1. Interdomain Routing Jennifer Rexford Advanced Computer Networks http://www.cs.princeton.edu/courses/archive/fall08/cos561/ Tuesdays/Thursdays 1:30pm-2:50pm

2. Outline Interdomain routing –Autonomous Systems (ASes) Path-vector routing –Loop detection and convergence –Flexible path selection Business relationships –Customer-provider and peer-peer –Hierarchy from tier-1 ASes to stubs Border Gateway Protocol (BGP) –Announcements and withdrawals –Import and export policies Discussion of Paxson96 and Labovitz97

3. Interdomain Routing: Between Networks AS-level topology –Nodes are Autonomous Systems (ASes) –Destinations are prefixes (e.g., 12.0.0.0/8) –Edges are links and business relationships 1 2 3 4 5 6 7 ClientWeb server

4. AS Numbers (ASNs) ASNs are 16 bit values. 64512 through 65535 are “private” Level 3: 1 MIT: 3 Harvard: 11 Yale: 29 Princeton: 88 AT&T: 7018, 6341, 5074, … UUNET: 701, 702, 284, 12199, … Sprint: 1239, 1240, 6211, 6242, … … ASNs represent units of routing policy Currently around 30,000 in use.

5. Challenges for Interdomain Routing Scale –Prefixes: 250,000, and growing –ASes: 30,000, and growing –Routers: at least in the millions… Privacy –ASes don’t want to divulge internal topologies –… or their business relationships with neighbors Policy –No Internet-wide notion of a link cost metric –Need control over where you send traffic –… and who can send traffic through you

6. Policy-Based Path-Vector Routing

7. Path-Vector Routing Extension of distance-vector routing –Support flexible routing policies –Reduce convergence time (avoid count-to-infinity) Key idea: advertise the entire path –Distance vector: send distance metric per dest d –Path vector: send the entire path for each dest d 3 2 1 d “d: path (2,1)” “d: path (1)” data traffic

8. Faster Loop Detection Node can easily detect a loop –Look for its own node identifier in the path –E.g., node 1 sees itself in the path “3, 2, 1” Node can simply discard paths with loops –E.g., node 1 simply discards the advertisement 3 2 1 “d: path (2,1)” “d: path (1)” “d: path (3,2,1)”

9. Flexible Policies Each node can apply local policies –Path selection: Which path to use? –Path export: Whether to advertise the path? Examples –Node 2 may prefer the path “2, 3, 1” over “2, 1” –Node 1 may not let node 3 hear the path “1, 2” 2 3 1 2 3 1

10. Business Relationships

11. Neighboring ASes have business contracts –How much traffic to carry –Which destinations to reach –How much money to pay Common business relationships –Customer-provider E.g., Princeton is a customer of USLEC E.g., MIT is a customer of Level3 –Peer-peer E.g., UUNET is a peer of Sprint E.g., Harvard is a peer of Harvard Business School

12. Customer-Provider Relationship Customer needs to be reachable from everyone –Provider tells all neighbors how to reach the customer Customer does not want to provide transit service –Customer does not let its providers route through it d d provider customer provider Traffic to the customerTraffic from the customer announcements traffic

13. Multi-Homing: Two or More Providers Motivations for multi-homing –Extra reliability, survive single ISP failure –Financial leverage through competition –Better performance by selecting better path –Gaming the 95 th -percentile billing model Provider 1 Provider 2

14. Princeton Example Internet: customer of USLEC and Patriot Research universities/labs: customer of Internet2 Local non-profits: provider for several non-profits Patriot USLEC Internet2

15. Peer-Peer Relationship Peers exchange traffic between customers –AS exports only customer routes to a peer –AS exports a peer’s routes only to its customers –Often the relationship is settlement-free (i.e., no $$$) peer Traffic to/from the peer and its customers d announcements traffic

16. AS Structure: Tier-1 Providers Tier-1 provider –Has no upstream provider of its own –Typically has a national or international backbone Top of the Internet hierarchy of ~10 ASes –AOL, AT&T, Global Crossing, Level3, UUNET, NTT, Qwest, SAVVIS (formerly Cable & Wireless), and Sprint –Full peer-peer connections between tier-1 providers

17. AS Structure: Other ASes Other providers –Provide transit service to downstream customers –… but, need at least one provider of their own –Typically have national or regional scope –Includes several thousand ASes Stub ASes –Do not provide transit service to others –Connect to one or more upstream providers –Includes vast majority (e.g., 85-90%) of the ASes

18. Border Gateway Protocol

19. Prefix-based path-vector protocol Policy-based routing based on AS Paths Evolved during the past 18 years 1989 : BGP-1 [RFC 1105], replacement for EGP 1990 : BGP-2 [RFC 1163] 1991 : BGP-3 [RFC 1267] 1995 : BGP-4 [RFC 1771], support for CIDR 2006 : BGP-4 [RFC 4271], update Border Gateway Protocol

20. BGP Operations Establish session on TCP port 179 Exchange all active routes Exchange incremental updates AS1 AS2 While connection is ALIVE exchange route UPDATE messages BGP session

21. Incremental Protocol A node learns multiple paths to destination –Stores all of the routes in a routing table –Applies policy to select a single active route –… and may advertise the route to its neighbors Incremental updates –Announcement Upon selecting a new active route, add node id to path … and (optionally) advertise to each neighbor –Withdrawal If the active route is no longer available … send a withdrawal message to the neighbors

22. ASPATH Attribute AS7018 128.112.0.0/16 AS Path = 88 AS 1239 Sprint AS 1755 Ebone AT&T AS 3549 Global Crossing 128.112.0.0/16 AS Path = 7018 88 128.112.0.0/16 AS Path = 3549 7018 88 AS 88 128.112.0.0/16 Princeton Prefix Originated AS 12654 RIPE NCC RIS project AS 1129 Global Access 128.112.0.0/16 AS Path = 7018 88 128.112.0.0/16 AS Path = 1239 7018 88 128.112.0.0/16 AS Path = 1129 1755 1239 7018 88 128.112.0.0/16 AS Path = 1755 1239 7018 88

23. BGP Policy: Applying Policy to Routes Import policy –Filter unwanted routes from neighbor E.g. prefix that your customer doesn’t own –Manipulate attributes to influence path selection E.g., assign local preference to favored routes Export policy –Filter routes you don’t want to tell your neighbor E.g., don’t tell a peer a route learned from other peer –Manipulate attributes to control what they see E.g., make a path look artificially longer than it is

24. BGP Policy: Influencing Decisions Best Route Selection Apply Import Policies Best Route Table Apply Export Policies Install forwarding Entries for best Routes. Receive BGP Updates Best Routes Transmit BGP Updates Apply Policy = filter routes & tweak attributes Based on Attribute Values IP Forwarding Table Apply Policy = filter routes & tweak attributes Open ended programming. Constrained only by vendor configuration language

25. Import Policy: Local Preference Favor one path over another –Override the influence of AS path length –Apply local policies to prefer a path Example: prefer customer over peer AT&T Sprint Yale Tier-2 Tier-3 Local-pref = 100 Local-pref = 90

26. Import Policy: Filtering Discard some route announcements –Detect configuration mistakes and attacks Examples on session to a customer –Discard route if prefix not owned by the customer –Discard route that contains other large ISP in AS path Patriot Princeton USLEC 128.112.0.0/16

27. Export Policy: Filtering Discard some route announcements –Limit propagation of routing information Examples –Don’t announce routes from one peer to another AT&T Sprint UUNET

28. Export Policy: Filtering Discard some route announcements –Limit propagation of routing information Examples –Don’t announce routes for network-management hosts or the underlying routers themselves USLEC Princeton network operator

29. Export Policy: Attribute Manipulation Modify attributes of the active route –To influence the way other ASes behave Example: AS prepending –Artificially inflate the AS path length seen by others –To convince some ASes to send traffic another way Patriot Princeton USLEC 128.112.0.0/16 Sprint 88

30. BGP Policy Configuration Routing policy languages are vendor-specific –Not part of the BGP protocol specification –Different languages for Cisco, Juniper, etc. Still, all languages have some key features –Policy as a list of clauses –Each clause matches on route attributes –… and either discards or modifies matching routes Configuration often done by human operators –Implementing the policies of their AS –Biz relationships, traffic engineering, security, …

31. Measuring Internet Routing

32. Motivations for Measuring the Routing System Characterizing the Internet –Internet path properties –Demands on Internet routers –Routing convergence Improving Internet health –Protocol design problems –Protocol implementation problems –Configuration errors or attacks Operating a network –Detecting and diagnosing routing problems –Traffic shifts, routing attacks, flaky equipment, …

33. Techniques for Measuring Internet Routing Active probing –Inject probes along path through the data plane –E.g., using traceroute Passive route monitoring –Capture control-plane messages between routers –E.g., using tcpdump or a software router –E.g., dumping the routing table on a router Injecting network events –Cause failure/recovery at planned time and place –E.g., BGP route beacon, or planned maintenance

34. Internet Routing is Hard to Measure Nobody knows the Internet topology –No central registry of the AS-level graph –Little public information about intra-AS topologies Deploying monitoring infrastructure is hard –Forwarding: active probes of end-to-end paths –Routing: passive monitoring of routing messages Many measurement challenges –Network conditions vary by location –Network conditions change over time –One-way measurements are hard to collect –Controlled experiments are hard to do

35. Two Papers for Today Both early measurement studies of routing –Initially appeared at SIGCOMM’96 and ’97 –Both won the “best student paper” award And recently won the SIGCOMM “test of time” award! –Early glimpses into the health of Internet routing –Early wave of papers on Internet measurement Differences in emphasis –Paxson96: end-to-end active probing to measure the characteristics of the data plane –Labovitz97: passive monitoring of BGP update messages from several ISPs to characterize (in)stability of the interdomain routing system

36. Backup Slides Summarizing Paxson96 and Labovitz97

37. Active Measurement: Traceroute Time-To-Live field in IP packet header –Source sends a packet with a TTL of n –Each router along the path decrements the TTL –“TTL exceeded” sent when TTL reaches 0 Traceroute tool exploits this TTL behavior source destination TTL=1 Time exceeded TTL=2 Send packets with TTL=1, 2, 3, … and record source of “time exceeded” message

38. Paxson Study: Forwarding Loops Forwarding loop –Packet returns to same router multiple times May cause traceroute to show a loop –If loop lasted long enough –So many packets traverse the loopy path Traceroute may reveal false loops –Path change that leads to a longer path –Causing later probe packets to hit same nodes Heuristic solution –Require traceroute to return same path 3 times

39. Paxson Study: Causes of Loops Transient vs. persistent –Transient: routing-protocol convergence –Persistent: likely configuration problem Challenges –Appropriate time boundary between the two? –What about flaky equipment going up and down? –Determining the cause of persistent loops? Causes of persistent loops –E.g., misconfiguration 0.0.0.0/012.1.2.0/24

40. Paxson Study: Path Fluttering Rapid changes between paths –Multiple paths between a pair of hosts –Load balancing policies inside the network Packet-based load balancing –Round-robin or random –Multiple paths for packets in a single flow Flow-based load balancing –Hash of some fields in the packet header –E.g., IP addresses, port numbers, etc. –To keep packets in a flow on one path

41. Paxson Study: Routing Stability Route prevalence –Likelihood of observing a particular route –Relatively easy to measure with sound sampling –Poisson arrivals see time averages (PASTA) –Most host pairs have a dominant route Route persistence –How long a route endures before a change –Much harder to measure through active probes –Look for cases of multiple observations –Typical host pair has path persistence of a week

42. Paxson Study: Route Asymmetry Hot-potato routingOther causes –Asymmetric link weights in intradomain routing –Cold-potato routing, where AS requests traffic enter at particular place Consequences –Lots of asymmetry –One-way delay is not necessarily half of the round-trip time Customer A Customer B multiple peering points Provider A Provider B Early-exit routing

43. Labovitz Study: Surprising Statistics BGP is an incremental protocol –In theory, no update messages in steady state Two kinds of update messages –Announcement: advertising a new route –Withdrawal: withdrawing an old route Study saw an alarming number of updates –At the time, Internet had around 45,000 prefixes –Routers were exchanging 3-6 million updates/day –Sometimes as high as 30 million in a day Placing a very high load on the routers

44. Labovitz Study: Classifying Update Messages Analyze update messages –For each (prefix, peer) tuple –Classify the kinds of routing changes Forwarding instability –WADiff: explicit withdraw, replaced by alternate –AADiff: implict withdraw, replaced by alternate Pathological –WADup: explicit withdraw, and then reanounced –AADup: duplicate announcement –WWDup: duplicate withdrawal

45. Labovitz Study: Duplicate Withdrawals Time-space trade-off in router implementation –Common system building technique –Trade one resource for another –Can have surprising side effects The gory details –Ideally, you should not send a withdrawal if you never sent a neighbor a corresponding announcement –Requires remembering what update message you sent to each neighbor –Easier to just send everyone a withdrawal when your route goes away

46. Labovitz Study: Practical Impact “Stateless BGP” is compliant with the standard –But, it forces other routers to handle more load –So that you don’t have to maintain state –Arguably very unfair, and bad for global Internet One router vendor was largely at fault –Router vendor modified its implementation –ISPs then deployed the updated software

47. Labovitz Study: Still Hard to Diagnose Problems Despite having very detailed view into BGP –Some pathologies were very hard to diagnose Possible causes –Flaky equipment –Synchronization of BGP timers –Interaction between BGP and intradomain routing –Policy oscillation Topics addressed more in subsequent work