06/02/ Local & Metropolitan Area Networks. Overview. Routing algorithm ACOE322. Lecture 6 Routing

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1 Local & Metropolitan rea Networks OE3 Lecture 6 Routing r. L. hristofi Overview The main function of the network layer is routing packets from the source to the destination machine. The only exception is for broadcasting networks In broadcasting routing a packet is sent simultaneously to all destinations Still routing is an issue if the source and destination are not on the same network r. L. hristofi Routing algorithm B source router R R3 destination router How to find the best path from to F? How does R chooses the best route to R4? The routing algorithm is that part of the network layer software responsible for deciding which output line an incoming packet should be transmitted on. r. L. hristofi 3 R R4 F E 06/0/006

2 Routing & forwarding Not the same thing! Routing means filling in and updating the routing tables Forwarding means handling the packets based on routing tables Routing differs in datagram and virtual-circuit networks r. L. hristofi 4 Routing - properties ertain properties are desirable in a routing algorithm:. correctness. simplicity 3. robustness updating possibility should cope with changes in the topology and traffic 4. stability must converge to equilibrium 5. fairness 6. optimality min mean packet delay max total network throughput 5 & 6 often contradictory r. L. hristofi 5 Routing algorithms YNMI (adaptive) change routing decisions to reflect changes in the topology adapt for changes in the traffic (load change) LGORITHMS: where routers get the information from? locally from adjacent routers from all routers LGORITHMS: when they change their routes? every T sec when the load changes when topology changes STTI (non-adaptive) routes computed in advance node failures, current load etc. not taken into account Note that both adaptive & non-adaptive algorithms can be either load-sensitive or load-insensitive r. L. hristofi 6 06/0/006

3 3 Global & decentralized routing algorithms. Global routing algorithm least-cost path calculated using global knowledge about network input: connectivity between all nodes & link costs nodes Link-state algorithms. ecentralized routing algorithm least-cost path calculated in an iterative, distributed manner no node has complete info about the cost of all network links begins with cost of directly attached links info exchange with neighbouring nodes istance-vector algorithms r. L. hristofi etermining the path Build a graph of the subnet: each router represented by a node node connected by a link (communication line) 5 B cost: number of hops, geographic distance in km, queuing delay, transmission delay, bandwidth, reliability, price least-cost path the minimum sum of the cost of the links shortest path crossing the smallest number of links r. L. hristofi E 5 F Static algorithms Shortest Path routing ijkstra s algorithm computes the least-cost path (route) from one node to all the other nodes Flooding omputes the shortest path (route) from one node to all the other nodes (inverse tree) r. L. hristofi 9 06/0/006 3

4 4 Shortest Path routing () Metrics (criteria for routing) istance path length = the number of hops Geographic distance in km Bandwidth elay verage traffic ommunication cost Mean queue length Measured delay By changing the weighting function, the algorithm can compute the shortest path measured according to any one of a number of criteria or to a combination of criteria r. L. hristofi 0 Shortest Path routing () Best known algorithm to compute the shortest path between two nodes is ijkstra (959) Each node is labeled with its distance from the source node along the best known path Initially, no paths are known, so all nodes are labeled with infinity s the algorithm proceeds and paths are found, the labels may change, reflecting better paths label may be either tentative or permanent Initially all labels are tentative When it is discovered that a label represents the shortest possible path from the source to that node, it is made permanent and never changed thereafter r. L. hristofi How labeling works () 6 B G E 4 F 3 The weights represent, for example, distance We want to find the shortest path from to We start by marking node as permanent (filled-in circle) Then we examine each of the nodes adjacent to, relabeling each one with the distance to H 3 r. L. hristofi 06/0/006 4

5 5 How labeling works () B (, ) E(,-) 6 G(6,) 4 (,-) 3 3 F(,-) (,-) H(,-) Whenever a node is relabeled we also label it with the node from which the probe was made so that we can reconstruct the final path later Then we examine all the tentatively labeled nodes and make the one with the smallest label permanent. This one becomes the new working node (B, in this case) We now start at B and examine all nodes adjacent to it. If the sum of the label on B and the distance from B to the node being considered is less than the label on that node, we have a shorter path, so the node is relabeled r. L. hristofi 3 How labeling works (3) B (, ) E(4,B) 6 G(6,) 4 (9,B) 3 3 F(,-) (,-) H(,-) fter all nodes adjacent to the working node (B) have been inspected and the tentative labels changed if possible, then a search is made to find the tentatively-labeled node with the smallest value This node is made permanent and becomes the new working node for the next round (node E) r. L. hristofi 4 How labeling works (4) 6 B (, ) 6 G(5,E) B (, ) G(5,E) B (, ) 6 E(4,B) 4 E(4,B) 4 E(4,B) F(6,E) (9,B) 3 3 H(,-) F(6,E) (9,B) 3 3 H(9,G) F(6,E) (9,B) 3 3 (,-) (,-) (,-) The process repeats until the shortest path is found, which is -B-E-F-H- r. L. hristofi 5 G(5,E) 4 H(8,F) 06/0/006 5

6 6 Flooding () B nother static algorithm Every incoming packet is sent out to every outgoing line except the one that the packet arrived on PROBLEM: F G E Ideally, the hop counter should be initialized to the length of the path r. L. hristofi 6 from source to destination H large number of duplicated packets consumes bandwidth SOLUTION: Have a hop counter in the header of each packet, which is decremented at each hop When counter reaches zero, the packet is discarded Flooding () Flooding always chooses the shortest path because it chooses every possible path in parallel. Flooding is not practical in most applications, but it has several important uses:. In military applications, where large numbers of routers may be blown at any instant, the tremendous robustness of flooding is highly desirable. In distributed database applications, it is sometimes necessary to update all the databases concurrently 3. In wireless networks all messages transmitted by a station can be received by all other stations within its radio range 4. metric against which other routing algorithms can be compared In selective flooding, a router sends packets out only on those lines in the general direction of the destination. That is, don't send packets out on lines that clearly lead in the wrong direction. r. L. hristofi ynamic algorithms istance-vector Routing used in the RPNET until 99 Link-State Routing used in the newer Internet Open Short Path First (OSPF) protocol r. L. hristofi 8 06/0/006 6

7 destination nodes The istance Vector Routing Operates by having each router maintain a table (vector) giving the best known distance to each destination and which line to use to get there dynamic algorithm takes current network load into account distributed each node receives information from its directly attached neighbours, performs a calculation, distribute the results back to neighbours the last one introduces overhead iterative algorithm performed in steps until no more information to change initially, each node knows only about its adjacent nodes asynchronous nodes do not operate in lockstep with each other r. L. hristofi 9 The istance Vector Routing distance tables from neighbors E () B 0 B 0 0 c( E, ) = c( E, B) = 8 c( E, ) = intermediate distance table B E s distance vector, 8,B 4, Note that this is not the final vector! B E 8, node E sends this distance vector to its neighbors re these paths X Z ( Y ) = min Z N ( X )( c( X, Z ) + ( Y )) shortest possible? r. L. hristofi 0 The count-to-infinity problem VR good news spread rapidly, bad news slowly Suppose all distance vectors sent at once Suppose that was down (link cost = ) and it just came up a metric is the number of hosts They still think that is down If node X tells Y that it has a path somewhere, Y has no way of knowing whether it itself is on the path. How can we avoid this problem? r. L. hristofi 06/0/006

8 destination nodes 8 void looping Split horizon Never send information about the routing for a particular packet in the direction from which it was received an be achieved by means of a technique called poison reverse. informing all routers that the path back to the originating node for a particular packet has an infinite metric Performance: Split horizon with poison reverse, is more effective in networks with multiple routing paths r. L. hristofi The Split horizon with poison reverse if a path to a dest node Y is through neighboring node X report to node X for destination node distance tables from neighbors E () B 0 B 0 0 intermediate distance table B c ( E, ) = ; c( E, B) = 8; c( E, ) = E s distance vector B E Note that this is not the final vector!, To : To B: 8,B 4, B 8 B, 4 4 E 0 E 0 8 To : B 8 E 0 r. L. hristofi 3 The distance vector routing Two problems. Link bandwidth not taken into account for metric, only the queue length all the lines at that time 56 Kbps. Too long time to converge QUESTION: when the algorithm converges? NSWER: when every node knows about all other nodes and networks and computes the shortest path to them will the nodes know the exact network topology by then? r. L. hristofi 4 06/0/006 8

9 9 ynamic algorithms istance Vector Routing Link State Routing r. L. hristofi 5 Link-state routing algorithm link state broadcast node learn about path costs from its neighbors inform the neighbors whenever the link cost changes hence the name link state r. L. hristofi 6 Link state routing Each router does the following (repeatedly): discover neighbors, particularly, learn their network addresses router learns about its neighbours by sending a special HELLO packet to each point-to-point line. Routers on the other end send a reply measure cost to each neighbor e.g. by exchanging a series of packets sending EHO packets and measuring the average roundtrip-time include traffic-induced delay? construct a link state packets send this packet to all other routers using what route information? chicken / egg what if re-ordered? or delayed? compute locally the shortest path to every other router when this information is received r. L. hristofi 06/0/006 9

10 0 onstructing link state packets sender subnet link state packets for this subnet When to build these packets? at regular time intervals on occurrence of some significant event link goes down (or comes back), cost change appreciably r. L. hristofi 8 istributing the link state packets Typically, flooding routers recognize packets passed earlier sequence number incremented for each new packet sent routers keep track of the (source router, sequence) pair thus avoiding the exponential packet explosion first receivers start changes already while changes are being reported sequence numbers wrap around or might be corrupted (a bit inversed instead of 4) 3 bit sequence number (3 years to wrap) To avoid corrupted sequences (or a router reboot) and therefore prevent any update, the state at each router has an age field that is decremented once a second but, need additional robustness in order to deal with errors on router-to-router lines acknowledgements r. L. hristofi 9 Routing in the Internet What would happen if hundreds of millions of routers execute the same routing algorithm to compute routing paths through the network? Scale large overhead enormous memory space in the routers no bandwidth left for data transmission would V algorithm converge? dministrative autonomy an organization should run and administer its networks as wishes but must be able to connect it to outside networks r. L. hristofi 30 06/0/006 0

11 Hierarchical routing The Internet uses hierarchical routing it is split into utonomous Systems (S) routers at the border: gateways gateways must run both intra & inter S routing protocols routers within S run the same routing algorithm the administrator can chose any Interior Gateway Protocol Routing Information Protocol (RIP) Open Shortest Path First (OSPF) between S gateways use Exterior Gateway Protocol Border Gateway Protocol (BGP) Why do we have different protocols for inter & intra S routing? r. L. hristofi 3 utonomous systems gateway H network router BGP B RIP & BGP OSPF H BGP gateways (R, R, R3, R4) use both interior & exterior routing other routers use only interior routing Note: S routing protocols in, B, & not need to be the same! r. L. hristofi 3 Routing within S The gateways are exit points routers use default routing each router knows all netid s within S packets destined to another S are sent to the default router default router is the border gateway to the next S r. L. hristofi 33 06/0/006

12 Routing Information Protocol Based on istance Vector Routing istance metric = hop count each link have cost = maximum cost path = 5 limited to S < 5 hops in diameter. each router shares its knowledge about the entire S it is unimportant how much it knows, it sends whatever it has. sharing only with neighbours 3. updates exchanged among neighbours every 30 sec RIP response message Send the distance to networks within S r. L. hristofi 34 RIP routing table estination Hop ount Next Router Other information Other information subnet mask the time a table was updated r. L. hristofi 35 RIP updating algorithm Receive: a response RIP message. dd one hop to the hop count for each advertised destination.. Repeat the following steps for each advertised destination: a. If (destination not in the routing table) I. dd the advertised information to the table. b. Else I. If (next-hop field is the same) i. Replace entry in the table with the advertised one. II. Else i. If (advertised hop count smaller than one in the table) - Replace entry in the routing table. 3. Return. r. L. hristofi 36 06/0/006

13 3 RIP updating the table r. L. hristofi 3 RIP an example destination hop next counter router initial routing tables r. L. hristofi 38 RIP an example (cnt d) destination hop next counter router final routing tables r. L. hristofi 39 06/0/006 3

14 4 Routing protocols r. L. hristofi 40 Open Shortest Path First (OSPF) Open - resources assumed to be freely usable Uses Link State algorithm Link state (LS) packet spreading Topology map at each node Route computation using ijkstra algorithm link costs set up by the administrator Separates policy from mechanism r. L. hristofi 4 OSPF advances to RIP Security: all messages between routers (for example link state updates) are authenticated Multiple same-cost path: allowed Multiple cost metric: for each link, multiple cost for each type of link (satellite connection, fiber, etc.) Support for hierarchy: S is divided into areas to handle routing efficiently r. L. hristofi 4 06/0/006 4

15 5 reas in S intra area routing involves only routers within the same area area border router routs the packet outside the area exactly area configured to be backbone area backbone routers run OSPF within backbone area S bound. router exchanges routing info with routers in other S s r. L. hristofi 43 Routing protocols Intra S routing Inter S routing r. L. hristofi 44 Inter S routing Border Gateway Protocol it is de facto standard interdomain routing protocol in today s Internet gateway H network router BGP B RIP & BGP OSPF H BGP r. L. hristofi 45 06/0/006 5

16 6 BGP Why are istance Vector Routing & Link State Routing not good candidates? route with the smallest hop count not the preferred one S not secure VR: only number of hops known to destination not path to get there LSR: Internet too big for this routing method huge databases long time to run ijsktra s algorithm r. L. hristofi 46 BGP- (cnt d) Path Vector Routing (V based) offers control to the administrator! Network Next Router Path N0 R0 S4, S3, S6 IRized destination network address N0 N03 R05 R06 S, S6, S05, S89 S6, S89, S09, S34 (8.9.40/4) N04 R S6, S0, S09 path: an ordered list of S that a packet should travel through to reach the destination Path information rather than cost information! S # s assigned by Internet orporation for ssigned Names and Numbers (INN) regional registries r. L. hristofi 4 BGP- path vector messages network next router path. router R sends a path vector advertising the detachability of N. router R receives the message, updates its table, replaces the router # with its own, adds its S # and sends a message to R3 3. r. L. hristofi 48 06/0/006 6

17 BGP activities. receiving & filtering route advertisement from directly attached neighbors Filtering: ignore advs that contain its own number in the S path (avoid looping). route selection distinguish between routing mechanism & routing policy 3. sending its route advertisement to neighbors only provides mechanism not policy r. L. hristofi 49 BGP an example S B S W X Y provider network (ISP) customer network W, X, Y source/destination off all traffic leaving/entering S How will X be prevented from forwarding traffic from B to? controlled routes advertisement X advertises to its neighbors B & that it has no paths to or Y even though he knows that path! B will not send packets for through X Should B advertise path W via B to or only to X? r. L. hristofi Traffic from should go directly via 50 Types of BGP packets Open: create a neighbor relationship a router running BGP opens a connection and sends an open message if a neighbour accepts the relationship its responds with a keep-alive Update: heart of BGP used to redraw destinations advertised previously Keep-alive: routers tell each other that they are active Notification: in case of error or when router wants to close the connection r. L. hristofi 5 06/0/006

18 8 Network ddress Translation (NT) Number of home users and small business that want to use the Internet ever increases always on-line (SL, cable, ) IPv4 address space limited Solution: NT large number of internal addresses and limited number of external addresses ddresses for private use (no permission required) Private address range to to to Total addresses r. L. hristofi 5 NT (cnt d) address translation r. L. hristofi 53 NT (cnt d) communication is always initiated by the private network only private-network host can access the same external host r. L. hristofi 54 06/0/006 8

19 9 NT (cnt d) Using pool of addresses (example: 4 external addresses instead of ) drawback: no more than 4 connections can be made to the same destination Using both IP addresses and port numbers Private ddress Private Port External ddress External Port Transport Protocol TP TP... r. L. hristofi 55 Exercises. How can flooding and broadcast be said to be similar to each other? How do they differ? Name one way in which they are similar/different.. Explain how looping can be avoided in distance-vector routing. 3. How does static routing differs from dynamic routing? Name two static and two dynamic algorithms used in routing packets. 4. Explain the operation of ijkstra s algorithm. 5. By means of appropriate diagrams explain how labeling in shortest path routing works. 6. Which problems are encountered with distance-vector routing?. Which actions does a router perform in link-state routing? 8. ontrast RIP, OSPF and BGP routing algorithms. 9. What is NT and why is it used? r. L. hristofi 56 References.S. Tanenbaum, omputer Networks, 4 th edition, Pearson Education International, 003 F. Halsall, ata ommunications, omputer Networks and Open Systems, 4 th edition, ddison Wesley, 995 r. L. hristofi 5 06/0/006 9