RE: Problem with use of NotUnderstood in negotiations

[pat_thaler@agilent.com]

Title: RE: Problem with use of NotUnderstood in negotiations

Bart,

 

So the question is, in what order does a device do the checking.

 

There are many possibilites for handling a received key that is

unknown and comes in with the value unknown:

 

A) Check key

       if key unknown, send key=NotUnderstood

         else ...process key=value pair

 

B) Check key

       if key unknown

         if value=NotUnderstood silently drop (or close connection)

           else send key=NotUnderstood

 

I expect case A is more likely to get implemented in the absence

of an explicit statement. There would normally be no need to

examine the value of a key when one doesn't understand the key.

In this case, the receiver never does a test that detects the

apparent protocol violation of making an offer with a value

NotUnderstood.

 

So, if we want to stop the loop that Bill has found, we should

put in an explicit requirement to test the value for NotUnderstood

before responding to a key with NotUnderstood.

 

The alternative is to leave things as they are and count on

implementations to abort the negotiation based on a timeout

or a count of excessive number of negotiation PDU exchanges.

 

Regards,

Pat

 

-----Original Message-----
From: Bart Crane [mailto:bcrane@iready.com]
Sent: Friday, August 09, 2002 12:33 PM
To: ips@ece.cmu.edu
Subject: RE: Problem with use of NotUnderstood in negotiations

This new rule is not necessary.

Sec. 4.2 says:

   The constants None, Reject, Irrelevant, and NotUnderstood
   are reserved and must only be used as described here.

In the situation you describe, the sender will be expecting
a response to "keyA", but the key-name was corrupted to "keyB".

The receiver does not understand "keyB", and so responds with
"keyB=NotUnderstood".

To the sender, this appears to be the start of negotiating a
new key, "keyB", with the value "NotUnderstood". 

But, this is not a valid use of the value "NotUnderstood",
so it is a protocol error.

So, the new rule of not-responding to keys with the value
"NotUnderstood" is not necessary.

Bart.

-----Original Message-----
From: Bill Studenmund [mailto:wrstuden@wasabisystems.com]
Sent: Friday, August 09, 2002 9:12 AM
To: ips@ece.cmu.edu
Subject: Problem with use of NotUnderstood in negotiations

I encountered a problem with how draft 15 specifies using NotUnderstood as
a reply to keys that aren't understood. Namely if something injects
garbage into the negotiation stream we can end up with a key BOTH sides
don't understand, and so they both sit there sending "foo=NotUnderstood"
back and forth to each other.

Yes, well-behaved negotiators won't offer a key they don't understand. But
the data stream can be corrupted, and all it would take is a single-bit
error in a key name and we encounter the above.

I propose we change the text to:

Any key not understood by the acceptor may be ignored by the acceptor
without affecting the basic function. However, unless the value for the
key was "NotUnderstood", the answer for a key not understood MUST be
key=NotUnderstood. If the value for the key was "NotUnderstood", the
acceptor MUST not answer the key.

Note: I can easily see closing the connection with an error in the above
case too.

Thoughts?

Take care,

Bill