VCs, MaxNumberVCs, SessionKey

Richard Sharpe rsharpe at
Fri Jun 28 10:15:02 GMT 2002

On Fri, 28 Jun 2002, Raghu Iyer wrote:

> Can a SMB expert help me understand how VcNumber, 
> MaxNumberVCs, SessionKey are used in the SMB protocol
> with some examples and commands that can be issued
> from a NT/W2K client to a NT/W2K/Samba Server to
> experiment with.

I dodn't think it is used, except with Terminal Server-like products.

It seems to have been an attempt to allow multiple security contexts over 
a single NBT session. However, it seems not well thought out, and you must 
be prepared to accept multiple SessSesup&X requests in anycase, and there 
is sufficient info in each SMB to keep this stuff separate (UID, MID, 
> Is there a good reference that can illustrate the concepts
> behind these parameters ? I have gone through the
> CIFS mailing list on and have a
> very hazy idea. I have not been able to produce sessions 
> where - MaxNumberVCs is > 1 OR where SessionKey 
> is != 0 OR where VcNumber is != 0. So I fail to appreciate
> under what circumstances these parameters change.

They are not discussed, except in passing, in CIFS-TR-1.0. If you do a 
goodle search on SMB VCnumber or some such, you will find some discussion.

If you or anyone can send some TerminalServer traces, it would be useful.

Richard Sharpe, rsharpe at, rsharpe at, 
sharpe at

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