NLM (NFS Lock Manager) premature TCP close

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

(I'm unsure whether NFS issues should go to tech-kern or tech-net)

I'm facing a strange NLM problem where the (Linux) client sends two unlock 
calls in quick succession and the (4.0/amd64) server then closes down the TCP 
connection without sending an unlock reply to any of the two calls.
As I'm more used to RPC-over-UDP than -over-TCP, I'm not certain whether 
interleaving calls (i.e., sending a second call without having received the 
reply to the first one) is permitted.
So, is this a client error or server error? If server, is it a known one? If 
not, I've probably make myself familiar with the NLM code again I last worked 
on five years ago.