20.01.2014 Views

Lectures notes for 2010 - KTH

Lectures notes for 2010 - KTH

Lectures notes for 2010 - KTH

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

Round-Trip Time Measurement<br />

Fundamental to TCP’s timeout and retransmission is the measurement (M) of the<br />

round-trip time (RTT). As the RTT changes TCP should modify its timeouts.<br />

Originally TCP specificed:<br />

R ← αR+ ( 1–<br />

α)M<br />

αa smoothing factor, with a recommended value of 0.9<br />

RTO<br />

=<br />

Rβ<br />

βa delay variance factor, with a recommended value of 2<br />

RTO == retransmission timout time<br />

Van Jacobson found that this could not keep up with wide fluctuations in RTT,<br />

which leads to more retransmissions, when the network is already loaded! So he<br />

proposed tracking the variance in RTT and gave <strong>for</strong>mulas which compute the RTO<br />

based on the mean and variance in RTT and can be easily calculated using integer<br />

arithmetic (see Stevens, Vol. 1, pg. 300 <strong>for</strong> details).<br />

Maguire TCP header continued 5: 27 of 77<br />

maguire@kth.se <strong>2010</strong>.03.21 Internetworking/Internetteknik

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!