SWiFT Status Report

Dan Berger <dberger@cs.ucr.edu>

060203

Outline

Split TCP: The AftermathFalse Link Failures: ns-2, The RevengeCurrent Work: 802.11 RendezvousConclusion

Split TCP: The Aftermath

Recall my last update: SplitTCP, while intuitively attractive; did

not perform as expected when modeled accurately.

Since then a handful of additional experiments were performed to strengthen understanding.This work is now indefinitely on hold.

False Link Failures

Subsequent discussions focused attention on false link failures. Incorrect link abandonment due to

congestion.

Extensive simulations were performed: Static string of various lengths > 3* Looking at the number and distribution

of false link failures

* This becomes important – stay tuned

ns-2, The Revenge

Just when you thought it was safe to run a simulation…Analysis of the simulation results revealed surprising patterns. FLF’s were occurring with almost

predictable periodicity. Theories abounded: Long range dependence? Invaders from Planet X?

A Mystery Solved

A simulation of a three node provided a key insight: FLF’s shouldn’t occur – but were.

Something was rotten in Denmark.After consultation with Vikram, we isolated the problem to a recent (2/03) change to the NS 802.11 code.

Nothing to see here, move along…

A fix was developed and tested – once we were satisfied it was submitted to the ns-2 maintainers.Unfortunately they still haven’t applied it to the official sources.It’s available on my cs web page.

Back to your regularly scheduled programming

Attention was returned to False Link Failures. They’re not, as far as we can tell,

periodic But they are hugely destabilizing.

Eliminating* FLF’s saw a 100% improvement in goodput.

*Via a static topology, simulation-only hack

Current Work: Rendezvous

During discussion, Prasun suggested modifying the MAC to exchange packets in an effort to reduce channel contention. Total data transmission time is bounded.

An initial implementation has been done, and showed positive results (30% improvement @ 20 hops).

Rendezvous802.11 802.11 Rendezvous*

RTS

CTS

Data

ACK

RTS

CTS

ACK

Data(1)

Data(2)

ACK

Rendezvous: Current State

Similar results were published in ACM Mobile Networks & Applications in 2000. We’re assessing the appropriate

reaction.

The model has been iterated on once, and has (at least) one more iteration before it’s general.

Conclusion

This, and similar work by Frank is being targeted for an Infocomm paper.

Questions?