1. 1 Internet Routing 4/12/2012

2. Admin. r Exam 2 date: m Wednesday, May 2 at 2:00 p.m. m If you want to take the exam in another day (e.g. due to travel), please talk to me r Written assignment—Routing posted. 2

3. Recap r Basic routing protocols m Distance vector protocols m Link state protocols r Remaining question: how to design the routing infrastructure for the global Internet? 3

4. b a AS C (RIP intra routing) Recap: Routing in the Internet c AS B (OSPF intra routing) AS A (OSPF intra routing) eBGP iBGP Border (exterior gateway) routers participate in intradomain to learn internal routes. Border (exterior gateway) routers summarize internal routes and exchange routing w/ BGP peers i e f g h d r Internet routing is divided into intra-AS (intradomain) routing and inter-AS (interdomain) routing

5. Multiple Routing Processes on a Border Router Forwarding Table OSPF domain RIP domain BGP OS kernel RIP process RIP routing table Forwarding Table Manager OSPF process OSPF Routing table BGP process BGP routing table

6. Outline  Routing in the Internet o overview  BGP

7. BGP Setup

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

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

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

11. Benefits of Including Path Vector r Path vector prevents counting-to-infinity problem r Path vector allows an AS to define local policies on the selection of AS paths for each destination

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

13. BGP Route Selection/Export Policies r Route selection policy m algorithm to select the best currently available route from routing cache (routing information base) m typical (Cisco) route selection policy highest local pref shortest AS path length prefer eBGP over iBGP … r Route export policy: m 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 QwestAT&T Internet2

14. 14 Routing: Example 1 AS A (OSPF) AS B (OSPF intra routing) AS D AS C i b b->i: I can reach hosts in D; my path: BCD a1 a2 d d->a2: I can reach hosts in D; my path: D a1->i: I can reach hosts in D; my path: AD E F Export to E: i->e: I can reach hosts in D; path: IAD AS I a2->a1: I can reach hosts in D; path: D Route selection policy: - Shortest AS Path policy: b->i2: I can reach hosts in D; my path: BCD i2 i2->i: I can reach hosts in D; path: BCD No Export to F Choose AD using a1

15. 15 Routing: Example 2 AS A (OSPF) AS B (OSPF intra routing) AS D AS C i b b->i: I can reach hosts in D; my path: BCD a1 a2 d d->a2: I can reach hosts in D; my path: D a1->i: I can reach hosts in D; my path: AD E F Export to E: i->e: I can reach hosts in D; path: IBCD AS I a2->a1: I can reach hosts in D; path: D Selection policy: - Low local_pref for A - Shortest AS Path - Prefer eBGP (hot potato) b->i2: I can reach hosts in D; my path: BCD i2 i2->i: I can reach hosts in D; path: BCD Choose BCD using b

16. 16 Routing: Example 3 AS A (OSPF) AS B (OSPF intra routing) AS D AS C i b b->i: I can reach hosts in D; my path: BCD a1 a2 d d->a2: I can reach hosts in D; my path: D a1->i: I can reach hosts in D; my path: AD E F Export to E: i->e: I can reach hosts in D; path: IBCD AS I a2->a1: I can reach hosts in D; path: D Selection policy: - Low local_pref A - Shortest AS Path - Prefer iBGP (cold potato) b->i2: I can reach hosts in D; my path: BCD i2 i2->i: I can reach hosts in D; path: BCD Choose BCD using i2

17. Observing BGP Paths r Using one of the looking glass servers: http://www.bgp4.as/looking-glasses 17

18. Summary of Interdomain Routing r Key features o Hierarchical routing o Policy routing

19. Benefits of Hierarchical Routing o ASes have flexibility to choose their own intra-AS routing protocols - allows autonomy o Only a small # of routers (gateways) from each AS in the inter-AS level - improves scalability o Inter-AS route represented as a list of ASNs instead of detailed routers - improves scalability/privacy

20. Issue 1: Hierarchical Routing May Pay a Price for Path Quality AS 4 AS 3 AS 2 AS 1

21. Issue 2: BGP Instability 2 0 3 1 2 1 0 2 0 1 3 0 1 0 3 2 0 3 0 4 3 preferred less preferred 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 !

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

23. Partial Order Graph and BGP Convergence  If the P-graph has no loop, then BGP policy converges.  intuition: choose the path node from the partial order graph with no out-going edge to non-fixed path nodes, fix the path node, eliminate all no longer feasible; continue  Example: suppose we swap the order of 30 and 320 2 1 0 2 0 1 0 3 0 1 3 0 3 2 0 2 0 1 3 30 320 210 20 130 10

24. Question Do we often see path instability in the Internet? Preview: the current Internet ISP economy implies no loop in P-graph !

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

26. Implication of Business Relationship: Route Selection Policies r Route selection (ranking) policy: m the typical route selection policy is to prefer customers over peers/providers to reach a destination, i.e., Customer > pEer/Provider provider customer peer

27. customer provider customer peer Implication of Business Relationship: Export Policies provider customer peer provider customer peer routes learned from a customer are sent to all other neighbors routes learned from a provider are sent only to customers routes learned from a peer are sent only to customers case 1: routes learned from customer case 2: routes learned from provider case 3: routes learned from peer

28. Example: Typical Export -> No-Valley Routing P1P1 A P2P2 IP traffic A advertises path to C, but not P 2 A advertises to C paths to P 1 and P 2 A advertises path to C, but not P 1 Suppose P 1 and P 2 are providers of A; A is a provider of C C A learns paths to C, P1, P2

29. Typical Export Policies Imply Patterns of Routes r Assume a BGP path SABCD to destination AS D. Consider the business relationship between each pair: r Three types of business relationships: m PC (provider-customer) m CP (customer-provider) m PP (peer-peer) 29 S A B C D

30. Typical Export Policies Imply Patterns of Routes r Invariant 1 of valid BGP routes (with labels representing business relationship) P C ? Dest P C Reasoning: only route learned from customer is sent to provider; thus after a PC, it is always PC to the destination

31. Typical Export Policies Imply Patterns of Routes r Invariant 2 of valid BGP routes (with labels representing business relationship) CP/PP ? CP ······ Dest Reasoning: routes learned from peer or provider are sent to only customers; thus all relationship before is CP

32. Stability of BGP Routing r Suppose 1. there is no loop formed by provider-customer relationship in the Internet 2. each AS uses typical route selection policy: C > E/P 3. each AS uses the typical export policies  Then BGP policy routing always converges.

33. Case 1: A Link is PC PC Proof by contradiction. Assume a loop in P-graph. Consider a fixed link. in the loop PC AS 1AS 2 AS 3AS 4

34. Case 2: Link is CP/PP CP/PP CP CP/PP CP AS 1AS 2 AS 3AS 4

35. Summary: BGP Policy Routing  Advantage - satisfies real demand  Issue - policy dispute can lead to instability -current Internet economy provides a stability framework, but if the framework changes, we may see instability  Comment: -a policy routing framework is a preference (ranking) aggregation framework -a fundamental negative result is Arrow’s Impossibility Theorem (http://en.wikipedia.org/wiki/Arrow's_impossibi lity_theorem)

36. 36 Routing: Example AS A (OSPF) AS B (OSPF intra routing) AS D AS C i b How to specify? a1 a2 d a1->i: I can reach hosts in D; my path: AD E F AS I d1 d2

37. Backup Slides 37

38. 38 Another Method for Checking Convergence: Dispute Wheels r A dispute wheel m u 1 prefers R 1 Q 2 over Q 1 m u 2 prefers R 2 Q 3 over Q 2 m etc u2u2 d u3u3 u1u1 Q2Q2 Q3Q3 Q1Q1 R2R2 R3R3 R1R1 RnRn 2 0 1 3 320 30 210 20 130 10 unun QnQn

39. 39 Patterns of Valid Routes r Consider how a path is extended according to the export policies m case 1 (format: Destination, link type, …) Dest, …CP  Dest, … CP CP Dest, …CP  Dest, … CP PC Dest, …CP  Dest, … CP PP m case 2 Dest, …PC  Dest, … PC PC m case 3 Dest, …PP  Dest, … PP PC r Valid consecutive link types (starting from source to destination, i.e., reserve) m PC PC m CP PC m PP PC m CP CP m CP PP

40. 40 Case 1: The first link of Q 1 is a PC link r Then the first link of R 1 Q 2 must be a PC link m because u 1 chooses R 1 Q 2 and C > E/P r Thus all remaining links along R 1 Q 2 are PC links m because only PC follows PC r Thus the first link of Q 2 is a PC link r …… r All links along R 1, …, R n are PC links r The network has a PC loop. contradiction ! u1u1 d u2u2 unun Q1Q1 Q2Q2 QnQn R1R1 R2R2 RnRn R n-1 PC

41. 41 Case 2: The first link of Q 1 is a CP/PP link u1u1 d u2u2 unun Q1Q1 Q2Q2 QnQn R2R2 RnRn R n-1 r All links along R n are CP links because all links before CP/PP are CP r The first link of Q n must be a CP/PP link m because C > E/P r …… r All links along R1, …, Rn are CP links r The network has a PC loop. contradiction ! CP/PP CP CP/PP

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