<div dir="ltr"><div class="gmail_extra"><div class="gmail_quote">On Sun, Nov 9, 2014 at 8:10 PM, Jeroen Massar <span dir="ltr"><<a href="mailto:jeroen@massar.ch" target="_blank">jeroen@massar.ch</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><span class="">> Another fun question is why folks are relying on PMTUD instead of<br>
> adjusting their MTU settings (e.g., via RAs).<br>
<br>
</span>Because why would anybody want to penalize their INTERNAL network?<br></blockquote><div><br></div><div>Lowering the MTU from 1500 to 1280 is only a 1% penalty in throughput. I'd argue that that 1% is way less important than the latency penalty.</div><div><br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Because you can't know if that is always the case.<br></blockquote><div><br></div><div>I'm not saying that PMTUD shouldn't work. I'm saying that if you know that your Internet connection has an MTU of 1280, setting an MTU of 1500 on your host is a bad idea, because you know for sure that you will experience a 1-RTT delay every time you talk to a new destination.</div><div><br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">As you work at Google, ever heard of this QUIC protocol that does not<br></blockquote><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
use TCP?<br>
<br>
Maybe you want to ask your colleagues about that :)<br></blockquote><div><br></div><div>Does QUIC work from behind your tunnel? If so, maybe my colleagues have already solved that problem.</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><span class="">> (Some parts of) Google infrastructure do not do<br>
> PMTUD for the latency reasons above and for reasons similar to those<br>
> listed<br>
> in <a href="https://tools.ietf.org/html/draft-v6ops-jaeggli-pmtud-ecmp-problem-00" target="_blank">https://tools.ietf.org/html/draft-v6ops-jaeggli-pmtud-ecmp-problem-00</a> .<br>
<br>
</span>As such, you are ON PURPOSE breaking PMTUD, instead trying to fix it<br>
with some other bandaid.<br></blockquote><div><br></div><div>The draft explains some of the reasons why infrastructure is often built this way.</div></div></div></div>