1. Improving Slow Start
InKwan Yu

2. 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

3. 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

4. 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.

5. 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

6. Related Work
Hu’s Method

7. 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

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

9. 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 ?

10. 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

11. 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

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

13. 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.