Basic Concepts and Terminology - Lecture Slides | CSC 573, Study notes of Computer Science

Download Basic Concepts and Terminology - Lecture Slides | CSC 573 and more Study notes Computer Science in PDF only on Docsity! 1 ROUTING, PART 1 Internet Protocols CSC / ECE 573 Fall, 2005 N. C. State University copyright 2005 Douglas S. Reeves 2 Today’s Lecture I. Basic concepts and terminology II. Distance-Vector protocols III. Link-State protocols copyright 2005 Douglas S. Reeves 3 Announcements Exam will be returned on Tuesday HW4 online – due Tuesday help! Project Part II online BASIC CONCEPTS AND TERMINOLOGY copyright 2005 Douglas S. Reeves 5 Routing Protocols • 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 1. routers discover their neighbors 2. routers exchange information about the network 3. routers compute forwarding tables copyright 2005 Douglas S. Reeves 6 Metrics For Routing Algorithms 1. Find a route if one exists 2. Avoid routing loops 3. Support for enforcement of routing “policy” 4. Scale to large size networks 5. Adapt quickly to changes in network state 6. Recover quickly from temporary failures and mistakes 7. Minimize size of routing tables 2 copyright 2005 Douglas S. Reeves 7 What is the “Best” Route? copyright 2005 Douglas S. Reeves 8 Autonomous Systems (RFC 1930) • 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 16 bit ID number – there are currently ~17,000 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 2005 Douglas S. Reeves 9 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 copyright 2005 Douglas S. Reeves 10 Internet Routing Architecture (cont’d) • Facilities where ISPs interconnect and exchange traffic are Internet Exchanges – slightly less than 100 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 2005 Douglas S. Reeves 11 Autonomous Systems Example Default-free Provider Default-free Provider Mid-level Provider Mid-level Provider Dial-up Provider Dial-up Provider Company Company Route Server Route Server Internet Exchange Internet Exchange copyright 2005 Douglas S. Reeves 12 The Internet AS Topology, Again 5 copyright 2005 Douglas S. Reeves 25 Propagating Distance Information Distance to network N1  N1 N2 N3 N4 N5 R1 R2 R3 R4 R5 InitiallyInfInfInfInf After 1 exchange (DV’s)InfInfInf1 After 2 exchangesInfInf21 After 3 exchangesInf321 After 4 exchanges4321 • “Good news travels fast” = availability of a new link or router copyright 2005 Douglas S. Reeves 26 The “Count to Infinity” Problem Distance to network N1  N1 N2 N3 N4 N5 R1 R2 R3 R4 R5 • “Bad news travels slowly” = failure of an existing link or router Initially4321 link fails copyright 2005 Douglas S. Reeves 27 The “Count to Infinity” Problem Distance to network N1  N1 N2 N3 N4 N5 R1 R2 R3 R4 R5 After 1 exchanges4323 After 2 exchanges4343 After 3 exchanges4545 • “Bad news travels slowly” = failure of an existing link or router Initially4321 After 4 exchanges6565 After 5 exchanges6767 link fails LINK-STATE PROTOCOLS copyright 2005 Douglas S. Reeves 29 “Link State” (LS) Routing • 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 2005 Douglas S. Reeves 30 LS Database Example 6 copyright 2005 Douglas S. Reeves 31 Computing Routes from 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 2005 Douglas S. Reeves 32 Example: LSAs copyright 2005 Douglas S. Reeves 33 Comparison of LS and DV Algorithms –+Amount of data exchanged –+Simplicity ??Robustness +–Flexibility (metrics) +–Scalability +–Convergence Time LSDVProperty copyright 2005 Douglas S. Reeves 34 Also… • Flooding and broadcasting • Hierarchical routing copyright 2005 Douglas S. Reeves 38 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 copyright 2005 Douglas S. Reeves 39 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 7 copyright 2005 Douglas S. Reeves 40 Next Lecture • Routing, Part 2 (RIP and OSPF)