GROW T. Griffin
Internet-Draft University of Cambridge
Expires: September 27, 2004 G. Huston
APNIC
March 29, 2004
BGP Wedgies
draft-ietf-grow-bgp-wedgies-01.txt
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Copyright (C) The Internet Society (2004).
Abstract
It has commonly been assumed that the Border Gateway Protocol (BGP)
is a tool for distributing reachability information in a manner that
creates forwarding paths in a deterministic manner. In this memo we
will describe a class of BGP configurations for which there is more
than one potential outcome, and where forwarding states other than
the intended state are equally stable, and that the stable state
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where BGP converges may be selected by BGP in a non-deterministic
manner. These stable, but unintended, BGP states are termed here
"BGP Wedgies".
1. Introduction
It has commonly been assumed that the Border Gateway Protocol (BGP)
[RFC1771] is a tool for distributing reachability information in a
manner that creates forwarding paths in a deterministic manner. This
is a 'problem statement' memo that describes a class of BGP
configurations for which there is more than one stable forwarding
state. In this class of configurations forwarding states other than
the intended state are equally stable, and the stable state where BGP
converges may be selected by BGP in a non-deterministic manner.
These stable, but unintended, BGP states are termed here "BGP
Wedgies".
2. Describing BGP Routing Policy
BGP routing policies generally reflect each network administrator's
objective to optimize their position with respect to their network's
cost, performance and reliability.
With respect to cost optimization, the local network's default
routing policy often reflects a local preference to prefer routes
learned from a customer to routes learned from some form of peering
exchange. In the same vein the local network is often configured to
prefer routes learned from a peer or a customer over those learned
from a directly connected upstream transit provider. These
preferences may be expressed via a local preference configuration
setting, where the local preference overrides the AS path length
metric of the base BGP operation.
In terms of engineering reliability in the inter-domain routing
environment it is commonly the case that a service provider may enter
into arrangements with two or more upstream transit providers,
passing routes to both providers , and receiving traffic from both
sources. If the path to one upstream fails the traffic will switch
to other links, and once the path is recovered, the traffic should
switch back.
In such situations of multiple upstream providers it is also
commonplace to place a relative preference on the providers, so that
one connection is regarded as a preferred, or "primary" connection,
and other connections are regarded as less preferred, or "backup"
connections. The intent is typically that the backup connections
will be used for traffic only for the duration of a failure in the
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primary connection.
It is possible to express this primary / backup policy using local AS
path prepending, where the AS path is artificially lengthened towards
the backup providers, using additional instances of the local AS.
This is not a deterministic selection algorithm, as the selected
primary provider may in turn be using AS path prepending to its
backup upstream provider, and in certain cases the path through the
backup provider may still be selected as the shortest AS path length.
An alternative approach to routing policy specification uses BGP
communities [RFC1997]. In this case the provider publishes a set of
community values that allows the client to select the provider's
local preference setting. The client can use a community to mark a
route as "backup only" towards the backup provider, and "primary
preferred' to the primary provider, assuming both providers suppoprt
community values with such semantics. In this case the local
preference overrides the AS path length metric, so that if the route
is marked "backup only", the route will be selected only when there
is no other source of the route.
3. BGP Wedgies
The richness of local policy expression through the use of
communities, when coupled with the behavior of a distance vector
protocol like BGP leads to the observation that certain
configurations have more than one "solution", or more than one stable
BGP state. An example of such a situation is indicated in Figure 1.
+----+ +----+
|AS 3|----------------|AS 4|
+----+ peer peer +----+
|provider |provider
| |
|customer |
+----+ |
|AS 2| |
+----+ |
|provider |
| |
|customer |customer
+-------+ +----------+
backup| |primary
+----+
|AS 1|
+----+
Figure 1
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In this case AS1 has marked its advertisement of prefixes to AS2 as
"backup only", and its advertisement of prefixes to AS4 as "primary".
AS3 will hear AS4's advertisement across the peering link, and pick
of AS1's prefixes with the path "AS4, AS1". AS3 will advertise this
to AS2. AS2 will hear two paths to AS1, the first is by the direct
connection to AS1, and the second is via the path "AS3, AS4, AS1".
AS2 will prefer the longer path as the directly connected routes are
marked "backup only", and AS2's local preference decision will prefer
the AS3 advertisement over the AS1 advertisement.
This is the intended outcome of AS1's policy settings, where no
traffic passes from AS2 to AS1, and AS2, reaches AS1 via a path that
transits AS3 and AS4.
This intended outcome is achieved as long as AS1 announces its routes
on the primary path, to AS4, before announcing its backup routes to
AS2.
If the AS1 - AS4 path is broken, causing aBGP sesssion failure
between AS1 and AS4, then AS4 will withdraw its advertisement of
AS1's routes to AS3, who, in turn will send a withdrawal to AS2.
As2, will then select the backup path to AS1. AS2 will advertise
this path to AS3, and AS3 will advertise this path to AS4. Again,
this is part of the intended operation of the primary / backup policy
setting.
When connectivity between AS4 and AS1 is restored the BGP state will
not revert to the original state. AS4 will learn the primary path to
AS1, and readvertise this to AS3 using the path "AS4, AS1". AS3,
using a default preference of preferring customer-advertised routes
over peer routes will continue to prefer the "AS2, AS1" path. AS3
will not pass any updates to AS2. After the restoration of the
circuit traffic from AS3 to AS1 and from AS2 to AS1 will be presented
to AS1 via the backup path, even through the primary path via AS4 is
in service.
The intended forwarding state can only be restored by AS1
deliberately bringing down its eBGP session with AS2, even though it
is carrying traffic. This will cause the BGP state to revert to the
intended configuration.
It is often the case that an AS will attempt to balance incoming
traffic across multiple providers, again using the primary / backup
mechanism. For some prefixes one link is configured as the primary
link, and the others as the backup link, while for other prefixes
another link is selected as the primary link. An example is shown in
Figure 2.
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+----+ +----+
|AS 3|----------------|AS 4|
+----+ peer peer +----+
|provider |provider
| |
|customer |customer
+----+ +----+
|AS 2| |AS 5|
+----+ +----+
|provider |provider
| |
|customer |customer
+-------+ +----------+
backup| |primary for 192.9.200.0/25
primary| |backup for 192.9.200.128/25
+----+
|AS 1|
+----+
Figure 2
The intended configuration has all incoming traffic for addresses in
the range 192.9.200.0/25 via the link from AS5, and all incoming
traffic for addresses in the range 192.9.200.128/25 from AS2.
In this case if the link between AS3 and AS4 is reset, AS3 will learn
both routes from AS2, and AS4 will learn both routes from AS5. As
these customer routes are preferred over peer routes, when the link
between AS3 and AS4 is restored, neither AS will alter its routing
behavior with respect to AS1's routes. This situation is now wedged,
in that there is no eBGP peering that can be reset that will flip BGP
back to the intended state. This is an instance of a BGP Wedgie.
The restoration path here is that AS1 has to withdraw the backup
advertisements on both paths and operate for an interval without
backup, and then readvertise the backup prefix advertisements. The
length of the interval cannot be readily determined in advance, as it
has to be sufficiently long so as to allow AS2 and AS5 to learn of an
alternate path to AS1. At this stage the backup routes can be
readvertised.
4. Multi-Party BGP Wedgies
This situation can be more complex when three or more parties provide
upstream transit services to an AS. An example is indicated in
Figure 3.
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+----+ +----+
|AS 3|----------------|AS 4|
+----+ peer peer +----+
||provider |provider
|+-----------+ |
|customer |customer |
+----+ +----+ |
|AS 2|-------|AS 5| |
+----+ peer +----+ |
|provider |provider |
| | |
|customer +-+customer |customer
+-------+ |+----------+
backup| ||primary
+----+
|AS 1|
+----+
Figure 3
In this example the intended state is that AS2 and AS5 are both
backup providers, and AS4 is the primary provider. When the link
between AS1 and AS4 breaks and is subsequently restored, AS3 will
continue to direct traffic to AS1 via AS2 or AS5. In this case a
single reset of the link between AS2 and AS1 will not restore the
original intended BGP state, as the BGP-selected best route to AS1
will switch to AS5, and AS2 and AS3 will learn a path to AS1 via AS5.
What AS1 is observing is incoming traffic on the backup link from
AS2. Resetting this connection will not restore traffic back to the
primary path, but instead will switch incoming traffic over to AS5.
The action required to correct the situation is to simultaneously
reset both the link to AS2, and also the link to AS5. This is not
necessarily an intuitive solution, as at any point on time only one
of these links will be carrying backup traffic, yet both BGP sessions
need to be brought down at the same time in order to commence
restoration of the intended primary and backup state.
5. BGP and Determinism
BGP does not behave deterministically in all cases, and, as a
consequence, there is intended and unintended non-determinism in BGP.
For example, the default final tie break in some implementations of
BGP is to prefer the longest-lived route. To achieve determinism in
this last step it would be necessary to use a comparison operator
that has a predictable outcome, such as a comparison of router
identifiers. This class of non-deterministic behavior is termed here
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"intended" non-determinism, in that the policy interactions are, to
some extent, predictable by network administrators.
BGP is also able to generate outcomes that can be described as
"unintended non- determinism" that can result from unexpected policy
interactions. These outcomes do not represent misconfiguration in
the standard sense, since all policies may look completely rational
locally, but their interaction across multiple routing entities can
cause unintended outcomes, and BGP may reach a state that includes
such unintended outcomes in a non-deterministic manner.
Unintended non-determinism in BGP would not be as critical an issue
if all stable routings were guaranteed to be consistent with the
policy writer's intent. However, this is not always the case. The
above examples indicate that the operation of BGP allows multiple
stable states to exist from a single configuration state, where some
of these states are not consistent with the policy writer's intent.
These particular examples can be described as a form of "route
pinning", where the route is pinned to a non-preferred path.
The challenge for the network administrator is to ensure that an
intended state is maintained. Under certain circumstances this can
only be achieved by deliberate service disruption, involving the
withdrawal of routes being used to forward traffic, and
re-advertising routes in a certain sequence in order to induce an
intended BGP state. However, the knowledge that is required by any
single network operator administrator in order to understand the
reason why BGP has stabilized to an unintended state requires BGP
policy configuration knowledge of remote networks. In effect there
is insufficient local information for any single network
administrator to correctly identify the root cause of the unintended
BGP state, nor is there sufficient information to allow any single
network administrator to undertake a sequence of steps to rectify the
situation back to the intended routing state.
It is reasonable to anticipate that as the density of interconnection
increases, and also that the capability for policy-based preference
setting of learned and re-advertised routes will become more
expressive. It is therefore reasonable to anticipate that the
incidence of unintended BGP states will increase, and the ability to
understand the necessary sequence of route withdrawals and
re-advertisements will become more challenging to determine in
advance.
Whether this could lead to BGP routing system reaching a point where
each network consistently cannot direct traffic in a deterministic
manner is at this stage a matter of speculation. BGP Wedgies are an
illustration that a sufficiently complex interconnection topology,
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coupled with a sufficiently expressive set of policy constructs, can
lead to a number of stable BGP states, rather than a single intended
state. As the topology complexity increases it is not possible to
deterministically predict which state the BGP routing system may
converge to. Paradoxically, the demands of inter-domain traffic
engineering appear to require both greater levels of expressive
capability in policy-based routing directives, operating across
denser interconnectivity topologies in a deterministic manner. This
may not be a sustainable outcome in BGP-based routing systems.
6. Security Considerations
BGP is a relaying protocol, where route information is received,
processed and forwarded. BGP contains no specific mechanisms to
prevent the unauthorized modification of the information by a
forwarding agent, allowing routing information to be modified,
deleted or false information to be inserted without the knowledge of
the originator of the routing information or any of the recipients.
The memo proposes no modifications to the BGP protocol, nor does it
propose any changes to the manner of deployment of BGP, and therefore
introduces no new factors in terms of the security and integrity of
inter-domain routing.
The memo illustrates that in attempting to create policy-based
outcomes relating to path selection for incoming traffic it is
possible to generate BGP configurations where there are multiple
stable outcomes, rather than a single outcome. Furthermore, of these
instances of multiple outcomes, there are cases where the BGP
selection of a particular outcome is not a deterministic selection.
7. References
7.1 Normative References
[RFC1771] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC 1771, March 1995.
7.2 Informative References
[RFC1997] Chandrasekeran, R., Traina, P. and T. Li, "BGP Communities
Attribute", RFC 1997, August 1996.
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Authors' Addresses
Tim Griffin
University of Cambridge
EMail: Timothy.Griffin@cl.cam.ac.uk
Geoff Huston
Asia Pacific Network Information Centre
EMail: gih@apnic.net
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