ROUTING IN THE INTERNET AUTONOMOUS SYSTEM AS Collections

ROUTING IN THE INTERNET

AUTONOMOUS SYSTEM (AS) Collections of routers that has the same protocol, administative and technical control Intra-AS routing Inter-AS routing 3 c 3 b 3 a AS 3 2 a 1 c 1 a 1 d 2 c AS 2 1 b AS 1 2 b

INTRA-AS ROUTING also known as Interior Gateway Protocols (IGP) most common Intra-AS routing protocols: � RIP: Routing Information Protocol � OSPF: Open Shortest Path First

RIP ( ROUTING INFORMATION PROTOCOL) Deployed in lover-tier ISPs & enterprise networks Hop is the number of subnets traversed from source to destination Maximum 15 hops Response message (advertisment) every 30’s second UDP & port 520 From router A to subnets: u v A z C B w x D y destination hops u 1 v 2 w 2 x 3 y 3 z 2

RIP: EXAMPLE z w A x y D B C Destination Network Next Router Num. of hops to dest. w y z x A B B -- 2 2 7 1 …. . . Routing/Forwarding table in D 4 -5

OSPF (OPEN SHORTEST PATH FIRST) Deployed in upper-tier ISPs “open” - publicly available Djikstra’s shortest-path algorithm Broadcasts routing information, carried by IP Divides routers in hierarchical areas.

HIERARCHICAL OSPF

ADVANCES EMBODIED IN OSPF Robustness Secure Multiple same-cost path Integrated support for unicast and multicast routing (MOSPF) Support for hierarchy within a single routing domain

INTERNET INTER-AS ROUTINGBGP : BGP (Border Gateway Protocol) The protocol that glue internet together BGP provides each AS a means to: Obtain subnet reachability information from neighboring ASs. 2. Propagate reachability information to all AS-internal routers. 3. Determine “good” routes to subnets based on reachability information and policy. 4. Allow each subnet to advertise its existence to the rest of the internet, “I am here” 1.

Uses semi permanent TCP, port 179 BGP peers, two routers that’s connected BGP session BGP defines ASs through ASN e. BGP session 3 c 3 a 3 b AS 3 1 a AS 1 i. BGP session 2 c 2 a 1 c 1 d 2 b AS 2 1 b

IMPORTANT ATTRIBUTES AS-PATH – cotains the ASs through which the adsvertisement for the prefix has passed NEXT-HOP – begins the AS-PATH also provides the critical link between inter- and intra-AS routing protocols Import policy – Decides whether the router should accept of filter the route.

BGP ROUTE SELECTION Router may learn about more than 1 route to some prefix. Router must select route. Elimination rules: 1. 2. 3. 4. local preference value attribute: policy decision shortest AS-PATH closest NEXT-HOP router: hot potato routing additional criteria

BGP ROUTING POLICY legend: B W provider network X A customer network: C Y r A, B, C are provider networks r X, W, Y are customer (of provider networks) r X is dual-homed: attached to two networks m. X does not want to route from B via X to C m. . so X will not advertise to B a route to C Network Layer 4 -13