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 – 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 2

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 3b 1d 3a 1c 2a AS3 AS1 AS2 1a 2c 2b 1b 3c eBGP session iBGP session CPE 401/601 Lecture 10 : Border Gateway Protocol 3

Distributing reachability info 3b 1d 3a 1c 2a AS3 AS1 AS2 1a 2c 2b 1b 3c eBGP session iBGP session 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 CPE 401/601 Lecture 10 : Border Gateway Protocol 4

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 5

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 6

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 7

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 A B C W X Y legend : customer network: provider network CPE 401/601 Lecture 10 : Border Gateway Protocol 8

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! A B C W X Y legend : customer network: provider network CPE 401/601 Lecture 10 : Border Gateway Protocol 9

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 10

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

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 12

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 13

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

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 15

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

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 17

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

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 RTT C. N CPE 401/601 Lecture 11 : Router Architectures 19

Bridges

Bridge Software A bridge connects networks and forwards frames from one network to another. 21 BRIDGE AB CD EF GH PORTS CPE 401/601 Lecture 11 : Bridges

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 22 BRIDGE AB CD EF GH CPE 401/601 Lecture 11 : Bridges

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. 23 Hard to add new computers Some loss of efficiency CPE 401/601 Lecture 11 : Bridges

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. 24 CPE 401/601 Lecture 11 : Bridges

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

What is the problem ? 26 Bridge 1 10 Bridge 2 10 A C B D F E CPE 401/601 Lecture 11 : Bridges

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. 27 CPE 401/601 Lecture 11 : Bridges

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 28 CPE 401/601 Lecture 11 : Bridges

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