Presentation on theme: "Multihoming and Multi-path Routing"— Presentation transcript:
1 Multihoming and Multi-path Routing CS 4251: Computer Networking II Nick Feamster Spring 2008
2 Notes on Assignment OS Image: FC6-STD Don’t use static routes for any part of this assignmentPart 2 (Click): You should be able to do this entirely running Click in user spacePart 3: Don’t use a LAN for this part. OSPF over point-to-point links should work
3 Today’s Topic IP-Based Multihoming Problems with IP-based multihoming What is it?What problem is it solving? (Why multihome?)How is it implemented today (in IP)?Traffic EngineeringHow many upstream ISPs are enough?Problems with IP-based multihomingInbound route controlRouting table growthAnother approach: host-based multihoming
4 What is Multihoming?The use of redundant network links for the purposes of external connectivityCan be achieved at many layers of the protocol stack and many places in the networkMultiple network interfaces in a PCAn ISP with multiple upstream interfacesCan refer to having multiple connections toThe same ISPMultiple ISPs
5 Why Multihome? Redundancy Availability Performance Cost Interdomain traffic engineering: the process by which a multihomed network configures its network to achieve these goals
6 Redundancy Maintain connectivity in the face of: Physical connectivity problems (fiber cut, device failures, etc.)Failures in upstream ISP
7 PerformanceUse multiple network links at once to achieve higher throughput than just over a single link.Allows incoming traffic to be load-balanced.30% of traffic70% of traffic
8 Multihoming in IP Networks Today Stub AS: no transit service for other ASesNo need to use BGPMulti-homed stub AS: has connectivity to multiple immediate upstream ISPsNeed BGPNo need for a public AS numberNo need for IP prefix allocationMulti-homed transit AS: connectivity to multiple ASes and transit serviceNeed BGP, public AS number, IP prefix allocation
9 BGP or no? Advantages of static routing Advantages of BGP Cheaper/smaller routers (less true nowadays)Simpler to configureAdvantages of BGPMore control of your destiny (have providers stop announcing you)Faster/more intelligent selection of where to send outbound packets.Better debugging of net problems (you can see the Internet topology now)
10 Same Provider or Multiple? If your provider is reliable and fast, and affordably, and offers good tech-support, you may want to multi-home initially to them via some backup path (slow is better than dead).Eventually you’ll want to multi-home to different providers, to avoid failure modes due to one provider’s architecture decisions.
11 Multihomed Stub: One Link Multiple links between same pair of routers.Default routes to “border”“Stub” ISPUpstream ISPDownstream ISP’s routers configure default (“static”) routes pointing to border router.Upstream ISP advertises reachability
12 Multihomed Stub: Multiple Links Multiple links to different upstream routersBGP for load balance at edge“Stub” ISPUpstream ISPInternal routing for “hot potato”Use BGP to share loadUse private AS number (why is this OK?)As before, upstream ISP advertises prefix
13 Multihomed Stub: Multiple ISPs Upstream ISP 1“Stub” ISPUpstream ISP 2Many possibilitiesLoad sharingPrimary-backupSelective use of different ISPsRequires BGP, public AS number, etc.
14 Multihomed Transit Network ISP 1Transit ISPISP 3ISP 2BGP everywhereIncoming and outcoming trafficChallenge: balancing load on intradomain and egress links, given an offered traffic load
15 Interdomain Traffic Engineering The process by which a network operator configures the network to achieveTraffic load balanceRedundancy (primary/backup), etc.Two tasksOutbound traffic controlInbound traffic controlKey Problems: Predictability and Scalability
16 Outbound Traffic Control Easier to control than inbound trafficDestination-based routing: sender determines where the packets goControl over next-hop AS onlyCannot control selection of the entire pathProvider 1Provider 2Control with local preference
17 Outbound Traffic: Load Balancing Control routes to provider per-prefixAssign local preference across destination prefixesChange the local preference assignments over timeUseful inputs to load balancingEnd-to-end path performance dataOutbound traffic statistics per destination prefixChallenge: Getting from traffic volumes to groups of prefixes that should be assigned to each linkPremise of “intelligent route control” preoducts.
18 Inbound Traffic Control More difficult: no control over neighbors’ decisions.Three common techniques (previously discussed)AS path prependingCommunities and local preferencePrefix splittingHow does today’s paper (MONET) control inbound traffic?
19 AS Path Length Hack: Prepending TrafficAS 2AS 3AS Path: “1”AS Path: “1 1”AS 1DAttempt to control inbound trafficMake AS path length look artificially longerHow well does this work in practice vs. e.g., hacks on longest-prefix match?