RTT of TCP

[Lloyd Wood]

On Tue, 31 Aug 1999, Xiao Hannan wrote:

> What's the unit of variables for RTT estimations of TCP in NS?

read ~ns/tcp.h.

/*
 * tcp_tick_:
 * default 0.1,
 * 0.3 for 4.3 BSD, 
 * 0.01 for new window algorithms,
 */

...in seconds.

> From the
> documentation, the RTT samples are computed as the difference between the
> current time and a "time echo" field in the ACK packet.

That's SACK/FACK only; otherwise you're relying on time of send/time
of receipt and Karn's algorithm.


> So is RTT in unit of time? 
> 
> I let a TCP session run over a link with 20ms delay. Then I  monitor that
> the srtt_ of TCP is 8, and rttvar_ is 2 during a simulation time of 10s.
> Then, what's the relationship between these value?  

rttvar_ is 2 x 0.01s
 
See e.g. RFC793 for srtt calculations.

L.


> Thanks a lot
> Hannan
> 
> 

<L.Wood@surrey.ac.uk>PGP<http://www.ee.surrey.ac.uk/Personal/L.Wood/>