Dove siamo? Architecture of Dynamic Routing

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1 Dove siamo? Algoritmi di routing Protocolli di routing» Intra dominio (IGP)» Inter dominio (EGP) Le slides relative a questo argomenti sono tratte da Interdomain Routing and The Border Gateway Protocol (BGP) courtesy of Timothy G. Griffin Intel Research, Cambridge UK, [ 1 Architecture of Dynamic Routing IGP AS 1 EGP (= BGP) IGP = Interior Gateway Protocol Metric based: OSPF, IS-IS, RIP, EIGRP (cisco) EGP = Exterior Gateway Protocol Policy based: BGP AS 2 IGP The Routing Domain of BGP is the entire Internet 2

2 Technology of Distributed Routing Link State Topology information is flooded within the routing domain Best end-to-end paths are computed locally at each router. Best end-to-end paths determine next-hops. Based on minimizing some notion of distance Works only if policy is shared and uniform Examples: OSPF, IS-IS Vectoring Each router knows little about network topology Only best next-hops are chosen by each router for each destination network. Best end-to-end paths result from composition of all next-hop choices Does not require any notion of distance Does not require uniform policies at all routers Examples: RIP, BGP 3 Comparison of protocols IGP Link State OSPF IS-IS Vectoring RIP EGP BGP 4

3 BGP-4 for interdomain routing Tier-1 AS or Tier-1 ISP AS3 AS1 ISP: Internet Service Provider transit domain transit domain AS3 BGP session AS2 AS4 AS5 stub domain 5 BGP-4 BGP = Border Gateway Protocol Is a Policy-Based routing protocol Is the de facto EGP of today s global Internet Relatively simple protocol, but configuration is complex and the entire world can see, and be impacted by, your mistakes. 6

4 BGP Operations (Simplified) Establish session on TCP port 179 AS1 Exchange all active routes BGP session AS2 Exchange incremental updates While connection is ALIVE exchange route UPDATE messages 7 Four Types of BGP Messages Open : Establish a peering session. Keep Alive : Handshake at regular intervals. Notification : Shuts down a peering session. Update : Announcing new routes or withdrawing previously announced routes. announcement = prefix + attributes values 8

5 AS Numbers (ASNs) ASNs are 16 bit values through are private Genuity: 1 MIT: 3 JANET: 786 UC San Diego: 7377 AT&T: 7018, 6341, 5074, UUNET: 701, 702, 284, 12199, Sprint: 1239, 1240, 6211, 6242, ASNs represent units of routing policy in BGP-4 BGP Routing Tables 9 show ip bgp BGP table version is , local router ID is Status codes: s suppressed, d damped, h history, * valid, > best,, i - internal Origin codes: i - IGP, e - EGP,? - incomplete Network Next Hop Metric LocPrf Weight Path... *>i i *>i i *>i i *>i i *>i i *>i i *>i i *>i i *>i i *>i / i... Thanks to Geoff Huston. on July 6, 2001 Use whois queries to associate an ASN with owner (for example, = AT&T Worldnet, 701 =Uunet, 3561 = Cable & Wireless, 10

6 AS Graphs Do Not Show Topology! BGP was designed to throw away information! The AS graph may look like this. Reality may be closer to this AS Graphs Can Be Fun 11 The subgraph showing all ASes that have more than 100 neighbors in full graph of 11,158 nodes. July 6, Point of view: AT&T route-server 12

7 How Many ASNs are there today? 18,217 Thanks to Geoff Huston. on October 26, How Many ASNs are there today? Thanks to Geoff Huston. on November 18,

8 How many prefixes today? 179,903 Note: numbers actually depends point of view Thanks to Geoff Huston. on October 26, How many prefixes today? Thanks to Geoff Huston. on November,

9 Attributes are Used to Select Best Routes pick me! pick me! pick me! pick me! Given multiple routes to the same prefix, a BGP speaker must pick at most one best route (Note: it could reject them all!) 17 AS PATH attribute /16 AS Path = AS 1129 Global Access /16 AS Path = AS 1239 Sprint AS 1755 Ebone /16 AS Path = /16 AS Path = AS RIPE NCC RIS project /16 AS Path = 6341 AS 6341 AT&T Research /16 Prefix Originated AS7018 AT&T /16 AS Path = /16 AS Path = AS 3549 Global Crossing 18

10 Policy-Based vs. Distance-Based Routing? Minimizing hop count can violate commercial relationships that constrain interdomain routing. Cust1 ISP1 Host 1 YES NO ISP3 ISP2 Host 2 Cust3 Cust2 19 Why not minimize AS hop count? National ISP1 National ISP2 YES NO Regional ISP3 Regional ISP2 Regional ISP1 Cust3 Cust2 Cust1 Shortest path routing is not compatible with commercial relations 20

11 Customers and Providers IP traffic Customer pays for access to the Internet The Peering Relationship 21 peer traffic allowed peer traffic NOT allowed Peers provide transit between their respective s Peers do not provide transit between peers Peers (often) do not exchange $$$ 22

12 Peering Provides Shortcuts Peering also allows connectivity between the s of Tier 1 s. peer peer 23 Peering Wars Peer Reduces upstream transit costs Can increase end-to-end performance May be the only way to connect your s to some part of the Internet ( Tier 1 ) Don t Peer You would rather have s Peers are usually your competition Peering relationships may require periodic renegotiation Peering struggles are by far the most contentious issues in the ISP world! Peering agreements are often confidential. 24

13 Implementing Customer/Provider and Peer/Peer relationships Two parts: Enforce transit relationships» Outbound route filtering (to avoid being used as transit if not desired) Enforce order of route preference» < peer < 25 Import Routes route peer route route ISP route From From From peer From peer From From 26

14 Export Routes route peer route route ISP route To From To peer To peer To To filters block 27 BGP Route Processing Each operator defines its own policies! Receive BGP Updates Apply Policy = filter routes & tweak attributes Based on Attribute Values Best Routes Apply Policy = filter routes & tweak attributes Transmit BGP Updates Apply Import Policies Best Route Selection Best Route Table Apply Export Policies Install forwarding Entries for best Routes. IP Forwarding Table 28

15 Shorter Doesn t Always Mean Shorter Mr. BGP says that path 4 1 is better than path Duh! AS 4 AS 3 AS 2 AS 1 Non è possibile fare di meglio : se si esportasse lo stato interno degli autonomous systems la soluzione non sarebbe scalabile! 29 Routing Example 1 A = planetlab1.xeno.cl.cam.ac.uk B = planetlab1.info.ucl.ac.be C = planetlab1.cs.cornell.edu A JANET BELNET B NY UK NL BE GEANT New York Chicago Abilene NYSERNet C Cornell 30

16 Routing Example 2 A = planetlab1.xeno.cl.cam.ac.uk B = phys0bha 4a.chem.msu.ru C = planetlab1.ie.cuhk.edu.hk AS20965 GEANT AS11537 ABILENE ASCC AS9264 AS5568 RBNET AS3491 PCCW AS4637 REACH AS6453 TeleGlobe B MSU A JANET C CUHK AS2848 AS786 AS Tweak Tweak Tweak For inbound traffic» Filter outbound routes» Tweak attributes on outbound routes in the hope of influencing your neighbor s best route selection For outbound traffic» Filter inbound routes» Tweak attributes on inbound routes to influence best route selection In general, an AS has more control over outbound traffic inbound traffic outbound traffic outbound routes inbound routes Tweak: letteralmente pizzicare, in slang mettere a punto 32

17 Implementing Backup Links with Local Preference (Outbound Traffic) AS 1 primary link backup link Set Local Pref = 100 for all routes from AS 1 AS Set Local Pref = 50 for all routes from AS 1 Forces outbound traffic to take primary link, unless link is down. Multihomed Backups (Outbound Traffic) 33 AS 1 AS 3 primary link backup link Set Local Pref = 100 for all routes from AS 1 AS 2 Set Local Pref = 50 for all routes from AS 3 Forces outbound traffic to take primary link, unless link is down. 34

18 Controlling Inbound Traffic with ASPATH Prepending AS 1 Prepending will (usually) force inbound traffic from AS 1 to take primary link ASPATH = 2 ASPATH = primary AS 2 backup 35 But it does not always work AS 1 AS 3 ASPATH = 2 ASPATH = primary AS 2 backup AS 3 will send traffic on backup link because it prefers routes and local preference is considered before ASPATH length! 36

19 BGP issues There is no guarantee that a BGP configuration has a unique routing solution.» When multiple solutions exist, the (unpredictable) order of updates will determine which one is wins. There is no guarantee that a BGP configuration has any solution!» And checking configurations NP-Complete [GW1999] Complex policies increase chances of routing anomalies.» yet this is the current trend! 37

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