CPS-356 Network Layer: Inter-domain Routing

1. CPS-356Network Layer:Inter-domain Routing Theophilus Benson Based partly on lecture notes by Xiaowei Yang, Rodrigo Foncesa, Rob Sherwood, David Mazières, Phil Levis, John Jannotti

2. Administrivia • Assignment #2 is out • You’ll be making your own router • Implementing routing: RIP • Implementing forwarding: IP-forwarding • Due in about three weeks (March 02, 2015) • Groups of 2

3. Today • Inter-Domain Routing (EGP) • BGP • Implementation details • BGP Challenges

4. Why study BGP? • Critical protocol: makes the Internet run • Only widely deployed EGP • Active area of problems! • Efficiency • Cogent vs. Level3: Internet Partition • Spammers use prefix hijacking • Pakistan accidentally took down YouTube • Egypt disconnected for 5 days

5. Why Inter vs. Intra • Why not just use OSPF everywhere? • E.g., hierarchies of OSPF areas? • Hint: scaling is not the only limitation • BGP is a policy control and information hiding protocol • Autonomous System (AS) owner of a network • intra == trusted, inter == untrusted • Different policies by different ASs • Different costs by different ASs

6. Types of ASs • Local Traffic – source or destination in local AS • Transit Traffic – passes through an AS • Stub AS • Connects to only a single other AS • Multihomed AS • Connects to multiple ASs • Carries no transit traffic • Transit AS • Connects to multiple ASs and carries transit traffic

7. Duke AT&T (Transit) Abilene (Transit) Comcast (Stub) BGP Multihomed Cogent (Transit) All ASes are not equal

8. AS relationships Very complex economic landscape Simplifying a bit: Transit: “I pay you to carry my packets to everywhere” (provider-customer) Peering: “For free, I carry your packets to my customers only.” (peer-peer) Technical definition of tier-1 ISP: In the “default-free” zone. No transit. Note that other “tiers” are marketing, but convenient. “Tier 3” may connect to tier-1.

9. Zooming in 4x Tier 1 ISP Tier 1 ISP Default free, Has information on every prefix $$ Default: provider Tier 2 $$ Tier 2 Regional Tier 2 $$ Tier 3 (local) Tier 2: Regional/National Tier 3: Local

10. Who pays whom? Transit: Customer pays the provider Who is who? Usually, the one who can “live without” the other. AT&T does not need Duke, but Duke needs some ISP. What if both need each other? Free Peering. Instead of sending packets over $$ transit, set up a direct connection and exchange traffic for free! http://vijaygill.wordpress.com/2009/09/08/peering-policy-analysis/

11. Tier 1s must all peer with each other by definition Tier 1s form a full mesh Internet core Peering can give: Better performance Lower cost More “efficient” routing (keeps packets local) But negotiating can be very tricky!

12. Tussles between Business and Peering Cooperative competition (competition) Much more desirable to have your peer’s customers Much nicer to get paid for transit Peering “tiffs” are relatively common

13. Peering Drama • Cogent vs. Level3 were peers • In 2003, Level3 decided to start charging Cogent • Cogent said no • Internet partition: Cogent’s customers couldn’t get to Level3’s customers and vice-versa • Other ISPs were affected as well • Took 3 weeks to reach an undisclosed agreement

14. How do you choose a routing protocol for the WAN? • Constraints • Respect $$$$$$ • Autonomy (policy and privacy) • Scaling

15. Sometimes you need to lie. Tier 1 ISP Tier 1 ISP Default free, Has information on every prefix $$ $$ $$ Default: provider Tier 2 $$ $$ Tier 2 Regional Tier 2 $$ Tier 3 (local) Tier 3 (local)

16. Choice of Routing Algorithm • Link-state? • Requiressharing of complete information • Information exchange does not scale • Hence areas …. • Can’t express policy • Distance Vector? • Scales and retains privacy • Can’t implement policy • Can’t avoid loops if shortest path not taken • Hence Count-to-infinity ..

17. Path Vector Protocol • Distance vector algorithm with extra information • For each route, store the complete path (ASs) • No extra computation, just extra storage (and traffic) • Advantages • Can make policy choices based on set of ASs in path • Can easily avoid loops

18. BGP

19. BGP - High Level • Single EGP protocol in use today • Abstract each AS to a single node • Destinations are CIDR prefixes • Exchange prefix reachability with all neighbors • E.g., “I can reach prefix 128.148.0.0/16 through ASes 44444 3356 14325 11078” • Select a single path by routing policy • Critical: learn many paths, propagate one • Add your ASN to advertised path

20. Terms Route: a network prefix plus path attributes Customer/provider/peer routes: Route advertisements heard from customers/providers/peers Transit service: If A advertises a route to B, it implies that A will forward packets coming from B to any destination in the advertised prefix 152.2.3.4 AS 1 AS 2 AS 3 152.3/16 152.3/16 UNC Duke NC RegNet 152.2.3.4 152.3.137.179

21. BGP: Distance Vector+Paths Session (over TCP) Autonomous Systems (ASes) Route Advertisement Traffic BGP peers

22. BGP: Distance Vector+Paths Autonomous Systems (ASes) Why should I let othersknow of the Route Advertisements? Route Advertisement Traffic Which of two paths to use?

23. Enforcing relationships Two mechanisms Route export filters Control what routes you send to neighbors Route import ranking Controls which route you prefer of those you hear. “LOCALPREF” – Local Preference. More later.

24. Export Policies Provider  Customer All routes so as to provide transit service Customer  Provider Only customer routes Why? Only transit for those that pay Peer  Peer Only customer routes

25. Sometimes you need to lie. Tier 1 ISP Tier 1 ISP Default free, Has information on every prefix $$ $$ $$ Default: provider Tier 2 $$ $$ Tier 2 Regional Tier 2 $$ Tier 3 (local) Tier 3 (local)

26. Import policies Same routes heard from providers, customers, and peers, whom to choose? customer > peer > provider Why? Choose the most economic routes! Customer route: charge $$  Peer route: free Provider route: pay $$ 

27. Valley Free Routing • Number links as(+1,0,-1) for provider, peer and customer • In any valid path should only see sequence of+1 , followed by at most one 0, followed by sequence of -1 – Why? • Consider the economics of the situation

28. Sometimes you need to lie. Tier 1 ISP Tier 1 ISP 0 Default free, Has information on every prefix $$ -1 $$ $$ Default: provider Tier 2 +1 $$ $$ Tier 2 Regional Tier 2 +1 $$ Tier 3 (local) Tier 3 (local)

29. Sometimes you need to lie. Tier 1 ISP Tier 1 ISP 0 Default free, Has information on every prefix $$ -1 $$ -1 $$ Default: provider Tier 2 +1 $$ $$ Tier 2 Regional Tier 2 +1 $$ Tier 3 (local) Tier 3 (local)

30. BGP Implementation Details

31. BGP BGP = Border Gateway Protocol Currently in version 4, specified in RFC 1771. (~ 60 pages) Inter-domain routing protocol for routing between autonomous systems Uses TCP to establish a BGP session and to send routing messages over the BGP session BGP is a path vector protocol Similar to distance vector routing, but routing messages in BGP contain complete paths Network administrators can specify routing policies

32. BGP policy routing BGP’s goal is to find any path (not an optimal one) Since the internals of the AS are never revealed, finding an optimal path is not feasible Network administrator sets BGP’s policies to determine the “best path” to reach a destination network

33. BGP messages • OPEN • UPDATE • Announcements • Dest Next-hop AS Path … other attributes … • 128.2.0.0/16 196.7.106.245 2905 701 1239 5050 9 • Withdrawals • KEEPALIVE • Keepalive timer / hold timer • Key thing: The Next Hop attribute

34. Path Vector • ASPATH Attribute • Records what ASes a route goes through • Loop avoidance: Immediately discard • Shortest path heuristics • Like distance vector, but fixes the count-to-infinity problem

35. I can reach d via B,D A B D I can reach d Via A,B,D I can reach d Via C,A,B,D d C

36. Common BGP path attributes Origin: indicates how BGP learned about a particular route IGP (internal gateway protocol) EGP (external gateway protocol) Incomplete AS path : When a route advertisement passes through an autonomous system, the AS number is added to an ordered list of AS numbers that the route advertisement has traversed Next hop Multi_Exit_Disc (MED, multiple exit discriminator): -used as a suggestion to an external AS regarding the preferred route into the AS Local_pref: is used to prefer an exit point from the local autonomous system Community: apply routing decisions to a group of destinations

37. BGP route selection process Input/output engine may filter routes or manipulate their attributes Routes sent to peers Routes recved from peers Best routes Decision process Input Policy Engine Out Policy Engine

38. Decision Engine:Best path selection algorithm If next hop is inaccessible, ignore routes Prefer the route with the largest local preference value. If local prefs are the same, prefer route with the shortest AS path If AS_path is the same, prefer route with lowest origin (IGP < EGP < incomplete) If origin is the same, prefer the route with lowest MED IF MEDs are the same, prefer eBGP paths to iBGP paths If all the above are the same, prefer the route that can be reached via the closest IGP neighbor. If the IGP costs are the same, prefer the router with lowest router id.

39. Some BGP Challenges • Convergence • Traffic engineering: Load Balancing • How to assure certain routes are selected • Scaling (route reflectors) • Security (Next class)

40. Convergence • Some reasons for change • Topology changes • BGP session failures • Changes in policy • Conflicts between policies can cause oscillation • Given a change, how long until the network re-stabilizes? • Depends on the change: sometimes never • Depends on the topology • Depends on the time of updates • Open research problem: “tweak and pray”

41. Change: D goes down. I can reach d via E, B,D E I can reach d via B,D A B I can reach d Via A,E,B,D I can reach d Via A,E,B,D I can reach d Via A,B,D I can reach d Via C,A,B,D D I can reach d Via C,A,E,B,D d C

42. Avoiding BGP Instabilities • Detecting conflicting policies • Centralized: NP-Complete problem! • Distributed: open research problem • Requires too much cooperation • Detecting oscillations • Monitoring for repetitive BGP messages • Restricted routing policies and topologies • Some topologies / policies proven to be safe* * Gao& Rexford, “Stable Internet Routing without Global Coordination”, IEEE/ACM ToN, 2001

43. Some BGP Challenges • Convergence • Traffic engineering: Load Balancing • How to assure certain routes are selected • Scaling (route reflectors) • Security (Next class)

44. Traffic Engineering: Load balancing Same route from two providers Outbound is “easy” (you have control) Set localpref according to goals Inbound is tough (nobody has to listen) AS path prepending MEDs Hot and Cold Potato Routing (picture) Often ignored unless contracts involved Practical use: tier-1 peering with a content provider

45. Hot-Potato Routing (early exit) Foo 12/8 12/8 NYC SF AT&T 12.0.0.1 12//8 NYC SF Sprint 12/8 Bar

46. Why is Hot-Potato Bad? • Forward and reverse paths have different properties • Different latencies and loss rates affect TCP • inaccuracies in measurements of the forwarding system • packet loss due to forwarding loops

47. Cold-Potato Routing (MED) NYC SF Akamai Med=100 Med=200 NYC Sprint SF

48. Other Traffic Engineering • Want to: • Helps Improve revenue • Route filtering • Avoid Hot-potato routing? • Setting MED weights • Avoid other ISPs using you to get to ISP X? • AS prepending: “477 477 477 477” • People prefer shortest routes • More of an art than science

49. Some BGP Challenges • Convergence • Traffic engineering: Load Balancing • How to assure certain routes are selected • Scaling (route reflectors) • Security (Next class)

50. Routing table scalability with Classful IP Addresses Fast growing routing table size Classless inter-domain routing aims to address this issue