ROUTING, PART 1. Today s Lecture. Announcements BASIC CONCEPTS AND TERMINOLOGY. Metrics For Routing Algorithms. Routing Protocols

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1 Today s Lecture I. Basic concepts and terminology ROUTING, PART Internet Protocols CSC / ECE 7 Fall, 00 N. C. State University II. III. Distance-Vector protocols Link-State protocols copyright 00 Douglas S. Reeves Announcements Exam will be returned on Tuesday HW online due Tuesday help! Project Part II online BASIC CONCEPTS AND TERMINOLOGY copyright 00 Douglas S. Reeves Routing Protocols Metrics For Routing Algorithms Goal: configure router forwarding tables so datagrams from anywhere to anywhere will be delivered by the best path what is best? what info does each router need? how does the routing algorithm work? how ensure routers make consistent decisions? Steps. routers discover their neighbors. routers exchange information about the network. routers compute forwarding tables copyright 00 Douglas S. Reeves. Find a route if one exists. Avoid routing loops. Support for enforcement of routing policy. Scale to large size networks. Adapt quickly to changes in network state. Recover quickly from temporary failures and mistakes 7. Minimize size of routing tables copyright 00 Douglas S. Reeves

2 What is the Best Route? Autonomous Systems (RFC 90) An autonomous system (AS) is an independent routing domain within which all routers agree on the same external routing policy under control of a single administrative entity each AS gets a bit ID number there are currently ~7,000 ASes AS es are arranged in a hierarchy; those at top have largest routing tables have most specific routes do not use a default route copyright 00 Douglas S. Reeves 7 copyright 00 Douglas S. Reeves 8 Internet Routing Architecture Regional registries (ARIN, RIPE, LACNIC, etc.) manage allocation of CIDR blocks manage allocation of AS numbers manage allocation of top-level domain names Interconnection of ISPs occurs by peering or traffic exchange agreement either they exchange routing tables, and carry traffic for each other, or one network serves as transit network for another Internet Routing Architecture (cont d) Facilities where ISPs interconnect and exchange traffic are Internet Exchanges slightly less than 00 in the world today A route server collects route information from all peers at an exchange, and redistributes to them The Internet Routing Registry is a distributed database where operators register their routing information allows consistency checking of routing policies copyright 00 Douglas S. Reeves 9 copyright 00 Douglas S. Reeves 0 Autonomous Systems Example The Internet AS Topology, Again Route Server Internet Exchange Default-free Route Server Internet Exchange Default-free Mid-level Company Dial-up Dial-up Mid-level Company copyright 00 Douglas S. Reeves copyright 00 Douglas S. Reeves

3 Distribution of AS Degree (00) 0000 Degree Histogram of ASes on AS-level Internet Topology (8 ASes, 9 Links / Skitter 8/00) Interior vs. Exterior Gateway Protocols Interior (intra-domain): within a single AS single administration, single policy normal objectives: efficiency / simplicity / stability 000 Exterior (inter-domain): between ASes Frequency Number of Peers of the Autonomous System many organizations, each with own policy objectives: configurability, scalability, robustness, security Exchange of info between inter-domain, intradomain routing occurs at border gateways copyright 00 Douglas S. Reeves copyright 00 Douglas S. Reeves Distance Vector (DV) Routing DISTANCE VECTOR PROTOCOLS Routers exchange information in the following form: <destination, min_cost to destination>, <destination, min_cost to destination>, <destination, min_cost to destination>,... Routers do not know the complete network topology Exchange info with directly-attached neighbors only multiple steps of exchange needed to propagate information across the network copyright 00 Douglas S. Reeves DV Route Computation Asynchronous, iterative, distributed computation For each destination, find cost of least cost path first hop (neighbor) router on least cost path Computation based on Bellman-Ford algorithm DV Route Computation (cont d) In words: Least cost path to destination j is the path starting from any neighbor k for which (cost of link to k path cost from k to j) is the least copyright 00 Douglas S. Reeves 7 copyright 00 Douglas S. Reeves 8

4 DV Routing Example DV Routing Example (cont'd) copyright 00 Douglas S. Reeves 9 copyright 00 Douglas S. Reeves 0 DV Routing Example (cont'd) The Spanning Tree from A copyright 00 Douglas S. Reeves copyright 00 Douglas S. Reeves Results of DV Computation DV Routing Example -- Link Failure Guaranteed non-looping LC path between every pair of nodes, after convergence But, while converging, or when changes occur, there may be temporary routing loops or inefficient paths Source of the problem: with distance vectors, you can t tell (locally) what path is being taken to a destination copyright 00 Douglas S. Reeves i.e., D s distance vector has not been updated (yet), so D s LC path to B goes through A, and A s LC path to B goes through D

5 Propagating Distance ormation N N N N N The Count to inity Problem link fails N N N N N R R R R R R R R R R Distance to network N Initially After exchange (DV s) Distance to network N Initially After exchanges After exchanges After exchanges Good news travels fast = availability of a new link or router copyright 00 Douglas S. Reeves Bad news travels slowly = failure of an existing link or router copyright 00 Douglas S. Reeves The Count to inity Problem link fails N N N N N R R R R R Distance to network N Initially After exchanges LINK-STATE PROTOCOLS After exchanges After exchanges After exchanges 7 7 After exchanges Bad news travels slowly = failure of an existing link or router copyright 00 Douglas S. Reeves 7 Link State (LS) Routing LS Database Example A link-state database is a map of the entire network topology, describing nodes and links Each router maintains an identical copy of this database routers broadcast link-state advertisements (LSAs) to update the database of other routers each router is responsible for originating an LSA describing its working interfaces copyright 00 Douglas S. Reeves 9 copyright 00 Douglas S. Reeves 0

6 Computing Routes from LSAs Example: LSAs Each router computes the least cost path from itself to all networks normally computed by Dijkstra's shortest-path algorithm no convergence issues, no count to infinity issues only issue: how long to propagate LSAs throughout entire network? With complete network topology information, there is more flexibility about how to optimize routes copyright 00 Douglas S. Reeves copyright 00 Douglas S. Reeves Comparison of LS and DV Algorithms Also Property Convergence Time Scalability Flexibility (metrics) Robustness Simplicity Amount of data exchanged DV? LS? Flooding and broadcasting Hierarchical routing copyright 00 Douglas S. Reeves copyright 00 Douglas S. Reeves Summary The Internet routing architecture consists of routers, Internet exchanges, routing databases The top level of the Internet organization is the Autonomous Systems level Exterior gateway protocols exchange routing information between ASes; interior protocols are used inside an AS Summary (cont d) The two classes of routing algorithms are distance-vector and link-state link-state is more powerful, converges faster, and scales better distance vector is simpler copyright 00 Douglas S. Reeves 8 copyright 00 Douglas S. Reeves 9

7 Next Lecture Routing, Part (RIP and OSPF) copyright 00 Douglas S. Reeves 0 7