1. 15-744: Computer Networking L-7 Routing Issues

2. L -7; 2-6-02© Srinivasan Seshan, 20022 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

3. L -7; 2-6-02© Srinivasan Seshan, 20023 Outline Multi-Homing Stability Issues Overlay Routing Active Networks

4. L -7; 2-6-02© Srinivasan Seshan, 20024 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

5. L -7; 2-6-02© Srinivasan Seshan, 20025 Multi-homing to Multiple Providers Major issues: Addressing Aggregation Customer address space: Delegated by ISP1 Delegated by ISP2 Delegated by ISP1 and ISP2 Obtained independently ISP1ISP2 ISP3 Customer

6. L -7; 2-6-02© Srinivasan Seshan, 20026 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? 138.39/16 138.39.1/24 ISP1ISP2 ISP3 Customer

7. L -7; 2-6-02© Srinivasan Seshan, 20027 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. 138.39.0/19 138.39/16 ISP1ISP2 ISP3 Customer 138.39.1/24

8. L -7; 2-6-02© Srinivasan Seshan, 20028 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. 138.39.1/24 204.70.1/24 ISP1ISP2 ISP3 Customer

9. L -7; 2-6-02© Srinivasan Seshan, 20029 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 ISP1ISP2 ISP3 Customer

10. L -7; 2-6-02© Srinivasan Seshan, 200210 Outline Multi-Homing Stability Issues Overlay Routing Active Networks

11. L -7; 2-6-02© Srinivasan Seshan, 200211 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.

12. L -7; 2-6-02© Srinivasan Seshan, 200212 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!

13. L -7; 2-6-02© Srinivasan Seshan, 200213 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

14. L -7; 2-6-02© Srinivasan Seshan, 200214 BGP Limitations: Oscillations AS 0 AS 2 AS 1 R (*R,1R,2R) (0R,*R,2R)(0R,1R,*R)

15. L -7; 2-6-02© Srinivasan Seshan, 200215 BGP Limitations: Oscillations AS 0 AS 2 AS 1 R (-,*1R,2R) (*0R,-,2R) (*0R,1R,-) W W W    (*R,1R,2R) (0R,*R,2R) (0R,1R,*R)

16. L -7; 2-6-02© Srinivasan Seshan, 200216 BGP Limitations: Oscillations AS 0 AS 2 AS 1 R (-,*1R,2R) (-,-,*2R) (01R,*1R,-) 01R    (-,*1R,2R) (*0R,-,2R) (*0R,1R,-)

17. L -7; 2-6-02© Srinivasan Seshan, 200217 BGP Limitations: Oscillations AS 0 AS 2 AS 1 R (-,-,*2R) (*01R,10R,-) 10R    (-,*1R,2R) (-,-,*2R) (01R,*1R,-)

18. L -7; 2-6-02© Srinivasan Seshan, 200218 BGP Limitations: Oscillations AS 0 AS 2 AS 1 R (-,-,-) (-,-,*20R) (*01R,10R,-) 20R    (-,-,*2R) (*01R,10R,-)

19. L -7; 2-6-02© Srinivasan Seshan, 200219 BGP Limitations: Oscillations AS 0 AS 2 AS 1 R (-,*12R,-) (-,-,*20R) (*01R,-,-) 12R    (-,-,-) (-,-,*20R)(*01R,10R,-)

20. L -7; 2-6-02© Srinivasan Seshan, 200220 BGP Limitations: Oscillations AS 0 AS 2 AS 1 R (-,*12R,21R) (-,-,-)(*01R,-,-) 21R (-,*12R,-) (-,-,*20R)(*01R,-,-)  

21. L -7; 2-6-02© Srinivasan Seshan, 200221 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

22. L -7; 2-6-02© Srinivasan Seshan, 200222 Outline Multi-Homing Stability Issues Overlay Routing Active Networks

23. L -7; 2-6-02© Srinivasan Seshan, 200223 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

24. L -7; 2-6-02© Srinivasan Seshan, 200224 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.

25. L -7; 2-6-02© Srinivasan Seshan, 200225 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

26. L -7; 2-6-02© Srinivasan Seshan, 200226 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

27. L -7; 2-6-02© Srinivasan Seshan, 200227 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

28. L -7; 2-6-02© Srinivasan Seshan, 200228 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

29. L -7; 2-6-02© Srinivasan Seshan, 200229 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

30. L -7; 2-6-02© Srinivasan Seshan, 200230 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

31. L -7; 2-6-02© Srinivasan Seshan, 200231 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

32. L -7; 2-6-02© Srinivasan Seshan, 200232 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 N 2 links – which links to create? Without direct knowledge of underlying topology how to know what’s nearby and what is efficient?

33. L -7; 2-6-02© Srinivasan Seshan, 200233 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

34. L -7; 2-6-02© Srinivasan Seshan, 200234 Outline Multi-Homing Stability Issues Overlay Routing Active Networks

35. L -7; 2-6-02© Srinivasan Seshan, 200235 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

36. L -7; 2-6-02© Srinivasan Seshan, 200236 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

37. L -7; 2-6-02© Srinivasan Seshan, 200237 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

38. L -7; 2-6-02© Srinivasan Seshan, 200238 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

39. L -7; 2-6-02© Srinivasan Seshan, 200239 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

40. L -7; 2-6-02© Srinivasan Seshan, 200240 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

41. L -7; 2-6-02© Srinivasan Seshan, 200241 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

42. L -7; 2-6-02© Srinivasan Seshan, 200242 Capsules Active Node IP Router Active Node Capsule IP HeaderVersionDataType Previous Address Type Dependent Header Files ANTS-specific header Capsules are forwarded past normal IP routers

43. L -7; 2-6-02© Srinivasan Seshan, 200243 Capsules Active Node 1 IP Router Active Node 2 Capsule Request for code 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

44. L -7; 2-6-02© Srinivasan Seshan, 200244 Capsules Active Node 1 IP Router Active Node 2 Capsule Code Sent 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

45. L -7; 2-6-02© Srinivasan Seshan, 200245 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?

46. L -7; 2-6-02© Srinivasan Seshan, 200246 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

47. L -7; 2-6-02© Srinivasan Seshan, 200247 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

48. L -7; 2-6-02© Srinivasan Seshan, 200248 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.

49. L -7; 2-6-02© Srinivasan Seshan, 200249 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?

50. L -7; 2-6-02© Srinivasan Seshan, 200250 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 Multi- Hop Wireless Ad Hoc Routing Protocols