Communication Networks NETW 501 Lecture 11 Interconnecting Networks: Routers Course Instructor: Dr.-Ing. Maggie Mashaly maggie.ezzat@guc.edu.eg C3.220

Bridges vs. Routers Both store and forward devices Routers: Network layer devices Bridges: Datalink layer devices Routers maintain routing tables, implement routing algorithms Bridges maintain forwarding tables, implement filtering, learning and spanning tree algorithms BRIDGES do well in SMALL Networks (few hundred hosts) while ROUTERS used in LARGE Networks (thousands of hosts)

Bridges Characteristics Advantages: Bridge operation is simpler requiring less processing bandwidth Limitations: Homogeneous link layer (e.g., all Ethernet) is desirable for transparent bridging Topologies are restricted with bridges: a spanning tree must be built to avoid cycles As a result of the spanning tree algorithm the root bridge may represent a bottleneck Bridges do not offer protection from broadcast storms (endless broadcasting by a host will be forwarded by a bridge)

Routers Characteristics Goal: Scalable interconnection of a large numbers of networks of different types Advantages: Arbitrary topologies can be supported, cycling is limited by TTL (time-to-live counters) and good routing protocols. Provides firewall protection against broadcast storms Disadvantages: Require IP address configuration (not plug and play) Require higher processing bandwidth

Router Functions Forwarding Routing Queuing Buffer Management Move packet from input link to the appropriate output link Purely local computation Must go by very fast (executed for ever packet) Routing Keep track of network topology so you know where to forward packets Queuing Buffer (i.e., store) packets if router can’t forward it right now Buffer Management Drop packet if you run out of buffer space

Routing Algorithm Classification 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 Static or dynamic? Static routes change slowly over time Dynamic routes change more quickly periodic update in response to link cost changes

Routing Approaches Static Distance vector Link state Type in the right answers and hope they are always true Distance vector Tell your neighbors what you know about everyone Link state Tell everyone what you know about your neighbors

Bellman-Ford Distance Vector Routing Algorithms Assumption Each router knows own address & cost to reach neighbors Objective Calculate routing table containing next-hop information for every destination at each router Distributed Bellman-Ford algorithm Each router maintains a vector of costs to all destinations Initialize neighbors with known cost, others with infinity Periodically send copy of distance vector to neighbors On reception of a vector, If neighbor’s path to a destination is shorter, switch to it

Note: (a,b)=(next hop, total cost to destination) Bellman-Ford Algorithm: Shortest Path to Node 6 Note: (a,b)=(next hop, total cost to destination) Step (Iteration) Node 1 Node 2 Node 3 Node 4 Node 5 Initial (-1,∞) 1 (6,1) (6,2) 2 (3,3) (5,6) 3 (4,4) 4

Bellman-Ford Algorithm Iteration Node 1 Node 2 Node 3 Node 4 Node 5 Initial (-1, ) 1 2 3 Table entry @ node 1 for dest SJ Table entry @ node 3 for dest SJ 3 1 5 4 6 2 San Jose

Bellman-Ford Algorithm Iteration Node 1 Node 2 Node 3 Node 4 Node 5 Initial (-1, ) 1 (6,1) (6,2) 2 3 D3=D6+1 n3=6 D6=0 1 3 1 5 4 6 2 San Jose 2 D6=0 D5=D6+2 n5=6

Bellman-Ford Algorithm Iteration Node 1 Node 2 Node 3 Node 4 Node 5 Initial (-1, ) 1 (6, 1) (6,2) 2 (3,3) (5,6) 3 3 1 3 1 5 4 6 2 3 San Jose 6 2

Bellman-Ford Algorithm Iteration Node 1 Node 2 Node 3 Node 4 Node 5 Initial (-1, ) 1 (6, 1) (6,2) 2 (3,3) (5,6) 3 (4,4) 1 3 3 1 5 4 6 2 3 San Jose 6 4 2

Network disconnected; Loop created between nodes 3 and 4 Bellman-Ford Algorithm Iteration Node 1 Node 2 Node 3 Node 4 Node 5 Initial (3,3) (4,4) (6, 1) (6,2) 1 (4, 5) 2 3 1 5 3 3 1 5 4 6 2 3 San Jose 4 2 Network disconnected; Loop created between nodes 3 and 4

Node 4 could have chosen 2 as next node because of tie Bellman-Ford Algorithm Iteration Node 1 Node 2 Node 3 Node 4 Node 5 Initial (3,3) (4,4) (6, 1) (6,2) 1 (4, 5) 2 (3,7) (5,5) 3 5 7 3 3 1 5 4 6 2 5 3 San Jose 2 4 Node 4 could have chosen 2 as next node because of tie

Bellman-Ford Algorithm Iteration Node 1 Node 2 Node 3 Node 4 Node 5 Initial (3,3) (4,4) (6, 1) (6,2) 1 (4, 5) 2 (3,7) (5,5) 3 (4,6) (4, 7) 5 7 7 3 1 5 4 6 2 5 San Jose 2 4 6 Node 2 could have chosen 5 as next node because of tie

Bellman-Ford Algorithm Iteration Node 1 Node 2 Node 3 Node 4 Node 5 1 (3,3) (4,4) (4, 5) (6,2) 2 (3,7) (2,5) 3 (4,6) (4, 7) (5,5) 4 (2,9) 7 7 9 3 5 4 6 2 1 5 San Jose 6 2 Node 1 could have chose 3 as next node because of tie

Counting to Infinity Problem 3 1 2 4 X (a) (b) Nodes believe best path is through each other (Destination is node 4) Update Node 1 Node 2 Node 3 Before break (2,3) (3,2) (4, 1) After break 1 (3,4) 2 (2,5) 3 (3,6) 4 (2,7) 5 (3,8) …

Problem: Bad News Travel Slowly Solutions: Split Horizon Do not report route to a destination to the neighbor from which route was learned Poisoned Reverse Report route to a destination to the neighbor from which route was learned, but with infinite distance Breaks erroneous direct loops immediately Does not work on some indirect loops

Nodes believe best path is through each other Split Horizon with Poison Reverse 3 1 2 4 X (a) (b) Nodes believe best path is through each other Update Node 1 Node 2 Node 3 Before break (2, 3) (3, 2) (4, 1) After break (-1, ) Node 2 advertizes its route to 4 to node 3 as having distance infinity; node 3 finds there is no route to 4 1 Node 1 advertizes its route to 4 to node 2 as having distance infinity; node 2 finds there is no route to 4 2 Node 1 finds there is no route to 4

Dijkstra Algorithm 3 1 5 4 6 2 Iteration N Node 2 Node 3 Node 4 Node 5 Initial {1} 3 2 5 ∞ 1 {1,3} 4 {1,2,3} 7 {1,2,3,6} {1,2,3,4,6} {1,2,3,4,5,6}

References NETW 501 Lectures slides by Assoc. Prof. Tallal El-Shabrawy “Communication Networks 2nd Edition”, A. Leon-Garcia and I. Widjaja, McGraw Hill, 2013 “Computer Networks 4th Edition”, A. S. Tanenbaum, Pearson International