CS 640: Introduction to Computer Networks

Transcription

1 S : Introduction to omputer Networks ditya kella Lecture - Intra-omain Routing rom previous lecture. Three Types of Switching abrics Switching Via a Memory irst generation routers looked like Ps Packet copied by system s (single) PU Speed limited by memory bandwidth ( bus crossings per datagram) Input Port Memory Output Port System us Most modern routers switch via memory, but Input port processor performs lookup, copy into memory isco atalyst 8

2 Switching Via a us atagram from input port memory to output port memory via a shared bus us contention: switching speed limited by bus bandwidth Gbps bus, isco 9: sufficient speed for access and enterprise routers (not regional or backbone) Switching Via an Interconnection Network Overcome bus and memory bandwidth limitations rossbar provides full NxN interconnect xpensive Uses N buses isco : switches Gbps through the interconnection network Routing Past two lectures IP addresses are structured IP packet headers carry these addresses When packet arrives at router xamine header for intended destination Look up next hop in table Send packet out appropriate port Router This lecture: How these forwarding tables are built? Routing algorithms

3 Model of the Problem Network as a Graph: Represent each router as node irect link between routers represented by edge Symmetric links undirected graph dge cost c(x,y) denotes measure of difficulty of using link delay, $ cost, or congestion level Task etermine least cost path from every node to every other node Path cost d(x,y) = sum of link costs Ways to ompute Shortest Paths entralized ollect graph structure in one place Use standard graph algorithm isseminate routing tables istributed Routers perform local computation onverge to a globally consistent routing state Global : Link-state very node collects complete graph structure ach computes shortest paths from it ach generates own routing table Local: istance-vector No one has copy of graph s construct their own tables iteratively ach sends information about its table to neighbors 8 istance-vector Method Initial Table for est ost Nex t Idea t any time, have cost/next hop of best known path to destination Use cost when no path known Initially Only have entries for directly 9 connected nodes

4 lgorithm ach node x stores: c(x,v) for each neighbor v istance vector of node x: estimate of d(x,y) for all y istance vectors heard from each neighbor Initialization:. d(x,y) = c(x,y) for all y.. Send distance vector to each neighbor Repeat: Whenever link cost to neighbor changes or distance vector received from neighbor or every neighbor z or every destination y d(x,y) Update(x,y,z) If d(x,y) changed for any y, send distance vector to all neighbors istance-vector Update c(x,z) z d(z,y) x d(x,y) y Update(x,y,z) d c(x,z) + d(z,y) /* ost of path from x to y with first hop z */ if d < d(x,y) /* ound better path */ return d,z /* Updated cost / next hop */ else return d(x,y), nexthop(x,y) /* xisting cost / next hop */ Start Optimum -hop paths Table for st st Table for st st Table for Table for Table for Table for st st st st st st st st

5 Optimum -hop paths Iteration # Table for st st Table for st st Table for Table for Table for Table for st st st st st st st st Optimum -hop paths Iteration # Table for st st Table for st st Table for Table for Table for Table for st st st st st st st st istance Vector: Link ost hanges Link cost changes: ad news travels slow - count to infinity problem! X Y Z algorithm continues on!

6 istance Vector: Poison 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 completely solve count to infinity problem? X Y Z algorithm terminates Poison Reverse ailures st Table for st st st st Table for st Table for st st Table for st 8 orced Update Table for orced Update st st Table for st 9 etter Route orced Update st Table for st 9 orced Update Table for orced st st Update st st Table for st Table for st Iterations don t converge ount to infinity Solution Make infinity smaller What is upper bound on maximum path length? Routing Information Protocol (RIP) arliest IP routing protocol (98 S) urrent standard is version (R ) eatures very link has cost count Infinity = Limits to networks where everything reachable within hops Sending Updates very router listens for updates on UP port Triggered When every entry changes, send copy of entry to neighbors xcept for one causing update (split horizon rule) Periodic very seconds, router sends copy of its table to each neighbor 8

7 Link State Protocol oncept very node gets complete copy of graph very node floods network with data about its outgoing links very node computes routes to every other node Using single-source, shortest-path algorithm Process performed whenever needed When interconnections die / reappear 9 Sending Link States by looding X wants to send information Sends on all outgoing links When node Y receives information from Z Resend on all links other than Z X (a) X (c) X (b) X (d) ijkstra s lgorithm Given Graph with source node s and edge costs c(u,v) etermine least cost path from s to every node v Single source shortest Path lgorithm Traverse graph in order of least cost from source

8 ijkstra s lgorithm Horizon Sets one lready have least cost path to it Horizon: Reachable in hop from node in one Unseen: annot reach directly from node in one one Label d(v) = path cost rom s to v Path Unseen urrent Path osts Keep track of last link in path ijkstra s lgorithm: Initially Horizon one urrent Path osts Unseen No nodes done in horizon ijkstra s lgorithm: Initially one Horizon urrent Path osts Unseen d(v) to node shown in red Only consider links from done nodes 8

9 ijkstra s lgorithm one Horizon urrent Path osts Unseen Select node v in horizon with minimum d(v) dd link used to add node to shortest path tree Update d(v) information ijkstra s lgorithm one Horizon urrent Path osts Unseen Repeat ijkstra s lgorithm one Horizon Unseen urrent Path osts ddition of node can add new nodes to horizon 9

10 ijkstra s lgorithm inal tree shown in green 8 Link State haracteristics With consistent LSs*, all nodes compute consistent loop-free paths X an still have transient loops *Link State ata ase Packet from may loop around if knows about failure and & do not 9 OSP Routing Protocol Open Open standard created by IT More prevalent than RIP

11 OSP Messages Transmit link state advertisements Originating router Typically, IP address for router Link I I of router at other end of link Metric ost of link Sequence number Incremented each time sending new link information OSP looding Operation X Receives LS from Y With Sequence Number q Looks for entry with same origin/link I ases No entry present dd entry, propagate to all neighbors other than Y ntry present with sequence number p < q Update entry, propagate to all neighbors other than Y ntry present with sequence number p > q Send entry back to Y To tell Y that it has out-of-date information ntry present with sequence number p = q Ignore it looding Issues When should it be performed Periodically When status of link changes etected by connected node ongestion, lack of electric or optical signal What happens when router goes down & back up Sequence number reset to Other routers may have entries with higher sequence numbers Router will send out LSs with number Will get back LSs with last valid sequence number p Router sets sequence number to p+ & resends

12 doption of OSP RIP viewed as outmoded Good when networks small and routers had limited memory & computational power OSP dvantages ast convergence when configuration changes ull topology map helps omparison of LS and V lgorithms Message complexity LS: with n nodes, v neighbors, O(nv) messages per node V: exchange between neighbors only Speed of onvergence LS: omplex computation ut can forward before computation may have oscillations V: convergence time varies may be routing loops count-to-infinity problem (faster with triggered updates) omparison of LS and V lgorithms Robustness: what happens if router malfunctions? LS: node can advertise incorrect link cost each node computes only its own table V: V node can advertise incorrect path cost each node s table used by others errors propagate thru network Other tradeoffs Making LSP flood reliable difficult Prioritize routing packets?