The tech report about developing FAST TCP.

Standard AIMD: Window of a source increase by 1 packet per RTT and decrease per unit time by packets. The 4/3 is the peak window size before seeing loss and 1/2 is the MD factor. The rate is denoted by (window size over RTT) and the loss probability is . Thus the flow-level model of AIMD can be expressed as the differential equation:

From the D.E., the window size in equilibrium is and this means on average we have packets lost per RTT. Moreover, the D.E. can be expressed as:

which it is found that all TCP have such structure, different variants with different gain function , utility function and congestion measure (loss or delay) .

The dynamic of different TCP is illustrated in P.7: Reno oscillates about the point that is marginal to loss and with large queueing delay, while FAST and Vegas tries to stabilize at the knee of minimal queueing delay.

Loss-based TCP includes Reno, HSTCP and Scalable TCP. The HSTCP defines the equilibrium relation between e2e loss prob and window size as , which then makes the AIMD of cwnd follows

respectively with the values of functions and depend on window size. Scalable TCP is MIMD:

with and equals to 0.01 and 0.125 respectively.


Bibliographic data

   title = "FAST TCP: Motivation, Architecture, Algorithms, Performance",
   author = "Cheng Jin and David X. Wei and Steven H. Low",
   institution = "Caltech",
   number = "CS Technical Report CaltechCSTR:2003.010",
   howpublished = "Caltech CS TR",
   year = "2003",