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<pre>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
<span class="h1">Bidirectional Forwarding Detection (BFD) Multipoint Active Tails</span>
Abstract
This document describes active tail extensions to the Bidirectional
Forwarding Detection (BFD) protocol for multipoint networks.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in <a href="./rfc7841#section-2">Section 2 of RFC 7841</a>.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
<a href="https://www.rfc-editor.org/info/rfc8563">https://www.rfc-editor.org/info/rfc8563</a>.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
Provisions Relating to IETF Documents
(<a href="https://trustee.ietf.org/license-info">https://trustee.ietf.org/license-info</a>) 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.
<span class="grey">Katz, et al. Standards Track [Page 1]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-2" ></span>
<span class="grey"><a href="./rfc8563">RFC 8563</a> BFD Multipoint Active Tails April 2019</span>
Table of Contents
<a href="#section-1">1</a>. Introduction . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-3">3</a>
<a href="#section-2">2</a>. Terminology and Acronyms . . . . . . . . . . . . . . . . . . <a href="#page-3">3</a>
<a href="#section-3">3</a>. Keywords . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-3">3</a>
<a href="#section-4">4</a>. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-4">4</a>
<a href="#section-5">5</a>. Operational Scenarios . . . . . . . . . . . . . . . . . . . . <a href="#page-5">5</a>
<a href="#section-5.1">5.1</a>. No Head Notification . . . . . . . . . . . . . . . . . . <a href="#page-5">5</a>
<a href="#section-5.2">5.2</a>. Head Notification . . . . . . . . . . . . . . . . . . . . <a href="#page-5">5</a>
<a href="#section-5.2.1">5.2.1</a>. Head Notification without Polling . . . . . . . . . . <a href="#page-5">5</a>
5.2.2. Head Notification and Tail Solicitation with
Multipoint Polling . . . . . . . . . . . . . . . . . <a href="#page-6">6</a>
<a href="#section-5.2.3">5.2.3</a>. Head Notification with Composite Polling . . . . . . <a href="#page-6">6</a>
<a href="#section-6">6</a>. Protocol Details . . . . . . . . . . . . . . . . . . . . . . <a href="#page-7">7</a>
<a href="#section-6.1">6.1</a>. Multipoint Client Session . . . . . . . . . . . . . . . . <a href="#page-8">8</a>
<a href="#section-6.2">6.2</a>. Multipoint Client Session Failure . . . . . . . . . . . . <a href="#page-8">8</a>
<a href="#section-6.3">6.3</a>. State Variables . . . . . . . . . . . . . . . . . . . . . <a href="#page-8">8</a>
<a href="#section-6.3.1">6.3.1</a>. New State Variables . . . . . . . . . . . . . . . . . <a href="#page-8">8</a>
<a href="#section-6.3.2">6.3.2</a>. New State Variable Value . . . . . . . . . . . . . . <a href="#page-9">9</a>
<a href="#section-6.3.3">6.3.3</a>. State Variable Initialization and Maintenance . . . . <a href="#page-10">10</a>
<a href="#section-6.4">6.4</a>. Controlling Multipoint BFD Options . . . . . . . . . . . <a href="#page-11">11</a>
<a href="#section-6.5">6.5</a>. State Machine . . . . . . . . . . . . . . . . . . . . . . <a href="#page-11">11</a>
<a href="#section-6.6">6.6</a>. Session Establishment . . . . . . . . . . . . . . . . . . <a href="#page-12">12</a>
<a href="#section-6.7">6.7</a>. Discriminators and Packet Demultiplexing . . . . . . . . <a href="#page-12">12</a>
<a href="#section-6.8">6.8</a>. Controlling Tail Packet Transmission . . . . . . . . . . <a href="#page-12">12</a>
<a href="#section-6.9">6.9</a>. Soliciting the Tails . . . . . . . . . . . . . . . . . . <a href="#page-13">13</a>
<a href="#section-6.10">6.10</a>. Verifying Connectivity to Specific Tails . . . . . . . . <a href="#page-13">13</a>
<a href="#section-6.11">6.11</a>. Detection Times . . . . . . . . . . . . . . . . . . . . . <a href="#page-14">14</a>
<a href="#section-6.12">6.12</a>. MultipointClient Down/AdminDown Sessions . . . . . . . . <a href="#page-15">15</a>
6.13. Base BFD for Multipoint Networks Specification Text
Replacement . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-15">15</a>
<a href="#section-6.13.1">6.13.1</a>. Reception of BFD Control Packets . . . . . . . . . . <a href="#page-15">15</a>
<a href="#section-6.13.2">6.13.2</a>. Demultiplexing BFD Control Packets . . . . . . . . . <a href="#page-16">16</a>
<a href="#section-6.13.3">6.13.3</a>. Transmitting BFD Control Packets . . . . . . . . . . <a href="#page-16">16</a>
<a href="#section-7">7</a>. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-17">17</a>
<a href="#section-8">8</a>. Operational Considerations . . . . . . . . . . . . . . . . . <a href="#page-18">18</a>
<a href="#section-9">9</a>. IANA Considerations . . . . . . . . . . . . . . . . . . . . . <a href="#page-18">18</a>
<a href="#section-10">10</a>. Security Considerations . . . . . . . . . . . . . . . . . . . <a href="#page-18">18</a>
<a href="#section-11">11</a>. Normative References . . . . . . . . . . . . . . . . . . . . <a href="#page-19">19</a>
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-19">19</a>
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-19">19</a>
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-20">20</a>
<span class="grey">Katz, et al. Standards Track [Page 2]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-3" ></span>
<span class="grey"><a href="./rfc8563">RFC 8563</a> BFD Multipoint Active Tails April 2019</span>
<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Introduction</span>
This application of BFD is an extension to Multipoint BFD [<a href="./rfc8562" title=""Bidirectional Forwarding Detection (BFD) for Multipoint Networks"">RFC8562</a>],
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.
The goal of this application is for the head to have reasonably rapid
knowledge of tails that have lost connectivity from the head.
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.
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.
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.
This document effectively modifies and adds to Sections <a href="#section-5.12">5.12</a> and <a href="#section-5.13">5.13</a>
of the base BFD multipoint networks specification [<a href="./rfc8562" title=""Bidirectional Forwarding Detection (BFD) for Multipoint Networks"">RFC8562</a>].
<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. Terminology and Acronyms</span>
BFD: Bidirectional Forwarding Detection
c-poll: Composite Poll
m-poll: Multipoint Poll
<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. Keywords</span>
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
<a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a> [<a href="./rfc2119" title=""Key words for use in RFCs to Indicate Requirement Levels"">RFC2119</a>] [<a href="./rfc8174" title=""Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words"">RFC8174</a>] when, and only when, they appear in all
capitals, as shown here.
<span class="grey">Katz, et al. Standards Track [Page 3]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-4" ></span>
<span class="grey"><a href="./rfc8563">RFC 8563</a> BFD Multipoint Active Tails April 2019</span>
<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. Overview</span>
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 <a href="#section-5.2.1">Section 5.2.1</a>, the tails can send unsolicited unicast
BFD Control packets to the head when the path fails, as described in
<a href="#section-6.4">Section 6.4</a>.
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
<a href="#section-5.2.2">Section 5.2.2</a>). 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).
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 <a href="#section-5.2.3">Section 5.2.3</a>). 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).
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.
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.
<span class="grey">Katz, et al. Standards Track [Page 4]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-5" ></span>
<span class="grey"><a href="./rfc8563">RFC 8563</a> BFD Multipoint Active Tails April 2019</span>
<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. Operational Scenarios</span>
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 <a href="#section-6.3">Section 6.3</a>).
<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>. No Head Notification</span>
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 [<a href="./rfc8562" title=""Bidirectional Forwarding Detection (BFD) for Multipoint Networks"">RFC8562</a>]. In this mode, bfd.SessionType is
MultipointTail, and the variable bfd.SilentTail (see <a href="#section-6.3.1">Section 6.3.1</a>)
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.
<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>. Head Notification</span>
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:
o if bfd.SessionType is MultipointTail, the variable bfd.SilentTail
(see <a href="#section-6.3.1">Section 6.3.1</a>) MUST be set to zero;
o if bfd.SessionType is MultipointHead or MultipointClient,
bfd.ReportTailDown MUST be set to 1;
o the head MUST set bfd.RequiredMinRxInterval to nonzero and thus
allow tails to send BFD Control packets.
<span class="h4"><a class="selflink" id="section-5.2.1" href="#section-5.2.1">5.2.1</a>. Head Notification without Polling</span>
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.
<span class="grey">Katz, et al. Standards Track [Page 5]</span></pre>
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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).
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.
<span class="h4"><a class="selflink" id="section-5.2.2" href="#section-5.2.2">5.2.2</a>. Head Notification and Tail Solicitation with Multipoint Polling</span>
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.
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).
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.
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.
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.
<span class="h4"><a class="selflink" id="section-5.2.3" href="#section-5.2.3">5.2.3</a>. Head Notification with Composite Polling</span>
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 <a href="#section-6.3.1">Section 6.3.1</a>) MUST be set to zero.
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
<span class="grey">Katz, et al. Standards Track [Page 6]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-7" ></span>
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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).
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.
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).
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.
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.
<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>. Protocol Details</span>
This section describes the operation of the BFD Multipoint active
tail in detail. This section modifies <a href="./rfc8562#section-4">Section 4 of [RFC8562]</a> as
follows:
o <a href="#section-6.3">Section 6.3</a> introduces new state variables and modifies the usage
of a few existing ones;
o <a href="#section-6.13">Section 6.13</a> replaces the corresponding sections in the base BFD
for multipoint networks specification.
<span class="grey">Katz, et al. Standards Track [Page 7]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-8" ></span>
<span class="grey"><a href="./rfc8563">RFC 8563</a> BFD Multipoint Active Tails April 2019</span>
<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>. Multipoint Client Session</span>
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.
<span class="h3"><a class="selflink" id="section-6.2" href="#section-6.2">6.2</a>. Multipoint Client Session Failure</span>
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.
<span class="h3"><a class="selflink" id="section-6.3" href="#section-6.3">6.3</a>. State Variables</span>
BFD Multipoint active tail introduces new state variables and
modifies the usage of a few existing ones defined in <a href="./rfc8562#section-5.4">Section 5.4 of
[RFC8562]</a>.
<span class="h4"><a class="selflink" id="section-6.3.1" href="#section-6.3.1">6.3.1</a>. New State Variables</span>
A few state variables are added in support of multipoint BFD active
tail.
bfd.SilentTail
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.
<span class="grey">Katz, et al. Standards Track [Page 8]</span></pre>
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This variable is only pertinent when bfd.SessionType is
MultipointTail and SHOULD NOT be modified after the
MultipointTail session has been created.
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.
This variable is only pertinent when bfd.SessionType is
MultipointHead or MultipointClient.
bfd.UnicastRcvd
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.
This variable MUST be initialized to zero.
This variable is only pertinent when bfd.SessionType is
MultipointTail.
<span class="h4"><a class="selflink" id="section-6.3.2" href="#section-6.3.2">6.3.2</a>. New State Variable Value</span>
A new state variable value being added to:
bfd.SessionType
The type of this session as defined in [<a href="./rfc7880" title=""Seamless Bidirectional Forwarding Detection (S-BFD)"">RFC7880</a>]. A new value
introduced is:
MultipointClient: A session on the head that tracks the state
of an individual tail, when desirable.
This variable MUST be initialized to the appropriate type when the
session is created, according to the rules in <a href="./rfc8562#section-5.4">Section 5.4 of
[RFC8562]</a>.
<span class="grey">Katz, et al. Standards Track [Page 9]</span></pre>
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<span class="h4"><a class="selflink" id="section-6.3.3" href="#section-6.3.3">6.3.3</a>. State Variable Initialization and Maintenance</span>
Some state variables defined in <a href="./rfc5880#section-6.8.1">Section 6.8.1 of [RFC5880]</a> 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 <a href="./rfc8562#section-5.4.2">Section 5.4.2 of
[RFC8562]</a>).
bfd.LocalDiscr
For session type MultipointClient, this variable MUST always
match the value of bfd.LocalDiscr in the associated
MultipointHead session.
bfd.DesiredMinTxInterval
For session type MultipointClient, this variable MUST always
match the value of bfd.DesiredMinTxInterval in the associated
MultipointHead session.
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.
bfd.DemandMode
This variable MUST be initialized to 1 for session types
MultipointClient.
bfd.DetectMult
For session type MultipointClient, this variable MUST always
match the value of bfd.DetectMult in the associated
MultipointHead session.
<span class="grey">Katz, et al. Standards Track [Page 10]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-11" ></span>
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<span class="h3"><a class="selflink" id="section-6.4" href="#section-6.4">6.4</a>. Controlling Multipoint BFD Options</span>
The state variables defined above are used to choose which
operational options are active.
The most basic form of the BFD operation in multipoint networks is
explained in [<a href="./rfc8562" title=""Bidirectional Forwarding Detection (BFD) for Multipoint Networks"">RFC8562</a>]. 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 (<a href="#section-5.1">Section 5.1</a>).
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 <a href="#section-5.2.2">Section 5.2.2</a>).
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.
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 <a href="#section-6.13.3">Section 6.13.3</a>, 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.
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.
<span class="h3"><a class="selflink" id="section-6.5" href="#section-6.5">6.5</a>. State Machine</span>
Though the state transitions for the state machine, as defined in
<a href="./rfc8562#section-5.5">Section 5.5 of [RFC8562]</a>, 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.
<span class="grey">Katz, et al. Standards Track [Page 11]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-12" ></span>
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<span class="h3"><a class="selflink" id="section-6.6" href="#section-6.6">6.6</a>. Session Establishment</span>
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.
<span class="h3"><a class="selflink" id="section-6.7" href="#section-6.7">6.7</a>. Discriminators and Packet Demultiplexing</span>
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.
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.
<span class="h3"><a class="selflink" id="section-6.8" href="#section-6.8">6.8</a>. Controlling Tail Packet Transmission</span>
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.
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.
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
<a href="#section-6.13.3">Section 6.13.3</a>, prior to transmission from the tail that would
otherwise be inserted, reducing the latency of reporting a failure to
the head.
<span class="grey">Katz, et al. Standards Track [Page 12]</span></pre>
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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).
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.
<span class="h3"><a class="selflink" id="section-6.9" href="#section-6.9">6.9</a>. Soliciting the Tails</span>
If the head wishes to know of the active tails, the MultipointHead
session can send a BFD Control packet as specified in <a href="#section-6.13.3">Section 6.13.3</a>,
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.
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.
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.
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 <a href="#section-6.11">Section 6.11</a>
for more details.
<span class="h3"><a class="selflink" id="section-6.10" href="#section-6.10">6.10</a>. Verifying Connectivity to Specific Tails</span>
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.
The interval between transmitted packets in the Poll Sequence MUST be
calculated as specified in the base BFD specification [<a href="./rfc5880" title=""Bidirectional Forwarding Detection (BFD)"">RFC5880</a>] (the
greater of bfd.DesiredMinTxInterval and bfd.RemoteMinRxInterval).
<span class="grey">Katz, et al. Standards Track [Page 13]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-14" ></span>
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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).
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).
<span class="h3"><a class="selflink" id="section-6.11" href="#section-6.11">6.11</a>. Detection Times</span>
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 [<a href="./rfc5880" title=""Bidirectional Forwarding Detection (BFD)"">RFC5880</a>], 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.
<span class="grey">Katz, et al. Standards Track [Page 14]</span></pre>
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<span class="h3"><a class="selflink" id="section-6.12" href="#section-6.12">6.12</a>. MultipointClient Down/AdminDown Sessions</span>
If the MultipointHead session is in Down/AdminDown state (which only
happens administratively), all associated MultipointClient sessions
SHOULD be destroyed as they are superfluous.
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.
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.
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.
<span class="h3"><a class="selflink" id="section-6.13" href="#section-6.13">6.13</a>. Base BFD for Multipoint Networks Specification Text Replacement</span>
The following sections are meant to extend the corresponding sections
in the base BFD for multipoint networks specification [<a href="./rfc8562" title=""Bidirectional Forwarding Detection (BFD) for Multipoint Networks"">RFC8562</a>].
<span class="h4"><a class="selflink" id="section-6.13.1" href="#section-6.13.1">6.13.1</a>. Reception of BFD Control Packets</span>
The following procedure modifies parts of <a href="./rfc8562#section-5.13.1">Section 5.13.1 of
[RFC8562]</a>.
When a BFD Control packet is received, the procedure defined in
<a href="./rfc8562#section-5.13.1">Section 5.13.1 of [RFC8562]</a> 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.
If bfd.SessionType is MultipointTail
If bfd.UnicastRcvd is zero or the Multipoint (M) bit is clear,
set bfd.RemoteMinRxInterval to the value of Required Min RX
Interval.
If the Multipoint (M) bit is clear, set bfd.UnicastRcvd to 1.
<span class="grey">Katz, et al. Standards Track [Page 15]</span></pre>
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<span class="grey"><a href="./rfc8563">RFC 8563</a> BFD Multipoint Active Tails April 2019</span>
Else (not MultipointTail)
Set bfd.RemoteMinRxInterval to the value of Required Min RX
Interval.
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 <a href="#section-6.13.3">Section 6.13.3</a>).
<span class="h4"><a class="selflink" id="section-6.13.2" href="#section-6.13.2">6.13.2</a>. Demultiplexing BFD Control Packets</span>
This section is part of the addition to <a href="./rfc8562#section-5.13.2">Section 5.13.2 of [RFC8562]</a>,
separated for clarity.
If the Multipoint (M) bit is clear
If the Your Discriminator field is nonzero:
Select a session based on the value of Your Discriminator.
If no session is found, the packet MUST be discarded.
If bfd.SessionType is 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.
If bfd.SessionType is not MultipointClient, the packet
MUST be discarded.
<span class="h4"><a class="selflink" id="section-6.13.3" href="#section-6.13.3">6.13.3</a>. Transmitting BFD Control Packets</span>
A system MUST NOT periodically transmit BFD Control packets if
bfd.SessionType is MultipointClient and a Poll Sequence is not being
transmitted.
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).
<span class="grey">Katz, et al. Standards Track [Page 16]</span></pre>
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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.
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.
Content of the transmitted packet MUST be as explained in
<a href="./rfc8562#section-5.13.3">Section 5.13.3 of [RFC8562]</a>.
<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>. Assumptions</span>
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.
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.
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.
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.
<span class="grey">Katz, et al. Standards Track [Page 17]</span></pre>
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<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>. Operational Considerations</span>
<a href="./rfc5880#section-7">Section 7 of [RFC5880]</a> 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
<a href="#section-5.1">Section 5.1</a>, the head MUST limit the packet transmission rate to no
higher than one BFD packet per second (see <a href="./rfc8562#section-5.13.3">Section 5.13.3 of
[RFC8562]</a>). When the BFD uses one of the notifications by the tails
to the head mechanisms described in <a href="#section-5.2">Section 5.2</a>, 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.
As noted in <a href="./rfc5880#section-7">Section 7 of [RFC5880]</a>, "any mechanism that increases the
transmit or receive intervals will increase the Detection Time for
the session".
<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>. IANA Considerations</span>
This document has no IANA actions.
<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>. Security Considerations</span>
The same security considerations as those described in [<a href="./rfc5880" title=""Bidirectional Forwarding Detection (BFD)"">RFC5880</a>] and
[<a href="./rfc8562" title=""Bidirectional Forwarding Detection (BFD) for Multipoint Networks"">RFC8562</a>] apply to this document.
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.
When the number of MultipointClient sessions exceeds the number of
expected tails, the implementation should generate an alarm to
users to indicate the anomaly.
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.
This specification does not raise any additional security issues
beyond those of the specifications referred to in the list of
normative references.
<span class="grey">Katz, et al. Standards Track [Page 18]</span></pre>
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<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>. Normative References</span>
[<a id="ref-RFC2119">RFC2119</a>] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>,
DOI 10.17487/RFC2119, March 1997,
<<a href="https://www.rfc-editor.org/info/rfc2119">https://www.rfc-editor.org/info/rfc2119</a>>.
[<a id="ref-RFC5880">RFC5880</a>] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", <a href="./rfc5880">RFC 5880</a>, DOI 10.17487/RFC5880, June 2010,
<<a href="https://www.rfc-editor.org/info/rfc5880">https://www.rfc-editor.org/info/rfc5880</a>>.
[<a id="ref-RFC7880">RFC7880</a>] Pignataro, C., Ward, D., Akiya, N., Bhatia, M., and
S. Pallagatti, "Seamless Bidirectional Forwarding
Detection (S-BFD)", <a href="./rfc7880">RFC 7880</a>, DOI 10.17487/RFC7880, July
2016, <<a href="https://www.rfc-editor.org/info/rfc7880">https://www.rfc-editor.org/info/rfc7880</a>>.
[<a id="ref-RFC8174">RFC8174</a>] Leiba, B., "Ambiguity of Uppercase vs Lowercase in <a href="./rfc2119">RFC</a>
<a href="./rfc2119">2119</a> Key Words", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc8174">RFC 8174</a>, DOI 10.17487/RFC8174,
May 2017, <<a href="https://www.rfc-editor.org/info/rfc8174">https://www.rfc-editor.org/info/rfc8174</a>>.
[<a id="ref-RFC8562">RFC8562</a>] Katz, D., Ward, D., Pallagatti, S., Ed., and G. Mirsky,
Ed., "Bidirectional Forwarding Detection (BFD) for
Multipoint Networks", <a href="./rfc8562">RFC 8562</a>, DOI 10.17487/RFC8562,
April 2019, <<a href="https://www.rfc-editor.org/info/rfc8562">https://www.rfc-editor.org/info/rfc8562</a>>.
Acknowledgments
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.
Contributors
Rahul Aggarwal of Juniper Networks and George Swallow of Cisco
Systems provided the initial idea for this specification and
contributed to its development.
<span class="grey">Katz, et al. Standards Track [Page 19]</span></pre>
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Authors' Addresses
Dave Katz
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, California 94089-1206
United States of America
Email: dkatz@juniper.net
Dave Ward
Cisco Systems
170 West Tasman Dr.
San Jose, California 95134
United States of America
Email: wardd@cisco.com
Santosh Pallagatti (editor)
VMware
Email: santosh.pallagatti@gmail.com
Greg Mirsky (editor)
ZTE Corp.
Email: gregimirsky@gmail.com
Katz, et al. Standards Track [Page 20]
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