Re: [nfsv4] Review of draft-ietf-nfsv4-rfc5667bis-06

[Chuck Lever <[e-mail]>]

> On Feb 25, 2017, at 4:57 AM, David Noveck <[e-mail]> wrote:
> 
> 3.  Upper Layer Binding for NFS Versions 2 and 3
> The penultimate paragraph in this section is new in this version and I don't understand the need for a new version 2/3 restriction so late in the game.

RFC 5667 Section 4 says:

> Similarly, a single RDMA Read list entry MAY be posted by the client
> to supply the opaque file data for a WRITE request or the pathname
> for a SYMLINK request.  The server MUST ignore any Read list for
> other NFS procedures, as well as additional Read list entries beyond
> the first in the list.

I take "Read list entry" to mean Read chunk, composed of
multiple list entries that share the same XDR position.
This comports with similar language describing Write
chunks where a single list entry is indeed allowed to
have multiple segments.

However, the original intent might have been "single
Read segment". In that case, this language clearly states
that Legacy clients can send only one contiguous memory
region in Legacy NFS READ and SYMLINK requests.

At your behest I replaced the RFC 5667 language with
just the definitions of which XDR data items are DDP
eligible, and removed the discussion about Read list
entries.

This new language permits a Legacy client to reduce the
Payload stream of an NFS READ or SYMLINK request and put
that stream into a Position-Zero Read chunk. The client
would then send the PZRC and the non-zero position chunk:
two distinct Read chunks, which is not allowed by the
RFC 5667 language discussing NFS READ or SYMLINK.

To remain interoperable with RFC 5667-compliant
implementations, only one Read chunk is allowed, which
excludes the use of reduced Payload streams in a PZRC,
even though RPC-over-RDMA Version One allows this
construction.

There isn't a need to support sending a reduced Payload
stream this way in these NFS versions, so I added the new
paragraph to forbid it explicitly. This should now have
the same basic meaning as Section 4 of RFC 5667.

RFC 5667 does not restrict sending a Write chunk and a
Reply chunk together. However IIRC current implementations
do not support this (at least with NFSv2 and NFSv3) and
there shouldn't ever be a need to for these NFS versions.


> If this is not a new restriction, the intent would be clearer if it was rewritten as follows:
> The structure of the NFS version 2 and  3 protocols and limitations on DDP-eligible data items given above are such that:
> 	• It is impossible for a Legacy  NFS client to validly send a reduced Payload stream in a Long Call.  
> 	• It is impossible for a Legacy  NFS client to validly enable a Legacy NFS server to send a reduced Payload stream in a Long Reply.


--
Chuck Lever