TCP with Variance Control for Multihop IEEE Wireless Networks Jiwei Chen, Mario Gerla, Yeng-zhong Lee

Introduction “Ad hoc” wireless TCP problems still unresolved –Prone to Congestion,Capture, efficiency, fairness Focus here is on Congestion –Existing solutions that work use cross layers (MAC, net layer, TCP) TCP Adaptive pacing –E2E current solutions TCP Small (unfair); We follow E2E –New approach: use NOT only pkt loss, but throughput variance as early congestion indication.

Problems TCP typically overloads multihop wireless networks and causes high IEEE MAC contention Approaches: –Optimal congestion window setting –Small window increase TCP fairness in wireless networks is typically bad –Rate control is helpful to alleviate the problem

How does TCP work? TCP protocol dynamically increases the congestion window CWIN at each round –However, if congestion is detected, the TCP source reduces CWIN (by half, in TCP Reno) –In most TCP varieties, congestion is “signalled” to source by packet loss This strategy is effective in the Wired Internet: –Congestion immediately causes buffer overflow and packet loss –The loss is quickly reported to source

Why packet loss not enough in wireless nets? In wireless (802.11), we have ARQ: –Packet is retx up to 5 times; timer doubled each time –Only after 5 retx the packet is dropped and source learns about it –Packet drop comes too late! –Pkt drop feedback latency leads to “oscillatory” behavior, slow convergence, unfairness, capture etc Experimental result on wired and wireless links in next 2 slides

CBR experiment: Throughput Variance in Wired Network Instantaneous CBR throughput vs. different rates on a wired link with 500 Kbps capacity

Rate Variance in Wireless Networks Instantaneous CBR throughput vs. different rate on a 4 hop chain. Channel data rate = 2Mbps

Why High Rate Variance in nets? When congestion builds up (due to excessive self rate or too many connections): –Packets are dropped due to collisions and are retransmitted –Delay between pkts and ACKs fluctuates –Observed data rate fluctuates –And, data rate variance increases –All this happens before any packet is lost! Rate variance more timely congestion indicator than packet loss Note: Variance increases also when packets are “randomly” dropped, say, because of jamming - more later..

TCP-VAR Combined: CWND Cong Control + Rate Control Congestion Control: –Linearly Increase cwnd if throughput variance is small –Multiplicatively decrease cwnd if throughput variance is large –If packet loss detected, cut cwnd by half Rate control: –Use pacing with rate given by:

Performance Evaluation Compare TCP-VAR, TCP adaptive pacing, TCP small and TCP NewReno One or multiple TCP flow(s) over a chain topology Channel bandwidth 2 Mbps Packet size 1460 bytes AODV routing is used.

Comparison set Small window increase (TCP-Small) Congestion Window Limit based on the round trip hop count (1 pkt every 4 hops..) The linear increase parameter is a small value, say 0.01, i.e w=w+0.01/w (Mobihoc 05) TCP Adaptive Pacing (TCP-AP) 4 hop pacing (Mobihoc 05) Same assumptions as above.

Related Work TCP Optimal Window Concept –Congestion Window Limit based on the round trip hop count –Assumptions of the shortest path and ratio of Interference/Transmission range Small window increase (TCP-Small) Congestion Window Limit based on the round trip hop count (1 pkt every 4 hops..) The linear increase parameter is a small value, say 0.01, i.e w=w+0.01/w (Mobihoc 05) TCP Adaptive Pacing (TCP-AP) 4 hop pacing (Mobihoc 05) Same assumptions as above.

One TCP on 4 hop Chain

Packet Losses ProtocolPacket Loss TCP NewReno1655 TCP Small (0.01)45 TCP AP5 TCP VAR0

1 TCP Flow on Varying Hop Length Hop Count Aggregate Throughput

4 TCP Flows on Varying Hop Length Hop Count Aggregate Throughput

Fairness and Efficiency TCP-VAR keeps best fairness and efficiency 4 TCP flows on 4 hop chain4 TCP flows on 6 hop chain

Routing Impact If the path is not the shortest path (or equivalently, interference/Tx range is different) –An example: Interference/Tx ratio = 7 –TCP adaptive pacing would be inefficiency since it assumes the knowledge of the interference/Tx ratio

Independence of Low Layer Time (second) Instantaneous Throughput (Kbps) Time (second) TCP APTCP VAR

Conclusions TCP-VAR is purely end-to-end: –independent of routing protocols, –hop distance computation and –ratio of Interference/Tx ratio. Novel congestion detection and congestion control algorithm; –It combines CWND and Rate adjustment TCP-VAR improves both efficiency and fairness.

Future Work Differentiate random loss and congestion loss Apply variance concept to other metrics –Eg, RTT Friendliness issue –Parameter adaptation to make it friendly to standard TCP. Wired and wireless network integration