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Internet Engineering Task Force (IETF)                          J. ArkkoRequest for Comments: 6619                                      EricssonCategory: Standards Track                                      L. EggertISSN: 2070-1721                                                   NetApp                                                             M. Townsley                                                                   Cisco                                                               June 2012Scalable Operation of Address Translators with Per-Interface BindingsAbstract   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   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   Internet Standards is available inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6619.Copyright Notice   Copyright (c) 2012 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Arkko, et al.                Standards Track                    [Page 1]

RFC 6619                      Scalable NATs                    June 20121.  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 were 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 this   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.  It would result in   significant operational and management overheads, and it 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 coexist, address or protocol translation will   be needed in order to reach IPv4 destinations.   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,   andSection 5 discusses the use of this technique with IPv6.Arkko, et al.                Standards Track                    [Page 2]

RFC 6619                      Scalable NATs                    June 20122.  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   addresses.   In many 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 private IPv4 address that is   unique within this network.  In such deployments, it is natural also   to implement the NAT functionality such that it uses the private IPv4   address when looking up which mapping should be used to translate a   given communication session.   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 that can obtain the same   benefits by establishing new tunnels over any IP network [RFC6333].   However, where the point-to-point links already exist, creating an   additional layer of tunneling is unnecessary (and even potentially   harmful due to effects on 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 on hosts behind it.   In addition, as no additional layers of tunneling are introduced,   there is no effect on 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.Arkko, et al.                Standards Track                    [Page 3]

RFC 6619                      Scalable NATs                    June 2012   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 used network technologies 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 in 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",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described in [RFC2119].   "NAT" in this document includes both "Basic NAT" and "Network Address   Port Translation (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      representations.   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   interface on which the user device is connected to the NAT.  ThisArkko, et al.                Standards Track                    [Page 4]

RFC 6619                      Scalable NATs                    June 2012   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.   All address translators make it hard to address devices behind them.   The same is true of the particular NAT variant described in this   document.  An additional constraint is caused by the use of the same   address space for different devices behind the NAT, which prevents   the use of unique private addresses for communication between devices   behind the same NAT.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.Arkko, et al.                Standards Track                    [Page 5]

RFC 6619                      Scalable NATs                    June 2012   The second deployment model involves the use of address and protocol   translation, such as the one defined in [RFC6146].  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 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   The practices outlined in this document do not affect the security   properties of address translation.  The binding method specified in   this document is not observable to a device that is on the outside of   the NAT; i.e., a regular NAT and a NAT specified here cannot be   distinguished.  However, the use of point-to-point links implies   naturally that the devices behind the NAT cannot communicate with   each other directly without going through the NAT (or a router).  The   use of the same address space for different devices implies in   addition that a NAT operation must occur between two devices in order   for them to communicate.   The security implications of address translation in general have been   discussed in many previous documents, including [RFC2663], [RFC2993],   [RFC4787], and [RFC5382].7.  References7.1.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.7.2.  Informative References   [L2NAT]    Miles, D., Ed., and M. Townsley,"Layer2-Aware NAT", Work              in Progress, March 2009.   [RFC1661]  Simpson, W., Ed., "The Point-to-Point Protocol (PPP)",              STD 51,RFC 1661, July 1994.   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., de 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.Arkko, et al.                Standards Track                    [Page 6]

RFC 6619                      Scalable NATs                    June 2012   [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address              Translator (NAT) Terminology and Considerations",RFC 2663, August 1999.   [RFC2993]  Hain, T., "Architectural Implications of NAT",RFC 2993,              November 2000.   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms              for IPv6 Hosts and Routers",RFC 4213, October 2005.   [RFC4787]  Audet, F., Ed., and C. Jennings, "Network Address              Translation (NAT) Behavioral Requirements for Unicast              UDP",BCP 127,RFC 4787, January 2007.   [RFC5382]  Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and P.              Srisuresh, "NAT Behavioral Requirements for TCP",BCP 142,RFC 5382, October 2008.   [RFC6127]  Arkko, J. and M. Townsley, "IPv4 Run-Out and IPv4-IPv6              Co-Existence Scenarios",RFC 6127, May 2011.   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful              NAT64: Network Address and Protocol Translation from IPv6              Clients to IPv4 Servers",RFC 6146, April 2011.   [RFC6333]  Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-              Stack Lite Broadband Deployments Following IPv4              Exhaustion",RFC 6333, August 2011.   [TRILOGY]  "Trilogy Project", <http://www.trilogy-project.org/>.Arkko, et al.                Standards Track                    [Page 7]

RFC 6619                      Scalable NATs                    June 2012Appendix A.  Contributors   The ideas in this document were first presented in [RFC6333].  This   document is also indebted to [RFC6127] and [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 per-interface NAT mappings.  However, 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 space.   Lars Eggert is partly funded by the Trilogy Project [TRILOGY], a   research project supported by the European Commission under its   Seventh Framework Program.Arkko, et al.                Standards Track                    [Page 8]

RFC 6619                      Scalable NATs                    June 2012Authors' Addresses   Jari Arkko   Ericsson   Jorvas  02420   Finland   EMail: jari.arkko@piuha.net   Lars Eggert   NetApp   Sonnenallee 1   85551 Kirchheim   Germany   Phone: +49 151 12055791   EMail: lars@netapp.com   URI:http://eggert.org/   Mark Townsley   Cisco   Paris  75006   France   EMail: townsley@cisco.comArkko, et al.                Standards Track                    [Page 9]