Internet DRAFT - draft-huston-nopeer
draft-huston-nopeer
Internet Engineering Task Force Geoff. Huston
Internet Draft Telstra
Document: draft-huston-nopeer-00.txt August 2001
Expires: February 2002
NOPEER community for BGP route scope control
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [1].
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Comments on this draft should be directed to gih@telstra.net.
Abstract
This document proposes the use of a scope control BGP community.
This new well-known advisory transitive community is intended to
allow an origin AS to specify the extent to which a specific route
should be externally propagated. In particular this community,
termed here as NOPEER, allows the origin AS to specify that a route
with this attribute need not be advertised across bilateral peer
connections.
1. Introduction
BGP today has a limited number of commonly defined mechanisms that
allow a route to be propagated across some subset of the routing
system. The NOEXPORT community allows a BGP speaker to specify that
redistribution should extend only to the neighbouring AS. Providers
commonly define a number of communities that allow their neighbours
to specify how advertised routes should be re-advertised. Current
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operational practice is that such communities are defined on as AS
by AS basis, and while they allow an AS to influence the re-
advertisement behaviour of routes passed from a neighbouring AS,
they do not allow this scope definition ability to be passed in a
transitive fashion to a remote AS.
Advertisement scope specification is of most use in specifying the
boundary conditions of route propagation. The specification can take
on a number of forms, including as AS transit hop count, a set of
target ASs, the presence of a particular route object, or a
particular characteristic of the inter-AS connection.
There are a number of motivations for controlling the scope of
advertisement of route prefixes, including support of limited
transit services where advertisements are restricted to certain
transit providers, and various forms of selective transit in a
multi-homed environment.
This proposal does not attempt to address all such motivations of
scope control, and addresses in particular the situation of both
multi-homing and traffic engineering. The commonly adopted
operational technique is that the originating AS advertises an
encompassing aggregate route to all multi-home neighbours, and also
selectively advertises a collection of more specific routes. This
implements a form of destination-based traffic engineering with some
level of fail over protection. The more specific routes typically
cease to lever any useful traffic engineering outcome beyond a
certain radius of redistribution, and a means of advising that such
routes need not to be distributed beyond such a point is of some
value in moderating one of the factors of continued route table
growth.
Analysis of the BGP routing tables reveals a significant use of the
technique of advertising more specific prefixes in addition to
advertising a covering aggregate. In an effort to ameliorate some of
the effects of this practice, in terms of overall growth of the BGP
routing tables in the Internet and the associated burden of global
propagation of dynamic changes in the reachability of such more
specific address prefixes, this draft proposes the use of a
transitive BGP route attribute that is intended to allow more
specific route tables entries to be discarded from the BGP tables
under appropriate conditions. Specifically, this attribute, NOPEER,
allows a remote AS not to advertise a route object to a neighbour AS
when the two AS's are interconnected under the conditions of some
form of sender keep all arrangement, as distinct from some form of
provider / customer arrangement.
2. Proposal
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The proposal is to add a well-known transitive community, NOPEER.
The intended semantics of this attribute is to allow an AS to
interpret the presence of this community as an advisory
qualification to re advertisement of a route prefix, permitting an
AS not to re advertise the route prefix to all external bilateral
peer neighbour AS's. It is consistent with the intended semantics
that an AS may filter received prefixes that are received across a
peering session that the receiver regards as a bilateral peer
sessions.
3. Motivation
The size of the BGP routing table has been increasing at an
accelerating rate since late 1998. At the time of writing (August
2001) the BGP forwarding table contains over 100,000 entries, and
the three year growth rate of this table shows a trend rate which
can be correlated to a compound growth rate of no less than 40% per
year [2].
One of the aspects of the current BGP routing table is the
widespread use of the technique of advertising both an aggregate and
a number of more specific address prefixes. For example, the table
may contain a routing entry for the prefix 10.0.0.0/23 and also
contain entries for the prefixes 10.0.0.0/24 and 10.0.1.0/24. In
this example the specific routes fully cover the aggregate
announcement. Sparse coverage of aggregates with more specifics is
also observed, where, for example, routing entries for 10.0.0.0/8
and 10.0.1.0/24 both exist in the routing table. In total, these
more specific route entries occupy some 52% of the routing table[3],
so that more than one half of the routing table does not add
additional address reachability information into the routing system,
but instead is used to impose a finer level of detail on existing
reachability information.
There are a number of motivations for having both an aggregate route
and a number of more specific routes in the routing table, including
various forms of multi-homed configurations, where there is a
requirement to specify a different reachability policy for a part of
the advertised address space.
One of the observed common requirements in the multi-homed network
configuration is that of undertaking some form of load balancing of
incoming traffic across a number of external connections to a number
of different neighbouring ASs. If, for example, an AS wishes to use
a multi-homed configuration for routing-based load balancing and
some form of mutual fail over between the multiple access
connections for incoming traffic, then one approach is for the AS to
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advertise the same aggregate address prefix to a number of its
upstream transit providers, and then advertise a number of more
specifics to individual upstream providers. In such a case all of
the traffic destined to the more specific address prefixes will be
received only over those connections where the more specific has
been advertised. If the neighbour BGP peering session of the more
specific advertisement fails, the more specific will cease to be
announced and incoming traffic will then be passed to the
originating network based on the path associated with the
advertisement of the encompassing aggregate. In this situation the
more specific routes are not automatically subsumed by the presence
of the aggregate at any remote AS. Both the aggregate and the
associated more specifics are redistributed across the entire
external BGP routing domain. In many cases, particularly those
associated with desire to undertake traffic engineering and service
resilience, the more specific routes are redistributed well beyond
the scope where there is any outcomes in terms of traffic
differentiation.
To the extent that remote analysis of BGP tables can observe this
form of configuration, the number of entries in the BGP forwarding
table where more specific entries share a common origin AS with
their immediately enclosing aggregates comprise some 20% of the
total number of FIB entries. Using a slightly stricter criteria
where the AS path of the more specific route matches the immediately
enclosing aggregate, the number of more specific routes comprises
some 13% of the number of FIB entries [3].
One protocol mechanism that could be useful in this context is to
allow the originator of an advertisement to state some additional
qualification on the redistribution of the advertisement, allowing a
remote AS to suppress further redistribution under some originator-
specified criteria.
The redistribution qualification condition can be specified either
by enumeration or by classification. Enumeration would encompass the
use of a well-known transitive extended community to specify a list
of remote AS's where further redistribution is not advised. The
weakness of this approach is that the originating AS would need to
constantly revise this enumerated AS list to reflect the changes in
inter-AS topology, as, otherwise, the more specific routes would
leak beyond the intended redistribution scope. An approach of
classification allows an originating AS to specify the conditions
where further redistribution is not advised without having to refer
to the particular AS's where a match to such conditions are
anticipated.
The approach proposed here to specifying the redistribution boundary
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condition is one based on the type of bilateral inter-AS peering.
Where one AS can be considered as a customer, and the other AS can
be considered as a contracted agent of the customer, or provider,
then the relationship is one where the provider, as an agent of the
customer, carries the routes and associated policy associated with
the routes. Where neither AS can be considered as a customer of the
other, then the relationship is one of bilateral peering, and
neither AS can be considered as an agent of the other in
redistributing policies associated with routes. This latter
arrangement is commonly referred to as a "sender keep all peer"
relationship, or "peering". This peer boundary can be regarded as a
logical point where the redistribution of additional reachability
policy imposed by the origin AS on a route is no longer an imposed
requirement.
This approach allows an originator of a prefix to attach a commonly
defined policy to a route prefix, indicate that a route should be
re-advertised conditionally, based on the characteristics of the
inter-AS connection.
4. IANA considerations
Adoption of this proposal would imply the request to IANA for the
registration of a new BGP well-known transitive community field from
IANA.
5. Security considerations
This proposal has the capability to introduce additional security
concerns into BGP by allowing the potential for denial of service
attacks for an address prefix range being launched by a remote AS.
Unauthorized addition of this community to a route prefix by a
transit provider where this is no covering aggregate route prefix
may cause a denial of service attack based on denial of reachability
to the prefix. Even in the case that there is a covering aggregate,
if the more specific route has a different origin AS than the
aggregate, the addition of this community by a transit AS may cause
a denial of service attack on the origin AS of the more specific
prefix.
BGP is already vulnerable to a denial of service attack based on the
injection of false routing information. It is possible to use this
community to limit the redistribution of a false route entry such
that its visibility can be limited and detection and rectification
of the problem can be more difficult under the circumstances of
limited redistribution.
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References
[1] RFC 2026
[2] draft-iab-bgparch-01.txt - Work in Progress
[3] Analysis of BGP table data - http://www.telstra.net/ops/bgp
Author's Address
Geoff Huston
Telstra
5/490 Northbourne Ave, Dickson, ACT Australia
Email: gih@telstra.net
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