Network Working Group                                            C. Aoun
Request for Comments: 4966                                Energize Urnet
Obsoletes: 2766                                                E. Davies
Category: Informational                                 Folly Consulting
                                                               July 2007
Network Working Group                                            C. Aoun
Request for Comments: 4966                                Energize Urnet
Obsoletes: 2766                                                E. Davies
Category: Informational                                 Folly Consulting
                                                               July 2007

Reasons to Move the Network Address Translator - Protocol Translator (NAT-PT) to Historic Status


Status of This Memo


This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.


Copyright Notice


Copyright (C) The IETF Trust (2007).




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 Proposed Standard to Historic status.

本文档讨论由RFC 2766中定义的网络地址转换器-协议转换器(NAT-PT)实现的IPv6-IPv4协议转换机制的具体形式的问题。这些问题非常严重,建议将RFC 2766作为通用过渡机制已不再可取,本文件建议IETF将RFC 2766从拟议标准重新分类为历史状态。

Table of Contents


   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Issues Unrelated to an DNS-ALG . . . . . . . . . . . . . . . .  7
     2.1.  Issues with Protocols Embedding IP Addresses . . . . . . .  7
     2.2.  NAPT-PT Redirection Issues . . . . . . . . . . . . . . . .  8
     2.3.  NAT-PT Binding State Decay . . . . . . . . . . . . . . . .  8
     2.4.  Loss of Information through Incompatible Semantics . . . .  9
     2.5.  NAT-PT and Fragmentation . . . . . . . . . . . . . . . . . 10
     2.6.  NAT-PT Interaction with SCTP and Multihoming . . . . . . . 11
     2.7.  NAT-PT as a Proxy Correspondent Node for MIPv6 . . . . . . 12
     2.8.  NAT-PT and Multicast . . . . . . . . . . . . . . . . . . . 12
   3.  Issues Exacerbated by the Use of DNS-ALG . . . . . . . . . . . 13
     3.1.  Network Topology Constraints Implied by NAT-PT . . . . . . 13
     3.2.  Scalability and Single Point of Failure Concerns . . . . . 14
     3.3.  Issues with Lack of Address Persistence  . . . . . . . . . 15
     3.4.  DoS Attacks on Memory and Address/Port Pools . . . . . . . 16
   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 . . . . . . . 18
     4.3.  Inappropriate Translation of Responses to A Queries  . . . 19
     4.4.  DNS-ALG and Multi-Addressed Nodes  . . . . . . . . . . . . 19
     4.5.  Limitations on Deployment of DNS Security Capabilities . . 19
   5.  Impact on IPv6 Application Development . . . . . . . . . . . . 20
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   7.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 21
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 22
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 22
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 23
   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Issues Unrelated to an DNS-ALG . . . . . . . . . . . . . . . .  7
     2.1.  Issues with Protocols Embedding IP Addresses . . . . . . .  7
     2.2.  NAPT-PT Redirection Issues . . . . . . . . . . . . . . . .  8
     2.3.  NAT-PT Binding State Decay . . . . . . . . . . . . . . . .  8
     2.4.  Loss of Information through Incompatible Semantics . . . .  9
     2.5.  NAT-PT and Fragmentation . . . . . . . . . . . . . . . . . 10
     2.6.  NAT-PT Interaction with SCTP and Multihoming . . . . . . . 11
     2.7.  NAT-PT as a Proxy Correspondent Node for MIPv6 . . . . . . 12
     2.8.  NAT-PT and Multicast . . . . . . . . . . . . . . . . . . . 12
   3.  Issues Exacerbated by the Use of DNS-ALG . . . . . . . . . . . 13
     3.1.  Network Topology Constraints Implied by NAT-PT . . . . . . 13
     3.2.  Scalability and Single Point of Failure Concerns . . . . . 14
     3.3.  Issues with Lack of Address Persistence  . . . . . . . . . 15
     3.4.  DoS Attacks on Memory and Address/Port Pools . . . . . . . 16
   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 . . . . . . . 18
     4.3.  Inappropriate Translation of Responses to A Queries  . . . 19
     4.4.  DNS-ALG and Multi-Addressed Nodes  . . . . . . . . . . . . 19
     4.5.  Limitations on Deployment of DNS Security Capabilities . . 19
   5.  Impact on IPv6 Application Development . . . . . . . . . . . . 20
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   7.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 21
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 22
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 22
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 23
1. Introduction
1. 介绍

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 the 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. Proposed solutions to the issues are mentioned and any resulting need for changes to the specification is identified.

以前的许多文件都对NAT-PT提出了问题。本文档将总结这些问题,注意传统IPv4 NAT遗留的几个其他问题,并确定一些其他地方未讨论的其他问题。提出了问题的建议解决方案,并确定了规范变更的任何需求。

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 revised, possibly restricted version of NAT-PT can be a suitable solution; accordingly, this document recommends that the IETF should reclassify RFC 2766 from Proposed Standard to Historic status to

尽管包转换的[RFC2766]形式通常不适用,但在某些情况下,仅支持IPv4的节点可能需要与仅支持IPv6的节点通信;这需要某种翻译机制。尽管这可以由应用程序级代理或传输层转换器更好地执行,但仍可能存在这样的场景,即NAT-PT的修订版(可能是受限版本)是合适的解决方案;因此,本文件建议IETF将RFC 2766从拟定标准重新分类为历史状态,以

avoid it from being used in inappropriate scenarios while any replacement is developed.


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 网络地址转换-协议转换(NAT-PT)[RFC2766]

o Stateless IP/ICMP Translation Algorithm (SIIT) [RFC2765]

o 无状态IP/ICMP转换算法(SIIT)[RFC2765]

o NAT-PT Applicability Statement [NATP-APP]


o Issues with NAT-PT DNS ALG (Application Layer Gateway) in RFC 2766 [DNS-ALG-ISSUES]

o RFC 2766中NAT-PT DNS ALG(应用层网关)的问题[DNS-ALG-Issues]



o NAT-PT Security Considerations [NATPT-SEC]


o Issues when Translating between IPv4 and IPv6 [TRANS-ISSUES]

o 在IPv4和IPv6之间转换时出现的问题[转换问题]

o IPv6-IPv4 Translation Mechanism for SIP-Based Services in Third Generation Partnership Project (3GPP) Networks [3GPP-TRANS]

o 第三代合作伙伴计划(3GPP)网络中基于SIP的服务的IPv6-IPv4转换机制[3GPP-TRANS]

o Analysis on IPv6 Transition in 3GPP Networks [RFC4215]

o 3GPP网络IPv6过渡分析[RFC4215]

o Considerations for Mobile IP Support in NAT-PT [NATPT-MOB]

o NAT-PT[NATPT-MOB]中移动IP支持的考虑因素

o An IPv6-IPv4 Multicast Translator based on Internet Group Management Protocol / Multicast Listener Discovery (IGMP/MLD) Proxying (mtp) [MTP]

o 基于Internet组管理协议/多播侦听器发现(IGMP/MLD)代理(mtp)[mtp]的IPv6-IPv4多播转换器

o An IPv4-IPv6 Multicast Gateway [MCASTGW]

o IPv4-IPv6多播网关[MCASTGW]

o Scalable mNAT-PT Solution [MUL-NATPT]

o 可扩展mNAT PT解决方案[MUL-NATPT]

Because the majority of the documents containing discussions of the issues are documents that 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:

从“传统”IPv4 NAT中已知的相应问题可以推断出一些其他问题。以下文件是相关的:

o Protocol Complications with the IP Network Address Translator [RFC3027]

o IP网络地址转换器的协议复杂性[RFC3027]

o IP Network Address Translator (NAT) Terminology and Considerations [RFC2663]

o IP网络地址转换器(NAT)术语和注意事项[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.


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 a 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 an IPv6-IPv4 translator:

o 独立于DNS-ALG使用的问题,因此适用于任何形式的IPv6-IPv4转换器:

* Disruption of all protocols that embed IP addresses (and/or ports) in packet payloads or apply integrity mechanisms using IP addresses (and ports).

* 中断在数据包有效负载中嵌入IP地址(和/或端口)或使用IP地址(和端口)应用完整性机制的所有协议。

* Inability to redirect 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.

* 在一个NAPT-PT正在为多个IPv6主机进行转换的情况下,无法重定向缺少解复用功能或未构建在特定传输层协议之上的协议的流量。

* Requirement for applications to use keepalive mechanisms to workaround connectivity issues caused by premature NAT-PT state timeout.

* 要求应用程序使用keepalive机制来解决NAT-PT状态过早超时导致的连接问题。

* Loss of information due to incompatible semantics between IPv4 and IPv6 versions of headers and protocols.

* 由于标头和协议的IPv4和IPv6版本之间的语义不兼容而导致信息丢失。

* Need for additional state and/or packet reconstruction in NAPT-PT translators dealing with packet fragmentation.

* 在处理数据包碎片的NAPT-PT转换器中需要额外的状态和/或数据包重建。

* Interaction with SCTP and multihoming.

* 与SCTP和多归宿的交互。

* Need for NAT-PT to act as proxy for correspondent node when IPv6 node is mobile, with consequent restrictions on mobility.

* 当IPv6节点是移动的时,NAT-PT需要充当对应节点的代理,从而限制了移动性。

* NAT-PT not being able to handle multicast traffic.

* NAT-PT无法处理多播流量。

o Issues that are exacerbated by the use of a DNS-ALG and are, therefore, also applicable to any form of an IPv6-IPv4 translator:

o 由于使用DNS-ALG而加剧的问题,因此也适用于任何形式的IPv6-IPv4转换器:

* Constraints on network topology.

* 网络拓扑的约束。

* Scalability concerns together with introduction of a single point of failure and a security attack nexus.

* 可扩展性问题,以及引入单点故障和安全攻击关系。

* 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 that applications must start using the address shortly after the mapping is created, as well as keep it alive once they start using it.

* 缺少地址映射持久性:某些应用程序需要会话之间的地址保留。如果使用不同的映射,用户通信将中断。使用DNS-ALG创建具有有限生存期的地址映射意味着应用程序必须在创建映射后不久开始使用该地址,并在开始使用该地址后保持其活动状态。

* Creation of a DoS (Denial of Service) threat relating to exhaustion of memory and address/port pool resources on the translator.

* 创建与转换器上内存和地址/端口池资源耗尽有关的DoS(拒绝服务)威胁。

o Issues that result from the use of a DNS-ALG and are, therefore, specific to NAT-PT as defined in [RFC2766]:

o 由于使用DNS-ALG而产生的问题,因此,这些问题特定于[RFC2766]中定义的NAT-PT:

* Address selection issues when either the internal or external hosts implement both IPv4 and IPv6.

* 内部或外部主机同时实现IPv4和IPv6时的地址选择问题。

* Restricted validity of translated DNS records: a translated record may be forwarded to an application that cannot use it.

* 翻译后DNS记录的有效性受限:翻译后的记录可能会转发给无法使用它的应用程序。

* Inappropriate translation of responses to A queries from IPv6 nodes.

* 对来自IPv6节点的查询的响应转换不正确。

* Address selection issues and resource consumption in a DNS-ALG with multi-addressed nodes.

* 具有多地址节点的DNS-ALG中的地址选择问题和资源消耗。

* Limitations on DNS security capabilities when using a DNS-ALG.

* 使用DNS-ALG时对DNS安全功能的限制。

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 an DNS-ALG
2. 与DNS-ALG无关的问题
2.1. Issues with Protocols Embedding IP Addresses
2.1. 嵌入IP地址的协议问题

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 packet 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.

在IPv4 NAT上的工作(参见[RFC2663]和[RFC3027]的第8节)众所周知,在有效负载中嵌入数字IP地址的大型协议要么无法通过NAT工作,要么需要特定的ALG作为助手根据地址和端口转换来转换有效负载。同一组协议不能通过NAT-PT。由于IPv6和IPv4地址具有不同的长度,因此数据包长度和数据包内容都会发生变化,这使得问题更加严重。[RFC2766]将后果描述为FTP ALG描述的一部分。对于通过TCP传输运行的具有嵌入式IP地址的所有协议,都需要类似的解决方法。

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


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 colocated 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 provide additional points of security vulnerability.


Unless UDP encapsulation is used for IPsec [RFC3498], traffic using IPsec AH (Authentication Header), in transport and tunnel mode, and IPsec ESP (Encapsulating Security Payload), 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.

除非UDP封装用于IPsec[RFC3498],否则在传输和隧道模式下使用IPsec AH(身份验证标头)的通信量以及在传输模式下使用IPsec ESP(封装安全有效负载)的通信量无法通过NAT-PT进行,而必须终止NAT-PT上的安全关联,由于使用了密码完整性保护。

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


2.2. NAPT-PT Redirection Issues
2.2. NAPT-PT重定向问题

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 anywhere, which could lead to non-deterministic and 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
2.3. NAT-PT结合态衰变

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 are used for dynamically created address bindings. This allows all types of NAT-PT boxes 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 keepalive mechanism, which may not be possible for legacy systems.


Also, [RFC2766] does not specify how to choose timeouts for bindings. As 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
2.4. 通过不兼容的语义丢失信息

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.

o 在IPv4中,IPv6标头的流标签字段没有等效项。因此,任何基于流标签模式的数据包特殊处理都不能传播到IPv4域。

o IPv6 extension headers provide flexibility for future improvements in the IP protocol suite and new headers that do not have equivalents in IPv4 may be defined. In practice, some existing extensions such as routing headers and mobility extensions are not translatable.

o IPv6扩展头为IP协议套件的未来改进提供了灵活性,并且可以定义在IPv4中没有等价物的新头。在实践中,一些现有的扩展(如路由头和移动性扩展)是不可翻译的。

o As described in Section 2.2 of [NATP-APP], 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 the 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 by arbitrary packets. If the NAT-PT is

o 如[NATP-APP]第2.2节所述,IPv6中的某些ICMP(v4)消息没有等价物,而其他消息(尤其是“参数问题”消息)的语义不等价。翻译此类信息可能会导致信息丢失。然而,这个问题可能不是很严重,因为错误消息与NAT-PT转换的数据包有关,而不是与任意数据包有关。如果NAT-PT是

functioning correctly, there is, for example, no reason why IPv6 packets with unusual extension headers or options should be generated.


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
2.5. NAT-PT与碎片化

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, in general, fragmentation cannot be allowed for any traffic that traverses a NAPT-PT. One attempted workaround requires the NAPT-PT to maintain state information derived from the first fragment until all fragments of the packet have transited the NAPT-PT. This is not a complete solution because fragment misordering could lead to the first fragment appearing at the NAPT-PT after later fragments. Consequently, the NAPT-PT would 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 many forms of stateful packet inspection applied to IPv6 packets. The current specifications of IPv6 do not mandate (1) any minimum packet size 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 a significant delay to the packet, especially if it has to be re-fragmented before transmission on the IPv6 side.

当不具有传输层校验和的分段IPv4 UDP数据包穿越任何类型的NAT-PT盒时,会出现另一个问题。如[RFC2766]所述,NAT-PT必须重构整个数据包,以便能够计算转换后的IPv6数据包所需的校验和。这可能会导致数据包严重延迟,尤其是在IPv6端传输之前必须对其进行重新分段的情况下。

If NAT-PT boxes reassembled all incoming fragmented packets (both from the IPv4 and IPv6 directions) in the same way they have to 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).

如果NAT-PT盒将所有传入的碎片数据包(来自IPv4和IPv6方向)以与未选中的IPv4 UDP数据包相同的方式重新组装,这将是第一个问题的解决方案。如果传出数据包必须重新分段,那么除了潜在的延迟问题外,资源成本将相当可观。在任何情况下,碎片将意味着NAT-PT将消耗额外的内存和CPU资源,使NAT-PT的可伸缩性更低(参见第3.2节)。

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
2.6. NAT-PT与SCTP和多归宿的相互作用

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


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 that state would have to be maintained to relate packets transmitted using the different IP addresses associated with the connection.


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 that state is required, the state would have to be 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
2.7. NAT-PT作为MIPv6的代理对应节点

As discussed in [NATPT-MOB], it is not possible to propagate Mobile IPv6 (MIPv6) control messages into the IPv4 domain. According to the IPv6 Node Requirements [RFC4294], 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 NAT-PT可以丢弃与转交地址更改相关的消息和头,包括反向路由检查。与移动节点的通信将通过归属代理继续进行,而无需进行路由优化。这显然是次优的,但沟通仍有可能。

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.

o 可以在NAT-PT中实现其他功能,使其能够作为其绑定的所有IPv4节点的代理对应节点。该方案大大增加了NAT-PT的复杂性。根据用于转换IPv4地址的IPv6前缀的可路由性,它还可能限制移动节点的移动范围:到IPv4目的地的所有通信都必须通过相同的NAT-PT,即使移动节点移动到与NAT-PT没有直接IPv6连接的网络。

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.


2.8. NAT-PT and Multicast
2.8. NAT-PT与多播

SIIT [RFC2765] cannot handle the 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 work uses a separate gateway that understands some or all of the relevant multicast control and routing protocols in each domain. It has not yet been carried through into standards.


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)' [MTP]. A more comprehensive approach, which includes proxying of the multicast routing protocols, is described in 'An IPv4 - IPv6 multicast gateway' [MCASTGW]. Both approaches have several of the issues described in this section, notably issues with embedded addresses.


[NATPT-SEC] 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. 使用DNS-ALG加剧的问题
3.1. Network Topology Constraints Implied by NAT-PT
3.1. NAT-PT隐含的网络拓扑约束

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 will not be appropriate.


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 [DNS-ALG-ISSUES].


[DNS-ALG-SOL] 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.

主要建议是,仅限IPv6的客户端应将此DNS服务器用于所有查询。这在NAT-PT资源上非常昂贵,因为与具有本机IPv6地址的主机相关的请求也将使用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 the DNS RR (Resource Record) [RFC1035] translation.

不幸的是,对于另一种建议,没有一种先验的方式,发起者可以决定将哪个DNS服务器用于特定的查询。如果启动器做出错误的选择,DNS查询将返回一个空列表,而不是无法响应。使用标准DNS逻辑,启动器将不会尝试其他DNS服务器,因为它已收到响应。这意味着解决方案将包括始终使用具有NAT-PT前缀的DNS服务器。这就增加了总是需要DNS RR(资源记录)[RFC1035]转换的负担。

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 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
3.2. 可扩展性和单点故障问题

As with traditional NAT, NAT-PT is a bottleneck in the network with significant scalability concerns. Furthermore, 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 [MUL-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
3.3. 缺少地址持久性的问题

Using the DNS-ALG to create address bindings requires that the translated address returned by the DNS query is used for communications 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 binding lifetimes 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 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
3.4. 对内存和地址/端口池的DoS攻击

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 [NATPT-SEC] discuss the possible denial of service (DoS) attacks on basic NAT-PT and NAPT-PT that would result in a 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 [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.


[DNS-ALG-SOL] 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.


4. Issues Directly Related to Use of DNS-ALG
4. 与DNS-ALG使用直接相关的问题

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

4.1. 与双栈端主机通信时的地址选择问题

[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.

[DNS-ALG-ISSUES]讨论与地址选择有关的NAT-PT DNS-ALG问题。如[RFC2766]中所述,DNS-ALG将两个可能来源的AAAA资源记录(RRs)返回给进行AAAA DNS查询的IPv6主机。

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

如果查询与双堆栈主机相关,则查询将返回AAAA RRs中的本机IPv6地址和转换后的IPv4地址。如果没有其他信息,IPv6主机地址

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.


[DNS-ALG-SOL] 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 as described below:


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. The following two results can occur:

o 当DNS AAAA查询遍历NAT-PT DNS-ALG时,NAT-PT将不加更改地将查询转发到IPv4域中的DNS服务器,但使用IPv4传输。可能出现以下两种结果:

* 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.

* 如果权威DNS服务器有一个或多个AAAA记录,它将返回这些记录。然后,DNS-ALG将此响应转发给IPv6主机,并且不会像标准NAT-PT那样发送查询。

* 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.

* 否则,如果DNS服务器不理解AAAA查询或没有主机的AAAA条目,它将返回错误。NAT-PT DNS-ALG将截获错误或空返回,并发送同一主机的查询。如果此查询返回IPv4地址,ALG将创建绑定并合成相应的AAAA记录,并将其发送回IPv6主机。

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.

o 因此,NAT-PT将第一次成功的DNS响应的结果转发回终端主机,如果两者均未成功,则转发错误。因此,将只提供来自一个源的AAAA RRs,而不是[RFC2766]中指定的两个,并且它将包含双堆栈或仅IPv6查询器的最合适地址。

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


o The DNS client may timeout the query if it doesn't receive a response in time. This is more likely than for queries not passing through a DNS ALG 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.

o 如果DNS客户端没有及时收到响应,它可能会使查询超时。这比不通过DNS ALG的查询更有可能,因为NAT-PT可能必须进行两个独立的顺序查询,而客户机不知道这两个查询。若AAAA返回一个或多个地址,可以通过并行发送两个查询并忽略查询结果来减少响应时间。但是,在接收到第一个响应后仍然需要延迟以确定是否有第二个响应,这可能仍然会触发DNS客户端超时。

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 use the NAT-PT PREFIX [RFC2766] within its address selection policies 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 a configuration file download (in case the host or Dynamic Host Configuration Protocol for IPv6 (DHCPv6) server did not support vendor options and to avoid a 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 [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
4.2. 已翻译RR记录的非全局有效性

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 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
4.3. 对问题的回答翻译不当

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 the IPv4 side. The default action is to translate the response.

在双堆栈节点上运行的某些应用程序可能希望查询目标的IPv4地址。如果生成的A查询通过NAT-PT DNS-ALG,DNS-ALG将使用转换后的地址不适当地将响应转换为AAAA记录。发生这种情况是因为[RFC2766]中指定的DNS-ALG无状态运行,因此没有在IPv4端引发A请求的IPv6查询的内存。默认操作是转换响应。

The specification of NAT-PT could be modified to maintain a 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
4.4. DNS-ALG和多寻址节点

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.

在多址网络中使用SCTP时,如果多个NAT PT转换多个地址,则问题会加剧。此外,还不清楚SCTP是否会通过DNS查找所有目标IP地址,因此当数据包到达时绑定可能不到位。

4.5. Limitations on Deployment of DNS Security Capabilities
4.5. DNS安全功能部署的限制

Secure DNS (DNSSEC) [RFC4033] 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 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
5. 对IPv6应用程序开发的影响

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.

如果部署了NAT-PT,则假定无NAT IPv6环境的应用程序可能会在流量通过NAT-PT时中断。这已经够糟糕的了,但要求开发人员包含特殊功能,以解决所谓的临时过渡“辅助”问题,这就更糟糕了。最后,NAT-PT的部署可能会抑制IPv6数据包中灵活扩展报头系统启用的额外IPv6功能的开发和使用。

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.


6. Security Considerations
6. 安全考虑

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 第2.1节讨论了NAT-PT如何破坏IPsec AH(传输和隧道模式)和IPsec ESP传输模式(当未使用IPsec UDP封装[RFC3498]时),身份验证和加密通常与NAT-PT不兼容。

o Section 2.5 discusses possible fragmentation related security attacks on NAT-PT.

o 第2.5节讨论了NAT-PT上可能存在的碎片相关安全攻击。

o Section 2.8 discusses security issues related to multicast addresses and NAT-PT.

o 第2.8节讨论了与多播地址和NAT-PT相关的安全问题。

o Section 3.3 highlights that NAT-PT is an enticing nexus for security attacks.

o 第3.3节强调了NAT-PT是安全攻击的诱人纽带。

o Section 3.4 discusses possible NAT-PT DoS attacks on both memory and address/port pools.

o 第3.4节讨论了可能对内存池和地址/端口池的NAT-PT DoS攻击。

o Section 4.5 discusses why NAT-PT is incompatible with DNSSEC [RFC4033] and how deployment of NAT-PT may inhibit deployment of DNSSEC.

o 第4.5节讨论了NAT-PT与DNSSEC[RFC4033]不兼容的原因,以及NAT-PT的部署如何抑制DNSSEC的部署。

7. Conclusion
7. 结论

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 NAT-PT is envisaged as a potential mechanism to allow communication between an IPv6-only host and an IPv4-only host as discussed in the 3GPP IPv6 transition analysis [RFC4215]. But RFC 4215 recommends that the generic form of NAT-PT should not be used and that modified forms should only be used under strict conditions. Moreover, it documents a number of caveats and security issues specific to 3GPP. In addition, NAT-PT has seen some limited usage for other purposes.

本文件讨论了[RFC2766]中定义的NAT-PT的许多重要问题。从部署角度来看,3GPP网络是目前唯一的“标准化”场景,其中NAT-PT被设想为一种潜在机制,允许仅IPv6主机和仅IPv4主机之间的通信,如3GPP IPv6过渡分析[RFC4215]中所述。但RFC 4215建议不应使用NAT-PT的通用形式,修改后的形式应仅在严格条件下使用。此外,它还记录了一些针对3GPP的警告和安全问题。此外,NAT-PT在其他方面的使用也很有限。

Although some of the issues identified with NAT-PT appear to have solutions, many of the solutions proposed require significant alterations to the existing specification and would likely 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 application proxies in 3GPP scenarios [RFC4215]


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:


o the IETF no longer suggest its usage as a general IPv4-IPv6 transition mechanism in the Internet, and

o IETF不再建议将其用作Internet中的通用IPv4-IPv6转换机制,以及

o RFC 2766 is moved to Historic status to limit the possibility of it being deployed inappropriately.

o RFC 2766已移至历史状态,以限制其不适当部署的可能性。

8. Acknowledgments
8. 致谢

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.


9. References
9. 工具书类
9.1. Normative References
9.1. 规范性引用文件

[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.

[RFC2766]Tsirtsis,G.和P.Srisuresh,“网络地址转换-协议转换(NAT-PT)”,RFC 2766,2000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。

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

[RFC3027]Holdrege,M.和P.Srisuresh,“IP网络地址转换器的协议复杂性”,RFC 3027,2001年1月。

[RFC3314] Wasserman, M., "Recommendations for IPv6 in Third Generation Partnership Project (3GPP) Standards", RFC 3314, September 2002.

[RFC3314]Wasserman,M.,“第三代合作伙伴项目(3GPP)标准中IPv6的建议”,RFC 3314,2002年9月。

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

[RFC3484]Draves,R.,“互联网协议版本6(IPv6)的默认地址选择”,RFC 3484,2003年2月。

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

[RFC1035]Mockapetris,P.,“域名-实现和规范”,STD 13,RFC 1035,1987年11月。

[RFC4294] Loughney, J., "IPv6 Node Requirements", RFC 4294, April 2006.

[RFC4294]Loughney,J.,“IPv6节点要求”,RFC 42942006年4月。

[RFC4215] Wiljakka, J., "Analysis on IPv6 Transition in Third Generation Partnership Project (3GPP) Networks", RFC 4215, October 2005.

[RFC4215]Wiljakka,J.,“第三代合作伙伴计划(3GPP)网络中IPv6过渡的分析”,RFC 4215,2005年10月。

[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, March 2005.

[RFC4033]Arends,R.,Austein,R.,Larson,M.,Massey,D.,和S.Rose,“DNS安全介绍和要求”,RFC 4033,2005年3月。

9.2. Informative References
9.2. 资料性引用

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

[RFC1858]Ziemba,G.,Reed,D.,和P.Trana,“IP片段过滤的安全考虑”,RFC 1858,1995年10月。

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

[RFC3128]Miller,I.,“防止微小碎片攻击的变体(RFC 1858)”,RFC 31281001年6月。

[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.

[RFC2960]Stewart,R.,Xie,Q.,Morneault,K.,Sharp,C.,Schwarzbauer,H.,Taylor,T.,Rytina,I.,Kalla,M.,Zhang,L.,和V.Paxson,“流控制传输协议”,RFC 29602000年10月。

[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.

[RFC3498]Kuhfeld,J.,Johnson,J.,和M.Thatcher,“同步光网络(SONET)线性自动保护交换(APS)体系结构的受管对象定义”,RFC 3498,2003年3月。

[NATP-APP] Satapati, S., "NAT-PT Applicability", Work in Progress, October 2003.


[DNS-ALG-ISSUES] Durand, A., "Issues with NAT-PT DNS ALG in RFC2766", Work in Progress, February 2002.

[DNS-ALG-ISSUES]Durand,A.,“RFC2766中NAT-PT DNS ALG的问题”,正在进行的工作,2002年2月。

[DNS-ALG-SOL] Hallingby, P. and S. Satapati, "NAT-PT DNS ALG solutions", Work in Progress, July 2002.

[DNS-ALG-SOL]Hallingby,P.和S.Satapati,“NAT-PT DNS ALG解决方案”,正在进行的工作,2002年7月。

[NATPT-MOB] Shin, M. and J. Lee, "Considerations for Mobility Support in NAT-PT", Work in Progress, July 2005.


[NATPT-SEC] Okazaki, S. and A. Desai, "NAT-PT Security Considerations", Work in Progress, June 2003.


[TRANS-ISSUES] Pol, R., Satapati, S., and S. Sivakumar, "Issues when translating between IPv4 and IPv6", Work in Progress, January 2003.


[3GPP-TRANS] Malki, K., "IPv6-IPv4 Translation mechanism for SIP-based services in Third Generation Partnership Project (3GPP) Networks", Work in Progress, December 2003.


[MTP] Tsuchiya, K., Higuchi, H., Sawada, S., and S. Nozaki, "An IPv6/IPv4 Multicast Translator based on IGMP/MLD Proxying (mtp)", Work in Progress, February 2003.


[MCASTGW] Venaas, S., "An IPv4 - IPv6 multicast gateway", Work in Progress, February 2003.


[MUL-NATPT] Park, S., "Scalable mNAT-PT Solution", Work in Progress, May 2003.

[MUL-NATPT]Park,S.,“可扩展的mNAT PT解决方案”,正在进行的工作,2003年5月。

Authors' Addresses


Cedric Aoun Energize Urnet Paris France

塞德里克·奥恩(Cedric Aoun)为法国巴黎瓮城(Urnet Paris)注入活力


Elwyn B. Davies Folly Consulting Soham, Cambs UK

Elwyn B.Davies Folly Consulting Soham,英国剑桥

   Phone: +44 7889 488 335
   Phone: +44 7889 488 335

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