1266 IEEE/ACM TRANSACTIONS ON NETWORKING, VOL. 15, NO. 6, DECEMBER 2007
Implications of Autonomy for the Expressiveness
of Policy Routing
Nick Feamster, Ramesh Johari, Member, IEEE, and Hari Balakrishnan
Abstract—Thousands of competing autonomous systems must
cooperate with each other to provide global Internet connectivity.
Each autonomous system (AS) encodes various economic, busi-
ness, and performance decisions in its routing policy. The current
interdomain routing system enables each AS to express policy
using rankings that determine how each router in the AS chooses
among different routes to a destination, and filters that determine
which routes are hidden from each neighboring AS. Because the
Internet is composed of many independent, competing networks,
the interdomain routing system should provide autonomy, allowing
network operators to set their rankings independently, and to
have no constraints on allowed filters. This paper studies routing
protocol stability under these conditions. We first demonstrate
that “next-hop rankings,” commonly used in practice, may not
ensure routing stability. We then prove that, when providers
can set rankings and filters autonomously, guaranteeing that the
routing system will converge to a stable path assignment imposes
strong restrictions on the rankings ASes are allowed to choose.
We discuss the implications of these results for the future of
interdomain routing.
Index Terms—Automony, BGP, interdomain routing, policy,
safety.
I. INTRODUCTION
I N Internet routing, independently operated autonomous sys-tems (ASes) must cooperate to exchange global information;
nevertheless, this cooperation occurs in a landscape where these
independent networks compete to provide Internet service. BGP
facilitates this “competitive cooperation” by enabling network
operators to express routing policies that are consistent with de-
sired economic, business, and performance goals.
Ranking and filtering are the two main mechanisms that oper-
ators use to implement their policies. Ranking determines which
of many possible routes to a destination should be used, thus
providing an AS the flexibility to specify preferences over mul-
tiple candidate paths to a destination (e.g., specifying a primary
Manuscript received October 25, 2005; revised May 14, 2006; approved by
IEEE/ACM TRANSACTIONS ON NETWORKING Editor O. Bonaventure. This
work was supported by the National Science Foundation (NSF) under Grants
CNS-0225660 and CNS-0520241 and by a Cisco URP grant. The work of N.
Feamster was supported in part by an NSF Graduate Research Fellowship.
The work of R. Johari was supported in part by the Okawa Foundation. A
preliminary version of this paper appeared in the Proceedings of the ACM
SIGCOMM, August 2005.
N. Feamster is with the School of Computer Science, Georgia Institute of
Technology, Atlanta, GA 30332 USA (e-mail: feamster@cc.gatech.edu).
R. Johari is with the Department of Management Science and Engineering,
Stanford University, Stanford, CA 94305-4026 USA (e-mail: ramesh.jo-
hari@stanford.edu).
H. Balakrishnan is with the Electrical Engineering and Computer Science
Department and the Computer Science and Artificial Intelligence Laboratory
(CSAIL), Massachusetts Institute of Technology (MIT), Cambridge, MA 02139
USA (e-mail: hari@csail.mit.edu).
Digital Object Identifier 10.1109/TNET.2007.896531
Fig. 1. Instability can arise when each AS independently specifies rankings
[14], [22]. Each circle represents an AS. AS 0 is the destination. The listing of
paths beside each node denotes a ranking over paths.
and a backup path). Filtering allows an AS to selectively adver-
tise routes to some ASes and hide routes from others, thereby
controlling which neighboring ASes send traffic over its infra-
structure.
There are two important characteristics of policy routing: au-
tonomy and expressiveness. Autonomy is the ability of each AS
to set its rankings and filters independent of the others. Expres-
siveness refers to the flexibility that the routing protocol pro-
vides an operator for specifying rankings and filters. Ranking
expressiveness determines what classes of rankings over routes
are permitted by the protocol, while filtering expressiveness de-
termines the range of route filters that are allowed.
The combination of expressiveness and autonomy has, in
large part, been the reason for the success of BGP over the
past decade. We contend that both autonomy and filtering
expressiveness will be requirements for policy routing for the
foreseeable future. Previous studies of routing stability assume
that ASes are willing to compromise some degree of autonomy,
filtering expressiveness, or both (see Section II). Autonomy,
however, preserves each AS’s ability to set its policies without
coordinating with any other AS. Filtering expressiveness gives
an AS flexibility in how it establishes contracts with another
AS, a task that should be unconstrained.
Ideally, an interdomain routing system should preserve au-
tonomy, filtering expressiveness, and ranking expressiveness.
However, the ability to specify highly expressive rankings
comes at considerable cost to system robustness: as has been
observed by Varadhan et al. and Griffin et al., among others, if
there are no constraints on the rankings that an AS can specify,
BGP may violate safety (i.e., oscillate forever) [14], [22].
Example 1: Consider Fig. 1 [14], [22]. ASes 1, 2, and 3
each prefer the indirect path through their neighboring AS in
the clockwise direction over the direct path to the destination,
0. All other paths are filtered. This configuration has no stable
path assignment (i.e., a path assignment from which no node
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