Advance Computer Networks Lecture#08 Instructor: Engr. Muhammad Mateen Yaqoob

MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE IP destination address in arriving packet’s header routing algorithm local forwarding table dest address output link address-range 1 address-range 2 address-range 3 address-range Interplay between routing, forwarding routing algorithm determines end-end-path through network forwarding table determines local forwarding at this router

MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE u y x wv z graph: G = (N,E) N = set of routers = { u, v, w, x, y, z } E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) } Graph abstraction aside: graph abstraction is useful in other network contexts, e.g., P2P, where N is set of peers and E is set of TCP connections

MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE Graph abstraction: costs u y x wv z c(x,x’) = cost of link (x,x’) e.g., c(w,z) = 5 cost could always be 1, or inversely related to bandwidth, or inversely related to congestion cost of path (x 1, x 2, x 3,…, x p ) = c(x 1,x 2 ) + c(x 2,x 3 ) + … + c(x p-1,x p ) key question: what is the least-cost path between u and z ? routing algorithm: algorithm that finds that least cost path

MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE Routing algorithm classification Q: global or decentralized information? global: all routers have complete topology, link cost info “link state” algorithms decentralized: router knows physically-connected neighbors, link costs to neighbors iterative process of computation, exchange of info with neighbors “distance vector” algorithms Q: static or dynamic? static:  routes change slowly over time dynamic:  routes change more quickly  periodic update  in response to link cost changes

Routing in Packet Switched Network key design issue for (packet) switched networks select route across network between end nodes characteristics required: ◦correctness ◦simplicity ◦robustness ◦stability ◦fairness ◦optimality ◦efficiency MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Performance Criteria used for selection of route simplest is “minimum hop” can be generalized as “least cost” MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Decision Time and Place time ◦packet or virtual circuit basis ◦fixed or dynamically changing place ◦distributed - made by each node ◦centralized ◦source MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Network Information Source and Update Timing routing decisions usually based on knowledge of network (not always) ◦distributed routing ◦using local knowledge, info from adjacent nodes, info from all nodes on a potential route ◦central routing ◦collect info from all nodes issue of update timing ◦when is network info held by nodes updated ◦fixed - never updated ◦adaptive - regular updates MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Routing Strategies - Fixed Routing use a single permanent route for each source to destination pair determined using a least cost algorithm route is fixed ◦at least until a change in network topology ◦hence cannot respond to traffic changes advantage is simplicity disadvantage is lack of flexibility MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Routing Strategies - Flooding packet sent by node to every neighbor eventually multiple copies arrive at destination no network info required each packet is uniquely numbered so duplicates can be discarded need some way to limit incessant retransmission ◦nodes can remember packets already forwarded to keep network load in bounds ◦or include a hop count in packets MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Properties of Flooding all possible routes are tried ◦very robust at least one packet will have taken minimum hop count route ◦can be used to set up virtual circuit all nodes are visited ◦useful to distribute information (eg. routing) disadvantage is high traffic load generated MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Routing Strategies - Random Routing simplicity of flooding with much less load node selects one outgoing path for retransmission of incoming packet selection can be random or round robin a refinement is to select outgoing path based on probability calculation no network info needed but a random route is typically neither least cost nor minimum hop MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Routing Strategies - Adaptive Routing used by almost all packet switching networks routing decisions change as conditions on the network change due to failure or congestion requires info about network disadvantages: ◦decisions more complex ◦tradeoff between quality of network info and overhead ◦reacting too slowly means info may be irrelevant MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Adaptive Routing - Advantages improved performance aid congestion control but since is a complex system, may not realize theoretical benefits ◦cf. outages on many packet-switched nets MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Classification of Adaptive Routing Startegies based on information sources ◦local (isolated) ◦route to outgoing link with shortest queue ◦can include bias for each destination ◦Rarely used - does not make use of available info ◦adjacent nodes ◦takes advantage on delay / outage info ◦distributed or centralized ◦all nodes ◦like adjacent MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm finds shortest paths from given source node s to all other nodes by developing paths in order of increasing path length algorithm runs in stages each time adding node with next shortest path algorithm terminates when all nodes processed by algorithm (in set T) MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm finds the shortest path from the start vertex to every other vertex in the network. We will find the shortest path from A to G A C D B F E G MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm 1.Label the start vertex with permanent label 0 and order label 1 2Assign temporary labels to all the vertices that can be reached directly from the start 3Select the vertex with the smallest temporary label and make its label permanent. Add the correct order label. 4Put temporary labels on each vertex that can be reached directly from the vertex you have just made permanent. The temporary label must be equal to the sum of the permanent label and the direct distance from it. If there is an existing temporary label at a vertex, it should be replaced only if the new sum is smaller. 5Select the vertex with the smallest temporary label and make its label permanent. Add the correct order label. 6Repeat until the finishing vertex has a permanent label. 7To find the shortest paths(s), trace back from the end vertex to the start vertex. Write the route forwards and state the length. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm Order in which vertices are labelled. Distance from A to vertex Working A C D B F E G Label vertex A 1 as it is the first vertex labelled MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm A C D B F E G We update each vertex adjacent to A with a ‘working value’ for its distance from A. 1 0 MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm A C D B F E G Vertex C is closest to A so we give it a permanent label 3. C is the 2 nd vertex to be permanently labelled. 1 0 MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm We update each vertex adjacent to C with a ‘working value’ for its total distance from A, by adding its distance from C to C’s permanent label of A C D B F E G < 7 so replace the t-label here MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm A C D B F E G The vertex with the smallest temporary label is B, so make this label permanent. B is the 3 rd vertex to be permanently labelled. 3 4 MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm A C D B F E G We update each vertex adjacent to B with a ‘working value’ for its total distance from A, by adding its distance from B to B’s permanent label of < 6 so replace the t-label here MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm A C D B F E G The vertex with the smallest temporary label is D, so make this label permanent. D is the 4 th vertex to be permanently labelled. 4 5 MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm A C D B F E G We update each vertex adjacent to D with a ‘working value’ for its total distance from A, by adding its distance from D to D’s permanent label of 5. 7 < 8 so replace the t-label here < 8 so replace the t-label here 7 MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm A C D B F E G The vertices with the smallest temporary labels are E and F, so choose one and make the label permanent. E is chosen - the 5 th vertex to be permanently labelled. 5 7 MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm A C D B F E G We update each vertex adjacent to E with a ‘working value’ for its total distance from A, by adding its distance from E to E’s permanent label of 7. 9 < 12 so replace the t-label here 9 MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm A C D B F E G The vertex with the smallest temporary label is F, so make this label permanent.F is the 6 th vertex to be permanently labelled MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm A C D B F E G We update each vertex adjacent to F with a ‘working value’ for its total distance from A, by adding its distance from F to F’s permanent label of > 9 so do not replace the t-label here MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm A C D B F E G G is the final vertex to be permanently labelled. 7 9 MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra’s Algorithm A C D B F E G To find the shortest path from A to G, start from G and work backwards, choosing arcs for which the difference between the permanent labels is equal to the arc length. The shortest path is ABDEG, with length 9. MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Dijkstra's ◦each node needs complete topology ◦must know link costs of all links in network ◦must exchange information with all other nodes MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Evaluation dependent on ◦processing time of algorithms ◦amount of information required from other nodes implementation specific under static topology and costs resulting to same solution as other algorithms if link costs change, algs attempt to catch up if link costs depend on traffic, which depends on routes chosen, may have feedback instability MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

Hierarchical routing scale: with 600 million destinations: can’t store all dest’s in routing tables! routing table exchange would swamp links! administrative autonomy  internet = network of networks  each network admin may want to control routing in its own network our routing study thus far - idealization  all routers identical  network “flat” … not true in practice

Hierarchical routing aggregate routers into regions, “autonomous systems” (AS) routers in same AS run same routing protocol ◦“intra-AS” routing protocol ◦routers in different AS can run different intra-AS routing protocol gateway router: at “edge” of its own AS has link to router in another AS MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE

3b 1d 3a 1c 2a AS3 AS1 AS2 1a 2c 2b 1b Intra-AS Routing algorithm Inter-AS Routing algorithm Forwarding table 3c Interconnected ASs  forwarding table configured by both intra- and inter-AS routing algorithm  intra-AS sets entries for internal dests  inter-AS & intra-AS sets entries for external dests

Inter-AS tasks  suppose router in AS1 receives datagram destined outside of AS1:  router should forward packet to gateway router, but which one? AS1 must: 1.learn which dests are reachable through AS2, which through AS3 2.propagate this reachability info to all routers in AS1 job of inter-AS routing! MATEEN YAQOOB DEPARTMENT OF COMPUTER SCIENCE AS3 AS2 3b 3c 3a AS1 1c 1a 1d 1b 2a 2c 2b other networks other networks