TCP fairness discussions are quite frequent for the Linux network maillingslist (netdev). This time Tom Herbert from google posted a patch where the TCP Congestion Window (CWND) can be modified through setsockopt(3). It is not easy to explain the impact of this change because it requires a lot of background information about how TCP works. To shorten the story: this option can - if employed to aggressive - lead to serious performance problems of the Internet. Network components these days are forced to behave good natured. If network components ignore the feedback or information from the network the Internet can collapse. This knowledge is not new at all - in contrast - the Internet was collapsed in the 80s because no congestion control mechanism was integrated. Since this time the cooperative behavior is a basic requirement for the functioning of the Internet.

Over time people tend to neglect the obvious and current network stack implementation soften the requirements. Some months ago another discussion about the plugable congestion control algorithms where similar. Some congestion control algorithms are unfair by default (e.g. BIC in environment with small RTT). I posted a patch to restrict the unfair behavior to root and provide only superuser the capabilities and freedom to do what he want. This patch effective disabled any changes by any other user. Stephen Hemminger negate and pushed another patch. Now the situation is that user who can compile and replace the kernel can enforce a strict or less-strict behavior. Less strict means that every user can select a congestion controll algorithm via setsockopt() his own congestion control algorithm, strict means that the user is forced to use the default congestion control algorithm (e.g. NewReno, Westwood). By the way: congestion control algorithms aren't the only place where unfair protocol behavior is possible.

But back to the current debate: Tom Herbert patch where declined and some fundamental Internet philosophies where discussed. The most impressive post for me was from Denys Fedorysychenko:

In Lebanon i have around 30k users behind few IP addresses(around 6, for web). Because backbone here $1200/Mbit, and satellites mostly(rtt 400+ ms)... so TCP accelerators and caching proxy a must. Tproxy doesn't work well yet to use full set of ip's.

The whole debate can is archived here: "":