1 / 45

Distance Vector Routing Protocols

Distance Vector Routing Protocols. Apr. 5, 2012. Control protocols error reporting e.g. ICMP. Control protocols - router “ signaling ” e.g. RSVP. Network layer protocol (e.g., IP) addressing conventions packet format packet handling conventions. Routing protocols path selection

trella
Télécharger la présentation

Distance Vector Routing Protocols

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Distance Vector Routing Protocols Apr. 5, 2012

  2. Control protocols • error reportinge.g. ICMP Control protocols - router “signaling”e.g. RSVP • Network layer protocol (e.g., IP) • addressing conventions • packet format • packet handling conventions • Routing protocols • path selection • e.g., RIP, OSPF, BGP forwarding Recap: Internet Network Layer Protocols Transport layer Network layer Link layer physical layer

  3. 5 3 5 2 2 1 3 1 2 1 A D E B F C Routing Recap: Routing Goal: determine “good” paths (sequences of routers) thru network from source to dest. Graph abstraction for the routing problem: • graph nodes are routers • graph edges are physical links • links have properties: delay, capacity, $ cost, policy

  4. Basis of RIP, IGRP, EIGRP routing protocols Positive costs assigned to network links Distributedshortestrouting conceptually, runs for each destination separately hence we consider one dest only state: each node maintains a current estimate to the destination di denotes the distance estimation from node i to dest update rule: based on Bellman-Ford alg. 5 3 5 2 2 1 3 1 2 1 A D B E F C Distance Vector Routing

  5. At node i, the basic update rule where - di denotes the distance estimation from i to the destination, - N(i) is set of neighbors of node i, and - dij is the distance of the direct link from i to j Distance Vector Routing: Update destination j i

  6. A D E B C Synchronous Bellman-Ford (SBF) 1 7 2 8 10 2 • Nodes update in rounds: • there is a global clock; • at the beginning of each round, each node sends its estimate to dest to all of its neighbors; • at the end of the round, updates its estimation A B E C D

  7. Outline • Network overview • Control plane: routing overview • Distance vector protocols • synchronous Bellman-Ford (SBF) • - SBF/

  8. A D E B C SBF/ 1 7 2 8 10 2 • Initialization (time 0):

  9. 1 7 2 8 10 2 1232 2 0 2 2 0 1032 2 0 1032 2 0   0 d(0) d(4) d(2) d(3) d(1) A D B E C Example Consider D as destination; d(t) is a vector consisting of estimation of each node at round t A B C E D Observation: d(0)  d(1)  d(2)  d(3)  d(4) =d*

  10. A Nice Property of SBF: Monotonicity • Consider two configurations d(t) and d’(t) • If d(t)  d’(t) • i.e., each node has a higher estimate in d than in d’, • then d(t+1)  d’(t+1) • i.e., each node has a higher estimate in d than in d’ after one round of synchronous update.

  11. Correctness of SBF/ • Claim: di(h) is the length Li(h) of a shortest path from i to the destination using ≤ h hops • base case: h = 0 is trivially true • assume true for ≤ h, i.e., Li(h)=di(h), Li(h-1)=di(h-1), …

  12. since di(h) ≤ di(h-1) Correctness of SBF/ • consider ≤ h+1 hops:

  13. Bellman Equation • We referred to the equations as Bellman equations (BE): where dD = 0. • SBF/ solves the equations in a distributed way • Does the equation have a unique solution (i.e., the shortest path one)?

  14. 1 7 2 8 10 2 A D E B C 1 1 1 1 Uniqueness of Solution to BE • Assume another solution d, we will show that d = d* case 1: we show d  d* Since d is a solution to BE, we can construct paths as follows: for each i, pick a j which satisfies the equation; since d* is shortest, d  d* 2 3 Dest. 10 2

  15. Uniqueness of Solution to BE Case 2: we show d ≤ d* assume we run SBF with two initial configurations: • one is d • another is SBF/ (d), -> monotonicity and convergence of SBF/ imply that d ≤ d*

  16. Outline • Network overview • Control plane: routing overview • Distance vector protocols • synchronous Bellman-Ford (SBF) • SBF/ • SBF/-1

  17. A D E B C SBF at another Initial Configuration: SBF/-1 1 7 2 8 10 2 • Initialization (time 0):

  18. 1 7 2 8 10 2 -1-1 -1 -1 0 103 3 2 0 9 3 3 2 0 8 2 2 2 0 7 1 1 2 0 6 0 0 2 0 103 3 2 0 d(5) d(4) d(3) d(6) d(1) d(0) d(2) A D B E C Example Consider D as destination A B C E D Observation: d(0)  d(1)  d(2)  d(3)  d(4)  d(5) = d(6) = d*

  19. Bellman Equation and Correctness of SBF/-1 • SBF/-1 converges due to monotonicity • At equilibrium, SBF/-1 satisfies the set of equations called Bellman equations (BE) where dD = 0. • Another solution is shortest path solution d* • Since there is a unique solution to the BE equations; thus SBF/-1 converges to shortest path Question: will SBF converge under other non-negative initial conditions? Problems of running synchronous BF?

  20. Outline • Network overview • Control plane: routing overview • Distance vector protocols • synchronous Bellman-Ford (SBF) • asynchronous Bellman-Ford (ABF)

  21. Asynchronous Bellman-Ford (ABF) • No notion of global iterations • each node updates at its own pace • Asynchronously each node i computes using last received value dij from neighbor j. • Asyncrhonously node j sends its estimate to its neighbor i: • there is an upper bound on the delay of estimate packets (no worry for out of order)

  22. j i Asynchronous Bellman-Ford (ABF) • In general, nodes are using different andpossibly inconsistent estimates dj dij di

  23. 1 distance table E sends to its neighbors 7 A: 10 B: 8 C: 4 D: 2 E: 0 2 8 10 2 A D E B C Distance Table: Example Below is just one step! The protocol repeats forever! distance tablesfrom neighbors computation E’s distance table d () A B C D A B D A B D E 10 15  0 7  A: 10 B: 8 7 0  17 8  destinations D: 4 1 2 9 4 0 2 D: 2 10 8 2

  24. Asynchronous Bellman-Ford (ABF) • ABF will eventually converge tothe shortest path • links can go down and come up – but if topology is stabilized after some timet, ABF will eventually converge to theshortest path ! • Ifthe network is connected, then ABF converges in finite amount of time, if conditions are met

  25. ABF Convergence • There are too many different “runs” of ABF, so need to use monotonicity • Consider two sequences: • SBF/; call the sequence U() • SBF/-1; call the sequence L()

  26. j i System State where can distance estimate from node j appear?

  27. dj di System State • three types of distance estimates from node j: • dj: current distance estimate at node j • - dij: last dj that neighbor i received • - dij: those dj that are still in transit to neighbor i dij dij

  28. ABF Convergence • Consider the time when the topology is stabilized as time 0 • U(0) and L(0) provide upper and lower bound at time 0 on all corresponding elements of states • Lj (0) ≤ dj≤ Uj (0) for all dj state at node j • Lj (0) ≤ dij≤ Uj (0) • Lj (0) ≤ update messages dij≤ Uj (0)

  29. dj di ABF Convergence • dj • after at least one update at node j: dj falls between Lj (1) ≤ dj ≤ Uj (1) • dij : • eventually all dij that are only bounded by Lj (0) and Uj (0) are replaced with inLj(1) and Uj(1) dij dij

  30. Distributed:each node communicates its routing table to its directly-attached neighbors Iterative:continues periodically or when link changes, e.g. detects a link failure Asynchronous:nodes need not exchange info/iterate in lock step! Convergence in finite steps, independent of initial condition if network is connected Asynchronous Bellman-Ford: Summary

  31. Properties of Distance-Vector Algorithms • Good news propagate fast

  32. Properties of Distance-Vector Algorithms • Bad news propagate slowly (link A-B broke) • This is called the counting-to-infinity problem Question: why does counting-to-infinity happen?

  33. What is a Routing Loop? • A routing loop is a global state (consisting of the nodes’ local states) at a global moment (observed by an oracle) such that there exist nodes A, B, C, … E such that A (locally) thinks B as down stream, B thinks C as down stream, … E thinks A as down stream • Counting-to-infinity because of routing loops

  34. 1 7 2 8 1 2 A D E B C The Reverse-Poison (Split-horizon) Hack If the path to dest is through neighbor h, report  to neighbor h for dest. distance tables from neighbors computation E’s distance table distance table E sends to its neighbors E D () A B C D A 0 7   1 c(E,A) B 7 0 1  8 c(E,B) A 1 8   D   2 0 2 c(E,D) B 15 8 9  D   4 2 1, A 8, B 4, D 2, D To B A: 1 B:  C: 4 D: 2 E: 0 To A A:  B: 8 C: 4 D: 2 E: 0 To D A: 1 B: 8 C:  D:  E: 0 destinations distance through neighbor

  35. An Example Where Split-horizon Fails 1 1 1 1 • When the link between C and D fails, C will set its distance to D as  • However, unfortunate timing can cause problem • A receives the bad news () from C, A will use B to go to D • A sends the news to C • C sends the news to B Question: what is the routing loop formed?

  36. Example: RIP ( Routing Information Protocol) • Distance vector • Included in BSD-UNIX Distribution in 1982 • Link cost: 1 • Distance metric: # of hops • Distance vectors • exchanged every 30 sec via Response Message (also called advertisement) using UDP • each advertisement: route to up to 25 destination nets

  37. RIP (Routing Information Protocol) z ... w x y A I B C Destination Network Next Router Num. of hops to dest. w A 2 y B 2 z B 7 x -- 1 …. …. .... Routing table in I

  38. RIP: Link Failure and Recovery If no advertisement heard after 180 sec --> neighbor/link declared dead • routes via neighbor invalidated • new advertisements sent to neighbors • neighbors in turn send out new advertisements (if tables changed) • link failure info quickly propagates to entire net • reverse-poison used to prevent ping-pong loops • set infinite distance = 16 hops (why?)

  39. EIGRP Neighbor Discovery • EIGRP routers actively establish relationships with their neighbors • EIGRP routers establish adjacencies with neighbor routers by using small hello packets. • The Hello protocol uses a multicast address of 224.0.0.10, and all routers periodically send hellos.

  40. EIGRP Neighbor Discovery • On hearing hellos, the router creates a table of its neighbors. • The continued receipt of these packets maintains the neighbor table By forming adjacencies, EIGRP routers do the following: • Dynamically learn of new routes that join their network • Identify routers that become either unreachable or inoperable • Rediscover routers that had previously been unreachable

  41. Neighbor Discovery - 3

  42. Default Hello Intervals and Hold Time for EIGRP

  43. Backup Slides

  44. Using Virtual Circuit to Implement Network Services • In order to provide some functionalities, a network may choose virtual circuit, e.g., Virtual Private Network (VPN)

  45. Using Datagram to Implement the Most Basic Network Service • A datagram network generally provides simple services: the forwarding of packets from src to dest. • We will focus on datagram networks which provide best effort service • extensions to provide more services will be discussed in the multimedia networking part of the course

More Related