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Part 2.6

Part 2.6. Internetwork Routing (Static and automatic routing; route propagation; BGP, RIP, OSPF; multicast routing). Robert L. Probert & Os Monkewich. Terminology. Forwarding Refers to datagram transfer Performed by host or router Uses routing table Routing

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Part 2.6

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  1. Part 2.6 Internetwork Routing (Static and automatic routing; route propagation; BGP, RIP, OSPF; multicast routing) Robert L. Probert & Os Monkewich CSI 4118 R.L.Probert

  2. Terminology • Forwarding • Refers to datagram transfer • Performed by host or router • Uses routing table • Routing • Refers to propagation of routing information • Performed by routers • Inserts / changes values in routing table Fall 2004

  3. Two Forms of Internet Routing • Static routing • Table initialized when system boots • No further changes unless error is detected • Automatic routing • Table initialized when system boots • Initialization is started the same way as static • But, rout propagation (routing) software is also started • Routing software learns optimal routes and updates routing table • Continuous changes possible Fall 2004

  4. Static Routing • No routing software • Does not consume bandwidth or CPU time • It cannot accomodate network failures • It cannot accommodate changes in network topology • Used when host is on one network and sees the Internet through a router • Typical routing table has two entries: • Local network → direct delivery • Default→ nearest router Fall 2004

  5. Routing Table of One Host (E.g. H1) Direct to H2 To the Internet Example of Static Routing Fall 2004

  6. A Note on Addressing • 129.52.18.6/20 means • 20 leading bits are for network addresses • the remaining 12 bits are for host addresses • This means that some of the bits making up .18. • belong to network addressing • belong to host addressing • Need to expand back to binary to resolve • expand 10000001 00110100 00010010 00000110 • network 10000001 00110100 0001 (address prefix) • host 0010 00000110 • Need a subnet mask • 11111111 11111111 11110000 00000000 or 255.255.220.0 Fall 2004

  7. Automatic Routing • Used by IP routers • Requires special software • Each router communicates with neighbors • Pass routing information • Use route propagation protocol Fall 2004

  8. Example of Route Propagation • Router R1 does not know about Net 3 • Router R2 does not know about Net 2 • If static tables with 100 or more hosts on Net 2 and Net 3 • manual routing table updates become impractical • each time a new host is added, ISP1 needs to inform ISP1 to update • Need to automate - add routing software Fall 2004

  9. Example of Route Propagation • Routers R1 and R2 each run routing software • Routing software on R1 installs the route to Net 3 in its routing table • Routing software on R2 installs the route to Net 2 in its routing table • Each router advertises destinations that lie beyond it of its link state • If Net 3 goes down, R1 routing software removes Net 3 from its table • If Net 3 comes back up, R1 routing software places Net 3 into its table Fall 2004

  10. The Point of Routing Exchange Each router runs routing software that learns about destinations other routers can reach, and informs other routers about destinations that it can reach. The routing software uses incoming information to update the local routing table continuously. Fall 2004

  11. Routing in the Global Internet • The Internet is subdivided into routing areas to reduce routing traffic • At least one router in an area summarizes the routing information and passes it to other areas • Main considerations • size of routing areas • routing protocol within a routing area • how routing information is represented • what protocol is used between routing areas Fall 2004

  12. Autonomous System Concept • Set of networks and routers under one administrative authority • Flexible, soft definition depending on • cost • administrative convenience • capability of routing protocol chosen • Intuition: a single corporation, university • Needed because • no routing protocol can scale to entire Internet • table update traffic would be to great • Each AS chooses a routing protocol Fall 2004

  13. Classifications of Inernet Routing Protocols • Two broad classes • Interior Gateway Protocols (IGPs) • Used among routers within autonomous system • Destinations lie within IGP • Example of IGP - OSPF • Exterior Gateway Protocols (EGPs) • Exchange routing information with routers in other autonomous systems • Destinations lie throughout Internet • Example of EGP - BGP4 Fall 2004

  14. Illustration of IGP / EGP Use • R1 summarizes routing information from AS1 and sends the summary to R4 • R1 accepts AS2 summary from R4 Fall 2004

  15. Optimal Routes • Optimal depends on need • interacitve login - least delay is optimal • browser downloading - max. throughput is optimal • real-time audio - least jitter is optimal • Most Internet routing uses other metrics • administrative cost (corporate policy to control traffic) • hop count (fewer hops for customer, more for internal) • EGP does not use metrics • cannot compare routing metrics from different Autonomous Systems Fall 2004

  16. The Concept of Route and Data Traffic ISP1 advertises routes to its customers in this direciton Each ISP is an autonomous system that uses an Exterior Gateway Protocol to advertise its customers’ networks to other ISPs. After an ISP advertises destination D, datagrams destined for D can begin to arrive Datagrams to ISP1 customers flow in this direciton Fall 2004

  17. Specific Internet Routing Protocols • Border Gateway Protocol (BGP) • Routing Information Protocol (RIP) • Open Shortest Path First Protocol (OSPF) Fall 2004

  18. Border Gateway Protocol (BGP) • Provides routing among autonomous systems (EGP) • Only two routers are involved • A BGP session • open BGP session over TCP to inform neighbour • of new routes that are active (in terms of prefix) • of old routes no longer active • that this connection is still viable • of any unusual conditions • Policies to control routes advertised • Gives path of autonomous systems for each destination • EGP of choice in the Internet is BGP ( BGP-4) Fall 2004

  19. BGP link BGP link AS 1 AS 4 AS 5 AS 3 AS 2 BGP link BGP link BGP link BGP AS Links and Path Tree • BGP provides routes to other ASs (address prefixes) • BGP builds a graph of ASs • Graph derived from routing information • BGP sees the entire Internet as a graph • BGP can skip intrmediate routers in AS Fall 2004

  20. BGP link BGP link BGP link BGP link AS 2 AS 1 AS 5 AS 3 AS 4 BGP link BGP Advertising 192.168.0.0/16 • 192.168.0.0/16 is Reachable throug this AS • AS 1 advertises: 192.168.0.0/16 through AS 1 • AS 2 advertises: 192.168.0.0/16 through AS 1 and AS 2 • AS 3 advertises: 192.168.0.0/16 through AS 1 and AS 2 and AS 3 • AS 5 sees: 192.168.0.0/16 through AS 1 and AS 2 • AS 5 sees: 192.168.0.0/16 through AS 1 and AS 2 and AS 3 Fall 2004

  21. The Routing Information Protocol (RIP) • Routing within an autonomous system (IGP) • Hop count metric • Unreliable transport (uses UDP) • Broadcast or multicast delivery • Distance vector algorithm • Can propagate a default route • Implemented by Unix program routed Fall 2004

  22. Illustration of RIP Packet Format Fall 2004

  23. The Open Shortest Path First Protocol (OSPF) • Routing within an autonomous system (IGP) • Full CIDR address scheme and subnet support • Authenticated message exchange • Allows routes to be imported from outside the autonomous system • Uses link-status (SPF) algorithm • Support for multi-access networks (e.g., Ethernet) Fall 2004

  24. OSPF Areas and Efficiency • Allows subdivision of an AS into Areas • Link-status information propagated within one Area • One router in each Area is designated to communicate with one or more other Areas • Routing information is summarized before being propagated to another Area • Reduces overhead (less broadcast traffic) • Able to scale to large or small networks Fall 2004

  25. Link-Status in the Internet • Router corresponds to node in graph • Network corresponds to edge • Adjacent pair of routers periodically • Test connectivity • Broadcast link-status information to area • Each router uses link-status messages to compute shortest paths Fall 2004

  26. LSA Flooding R1 LSA Data Base LSA1, LSA2, LSA3, LSA4, LSA5, LSA6, LASa, LSAb, LSAc, LSAd, LSAe,LSAf, LSAg, LSAh AREA with 2 Routers 2 Links to R1: a, b R1 LSA1, LSA2 LSA1, LSA2 LSAa, LSAb 1 2 LSAa, LSAb LSAf, LSAg, LSAh LSAc, LSAd, LSAe R2 LSA Data Base LSA1, LSA2, LSA3, LSA4, LSA5, LSA6, LASa, LSAb, LSAc, LSAd, LSAe,LSAf, LSAg, LSAh R3 LSA Data Base LSA1, LSA2, LSA3, LSA4, LSA5, LSA6, LASa, LSAb, LSAc, LSAd, LSAe,LSAf, LSAg, LSAh LSA5, LSA6 LSA3, LSA4 5 3 4 R2 R3 6 LSA5, LSA6 LSA3, LSA4 AREA with 3 Routers 3 Links to R3: c, d, e AREA with 3 Routers 3 Links to R2: f, g, h LSAf, LSAg, LSAh LSAc, LSAd, LSAe Fall 2004

  27. LS Age Options LS Type = 1 Link State ID Advertising Router LS Sequence Number Length LS Checksum LSA Header 0 E B 0 # links V Link ID Link Data Type # TOS metric . . . 0 TOS # TOS Link ID Link Data . . . Inside the Router-LSA LSA Header Router-LSAs Fall 2004

  28. Illustration of OSPF Graph • (a) an interconnect of routers and networks, and • (b) an equivalent OSPF graph • Router corresponds to a node in the graph Fall 2004

  29. OSPF and Scale Because it allows a manager to partition the routers and networks in an autonomous system into multiple areas, OSPF can scale to handle a much larger number of routers than other IGPs Fall 2004

  30. Internet Multicast Routing • Difficult because Internet multicast allows • Arbitrary computer to join multicast group at any time • Arbitrary member to leave multicast group at any time • Arbitrary computer to send message to a group (even if not a member) • Internet Group Multicast Protocol (IGMP) • Used between computer and local router • Specifies multicast group membership Fall 2004

  31. Multicast Routing Protocols • Several protocols exist • Distance Vector Multicast Routing Protocol (DVMRP) • Core Based Trees (CBT) • Protocol Independent Multicast – Sparse Mode (PIM-SM) • Protocol Independent Multicast – Dense Mode (PIM-DM) • Multicast extensions to the Open Shortest Path First (MOSPF) • None best in all circumstances Fall 2004

  32. Summary • Static routing used by hosts • Routers require automatic routing • Internet divided into autonomous systems • Two broad classes of routing protocols • Interior Gateway Protocols (IGPs) provide routing within an autonomous system • Exterior Gateway Protocols (EGPs) provide routing among autonomous systems Fall 2004

  33. Summary (continued) • Border Gateway Protocol (BGP) is current EGP used in Internet • Interior Gateway Protocols include: • Routing Information Protocol (RIP) • Open Shortest Path First protocol (OSPF) • Internet multicast routing difficult • Protocols proposed include: DVMRP, PIM-SM, PIM-DM, MOSPF Fall 2004

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