Bidirectional Forwarding Detection
Internet Draft A. Palanivelan
Category: Standards Track Cisco Systems
Expires: June 2009 December 15, 2008
BFD with Graceful Restart
draft-palanivelan-bfd-v2-gr-00.txt
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Abstract
This document proposes an extension for Bidirectional Forwarding
Detection (BFD) to support Graceful restart, in complementing
Graceful restart support of the underlying protocol.This shall
work consistently irespective of the bfd mode or protocol or
the type of restart.This document describes the challenges to bfd
in surviving a graceful restart and a generic solution to succeed.
Conventions used in this draft
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].
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Table of Contents
1 INTRODUCTION ............................................. 3
2 OVERVIEW ............................................. 3
3 MOTIVATIONS ............................................. 3
3.1 Planned and Unplanned Restarts with control protocols .... 4
3.2 BFD Co-existing with BB configs .......................... 4
4 Extensions to BFD ........................................ 5
4.1 Version (Vers) ........................................... 5
4.2 Diagnostic (Diag) ........................................ 5
4.3 My Restart Interval ...................................... 6
4.4 Your Restart Interval ................................... 6
5 Theory of operation ..................................... 6
6 Security Consderations .................................. 8
7 IANA Considerations ...................................... 8
8 References .............................................. 9
9 Author's address ......................................... 9
10 Intellectual Property Statement ......................... 9
11 Full Copyright Statement ................................ 10
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1.Introduction
The Bidirectional Forwarding Detection protocol [BFD] provides a
mechanism for liveness detection of arbitrary paths between systems.
It is intended 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.
It operates independently of media,data protocols,and routing protocols.
An additional goal is to provide a single mechanism that can be used for
liveness detection over any media, at any protocol layer, with a wide
range of detection times and overhead, to avoid a proliferation of
different methods.
The extensions introduced in this draft for bfd shall aid in bfd
complementing the GR capabilities of protocols such as ospf and also in
providing a consistent behavior for planned/unplanned restarts
irrespective of the underlying protocols.
2.Overview
The Bidirectional Forwarding Detection (BFD) specification defines a
protocol with simple and specific semantics. Its sole purpose is to
verify connectivity between a pair of systems, for a particular data
protocol across a path (which may be of any technology, length, or
OSI layer). The promptness of the detection of a path failure can be
controlled by trading off protocol overhead and system load with
detection times.
The extensions introduced in this draft for bfd shall aid in bfd
complementing the GR capabilities of protocols such as ospf and also
in providing a consistent behavior for planned/unplanned restarts
for the underlying protocols.
3.Motivations
Though the existing drafts discuss bfd interactions with applications
with Graceful Restart and ways of implementing in serving successful GR,
the drafts itself have some exceptions and caveats applied.
This draft in particular discusses the issues in the following scenarios
and provides a generic solution that would scale for future applications.
* Unplanned restart.
* Planned restart with a control protocol such as ISIS,
which cannot signal GR.
* BFD co-existing with BB configs
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This document tries to address the above issues in specific and Graceful
restart mechanism in general, for bfd.
3.1 Planned and Unplanned Restarts with control protocols
The existing bfd drafts suggest administratively disabling bfd prior
to the start of GR. But, this works only for planned restarts and not
for unplanned restarts. This also does not work for a protocol such
as isis that cannot signal a planned restart.
For a Planned restart where a control protocol can signal before
restarting, if a BFD session failure occurs during the restart, it is
recommended in the existing draft(s) that, such a planned restart
SHOULD NOT be aborted and the session failure SHOULD NOT result in a
topology change being signaled in the control protocol.
Control protocols that cannot signal a planned restart depend on the
recently restarted system to signal the Graceful Restart prior to the
control protocol adjacency timeout. In most cases, whether the
restart is planned or unplanned, it is likely that the BFD session
will time out prior to the onset of Graceful Restart, and a topology
change SHALL be signaled.
This type of implementation shall impact non-stop routing and non-stop
forwarding support using GR-enabled protocols and provides an
opportunity to review the existing bfd implementations and improve.
3.2 BFD Co-existing with BB configs
In a real time scenario with Broadband configurations,it is highly
likely that the bfd sessions do not survive a Graceful restart.
Assume a router at PE that has active DHCP sessions with a large
number of clients (say 16k). During a planned restart, it is also
likely that the DHCP clients request for renewal of IP address to the
server (restarting router) at that time. When the router is
restarting, these requests do not reach the router. But, when these
requests reach the router when the router has just come up,
it will treat these requests at a high priority and responds to them.
When we have thousands of such requests to the restarting router,
the router shall spend a major part of its first second of uptime
in addressing these requests. In this scenario, a control protocol
like ospfv2 that has GR enabled, shall withstand the restart for the
specified restart interval (as it will be in seconds) and it is
likely to survive the restart in maintaining its forwarding plane.
In the same scenario, if bfd is enabled for ospfv2, for an unplanned
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restart, the (bfd) neighbor router will be expecting bfd control
packets in milliseconds interval and during the restart process,
is likely to timeout, also impacting the associated ospfv2 adjacency
and resulting in loss of traffic.
The scenario will be the same for bfd with a protocol such as is-is,
where the problem is likely to be seen for a planned/unplanned
restarts.
4.Extensions to BFD
This draft introduces a new diag value to indicate that the neighbor is
restarting and provisions to configure graceful restart timers.The modified
Generic BFD Control Packet Format shown below introduces two additional
sections "My Restart Interval" and "Your Restart Interval".
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Vers | Diag |Sta|P|F|C|A|D|M| Detect Mult | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| My Discriminator |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Your Discriminator |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Desired Min TX Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Required Min RX Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Required Min Echo RX Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| My Restart Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Your Restart Interval |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.1 Version (Vers)
The version of bfd defined by this draft, that has support for gr
configuration and a diag for neighbor restarting state, shall have
a value of 2.
4.2 Diagnostic (Diag)
A diagnostic code specifying the local system's reason for the last
change in session state.
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A new diag value 9 for "Neighbor Restarting" is introduced in this
draft.Values are:
0 -- No Diagnostic
1 -- Control Detection Time Expired
2 -- Echo Function Failed
3 -- Neighbor Signaled Session Down
4 -- Forwarding Plane Reset
5 -- Path Down
6 -- Concatenated Path Down
7 -- Administratively Down
8 -- Reverse Concatenated Path Down
9 - Neighbor Restarting
10-31 -- Reserved for future use
This field allows remote systems to determine the reason that the
previous session failed.
4.3 My Restart Interval
This is the restart interval,in microseconds, of the transmitting
system advertised to the remote system. In the case of a restart (of
transmitting system), the remote system is xpected to keep the bfd
session up for this duration of time.
4.4 Your Restart Interval
The restart interval,in microseconds, received from the corresponding
remote system. In the case of a restart (of remote system), the
transmitting system is expected to keep the bfd session up for this
duration of time.
5.Theory of Operation
The system that has support for high-availability, when using a routing
protocol that is GR enabled, shall continue to forward traffic during a
restart.when bfd is enabled on such a protocol, it is expected to assist
the process than disturb it.
With current bfd implementations, the bfd sessions do not survive a restart
under different conditions.An Unplanned restart or a planned restart with a
protocol such as isis that cannot signal about restart, are some of the
conditions where bfd config is set to impact a high-availability situation.
Though there are certain implementations adopted by various companies to
make bfd survive restarts, there is no uniform method of achieving this and
is likely to fail when interop with routers from other companies.This draft
proposes a standard way of achieving this objective.
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This draft recommends the introduction of a new diag value (9 for "Neighbor
restarting"), new version (2 for gr supported bfd) and two additional
sections to the bfd packet format.This design is expected to provide a
capability to bfd in withstanding restart scenarios, in complementing the
associated protocol.This shall work consistently irespective of the bfd
mode or protocol or the type of restart.
This bfd implementation shall have its version field set to 2, that
indicates the support for GR.The two new sections to the bfd packet
format,"My Restart Interval" and "Your Restart interval" shall be used to
exchange the GR timers info. between the systems.
"My Restart Interval" is the time interval in microseconds,that this system
expects its remote system to wait for, before bringing down its bfd session
with this system.
"Your Restart Interval" is the time interval in microseconds, specified by
the remote system, that it expects this system, to wait for, before bringing
down its bfd session with this system.
The initial bfd packet exhange from the system to remote system shall have
the configured value for the "My Restart Interval" or 0.The "Your Restart
Interval" will reflect the value received in "My Restart Inteval" from the
corresponding remote system or is Zero if value is unknown.A value of Zero
for "Your Restart Interval" shall mean that the bfd gr is diabled at the
remote end and similarly a value of Zero for "My Restart Interval" shall
mean that bfd gr is disabled at the transmitting system.This effectively
indicates to the other system if there is a requirement for it to wait for
"restart interval" before timer expiry or not,but doesn't limit the system
to act as a gr-helper.In other words, all systems with bfdv2 configurations
shall act as GR helpers.
Once the packet exchanges are complete and the bfd sessions are up,every bfd
session will have info, about the time interval, its remote system will wait
during a Restart and also the time interval this system has to wait,when the
remote system restarts.The "My restart interval" value can be modified after
the session is up,and in this case, the packet exchanges shall sync up the
restart interval times (My and Your) on both the sides appropriately.
The "My Restart Interval" configured shall have a value of 0 or a value more
than its (Tx interval * Multiplier) value.The system shall not allow
configuration of any value between these values.
For Planned restarts, with bfd sessions succesfully up with a protocol such
as ospfv2 and gr enabled,ospf shall signal its adjacent neighbor (al o a
ospfv2-bfd neighbor) that a restart is about to begin. The bfd diag at the
transmitting system (Restarter) shall be set to a value of 9 (Neighbor
Restarting) if its "My Restart Interval" has a value other than 0, and bfd
shall maintain its neighborship with the remote system for "Your Restart
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Interval" time(at remote neighbor) and the remote system shallnot bring down
the adjacency,knowing that the neighbor is restarting (known through the
diag value).
For Planned restarts, with bfd sessions succesfully up with a protocol such
as isis and gr enabled,isis shall not signal its adjacent neighbor (also a
isis-bfd neighbor) that a restart is about to begin.Here again, the remote
system shall get to know that the other end is restarting,only after the
restart begins.The bfd diag at the transmitting system (Restarter) shall be
set to a value of 9 (Neighbor Restarting) if its "My Restart Interval" has
a value other than 0, and bfd shall maintain its neighborship with the
remote system for "Your Restart Interval" time (at remote neighbor) and the
remote system shall not bring down the adjacency,knowing that the neighbor
is restarting (known through the diag value).
For Unplanned restarts,the system cannot signal to its neighbor that a
restart is about to begin.In this scenario, Once the restart begins, the
system will signal the restart,at the first possible instance.The bfd diag
at the transmitting system(Restarter) shall be set to a value of 9
(Neighbor Restarting)if its "My Restart Interval" has a value other than 0,
and bfd shall maintain its neighborship with the remote system for "Your
Restart Interval" time (at remote neighbor) and the remote system shall not
bring down the adjacency,knowing that the neighbor is restarting (known
through the diag value). It may also be noted that attempting a GR for
unplanned restart may not be a good idea, since the router may not properly
prepare for a restart.The implementators,in this case,shall optionally
provide a knob to turn the option off.
6.Security Considerations
Security considerations discussed in [BFD], [BFD-1HOP] and [BFD-MHOP]
apply to this document.
7.IANA Considerations
This document currently defines a Diag value of 9 to be used to specify
"Neighbor Restarting"
This document introduces two new sections "My Restart Interval" and "Your
Restart Interval" to the bfd generic packet format.
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8.References
[BFD] Katz, D., et al., " Bidirectional Forwarding Detection ",
draft-ietf-bfd-base-08.txt, {work in progress}.
[BFD-1HOP] Katz, D., Ward, D.,'' BFD for IPv4 and IPv6 (Single Hop)'',
draft-ietf-bfd-v4v6-1hop-08.txt, {work in progress}.
[BFD-MULTI] Katz, D., and Ward, D., "BFD for Multihop Paths", draft-
ietf-bfd-multihop-06.txt, January, 2008.
9.Authors' Addresses
Palanivelan A
Cisco Systems,
Bangalore,India.
Email: apvelan@cisco.com
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11.Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
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This document and the information contained herein are provided on an
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THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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