Computer Networks Lecture #6 PDF

Computer Networks Lecture #6 In the last lecture Introduction to control plane in network layer Routing protocols Link states Distance vectors Today Introduction to routing protocol in practice, Intra-ISP routing (OSPF) Routing among ISPs: BGP Introduction to routing protocols in...

Computer Networks Lecture #6 In the last lecture Introduction to control plane in network layer Routing protocols Link states Distance vectors Today Introduction to routing protocol in practice, Intra-ISP routing (OSPF) Routing among ISPs: BGP Introduction to routing protocols in practice, Intra-AS routing (OSPF) Making routing scalable Our routing study thus far - idealized on the assumption that All routers are “identical” Network is “ at” … not true in practice In practice Scale: billions of destination Cannot store all destination in routing table Routing table exchange would swamp links Administrative autonomy Internet is a network of networks Each network admin may want to control routing in its own network fl Internet approach to scalable routing Aggregate routers into regions known as “autonomous systems” (AS) (a.k.a domains) Intra-AS (a.k.a intra-domain) Routing within the same AS (domain) All routers in the same AS must run the same intra-domain routing protocol Routers in the di erent ASes can run di erent intra-domain routing protocols Gateway router Locates at the edge of its own AS Has link(s) to router(s) in other CASes Inter-AS (a.k.a inter-domain) Routing among ASes Gateways perform inter-domain routing (as well as intra-domain routing) ff ff Interconnected ASes Forwarding table is con gured by intra- and inter-AS routing algorithms fi Interconnected ASes Forwarding table is con gured by intra- and inter-AS routing algorithms intra-AS routing determines entries for destinations within AS fi Interconnected ASes Forwarding table is con gured by intra- and inter-AS routing algorithms intra-AS routing determines entries for destinations within AS inter-AS and intra-AS routing determines entries for external destinations fi Inter-AS routing Suppose router in AS1 receives datagram destined outside of AS1. Then, router should forward packet to gateway router of AS1, but which one? AS1 inter-domain routing must: Learn which destinations reachable through AS2, which through AS3 Propagate this reachability info to all routers in AS1 Intra-AS routing: routing within an AS Most common intra-AS routing protocols Also known as Routing Information Protocol (RIP), [RFC 1723] Interior Gateway Protocol (IGP) Classic DV : DVs exchanged every 30secs No longer widely used Enhanced Interior Gateway Routing Protocol (EIGRP) DV-based Formerly Cisco-proprietary for decades (became open in 2013 [RFC 7868]) Open Shortest Path First (OSPF) [RFC 2328] Link-state routing IS-IS protocol (ISO standard, not RFC standard) essentially same as OSFP Open Shortest Path First (OSPF) “Open” Publicly available Class link-state Each router oods OSPF link-state advertisements (directly over IP rather than using TCP/ UDP) to all other routers in entire AS Multiple link costs metrics possible (e.g, bandwidth, delay, etc.) Each router has full topology, uses Dijkstra’s algorithm to compute forwarding table Security All OSPF messages authenticated (to prevent malicious intrusion) fl Hierarchical OSPF Two-level hierarchy Local area, backbone Link-state advertisement ooded only in area, or backbone Each node has detailed area topology; only knows direction to reach other destination fl Routing among ISPs : BGP Internet Inter-AS routing: BGP Border Gateway Protocol (BGP) de facto inter-domain routing protocol “glue that holds the Internet together” Allows subnet to advertise its existence, and the destination it can reach, to rest of Internet → “I am here, here is who I can reach, and how” BGP provides each AS a means to Obtain subnet reachability information from neighboring ASes (eBGP) Propagate reachability information to all AS-internal routers (iBGP) Determine good routes to other networks based on reachability information and policy eBGP, iBGP BGP basics BGP session Two BGP routers (“peers”) exchange BGP messages over semi-permanent TCP connection Advertising paths to di erent destination network pre xes (BGP is a “path vector” protocol) When AS3 gateway 3a advertises path AS3,X to AS2 gateway 2c AS3 promises to AS2 it will forward datagrams toward X ff fi  Path attributes and BGP routes BGP advertised route: pre x + attributes Pre x: destination being advertised Two important attributes AS-PATH: list of ASes through which pre x advertisement has passed NEXT-HOP: indicate speci c internal-AS router to next-hop AS Policy-based routing Gateway receiving route advertisement uses import policy to accept/decline path (e.g., never route through AS Y) AS policy also determines whether to advertise path to other neighboring ASes fi fi fi fi BGP path advertisement AS2 router 2c receives path advertisement AS3,X (via eBGP) from AS3 router 3a Based on AS2 policy, AS2 router 2c accepts path AS3,X, propagates (via iBGP) to all AS2 routers Based on AS2 policy, AS2 router 2a advertises (via eBGP) path AS2,AS3,X to AS1 router 1c BGP path advertisement Gateway router may learn about multiple paths to destination AS1 gateway router c1 learns path AS2,AS3,X from 2a AS1 gateway router c1 learns path AS3,X from 3a Based on policy, A1 gateway router c1 chooses path AS3,X and advertises path within AS1 via iBGP BGP messages BGP messages are exchanged between peers over TCP connection BGP messages OPEN Opens TCP connection to remote BGP peer and authenticates sending BGP peer UPDATE Advertises new path (or withdraws old) KEEPALIVE Keeps connection alive in absence of UPDATE Also used to ack OPEN message from peer (that is, in the order of OPEN to peer, OPEN to speaker, KEEPALIVE) NOTIFICATION Reports errors in previous message Also used to close connection BGP path advertisement Recall: 1a, 1b, 1d learn via iBGP from 1c: “path to X goes through 1c” at 1d iBGP says: to get to X, use interface 1 OSPF intra-domain routing tells: to get to 1c use interface 1 BGP path advertisement Recall: 1a, 1b, 1d learn via iBGP from 1c: “path to X goes through 1c” at 1a iBGP says: to get to X, use interface 2 OSPF intra-domain routing tells: to get to 1c use interface 2 Why different intra-, inter-AS routing? Policy Inter-AS : admin wants control over how its tra c is routed, who routes through its network Intra-AS : single admin, so policy is less of an issue Scale Hierarchical routing saves table sizes, reduced update tra c Performance Inter-AS : policy dominates over performance Intra-AS : can focus on performance ffi ffi Hot potato routing 2d learns (via iBGP) it can route to X via 2a or 2c Hot potato routing Choose local gateway that has least intra-domain cost (e.g., 2d chooses 2a, even though more AS hop to X) Don’t worry about inter-domain cost BGP: implementing policy via advertisement ISP only wants to route tra c to/from its customer networks Does not want to carry transit tra c between other ISP (e.g., B ↛ C in Fig. Above) A typical real-world policy ffi ffi BGP: implementing policy via advertisement Example scenario I A advertises path A,w to B and to C B chooses not to advertises B,A,w to C B gets no revenue for routing C→B→A→w, since none of C, A, w are B’s customers C does not learn about the path C,B,A,w BGP: implementing policy via advertisement Example scenario II A, B, C are provider networks x, w, y are customers (of provider networks) x is dual-homed (i.e., attached to two networks) Policy to enforce : x does not want to route from B to C via x → x will not advertise to B a route to C BGP route selection Router may learn about more than one route to destination AS Route selection is performed based on Local preference value attribute: policy decision Shortest AS-PATH Closest NEXT-HOP router: hot potato routing Additional criteria e.g, cisco’s best path selection algorithm https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html Questions?