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Internet Engineering Task Force (IETF) D. Katz
Request for Comments: 8562 Juniper Networks
Updates: 5880 D. Ward
Category: Standards Track Cisco Systems
ISSN: 2070-1721 S. Pallagatti, Ed.
VMware
G. Mirsky, Ed.
ZTE Corp.
April 2019
Bidirectional Forwarding Detection (BFD) for Multipoint Networks
Abstract
This document describes extensions to the Bidirectional Forwarding
Detection (BFD) protocol for its use in multipoint and multicast
networks.
This document updates RFC 5880.
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 Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8562.
Katz, et al. Standards Track [Page 1]
RFC 8562 BFD for Multipoint Networks April 2019
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Katz, et al. Standards Track [Page 2]
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Keywords . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Protocol Details . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Multipoint BFD Control Packets . . . . . . . . . . . . . 6
5.2. Session Model . . . . . . . . . . . . . . . . . . . . . . 6
5.3. Session-Failure Semantics . . . . . . . . . . . . . . . . 6
5.4. State Variables . . . . . . . . . . . . . . . . . . . . . 6
5.4.1. New State Variable Values . . . . . . . . . . . . . . 6
5.4.2. State Variable Initialization and Maintenance . . . . 7
5.5. State Machine . . . . . . . . . . . . . . . . . . . . . . 7
5.6. Session Establishment . . . . . . . . . . . . . . . . . . 8
5.7. Discriminators and Packet Demultiplexing . . . . . . . . 8
5.8. Packet Consumption on Tails . . . . . . . . . . . . . . . 9
5.9. Bringing Up and Shutting Down Multipoint BFD Service . . 9
5.10. Timer Manipulation . . . . . . . . . . . . . . . . . . . 10
5.11. Detection Times . . . . . . . . . . . . . . . . . . . . . 10
5.12. State Maintenance for Down/AdminDown Sessions . . . . . . 11
5.12.1. MultipointHead Sessions . . . . . . . . . . . . . . 11
5.12.2. MultipointTail Sessions . . . . . . . . . . . . . . 11
5.13. Base Specification Text Replacement . . . . . . . . . . . 11
5.13.1. Reception of BFD Control Packets . . . . . . . . . . 12
5.13.2. Demultiplexing BFD Control Packets . . . . . . . . . 15
5.13.3. Transmitting BFD Control Packets . . . . . . . . . . 16
6. Congestion Considerations . . . . . . . . . . . . . . . . . . 19
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
8. Security Considerations . . . . . . . . . . . . . . . . . . . 20
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
9.1. Normative References . . . . . . . . . . . . . . . . . . 21
9.2. Informative References . . . . . . . . . . . . . . . . . 22
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 22
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
Katz, et al. Standards Track [Page 3]
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1. Introduction
The Bidirectional Forwarding Detection (BFD) protocol [RFC5880]
specifies a method for verifying unicast connectivity between a pair
of systems. This document updates [RFC5880] by defining a new method
for using BFD. This new method provides verification of multipoint
or multicast connectivity between a multipoint sender (the "head")
and a set of one or more multipoint receivers (the "tails").
As multipoint transmissions are inherently unidirectional, this
mechanism purports only to verify this unidirectional connectivity.
Although this seems in conflict with the "Bidirectional" in BFD, the
protocol is capable of supporting this use case. Use of BFD in
Demand mode allows a tail to monitor the availability of a multipoint
path even without the existence of some kind of a return path to the
head. As an option, if a return path from a tail to the head exists,
the tail may notify the head of the lack of multipoint connectivity.
Details of tail notification to the head are outside the scope of
this document and are discussed in [RFC8563].
This application of BFD allows for the tails to detect a lack of
connectivity from the head. For some applications, such detection of
the failure at the tail is useful, for example, the use of multipoint
BFD to enable fast failure detection and faster failover in multicast
VPN as described in [MVPN-FAILOVER]. Due to its unidirectional
nature, virtually all options and timing parameters are controlled by
the head.
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 forwarding path
between the sender and the receiver.
This document effectively updates and extends the base BFD
specification [RFC5880].
2. Keywords
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Katz, et al. Standards Track [Page 4]
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3. Goals
The primary goal of this mechanism is to allow tails to rapidly
detect the fact that multipoint connectivity from the head has
failed.
Another goal is for the mechanism to work on any multicast
technology.
A further goal is to support multiple, overlapping point-to-
multipoint paths, as well as multipoint-to-multipoint paths, and to
allow point-to-point BFD sessions to operate simultaneously among the
systems participating in multipoint BFD.
It is not a goal for this protocol to verify point-to-point
bidirectional connectivity between the head and any tail. This can
be done independently (and with no penalty in protocol overhead) by
using point-to-point BFD.
4. Overview
The heart of this protocol is the periodic transmission of BFD
Control packets along a multipoint path, from the head to all tails
on the path. The contents of the BFD packets provide the means for
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
tail declares that the path has failed. For some applications, this
is the only mechanism necessary; the head can remain ignorant of the
status of connectivity to the tails.
The head of a multipoint BFD session may wish to be alerted to the
tails' connectivity (or lack thereof). Details of how the head keeps
track of tails and how tails alert their connectivity to the head are
outside the scope of this document and are discussed in [RFC8563].
Although this document describes a single head and a set of tails
spanned by a single multipoint path, the protocol is capable of
supporting (and discriminating between) more than one multipoint path
at both heads and tails, as described in Sections 5.7 and 5.13.2.
Furthermore, the same head and tail may share multiple multipoint
paths, and a multipoint path may have multiple heads.
5. Protocol Details
This section describes the operation of Multipoint BFD in detail.
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5.1. Multipoint BFD Control Packets
Multipoint BFD Control packets (packets sent by the head over a
multipoint path) are explicitly marked as such, via the setting of
the Multipoint (M) bit [RFC5880]. This means that multipoint BFD
does not depend on the recipient of a packet to know whether the
packet was received over a multipoint path. This can be useful in
scenarios where this information may not be available to the
recipient.
5.2. Session Model
Multipoint BFD is modeled as a set of sessions of different types.
The elements of procedure differ slightly for each type.
The head has a session of type MultipointHead, as defined in
Section 5.4.1, that is bound to a multipoint path. Multipoint BFD
Control packets are sent by this session over the multipoint path,
and no BFD Control packets are received by it.
Each tail has a session of type MultipointTail, as defined in
Section 5.4.1, associated with a multipoint path. These sessions
receive BFD Control packets from the head over the multipoint path.
5.3. Session-Failure Semantics
The semantics of session failure is subtle enough to warrant further
explanation.
MultipointHead sessions cannot fail (since they are controlled
administratively).
If a MultipointTail session fails, it means that the tail definitely
has lost contact with the head (or the head has been administratively
disabled), and the tail may use mechanisms other than BFD, e.g.,
logging or NETCONF [RFC6241], to send a notification to the user.
5.4. State Variables
Multipoint BFD introduces some new state variables and modifies the
usage of a few existing ones.
5.4.1. New State Variable Values
A number of new values of the state variable bfd.SessionType are
added to the base BFD [RFC5880] and base Seamless Bidirectional
Forwarding Detection (S-BFD) [RFC7880] specifications in support of
multipoint BFD.
Katz, et al. Standards Track [Page 6]
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bfd.SessionType
The type of this session as defined in [RFC7880]. Newly added
values are:
PointToPoint: Classic point-to-point BFD, as described in
[RFC5880].
MultipointHead: A session on the head responsible for the
periodic transmission of multipoint BFD Control packets
along the multipoint path.
MultipointTail: A multipoint session on a tail.
This variable MUST be initialized to the appropriate type when
the session is created.
5.4.2. State Variable Initialization and Maintenance
Some state variables defined in Section 6.8.1 of [RFC5880] need to be
initialized or manipulated differently depending on the session type.
bfd.RequiredMinRxInterval
This variable MUST be initialized to zero for session type
MultipointHead.
bfd.DemandMode
This variable MUST be initialized to 1 for session type
MultipointHead and MUST be initialized to zero for session type
MultipointTail.
5.5. State Machine
There are slight differences in how the BFD state machine works in
the multipoint application. In particular, since there is a many-to-
one mapping, three-way handshakes for session establishment and
teardown are neither possible nor appropriate. As such, there is no
Init state. Sessions of type MultipointHead MUST NOT send BFD
Control packets with the State field being set to INIT, and those
packets MUST be ignored on receipt.
The following diagram provides an overview of the state machine for
session type MultipointTail. The notation on each arc represents the
state of the remote system (as received in the State field in the BFD
Control packet) or indicates the expiration of the Detection Timer.
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DOWN, ADMIN DOWN,
+------+ TIMER +------+
+----| |<---------------------| |----+
DOWN,| | DOWN | | UP | |UP
ADMIN DOWN,+--->| |--------------------->| |<---+
TIMER +------+ UP +------+
Sessions of type MultipointHead never receive packets and have no
Detection Timer; as such, all state transitions are administratively
driven.
5.6. Session Establishment
Unlike point-to-point BFD, multipoint BFD provides a form of the
discovery mechanism that enables tails to discover the head. The
minimum amount of a priori information required both on the head and
tails is the binding to the multipoint path over which BFD is
running. The head transmits multipoint BFD packets on that path, and
the tails listen for BFD packets on that path. All other information
can be determined dynamically.
A session of type MultipointHead is created for each multipoint path
over which the head wishes to run BFD. This session runs in the
Active role, per Section 6.1 of [RFC5880]. Except when
administratively terminating BFD service, this session is always in
state Up and always operates in Demand mode. No received packets are
ever demultiplexed to the MultipointHead session. In this sense, it
is a degenerate form of a session.
Sessions on the tail MAY be established dynamically, based on the
receipt of a multipoint BFD Control packet from the head, and are of
type MultipointTail. Tail sessions always take the Passive role, per
Section 6.1 of [RFC5880].
5.7. Discriminators and Packet Demultiplexing
The use of discriminators is somewhat different in multipoint BFD
than in point-to-point BFD.
The head sends multipoint BFD Control packets over the multipoint
path via the MultipointHead session with My Discriminator set to a
value bound to the multipoint path and with Your Discriminator set to
zero.
IP and MPLS multipoint tails MUST demultiplex BFD packets based on a
combination of the source address, My Discriminator, and the identity
of the multipoint path that the multipoint BFD Control packet was
received from. Together they uniquely identify the head of the
Katz, et al. Standards Track [Page 8]
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multipoint path. Bootstrapping a BFD session to multipoint MPLS
Label Switched Path (LSP) may use the control plane, e.g., as
described in [MVPN-FAILOVER], and is outside the scope of this
document.
Note that, unlike point-to-point sessions, the My Discriminator value
on the MultipointHead session MUST NOT be changed during the life of
a session. This is a side effect of the more complex demultiplexing
scheme.
5.8. Packet Consumption on Tails
BFD packets received on tails for an IP multicast group MUST be
consumed by tails and MUST NOT be forwarded to receivers. Nodes with
the BFD session of type MultipointTail MUST identify packets received
on an IP multipoint path as a BFD Control packet if the destination
UDP port value equals 3784.
For multipoint LSPs, when IP/UDP encapsulation of BFD Control packets
is used, MultipointTail MUST expect destination UDP port 3784. The
destination IP address of a BFD Control packet MUST be in the
127.0.0.0/8 range for IPv4 or in the 0:0:0:0:0:FFFF:7F00:0/104 range
for IPv6. The use of these destination addresses is consistent with
the explanations and usage in [RFC8029]. Packets identified as BFD
packets MUST be consumed by MultipointTail and demultiplexed as
described in Section 5.13.2. Use of other types of encapsulation of
the BFD control message over multipoint LSP is outside the scope of
this document.
5.9. Bringing Up and Shutting Down Multipoint BFD Service
Because there is no three-way handshake in multipoint BFD, a newly
started head (that does not have any previous state information
available) SHOULD start with bfd.SessionState set to Down, and
bfd.RequiredMinRxInterval MUST be set to zero in the MultipointHead
session. The session SHOULD remain in this state for a time equal to
(bfd.DesiredMinTxInterval * bfd.DetectMult). This will ensure that
all MultipointTail sessions are reset (so long as the restarted head
is using the same or a larger value of bfd.DesiredMinTxInterval than
it did previously).
Multipoint BFD service is brought up by administratively setting
bfd.SessionState to Up in the MultipointHead session.
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The head of a multipoint BFD session may wish to shut down its BFD
service in a controlled fashion. This is desirable because the tails
need not wait for a Detection Time prior to declaring the multipoint
session to be down (and taking whatever action is necessary in that
case).
To shut down a multipoint session in a controlled fashion, the head
MUST administratively set bfd.SessionState in the MultipointHead
session to either Down or AdminDown and SHOULD set
bfd.RequiredMinRxInterval to zero. The session SHOULD send BFD
Control packets in this state for a period equal to
(bfd.DesiredMinTxInterval * bfd.DetectMult). Alternatively, the head
MAY stop transmitting BFD Control packets and not send any more BFD
Control packets with the new state (Down or AdminDown). Tails will
declare the multipoint session down only after the Detection Time
interval runs out.
5.10. Timer Manipulation
Because of the one-to-many mapping, a session of type MultipointHead
SHOULD NOT initiate a Poll Sequence in conjunction with timer value
changes. However, to indicate a change in the packets, a
MultipointHead session MUST send packets with the P bit set. A
MultipointTail session MUST NOT reply if the packet has the M and P
bits set and bfd.RequiredMinRxInterval set to zero. Because the Poll
Sequence is not used, the tail cannot negotiate down MultpointHead's
transmit interval. If the value of Desired Min TX Interval in the
BFD Control packet received by MultipointTail is too high (that
determination may change in time based on the current environment),
it must be handled by the implementation and may be controlled by
local policy, e.g., close the MultipointTail session.
The MultipointHead, when changing the transmit interval to a higher
value, MUST send BFD Control packets with the P bit set at the old
transmit interval before using the higher value in order to avoid
false detection timeouts at the tails. A MultipointHead session MAY
also wait some amount of time before making the changes to the
transmit interval (through configuration).
Change in the value of bfd.RequiredMinRxInterval is outside the scope
of this document and is discussed in [RFC8563].
5.11. Detection Times
Multipoint BFD is inherently asymmetric. As such, each session type
has a different approach to Detection Times.
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RFC 8562 BFD for Multipoint Networks April 2019
Since MultipointHead sessions never receive packets, they do not
calculate a Detection Time.
MultipointTail sessions cannot influence the transmission rate of the
MultipointHead session using the Required Min Rx Interval field
because of its one-to-many nature. As such, the Detection Time
calculation for a MultipointTail session does not use
bfd.RequiredMinRxInterval. The Detection Time is calculated as the
product of the last received values of Desired Min TX Interval and
Detect Mult.
The value of bfd.DetectMult may be changed at any time on any session
type.
5.12. State Maintenance for Down/AdminDown Sessions
The length of time the session state is kept after the session goes
down determines how long the session will continue to send BFD
Control packets (since no packets can be sent after the session is
destroyed).
5.12.1. MultipointHead Sessions
When a MultipointHead session transitions to states Down or
AdminDown, the state SHOULD be maintained for a period equal to
(bfd.DesiredMinTxInterval * bfd.DetectMult) to ensure that the tails
more quickly detect the session going down (by continuing to transmit
BFD Control packets with the new state).
5.12.2. MultipointTail Sessions
MultipointTail sessions MAY be destroyed immediately upon leaving Up
state, since the tail will transmit no packets.
Otherwise, MultipointTail sessions SHOULD be maintained as long as
BFD Control packets are being received by it (which by definition
will indicate that the head is not Up).
5.13. Base Specification Text Replacement
The following sections are meant to replace the corresponding
sections in the base specification [RFC5880] to support BFD for
multipoint networks while not changing processing for point-to-point
BFD.
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5.13.1. Reception of BFD Control Packets
The following procedure replaces Section 6.8.6 of [RFC5880] entirely.
When a BFD Control packet is received, the following procedure 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.
If the version number is not correct (1), the packet MUST be
discarded.
If the Length field is less than the minimum correct value (24 if
the A bit is clear, or 26 if the A bit is set), the packet MUST be
discarded.
If the Length field is greater than the payload of the
encapsulating protocol, the packet MUST be discarded.
If the Detect Mult field is zero, the packet MUST be discarded.
If the My Discriminator field is zero, the packet MUST be
discarded.
Demultiplex the packet to a session according to Section 5.13.2.
The result is either a session of the proper type, or the packet
is discarded (and packet processing MUST cease).
If the A bit is set and no authentication is in use (bfd.AuthType
is zero), the packet MUST be discarded.
If the A bit is clear and authentication is in use (bfd.AuthType
is nonzero), the packet MUST be discarded.
If the A bit is set, the packet MUST be authenticated under the
rules of Section 6.7 of [RFC5880], based on the authentication
type in use (bfd.AuthType). This may cause the packet to be
discarded.
Set bfd.RemoteDiscr to the value of My Discriminator.
Set bfd.RemoteState to the value of the State (Sta) field.
Set bfd.RemoteDemandMode to the value of the Demand (D) bit.
Set bfd.RemoteMinRxInterval to the value of Required Min RX
Interval.
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If the Required Min Echo RX Interval field is zero, the
transmission of Echo packets, if any, MUST cease.
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
MUST be terminated.
If bfd.SessionType is PointToPoint, update the transmit interval
as described in Section 6.8.2 of [RFC5880].
If bfd.SessionType is PointToPoint, update the Detection Time as
described in Section 6.8.4 of [RFC5880].
Else
If bfd.SessionType is MultipointTail, then update the Detection
Time as the product of the last received values of Desired Min
TX Interval and Detect Mult, as described in Section 5.11 of
this specification.
If bfd.SessionState is AdminDown
Discard the packet
If the received State is AdminDown
If bfd.SessionState is not Down
Set bfd.LocalDiag to 3 (Neighbor signaled session down)
Set bfd.SessionState to Down
Else
If bfd.SessionState is Down
If bfd.SessionType is PointToPoint
If received State is Down
Set bfd.SessionState to Init
Else if received State is Init
Set bfd.SessionState to Up
Katz, et al. Standards Track [Page 13]
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Else (bfd.SessionType is not PointToPoint)
If received State is Up
Set bfd.SessionState to Up
Else if bfd.SessionState is Init
If received State is Init or Up
Set bfd.SessionState to Up
Else (bfd.SessionState is Up)
If received State is Down
Set bfd.LocalDiag to 3 (Neighbor signaled session down)
Set bfd.SessionState to Down
Check to see if Demand mode should become active or not (see
[RFC5880], Section 6.6).
If bfd.RemoteDemandMode is 1, bfd.SessionState is Up, and
bfd.RemoteSessionState is Up, Demand mode is active on the remote
system and the local system MUST cease the periodic transmission
of BFD Control packets (see Section 5.13.3).
If bfd.RemoteDemandMode is zero, bfd.SessionState is not Up, or
bfd.RemoteSessionState is not Up, Demand mode is not active on the
remote system and the local system MUST send periodic BFD Control
packets (see Section 5.13.3).
If the Poll (P) bit is set, and bfd.SessionType is PointToPoint,
send a BFD Control packet to the remote system with the Poll (P)
bit clear, and the Final (F) bit set (see Section 5.13.3).
If the packet was not discarded, it has been received for purposes
of the Detection Time expiration rules in Section 6.8.4 of
[RFC5880].
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5.13.2. Demultiplexing BFD Control Packets
This section is part of the replacement for Section 6.8.6 of
[RFC5880]; it is separated for clarity.
If the Multipoint (M) bit is set
If the Your Discriminator field is nonzero, the packet MUST be
discarded.
Select a session based on the source address, My Discriminator,
and the identity of the multipoint path on which the multipoint
BFD Control packet was received.
If a session is found, and bfd.SessionType is not
MultipointTail, the packet MUST be discarded.
Else
If a session is not found, a new session of type
MultipointTail MAY be created, or the packet MAY be
discarded. This choice can be controlled by the local
policy, e.g., by setting a maximum number of MultipointTail
sessions. Use of the local policy and the exact mechanism
of it are outside the scope of this specification.
Else (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.
Else (Your Discriminator is zero)
If the State field is not Down or AdminDown, the packet MUST
be discarded.
Otherwise, the session MUST be selected based on some
combination of other fields, possibly including source
addressing information, the My Discriminator field, and the
interface over which the packet was received. The exact
method of selection is application specific and is thus
outside the scope of this specification.
If a matching session is found, and bfd.SessionType is not
PointToPoint, the packet MUST be discarded.
Katz, et al. Standards Track [Page 15]
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If a matching session is not found, a new session of type
PointToPoint MAY be created, or the packet MAY be discarded.
This choice MAY be controlled by a local policy and is
outside the scope of this specification.
If the State field is Init and bfd.SessionType is not
PointToPoint, the packet MUST be discarded.
5.13.3. Transmitting BFD Control Packets
The following procedure replaces Section 6.8.7 of [RFC5880] entirely.
With the exceptions listed in the remainder of this section, a system
MUST NOT transmit BFD Control packets at an interval less than the
larger of bfd.DesiredMinTxInterval and bfd.RemoteMinRxInterval, less
applied jitter (see below). In other words, the system reporting the
slower rate determines the transmission rate.
The periodic transmission of BFD Control packets MUST be jittered on
a per-packet basis by up to 25%; that is, the interval MUST be
reduced by a random value of 0 to 25%, in order to avoid self-
synchronization with other systems on the same subnetwork. Thus, the
average interval between packets will be roughly 12.5% less than that
negotiated.
If bfd.DetectMult is equal to 1, the interval between transmitted BFD
Control packets MUST be no more than 90% of the negotiated
transmission interval and MUST be no less than 75% of the negotiated
transmission interval. This is to ensure that, on the remote system,
the calculated Detection Time does not pass prior to the receipt of
the next BFD Control packet.
A system MUST NOT transmit any BFD Control packets if bfd.RemoteDiscr
is zero and the system is taking the Passive role.
A system MUST NOT transmit any BFD Control packets if bfd.SessionType
is MultipointTail.
A system MUST NOT periodically transmit BFD Control packets if Demand
mode is active on the remote system (bfd.RemoteDemandMode is 1,
bfd.SessionState is Up, and bfd.RemoteSessionState is Up), and a Poll
Sequence is not being transmitted.
A system MUST NOT periodically transmit BFD Control packets if
bfd.RemoteMinRxInterval is zero.
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If bfd.SessionType is MultipointHead, the transmit interval MUST be
set to bfd.DesiredMinTxInterval (this should happen automatically, as
bfd.RemoteMinRxInterval will be zero).
If bfd.SessionType is not MultipointHead, the transmit interval MUST
be recalculated whenever bfd.DesiredMinTxInterval changes, or
whenever bfd.RemoteMinRxInterval changes, and is equal to the greater
of those two values. See Sections 6.8.2 and 6.8.3 of [RFC5880] for
details on transmit timers.
A system MUST NOT set the Demand (D) bit if bfd.SessionType is
MultipointTail.
A system MUST NOT set the Demand (D) bit if bfd.SessionType is
PointToPoint unless bfd.DemandMode is 1, bfd.SessionState is Up, and
bfd.RemoteSessionState is Up.
If bfd.SessionType is PointToPoint or MultipointHead, a BFD Control
packet SHOULD be transmitted during the interval between periodic
Control packet transmissions when the contents of that packet would
differ from that in the previously transmitted packet (other than the
Poll (P) and Final (F) bits) in order to more rapidly communicate a
change in state.
The contents of transmitted BFD Control packets MUST be set as
follows:
Version
Set to the current version number (1).
Diagnostic (Diag)
Set to bfd.LocalDiag.
State (Sta)
Set to the value indicated by bfd.SessionState.
Poll (P)
Set to 1 if the local system is sending a Poll Sequence or is a
session of type MultipointHead soliciting the identities of the
tails, or zero if not.
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Final (F)
Set to 1 if the local system is responding to a BFD Control
packet received with the Poll (P) bit set, or zero if not.
Control Plane Independent (C)
Set to 1 if the local system's BFD implementation is
independent of the control plane (it can continue to function
through a disruption of the control plane).
Authentication Present (A)
Set to 1 if authentication is in use in this session
(bfd.AuthType is nonzero), or zero if not.
Demand (D)
Set to bfd.DemandMode if bfd.SessionState is Up and
bfd.RemoteSessionState is Up. Set to 1 if bfd.SessionType is
MultipointHead. Otherwise, it is set to zero.
Multipoint (M)
Set to 1 if bfd.SessionType is MultipointHead. Otherwise, it
is set to zero.
Detect Mult
Set to bfd.DetectMult.
Length
Set to the appropriate length, based on the fixed header length
(24) plus any Authentication Section.
My Discriminator
Set to bfd.LocalDiscr.
Your Discriminator
Set to bfd.RemoteDiscr.
Desired Min TX Interval
Set to bfd.DesiredMinTxInterval.
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Required Min RX Interval
Set to bfd.RequiredMinRxInterval.
Required Min Echo RX Interval
Set to zero if bfd.SessionType is MultipointHead or
MultipointTail. Otherwise, set to the minimum required Echo
packet receive interval for this session. If this field is set
to zero, the local system 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
Included and set according to the rules in Section 6.7 of
[RFC5880] if authentication is in use (bfd.AuthType is
nonzero). Otherwise, this section is not present.
6. Congestion Considerations
As a foreword, although congestion can occur because of a number of
factors, it should be noted that high transmission rates are by
themselves subject to creating congestion either along the path or at
the tail end(s). As such, as stated in [RFC5883]:
it is required that the operator correctly provision the rates at
which BFD is transmitted to avoid congestion (e.g link, I/O, CPU)
and false failure detection.
Use of BFD in multipoint networks, as specified in this document,
over multiple hops requires consideration of the mechanisms to react
to network congestion. Requirements stated in Section 7 of the BFD
base specification [RFC5880] equally apply to BFD in multipoint
networks and are repeated here:
When BFD is used across multiple hops, a congestion control
mechanism MUST be implemented, and when congestion is detected,
the BFD implementation MUST reduce the amount of traffic it
generates.
The mechanism to control the load of BFD traffic MAY use BFD's
configuration interface to control BFD state variable
bfd.DesiredMinTxInterval. However, such a control loop does not form
part of the BFD protocol itself, and its specification is thus
outside the scope of this document.
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Additional considerations apply to BFD in multipoint networks, as
specified in this document. Indeed, because a tail does not transmit
any BFD Control packets to the head of the BFD session, such a head
node has no BFD-based mechanism and thus is not aware of the state of
the session at the tail. In the absence of any other mechanism, the
head of the session could thus continue to send packets towards the
tail(s) even though a link failure has happened. In such a scenario,
when it is required for the head of the session to be aware of the
state of the tail of the session, it is RECOMMENDED to implement the
extension described in [RFC8563].
7. IANA Considerations
This document has no IANA actions.
8. Security Considerations
The same security considerations as those described in [RFC5880]
apply to this document. Additionally, implementations that create
MultpointTail sessions dynamically upon receipt of multipoint BFD
Control packets MUST implement protective measures to prevent an
infinite number of MultipointTail sessions from being created. Below
are some points to consider in such implementations.
If a multipoint BFD Control packet did not arrive on a multicast
path (e.g., on the expected interface, with the expected MPLS
label, etc.), a MultipointTail session should not be created.
If redundant streams are expected for a given multicast stream,
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,
the implementation should generate an alarm to users to indicate
the anomaly.
The implementation should have a reasonable upper bound on the
number of MultipointHead sessions that can be created, with the
upper bound potentially being computed based on the load these
would generate.
The implementation should have a reasonable upper bound on the
number of MultipointTail sessions that can be created, with the
upper bound potentially being computed based on the number of
multicast streams that the system is expecting.
If authentication is in use, the head and all tails may be configured
to have a common authentication key in order for the tails to
validate multipoint BFD Control packets.
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Shared keys in multipoint scenarios allow any tail to spoof the head
from the viewpoint of any other tail. For this reason, using shared
keys to authenticate BFD Control packets in multipoint scenarios is a
significant security exposure unless all tails can be trusted not to
spoof the head. Otherwise, asymmetric message authentication would
be needed, e.g., protocols that use Timed Efficient Stream Loss-
Tolerant Authentication (TESLA) as described in [RFC4082].
Applicability of the asymmetric message authentication to BFD for
multipoint networks is outside the scope of this specification and is
for further study.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<https://www.rfc-editor.org/info/rfc5880>.
[RFC7880] Pignataro, C., Ward, D., Akiya, N., Bhatia, M., and
S. Pallagatti, "Seamless Bidirectional Forwarding
Detection (S-BFD)", RFC 7880, DOI 10.17487/RFC7880, July
2016, <https://www.rfc-editor.org/info/rfc7880>.
[RFC8029] Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N.,
Aldrin, S., and M. Chen, "Detecting Multiprotocol Label
Switched (MPLS) Data-Plane Failures", RFC 8029,
DOI 10.17487/RFC8029, March 2017,
<https://www.rfc-editor.org/info/rfc8029>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
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9.2. Informative References
[MVPN-FAILOVER]
Morin, T., Ed., Kebler, R., Ed., and G. Mirsky, Ed.,
"Multicast VPN fast upstream failover", Work in Progress,
draft-ietf-bess-mvpn-fast-failover-05, February 2019.
[RFC4082] Perrig, A., Song, D., Canetti, R., Tygar, J., and
B. Briscoe, "Timed Efficient Stream Loss-Tolerant
Authentication (TESLA): Multicast Source Authentication
Transform Introduction", RFC 4082, DOI 10.17487/RFC4082,
June 2005, <https://www.rfc-editor.org/info/rfc4082>.
[RFC5883] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD) for Multihop Paths", RFC 5883, DOI 10.17487/RFC5883,
June 2010, <https://www.rfc-editor.org/info/rfc5883>.
[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
<https://www.rfc-editor.org/info/rfc6241>.
[RFC8563] Katz, D., Ward, D., Pallagatti, S., Ed., and G. Mirsky,
Ed., "Bidirectional Forwarding Detection (BFD) Multipoint
Active Tails", RFC 8563, DOI 10.17487/RFC8563, April 2019,
<https://www.rfc-editor.org/info/rfc8563>.
Acknowledgments
The authors would like to thank Nobo Akiya, Vengada Prasad Govindan,
Jeff Haas, Wim Henderickx, Gregory Mirsky, 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.
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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 23]