[RRG] Perplexing PMTUD and packet length observations

[Robin Whittle <rw@firstpr.com.au>]

I expected that most hosts would send packets with DF=1, using RFC
1191 PMTUD, since it has been around since 1990 . . . and that it
would be considered antisocial to send any longish packets into the
Net with DF=0, saying the Network should do extra work if the
packets are too long for it.

In fact, some servers - including at least some Google servers, send
only DF=0 packets, and therefore never do any PMTUD.  They won't be
tempted by the client having a higher MSS than their own 1430, but
as long as the client's MSS is this or above, the server frequently
sends 1470 byte packets with DF=0.
 http://www.firstpr.com.au/ip/ivip/ipv4-bits/actual-packets.html#google-no-pmtud

I find this interesting:

  1 - I guess Google figure 1470 is a packet size which generally
      works in the core.

  2 - I guess they figure that if the client advertises an MSS and
      when 40 (IP and TCP headers) is added to that figure if the
      result is larger than any MTU limit between the client and
      Google's servers, then it is the client's problem.

If lots of hosts are sending long packets with DF=0, we need to cope
with then in any map-encap scheme.

We don't want to have to fragment these packets before
encapsulation, or after, but we have no way of telling the host to
send smaller ones.


Another perplexing discovery is that my web server in Texas does
ordinary TCP SYN handshakes with MSS values such as 1460 from both
sides and then quite merrily cranks up to packets of 7260 bytes and
longer.

http://www.firstpr.com.au/ip/ivip/ipv4-bits/actual-packets.html#jumbo1

I have no idea why this occurs, but at least it does it all with
DF=1.  These packets are received and acknowledged by clients who
are presumably on some kind of xDSL service.  So jumboframes are
alive and well in the core.

   - Robin


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