UDP Fragmentation and DF bit..

["Vincent, Pradeep" <pradeepv@amazon.com>]

After running into issues with UDP in multi-MTU network environment, I
started digging through the code and found somewhat inconsistent behavior
(if I read the code right)

OMan 7 ip¹ declares that ³The  system-wide  default  is  controlled  by  the
ip_no_pmtu_disc  sysctl  for SOCK_STREAM  sockets,  and  disabled  on all
others.² which led me to think ODF¹ bit will not be set for UDP packets.
But..

The code in ip_output.c seems to do the following,

1. If packet size <= current PMTU, then set the DF=1 and send the packet
out.
2. If packet size > current PMTU, then set DF=0 and send the packet out
after fragmentation.

In a network environment where MTU-big and MTU-small co-exist (and have
router¹s fragmentation turned off in favor of PMTU discovery), UDP packets
that are > MTU-small and < MTU-big find the PMTU effectively but UDP packets
that are > MTU-big get dropped. This looks like inconsistent behavior to me
and doesn¹t seem to match the advertised behavior.

Is there a reason why PMTU support for UDP is somewhat inconsistent ?

Is there a reason ODF¹ bit cannot be set on fragmented packets on UDP
transmission ? I couldn¹t find anything in RFC for IP protocol that
prohibited DF bit on fragmented packets. Did I miss
something here ?

Would it be reasonable if PMTU discovery is performed (DF bit set +
appropriate icmp logic) even for locally fragmented packets ? I think this
will be a great help for UDP users that don¹t have PMTU handling logic in
the application (most udp applications belong to this category in my
experience). Thoughts ?

Thanks,

- Pradeep Vincent