Subject: [Capwap] Reserved bits and fields

From: "Bob O'Hara (boohara)" <[email-masked]>
Date: Mon, 14 Aug 2006 09:02:31 -0700

I believe that we are overspecifying the use of the reserved bits and
fields in the draft.  For example, we place a requirement on the
transmitter that these bits and fields MUST be zero ("All
implementations complying with version zero of this protocol MUST set
these bits to zero.").  However, we do not specify any behavior by the
receiver.  I believe this is backwards.

Because the version field is available to all receivers of a CAPWAP
packet and that version field will unambiguously identify all the
reserved bits and fields, I believe we should change the requirement to
be that the receiver MUST ignore any value received in a reserved bit or
field.  This way, should a noncompliant transmitter send something in
these fields, the receiver will not be confused by it.

 -Bob

Bob O'Hara
Cisco Systems - WNBU

Phone:  +1 408 853 5513
Mobile: +1 408 218 4025
 
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From: Scott G Kelly <[email-masked]>
Date: Tue, 15 Aug 2006 07:03:12 -0700

Hi Bob,

> these fields, the receiver will not be confused by it.

I agree that we should say the receiver is to ignore non-zero reserved 
fields. This is consistent with the general IETF protocol philosophy of 
being strict in what you send, and liberal in what you accept. However, 
it is also common to require the sender to zeroize reserved fields 
(strict in what you send).

I'm not sure how/when this convention originated, but one 
security-related benefit is that it prevents the use of the reserved 
fields as a covert channel.

Do you feel strongly that the sender should be allowed to set reserved 
bits, or are you okay with zeroize at the sender, ignore at the receiver?

Scott