Internet DRAFT - draft-spallagatti-bfd-multipoint-active-tail
draft-spallagatti-bfd-multipoint-active-tail
Internet Engineering Task Force D. Katz
Internet-Draft Juniper Networks
Intended status: Standards Track D. Ward
Expires: August 6, 2015 Cisco Systems
S. Pallagatti, Ed.
Juniper Networks
February 2, 2015
BFD Multipoint Active Tails.
draft-spallagatti-bfd-multipoint-active-tail-00
Abstract
This document describes active tail extensions to the Bidirectional
Forwarding Detection (BFD) protocol for multipoint and multicast
networks. Comments on this draft should be directed to rtg-
bfd@ietf.org.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 6, 2015.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Protocol Details . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Multipoint Client Session . . . . . . . . . . . . . . . . 4
3.2. Multipoint Client Session Failure . . . . . . . . . . . . 4
3.3. State Variables . . . . . . . . . . . . . . . . . . . . . 5
3.3.1. New State Variables . . . . . . . . . . . . . . . . . 5
3.3.2. State Variable Initialization and Maintenance . . . . 6
3.4. Controlling Multipoint BFD Options . . . . . . . . . . . 7
3.5. State Machine . . . . . . . . . . . . . . . . . . . . . . 7
3.6. Session Establishment . . . . . . . . . . . . . . . . . . 7
3.7. Discriminators and Packet Demultiplexing . . . . . . . . 8
3.8. Controlling Tail Packet Transmission . . . . . . . . . . 8
3.9. Soliciting the Tails . . . . . . . . . . . . . . . . . . 9
3.10. Verifying Connectivity to Specific Tails . . . . . . . . 9
3.11. Detection Times . . . . . . . . . . . . . . . . . . . . . 10
3.12. MultipointClient Down/AdminDown Sessions . . . . . . . . 10
3.13. Base Specification Text Replacement . . . . . . . . . . . 11
3.13.1. Reception of BFD Control Packets . . . . . . . . . . 11
3.13.2. Demultiplexing BFD Control Packets . . . . . . . . . 11
3.13.3. Transmitting BFD Control Packets . . . . . . . . . . 12
4. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 13
5. Operational Scenarios . . . . . . . . . . . . . . . . . . . . 13
5.1. No Head Notification . . . . . . . . . . . . . . . . . . 13
5.2. Unreliable Head Notification . . . . . . . . . . . . . . 14
5.3. Semi-reliable Head Notification and Tail Solicitation . . 14
5.4. Reliable Head Notification . . . . . . . . . . . . . . . 14
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
7. Security Considerations . . . . . . . . . . . . . . . . . . . 15
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 16
9. Normative References . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
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1. Introduction
This application of BFD is an extension to Multipoint BFD
[I-D.ietf-bfd-multipoint] which allows tail to unreliably notify the
head of the lack of multipoint connectivity. As a further 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 base document [I-D.ietf-bfd-multipoint] describes
procedures to verify 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.)
Goal of this application is for the head to reasonably rapidly have
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 required that the head be in control of all
timing aspects of the mechanism, and that BFD packets from the tails
to the head not be synchronized.
This document effectively modifies and adds to the base BFD
specification and base BFD multipoint document.
2. Overview
Head may wish 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
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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
control packets to the head, even when the head wishes notification
of path failure from the tail. Such tails will never be known to the
head, but will still be able to detect multipoint path failures from
the head.
3. Protocol Details
This section describes the operation of BFD Multipoint active tail in
detail. This section is update to section 4 of
[I-D.ietf-bfd-multipoint]
3.1. Multipoint Client Session
If the head is keeping track of some or all of the tails, it has a
session of type MultipointClient per tail that it cares about. All
of the MultipointClient sessions for tails on a particular multipoint
path are 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.)
3.2. Multipoint Client Session Failure
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.
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3.3. State Variables
BFD Multipoint active tail introduces new state variables and
modifies the usage of a few existing ones defined in section 4.4 of
[I-D.ietf-bfd-multipoint].
3.3.1. New State Variables
Few state variables are added and modified support of Multipoint BFD
active tail.
bfd.SessionType
The type of this session as defined in
[I-D.ietf-bfd-multipoint]. A new value introduced is:
MultipointClient: A session on the head that tracks the
state of an individual tail, when desirable.
This variable MUST be initialized to the appropriate type when
the session is created, according to the rules in section 4.13
of [I-D.ietf-bfd-multipoint].
bfd.SilentTail
If 0, a tail may send packets to the head according to other
parts of this specification. Setting this to 1 allows tails to
be provisioned to always be silent, even when the head is
soliciting traffic from the tails. This can be useful, for
example, in deployments of a large number of tails when the
head wishes to track the state of a subset of them. This
variable MUST be initialized based on configuration.
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
MultipointHead or MultipointClient.
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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.
3.3.2. State Variable Initialization and Maintenance
Some state variables defined in section 6.8.1 of the [RFC5880] needs
to be initialized or manipulated differently depending on the session
type (see section 4.4.2 of [I-D.ietf-bfd-multipoint]).
bfd.LocalDiscr
For session type MultipointClient, this variable MUST always
match the value of bfd.LocalDiscr in the associated
MultipointHead session.
bfd.DesiredMinTxInterval
For session type MultipointClient, this variable MUST always
match the value of bfd.DesiredMinTxInterval in the associated
MultipointHead session.
bfd.RequiredMinRxInterval
It 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
This variable MUST be initialized to 1 for session types
MultipointClient.
bfd.DetectMult
For session type MultipointClient, this variable MUST always
match the value of bfd.DetectMult in the associated
MultipointHead session.
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3.4. Controlling Multipoint BFD Options
The state variables defined above are used to choose which
operational options are active.
The most basic form of operation as explained in
[I-D.ietf-bfd-multipoint], 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.
3.5. State Machine
State machine for session of type MultipointClient is same as defined
in section 4.5 of [I-D.ietf-bfd-multipoint].
3.6. Session Establishment
If BFD Control packets are received at the head, they are
demultiplexed to sessions of type MultipointClient, which represent
the set of tails that the head is interested in tracking. These
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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.
3.7. Discriminators and Packet Demultiplexing
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
MultipointClient session, the value of Your Discr will be valid, and
the tail MUST demultiplex the packet based solely on Your Discr.
3.8. Controlling Tail Packet Transmission
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.)
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Any tail may be provisioned to never send *any* BFD Control packets
to the head by setting bfd.SilentTail to 1. This provides a
mechanism by which only a subset of tails report their session status
to the head.
3.9. Soliciting the Tails
If the head wishes to know the identities of the tails, the
MultipointHead session MAY send a BFD Control packet as specified in
Section 3.13.3, with the Poll (P) bit set to 1. This will cause all
of the tails to reply with a unicast BFD Control Packet, randomized
across one packet interval.
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 3.11
for more details.
3.10. 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
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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.)
3.11. Detection Times
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.
3.12. MultipointClient Down/AdminDown 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.
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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.
3.13. Base Specification Text Replacement
The following sections are meant to replace the corresponding
sections in the base specification.
3.13.1. Reception of BFD Control Packets
The following procedure replaces section 6.8.6 of [RFC5880].
When a BFD Control packet is received, procedure defined in section
4.13.1 of [I-D.ietf-bfd-multipoint] MUST be followed, in the order
specified. If the packet is discarded according to these rules,
processing of the packet MUST cease at that point. In addition to
that if tail tracking is desired by head below procedure MUST be
applied.
If bfd.SessionType is MultipointTail
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 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 3.13.3.)
3.13.2. Demultiplexing BFD Control Packets
This section is part of the replacement for [RFC5880] section 6.8.6
and addition to section 4.13.2 of [I-D.ietf-bfd-multipoint],
separated for clarity.
If Multipoint (M) bit is clear
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If the Your Discriminator field is nonzero
Select a session based on the value of Your Discriminator.
If no session is found, the packet MUST be discarded.
If bfd.SessionType is MulticastHead
Find a MultipointClient session grouped to this
MulticastHead session, based on the source address and
the value of Your Discriminator. If a session is found
and is not MulticastClient, the packet MUST be discarded.
If no session is found, a new session of type
MultipointClient MAY be created, or the packet MAY be
discarded. This choice is outside the scope of this
specification.
If bfd.SessionType is not MulticastClient, the packet
MUST be discarded.
3.13.3. Transmitting BFD Control Packets
The following procedure replaces section 6.8.7 of [RFC5880].
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 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,
the receiving system MUST transmit a BFD Control packet with the Poll
(P) bit clear and the Final (F) bit, without respect to the
transmission timer or any other transmission limitations, without
respect to the session state, and without respect to whether Demand
mode is active on either system. A system MAY limit the rate at
which such packets are transmitted. If rate limiting is in effect,
the advertised value of Desired Min TX Interval MUST be greater than
or equal to the interval between transmitted packets imposed by the
rate limiting function. If the Multipoint (M) bit is set in the
received packet, the packet transmission MUST be delayed by a random
amount of time between zero and (0.9 * bfd.RemoteMinRxInterval).
Otherwise, the packet MUST be transmitted as soon as practicable.
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A system MUST NOT set the Demand (D) bit if bfd.SessionType is
MultipointClient unless bfd.DemandMode is 1, bfd.SessionState is Up,
and bfd.RemoteSessionState is Up.
Contents of transmitted packet MUST be as explained in section 4.13.3
of [I-D.ietf-bfd-multipoint].
4. 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.
5. Operational Scenarios
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.
5.1. 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.
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5.2. 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.
5.3. 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.
5.4. 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
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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,
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
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.
6. IANA Considerations
This document has no actions for IANA.
7. Security Considerations
This specification does not raise any additional security issues
beyond those of the specifications referred to in the list of
normative references.
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8. Contributors
Rahul Aggarwal of Juniper Networks and George Swallow of Cisco
Systems provided the initial idea for this specification and
contributed to its development.
9. Normative References
[I-D.ietf-bfd-multipoint]
Katz, D., Ward, D., and J. Networks, "BFD for Multipoint
Networks", draft-ietf-bfd-multipoint-05 (work in
progress), January 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, June 2010.
Authors' Addresses
Dave Katz
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, California 94089-1206
USA
Email: dkatz@juniper.net
Dave Ward
Cisco Systems
170 West Tasman Dr.
San Jose, California 95134
USA
Email: wardd@cisco.com
Santosh Pallagatti (editor)
Juniper Networks
Embassy Business Park
Bangalore, KA 560093
India
Email: santoshpk@juniper.net
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