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Transcript
15-744: Computer Networking L-7 Routing Issues New Routing Ideas • • • • Border Gateway Protocol (BGP) cont. Overlay networks Active networks Assigned reading • [S+99] The End-to-End Effects of Internet Path Selection • [W99] Active network vision and reality: lessons from a capsule-based system © Srinivasan Seshan, 2002 L -7; 2-6-02 2 Outline • Multi-Homing • Stability Issues • Overlay Routing • Active Networks © Srinivasan Seshan, 2002 L -7; 2-6-02 3 Multi-homing • With multi-homing, a single network has more than one connection to the Internet. • Improves reliability and performance: • Can accommodate link failure • Bandwidth is sum of links to Internet • Challenges • Getting policy right (MED, etc..) • Addressing © Srinivasan Seshan, 2002 L -7; 2-6-02 4 Multi-homing to Multiple Providers • Major issues: • Addressing • Aggregation ISP3 • Customer address space: ISP1 • Delegated by ISP1 • Delegated by ISP2 • Delegated by ISP1 and ISP2 • Obtained independently © Srinivasan Seshan, 2002 ISP2 Customer L -7; 2-6-02 5 Address Space from one ISP • Customer uses address space from ISP1 • ISP1 advertises /16 aggregate • Customer advertises /24 route to ISP2 • ISP2 relays route to ISP1 and ISP3 • ISP2-3 use /24 route • ISP1 routes directly • Problems with traffic load? © Srinivasan Seshan, 2002 ISP3 138.39/16 ISP1 ISP2 Customer 138.39.1/24 L -7; 2-6-02 6 Pitfalls • ISP1 aggregates to a /19 at border router to reduce internal tables. • ISP1 still announces /16. • ISP1 hears /24 from ISP2. • ISP1 routes packets for customer to ISP2! • Workaround: ISP1 must inject /24 into I-BGP. ISP3 138.39/16 ISP1 ISP2 138.39.0/19 Customer 138.39.1/24 © Srinivasan Seshan, 2002 L -7; 2-6-02 7 Address Space from Both ISPs • ISP1 and ISP2 continue to announce aggregates • Load sharing depends on traffic to two prefixes • Lack of reliability: if ISP1 link goes down, part of customer becomes inaccessible. • Customer may announce prefixes to both ISPs, but still problems with longest match as in case 1. © Srinivasan Seshan, 2002 L -7; 2-6-02 ISP3 ISP1 138.39.1/24 ISP2 204.70.1/24 Customer 8 Address Space Obtained Independently • Offers the most control, but at the cost of aggregation. • Still need to control paths • Many ISP’s ignore advertisements of less than /19 © Srinivasan Seshan, 2002 ISP3 ISP1 ISP2 Customer L -7; 2-6-02 9 Outline • Multi-Homing • Stability Issues • Overlay Routing • Active Networks © Srinivasan Seshan, 2002 L -7; 2-6-02 10 Signs of Routing Instability • Record of BGP messages at major exchanges • Discovered orders of magnitude larger than expected updates • Bulk were duplicate withdrawals • Stateless implementation of BGP – did not keep track of information passed to peers • Impact of few implementations • Strong frequency (30/60 sec) components • Interaction with other local routing/links etc. © Srinivasan Seshan, 2002 L -7; 2-6-02 11 Route Flap Storm • Overloaded routers fail to send Keep_Alive message and marked as down • I-BGP peers find alternate paths • Overloaded router re-establishes peering session • Must send large updates • Increased load causes more routers to fail! © Srinivasan Seshan, 2002 L -7; 2-6-02 12 Route Flap Dampening • Routers now give higher priority to BGP/Keep_Alive to avoid problem • Associate a penalty with each route • Increase when route flaps • Exponentially decay penalty with time • When penalty reaches threshold, suppress route © Srinivasan Seshan, 2002 L -7; 2-6-02 13 BGP Limitations: Oscillations AS 0 (*R,1R,2R) R AS 1 AS 2 (0R,1R,*R) © Srinivasan Seshan, 2002 (0R,*R,2R) L -7; 2-6-02 14 BGP Limitations: Oscillations AS 0 (-,*1R,2R) (*R,1R,2R) W R W W AS 1 AS 2 (*0R,-,2R) (0R,*R,2R) (0R,1R,*R) (*0R,1R,-) © Srinivasan Seshan, 2002 L -7; 2-6-02 15 BGP Limitations: Oscillations AS 0 (-,*1R,2R) (-,*1R,2R) 01R 01R R AS 1 AS 2 (-,-,*2R) (*0R,-,2R) (*0R,1R,-) (01R,*1R,-) © Srinivasan Seshan, 2002 L -7; 2-6-02 16 BGP Limitations: Oscillations AS 0 (-,-,*2R) (-,*1R,2R) 10R R AS 1 AS 2 (01R,*1R,-) (*01R,10R,-) © Srinivasan Seshan, 2002 10R L -7; 2-6-02 (-,-,*2R) (-,-,*2R) 17 BGP Limitations: Oscillations AS 0 (-,-,-) (-,-,*2R) 20R R AS 1 AS 2 (*01R,10R,-) (*01R,10R,-) © Srinivasan Seshan, 2002 20R L -7; 2-6-02 (-,-,*20R) (-,-,*2R) 18 BGP Limitations: Oscillations AS 0 (-,*12R,-) (-,-,-) 12R R AS 1 AS 2 (*01R,10R,-) (*01R,-,-) © Srinivasan Seshan, 2002 12R L -7; 2-6-02 (-,-,*20R) (-,-,*20R) 19 BGP Limitations: Oscillations AS 0 (-,*12R,21R) (-,*12R,-) 21R R AS 1 AS 2 (*01R,-,-) (*01R,-,-) © Srinivasan Seshan, 2002 21R L -7; 2-6-02 (-,-,-) (-,-,*20R) 20 BGP Oscillations • Can possible explore every possible path through network (n-1)! Combinations • Limit between update messages (MinRouteAdver) reduces exploration • Forces router to process all outstanding messages • Typical Internet failover times • New/shorter link 60 seconds • Results in simple replacement at nodes • Down link 180 seconds • Results in search of possible options • Longer link 120 seconds • Results in replacement or search based on length © Srinivasan Seshan, 2002 L -7; 2-6-02 21 Outline • Multi-Homing • Stability Issues • Overlay Routing • Active Networks © Srinivasan Seshan, 2002 L -7; 2-6-02 22 Overlay Routing • Basic idea: • Treat multiple hops through IP network as one hop in overlay network • Run routing protocol on overlay nodes • Why? • For performance – can run more clever protocol on overlay • For efficiency – can make core routers very simple • For functionality – can provide new features such as multicast, active processing, IPv6 © Srinivasan Seshan, 2002 L -7; 2-6-02 23 Overlay for Features • How do we add new features to the network? • Does every router need to support new feature? • Choices • Reprogram all routers active networks • Support new feature within an overlay • Basic technique: tunnel packets • Tunnels • IP-in-IP encapsulation • Poor interaction with firewalls, multi-path routers, etc. © Srinivasan Seshan, 2002 L -7; 2-6-02 24 Examples • IP V6 & IP Multicast • Tunnels between routers supporting feature • Mobile IP • Home agent tunnels packets to mobile host’s location • QOS • Needs some support from intermediate routers © Srinivasan Seshan, 2002 L -7; 2-6-02 25 Overlay for Efficiency • Multi-path routing • More efficient use of links or QOS • Need to be able to direct packets based on more than just destination address can be computationally expensive • What granularity? Per source? Per connection? Per packet? • Per packet re-ordering • Per source, per flow coarse grain vs. fine grain • Take advantage of relative duration of flows • Most bytes on long flows © Srinivasan Seshan, 2002 L -7; 2-6-02 26 Edge vs. Core Routers • Island of routers • Edges can perform complex computation to classify flow • Cores do extremely simple forwarding • How to communicate computation results • MPLS • Dynamic packet state © Srinivasan Seshan, 2002 L -7; 2-6-02 27 Overlay for Performance [S+99] • Why would IP routing not give good performance? • Policy routing – limits selection/advertisement of routes • Early exit/hot-potato routing – local not global incentives • Lack of performance based metrics – AS hop count is the wide area metric • How bad is it really? • Look at performance gain an overlay provides © Srinivasan Seshan, 2002 L -7; 2-6-02 28 Quantifying Performance Loss • Measure round trip time (RTT) and loss rate between pairs of hosts • ICMP rate limiting • Alternate path characteristics • 30-55% of hosts had lower latency • 10% of alternate routes have 50% lower latency • 75-85% have lower loss rates © Srinivasan Seshan, 2002 L -7; 2-6-02 29 Bandwidth Estimation • RTT & loss for multi-hop path • RTT by addition • Loss either worst or combine of hops – why? • Large number of flows combination of probabilities • Small number of flows worst hop • Bandwidth calculation • TCP bandwidth is based primarily on loss and RTT • 70-80% paths have better bandwidth • 10-20% of paths have 3x improvement © Srinivasan Seshan, 2002 L -7; 2-6-02 30 Possible Sources of Alternate Paths • A few really good or bad AS’s • No, benefit of top ten hosts not great • Better congestion or better propagation delay? • How to measure? • Propagation = 10th percentile of delays • Both contribute to improvement of performance © Srinivasan Seshan, 2002 L -7; 2-6-02 31 Overlay Challenges • “Routers” no longer have complete knowledge about link they are responsible for • How do you build efficient overlay • Probably don’t want all N2 links – which links to create? • Without direct knowledge of underlying topology how to know what’s nearby and what is efficient? © Srinivasan Seshan, 2002 L -7; 2-6-02 32 Future of Overlay • Application specific overlays • Why should overlay nodes only do routing? • Caching • Intercept requests and create responses • Transcoding • Changing content of packets to match available bandwidth • Peer-to-peer applications © Srinivasan Seshan, 2002 L -7; 2-6-02 33 Outline • Multi-Homing • Stability Issues • Overlay Routing • Active Networks © Srinivasan Seshan, 2002 L -7; 2-6-02 34 Why Active Networks? • Traditional networks route packets looking only at destination • Also, maybe source fields (e.g. multicast) • Problem • Rate of deployment of new protocols and applications is too slow • Solution • Allow computation in routers to support new protocol deployment © Srinivasan Seshan, 2002 L -7; 2-6-02 35 Active Networks • Nodes (routers) receive packets: • Perform computation based on their internal state and control information carried in packet • Forward zero or more packets to end points depending on result of the computation • Users and apps can control behavior of the routers • End result: network services richer than those by the simple IP service model © Srinivasan Seshan, 2002 L -7; 2-6-02 36 Why not IP? • Applications that do more than IP forwarding • • • • • • • • Firewalls Web proxies and caches Transcoding services Nomadic routers (mobile IP) Transport gateways (snoop) Reliable multicast (lightweight multicast, PGM) Online auctions Sensor data mixing and fusion • Active networks makes such applications easy to develop and deploy © Srinivasan Seshan, 2002 L -7; 2-6-02 37 Variations on Active Networks • Programmable routers • More flexible than current configuration mechanism • For use by administrators or privileged users • Active control • Forwarding code remains the same • Useful for management/signaling/measurement of traffic • “Active networks” • Computation occurring at the network (IP) layer of the protocol stack capsule based approach • Programming can be done by any user • Source of most active debate © Srinivasan Seshan, 2002 L -7; 2-6-02 38 Case Study: MIT ANTS System • Conventional Networks: • All routers perform same computation • Active Networks: • Routers have same runtime system • Tradeoffs between functionality, performance and security © Srinivasan Seshan, 2002 L -7; 2-6-02 39 System Components • Capsules • Active Nodes: • Execute capsules of protocol and maintain protocol state • Provide capsule execution API and safety using OS/language techniques • Code Distribution Mechanism • Ensure capsule processing routines automatically/dynamically transfer to node as needed © Srinivasan Seshan, 2002 L -7; 2-6-02 40 Capsules • Each user/flow programs router to handle its own packets • Code sent along with packets • Code sent by reference • Protocol: • Capsules that share the same processing code • May share state in the network • Capsule ID is MD5 of code © Srinivasan Seshan, 2002 L -7; 2-6-02 41 Capsules Active Node IP Router Capsule IP Header Version Active Node Capsule Type Previous Address Type Dependent Header Files Data ANTS-specific header • Capsules are forwarded past normal IP routers © Srinivasan Seshan, 2002 L -7; 2-6-02 42 Capsules Request for code Active Node 1 IP Router Capsule Active Node 2 Capsule • When node receives capsule uses “type” to determine code to run • If no code at node requests code from “previous address” node • Likely to have code since it was recently used © Srinivasan Seshan, 2002 L -7; 2-6-02 43 Capsules Code Sent Active Node 1 IP Router Active Node 2 Capsule Capsule • Code is transferred from previous node • Size limited to 16KB • Code is signed by trusted authority (e.g. IETF) to guarantee reasonable global resource use © Srinivasan Seshan, 2002 L -7; 2-6-02 44 Research Questions • Execution environments • What can capsule code access/do? • Safety, security & resource sharing • How isolate capsules from other flows, resources? • Performance • Will active code slow the network? • Applications • What type of applications/protocols does this enable? © Srinivasan Seshan, 2002 L -7; 2-6-02 45 Functions Provided by Capsule • Environment Access • Querying node address, time, routing tables • Capsule Manipulation • Access header and payload • Control Operations • Create, forward and suppress capsules • How to control creation of new capsules? • Storage • Soft-state cache of app-defined objects © Srinivasan Seshan, 2002 L -7; 2-6-02 46 Safety, Resource Mgt, Support • Safety: • Provided by mobile code technology (e.g. Java) • Resource Management: • Node OS monitors capsule resource consumption • Support: • If node doesn’t have capsule code, retrieve from somewhere on path © Srinivasan Seshan, 2002 L -7; 2-6-02 47 Applications/Protocols • Limitations • Expressible limited by execution environment • Compact less than 16KB • Fast aborted if slower than forwarding rate • Incremental not all nodes will be active • Proof by example • Host mobility, multicast, path MTU, Web cache routing, etc. © Srinivasan Seshan, 2002 L -7; 2-6-02 48 Discussion • Active nodes present lots of applications with a desirable architecture • Key questions • Is all this necessary at the forwarding level of the network? • Is ease of deploying new apps/services and protocols a reality? © Srinivasan Seshan, 2002 L -7; 2-6-02 49 Next Lecture: Mobile Routing • Mobile IP • Ad-hoc networks • Assigned reading • [Joh96] Scalable Support for Transparent Mobile Host Internetworking • [BMJ+98] A Performance Comparison of MultiHop Wireless Ad Hoc Routing Protocols © Srinivasan Seshan, 2002 L -7; 2-6-02 50