One document matched: draft-white-bounded-longest-match-02.txt
Differences from draft-white-bounded-longest-match-01.txt
Network Working Group R. White
Internet-Draft Cisco Systems
Intended status: Experimental S. Hares
Expires: February 1, 2009 NextHop Technologies
T. Hardie
July 31, 2008
Bounding Longer Routes to Remove TE
draft-white-bounded-longest-match-02
Status of this Memo
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Abstract
Some ASes currently use length-based filters to manage the size of
the routing table they use and propagate. This draft explores an
alternative to length-based filters which allows for more automatic
configuration and which provides for better redundancy.
Rather than use a filter, this draft proposes a method of modifying
the BGP [RFC1771] longest match algorithm by setting a bound on the
prefix lengths eligible for preference. A bound would operate on
long prefixes when covering route announcements are available; in
certain circumstances it would cause a router to prefer an aggregate
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over a more specific route announcement.
1. Introduction
Many routes injected into the global default free zone of the
Internet today are injected to steer traffic (or provide traffic
engineering), rather than to provide reachability information
directly. In several recent discussions, it has been asserted that
this table growth due to routes injected to provide traffic
engineering is causing many problems within the default free zone,
including more table instability, as these routes appear to change
state more often than shorter prefix aggregate routes.
While filtering all routes at some predetermined length is an
attractive option, it can be difficult to maintain and manage large
filter sets built around a constantly changing database. It appears
a more fruitful approach would be to detect routes injected for
traffic engineering purposes, and remove them from the routing system
automatically once they are beyond the point in the network where
they are useful. This draft proposes a mechanism to perform just
this task. When two routes with overlapping prefixes are detected,
they are marked, and removed from the routing system, at a point
where they are no longer needed. This mechanism does not suffer from
any problems from route withdraws or failures, since routing will
naturally take care of any connectivity changes. Various estimates
have stated that removing the longer prefix routes within the routing
table could reduce the table size by 25%.
No actual changes to the operation of the BGP protocol at the packet
or peering levels are required to implement this draft. A new well
known non-transitive community is proposed.
2. Proposed Enhancements
Two enhancements are proposed by this draft: three new communities,
and a new way of handling overlapping prefixes received from an
external peer.
As each prefix is received by a BGP speaker from an external peer, it
would be evaluated in the light of other prefies already received.
If two prefixes overlap in space (such as 192.168.0.0/16 and
192.168.1.0/24), the longer prefix would be marked with a new BOUNDED
community, and the local preference set to a very high number so that
it would always win in any best path computations within the
autonomous system. The longer prefix may also be marked with a new
community, NO_INSTALL.
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Routes marked with the new BOUNDED community MAY be filtered at the
autonomous system edge to reduce the number of routes advertised by
an AS.
2.1. Example of Bounding the Longer Prefix
Assume the following configuration of autonomous systems:
( )
/-------( AS2 )--------\
( ) / ( ) \ ( ) ( )
( AS1 ) ( AS4 )-----( AS5 )
( ) \ ( ) / ( ) ( )
\-------( AS3 )--------/
( )
o AS1 is advertising 192.168.1.0/24 to both AS2 and AS3.
o AS2 is advertising both 192.168.1.0/24 and 192.168.0.0/16 into
AS4.
o AS3 is advertising 192.168.1.0/24 into AS4
o Each connection (session) is handled by a seperate router within
each AS (for instance, AS4 peers with AS2 and AS3 on a seperate
routers).
When the peering router in AS4 between AS4 and AS2 receives both the
192.168.1.0/24 and the 192.168.0.0/16 prefixes, it will mark the
192.168.1.0/24 as BOUNDED, and set the local preference high, based
on its router ID, as described in the section Setting the Local
Preference, below, and will then propogate this through AS4.
The border router between AS4 and AS3 will receive the longer prefix
from AS3, and the iBGP prefix with the high local preference with
BOUNDED set. Given it does not see the overlapping prefix, it will
compare the default (lower) local preference of the externally
learned route with the higher local preference set by the AS2/AS4
border router, and will not advertise the 192.168.1.0/24 prefix into
AS4 at all.
The AS3/AS4 border router may also, on detecting the overlap, mark
the longer prefix with a new community, NO_INSTALL, which is non-
transitive and optional. Router which understand this community may
choose not to install this prefix into the local RIB, in order to
reduce memory consumption.
If the link between AS1 and AS2 fails, the longer length prefix will
be withdrawn from AS2, and thus the peering point between AS2 and AS4
will no longer have an overlapping set of prefixes. Within AS4, the
border router which peers with AS2 will cease advertising the
192.168.1.0/24 prefix, which allows the AS3/AS4 border router to
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being advertising it into AS4, and through AS4 into AS5, restoring
connectivity to AS1.
2.2. Setting the Local Preference
Since there could be multiple points at which an autonomous system
may receive the same pair of overlapping prefixes, there must be some
way to ensure that one of the longer prefixes wins in the [BGP]
decision algorithm consistently. In practice, this means that each
BGP speaker which receives an overlapping set of routes should set
the local preference on the set of longer prefixes so there won't be
two longer prefixes with matching local preferences.
The easiest way to ensure this within an autonomous system is to set
the local preference for longer prefixes based on some unique number
assigned to each BGP speaker. Given the router ID and the local
preference are both 32 bit numbers, an ideal solution appears to be
to simply set the local preference to the router ID of the BGP
speaker. The primary problem with this is that in some cases, the
router ID of the device may be lower than some standard Local
Preference, perhaps even lower than a standard Local Prference used
by default throughout a network.
To alleviate this problem, the local preference of longer prefixes
which overlap with shorter prefixes should be set to the router ID of
the BGP speaker, and then the high order bit of the Local Preference
should be set, so the setting will be gauranteed to be at least above
64,000.
2.3. The NO_INSTALL Community
An optional optimization to bounding longer prefixes by marking them
with a high Local Preference and the BOUNDED community is to also
mark them with a new, non-trasitive, optional community, NO_INSTALL.
The effect of this community would be for any BGP speaker receiving a
prefix with this community set to treat the prefix normally in the
BGP bestpath computation, and to forward bestpaths marked as
NO_INSTALL to iBGP peers, but to simply fail to install such prefixes
in the local routing table.
This would result in a some small amount of information stored and
maintained in the local routing table, and the local forwarding
tables built from the local routing table. If there are enough
prefixes thus marked, the memory and computation savings could be
significant. BGP sepakers which receive a prefix marked with
NO_INSTALL, and which do not understand this community, simply ignore
the community.
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3. The NO_BOUNDING Community
In some situations, the originator of a longer length prefix might
determine their routing will not work properly if their prefix is
bounded at a point where it overlaps with a shorter prefix aggregate.
To resolve this case, we propose a new transitive optional extended
community, NO_BOUNDING.
The NO_BOUNDING extended community consists of a type, to be
determined through the IANA process, and a value containing the
minimum AS Path length before which the route should not be bounded.
If a BGP speaker determines a route could be bounded, but the route
is marked with NO_BOUNDING, and the AS Path length is shorter than
the minumum AS Path length noted in the NO_BOUNDING extended
community, they speaker SHOULD NOT mark the route for bounding.
This allows the originator of a prefix to control the bounding
properties of the prefix.
4. Benefits and Risks
The benefits and risks associated with this proposal are discussed in
the sections below.
4.1. Advantages to the Service Provider
AS4, in each of the situations, reduces the number of prefixes carred
through the autonomous system by the number of longer prefixes that
overlap with aggregates of those prefixes. While one copy of the
prefix continues to be carried through the autonomous system, this
entry can be marked with the optional NO_INSTALL community, so it is
not placed in the forwarding table, nor is it propogated outside the
autonomous system.
AS5 receives one prefix instead of two (or possibly more).
4.2. Advantages to the Customer
In this case, the customer is respresented as AS1. The customer will
continue to receive some amount of traffic over both peering
sessions, and dual homing through two Service Providers is still
effective. If the customer's primary link fails, the alternate link
through AS3 will take over receving all inbound traffic
automatically. With most other schemes presented to this point, the
customer loses all impact of dual-homing into the Internet, unless
both connections are through one Service Provider.
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4.3. Advantages to the Internet
Beyond the second AS hop, aggregation is preserved in all cases.
While this would not reduce the backbone routing table by the
dramatic amounts that other methods might, the advantages to the
community are great, and at greatly reduced risk to customers.
4.4. Implications for Router processing
This proposal clearly adds to the work which needs to be done during
overall [BGP] processing. Because a check needs to be done for both
covered and covering routes, some part of this work is required for
routes of lengths on either side of the bound. Should this become
common, however, the rate of growth in the number of routes should be
smaller and a balance should be struck between the extra processing
per route and the number of routes.
4.5. Implications for Traffic engineering
The implementation of a bound risks magnifying or removing the effect
of certain widely deployed traffic engineering methods. If, for
example, an AS chose to prepend its own route to an announcement in
order to alter the preference for that route, a BGP neighbor using a
bounded longest match might now see that route as eligible for
discard in favor of an aggregate. While it is fairly easy to code
around that particular problem, to avoid this class of problems it
might be preferable to allow this to apply to specific AS Sets as
well as to all BGP neighbors.
4.6. Implications for Convergence Time
If the route to the AS providing the route to the aggregate should be
lost, the more-specific must propagate into the ASes which had
formerly heard only the aggregate. This increases convergence time
and may create situations in which reachability is temporarily
compromised. Unlike the filter case, however, normal BGP behavior
should restore reachability without changes to the router
configuration. There is a also a risk that during a pathological
event the increased processing required by this change will degrade
propagation times during those events. This depends on both the
speed of specific implementations and the character of the topology.
5. Acknowledgements
Cengiz Alaentinoglu, Alvaro Retana, Daniel Walton, David Ball, and
Barry Greene gave valuable comments on this draft. Jeff Hass
suggested the NO_BOUNDING community, along with the AS Path length
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limit described in the NO_BOUNDING section. A number of colleagues
also gave the author valuable comments on the white board markings
that gave rise to this paper; among them are Lane Patterson, Ian
Cooper, Gerd Besch, Bill Norton, Diarmuid Flynn, and Sean Donelan.
6. Security Considerations
This document presumes that the implementation of bounded longest
match is a knob inside a router config. Since the use of the knob
affects route announcements not originating within the router's AS or
its direct neighbors, the new behavior may result in surprises to the
announcing AS. It is possible that this behavior might be considered
a denial of service or mistaken for a denial of service by systems
designed to detect black-holing on behalf of the origin AS.
7. IANA Considerations
This draft proposes three new communities, BOUNDED, NO_BOUNDING, and
NO_INSTALL, for which new community values would need to be assigned.
These should be assigned as described in EXT-COMM.
8. Informative References
[BGP-TABLE]
Bush, R., "Plenary, IETF 51.
http://www.ietf.org/proceedings/01aug/".
[EXT-COMM]
Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
Communities Attribute",
draft-ietf-idr-bgp-ext-communities-09 (work in progress),
January 2006.
[RFC1771] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC 1771, March 1995.
Authors' Addresses
Russ White
Cisco Systems
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Susan Hares
NextHop Technologies
825 Victors Way
Ann Arbor, MI 48108
Phone: 734-222-1610
Fax:
Email: skh@nexthop.com
URI:
Ted Hardie
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