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Understanding Internet Routing: Principles, Challenges, and Distance-Vector Algorithms

This lecture delves into Internet routing with a focus on achieving loop-free routes and the complexities of global optimization influenced by economic and political factors. It covers distance-vector routing, link failures, and split horizon techniques to prevent routing loops. The discussion includes practical examples illustrating the behavior of routing tables during link failures and advertisement updates. Additionally, it explores the importance of subnetting, Classless Interdomain Routing (CIDR), and the role of Border Gateway Protocol (BGP) in managing a vast number of network addresses effectively.

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Understanding Internet Routing: Principles, Challenges, and Distance-Vector Algorithms

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  1. Lecture 18 Internet Routing: The goal is to find any route that is loop free-Global optimization is a distant dream depending on economic and political drivers Homework: 4.1-35, 37-45

  2. Distance-vector routing: A’s routing table just after link A-E failure

  3. A What happens when G advertises its table to D? How does D get the message that A-E has failed? F G

  4. A advertises [E, ¥] to B and B advertises [E, 2] to A (they cross intransit) A advertises [E,3] and B advertises [E, ¥] etc. B resets [E, ¥, -] (since A is next hop) A updates [E, 3, B] since 3<¥ B updates [E, 4,A] and A resets [E, ¥, -] since B is next hop etc. Loops -A Split horizon: B should not advertise a route [E, 2] it got from A (in the first step)

  5. A advertises [E, ¥] to B & C Advertisement to C is delayed B may advertise [E, ¥] to C C advertises [E, 2] to B A’s advertisement [E, ¥] arrives at C B advertises [E,3] to A C advertises [E, ¥] to B A advertises [E,4] to C B advertises [E, ¥] to A C advertises [E,5] to B B resets [E, ¥, -] since A is next hop C will not update (why) B updates [E, 3, C] since 3<¥ C updates [E, ¥, -] since A is next hop A updates [E,4, B] since 4<¥ B updates [E, ¥, -] since C is next hop C updates [E, 5, A] since 5<¥ A updates [E, ¥, -] since B is next hop B updates [E, 6, A] since 6<¥ Loops -B Split horizon will not solve this problem

  6. Link State Routing You have a global view: routing table is spanning tree as seen from root node

  7. The Internet circa 1990: A hierarchical collection of autonomous systems (AS)

  8. Today’s multiple backbone Stub AS Multi-homed AS Transit AS

  9. Inefficiency of IP address classes • If you have 257 end users, you need class B and then you have 16K addresses. We need finer distinctions. • Two issues: • How do you give different network addresses to physical networks within 1 class A, B or C network---subnetting • How do you aggregate networks within an domain to simplifier routing outside the domain--supernetting

  10. Forwarding Table of R1

  11. More Subnetting • You can break the same physical network into subnets—forcing hosts to speak through a router • Each host has it’s own subnet in the new engineering network • Subnet mask=IP address---non-contiguous 1’s and 0’s

  12. Classless Interdomain Routing (CIDR) • We can get better utilization if we hand out Class C addresses • This would increase the size of forwarding tables • We aggregate contiguous class C blocks

  13. Aggregation of 16 Class C network addresses into a single 20 bit CIDER address

  14. Intradomain Routing Border Gateway Routers—Default router for outbound traffic

  15. Intradomain routing issues • Scale—100,000 network addresses • Autonomy • Trust • Flexibility—hot potato routing

  16. BGP example 1 BGP speaker/AS: Advertise routes-prevents looping Withdrawn broken routes

  17. Routing Areas in OSPF

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