Dries Schellekens wrote:
On Tue, 11 Feb 2003, Damien Miller wrote:
Quite possibly the final word on the matter:
https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=58084
Daniel finally figured out why they use DF on fragments:
"The missing piece in the puzzle was the fact that certain protocols like
NFS can't split transactions/operations into smaller packets, they need to
send the entire transaction in one single (complete) IP packet. This size
might exceed any real MTU, so it will get fragmented first. And only
afterwards PMTU discovery gets applied to the fragments. Hence, DF on
fragments. This scheme is not explicitely covered by the RFCs, but I agree
that it's a logical conclusion."
Actually, the NFS case is explicitly covered in RFC1191:
Also, since a single NFS operation cannot be split across several UDP
datagrams, certain operations (primarily, those operating on file
names and directories) require a minimum datagram size that may be
larger than the PMTU. NFS implementations should not reduce the
datagram size below this threshold, even if PMTU Discovery suggests a
lower value. ***(Of course, in this case datagrams should not be
sent with DF set.)***
(section 6.5, page 14, emphasis mine)
-d
