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Lecture 10 Overview

Lecture 10 Overview. Border Gateway Protocol(BGP). De facto standard for Internet inter-AS routing allows subnet to advertise its existence to rest of Internet: “ I am here ” BGP provides each AS a means to: Obtain subnet reachability information from neighboring ASs

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Lecture 10 Overview

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  1. Lecture 10 Overview

  2. Border Gateway Protocol(BGP) • De facto standard for Internet inter-AS routing • allows subnet to advertise its existence to rest of Internet: “I am here” • BGP provides each AS a means to: • Obtain subnet reachability information from neighboring ASs • Propagate reachability information to all AS-internal routers • Determine “good” routes to subnets based on reachability information and policy CPE 401/601 Lecture 10 : Border Gateway Protocol

  3. 2c 2b 1b 1d 1c 3c BGP basics • Pairs of routers (BGP peers) exchange routing info over semi-permanent TCP connections: BGP sessions • BGP sessions need not correspond to physical links • when AS2 advertises a prefix to AS1: • AS2 promises it will forward datagrams towards that prefix • AS2 can aggregate prefixes in its advertisement eBGP session iBGP session 3a 3b 2a AS3 AS2 1a AS1 CPE 401/601 Lecture 10 : Border Gateway Protocol

  4. 2c 2b 1b 1d 1c 3c Distributing reachability info • using eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS1 • 1c can then use iBGP do distribute new prefix info to all routers in AS1 • 1b can then re-advertise new reachability info to AS2 over 1b-to-2a eBGP session eBGP session iBGP session 3a 3b 2a AS3 AS2 1a AS1 CPE 401/601 Lecture 10 : Border Gateway Protocol

  5. Path attributes & BGP routes • advertised prefix includes BGP attributes • prefix + attributes = “route” • two important attributes • AS-PATH: contains ASs through which prefix advertisement has passed: e.g, AS 67, AS 17 • NEXT-HOP: indicates specific internal-AS router to next-hop AS • may be multiple links from current AS to next-hop-AS • when gateway router receives advertisement, uses import policy to accept/decline CPE 401/601 Lecture 10 : Border Gateway Protocol

  6. BGP route selection • router may learn about more than 1 route to some prefix • Router must select route • elimination rules • local preference value attribute: policy decision • shortest AS-PATH • closest NEXT-HOP router: hot potato routing • additional criteria CPE 401/601 Lecture 10 : Border Gateway Protocol

  7. BGP messages • BGP messages exchanged using TCP • BGP messages: • OPEN: opens TCP connection to peer and authenticates sender • 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 CPE 401/601 Lecture 10 : Border Gateway Protocol

  8. BGP routing policy • A,B,C are provider networks • X,W,Y are customer (of provider networks) • X is dual-homed: attached to two networks • X does not want to route from B via X to C • .. so X will not advertise to B a route to C legend: provider B network X W A customer network: C Y CPE 401/601 Lecture 10 : Border Gateway Protocol

  9. BGP routing policy (2) • A advertises path AW to B • B advertises path BAW to X • Should B advertise path BAW to C? • No way! B gets no “revenue” for routing CBAW since neither W nor C are B’s customers • B wants to force C to route to w via A • B wants to route only to/from its customers! legend: provider B network X W A customer network: C Y CPE 401/601 Lecture 10 : Border Gateway Protocol

  10. Why different Intra- and Inter-AS routing ? • Policy: • Inter-AS: admin wants control over how its traffic routed, who routes through its net • Intra-AS: single admin, no need for policy decisions • Scale: • hierarchical routing saves table size, reduced update traffic • Performance: • Intra-AS: can focus on performance • Inter-AS: policy may dominate over performance CPE 401/601 Lecture 10 : Border Gateway Protocol

  11. Lecture 11Router Architectures CPE 401 / 601 Computer Network Systems slides are modified from Dave Hollinger slides are modified from Dave Hollinger

  12. Router Architecture Overview • Two key router functions: • run routing algorithms/protocol (RIP, OSPF, BGP) • forwarding datagrams from incoming to outgoing link CPE 401/601 Lecture 11 : Router Architectures

  13. Input Port Functions • Goal: complete input port processing at ‘line speed’ • Decentralized switching: • given datagram dest., lookup output port using forwarding table in input port memory CPE 401/601 Lecture 11 : Router Architectures

  14. Input Port Functions Physical layer: bit-level reception Network layer: Routing & forwarding Data link layer: e.g., Ethernet CPE 401/601 Lecture 11 : Router Architectures

  15. Input Port Queuing • Queuing when datagrams arrive faster than forwarding rate into switch fabric • queuing delay and loss due to input buffer overflow! • Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward CPE 401/601 Lecture 11 : Router Architectures

  16. Three types of switching fabrics CPE 401/601 Lecture 11 : Router Architectures

  17. Output Ports • Buffering required when datagrams arrive from fabric faster than the transmission rate • Scheduling discipline chooses among queued datagrams for transmission CPE 401/601 Lecture 11 : Router Architectures

  18. Output port queueing • Queuing (delay) and loss due to output port buffer overflow! CPE 401/601 Lecture 11 : Router Architectures

  19. . RTT C N How much buffering? • RFC 3439 rule of thumb: • buffering equal to typical RTT (say 250 msec) times link capacity C • e.g., C = 10 Gps link: • 2.5 Gbit buffer • Recent recommendation: • with N flows, buffering equal to CPE 401/601 Lecture 11 : Router Architectures

  20. Bridges

  21. Bridge Software • A bridge connects networks and forwards frames from one network to another. PORTS A B E F BRIDGE C D G H CPE 401/601 Lecture 11 : Bridges

  22. Selective Forwarding • If A sends a frame to E • the frame must be forwarded by the bridge • If A sends a frame to B • there is no reason to forward the frame A B E F BRIDGE C D G H CPE 401/601 Lecture 11 : Bridges

  23. Bridge Database • The bridge needs a database that contains information about which hosts are on which network. • The realistic options are: • The system administrator can create and maintain the database. • The bridge can acquire the database on the fly. Hard to add new computers Some loss of efficiency CPE 401/601 Lecture 11 : Bridges

  24. Learning the host mapping • The bridge forwards packets for which it does not know which network the destination is on • Every time the bridge forwards a packet it can record the network on which the sender is located • Each host mapping expires when it is unused for a “long” period of time. CPE 401/601 Lecture 11 : Bridges

  25. “Learning” Bridge • A host can be moved to another network. • New hosts can be added at any time. • Requires no setup information from humans. CPE 401/601 Lecture 11 : Bridges

  26. What is the problem ? Bridge 1 0 1 D A E B F C Bridge 2 0 1 CPE 401/601 Lecture 11 : Bridges

  27. Possible Solutions • Forget about smart bridges • Tell customers to avoid topologies that include loops • design bridges so that they detect loops and scream at the customer • Design bridges that prune the network topology into something with no loops. CPE 401/601 Lecture 11 : Bridges

  28. Spanning Tree Creation • The bridges must communicate! • They send configuration bridge protocol data units (BPDUs) • Multicast: special data link address • Each bridge has a unique ID • Use an algorithm to construct a spanning tree based on local messages CPE 401/601 Lecture 11 : Bridges

  29. Spanning Tree B B B B B B B B Prune B B B B network B bridge CPE 401/601 Lecture 11 : Bridges

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