TCP with Variance Control for Multihop IEEE 802.11 Wireless

Networks

Jiwei Chen, Mario Gerla, Yeng-zhong Lee

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 802.11 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.

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

Protocol Packet Loss

TCP NewReno 1655

TCP Small (0.01) 45

TCP AP 5

TCP VAR 0

1 TCP Flow on Varying Hop Length

Hop Count

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4 TCP Flows on Varying Hop Length

Hop Count

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Fairness and Efficiency

TCP-VAR keeps best fairness and efficiency

4 TCP flows on 4 hop chain 4 TCP flows on 6 hop chain

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