Hamilton Institute TCP over e Doug Leith & Peter Clifford Hamilton Institute, Ireland.

Hamilton Institute Overview Known that b TCP uploads exhibit unfairness and sustained lockout. Known that b unfairness exists between uploads and downloads. Setting e parameters correctly resolves these problems. Analytic model of the MAC behaviour of the solution. Model TCP dynamics in an arbitrary WLAN. Round Trip Time unfairness for uploads disappears when WLAN is bottleneck. Convergence rate calculations for competing TCP flows possible. Setup b/e, single cell, infrastructure mode, DCF, TCP SACK, saturated TCP. Applicable to any situation where WLAN is the bottleneck.

Hamilton Institute TCP Upload Unfairness in b Upload ACKs Upload data AP Usual b unfairness between upload data and upload ACKs: MAC ensures fair channel access: Upload data wins n/(n+1) of transmission opportunities, Upload ACKs win only 1/(n+1)  Very few ACKs get through. AP queue (which is relatively underprovisioned) overflows, drops occur. [Pilosof, Ramjee, Shavitt, Sinha, INFOCOM 2003] TCP specific unfairness between competing flows in the same direction: ACK clocking disrupted, burstiness increased Congestion window growth hampered Possible exponential increase in retry timer  flow gets locked out. [Detti, Graziosi, Minichiello, Salsano, Sangregorio]

Hamilton Institute TCP Upload Unfairness in b

Hamilton Institute Restoring Upload Fairness using e Prevent loss of much more ACKs than data: Use separate queues for ACKs and data, Set e AIFS parameter to highly prioritise the queue containing ACKs. No ACK overload as they are created & limited by data. High priority will prevent any ACK queues from filling and avoid concerns about correct queue provisioning.

Hamilton Institute Restoring Upload Fairness using e

Hamilton Institute Advantages of the solution ACK prioritisation essentially decouples MAC layer contention from transport layer congestion control. Throughput only slightly reduced. Expected normal TCP behaviour restored. CWmin parameter still available for differential prioritisation of TCP flows.

Hamilton Institute Although TCP traffic is not saturated, the rate of ACKs is matched to the rate of data packets. Can use this fact to extend Bianchi type model fairly easily: Assume that a data packet is immediately followed by its TCP ACK, and that the TCP ACK is never involved in a collision. Agrees well with simulation ACKs never colliding is not a legitimate assumption (e.g. ACK arrives after data packet has already counted down a lot) Better to extend Battiti & Li model by using a hold state which data must count down through if there is a TCP ACK present. Replace TCP ACK saturation assumption with rate matching assumption. Both models easily adjusted to delayed ACK case. Analytic MAC Model

Hamilton Institute Analytic MAC Model

Hamilton Institute Analytic MAC Model

Hamilton Institute TCP Unfairness between uploads and downloads in b

Hamilton Institute Restoring Fairness between uploads and downloads using e At AP: Use separate queues for TCP data, Determine current number n d of destination stations, Use e TxOP burst parameter to send first packet to each of the destination stations whenever the AP gains medium access. Must prioritise the download ACKs using AIFS to avoid same problems as before. Accommodates both bursty short lived traffic and long lived traffic. If time sensitive traffic present can use CWmin parameter also to give higher priority smaller bursts. Can apply Battiti & Li model to calculate correct value of CWmin for the download ACKs.

Hamilton Institute Restoring Fairness between uploads and downloads using e

Hamilton Institute TCP Congestion Window Model Assumptions Station interface queues are large enough so that they do not empty when a TCP flow backs off its congestion window following packet loss: Wireless stations always saturated. Activity on medium is decoupled from the congestion window size. The wireless link provides stochastic service: Random packet losses. Service and loss rates independent of congestion window size.

Hamilton Institute TCP Congestion Window Evolution Model [A. Berman, R. Shorten, D. Leith, “Positive Matrices Associated with Synchronised Communications Networks”, Linear Algebra and its Applications 393, 2004]

Hamilton Institute TCP Congestion Window Evolution Model

Hamilton Institute TCP congestion window evolution model Positive real eigenvalues, Unique largest eigenvalue = 1, Network dynamics globally exponentially stable, Equilibrium point equal to eigenvector of eigenvalue 1, Equilibrium value of congestion window of flow i is Convergence rate of mean congestion windows bounded above by largest mean backoff factor.

Hamilton Institute WLAN bottleneck: Throughput When the WLAN is the bottleneck, the throughput achieved by the competing TCP upload flows is independent of the TCP AIMD parameters:

Hamilton Institute Varying bottleneck: Upload RTT Fairness When the bottleneck is the WLAN, normal upload TCP Round Trip Time unfairness does not occur.

Hamilton Institute Varying bottleneck: Changing upload fairness When the bottleneck varies in location, a sharp transition in behaviour occurs.

Hamilton Institute Convergence Rate: Downloads Convergence rate is the time that the network takes to reach steady state following a disturbance. Download TCP flows’ convergence rate is determined by the AIMD backoff factor. Usual TCP backoff factor.5: Model predicts 4 congestion epochs to convergence.

Hamilton Institute Convergence Rate: Downloads Unusual TCP backoff factor.8: Model predicts 14 congestion epochs to convergence.

Hamilton Institute Convergence Rate: Uploads, WLAN bottleneck Instant convergence independent of backoff factor. E.g. backoff factor.5:

Hamilton Institute Convergence Rate: Uploads, WLAN bottleneck Instant convergence independent of backoff factor. E.g. backoff factor.95:

Hamilton Institute Conclusions Using the power of e and the correct choice of parameters we can restore TCP fairness and expected TCP behaviour to both uploads and downloads. Analytic MAC model presented. In any similar asymmetric TCP situation, ACKs should be prioritised. Modelled TCP dynamics in an arbitrary WLAN. Round Trip Time unfairness for uploads disappears when WLAN is the bottleneck. Convergence rate calculations for competing TCP flows possible. Instant convergence for uploads when WLAN is the bottleneck. Publications Modelling TCP Dynamics in Wireless Networks [Wirelesscom 2005]. TCP Fairness in e WLANs [Wirelesscom 2005]. Using the e EDCF to Achieve TCP Upload Fairness Over WLAN Links [WiOpt 05].

Hamilton Institute Ongoing Work Mixed voice and data. Results verified experimentally. [D. Malone, A. Ng, D. Leith, SIGCOMM E-WIND ‘05] Prioritisation using CWmin: