back channel flags, CREATE_SESSION, BIND_CONN_TO_SESSION

["Matt W. Benjamin" <matt@linuxbox.com>]

Hi,

There seem to be legitimate reasons for an (NFSv4.1) client and/or server to prefer a dedicated callback channel.

If a server wants this result, it seems from the language of 18.36.3 that it should indicate it by not setting CREATE_SESSION4_FLAG_CONN_BACK_CHAN in csr_flags in the CREATE_SESSION response, presuming the flag is set in the corresponding csa_flags argument (it's not allowed to set it otherwise).  The client may respond with BIND_CONN_TO_SESSION on a new channel, setting bctsa_dir to CDFC4_BACK.

Currently, the Linux and I believe also the CITI Windows client always propose channels in both directions.  The Linux mainline Linux client doesn't know how to BIND_CONN_TO_SESSION, so trivially it won't negotiate any back channel if a server didn't agree to both directions today, either.  I've experimentally implemented a "fallback" model in a Linux client and (partly in a) Ganesha server.  I'd appreciate any feedback on the idea.

Thanks,

Matt

-- 

Matt Benjamin

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