Subject: Re: RFC describing exponential backoff for TCP retransmission?
To: Kevin M. Lahey <>
From: Perry E. Metzger <>
List: tech-net
Date: 03/16/1998 22:42:53
"Kevin M. Lahey" writes:
> I think I should know this, but I don't.  Can anyone tell me what
> RFC documents exponential backoff for TCP retransmissions?
> RFC1122 seems to refer to an exponential backoff, but doesn't
> really specify how it should be done.

RFC2001 documents what you are looking for, I believe.

However, you probably should look at the current drafts from the
tcp-impl working group for better data. The drafts are more up to


> Using NetBSD-current on the local LAN, I find that I get
> 12 retransmissions, each one second apart, and then the connection
> gets dropped.  The other OS I had close at hand, IRIX, does
> exponential backoff, pausing 3 seconds, then 6, then 12, and so on.  
> Stevens suggests that systems ought to do this kind of exponential 
> backoff.
> Arguably, if the RTT is 12ms and the varience is 1ms, then 
> the RTO should be something like 16ms, and we're just rounding
> it up to a second, and all is copacetic.  I'm not sure I buy that,
> and I'm not sure I want my system to work that way on the LAN.
> I'd like to fix NetBSD so it works right, but I'd like to see
> what "right" is supposed to be before I fix it.
> Thanks,
> Kevin