Stealth Multicast - A New Paradigm for Incremental Multicast Deployment

1. Stealth Multicast - A New Paradigm for Incremental Multicast Deployment Dr. Aaron Striegel Dept. of Computer Science & Engineering University of Notre Dame

2. Talk Overview • Information Dissemination • Motivation • Stealth Multicast • Basic Architecture • Recent work: Dynamic groups • Preliminary Results • Future Research • Inter-domain Peering • Network stack enhancement

3. Information Dissemination • Distribute rich content in a timely fashion to users • Problem: Internet evolved as point-to-point • Inefficient but currently manageable via unicasts • Two main approaches • Active involvement - Multicast • Close temporal proximity • Applications, network can participate • Community participation -> network efficiency • Passive involvement - Caching • Multiply-accessed data over time • No required participation of apps/network • Exploit existing characteristics of network • HTTP Caching • Packet-level caching

4. Multicast • Operation • Reduces packet transmissionto an efficient tree • Relies on network state forreplication • Benefits • Reduced bandwidth • N receivers << N bandwidth • Bottleneck relief • Relief close to source • Simplifies sender management • Send to group vs. individuals

5. Caching vs. Multicast • Caching • Cannot handle rapidly changing data • Data w/close temporal proximity • Easy deployment • No global participation • Multicast • Deployment problems • Global participation • Addressed by ALM • Delay-sensitive traffic, rich user base • Economic incentive • Bandwidth glut, ISP benefit • Can handle close temporal data • Group-oriented activities - synchrony is an issue

6. Current State • Caching: Yes Multicast: ?? • Several recent studies [2000, 2003] • Lackluster adoption 150 groups (1999) -> 250 (2001) • Most groups are single source (SSM) • Why have *, G, CBTs, etc.? • Harder form of multicast anyway • Key lesson from caching • Incremental deployment is key • Big-bang theory is impossible • Transition as easy as possible (FUD inertia) • Immediate benefit • Large benefit with minimal investment • Directable economic benefit • Avoid “If you build it, they will come…”

7. Motivation • Research premise • Transparent bandwidth conservation technique • Change the paradigm of multicast • Incremental deployment • Zero dependence on external forces • Immediate benefit • Exploit the redundancy in the network • Economics • Offer a significant and quantifiable benefit • Stealth Multicast • Dynamically convert packets to/from multicast • Target • Small to medium group-oriented apps 5-500 users • Delay-sensitive apps • On-line gaming, video streaming • Improve ALM-based apps

8. Stealth Multicast • Two governing principles • Externally transparent • Zero modification - application (server/client) • Zero modification - external Internet • Seamless operation • Negligible QoS impact? • Should not noticeably impact QoS • What are noticeable QoS changes? • Depends upon application • Large buffer - streaming video • On-line game - zero buffer • Informal definition • Additional delay should not make the applicationunusable versus separate unicasts

9. Stealth Multicast Model Conversion Other Domains Servers ISP Domain Clients Company LAN (Content Provider) Unicast Multicast Unicast

10. Multicast Detection VGDM - Virtual Group Detection Manager Virtual Group Mgr Disp Checksum Calculation Tree Construction H State Management Rules Filter Edge Router Incoming Traffic (Unicast only) Outgoing Traffic (Unicast+Multicast)

11. Further Examination • Benefits • Dynamically convert • Zero modification • Multicast transport via virtual groups • Exact billing • Drawbacks • Non-zero queuing delay • Aggregation effects • Imperfect virtual groups • Not a universal solution Benefit Delay Virtual Group Delay Minimum gain Maximum delay

12. Multicast Transition Options

13. Talk Overview • Information Dissemination • Motivation • Stealth Multicast • Basic Architecture • Recent work: Dynamic groups • Preliminary Results • Future Research • Inter-domain Peering • Network stack enhancement

14. Dynamic Trees • Implementation of stealth multicast • Dynamically grow/shrink physical multicast groups • Virtual group - snapshot at current time • Physical group - superset of potential clients • Defines key issues of stealth multicast • Triggers- Virtual group release • Transport - Dynamic groups • State management - Where to place state

15. Application State

16. Virtual Group Triggers • Trigger • Dilemma: Gain for waiting • When to release the virtual group • MHT - Maximum Hold Time • TSW - Time Search Width • PSW - Packet Search Width Target packet MHT TSW PSW time

17. Triggers - Continued • Triggers/thresholds • MinGS - Minimum group size • MaxGS - Maximum group size • MVG - Maximum virtual groups New group Multicast Unicast VG 0 .. MVG MaxGS MinGS VG N

18. System Balance • VGDM Limit • MVG - Maximum Virtual Groups • Hard limit - should be avoided • No multicast benefit - overloaded • Inputs • Filter effectiveness • Eliminate non-candidate traffic • Triggers - dispatch • PSW, TSW, MHT • MaxGS • Tradeoff • Capacity, QoS vs. efficiency

19. Scalability & Storage • Examine worst case constraints • Worst case delay is MHT • 10% of an RTT of 50 ms • 5 ms MHT • Actual delay is MHT / 2 • Worst case storage • PSW and TSW yield MHT, no matches • 1 Gb/s link, 1000 byte group overhead • 1 Gb/s @ 8 bit/byte * 5 ms = 625 kB • 625 kB/sec / 64 bytes = 9765 packets • 625 kB + 9765 * 1000 = ~ 11 MB

20. Multicast Transport • Issue • Members (clients) not known a priori • Dynamically construct tree • Approaches • Exhaustive tree construction • All variations, all egress points • Broadcast/hold • Costly - queuing at edge • Encapsulation-based • Embed tree inside the packet • Dynamic tree construction • Grow/shrink tree as necessary

21. Application State

22. Egress Node State

23. Control Messages

24. State Management • Issue • Unique portions of packet • Compress multiple packets for different destinations into a single packet • Dest port, dest IP • Who is responsible for exporting? • Egress A vs. Egress B vs. Egress C • Approaches • Include in packet • Similar to encapsulation-based approach • Distributed knowledge • Egress points share knowledge

25. Application-Assisted Method • Virtual group detection • Imprecise nature - best guess • Application-assist • Application knows about VGDM • Application sends 1 packet w/state to VGDM • VGDM constructs tree • Benefits • No change to the client - deployment • Precise group construction • Issues • Billing • Requires change to server app

26. Other Issues - TCP, IPSec, IPv6 • TCP • Limited benefit • Why? • Extra state • Retransmit of lost packets • Potential benefit • Web serving - initial request • Assume no cookies • CNN on 9/11 • IPSec / VPNs • IPv6

27. Simulation Studies • Simulation setup • Ns-2 simulator • Freely available simulator • GenMCast module for ns-2, GIPSE- simulation management • Network setup • Medium-sized multicast groups • On-line gaming apps - 8 to 64 clients • UDP traffic - 40 server apps • Compare various approaches • Based on VGDM location + others • Local, Stealth, None, App-Assist, ALM • Evaluation metrics • Bandwidth savings • End-user QoS

28. Effect of Clients - Link BW No savings, unicast Higher up-link cost

29. Effect of clients - Domain BW Increasing savings vs. unicast Trades B/W for client B/W

30. Effect of Clients - QoS (Delay) Limited impact of queuing

31. Other Results • Other aspect • Out of order delivery • VGDM Overload • Traffic aggregation • OS/app effect • Spacing between packets • Live traffic • Work in progress on prototype

32. Talk Overview • Information Dissemination • Motivation • Stealth Multicast • Basic Architecture • Dynamic Groups • Preliminary Results • Future Research • Inter-domain Peering • Network stack enhancement

33. Immediate Research Areas • Practical transport • Encapsulation • Dynamic groups • Compare various approaches • Fixed grouping w/hierarchy • How to find the group that maps to the egress points • Combination of groups • Broadcast w/hold • Impact of egress point sparsity • ALM • Apply ALM on a domain-wise level • Fixed vs. dynamic groups

34. Future Research Areas • Inter-domain peering • Transparent bandwidth conservation • Packet caching and stealth multicast • Edge routers of domains exchange info • Stealth multicast • Avoid conversion to/from multicast/unicast • Construct tree for new domain • Packet caching • Share cache in other domains • Issues • Billing, QoS • Resource management • Protocol / security

35. Future Research Areas • Network stack modification • Present: Minimize overhead • Avoid extra IP/TCP or IP/UDP headers • Premise • Can we increase redundancy but increase overall system performance? • Enhance network stack • Add End of Data marker - TCP • Modify sendmail / Apache to use • Issues • Benefit to the network • Downstream impact -> net system impact

36. Conclusions • Stealth multicast • New paradigm for multicast • Offers several key benefits • Solves multicast deployment issue • Zero modification outside of the domain • Inherent resource management • Offers directable economic benefit • Interesting research problems • Transport, state management • Inter-domain peering, stack optimization

37. Questions? striegel@cse.nd.edu http://www.cse.nd.edu/~striegel GenMCast Package (ns-2) http://www.cse.nd.edu/~striegel/GenMCast