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Split-TCP: State of the Union Address

Split-TCP: State of the Union Address. Dan Berger 03/03/03. Adgenda. Proxied or “Split” TCP Conceptual Overview Previous Investigation Current Work Results and Analysis Conclusion. Conceptual Overview. The Problem(s) TCP can’t distinguish link failure from congestion.

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Split-TCP: State of the Union Address

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  1. Split-TCP: State of the Union Address Dan Berger 03/03/03

  2. Adgenda • Proxied or “Split” TCP Conceptual Overview • Previous Investigation • Current Work • Results and Analysis • Conclusion

  3. Conceptual Overview • The Problem(s) • TCP can’t distinguish link failure from congestion. • Results in lowered throughput • Effect is magnified on longer (hop-count) connections

  4. Concepts (Cont.) • The Proposal • Designate nodes along the path to be packet-level proxies. • These proxies buffer segments until they’ve been received (and acknowledged) by the next proxy (or destination) • If the packet isn’t ack’d in “reasonable” time – they resend.

  5. Details, Details… • Proxy selection must be distributed and dynamic, to accommodate mobility in the network. • The proxies should be as stateless as possible – specifically, keeping per-flow state is bad.

  6. Previous Investigation • In a simple string topology “Split-TCP is able to provide a better overall throughput than TCP, up to about 15%” • In a complex topology with mobility, “the total throughput improves with the use of proxies by about 5% to about 30%”

  7. Current Work • The initial goal was to implement a linux kernel implementation. • This work was stymied by a lack of detailed understanding of the desired behavior of the protocol. • Work done last quarter found issues with the current simulation model which suggested it needed to be revisited.

  8. Forward, march! • We decided to turn our attention to building a more accurate and usable simulation model in ns. • The specific issues in the existing model we wanted to address were: • Lack of end-to-end window management • Lack of dynamic proxy selection

  9. s t Proxy-To-Proxy Window Mgmt. • The initial simulation model literally decomposed a long TCP connection into multiple shorter connections. • This had undesired (and unforeseen) consequences.

  10. Static Proxy Selection • Additionally, this decomposition was done using global routing knowledge. • To accommodate mobility, the proxy selection code was re-run periodically. • This meant that new TCP connections were established, and the old ones allowed to “drain.”

  11. The New Model • The new simulation model consists of three components: • SplitTCPSource • SplitTCPWedge • SplitTCPSink • The Source and Sink are simply specializations of TCPSource and TCPSink, respectively.

  12. SplitTCPWedge • In order to perform dynamic proxy selection, one needed to be able to inspect each packet received by a node. • Then the decision to proxy can be based on whatever criteria seem appropriate. • All “Split-TCP enabled” nodes run the wedge code.

  13. An Anonymous NS Node

  14. Better, Stronger, Faster SplitTCPWedge

  15. Model Summary • This new simulation model uses a TCP session established between the source and target, and dynamic proxy selection is performed along the path. s t

  16. End-To-End Window Mgmt. • The TCP session performs window management “as usual” – but control can be tweaked both manually as well as by the Wedge. • For instance, end-to-end ACKs can be skipped, the congestion window can be managed based on inter-proxy ACKs, etc.

  17. Proxy Selection • Each node decides, independently, (currently based on hops since last proxy and their current backlog) if they should proxy a packet. • Proxying does not keep per-flow state. • Retransmissions are scheduled based on the current inter-proxy round trip time estimate.

  18. Results • Current simulations show a  10% degradation in throughput vs. Standard TCP. • With parameter tweaking, this degradation varies, but Split TCP never equals standard TCP.

  19. Analysis • One identified cause of the dramatically different behavior of the old and new simulation models relates to out-of-order packet delivery. • In the old model, the packet stream was “reassembled” at each proxy – and bytes were only forwarded once they arrived in order. • Per-flow state at each proxy. • In the new model, this is not the case.

  20. Parameterization • By tuning various parameters – including initial congestion window, window growth factors, etc – the throughput of Split TCP can be improved. • There are over 150 tuning parameters in the “stock” NS TCP code. If they were all just binary parameters that’s still 2150 possible configurations!

  21. Conclusion • We’re trying to determine rational next steps: • Do we continue the search for the “magic” set of parameters? • Do we examine the behavior of the new model in scenarios with mobility? • Few answers, many more questions.

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