Internet Engineering Task Force (IETF) D. Katz
Request for Comments: 8563 Juniper Networks
Category: Standards Track D. Ward
ISSN: 2070-1721 Cisco Systems
S. Pallagatti, Ed.
VMware
G. Mirsky, Ed.
ZTE Corp.
April 2019
Internet Engineering Task Force (IETF) D. Katz
Request for Comments: 8563 Juniper Networks
Category: Standards Track D. Ward
ISSN: 2070-1721 Cisco Systems
S. Pallagatti, Ed.
VMware
G. Mirsky, Ed.
ZTE Corp.
April 2019
Bidirectional Forwarding Detection (BFD) Multipoint Active Tails
双向转发检测（BFD）多点主动跟踪
Abstract
摘要
This document describes active tail extensions to the Bidirectional Forwarding Detection (BFD) protocol for multipoint networks.
本文档描述了用于多点网络的双向转发检测（BFD）协议的主动尾部扩展。
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 https://www.rfc-editor.org/info/rfc8563.
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https://www.rfc-editor.org/info/rfc8563.
Copyright Notice
版权公告
Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.
版权（c）2019 IETF信托基金和被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 (https://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(https://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。
Table of Contents
目录
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology and Acronyms . . . . . . . . . . . . . . . . . . 3
3. Keywords . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Operational Scenarios . . . . . . . . . . . . . . . . . . . . 5
5.1. No Head Notification . . . . . . . . . . . . . . . . . . 5
5.2. Head Notification . . . . . . . . . . . . . . . . . . . . 5
5.2.1. Head Notification without Polling . . . . . . . . . . 5
5.2.2. Head Notification and Tail Solicitation with
Multipoint Polling . . . . . . . . . . . . . . . . . 6
5.2.3. Head Notification with Composite Polling . . . . . . 6
6. Protocol Details . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Multipoint Client Session . . . . . . . . . . . . . . . . 8
6.2. Multipoint Client Session Failure . . . . . . . . . . . . 8
6.3. State Variables . . . . . . . . . . . . . . . . . . . . . 8
6.3.1. New State Variables . . . . . . . . . . . . . . . . . 8
6.3.2. New State Variable Value . . . . . . . . . . . . . . 9
6.3.3. State Variable Initialization and Maintenance . . . . 10
6.4. Controlling Multipoint BFD Options . . . . . . . . . . . 11
6.5. State Machine . . . . . . . . . . . . . . . . . . . . . . 11
6.6. Session Establishment . . . . . . . . . . . . . . . . . . 12
6.7. Discriminators and Packet Demultiplexing . . . . . . . . 12
6.8. Controlling Tail Packet Transmission . . . . . . . . . . 12
6.9. Soliciting the Tails . . . . . . . . . . . . . . . . . . 13
6.10. Verifying Connectivity to Specific Tails . . . . . . . . 13
6.11. Detection Times . . . . . . . . . . . . . . . . . . . . . 14
6.12. MultipointClient Down/AdminDown Sessions . . . . . . . . 15
6.13. Base BFD for Multipoint Networks Specification Text
Replacement . . . . . . . . . . . . . . . . . . . . . . . 15
6.13.1. Reception of BFD Control Packets . . . . . . . . . . 15
6.13.2. Demultiplexing BFD Control Packets . . . . . . . . . 16
6.13.3. Transmitting BFD Control Packets . . . . . . . . . . 16
7. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 17
8. Operational Considerations . . . . . . . . . . . . . . . . . 18
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
10. Security Considerations . . . . . . . . . . . . . . . . . . . 18
11. Normative References . . . . . . . . . . . . . . . . . . . . 19
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 19
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology and Acronyms . . . . . . . . . . . . . . . . . . 3
3. Keywords . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Operational Scenarios . . . . . . . . . . . . . . . . . . . . 5
5.1. No Head Notification . . . . . . . . . . . . . . . . . . 5
5.2. Head Notification . . . . . . . . . . . . . . . . . . . . 5
5.2.1. Head Notification without Polling . . . . . . . . . . 5
5.2.2. Head Notification and Tail Solicitation with
Multipoint Polling . . . . . . . . . . . . . . . . . 6
5.2.3. Head Notification with Composite Polling . . . . . . 6
6. Protocol Details . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Multipoint Client Session . . . . . . . . . . . . . . . . 8
6.2. Multipoint Client Session Failure . . . . . . . . . . . . 8
6.3. State Variables . . . . . . . . . . . . . . . . . . . . . 8
6.3.1. New State Variables . . . . . . . . . . . . . . . . . 8
6.3.2. New State Variable Value . . . . . . . . . . . . . . 9
6.3.3. State Variable Initialization and Maintenance . . . . 10
6.4. Controlling Multipoint BFD Options . . . . . . . . . . . 11
6.5. State Machine . . . . . . . . . . . . . . . . . . . . . . 11
6.6. Session Establishment . . . . . . . . . . . . . . . . . . 12
6.7. Discriminators and Packet Demultiplexing . . . . . . . . 12
6.8. Controlling Tail Packet Transmission . . . . . . . . . . 12
6.9. Soliciting the Tails . . . . . . . . . . . . . . . . . . 13
6.10. Verifying Connectivity to Specific Tails . . . . . . . . 13
6.11. Detection Times . . . . . . . . . . . . . . . . . . . . . 14
6.12. MultipointClient Down/AdminDown Sessions . . . . . . . . 15
6.13. Base BFD for Multipoint Networks Specification Text
Replacement . . . . . . . . . . . . . . . . . . . . . . . 15
6.13.1. Reception of BFD Control Packets . . . . . . . . . . 15
6.13.2. Demultiplexing BFD Control Packets . . . . . . . . . 16
6.13.3. Transmitting BFD Control Packets . . . . . . . . . . 16
7. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 17
8. Operational Considerations . . . . . . . . . . . . . . . . . 18
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
10. Security Considerations . . . . . . . . . . . . . . . . . . . 18
11. Normative References . . . . . . . . . . . . . . . . . . . . 19
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 19
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
This application of BFD is an extension to Multipoint BFD [RFC8562], which allows tails to notify the head of the lack of multipoint connectivity. As a further option, heads can request a notification from the tails by means of a polling mechanism. Notification to the head can be enabled for all tails, or for only a subset of the tails.
BFD的这个应用是对多点BFD[RFC8562]的扩展，它允许TAIL通知头部缺少多点连接。作为进一步的选择，头部可以通过轮询机制从尾部请求通知。可以为所有尾部或仅为尾部的子集启用头部通知。
The goal of this application is for the head to have reasonably rapid knowledge of tails that have lost connectivity from the head.
此应用程序的目标是使头部能够合理快速地了解与头部失去连接的尾部。
Since scaling is a primary concern (particularly state explosion toward the head), it is required that the head be in control of all timing aspects of the mechanism and that BFD packets from the tails to the head not be synchronized.
由于缩放是一个主要问题（尤其是朝向头部的状态爆炸），因此要求头部控制机制的所有定时方面，并且不同步从尾部到头部的BFD数据包。
Throughout this document, the term "multipoint" is defined as a mechanism by which one or more systems receive packets sent by a single sender. This specifically includes such things as IP multicast and point-to-multipoint MPLS.
在本文件中，术语“多点”定义为一个或多个系统接收单个发送方发送的数据包的机制。这具体包括IP多播和点对多点MPLS。
The term "connectivity" in this document is not being used in the context of connectivity verification in a transport network but as an alternative to "continuity", i.e., the existence of a path between the sender and the receiver.
本文件中的术语“连接性”不用于传输网络中的连接性验证，而是作为“连续性”的替代，即发送方和接收方之间存在路径。
This document effectively modifies and adds to Sections 5.12 and 5.13 of the base BFD multipoint networks specification [RFC8562].
本文件对基本BFD多点网络规范[RFC8562]第5.12节和第5.13节进行了有效修改和补充。
BFD: Bidirectional Forwarding Detection
双向转发检测
c-poll: Composite Poll
c投票：综合投票
m-poll: Multipoint Poll
多点轮询
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
本文件中的关键词“必须”、“不得”、“必需”、“应”、“不应”、“建议”、“不建议”、“可”和“可选”在所有大写字母出现时（如图所示）应按照BCP 14[RFC2119][RFC8174]所述进行解释。
A head may wish to be alerted of the tails' connectivity (or lack thereof), and there are a number of options to achieve that. First, if all that is needed is a best-effort failure notification, as discussed in Section 5.2.1, the tails can send unsolicited unicast BFD Control packets to the head when the path fails, as described in Section 6.4.
头部可能希望收到尾部连通性（或缺乏连通性）的警报，实现这一点有多种选择。首先，如第5.2.1节所述，如果所需的只是尽力而为的失败通知，则尾部可以在路径失败时向头部发送未经请求的单播BFD控制数据包，如第6.4节所述。
If the head wishes to know of the active tails on the multipoint path, it may send a multipoint BFD Control packet with the Poll (P) bit set, which will induce the tails to return a unicast BFD Control packet with the Final (F) bit set (see a detailed description in Section 5.2.2). The head can then create BFD session state for each of the tails that have multipoint connectivity. If the head sends such a packet on occasion, it can keep track of which tails answer, thus providing a more deterministic mechanism for detecting which tails fail to respond (implying a loss of multipoint connectivity). In this document, this method is referred to as the Multipoint Poll (m-poll).
如果头部希望知道多点路径上的活动尾部，它可以发送带有轮询（P）位集的多点BFD控制包，这将促使尾部返回带有最终（F）位集的单播BFD控制包（见第5.2.2节中的详细说明）。然后，头部可以为具有多点连接的每个尾部创建BFD会话状态。如果头部偶尔发送这样的数据包，它可以跟踪哪个尾部应答，从而提供更确定的机制来检测哪些尾部没有响应（意味着失去多点连接）。在本文档中，此方法称为多点轮询（m-Poll）。
If the head wishes the definite indication of the tails' connectivity, it may do all of the above, but if it detects that a tail did not answer the previous multipoint poll, it may initiate a Demand mode Poll Sequence as a unicast to that tail (see a detailed description in Section 5.2.3). This covers the case where either the multipoint poll or the single reply is also lost in transit. If desired, the head may Poll one or more tails proactively to track the tails' connectivity. In this document, the method that combines the use of multipoint and unicast polling of tails by the head is referred to as the Composite Poll (c-poll).
如果头部希望明确指示尾部的连接性，它可以执行上述所有操作，但如果它检测到尾部没有响应之前的多点轮询，它可以启动一个请求模式轮询序列，作为对该尾部的单播（见第5.2.3节中的详细描述）。这包括多点轮询或单个回复在传输过程中丢失的情况。如果需要，头部可以主动轮询一个或多个尾部，以跟踪尾部的连接性。在本文档中，结合使用多点轮询和头部对尾部进行单播轮询的方法称为复合轮询（c-Poll）。
If the awareness of the state of some nodes is more important for the head, in the sense that the head needs to detect the lack of multipoint connectivity to a subset of tails at a different rate, the head may transmit unicast BFD Polls to that subset of tails. In this case, the timing may be independent on a tail-by-tail basis.
如果对某些节点的状态的感知对于头部更为重要，在头部需要以不同速率检测到与尾部子集的多点连接的缺乏的意义上，头部可以向尾部子集发送单播BFD轮询。在这种情况下，定时可能在逐尾的基础上是独立的。
Individual tails may be configured so that they never send BFD Control packets to the head. Such tails will never be known to the head but will still be able to detect multipoint path failures from the head.
可以配置单个尾部，以便它们从不向头部发送BFD控制数据包。头部永远不会知道这样的尾部，但仍然能够从头部检测到多点路径故障。
It is worth analyzing how this protocol reacts to various scenarios. There are three path components present: namely, the multipoint path, the forward unicast path (from the head to a particular tail), and the reverse unicast path (from a tail to the head). There are also four options as to how the head is notified about failures from the tail. For the different modes described below, the setting of new state variables are given even if these are only introduced later in the document (see Section 6.3).
值得分析该协议对各种场景的反应。存在三个路径组件：即多点路径、正向单播路径（从头部到特定尾部）和反向单播路径（从尾部到头部）。关于如何从尾部通知头部故障，还有四个选项。对于下面描述的不同模式，给出了新状态变量的设置，即使这些变量仅在本文件后面介绍（见第6.3节）。
In this scenario, only the multipoint path is used and none of the others matter. A failure in the multipoint path will result in the tail noticing the failure within a Detection Time, and the head will remain ignorant of the tail state. This mode emulates the behavior described in [RFC8562]. In this mode, bfd.SessionType is MultipointTail, and the variable bfd.SilentTail (see Section 6.3.1) MUST be set to 1. If bfd.SessionType is MultipointHead or MultipointClient, bfd.ReportTailDown MUST be set to zero. The head MAY set bfd.RequiredMinRxInterval to zero and thus suppress tails sending any BFD Control packets.
在这种情况下，只使用多点路径，其他路径都不重要。多点路径中的故障将导致尾部在检测时间内注意到故障，头部将不知道尾部状态。此模式模拟[RFC8562]中描述的行为。在此模式下，bfd.SessionType为MultipointTail，变量bfd.silentail（参见第6.3.1节）必须设置为1。如果bfd.SessionType是MultiPointAD或MultipointClient，则bfd.ReportTailDown必须设置为零。头部可以将bfd.RequiredMinRxInterval设置为零，从而抑制尾部发送任何bfd控制数据包。
In these scenarios, the tail sends unsolicited or solicited BFD packets in response to the detection of a multipoint path failure. All these scenarios have common settings:
在这些场景中，tail发送未经请求或请求的BFD数据包，以响应检测到的多点路径故障。所有这些场景都有共同的设置：
o if bfd.SessionType is MultipointTail, the variable bfd.SilentTail (see Section 6.3.1) MUST be set to zero;
o 如果bfd.SessionType为MultipointTail，则变量bfd.SilentTail（参见第6.3.1节）必须设置为零；
o if bfd.SessionType is MultipointHead or MultipointClient, bfd.ReportTailDown MUST be set to 1;
o 如果bfd.SessionType为multipointaiad或MultipointClient，则bfd.ReportTailDown必须设置为1；
o the head MUST set bfd.RequiredMinRxInterval to nonzero and thus allow tails to send BFD Control packets.
o 头部必须将bfd.RequiredMinRxInterval设置为非零，从而允许尾部发送bfd控制数据包。
In this scenario, the tail sends unsolicited BFD packets in response to the detection of a multipoint path failure. It uses the reverse unicast path, but not the forward unicast path.
在这种情况下，tail发送未经请求的BFD数据包，以响应多点路径故障的检测。它使用反向单播路径，但不使用正向单播路径。
If the multipoint path fails but the reverse unicast path stays up, the tail will detect the failure within a Detection Time, and the head will know about it within one reverse packet time (since the notification is delayed).
如果多点路径失败，但反向单播路径保持不变，则尾部将在检测时间内检测到故障，头部将在一个反向数据包时间内知道故障（因为通知延迟）。
If both the multipoint path and the reverse unicast paths fail, the tail will detect the failure, but the head will remain unaware of it.
如果多点路径和反向单播路径都失败，尾部将检测到失败，但头部仍不知道。
In this scenario, the head sends occasional multipoint Polls in addition to (or in lieu of) non-Poll multipoint BFD Control packets, expecting the tails to reply with Final. This also uses the reverse unicast path, but not the forward unicast path.
在这种情况下，除了（或代替）非轮询多点BFD控制数据包之外，头部偶尔发送多点轮询，期望尾部使用Final进行回复。这也使用反向单播路径，但不使用正向单播路径。
If the multipoint path fails but the reverse unicast path stays up, the tail will detect the failure within a Detection Time, and the head will know about it within one reverse packet time (the notification is delayed to avoid synchronization of the tails).
如果多点路径失败，但反向单播路径保持不变，则尾部将在检测时间内检测到故障，头部将在一个反向数据包时间内知道故障（延迟通知以避免尾部同步）。
If both the multipoint path and the reverse unicast paths fail, the tail will detect the failure, but the head will remain unaware of this fact.
如果多点路径和反向单播路径都失败，尾部将检测到失败，但头部将不知道这一事实。
If the reverse unicast path fails but the multipoint path stays up, the head will see the BFD session fail, but the state of the multipoint path will be unknown to the head. The tail will continue to receive multipoint data traffic.
如果反向单播路径失败，但多点路径保持不变，则头部将看到BFD会话失败，但头部将不知道多点路径的状态。tail将继续接收多点数据通信。
If either the multipoint Poll or the unicast reply is lost in transit, the head will see the BFD session fail, but the state of the multipoint path will be unknown to the head. The tail will continue to receive multipoint data traffic.
如果多点轮询或单播应答在传输过程中丢失，则头部将看到BFD会话失败，但头部不知道多点路径的状态。tail将继续接收多点数据通信。
In this scenario, the head sends occasional multipoint Polls in addition to (or in lieu of) non-Poll multipoint BFD Control packets, expecting the tails to reply with Final. If a tail that had previously replied to a multipoint Poll fails to reply (or if the head simply wishes to verify tail connectivity), the head issues a unicast Poll Sequence to the tail. This scenario makes use of all three paths. In this mode for bfd.SessionType of MultipointTail, variable bfd.SilentTail (see Section 6.3.1) MUST be set to zero.
在这种情况下，除了（或代替）非轮询多点BFD控制数据包之外，头部偶尔发送多点轮询，期望尾部使用Final进行回复。如果先前回复多点轮询的尾部未能回复（或者如果头部只是希望验证尾部连接），则头部会向尾部发出单播轮询序列。此场景使用所有三条路径。在此模式下，对于多点尾的bfd.SessionType，变量bfd.SilentTail（参见第6.3.1节）必须设置为零。
If the multipoint path fails but the two unicast paths stay up, the tail will detect the failure within a Detection Time, and the head will know about it within one reverse packet time (since the
如果多点路径失败，但两条单播路径保持不变，则尾部将在检测时间内检测到故障，头部将在一个反向数据包时间内（自
notification is delayed). Note that the reverse packet time may be smaller in this case if the head has previously issued a unicast Poll (since the tail will not delay transmission of the notification in this case).
通知延迟）。注意，在这种情况下，如果头部先前发出了单播轮询，则反向分组时间可能更小（因为在这种情况下，尾部不会延迟通知的传输）。
If both the multipoint path and the reverse unicast paths fail (regardless of the state of the forward unicast path), the tail will detect the failure, but the head will remain unaware of this fact. The head will detect a BFD session failure to the tail but cannot make a determination about the state of the tail's multipoint connectivity.
如果多点路径和反向单播路径都失败（无论前向单播路径的状态如何），尾部将检测到失败，但头部将不知道这一事实。头部将检测到尾部的BFD会话故障，但无法确定尾部多点连接的状态。
If the forward unicast path fails but the reverse unicast path stays up, the head will detect a BFD session failure to the tail if it happens to send a unicast Poll sequence but cannot make a determination about the state of the tail's multipoint connectivity. If the multipoint path to the tail fails prior to any unicast Poll being sent, the tail will detect the failure within a Detection Time, and the head will know about it within one reverse packet time (since the notification is delayed).
如果正向单播路径失败，但反向单播路径保持不变，则如果头部碰巧发送单播轮询序列，但无法确定尾部的多点连接状态，则头部将检测到尾部的BFD会话失败。如果到尾部的多点路径在发送任何单播轮询之前失败，尾部将在检测时间内检测到失败，头部将在一个反向包时间内知道失败（因为通知延迟）。
If the multipoint path stays up but the reverse unicast path fails, the head will see the particular MultipointClient session fail if it happens to send a Poll Sequence, but the state of the multipoint path will be unknown to the head. The tail will continue to receive multipoint data traffic.
如果多点路径保持不变，但反向单播路径失败，则如果碰巧发送轮询序列，则头部将看到特定的多点客户端会话失败，但头部将不知道多点路径的状态。tail将继续接收多点数据通信。
If the multipoint path and the reverse unicast path both stay up but the forward unicast path fails, neither side will notice this failure as long as a unicast Poll Sequence is never sent by the head. If the head sends a unicast Poll Sequence, the head will detect the failure in the forward unicast path. The state of the multipoint path will be determined by the multipoint Poll. The tail will continue to receive multipoint data traffic.
如果多点路径和反向单播路径都保持正常，但正向单播路径失败，则只要头部从未发送单播轮询序列，任何一方都不会注意到此失败。如果头部发送单播轮询序列，则头部将检测到前向单播路径中的故障。多点路径的状态将由多点轮询决定。tail将继续接收多点数据通信。
This section describes the operation of the BFD Multipoint active tail in detail. This section modifies Section 4 of [RFC8562] as follows:
本节详细描述了BFD多点主动尾翼的操作。本节对[RFC8562]第4节进行如下修改：
o Section 6.3 introduces new state variables and modifies the usage of a few existing ones;
o 第6.3节引入了新的状态变量，并修改了一些现有变量的用法；
o Section 6.13 replaces the corresponding sections in the base BFD for multipoint networks specification.
o 第6.13节取代了多点网络规范基本BFD中的相应章节。
If the head is keeping track of some or all of the tails, it has a session of type MultipointClient per tail that it cares about. All of the MultipointClient sessions for tails on a particular multipoint path are associated with the MultipointHead session to which the clients are listening. A BFD Poll Sequence may be sent over a MultipointClient session to a tail if the head wishes to verify connectivity. These sessions receive any BFD Control packets sent by the tails and MUST NOT transmit periodic BFD Control packets other than Poll Sequences (since periodic transmission is always done by the MultipointHead session). Note that the settings of all BFD variables in a MultipointClient session for a particular tail override the corresponding settings in the MultipointHead session.
如果头部跟踪部分或全部尾部，则它所关心的会话类型为MultipointClient per tail。特定多点路径上尾部的所有多点客户端会话都与客户端正在侦听的多点客户端会话相关联。如果头部希望验证连接性，则BFD轮询序列可以通过多点客户端会话发送到尾部。这些会话接收尾部发送的任何BFD控制数据包，并且不得发送除轮询序列以外的周期性BFD控制数据包（因为周期性传输始终由多任务会话完成）。请注意，特定尾部的多点客户端会话中所有BFD变量的设置将覆盖多点客户端会话中的相应设置。
If a MultipointClient session receives a BFD Control packet from the tail with state Down or AdminDown, the head reliably knows that the tail has lost multipoint connectivity. If the Detection Time expires on a MultipointClient session, it is ambiguous as to whether the multipoint connectivity failed or whether there was a unicast path problem in one direction or the other, so the head does not reliably know the tail's state.
如果多点客户端会话从tail接收到状态为Down或AdminDown的BFD控制数据包，则头部可靠地知道tail已失去多点连接。如果检测时间在多点客户端会话上过期，则多点连接是否失败或在一个方向或另一个方向上是否存在单播路径问题是不明确的，因此头部无法可靠地知道尾部的状态。
BFD Multipoint active tail introduces new state variables and modifies the usage of a few existing ones defined in Section 5.4 of [RFC8562].
BFD多点主动尾翼引入了新的状态变量，并修改了[RFC8562]第5.4节中定义的一些现有变量的用法。
A few state variables are added in support of multipoint BFD active tail.
添加了一些状态变量以支持多点BFD活动尾。
bfd.SilentTail
白鳍豚
If zero, a tail may send packets to the head according to other parts of this specification. Setting this to 1 allows tails to be provisioned to always be silent, even when the head is soliciting traffic from the tails. This can be useful, for example, in deployments of a large number of tails when the head wishes to track the state of a subset of them. This variable MUST be initialized based on configuration. The default value MUST be 1.
如果为零，则尾部可以根据本规范的其他部分向头部发送数据包。将此设置为1允许将尾部设置为始终保持静默，即使头部正在从尾部请求流量。例如，当头部希望跟踪尾部子集的状态时，这在部署大量尾部时非常有用。必须根据配置初始化此变量。默认值必须为1。
This variable is only pertinent when bfd.SessionType is MultipointTail and SHOULD NOT be modified after the MultipointTail session has been created.
仅当bfd.SessionType为MultipointTail时，此变量才相关，并且在创建MultipointTail会话后不应修改此变量。
bfd.ReportTailDown
bfd.ReportTailDown
Set to 1 if the head wishes tails to notify the head, via periodic BFD Control packets, when they see the BFD session fail. If zero, the tail will never send periodic BFD Control packets, and the head will not be notified of session failures by the tails. This variable MUST be initialized based on configuration. The default value MUST be zero.
如果头部希望尾部在看到BFD会话失败时通过定期BFD控制数据包通知头部，则设置为1。如果为零，尾部将永远不会发送周期性BFD控制数据包，并且尾部不会通知头部会话失败。必须根据配置初始化此变量。默认值必须为零。
This variable is only pertinent when bfd.SessionType is MultipointHead or MultipointClient.
此变量仅在bfd.SessionType为MultiPointAD或MultipointClient时相关。
bfd.UnicastRcvd
bfd.cvd
Set to 1 if a tail has received a unicast BFD Control packet from the head while being in Up state. This variable MUST be set to zero if the session transitions from Up state to some other state.
如果尾部在向上状态下从头部接收到单播BFD控制数据包，则设置为1。如果会话从Up状态转换到其他状态，则必须将此变量设置为零。
This variable MUST be initialized to zero.
此变量必须初始化为零。
This variable is only pertinent when bfd.SessionType is MultipointTail.
此变量仅在bfd.SessionType为MultipointTail时相关。
A new state variable value being added to:
将新的状态变量值添加到：
bfd.SessionType
bfd.SessionType
The type of this session as defined in [RFC7880]. A new value introduced is:
[RFC7880]中定义的此会话的类型。引入的新值是：
MultipointClient: A session on the head that tracks the state of an individual tail, when desirable.
多点客户端：头部的会话，在需要时跟踪单个尾部的状态。
This variable MUST be initialized to the appropriate type when the session is created, according to the rules in Section 5.4 of [RFC8562].
根据[RFC8562]第5.4节中的规则，创建会话时必须将此变量初始化为适当的类型。
Some state variables defined in Section 6.8.1 of [RFC5880] need to be initialized or manipulated differently depending on the session type. The values of some of these variables relate to those of the same variables of a MultipointHead session (see Section 5.4.2 of [RFC8562]).
[RFC5880]第6.8.1节中定义的一些状态变量需要根据会话类型进行不同的初始化或操作。其中一些变量的值与多天线会话中相同变量的值相关（见[RFC8562]第5.4.2节）。
bfd.LocalDiscr
bfd.LocalDiscr
For session type MultipointClient, this variable MUST always match the value of bfd.LocalDiscr in the associated MultipointHead session.
对于会话类型MultipointClient，此变量必须始终与关联MultiPointAD会话中的bfd.LocalDiscr值匹配。
bfd.DesiredMinTxInterval
bfd.DesiredMinTxInterval
For session type MultipointClient, this variable MUST always match the value of bfd.DesiredMinTxInterval in the associated MultipointHead session.
对于会话类型MultipointClient，此变量必须始终与关联MultiPointAD会话中的bfd.DesiredIntxinterval值匹配。
bfd.RequiredMinRxInterval
bfd.RequiredMinRxInterval
It MAY be set to zero at the head BFD system to suppress traffic from the tails. Setting it to zero in the MultipointHead session suppresses traffic from all tails; the setting in a MultipointClient session suppresses traffic from a single tail.
可在前端BFD系统将其设置为零，以抑制来自尾部的流量。在多线程会话中将其设置为零会抑制来自所有尾部的流量；多点客户端会话中的设置会抑制来自单个尾部的流量。
bfd.DemandMode
需求模式
This variable MUST be initialized to 1 for session types MultipointClient.
对于会话类型MultipointClient，此变量必须初始化为1。
bfd.DetectMult
探测结果
For session type MultipointClient, this variable MUST always match the value of bfd.DetectMult in the associated MultipointHead session.
对于会话类型MultipointClient，此变量必须始终与关联的MultipointClient会话中的bfd.DetectMult值匹配。
The state variables defined above are used to choose which operational options are active.
上面定义的状态变量用于选择哪些操作选项处于活动状态。
The most basic form of the BFD operation in multipoint networks is explained in [RFC8562]. In this scenario, BFD Control packets flow only from the head, and no tracking of tail state at the head is desired. That can be accomplished by setting bfd.ReportTailDown to zero in the MultipointHead session (Section 5.1).
[RFC8562]解释了多点网络中BFD操作的最基本形式。在这种情况下，BFD只控制来自头部的数据包流，不需要跟踪头部的尾部状态。这可以通过在多线程会话中将bfd.ReportTailDown设置为零来实现（第5.1节）。
If the head wishes to know of active tails, it sends multipoint Polls as needed. Previously known tails that don't respond to the Polls will be detected (as per Section 5.2.2).
如果头部希望知道活动尾部，它会根据需要发送多点轮询。将检测到之前已知的不响应投票的尾部（根据第5.2.2节）。
If the head wishes to request a notification from the tails when they lose connectivity, it sets bfd.ReportTailDown to 1 in either the MultipointHead session (if such notification is desired from all tails) or the MultipointClient session (if notification is desired from a particular tail). Note that the setting of this variable in a MultipointClient session for a particular tail overrides the setting in the MultipointHead session.
如果头部希望在尾部失去连接时从尾部请求通知，则它会在MultiPointIn AD会话（如果需要从所有尾部发出通知）或MultipointClient会话（如果需要从特定尾部发出通知）中将bfd.ReportTailDown设置为1。请注意，在多点客户端会话中为特定尾部设置此变量将覆盖多点客户端会话中的设置。
If the head wishes to verify the state of a tail on an ongoing basis, it sends a Poll Sequence from the MultipointClient session associated with that tail as needed. This has the effect of eliminating the initial delay, as described in Section 6.13.3, that the tail would otherwise insert prior to transmission of the packet; thus, the head may have notification of the session failure more quickly when comparing with use of m-poll.
如果头部希望持续验证尾部的状态，则根据需要从与该尾部关联的多点客户端会话发送轮询序列。这具有消除初始延迟的效果，如第6.13.3节所述，在传输数据包之前，尾部将插入初始延迟；因此，与使用m-poll相比，头部可以更快地收到会话失败的通知。
If a tail wishes to operate silently (sending no BFD Control packets to the head), it sets bfd.SilentTail to 1 in the MultipointTail session. This allows a tail to be silent independent of the settings on the head.
如果尾部希望静默操作（不向头部发送BFD控制数据包），则在多点尾部会话中将BFD.SilentTail设置为1。这允许尾巴保持安静，而不受头部设置的影响。
Though the state transitions for the state machine, as defined in Section 5.5 of [RFC8562], for a session type MultipointHead are only administratively driven, the state machine for a session of type MultipointClient is the same, and the diagram is applicable.
尽管状态机的状态转换（如[RFC8562]第5.5节中所定义）对于会话类型MultiPointAD而言仅受管理驱动，但对于会话类型MultipointClient而言，状态机是相同的，并且该图适用。
If BFD Control packets are received at the head, they are demultiplexed to sessions of type MultipointClient, which represent the set of tails that the head is interested in tracking. These sessions will typically also be established dynamically based on the receipt of BFD Control packets. The head has broad latitude in choosing which tails to track, if any, without affecting the basic operation of the protocol. The head directly controls whether or not tails are allowed to send BFD Control packets back to the head by setting bfd.RequiredMinRxInterval to zero in a MultipointHead or a MultipointClient session.
如果在头部接收到BFD控制数据包，则将其解复用到MultipointClient类型的会话，该会话表示头部感兴趣跟踪的尾部集。这些会话通常也将根据BFD控制数据包的接收动态建立。头部在选择跟踪哪条尾巴（如果有的话）方面有很大的自由度，而不影响协议的基本操作。通过在MultiPointAD或MultipointClient会话中将BFD.RequiredMinRxInterval设置为零，头部直接控制是否允许尾部将BFD控制数据包发送回头部。
When the tails send BFD Control packets to the head from the MultipointTail session, the contents of Your Discriminator (the discriminator received from the head) will not be sufficient for the head to demultiplex the packet, since the same value will be received from all tails on the multicast tree. In this case, the head MUST demultiplex packets based on the source address and the value of Your Discriminator, which together uniquely identify the tail and the multipoint path.
当尾部从多点尾部会话向头部发送BFD控制数据包时，您的鉴别器（从头部接收的鉴别器）的内容将不足以使头部解复用数据包，因为将从多播树上的所有尾部接收相同的值。在这种情况下，头部必须基于源地址和鉴别器的值来解复用数据包，这两者共同唯一地标识尾部和多点路径。
When the head sends unicast BFD Control packets to a tail from a MultipointClient session, the value of Your Discriminator will be valid, and the tail MUST demultiplex the packet based solely on Your Discriminator.
当头部从多点客户端会话向尾部发送单播BFD控制数据包时，您的鉴别器的值将有效，尾部必须仅基于您的鉴别器对数据包进行解复用。
As the fan-in from the tails to the head may be very large, it is critical that the flow of BFD Control packets from the tails is controlled.
由于从尾部到头部的扇入可能非常大，因此控制来自尾部的BFD控制数据包的流是至关重要的。
The head always operates in Demand mode. This means that no tail will send an asynchronous BFD Control packet as long as the session is Up.
头部始终在需求模式下工作。这意味着，只要会话启动，tail就不会发送异步BFD控制数据包。
The value of Required Min Rx Interval received by a tail in a unicast BFD Control packet, if any, always takes precedence over the value received in multipoint BFD Control packets. This allows the packet rate from individual tails to be controlled separately as desired by sending a BFD Control packet from the corresponding MultipointClient session. This also eliminates the random delay, as discussed in Section 6.13.3, prior to transmission from the tail that would otherwise be inserted, reducing the latency of reporting a failure to the head.
单播BFD控制数据包中尾部接收到的所需最小接收间隔值（如有）始终优先于多点BFD控制数据包中接收到的值。这允许通过从相应的多点客户端会话发送BFD控制数据包，根据需要单独控制来自各个尾部的数据包速率。这也消除了随机延迟，如第6.13.3节所述，在从尾部传输之前（否则将插入），减少了向头部报告故障的延迟。
If the head wishes to suppress traffic from the tails when they detect a session failure, it MAY set bfd.RequiredMinRxInterval to zero, which is a reserved value that indicates that the sender wishes to receive no periodic traffic. This can be set in the MultipointHead session (suppressing traffic from all tails), or it can be set in a MultipointClient session (suppressing traffic from only a single tail).
如果头部希望在检测到会话失败时抑制来自尾部的通信量，则可以将bfd.RequiredMinRxInterval设置为零，这是一个保留值，表示发送方希望不接收定期通信量。这可以在多点客户端会话中设置（禁止来自所有尾部的流量），也可以在多点客户端会话中设置（仅禁止来自单个尾部的流量）。
Any tail may be provisioned to never send *any* BFD Control packets to the head by setting bfd.SilentTail to 1. This provides a mechanism by which only a subset of tails reports their session status to the head.
通过将BFD.silentail设置为1，可以将任何尾部设置为从不向头部发送*Any*BFD控制数据包。这提供了一种机制，通过该机制，只有尾部的子集向头部报告其会话状态。
If the head wishes to know of the active tails, the MultipointHead session can send a BFD Control packet as specified in Section 6.13.3, with the Poll (P) bit set to 1. This will cause all of the tails to reply with a unicast BFD Control Packet, randomized across one packet interval.
如果头部希望知道活动的尾部，则MultiPioneTad会话可以按照第6.13.3节的规定发送BFD控制数据包，轮询（P）位设置为1。这将导致所有尾部使用单播BFD控制数据包进行应答，该数据包在一个数据包间隔内随机化。
The decision as to when to send a multipoint Poll is outside the scope of this specification. However, it MUST NOT be sent more often than the regular multipoint BFD Control packet. Since the tail will treat a multipoint Poll like any other multipoint BFD Control packet, Polls may be sent in lieu of non-Poll packets.
关于何时发送多点轮询的决定超出了本规范的范围。但是，发送频率不得超过常规多点BFD控制数据包。由于尾部将像对待任何其他多点BFD控制数据包一样对待多点轮询，因此可以发送轮询来代替非轮询数据包。
Soliciting the tails also starts the Detection Timer for each of the associated MultipointClient sessions, which will cause those sessions to time out if the associated tails do not respond.
请求尾部还将启动每个相关联的多点客户端会话的检测计时器，如果相关联的尾部没有响应，这将导致这些会话超时。
Note that for this mechanism to work properly, the Detection Time (which is equal to bfd.DesiredMinTxInterval) MUST be greater than the round-trip time of BFD Control packets from the head to the tail (via the multipoint path) and back (via a unicast path). See Section 6.11 for more details.
请注意，为了使该机制正常工作，检测时间（等于bfd.DesiredIntxinterval）必须大于bfd控制数据包从头部到尾部（通过多点路径）以及从后部（通过单播路径）的往返时间。详见第6.11节。
If the head wishes to verify connectivity to a specific tail, the corresponding MultipointClient session can send a BFD Poll Sequence to said tail. This might be done in reaction to the expiration of the Detection Timer (the tail didn't respond to a multipoint Poll), or it might be done on a proactive basis.
如果头部希望验证与特定尾部的连接，则相应的多点客户端会话可以向所述尾部发送BFD轮询序列。这可以在检测计时器过期时进行（尾部没有响应多点轮询），也可以在主动的基础上进行。
The interval between transmitted packets in the Poll Sequence MUST be calculated as specified in the base BFD specification [RFC5880] (the greater of bfd.DesiredMinTxInterval and bfd.RemoteMinRxInterval).
轮询序列中传输的数据包之间的间隔必须按照基本BFD规范[RFC5880]中的规定进行计算（BFD.DesiredMinxinterval和BFD.RemoteMinRxInterval中的较大者）。
The value transmitted in Required Min RX Interval will be used by the tail (rather than the value received in any multipoint packet) when it transmits BFD Control packets to the head to notify it of a session failure, and the transmitted packets will not be delayed. This value can potentially be set much lower than in the multipoint case, in order to speed up a notification to the head, since the value will be used only by the single tail. This value (and the lack of delay) are "sticky", in that once the tail receives it, it will continue to use it indefinitely. Therefore, if the head no longer wishes to single out the tail, it SHOULD reset the timer to the default by sending a Poll Sequence with the same value of Required Min Rx Interval as is carried in the multipoint packets, or it MAY reset the tail session by sending a Poll Sequence with state AdminDown (after the completion of which the session will come back up).
当尾部向头部发送BFD控制数据包以通知其会话失败时，在要求的最小接收间隔内发送的值（而不是在任何多点数据包中接收的值）将被尾部使用，并且发送的数据包不会被延迟。该值可能会设置得比多点情况下低得多，以加快向头部的通知，因为该值将仅由单个尾部使用。这个值（以及没有延迟）是“粘性的”，因为一旦尾部接收到它，它将无限期地继续使用它。因此，如果头部不再希望单独选择尾部，则应通过发送与多点数据包中携带的所需最小Rx间隔值相同的轮询序列，将计时器重置为默认值，或者通过发送状态为AdminDown的轮询序列，重置尾部会话（完成后，会话将返回）。
Note that a failure of the head to receive a response to a Poll Sequence does not necessarily mean that the tail has lost multipoint connectivity, though a reply to a Poll Sequence does reliably indicate connectivity or lack thereof (by virtue of the tail's state not being Up in the BFD Control packet).
注意，头部未能接收到对轮询序列的响应并不一定意味着尾部已失去多点连接，尽管对轮询序列的响应确实可靠地指示连接或缺乏连接（由于尾部的状态在BFD控制分组中未处于上升状态）。
MultipointClient sessions at the head are always in the Demand mode, and as such only care about Detection Time in two cases. First, if a Poll Sequence is being sent on a MultipointClient session, the Detection Time on this session is calculated according to the base BFD specification [RFC5880], that is, the transmission interval multiplied by bfd.DetectMult. Second, when a multipoint Poll is sent to solicit tail replies, the Detection Time on all associated MultipointClient sessions that aren't currently sending Poll Sequences is set to a value greater than or equal to bfd.RequiredMinRxInterval (one packet time). This value can be made arbitrarily large in order to ensure that the Detection Time is greater than the round-trip time of a BFD Control packet between the head and the tail with no ill effects, other than delaying the detection of unresponsive tails. Note that a Detection Time expiration on a MultipointClient session at the head, while indicating a BFD session failure, cannot be construed to mean that the tail is not hearing multipoint packets from the head.
头部的多点客户端会话始终处于请求模式，因此在两种情况下只关心检测时间。首先，如果在多点客户端会话上发送轮询序列，则根据基本BFD规范[RFC5880]计算该会话上的检测时间，即传输间隔乘以BFD.DetectMult。其次，当发送多点轮询以请求尾部应答时，当前未发送轮询序列的所有关联多点客户端会话上的检测时间设置为大于或等于bfd.RequiredMinRxInterval（一个数据包时间）的值。该值可以任意大，以确保检测时间大于头部和尾部之间BFD控制数据包的往返时间，而不会产生不良影响，除了延迟检测无响应的尾部。注意，头部多点客户端会话上的检测时间到期，虽然指示BFD会话失败，但不能解释为尾部没有听到来自头部的多点数据包。
If the MultipointHead session is in Down/AdminDown state (which only happens administratively), all associated MultipointClient sessions SHOULD be destroyed as they are superfluous.
如果MultiPointAD会话处于关闭/管理关闭状态（仅以管理方式发生），则应销毁所有关联的MultipointClient会话，因为它们是多余的。
If a MultipointClient session goes down due to the receipt of an unsolicited BFD Control packet from the tail with state Down or AdminDown (not in response to a Poll), and tail connectivity verification is not being done, the session MAY be destroyed. If verification is desired, the session SHOULD send a Poll Sequence and the session SHOULD be maintained.
如果多点客户端会话由于从tail接收到未经请求的BFD控制数据包（状态为down或AdminDown，不响应轮询）而中断，并且tail连接验证未完成，则会话可能会被销毁。如果需要验证，会话应发送轮询序列，并应维护会话。
If the tail replies to a Poll Sequence with state Down or AdminDown, it means that the tail session is definitely down. In this case, the session MAY be destroyed.
如果tail以state Down或AdminDown回复轮询序列，则表示tail会话肯定已关闭。在这种情况下，会话可能会被破坏。
If the Detection Time expires on a MultipointClient session (meaning that the tail did not reply to a Poll Sequence), the session MAY be destroyed.
如果检测时间在多点客户端会话上过期（意味着尾部没有回复轮询序列），则会话可能会被销毁。
The following sections are meant to extend the corresponding sections in the base BFD for multipoint networks specification [RFC8562].
以下章节旨在扩展多点网络基本BFD规范[RFC8562]中的相应章节。
The following procedure modifies parts of Section 5.13.1 of [RFC8562].
以下程序修改了[RFC8562]第5.13.1节的部分内容。
When a BFD Control packet is received, the procedure defined in Section 5.13.1 of [RFC8562] MUST be followed, in the order specified. If the packet is discarded according to these rules, processing of the packet MUST cease at that point. In addition to that, if tail tracking is desired by the head, the following procedure MUST be applied.
当收到BFD控制数据包时，必须按照[RFC8562]第5.13.1节规定的顺序执行程序。如果根据这些规则丢弃数据包，则数据包的处理必须在该点停止。除此之外，如果头部需要尾部跟踪，则必须采用以下程序。
If bfd.SessionType is MultipointTail
如果bfd.SessionType为MultipointTail
If bfd.UnicastRcvd is zero or the Multipoint (M) bit is clear, set bfd.RemoteMinRxInterval to the value of Required Min RX Interval.
如果bfd.unicastcvd为零或多点（M）位清除，则将bfd.RemoteMinRxInterval设置为所需的最小接收间隔值。
If the Multipoint (M) bit is clear, set bfd.UnicastRcvd to 1.
如果多点（M）位清除，则将bfd.CVD设置为1。
Else (not MultipointTail)
Else（非多点尾）
Set bfd.RemoteMinRxInterval to the value of Required Min RX Interval.
将bfd.RemoteMinRxInterval设置为所需的最小接收间隔值。
If the Poll (P) bit is set, and bfd.SilentTail is zero, send a BFD Control packet to the remote system with the Poll (P) bit clear and the Final (F) bit set (see Section 6.13.3).
如果设置了轮询（P）位，且bfd.SilentTail为零，则向远程系统发送一个bfd控制数据包，清除轮询（P）位并设置最终（F）位（见第6.13.3节）。
This section is part of the addition to Section 5.13.2 of [RFC8562], separated for clarity.
本节是[RFC8562]第5.13.2节新增内容的一部分，为清晰起见，将其分开。
If the Multipoint (M) bit is clear
如果多点（M）位清除
If the Your Discriminator field is nonzero:
如果您的鉴别器字段为非零：
Select a session based on the value of Your Discriminator. If no session is found, the packet MUST be discarded.
根据鉴别器的值选择会话。如果找不到会话，则必须丢弃数据包。
If bfd.SessionType is MultipointHead:
如果bfd.SessionType为MultipointHead：
Find a MultipointClient session grouped to this MultipointHead session, based on the source address and the value of Your Discriminator. If a session is found and is not MultipointClient, the packet MUST be discarded. If no session is found, a new session of type MultipointClient MAY be created, or the packet MAY be discarded. This choice is outside the scope of this specification.
根据源地址和鉴别器的值，查找分组到此MultiPointIn AD会话的MultipointClient会话。如果找到一个会话且该会话不是多点客户端，则必须丢弃该数据包。如果未找到会话，则可能会创建MultipointClient类型的新会话，或者丢弃数据包。此选择不在本规范的范围内。
If bfd.SessionType is not MultipointClient, the packet MUST be discarded.
如果bfd.SessionType不是MultipointClient，则必须丢弃该数据包。
A system MUST NOT periodically transmit BFD Control packets if bfd.SessionType is MultipointClient and a Poll Sequence is not being transmitted.
如果BFD.SessionType为MultipointClient且未传输轮询序列，则系统不得定期传输BFD控制数据包。
If the bfd.SessionType value is MultipointTail and the periodic transmission of BFD Control packets is just starting (due to Demand mode not being active on the remote system), the first packet to be transmitted MUST be delayed by a random amount of time between zero and (0.9 * bfd.RemoteMinRxInterval).
如果bfd.SessionType值为MultipointTail，并且bfd控制数据包的定期传输刚刚开始（由于远程系统上的请求模式未处于活动状态），则要传输的第一个数据包必须延迟0到（0.9*bfd.RemoteMinRxInterval）之间的随机时间量。
If a BFD Control packet is received with the Poll (P) bit set to 1, the receiving system MUST transmit a BFD Control packet with the Poll (P) bit clear and the Final (F) bit, without respect to the transmission timer or any other transmission limitations, the session state, and whether Demand mode is active on either system. A system MAY limit the rate at which such packets are transmitted. If rate limiting is in effect, the advertised value of Desired Min TX Interval MUST be greater than or equal to the interval between transmitted packets imposed by the rate-limiting function. If the Multipoint (M) bit is set in the received packet, the packet transmission MUST be delayed by a random amount of time between zero and (0.9 * bfd.RemoteMinRxInterval). Otherwise, the packet MUST be transmitted as soon as practicable.
如果在轮询（P）位设置为1的情况下接收BFD控制数据包，则接收系统必须发送轮询（P）位清除且最终（F）位为的BFD控制数据包，而不考虑传输计时器或任何其他传输限制、会话状态以及任一系统上的请求模式是否处于活动状态。系统可限制此类分组的传输速率。如果速率限制生效，则所需最小TX间隔的公布值必须大于或等于速率限制功能施加的传输数据包之间的间隔。如果在接收的数据包中设置了多点（M）位，则数据包传输必须延迟0到（0.9*bfd.RemoteMinRxInterval）之间的随机时间量。否则，必须尽快发送数据包。
A system MUST NOT set the Demand (D) bit if bfd.SessionType is MultipointClient unless bfd.DemandMode is 1, bfd.SessionState is Up, and bfd.RemoteSessionState is Up.
如果bfd.SessionType为MultipointClient，则系统不得设置需求（D）位，除非bfd.DemandMode为1，bfd.SessionState为Up，bfd.RemoteSessionState为Up。
Content of the transmitted packet MUST be as explained in Section 5.13.3 of [RFC8562].
传输数据包的内容必须如[RFC8562]第5.13.3节所述。
If the head notification is to be used, it is assumed that a multipoint BFD packet encapsulation contains enough information so that a tail can address a unicast BFD packet to the head.
如果要使用头部通知，则假定多点BFD包封装包含足够的信息，以便尾部可以将单播BFD包寻址到头部。
If the head notification is to be used, it is assumed that there is bidirectional unicast communication available (at the same protocol layer within which BFD is being run) between the tail and head.
如果要使用头部通知，则假定尾部和头部之间存在可用的双向单播通信（在运行BFD的同一协议层）。
For the head to know reliably that a tail has lost multipoint connectivity, the unicast paths in both directions between that tail and the head must remain operational when the multipoint path fails. It is thus desirable that unicast paths not share fate with the multipoint path to the extent possible if the head wants more definite knowledge of the tail state.
为了让头部可靠地知道尾部已失去多点连接，当多点路径出现故障时，尾部和头部之间的双向单播路径必须保持工作状态。因此，如果头部想要更明确地了解尾部状态，则希望单播路径尽可能不与多点路径共享命运。
Since the normal BFD three-way handshake is not used in this application, a tail transitioning from state Up to Down and back to Up again may not be reliably detected at the head.
由于此应用中未使用正常的BFD三向握手，因此在头部可能无法可靠地检测到从状态向上到向下再从状态向上转换的尾部。
Section 7 of [RFC5880] includes the requirements for implementation of a congestion control mechanism when BFD is used across multiple hops and a mechanism that uses congestion detection to reduce the amount of BFD packets the system generates. These requirements are also applicable to this specification. When this specification is used in the mode with no head notifications by tails, as discussed in Section 5.1, the head MUST limit the packet transmission rate to no higher than one BFD packet per second (see Section 5.13.3 of [RFC8562]). When the BFD uses one of the notifications by the tails to the head mechanisms described in Section 5.2, Min RX Interval can be used by the tail to control the packet transmission rate of the head. The exact mechanism of processing changes in the Min RX Interval value in the received from the tail response to multicast or the unicast Poll BFD packet is outside the scope of this document.
[RFC5880]第7节包括在多个跃点之间使用BFD时实施拥塞控制机制的要求，以及使用拥塞检测减少系统生成的BFD数据包数量的机制。这些要求也适用于本规范。如第5.1节所述，当本规范用于无尾部头部通知的模式时，头部必须将数据包传输速率限制为不高于每秒一个BFD数据包（见[RFC8562]第5.13.3节）。当BFD使用第5.2节中描述的尾部到头部机制的通知之一时，尾部可以使用最小RX间隔来控制头部的数据包传输速率。处理从多播尾部响应或单播轮询BFD数据包接收的最小RX间隔值变化的确切机制不在本文档范围内。
As noted in Section 7 of [RFC5880], "any mechanism that increases the transmit or receive intervals will increase the Detection Time for the session".
如[RFC5880]第7节所述，“任何增加发送或接收间隔的机制都会增加会话的检测时间”。
This document has no IANA actions.
本文档没有IANA操作。
The same security considerations as those described in [RFC5880] and [RFC8562] apply to this document.
[RFC5880]和[RFC8562]中所述的安全注意事项同样适用于本文件。
Additionally, implementations that create MultpointClient sessions dynamically upon receipt of a BFD Control packet from a tail MUST implement protective measures to prevent a number of MultipointClient sessions from being created and growing out of control. Below are some points to be considered in such implementations.
此外，在从tail接收BFD控制数据包时动态创建多点客户端会话的实现必须实施保护措施，以防止多点客户端会话被创建和失控。下面是在这种实现中需要考虑的一些要点。
When the number of MultipointClient sessions exceeds the number of expected tails, the implementation should generate an alarm to users to indicate the anomaly.
当多点客户端会话数超过预期尾部数时，实现应向用户生成警报，以指示异常。
The implementation should have a reasonable upper bound on the number of MultipointClient sessions that can be created, with the upper bound potentially being computed based on the number of multicast streams that the system is expecting.
实现应该对可以创建的多点客户端会话的数量有一个合理的上限，该上限可能基于系统期望的多播流的数量进行计算。
This specification does not raise any additional security issues beyond those of the specifications referred to in the list of normative references.
除了规范性参考文献列表中提及的规范外，本规范不会提出任何其他安全问题。
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://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<https://www.rfc-editor.org/info/rfc2119>.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010, <https://www.rfc-editor.org/info/rfc5880>.
[RFC5880]Katz，D.和D.Ward，“双向转发检测（BFD）”，RFC 5880，DOI 10.17487/RFC5880，2010年6月<https://www.rfc-editor.org/info/rfc5880>.
[RFC7880] Pignataro, C., Ward, D., Akiya, N., Bhatia, M., and S. Pallagatti, "Seamless Bidirectional Forwarding Detection (S-BFD)", RFC 7880, DOI 10.17487/RFC7880, July 2016, <https://www.rfc-editor.org/info/rfc7880>.
[RFC7880]Pignataro，C.，Ward，D.，Akiya，N.，Bhatia，M.，和S.Pallagati，“无缝双向转发检测（S-BFD）”，RFC 7880，DOI 10.17487/RFC78802016年7月<https://www.rfc-editor.org/info/rfc7880>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8174]Leiba，B.，“RFC 2119关键词中大写与小写的歧义”，BCP 14，RFC 8174，DOI 10.17487/RFC8174，2017年5月<https://www.rfc-editor.org/info/rfc8174>.
[RFC8562] Katz, D., Ward, D., Pallagatti, S., Ed., and G. Mirsky, Ed., "Bidirectional Forwarding Detection (BFD) for Multipoint Networks", RFC 8562, DOI 10.17487/RFC8562, April 2019, <https://www.rfc-editor.org/info/rfc8562>.
[RFC8562]Katz，D.，Ward，D.，Pallagatti，S.，Ed.，和G.Mirsky，Ed.，“多点网络的双向转发检测（BFD）”，RFC 8562，DOI 10.17487/RFC8562，2019年4月<https://www.rfc-editor.org/info/rfc8562>.
Acknowledgments
致谢
The authors would like to thank Nobo Akiya, Vengada Prasad Govindan, Jeff Haas, Wim Henderickx, and Mingui Zhang who have greatly contributed to this document.
作者要感谢Nobo Akiya、Vengada Prasad Govindan、Jeff Haas、Wim Henderickx和Mingui Zhang，他们对本文件做出了巨大贡献。
Contributors
贡献者
Rahul Aggarwal of Juniper Networks and George Swallow of Cisco Systems provided the initial idea for this specification and contributed to its development.
Juniper Networks的Rahul Aggarwal和Cisco Systems的George Swallow为该规范提供了最初的想法，并为其开发做出了贡献。
Authors' Addresses
作者地址
Dave Katz Juniper Networks 1194 N. Mathilda Ave. Sunnyvale, California 94089-1206 United States of America
Dave Katz Juniper Networks美国加利福尼亚州桑尼维尔市马蒂尔达大道北1194号，邮编94089-1206
Email: dkatz@juniper.net
Email: dkatz@juniper.net
Dave Ward Cisco Systems 170 West Tasman Dr. San Jose, California 95134 United States of America
Dave Ward Cisco Systems 170 West Tasman Dr.San Jose，California 95134美利坚合众国
Email: wardd@cisco.com
Email: wardd@cisco.com
Santosh Pallagatti (editor) VMware
桑托什·帕拉加蒂（编辑）VMware
Email: santosh.pallagatti@gmail.com
Email: santosh.pallagatti@gmail.com
Greg Mirsky (editor) ZTE Corp.
格雷格·米尔斯基（编辑）中兴通讯公司。
Email: gregimirsky@gmail.com
Email: gregimirsky@gmail.com