Network Working Group                                           J. Arkko
Internet-Draft                                                  Ericsson
Intended status: Standards Track                               L. Eggert
Expires: April 28, 2011                                            Nokia
                                                        October 25, 2010

 Scalable Operation of Address Translators with Per-Interface Bindings


   This document explains how to employ address translation in networks
   that serve a large number of individual customers without requiring a
   correspondingly large amount of private IPv4 address space.

Status of this Memo

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   This Internet-Draft will expire on April 28, 2011.

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1.  Introduction

   This document explains how to employ address translation without
   consuming a large amount of private address space.  This is important
   in networks that serve a large number of individual customers.
   Networks that serve more than 2^24 (16 million) users cannot assign a
   unique private IPv4 address to each user, because the largest
   reserved private address block reserved is 10/8 [RFC1918].  Many
   networks are already hitting these limits today, for instance, in the
   consumer Internet service market.  Even some individual devices may
   approach these limits, for instance, cellular network gateways or
   mobile IP home agents.

   If ample IPv4 address space was available, this would be a non-issue,
   because the current practice of assigning public IPv4 addresses to
   each user would remain viable, and the complications associated with
   using the more limited private address space could be avoided.
   However, as the IPv4 address pool is becoming depleted, this practice
   is becoming increasingly difficult to sustain.

   It has been suggested that more of the unassigned IPv4 space should
   be converted for private use, in order to allow the provisioning of
   larger networks with private IPv4 address space.  At the time of
   writing, the IANA "free pool" contained only 12 unallocated unicast
   IPv4 /8 prefixes.  Although reserving a few of those for private use
   would create some breathing room for such deployments, it would not
   result in a solution with long-term viability, would result in
   significant operational and management overheads, and would further
   reduce the number of available IPv4 addresses.

   Segmenting a network into areas of overlapping private address space
   is another possible technique, but it severely complicates the design
   and operation of a network.

   Finally, the transition to IPv6 will eventually eliminate these
   addressing limitations.  However, during the migration period when
   IPv4 and IPv6 have to co-exist, there will be the need to reach IPv4
   destinations, which involves the use of address or protocol

   The rest of this document is organized as follows.  Section 2 gives
   an outline of the solution, Section 3 introduces some terms,
   Section 4 specifies the required behavior for managing NAT bindings
   and Section 5 discusses the use of this technique with IPv6.

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2.  Solution Outline

   The need for address or protocol translation during the migration
   period to IPv6 creates the opportunity to deploy these mechanisms in
   a way that allows the support of a large user base without the need
   for a correspondingly large IPv4 address block.

   A Network Address Translator (NAT) is typically configured to connect
   a network domain that uses private IPv4 addresses to the public
   Internet.  The NAT device - which is configured with a public IPv4
   address - creates and maintains a mapping for each communication
   session from a device inside the domain it serves to devices in the
   public Internet.  It does that by translating the packet flow of each
   session such that the externally visible traffic uses only public

   In most NAT deployments, the network domain connected by the NAT to
   the public Internet is a broadcast network sharing the same media,
   where each individual device must have a unique IPv4 address.  In
   such deployments it is natural to also implement the NAT
   functionality such that it uses this unique IPv4 address when looking
   up which mapping should be used to translate a given communication

   It is important to note, however, that this is not an inherent
   requirement.  When other methods of identifying the correct mapping
   are available, and the NAT is not connecting a shared-media broadcast
   network to the Internet, there is no need to assign each device in
   the domain a unique IPv4 address.

   This is the case, for example, when the NAT connects devices to the
   Internet that connect to it with individual point-to-point links.  In
   this case, it becomes possible to use the same private addresses many
   times, making it possible to support any number of devices behind a
   NAT using very few IPv4 addresses.

   There are tunneling-based techniques to reach the same benefits, by
   establishing new tunnels over any IP network
   [I-D.ietf-softwire-dual-stack-lite].  However, where the point-to-
   point links already exists, creating an additional layer of tunneling
   is unnecessary (and even potentially harmful due to effects to the
   Maximum Transfer Unit) MTU settings).  The approach described in this
   document can be implemented and deployed within a single device and
   has no effect to hosts behind it.  In addition, as no additional
   layers of tunneling are introduced, there is no effect to the MTU.
   It is also unnecessary to implement tunnel endpoint discovery,
   security mechanisms or other aspects of a tunneling solution.  In
   fact, there are no changes to the devices behind the NAT.

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   Note, however, that existing tunnels are a common special case of
   point-to-point links.  For instance, cellular network gateways
   terminate a large number of tunnels that are already needed for
   mobility management reasons.  Implementing the approach described in
   this document is particularly attractive in such environments, given
   that no additional tunneling mechanisms, negotiation, or host changes
   are required.  In addition, since there is no additional tunneling,
   packets continue to take the same path as they would normally take.
   Other commonly appearing network technology that may be of interest
   include Point-to-Point Protocol (PPP) [RFC1661] links, PPP over
   Ethernet (PPPoE) [RFC2516] encapsulation, Asynchronous Transfer Mode
   (ATM) Permanent Virtual Circuits (PVCs), and per-subscriber virtual
   LAN (VLAN) allocation in consumer broadband networks.

   The approach described here also results overlapping private address
   space, like the segmentation of the network to different areas.
   However, this overlap is applied only at the network edges, and does
   not impact routing or reachability of servers in a negative way.

3.  Terms

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   "NAT" in this document includes both "Basic NAT" and "Network
   Address/Port Translator (NAPT)" as defined by [RFC2663].  The term
   "NAT Session" is adapted from [RFC5382] and is defined as follows.

   NAT Session - A NAT session is an association between a transport
   layer session as seen in the internal realm and a session as seen in
   the external realm, by virtue of NAT translation.  The NAT session
   will provide the translation glue between the two session

   This document uses the term mapping as defined in [RFC4787] to refer
   to state at the NAT necessary for network address and port
   translation of sessions.

4.  Per-Interface Bindings

   To support a mode of operation that uses a fixed number of IPv4
   addresses to serve an arbitrary number of devices, a NAT MUST manage
   its mappings on a per-interface basis, by associating a particular
   NAT session not only with the five tuples used for the transport
   connection on both sides of the NAT, but also with the internal

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   interface on which the user device is connected to the NAT.  This
   approach allows each internal interface to use the same private IPv4
   address range.  Note that the interface need not be physical, it may
   also correspond to a tunnel, VLAN, or other identifiable
   communications channel.

   For deployments where exactly one user device is connected with a
   separate tunnel interface and all tunnels use the same IPv4 address
   for the user devices, it is redundant to store this address in the
   mapping in addition to the internal interface identifier.  When the
   internal interface identifier is shorter than a 32-bit IPv4 address,
   this may decrease the storage requirements of a mapping entry by a
   small measure, which may aid NAT scalability.  For other deployments,
   it is likely necessary to store both the user device IPv4 address and
   the internal interface identifier, which slightly increases the size
   of the mapping entry.

   This mode of operation is only suitable in deployments where user
   devices connect to the NAT over point-to-point links.  If supported,
   this mode of operation SHOULD be configurable, and it should be
   disabled by default in general-purpose NAT devices.

5.  IPv6 Considerations

   Private address space conservation is important even during the
   migration to IPv6, because it will be necessary to communicate with
   the IPv4 Internet for a long time.  This document specifies two
   recommended deployment models for IPv6.  In the first deployment
   model the mechanisms specified in this document are useful.  In the
   second deployment model no additional mechanisms are needed, because
   IPv6 addresses are already sufficient to distinguish mappings from
   each other.

   The first deployment model employs dual stack [RFC4213].  The IPv6
   side of dual stack operates based on global addresses and direct end-
   to-end communication.  However, on the IPv4 side private addressing
   and NATs are a necessity.  The use of per-interface NAT mappings is
   RECOMMENDED for the IPv4 side under these circumstances.  Per-
   interface mappings help the NAT scale, while dual stack operation
   helps reduce the pressure on the NAT device by moving key types of
   traffic to IPv6, eliminating the need for NAT processing.

   The second deployment model involves the use of address and protocol
   translation, such as the one defined in
   [I-D.ietf-behave-v6v4-xlate-stateful].  In this deployment model
   there is no IPv4 in the internal network at all.  This model is
   applicable only in situations where all relevant devices and

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   applications are IPv6-capable.  In this situation, per-interface
   mappings could be employed as specified above, but they are generally
   unnecessary as the IPv6 address space is large enough to provide a
   sufficient number of mappings.

6.  Security Considerations

   This practices outlined in this document do not affect the security
   properties of address translation.

7.  IANA Considerations

   This document has no IANA implications.

8.  References

8.1.  Normative References

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

8.2.  Informative References

   [RFC1661]  Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
              RFC 1661, July 1994.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC2516]  Mamakos, L., Lidl, K., Evarts, J., Carrel, D., Simone, D.,
              and R. Wheeler, "A Method for Transmitting PPP Over
              Ethernet (PPPoE)", RFC 2516, February 1999.

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

   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 4213, October 2005.

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

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

              Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
              Stack Lite Broadband Deployments Following IPv4
              Exhaustion", draft-ietf-softwire-dual-stack-lite-06 (work
              in progress), August 2010.

              Arkko, J. and M. Townsley, "IPv4 Run-Out and IPv4-IPv6 Co-
              Existence Scenarios", draft-arkko-townsley-coexistence-06
              (work in progress), October 2010.

              Bagnulo, M., Matthews, P., and I. Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers",
              draft-ietf-behave-v6v4-xlate-stateful-12 (work in
              progress), July 2010.

              Miles, D. and M. Townsley, "Layer2-Aware NAT",
              draft-miles-behave-l2nat-00 (work in progress),
              March 2009.

   [TRILOGY]  "Trilogy Project",

Appendix A.  Contributors

   The ideas in this draft were first presented in
   [I-D.ietf-softwire-dual-stack-lite].  This document also in debt to
   [I-D.arkko-townsley-coexistence] and [I-D.miles-behave-l2nat].
   However, all of these documents focused on additional components,
   such as tunneling protocols or the allocation of special IP address
   ranges.  We wanted to publish a specification that just focuses on
   the core functionality of a per-interface NAT mappings.  However,
   Mark Townsley, David Miles, and Alain Durand should be credited with
   coming up with the ideas discussed in this memo.

Appendix B.  Acknowledgments

   The authors would also like to thank Randy Bush, Fredrik Garneij, Dan
   Wing, Christian Vogt, Marcelo Braun, Joel Halpern, Wassim Haddad,
   Alan Kavanaugh and others for interesting discussions in this problem

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   Lars Eggert is partly funded by the Trilogy Project [TRILOGY], a
   research project supported by the European Commission under its
   Seventh Framework Program.

Authors' Addresses

   Jari Arkko
   Jorvas  02420


   Lars Eggert
   Nokia Research Center
   P.O. Box 407
   Nokia Group  00045

   Phone: +358 50 48 24461

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