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This bugzilla has been raised for the NFS Client part.

+++ This bug was initially created as a clone of Bug #606260 +++

At the moment on RHEL 6 beta, we support NFSv4 over UDP. 

RFC3530 states

   Where an NFS version 4 implementation supports operation over the IP
   network protocol, the supported transports between NFS and IP MUST be
   among the IETF-approved congestion control transport protocols, which
   include TCP and SCTP

UDP doesn't perform congestion control. This combination is thus not standards compliant.

I came across this thread upstream.
http://linux-nfs.org/pipermail/nfsv4/2006-February/003480.html

This request was evaluated by Red Hat Product Management for inclusion in a Red
Hat Enterprise Linux major release.  Product Management has requested further
review of this request by Red Hat Engineering, for potential inclusion in a Red
Hat Enterprise Linux Major release.  This request is not yet committed for
inclusion.

This issue has been proposed when we are only considering blocker
issues in the current Red Hat Enterprise Linux release. It has
been denied for the current Red Hat Enterprise Linux release.

** If you would still like this issue considered for the current
release, ask your support representative to file as a blocker on
your behalf. Otherwise ask that it be considered for the next
Red Hat Enterprise Linux release. **