Re: TCP timestamp option

Andi Kleen (
21 May 1998 18:09:26 +0200

Meelis Roos <> writes:

> Why do 2 "nop" tcp options usually precede the timestamp option in tcp
> packets generated bly linux tcp stack when the timestamps are enabled?
> Is the ip header padding done in options block this way, or is it
> something other?

To align the timestamp option to a 32bit boundary. Many CPUs get
a significant slowdown while accessing non aligned data.

It is padded in the TCP option space, which is different from the IP

> Also, what's the use of the timestamp option?

More accurate Round Trip Time mesasurement. An accurate RTT estimate
is cruicial for TCP's performance to prevent unneeded packet retransmissions.

It also prevents TCP against old packets when very large windows are used.


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