Internet Routing

1. Internet Routing 11/11/2009

2. Admin. • Assignment 3

3. Recap • Distance vector protocol • Link state protocol

4. Outline • Routing in the Internet • overview

5. Routing in the Internet • The Global Internet consists of Autonomous Systems (AS) interconnected with each other • An AS is identified by an AS Number (ASN), e.g. Yale ASN is 29 • try %whois -h whois.arin.net “a Yale“

6. Internet ISP Connectivity

7. Challenge of the Internet: Characterizing Internet Topology

8. Routing with AS • Internet routing is divided into intra-AS routing and inter-AS routing • Intra-AS • A protocol running insides an AS is called an Interior Gateway Protocol (IGP) • RIP: Routing Information Protocol • E/IGRP: Interior Gateway Routing Protocol (Cisco) • OSPF: Open Shortest Path First • IS-IS: very similar to OSPF (or should we say OSPF is very similar to IS-IS?) • Inter-AS • a protocol runs among AS’s is also called an Exterior Gateway Protocol (EGP) • for global connectivity, a single interdomain routing protocol

9. b c a a C b B b c a d A A.a A.c C.b B.a Intra-AS and Inter-AS: Example border (exterior gateway) routers interior (gateway) routers

10. AS C (RIP intra AS B (OSPF intra AS A (OSPF intra routing) routing) routing) a b Routing in the Internet: Example Gateway routers participate in intradomain to learn internal routes. gateway routers of same AS share learned externalroutes using iBGP. i e eBGP iBGP g Gateway routers of diff. AS’s exchange routes using eBGP f d h c

11. RIP process OSPF process BGP process RIP routing table BGP routing table Many Routing Processes on a Single Router BGP OSPF Routing table OS kernel RIP domain OSPF domain Forwarding Table Manager Forwarding Table

12. inter-AS routing between A and B b c a a C b B b a c d Host h1 A A.a A.c C.b B.a Intra-AS and Inter-AS Routing border (exterior gateway) routers Host h2 intra-AS routing within AS B Intra-AS routing within AS A interior (gateway) routers

13. Why Partition into Intra- and Inter-AS Routing? • This partition allows ASes flexibility to choose their own intra-AS routing protocols • autonomy • By aggregating many destinations inside an AS into a single destination in interdomain routing, it improves scalability • the partition is a type of hierarchical routing • hierarchical routing improves scalability: only a small number of routers are involved with outside

14. Internet2 AT&T Qwest Yale Internet Connectivity Yale default routes 0.0.0.0/0 pointing to provider. 132.130.0.0/16 128.36.0.0/16

15. Outline • Routing in the Internet • overview • BGP

16. BGP Setup

17. Internet Interdomain Routing: BGP • BGP (Border Gateway Protocol):the de facto standard • Path Vector protocol: • similar to Distance Vector protocol • a border gateway sends to a neighbor entire path (i.e., a sequence of ASes) to a destination, e.g., • gateway X sends to neighbor N its path to dest. Z: path (X,Z) = X,Y1,Y2,Y3,…,Z • if N selects path(X, Z) advertised by X, then: path (N,Z) = N, path (X,Z) Z N X

18. BGP Operations (Simplified) Establish session on TCP port 179 AS1 BGP session Exchange all active routes AS2 while (connection is ALIVE) exchange UPDATE messageselect best available route if route changes, export to neigh. Exchange incremental updates

19. BGP Messages • Four types of messages • OPEN: opens TCP connection to peer and authenticates sender • UPDATE: advertises new path (or withdraws old) • KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request • NOTIFICATION: reports errors in previous msg; also used to close connection

20. Why Path Vector? • Path vector prevents counting-to-infinity problem • Path vector allows an AS to define local policies on the ASes of a given path

21. BGP Routing Decision Process route selection policy: rank paths select best path routing cache export policy: which paths export to which neighbors export path to neighbors

22. Internet2 AT&T Qwest BGP Route Selection/Export Policies • Route selection • algorithm to select the best currently available route from routing cache (routing information base) • typical (Cisco) route selection policy • highest local pref • shortest AS path length • prefer eBGP over iBGP • … • Route export policy: • since the export of a path to a neighbor is an indication that the AS is willing to transport traffic for the neighbor, an AS may not export some routes to some neighbors (more later) Yale

23. AS B (OSPF intra AS D AS A (OSPF) routing) Routing: Example d E d->a2: I can reach hosts in D; my path: D Export to E: i->e: I can reach hosts in D; path: IBCD a2 a2->a1: I can reach hosts in D; path: D a1->i: I can reach hosts in D; my path: AD a1 No Exportto F i F AS C choose BCD using i2 i2->i: I can reach hosts in D; path: BCD b->i: I can reach hosts in D; my path: BCD i2 b b->i2: I can reach hosts in D; my path: BCD AS I

24. Hierarchical Routing May Pay a Price for Path Quality AS 4 AS 3 AS 2 AS 1

25. Outline • Admin. • BGP • Overview • BGP instability and stability condition

26. 2 1 0 2 0 2 4 0 preferred 3 2 0 3 0 1 3 0 1 0 3 3 1 less preferred BGP Policy Interactions The BAD GADGET example: - 0 is the destination - the route selection policy of each AS is to prefer its counter clock-wise neighbor Policy interaction causes routing instability !

27. Nodes in P-graph are feasible paths A directed edge from path N1P1 to P1 intuition: to let N1 choose N1P1, P1 must be chosen and exported to N1 A directed edge from a lower ranked path to a higher ranked path intuition: the higher ranked path should be considered first 210 20 2 320 30 130 10 0 3 1 2 1 0 3 2 0 1 3 0 3 0 2 0 1 0 Understanding Instability: P-Graph P-graph

28. If the P-graph has no loop, then BGP policy converges. intuition: choose the node (i.e., a path) from the partial order graph with no out-going edge, choose the path, remove the path and all other lowered ranked paths of the same node; remove nodes if suffix removed; continue Example: suppose we swap the order of 30 and 320 210 20 2 30 320 130 10 0 3 1 2 1 0 3 0 1 3 0 3 2 0 2 0 1 0 Partial Order Graph and Convergence

29. Partial Order Graph and BGP Convergence Preview: A reason we do not often see instability in the Internet is that: the current Internet ISP economy implies no loop in P-graph !

30. Customer provider relationship a provider is an AS that connects the customer to the rest of the Internet customer pays the provider for the transit service e.g., Yale is a customer of AT&T and QWEST Peer-to-peer relationship mutually agree to exchange traffic between their respective customers there is no payment between peers peer peer Internet Economy: Two Types of Business Relationship provider provider customer provider to customer

31. Implication of Business Relationship on Policies • Route selection (ranking) policy: • the typical route selection policy is to prefer customers over peers/providers to reach a destination, i.e., Customer > pEer/Provider

32. provider provider provider customer provider peer customer customer customer peer peer peer Typical Export Policies case 1: routes learned from customer routes learned from acustomer are sent toall other neighbors case 2: routes learned from provider case 3: routes learned from peer routes learned from a peer are sent only to customers routes learned from a provider are sentonly to customers

33. A advertises to C paths to P1 and P2 A advertises path to C, but not P2 A advertises path to C, but not P1 IP traffic Typical Export -> No-Valley Routing P1 P2 A learns paths toC, P1, P2 A C Suppose P1 and P2 are providers of A; A is a provider of C

34. Internet2 AT&T Qwest Yale Internet Connectivity Yale default routes 0.0.0.0/0 pointing to provider. 132.130.0.0/16 128.36.0.0/16

35. Typical Export Policies Imply Patterns of Routes • Assume a BGP path SABCD to destination AS D. Consider the business relationship between each pair: • Three types of business relationships: • PC (provider-customer) • CP (customer-provider) • PP (peer-peer) S A B C D