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Slide 1Network OverlaysUses of overlaysEarly OverlaysTunneling IllustratedWidely known overlaysVirtual Dial-up Example (1)Virtual Dial-up Example (2)Logical Network CreationOther uses for overlaysBasic idea…ComponentsFault-toleranceWhy an overlay?Outcome?Resilient Overlay NetworksFinal example for today: P6PHow P6P worksHow P6P worksHow well does it work?Summary: OverlaysNetwork Overlays.Ken BirmanCornell University. CS5410 Fall 2008.Network OverlaysConsider the InternetIt creates the illusion of a fully connected n x n world of addressable endpointsIn reality, packets must route through a complex infrastructure, but the end user doesn’t see that infrastructureOverlay concept takes this one step furtherWe focus on some application… and create a dedicated personal internet just for itThe dedicated network might have special propertiesUses of overlaysLoad balancing, other forms of quality of serviceDistributing files or data down some form of tree structure (allows massive fanouts without forcing any single node to send huge numbers of copies)Route around congestionContent routing: packets routed on the basis of the data inside them (could look at fields, or might do a whole xquery)Publish subscribe: packets route on the basis of topicDHT: In fact, even a DHT is an overlay!Early OverlaysThe first overlays were really Internet “tunnels”Idea was to encapsulate IP packets in some other network standard… then route them over a link that used non-IP technology… then unpack them and drop them back into IP-landThen we started to see fancier tunnelsIP multicast over TCPIPv6 over IPv4Tunneling IllustratedRouter AWorkstation XRouter BWorkstation YOriginal IPpacket dest YStep 1.Original, unroutableIP Packet sent to routerStep 2Original IPpacketencapsulatedin another IPpacketOriginal IPpacketNew IPPacketTunnelStep 3Original packetextracted, sentto destinationOriginal IPpacket dest YTunnelWidely known overlaysVirtual private networksEnd point computers need to have some form of certificate that they use to identify themselvesTypically: each machine has a private key and a public keyWith this can send “unforgeable” encrypted dataSo: edge machine authenticates itself to the VPN server, which sends back the current secret key of the VPN (a symmetric key)The edge machine tunnels traffic encrypted with the VPN key via the VPN server, which acts as a routerVirtual Dial-up Example (1)Worker dials ISP to get basic IP serviceWorker creates his own tunnel to Home NetworkInternetTunnelGatewayGateway(NAS)Internet Service ProviderPublic Switched Telephone Network (PSTN)WorkerMachineHome NetworkVirtual Dial-up Example (2)Remote worker connects to Home Network through ISP created tunnelAllows wholesale dial-upInternetTunnelGateway(NAC)Gateway(NAS)Internet Service ProviderPublic Switched Telephone Network (PSTN)Home NetworkLogical Network CreationRemote networks 1 and 2 create a logical networkSecure communication at lowest levelInternetTunnelGateway(NAC)Gateway(NAS)Network 1Network 2Other uses for overlaysNew York Stock Exchange Quote Distribution SystemBuilt around 1995Issue: needed a customizable way to route quotes to overhead displays over internal networkRequired fault-toleranceContent sources ran at higher speeds than most display end systems could sustainBasic idea…Build a routing tree for quotesThen replicate it for fault-tolerancesource    source      ComponentsThe source systems were the five or six “clearing” machines used by the NYSE to capture trades, bids, offered pricesThe routers were inexpensive dedicated computers with dual ethernet cards, one for each networkEach network was a separate ethernet with distinct IP addresses and no automated routingThe overhead displays were basically workstationsFault-toleranceThey used a virtual synchrony package (Isis) to replicate state within router pairs, and to track subscription patterns… lots of groupssource       Replicate router stateReplicate subscription patternsWhy an overlay?Isis wasn’t capable of supporting very large groups with very high data ratesSo sending the actual trades/quotes wasn’t feasibleTotal number of routers was about 75… serving 1000 or more display systemsBy building a TCP-based overlay and using the Isis groups “out of band”, Isis wasn’t on the critical pathIsis knew about the dual IP network… TCP didn’t.Outcome?The solution was completely robust and was used from 1995 until mid 2006During that decade there were many failures and even entire network outages But the NYSE “rode them all out” absolutely unperturbed: traders saw no glitches at allSo here the overlay plays two rolesOverlay carries the heavy communication burdenOne overlay for each IP networkResilient Overlay NetworksRon Slideshttp://nms.lcs.mit.edu/ron/Final example for today: P6PResearch by Li Dong Zhou and Van RenesseIssue addressed by this workPeople want to use IPv6But the Internet itself is locked into IPv4So idea is to support IPv6 as an overlayFeatures of IPv6?Very long addresses (64 bits)Address doesn’t reveal location (unlike IPv4)How P6P worksAssumes two worldsAn IPv6 world, invisible to themAn IPv4 world, where P6P livesSome IPv6 nodes live in both, call them “internal gateway nodes”These have both an IPv6 and an IPv4 addressP6P itself implemented by what they call “external gateway” nodes that run in the IPv4 networkHow P6P worksThey designed a DHT based on ChordEach IPv6 node must have an associated IG So treat the (IPv6,IPv4) tuple as a (key,value) pair!IPv6 address is an index into ChordNew IPv6 node would create a new (key,value) pairTo send an IPv6 packet, look up the IPv4 helper node, then forward the IPv6 packet to the helperCache information for reusePlus many optimizations, and a security architecture…How well does it work?They designed a detailed simulation and looked at random traffic (perhaps unrealistic…)In this model, P6P performed extremely wellRapid routingFairly quick response when mobile nodes changed their associated IG nodeSome false routing, but then automatically recoversSeems to be a very practical way to roll IPv6 out…Summary: OverlaysWe’ve seen a


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CORNELL CS 5410 - Network Overlays

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