Analyzing Cross-layer Interaction in Overlay Networks

1. Analyzing Cross-layer Interactionin Overlay Networks Srinivasan Seetharaman September 2007

2. Overlay Networks • Overlay networking helpsovercome functionalitylimitations of the Internetby forming a virtual networkover the native IP networkthat is: • Independent • Customizable

3. Service Overlay Networks Offer enhanced or new services by deploying intelligent routing schemes. Overlay link Relaying

4. Service Overlay Networks (contd.) • Characteristics: • Nodes and links are persistent • Perform overlay routing independent of native layer routing • Each Overlay path comprises one or more Overlay links, based on a certain selfish objective • Many types of services can be offered • Multicast (e.g. ESM, Overcast) • QoS (e.g. OverQoS, SON) • Security (e.g. DynaBone, SOS) • Better routes (e.g. RON, Detour, X-Bone) … and much more

5. Cross-Layer Interaction Performing dynamic routing at both overlay and native IP layers leads to: • Conflict due to mismatch or misalignment of routing objectives • Contention for limited physical resources • Functionality overlap (Both overlay layer and IP layer perform similar set of functions)

6. Cross-Layer Interaction (contd.) These issues are amplified in the presence of • Selfish motives and aggressive behavior • Lack of information about other layer • Increasing impact ( #overlays  |Traffic| )

7. Context • Native network topology • Intra-domain • Inter-domain • Attitude of native network • Restrictive • Oblivious • Cooperative

8. Thesis Organization

9. INTERACTION BETWEEN FAILURE RECOVERY IN THE NATIVEAND OVERLAY LAYERS Chapter III

10. Dual Rerouting • Each layer performs rerouting, with no knowledge of which layer leads to optimal restoration Overlay rerouting C OVERLAY1 LAYER F E H A G D A B A C E F H X Failure B NATIVE IP LAYER D G Native rerouting

11. Downside to Dual Rerouting • Overlap of functionality between layers causing • Unnecessary route changes (esp when connectivity in native network is very dynamic) • Increased probing overhead • Unawareness of other layer’s decisions leading to • Multiple simultaneous failures • Lack of flexibility and control

12. Tuning Dual Rerouting • Intra-domain (keepAlive-time = 1 sec, hold-time = 3 secs)

13. Further Improving Recovery Adjust the functioning of native layer: • Tuning the native layer keepAlive-time: keepAlive-time keepAlive-time This produces the best tradeoff between # of route changes, stabilization time and recovery time Tuning

14. INTERACTION BETWEEN OVERLAY ROUTING AND TRAFFIC ENGINEERING Chapter V

15. Repeated Non-Cooperative Game Player1: Overlay Routing - Latency-optimized paths between nodes Player2: Traffic Engineering - Optimal load-balanced routes Overlay Routing Overlay Link Latencies Overlay routes Overlay layer traffic  Native link delays  Traffic on each overlay link Traffic Engineering Native routes Background traffic  TM

16. Simulation Results TEobjective Overlayobjective Overallstability Round

17. Our goal .. is to propose strategies that • obtain the best possible performance for a particular layer • while steering the system towards a stable state.

18. Resolving Conflict – Our Approach • Assume: Each layer has a general notion of the other layer’s selfish objective • Designate leader / follower • Operate leader such that • Follower has no desire to change  Friendly • Follower has no alternative to pick  Hostile • Use history to learn desired action gradually.

19. Performance of Preemptive Strategies • We proposed four strategies that improve performance for one layer and achieve a stable operating point • Inflation factor = Steady state obj value with strategy Best obj value achieved Inflation

20. CROSS-LAYER INTERACTION OF PERFORMANCE-AWARE OVERLAYAPPLICATIONS Chapter VI

21. BitTorrent File-Sharing • Popular file-sharing application that generates a large volume of Internet traffic • Characteristics: • Service capacity increases with demand • Centralized tracker regulating neighborhood • Dynamically change active peers by choke/unchoke protocol

22. Comparison to Overlay Routing Data2 Data1 B1 B2 A3 A2 A1 AX BY

23. BitTorrent Protocol • Tit-for-tat based incentive for uploading decisions • Leecher: Unchoke the fastest uploaders • Seed: Unchoke the fastest downloaders • Popular strategy to improve performance • Optimistic unchoke: periodically look for faster peers

24. BitTorrent Dynamics B D A E Choke Choke C When bottlenecked on link L1 L2 Unchoke Request Unchoke L1 X Load distribution across links is balanced BitTorrent apps use all available b/w

25. BitTorrent Dynamics B D A E Choke Choke C When NOT bottlenecked on link L1 L2 Unchoke L1 X Load distribution across links is unbalanced

26. Cross-Layer Interaction • Operating BitTorrent disrupts load balance and can result in high max util: • This can be a problem for background traffic • Objective of native layer: • Minimize ( Max Util.) • Objective of BitTorrent: • Minimize (Overall finish time)

27. Simulation Setup Pick 100 ASes with 60% of them being non-stub ASes

28. Simulation Setup L1 D F F B C A E L2 Generate 1-50 peers. Each associating with 1-3 torrents

29. Simulation Performance Metrics • Max util across access links= MaxaE ( Xa/Ca ), E is set of all links X is the load, C is the capacity • Average finish time inflation of leechers= 1/Nl ( ’i / i ) Nl is # of leechers ’ is finish time after strategy Nli=1

30. Reducing Impact – Traffic Engg • TE can be performed across inter-domain access links, in order to minimize (Max util) • Two flavors: Ingress / Egress • Determines which access link to pick for a certain destination or source IP address

31. Reducing Impact – Traffic Engg (contd.) • Performance of a random AS (Focus AS) Applying TE does not make much difference

32. Reducing Impact – Tuning BitTorrent • Alter certain BitTorrent protocol components or tune the associated parameters • Minimal reduction of the max util • Significant inflation of finish time • Specifically, we tried each of the following: • Make peer selection random • Make piece selection random • Reduce duration of optimistic unchoking • Freeze list of unchoked peers after 10 mins • Tune the unchoking timers

33. Reducing Impact – Locality-awareness Locality-based traffic management • Give priority to peers within AS • No change to BitTorrent clients • Also try caching of requests sent outside AS

34. Reducing Impact – Bandwidth Throttling Limit bandwidth consumed by BT traffic • Popular strategy among most ASes • Involves lesser infrastructure cost

35. Cross-layer Conflict • Native layer and BitTorrent layer constantly retaliate to other layer’s disruptive behavior • Peers deploy BitTorrent Protocol Encryption to avoid detection by native layer • We develop two “friendly” BitTorrent strategies that achieve a mutually agreeable point by reducing peak load

36. A. Limit # of parallel downloads • The unchoking protocol and their timeline is uncoordinated across neighbors Average

37. A. Limit # of parallel downloads (contd.) • Reduces peak load from 0.94 to 0.852Finish time inflation is 1.1501

38. B. Avoiding common neighbors • Problem is that two peers in same AS often contact same peer outside AS • Algorithm • Perform bilateral info exchange where each peer A finds out if its neighbor B has a neighbor C inside its own AS • If yes, toss a coin to determine if we can download from this peer B (Randomization acts as a load balancing strategy)

39. B. Avoiding common neighbors (contd.) • Reduces max util from 0.94 to 0.85Finish time inflation is 1.187

40. ANALYZING INTER-DOMAIN POLICY VIOLATIONS IN OVERLAYROUTES Chapter IV

41. Inter-Domain Policy Violations Two types of violations exist Provider 2 Provider 1 Peer Client 1 Legitimate native route $$ $ Client3 A Client 2 Overlay route B C Transitviolation Exit violation

42. Measurement Results • Each transit violation has a corresponding exit violation upstream • Extent of exit policy violations in multihop paths

43. Policy Enforcement by Native Layer • As ISPs become aware of the negative impact of overlays and commence filtering, this leads to • drastic deterioration in overlay route performance • commensurate with the number of ASes enforcing policy

44. Resolving Conflict • Overlay Service Provider (OSP) adopts a combination of the following strategies for achieving good legitimate paths: • Obtain transit permit from certain AS for $T • Add new node to certain provider AS for $N • Obtain exit permit from certain AS for $E

45. Illustration of Mitigation Strategy • With no filtering, 11 13 Tier-1 provider Tier-2 provider Stub customer AS hosting overlay node Cust-Prov relation 23 21 Peering relation 22 32 31 33 Transitviolation

46. Illustration of Mitigation Strategy (contd.) • With filtering, we have no multi-hop paths 11 13 Tier-1 provider Tier-2 provider Stub customer AS hosting overlay node Cust-Prov relation 23 21 Peering relation 22 32 31 33

47. Illustration of Mitigation Strategy (contd.) Option 1: Add new overlay node to provider AS 22 Option 2: Obtain transit permit from stub AS 32 11 13 Tier-1 provider Tier-2 provider Stub customer AS hosting overlay node Cust-Prov relation 23 21 Peering relation 22 22 32 31 33

48. Objective of Mitigation Strategy For a certain budget, determine which ASes • to obtain transit permit from • to add new node to • to obtain exit permit from … so as to achieve the best possible gain Gain = Native route latency – Overlay path latency Native route latency

49. Mitigation Results • When all permit fee = P, new node fee = N Permit Add new node

50. Summary of Cross-Layer Interaction • Overlays offer valuable services needed by end-systems. But, lead to complex cross-layer interaction with potentially detrimental effects • Layer awareness is essential to reduce negative effects and to improve performance of both layers. We propose simple strategies that achieve this goal in an effective manner.