peerfinder/templates/about.html

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<h2>Why look at latency?</h2>

<h3>The problem of peering quality</h3>

<p>Determining what is a "good" peering in dn42 is quite difficult: many
criteria come into play, such as latency, jitter, capacity, packet loss,
stability, or even the price your ISP will pay (peering vs. transit).</p>

<p>On the other hand, a "bad" peering is easy to picture: if you are in
Paris and peer with somebody in Australia, then you might end up doing
Paris → Australia → Hamburg if you want to send packets to Germany.  This
does not feel very efficient.  People usually solve this problem with
policy routing (local preference and path-prepending).  But it's still a
good idea to build good links and avoid terrible links.</p>

<p>Of course, you need to build long-distance links sometimes.  Otherwise,
dn42 would be made of small, independent islands.  This tool can also help
you to choose the best long-distance links.</p>

<h3>Looking at latency</h3>

<p>Latency is actually a good enough indicator of "distance".  For
instance, two machines located at the same ISP are expected to have low
latency towards each other.  On the other hand, a latency above 200 ms
usually indicates that the two machines are quite far away geographically
(but not always).</p>

<p>Additionally, latency can vary widely for long-distance links,
depending on the quality of transit and peering agreements between ISPs.
For instance, to reach a specific destination in Singapore from France, we
have the following latency as of September 2014:</p>

<table>
  <tr>
    <th>ISP</th>
    <th>Latency</th>
  </tr>
  <tr>
    <td>Online</td>
    <td>177 ms</td>
  </tr>
  <tr>
    <td>tetaneutral.net</td>
    <td>264 ms</td>
  </tr>
  <tr>
    <td>SFR</td>
    <td>267 ms</td>
  </tr>
  <tr>
    <td>Free</td>
    <td>365 ms</td>
  </tr>
  <tr>
    <td>OVH</td>
    <td>402 ms</td>
  </tr>
</table>

<p>Of course, this is only a snapshot, and reflects the situation for a
specific source and destination.  Still, the latency more than doubles
depending on the ISP, which in this case strongly favours a peering with
somebody hosted by Online instead of OVH.</p>

<h3>Situations this tool aims to solve</h3>

<p>To sum up, this tool can help in several situations:
  <ol>
    <li>Detecting when somebody is in the same datacenter as you, so that
    it's mostly free to peer</li>
    <li>When you are far from everybody, find the peering with the lowest
    latency</li>
  </ol>
</p>

<p>By building low-latency links in dn42, it's actually possible to have
lower latency in dn42 than over the Internet, for the same destination
(it's called <em>detour routing</em>).</p>

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