Improving Slow Start InKwan Yu

Slow Start? Slow Start Three cases of Slow Start Grows the congestion window size exponentially. Terminates when ssthresh (64K) is reached or a packet is lost Three cases of Slow Start Initial : when a TCP connection is established Idle : packets are not sent for a few minutes Timeout : transmission timeout

Related Work Estimating Bandwidth Hoe’s Approach First packet pair to estimate ssthresh RTT / Inter packet gap = cwd (initial congestion window size or ssthresh) Network Inter Packet Gap s t

Related Work Caching Source has cache of ssthresh values of destinations When a new connection is from a cached destination, use the ssthresh of the cache.

Related Work Hu’s Method Congestion : input inter packet gap is smaller than output packet gap No congestion : input inter packet gap is larger Binary search using about 3 to 4 rounds

Related Work Hu’s Method

Goals Faster Start Better ssthresh value In a large bandwidth*delay path, slow start is not fast enough due to TCP’s self clocking Better ssthresh value Overestimated ssthresh value will cause packet drops

New Methods History based With fair TCP, the equal amount of bandwidth will be shared 1Mb s t 512Kb s t 512Kb

New Methods History based When there are already k connections, a new connection will have B*1/(k+1) share from B total available bandwidth 1/k …… s …… t 1/k ?

New Methods History based Extension to a subnet, inside of which delay is negligible and there is no congestion There is a common path from the source to the subnet t Subnet t s Host Host t

New Methods Estimation based Weakness of Hu’s method Modification The binary search method can be misleading with the first few errors The cwd growth rate is to the 2nd power (slow) The delayed acknowledgement Modification Regression based method Confidence interval Sequence numbers of packet pair are reversed

New Methods Estimation based y=x Arriving Inter Packet Gap (ACK gap) Departing Inter Packet Gap

Reference Ningning Hu, P. Steenkiste, Improving TCP startup performance using active measurements: algorithm and evaluation, ICNP 2003. J. Hoe, Improving the start-up behavior of a congestion control scheme for TCP, SIGCOMM 1996.