draft-ietf-bfd-multipoint-04.txt   draft-ietf-bfd-multipoint-05.txt 
Internet Engineering Task Force D. Katz Internet Engineering Task Force D. Katz
Internet-Draft Juniper Networks Internet-Draft Juniper Networks
Intended status: Standards Track D. Ward Intended status: Standards Track D. Ward
Expires: February 13, 2015 Cisco Systems Expires: July 16, 2015 Cisco Systems
S. Pallagatti, Ed. S. Pallagatti, Ed.
Juniper Networks Juniper Networks
August 12, 2014 January 12, 2015
BFD for Multipoint Networks BFD for Multipoint Networks
draft-ietf-bfd-multipoint-04 draft-ietf-bfd-multipoint-05
Abstract Abstract
This document describes extensions to the Bidirectional Forwarding This document describes extensions to the Bidirectional Forwarding
Detection (BFD) protocol for its use in multipoint and multicast Detection (BFD) protocol for its use in multipoint and multicast
networks. Comments on this draft should be directed to rtg- networks. Comments on this draft should be directed to rtg-
bfd@ietf.org. bfd@ietf.org.
Requirements Language Requirements Language
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 13, 2015. This Internet-Draft will expire on July 16, 2015.
Copyright Notice Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Protocol Details . . . . . . . . . . . . . . . . . . . . . . 5 4. Protocol Details . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Multipoint BFD Control Packets . . . . . . . . . . . . . 5 4.1. Multipoint BFD Control Packets . . . . . . . . . . . . . 4
4.2. Session Model . . . . . . . . . . . . . . . . . . . . . . 6 4.2. Session Model . . . . . . . . . . . . . . . . . . . . . . 4
4.3. Session Failure Semantics . . . . . . . . . . . . . . . . 6 4.3. Session Failure Semantics . . . . . . . . . . . . . . . . 5
4.4. State Variables . . . . . . . . . . . . . . . . . . . . . 7 4.4. State Variables . . . . . . . . . . . . . . . . . . . . . 5
4.4.1. New State Variables . . . . . . . . . . . . . . . . . 7 4.4.1. New State Variables . . . . . . . . . . . . . . . . . 5
4.4.2. State Variable Initialization and Maintenance . . . . 8 4.4.2. State Variable Initialization and Maintenance . . . . 6
4.5. Controlling Multipoint BFD Options . . . . . . . . . . . 9 4.5. State Machine . . . . . . . . . . . . . . . . . . . . . . 6
4.6. State Machine . . . . . . . . . . . . . . . . . . . . . . 10 4.6. Session Establishment . . . . . . . . . . . . . . . . . . 7
4.7. Session Establishment . . . . . . . . . . . . . . . . . . 10 4.7. Discriminators and Packet Demultiplexing . . . . . . . . 7
4.8. Discriminators and Packet Demultiplexing . . . . . . . . 11 4.8. Packet consumption on tails . . . . . . . . . . . . . . . 7
4.9. Controlling Tail Packet Transmission . . . . . . . . . . 11 4.9. Bringing Up and Shutting Down Multipoint BFD Service . . 8
4.10. Bringing Up and Shutting Down Multipoint BFD Service . . 12 4.10. Timer Manipulation . . . . . . . . . . . . . . . . . . . 8
4.11. Soliciting the Tails . . . . . . . . . . . . . . . . . . 13 4.11. Detection Times . . . . . . . . . . . . . . . . . . . . . 9
4.12. Verifying Connectivity to Specific Tails . . . . . . . . 13 4.12. State Maintenance for Down/AdminDown Sessions . . . . . . 9
4.13. Timer Manipulation . . . . . . . . . . . . . . . . . . . 14 4.12.1. MultipointHead Sessions . . . . . . . . . . . . . . 9
4.14. Detection Times . . . . . . . . . . . . . . . . . . . . . 14 4.12.2. MultipointTail Sessions . . . . . . . . . . . . . . 9
4.15. State Maintenance for Down/AdminDown Sessions . . . . . . 15 4.13. Base Specification Text Replacement . . . . . . . . . . . 9
4.15.1. MultipointHead Sessions . . . . . . . . . . . . . . 15 4.13.1. Reception of BFD Control Packets . . . . . . . . . . 10
4.15.2. MultipointTail Sessions . . . . . . . . . . . . . . 15 4.13.2. Demultiplexing BFD Control Packets . . . . . . . . . 12
4.15.3. MultipointClient Sessions . . . . . . . . . . . . . 16 4.13.3. Transmitting BFD Control Packets . . . . . . . . . . 13
4.16. Base Specification Text Replacement . . . . . . . . . . . 16 5. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.16.1. Reception of BFD Control Packets . . . . . . . . . . 16 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
4.16.2. Demultiplexing BFD Control Packets . . . . . . . . . 19 7. Security Considerations . . . . . . . . . . . . . . . . . . . 16
4.16.3. Transmitting BFD Control Packets . . . . . . . . . . 20 8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 17
5. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 24 9. Normative References . . . . . . . . . . . . . . . . . . . . 17
6. Operational Scenarios . . . . . . . . . . . . . . . . . . . . 24 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
6.1. No Head Notification . . . . . . . . . . . . . . . . . . 24
6.2. Unreliable Head Notification . . . . . . . . . . . . . . 24
6.3. Semi-reliable Head Notification and Tail Solicitation . . 25
6.4. Reliable Head Notification . . . . . . . . . . . . . . . 25
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
8. Security Considerations . . . . . . . . . . . . . . . . . . . 26
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 26
10. Normative References . . . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
1. Introduction 1. Introduction
The Bidirectional Forwarding Detection protocol [RFC5880] specifies a The Bidirectional Forwarding Detection protocol [RFC5880] specifies a
method for verifying unicast connectivity between a pair of systems. method for verifying unicast connectivity between a pair of systems.
This document defines a method for using BFD to provide verification This document defines a method for using BFD to provide verification
of multipoint or multicast connectivity between a multipoint sender of multipoint or multicast connectivity between a multipoint sender
(the "head") and a set of one or more multipoint receivers (the (the "head") and a set of one or more multipoint receivers (the
"tails"). "tails").
As multipoint transmissions are inherently unidirectional, this As multipoint transmissions are inherently unidirectional, this
mechanism purports only to verify this unidirectional connectivity. mechanism purports only to verify this unidirectional connectivity.
Although this seems in conflict with the "Bidirectional" in BFD, it Although this seems in conflict with the "Bidirectional" in BFD, it
is a natural fit for that protocol. is a natural fit for that protocol.
This application of BFD allows for the tails to detect a lack of This application of BFD allows for the tails to detect a lack of
connectivity from the head. As an option, the tail may unreliably connectivity from the head. Due to unidirectional nature, virtually
notify the head of the lack of multipoint connectivity. As a further all options and timing parameters are controlled by the head.
option, this notification can be made reliable. Notification to the
head can be enabled for all tails, or for only a subset of the tails.
Multipoint BFD verifies only the head-to-tail connectivity over the
multipoint path. Although it may use unicast paths in both
directions, Multipoint BFD does not verify those paths (and in fact
it is preferable if unicast paths share as little fate with the
multipoint path as is feasible.)
Virtually all options and timing parameters are controlled by the As an option, the tail may notify the head of the lack of multipoint
head. This is particularly important if head notifications are connectivity. Details of tail notification to head are outside the
enabled, since there are obvious scaling concerns in that case. scope of this document.
Throughout this document, the term "multipoint" is defined as a Throughout this document, the term "multipoint" is defined as a
mechanism by which one or more systems receive packets sent by a mechanism by which one or more systems receive packets sent by a
single sender. This specifically includes such things as IP single sender. This specifically includes such things as IP
multicast and point-to-multipoint MPLS. multicast and point-to-multipoint MPLS.
This document effectively modifies and adds to the base BFD This document effectively modifies and adds to the base BFD
specification. It is the intention of the authors to fold these specification. It is the intention of the authors to fold these
extensions into the base specification at the appropriate time. extensions into the base specification at the appropriate time.
2. Goals 2. Goals
The primary goal of this mechanism is to allow tails to rapidly The primary goal of this mechanism is to allow tails to rapidly
detect the fact that multipoint connectivity from the head has detect the fact that multipoint connectivity from the head has
failed. An optional goal is for the head to reasonably rapidly have failed.
knowledge of tails that have lost connectivity from the head.
Since scaling is a primary concern (particularly state implosion
toward the head), it is a goal 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.
Another goal is for the mechanism to work on any multicast or Another goal is for the mechanism to work on any multicast or
multipoint medium. multipoint medium.
A further goal is to support multiple, overlapping multipoint paths, A further goal is to support multiple, overlapping multipoint paths,
as well as multipoint paths with multiple heads, and to allow point- as well as multipoint paths with multiple heads, and to allow point-
to-point BFD sessions to operate simultaneously among the systems to-point BFD sessions to operate simultaneously among the systems
participating in Multipoint BFD. participating in Multipoint BFD.
A final goal is to integrate multipoint operation into the base A final goal is to integrate multipoint operation into the base
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3. Overview 3. Overview
The heart of this protocol is the periodic transmission of BFD The heart of this protocol is the periodic transmission of BFD
Control packets along a multipoint path, from the head to all tails Control packets along a multipoint path, from the head to all tails
on the tree. The contents of the BFD packets provide the means for on the tree. The contents of the BFD packets provide the means for
the tails to calculate the detection time for path failure. If no the tails to calculate the detection time for path failure. If no
BFD Control packets are received by a tail for a detection time, the BFD Control packets are received by a tail for a detection time, the
tail declares the path to have failed. For some applications this is tail declares the path to have failed. For some applications this is
the only mechanism necessary; the head can remain ignorant of the the only mechanism necessary; the head can remain ignorant of the
tails. In this mode, the tails never send any BFD traffic to the tails.
head.
If the head wishes to be alerted to the tails' connectivity (or lack
thereof), there are a number of options. First, if all that is
needed is an unreliable failure notification, the head can direct the
tails to transmit unicast BFD Control packets back to the head when
the path fails.
If the head wishes to know the identity of the tails on the
multipoint path, it may solicit membership by sending 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. 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 somewhat reliable mechanism for detecting which tails
fail to respond (implying a loss of multipoint connectivity.)
If the head wishes a reliable 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 the tail. This covers the case where
either the multipoint poll or the single reply thereto is lost in
transit. If desired, the head may Poll one or more tails proactively
to track the tails' connectivity.
If some tails are more equal than others, 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 Head may wish to be alerted to the tails' connectivity (or lack
control packets to the head, even when the head wishes notification thereof). There may be a number of options, details of which are
of path failure from the tail. Such tails will never be known to the outside scope of this document.
head, but will still be able to detect multipoint path failures from
the head.
Although this document describes a single head and a set of tails Although this document describes a single head and a set of tails
spanned by a single multipoint path, the protocol is capable of spanned by a single multipoint path, the protocol is capable of
supporting (and discriminating between) more than one multipoint path supporting (and discriminating between) more than one multipoint path
at both heads and tails. Furthermore, the same head and tail may at both heads and tails. Furthermore, the same head and tail may
share multiple multipoint paths, and a multipoint path may have share multiple multipoint paths, and a multipoint path may have
multiple heads. multiple heads.
4. Protocol Details 4. Protocol Details
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The elements of procedure differ slightly for each type. The elements of procedure differ slightly for each type.
Point-to-point sessions, as described in [BFD], are of type Point-to-point sessions, as described in [BFD], are of type
PointToPoint. PointToPoint.
The head has a session of type MultipointHead that is bound to a The head has a session of type MultipointHead that is bound to a
multipoint path. Multipoint BFD Control packets are sent by this multipoint path. Multipoint BFD Control packets are sent by this
session over the multipoint path, and no BFD Control packets are session over the multipoint path, and no BFD Control packets are
received by it. received by it.
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 particular
multipoint path are grouped with the MultipointHead session to which
the clients are listening. A BFD Poll Sequence may be sent over such
a session to a tail if the head wishes to verify connectivity. These
sessions receive any BFD Control packets sent by the tails, and never
transmit periodic BFD Control packets other than Poll Sequences
(since periodic transmission is always done by the MultipointHead
session.)
Each tail has a session of type MultipointTail associated with a Each tail has a session of type MultipointTail associated with a
multipoint path. These sessions receive BFD Control packets from the multipoint path. These sessions receive BFD Control packets from the
head, both as multipoint packets (the MultipointHead session) and as head over multipoint path.
unicast packets (the MultipointClient session, if it exists.) Any
BFD Control packets sent to the head are sent over this session.
4.3. Session Failure Semantics 4.3. Session Failure Semantics
The semantics of session failure are subtle enough to warrant further The semantics of session failure are subtle enough to warrant further
explanation. explanation.
MultipointHead sessions cannot fail (since they are controlled MultipointHead sessions cannot fail (since they are controlled
administratively.) administratively.)
If a MultipointTail session fails, it means that the tail definitely If a MultipointTail session fails, it means that the tail definitely
has lost contact with the head (or the head has been administratively has lost contact with the head (or the head has been administratively
disabled) and the tail should take appropriate action. disabled) and the tail should take appropriate action.
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 state.
4.4. State Variables 4.4. State Variables
Multipoint BFD introduces some new state variables, and modifies the Multipoint BFD introduces some new state variables, and modifies the
usage of a few existing ones. usage of a few existing ones.
4.4.1. New State Variables 4.4.1. New State Variables
A number of state variables are added to the base specification in A number of state variables are added to the base specification in
support of Multipoint BFD. support of Multipoint BFD.
bfd.SessionType bfd.SessionType
The type of this session. Allowable values are: The type of this session. Allowable values are:
PointToPoint: Classic point-to-point BFD. PointToPoint: Classic point-to-point BFD.
MultipointHead: A session on the head responsible for the MultipointHead: A session on the head responsible for the
periodic transmission of multipoint BFD Control packets periodic transmission of multipoint BFD Control packets
along the multipoint path. along the multipoint path.
MultipointClient: A session on the head that tracks the
state of an individual tail, when desirable.
MultipointTail: A multipoint session on a tail. MultipointTail: A multipoint session on a tail.
This variable MUST be initialized to the appropriate type when This variable MUST be initialized to the appropriate type when
the session is created, according to the rules in section 4.16. the session is created, according to the rules in section 4.16.
bfd.SilentTail bfd.SilentTail
Always set to 1, a tail will never transmit any BFD Control
If 1, a tail will never transmit any BFD Control packets to the packets to the head under any circumstances. Setting to 0 is
head under any circumstances. If 0, a tail may send packets to outside the scope of this document.
the head according to other parts of this specification. This
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.
This variable is only pertinent when bfd.SessionType is
MultipointTail.
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 0, 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.
This variable is only pertinent when bfd.SessionType is This variable is only pertinent when bfd.SessionType is
MultipointHead or MultipointClient.
bfd.UnicastRcvd
Set to 1 if a tail receives a unicast BFD Control packet from
the head. 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. MultipointTail.
4.4.2. State Variable Initialization and Maintenance 4.4.2. State Variable Initialization and Maintenance
Some state variables defined in section 6.8.1 of the BFD base Some state variables defined in section 6.8.1 of the BFD base
specification need to be initialized or manipulated differently specification need to be initialized or manipulated differently
depending on the session type. depending on the session type.
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 bfd.RequiredMinRxInterval
This variable MUST always be 0 for session type MultipointHead This variable MUST be set to 0 for session type MultipointHead.
if bfd.ReportTailDown is 0.
It should be noted that for sessions of type MultipointTail,
this variable only affects the rate of unicast Polls sent by
the head; the rate of multipoint packets is necessarily
unaffected by it.
bfd.DemandMode bfd.DemandMode
This variable MUST be initialized to 1 for session types This variable MUST be initialized to 1 for session type
MultipointHead and MultipointClient, and MUST be initialized to MultipointHead and MUST be initialized to 0 for session type
0 for session type MultipointTail. MultipointTail.
bfd.DetectMult
For session type MultipointClient, this variable MUST always
match the value of bfd.DetectMult in the associated
MultipointHead session.
4.5. Controlling Multipoint BFD Options
The state variables defined above are used to choose which
operational options are active.
The most basic form of operation, in which BFD Control packets flow
only from the head and no tracking is desired of tail state at the
head, is accomplished by setting bfd.ReportTailDown to 0 in the
MultipointHead session.
If the head wishes to know the identity of the tails, it sends
multipoint Polls as needed. Previously known tails that don't
respond to the Polls will be detected.
If the head wishes to be notified by 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 in 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.
If the head wants to more quickly be alerted to a session failure
from a particular tail, it sends a BFD Control packet from the
MultipointClient session associated with that tail. This has the
effect of eliminating the initial delay that the tail would otherwise
insert prior to transmission of the packet.
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.
4.6. State Machine 4.5. State Machine
The BFD state machine works slightly differently in the multipoint The BFD state machine works slightly differently in the multipoint
application. In particular, since there is a many-to-one mapping, application. In particular, since there is a many-to-one mapping,
three-way handshakes for session establishment and teardown are three-way handshakes for session establishment and teardown are
neither possible nor appropriate. As such there is no Init state. neither possible nor appropriate. As such there is no Init state.
The following diagram provides an overview of the state machine for The following diagram provides an overview of the state machine for
session types MultipointClient and MultipointTail. The notation on session type MultipointTail. The notation on each arc represents the
each arc represents the state of the remote system (as received in state of the remote system (as received in the State field in the BFD
the State field in the BFD Control packet) or indicates the Control packet) or indicates the expiration of the Detection Timer.
expiration of the Detection Timer.
DOWN, ADMIN DOWN, DOWN, ADMIN DOWN,
+------+ TIMER +------+ +------+ TIMER +------+
+----| |<---------------------| |----+ +----| |<---------------------| |----+
DOWN,| | DOWN | | UP | |UP DOWN,| | DOWN | | UP | |UP
ADMIN DOWN,+--->| |--------------------->| |<---+ ADMIN DOWN,+--->| |--------------------->| |<---+
TIMER +------+ UP +------+ TIMER +------+ UP +------+
Sessions of type MultipointHead never receive packets and have no Sessions of type MultipointHead never receive packets and have no
Detection Timer, and as such all state transitions are Detection Timer, and as such all state transitions are
administratively driven. administratively driven.
4.7. Session Establishment 4.6. Session Establishment
Unlike Point-to-point BFD, Multipoint BFD provides a form of Unlike Point-to-point BFD, Multipoint BFD provides a form of
discovery mechanism for tails to discover the head, and vice versa. discovery mechanism for tails to discover the head. The minimum
The minimum amount of a priori information required both on the head amount of a priori information required both on the head and tails is
and tails is the binding to the multipoint path over which BFD is the binding to the multipoint path over which BFD is running. The
running. The head transmits Multipoint BFD packets on that tree, and head transmits Multipoint BFD packets on that tree, and the tails
the tails listen for BFD packets on that tree. All other information listen for BFD packets on that tree. All other information MAY be
MAY be determined dynamically. determined dynamically.
A session of type MultipointHead is created for each multipoint path A session of type MultipointHead is created for each multipoint path
over which the head wishes to run BFD. This session runs in the over which the head wishes to run BFD. This session runs in the
Active role. Except when terminating BFD service, this session is Active role. Except when terminating BFD service, this session is
always in state Up and always operates in Demand mode. No received always in state Up and always operates in Demand mode. No received
packets are ever demultiplexed to the MultipointHead session. In packets are ever demultiplexed to the MultipointHead session. In
this sense it is a degenerate form of a session. this sense it is a degenerate form of a session.
Sessions on the tail MAY be established dynamically, based on the Sessions on the tail MAY be established dynamically, based on the
receipt of a Multipoint BFD Control packet from the head, and are of receipt of a Multipoint BFD Control packet from the head, and are of
type MultipointTail. Tail sessions always take the Passive role. type MultipointTail. Tail sessions always take the Passive role.
If BFD Control packets are received at the head, they are 4.7. Discriminators and Packet Demultiplexing
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.
4.8. Discriminators and Packet Demultiplexing
The use of Discriminators is somewhat different in Multipoint BFD The use of Discriminators is somewhat different in Multipoint BFD
than in Point-to-point BFD. than in Point-to-point BFD.
The head sends Multipoint BFD Control packets over the MultipointHead The head sends Multipoint BFD Control packets over the MultipointHead
session with My Discr set to a value bound to the multipoint path, session with My Discr set to a value bound to the multipoint path,
and with Your Discr set to zero. The tails MUST demultiplex these and with Your Discr set to zero.
packets based on a combination of the source address and My Discr,
which together uniquely identify the head and the multipoint path.
When the tails send BFD Control packets to the head from the
MultipointTail session, the contents of Your Discr (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
Discr, which together uniquely identify the tail and the multipoint
path.
When the head sends unicast BFD Control packets to a tail from a IP and MPLS multipoint tails MUST demultiplex BFD packets based on a
MultipointClient session, the value of Your Discr will be valid, and combination of the source address, My Discriminator and the identity
the tail MUST demultiplex the packet based solely on Your Discr. of the multipoint tree which the Multipoint BFD Control packet was
received from. Together they uniquely identify the head of the
multipoint path. Bootstrapping BFD session to a multipoint LSP in
case of penultimate hop popping is outside the scope of this
document.
Note that, unlike PointToPoint sessions, the discriminator values on Note that, unlike PointToPoint sessions, the discriminator values on
all multipoint session types MUST NOT be changed during the life of a all multipoint session types MUST NOT be changed during the life of a
session. This is a side effect of the more complex demultiplexing session. This is a side effect of the more complex demultiplexing
scheme. scheme.
4.9. Controlling Tail Packet Transmission 4.8. Packet consumption on tails
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 prior to transmission
from the tail that would otherwise be inserted, reducing the latency
of reporting a failure to the head.
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 Tail MUST consume packet with destination UDP port number "3784" on
to the head by setting bfd.SilentTail to 1. This provides a IP multipoint path. For multipoint LSP, tail MUST use destination
mechanism by which only a subset of tails report their session status UDP port "3784" and IP "127.0.0.0/8" range.
to the head.
4.10. Bringing Up and Shutting Down Multipoint BFD Service 4.9. Bringing Up and Shutting Down Multipoint BFD Service
Because there is no three-way handshake in Multipoint BFD, a newly Because there is no three-way handshake in Multipoint BFD, a newly
started head (that does not have any previous state information started head (that does not have any previous state information
available) SHOULD start with bfd.SessionState set to Down and with available) SHOULD start with bfd.SessionState set to Down and with
bfd.RequiredMinRxInterval set to zero in the MultipointHead session. bfd.RequiredMinRxInterval set to zero in the MultipointHead session.
The session SHOULD remain in this state for a time equal to The session SHOULD remain in this state for a time equal to
(bfd.DesiredMinTxInterval * bfd.DetectMult). This will ensure that (bfd.DesiredMinTxInterval * bfd.DetectMult). This will ensure that
all MultipointTail sessions are reset (so long as the restarted head all MultipointTail sessions are reset (so long as the restarted head
is using the same or larger value of bfd.DesiredMinTxInterval than it is using the same or larger value of bfd.DesiredMinTxInterval than it
did previously.) did previously.)
skipping to change at page 12, line 52 skipping to change at page 8, line 30
A head may wish to shut down its BFD service in a controlled fashion. A head may wish to shut down its BFD service in a controlled fashion.
This is desirable because the tails need not wait a detection time This is desirable because the tails need not wait a detection time
prior to declaring the multipoint session to be down (and taking prior to declaring the multipoint session to be down (and taking
whatever action is necessary in that case.) whatever action is necessary in that case.)
To shut down a multipoint session a head MUST administratively set To shut down a multipoint session a head MUST administratively set
bfd.SessionState in the MultipointHead session to either Down or bfd.SessionState in the MultipointHead session to either Down or
AdminDown and SHOULD set bfd.RequiredMinRxInterval to zero (to keep AdminDown and SHOULD set bfd.RequiredMinRxInterval to zero (to keep
the tails from sending any BFD Control packets back.) The session the tails from sending any BFD Control packets back.) The session
SHOULD send BFD Control packets in this state for a period equal to SHOULD send BFD Control packets in this state for a period equal to
(bfd.DesiredMinTxInterval * bfd.DetectMult). The tail SHOULD destroy (bfd.DesiredMinTxInterval * bfd.DetectMult).
all MultipointClient sessions associated with the MultipointHead
session.
The semantic difference between Down and AdminDown state is for The semantic difference between Down and AdminDown state is for
further discussion. further discussion.
4.11. Soliciting the Tails 4.10. Timer Manipulation
If the head wishes to know the identities of the tails, the
MultipointHead session MAY send a BFD Control packet as specified in
section 4.16.3, 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.
The decision as to when to send a multipoint Poll is outside the
scope of this specification. However, it must never 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
associated MultipointClient session, 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 section 4.14
for more details.
4.12. Verifying Connectivity to Specific Tails
If the head wishes to verify connectivity to a specific tail, the
corresponding MultipointClient session MAY 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 specification (the greater of
bfd.DesiredMinTxInterval and bfd.RemoteMinRxInterval.)
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 notifying 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 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.)
4.13. Timer Manipulation
Because of the one-to-many mapping, a session of type MultipointHead Because of the one-to-many mapping, a session of type MultipointHead
SHOULD NOT initiate a Poll Sequence in conjunction with timer value SHOULD NOT initiate a Poll Sequence in conjunction with timer value
changes. As such, such a session cannot wait for a Final before changes. However to indicate change in packet MultipointHead session
increasing the transmit interval; such a session SHOULD send MUST send packet with P bit set. MultipointTail session MUST NOT
bfd.DetectMult packets at the old transmit interval before using the reply if packet has M, P bit set and bfd.RequiredMinRxInterval set to
higher value in order to avoid false detection timeouts at the tails. 0.
Since MultipointHead sessions do not calculate detection times, the The MultipointHead MUST send bfd.DetectMult packets with P bit set at
value of bfd.RequiredMinRxInterval may be changed at any time. the old transmit interval before using the higher value in order to
avoid false detection timeouts at the tails. MultipointHead May also
wait some amount of time before making the changes to the transmit
interval (through configuration).
4.14. Detection Times Change in the value of bfd.RequiredMinRxInterval is outside the scope
of this document.
4.11. Detection Times
Multipoint BFD is inherently asymmetric. As such, each session type Multipoint BFD is inherently asymmetric. As such, each session type
has a different approach to detection times. has a different approach to detection times.
Since the MultipointHead session never receives packets, it does not Since the MultipointHead session never receives packets, it does not
calculate a detection time. calculate a detection time.
MultipointClient sessions at the head are always in 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
specification, 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 BFD round trip time
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.
MultipointTail sessions cannot influence the transmission rate of the MultipointTail sessions cannot influence the transmission rate of the
MultipointHead session using the Required Min Rx Interval field MultipointHead session using the Required Min Rx Interval field
because of its one-to-many nature. As such, the Detection Time because of its one-to-many nature. As such, the Detection Time
calculation for a MultipointTail session does not use calculation for a MultipointTail session does not use
bfd.RequiredMinRxInterval in the calculation. The detection time is bfd.RequiredMinRxInterval in the calculation. The detection time is
calculated as the product of the last received values of Desired Min calculated as the product of the last received values of Desired Min
TX Interval and Detect Mult. TX Interval and Detect Mult.
The value of bfd.DetectMult may be changed at any time on any session The value of bfd.DetectMult may be changed at any time on any session
type. type.
4.15. State Maintenance for Down/AdminDown Sessions 4.12. State Maintenance for Down/AdminDown Sessions
The length of time session state is kept after the session goes down The length of time session state is kept after the session goes down
determines how long the session will continue to send BFD Control determines how long the session will continue to send BFD Control
packets (since no packets can be sent after the session is packets (since no packets can be sent after the session is
destroyed.) destroyed.)
4.15.1. MultipointHead Sessions 4.12.1. MultipointHead Sessions
When a MultipointHead session transitions to states Down or When a MultipointHead session transitions to states Down or
AdminDown, the state SHOULD be maintained for a period equal to AdminDown, the state SHOULD be maintained for a period equal to
(bfd.DesiredMinTxInterval * bfd.DetectMult) to ensure that the tails (bfd.DesiredMinTxInterval * bfd.DetectMult) to ensure that the tails
more quickly detect the session going down (by continuing to transmit more quickly detect the session going down (by continuing to transmit
BFD Control packets with the new state.) BFD Control packets with the new state.)
4.15.2. MultipointTail Sessions 4.12.2. MultipointTail Sessions
If bfd.SilentTail is 1, or bfd.RemoteMinRxInterval is zero,
MultipointTail sessions MAY be destroyed immediately upon leaving Up MultipointTail sessions MAY be destroyed immediately upon leaving Up
state, since they will transmit no further packets. state, since tail will transmit no packets.
Otherwise, MultipointTail sessions MUST be maintained as long as BFD Otherwise, MultipointTail sessions MUST be maintained as long as BFD
Control packets are being received by it (which by definition will Control packets are being received by it (which by definition will
indicate that the head is not Up.) indicate that the head is not Up.)
MultipointTail sessions MUST be maintained after a Detection Time 4.13. Base Specification Text Replacement
expiration for at least the longer of an additional Detection Time
and the transmission of the first (delayed) BFD Control packet to the
head. The state MAY be maintained longer than this, but the session
MUST NOT transmit periodic BFD Control packets for a period longer
than the negotiated transmit interval multiplied by bfd.DetectMult;
after this time either the session MUST be destroyed or
bfd.RemoteMinRxInterval MUST be set to zero to suppress packet
transmission.
4.15.3. MultipointClient Sessions
If the MultipointHead session is going down (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.
4.16. Base Specification Text Replacement
The following sections are meant to replace the corresponding The following sections are meant to replace the corresponding
sections in the base specification. sections in the base specification.
4.16.1. Reception of BFD Control Packets 4.13.1. Reception of BFD Control Packets
The following procedure replaces section 6.8.6 of [RFC5880]. The following procedure replaces section 6.8.6 of [RFC5880].
When a BFD Control packet is received, the following procedure MUST When a BFD Control packet is received, the following procedure MUST
be followed, in the order specified. If the packet is discarded be followed, in the order specified. If the packet is discarded
according to these rules, processing of the packet MUST cease at that according to these rules, processing of the packet MUST cease at that
point. point.
If the version number is not correct (1), the packet MUST be If the version number is not correct (1), the packet MUST be
discarded. discarded.
skipping to change at page 17, line 30 skipping to change at page 10, line 49
If the A bit is set, the packet MUST be authenticated under the If the A bit is set, the packet MUST be authenticated under the
rules of section 6.7, based on the authentication type in use rules of section 6.7, based on the authentication type in use
(bfd.AuthType.) This may cause the packet to be discarded. (bfd.AuthType.) This may cause the packet to be discarded.
Set bfd.RemoteDiscr to the value of My Discriminator. Set bfd.RemoteDiscr to the value of My Discriminator.
Set bfd.RemoteState to the value of the State (Sta) field. Set bfd.RemoteState to the value of the State (Sta) field.
Set bfd.RemoteDemandMode to the value of the Demand (D) bit. Set bfd.RemoteDemandMode to the value of the Demand (D) bit.
If bfd.SessionType is MultipointTail Set bfd.RemoteMinRxInterval to the value of Required Min RX
Interval.
If bfd.UnicastRcvd is 0 or the M bit is clear, set
bfd.RemoteMinRxInterval to the value of Required Min RX
Interval.
If the M bit is clear, set bfd.UnicastRcvd to 1.
Else (not MultipointTail)
Set bfd.RemoteMinRxInterval to the value of Required Min RX
Interval.
If the Required Min Echo RX Interval field is zero, the If the Required Min Echo RX Interval field is zero, the
transmission of Echo packets, if any, MUST cease. transmission of Echo packets, if any, MUST cease.
If a Poll Sequence is being transmitted by the local system and If a Poll Sequence is being transmitted by the local system and
the Final (F) bit in the received packet is set, the Poll Sequence the Final (F) bit in the received packet is set, the Poll Sequence
MUST be terminated. MUST be terminated.
If bfd.SessionType is PointToPoint or MultipointClient, update the If bfd.SessionType is PointToPoint, update the transmit interval
transmit interval as described in [BFD] section 6.8.2. as described in [BFD] section 6.8.2.
If bfd.SessionType is PointToPoint, update the Detection Time as If bfd.SessionType is PointToPoint, update the Detection Time as
described in [BFD] section 6.8.4. Otherwise, update the Detection described in [BFD] section 6.8.4. Otherwise, update the Detection
Time as described in section 4.14 above. Time as described in section 4.14 above.
If bfd.SessionState is AdminDown If bfd.SessionState is AdminDown
Discard the packet Discard the packet
If received state is AdminDown If received state is AdminDown
skipping to change at page 19, line 19 skipping to change at page 12, line 29
If bfd.RemoteDemandMode is 1, bfd.SessionState is Up, and If bfd.RemoteDemandMode is 1, bfd.SessionState is Up, and
bfd.RemoteSessionState is Up, Demand mode is active on the remote bfd.RemoteSessionState is Up, Demand mode is active on the remote
system and the local system MUST cease the periodic transmission system and the local system MUST cease the periodic transmission
of BFD Control packets (see section 4.16.3.) of BFD Control packets (see section 4.16.3.)
If bfd.RemoteDemandMode is 0, or bfd.SessionState is not Up, or If bfd.RemoteDemandMode is 0, or bfd.SessionState is not Up, or
bfd.RemoteSessionState is not Up, Demand mode is not active on the bfd.RemoteSessionState is not Up, Demand mode is not active on the
remote system and the local system MUST send periodic BFD Control remote system and the local system MUST send periodic BFD Control
packets (see section 4.16.3.) packets (see section 4.16.3.)
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 4.16.3.)
If the packet was not discarded, it has been received for purposes If the packet was not discarded, it has been received for purposes
of the Detection Time expiration rules in [BFD] section 6.8.4. of the Detection Time expiration rules in [BFD] section 6.8.4.
4.16.2. Demultiplexing BFD Control Packets 4.13.2. Demultiplexing BFD Control Packets
This section is part of the replacement for [RFC5880] section 6.8.6, This section is part of the replacement for [RFC5880] section 6.8.6,
separated for clarity. separated for clarity.
If the Multipoint (M) bit is set If the Multipoint (M) bit is set
If the Your Discriminator field is nonzero, the packet MUST be If the Your Discriminator field is nonzero, the packet MUST be
discarded. discarded.
Select a session based on the source address and the My Select a session based on the source address and the My
skipping to change at page 19, line 46 skipping to change at page 13, line 4
Select a session based on the source address and the My Select a session based on the source address and the My
Discriminator field. If a session is found, and Discriminator field. If a session is found, and
bfd.SessionType is not MultipointTail, the packet MUST be bfd.SessionType is not MultipointTail, the packet MUST be
discarded. If a session is not found, a new session of type discarded. If a session is not found, a new session of type
MultipointTail MAY be created, or the packet MAY be discarded. MultipointTail MAY be created, or the packet MAY be discarded.
This choice is outside the scope of this specification. This choice is outside the scope of this specification.
Else (Multipoint bit is clear) Else (Multipoint bit is clear)
If the Your Discriminator field is nonzero If the Your Discriminator field is nonzero
Select a session based on the value of Your Discriminator. Select a session based on the value of Your Discriminator.
If no session is found, the packet MUST be discarded. If no session is found, the packet MUST be discarded.
If bfd.SessionType is MulticastHead
Select a session based on the source address and the
value of Your Discriminator. 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 MulticastClient, the packet
MUST be discarded.
Else (Your Discriminator is zero) Else (Your Discriminator is zero)
If the State field is not Down or AdminDown, the packet MUST If the State field is not Down or AdminDown, the packet MUST
be discarded. be discarded.
Otherwise, the session MUST be selected based on some Otherwise, the session MUST be selected based on some
combination of other fields, possibly including source combination of other fields, possibly including source
addressing information, the My Discriminator field, and the addressing information, the My Discriminator field, and the
interface over which the packet was received. The exact interface over which the packet was received. The exact
method of selection is application-specific and is thus method of selection is application-specific and is thus
skipping to change at page 20, line 35 skipping to change at page 13, line 29
If a matching session is found, and bfd.SessionType is not If a matching session is found, and bfd.SessionType is not
PointToPoint, the packet MUST be discarded. PointToPoint, the packet MUST be discarded.
If a matching session is not found, a new session of type If a matching session is not found, a new session of type
PointToPoint may be created, or the packet may be discarded. PointToPoint may be created, or the packet may be discarded.
This choice is outside the scope of this specification. This choice is outside the scope of this specification.
If the State field is Init and bfd.SessionType is not If the State field is Init and bfd.SessionType is not
PointToPoint, the packet MUST be discarded. PointToPoint, the packet MUST be discarded.
4.16.3. Transmitting BFD Control Packets 4.13.3. Transmitting BFD Control Packets
The following procedure replaces section 6.8.7 of [RFC5880]. The following procedure replaces section 6.8.7 of [RFC5880].
BFD Control packets MUST be transmitted periodically at the rate BFD Control packets MUST be transmitted periodically at the rate
determined according to [BFD] section 6.8.2, except as specified in determined according to [BFD] section 6.8.2, except as specified in
this section. this section.
A system MUST NOT transmit any BFD Control packets if bfd.RemoteDiscr A system MUST NOT transmit any BFD Control packets if bfd.RemoteDiscr
is zero and the system is taking the Passive role. is zero and the system is taking the Passive role.
skipping to change at page 21, line 10 skipping to change at page 14, line 5
is 1. is 1.
A system MUST NOT periodically transmit BFD Control packets if Demand A system MUST NOT periodically transmit BFD Control packets if Demand
mode is active on the remote system (bfd.RemoteDemandMode is 1, mode is active on the remote system (bfd.RemoteDemandMode is 1,
bfd.SessionState is Up, and bfd.RemoteSessionState is Up) and a Poll bfd.SessionState is Up, and bfd.RemoteSessionState is Up) and a Poll
Sequence is not being transmitted. Sequence is not being transmitted.
A system MUST NOT periodically transmit BFD Control packets if A system MUST NOT periodically transmit BFD Control packets if
bfd.RemoteMinRxInterval is zero. bfd.RemoteMinRxInterval is zero.
A system MUST NOT periodically transmit BFD Control packets if
bfd.SessionType is MulticastClient and a Poll Sequence is not being
transmitted.
If bfd.SessionType is MultipointHead, the transmit interval MUST be If bfd.SessionType is MultipointHead, the transmit interval MUST be
set to bfd.DesiredMinTxInterval (this should happen automatically, as set to bfd.DesiredMinTxInterval (this should happen automatically, as
bfd.RemoteMinRxInterval will be zero.) bfd.RemoteMinRxInterval will be zero.)
If bfd.SessionType is not MultipointHead, the transmit interval MUST If bfd.SessionType is not MultipointHead, the transmit interval MUST
be recalculated whenever bfd.DesiredMinTxInterval changes, or be recalculated whenever bfd.DesiredMinTxInterval changes, or
whenever bfd.RemoteMinRxInterval changes, and is equal to the greater whenever bfd.RemoteMinRxInterval changes, and is equal to the greater
of those two values. See [BFD] sections 6.8.2 and 6.8.3 for details of those two values. See [BFD] sections 6.8.2 and 6.8.3 for details
on transmit timers. on transmit timers.
If bfd.SessionType is MulticastTail and 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).
If a BFD Control packet is received with the Poll (P) bit set to 1, 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 the receiving system MUST transmit a BFD Control packet with the Poll
(P) bit clear and the Final (F) bit, without respect to the (P) bit clear and the Final (F) bit, without respect to the
transmission timer or any other transmission limitations, without transmission timer or any other transmission limitations, without
respect to the session state, and without respect to whether Demand respect to the session state, and without respect to whether Demand
mode is active on either system. A system MAY limit the rate at mode is active on either system. A system MAY limit the rate at
which such packets are transmitted. If rate limiting is in effect, which such packets are transmitted. If rate limiting is in effect,
the advertised value of Desired Min TX Interval MUST be greater than the advertised value of Desired Min TX Interval MUST be greater than
or equal to the interval between transmitted packets imposed by the or equal to the interval between transmitted packets imposed by the
rate limiting function. If the Multipoint (M) bit is set in the rate limiting function. If the Multipoint (M) bit is set in the
received packet, the packet transmission MUST be delayed by a random received packet, the packet transmission MUST be delayed by a random
amount of time between zero and (0.9 * bfd.RemoteMinRxInterval). amount of time between zero and (0.9 * bfd.RemoteMinRxInterval).
Otherwise, the packet MUST be transmitted as soon as practicable. Otherwise, the packet MUST be transmitted as soon as practicable.
A system MUST NOT set the Demand (D) bit if bfd.SessionType is A system MUST NOT set the Demand (D) bit if bfd.SessionType is
MultipointTail. MultipointTail.
A system MUST NOT set the Demand (D) bit if bfd.SessionType is A system MUST NOT set the Demand (D) bit if bfd.SessionType
MultipointClient or PointToPoint unless bfd.DemandMode is 1, PointToPoint unless bfd.DemandMode is 1, bfd.SessionState is Up, and
bfd.SessionState is Up, and bfd.RemoteSessionState is Up. bfd.RemoteSessionState is Up.
If bfd.SessionType is PointToPoint or MultipointHead, a BFD Control If bfd.SessionType is PointToPoint or MultipointHead, a BFD Control
packet SHOULD be transmitted during the interval between periodic packet SHOULD be transmitted during the interval between periodic
Control packet transmissions when the contents of that packet would Control packet transmissions when the contents of that packet would
differ from that in the previously transmitted packet (other than the differ from that in the previously transmitted packet (other than the
Poll and Final bits) in order to more rapidly communicate a change in Poll and Final bits) in order to more rapidly communicate a change in
state. state.
The contents of transmitted BFD Control packets MUST be set as The contents of transmitted BFD Control packets MUST be set as
follows: follows:
skipping to change at page 23, line 4 skipping to change at page 15, line 32
Set to 1 if the local system's BFD implementation is Set to 1 if the local system's BFD implementation is
independent of the control plane (it can continue to function independent of the control plane (it can continue to function
through a disruption of the control plane.) through a disruption of the control plane.)
Authentication Present (A) Authentication Present (A)
Set to 1 if authentication is in use on this session Set to 1 if authentication is in use on this session
(bfd.AuthType is nonzero), or 0 if not. (bfd.AuthType is nonzero), or 0 if not.
Demand (D) Demand (D)
Set to bfd.DemandMode if bfd.SessionState is Up and Set to bfd.DemandMode if bfd.SessionState is Up and
bfd.RemoteSessionState is Up. Set to 1 if bfd.SessionType is bfd.RemoteSessionState is Up. Set to 1 if bfd.SessionType is
MultipointHead or MultipointClient. Otherwise it is set to 0. MultipointHead. Otherwise it is set to 0.
Multipoint (M) Multipoint (M)
Set to 1 if bfd.SessionType is MultipointHead. Otherwise it is Set to 1 if bfd.SessionType is MultipointHead. Otherwise it is
set to 0. set to 0.
Detect Mult Detect Mult
Set to bfd.DetectMult. Set to bfd.DetectMult.
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Desired Min TX Interval Desired Min TX Interval
Set to bfd.DesiredMinTxInterval. Set to bfd.DesiredMinTxInterval.
Required Min RX Interval Required Min RX Interval
Set to bfd.RequiredMinRxInterval. Set to bfd.RequiredMinRxInterval.
Required Min Echo RX Interval Required Min Echo RX Interval
Set to the minimum required Echo packet receive interval for Set to 0 if bfd.SessionType is MultipointHead or
this session. If this field is set to zero, the local system MultipointTail.
is unwilling or unable to loop back BFD Echo packets to the
remote system, and the remote system will not send Echo
packets.
Authentication Section Authentication Section
Included and set according to the rules in section 6.7 if Included and set according to the rules in section 6.7 if
authentication is in use (bfd.AuthType is nonzero.) Otherwise authentication is in use (bfd.AuthType is nonzero.) Otherwise
this section is not present. this section is not present.
5. Assumptions 5. Assumptions
If 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 head notification is to be used, it is assumed that is 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 reliable
knowledge of 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.
If authentication is in use, all tails must be configured to have a If authentication is in use, all tails must be configured to have a
common authentication key in order to receive the multipoint BFD common authentication key in order to receive the multipoint BFD
Control packets. Control packets.
6. Operational Scenarios 6. IANA Considerations
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 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.
6.1. No Head Notification
Since the only path used in this scenario is the multipoint path,
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.
6.2. Unreliable Head Notification
In this scenario, the tail sends back unsolicicted BFD packets in
response to the detection of a multipoint path failure. It 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 (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.
6.3. Semi-reliable Head Notification and Tail Solicitation
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.
6.4. Reliable Head Notification
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.
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
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, This document has no actions for IANA.
the head will see the BFD 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 7. Security Considerations
the forward unicast path fails, neither side will notice so long as a
unicast Poll Sequence is never sent by the head. If the head sends a
unicast Poll Sequence, 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.
7. IANA Considerations Implementations that creates MultpointTail sessions dynamically upon
receipt of Multipoint BFD Control packets MUST implement protective
measures to prevent infinite number of MultipointTail session being
created. Below lists some points to be considered in such
implementations.
This document has no actions for IANA. If a Multipoint BFD Control packet did not arrive on a multicast
tree (ex: on expected interface, with expected MPLS label, etc),
then a MultipointTail session should not be created.
8. Security Considerations If redundant streams are expected for a given multicast stream,
then the implementations should not create more MultipointTail
sessions than the number of streams. Additionally, when the
number of MultipointTail sessions exceeds the number of expected
streams, then the implementation should generate an alarm to users
to indicate the anomaly.
This specification does not raise any additional security issues The implementation should have a reasonable upper bound on the
beyond those of the specifications referred to in the list of number of MultipointTail sessions that can be created, with the
normative references. upper bound potentially being computed based on the number of
multicast streams that the system is expecting.
9. Contributors 8. Contributors
Rahul Aggarwal of Juniper Networks and George Swallow of Cisco Rahul Aggarwal of Juniper Networks and George Swallow of Cisco
Systems provided the initial idea for this specification and Systems provided the initial idea for this specification and
contributed to its development. contributed to its development.
10. Normative References 9. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection [RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, June 2010. (BFD)", RFC 5880, June 2010.
Authors' Addresses Authors' Addresses
Dave Katz Dave Katz
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