CS 453 Computer Networks Lecture 24 IP Routing

See…

Routing In layer 1 and layer 2 we, logically and conceptually anyway, treated communications links as point-to-point… While that was not technically true is a viable was to organize networks because The layer hardware and protocols took care of the point-to-pointness for us

Routing Remember that the Internet is a network of networks… … of autonomous networks (or automous systems – we get back to this idea later)… With a conglomerations of networks – layer 3) we introduce … A number of new issues and complications

Routing Notably – paths and routes Recall one of the principles of the internet is that … Any node is reachable (in principle) from any other node

Routing Our packets and messages must be able to navigator through an immensely complete irregular mesh of nodes and links to get from A to B But A and B don’t know how to do this …so in reality the packets and message are navigated through the network… …by the network This is Routing

Routing Devices (routers) within the network are waypoints for messages… …and must make decisions about the “best” path to get the messages from A to B… … and, then, move the messages along that path

Routing Routing is the job of moving message across the network…..by the routers A B How to get from A to B?

Routing It can get very complex How many routes Which one to use A B

Routing Non-adaptive Routing (Static Routing) Routes from any source to any destination… Predefined Fixed Established when routers boot Does reflect router/link failures, loads, etc

Routing Adaptive Routing Routes are dynamic Routers “teach” other routers routes Routers swap routing information to reflect changes in available routes, down links, …many other things

Routing Shortest Path Routing Conceptualize a network as an undirected graph (easy to do!) Better yet, a weighted undirected graph Shortest Path algorithm finds the short path through this graph from A to B Shortest, that is distance can be defined in a number of ways Hops, physical distance, delay, etc. Such metrics are the weights Most common: Dijkstra’s Shortest Path Algorithm

Routing Flooding Algorithm Router sends message out on every port except the one that it came in on – concurrently So, the same message “flying” on every route but one Selective routing – flood routes that make sense …all routes to Europe, rather than all routes

Routing Flooding Algorithm Very inefficient Wastes a lot of bandwidth Resilient Good for high failure networks Military – combat deployments Catastrophe deployment

Routing Distance Vector Routing Algorithms Bellman-Ford algorithm Each router maintains a table of routes Initially each router table has entry or each neighbor and distance to that neighbor Periodically each router swaps with its neighbor all of the routes and distances to every location that it knows about

Routing Distance Vector Routing Algorithms Over time, due to this swapping, each router builds and maintains a table of routes and distances to all destinations Networks changes (link or router failures) are reflected in these route swaps “Distance” in this algorithm can be any defined network metric Hops, delay, cost,…

Routing Distance Vector Routing Algorithms From: Tanenbaum (2003) pg. 358

Routing Count-to-infinity problem ABCD Consider – each link = 1 hop B has message for A – distance = 1 A-B link goes down, B knows this and looks for another route to A B has route to C and C has route to A – distance 2 + 1(from B)… So update the routing table in B, C, D in this scenario

Routing Link State Algorithms Replaced distance vector routing on Internet in 1979 Each router – Floods the network to discover its neighbors Measures the cost/delay to discovered neighbors Floods network with information about every node it can reach (LSA) Constructs a topology map with this information Calculates shortest path to every router/node in network (Dijkstra’s Shortest Path algorithm

Routing Link State Algorithms Result Tree graph with each router as the root … … and the least cost path to every other node Build a routing table with best next hop for from current node to every other node in network

Routing Each router is responsible for maintaining its own route tables Although it may follow principles, there is no single router or control point responsible for selecting routes In a sense it is like politics All routing is local routing

Routing Hot-potato routing Carrier X Carrier Y 125 ms latency 100 ms latency AC BD 10 ms latency5 ms latency Router A needs to get a packet to Route D What is the best way to get it there? …but…

Routing Internal Gateway Routing Protocol

Routing Remember the idea of Autonomous Systems One of the key principles that makes the Internet work is that it is a network of Autonomous Systems (AS) Issues associated with routing within an AS (interior) are different from routing between AS (exterior)

Routing Intra-AS routing RIP – Routing Information Protocol One the earliest interior routing protocols Distance Vector algorithm Distance = hop count Hops = number of subnets to tranverse to get from source router to destination subnet Hops max limited to 15

Routing Intra-AS routing RIP – Routing Information Protocol DV algorithm – neighbors exchange distance vectors RIP – distance vector messages are called RIP advertisements In RIP – RIP advertisements occur every 30 seconds Collections of RIP information used for routing tables at each router

Routing Intra-AS routing OSPF – Open Shortest Path First protocol OSPF successor to RIP Link-State algorithm Avoids count-to-infinity problem

Routing Intra-AS routing OSPF – Open Shortest Path First protocol OSPF – floods AS with Link State information – Called Link State Advertisements (LSAs) Each router uses LSA to build a complete map (graph) of the AS Router then runs Dijkstra’s Shortest Path algorithm to find shortest to each destination

Routing Intra-AS routing OSPF – Open Shortest Path First protocol AS are organized in to areas Areas have boundary routers to route from one area to another (can have more than one inter area router)

Routing Border Gateway Protocol – BGP Exterior Gateway Routing Protocol Inter- AS routing DV algorithm, except rather than exchanging costs… BGP exchanges complete routes After all paths come in from neighboring routers… BGP router examines them and discards useless ones…

Routing Border Gateway Protocol – BGP OSPF does not consider politics BGP deals with politics …through routing policies

Routing Border Gateway Protocol – BGP …then scores each possible route from x to y Scoring function can be anything… Determined by network administrator

Routing Border Gateway Protocol – BGP See

Routing From: Tanenbaum (2003) pg. 458