View source for H-TCP

{{Short description|TCP congestion avoidance algorithm}}
{{hatnote|For Hypertext Caching Protocol, see [[Hypertext caching protocol]]}}
{{More citations needed|date=February 2016}}
{{Use American English|date=January 2019}}
{{Use mdy dates|date=January 2019}}
'''H-TCP''' is another implementation of [[Transmission Control Protocol|TCP]] with an optimized [[congestion control algorithm]] for high speed networks with high latency (LFN: Long Fat Networks). It was created by researchers at the [[Hamilton Institute]] in Ireland.
H-TCP is an optional module in [[Linux]] since [[kernel (operating system)|kernel]] version 2.6, and has been implemented for FreeBSD 7.<ref>{{Cite journal|url=http://dl.acm.org/citation.cfm?doid=1384609.1384613|doi=10.1145/1384609.1384613|title=An independent H-TCP implementation under FreeBSD 7.0|year=2008|last1=Armitage|first1=Grenville|last2=Stewart|first2=Lawrence|last3=Welzl|first3=Michael|last4=Healy|first4=James|journal=ACM SIGCOMM Computer Communication Review|volume=38|issue=3|pages=27–38|s2cid=7175592}}</ref>
==Principles of operation==
H-TCP is a [[Packet loss|loss-based]] algorithm, using additive-increase/multiplicative-decrease (AIMD) to control TCP's congestion window.  It is one of many [[TCP congestion control|TCP congestion avoidance algorithms]] which seeks to increase the aggressiveness of TCP on high bandwidth-delay product (BDP) paths, while maintaining "TCP friendliness" for small BDP paths.  H-TCP increases its aggressiveness (in particular, the rate of additive increase) as the time since the previous loss increases.  This avoids the problem encountered by [[HSTCP]] and [[BIC TCP]] of making flows more aggressive if their windows are already large. Thus, new flows can be expected to converge to fairness faster under HTCP than HSTCP and BIC TCP.
==Strengths and weaknesses==
A side effect of increasing the rate of increase as the time since the last packet loss increases, is that flows which happen not to lose a packet when other flows do, can then take an unfair portion of the [[bandwidth (computing)|bandwidth]].  Techniques to overcome this are currently in the research phase.
The Linux implementation of H-TCP also has an option for avoiding "[[Round-trip delay time|RTT]] unfairness", which occurs in [[TCP Reno]], but is a particular problem for most high speed variants of TCP (although not [[FAST TCP]]).
==Name==
The algorithm was initially introduced as H-TCP, without mention of what the 'H' stands for.  However, it is often called "Hamilton TCP", for the Hamilton Institute where it was created.
==See also==
* [[TCP congestion control]]
* [[Transmission Control Protocol#Historical origin]]
==References==
{{Reflist}}
==External links==
*[http://www.hamilton.ie/net/htcp/ H-TCP] Home Page
[[Category:TCP congestion control]]