFC4963] .

   *  Setting a low timeout for SCTP mapping entries to cause failures
      to deliver incoming SCTP packets.



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   *  Instantiating mapping entries to cause NAT collision.

10.  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>.

11.  Informative References

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

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022,
              DOI 10.17487/RFC3022, January 2001,
              <https://www.rfc-editor.org/info/rfc3022>.

   [RFC4787]  Audet, F., Ed. and C. Jennings, "Network Address
              Translation (NAT) Behavioral Requirements for Unicast
              UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January
              2007, <https://www.rfc-editor.org/info/rfc4787>.



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   [RFC4963]  Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
              Errors at High Data Rates", RFC 4963,
              DOI 10.17487/RFC4963, July 2007,
              <https://www.rfc-editor.org/info/rfc4963>.

   [RFC5382]  Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and P.
              Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
              RFC 5382, DOI 10.17487/RFC5382, October 2008,
              <https://www.rfc-editor.org/info/rfc5382>.

   [RFC5508]  Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT
              Behavioral Requirements for ICMP", BCP 148, RFC 5508,
              DOI 10.17487/RFC5508, April 2009,
              <https://www.rfc-editor.org/info/rfc5508>.

   [RFC6056]  Larsen, M. and F. Gont, "Recommendations for Transport-
              Protocol Port Randomization", BCP 156, RFC 6056,
              DOI 10.17487/RFC6056, January 2011,
              <https://www.rfc-editor.org/info/rfc6056>.

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
              April 2011, <https://www.rfc-editor.org/info/rfc6146>.

   [RFC6269]  Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and
              P. Roberts, "Issues with IP Address Sharing", RFC 6269,
              DOI 10.17487/RFC6269, June 2011,
              <https://www.rfc-editor.org/info/rfc6269>.

   [RFC6333]  Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
              Stack Lite Broadband Deployments Following IPv4
              Exhaustion", RFC 6333, DOI 10.17487/RFC6333, August 2011,
              <https://www.rfc-editor.org/info/rfc6333>.

   [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>.






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   [RFC7857]  Penno, R., Perreault, S., Boucadair, M., Ed., Sivakumar,
              S., and K. Naito, "Updates to Network Address Translation
              (NAT) Behavioral Requirements", BCP 127, RFC 7857,
              DOI 10.17487/RFC7857, April 2016,
              <https://www.rfc-editor.org/info/rfc7857>.

   [RFC8900]  Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
              and F. Gont, "IP Fragmentation Considered Fragile",
              BCP 230, RFC 8900, DOI 10.17487/RFC8900, September 2020,
              <https://www.rfc-editor.org/info/rfc8900>.

Acknowledgments

   The author wishes to thank Michael Tuxen , and Magnus Westerlund for
   their invaluable comments.

   In addition, the author wishes to thank Sriram Yagnaraman , for their
   suggestions.

   The author also wishes to thank the authors of draft-ietf-tsvwg-
   natsupp-22 which this document is based.

Author's Address

   Claudio Porfiri
   Ericsson AB
   Torshamnsgatan 21
   16440 Stockholm
   Sweden
   Email: claudio.porfiri@ericsson.com





















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