Network Layer Control Plane COMPSCI 453 Computer Networks

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Network Layer: Control Plane COMPSCI 453 Computer Networks Professor Jim Kurose College of Information

Network Layer: Control Plane COMPSCI 453 Computer Networks Professor Jim Kurose College of Information and Computer Sciences University of Massachusetts § introduction § routing algorithms § link state § distance vector § intra-ISP routing: OSPF § routing among ISPs: BGP § SDN control plane § Internet Control Message Protocol § Network management, configuration Class textbook: Computer Networking: A Top. Down Approach (8 th ed. ) J. F. Kurose, K. W. Ross Pearson, 2020 http: //gaia. cs. umass. edu/kurose_ross

Internet inter-AS routing: BGP § BGP (Border Gateway Protocol): the de facto inter-domain routing

Internet inter-AS routing: BGP § BGP (Border Gateway Protocol): the de facto inter-domain routing protocol • “glue that holds the Internet together” § allows subnet to advertise its existence, and the destinations 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 destination network reachability info from neighboring ASes (e. BGP) • determine routes to other networks based on reachability information and policy • propagate reachability information to all AS-internal routers (i. BGP) • advertise (to neighboring networks) destination reachability info

e. BGP, i. BGP connections 2 b 2 a 1 b 1 a 1

e. BGP, i. BGP connections 2 b 2 a 1 b 1 a 1 c 2 d 3 b ∂ 3 a AS 2 1 d AS 1 1 c 2 c ∂ e. BGP connectivity logical i. BGP connectivity 3 c 3 d AS 3 gateway routers run both e. BGP and i. BGP protocols

BGP basics § BGP session: two BGP routers (“peers, speakers”) exchange BGP messages over

BGP basics § BGP session: two BGP routers (“peers, speakers”) exchange BGP messages over semi-permanent TCP connection: • advertising paths to different destination network prefixes (e. g. , to a destination /16 network) • BGP is a “path vector” protocol § when AS 3 gateway 3 a advertises path AS 3, X to AS 2 gateway 2 c: • AS 3 promises to AS 2 it will forward datagrams towards X AS 1 AS 3 1 b 1 a 1 c 1 d 3 b 3 a AS 2 2 b 2 a 3 d 2 c 2 d 3 c BGP advertisement: AS 3, X X

BGP protocol messages § BGP messages exchanged between peers over TCP connection § BGP

BGP protocol messages § BGP messages exchanged between peers over TCP connection § BGP messages [RFC 4371]: • 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 UPDATES; also ACKs OPEN request • NOTIFICATION: reports errors in previous msg; also used to close connection

Path attributes and BGP routes § BGP advertised path: prefix + attributes • path

Path attributes and BGP routes § BGP advertised path: prefix + attributes • path prefix: destination being advertised • two important attributes: • AS-PATH: list of ASes through which prefix advertisement has passed • NEXT-HOP: indicates specific internal-AS router to next-hop AS § policy-based routing: • router receiving route advertisement to destination X uses policy to accept/reject a path (e. g. , never route through AS W, or country Y). • router uses policy to decide whether to advertise a path to neighboring AS Z (does router want to route traffic forwarded from Z destined to X? )

BGP path advertisement AS 3 AS 1 1 b 1 a 3 a 1

BGP path advertisement AS 3 AS 1 1 b 1 a 3 a 1 c 1 d 3 b AS 2, AS 3, X AS 2 3 d 2 b 2 a 3 c 2 c X AS 3, X 2 d § AS 2 router 2 c receives path advertisement AS 3, X (via e. BGP) from AS 3 router 3 a § based on AS 2 policy, AS 2 router 2 c accepts path AS 3, X, propagates (via i. BGP) to all AS 2 routers § based on AS 2 policy, AS 2 router 2 a advertises (via e. BGP) path AS 2, AS 3, X to AS 1 router 1 c

BGP path advertisement: multiple paths AS 3 AS 1 1 a 1 b AS

BGP path advertisement: multiple paths AS 3 AS 1 1 a 1 b AS 3, X 1 d AS 3, X 1 c AS 3, X AS 2, AS 3, X 3 b 3 a AS 2 3 d 2 b 2 a 3 c 2 c X AS 3, X 2 d gateway routers may learn about multiple paths to destination: § AS 1 gateway router 1 c learns path AS 2, AS 3, X from 2 a § AS 1 gateway router 1 c learns path AS 3, X from 3 a § based on policy, AS 1 gateway router 1 c chooses path AS 3, X and advertises path within AS 1 via i. BGP

BGP: achieving policy via advertisements A, w w A A, w B C provider

BGP: achieving policy via advertisements A, w w A A, w B C provider network x legend: y customer network: ISP only wants to route traffic to/from its customer networks (does not want to carry transit traffic between other ISPs – a typical “real world” policy) § A advertises path Aw to B and to C § B chooses not to advertise BAw to C! § B gets no “revenue” for routing CBAw, since none of C, A, w are B’s customers § C does not learn about CBAw path § C will route CAw (not using B) to get to w

BGP: achieving policy via advertisements (more) B w A C provider network x legend:

BGP: achieving policy via advertisements (more) B w A C provider network x legend: y customer network: ISP only wants to route traffic to/from its customer networks (does not want to carry transit traffic between other ISPs – a typical “real world” policy) § A, B, C are provider networks § x, w, y are customer (of provider networks) § x is dual-homed: attached to two networks § policy to enforce: x does not want to route from B to C via x §. . so x will not advertise to B a route to C

BGP: populating forwarding tables AS 3 AS 1 1 b 1 AS 3, X

BGP: populating forwarding tables AS 3 AS 1 1 b 1 AS 3, X 1 a local link interfaces at 1 a, 1 d AS 3, X 2 2 1 d 1 AS 3, X 1 c AS 3, X AS 2, AS 3, X 3 b 3 a AS 2 3 d 2 b 2 a 3 c 2 c X AS 3, X 2 d dest interface … … 1 1 c X 1 … … § recall: 1 a, 1 b, 1 d learn via i. BGP from 1 c: “path to X goes through 1 c” § at 1 d: OSPF intra-domain routing: to get to 1 c, use interface 1 § at 1 d: to get to X, use interface 1

BGP: populating forwarding tables AS 3 AS 1 1 b 3 a 1 1

BGP: populating forwarding tables AS 3 AS 1 1 b 3 a 1 1 a 1 c 2 3 b 1 d AS 2 3 d 2 b 2 a 3 c X 2 c 2 d dest interface … … 2 1 c X 2 … … § § § recall: 1 a, 1 b, 1 d learn via i. BGP from 1 c: “path to X goes through 1 c” at 1 d: OSPF intra-domain routing: to get to 1 c, use interface 1 at 1 d: to get to X, use interface 1 at 1 a: OSPF intra-domain routing: to get to 1 c, use interface 2 at 1 a: to get to X, use interface 2

Hot potato routing AS 3 AS 1 1 b 1 a 3 a 1

Hot potato routing AS 3 AS 1 1 b 1 a 3 a 1 c 1 d 3 b AS 1, AS 3, X AS 2 2 a 2 b 201 2 d 3 d 112 263 3 c 2 c X AS 3, X OSPF link weights § 2 d learns (via i. BGP) it can route to X via 2 a or 2 c § hot potato routing: choose local gateway that has least intra-domain cost (e. g. , 2 d chooses 2 a, even though more AS hops to X): don’t worry about inter-domain cost!

Why different Intra-, Inter-AS routing ? policy: § inter-AS: admin wants control over how

Why different Intra-, Inter-AS routing ? policy: § inter-AS: admin wants control over how its traffic routed, who routes through its network § intra-AS: single admin, so policy less of an issue scale: reducing forwarding table size, routing update traffic § hierarchical routing: limiting the scope of full topological information § BGP routing to CIDRized destination networks (summarized routes) performance: § intra-AS: can focus on performance § inter-AS: policy dominates over performance

Network Layer: Data Plane COMPSCI 453 Computer Networks Professor Jim Kurose College of Information

Network Layer: Data Plane COMPSCI 453 Computer Networks Professor Jim Kurose College of Information and Computer Sciences University of Massachusetts § introduction § routing algorithms § link state § distance vector § intra-ISP routing: OSPF § routing among ISPs: BGP § SDN control plane § Internet Control Message Protocol § Network management, configuration Class textbook: Computer Networking: A Top. Down Approach (8 th ed. ) J. F. Kurose, K. W. Ross Pearson, 2020 http: //gaia. cs. umass. edu/kurose_ross Video: 2020, J. F. Kurose, All Rights Reserved Powerpoint: 1996 -2020, J. F. Kurose, K. W. Ross, All Rights Reserved