S ufficient C onditions to G uarantee P ath V isibility Akeel ur Rehman Faridee

The Internet (General Picture) End-hosts Routers

The Internet (Original Picture) End- hosts Internet is actually divided into different Autonomous Systems (AS’s), governed by different organization entities.

Autonomous Systems (ASes) An autonomous system is a region of the Internet that is administered by a single entity and that has a unified routing policy RFC 1930: Guidelines for creation, selection, and registration of an Autonomous System Economically independent All must cooperate to ensure reachability Each AS is assigned an autonomous number (ASN) Interdomain routing is concerned with determining paths b/w AS’s

5 Interdomain vs Intradomain Intradomain routing Routing is done based on metrics Routing domain is one autonomous system Interdomain routing Routing is done based on policies Routing domain is the entire Internet

What Problem is BGP Solving? Underlying problem Shortest Paths Distributed means of computing a solution. X? RIP, OSPF, IS-IS BGP Reachability Information

7 BGP Prefixes reachable from AS 1 Prefixes reachable from AS 3

Two Flavors of BGP External BGP (eBGP): exchanging routes between ASes Internal BGP (iBGP): disseminating routes to external destinations among the routers within an AS eBGP iBGP

Two flavors? Most ASes have more than one “border” router that talks to other peers Must disseminate information inside the AS and through the AS. –Must have complete visibility. COMPLETE VISIBILITY: For every external destination, each router picks the same route that it would have picked had it seen the best routes from each eBGP router in the AS.

iBGP iBGP sessions run on TCP. Overlay over intra-domain routing protocol. Routing messages and data packets forwarded via IGP within AS. Routes from iBGP session not propagated to another iBGP session. Originally FULL MESH – All routers see all routes – causing scaling problems Route Reflectors and Confederations are the solution.

11 iBGP Mesh Does Not Scale eBGP update iBGP updates N border routers means N(N-1)/2 peering sessions Each router must have N-1 iBGP sessions configured Size of iBGP routing table can be order N larger than number of best routes (remember alternate routes!) Each router has to listen to update noise from each neighbor

Problems with Route Reflectors Single point of failure – can be fixed easily All clients take same path – compromises complete visibility Some configurations may provide complete visibility BUT not all ! Lack of complete visibility: every router is not guaranteed to see its best available route.

13 Route reflectors can pass on iBGP updates to clients Each RR passes along ONLY best routes ORIGINATOR_ID and CLUSTER_LIST attributes are needed to avoid loops RR Route Reflectors

BGP Blackhole A missing iBGP session can create network partition even the underlying IP topology is connected. A missing iBGP session might keep one router from receiving route information for some prefixes, making it a blackhole for that prefix. Knowing the sufficient conditions to gaurantee the VISIBILITY of all available path to prevent the blackhole, both in case of partitions and common case, is of greater interset. Blackhole: A router that drops all the packets of a particular prefix (because it doesn’t have the reachability information for this prefix) is called the black hole for this prefix.

Selected BGP RFCs IDR : RFC 1771 A Border Gateway Protocol 4 (BGP-4) RFC 1772 Application of the Border Gateway Protocol in the Internet RFC 1773 Experience with the BGP-4 protocol RFC 1774 BGP-4 Protocol Analysis RFC 2796 BGP Route Reflection An alternative to full mesh IBGP RFC 3065 Autonomous System Confederations for BGP Internet Engineering Task Force (IETF)

Acknowledgements Some of the slides/figures are taken from Hari Balakrishnan and Nick Feamster website.

References [1] Hari Balakrishnan, , and Nick Feamster, Chapter 4: Interdomain Internet Routing, 2005, [2] M Vutukuru, P Valiant, S Kopparty, Hari Balakrishnan How to Construct a correct and Scalable iBGP Confuguration, Proceedings of IEEE INFOCOM, 2006 [3] T. Bates, R. Chandra, and E. H. Chen. BGP Route Reflection – An Alternative to Full Mesh iBGP, April 2000 [4] Timothy Griffin and Gordon T. Wilfong. On the Correctness of iBGP Configuration. In Proc. ACM SIGCOM, pages 17-29, Pittsburgh, PA, August 2002 [5] FEAMSTER, N., AND BALAKRISHNAN, H. Verifying the correctness of wide- area Internet routing. Tech. Rep. MIT-LCS-TR-948, Massachusetts Inst. of Tech., May 2004 [6] Nick Feamster, Jared Winick, and Jennifer Rexford. A Model of BGP Routing for Network Engineering. In ACM Sigmetrics - Performance 2004, New York, NY, June 2004.