CMPE 150: Introduction to Computer Networks

Transcription

1 CMPE 150: Introduction to Computer Networks Dr. Chane L. Fullmer Spring 2003 UCSC cmpe150 1

2 Homework Assignments Homework assignment #3 Chapter Four Due by May 22 Spring 2003 UCSC cmpe150 2

3 Thursday s Lecture Guest lecturer Dr. Soumya oy On Demand Link State outing in Ad Hoc Networks Will speak on IP multicasting and related issues (Yes, his lecture will be on the exam) Spring 2003 UCSC cmpe150 3

4 CMPE 150: Introduction to Computer Networks Set 12: Internet outing Spring 2003 UCSC cmpe150 4

5 Internet outing Protocols Intra-domain routing: IP, OSPF, EIGP Single-path routing protocols, static link costs Performance (shortest path) Inter-domain routing: Border Gateway Protocol (BGP) Single path Policy based Spring 2003 UCSC cmpe150 5

6 IP (v1) Based on DBF Used in small internets Problems: Counting to infinity and looping, single-path routing, link cost should be 1 or infinity Update specifies only a destination network and a distance to it; hence, no variable subnet masks are allowed in local internet and a static subnetting convention must be used for all routers outer sends its routing table to its neighbors every 30 sec. or when it must update its routing table. uns on top of UDP. Spring 2003 UCSC cmpe150 6

7 IPv2 Adds the next hop to a destination and subnet mask in each update. Variable subnets are allowed. Performance does not improve much. Spring 2003 UCSC cmpe150 7

8 OSPF: Open Shortest Path First Dijkstra s SPF used to compute shortest paths locally based on topology map. Flooding is used to disseminate topology maps. Sequence numbers and age fields are used to validate link-state updates. uns on top of IP and implements its own reliable transmission of link-state updates. Designated routers are used to reduce overhead within a LAN, and areas connected by a backbone are used to reduce overhead across LANs. HELLOs used to identify neighbors. Spring 2003 UCSC cmpe150 8

9 OSPF A1 A2 backbone not allowed A3 A4 Areas must be connected by a connected backbone (area 0) Spring 2003 UCSC cmpe150 9

10 OSPF internal router A1 A2 To other domains EA1 EA2 EAn boundary router, backbone router A3 area border router A4 Spring 2003 UCSC cmpe150 10

11 OSPF Areas need unique IDS, an IP address. Zero or more address ranges can be reached in an area. Different types of routers have different views of topology. End result is a hybrid of link-state and distance information. Spring 2003 UCSC cmpe150 11

12 Topology Information at Backbone outer A2 A1 EA1 EA2 EAn A3 A4 Spring 2003 UCSC cmpe150 12

13 Topology Information at Area Border outer A1 A2 EA1 EA2 EAn A3 A4 Spring 2003 UCSC cmpe150 13

14 Topology Information at Internal outer A2 A1 EA1 EA2 EAn A3 A4 Spring 2003 UCSC cmpe150 14

15 OSPF In a broadcast LAN, designated router eliminates too many link state updates. LSUs, HELLOs and topology updates sent unicast to designated router, which keeps all routers in LAN updated. Spring 2003 UCSC cmpe150 15

16 Inter-AS routing in the Internet: BGP 5 AS1 (IP intra-as routing) 1 BGP 2 3 BGP AS2 (OSPF intra-as routing) 4 AS3 (OSPF intra-as routing) Figure new2: BGP use for inter-domain routing Spring 2003 UCSC cmpe150 16

17 BGP BGP (Border Gateway Protocol): the de facto standard for Internet inter-as routing. Path Vector protocol: Similar to Distance Vector protocol Each Border Gateway broadcast to neighbors (peers) entire path (i.e., sequence of AS s) to destination BGP routes to networks (ASs), not individual hosts e.g., Gateway X may send its path to dest. Z: Path (X,Z) = X,Y1,Y2,Y3,,Z Spring 2003 UCSC cmpe150 17

18 BGP Gateway X send its path to peer gateway W W may or may not select path offered by X cost, policy (don t route via competitors AS), loop prevention reasons. If W selects path advertised by X, then: Path (W,Z) = w, Path (X,Z) Note: X can control incoming traffic by controlling its route advertisements to peers: e.g., do not want to route traffic to Z -> do not advertise any routes to Z Spring 2003 UCSC cmpe150 18

19 Controlling Who outes to You B legend: provider network W A C X Do not allow customer network: Y Figure 4.5-BGPnew: a simple BGP scenario 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 Spring 2003 UCSC cmpe150 19

20 Controlling Who outes to You B legend: provider network W A C X customer network: Y Figure 4.5-BGPnew: a simple BGP scenario A advertises to B the path AW B advertises to X the path BAW Should B advertise to C the path BAW? No... 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! Spring 2003 UCSC cmpe150 20

21 BGP Operation eceiving and filtering route advertisements from directly attached neighbor(s). oute selection. To route to destination X, which path (of several advertised) will be taken? Sending route advertisements to neighbors. Spring 2003 UCSC cmpe150 21

22 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 Spring 2003 UCSC cmpe150 22

23 Differences between Intra- and Inter-AS routing Policy: Inter-AS: admin wants control over how its traffic routed, who routes through its net. Intra-AS: single administration, so no policy decisions needed (usually; changing with traffic engineering) Scale: Hierarchical routing saves table size, reduced update traffic Performance: Intra-AS: can focus on performance Inter-AS: policy dominates over performance Spring 2003 UCSC cmpe150 23

24 Supporting Mobility in outing Host moves and new connections are established Use DHCP to obtain a temporary new IP address Host moves and same IP address must be used Use mobile IP Hosts and routers move within the same domain (I.e., ad hoc wireless nets) Use ad-hoc routing protocol Spring 2003 UCSC cmpe150 24

25 outing in Ad Hoc Networks The problem addressed is host and router mobility. IETF MANET Working Group is defining which approach to evolve into a standard. Proposals: On-demand routing approaches: DS, AODV, TOA, and many others. Table-driven and hybrid approaches: STA and approaches based on topology broadcast. Spring 2003 UCSC cmpe150 25

26 On-Demand outing outers maintain routing-table entries for only those destinations that they need to reach. To reach a destination with an unknown route, source sends a flood search packet, just like in source routing bridges. Flood search packet reaches the destination one or multiple times. Destination sends a reply one or multiple times with the desired routing information. DS uses source routes in flood searches and replies; AODV uses destination sequence numbers. Spring 2003 UCSC cmpe150 26

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