1. EEC-484 Computer Networks Lecture 10 Wenbing Zhao wenbingz@gmail.com (Part of the slides are based on Drs. Kurose & Ross ’ s slides for their Computer Networking book) 3/5/2016 EEC-484: Computer Networks 1

2. 3/5/2016 Outline Survey result Reminder  Time to start working on the project#1! Routing algorithms  Link state routing  Distance vector routing Internet protocol v4  Header  Fragmentation (omitted) Internet Protocol v6 (omitted) EEC-484: Computer Networks 2

3. 3/5/2016 Survey Result and Actions Main issues:  Homework and labs are not very useful for exams  Inadequate examples  Mismatch between examples and exam questions Adjustments  => Reformat the homework format  => Eliminate some non-essential topics to give room for more examples  => More questions towards students, perhaps calling students randomly to answer questions  => Reduce project from two to one. Second project as extra credit project EEC-484: Computer Networks 3

4. 3/5/2016 Link State Routing Basic idea Assumes net topology & link costs known to all nodes  Accomplished via “link state broadcast”  All nodes have same info Computes least cost paths from one node (‘source”) to all other nodes, using Dijkstra ’ s Algorithm  Gives forwarding table for that node EEC-484: Computer Networks 4

5. 3/5/2016 Dijkstra ’ s Algorithm Each node labeled with distance from source node along best known path Initially, no paths known so all nodes labeled with infinity As algorithm proceeds, labels may change reflecting shortest path Label may be tentative or permanent, initially, all tentative 5 5

6. 3/5/2016 EEC-484: Computer Networks Compute Shortest Path from A to D Start with node A as the initial working node Examine each of the nodes adjacent to A, i.e., B and G, relabeling them with the distance to A Examine all the tentatively labeled nodes in the whole graph and make the one with the smallest label permanent, i.e., B. B is the new working node 6

7. 3/5/2016 EEC-484: Computer Networks Compute Shortest Path from A to D 7

8. 3/5/2016 EEC-484: Computer Networks Step Permanently labeled BGECFHD 1 A 2,A6,A∞∞∞∞∞ 2 AB 6,A4,B9,B∞∞∞ 3 ABE 5,E9,B6,E∞∞ 4 ABEG 9,B6,E9,G∞ 5 ABEGF 9,B8,F∞ 6 ABEGFH 9,B10,H 7 ABEGFHC 10,H 8 ABEGFHCD 8

9. 3/5/2016 EEC-484: Computer Networks Computation Results BCDEFGHBCDEFGH (A,B) Destination link A B C D E F G H Routing Table in A 9

10. 3/5/2016 EEC-484: Computer Networks Distance Vector Routing Also called Bellman-Ford or Ford-Fulkerson Each router maintains a table, giving best known distance to each destination and which line to use to get there  Table is updated by exchanging info with neighbors  Table contains one entry for each router in network with Preferred outgoing line to that destination Estimate of time or distance to that destination  Once every T msec, router sends to each neighbor a list of estimated delays to each destination and receives same from those neighbors 10

11. 3/5/2016 EEC-484: Computer Networks Distance Vector Routing: How each entry is updated d(A,X) d(A,Y) A X Z d(Y,Z) d(X,Z) At router A, for Z Compute d(A,X) + d(X,Z) and d(A,Y) + d(Y,Z), take minimum Y d(A,Z) = min {d(A,v) + d(v,Z) } where min is taken over all neighbors v of A 11

12. 3/5/2016 EEC-484: Computer Networks x y z x y z 0 2 7 ∞∞∞ ∞∞∞ from cost to from x y z x y z 0 from cost to x y z x y z ∞∞ ∞∞∞ cost to x y z x y z ∞∞∞ 710 cost to ∞ 2 0 1 ∞ ∞ ∞ 2 0 1 7 1 0 time x z 1 2 7 y node x table node y table node z table d(x,y) = min{d(x,y) + d(y,y), d(x,z) + d(z,y)} = min{2+0, 7+1} = 2 d(x,z) = min{d(x,y) + d(y,z), d(x,z) + d(z,z)} = min{2+1, 7+0} = 3 32 Each node keeps track of the following info: 1.Its own distance vector: least-cost to each of other routers 2.Each of its neighbor’s distance vector received most recently If there is a change in distance vector, a node sends the update to all its neighbors 12

13. 3/5/2016 EEC-484: Computer Networks x y z x y z 0 2 7 ∞∞∞ ∞∞∞ from cost to from x y z x y z 0 2 3 from cost to x y z x y z 0 2 3 from cost to x y z x y z ∞∞ ∞∞∞ cost to x y z x y z 0 2 7 from cost to x y z x y z 0 2 3 from cost to x y z x y z 0 2 3 from cost to x y z x y z 0 2 7 from cost to x y z x y z ∞∞∞ 710 cost to ∞ 2 0 1 ∞ ∞ ∞ 2 0 1 7 1 0 2 0 1 7 1 0 2 0 1 3 1 0 2 0 1 3 1 0 2 0 1 3 1 0 2 0 1 3 1 0 time x z 1 2 7 y node x table node y table node z table d(x,y) = min{d(x,y) + d(y,y), d(x,z) + d(z,y)} = min{2+0, 7+1} = 2 d(x,z) = min{d(x,y) + d(y,z), d(x,z) + d(z,z)} = min{2+1, 7+0} = 3 13

14. 3/5/2016 EEC-484: Computer Networks Distance Vector Routing Distance from A to B 12ms, to C 25ms, to D 40ms, to G 18ms Distance from J to A 8ms, to I 10ms, to H 12ms, to K 6ms Distance from J to A to G 8+18 = 26ms to I to G 10+31 = 41ms to H to G 12+6=18ms to K to G 6+31=37ms 14

15. Homework#3 Objective 1: Able to compute the forwarding table using the link state routing method Important concepts/knowledge (please elaborate each)  Computation objective, and Information needed for the computation  Dijkstra’s Algorithm  Shortest-path tree Key points  Never replace a temporarily labeled route by another route of longer or the same distance! Problem: Given the subnet shown below, using the Dijkstra’s Algorithm, determine the shortest path tree from node u and its routing table. Please show all intermediate steps! Extra credit question (1% of the course): Write in your own words the bio of Dr. Edsger W. Dijkstra and his contributions to networking. Find 3 of his papers related to networking and summarize them (2-3 pages long) 3/5/2016 EEC-484: Computer Networks 15

16. Homework#3 Objective 2: Able to compute the forwarding table using the distance vector routing method Important concepts/knowledge (please elaborate each)  Computation objective  Information exchanged between neighboring nodes  Algorithm used to compute/update forwarding table Key points  Never compute/update the cost/outgoing link for the entry corresponding to the router itself! Problem: Consider the subnet shown below. Distance vector routing is used, and the following vectors have just come in to router C: from B: (5, 0, 8, 12, 6, 2); from D: (16, 12, 6, 0, 9, 10); and from E: (7, 6, 3, 9, 0, 4). The measured delays to B, D, and E, are 6, 3, and 5, respectively. What is C's new routing table? Give both the outgoing line to use and the expected delay. Please show all intermediate steps! 3/5/2016 EEC-484: Computer Networks 16