Internet Engineering Task Force (IETF) J. McCann
Request for Comments: 8201 Digital Equipment Corporation
STD: 87 S. Deering
Obsoletes: 1981 Retired
Category: Standards Track J. Mogul
ISSN: 2070-1721 Digital Equipment Corporation
R. Hinden, Ed.
Check Point Software
July 2017
Internet Engineering Task Force (IETF) J. McCann
Request for Comments: 8201 Digital Equipment Corporation
STD: 87 S. Deering
Obsoletes: 1981 Retired
Category: Standards Track J. Mogul
ISSN: 2070-1721 Digital Equipment Corporation
R. Hinden, Ed.
Check Point Software
July 2017
Path MTU Discovery for IP version 6
IP版本6的路径MTU发现
Abstract
摘要
This document describes Path MTU Discovery (PMTUD) for IP version 6. It is largely derived from RFC 1191, which describes Path MTU Discovery for IP version 4. It obsoletes RFC 1981.
本文档介绍IP版本6的路径MTU发现（PMTUD）。它主要源自RFC1191，它描述了IP版本4的路径MTU发现。它废除了RFC 1981。
Status of This Memo
关于下段备忘
This is an Internet Standards Track document.
这是一份互联网标准跟踪文件。
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 7841.
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 7841第2节。
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc8201.
有关本文件当前状态、任何勘误表以及如何提供反馈的信息，请访问http://www.rfc-editor.org/info/rfc8201.
Copyright Notice
版权公告
Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved.
版权所有（c）2017 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 (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.
本文件受BCP 78和IETF信托有关IETF文件的法律规定的约束(http://trustee.ietf.org/license-info)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。
This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.
本文件可能包含2008年11月10日之前发布或公开的IETF文件或IETF贡献中的材料。控制某些材料版权的人员可能未授予IETF信托允许在IETF标准流程之外修改此类材料的权利。在未从控制此类材料版权的人员处获得充分许可的情况下，不得在IETF标准流程之外修改本文件，也不得在IETF标准流程之外创建其衍生作品，除了将其格式化以RFC形式发布或将其翻译成英语以外的其他语言。
Table of Contents
目录
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 6
4. Protocol Requirements . . . . . . . . . . . . . . . . . . . . 7
5. Implementation Issues . . . . . . . . . . . . . . . . . . . . 8
5.1. Layering . . . . . . . . . . . . . . . . . . . . . . . . 8
5.2. Storing PMTU Information . . . . . . . . . . . . . . . . 9
5.3. Purging Stale PMTU Information . . . . . . . . . . . . . 11
5.4. Packetization Layer Actions . . . . . . . . . . . . . . . 12
5.5. Issues for Other Transport Protocols . . . . . . . . . . 13
5.6. Management Interface . . . . . . . . . . . . . . . . . . 14
6. Security Considerations . . . . . . . . . . . . . . . . . . . 14
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.1. Normative References . . . . . . . . . . . . . . . . . . 15
8.2. Informative References . . . . . . . . . . . . . . . . . 15
Appendix A. Comparison to RFC 1191 . . . . . . . . . . . . . . . 17
Appendix B. Changes Since RFC 1981 . . . . . . . . . . . . . . . 17
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 6
4. Protocol Requirements . . . . . . . . . . . . . . . . . . . . 7
5. Implementation Issues . . . . . . . . . . . . . . . . . . . . 8
5.1. Layering . . . . . . . . . . . . . . . . . . . . . . . . 8
5.2. Storing PMTU Information . . . . . . . . . . . . . . . . 9
5.3. Purging Stale PMTU Information . . . . . . . . . . . . . 11
5.4. Packetization Layer Actions . . . . . . . . . . . . . . . 12
5.5. Issues for Other Transport Protocols . . . . . . . . . . 13
5.6. Management Interface . . . . . . . . . . . . . . . . . . 14
6. Security Considerations . . . . . . . . . . . . . . . . . . . 14
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.1. Normative References . . . . . . . . . . . . . . . . . . 15
8.2. Informative References . . . . . . . . . . . . . . . . . 15
Appendix A. Comparison to RFC 1191 . . . . . . . . . . . . . . . 17
Appendix B. Changes Since RFC 1981 . . . . . . . . . . . . . . . 17
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
When one IPv6 node has a large amount of data to send to another node, the data is transmitted in a series of IPv6 packets. These packets can have a size less than or equal to the Path MTU (PMTU). Alternatively, they can be larger packets that are fragmented into a series of fragments each with a size less than or equal to the PMTU.
当一个IPv6节点有大量数据要发送到另一个节点时，数据将以一系列IPv6数据包的形式传输。这些数据包的大小可以小于或等于路径MTU（PMTU）。或者，它们可以是更大的分组，这些分组被分割成一系列片段，每个片段的大小小于或等于PMTU。
It is usually preferable that these packets be of the largest size that can successfully traverse the path from the source node to the destination node without the need for IPv6 fragmentation. This packet size is referred to as the Path MTU, and it is equal to the minimum link MTU of all the links in a path. This document defines a standard mechanism for a node to discover the PMTU of an arbitrary path.
通常，这些数据包最好具有最大的大小，能够在不需要IPv6分段的情况下成功地穿过从源节点到目标节点的路径。此数据包大小称为路径MTU，它等于路径中所有链路的最小链路MTU。本文档定义了一种标准机制，用于节点发现任意路径的PMTU。
IPv6 nodes should implement Path MTU Discovery in order to discover and take advantage of paths with PMTU greater than the IPv6 minimum link MTU [RFC8200]. A minimal IPv6 implementation (e.g., in a boot ROM) may choose to omit implementation of Path MTU Discovery.
IPv6节点应实现路径MTU发现，以便发现和利用PMTU大于IPv6最小链路MTU的路径[RFC8200]。最小IPv6实现（例如，在引导ROM中）可以选择省略路径MTU发现的实现。
Nodes not implementing Path MTU Discovery must use the IPv6 minimum link MTU defined in [RFC8200] as the maximum packet size. In most cases, this will result in the use of smaller packets than necessary, because most paths have a PMTU greater than the IPv6 minimum link MTU. A node sending packets much smaller than the Path MTU allows is wasting network resources and probably getting suboptimal throughput.
未实现路径MTU发现的节点必须使用[RFC8200]中定义的IPv6最小链路MTU作为最大数据包大小。在大多数情况下，这将导致使用比所需更小的数据包，因为大多数路径的PMTU大于IPv6最小链路MTU。发送比MTU允许的路径小得多的数据包的节点正在浪费网络资源，并且可能获得次优吞吐量。
Nodes implementing Path MTU Discovery and sending packets larger than the IPv6 minimum link MTU are susceptible to problematic connectivity if ICMPv6 [ICMPv6] messages are blocked or not transmitted. For example, this will result in connections that complete the TCP three-way handshake correctly but then hang when data is transferred. This state is referred to as a black-hole connection [RFC2923]. Path MTU Discovery relies on ICMPv6 Packet Too Big (PTB) to determine the MTU of the path.
如果ICMPv6[ICMPv6]消息被阻止或未传输，则实现路径MTU发现和发送大于IPv6最小链路MTU的数据包的节点容易出现连接问题。例如，这将导致连接正确完成TCP三方握手，但在传输数据时挂起。这种状态称为黑洞连接[RFC2923]。路径MTU发现依赖于ICMPv6数据包太大（PTB）来确定路径的MTU。
An extension to Path MTU Discovery defined in this document can be found in [RFC4821]. RFC 4821 defines a method for Packetization Layer Path MTU Discovery (PLPMTUD) designed for use over paths where delivery of ICMPv6 messages to a host is not assured.
本文档中定义的路径MTU发现的扩展可在[RFC4821]中找到。RFC 4821定义了一种用于包化层路径MTU发现（PLPMTUD）的方法，该方法设计用于无法确保向主机传递ICMPv6消息的路径。
Note: This document is an update to [RFC1981] that was published prior to [RFC2119] being published. Consequently, although RFC 1981 used the "should/must" style language in upper and lower case, this document does not cite the RFC 2119 definitions and only uses lower case for these words.
注：本文件是对[RFC1981]的更新，该更新是在[RFC2119]发布之前发布的。因此，尽管RFC 1981使用了大写和小写形式的“应该/必须”语言，但本文件未引用RFC 2119定义，仅对这些单词使用小写形式。
node a device that implements IPv6.
节点实现IPv6的设备。
router a node that forwards IPv6 packets not explicitly addressed to itself.
路由器转发未显式寻址到自身的IPv6数据包的节点。
host any node that is not a router.
托管不是路由器的任何节点。
upper layer a protocol layer immediately above IPv6. Examples are transport protocols such as TCP and UDP, control protocols such as ICMPv6, routing protocols such as OSPF, and internet-layer or lower-layer protocols being "tunneled" over (i.e., encapsulated in) IPv6 such as Internetwork Packet Exchange (IPX), AppleTalk, or IPv6 itself.
上层IPv6之上的协议层。例如，传输协议（如TCP和UDP）、控制协议（如ICMPv6）、路由协议（如OSPF）以及通过（即封装在）IPv6（如网络间数据包交换（IPX）、AppleTalk或IPv6本身）“隧道”的互联网层或较低层协议。
link a communication facility or medium over which nodes can communicate at the link layer, i.e., the layer immediately below IPv6. Examples are Ethernets (simple or bridged); PPP links; X.25, Frame Relay, or ATM networks; and internet-layer or higher-layer "tunnels", such as tunnels over IPv4 or IPv6 itself.
链路节点可在链路层（即IPv6正下方的层）上进行通信的通信设施或介质。例如以太网络（简单或桥接）；PPP链接；X.25、帧中继或ATM网络；互联网层或更高层的“隧道”，如IPv4或IPv6本身上的隧道。
interface a node's attachment to a link.
将节点的附件连接到链接。
address an IPv6-layer identifier for an interface or a set of interfaces.
寻址一个或一组接口的IPv6层标识符。
packet an IPv6 header plus payload. The packet can have a size less than or equal to the PMTU. Alternatively, this can be a larger packet that is fragmented into a series of fragments each with a size less than or equal to the PMTU.
数据包包含IPv6标头和有效负载。数据包的大小可以小于或等于PMTU。或者，这可以是更大的数据包，该数据包被分割成一系列片段，每个片段的大小小于或等于PMTU。
link MTU the maximum transmission unit, i.e., maximum packet size in octets, that can be conveyed in one piece over a link.
链路MTU——最大传输单元，即以八位字节为单位的最大数据包大小，可在链路上以一段方式传输。
path the set of links traversed by a packet between a source node and a destination node.
数据包在源节点和目标节点之间穿过的链路集。
path MTU the minimum link MTU of all the links in a path between a source node and a destination node.
路径MTU源节点和目标节点之间路径中所有链路的最小链路MTU。
PMTU path MTU.
PMTU路径MTU。
Path MTU Discovery the process by which a node learns the PMTU of a path.
路径MTU发现节点学习路径的PMTU的过程。
EMTU_S Effective MTU for sending; used by upper-layer protocols to limit the size of IP packets they queue for sending [RFC6691] [RFC1122].
EMTU_用于发送的有效MTU；上层协议用于限制其排队发送的IP数据包的大小[RFC6691][RFC1122]。
EMTU_R Effective MTU for receiving; the largest packet that can be reassembled at the receiver [RFC1122].
EMTU\R接收的有效MTU；可在接收器处重新组装的最大数据包[RFC1122]。
flow a sequence of packets sent from a particular source to a particular (unicast or multicast) destination for which the source desires special handling by the intervening routers.
流从特定源发送到特定（单播或多播）目的地的数据包序列，该目的地的源需要中间路由器对其进行特殊处理。
flow id a combination of a source address and a non-zero flow label.
流id源地址和非零流标签的组合。
This memo describes a technique to dynamically discover the PMTU of a path. The basic idea is that a source node initially assumes that the PMTU of a path is the (known) MTU of the first hop in the path. If any of the packets sent on that path are too large to be forwarded by some node along the path, that node will discard them and return ICMPv6 Packet Too Big messages. Upon receipt of such a message, the source node reduces its assumed PMTU for the path based on the MTU of the constricting hop as reported in the Packet Too Big message. The decreased PMTU causes the source to send smaller packets or change EMTU_S to cause the upper layer to reduce the size of IP packets it sends.
本备忘录描述了一种动态发现路径PMTU的技术。基本思想是，源节点最初假定路径的PMTU是路径中第一跳的（已知）MTU。如果在该路径上发送的任何数据包过大，无法由该路径上的某个节点转发，则该节点将丢弃它们并返回ICMPv6数据包过大消息。在接收到这样的消息时，源节点根据分组过大消息中报告的压缩跳的MTU减少其对路径的假定PMTU。减少的PMTU导致源发送较小的数据包，或更改EMTU以使上层减少其发送的IP数据包的大小。
The Path MTU Discovery process ends when the source node's estimate of the PMTU is less than or equal to the actual PMTU. Note that several iterations of the packet-sent/Packet-Too-Big-message-received cycle may occur before the Path MTU Discovery process ends, as there may be links with smaller MTUs further along the path.
当源节点对PMTU的估计值小于或等于实际PMTU时，路径MTU发现过程结束。注意，在路径MTU发现过程结束之前，可能会发生分组发送/分组太大消息接收周期的几次迭代，因为路径上可能存在具有更小MTU的链路。
Alternatively, the node may elect to end the discovery process by ceasing to send packets larger than the IPv6 minimum link MTU.
或者，节点可以选择通过停止发送大于IPv6最小链路MTU的分组来结束发现过程。
The PMTU of a path may change over time, due to changes in the routing topology. Reductions of the PMTU are detected by Packet Too Big messages. To detect increases in a path's PMTU, a node periodically increases its assumed PMTU. This will almost always result in packets being discarded and Packet Too Big messages being
由于路由拓扑的变化，路径的PMTU可能会随时间而变化。PMTU的减少通过数据包太大的消息来检测。为了检测路径PMTU的增加，节点周期性地增加其假定的PMTU。这几乎总是导致数据包被丢弃，数据包太大的消息被丢弃
generated, because in most cases the PMTU of the path will not have changed. Therefore, attempts to detect increases in a path's PMTU should be done infrequently.
生成，因为在大多数情况下，路径的PMTU不会更改。因此，不应经常尝试检测路径PMTU的增加。
Path MTU Discovery supports multicast as well as unicast destinations. In the case of a multicast destination, copies of a packet may traverse many different paths to many different nodes. Each path may have a different PMTU, and a single multicast packet may result in multiple Packet Too Big messages, each reporting a different next-hop MTU. The minimum PMTU value across the set of paths in use determines the size of subsequent packets sent to the multicast destination.
路径MTU发现支持多播和单播目的地。在多播目的地的情况下，分组的副本可以穿过许多不同的路径到达许多不同的节点。每个路径可能具有不同的PMTU，并且单个多播数据包可能导致多个数据包过大消息，每个数据包报告不同的下一跳MTU。使用中的路径集上的最小PMTU值确定发送到多播目的地的后续数据包的大小。
Note that Path MTU Discovery must be performed even in cases where a node "thinks" a destination is attached to the same link as itself, as it might have a PMTU lower than the link MTU. In a situation such as when a neighboring router acts as proxy [ND] for some destination, the destination can appear to be directly connected, but it is in fact more than one hop away.
请注意，即使在节点“认为”目的地与自身连接到同一链路的情况下，也必须执行路径MTU发现，因为它的PMTU可能低于链路MTU。在诸如当相邻路由器充当某个目的地的代理[ND]时的情况下，该目的地可以看起来是直接连接的，但实际上距离它不止一跳。
As discussed in Section 1, IPv6 nodes are not required to implement Path MTU Discovery. The requirements in this section apply only to those implementations that include Path MTU Discovery.
如第1节所述，实现路径MTU发现不需要IPv6节点。本节中的要求仅适用于包括路径MTU发现的实现。
Nodes should appropriately validate the payload of ICMPv6 PTB messages to ensure these are received in response to transmitted traffic (i.e., a reported error condition that corresponds to an IPv6 packet actually sent by the application) per [ICMPv6].
节点应根据[ICMPv6]适当验证ICMPv6 PTB消息的有效负载，以确保这些消息是响应传输的流量（即，与应用程序实际发送的IPv6数据包相对应的报告错误条件）而接收的。
If a node receives a Packet Too Big message reporting a next-hop MTU that is less than the IPv6 minimum link MTU, it must discard it. A node must not reduce its estimate of the Path MTU below the IPv6 minimum link MTU on receipt of a Packet Too Big message.
如果节点接收到一个数据包太大的消息，报告下一跳MTU小于IPv6最小链路MTU，则必须放弃该消息。在接收到数据包过大的消息时，节点不得将其对路径MTU的估计降低到IPv6最小链路MTU以下。
When a node receives a Packet Too Big message, it must reduce its estimate of the PMTU for the relevant path, based on the value of the MTU field in the message. The precise behavior of a node in this circumstance is not specified, since different applications may have different requirements, and since different implementation architectures may favor different strategies.
当节点接收到数据包过大的消息时，它必须根据消息中MTU字段的值，减少对相关路径的PMTU的估计。由于不同的应用程序可能有不同的需求，并且由于不同的实现架构可能支持不同的策略，因此没有指定节点在这种情况下的精确行为。
After receiving a Packet Too Big message, a node must attempt to avoid eliciting more such messages in the near future. The node must reduce the size of the packets it is sending along the path. Using a PMTU estimate larger than the IPv6 minimum link MTU may continue to elicit Packet Too Big messages. Because each of these messages (and
在接收到数据包太大的消息后，节点必须尝试避免在不久的将来引发更多此类消息。节点必须减小沿路径发送的数据包的大小。使用大于IPv6最小链路MTU的PMTU估计值可能会继续引发数据包过大的消息。因为这些消息（和
the dropped packets they respond to) consume network resources, nodes using Path MTU Discovery must detect decreases in PMTU as fast as possible.
它们响应的丢弃的数据包消耗网络资源，使用路径MTU发现的节点必须尽可能快地检测到PMTU中的减少。
Nodes may detect increases in PMTU, but because doing so requires sending packets larger than the current estimated PMTU, and because the likelihood is that the PMTU will not have increased, this must be done at infrequent intervals. An attempt to detect an increase (by sending a packet larger than the current estimate) must not be done less than 5 minutes after a Packet Too Big message has been received for the given path. The recommended setting for this timer is twice its minimum value (10 minutes).
节点可能会检测到PMTU的增加，但因为这样做需要发送比当前估计的PMTU更大的数据包，并且因为PMTU不会增加的可能性，所以必须在不频繁的时间间隔内进行。对于给定路径，在收到数据包过大消息后5分钟内，不得尝试检测数据包增加（通过发送大于当前估计值的数据包）。此计时器的建议设置是其最小值（10分钟）的两倍。
A node must not increase its estimate of the Path MTU in response to the contents of a Packet Too Big message. A message purporting to announce an increase in the Path MTU might be a stale packet that has been floating around in the network, a false packet injected as part of a denial-of-service (DoS) attack, or the result of having multiple paths to the destination, each with a different PMTU.
节点不得增加其路径MTU的估计值，以响应数据包过大消息的内容。声称宣布路径MTU增加的消息可能是在网络中漂浮的过时数据包、作为拒绝服务（DoS）攻击的一部分注入的虚假数据包，或者是具有多条到目的地的路径的结果，每条路径具有不同的PMTU。
This section discusses a number of issues related to the implementation of Path MTU Discovery. This is not a specification, but rather a set of notes provided as an aid for implementers.
本节讨论与路径MTU发现的实现相关的一些问题。这不是一个规范，而是为实现者提供的一组注释。
The issues include:
这些问题包括：
- What layer or layers implement Path MTU Discovery?
- 哪一层或哪几层实现了路径MTU发现？
- How is the PMTU information cached?
- 如何缓存PMTU信息？
- How is stale PMTU information removed?
- 如何删除过时的PMTU信息？
- What must transport and higher layers do?
- 传输层和更高层必须做什么？
In the IP architecture, the choice of what size packet to send is made by a protocol at a layer above IP. This memo refers to such a protocol as a "packetization protocol". Packetization protocols are usually transport protocols (for example, TCP) but can also be higher-layer protocols (for example, protocols built on top of UDP).
在IP体系结构中，要发送的数据包大小由IP上一层的协议决定。本备忘录将此类协议称为“打包协议”。打包协议通常是传输协议（例如TCP），但也可以是更高层的协议（例如，构建在UDP之上的协议）。
Implementing Path MTU Discovery in the packetization layers simplifies some of the inter-layer issues but has several drawbacks: the implementation may have to be redone for each packetization protocol, it becomes hard to share PMTU information between different
在包化层中实现路径MTU发现简化了一些层间问题，但有几个缺点：对于每个包化协议，可能必须重新实现，在不同的包之间共享PMTU信息变得很困难
packetization layers, and the connection-oriented state maintained by some packetization layers may not easily extend to save PMTU information for long periods.
打包层，以及某些打包层维护的面向连接的状态可能不容易扩展以长期保存PMTU信息。
It is therefore suggested that the IP layer store PMTU information and that the ICMPv6 layer process received Packet Too Big messages. The packetization layers may respond to changes in the PMTU by changing the size of the messages they send. To support this layering, packetization layers require a way to learn of changes in the value of MMS_S, the "maximum send transport-message size" [RFC1122].
因此，建议IP层存储PMTU信息，ICMPv6层进程接收的数据包太大。分组化层可以通过更改其发送的消息的大小来响应PMTU中的更改。为了支持这种分层，打包层需要一种方法来了解MMS_S值的变化，即“最大发送传输消息大小”[RFC1122]。
MMS_S is a transport message size calculated by subtracting the size of the IPv6 header (including IPv6 extension headers) from the largest IP packet that can be sent, EMTU_S. MMS_S is limited by a combination of factors, including the PMTU, support for packet fragmentation and reassembly, and the packet reassembly limit (see "Fragment Header", Section 4.5 of [RFC8200]). When source fragmentation is available, EMTU_S is set to EMTU_R, as indicated by the receiver using an upper-layer protocol or based on protocol requirements (1500 octets for IPv6). When a message larger than PMTU is to be transmitted, the source creates fragments, each limited by PMTU. When source fragmentation is not desired, EMTU_S is set to PMTU, and the upper-layer protocol is expected to either perform its own fragmentation and reassembly or otherwise limit the size of its messages accordingly.
MMS_S是通过从可发送的最大IP数据包EMTU_S中减去IPv6报头（包括IPv6扩展报头）的大小计算得出的传输消息大小。MMS_S受到多种因素的限制，包括PMTU、对数据包分段和重组的支持以及数据包重组限制（请参阅“分段报头”），第4.5节[RFC8200]）。当源碎片可用时，EMTU_S被设置为EMTU_R，如接收器使用上层协议或基于协议要求（1500个八位字节用于IPv6）所示。当要传输大于PMTU的消息时，源会创建片段，每个片段都受到PMTU的限制。当不需要源碎片时，EMTU_S被设置为PMTU，上层协议将执行其自身的碎片和重组，或者相应地限制其消息的大小。
However, packetization layers are encouraged to avoid sending messages that will require source fragmentation (for the case against fragmentation, see [FRAG]).
但是，鼓励打包层避免发送需要源碎片的消息（有关碎片的情况，请参见[FRAG]）。
Ideally, a PMTU value should be associated with a specific path traversed by packets exchanged between the source and destination nodes. However, in most cases a node will not have enough information to completely and accurately identify such a path. Rather, a node must associate a PMTU value with some local representation of a path. It is left to the implementation to select the local representation of a path. For nodes with multiple interfaces, Path MTU information should be maintained for each IPv6 link.
理想情况下，PMTU值应该与源节点和目标节点之间交换的数据包所经过的特定路径相关联。然而，在大多数情况下，节点将没有足够的信息来完全准确地识别这样的路径。相反，节点必须将PMTU值与路径的某些局部表示相关联。由实现选择路径的本地表示形式。对于具有多个接口的节点，应为每个IPv6链路维护路径MTU信息。
In the case of a multicast destination address, copies of a packet may traverse many different paths to reach many different nodes. The local representation of the "path" to a multicast destination must represent a potentially large set of paths.
在多播目的地地址的情况下，分组的副本可以穿过许多不同的路径到达许多不同的节点。到多播目的地的“路径”的本地表示必须表示一组可能很大的路径。
Minimally, an implementation could maintain a single PMTU value to be used for all packets originated from the node. This PMTU value would be the minimum PMTU learned across the set of all paths in use by the node. This approach is likely to result in the use of smaller packets than is necessary for many paths. In the case of multipath routing (e.g., Equal-Cost Multipath Routing (ECMP)), a set of paths can exist even for a single source and destination pair.
最低限度，一个实现可以维护一个PMTU值，用于来自该节点的所有数据包。该PMTU值将是在节点使用的所有路径集合中学习到的最小PMTU。这种方法可能导致使用比许多路径所需的更小的数据包。在多路径路由（例如，等成本多路径路由（ECMP））的情况下，即使对于单个源和目标对，也可以存在一组路径。
An implementation could use the destination address as the local representation of a path. The PMTU value associated with a destination would be the minimum PMTU learned across the set of all paths in use to that destination. This approach will result in the use of optimally sized packets on a per-destination basis. This approach integrates nicely with the conceptual model of a host as described in [ND]: a PMTU value could be stored with the corresponding entry in the destination cache.
实现可以使用目标地址作为路径的本地表示。与目的地关联的PMTU值将是在该目的地使用的所有路径集合中学习到的最小PMTU。这种方法将导致在每个目的地的基础上使用最佳大小的数据包。这种方法与[ND]中描述的主机概念模型很好地集成：PMTU值可以与目标缓存中的相应条目一起存储。
If flows [RFC8200] are in use, an implementation could use the flow id as the local representation of a path. Packets sent to a particular destination but belonging to different flows may use different paths, as with ECMP, in which the choice of path might depend on the flow id. This approach might result in the use of optimally sized packets on a per-flow basis, providing finer granularity than PMTU values maintained on a per-destination basis.
如果正在使用流[RFC8200]，则实现可以使用流id作为路径的本地表示形式。发送到特定目的地但属于不同流的数据包可以使用不同的路径，如ECMP，路径的选择可能取决于流id。这种方法可能导致在每个流的基础上使用最佳大小的数据包，提供比在每个目的地基础上维护的PMTU值更细的粒度。
For source-routed packets (i.e. packets containing an IPv6 Routing header [RFC8200]), the source route may further qualify the local representation of a path.
对于源路由分组（即，包含IPv6路由报头[RFC8200]的分组），源路由可进一步限定路径的本地表示。
Initially, the PMTU value for a path is assumed to be the (known) MTU of the first-hop link.
最初，假定路径的PMTU值为第一跳链路的（已知）MTU。
When a Packet Too Big message is received, the node determines which path the message applies to based on the contents of the Packet Too Big message. For example, if the destination address is used as the local representation of a path, the destination address from the original packet would be used to determine which path the message applies to.
当接收到数据包过大消息时，节点根据数据包过大消息的内容确定消息应用于哪个路径。例如，如果目的地地址用作路径的本地表示，则来自原始分组的目的地地址将用于确定消息应用于哪个路径。
Note: if the original packet contained a Routing header, the Routing header should be used to determine the location of the destination address within the original packet. If Segments Left is equal to zero, the destination address is in the Destination Address field in the IPv6 header. If Segments Left is greater than zero, the destination address is the last address (Address[n]) in the Routing header.
注意：如果原始数据包包含路由报头，则应使用路由报头确定原始数据包中的目标地址位置。如果“左段”等于零，则目标地址位于IPv6标头的“目标地址”字段中。如果剩余的段数大于零，则目标地址是路由标头中的最后一个地址（地址[n]）。
The node then uses the value in the MTU field in the Packet Too Big message as a tentative PMTU value or the IPv6 minimum link MTU if that is larger, and compares the tentative PMTU to the existing PMTU. If the tentative PMTU is less than the existing PMTU estimate, the tentative PMTU replaces the existing PMTU as the PMTU value for the path.
然后，节点使用数据包过大消息中MTU字段中的值作为暂定PMTU值，或者如果该值较大，则使用IPv6最小链路MTU，并将暂定PMTU与现有PMTU进行比较。如果暂定PMTU小于现有PMTU估计值，则暂定PMTU将替换现有PMTU作为路径的PMTU值。
The packetization layers must be notified about decreases in the PMTU. Any packetization layer instance (for example, a TCP connection) that is actively using the path must be notified if the PMTU estimate is decreased.
必须通知包装层PMTU的减少情况。如果PMTU估计值降低，则必须通知正在使用该路径的任何打包层实例（例如TCP连接）。
Note: even if the Packet Too Big message contains an Original Packet Header that refers to a UDP packet, the TCP layer must be notified if any of its connections use the given path.
注意：即使“数据包太大”消息包含引用UDP数据包的原始数据包头，如果TCP层的任何连接使用给定路径，也必须通知TCP层。
Also, the instance that sent the packet that elicited the Packet Too Big message should be notified that its packet has been dropped, even if the PMTU estimate has not changed, so that it may retransmit the dropped data.
此外，发送导致数据包过大消息的数据包的实例应被通知其数据包已被丢弃，即使PMTU估计值没有改变，以便它可以重新传输丢弃的数据。
Note: An implementation can avoid the use of an asynchronous notification mechanism for PMTU decreases by postponing notification until the next attempt to send a packet larger than the PMTU estimate. In this approach, when an attempt is made to SEND a packet that is larger than the PMTU estimate, the SEND function should fail and return a suitable error indication. This approach may be more suitable to a connectionless packetization layer (such as one using UDP), which (in some implementations) may be hard to "notify" from the ICMPv6 layer. In this case, the normal timeout-based retransmission mechanisms would be used to recover from the dropped packets.
注意：通过将通知延迟到下一次尝试发送大于PMTU估计值的数据包之前，实现可以避免对PMTU使用异步通知机制。在这种方法中，当尝试发送大于PMTU估计值的数据包时，发送功能应失败并返回适当的错误指示。这种方法可能更适合于无连接的打包层（例如使用UDP的打包层），这（在某些实现中）可能很难从ICMPv6层“通知”。在这种情况下，将使用基于正常超时的重传机制从丢弃的数据包中恢复。
It is important to understand that the notification of the packetization layer instances using the path about the change in the PMTU is distinct from the notification of a specific instance that a packet has been dropped. The latter should be done as soon as practical (i.e., asynchronously from the point of view of the packetization layer instance), while the former may be delayed until a packetization layer instance wants to create a packet.
重要的是要理解，使用关于PMTU中的更改的路径的分组层实例的通知不同于特定实例的分组已被丢弃的通知。后者应尽可能快地完成（即，从分组化层实例的角度来看是异步的），而前者可能会延迟到分组化层实例想要创建分组为止。
Internetwork topology is dynamic; routes change over time. While the local representation of a path may remain constant, the actual path(s) in use may change. Thus, PMTU information cached by a node can become stale.
网络拓扑是动态的；路线随时间而变化。虽然路径的局部表示可能保持不变，但实际使用的路径可能会发生变化。因此，节点缓存的PMTU信息可能会过时。
If the stale PMTU value is too large, this will be discovered almost immediately once a large enough packet is sent on the path. No such mechanism exists for realizing that a stale PMTU value is too small, so an implementation should "age" cached values. When a PMTU value has not been decreased for a while (on the order of 10 minutes), it should probe to find if a larger PMTU is supported.
如果过时的PMTU值太大，则在路径上发送足够大的数据包后，几乎会立即发现该值。不存在这样的机制来实现过时的PMTU值太小，因此实现应该“老化”缓存的值。当PMTU值有一段时间没有减小（大约10分钟）时，它应该探测以确定是否支持更大的PMTU。
Note: an implementation should provide a means for changing the timeout duration, including setting it to "infinity". For example, nodes attached to a link with a large MTU that is then attached to the rest of the Internet via a link with a small MTU are never going to discover a new non-local PMTU, so they should not have to put up with dropped packets every 10 minutes.
注意：实现应提供更改超时持续时间的方法，包括将其设置为“无限”。例如，连接到具有大MTU的链路的节点，然后通过具有小MTU的链路连接到Internet的其余部分，它们永远不会发现新的非本地PMTU，因此它们不必每10分钟忍受丢弃的数据包。
A packetization layer (e.g., TCP) must use the PMTU for the path(s) in use by a connection; it should not send segments that would result in packets larger than the PMTU, except to probe during PMTU Discovery (this probe packet must not be fragmented to the PMTU). A simple implementation could ask the IP layer for this value each time it created a new segment, but this could be inefficient. An implementation typically caches other values derived from the PMTU. It may be simpler to receive asynchronous notification when the PMTU changes, so that these variables may be also updated.
打包层（如TCP）必须将PMTU用于连接使用的路径；它不应发送导致数据包大于PMTU的数据段，除非在PMTU发现期间进行探测（此探测数据包不得分段到PMTU）。一个简单的实现可以在每次创建新的网段时向IP层请求该值，但这可能是低效的。实现通常缓存从PMTU派生的其他值。当PMTU更改时，接收异步通知可能更简单，因此这些变量也可以更新。
A TCP implementation must also store the Maximum Segment Size (MSS) value received from its peer, which represents the EMTU_R, the largest packet that can be reassembled by the receiver, and must not send any segment larger than this MSS, regardless of the PMTU.
TCP实现还必须存储从其对等方接收的最大段大小（MSS）值，该值表示EMTU_R，这是可由接收器重新组装的最大数据包，并且无论PMTU如何，都不得发送任何大于该MSS的段。
The value sent in the TCP MSS option is independent of the PMTU; it is determined by the receiver reassembly limit EMTU_R. This MSS option value is used by the other end of the connection, which may be using an unrelated PMTU value. See Section 5, "Packet Size Issues", and Section 8.3, "Maximum Upper-Layer Payload Size", of [RFC8200] for information on selecting a value for the TCP MSS option.
TCP MSS选项中发送的值独立于PMTU；它由接收器重新装配极限EMTU_R确定。连接的另一端使用此MSS选项值，可能使用不相关的PMTU值。有关选择TCP MSS选项值的信息，请参见[RFC8200]第5节“数据包大小问题”和第8.3节“最大上层有效负载大小”。
Reception of a Packet Too Big message implies that a packet was dropped by the node that sent the ICMPv6 message. A reliable upper-layer protocol will detect this loss by its own means, and recover it by its normal retransmission methods. The retransmission could result in delay, depending on the loss detection method used by the upper-layer protocol. If the Path MTU Discovery process requires several steps to find the PMTU of the full path, this could finally delay the retransmission by many round-trip times.
接收数据包过大消息意味着发送ICMPv6消息的节点丢弃了数据包。可靠的上层协议将通过自己的方式检测这种丢失，并通过其正常的重传方法进行恢复。根据上层协议使用的丢失检测方法，重传可能导致延迟。如果路径MTU发现过程需要几个步骤来找到完整路径的PMTU，这最终可能会将重传延迟许多往返时间。
Alternatively, the retransmission could be done in immediate response to a notification that the Path MTU was decreased, but only for the specific connection specified by the Packet Too Big message. The packet size used in the retransmission should be no larger than the new PMTU.
或者，可以立即响应路径MTU减少的通知而进行重传，但仅针对由分组过大消息指定的特定连接。重传中使用的数据包大小不应大于新的PMTU。
Note: A packetization layer that determines a probe packet is lost needs to adapt the segment size of the retransmission. Using the reported size in the last Packet Too Big message, however, can lead to further losses as there might be smaller PMTU limits at the routers further along the path. This would lead to loss of all retransmitted segments and therefore cause unnecessary congestion as well as additional packets to be sent each time a new router announces a smaller MTU. Any packetization layer that uses retransmission is therefore also responsible for congestion control of its retransmissions [RFC8085].
注意：确定探测包丢失的分组层需要调整重传的段大小。然而，在最后一个数据包中使用报告的大小太大消息可能会导致进一步的损失，因为在路径上更远的路由器上可能存在更小的PMTU限制。这将导致丢失所有重新传输的段，从而导致不必要的拥塞，以及每次新路由器宣布较小的MTU时发送额外的数据包。因此，任何使用重传的分组化层也负责其重传的拥塞控制[RFC8085]。
A loss caused by a PMTU probe indicated by the reception of a Packet Too Big message must not be considered as a congestion notification, and hence the congestion window may not change.
由于接收到数据包过大消息而导致的PMTU探测造成的丢失不得视为拥塞通知，因此拥塞窗口可能不会改变。
Some transport protocols are not allowed to repacketize when doing a retransmission. That is, once an attempt is made to transmit a segment of a certain size, the transport cannot split the contents of the segment into smaller segments for retransmission. In such a case, the original segment can be fragmented by the IP layer during retransmission. Subsequent segments, when transmitted for the first time, should be no larger than allowed by the Path MTU.
某些传输协议在进行重传时不允许重新打包。也就是说，一旦尝试传输某个大小的段，传输就不能将该段的内容分割成更小的段进行重新传输。在这种情况下，原始段可以在重传期间被IP层分段。第一次传输时，后续段不应大于路径MTU允许的值。
Path MTU Discovery for IPv4 [RFC1191] used NFS as an example of a UDP-based application that benefits from PMTU Discovery. Since then, [RFC7530] states that the supported transport layer between NFS and IP must be an IETF standardized transport protocol that is specified to avoid network congestion; such transports include TCP, Stream Control Transmission Protocol (SCTP) [RFC4960], and the Datagram Congestion Control Protocol (DCCP) [RFC4340]. In this case, the transport is responsible for ensuring that transmitted segments (except probes) conform to the Path MTU, including supporting PMTU Discovery probe transmissions as needed.
IPv4的路径MTU发现[RFC1191]使用NFS作为基于UDP的应用程序的示例，该应用程序受益于PMTU发现。此后，[RFC7530]指出，NFS和IP之间受支持的传输层必须是一个IETF标准化传输协议，指定该协议以避免网络拥塞；此类传输包括TCP、流控制传输协议（SCTP）[RFC4960]和数据报拥塞控制协议（DCCP）[RFC4340]。在这种情况下，传输负责确保传输段（探头除外）符合路径MTU，包括根据需要支持PMTU发现探头传输。
It is suggested that an implementation provides a way for a system utility program to:
建议实施为系统实用程序提供一种方法，以：
- Specify that Path MTU Discovery not be done on a given path.
- 指定不在给定路径上进行路径MTU发现。
- Change the PMTU value associated with a given path.
- 更改与给定路径关联的PMTU值。
The former can be accomplished by associating a flag with the path; when a packet is sent on a path with this flag set, the IP layer does not send packets larger than the IPv6 minimum link MTU.
前者可以通过将标志与路径相关联来实现；当在设置了此标志的路径上发送数据包时，IP层不会发送大于IPv6最小链路MTU的数据包。
These features might be used to work around an anomalous situation or by a routing protocol implementation that is able to obtain Path MTU values.
这些功能可用于解决异常情况，或由能够获取路径MTU值的路由协议实现使用。
The implementation should also provide a way to change the timeout period for aging stale PMTU information.
该实现还应提供一种方法来更改老化过时PMTU信息的超时时间。
This Path MTU Discovery mechanism makes possible two DoS attacks, both based on a malicious party sending false Packet Too Big messages to a node.
这种路径MTU发现机制使得两种DoS攻击成为可能，这两种攻击都是基于恶意方向节点发送虚假数据包太大的消息。
In the first attack, the false message indicates a PMTU much smaller than reality. In response, the victim node should never set its PMTU estimate below the IPv6 minimum link MTU. A sender that falsely reduces to this MTU would observe suboptimal performance.
在第一次攻击中，错误消息表示PMTU比实际值小得多。作为响应，受害节点不应将其PMTU估计值设置为低于IPv6最小链路MTU。错误地减少到该MTU的发送方将观察到次优性能。
In the second attack, the false message indicates a PMTU larger than reality. If believed, this could cause temporary blockage as the victim sends packets that will be dropped by some router. Within one round-trip time, the node would discover its mistake (receiving Packet Too Big messages from that router), but frequent repetition of this attack could cause lots of packets to be dropped. A node, however, must not raise its estimate of the PMTU based on a Packet Too Big message, so it should not be vulnerable to this attack.
在第二次攻击中，错误消息表示PMTU大于实际值。如果相信，这可能会导致暂时的阻塞，因为受害者发送的数据包将被某些路由器丢弃。在一个往返时间内，节点将发现其错误（从该路由器接收数据包太大的消息），但频繁重复这种攻击可能会导致大量数据包被丢弃。然而，节点不能基于数据包太大的消息来提高其对PMTU的估计，所以它不应该容易受到这种攻击。
Both of these attacks can cause a black-hole connection, that is, the TCP three-way handshake completes correctly but the connection hangs when data is transferred.
这两种攻击都可能导致黑洞连接，即TCP三方握手正确完成，但数据传输时连接挂起。
A malicious party could also cause problems if it could stop a victim from receiving legitimate Packet Too Big messages, but in this case there are simpler DoS attacks available.
恶意方也可能导致问题，如果它可以阻止受害者接收合法的数据包太大的消息，但在这种情况下，有更简单的DoS攻击可用。
If ICMPv6 filtering prevents reception of ICMPv6 Packet Too Big messages, the source will not learn the actual path MTU. "Packetization Layer Path MTU Discovery" [RFC4821] does not rely upon network support for ICMPv6 messages and is therefore considered more robust than standard PMTUD. It is not susceptible to "black-holed" connections caused by the filtering of ICMPv6 messages. See [RFC4890] for recommendations regarding filtering ICMPv6 messages.
如果ICMPv6过滤阻止接收ICMPv6数据包过大的消息，则源将不会学习实际路径MTU。“打包层路径MTU发现”[RFC4821]不依赖于ICMPv6消息的网络支持，因此被认为比标准PMTUD更健壮。它不易受到因过滤ICMPv6消息而导致的“黑洞”连接的影响。有关过滤ICMPv6消息的建议，请参阅[RFC4890]。
This document does not require any IANA actions.
本文件不要求IANA采取任何行动。
[ICMPv6] Conta, A., Deering, S., and M. Gupta, Ed., "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", STD 89, RFC 4443, DOI 10.17487/RFC4443, March 2006, <http://www.rfc-editor.org/info/rfc4443>.
[ICMPv6]Conta，A.，Deering，S.和M.Gupta，Ed.，“互联网协议版本6（IPv6）规范的互联网控制消息协议（ICMPv6）”，STD 89，RFC 4443，DOI 10.17487/RFC4443，2006年3月<http://www.rfc-editor.org/info/rfc4443>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, July 2017, <http://www.rfc-editor.org/info/rfc8200>.
[RFC8200]Deering，S.和R.Hinden，“互联网协议，第6版（IPv6）规范”，STD 86，RFC 8200，DOI 10.17487/RFC8200，2017年7月<http://www.rfc-editor.org/info/rfc8200>.
[FRAG] Kent, C. and J. Mogul, "Fragmentation Considered Harmful", In Proc. SIGCOMM '87 Workshop on Frontiers in Computer Communications Technology, DOI 10.1145/55483.55524, August 1987.
[FRAG]Kent，C.和J.Mogul，“碎片被认为是有害的”，在Proc。SIGCOMM'87计算机通信技术前沿研讨会，DOI 10.1145/55483.55524，1987年8月。
[ND] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, DOI 10.17487/RFC4861, September 2007, <http://www.rfc-editor.org/info/rfc4861>.
[ND]Narten，T.，Nordmark，E.，Simpson，W.，和H.Soliman，“IP版本6（IPv6）的邻居发现”，RFC 4861，DOI 10.17487/RFC48612007年9月<http://www.rfc-editor.org/info/rfc4861>.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, DOI 10.17487/RFC1122, October 1989, <http://www.rfc-editor.org/info/rfc1122>.
[RFC1122]Braden，R.，Ed.“互联网主机的要求-通信层”，STD 3，RFC 1122，DOI 10.17487/RFC1122，1989年10月<http://www.rfc-editor.org/info/rfc1122>.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, DOI 10.17487/RFC1191, November 1990, <http://www.rfc-editor.org/info/rfc1191>.
[RFC1191]Mogul，J.和S.Deering，“MTU发现路径”，RFC 1191，DOI 10.17487/RFC1191，1990年11月<http://www.rfc-editor.org/info/rfc1191>.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery for IP version 6", RFC 1981, DOI 10.17487/RFC1981, August 1996, <http://www.rfc-editor.org/info/rfc1981>.
[RFC1981]McCann，J.，Deering，S.，和J.Mogul，“IP版本6的路径MTU发现”，RFC 1981，DOI 10.17487/RFC19811996年8月<http://www.rfc-editor.org/info/rfc1981>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>.
[RFC2119]Bradner，S.，“RFC中用于表示需求水平的关键词”，BCP 14，RFC 2119，DOI 10.17487/RFC2119，1997年3月<http://www.rfc-editor.org/info/rfc2119>.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery", RFC 2923, DOI 10.17487/RFC2923, September 2000, <http://www.rfc-editor.org/info/rfc2923>.
[RFC2923]Lahey，K.，“路径MTU发现的TCP问题”，RFC 2923，DOI 10.17487/RFC2923，2000年9月<http://www.rfc-editor.org/info/rfc2923>.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram Congestion Control Protocol (DCCP)", RFC 4340, DOI 10.17487/RFC4340, March 2006, <http://www.rfc-editor.org/info/rfc4340>.
[RFC4340]Kohler，E.，Handley，M.和S.Floyd，“数据报拥塞控制协议（DCCP）”，RFC 4340，DOI 10.17487/RFC4340，2006年3月<http://www.rfc-editor.org/info/rfc4340>.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007, <http://www.rfc-editor.org/info/rfc4821>.
[RFC4821]Mathis，M.和J.Heffner，“打包层路径MTU发现”，RFC 4821，DOI 10.17487/RFC4821，2007年3月<http://www.rfc-editor.org/info/rfc4821>.
[RFC4890] Davies, E. and J. Mohacsi, "Recommendations for Filtering ICMPv6 Messages in Firewalls", RFC 4890, DOI 10.17487/RFC4890, May 2007, <http://www.rfc-editor.org/info/rfc4890>.
[RFC4890]Davies，E.和J.Mohacsi，“防火墙中过滤ICMPv6消息的建议”，RFC 4890，DOI 10.17487/RFC4890，2007年5月<http://www.rfc-editor.org/info/rfc4890>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol", RFC 4960, DOI 10.17487/RFC4960, September 2007, <http://www.rfc-editor.org/info/rfc4960>.
[RFC4960]Stewart，R.，Ed.“流控制传输协议”，RFC 4960，DOI 10.17487/RFC4960，2007年9月<http://www.rfc-editor.org/info/rfc4960>.
[RFC6691] Borman, D., "TCP Options and Maximum Segment Size (MSS)", RFC 6691, DOI 10.17487/RFC6691, July 2012, <http://www.rfc-editor.org/info/rfc6691>.
[RFC6691]Borman，D.，“TCP选项和最大段大小（MSS）”，RFC 6691，DOI 10.17487/RFC6691192012年7月<http://www.rfc-editor.org/info/rfc6691>.
[RFC7530] Haynes, T., Ed. and D. Noveck, Ed., "Network File System (NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530, March 2015, <http://www.rfc-editor.org/info/rfc7530>.
[RFC7530]Haynes，T.，Ed.和D.Noveck，Ed.，“网络文件系统（NFS）第4版协议”，RFC 7530，DOI 10.17487/RFC7530，2015年3月<http://www.rfc-editor.org/info/rfc7530>.
[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085, March 2017, <http://www.rfc-editor.org/info/rfc8085>.
[RFC8085]Eggert，L.，Fairhurst，G.和G.Shepherd，“UDP使用指南”，BCP 145，RFC 8085，DOI 10.17487/RFC8085，2017年3月<http://www.rfc-editor.org/info/rfc8085>.
RFC 1981 (obsoleted by this document) was based in large part on RFC 1191, which describes Path MTU Discovery for IPv4. Certain portions of RFC 1191 were not needed in RFC 1981:
RFC1981（已被本文档淘汰）在很大程度上基于RFC191，它描述了IPv4的路径MTU发现。RFC 1191的某些部分在RFC 1981中不需要：
router specification Packet Too Big messages and corresponding router behavior are defined in [ICMPv6]
[ICMPv6]中定义了路由器规范数据包过大消息和相应的路由器行为
Don't Fragment bit there is no DF bit in IPv6 packets
不分割位IPv6数据包中没有DF位
TCP MSS discussion selecting a value to send in the TCP MSS option is discussed in [RFC8200]
TCP MSS讨论在[RFC8200]中讨论了在TCP MSS选项中选择要发送的值
old-style messages all Packet Too Big messages report the MTU of the constricting link
旧式消息所有数据包过大的消息都报告压缩链路的MTU
MTU plateau tables not needed because there are no old-style messages
不需要MTU平台表，因为没有旧式消息
This document is based on RFC 1981 and has the following changes from RFC 1981:
本文件以RFC 1981为基础，与RFC 1981相比有以下变化：
o Clarified in Section 1, "Introduction", that the purpose of PMTUD is to reduce the need for IPv6 fragmentation.
o 在第1节“简介”中阐明，PMTUD的目的是减少对IPv6分段的需要。
o Added text to Section 1, "Introduction", about the effects on PMTUD when ICMPv6 messages are blocked.
o 在第1节“简介”中添加了有关阻止ICMPv6消息时对PMTUD的影响的文本。
o Added a "Note" to the introduction to document that this specification doesn't cite RFC 2119 and only uses lower case "should/must" language. Changed all upper case "should/must" to lower case.
o 在文档介绍中添加了一个“注释”，说明本规范未引用RFC 2119，仅使用小写“应该/必须”语言。将所有大写“应该/必须”改为小写。
o Added a short summary to Section 1, "Introduction", about PLPMTUD and a reference to RFC 4821 that defines it.
o 在第1节“简介”中添加了一个关于PLPMTUD的简短摘要，并引用了RFC 4821对其进行了定义。
o Aligned text in Section 2, "Terminology", to match current packetization layer terminology.
o 第2节“术语”中对齐的文本，以匹配当前的打包层术语。
o Added clarification in Section 4, "Protocol Requirements", that nodes should validate the payload of ICMP PTB messages per RFC 4443, and that nodes should detect decreases in PMTU as fast as possible.
o 在第4节“协议要求”中增加了澄清，即节点应根据RFC 4443验证ICMP PTB消息的有效负载，并且节点应尽快检测PMTU的减少。
o Removed a "Note" from Section 4, "Protocol Requirements", about a Packet Too Big message reporting a next-hop MTU that is less than the IPv6 minimum link MTU because this was removed from [RFC8200].
o 从第4节“协议要求”中删除了一条“注释”，内容是关于一条数据包太大的消息报告下一跳MTU小于IPv6最小链路MTU，因为该MTU已从[RFC8200]中删除。
o Added clarification in Section 5.2, "Storing PMTU Information", to discard an ICMPv6 Packet Too Big message if it contains an MTU less than the IPv6 minimum link MTU.
o 在第5.2节“存储PMTU信息”中增加了说明，如果ICMPv6数据包包含的MTU小于IPv6最小链路MTU，则丢弃过大的消息。
o Added clarification in Section 5.2, "Storing PMTU Information", that for nodes with multiple interfaces, Path MTU information should be stored for each link.
o 在第5.2节“存储PMTU信息”中增加了澄清，即对于具有多个接口的节点，应为每个链路存储路径MTU信息。
o Removed text in Section 5.2, "Storing PMTU Information", about Routing Header type 0 (RH0) because it was deprecated by RFC 5095.
o 删除了第5.2节“存储PMTU信息”中有关路由头类型0（RH0）的文本，因为RFC 5095不推荐该类型。
o Removed text about obsolete security classification from Section 5.2, "Storing PMTU Information".
o 删除了第5.2节“存储PMTU信息”中有关过时安全分类的文本。
o Changed the title of Section 5.4 to "Packetization Layer Actions" and changed the text in the first paragraph to generalize this section to cover all packetization layers, not just TCP.
o 将第5.4节的标题更改为“打包层操作”，并更改了第一段中的文本，以将本节概括为涵盖所有打包层，而不仅仅是TCP。
o Clarified text in Section 5.4, "Packetization Layer Actions", to use normal packetization layer retransmission methods.
o 澄清第5.4节“分组层动作”中的文本，以使用正常分组层重传方法。
o Removed text in Section 5.4, "Packetization Layer Actions", that described 4.2 BSD because it is obsolete, and removed reference to TP4.
o 删除了第5.4节“打包层操作”中描述4.2 BSD的文本，因为该文本已过时，并删除了对TP4的引用。
o Updated text in Section 5.5, "Issues for Other Transport Protocols", about NFS, including adding a current reference to NFS and removing obsolete text.
o 更新了第5.5节“其他传输协议的问题”中有关NFS的文本，包括添加对NFS的当前引用和删除过时文本。
o Added a paragraph to Section 6, "Security Considerations", about black-hole connections if PTB messages are not received and comparison to PLPMTUD.
o 在第6节“安全注意事项”中添加了一段，关于未接收PTB消息时的黑洞连接以及与PLPMTUD的比较。
o Updated "Acknowledgements".
o 更新了“确认”。
o Editorial Changes.
o 编辑上的改动。
Acknowledgements
致谢
We would like to acknowledge the authors of and contributors to [RFC1191], from which the majority of this document was derived. We would also like to acknowledge the members of the IPng Working Group for their careful review and constructive criticisms.
我们要感谢[RFC1191]的作者和贡献者，本文件的大部分内容都是从[RFC1191]获得的。我们还要感谢知识产权工作组成员的认真审查和建设性批评。
We would also like to acknowledge the contributors to this update of "Path MTU Discovery for IP Version 6". This includes members of the 6MAN Working Group, area directorate reviewers, the IESG, and especially Joe Touch and Gorry Fairhurst.
我们还要感谢“IP版本6的路径MTU发现”更新的贡献者。这包括6MAN工作组成员、地区董事会评审员、IESG，尤其是乔·图奇和戈里·费尔赫斯特。
Authors' Addresses
作者地址
Jack McCann Digital Equipment Corporation
杰克·麦肯数字设备公司
Stephen E. Deering Retired Vancouver, British Columbia Canada
斯蒂芬·迪林退休于加拿大不列颠哥伦比亚省温哥华
Jeffrey Mogul Digital Equipment Corporation
杰弗里·莫格尔数字设备公司
Robert M. Hinden (editor) Check Point Software 959 Skyway Road San Carlos, CA 94070 United States of America
Robert M.Hinden（编辑）美国加利福尼亚州圣卡洛斯市Skyway路959号检查点软件94070
Email: bob.hinden@gmail.com
Email: bob.hinden@gmail.com