University of Delaware CPEG 4191 zHomework due? zNext lecture is in 311 Pearson Hall (Studio A). Give me or the TA a sheet of paper with your name (attendance)

University of Delaware CPEG 4192 Connection vs. connectionless issuePacket switchingVirtual-circuit SetupNot neededneeded addressingEach packet has full addressJust VC number (label) State informationNothing about the flow is needed Each VC requires a some information RoutingEach packet routing independently Route is chosen at setup. Effect of router failurePerhaps little, a few packets lost until a new route is found. All VC that pass through router have to be re-setup Quality of ServiceDifficultEasy to implement, but hard to plan efficiently. Congestion controlDifficultEasy to implement, but difficult to plan efficiently.

University of Delaware CPEG 4193 zWe mostly look at connectionless routing. zBut connection-oriented is becoming more and more important (maybe).

University of Delaware CPEG 4194 Overview of Routing in the Global Internet Today zThe problem of scalability yEach router must know where to send a packet next. IP addressNext hop (interface #) The forwarding table are huge!!!

University of Delaware CPEG 4195 Subnetting Divide and conquer XX.XXX X.XXX XXX XXX.XXX A B XXX.XXX IP addressmask# ? IP addressmask# ? A B Must carefully distribute addresses AS (e.g. ATT) MCI GE

University of Delaware CPEG 4196 Hierarchical Routing zMust distribute address carefully zThen there are two problems yHow to route between subnets (interdomain routing) xBGP yHow to route in a subnet (intradomain routing) xDistance Vector (RIP) xLink State (OSPF)

University of Delaware CPEG 4197 Routing zBased on knowledge of network topology, choose appropriate paths from source to destination. zGoals yCorrectness ySimplicity (funny, aren’t we smart enough to make complicated things?) yRobustness – when a router or link goes down, the network must remain up. yStability – some routing algorithms never converge or take a very long time to converge after a small change. A small change in one part of the network should not lead to a large change. Currently, BGP does not meet this criteria. yFairness yOptimality

University of Delaware CPEG 4198 Fairness and optimality Total flow is maximized by just sending from 1->2, 3->4 and 5->6, but not so fair to 7->8

University of Delaware CPEG 4199 Adaptive and Non-adaptive Routing zNon-adaptive routing: yFixed routing, static routing. yDo not take current state of the network (e.g., load, topology). yRoutes are computed in advance, off-line, and downloaded to routers when booted. zAdaptive routing: yRoutes change dynamically as function of current state of network. yAlgorithms vary on how they get routing information, metrics used, and when they change routes.

University of Delaware CPEG Hot potato routing zJust send the packet somewhere. IP addressmask# ?

University of Delaware CPEG Flooding zEvery packet is sent to every neighboring router except the one it came on. yIn order to stop the packet from traveling forever, a counter of the number of hops is decremented at every hop. Once the counter hits zero, the packet is dropped. yUse sequence numbers so that if a router sees a packet twice, it will drop it. But then each router need to keep a list of packets it has seen so far. yEach packet could contain a list to routers visited so far. Then… zSelective flooding yFlood to those neighbors that are in the general right direction. zAs we shall see, flooding is used (but not for data). It was used in link layer routing.

University of Delaware CPEG Shortest Path Routing zShortest with respect to some metric yExample: xNumber of hops. xDelay. xBandwidth. xCost.

University of Delaware CPEG Bellman Optimality Principle zGeneral statement about optimal routes (topology, routing algorithm independent). zIf router J is on optimal path between I and K, then the optimal path from J to K also falls along the same route. yProof by contradiction. zCorollary: ySet of optimal routes from all sources to destination form a tree rooted at destination. ySink tree.

University of Delaware CPEG Distance Vector Routing – Dynamic Programming zLimited state information. Just the next hop and cost. A B D G F C H E addressNext hop cost AA0 BB1 CC1 DD1 EE2 FD2 GB2 HB3 A addressNext hop cost AA1 BB1 CA2 DD0 EB2 FF1 GB2 HB3 D

University of Delaware CPEG Distance Vector Routing – Dynamic Programming zSuppose a new node comes on line. A B D G F C H E I addressNext hop Cos t A?  B?  C?  D?  E?  F?  G?  H?  II0 I

University of Delaware CPEG Distance Vector Routing – Dynamic Programming zSuppose a new node comes on line. zSuppose I first talks to A. A B D G F C H E I addressNext hop cost AA0 BB1 CC1 DD1 EE2 FD2 GB2 HB3 A addressNext hop cost AA1 BA2 CA2 DA2 EA3 FA3 GA2 HA2 II0 I

University of Delaware CPEG Distance Vector Routing – Dynamic Programming zSuppose a new node comes on line. zSuppose I first talks to A. zNext I talks to D. A B D G F C H E I addressNext hop cost AA1 BA2 CA2 DD1 EA3 FD2 GA2 HA2 II0 I addressNext hop cost AA1 BB1 CA2 DD0 EB2 FF1 GB2 HB3 D

University of Delaware CPEG Distance Vector - Algorithm zStart with all destinations with infinite distance, except for the actual node, which is distance 0. zEvery 30 seconds (RIP), or when a change occurs in the table, send table to neighbors. zIf the distance to a prefix advertised by a neighbor is less plus the distance to the neighbor is less than known distance, reduce distance to prefix and route packets with that destination prefix to that neighbor.

University of Delaware CPEG Count to Infinity Problem ABCDE  initial 1  1 iteration 12  2 iterations 123  3 iterations iterations ABCDE 1234initial iteration iterations iterations iterations

University of Delaware CPEG Approaches to Mitigate Count Infinity Why is count to infinity a problem? It generates tons of routing updates – too much traffic The network should report that a route is unreachable. Put upper bound an upper bound the the diameter of the network. But what is the network grows (as it did). Split horizon. A router does not report a distance to the neighbor it learned the distance from. Split horizon with poison reverse. If A advertises the best cost to E to B, then B advertises a cost of infinity to E back to A. This only works for loops that involve two nodes. With larger loops, the mitigation is more difficult and these remedies reduce the rate of convergence. The way to fix it is to use link state routing.