Commit 599c2d94 authored by Leigh Stoller's avatar Leigh Stoller

Silly FAQ entry explaining why your packets take long to get where

they are going when you decrease the bandwidth.
parent 612f2200
......@@ -108,6 +108,8 @@
shows no packet-loss (or delay). Why?</a>
<li> <a href="#TR-4">I set a non-zero packet-loss (or delay) but 'ping'
shows a different packet-loss (or delay). Why?</a>
<li> <a href="#TR-5">I decreased the bandwidth on a link and now the
ping time between the nodes has increased. Why?</a>
</ul>
</ul>
......@@ -1378,4 +1380,40 @@
90.25%, or a round trip loss rate of 9.75% on a 5% lossy link.
</p>
<li><a NAME="TR-5"></a>
<font size='+1'><b>I decreased the bandwidth on a link and now the
ping time between the nodes has increased. Why?</b></font>
<p>
Short answer: Decreasing the bandwidth of a link means that your
bytes take longer to get where they are going!
</p>
<p>
Long Answer: A ping packet is 98 bytes of data; 56 bytes of data
plus 8 bytes of ICMP header plus 20 bytes of IP header plus 14
bytes of ethernet header. At 100Mbs those 98 bytes takes .0078ms
to traverse the wire, which is hardly noticeable! If you have set
the delay of your link to 10ms, then your ping packets will incur
10ms+0.0078ms of delay in each direction, for a 20ms roundtrip time.
</p>
<p>
Say you set the bandwith of your link to 250Kbs. The wire time for
those same 98 bytes is now 3ms. If your delay is 10ms like above,
then your ping packets will incur 10ms+3ms of delay in each
direction, for a 26ms roundtrip time! If you set the bandwith to
100Kbs, the wire time is now 7.8ms and your ping packets will
incur 10ms+7.8ms of delay in each direction, for a 35.6ms roundtrip
time!
</p>
<p>
Note: If you have a router connecting two nodes, then <em>each of
the two links</em> will incur the same wire time (and delay of
course). In the above 250Kbs example, each ping packet will incur
3ms of wire time to the router and <em>another</em> 3ms from the
router to the destination. The ping reply packet will see the same
6ms of wire time. If your delay is again 10ms, then the ping
roundtrip is 52ms.
</p>
</ul>
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