Network Working Group D. Katz
Internet Draft Juniper Networks
D. Ward
Cisco Systems
Category: Informational June, 2003
Expires: December, 2003
Bidirectional Forwarding Detection
draft-katz-ward-bfd-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
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Abstract
This document describes a protocol intended to detect faults in the
bidirectional path between two forwarding engines, including
interfaces, data link(s), and to the extent possible the forwarding
engines themselves, with potentially very low latency. It operates
independently of media, data protocols, and routing protocols.
Conventions used in this document
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 [KEYWORDS].
1. Introduction
An increasingly important feature of networking equipment is the
rapid detection of communication failures between adjacent systems,
in order to more quickly establish alternative paths. Currently,
detection can come fairly quickly in certain circumstances when data
link hardware comes into play (such as SONET alarms.) However, there
are media that do not provide this kind of signaling (such as
Ethernet), and some media may not detect certain kinds of failures in
the path, for example, failing interfaces or forwarding engine
components.
Networks use relatively slow "Hello" mechanisms, usually in routing
protocols, to detect failures when there is no hardware signaling to
help out. The detection times available in the existing protocols
are no better than a second, which is far too long for some
applications and represents a great deal of lost data at gigabit
rates. Furthermore, routing protocol Hellos are of no help when
those routing protocols are not in use, and the semantics of
detection are subtly different--they detect a failure in the path
between the two routing protocol engines.
The goal of BFD is to provide low-overhead, short-duration detection
of failures in the path between adjacent forwarding engines,
including the interfaces, data link(s), and to the extent possible
the forwarding engines themselves.
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2. Design
BFD is designed to detect failures in communication with a data plane
next hop. It is intended to be implemented in some component of the
forwarding engine of a system, in cases where the forwarding and
control engines are separated. This not only binds the protocol more
to the data plane, but decouples the protocol from the fate of the
routing protocol engine (making it useful in concert with various
"graceful restart" mechanisms for those protocols.)
BFD operates on top of any data protocol being forwarded between two
systems. It is always run in a unicast, point-to-point mode.
BFD can provide failure detection on any kind of path between
systems, including direct physical links, virtual circuits, tunnels,
MPLS LSPs, multihop routed paths, and unidirectional links (so long
as there is some return path, of course.) Multiple BFD sessions can
be established between the same pair of systems when multiple paths
between them are present in at least one direction, even if the same
path is used in one direction.
The BFD state machine implements a three-way handshake, both when
establishing a BFD session and when tearing it down for any reason,
to ensure that both systems are aware of the state change.
3. Protocol Overview
BFD is a simple, fixed-field, hello protocol that in many respects is
similar to the detection components of well-known routing protocols.
A pair of systems transmit BFD packets periodically over each path
between the two systems, and if a system stops receiving BFD packets
for long enough, some component in that particular bidirectional path
to the neighboring system is assumed to have failed.
A path is only declared to be operational when two-way communication
has been established between systems (though this does not
necessarily mean that a bidirectional link must be used.)
A separate BFD session is created for each communications path and
data protocol in use between two systems.
Each system estimates how quickly it can send and receive BFD packets
in order to come to an agreement with its neighbor about how rapidly
detection of failure will take place. These estimates can be
modified in real time in order to adapt to unusual situations. This
design also allows for fast systems on a shared medium with a slow
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system to be able to more rapidly detect failures between the fast
systems while allowing the slow system to participate to the best of
its ability.
BFD can operate in two different modes. The first mode is known as
Asynchronous mode. In this mode, each system sends a series of BFD
Control packets to one another, and if a number of those packets in a
row are not received by the other system, the session is declared to
be down.
The second mode is known as Echo mode. In echo mode, BFD Control
packets are sent at a relatively sedate rate, and additionally
streams of BFD Echo packets are transmitted in each direction in such
a way as to have the other system loop them back through its
forwarding path. If a number of packets in a row of either the
control stream or the echoed data stream are not received, the
session is declared to be down.
Asynchronous mode is advantageous in that it requires half as many
packets to achieve a particular detection time as does Echo mode. It
is also used when Echo mode cannot be supported for some reason.
Echo mode has the advantage of truly testing only the forwarding path
on the remote system, which may reduce round-trip jitter and thus
allow more aggressive detection times, as well as potentially
detecting some classes of failure that might not otherwise be
detected.
Echo mode is enabled only when both systems signal that they are
willing to do so.
4. Protocol Details
BFD packets are carried as the payload of whatever encapsulating
protocol is appropriate for the medium and network. Note that many
of the exact mechanisms are implementation dependent and will not
affect interoperability, and are thus outside the scope of this
specification. Those issues are so noted.
4.1. Timer Model
A timer is an entity that will measure an interval of time and
provide a notification when that time period expires. It has two
states, running and disarmed. A disarmed timer will not expire,
whereas a running timer will expire after the specified interval. A
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running timer can be restarted prior to its expiration to any
interval; it will then not expire until the new interval has passed.
Some timers may be jittered. This is a process where a random value
is subtracted from the interval (expressed as a percentage of the
interval) when the timer is started. Jitter is used to avoid the
self-synchronization of nominally independent timers.
4.2. State
BFD requires that a set of state elements be maintained for each
session with neighboring systems. The creation of this state is
outside the scope of this specification. This state description is
not intended to define implementation; any equivalent method can be
used.
st.SourceAddress
The source address information used when transmitting BFD Control
packets for this session, appropriate to the environment. The
setting of this value is outside the scope of this specification.
st.DestinationAddress
The destination address information used when transmitting BFD
Control packets for this session, appropriate to the environment.
The setting of this value is outside the scope of this
specification.
st.EchoSourceAddress
The source address information used when transmitting BFD Echo
packets for this session, appropriate to the environment, if Echo
mode is supported. This address MUST be an address associated
with the transmitting system, and MAY be part of a subnet other
than the one over which the packet is being sent (in order to
avoid the transmission of ICMP Redirects.) The setting of this
value is otherwise outside the scope of this specification.
st.EchoDestinationAddress
The destination address information used when transmitting BFD
Echo packets for this session, appropriate to the environment.
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This address MUST be an address associated with the transmitting
system, SHOULD be an address for which the remote system will
route packets back on the interface over which they are received,
and SHOULD be part of the subnet over which the packet is being
sent (if the link is subnetted.) The setting of this value is
otherwise outside the scope of this specification.
st.LocalDiscr
The local discriminator for this BFD session, used to uniquely
identify it. It MUST be unique on this system, and nonzero. The
value is otherwise outside the scope of this specification.
st.RemoteDiscr
The remote discriminator for this BFD session. This is the
discriminator chosen by the remote system, and is totally opaque
to the local system. This MUST be initialized to zero.
st.RemoteHeard
This field is set to 1 if the local system is actively receiving
BFD packets from the remote system, and is set to 0 if the local
system has not received BFD packets recently (within the detection
time) or if the local system is attempting to tear down the BFD
session. This MUST be initialized to zero.
st.SessionState
The perceived state of the session (Init, Up, Failing, or Down.)
The exact action taken when the session state changes is outside
the scope of this specification, though it is expected that this
state change (particularly to and from Up state) is reported to
other components of the system. This MUST be initialized to Down.
st.EchoModeDesired
A boolean stating whether or this system wishes to use Echo mode.
The setting of this value is outside the scope of this
specification.
st.EchoModeActive
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A boolean tracking whether or not Echo mode is active. This MUST
be initialized to FALSE.
st.LocalSessionDiagnostic
A diagnostic code specifying the reason the local session state
most recently transitioned from Up to some other state. This MUST
be initialized to zero.
st.RemoteSessionDiagnostic
A diagnostic code specifying the reason the remote session state
most recently transitioned from Up to some other state. This MUST
be initialized to zero.
st.DesiredMinAsyncTXInterval
The minimum interval, in microseconds, between transmitted BFD
Control packets that this system would like to use while operating
in Asynchronous mode when the session is up. The actual interval
is negotiated between the two systems. This value MUST be
nonzero, and is otherwise outside the scope of this specification.
st.DesiredMinSlowTXInterval
The minimum interval, in microseconds, between transmitted BFD
Control packets that this system would like to use while operating
in Echo mode, or when the session is not up. The actual interval
is negotiated between the two systems. This value MUST be
nonzero, and is otherwise outside the scope of this specification,
though it is suggested that this value SHOULD be at least one
second (1,000,000 usec.)
st.DesiredMinEchoTXInterval
The minimum interval, in microseconds, between transmitted BFD
Echo packets that this system would like to use while operating in
Echo mode. The actual interval is negotiated between the two
systems. If Echo mode is supported, this value MUST be nonzero,
and is otherwise outside the scope of this specification.
st.DesiredMinTXInterval
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The minimum interval, in microseconds, between transmitted BFD
Control packets that this system would like to use at the current
time. The actual interval is negotiated between the two systems.
This is set to either st.DesiredMinSlowTXInterval or
st.DesiredMinAsyncTXInterval depending on the session state. This
value MUST be initialized to st.DesiredMinSlowTXInterval.
st.RequiredMinRXInterval
The minimum interval, in microseconds, between received BFD
Control packets that this system requires. The setting of this
value is outside the scope of this specification.
st.RequiredMinEchoRXInterval
The minimum interval, in microseconds, between received BFD Echo
packets that this system requires. If this system supports Echo
mode, this value MUST be nonzero. If this system does not support
Echo mode this value MUST be zero. The setting of this value is
otherwise outside the scope of this specification.
st.TxInterval
The agreed BFD Control packet transmission interval, in
microseconds, for this session. This MUST be initialized to
st.DesiredMinTXInterval. Note that an independent transmit
interval may be used in each direction for a single BFD session.
st.EchoTxInterval
The agreed BFD Echo packet transmission interval, in microseconds,
for this session. This MUST be initialized to zero. Note that an
independent transmit interval may be used in each direction for a
single BFD session.
st.DetectMult
The desired detect time multiplier for BFD Control packets. The
negotiated Control packet transmission interval, multiplied by
this value, will be the detection time for this session (as seen
by the remote system.) This value MUST be a nonzero integer, and
is otherwise outside the scope of this specification.
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st.EchoDetectMult
The desired detect time multiplier for BFD Echo packets. The
negotiated Echo packet transmission interval, multiplied by this
value, will be the detection time for this session (as seen by the
local system.) This value MUST be a nonzero integer, and is
otherwise outside the scope of this specification.
st.DetectionTime
The detection time of the failure of this BFD session by virtue of
missing BFD Control packets, as seen by the local system, in
microseconds. It MUST be initialized to zero. Note that each
system determines its own detection time, and the values for each
system may not be the same.
st.DetectTimer
This timer is used to keep track of session liveness by tracking
the arrival of BFD Control packets. It MUST be initialized to the
disarmed state. When it expires, the session is deemed to have
failed.
st.EchoDetectionTime
The detection time of the failure of this BFD session by virtue of
missing BFD Echo packets, as seen by the local system, in
microseconds. It MUST be initialized to zero. Note that each
system determines its own detection time, and the values for each
system may not be the same.
st.EchoDetectTimer
This timer is used to keep track of session liveness by tracking
the arrival of BFD Echo packets. It MUST be initialized to the
disarmed state. When it expires, the session is deemed to have
failed.
st.TransmissionTimer
This timer triggers the transmission of a BFD Control packet. It
MUST be initialized to the running state, with an interval of
st.DesiredMinSlowTxInterval. A jitter of 25% SHOULD be applied to
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this timer.
st.EchoTransmissionTimer
This timer triggers the transmission of a BFD Echo packet. It
MUST be initialized to the disarmed state. A jitter of 25% SHOULD
be applied to this timer.
4.3. BFD Control Packet Format
BFD Control packets are sent in an encapsulation appropriate to the
environment. See "Encapsulation Specifics" below for the specifics
of particular environments.
The payload of a BFD Control packet has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version |H| Diagnostic | Detect Mult | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| My Discr |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Your Discr |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Desired Min TX Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Required Min RX Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Required Min Echo RX Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version
The version number of the protocol. This document defines
protocol version 0.
I Hear You (H)
This bit is set to 0 if the transmitting system either is not
receiving BFD packets from the remote system, or is in the process
of tearing down the BFD session for some reason (see the Elements
of Procedure below for more details.)
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Diagnostic
A diagnostic code specifying the local system's reason for the
last transition of the session from Up to some other state.
Values are:
0 -- No diagnostic
1 -- Control Detection time expired
2 -- Echo Detection time expired
3 -- Neighbor signaled session down
4 -- Forwarding plane reset
Detect Mult
Detect time multiplier. The negotiated transmission interval,
multiplied by this value, provides the detection time for the
transmitting system.
Length
Length of the BFD Control packet, in bytes.
My Discr
A unique, nonzero discriminator value generated by the
transmitting system, used to demultiplex multiple BFD sessions
between the same pair of systems.
Your Discr
The discriminator received from the corresponding remote
system. This field reflects back the received value of My
Discr, or is zero if that value is unknown.
Desired Min TX Interval
This is the minimum interval, in microseconds, that the local
system would like to use when transmitting BFD Control packets.
Required Min RX Interval
This is the minimum interval, in microseconds, between received
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BFD Control packets that this system is capable of supporting.
Required Min Echo RX Interval
This is the minimum interval, in microseconds, between received
BFD Echo packets that this system is capable of supporting. If
this value is zero, the transmitting system does not support
BFD Echo packets.
4.4. BFD Echo Packet Format
BFD Echo packets are sent in an encapsulation appropriate to the
Environment. See "Encapsulation Specifics" below for the specifics
of particular environments.
The payload of a BFD Echo packet is a local matter, since only the
sending system ever looks over the content. The only requirement is
that sufficient information is included to demultiplex the received
packet to the correct BFD session.
4.5. Elements of Procedure
4.5.1. Overview
A session begins with the periodic, slow transmission of BFD Control
packets. When bidirectional communication is achieved (by virtue of
the I Hear You field being nonzero, a three way handshake), the BFD
session comes up.
If both systems signal that they can support Echo mode, they continue
to send Control packets at the slow rate and start transmitting Echo
packets at the negotiated rate.
The mechanism for detecting lost Echo packets and determining the
detection time in Echo mode is outside of the scope of this
specification. The only normative aspect of Echo packets is that
they must not be sent more rapidly than the other system is willing
to accept them (according to the advertised Required Min Echo Rx
Interval.)
One possible mechanism for the handling of Echo mode is described
herein. Note that if the round trip time to the remote system is
greater than st.EchoDetectionTime, the algorithm described will
falsely declare a session failure when Echo mode is first enabled. A
system MAY decide not to negotiate Echo mode when the latency is high
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relative to the detection time, or it MAY set st.EchoDetectTimer to a
sufficiently large interval when it is first started (see
TurnOnEchoMode below), or it MAY choose to use a different mechanism
altogether (perhaps one that doesn't use timers at all) to determine
whether Echo packets have not arrived. The determination of whether
any of this is necessary is outside the scope of this specification.
If at least one system does not wish to or cannot support Echo mode,
the systems instead send Control packets at a higher rate.
If the session fails, the transmission of Echo packets (if any)
ceases, and the transmission of Control packets goes back to the slow
rate.
4.5.2. Reception of BFD Control Packets
When a BFD Control packet is received, the following procedure MUST
be followed, in the order specified:
If the version number is not correct (0), the packet MUST be
discarded.
If the length field is less than the correct value (24), the
packet MUST be discarded.
If the length field is greater than the payload of the
encapsulating protocol, the packet MUST be discarded.
The appropriate BFD state block is selected based on some
combination of source addressing information, the two
discriminator fields, and by the interface over which the packet
was received. The exact method for looking up a state block is
outside the scope of this specification. If a matching session is
not found, new state may be created, or the packet may be
discarded. This choice is outside the scope of this
specification.
If the value of st.RemoteDiscr is nonzero, it MUST match the value
of My Discr. If it does not, the packet MUST be discarded.
If the value of Your Discr is nonzero, it MUST match the value of
st.LocalDiscr. If it does not, the packet MUST be discarded.
If the value of st.RemoteDiscr is zero, set it to the value of My
Discr.
If st.EchoModeActive is TRUE and the received Required Min Echo RX
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Interval is zero, execute TurnOffEchoMode.
Set st.TxInterval to the greater of st.DesiredMinTxInterval and
the received Required Min Rx Interval.
Set st.EchoTxInterval to the greater of
st.DesiredMinEchoTXInterval and the received Required Min Echo Rx
Interval.
Set st.EchoDetectionTime to the value of st.EchoTxInterval
multiplied by the value of st.EchoDetectMult.
Set st.DetectionTime to the greater of st.RequiredMinRXInterval
and the received Desired Min TX Interval, multiplied by the
received Detect Multiplier.
(Re)start st.DetectTimer with an interval of st.DetectionTime.
Set st.RemoteSessionDiagnostic to the value of the received
Diagnostic.
If st.SessionState is Down
Set st.RemoteHeard to 1
If I Hear You is zero
Set st.SessionState to Init
Else
Set st.SessionState to Up
If st.EchoModeDesired is TRUE and Required Min Echo RX
Interval is nonzero
Execute TurnOnEchoMode
Else if st.SessionState is Init
If I Hear You is nonzero
Set st.SessionState to Up
If st.EchoModeDesired is TRUE and Required Min Echo RX
Interval is nonzero
Execute TurnOnEchoMode
Else if st.SessionState is Up
If I Hear You is zero
Set st.LocalSessionDiagnostic to 3 (Neighbor signaled
session down)
Execute TakeDownSession
Else if st.SessionState is Failing
If I Hear You is zero, set st.SessionState to Down
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TurnOffEchoMode:
Set st.EchoModeActive to FALSE
Disarm st.EchoDetectTimer
Disarm st.EchoTransmissionTimer
If st.SessionState is Up
Set st.DesiredMinTxInterval to st.DesiredMinAsyncTXInterval
Else
Set st.DesiredMinTxInterval to st.DesiredMinSlowTxInterval
TurnOnEchoMode:
Set st.EchoModeActive to TRUE
Set st.DesiredMinTxInterval to st.DesiredMinSlowTxInterval
Start st.EchoDetectTimer with an interval of st.EchoDetectionTime.
st.EchoDetectTimer MUST be started with an interval greater than
the link round trip time; if necessary, st.EchoDetectTimer MAY
be started with a value greater than st.EchoDetectionTime.
Start st.EchoTransmissionTimer with an interval of st.EchoTxInterval
TakeDownSession:
Disarm st.DetectTimer
Set st.SessionState to Failing
Set st.RemoteHeard to zero
Execute TurnOffEchoMode
4.5.3. st.TransmissionTimer Expiration
When st.TransmissionTimer expires, send a BFD Control packet, and
restart the timer with an interval of st.TxInterval. The packet is
sent with a source address of st.SourceAddress and a destination
address of st.DestinationAddress. The fields are set as follows:
Version
Set to the current version number (0).
I Hear You
Set to the value of st.RemoteHeard.
Diagnostic
Set to the value of st.LocalSessionDiagnostic.
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Detect Mult
Set to the value of st.DetectMult.
My Discr
Set to st.LocalDiscr.
Your Discr
If st.SessionState is Init or Up, set to st.RemoteDiscr.
Otherwise, set to zero.
Desired Min TX Interval
Set to st.DesiredMinTXInterval.
Required Min RX Interval
Set to st.RequiredMinRXInterval.
Required Min Echo RX Interval
Set to st.RequiredMinEchoRXInterval.
4.5.4. Reception of BFD Echo Packets
The processing of received Echo packets is outside of the scope of
this specification. However, when a BFD Echo packet is received, the
following procedure MAY be followed, in the order specified:
The appropriate BFD state block is selected based on some
combination of source addressing information, data placed in the
payload of the Echo packet, and the interface over which the
packet was received. If a matching session is not found, discard
the packet.
If st.EchoModeActive is FALSE, discard the packet.
Restart st.EchoDetectTimer with an interval of
st.EchoDetectionTime.
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4.5.5. st.EchoTransmissionTimer Expiration
When st.EchoTransmissionTimer expires, a BFD Echo packet MUST sent,
and the timer MUST be restarted with an interval of
st.EchoTxInterval. The packet is sent with a source address of
st.EchoSourceAddress and a destination address of
st.EchoDestinationAddress. The contents of the packet are outside
the scope of this specification.
4.5.6. st.DetectTimer Expiration
When st.DetectTimer expires, set st.LocalSessionDiagnostic to 1
(Control Detection time expired), set st.RemoteDiscr to zero, and
execute TakeDownSession.
4.5.7. st.EchoDetectTimer Expiration
When st.EchoDetectTimer expires, set st.LocalSessionDiagnostic to 2
(Echo Detection time expired), and execute TakeDownSession.
4.5.8. Min Rx Interval Change
When it is desired to change the rate at which BFD Control packets
arrive from the remote system, st.RequiredMinRxInterval can be
changed at any time to any value. The new value will be transmitted
at the next st.TransmissionTimer expiration, and the remote system
will adjust accordingly.
4.5.9. Min Tx Interval Change
When it is desired to change the rate at which BFD Control packets
are transmitted to the remote system (subject to the requirements of
the neighboring system), st.DesiredMinTxInterval can be changed at
any time to any value. The new value will be transmitted at the next
st.TransmissionTimer expiration. Note that st.TransmissionTimer
should not be touched; it will pick up the new value (if any) at its
next expiration. This is necessary when increasing the transmission
interval to avoid an expiration of the neighbor's detection timer.
If the first packet containing a new, larger value of the interval is
dropped, there is a chance that the detect timer will fire on the
remote system and take down the BFD session. An implementation MAY
continue to transmit BFD Control packets at the old, shorter interval
for up to st.DetectMult packets before using the new, longer
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interval.
4.5.10. Min Echo RX Interval Change
When it is desired to change the rate at which BFD Echo packets
arrive from the remote system, st.RequiredMinEchoRxInterval can be
changed at any time to any value. The new value will be transmitted
at the next st.TransmissionTimer expiration, and the remote system
will adjust accordingly.
4.5.11. Detect Multiplier Change
When it is desired to change the detect multiplier, the value of
st.DetectMult can be changed to any nonzero value. The new value
will be transmitted at the next st.TransmissionTimer expiration.
4.5.12. Forwarding Engine Reset
When the forwarding engine hardware is reset, set
st.LocalSessionDiagnostic to 4 (Forwarding plane reset), and execute
TakeDownSession.
4.5.13. Mode Change
If it is desired to switch between Async mode and Echo mode, this can
be done at any time (assuming that both systems are capable of
supporting Echo mode) by changing the value of
st.RequiredMinEchoRXInterval to zero or nonzero accordingly. If Echo
mode is enabled, Echo packets will be sent and the rate of Control
packets will be reduced, and the opposite will happen if Echo mode is
disabled.
4.6. Encapsulation Specifics
4.6.1. BFD for IPv4
In the case of IPv4, BFD Control packets are transmitted with source
and destination UDP port <TBD1> in an IPv4 packet. The source and
destination addresses MUST be associated with the local and remote
systems, respectively.
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BFD Echo packets are transmitted with source and destination UDP port
<TBD2> in an IPv4 packet. The source and destination addresses MUST
both be associated with the local system. The destination address
MUST be chosen in such a way as to cause the remote system to forward
the packet back to the local system.
4.6.2. BFD for IPv6
In the case of IPv6, BFD Control packets are transmitted with source
and destination UDP port <TBD1> in an IPv6 packet. The source and
destination addresses MUST be associated with the local and remote
systems, respectively.
BFD Echo packets are transmitted with source and destination UDP port
<TBD2> in an IPv6 packet. The source and destination addresses MUST
both be associated with the local system. The destination address
MUST be chosen in such a way as to cause the remote system to forward
the packet back to the local system.
4.6.3. BFD for IEEE 802 Networks
BFD can also be used directly on top of the datalink layer in IEEE
802 networks. In this case, BFD packets are transmitted in an
encapsulation appropriate for the particular IEEE 802 media, with
Ether Type <TBD3>. The source and destination addresses MUST be
unicast MAC addresses associated with the local and remote systems,
respectively.
BFD Echo packets are transmitted in an encapsulation appropriate for
the particular IEEE 802 media, with Ether Type <TBD4>. The source
and destination addresses MUST both be unicast MAC addresses
associated with the local system. The destination address MUST be
chosen in such a way as to cause the remote system to forward the
packet back to the local system.
Note that BFD Echo mode is not likely to be appropriate for use
directly over the data link layer, since most data link devices are
not able to forward frames out the interface over which they were
received.
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Contributors
Kireeti Kompella and Yakov Rekhter of Juniper Networks were also
significant contributors to this document.
Acknowledgments
This document was inspired by (and is intended to replace) the
Protocol Liveness Protocol draft, written by Kireeti Kompella.
The authors would also like to thank Mike Shand, John Scudder, and
Stewart Bryant for their substantive input.
Authors' Addresses
Dave Katz
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, California 94089-1206 USA
Phone: +1-408-745-2000
Email: dkatz@juniper.net
Dave Ward
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134 USA
Phone: +1-408-526-4000
Email: dward@cisco.com
Security Considerations
When BFD is run over network layer protocols, a significant denial-
of-service risk is created, as BFD packets may be trivial to spoof.
When the session is directly connected across a single link, the TTL
MUST be set to the maximum on transmit, and checked to be equal to
the maximum value on reception (and the packet dropped if this is not
the case.) If BFD is run across multiple hops, some alternative
mechanism MUST be used. One option would be to ensure that the
network addresses used for BFD are not routable outside of the
infrastructure in which BFD is running (and assuming there are no
users connected within that network.) Another option would be to
filter all packets carrying BFD's UDP ports at the edges of the
network. Still another option would be to use cryptographic methods,
though this is not likely to allow for very short detection times.
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IANA Considerations
Two well-known UDP port numbers need to be assigned to this protocol.
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Internet Draft Bidirectional Forwarding Detection June, 2003
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