Subject: [nfsv4] what defines a TCP connection as new, requiring BindConnectionToSession?

From: Rick Macklem <[email-masked]>
Date: Sun, 4 Apr 2021 03:21:34 +0000

Hi,

I have been doing testing of network partitioning between a
Linux client (5.2.10) and a FreeBSD server and have observed
the following:
- If I unplug a net cable for a few minutes and the plug it
   back in, the Linux client does a TCP "SYN", expects "SYN, ACK"
   and then continues on, using the same port# as before the
   network partitioning.
--> I had assumed the above created a "new connection" that
       will only be bound to the backchannel if a
       BindConnectionToSession is done on it.
Now I am not sure if this is considered a "new connection" or
the same connection (same port#) still bound to the back channel?

I am also wondering about this para. in RFC5661 pg. 493:
   Invoking BIND_CONN_TO_SESSION on a connection already associated with
   the specified session has no effect, and the server MUST respond with
   NFS4_OK, unless the client is demanding changes to the set of
   channels the connection is associated with.  If so, the server MUST
   return NFS4ERR_INVAL.
Since a "new connection" is bound to a session's fore channel by
sending an RPC with Sequence in it, does this imply that a
BindConnectionToSession done on a connection to bind a back channel
after an RPC with Sequence in it has been done on the connection,
must fail with NFS4ERR_INVAL?

As you can see, knowing what constitutes a new TCP connection matters.

Thanks in advance for any help with this, rick

---

From: Olga Kornievskaia <[email-masked]>
Date: Mon, 5 Apr 2021 11:42:52 -0400

> the same connection (same port#) still bound to the back channel?

My understanding is that the client has created a new connection
regardless if it reused the source port. At this point this connection
is not bound to anything until either the client sends something with
a SEQUENCE in it which will make it bound to the forechannel or the
client must send as the first operation BIND_CONN_TO_SESSION asking
for both and/or backchannel binding if the client wants to have this
connection bound to the callback channel.

> must fail with NFS4ERR_INVAL?

That's correct as per my understanding. If a client established a
connection, then say it sent something already with a SEQUENCE and
then tried to send BIND_CONN_TO_SESSION asking for the backchannel to
be bound to this existingconnection, the server must error with
ERR_INVAL. I had to fix the linux client for this as the client was
trying to change the backchannel binding after the channel was already
bound to just the forechannel.

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