CS 457 Networking and the Internet. What is Routing. Forwarding versus Routing 9/27/16. Fall 2016 Indrajit Ray. A famous quotation from RFC 791

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1 CS 457 Networking and the Internet Fall 2016 Indrajit Ray What is Routing A famous quotation from RFC 791 A name indicates what we seek An address indicates where it is A route indicates how we get there Jon Postel Forwarding versus Routing Forwarding: at the data plane Directing a packet to an outgoing link Individual router using a forwarding table Routing: at the control plane Computing paths the packet will follow Involves routers talking to each other Individual router creates a forwarding table 1

2 Routing vs. Forwarding Why Does Routing Matter End-to-end performance Quality of the path affects user performance Propagation delay, throughput, packet loss Use of network resources Balance of the traffic over routers and links Avoid congestion by directing traffic to lightly loaded links Transient disruptions during changes Failures, maintenance, load balancing Limiting packet loss and delay during changes Forwarding vs. Routing Forwarding table VS Routing table Forwarding table Used when a packet is being forwarded and so must contain enough information to accomplish the forwarding function A row in the forwarding table contains the mapping from a network number to an outgoing interface and some MAC information, such as Ethernet Address of the next hop Routing table Built by the routing algorithm as a precursor to build the forwarding table Generally contains mapping from network numbers to next hops 2

3 Forwarding Table vs. Routing Table Example rows from (a) routing and (b) forwarding tables Routing Network as a Graph The basic problem of routing is to find the lowest-cost path between any two nodes Where the cost of a path equals the sum of the costs of all the edges that make up the path Routing For a simple network, we can calculate all shortest paths and load them into some nonvolatile storage on each node. Such a static approach has several shortcomings It does not deal with node or link failures It does not consider the addition of new nodes or links It implies that edge costs cannot change What is the solution? Need a distributed and dynamic protocol Two main classes of protocols Distance Vector Link State 3

4 Routing Algorithm Classification Global or decentralized Global All routers have complete topology and link cost info link state algorithms Decentralized Routers knows about physically connected neighbors Iterative process of computation, exchange of info with neighbors distance vector algorithms Static or dynamic Static Manual configuration When routes very slowly over time after human intervention Dynamic When routes may change quickly Periodic update In response to link cost changes In response to link failures Distance Vector Each node constructs a one dimensional array (a vector) containing the distances (costs) to all other nodes and distributes that vector to its immediate neighbors Starting assumption is that each node knows the cost of the link to each of its directly connected neighbors Distance Vector Algorithm The Bellman-Ford Equation Define d x (y) := the cost of least-cost path from node x to node y Then d x (y) = min{c(x,v) + d v (y)} Where the min is taken over all v such that v is a neighbor of x 4

5 Bellman-Ford Example u v 2 x w y 1 Node that achieves minimum is next hop in shortest path routing table 5 2 z Note d v (z) = 5, d x (z) = 3, d w (z) = 3 B-F equation says: d u (z) = min { c(u,v) + d v (z), c(u,x) + d x (z), c(u,w) + d w (z) } = min {2 + 5, 1 + 3, 5 + 3} = 4 Distance Vector Algorithm D x (y) = estimate of least cost from x to y Distance vector: D x = [D x (y): y є N ] Node x knows cost to each neighbor v: c(x,v) Node x maintains D x = [D x (y): y є N ] Node x also maintains its neighbors distance vectors For each neighbor v, x maintains D v = [D v (y): y є N ] Example Distance Vector 5

6 Distance Vector Algorithm Basic idea: Each node periodically sends its own distance vector estimate to neighbors When node a node x receives new DV estimate from neighbor, it updates its own DV using B-F equation: D x (y) min v {c(x,v) + D v (y)} for each node y N Under minor, natural conditions, the estimate D x (y) converge to the actual least cost d x (y) Distance Vector Algorithm Iterative, asynchronous: each local iteration caused by: local link cost change DV update message from neighbor Distributed: each node notifies neighbors only when its DV changes neighbors then notify their neighbors if necessary Each node: wait for (change in local link cost of msg from neighbor) recompute estimates if DV to any dest has changed, notify neighbors 6

7 Example Initial Distances to Neighbors E Receives D s Routes 7

8 E Updates Cost to C A Receives B s Routes A Updates Cost to C 8

9 A Receives E s Routes A Updates Cost to C and D Final Distances 9

10 Final Distances After Link Failure View From a Node Distance Vector When a node detects a link failure F detects that link to G has failed F sets distance to G to infinity and sends update to A A sets distance to G to infinity since it uses F to reach G A receives periodic update from C with 2-hop path to G A sets distance to G to 3 and sends update to F F decides it can reach G in 4 hops via A 10

11 Distance Vector Example (Step 0) Distance Vector Example (step 2) Distance Vector Example (step 3) 11

12 The Bouncing Effect C Sends Route to B B Updates Distance to A 12

13 B Sends Route to C C Sends Route to B How Are These Loops Caused? Observation 1 B s cost metric increases Observation 2 C picks B as next hop to A But the implicit path from C to A includes itself 13

14 Solution 1: Holdowns If cost metric increases, delay propagating the information In our example, B delays advertisement C eventually think B s route is gone, picks its own route B then selects C as the next hop Adversely affects convergence Other Solutions Split horizon When a node sends a routing update to its neighbors, it does not send those routes it learned from each neighbor back to that neighbor B does not advertise route to C Poisoned reverse B advertises route to C with infinite distance Works for two node loops Does not work for loops with more nodes Example Where Split Horizon Fails 14

15 Avoiding the Bouncing Effect Select loop-free paths Have each route advertisement carry the entire path information If a router sees itself in a path, rejects the route BGP does it this way Space proportional to network diameter Distance Vector in Practice RIP and RIP2 Uses split-horizon with poison reverse BGP Propagates entire path Path also used for affecting policies Routing Information Protocol (RIP) Example Network running RIP RIPv2 Packet Format 15