Internet Engineering Task Force (IETF)                          J. Arkko
Request for Comments: 6619                                      Ericsson
Category: Standards Track                                      L. Eggert
ISSN: 2070-1721                                                   NetApp
                                                             M. Townsley
                                                               June 2012
Internet Engineering Task Force (IETF)                          J. Arkko
Request for Comments: 6619                                      Ericsson
Category: Standards Track                                      L. Eggert
ISSN: 2070-1721                                                   NetApp
                                                             M. Townsley
                                                               June 2012

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


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 in Section 2 of RFC 5741.

本文件是互联网工程任务组(IETF)的产品。它代表了IETF社区的共识。它已经接受了公众审查,并已被互联网工程指导小组(IESG)批准出版。有关互联网标准的更多信息,请参见RFC 5741第2节。

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at


Copyright Notice


Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.

版权所有(c)2012 IETF信托基金和确定为文件作者的人员。版权所有。

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents ( 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.

本文件受BCP 78和IETF信托有关IETF文件的法律规定的约束(自本文件出版之日起生效。请仔细阅读这些文件,因为它们描述了您对本文件的权利和限制。从本文件中提取的代码组件必须包括信托法律条款第4.e节中所述的简化BSD许可证文本,并提供简化BSD许可证中所述的无担保。

1. Introduction
1. 介绍

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, and Section 5 discusses the use of this technique with IPv6.


2. Solution Outline
2. 解决方案大纲

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.


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
3. 条款

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
4. 每个接口绑定

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. 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.


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
5. IPv6注意事项

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.

即使在迁移到IPv6的过程中,私有地址空间的保护也很重要,因为需要与IPv4 Internet进行长时间的通信。本文档指定了两种推荐的IPv6部署模型。在第一个部署模型中,本文档中指定的机制非常有用。在第二种部署模型中,不需要额外的机制,因为IPv6地址已经足以区分映射。

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 [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
6. 安全考虑

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.

本文档中概述的实践不会影响地址转换的安全属性。本文件中规定的绑定方法对于NAT外部的设备不可见;i、 例如,无法区分常规NAT和此处指定的NAT。然而,点到点链路的使用自然意味着NAT后面的设备不通过NAT(或路由器)就不能直接彼此通信。另外,为不同的设备使用相同的地址空间意味着两个设备之间必须进行NAT操作才能进行通信。

The security implications of address translation in general have been discussed in many previous documents, including [RFC2663], [RFC2993], [RFC4787], and [RFC5382].


7. References
7. 工具书类
7.1. Normative References
7.1. 规范性引用文件

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

[RFC2119]Bradner,S.,“RFC中用于表示需求水平的关键词”,BCP 14,RFC 2119,1997年3月。

7.2. Informative References
7.2. 资料性引用

[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.

[RFC1918]Rekhter,Y.,Moskowitz,R.,Karrenberg,D.,de Groot,G.,和E.Lear,“私人互联网地址分配”,BCP 5,RFC 1918,1996年2月。

[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.

[RFC2516]Mamakos,L.,Lidl,K.,Evarts,J.,Carrel,D.,Simone,D.,和R.Wheeler,“通过以太网传输PPP(PPPoE)的方法”,RFC 2516,1999年2月。

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

[RFC2663]Srisuresh,P.和M.Holdrege,“IP网络地址转换器(NAT)术语和注意事项”,RFC 2663,1999年8月。

[RFC2993] Hain, T., "Architectural Implications of NAT", RFC 2993, November 2000.

[RFC2993]Hain,T.,“NAT的建筑含义”,RFC 29932000年11月。

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

[RFC4213]Nordmark,E.和R.Gilligan,“IPv6主机和路由器的基本转换机制”,RFC 4213,2005年10月。

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

[RFC4787]Audet,F.,Ed.,和C.Jennings,“单播UDP的网络地址转换(NAT)行为要求”,BCP 127,RFC 4787,2007年1月。

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

[RFC5382]Guha,S.,Ed.,Biswas,K.,Ford,B.,Sivakumar,S.,和P.Srisuresh,“TCP的NAT行为要求”,BCP 142,RFC 5382,2008年10月。

[RFC6127] Arkko, J. and M. Townsley, "IPv4 Run-Out and IPv4-IPv6 Co-Existence Scenarios", RFC 6127, May 2011.

[RFC6127]Arkko,J.和M.Townsley,“IPv4耗尽和IPv4-IPv6共存场景”,RFC 6127,2011年5月。

[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.

[RFC6146]Bagnulo,M.,Matthews,P.,和I.van Beijnum,“有状态NAT64:从IPv6客户端到IPv4服务器的网络地址和协议转换”,RFC 61462011年4月。

[RFC6333] Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-Stack Lite Broadband Deployments Following IPv4 Exhaustion", RFC 6333, August 2011.

[RFC6333]Durand,A.,Droms,R.,Woodyatt,J.,和Y.Lee,“IPv4耗尽后的双栈Lite宽带部署”,RFC 63332011年8月。

[TRILOGY] "Trilogy Project", <>.


Appendix 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.

本文件中的想法首次在[RFC6333]中提出。本文件还应归功于[RFC6127]和[L2NAT]。然而,所有这些文档都关注附加组件,例如隧道协议或特殊IP地址范围的分配。我们想发布一个规范,该规范只关注每个接口NAT映射的核心功能。然而,大卫·迈尔斯(David Miles)和阿兰·杜兰德(Alain Durand)提出了这份备忘录中讨论的想法,这应该归功于他们。

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.

拉尔斯·艾格特(Lars Eggert)的部分资金来自三部曲项目(Trilogy Project)[Trilogy],这是一个由欧盟委员会根据其第七个框架计划支持的研究项目。

Authors' Addresses


Jari Arkko Ericsson Jorvas 02420 Finland



Lars Eggert NetApp Sonnenallee 1 85551 Kirchheim Germany


   Phone: +49 151 12055791
   Phone: +49 151 12055791

Mark Townsley Cisco Paris 75006 France