TCP initial congestion window (slow-start)

Question

I have noticed that the initial congestion window in my traces is 8920bytes~=6*1448. rfc3390 states the initial cwand should be max 4000 bytes(around 3*1448).

At first i thought it might be because i'm running my server on mac os x, so apple might have modified the tcp stack. Therefore I tried running the server on a Linux box (2.6.38) however i'm getting similar results 13032bytes=9*1448. Both server and client machines are running on a local network (via a router). Any ideas why the initial cwand is so large?

Answer 1

There's been a few articles on increasing the initcwnd lately - the default in Linux 2.3.69 (?) has been increased to 10

There are a few rules that can affect it pre this - see http://monolight.cc/2010/12/increasing-tcp-initial-congestion-window/ for more.

I presume you're running on one of the well know private IP ranges, whether this has any difference I don't know.

From past reading Google and MS are using 10 for their public facing webservers.