CEN 5501C - Computer Networks - Spring UF/CISE - Newman1 Computer Networks Chapter 12 – Routing Algorithm Concepts

CEN 5501C - Computer Networks - Spring UF/CISE - Newman2 Routing Algorithm Types Answer question: Given destination D, where do I send packet next? Distance Vector –Track cost to neighbor, neighbor cost to D –Bellman-Ford shortest path algorithm Link State –Track link costs –Run shortest path algorithm on whole graph

CEN 5501C - Computer Networks - Spring UF/CISE - Newman3 Distance Vector Each router has its own ID and link costs Initial DV: dist(self)=0, dist(other)=inf Tx DV to all neighbors, get neighbor DVs Compute new DV based on –dist(x)=min{DV[y,x], cost[y]} Tx new DV periodically and on changes –“triggered updates” Recompute when detect changes –(link down, different DV from y)

CEN 5501C - Computer Networks - Spring UF/CISE - Newman4 Count to Infinity Link dies, neighbors believe stale news ABC A:2,B B:1,B A:1,A C:1,C B:1,B C:2,B ABC A:2,B B:1,B A:1,A C:1,C B:1,B C:2,B A:1,A C:3,A B:1,B C:2,B A:1,A C:3,A B:1,B C:4,B A:1,A C:5,A B:1,B C:4,B Make infinity small

CEN 5501C - Computer Networks - Spring UF/CISE - Newman5 Count to Infinity Solutions Hold Down –If path to D goes down, don’t update for T s. –Advertise distance to D as infinity until T Report whole path –No loops, but $$$ Split Horizon (poison reverse) –If A uses B to get to D, tell B that DV[A,D]=inf Two metrics –Use both hops and other cost function

CEN 5501C - Computer Networks - Spring UF/CISE - Newman6 Split Horizon Problem ABD A:1,A C:1,C D:inf B:1,B C:1,C D:2,B A:1,A C:3,A D:inf B:1,B C:1,C D:3,C A:1,A C:1,C D:4,A B:1,B C:1,C D:inf A:1,A C:1,C D:4,A B:1,B C:1,C D:5,B Count to infinity slower…. C ABD A:1,A C:1,C D:1,D B:1,B C:1,C D:2,B C A:1,A B:1,B D:2,B A:1,A B:1,B D:3,A A:1,A B:1,B D:inf A:1,A B:1,B D:5,B A:1,A B:1,B D:2,B

CEN 5501C - Computer Networks - Spring UF/CISE - Newman7 Link State Protocol Each router has its own ID and link costs R meets neighbors, gets their names –Tx special “Hello” packet Tx Link State Packet (LSP) to all routers Compute least cost paths based on most recent LSPs from all other routers Tx new LSP when –New neighbor –Link cost changes –Link fails

CEN 5501C - Computer Networks - Spring UF/CISE - Newman8 Link State Protocol Broadcasting LSPs critical –Routers must have consistent graph info –Must avoid LSP storms How to route LSPs? –Chicken and egg (a.k.a. recursion) –Flooding with hop count or path –Constrained flooding (kill duplicate LSPs) Timestamps –Needed to determine most recent, duplicates –Sequence number/age schemes

CEN 5501C - Computer Networks - Spring UF/CISE - Newman9 Timestamps Knowing most recent LSP critical –Accidental use of large timestamp/seq # –Merging partitioned networks Timestamps –Sanity checks not possible without global time Sequence # + Aging –Sequence numbers wrap around – funny < –Age added to LSP, decrement while stored, discard when age reaches zero

CEN 5501C - Computer Networks - Spring UF/CISE - Newman10 ARPANET LSP Incident Source, seq#, age, neighbor list –Age was 3 bits, increments of 8 seconds –Routers required to send LSP every 60 sec –New router listens 90 seconds One bad router can spoil the whole bunch –Issued LSPs with sequence #s a, b, and c –But a<b<c<a in circular order! –Routers flooded with a, then b, then c, then a… Had to patch out bad source!

CEN 5501C - Computer Networks - Spring UF/CISE - Newman11 New LSP Distribution Source, seq#, age, neighbor list –Seq# is large (e.g., 32 bits) –No LSP accepted once seq# wraps until TRO Age field-- in storage & on forwarding Only store and flag to forward LSPs –Send flag and ACK flag per link per LSP –Check flags round robin –Only one LSP per source per link queue –Newer LSP overwrites LSP before sending –Send when bandwidth available on link

CEN 5501C - Computer Networks - Spring UF/CISE - Newman12 LSP with Age=0 Router deletes LSP data when age=0 –It has expired Router keeps LSP header to send to nbors Router deletes LSP when all nbors have ACKed the age=0 LSP Router must not accept LSP with age=0 unless it already has an LSP for that source –Indefinitely wandering age 0 LSPs…

CEN 5501C - Computer Networks - Spring UF/CISE - Newman13 New LSP Distribution Self-stabilizing –No matter how bad a state is reached, if defective/malicious nodes are disconnected, the network returns to normal “quickly” Efficient –Low frequency periodic LSP regeneration Responsive –Don’t have to wait to participate, except when sequence number wraps around (very rare!)

CEN 5501C - Computer Networks - Spring UF/CISE - Newman14 DV vs. LS Comparisons Assume n nodes, k n’bors each Memory –DV: DVs are O(n), memory is O(nk)*** –LS: LSPs are O(k), memory is O(kn) Bandwidth consumed –Mixed bag –DV: only propagate as far as effect change; only send to neighbors; DV size depends on n –LS: only router with changed link sends LSPs; LSP size depends only on k, not n; does not have CTI problem

CEN 5501C - Computer Networks - Spring UF/CISE - Newman15 DV vs. LS Comparisons Computation –Dijkstra: O(nk log n); single link change may cause many routing changes; incremental update may be possible…; hash-based sorting can reduce to O(nk) –Bellman-Ford: O(nk) per pass, but many passes; DV change to D only affects cost to D Functionality –LS gives topology, allows troubleshooting, gives support to source routing, more info overall

CEN 5501C - Computer Networks - Spring UF/CISE - Newman16 DV vs. LS Robustness Both can be killed by single bad router Misbehaving LS router might: –Claim to have a non-existent link –Claim not to have an existent link –Use strange LSP seq# progression –Fail to forward datagrams correctly –Fail to forward or to ACK LSPs –Corrupt data, seq#s in others’ LSPs Black Hole DV Router –DV is all 0’s

CEN 5501C - Computer Networks - Spring UF/CISE - Newman17 DV vs. LS Convergence Speed Convergence critical to minimize transients –Critical point of comparison DV: –Looping problem –Can’t propagate new DV until it is computed –Slower LS: –New LSP recognized and forwarded quickly –Converges faster

CEN 5501C - Computer Networks - Spring UF/CISE - Newman18 Other Issues Load splitting – multipath routing Dynamic link costs Routing algorithm migration –Run both –Manual –Translation LAN links TOS –Directives –Policy Routing –Filters Partition Repair –Virtual links and tunneling