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1 Internet Routing Instability and it's Origins Ilia Ferdman Lilia Tsvetinovich
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2 Abstract zProblems discussed yInternet Routing Instability yOrigins of Internet Routing Instability
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3 Internet Routing Instability zDefined as rapid fluctuation of network reachability and topology information zAlso referred as “route flap”
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4 Origins of Routing Instability zRouter configuration errors zTransient physical and data link problems zSoftware bugs
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5 Primary Effects zInstability can lead to yIncreased packet loss yDelays in the time for network convergence yAdditional resource overhead(memory, CPU) zImminent “death of the Internet”
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6 Internet Structure zComprised of interconnected regional and national backbones zLarge public exchange points are the “core” of the Internet
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7 Internet Structure (cont.) zBSP – Backbone service provider zEP – Exchange points BSP 1 BSP 1 EP2EP2 EP2EP2 EP4EP4 EP4EP4 EP3EP3 EP3EP3 EP1EP1 EP1EP1 BSP 3 BSP 3 BSP 2 BSP 2 BSP 5 BSP 5 BSP 7 BSP 7 BSP 4 BSP 4 BSP 6 BSP 6
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8 Internet Structure (cont.) zBackbone service providers exchange yTraffic yRouting information zBackbones in the core maintain default- free routing table
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9 Internet Structure (cont.) zAutonomous systems yDistinct routing policies yConnect to private or public exchange points zPeer border routers in AS exchange reachability information to prefixes zPrefixes – IP address blocks zExchange information through BGP
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10 Border Gateway Protocol zBGP yIncremental protocol yUses TCP yLimits distribution of routing information z IGRP, OSPF, etc yInterior protocols yUse datagram service yFlood network with all known routing table entries zBGP vs. IGRP & OSPF
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11 BGP (cont.) zAllows configuration for policy (MED) zMED – Multi Exit Descriptor zASPATH - list of AS numbers
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12 BGP (cont.) zAllows configuration for policy (MED) zMED – Multi Exit Descriptor zASPATH - list of AS numbers zBGP updates yAnnouncements yWithdrawals
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13 BGP updates - Withdrawals zExplicit Withdrawals zImplicit Withdrawals R1R1 R1R1 R2R2 R2R2 R3R3 R3R3 R1R1 R1R1 R2R2 R2R2
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14 BGP updates - Withdrawals zExplicit Withdrawals R1R1 R1R1 R2R2 R2R2 R3R3 R3R3
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15 BGP updates - Withdrawals zImplicit Withdrawals R1R1 R1R1 R2R2 R2R2
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16 BGP (cont.) zAllows configuration for policy (MED) zASPATH zBGP updates yAnnouncements yWithdrawals zStable wide-area networks performance expectations
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17 Methodology zSince January 1996, 9 months zRouting Arbiter project zPublic exchange points: AADS, Mae-East, Mae-West, PacBell, Sprint
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18 Methodology
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19 Methodology zMae-East backbone service providers: ANS, BBN, MCI, Sprint and UUNet zRAP – Routing Arbiter Project zRoute Servers used to collect information z12 gigabytes of compressed data
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20 Types of Routing Instability zBGP updates Instability rate zForwarding instability zRouting Policy Fluctuations zPathological updates zInstability – instance of forwarding instability or policy fluctuations
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21 Possible impacts zIncrease in cache misses zCPU & memory problems zRoute “flap storm” zForwarding loops
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22 Route Caching Architecture zRouting table cache of destination and next-hop lookups zRouting table is too big to keep it in main memory zInstability causes increase in cache misses zLoad on CPU
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23 Route Caching Architecture zPossible solution: yFull routing table in main memory
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24 Possible impacts zIncrease in cache misses zCPU & memory problems zRoute “flap storm” zForwarding loops
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25 CPU & Memory Problems zNormally could manage the router’s computational needs zInstability places large demands on a router’s CPU zKeep-Alive packets delayed
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26 Possible impacts zIncrease in cache misses zCPU & memory problems zRoute “flap storm” zForwarding loops
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27 Route “flap storm” Overloaded router marked as unreachable Peer routers choose alternative paths Peers update their peers “Down” router recovers and tries to re- initiate peering sessions Large state dump transmissions are generated Increased load causes more routers to fail
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28 Route “flap storm” (cont.) zPossible solution: yHigher priority to Keep-Alive messages
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29 Possible impacts zIncrease in cache misses zCPU & memory problems zRoute “flap storm” zForwarding loops
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30 Forwarding loops zDefined as steady-state cyclic transmission of user data between a set of peers zLoop verification by checking ASPATH zUnconstrained routing policies
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31 BGP Update Types zWA Different – WADiff zAA Different – AADiff zWA Duplicate – WADup zAA Duplicate – AADup zWW Duplicate – WWDup
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32 BGP Update Types - WADiff zExplicit withdrawal zUnreachable route is replaced by alternative route zASPATH or next-hop attribute differs zForwarding instability
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33 BGP Update Types - AADiff zImplicit withdrawal zRoute is unreachable zAlternative path becomes available zForwarding instability
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34 WADiff and AADiff zWADiff yExplicit withdrawal yForwarding instability z AADiff yImplicit withdrawal yForwarding instability zRoute is replaced by alternative one
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35 BGP Update Types - WADup zExplicit withdrawal zRoute explicitly withdrawn and then re- announced a reachable zTransient topological problems (link or router) zForwarding instability or Pathological behavior
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36 BGP Update Types - AADup zImplicit withdrawal zRoute is implicitly withdrawn and replaced by it’s duplicate zDuplicate route does not differ in ASPATH or next-hop attribute information zPolicy fluctuations and Pathological behavior
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37 WADup and AADup zWADup yExplicit withdrawal yPathological behavior Forwarding instability z AADup yImplicit withdrawal yPathological behavior Policy fluctuations
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38 BGP Update Types - WWDup zRepeated BGP withdrawals for a prefix that is unreachable zPathological behavior
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39 BGP Update Types - Summary Explicit Withdrawal Implicit Withdrawal Forwarding instability Policy Fluctuations Patholo gical Behavi or WADiff V V AADiff VV WADup V V V AADup V VV WWDup – – V
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40 BGP Update Types
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41 WW Duplicate zTransmitted by routers of AS that never previously announced reachability for the withdrawn prefixes
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42 Let’s have a break
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43 Internet Routing Instability and it's Origins Ilia Ferdman Lilia Tsvetinovich
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44 Instability Origins zHardware configuration problems zSoftware bugs problems zMulti – Homing sites zBGP implementation problems
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45 Instability Origins – Hardware configuration zInternet growth -> Traffic growth -> New hardware need zOld Hardware -> Increase in number of updates : yCPU overload yLink failures zSmall Service Providers use old hardware
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46 Instability Origins – Hardware configuration zCache architecture yNot all prefix table in memory yIncrease in number of updates -> Increase in number of cache misses
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47 Instability Origins – Software bugs zUse of old or not configured software is the reason for Routing Instability zSmall Service Providers use old software
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48 Instability Origins – Multi – Homing sites zEnd-sites connect to Internet via multiple Service Providers(SP) zMulti-Homed customer prefixes require global visibility zRouters maintain longer prefixes SP1 SP2 SP3 Site
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49 Instability Origins – BGP implementation zStateless BGP yAnnouncements or withdrawals are send without check yO (N*U) additional updates xN – number of routers xU – number of updates zThere are better implementations
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50 Instability Origins – BGP implementation zMisconfigured interaction between different gateway protocols R1 BGP R1 BGP R2 OSPF R2 OSPF
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51 Possible solutions zRoute Server zRoute Dampening Algorithm zAggregation
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52 Possible solutions – Route Servers zR1,R2,R3,R4,R5 – routers zR.S. – rout server zR.S. collects BGP information from routers R3R3 R3R3 R2R2 R2R2 R5R5 R5R5 R4R4 R4R4 R1R1 R1R1
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53 Possible solutions – Route Servers zR1,R2,R3,R4,R5 – routers zR.S. – route server zR.S. collects BGP information from routers R3R3 R3R3 R2R2 R2R2 R5R5 R5R5 R4R4 R4R4 R1R1 R1R1 R. S.
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54 Possible solutions – Route Servers zDo not forward network traffic zPeer with service providers zProvide aggregate BGP information zUnique platform for statistic collection and monitoring
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55 Possible solutions – Route Dampening Algorithm z“Hold-down” frequent updates zAnnouncements about new networks delayed zDraconian version of enforcing stability
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56 Possible solutions – Aggregation zAggregation also calls supernetting zConcept of Aggregation: Several networks to supernetwork
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57 Possible solutions – Aggregation Network 1 Network 1 Network 3 Network 3 Network 2 Network 2 Network 4 Network 4
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58 Possible solutions – Aggregation Super network
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59 Possible solutions – Aggregation (cont.) zAdvantage: ydecrease in number of global visible addresses ydecrease in number of updates zProblems: yNo correlation between Service Providers yMulti – Homing sites prevent aggregation
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60 Statistic zDefault – free table size : 45.000 prefixes zNumber of updates per day : between 3 to 6 millions z99 percent of routing information is pathologic zOnly 10 percent of routers send one or less WADiff per day zOnly 20 percent of routers send one or less AADiff per day
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61 Statistic – updates per period from 04/96 to 10/96
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62 Statistic – updates per week
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63 Results zToo many BGP updates exchanged zPathological updates are dominated zDaily and weekly cyclic trends zInstability happens to everybody zForwarding instability the main contributor
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64 Statistic – updates from 96 to 98
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65 Things that you don’t need to know about Routing Instability zRouting Instability is rapid fluctuation of network reachability and topology information zThere are three types of Instability: forwarding instability, policy fluctuation, pathological updates
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66 Things that you don’t need to know about Routing Instability (cont.) zInstability can lead to many unpleasant things such as yIncreased packet loss yDelays in the time for network convergence yAdditional resource overhead(memory, CPU)
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67 Things that you don’t need to know about Routing Instability (cont.) zThe possible origins of routing instability are yRouter configuration errors yTransient physical and data link problems ySoftware bugs
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68 Things that you don’t need to know about Routing Instability (cont.) zThere are several solutions: yRoute Server yRoute Dampening Algorithm yAggregation
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69 The END
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