Chapter 4 Distance Vector Problems, and Link-State Routing Professor Rick Han University of Colorado at Boulder

Prof. Rick Han, University of Colorado at Boulder Announcements Handing back HW #1, TA OH, solutions online later today Homework #2 available on Web site, due Feb. 26 Last week’s lectures are now on Web site Midterm for the week of March 12 Next, Distance vector problems, and link- state routing, …

Prof. Rick Han, University of Colorado at Boulder Recap of Previous Lecture Distributed Bellman-Ford = Distance Vector Bellman-Ford Equation D(i,j) = min {d(i,k) + D(k,j)} for all i<>j k neighbors Bellman-Ford Algorithm [Ford & Fulkerson] D(i,j)[h+1] = min {d(i,k) + D(k,j)[h]} for all i<>j, h=0,1, … k neighbors Distributed Bellman-Ford Algorithm Physically distribute the calculation of D(i,j)[h] to router i only, and Exchange the results of your D(i,j)[h] with neighboring routers at each iteration h

Prof. Rick Han, University of Colorado at Boulder RIP is simple 1.At each step, exchange distance vectors with each neighbor 2.Update distance table with new distance vector, adding one (all link costs are one) 3.Calculate minimum hop path to each destination by looking at minimum in the row A C B Aa Bb Cc Aa Bb Cc DestABC Aa++---a++ Bb++---b++ Cc++---c++ Distance table at B Via Port/Link

Prof. Rick Han, University of Colorado at Boulder Link Failure Causes “Bouncing” Effect A B C B C2 1 destcost X B B via A C1 1 destcost C A via A B1 2 destcost B B via

Prof. Rick Han, University of Colorado at Boulder B Notices A-B Link Failure A 25 1 B C B notices failure, resets cost via A to infinity in distance table (not shown), & knows cost via C is 26 B C2 1 destcost B B via A C1 26 destcost C C via A B1 2 destcost B B via

Prof. Rick Han, University of Colorado at Boulder C Sends Dist. Vector to B A 25 1 B C B C2 1 destcost B B via A C1 3 destcost C C via A B1 2 destcost B B via C sends routing update to B

Prof. Rick Han, University of Colorado at Boulder B Updates Distance to A A 25 1 B C Packet sent from C to A bounces between C and B until TTL=0! B C2 1 destcost B B via A C1 3 destcost C C via A B1 2 destcost B B via

Prof. Rick Han, University of Colorado at Boulder B Sends Dist. Vector to C A 25 1 B C C adds one to B’s advertised distance to A. (Why does C override its stored distance of 2 to A with 4, larger value?) B C2 1 destcost B B via A C1 3 destcost C C via A B1 4 destcost B B via

Prof. Rick Han, University of Colorado at Boulder C Sends Dist. Vector to B A 25 1 B C B adds one to C’s advertised distance to A. (overrides its stored distance of 3 to A with 5, larger value) B C2 1 destcost B B via A C1 5 destcost C C via A B1 4 destcost B B via

Prof. Rick Han, University of Colorado at Boulder Link Failure: Bad News Travels Slowly A 25 1 B C After 20+ exchanges, routing tables look like this: B C25 26 destcost C C via A C1 25 destcost C C via A B1 24 destcost B B via Assume A has advertised its link cost of 25 to C during B C exchanges. C stores this cost in its distance table (not shown)

Prof. Rick Han, University of Colorado at Boulder Bad News Travels Slowly (2) A 25 1 B C C increments B’s update by 1, and chooses 25 via A to A, instead of 26 Via B to A B C25 26 destcost C C via A C1 25 destcost C C via A B1 25 destcost B A via

Prof. Rick Han, University of Colorado at Boulder Bad News Travels Slowly (3) A 25 1 B C After 25 B-C exchanges, finally converge to stable routing B C25 26 destcost C C via A C1 26 destcost C C via A B1 25 destcost B A via

Prof. Rick Han, University of Colorado at Boulder Link Failure Causes “Counting to Infinity” Effect A B C B C2 1 destcost X B B via A C1 1 destcost C A via A B1 2 destcost B B via

Prof. Rick Han, University of Colorado at Boulder B Notices A-B Link Failure A 25 1 B C B notices failure, resets cost to 26 B C2 1 destcost B B via A C1 26 destcost C C via A B1 2 destcost B B via

Prof. Rick Han, University of Colorado at Boulder C Sends Dist. Vector to B A 25 1 B C B C2 1 destcost B B via A C1 3 destcost C C via A B1 2 destcost B B via C sends routing update to B

Prof. Rick Han, University of Colorado at Boulder A-C Link Fails A 1 B C C detects link to A has failed, but no change in C’s routing table (why?) A C1 3 destcost C C via A B1 2 destcost B B via

Prof. Rick Han, University of Colorado at Boulder Now, B and C Count to Infinity A 1 B C A C1 3 destcost C C via A B1 4 destcost B B via

Prof. Rick Han, University of Colorado at Boulder B and C Count to Infinity (2) A 1 B C A C1 5 destcost C C via A B1 4 destcost B B via

Prof. Rick Han, University of Colorado at Boulder Some “Solutions” Split horizon –C does not advertise route to B when it sends its distance vector Poisoned reverse –C advertises route to B with infinite distance in its distance vector Works for two node loops –Does not work for loops with more nodes

Prof. Rick Han, University of Colorado at Boulder B Notices A-B Link Failure A 25 1 B C B notices failure, resets cost to 26 B C2 1 destcost B B via A C1 26 destcost C C via A B1 2 destcost B B via

Prof. Rick Han, University of Colorado at Boulder Split Horizon A 25 1 B C B C2 1 destcost B B via A C1 26 destcost C C via A B1 2 destcost B B via C sends routing update to B B1 destcost No need to send dest A via B, since B should already know this

Prof. Rick Han, University of Colorado at Boulder Split Horizon With Poisoned Reverse A 25 1 B C B C2 1 destcost B B via A C1 ~ destcost C -- via A B1 2 destcost B B via C sends routing update to B A B1 ~ destcost If lowest cost path is via B, then when updating B send infinite cost

Prof. Rick Han, University of Colorado at Boulder Example Where Split Horizon Fails A When link breaks, C marks D as unreachable and reports that to A and B Suppose A learns it first –A now thinks best path to D is through B – A reports D unreachable to B and a route of cost=3 to C C thinks D is reachable through A at cost 4 and reports that to B B reports a cost 5 to A who reports new cost to C etc... X B C D

Prof. Rick Han, University of Colorado at Boulder Alternative Shortest Path Calc. Compute a shortest path tree Observation: shortest path to nodes further from the root must go through a branch of the shortest path tree closer to the root Strategy: expand outwards, calculating the shortest path tree from the root (B)

Prof. Rick Han, University of Colorado at Boulder Dijkstra’s Shortest Path Algorithm Let N = set of nodes in graph l(i,j) = link cost between i,j (= infinity if not neighbors) SPT = set of nodes in shortest path tree thus far S = source node C(n) = cost of path from S to node n

Prof. Rick Han, University of Colorado at Boulder Dijkstra’s Shortest Path Algorithm (2) Initialize shortest path tree SPT = {S} For each n not in SPT, C(n) = l(s,n) While (SPT<>N) SPT = SPT U {w} such that C(w) is minimum for all w in (N-SPT) For each n in (N-SPT) C(n) = MIN (C(n), C(w) + l(w,n))

Prof. Rick Han, University of Colorado at Boulder Dijkstra’s Shortest Path Algorithm (3) Initialize shortest path tree SPT = {B} For each n not in SPT, C(n) = l(s,n) C(E) = 1, C(A) = 3, C(C) = 4, C(others) = infinity Add closest node to the tree: SPT = SPT U {E} since C(E) is minimum for all w not in SPT. No shorter path to E can ever be found via some other roundabout path. Shortest path tree SPT = {B, E}

Prof. Rick Han, University of Colorado at Boulder Dijkstra’s Shortest Path Algorithm (4) Recalculate C(n) = MIN (C(n), C(E) + l(E,n)) for all nodes n not yet in SPT C(A) = MIN( C(A)=3, 1 + 1) = MIN(3,2) = 2 C(D) = MIN( infinity, 1 + 1) = 2 C(F) = MIN( infinity, 1 + 2) = 3 C(C ) = MIN( 4, 1 + infinity) = 4 Each new node in tree, could create a lower cost path, so redo costs

Prof. Rick Han, University of Colorado at Boulder Dijkstra’s Shortest Path Algorithm (5) Loop again, select node with the lowest cost path: C(A) = 2, C(D) = 2, C(F) = 3, C(C ) = 4 SPT = SPT U {A} = {B, E, A} No shorter path can be found from B to A, regardless of any new nodes added to tree Recalc: C(n) = MIN (C(n), C(A) + l(A,n)) for all n not yet in SPT C(D) = MIN(2, 2+inf) = 2 C(F) = 3, C(C) = 4

Prof. Rick Han, University of Colorado at Boulder Dijkstra’s Shortest Path Algorithm (6) Continue to loop, adding lowest cost node at each step and updating costs SPT crawls outward Remember to store the links in SPT as they are added (each node’s predecessor is stored) Each node has to store the entire topology, or database of link costs

Prof. Rick Han, University of Colorado at Boulder Link-State Routing = Reliable Flooding + Dijkstra SPT Start condition –Each node assumed to know state of links to its neighbors Step 1 –Each node broadcasts its state to all other nodes –Reliable flooding mechanism Step 2 –Each node locally computes shortest paths to all other nodes from global state –Dijkstra’s shortest path tree (SPT) algorithm

Prof. Rick Han, University of Colorado at Boulder Link State Packets (LSPs) Periodically, each node creates a link state packet containing: –Node ID –List of neighbors and link cost –Sequence number Needed to avoid stale information from flood –Time to live (TTL) –Node outputs LSP on all its links

Prof. Rick Han, University of Colorado at Boulder Reliable Flooding of LSPs When node J receives LSP from node K –If LSP is the most recent LSP from K that J has seen so far, J saves it in database and forwards a copy on all links except link LSP was received on –Otherwise, discard LSP How to tell more recent –Use sequence numbers –Same method as sliding window protocols

Prof. Rick Han, University of Colorado at Boulder OSPF – Open Shortest Path First A particular realization of link-state routing, used for intra-domain routing in the Internet Additional support for: –Authentication of routing updates –Support for broadcast networks –Different cost metrics Periodic and event-triggered flooding of LSP routing updates, like RIP