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Versions: (draft-bagnulo-behave-nat64) 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 6146

BEHAVE WG                                                     M. Bagnulo
Internet-Draft                                                      UC3M
Intended status: Standards Track                             P. Matthews
Expires: October 1, 2010                                  Alcatel-Lucent
                                                          I. van Beijnum
                                                          IMDEA Networks
                                                          March 30, 2010


   Stateful NAT64: Network Address and Protocol Translation from IPv6
                        Clients to IPv4 Servers
                draft-ietf-behave-v6v4-xlate-stateful-11

Abstract

   This document describes stateful NAT64 translation, which allows
   IPv6-only clients to contact IPv4 servers using unicast UDP, TCP, or
   ICMP.  The public IPv4 address can be shared among several IPv6-only
   clients.  When the stateful NAT64 is used in conjunction with DNS64
   no changes are usually required in the IPv6 client or the IPv4
   server.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   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
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   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on October 1, 2010.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the



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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
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   described in the BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.




























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Features of stateful NAT64 . . . . . . . . . . . . . . . .  5
     1.2.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  6
       1.2.1.  Stateful NAT64 solution elements . . . . . . . . . . .  6
       1.2.2.  Stateful NAT64 Behaviour Walkthrough . . . . . . . . .  8
       1.2.3.  Filtering  . . . . . . . . . . . . . . . . . . . . . . 10
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . . 11
   3.  Stateful NAT64 Normative Specification . . . . . . . . . . . . 13
     3.1.  Binding Information Bases  . . . . . . . . . . . . . . . . 14
     3.2.  Session Tables . . . . . . . . . . . . . . . . . . . . . . 15
     3.3.  Packet Processing Overview . . . . . . . . . . . . . . . . 17
     3.4.  Determining the Incoming tuple . . . . . . . . . . . . . . 18
     3.5.  Filtering and Updating Binding and Session Information . . 20
       3.5.1.  UDP Session Handling . . . . . . . . . . . . . . . . . 20
         3.5.1.1.  Rules for Allocation of IPv4 Transport
                   Addresses for UDP  . . . . . . . . . . . . . . . . 23
       3.5.2.  TCP Session Handling . . . . . . . . . . . . . . . . . 23
         3.5.2.1.  State definition . . . . . . . . . . . . . . . . . 24
         3.5.2.2.  State machine for TCP processing in the NAT64  . . 25
         3.5.2.3.  Rules for allocation of IPv4 transport
                   addresses for TCP  . . . . . . . . . . . . . . . . 32
       3.5.3.  ICMP Query Session Handling  . . . . . . . . . . . . . 33
       3.5.4.  Generation of the IPv6 Representations of IPv4
               Addresses  . . . . . . . . . . . . . . . . . . . . . . 36
     3.6.  Computing the Outgoing Tuple . . . . . . . . . . . . . . . 36
       3.6.1.  Computing the Outgoing 5-tuple for TCP and UDP . . . . 37
       3.6.2.  Computing the Outgoing 3-tuple for ICMP Query
               Messages . . . . . . . . . . . . . . . . . . . . . . . 37
     3.7.  Translating the Packet . . . . . . . . . . . . . . . . . . 38
     3.8.  Handling Hairpinning . . . . . . . . . . . . . . . . . . . 38
   4.  Protocol Constants . . . . . . . . . . . . . . . . . . . . . . 39
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 39
     5.1.  Implications on end-to-end security  . . . . . . . . . . . 39
     5.2.  Filtering  . . . . . . . . . . . . . . . . . . . . . . . . 39
     5.3.  Attacks on NAT64 . . . . . . . . . . . . . . . . . . . . . 41
     5.4.  Avoiding hairpinning loops . . . . . . . . . . . . . . . . 41
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 42
   7.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 42
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 43
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 43
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 43
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 44
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 45






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1.  Introduction

   This document specifies stateful NAT64, a mechanism for IPv4-IPv6
   transition and co-existence.  Together with DNS64
   [I-D.ietf-behave-dns64], these two mechanisms allow an IPv6-only
   client to initiate communications to an IPv4-only server.  They also
   enable peer-to-peer communication between an IPv4 and an IPv6 node,
   where the communication can be initiated by either end using
   existing, NAT-traversal, peer-to-peer communication techniques, such
   as ICE [I-D.ietf-mmusic-ice].  Stateful NAT64 also supports IPv4-
   initiated communications to a subset of the IPv6 hosts through
   statically configured bindings in the stateful NAT64.

   Stateful NAT64 is a mechanism for translating IPv6 packets to IPv4
   packets and vice-versa.  The translation is done by translating the
   packet headers according to the IP/ICMP Translation Algorithm defined
   in [I-D.ietf-behave-v6v4-xlate].  The IPv4 addresses of IPv4 hosts
   are algorithmically translated to and from IPv6 addresses by using
   the algorithm defined in [I-D.ietf-behave-address-format] and a
   prefix assigned to the stateful NAT64 for this specific purpose.  The
   IPv6 addresses of IPv6 hosts are translated to and from IPv4
   addresses by installing mappings in the normal NAPT manner [RFC3022].
   The current specification only defines how stateful NAT64 translates
   packets carrying TCP, UDP and ICMP traffic.  Other protocols,
   including SCTP, DCCP and IPsec are out of the scope of this
   specification.

   DNS64 is a mechanism for synthesizing AAAA resource records (RR) from
   A RR.  The IPv6 address contained in the synthetic AAAA RR is
   algorithmically generated from the IPv4 address and the IPv6 prefix
   assigned to a NAT64 device by using the same algorithm defined in
   [I-D.ietf-behave-address-format].

   Together, these two mechanisms allow an IPv6-only client (i.e. either
   a host with only IPv6 stack, or a host with both IPv4 and IPv6 stack,
   but only with IPv6 connectivity or a host running an IPv6 only
   application) to initiate communications to an IPv4-only server
   (analogous meaning to the IPv6-only host above).

   These mechanisms are expected to play a critical role in the IPv4-
   IPv6 transition and co-existence.  Due to IPv4 address depletion, it
   is likely that in the future, the new clients will be IPv6-only and
   they will want to connect to the existent IPv4-only servers.  The
   stateful NAT64 and DNS64 mechanisms are easily deployable, since they
   require no changes to either the IPv6 client nor the IPv4 server.
   For basic functionality, the approach only requires the deployment of
   the stateful NAT64 function in the devices connecting an IPv6-only
   network to the IPv4-only network, along with the deployment of a few



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   DNS64-enabled name servers accessible to the IPv6-only hosts.  An
   analysis of the application scenarios can be found in
   [I-D.ietf-behave-v6v4-framework].

   For brevity, in the rest of the document, we will refer to the
   stateful NAT64 either as stateful NAT64 or simply as NAT64.

1.1.  Features of stateful NAT64

   The features of NAT64 are:

   o  NAT64 is compliant with the recommendations for how NATs should
      handle UDP [RFC4787], TCP [RFC5382], and ICMP [RFC5508].  As such,
      NAT64 only supports Endpoint-Independent mappings and supports
      both Endpoint-Independent and Address-Dependent Filtering.
      Because of the compliance with the aforementioned requirements,
      NAT64 is compatible with current NAT traversal techniques, such as
      ICE [I-D.ietf-mmusic-ice] and compatible with other non-IETF-
      standard NAT traversal techniques.

   o  In the absence of any state in NAT64, only IPv6 nodes can initiate
      sessions to IPv4 nodes.  This works for roughly the same class of
      applications that work through IPv4-to-IPv4 NATs.

   o  Depending on the filtering policy used (Endpoint-Independent, or
      Address-Dependent), IPv4-nodes might be able to initiate sessions
      to a given IPv6 node, if the NAT64 somehow has an appropriate
      mapping (i.e.,state) for an IPv6 node, via one of the following
      mechanisms:

      *  The IPv6 node has recently initiated a session to the same or
         another IPv4 node. this is also the case if the IPv6 node has
         used a NAT-traversal technique (such as ICE) .

      *  If a statically configured mapping exists for the IPv6 node.

   o  IPv4 address sharing: NAT64 allows multiple IPv6-only nodes to
      share an IPv4 address to access the IPv4 Internet.  This helps
      with IPv4 forthcoming exhaustion.

   o  As currently defined in this NAT64 specification, only TCP/UDP/
      ICMP are supported.  Support for other protocols such as other
      transport protocols and IPsec are to be defined in separate
      documents.







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1.2.  Overview

   This section provides a non-normative introduction to NAT64.  This is
   achieved by describing the NAT64 behavior involving a simple setup,
   that involves a single NAT64 device, a single DNS64 and a simple
   network topology.  The goal of this description is to provide the
   reader with a general view of NAT64.  It is not the goal of this
   section to describe all possible configurations nor to provide a
   normative specification of the NAT64 behavior.  So, for the sake of
   clarity, only TCP and UDP are described in this overview; the details
   of ICMP, fragmentation, and other aspects of translation are
   purposefully avoided in this overview.  The normative specification
   of NAT64 is provided in Section 3.

   The NAT64 mechanism is implemented in a device which has (at least)
   two interfaces, an IPv4 interface connected to the IPv4 network, and
   an IPv6 interface connected to the IPv6 network.  Packets generated
   in the IPv6 network for a receiver located in the IPv4 network will
   be routed within the IPv6 network towards the NAT64 device.  The
   NAT64 will translate them and forward them as IPv4 packets through
   the IPv4 network to the IPv4 receiver.  The reverse takes place for
   packets generated by hosts connected to the IPv4 network for an IPv6
   receiver.  NAT64, however, is not symmetric.  In order to be able to
   perform IPv6-IPv4 translation, NAT64 requires state, binding an IPv6
   address and TCP/UDP port (hereafter called an IPv6 transport address)
   to an IPv4 address and TCP/UDP port (hereafter called an IPv4
   transport address).

   Such binding state is either statically configured in the NAT64 or it
   is created when the first packet flowing from the IPv6 network to the
   IPv4 network is translated.  After the binding state has been
   created, packets flowing in both directions on that particular flow
   are translated.  The result is that, in the general case, NAT64 only
   supports communications initiated by the IPv6-only node towards an
   IPv4-only node.  Some additional mechanisms (like ICE) or static
   binding configuration, can be used to provide support for
   communications initiated by an IPv4-only node to an IPv6-only node.

1.2.1.  Stateful NAT64 solution elements

   In this section we describe the different elements involved in the
   NAT64 approach.

   The main component of the proposed solution is the translator itself.
   The translator has essentially two main parts, the address
   translation mechanism and the protocol translation mechanism.

   Protocol translation from IPv4 packet header to IPv6 packet header



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   and vice-versa is performed according to the IP/ICMP Translation
   Algorithm [I-D.ietf-behave-v6v4-xlate].

   Address translation maps IPv6 transport addresses to IPv4 transport
   addresses and vice-versa.  In order to create these mappings the
   NAT64 has two pools of addresses: an IPv6 address pool (to represent
   IPv4 addresses in the IPv6 network) and an IPv4 address pool (to
   represent IPv6 addresses in the IPv4 network).

   The IPv6 address pool is one or more IPv6 prefixes assigned to the
   translator itself.  Hereafter we will call the IPv6 address pool as
   Pref64::/n, in the case there are more than one prefix assigned to
   the NAT64, the comments made about Pref64::/n apply to each of them.
   Pref64::/n will be used by the NAT64 to construct IPv4-Converted IPv6
   addresses as defined in [I-D.ietf-behave-address-format].  Due to the
   abundance of IPv6 address space, it is possible to assign one or more
   Pref64::/n, each of them being equal to or even bigger than the size
   of the whole IPv4 address space.  This allows each IPv4 address to be
   mapped into a different IPv6 address by simply concatenating a
   Pref64::/n with the IPv4 address being mapped and a suffix.  The
   provisioning of the Pref64::/n as well as the address format are
   defined in [I-D.ietf-behave-address-format].

   The IPv4 address pool is a set of IPv4 addresses, normally a small
   prefix assigned by the local administrator.  Since IPv4 address space
   is a scarce resource, the IPv4 address pool is small and typically
   not sufficient to establish permanent one-to-one mappings with IPv6
   addresses.  So, except for the static/manually created ones, mappings
   using the IPv4 address pool will be created and released dynamically.
   Moreover, because of the IPv4 address scarcity, the usual practice
   for NAT64 is likely to be the binding of IPv6 transport addresses
   into IPv4 transport addresses, instead of IPv6 addresses into IPv4
   addresses directly, enabling a higher utilization of the limited IPv4
   address pool.  This implies that NAT64 performs both address and port
   translation.

   Because of the dynamic nature of the IPv6 to IPv4 address mapping and
   the static nature of the IPv4 to IPv6 address mapping, it is far
   simpler to allow communications initiated from the IPv6 side toward
   an IPv4 node, whose address is algorithmically mapped into an IPv6
   address, than communications initiated from IPv4-only nodes to an
   IPv6 node in which case an IPv4 address needs to be associated with
   the IPv6 node's address dynamically.

   Using a mechanisms such as DNS64, an IPv6 client obtains an IPv6
   address that embeds the IPv4 address of the IPv4 server, and sends a
   packet to that IPv6 address.  The packets are intercepted by the
   NAT64 device, which associates an IPv4 transport address of its IPv4



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   pool to the IPv6 transport address of the initiator, creating binding
   state, so that reply packets can be translated and forwarded back to
   the initiator.  The binding state is kept while packets are flowing.
   Once the flow stops, and based on a timer, the IPv4 transport address
   is returned to the IPv4 address pool so that it can be reused for
   other communications.

   To allow an IPv6 initiator to do a DNS lookup to learn the address of
   the responder, DNS64 [I-D.ietf-behave-dns64] is used to synthesize
   AAAA RRs from the A RRs.  The IPv6 addresses contained in the
   synthetic AAAA RRs contain a Pref64::/n assigned to the NAT64 and the
   IPv4 address of the responder.  The synthetic AAAA RRs are passed
   back to the IPv6 initiator, which will initiate an IPv6 communication
   with an IPv6 address associated to the IPv4 receiver.  The packet
   will be routed to the NAT64 device, which will create the IPv6 to
   IPv4 address mapping as described before.

1.2.2.  Stateful NAT64 Behaviour Walkthrough

   In this section we provide a simple example of the NAT64 behaviour.
   We consider an IPv6 node located in an IPv6-only site that initiates
   a TCP connection to an IPv4-only node located in the IPv4 network.

   The scenario for this case is depicted in the following figure:


             +---------------------+         +---------------+
             |IPv6 network         |         |    IPv4       |
             |           |  +-------------+  |  Network      |
             |           |--| Name server |--|               |
             |           |  | with DNS64  |  |  +----+       |
             |  +----+   |  +-------------+  |  | H2 |       |
             |  | H1 |---|         |         |  +----+       |
             |  +----+   |      +-------+    |  192.0.2.1    |
             |2001:DB8::1|------| NAT64 |----|               |
             |           |      +-------+    |               |
             |           |         |         |               |
             +---------------------+         +---------------+

   The figure above shows an IPv6 node H1 with an IPv6 address 2001:
   DB8::1 and an IPv4 node H2 with IPv4 address 192.0.2.1 H2 has
   h2.example.com as FQDN.

   A NAT64 connects the IPv6 network to the IPv4 network.  This NAT64
   uses the Well-Known Prefix 64:FF9B::/96 defined in
   [I-D.ietf-behave-address-format] to represent IPv4 addresses in the
   IPv6 address space and a single IPv4 address 203.0.113.1 assigned to
   its IPv4 interface.  The routing is configured in such a way that the



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   IPv6 packets addressed to a destination address in 64:FF9B::/96 are
   routed to the IPv6 interface of the NAT64 device.

   Also shown is a local name server with DNS64 functionality.  The
   local name server uses the Well-Known prefix 64:FF9B::/96 to create
   the IPv6 addresses in the synthetic RRs.

   For this example, assume the typical DNS situation where IPv6 hosts
   have only stub resolvers and the local name server does the recursive
   lookups.

   The steps by which H1 establishes communication with H2 are:

   1.  H1 performs a DNS query for h2.example.com and receives the
       synthetic AAAA RR from the local name server that implements the
       DNS64 functionality.  The AAAA record contains an IPv6 address
       formed by the Well-Known Prefix and the IPv4 address of H2 (i.e.
       64:FF9B::192.0.2.1).

   2.  H1 sends a TCP SYN packet to H2.  The packet is sent from a
       source transport address of (2001:DB8::1,1500) to a destination
       transport address of (64:FF9B::192.0.2.1,80), where the ports are
       set by H1.

   3.  The packet is routed to the IPv6 interface of the NAT64 (since
       IPv6 routing is configured that way).

   4.  The NAT64 receives the packet and performs the following actions:

       *  The NAT64 selects an unused port (e.g. 2000) on its IPv4
          address 203.0.113.1 and creates the mapping entry (2001:DB8::
          1,1500) <--> (203.0.113.1,2000)

       *  The NAT64 translates the IPv6 header into an IPv4 header using
          the IP/ICMP Translation Algorithm
          [I-D.ietf-behave-v6v4-xlate].

       *  The NAT64 includes (203.0.113.1,2000) as source transport
          address in the packet and (192.0.2.1,80) as destination
          transport address in the packet.  Note that 192.0.2.1 is
          extracted directly from the destination IPv6 address of the
          received IPv6 packet that is being translated.  The
          destination port 80 of the translated packet is the same as
          the destination port of the received IPv6 packet.

   5.  The NAT64 sends the translated packet out its IPv4 interface and
       the packet arrives at H2.




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   6.  H2 node responds by sending a TCP SYN+ACK packet with destination
       transport address (203.0.113.1,2000) and source transport address
       (192.0.2.1,80).

   7.  Since the IPv4 address 203.0.113.1 is assigned to the IPv4
       interface of the NAT64 device, the packet is routed to the NAT64
       device, which will look for an existing mapping containing
       (203.0.113.1,2000).  Since the mapping (2001:DB8::1,1500) <-->
       (203.0.113.1,2000) exists, the NAT64 performs the following
       operations:

       *  The NAT64 translates the IPv4 header into an IPv6 header using
          the IP/ICMP Translation Algorithm
          [I-D.ietf-behave-v6v4-xlate].

       *  The NAT64 includes (2001:DB8::1,1500) as destination transport
          address in the packet and (64:FF9B::192.0.2.1,80) as source
          transport address in the packet.  Note that 192.0.2.1 is
          extracted directly from the source IPv4 address of the
          received IPv4 packet that is being translated.  The source
          port 80 of the translated packet is the same as the source
          port of the received IPv4 packet.

   8.  The translated packet is sent out the IPv6 interface to H1.

   The packet exchange between H1 and H2 continues and packets are
   translated in the different directions as previously described.

   It is important to note that the translation still works if the IPv6
   initiator H1 learns the IPv6 representation of H2's IPv4 address
   (i.e., 64:FF9B::192.0.2.1) through some scheme other than a DNS
   look-up.  This is because the DNS64 processing does NOT result in any
   state installed in the NAT64 and because the mapping of the IPv4
   address into an IPv6 address is the result of concatenating the Well-
   Known Prefix to the original IPv4 address.

1.2.3.  Filtering

   NAT64 may do filtering, which means that it only allows a packet in
   through an interface under certain circumstances.  The NAT64 can
   filter IPv6 packets based on the administrative rules to create
   entries in the binding and session tables.  The filtering can be
   flexible and general but the idea of the filtering is to provide the
   administrators necessary control to avoid DoS attacks that would
   result in exhaustion of the NAT64's IPv4 address, port, memory and
   CPU resources.  Filtering techniques of incoming IPv6 packets are not
   specific to the NAT64 and therefore are not described in this
   specification.



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   Filtering of IPv4 packets on the other hand is tightly coupled to the
   NAT64 state and therefore is described in this specification.  In
   this document, we consider that the NAT64 may do no filtering, or it
   may filter incoming IPv4 packets.

   NAT64 filtering of incoming IPv4 packets is consistent with the
   recommendations of [RFC4787], and the ones of [RFC5382].  Because of
   that, the NAT64 as specified in this document, supports both
   Endpoint-Independent Filtering and Address-Dependent Filtering, both
   for TCP and UDP as well as filtering of ICMP packets.

   If a NAT64 performs Endpoint-Independent Filtering of incoming IPv4
   packets, then an incoming IPv4 packet is dropped unless the NAT64 has
   state for the destination transport address of the incoming IPv4
   packet.

   If a NAT64 performs Address-Dependent Filtering of incoming IPv4
   packets, then an incoming IPv4 packet is dropped unless the NAT64 has
   state involving the destination transport address of the IPv4
   incoming packet and the particular source IP address of the incoming
   IPv4 packet.


2.  Terminology

   This section provides a definitive reference for all the terms used
   in this document.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   The following additional terms are used in this document:

   3-Tuple:  The tuple (source IP address, destination IP address, ICMP
      Identifier).  A 3-tuple uniquely identifies an ICMP Query session.
      When an ICMP Query session flows through a NAT64, each session has
      two different 3-tuples: one with IPv4 addresses and one with IPv6
      addresses.

   5-Tuple:  The tuple (source IP address, source port, destination IP
      address, destination port, transport protocol).  A 5-tuple
      uniquely identifies a UDP/TCP session.  When a UDP/TCP session
      flows through a NAT64, each session has two different 5-tuples:
      one with IPv4 addresses and one with IPv6 addresses.






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   BIB:  Binding Information Base.  A table of bindings kept by a NAT64.
      Each NAT64 has a session table for each translated protocol.  An
      implementation compliant to this document would have a session
      table for TCP, one for UDP and one for ICMP Queries.  Additional
      session tables would be added to support other protocols, such as
      SCTP.

   Endpoint-Independent Mapping:  In NAT64, using the same mapping for
      all the sessions involving a given IPv6 transport address of an
      IPv6 host (irrespectively of the transport address of the IPv4
      host involved in the communication).  Endpoint-independent Mapping
      is important for peer-to-peer communication.  See [RFC4787] for
      the definition of the different types of mappings in IPv4-to-IPv4
      NATs.

   Filtering, Endpoint-Independent:  The NAT64 only filters incoming
      IPv4 packets destined to a transport address for which there is no
      state in the NAT64, regardless of the source IPv4 transport
      address.  The NAT forwards any packets destined to any transport
      address for which it has state.  In other words, having state for
      a given transport address is sufficient to allow any packets back
      to the internal endpoint.  See [RFC4787] for the definition of the
      different types of filtering in IPv4-to-IPv4 NATs.

   Filtering, Address-Dependent:  The NAT64 filters incoming IPv4
      packets destined to a transport address for which there is no
      state (similar to the Endpoint-Independent Filtering).
      Additionally, the NAT64 will filter out incoming IPv4 packets
      coming from a given IPv4 address X and destined for a transport
      address that it has state for if the NAT64 has not sent packets to
      X previously (independently of the port used by X).  In other
      words, for receiving packets from a specific IPv4 endpoint, it is
      necessary for the IPv6 endpoint to send packets first to that
      specific IPv4 endpoint's IP address.

   Hairpinning:  Having a packet do a "U-turn" inside a NAT and come
      back out the same side as it arrived on.  If the destination IPv6
      address and its embedded IPv4 address are both assigned to the
      NAT64 itself, then the packet is being sent to another IPv6 host
      connected to the same NAT64.  Such a packet is called a 'hairpin
      packet'.  A NAT64 that forwards hairpin packets, back to the IPv6
      host are defined as supporting "hairpinning".  Hairpinning support
      is important for peer-to-peer applications, as there are cases
      when two different hosts on the same side of a NAT can only
      communicate using sessions that hairpin through the NAT.  Hairpin
      packets can be either TCP or UDP.  More detailed explanation of
      hairpinning and examples for the UDP case can be found in
      [RFC4787].



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   ICMP Query packet:  ICMP packets that are not ICMP error messages.
      For ICMPv6, ICMPv6 Query Messages are the ICMPv6 Informational
      messages as defined in [RFC4443].  For ICMPv4, ICMPv4 Query
      messages are all ICMPv4 messages that are not ICMPv4 error
      messages.

   Mapping or Binding:  A mapping between an IPv6 transport address and
      a IPv4 transport address or a mapping between an (IPv6 address,
      ICMPv6 Identifier) pair and an (IPv4 address, ICMPv4 Identifier)
      pair.  Used to translate the addresses and ports/ICMP Identifiers
      of packets flowing between the IPv6 host and the IPv4 host.  In
      NAT64, the IPv4 address and port/ICMPv4 Identifier is always one
      assigned to the NAT64 itself, while the IPv6 address and port/
      ICMPv6 Identifier belongs to some IPv6 host.

   Session:  The flow of packets between two different hosts.  This may
      be TCP, UDP or ICMP Queries.  In NAT64, typically one host is an
      IPv4 host, and the other one is an IPv6 host.  However, due to
      hairpinning, both hosts might be IPv6 hosts.

   Session table:  A table of sessions kept by a NAT64.  Each NAT64 has
      three session tables, one for TCP, one for UDP and one for ICMP
      Queries.

   Stateful NAT64:  A function that has per-flow state which translates
      IPv6 packets to IPv4 packets and vice-versa, for TCP, UDP, and
      ICMP.  The NAT64 uses binding state to perform the translation
      between IPv6 and IPv4 addresses.  In this document we also refer
      to stateful NAT64 simply as NAT64.

   Stateful NAT64 device:  The device where the NAT64 function is
      executed.  In this document we also refer to stateful NAT64 device
      simply as NAT64 device.

   Transport Address:  The combination of an IPv6 or IPv4 address and a
      port.  Typically written as (IP address, port)- e.g. (192.0.2.15,
      8001).

   Tuple:  Refers to either a 3-Tuple or a 5-tuple as defined above.

   For a detailed understanding of this document, the reader should also
   be familiar with NAT terminology [RFC4787].


3.  Stateful NAT64 Normative Specification

   A NAT64 is a device with at least one IPv6 interface and at least one
   IPv4 interface.  Each NAT64 device MUST have at least one unicast /n



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   IPv6 prefix assigned to it, denoted Pref64::/n.  Additional
   considerations about the Pref64::/n are presented in Section 3.5.4.
   A NAT64 MUST have one or more unicast IPv4 addresses assigned to it.

   A NAT64 uses the following conceptual dynamic data structures:

   o  UDP Binding Information Base

   o  UDP Session Table

   o  TCP Binding Information Base

   o  TCP Session Table

   o  ICMP Query Binding Information Base

   o  ICMP Query Session Table

   These tables contain information needed for the NAT64 processing.
   The actual division of the information into six tables is done in
   order to ease the description of the NAT64 behaviour.  NAT64
   implementations are free to use different data structures but they
   MUST store all the required information and the externally visible
   outcome MUST be the same as the one described in this document.

   The notation used is the following: upper case letters are IPv4
   addresses; upper case letters with a prime(') are IPv6 addresses;
   lower case letters are ports; IPv6 prefixes of length n are indicated
   by "P::/n", mappings are indicated as "(X,x) <--> (Y',y)".

3.1.  Binding Information Bases

   A NAT64 has three Binding Information Bases (BIBs): one for TCP, one
   for UDP and one for ICMP Queries.  In the case of UDP and TCP BIBs,
   each BIB entry specifies a mapping between an IPv6 transport address
   and an IPv4 transport address:

      (X',x) <--> (T,t)

   where X' is some IPv6 address, T is an IPv4 address, and x and t are
   ports.  T will always be one of the IPv4 addresses assigned to the
   NAT64.  The BIB has then two columns: the BIB IPv6 transport address
   and the BIB IPv4 transport address.  A given IPv6 or IPv4 transport
   address can appear in at most one entry in a BIB: for example, (2001:
   db8::17, 49832) can appear in at most one TCP and at most one UDP BIB
   entry.  TCP and UDP have separate BIBs because the port number space
   for TCP and UDP are distinct.  If the BIBs are implemented as
   specified in this document, it results in Endpoint-Independent



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   Mappings in the NAT64.  The information in the BIBs is also used to
   implement Endpoint-Independent Filtering.  (Address-Dependent
   Filtering is implemented using the session tables described below.)

   In the case of the ICMP Query BIB, each ICMP Query BIB entry
   specifies a mapping between an (IPv6 address, ICMPv6 Identifier) pair
   and an (IPv4 address, ICMPv4 Identifier) pair.

      (X',I1) <--> (T,I2)

   where X' is some IPv6 address, T is an IPv4 address, I1 is an ICMPv6
   Identifier and I2 is an ICMPv4 Identifier.  T will always be one of
   the IPv4 addresses assigned to the NAT64.  A given (IPv6 or IPv4
   address, ICMPv6 or ICMPv4 Identifier) pair can appear in at most one
   entry in the ICMP Query BIB.

   Entries in any of the three BIBs can be created dynamically as the
   result of the flow of packets as described in Section 3.5 but they
   can also be created manually by an administrator.  NAT64
   implementations SHOULD support manually configured BIB entries for
   any of the three BIBs.  Dynamically-created entries are deleted from
   the corresponding BIB when the last session associated with the BIB
   entry is removed from the session table.  Manually-configured BIB
   entries are not deleted when there is no corresponding session table
   entry and can only be deleted by the administrator.

3.2.  Session Tables

   A NAT64 also has three session tables: one for TCP sessions, one for
   UDP sessions, and one for ICMP Query sessions.  Each entry keeps
   information on the state of the corresponding session.  In the TCP
   and UDP session tables, each entry specifies a mapping between a pair
   of IPv6 transport addresses and a pair of IPv4 transport addresses:

      (X',x),(Y',y) <--> (T,t),(Z,z)

   where X' and Y' are IPv6 addresses, T and Z are IPv4 addresses, and
   x, y, z and t are ports.  T will always be one of the IPv4 addresses
   assigned to the NAT64.  Y' is always the IPv6 representation of the
   IPv4 address Z, so Y' is obtained from Z using the algorithm applied
   by the NAT64 to create IPv6 representations of IPv4 addresses. y will
   always be equal to z.

   For each TCP or UDP Session Table Entry (STE), there are then five
   columns.  The terminology used for the session table entry columns is
   from the perspective of an incoming IPv6 packet being translated into
   an outgoing IPv4 packet.  The columns are::




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      The STE source IPv6 transport address, (X',x) in the example
      above,

      The STE destination IPv6 transport address, (Y',y) in the example
      above,

      The STE source IPv4 transport address, (T,t) in the example above,
      and,

      The STE destination IPv4 transport address, (Z,z) in the example
      above.

      The STE lifetime.

   In the ICMP query session table, each entry specifies a mapping
   between a 3-tuple of IPv6 source address, IPv6 destination address
   and ICMPv6 Identifier and a 3-tuple of IPv4 source address, IPv4
   destination address and ICMPv4 Identifier:

      (X',Y',I1) <--> (T,Z,I2)

   where X' and Y' are IPv6 addresses, T and Z are IPv4 addresses, I1 is
   an ICMPv6 Identifier and I2 is an ICMPv4 Identifier.  T will always
   be one of the IPv4 addresses assigned to the NAT64.  Y' is always the
   IPv6 representation of the IPv4 address Z, so Y' is obtained from Z
   using the algorithm applied by the NAT64 to create IPv6
   representations of IPv4 addresses.

   For each ICMP Query Session Table Entry (STE), there are then seven
   columns:

      The STE source IPv6 address, X' in the example above,

      The STE destination IPv6 address, Y' in the example above,

      The STE ICMPv6 Identifier, I1 in the example above,

      The STE source IPv4 address, T in the example above,

      The STE destination IPv4 address, Z in the example above, and,

      The STE ICMPv4 Identifier, I2 in the example above.

      The STE lifetime.







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3.3.  Packet Processing Overview

   The NAT64 uses the session state information to determine when the
   session is completed, and also uses session information for Address-
   Dependent Filtering.  A session can be uniquely identified by either
   an incoming tuple or an outgoing tuple.

   For each TCP or UDP session, there is a corresponding BIB entry,
   uniquely specified by either the source IPv6 transport address (in
   the IPv6 --> IPv4 direction) or the destination IPv4 transport
   address (in the IPv4 --> IPv6 direction).  For each ICMP Query
   session, there is a corresponding BIB entry, uniquely specified by
   either the source IPv6 address and ICMPv6 Identifier (in the IPv6 -->
   IPv4 direction) or the destination IPv4 address and the ICMPv4
   Identifier (in the IPv4 --> IPv6 direction).  However, for all the
   BIBs, a single BIB entry can have multiple corresponding sessions.
   When the last corresponding session is deleted, if the BIB entry was
   dynamically created, the BIB entry is deleted.

   The NAT64 will receive packets through its interfaces.  These packets
   can be either IPv6 packets or IPv4 packets and they may carry TCP
   traffic, UDP traffic or ICMP traffic.  The processing of the packets
   will be described next.  In the case that the processing is common to
   all the aforementioned types of packets, we will refer to the packet
   as the incoming IP packet in general.  In case that the processing is
   specific to IPv6 packets, we will refer to the incoming IPv6 packet
   and similarly to the IPv4 packets.

   The processing of an incoming IP packet takes the following steps:

   1.  Determining the incoming tuple

   2.  Filtering and updating binding and session information

   3.  Computing the outgoing tuple

   4.  Translating the packet

   5.  Handling hairpinning

   The details of these steps are specified in the following
   subsections.

   This breakdown of the NAT64 behavior into processing steps is done
   for ease of presentation.  A NAT64 MAY perform the steps in a
   different order, or MAY perform different steps, but the externally
   visible outcome MUST be the same as the one described in this
   document.



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3.4.  Determining the Incoming tuple

   This step associates an incoming tuple with every incoming IP packet
   for use in subsequent steps.  In the case of TCP, UDP and ICMP error
   packets, the tuple is a 5-tuple consisting of source IP address,
   source port, destination IP address, destination port, transport
   protocol.  In case of ICMP Queries, the tuple is a 3-tuple consisting
   of the source IP address, destination IP address and ICMP Identifier.

   If the incoming IP packet contains a complete (un-fragmented) UDP or
   TCP protocol packet, then the 5-tuple is computed by extracting the
   appropriate fields from the received packet.

   If the incoming packet is a complete (un-fragmented) ICMP query
   message (i.e., an ICMPv4 Query message or an ICMPv6 Informational
   message), the 3-tuple is the source IP address, the destination IP
   address and the ICMP Identifier.

   If the incoming IP packet contains a complete (un-fragmented) ICMP
   error message containing a UDP or a TCP packet, then the 5-tuple is
   computed by extracting the appropriate fields from the IP packet
   embedded inside the ICMP error message.  However, the role of source
   and destination is swapped when doing this: the embedded source IP
   address becomes the destination IP address in the 5-tuple, the
   embedded source port becomes the destination port in the 5-tuple,
   etc.  If it is not possible to determine the 5-tuple (perhaps because
   not enough of the embedded packet is reproduced inside the ICMP
   message), then the incoming IP packet MUST be silently discarded.

   If the incoming IP packet contains a complete (un-fragmented) ICMP
   error message containing a ICMP error message, then the packet is
   silently discarded.

   If the incoming IP packet contains a complete (un-fragmented) ICMP
   error message containing an ICMP Query message, then the 3-tuple is
   computed by extracting the appropriate fields from the IP packet
   embedded inside the ICMP error message.  However, the role of source
   and destination is swapped when doing this: the embedded source IP
   address becomes the destination IP address in the 3-tuple, the
   embedded destination IP address becomes the source address in the
   3-tuple and the embedded ICMP Identifier is used as the ICMP
   Identifier of the 3-tuple.  If it is not possible to determine the
   3-tuple (perhaps because not enough of the embedded packet is
   reproduced inside the ICMP message), then the incoming IP packet MUST
   be silently discarded.

   If the incoming IP packet contains a fragment, then more processing
   may be needed.  This specification leaves open the exact details of



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   how a NAT64 handles incoming IP packets containing fragments, and
   simply requires that the external behavior of the NAT64 is compliant
   with the following conditions:

      The NAT64 MUST handle fragments.  In particular, NAT64 MUST handle
      fragments arriving out-of-order , conditioned on the following:

      *  The NAT64 MUST limit the amount of resources devoted to the
         storage of fragmented packets in order to protect from DoS
         attacks.

      *  As long as the NAT64 has available resources, the NAT64 MUST
         allow the fragments to arrive over a time interval.  The time
         interval SHOULD be configurable and the default value MUST be
         of at least FRAGMENT_MIN.

      *  The NAT64 MAY require that the UDP, TCP, or ICMP header be
         completely contained within the fragment that contains OFFSET
         equal to zero.

      For incoming packets carrying TCP or UDP fragments with non-null
      checksum, NAT64 MAY elect to queue the fragments as they arrive
      and translate all fragments at the same time.  In this case, the
      incoming tuple is determined as documented above to the un-
      fragmented packets.  Alternatively, a NAT64 MAY translate the
      fragments as they arrive, by storing information that allows it to
      compute the 5-tuple for fragments other than the first.  In the
      latter case, subsequent fragments may arrive before the first and
      the rules about how the NAT64 handles (out-of-order) fragments
      described in the bulleted list above apply.

      For incoming IPv4 packets carrying UDP packets with null checksum,
      if the NAT64 has enough resources, the NAT64 MUST reassemble the
      packets and MUST calculate the checksum.  If the NAT64 does not
      have enough resources, then it MUST silently discard the packets.

      Implementers of NAT64 should be aware that there are a number of
      well-known attacks against IP fragmentation; see [RFC1858] and
      [RFC3128].  Implementers should also be aware of additional issues
      with reassembling packets at high rates, described in [RFC4963].

   If the incoming packet is an IPv6 packet that contains a protocol
   other than TCP, UDP or ICMPv6 in the last Next Header, then the
   packet SHOULD be discarded and, if the security policy permits, the
   NAT64 SHOULD send an ICMPv6 Destination Unreachable error message
   with Code 3 (Destination Unreachable) to the source address of the
   received packet.  NOTE: This behaviour may be updated by future
   documents that define how other protocols such as SCTP or DCCP are



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   processed by NAT64.

   If the incoming packet is an IPv4 packet that contains a protocol
   other than TCP, UDP or ICMPv4, then the packet SHOULD discarded and,
   if the security policy permits, the NAT64 SHOULD send an ICMPv4
   Destination Unreachable error message with Code 2 (Protocol
   Unreachable) to the source address of the received packet.  NOTE:
   This behaviour may be updated by future documents that define how
   other protocols such as SCTP or DCCP are processed by NAT64.

3.5.  Filtering and Updating Binding and Session Information

   This step updates binding and session information stored in the
   appropriate tables.  This step may also filter incoming packets, if
   desired.

   Irrespective of the transport protocol used, the NAT64 MUST silently
   discard all incoming IPv6 packets containing a source address that
   contains the Pref64::/n.  This is required in order to prevent
   hairpinning loops as described in Section 5.  In addition, the NAT64
   MUST only process incoming IPv6 packets that contain a destination
   address that contains Pref64::/n.  Likewise, the NAT64 MUST only
   process incoming IPv4 packets that contain a destination address that
   belong to the IPv4 pool assigned to the NAT64.

   The details of this step depend on the protocol (UDP, TCP or ICMP
   Query).

3.5.1.  UDP Session Handling

   The following state information is stored for a UDP session:

      Binding:(X',x),(Y',y) <--> (T,t),(Z,z)

      Lifetime: a timer that tracks the remaining lifetime of the UDP
      session.  When the timer expires, the UDP session is deleted.  If
      all the UDP sessions corresponding to a dynamically created UDP
      BIB entry are deleted, then the UDP BIB entry is also deleted.

   An IPv6 incoming packet with an incoming tuple with source transport
   address (X',x) and destination transport address (Y',y) is processed
   as follows:

      The NAT64 searches for a UDP BIB entry that contains the BIB IPv6
      transport address that matches the IPv6 source transport address
      (X',x).  If such an entry does not exist, the NAT64 tries to
      create a new entry (if resources and policy permit).  The source
      IPv6 transport address of the packet (X',x) is used as BIB IPv6



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      transport address, and the BIB IPv4 transport address is set to
      (T,t) which is allocated using the rules defined in
      Section 3.5.1.1.  The result is a BIB entry as follows: (X',x)
      <--> (T,t).

      The NAT64 searches for the session table entry corresponding to
      the incoming 5-tuple.  If no such entry is found, the NAT64 tries
      to create a new entry (if resources and policy permit).  The
      information included in the session table is as follows:

      *  The STE source IPv6 transport address is set to (X',x), the
         source IPv6 transport addresses contained in the received IPv6
         packet,

      *  The STE destination IPv6 transport address is set to (Y',y),
         the destination IPv6 transport addresses contained in the
         received IPv6 packet,

      *  The STE source IPv4 transport address is extracted from the
         corresponding UDP BIB entry i.e. it is set to (T,t),

      *  The STE destination IPv4 transport is set to (Z(Y'),y), y being
         the same port as the STE destination IPv6 transport address and
         Z(Y') being algorithmically generated from the IPv6 destination
         address (i.e.  Y') using the reverse algorithm as specified in
         Section 3.5.4.

      The result is a Session table entry as follows: (X',x),(Y',y) <-->
      (T,t),(Z(Y'),y)

      The NAT64 sets (or resets) the timer in the Session Table Entry to
      the maximum session lifetime.  The maximum session lifetime MAY be
      configurable and the default SHOULD be at least UDP_DEFAULT.  The
      maximum session lifetime MUST NOT be less than UDP_MIN.  The
      packet is translated and forwarded as described in the following
      sections.

   An IPv4 incoming packet, with an incoming tuple with source IPv4
   transport address (Y,y) and destination IPv4 transport address (X,x)
   is processed as follows:

      The NAT64 searches for a UDP BIB entry that contains the BIB IPv4
      transport address matching (Y,y), (i.e., the IPv4 destination
      transport address in the incoming IPv4 packet).  If such an entry
      does not exist, the packet MUST be dropped.  An ICMP error message
      with type of 3 (Destination Unreachable) MAY be sent to the
      original sender of the packet, unless the discarded packet is
      itself an ICMP error message.



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      If the NAT64 applies Address-Dependent Filters on its IPv4
      interface, then the NAT64 checks to see if the incoming packet is
      allowed according to the Address-Dependent Filtering rule.  To do
      this, it searches for a session table entry with an STE source
      IPv4 transport address equal to (X,x), (i.e., the destination IPv4
      transport address in the incoming packet) and STE destination IPv4
      address equal to Y, (i.e., the source IPv4 address in the incoming
      packet).  If such an entry is found (there may be more than one),
      packet processing continues.  Otherwise, the packet is discarded.
      If the packet is discarded, then an ICMP error message MAY be sent
      to the original sender of the packet, unless the discarded packet
      is itself an ICMP message.  The ICMP error message, if sent, has a
      type of 3 (Destination Unreachable) and a code of 13
      (Communication Administratively Prohibited).

      In case the packet is not discarded in the previous processing
      (either because the NAT64 is not filtering or because the packet
      is compliant with the Address-Dependent Filtering rule), then the
      NAT64 searches for the session table entry corresponding
      containing the STE source IPv4 transport address equal to (X,x)
      and the STE destination IPv4 transport address equal to (Y,y).  If
      no such entry is found, the NAT64 tries to create a new entry (if
      resources and policy permit).  In case a new UDP session table
      entry is created, it contains the following information:

      *  The STE source IPv6 transport address is extracted from the
         corresponding UDP BIB entry.

      *  The STE destination IPv6 transport address is set to (Z'(Y),y),
         y being the same port y than the destination IPv4 transport
         address and Z'(Y) being the IPv6 representation of Y, generated
         using the algorithm described in Section 3.5.4.

      *  The STE source IPv4 transport address is set to (X,x) the
         destination IPv4 transport addresses contained in the received
         IPv4 packet.

      *  The STE destination IPv4 transport is set to (Y,y), the source
         IPv4 transport addresses contained in the received IPv4 packet.

      The NAT64 sets (or resets) the timer in the Session Table Entry to
      the maximum session lifetime.  The maximum session lifetime MAY be
      configurable and the default SHOULD be at least UDP_DEFAULT.  The
      maximum session lifetime MUST NOT be less than UDP_MIN.  The
      packet is translated and forwarded as described in the following
      sections.





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3.5.1.1.  Rules for Allocation of IPv4 Transport Addresses for UDP

   When a new UDP BIB entry is created for a source transport address of
   (S',s), then the NAT64 allocates an IPv4 transport address for this
   BIB entry as follows:

      If there exists some other BIB entry containing S' as the IPv6
      address and mapping it to some IPv4 address T, then the NAT64
      SHOULD use T as the IPv4 address.  Otherwise, use any IPv4 address
      of the IPv4 pool assigned to the NAT64 to be used for translation.

      If the port s is in the Well-Known port range 0-1023, and the
      NAT64 has an available port t in the same port range, then the
      NAT64 SHOULD allocate the port t.  If the NAT64 does not have a
      port available in the same range, the NAT64 MAY assign a port t
      from other range where it has an available port.  (This behavior
      is recommended in REQ 3-a of [RFC4787].)

      If the port s is in the range 1024-65535, and the NAT64 has an
      available port t in the same port range, then the NAT64 SHOULD
      allocate the port t.  If the NAT64 does not have a port available
      in the same range, the NAT64 MAY assign a port t from other range
      where it has an available port. (this behavior is recommended in
      REQ 3-a of [RFC4787])

      The NAT64 SHOULD preserve the port parity (odd/even), as per
      Section 4.2.2 of [RFC4787]).

      In all cases, the allocated IPv4 transport address (T,t) MUST NOT
      be in use in another entry in the same BIB, but MAY be in use in
      the other BIB (referring to the UDP and TCP BIBs).

   If it is not possible to allocate an appropriate IPv4 transport
   address or create a BIB entry, then the packet is discarded.  The
   NAT64 SHOULD send an ICMPv6 Destination Unreachable/Address
   unreachable (Code 3) message.

3.5.2.  TCP Session Handling

   In this section we describe how the TCP BIB and Session table are
   populated.  We do so by defining the state machine of the NAT64 uses
   for TCP.  We first describe the states and the information contained
   in them and then we describe the actual state machine and state
   transitions.







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3.5.2.1.  State definition

   The following state information is stored for a TCP session:

      Binding:(X',x),(Y',y) <--> (T,t),(Z,z)

      Lifetime: a timer that tracks the remaining lifetime of the TCP
      session.  When the timer expires, the TCP session is deleted.  If
      all the TCP sessions corresponding to a TCP BIB entry are deleted,
      then the dynamically created TCP BIB entry is also deleted.

   Because the TCP session inactivity lifetime is set to at least 2
   hours and 4 min (as per [RFC5382]), it is important that each TCP
   session table entry corresponds to an existent TCP session.  In order
   to do that, for each TCP session established through it, it tracks
   the corresponding state machine as follows.

   The states are the following ones:

      CLOSED: Analogous to [RFC0793], CLOSED is a fictional state
      because it represents the state when there is no state for this
      particular 5-tuple, and therefore, no connection.

      V4 SYN RCV: An IPv4 packet containing a TCP SYN was received by
      the NAT64, implying that a TCP connection is being initiated from
      the IPv4 side.  The NAT64 is now waiting for a matching IPv6
      packet containing the TCP SYN in the opposite direction.

      V6 SYN RCV: An IPv6 packet containing a TCP SYN was received,
      translated and forwarded by the NAT64, implying that a TCP
      connection is being initiated from the IPv6 side.  The NAT64 is
      now waiting for a matching IPv4 packet containing the TCP SYN in
      the opposite direction.

      ESTABLISHED: Represents an open connection, with data able to flow
      in both directions.

      V4 FIN RCV: An IPv4 packet containing a TCP FIN was received by
      the NAT64, data can still flow in the connection, and the NAT64 is
      waiting for a matching TCP FIN in the opposite direction.

      V6 FIN RCV: An IPv6 packet containing a TCP FIN was received by
      the NAT64, data can still flow in the connection, and the NAT64 is
      waiting for a matching TCP FIN in the opposite direction.

      V6 FIN + V4 FIN RCV: Both an IPv4 packet containing a TCP FIN and
      an IPv6 packet containing an TCP FIN for this connection were
      received by the NAT64.  The NAT64 keeps the connection state alive



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      and forwards packets in both directions for a short period of time
      to allow remaining packets (in particular the ACKs) to be
      delivered.

      4MIN: The lifetime of the state for the connection is set to 4
      minutes either because a packet containing a TCP RST was received
      by the NAT64 for this connection or simply because the lifetime of
      the connection has decreased and there are only 4 minutes left.
      The NAT64 will keep the state for the connection for 4 min. and if
      no other data packets for that connection are received, the state
      for this connection is then terminated.

3.5.2.2.  State machine for TCP processing in the NAT64

   The state machine used by the NAT64 for the TCP session processing is
   depicted next.  The described state machine handles all TCP segments
   received through the IPv6 and IPv4 interface.  There is one state
   machine per TCP connection that is potentially established through
   the NAT64.  After bootstrapping of the NAT64 device, all TCP sessions
   are in CLOSED state.  As we mention above, the CLOSED state is a
   fictional state when is no state for that particular connection in
   the NAT64.  It should be noted that there is one state machine per
   connection, so only packets belonging to a given connection are
   inputs to the state machine associated to that connection.  In other
   words, when in the state machine below we state that a packet is
   received, it is implicit that the incoming 5-tuple of the data packet
   matches to the one of the state machine.

   A TCP segment with the SYN flag set that is received through the IPv6
   interface is called a V6 SYN, similarly, V4 SYN, V4 FIN, V6 FIN, V6
   FIN + V4 FIN, V6 RST and V4 RST.




















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                                      +-----------------------------+
                                      |                             |
                                      V                             |
                       V6       +------+      V4                    |
                  +----SYN------|CLOSED|-----SYN------+             |
                  |             +------+              |             |
                  |                ^                  |             |
                  |                |4min T.O.         |             |
                  V                |                  V             |
              +-------+         +-------+          +-------+        |
              |V6 SYN |         |  4MIN |          |V4 SYN |        |
              |  RCV  |         +-------+          |  RCV  |        |
              +-------+          |    ^            +-------+        |
                 |         data pkt   |               |             |
                 |               |  V4 or V6 RST      |             |
                 |               |  2 hr. T.O.        |             |
              V4 SYN             V    |              V6 SYN         |
                 |          +--------------+          |             |
                 +--------->| ESTABLISHED  |<---------+             |
                            +--------------+                        |
                              |           |                         |
                          V4 FIN       V6 FIN                       |
                              |           |                         |
                              V           V                         |
                      +---------+       +----------+                |
                      | V4 FIN  |       |  V6 FIN  |                |
                      +---------+       +----------+                |
                              |           |                         |
                          V6 FIN       V4 FIN                     4 min
                              |           |                        T.O.
                              V           V                         |
                         +-------------------+                      |
                         | V4 FIN + V6 FIN   |----------------------+
                         +-------------------+



   We next describe the state information and the transitions.

   *** CLOSED ***

   If a V6 SYN is received with an incoming tuple with source transport
   address (X',x) and destination transport address (Y',y) (this is the
   case of a TCP connection initiated from the IPv6 side), the
   processing is as follows:

   1.  The NAT64 searches for a TCP BIB entry that matches the IPv6
       source transport address (X',x).



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          If such an entry does not exist, the NAT64 tries to create a
          new BIB entry (if resources and policy permit).  The BIB IPv6
          transport address is set to (X',x) (i.e., the source IPv6
          transport address of the packet).  The BIB IPv4 transport
          address is set to an IPv4 transport address allocated using
          the rules defined in Section 3.5.2.3 The processing of the
          packet continues as described in bullet 2.

          If the entry already exists, then the processing continues as
          described in bullet 2.

   2.  Then the NAT64 tries to create a new TCP session entry in the TCP
       session table (if resources and policy permit).  The information
       included in the session table is as follows:

          The STE source IPv6 transport address is set to (X',x) (i.e.
          the source transport address contained in the received V6 SYN
          packet,

          The STE destination IPv6 transport address is set to (Y',y)
          (i.e. the destination transport address contained in the
          received V6 SYN packet.

          The STE source IPv4 transport address is set to the BIB IPv4
          transport address of the corresponding TCP BIB entry.

          The STE destination IPv4 transport address contains the port y
          (i.e., the same port as the IPv6 destination transport
          address) and the IPv4 address that is algorithmically
          generated from the IPv6 destination address (i.e.  Y') using
          the reverse algorithm as specified in Section 3.5.4.

          The lifetime of the TCP session table entry is set to at least
          to TCP_TRANS (the transitory connection idle timeout as
          defined in [RFC5382]).

   3.  The state of the session is moved to V6 SYN RCV.

   4.  The NAT64 translates and forwards the packet as described in the
       following sections

   If a V4 SYN packet is received with an incoming tuple with source
   IPv4 transport address (Y,y) and destination IPv4 transport address
   (X,x) (this is the case of a TCP connection initiated from the IPv4
   side), the processing is as follows:

      If the security policy requires silently dropping externally
      initiated TCP connections, then the packet is silently discarded,



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      else,

      If the destination transport address contained in the incoming V4
      SYN (i.e., X,x) is not in use in the TCP BIB, then:

         The NAT64 tries to create a new session table entry in the TCP
         session table (if resources and policy permit), containing the
         following information:

         +  The STE source IPv4 transport address is set to (X,x) (i.e.
            the destination transport address contained in the V4 SYN)

         +  The STE destination IPv4 transport address is set to (Y,y)
            (i.e. the source transport address contained in the V4 SYN)

         +  The STE transport IPv6 source address is left unspecified
            and may be populated by other protocols out of the scope of
            this specification.

         +  The STE destination IPv6 transport address contains the port
            y (i.e. the same port than the destination IPv4 transport
            address) and the IPv6 representation of Y (i.e. the IPv4
            address of the destination IPv4 transport address),
            generated using the algorithm described in Section 3.5.4.

         The state is moved to V4 SYN RCV.

         The lifetime of the STE entry is set to TCP_INCOMING_SYN as per
         [RFC5382] and the packet is stored.  The result is that the
         NAT64 will not drop the packet based on the filtering, nor
         create a BIB entry.  Instead, the NAT64 will only create the
         session table entry and store the packet.  The motivation for
         this is to support simultaneous open of TCP connections.

      If the destination transport address contained in the incoming V4
      SYN (i.e., X,x) is in use in the TCP BIB, then:

         The NAT64 tries to create a new session table entry in the TCP
         session table (if resources and policy permit), containing the
         following information:

         +  The STE source IPv4 transport address is set to (X,x) (i.e.
            the destination transport address contained in the V4 SYN)

         +  The STE destination IPv4 transport address is set to (Y,y)
            (i.e. the source transport address contained in the V4 SYN)





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         +  The STE transport IPv6 source address is set to the IPv6
            transport address contained in the corresponding TCP BIB
            entry.

         +  The STE destination IPv6 transport address contains the port
            y (i.e. the same port than the destination IPv4 transport
            address) and the IPv6 representation of Y (i.e. the IPv4
            address of the destination IPv4 transport address),
            generated using the algorithm described in Section 3.5.4.

         The state is moved to V4 SYN RCV.

         If the NAT64 is performing Address-Dependent Filtering, the
         lifetime of the STE entry is set to TCP_INCOMING_SYN as per
         [RFC5382] and the packet is stored.  The motivation for
         creating the session table entry and storing the packet
         (instead of simply dropping the packet based on the filtering)
         is to support simultaneous open of TCP connections.

         If the NAT64 is not performing Address-Dependent Filtering, the
         lifetime of the STE is set to at least to TCP_TRANS (the
         transitory connection idle timeout as defined in [RFC5382]) and
         it translates and forwards the packet as described in the
         following sections.

   For any other packet belonging to this connection:

      If there is a corresponding entry in the TCP BIB other packets
      SHOULD be translated and forwarded if the security policy allows
      to do so.  The state remains unchanged.

      If there is no corresponding entry in the TCP BIB the packet is
      silently discarded.

   *** V4 SYN RCV ***

   If a V6 SYN is received with incoming tuple with source transport
   address (X',x) and destination transport address (Y',y).  The
   lifetime of the TCP session table entry is set to at least to the
   maximum session lifetime.  The value for the maximum session lifetime
   MAY be configurable but it MUST NOT be less than TCP_EST (the
   established connection idle timeout as defined in [RFC5382]).  The
   default value for the maximum session lifetime SHOULD be set to
   TCP_EST.  The packet is translated and forwarded.  The state is moved
   to ESTABLISHED.

   If the lifetime expires, an ICMP Port Unreachable error (Type 3, Code
   3) containing the IPv4 SYN packet stored is sent back to the source



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   of the v4 SYN, the session table entry is deleted and, the state is
   moved to CLOSED.

   For any other packet, other packets SHOULD be translated and
   forwarded if the security policy allows to do so.  The state remains
   unchanged.

   *** V6 SYN RCV ***

   If a V4 SYN is received (with or without the ACK flag set), with an
   incoming tuple with source IPv4 transport address (Y,y) and
   destination IPv4 transport address (X,x), then the state is moved to
   ESTABLISHED.  The lifetime of the TCP session table entry is set to
   at least to the maximum session lifetime.  The value for the maximum
   session lifetime MAY be configurable but it MUST NOT be less than
   TCP_EST (the established connection idle timeout as defined in
   [RFC5382]).  The default value for the maximum session lifetime
   SHOULD be set to TCP_EST.  The packet is translated and forwarded.

   If the lifetime expires, the session table entry is deleted and the
   state is moved to CLOSED.

   If a V6 SYN packet is received, the packet is translated and
   forwarded.  The lifetime of the TCP session table entry is set to at
   least to TCP_TRANS.  The state remains unchanged.

   For any other packet, other packets SHOULD be translated and
   forwarded if the security policy allows to do so.  The state remains
   unchanged.

   *** ESTABLISHED ***

   If a V4 FIN packet is received, the packet is translated and
   forwarded.  The state is moved to V4 FIN RCV.

   If a V6 FIN packet is received, the packet is translated and
   forwarded.  The state is moved to V6 FIN RCV.

   If a V4 RST or a V6 RST packet is received, the packet is translated
   and forwarded.  The lifetime is set to TCP_TRANS and the state is
   moved to 4MIN.  (Since the NAT64 is uncertain whether the peer will
   accept the RST packet, instead of moving the state to CLOSED, it
   moves to 4MIN, which has a shorter lifetime.  If no other packets are
   received for this connection during the short timer, the NAT64
   assumes that the peer has accepted the RST packet and moves to
   CLOSED.  If packets keep flowing, the NAT64 assumes that the peer has
   not accepted the RST packet and moves back to the ESTABLISHED state.
   This is described below in the 4MIN state processing description.)



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   If any other packet is received, the packet is translated and
   forwarded.  The lifetime of the TCP session table entry is set to at
   least to the maximum session lifetime.  The value for the maximum
   session lifetime MAY be configurable but it MUST NOT be less than
   TCP_EST (the established connection idle timeout as defined in
   [RFC5382]).  The default value for the maximum session lifetime
   SHOULD be set to TCP_EST.  The state remains unchanged as
   ESTABLISHED.

   If the lifetime expires then the NAT64 SHOULD send a probe packet (as
   defined next) to at least one of the endpoints of the TCP connection.
   The probe packet is a TCP segment for the connection with no data.
   The sequence number and the acknowledgment number are set to zero.
   All flags but the ACK flag are reset.  The state is moved to 4MIN.

      Upon the reception of this probe packet, the endpoint will reply
      with an ACK containing the expected sequence number for that
      connection.  It should be noted that, for an active connection,
      each of these probe packets will generate one packet from each end
      involved in the connection, since the reply of the first point to
      the probe packet will generate a reply from the other endpoint.

   *** V4 FIN RCV ***

   If a V6 FIN packet is received, the packet is translated and
   forwarded.  The lifetime is set to TCP_TRANS.  The state is moved to
   V6 FIN + V4 FIN RCV.

   If any packet other than the V6 FIN is received, the packet is
   translated and forwarded.  The lifetime of the TCP session table
   entry is set to at least to the maximum session lifetime.  The value
   for the maximum session lifetime MAY be configurable but it MUST NOT
   be less than TCP_EST (the established connection idle timeout as
   defined in [RFC5382]).  The default value for the maximum session
   lifetime SHOULD be set to TCP_EST.  The state remains unchanged as V4
   FIN RCV.

   If the lifetime expires, the session table entry is deleted and the
   state is moved to CLOSED.

   *** V6 FIN RCV ***

   If a V4 FIN packet is received, the packet is translated and
   forwarded.  The lifetime is set to TCT_TRANS.  The state is moved to
   V6 FIN + V4 FIN RCV.

   If any packet other than the V4 FIN is received, the packet is
   translated and forwarded.  The lifetime of the TCP session table



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   entry is set to at least to the maximum session lifetime.  The value
   for the maximum session lifetime MAY be configurable but it MUST NOT
   be less than TCP_EST (the established connection idle timeout as
   defined in [RFC5382]).  The default value for the maximum session
   lifetime SHOULD be set to TCP_EST.  The state remains unchanged as V6
   FIN RCV.

   If the lifetime expires, the session table entry is deleted and the
   state is moved to CLOSED.

   *** V6 FIN + V4 FIN RCV ***

   All packets are translated and forwarded.

   If the lifetime expires, the session table entry is deleted and the
   state is moved to CLOSED.

   *** 4MIN ***

   If a packet other than a RST packet is received, the lifetime of the
   TCP session table entry is set to at least to the maximum session
   lifetime.  The value for the maximum session lifetime MAY be
   configurable but it MUST NOT be less than TCP_EST (the established
   connection idle timeout as defined in [RFC5382]).  The default value
   for the maximum session lifetime SHOULD be set to TCP_EST.  The state
   is moved to ESTABLISHED.

   If the lifetime expires, the session table entry is deleted and the
   state is moved to CLOSED.

3.5.2.3.  Rules for allocation of IPv4 transport addresses for TCP

   When a new TCP BIB entry is created for a source transport address of
   (S',s), then the NAT64 allocates an IPv4 transport address for this
   BIB entry as follows:

      If there exists some other BIB entry containing S' as the IPv6
      address and mapping it to some IPv4 address T, then T SHOULD be
      used as the IPv4 address.  Otherwise, use any IPv4 address of the
      IPv4 pool assigned to the NAT64 to be used for translation.

      If the port s is in the Well-Known port range 0-1023, and the
      NAT64 has an available port t in the same port range, then the
      NAT64 SHOULD allocate the port t.  If the NAT64 does not have a
      port available in the same range, the NAT64 MAY assign a port t
      from another range where it has an available port.





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      If the port s is in the range 1024-65535, and the NAT64 has an
      available port t in the same port range, then the NAT64 SHOULD
      allocate the port t.  If the NAT64 does not have a port available
      in the same range, the NAT64 MAY assign a port t from another
      range where it has an available port.

      In all cases, the allocated IPv4 transport address (T,t) MUST NOT
      be in use in another entry in the same BIB, but MAY be in use in
      the other BIB (referring to the UDP and TCP BIBs).

   If it is not possible to allocate an appropriate IPv4 transport
   address or create a BIB entry, then the packet is discarded.  The
   NAT64 SHOULD send an ICMPv6 Destination Unreachable/Address
   unreachable (Code 3) message.

3.5.3.  ICMP Query Session Handling

   The following state information is stored for an ICMP Query session
   in the ICMP Query session table:

      Binding:(X',Y',I1) <--> (T,Z,I2)

      Lifetime: a timer that tracks the remaining lifetime of the ICMP
      Query session.  When the timer expires, the session is deleted.
      If all the ICMP Query sessions corresponding to a dynamically
      created ICMP Query BIB entry are deleted, then the ICMP Query BIB
      entry is also deleted.

   An incoming ICMPv6 Informational packet with IPv6 source address X',
   IPv6 destination address Y' and ICMPv6 Identifier I1, is processed as
   follows:

      If the local security policy determines that ICMPv6 Informational
      packets are to be filtered, the packet is silently discarded.
      Else, the NAT64 searches for an ICMP Query BIB entry that matches
      the (X',I1) pair.  If such entry does not exist, the NAT64 tries
      to create a new entry (if resources and policy permit) with the
      following data:

      *  The BIB IPv6 address is set to X' (i.e. the source IPv6 address
         of the IPv6 packet).

      *  The BIB ICMPv6 Identifier is set to I1 (i.e. the ICMPv6
         Identifier).

      *  If there exists another BIB entry containing the same IPv6
         address X' and mapping it to an IPv4 address T, then use T as
         the BIB IPv4 address for this new entry.  Otherwise, use any



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         IPv4 address assigned to the IPv4 interface.

      *  As the BIB ICMPv4 Identifier use any available value i.e. any
         identifier value for which no other entry exists with the same
         (IPv4 address, ICMPv4 Identifier) pair.

      The NAT64 searches for an ICMP query session table entry
      corresponding to the incoming 3-tuple (X',Y',I1).  If no such
      entry is found, the NAT64 tries to create a new entry (if
      resources and policy permit).  The information included in the new
      session table entry is as follows:

      *  The STE IPv6 source address is set to X' (i.e. the address
         contained in the received IPv6 packet),

      *  The STE IPv6 destination address is set to Y' (i.e. the address
         contained in the received IPv6 packet),

      *  The STE ICMPv6 Identifier is set to I1 (i.e. the identifier
         contained in the received IPv6 packet),

      *  The STE IPv4 source address is set to the IPv4 address
         contained in the corresponding BIB entry,

      *  The STE ICMPv4 Identifier is set to the IPv4 identifier
         contained in the corresponding BIB entry,

      *  The STE IPv4 destination address is algorithmically generated
         from Y' using the reverse algorithm as specified in
         Section 3.5.4.

      The NAT64 sets (or resets) the timer in the session table entry to
      the maximum session lifetime.  By default, the maximum session
      lifetime is ICMP_DEFAULT.  The maximum lifetime value SHOULD be
      configurable.  The packet is translated and forwarded as described
      in the following sections.

   An incoming ICMPv4 Query packet with source IPv4 address Y,
   destination IPv4 address X and ICMPv4 Identifier I2 is processed as
   follows:

      The NAT64 searches for an ICMP Query BIB entry that contains X as
      IPv4 address and I2 as the ICMPv4 Identifier.  If such an entry
      does not exist, the packet is dropped.  An ICMP error message MAY
      be sent to the original sender of the packet, unless the discarded
      packet is itself an ICMP error message.  The ICMP error message,
      if sent, has a type of 3 (Destination Unreachable).




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      If the NAT64 filters on its IPv4 interface, then the NAT64 checks
      to see if the incoming packet is allowed according to the Address-
      Dependent Filtering rule.  To do this, it searches for a session
      table entry with an STE source IPv4 address equal to X, an STE
      ICMPv4 Identifier equal to I2 and a STE destination IPv4 address
      equal to Y. If such an entry is found (there may be more than
      one), packet processing continues.  Otherwise, the packet is
      discarded.  If the packet is discarded, then an ICMP error message
      MAY be sent to the original sender of the packet, unless the
      discarded packet is itself an ICMP message.  The ICMP error
      message, if sent, has a type of 3 (Destination Unreachable) and a
      code of 13 (Communication Administratively Prohibited).

      In case the packet is not discarded in the previous processing
      steps (either because the NAT64 is not filtering or because the
      packet is compliant with the Address-dependent Filtering rule),
      then the NAT64 searches for a session table entry with an STE
      source IPv4 address equal to X, an STE ICMPv4 Identifier equal to
      I2 and a STE destination IPv4 address equal to Y. If no such entry
      is found, the NAT64 tries to create a new entry (if resources and
      policy permit) with the following information:

      *  The STE source IPv4 address is set to X,

      *  The STE ICMPv4 Identifier is set to I2,

      *  The STE destination IPv4 address is set to Y,

      *  The STE source IPv6 address is set to the IPv6 address of the
         corresponding BIB entry,

      *  The STE ICMPv6 Identifier is set to the ICMPv6 Identifier of
         the corresponding BIB entry, and,

      *  The STE destination IPv6 address is set to the IPv6
         representation of the IPv4 address of Y, generated using the
         algorithm described in Section 3.5.4.

      *  The NAT64 sets (or resets) the timer in the session table entry
         to the maximum session lifetime.  By default, the maximum
         session lifetime is ICMP_DEFAULT.  The maximum lifetime value
         SHOULD be configurable.  The packet is translated and forwarded
         as described in the following sections.








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3.5.4.  Generation of the IPv6 Representations of IPv4 Addresses

   NAT64 supports multiple algorithms for the generation of the IPv6
   representation of an IPv4 address.  The constraints imposed on the
   generation algorithms are the following:

      The algorithm MUST be reversible, i.e. it MUST be possible to
      derive the original IPv4 address from the IPv6 representation.

      The input for the algorithm MUST be limited to the IPv4 address,
      the IPv6 prefix (denoted Pref64::/n) used in the IPv6
      representations and optionally a set of stable parameters that are
      configured in the NAT64 (such as fixed string to be used as a
      suffix).

         If we note n the length of the prefix Pref64::/n, then n MUST
         be less or equal than 96.  If a Pref64::/n is configured
         through any means in the NAT64 (such as manually configured, or
         other automatic means not specified in this document), the
         default algorithm MUST use this prefix.  If no prefix is
         available, the algorithm SHOULD use the Well-Known Prefix (64:
         FF9B::/96) defined in [I-D.ietf-behave-address-format]

   NAT64 MUST support the algorithm for generating IPv6 representations
   of IPv4 addresses defined in Section 2.1 of
   [I-D.ietf-behave-address-format].  The aforementioned algorithm
   SHOULD be used as default algorithm.

3.6.  Computing the Outgoing Tuple

   This step computes the outgoing tuple by translating the IP addresses
   and port numbers or ICMP Identifier in the incoming tuple.

   In the text below, a reference to a BIB means either the TCP BIB, the
   UDP BIB or the ICMP Query BIB as appropriate.

      NOTE: Not all addresses are translated using the BIB.  BIB entries
      are used to translate IPv6 source transport addresses to IPv4
      source transport addresses, and IPv4 destination transport
      addresses to IPv6 destination transport addresses.  They are NOT
      used to translate IPv6 destination transport addresses to IPv4
      destination transport addresses, nor to translate IPv4 source
      transport addresses to IPv6 source transport addresses.  The
      latter cases are handled applying the algorithmic transformation
      described in Section 3.5.4.  This distinction is important;
      without it, hairpinning doesn't work correctly.





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3.6.1.  Computing the Outgoing 5-tuple for TCP and UDP

   The transport protocol in the outgoing 5-tuple is always the same as
   that in the incoming 5-tuple.

   When translating in the IPv6 --> IPv4 direction, let the incoming
   source and destination transport addresses in the 5-tuple be (S',s)
   and (D',d) respectively.  The outgoing source transport address is
   computed as follows: if the BIB contains a entry (S',s) <--> (T,t),
   then the outgoing source transport address is (T,t).

   The outgoing destination address is computed algorithmically from D'
   using the address transformation described in Section 3.5.4.

   When translating in the IPv4 --> IPv6 direction, let the incoming
   source and destination transport addresses in the 5-tuple be (S,s)
   and (D,d) respectively.  The outgoing source transport address is
   computed as follows:

      The outgoing source transport address is generated from S using
      the address transformation algorithm described in Section 3.5.4.

      The BIB table is searched for an entry (X',x) <--> (D,d), and if
      one is found, the outgoing destination transport address is set to
      (X',x).

3.6.2.  Computing the Outgoing 3-tuple for ICMP Query Messages

   When translating in the IPv6 --> IPv4 direction, let the incoming
   source and destination addresses in the 3-tuple be S' and D'
   respectively and the ICMPv6 Identifier be I1.  The outgoing source
   address is computed as follows: the BIB contains an entry (S',I1)
   <--> (T,I2), then the outgoing source address is T and the ICMPv4
   Identifier is I2.

   The outgoing IPv4 destination address is computed algorithmically
   from D' using the address transformation described in Section 3.5.4.

   When translating in the IPv4 --> IPv6 direction, let the incoming
   source and destination addresses in the 3-tuple be S and D
   respectively and the ICMPv4 Identifier is I2.  The outgoing source
   address is generated from S using the address transformation
   algorithm described in Section 3.5.4.  The BIB is searched for an
   entry containing (X',I1) <--> (D,I2) and if found the outgoing
   destination address is X' and the outgoing ICMPv6 Identifier is I1.






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3.7.  Translating the Packet

   This step translates the packet from IPv6 to IPv4 or vice-versa.

   The translation of the packet is as specified in Section 3 and
   Section 4 of the IP/ICMP Translation Algorithm
   [I-D.ietf-behave-v6v4-xlate], with the following modifications:

   o  When translating an IP header (Sections 3.1 and 4.1), the source
      and destination IP address fields are set to the source and
      destination IP addresses from the outgoing tuple as determined in
      Section 3.6.

   o  When the protocol following the IP header is TCP or UDP, then the
      source and destination ports are modified to the source and
      destination ports from the outgoing 5-tuple.  In addition, the TCP
      or UDP checksum must also be updated to reflect the translated
      addresses and ports; note that the TCP and UDP checksum covers the
      pseudo-header which contains the source and destination IP
      addresses.  An algorithm for efficiently updating these checksums
      is described in [RFC3022].

   o  When the protocol following the IP header is ICMP and it is an
      ICMP Query message, the ICMP Identifier is set to the one from the
      outgoing 3-tuple as determined in Section 3.6.2.

   o  When the protocol following the IP header is ICMP (Sections 3.4
      and 4.4) and it is an ICMP error message, the source and
      destination transport addresses in the embedded packet are set to
      the destination and source transport addresses from the outgoing
      5-tuple (note the swap of source and destination).

   The size of outgoing packets as well and the potential need for
   fragmentation is done according to the behavior defined in the IP/
   ICMP Translation Algorithm [I-D.ietf-behave-v6v4-xlate]

3.8.  Handling Hairpinning

   If the destination IP address of the translated packet is an IPv4
   address assigned to the NAT64 itself then the packet is a hairpin
   packet.  Hairpin packets are processed as follows:

   o  The outgoing 5-tuple becomes the incoming 5-tuple, and,

   o  the packet is treated as if it was received on the outgoing
      interface.





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   o  Processing of the packet continues at step 2 - Filtering and
      updating binding and session information described in Section 3.5.


4.  Protocol Constants

   UDP_MIN 2 minutes (as defined in [RFC4787])

   UDP_DEFAULT 5 minutes (as defined in [RFC4787])

   TCP_TRANS 4 minutes (as defined in [RFC5382])

   TCP_EST 2 hours (the minimum lifetime for an established TCP session
   defined in [RFC5382] is 2 hrs and 4 minutes, which is achieved adding
   the 2 hours with this timer and the 4 minutes with the TCP_TRANS
   timer)

   TCP_INCOMING_SYN 6 seconds (as defined in [RFC5382])

   FRAGMENT_MIN 2 seconds

   ICMP_DEFAULT 60 seconds (as defined in [RFC5508])


5.  Security Considerations

5.1.  Implications on end-to-end security

   Any protocols that protect IP header information are essentially
   incompatible with NAT64.  This implies that end-to-end IPsec
   verification will fail when AH is used (both transport and tunnel
   mode) and when ESP is used in transport mode.  This is inherent in
   any network-layer translation mechanism.  End-to-end IPsec protection
   can be restored, using UDP encapsulation as described in [RFC3948].
   The actual extensions to support IPsec are out of the scope of this
   document.

5.2.  Filtering

   NAT64 creates binding state using packets flowing from the IPv6 side
   to the IPv4 side.  In accordance with the procedures defined in this
   document following the guidelines defined in [RFC4787] a NAT64 MUST
   offer "Endpoint-Independent Mapping".  This means:

      for any IPv6 packet with source (S'1,s1) and destination (Pref64::
      D1,d1) that creates an external mapping to (S1,s1), (D1,d1),





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      for any subsequent external connection from S'1 to (D2,d2) within
      a given binding timer window,

      (S1,s1) = (S2,s2) for all values of D2,d2

   Implementations MAY also provide support for "Address-Dependent
   Mapping" as also defined in this document and following the
   guidelines defined in [RFC4787].

   The security properties however are determined by which packets the
   NAT64 filter allows in and which it does not.  The security
   properties are determined by the filtering behavior and filtering
   configuration in the filtering portions of the NAT64, not by the
   address mapping behavior.  For example,

      Without filtering - When "Endpoint-Independent Mapping" is used in
      NAT64, once a binding is created in the IPv6 ---> IPv4 direction,
      packets from any node on the IPv4 side destined to the IPv6
      transport address will traverse the NAT64 gateway and be forwarded
      to the IPv6 transport address that created the binding.  However,

      With filtering - When "Endpoint-Independent Mapping" is used in
      NAT64, once a binding is created in the IPv6 ---> IPv4 direction,
      packets from any node on the IPv4 side destined to the IPv6
      transport address will first be processed against the filtering
      rules.  If the source IPv4 address is permitted, the packets will
      be forwarded to the IPv6 transport address.  If the source IPv4
      address is explicitly denied -- or the default policy is to deny
      all addresses not explicitly permitted -- then the packet will be
      discarded.  A dynamic filter may be employed where by the filter
      will only allow packets from the IPv4 address to which the
      original packet that created the binding was sent.  This means
      that only the IPv4 addresses to which the IPv6 host has initiated
      connections will be able to reach the IPv6 transport address, and
      no others.  This essentially narrows the effective operation of
      the NAT64 device to an "Address-Dependent Mapping" behavior,
      though not by its mapping behavior, but instead by its filtering
      behavior.

   As currently specified, the NAT64 only requires filtering traffic
   based on the 5-tuple.  In some cases (e.g., statically configured
   mappings), this may make it easy for an attacker to guess.  An
   attacker need not be able to guess other fields, e.g. the TCP
   sequence number, to get a packet through the NAT64.  While such
   traffic might be dropped by the final destination, it does not
   provide additional mitigations against bandwidth/CPU attacks
   targeting the internal network.  To avoid this type of abuse, a NAT64
   MAY keep track of the sequence number of TCP packets in order to



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   verify that proper sequencing of exchanged segments, in particular,
   the SYNs and the FINs.

5.3.  Attacks on NAT64

   The NAT64 device itself is a potential victim of different types of
   attacks.  In particular, the NAT64 can be a victim of DoS attacks.
   The NAT64 device has a limited number of resources that can be
   consumed by attackers creating a DoS attack.  The NAT64 has a limited
   number of IPv4 addresses that it uses to create the bindings.  Even
   though the NAT64 performs address and port translation, it is
   possible for an attacker to consume all the IPv4 transport addresses
   by sending IPv6 packets with different source IPv6 transport
   addresses.  This attack can only be launched from the IPv6 side,
   since IPv4 packets are not used to create binding state.  DoS attacks
   can also affect other limited resources available in the NAT64 such
   as memory or link capacity.  For instance, it is possible for an
   attacker to launch a DoS attack on the memory of the NAT64 device by
   sending fragments that the NAT64 will store for a given period.  If
   the number of fragments is high enough, the memory of the NAT64 could
   be exhausted.  Similarly, A DoS attack against the NAT64 can be
   crafted by sending either V4 or V6 SYN packets that consume memory in
   the form of session and/or binding table entries.  NAT64 devices MUST
   implement proper protection against such attacks, for instance
   allocating a limited amount of memory for fragmented packet storage
   as specified in Section 3.4.

   Another consideration related to NAT64 resource depletion refers to
   the preservation of binding state.  Attackers may try to keep a
   binding state alive forever by sending periodic packets that refresh
   the state.  In order to allow the NAT64 to defend against such
   attacks, the NAT64 MAY choose not to extend the session entry
   lifetime for a specific entry upon the reception of packets for that
   entry through the external interface.  As described in the Framework
   document [I-D.ietf-behave-v6v4-framework], the NAT64 can be deployed
   in multiple scenarios, some of which the Internet side is the IPv6
   one and some of which the Internet side is the IPv4 one.  It is then
   important to properly set which is the Internet side of the NAT64 in
   each specific configuration.

5.4.  Avoiding hairpinning loops

   If an IPv6-only client can guess the IPv4 binding address that will
   be created, it can use the IPv6 representation of it as source
   address for creating this binding.  Then any packet sent to the
   binding's IPv4 address could loop in the NAT64.  This is prevented in
   the current specification by filtering incoming packets containing
   Pref64::/n in the source address as described next.



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   Consider the following example:

   Suppose that the IPv4 pool is 192.0.2.0/24

   Then the IPv6-only client sends this to NAT64:

      Source: [Pref64::192.0.2.1]:500

      Destination: whatever

   The NAT64 allocates 192.0.2.1:500 as IPv4 binding address.  Now
   anything sent to 192.0.2.1:500, be it a hairpinned IPv6 packet or an
   IPv4 packet, could loop.

   It is not hard to guess the IPv4 address that will be allocated.
   First the attacker creates a binding and use (for example) STUN to
   learn its external IPv4 address.  New bindings will always have this
   address.  Then it uses a source port in the range 1-1023.  This will
   increase the chances to 1/512 (since range and parity are preserved
   by NAT64 in UDP).

   In order to address this vulnerability, the NAT64 MUST drop IPv6
   packets whose source address is in Pref64::/n as defined in
   Section 3.5.


6.  IANA Considerations

   This document contains no actions for IANA.


7.  Contributors

   George Tsirtsis

      Qualcomm

      tsirtsis@googlemail.com

   Greg Lebovitz

      Juniper

      gregory.ietf@gmail.com

   Simon Parreault





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      Viagenie

      simon.perreault@viagenie.ca


8.  Acknowledgements

   Dave Thaler, Dan Wing, Alberto Garcia-Martinez, Reinaldo Penno,
   Ranjana Rao, Lars Eggert, Senthil Sivakumar, Zhen Cao, Xiangsong Cui,
   Mohamed Boucadair, Dong Zhang, Bryan Ford, Kentaro Ebisawa, Charles
   Perkins, Magnus Westerlund, Ed Jankiewicz, David Harrington and Joao
   Damas reviewed the document and provided useful comments to improve
   it.

   The content of the draft was improved thanks to discussions with
   Christian Huitema, Fred Baker and Jari Arkko.

   Marcelo Bagnulo and Iljitsch van Beijnum are partly funded by
   Trilogy, a research project supported by the European Commission
   under its Seventh Framework Program.


9.  References

9.1.  Normative References

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

   [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
              (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
              RFC 4787, January 2007.

   [RFC5382]  Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
              Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
              RFC 5382, October 2008.

   [RFC5508]  Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT
              Behavioral Requirements for ICMP", BCP 148, RFC 5508,
              April 2009.

   [I-D.ietf-behave-v6v4-xlate]
              Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
              Algorithm", draft-ietf-behave-v6v4-xlate-13 (work in
              progress), March 2010.

   [I-D.ietf-behave-address-format]
              Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.



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              Li, "IPv6 Addressing of IPv4/IPv6 Translators",
              draft-ietf-behave-address-format-06 (work in progress),
              March 2010.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.

9.2.  Informative References

   [I-D.ietf-behave-dns64]
              Bagnulo, M., Sullivan, A., Matthews, P., and I. Beijnum,
              "DNS64: DNS extensions for Network Address Translation
              from IPv6 Clients to IPv4 Servers",
              draft-ietf-behave-dns64-09 (work in progress), March 2010.

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

   [RFC1858]  Ziemba, G., Reed, D., and P. Traina, "Security
              Considerations for IP Fragment Filtering", RFC 1858,
              October 1995.

   [RFC3128]  Miller, I., "Protection Against a Variant of the Tiny
              Fragment Attack (RFC 1858)", RFC 3128, June 2001.

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022,
              January 2001.

   [I-D.ietf-mmusic-ice]
              Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols",
              draft-ietf-mmusic-ice-19 (work in progress), October 2007.

   [RFC4963]  Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
              Errors at High Data Rates", RFC 4963, July 2007.

   [I-D.ietf-behave-v6v4-framework]
              Baker, F., Li, X., Bao, C., and K. Yin, "Framework for
              IPv4/IPv6 Translation",
              draft-ietf-behave-v6v4-framework-08 (work in progress),
              March 2010.

   [RFC3948]  Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
              Stenberg, "UDP Encapsulation of IPsec ESP Packets",
              RFC 3948, January 2005.



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Authors' Addresses

   Marcelo Bagnulo
   UC3M
   Av. Universidad 30
   Leganes, Madrid  28911
   Spain

   Phone: +34-91-6249500
   Fax:
   Email: marcelo@it.uc3m.es
   URI:   http://www.it.uc3m.es/marcelo


   Philip Matthews
   Alcatel-Lucent
   600 March Road
   Ottawa, Ontario
   Canada

   Phone: +1 613-592-4343 x224
   Fax:
   Email: philip_matthews@magma.ca
   URI:


   Iljitsch van Beijnum
   IMDEA Networks
   Avda. del Mar Mediterraneo, 22
   Leganes, Madrid  28918
   Spain

   Email: iljitsch@muada.com


















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