INTERNET-DRAFT
Bidirectional Forwarding Detection A. Palanivelan
Category: EXPERIMENTAL Cisco Systems
Expires: Nov 2010 April 21, 2010
Bidirectional Forwarding Detection (BFD) with Graceful Restart
draft-palanivelan-bfd-v2-gr-04.txt
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A.Palanivelan [Page 1]
Internet Draft draft-palanivelan-bfd-v2-gr-04.txt Apr 2010
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.The intention of
this document is to provide a solution that works fine for all types
of bfd implementations.
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)..................................... 6
4.3 My Restart Interval.................................... 6
4.4 Your Restart Interval.................................. 6
5 Theory of operation......................................... 7
6 Security Consderations...................................... 9
7 IANA Considerations......................................... 8
8 References ................................................. 9
8.1 Normative References................................... 9
8.2 Informative References................................. 9
9 Author's address............................................ 10
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Internet Draft draft-palanivelan-bfd-v2-gr-04.txt Apr 2010
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.The intention of this document
is to provide a solution that works fine for all types of bfd
implementations.
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.
BFD is assumend to be working fine without a need for any GR support in
it.But, the deployments show that the different types of implementations
in the products and their inherent mechanisms lead to issues with bfd
especially in surviving GR.It is true that prioristising bfd would make
sure the other cpu intensive processes do not fail bfd, but this won't
be possible as there may be other higher priority processing that cant
be ignored.Example for this are the existing subscriber connections that
can't be given a lesser priority.
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 and to provide a solution that works fine for all types
of bfd implementations.
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Internet Draft draft-palanivelan-bfd-v2-gr-04.txt Apr 2010
* Unplanned restart.
* Planned restart with a control protocol such as ISIS,which cannot
signal GR.
* BFD co-existing with BB configs
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 [OSPF-GRACE], 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
[ISIS-GRACE], 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 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.
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4.2 Diagnostic (Diag)
A diagnostic code specifying the local system's reason for
the last change in session state. 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.
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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.
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 exchange 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
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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 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).A timer shall be started to wait till the "your restart
interval" and once the gr process is complete, shall be reset to "0".
The diag value is also reset to 0.
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.
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6. Security Considerations
Security considerations discussed in [BFD], [BFD-1HOP] 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.
8. References
8.1. Normative References
[BFD] Katz, D., and Ward, D., "Bidirectional Forwarding Detection",
draft-ietf-bfd-base-09.txt, February, 2009.
[BFD-1HOP] Katz, D., and Ward, D., "BFD for IPv4 and IPv6 (Single
Hop)", draft-ietf-bfd-v4v6-1hop-08.txt, February, 2009.
8.2. Informative References
[ISIS-GRACE] Shand, M., and Ginsberg, L., "Restart signaling for IS-
IS", RFC 5306, October 2008.
[OSPF-GRACE] Moy, J., et al, "Graceful OSPF Restart", RFC 3623,
November 2003.
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Authors' Addresses
Palanivelan A
Cisco Systems,
Bangalore,India.
Email: apvelan@cisco.com
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