Announcements. CS 5565 Network Architecture and Protocols. Count-To-Infinity. Poisoned Reverse. Distance Vector: Link Cost Changes.

Transcription

1 Announcements CS 6 Network Architecture and Protocols Lecture 20 Project 2B Part/ due Wed Apr 27 :9pm Part/2 due Wed Ma :9pm Current reading assignment: Chapter.6.7, Chapter Final Ma 0, 3:2pm, MCB 26 Godmar Back 2 CountToInfinit Before change: 6 to via 7 to via 8 to via Distance Vector: Link Cost Changes Link cost changes: good news travels fast bad news travels slow count to infinit problem! iterations before algorithm stabilies Poisoned reverse: If Z routes through Y to get to X : Z tells Y its (Z s) distance to X is infinite (so Y won t route to X via Z) will this completel solve count to infinit problem? 3 Poisoned Reverse After change: 6 to via Don t tell dist to to via Comparison of LS and DV algorithms Message compleit LS: with n nodes, E links, O(nE) msgs sent DV: echange between neighbors onl convergence time varies Speed of Convergence LS: O(n 2 ) algorithm requires O(nE) msgs ma have oscillations DV: convergence time varies ma be routing loops counttoinfinit problem Robustness: what happens if router malfunctions? LS: node can advertise incorrect link cost each node computes onl its own table DV: DV node can advertise incorrect path cost each node s table used b others error propagate thru network 6

2 Hierarchical Routing Hierarchical Routing Our routing stud thus far idealiation all routers identical network flat not true in practice scale: with 200 million destinations: can t store all dest s in routing tables! routing table echange would swamp links! administrative autonom internet = network of networks each network admin ma want to control routing in its own network aggregate routers into regions, autonomous sstems (AS) routers in same AS run same routing protocol intraas routing protocol routers in different AS ma run different intra AS routing protocols Gatewa router Direct link to router in another AS Eample: [2] 7 8 a Interconnected ASes c d b IntraAS Routing algorithm AS Forwarding table InterAS Routing algorithm Forwarding table is configured b both intra and interas routing algorithm IntraAS sets entries for internal dests InterAS & IntraAS sets entries for eternal dests 9 Suppose router in AS AS the gatewa routers, but a InterAS tasks c d b AS needs:. to learn which dests are reachable through and which through 2. to propagate this reachabilit info to all routers in AS Job of interas routing! AS 0 a InterAS tasks (cont d) Suppose router in AS AS the gatewa routers, but route to c d Scenario Destination is reachable through single AS InterAS must propagate this Least cost path to information inside AS c is interface l, add (, l) to routing table does not advertise route to Spread in AS: b AS use gatewa c to get to Suppose router in AS AS the gatewa routers, but a Hot Potato Routing c d b Scenario 2: Destination is reachable through multiple AS: Hot potato routing: send packet towards closest of two routers AS 2 2

3 Suppose router in AS AS the gatewa routers, but a Hot Potato Routing route to c d b Scenario 2: Destination is reachable through multiple AS: Hot potato routing: send packet b is closer than c, towards closest of two interface k leads to routers b, so add (, k) to routing table AS Spread in AS: c and b lead to route to 3 Summar Routing algorithms: Link State Distance Vector Hierarchical Routing AS: autonomous sstems Net: application to Internet/IP The Internet Network Laer Host, router network laer functions: Network laer Routing protocols path selection RIP, OSPF, BGP Transport laer: TCP, UDP forwarding table Link laer Phsical laer IP protocol addressing conventions datagram format packet handling conventions ICMP protocol error reporting router signaling IP protocol version number header (btes) tpe of data ma number remaining hops (decremented at each router) upper laer protocol to deliver paload to IP Datagram Format 32 bits ver head. tpe of len service 6bit identifier flgs fragment time to upper Internet live laer checksum 32 bit source IP address 32 bit destination IP address Options (if an) data (variable, tpicall a TCP or UDP segment) total datagram (btes) for fragmentation/ reassembl E.g. timestamp, record route taken, specif list of routers to visit. 6 IP Fragmentation & Reassembl network links have MTU (ma.transfer sie) largest possible linklevel frame. different link tpes, different MTUs large IP datagram divided ( fragmented ) within net one datagram becomes several datagrams reassembled onl at final destination IP header bits used to identif, order related fragments reassembl fragmentation: in: one large datagram out: 3 smaller datagrams 7 IP Fragmentation and Reassembl Eample 000 bte datagram MTU = 0 btes 80 btes in data field = 80/8 00 = = One large datagram becomes several smaller datagrams 0 = = = = =8 =

4 IP Addressing: Introduction IP address: 3it identifier for host, router interface interface: connection between host/router and phsical link router s tpicall have multiple interfaces host ma have multiple interfaces IP addresses associated with each interface = IP address: subnet part (high order bits) host part (low order bits) What s a subnet? device interfaces with same subnet part of IP address can phsicall reach each other without intervening router LAN network consisting of 3 subnets 20 Recipe To determine the subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet / / /2 Subnet mask: / How man? IP Addressing: CR CR: Classless InterDomain Routing subnet portion of address of arbitrar address format: a.b.c.d/, where is # bits in subnet portion of address Classfull Routing subnet host part part /23 23 A, B, C: Prett much onl of historical interest toda 2

5 Special IP Addresses Summar IP Addressing, fragmentation, subnets 2 26