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Versions: (draft-aoun-v6ops-natpt-deprecate) 00 01 02 03 draft-ietf-v6ops-natpt-to-historic

v6ops Working Group                                              C. Aoun
Internet-Draft                                            ZTE/ENST Paris
Updates: 2766 (if approved)                                    E. Davies
Expires: April 23, 2006                                       Consultant
                                                        October 20, 2005


                 Reasons to Move NAT-PT to Experimental
                 draft-ietf-v6ops-natpt-to-exprmntl-03

Status of this Memo

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   This Internet-Draft will expire on April 23, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document discusses issues with the specific form of IPv6-IPv4
   protocol translation mechanism implemented by the Network Address
   Translator - Protocol Translator (NAT-PT) defined in RFC 2766.  These
   issues are sufficiently serious that recommending RFC 2766 as a
   general purpose transition mechanism is no longer desirable, and this
   document recommends that the IETF should reclassify RFC 2766 from
   Standards Track to Experimental status.



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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Issues Unrelated to DNS-ALG  . . . . . . . . . . . . . . . . .  6
     2.1.  Issues with Protocols Embedding IP Addresses . . . . . . .  6
     2.2.  NAPT-PT Redirection Issues . . . . . . . . . . . . . . . .  7
     2.3.  NAT-PT Binding State Decay . . . . . . . . . . . . . . . .  8
     2.4.  Loss of Information through Incompatible Semantics . . . .  8
     2.5.  NAT-PT and Fragmentation . . . . . . . . . . . . . . . . .  9
     2.6.  NAT-PT Interaction with SCTP and Multihoming . . . . . . . 10
     2.7.  NAT-PT as a Proxy Correspondent Node for MIPv6 . . . . . . 11
     2.8.  NAT-PT and Multicast . . . . . . . . . . . . . . . . . . . 12
   3.  Issues exacerbated by the Use of DNS-ALG . . . . . . . . . . . 12
     3.1.  Network Topology Constraints Implied by NAT-PT . . . . . . 12
     3.2.  Scalability and Single Point of Failure Concerns . . . . . 14
     3.3.  Issues with Lack of Address Persistence  . . . . . . . . . 14
     3.4.  DoS Attacks on Memory and Address/Port Pools . . . . . . . 15
   4.  Issues Directly Related to Use of DNS-ALG  . . . . . . . . . . 16
     4.1.  Address Selection Issues when Communicating with
           Dual-Stack End-Hosts . . . . . . . . . . . . . . . . . . . 16
     4.2.  Non-global Validity of Translated RR Records . . . . . . . 17
     4.3.  Inappropriate Translation of Responses to A Queries  . . . 18
     4.4.  DNS-ALG and Multi-addressed Nodes  . . . . . . . . . . . . 18
     4.5.  Limitations on Deployment of DNS Security Capabilities . . 18
   5.  Impact on IPv6 Application Development . . . . . . . . . . . . 19
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
   8.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 20
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 21
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 21
     10.2. Informative References . . . . . . . . . . . . . . . . . . 22
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
   Intellectual Property and Copyright Statements . . . . . . . . . . 25

















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1.  Introduction

   The Network Address Translator - Protocol Translator (NAT-PT)
   document [RFC2766] defines a set of network-layer translation
   mechanisms designed to allow nodes that only support IPv4 to
   communicate with nodes that only support IPv6 during the transition
   to the use of IPv6 in the Internet.

   [RFC2766] specifies the basic NAT-PT in which only addresses are
   translated and Network Address Port Translator - Protocol Translator
   (NAPT-PT), which also translates transport identifiers, allowing for
   greater economy of scarce IPv4 addresses.  Protocol translation is
   performed using the Stateless IP/ICMP Translation Algorithm (SIIT)
   defined in [RFC2765].  In the following discussion, where the term
   "NAT-PT" is used unqualified, the discussion applies to both basic
   NAT-PT and NAPT-PT.  "Basic NAT-PT" will be used if points apply to
   the basic address-only translator.

   A number of previous documents have raised issues with NAT-PT.  This
   document will summarize these issues, note several other issues
   carried over from traditional IPv4 NATs, and identify some additional
   issues that have not been discussed elsewhere.  Where solutions to
   the issues have been proposed, these are mentioned and any resulting
   need for changes to the specification is identified.

   Whereas NAT is seen as an ongoing capability that is needed to work
   around the limited availability of globally unique IPv4 addresses,
   NAT-PT has a different status as a transition mechanism for IPv6.  As
   such, NAT-PT should not be allowed to constrain the development of
   IPv6 applications or impose limitations on future developments of
   IPv6.

   This document draws the conclusion that the technical and operational
   difficulties resulting from these issues, especially the possible
   future constraints on the development of IPv6 networks (see
   Section 5), make it undesirable to recommend NAT-PT as described in
   [RFC2766] as a general purpose transition mechanism for
   intercommunication between IPv6 networks and IPv4 networks.

   Although the [RFC2766] form of packet translation is not generally
   applicable, it is likely that in some circumstances a node that can
   only support IPv4 will need to communicate with a node that can only
   support IPv6; this needs a translation mechanism of some kind.
   Although this may be better carried out by an application-level proxy
   or transport-layer translator, there may still be scenarios in which
   a (possibly restricted) version of NAT-PT can be a suitable solution;
   accordingly, this document recommends that the IETF should reclassify
   RFC2766 from Standards Track to Experimental status.



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   The following documents relating directly to NAT-PT have been
   reviewed while drafting this document:

   o  Network Address Translation - Protocol Translation (NAT-PT)
      [RFC2766]
   o  Stateless IP/ICMP Translation Algorithm (SIIT) [RFC2765]
   o  NAT-PT applicability statement [I-D.satapati-v6ops-natpt-
      applicability]
   o  Issues with NAT-PT DNS ALG in RFC2766 [I-D.durand-natpt-dns-alg-
      issues]
   o  NAT-PT DNS ALG solutions [I-D.hallin-natpt-dns-alg-solutions]
   o  NAT-PT Security Considerations [I-D.okazaki-v6ops-natpt-security]
   o  Issues when translating between IPv4 and IPv6 [I-D.vanderpol-
      v6ops-translation-issues]
   o  IPv6-IPv4 Translation mechanism for SIP-based services in Third
      Generation Partnership Project (3GPP) Networks [I-D.elmalki-
      sipping-3gpp-translator]
   o  Analysis on IPv6 Transition in 3GPP Networks [I-D.ietf-v6ops-3gpp-
      analysis]
   o  Considerations for Mobile IP Support in NAT-PT [I-D.lee-v6ops-
      natpt-mobility]
   o  An IPv6/IPv4 Multicast Translator based on Internet Group
      Management Protocol / Multicast Listener Discovery (IGMP/MLD)
      Proxying (mtp) [I-D.tsuchiya-mtp]
   o  An IPv4 - IPv6 multicast gateway [I-D.venaas-mboned-v4v6mcastgw]
   o  Scalable mNAT-PT Solution [I-D.park-scalable-multi-natpt]

   Because the majority of the documents containing discussions of the
   issues are Internet Drafts which are unlikely to become RFCs, the
   issues are summarized here to avoid the need for normative
   references.

   Some additional issues can be inferred from corresponding issues
   known to exist in 'traditional' IPv4 NATs.  The following documents
   are relevant:

   o  Protocol Complications with the IP Network Address Translator
      [RFC3027]
   o  IP Network Address Translator (NAT) Terminology and Considerations
      [RFC2663]

   There is some ambiguity in [RFC2766] about whether the Application
   Layer Gateway (ALG) for DNS (referred to as DNS-ALG in this document)
   is an integral and mandatory part of the specification.  The
   ambiguity arises mainly from the first section of the applicability
   section (Section 8), which appears to imply that 'simple' use of
   NAT-PT could avoid the use of the DNS-ALG.




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   This is important because a number of the major issues arise from the
   interactions between DNS and NAT-PT.  However, detailed inspection of
   [RFC2766] shows that the 'simple' case has not been worked out and it
   is unclear how information about the address translation could be
   passed to the hosts in the absence of the DNS-ALG.  Therefore, this
   document assumes that the DNS-ALG is an integral part of NAT-PT;
   accordingly, issues with the DNS-ALG must be considered as issues for
   the whole specification.

   Note that issues not specifically related to the use of the DNS-ALG
   will apply to any network-layer translation scheme, including any
   based on the SIIT algorithm [RFC2765].  In the event that new forms
   of translator are developed as alternatives to NAT-PT, the generic
   issues relevant to all IPv6-IPv4 translators should be borne in mind.

   Issues raised with IPv6-IPv4 translators in general and NAT-PT in
   particular can be categorized as follows:

   o  Issues that are independent of the use of a DNS-ALG and are,
      therefore, applicable to any form of IPv6-IPv4 translator:
      *  Disruption of all protocols that embed IP addresses (and/or
         ports) in packet payloads or apply integrity mechanisms using
         IP addresses (and ports).
      *  Inability to re-direct traffic for protocols that lack
         demultiplexing capabilities or are not built on top of specific
         transport-layer protocols in situations where one NAPT-PT is
         translating for multiple IPv6 hosts.
      *  Requirement for applications to use keep alive mechanisms to
         workaround connectivity issues caused by premature NAT-PT state
         timeout.
      *  Loss of information due to incompatible semantics between IPv4
         and IPv6 versions of headers and protocols.
      *  Need for additional state and/or packet reconstruction in
         NAPT-PT translators dealing with packet fragmentation.
      *  Interaction with SCTP and multihoming.
      *  Need for NAT-PT to act as proxy for correspondent node when
         IPv6 node is mobile, with consequent restrictions on mobility.
      *  NAT-PT not being able to handle multicast traffic.

   o  Issues that are exacerbated by the use of a DNS-ALG and are,
      therefore, also applicable to any form of IPv6-IPv4 translator:
      *  Constraints on network topology.
      *  Scalability concerns together with introduction of single point
         of failure and security attack nexus.
      *  Lack of address mapping persistence: Some applications require
         address retention between sessions.  The user traffic will be
         disrupted if a different mapping is used.  The use of the DNS-
         ALG to create address mappings with limited lifetimes means



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         that applications must start using the address shortly after
         the mapping is created, as well as keeping it alive once they
         start using it.
      *  Creation of a DoS threat relating to exhaustion of memory and
         address/port pool resources on the translator.

   o  Issues that result from the use of a DNS-ALG and are, therefore,
      specific to NAT-PT as defined in [RFC2766]:
      *  Address selection issues when either the internal or external
         hosts implement both IPv4 and IPv6.
      *  Restricted validity of translated DNS records: a translated
         record may be forwarded to an application that cannot use it.
      *  Inappropriate translation of responses to A queries from IPv6
         nodes.
      *  Address selection issues and resource consumption in DNS-ALG
         with multi-addressed nodes.
      *  Limitations on DNS security capabilities when using DNS-ALG.

   Section 2, Section 3 and Section 4 discuss these groups of issues.
   Section 5 examines the consequences of deploying NAT-PT for
   application developers and the long term effects of NAT-PT (or any
   form of generally deployed IPv6-IPv4 translator) on the further
   development of IPv6.

   The terminology used in this document is defined in [RFC2663],
   [RFC2766], and [RFC3314].


2.  Issues Unrelated to DNS-ALG

2.1.  Issues with Protocols Embedding IP Addresses

   It is well known from work on IPv4 NATs (see Section 8 of [RFC2663]
   and [RFC3027]) that the large class of protocols that embed numeric
   IP addresses in their payloads either cannot work through NATs or
   require specific ALGs as helpers to translate the payloads in line
   with the address and port translations.  The same set of protocols
   cannot pass through NAT-PT.  The problem is exacerbated because the
   IPv6 and IPv4 addresses are of different lengths so that packet
   lengths as well as contents are altered.  [RFC2766] describes the
   consequences as part of the description of the FTP ALG: similar
   workarounds are needed for all protocols with embedded IP addresses
   that run over TCP transports.

   The issues raised in Sections 2 and 3 of [RFC2663], relating to
   authentication and encryption with NAT, are also applicable to
   NAT-PT.




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   Implementing a suite of ALGs requires that NAT-PT equipment includes
   the logic for each of the relevant protocols.  Most of these
   protocols are continuously evolving, requiring continual and
   coordinated updates of the ALGs to keep them in step.

   Assuming that the NAT-PT contains a co-located ALG for one of the
   relevant protocols, the ALG could replace the embedded IP addresses
   and ports.  However, this replacement can only happen if no
   cryptographic integrity mechanism is used and the protocol messages
   are sent in the clear (i.e., not encrypted).

   A possible workaround relies on the NAT-PT being party to the
   security association used to provide authentication and/or
   encryption.  NAT-PT would then be aware of the cryptographic
   algorithms and keys used to secure the traffic.  It could then modify
   and re-secure the packets; this would certainly complicate network
   operations and provides additional points of security vulnerability.

   Unless UDP encapsulation is used for IPsec [RFC3498], traffic using
   IPsec AH (in transport and tunnel mode) and IPsec ESP (in transport
   mode) is unable to be carried through NAT-PT without terminating the
   security associations on the NAT-PT, due to their usage of
   cryptographic integrity protection.

   A related issue with DNS security is discussed in Section 4.5.

2.2.  NAPT-PT Redirection Issues

   Section 4.2 of [RFC3027] discusses problems specific to RSVP and
   NATs, one of which is actually a more generic problem for all port
   translators.  When several end-hosts are using a single NAPT-PT box,
   protocols that do not have a demultiplexing capability similar to
   transport-layer port numbers may be unable to work through NAPT-PT
   (and any other port translator) because there is nothing for NAPT-PT
   to use to identify the correct binding.

   This type of issue affects IPsec encrypted packets where the
   transport port is not visible (although it might be possible to use
   the Security Parameter Index (SPI) as an alternative demultiplexer)
   and protocols, such as RSVP, which are carried directly in IP
   datagrams rather than using a standard transport-layer protocol such
   as TCP or UDP.  In the case of RSVP, packets going from the IPv4
   domain to the IPv6 domain do not necessarily carry a suitable
   demultiplexing field, because the port fields in the flow identifier
   and traffic specifications are optional.

   Several ad hoc workarounds could be used to solve the demultiplexing
   issues, however in most cases these solutions are not documented



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   anywhere, which could lead to non-deterministic, undesirable behavior
   (for example, such workarounds often assume particular network
   topologies, etc., in order to function correctly; if the assumptions
   are not met in a deployment, the workaround may not work as
   expected).

   This issue is closely related to the fragmentation issue described in
   Section 2.5.

2.3.  NAT-PT Binding State Decay

   NAT-PT will generally use dynamically created bindings to reduce the
   need for IPv4 addresses both for basic NAT-PT and NAPT-PT.  Both
   basic NAT-PT and NAPT-PT use soft state mechanisms to manage the
   address and, in the case of NAPT-PT, port pools used for dynamically
   created address bindings.  This allows all types of NAT-PT box to
   operate autonomously without requiring clients to signal, either
   implicitly or explicitly, that a binding is no longer required.  In
   any case, without soft state timeouts, network and application
   unreliability would inevitably lead to leaks, eventually causing
   address or port pool exhaustion.

   For a dynamic binding to persist for longer than the soft state
   timeout, packets must be sent periodically from one side of the
   NAT-PT to the other (the direction is not specified by the NAT-PT
   specification).  If no packets are sent in the proper direction, the
   NAT-PT binding will not be refreshed and the application connection
   will be broken.  Hence, all applications need to maintain their
   NAT-PT bindings during long idle periods by incorporating a keep-
   alive mechanism, which may not be possible for legacy systems.

   Also, [RFC2766] does not specify how to choose timeouts for bindings.
   As is discussed in [RFC2663] for traditional NATs, selecting suitable
   values is a matter of heuristics, and coordinating with application
   expectations may be impossible.

2.4.  Loss of Information through Incompatible Semantics

   NAT-PT reuses the SIIT header and protocol translations defined in
   [RFC2765].  Mismatches in semantics between IPv4 and IPv6 versions
   can lead to loss of information when packets are translated.  Three
   issues arising from this are:

   o  There is no equivalent in IPv4 for the flow label field of the
      IPv6 header.  Hence, any special treatment of packets based on
      flow label patterns cannot be propagated into the IPv4 domain.





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   o  IPv6 extension headers provide flexibility for improvements in the
      IP protocol suite in future.  In the future, new headers may be
      defined that do not have equivalents in IPv4.  In practice, some
      existing extensions such as routing headers and mobility
      extensions are not translatable.
   o  As described in Section 2.2 of [I-D.satapati-v6ops-natpt-
      applicability], there are no equivalents in IPv6 for some ICMP(v4)
      messages, while for others (notably the 'Parameter Problem'
      messages) the semantics are not equivalent.  Translation of such
      messages may lead to loss of information.  However, this issue may
      not be very severe because the error messages relate to packets
      that have been translated by NAT-PT rather than arbitrary packets.
      If the NAT-PT is functioning correctly, there is, for example, no
      reason why IPv6 packets with unusual extension headers or options
      should be generated.  This case is cited in [I-D.satapati-v6ops-
      natpt-applicability] as an example where the IPv6 error has no
      equivalent in IPv4 resulting in lost information.

   Loss of information in any of these cases could be a constraint to
   certain applications.

   A related matter concerns the propagation of the Differentiated
   Services Code Point (DSCP).  NAT-PT and SIIT simply copy the DSCP
   field when translating packets.  Accordingly, the IPv4 and IPv6
   domains must have equivalent Per-Hop Behaviors for the same code
   point, or alternative means must be in place to translate the DSCP
   between domains.

2.5.  NAT-PT and Fragmentation

   As mentioned in [RFC3027], simple port translators are unable to
   translate packet fragments, other than the first, from a fragmented
   packet, because subsequent fragments do not contain the port number
   information.

   This means that generally fragmentation cannot be allowed for any
   traffic that traverses a NAPT-PT.  One attempted workaround requires
   the NAPT-PT to maintain state about fragmented packets in transit.
   This is not a complete solution because fragment misordering could
   lead to the first fragment appearing at the NAPT-PT after later
   fragments.  The NAPT-PT would then not have the information needed to
   translate the fragments received before the first.

   Although it would not be expected in normal operation, NAPT-PT needs
   to be proofed against receiving short first fragments that don't
   contain the transport port numbers.  Note that such packets are a
   problem for IPv6 stateful packet inspection.  The current
   specifications of IPv6 do not mandate (1) any minimum packet size



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   beyond the need to carry the unfragmentable part (which doesn't
   include the transport port numbers) or (2) reassembly rules to
   minimize the effects of overlapping fragments.  Thus, IPv6 is open to
   the sort of attacks described in [RFC1858] and [RFC3128].

   An additional concern arises when a fragmented IPv4 UDP packet, which
   does not have a transport-layer checksum, traverses any type of
   NAT-PT box.  As described in [RFC2766], the NAT-PT has to reconstruct
   the whole packet so that it can calculate the checksum needed for the
   translated IPv6 packet.  This can result in significant delay to the
   packet, especially if it has to be re-fragmented before transmission
   on the IPv6 side.

   If NAT-PT boxes reassembled all incoming fragmented packets (both
   from the IPv4 and IPv6 directions) in the same way as they have to do
   for unchecksummed IPv4 UDP packets, this would be a solution to the
   first problem.  The resource cost would be considerable apart from
   the potential delay problem if the outgoing packet has to be re-
   fragmented.  In any case, fragmentation would mean that the NAT-PT
   would consume extra memory and CPU resources, making the NAT-PT even
   less scalable (see Section 3.2).

   Packet reassembly in a NAT-PT box also opens up the possibility of
   various fragment-related security attacks.  Some of these are
   analogous to attacks identified for IPv4.  Of particular concern is a
   DoS attack based on sending large numbers of small fragments without
   a terminating last fragment, which would potentially overload the
   reconstruction buffers and consume large amounts of CPU resources.

2.6.  NAT-PT Interaction with SCTP and Multihoming

   The Stream Control Transmission Protocol (SCTP) [RFC2960] is a
   transport protocol, which has been standardized since SIIT was
   specified.  SIIT does not explicitly cover translation of SCTP, but
   SCTP uses transport port numbers in the same way as UDP and TCP so
   that similar techniques could be used.

   However, SCTP also supports multihoming.  During connection setup,
   SCTP control packets carry embedded addresses that would have to be
   translated.  This would also require that the types of the options
   fields in the SCTP control packets be changed with consequent changes
   to packet length; the transport checksum would also have to be
   recalculated.  The ramifications of multihoming as it might interact
   with NAT-PT have not been fully explored.  Because of the 'chunked'
   nature of data transfer, it does not appear that state would have to
   be maintained to relate packets transmitted using the different IP
   addresses associated with the connection.




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   Even if these technical issues can be overcome, using SCTP in a
   NAT-PT environment may effectively nullify the multihoming advantages
   of SCTP if all the connections run through the same NAT-PT.  The
   consequences of running a multihomed network with separate NAT-PT
   boxes associated with each of the 'homes' have not been fully
   explored, but one issue that will arise is described in Section 4.4.
   SCTP will need an associated 'ALG' -- actually a Transport Layer
   Gateway -- to handle the packet payload modifications.  If it turns
   out that state is required, the state would have to distributed and
   synchronized across several NAT-PT boxes in a multihomed environment.

   SCTP running through NAT-PT in a multihomed environment is also
   incompatible with IPsec as described in Section 2.1.

2.7.  NAT-PT as a Proxy Correspondent Node for MIPv6

   As discussed in [I-D.lee-v6ops-natpt-mobility], it is not possible to
   propagate Mobile IPv6 control messages into the IPv4 domain.
   According to the IPv6 Node Requirements [I-D.ietf-ipv6-node-
   requirements], IPv6 nodes should normally be prepared to support the
   route optimization mechanisms needed in a correspondent node.  If
   communications from an IPv6 mobile node are traversing a NAT-PT, the
   destination IPv4 node will certainly not be able to support the
   correspondent node features needed for route optimization.

   This can be resolved in two ways:

   o  The NAT-PT can discard messages and headers relating to changes of
      care-of addresses, including reverse routing checks.
      Communications with the mobile node will continue through the home
      agent without route optimization.  This is clearly sub-optimal,
      but communication should remain possible.
   o  Additional functionality could be implemented in the NAT-PT to
      allow it to function as a proxy correspondent node for all IPv4
      nodes for which it has bindings.  This scheme adds considerably to
      the complexity of NAT-PT.  Depending on the routability of the
      IPv6 PREFIX used for translated IPv4 addresses, it may also limit
      the extent of mobility of the mobile node: all communications to
      the IPv4 destination have to go through the same NAT-PT, even if
      the mobile node moves to a network that does not have direct IPv6
      connectivity with the NAT-PT.

   In both cases, the existing NAT-PT specification would need to be
   extended to deal with IPv6 mobile nodes, and neither is a fully
   satisfactory solution.






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2.8.  NAT-PT and Multicast

   SIIT [RFC2765] cannot handle translation of multicast packets and
   NAT-PT does not discuss a way to map multicast addresses between IPv4
   and IPv6.  Some separate work has been done to provide an alternative
   mechanism to handle multicast.  This uses a separate gateway that
   understands some or all of the relevant multicast control and routing
   protocols in each domain.  This work has not been carried through
   into standards as yet.

   A basic mechanism, which involves only IGMP on the IPv4 side and MLD
   on the IPv6 side, is described in 'An IPv6/IPv4 Multicast Translator
   based on IGMP/MLD Proxying (mtp)' [I-D.tsuchiya-mtp].  A more
   comprehensive approach, which includes proxying of the multicast
   routing protocols, is described in 'An IPv4 - IPv6 multicast gateway'
   [I-D.venaas-mboned-v4v6mcastgw].  Both approaches have several of the
   issues described in this section, notably issues with embedded
   addresses.

   [I-D.okazaki-v6ops-natpt-security] identifies the possibility of a
   multiplicative reflection attack if the NAT-PT can be spoofed into
   creating a binding for a multicast address.  This attack would be
   very hard to mount because routers should not forward packets with
   multicast addresses in the source address field.  However, it
   highlights the possibility that a naively implemented DNS-ALG could
   create such bindings from spoofed DNS responses since [RFC2766] does
   not mention the need for checks on the types of addresses in these
   responses.

   The issues for NAT-PT and multicast reflect the fact that NAT-PT is
   at best a partial solution.  Completing the translation solution to
   cater for multicast traffic is likely to carry a similar set of
   issues to the current unicast NAT-PT and may open up significant
   additional security risks.


3.  Issues exacerbated by the Use of DNS-ALG

3.1.  Network Topology Constraints Implied by NAT-PT

   Traffic flow initiators in a NAT-PT environment are dependent on the
   DNS-ALG in the NAT-PT to provide the mapped address needed to
   communicate with the flow destination on the other side of the
   NAT-PT.  Whether used for flows initiated in the IPv4 domain or the
   IPv6 domain, the NAT-PT has to be on the path taken by the DNS query
   sent by the flow initiator to the relevant DNS server; otherwise, the
   DNS query will not be modified and the response type would not be
   appropriate.



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   The implication is that the NAT-PT box also has to be the default
   IPv6 router for the site so that the DNS-ALG is able to examine all
   DNS requests made over IPv6.  On sites with both IPv6 and dual-stack
   nodes, this will result in all traffic flowing through the NAT-PT
   with consequent scalability concerns.

   These constraints are described in more detail in [I-D.durand-natpt-
   dns-alg-issues].

   [I-D.hallin-natpt-dns-alg-solutions] proposes a solution for flows
   initiated from the IPv6 domain, but it appears that this solution
   still has issues.

   For IPv6-only clients, the solution requires the use of a DNS server
   in the IPv4 domain accessed via an IPv6 address which uses the NAT-PT
   PREFIX (see [RFC2766]).  Queries to this server would necessarily
   pass through the NAT-PT.  Dual-stack hosts would use a separate DNS
   server accessed through a normal IPv6 address.  This removes the need
   for the NAT-PT box to be the default IPv6 gateway for the domain.

   The primary proposal suggests that the IPv6-only clients should use
   this DNS server for all queries.  This is expensive on NAT-PT
   resources because requests relating to hosts with native IPv6
   addresses would also use the NAT-PT DNS-ALG.

   The alternate suggestion to reduce this burden appears to be flawed:
   if IPv6-only clients are provided with a list of DNS servers
   including both the server accessed via NAT-PT and server(s) accessed
   natively via IPv6, the proposal suggests that the client could avoid
   using NAT-PT for hosts that have native IPv6 addresses.

   Unfortunately for the alternate suggestion, there is no a priori way
   in which the initiator can decide which DNS server to use for a
   particular query.  In the event that the initiator makes the wrong
   choice, the DNS query will return an empty list rather than failing
   to respond.  With standard DNS logic, the initiator will not try
   alternative DNS servers because it has received a response.  This
   means that the solution would consist of always using DNS servers
   having the NAT-PT prefix.  This imposes the burden of always
   requiring DNS RR [RFC1035] translation.

   For flows initiated from the IPv4 network, the proposal recommends
   that the advertised DNS servers for the IPv6 network would have the
   IPv4 address of the NAT-PT.  Again there is no deterministic way to
   choose the correct DNS server for each query resulting in the same
   issues as were raised for flows initiated from the IPv6 domain.

   Although the engineering workaround, just described, provides a



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   partial solution to the topology constraints issue, it mandates that
   DNS queries and responses should still go through a NAT-PT even if
   there would normally be no reason to do so.  This mandatory passage
   through the NAT-PT for all DNS requests will exacerbate the other
   DNS-related issues discussed in Section 3.4 and Section 4.1.

3.2.  Scalability and Single Point of Failure Concerns

   As with traditional NAT, NAT-PT is a bottleneck in the network with
   significant scalability concerns and the anchoring of flows to a
   particular NAT-PT makes the NAT-PT a potential single point of
   failure in the network.  The addition of the DNS-ALG in NAT-PT
   further increases the scalability concerns.

   Solutions to both problems have been envisaged using collections of
   cooperating NAT-PT boxes, but such solutions require coordination and
   state synchronization, which has not yet been standardized and again
   adds to the functional and operational complexity of NAT-PT.  One
   such solution is described in [I-D.park-scalable-multi-natpt].

   As with traditional NAT, the concentration of flows through NAT-PT
   and the legitimate modification of packets in the NAT-PT make NAT-PTs
   enticing targets for security attacks.

3.3.  Issues with Lack of Address Persistence

   Using the DNS-ALG to create address bindings requires that the
   application uses the translated address returned by the DNS query
   before the NAT-PT binding state is timed out (see Section 2.3).
   Applications will not normally be aware of this constraint, which may
   be different from the existing lifetime of DNS query responses.  This
   could lead to "difficult to diagnose" problems with applications.

   Additionally, the DNS-ALG needs to determine the initial lifetime of
   bindings that it creates.  As noted in Section 2.3, this may need to
   be determined heuristically.  The DNS-ALG does not know which
   protocol the mapping is to be used for, and so needs another way to
   determine the initial lifetime.  This could be tied to the DNS
   response lifetime, but that might open up additional DoS attack
   possibilities if very long validities are allowed.  Also, the
   lifetime should be adjusted once the NAT-PT determines which protocol
   is being used with the binding.

   As with traditional NATs (see Section 2.5 of [RFC3027], NAT-PT will
   most likely break applications that require address mapping to be
   retained across contiguous sessions.  These applications require the
   IPv4 to IPv6 address mapping to be retained between sessions so the
   same mapped address may be reused for subsequent session



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   interactions.  NAT-PT cannot know this requirement and may reassign
   the previously used mapped address to different hosts between
   sessions.

   Trying to keep NAT-PT from discarding an address mapping would
   require either a NAT-PT extension protocol that would allow the
   application to request the NAT-PT device to retain the mappings, or
   an extended ALG (which has all the issues discussed in Section 2.1)
   that can interact with NAT-PT to keep the address mapping from being
   discarded after a session.

3.4.  DoS Attacks on Memory and Address/Port Pools

   As discussed in Section 2.3, a NAT-PT may create dynamic NAT
   bindings, each of which consumes memory resources as well as an
   address (or port if NAPT-PT is used) from an address (or port) pool.
   A number of documents, including [RFC2766] and [I-D.okazaki-v6ops-
   natpt-security] discuss possible denial of service (DoS) attacks on
   basic NAT-PT and NAPT-PT that result in resource depletion associated
   with address and port pools.  NAT-PT does not specify any
   authentication mechanisms; thus, an attacker may be able to create
   spurious bindings by spoofing addresses in packets sent through
   NAT-PT.  The attack is more damaging if the attacker is able to spoof
   protocols with long binding timeouts (typically used for TCP).

   The use of the DNS-ALG in NAT-PT introduces another vulnerability
   that can result in resource depletion.  The attack identified in
   [I-D.durand-natpt-dns-alg-issues] exploits the use of DNS queries
   traversing NAT-PT to create dynamic bindings.  Every time a DNS query
   is sent through the NAT-PT, the NAT-PT may create a new basic NAT-PT
   or NAPT-PT binding without any end-host authentication or
   authorization mechanisms.  This behavior could lead to a serious DoS
   attack on both memory and address or port pools.  Address spoofing is
   not required for this attack to be successful.

   [I-D.hallin-natpt-dns-alg-solutions] proposes to mitigate the DoS
   attack by using Access Control Lists (ACLs) and static binds, which
   increases the operational cost and may not always be practical.

   The ideal mitigation solution would be to disallow dynamically
   created binds until authentication and authorization of the end-host
   needing the protocol translation has been carried out.  This would
   require that the proper security infrastructure be in place to
   support the authentication and authorization, which increases the
   network operational complexity.






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4.  Issues Directly Related to Use of DNS-ALG

4.1.  Address Selection Issues when Communicating with Dual-Stack End-
      Hosts

   [I-D.durand-natpt-dns-alg-issues] discusses NAT-PT DNS-ALG issues
   with regard to address selection.  As specified in [RFC2766], the
   DNS-ALG returns AAAA resource records (RRs) from two possible sources
   to the IPv6 host that has made an AAAA DNS query.

   If the query relates to a dual-stack host, the query will return both
   the native IPv6 address(es) and the translated IPv4 address(es) in
   AAAA RRs.  Without additional information, the IPv6 host address
   selection may pick a translated IPv4 address instead of selecting the
   more appropriate native IPv6 address.  Under some circumstances, the
   address selection algorithms [RFC3484] will always prefer the
   translated address over the native IPv6 address; this is obviously
   undesirable.

   [I-D.hallin-natpt-dns-alg-solutions] proposes a solution that
   involves modification to the NAT-PT specification intended to return
   only the most appropriate address(es) to an IPv6 capable host:

   o  When a DNS AAAA query traverses the NAT-PT DNS-ALG, the NAT-PT
      will forward the query to the DNS server in the IPv4 domain
      unchanged, but using IPv4 transport:
      *  If the authoritative DNS server has one or more AAAA records,
         it returns them.  The DNS-ALG then forwards this response to
         the IPv6 host and does not send an A query as the standard
         NAT-PT would do.
      *  Otherwise, if the DNS server does not understand the AAAA query
         or has no AAAA entry for the host, it will return an error.
         The NAT-PT DNS-ALG will intercept the error or empty return and
         send an A query for the same host.  If this query returns an
         IPv4 address, the ALG creates a binding and synthesizes a
         corresponding AAAA record, which it sends back to the IPv6
         host.

   o  The NAT-PT thus forwards the result of the first successful DNS
      response back to the end-host or an error if neither succeeds.
      Consequently, only AAAA RRs from one source will be provided
      instead of two as specified in [RFC2766], and it will contain the
      most appropriate address for a dual-stack or IPv6-only querier.

   There is, however, still an issue with the proposed solution:






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   o  The DNS client may timeout the query if it doesn't receive a
      response in time.  This is more likely because the NAT-PT may have
      to make two separate, sequential queries of which the client is
      not aware.  It may be possible to reduce the response time by
      sending the two queries in parallel and ignoring the result of the
      A query if the AAAA returns one or more addresses.  However, it is
      still necessary to delay after receiving the first response to
      determine if a second is coming, which may still trigger the DNS
      client timeout.

   Unfortunately, the two queries cannot be combined in a single DNS
   request (all known DNS servers only process a single DNS query per
   request message because of difficulties expressing authoritativeness
   for arbitrary combinations of requests).

   An alternative solution would be to allow the IPv6 host to have,
   within its address selection policies, the NAT-PT PREFIX [RFC2766]
   used and to assign it a low selection priority.  This solution
   requires an automatic configuration of the NAT-PT PREFIX as well as
   its integration within the address selection policies.  The simplest
   way to integrate this automatic configuration would be through
   configuration file download (in case the host or Dynamic Host
   Configuration Protocol for IPv6 (DHCPv6) server did not support
   vendor options, to avoid standardization effort on the NAT-PT PREFIX
   option).  This solution does not require any modification to the
   NAT-PT specification.

   Neither of these solutions resolves a second issue related to address
   selection that is identified in [I-D.durand-natpt-dns-alg-issues].
   Applications have no way of knowing that the IPv6 address returned
   from the DNS-ALG is not a 'real' IPv6 address, but a translated IPv4
   address.  The application may therefore be led to believe that it has
   end-to-end IPv6 connectivity with the destination.  As a result, the
   application may use IPv6-specific options that are not supported by
   NAT-PT.  This issue is closely related to the issue described in
   Section 4.2 and the discussion in Section 5.

4.2.  Non-global Validity of Translated RR Records

   Some applications propagate information records retrieved from DNS to
   other applications.  The published semantics of DNS imply that the
   results will be consistent to any user for the duration of the
   attached lifetime.  RR records translated by NAT-PT violate these
   semantics because the retrieved addresses are only usable for
   communications through the translating NAT-PT.

   Applications that pass on retrieved DNS records to other applications
   will generally assume that they can rely on the passed on addresses



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   to be usable by the receiving application.  This may not be the case
   if the receiving application is on another node, especially if it is
   not in the domain served by the NAT-PT that generated the
   translation.

4.3.  Inappropriate Translation of Responses to A Queries

   Some applications running on dual-stack nodes may wish to query the
   IPv4 address of a destination.  If the resulting A query passes
   through the NAT-PT DNS-ALG, the DNS-ALG will translate the response
   inappropriately into a AAAA record using a translated address.  This
   happens because the DNS-ALG specified in [RFC2766] operates
   statelessly and hence has no memory of the IPv6 query that induced
   the A request on IPv4 side.  The default action is to translate the
   response.

   The specification of NAT-PT could be modified to maintain minimal
   state about queries passed through the DNS-ALG, and hence to respond
   correctly to A queries as well as AAAA queries.

4.4.  DNS-ALG and Multi-addressed Nodes

   Many IPv6 nodes, especially in multihomed situations but also in
   single homed deployments, can expect to have multiple global
   addresses.  The same may be true for multihomed IPv4 nodes.
   Responses to DNS queries for these nodes will normally contain all
   these addresses.  Since the DNS-ALG in the NAT-PT has no knowledge
   which of the addresses can or will be used by the application issuing
   the query, it is obliged to translate all of them.

   This could be a significant drain on resources in both basic NAT-PT
   and NAPT-PT, as bindings will have to be created for each address.

   When using SCTP in a multihomed network, the problem is exacerbated
   if multiple NAT-PTs translate multiple addresses.  Also, it is not
   clear that SCTP will actually look up all the destination IP
   addresses via DNS so that bindings may not be in place when packets
   arrive.

4.5.  Limitations on Deployment of DNS Security Capabilities

   Secure DNS (DNSSEC) [I-D.ietf-dnsext-dnssec-intro] uses public key
   cryptographic signing to authenticate DNS responses.  The DNS-ALG
   modifies DNS query responses traversing the NAT-PT in both directions
   which would invalidate the signatures as (partially) described in
   Section 7.5 of [RFC2766].

   Workarounds have been proposed, such as making the DNS-ALG behave



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   like a secure DNS server.  This would need to be done separately for
   both the IPv6 and IPv4 domains.  This is operationally very complex
   and there is a risk that the server could be mistaken for a
   conventional DNS server.  The NAT-PT specification would have to be
   altered to implement any such workaround.

   Hence DNSSEC is not deployable in domains that use NAT-PT as
   currently specified.  Widespread deployment of NAT-PT would become a
   serious obstacle to the large scale deployment of DNSSEC.


5.  Impact on IPv6 Application Development

   One of the major design goals for IPv6 is to restore the end-to-end
   transparency of the Internet.  Therefore, because IPv6 may be
   expected to remove the need for NATs and similar impediments to
   transparency, developers creating applications to work with IPv6 may
   be tempted to assume that the complex expedients that might have been
   needed to make the application work in a 'NATted' IPv4 environment
   are not required.

   Consequently, some classes of applications (e.g., peer-to-peer) that
   would need special measures to manage NAT traversal, including
   special encapsulations, attention to binding lifetime, and provision
   of keepalives, may build in assumptions on whether IPv6 is being used
   or not.  Developers would also like to exploit additional
   capabilities of IPv6 not available in IPv4.

   NAT-PT as specified in [RFC2766] is intended to work autonomously and
   be transparent to applications.  Therefore, there is no way for
   application developers to discover that a path contains a NAT-PT.

   If NAT-PT is deployed, applications that have assumed a NAT-free IPv6
   environment may break when the traffic passes through a NAT-PT.  This
   is bad enough, but requiring developers to include special
   capabilities to work around what is supposed to be a temporary
   transition 'aid' is even worse.  Finally, deployment of NAT-PT is
   likely to inhibit the development and use of additional IPv6
   capabilities enabled by the flexible extension header system in IPv6
   packets.

   Some of these deleterious effects could possibly be alleviated if
   applications could discover the presence of NAT-PT boxes on paths in
   use, allowing the applications to take steps to workaround the
   problems.  However, requiring applications to incorporate extra code
   to workaround problems with a transition aid still seems to be a very
   bad idea: the behavior of the application in native IPv6 and NAT-PT
   environments would be likely to be significantly different.



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6.  Security Considerations

   This document summarizes security issues related to the NAT-PT
   [RFC2766] specification.  Security issues are discussed in various
   sections:

   o  Section 2.1 discusses how IPsec AH (transport and tunnel mode) and
      IPsec ESP transport mode are broken by NAT-PT (when IPSEC UDP
      encapsulation is not used [RFC3498]); and authentication and
      encryption are generally incompatible with NAT-PT.
   o  Section 2.5 discusses possible fragmentation related security
      attacks on NAT-PT.
   o  Section 2.8 discusses security issues related to multicast
      addresses and NAT-PT.
   o  Section 3.3 highlights that NAT-PT is an enticing nexus for
      security attacks.
   o  Section 3.4 discusses possible NAT-PT DoS attacks on both memory
      and address/port pools.
   o  Section 4.5 discusses why NAT-PT is incompatible with DNSSEC
      [RFC2535] and how deployment of NAT-PT may inhibit deployment of
      DNSSEC.


7.  IANA Considerations

   There are no IANA considerations defined in this document.


8.  Conclusion

   This document has discussed a number of significant issues with
   NAT-PT as defined in [RFC2766].  From a deployment perspective, 3GPP
   networks are currently the only 'standardised' scenario where an
   IPv6-only host communicates with an IPv4-only host using NAT-PT as
   described in the 3GPP IPv6 transition analysis [I-D.ietf-v6ops-3gpp-
   analysis], but NAT-PT has seen some limited usage for other purposes.

   Although some of the issues identified with NAT-PT appear to have
   solutions, many of the solutions required significant alterations to
   the existing specification and would be likely to increase
   operational complexity.  Even if these solutions were applied, we
   have shown that NAT-PT still has significant, irresolvable issues and
   appears to have limited applicability.  The potential constraints on
   the development of IPv6 applications described in Section 5 are
   particularly undesirable.  It appears that alternatives to NAT-PT
   exist to cover the circumstances where NAT-PT has been suggested as a
   solution, such as the use of tunneling and header compression in 3GPP
   scenarios.



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   However, it is clear that in some circumstances an IPv6/IPv4 protocol
   translation solution may be a useful transitional solution,
   particularly in more constrained situations where the translator is
   not required to deal with traffic for a wide variety of protocols
   that are not determined in advance.  Therefore, it is possible that a
   more limited form of NAT-PT could be defined for use in specific
   situations.

   Accordingly, we recommend that the IETF no longer suggest its usage
   as a general IPv4/IPv6 transition mechanism in the Internet, but
   retain it as an experimental mechanism while further experience is
   gained and any future replacement is defined and deployed.
   Consequently, we recommend moving RFC2766 to experimental status.


9.  Acknowledgments

   This work builds on a large body of existing work examining the
   issues and applicability of NAT-PT: the work of the authors of the
   documents referred to in Section 1 has been extremely useful in
   creating this document.  Particular thanks are due to Pekka Savola
   for rapid and thorough review of the document.


10.  References

10.1.  Normative References

   [RFC2765]  Nordmark, E., "Stateless IP/ICMP Translation Algorithm
              (SIIT)", RFC 2765, February 2000.

   [RFC2766]  Tsirtsis, G. and P. Srisuresh, "Network Address
              Translation - Protocol Translation (NAT-PT)", RFC 2766,
              February 2000.

   [RFC2535]  Eastlake, D., "Domain Name System Security Extensions",
              RFC 2535, March 1999.

   [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address
              Translator (NAT) Terminology and Considerations",
              RFC 2663, August 1999.

   [RFC3027]  Holdrege, M. and P. Srisuresh, "Protocol Complications
              with the IP Network Address Translator", RFC 3027,
              January 2001.

   [RFC3314]  Wasserman, M., "Recommendations for IPv6 in Third
              Generation Partnership Project (3GPP) Standards",



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              RFC 3314, September 2002.

   [RFC3484]  Draves, R., "Default Address Selection for Internet
              Protocol version 6 (IPv6)", RFC 3484, February 2003.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [I-D.ietf-ipv6-node-requirements]
              Loughney, J., "IPv6 Node Requirements",
              draft-ietf-ipv6-node-requirements-11 (work in progress),
              August 2004.

   [I-D.ietf-v6ops-3gpp-analysis]
              Wiljakka, J., "Analysis on IPv6 Transition in 3GPP
              Networks", draft-ietf-v6ops-3gpp-analysis-11 (work in
              progress), October 2004.

   [I-D.ietf-dnsext-dnssec-intro]
              Arends, R., Austein, R., Massey, D., Larson, M., and S.
              Rose, "DNS Security Introduction and Requirements",
              draft-ietf-dnsext-dnssec-intro-13 (work in progress),
              October 2004.

10.2.  Informative References

   [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.

   [RFC2960]  Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
              Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
              Zhang, L., and V. Paxson, "Stream Control Transmission
              Protocol", RFC 2960, October 2000.

   [RFC3498]  Kuhfeld, J., Johnson, J., and M. Thatcher, "Definitions of
              Managed Objects for Synchronous Optical Network (SONET)
              Linear Automatic Protection Switching (APS)
              Architectures", RFC 3498, March 2003.

   [I-D.satapati-v6ops-natpt-applicability]
              Satapati, S., "NAT-PT Applicability",
              draft-satapati-v6ops-natpt-applicability-00 (work in
              progress), October 2003.




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   [I-D.durand-natpt-dns-alg-issues]
              Durand, A., "Issues with NAT-PT DNS ALG in RFC2766",
              draft-durand-natpt-dns-alg-issues-00 (work in progress),
              February 2002.

   [I-D.hallin-natpt-dns-alg-solutions]
              Hallingby, P. and S. Satapati, "NAT-PT DNS ALG solutions",
              draft-hallin-natpt-dns-alg-solutions-01 (work in
              progress), July 2002.

   [I-D.lee-v6ops-natpt-mobility]
              Shin, M. and J. Lee, "Considerations for Mobility Support
              in NAT-PT", draft-lee-v6ops-natpt-mobility-01 (work in
              progress), July 2005.

   [I-D.okazaki-v6ops-natpt-security]
              Okazaki, S. and A. Desai, "NAT-PT Security
              Considerations", draft-okazaki-v6ops-natpt-security-00
              (work in progress), June 2003.

   [I-D.vanderpol-v6ops-translation-issues]
              Pol, R., Satapati, S., and S. Sivakumar, "Issues when
              translating between IPv4 and IPv6",
              draft-vanderpol-v6ops-translation-issues-00 (work in
              progress), January 2003.

   [I-D.elmalki-sipping-3gpp-translator]
              Malki, K., "IPv6-IPv4 Translation mechanism for SIP-based
              services in Third Generation  Partnership Project (3GPP)
              Networks", draft-elmalki-sipping-3gpp-translator-00 (work
              in progress), December 2003.

   [I-D.tsuchiya-mtp]
              Tsuchiya, K., Higuchi, H., Sawada, S., and S. Nozaki, "An
              IPv6/IPv4 Multicast Translator based on IGMP/MLD Proxying
              (mtp)", draft-tsuchiya-mtp-01 (work in progress),
              February 2003.

   [I-D.venaas-mboned-v4v6mcastgw]
              Venaas, S., "An IPv4 - IPv6 multicast gateway",
              draft-venaas-mboned-v4v6mcastgw-00 (work in progress),
              February 2003.

   [I-D.park-scalable-multi-natpt]
              Park, S., "Scalable mNAT-PT Solution",
              draft-park-scalable-multi-natpt-00 (work in progress),
              May 2003.




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Authors' Addresses

   Cedric Aoun
   ZTE Corporation/ENST Paris
   France

   Email: aoun.cedric@zte.com.fr


   Elwyn B. Davies
   Consultant
   Soham, Cambs
   UK

   Phone: +44 7889 488 335
   Email: elwynd@dial.pipex.com



































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Aoun & Davies            Expires April 23, 2006                [Page 25]


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