Subject: [rohc] Question: CRC coverage of ROHC RTP feedback messages

From: Alan Kennington <[email-masked]>
Date: Sat, 5 Jul 2003 08:14:55 +0930

There seems to be ambiguity in the specification of the CRC coverage
for ROHC feedback messages.
Can anyone tell me what the correct interpretation is?
Could this be incorporated into the ROHC implementer's guide?

The issue arises in RFC 3095, section 5.7.6.3, page 92:

=========================================================
5.7.6.3.  The CRC option

   The CRC option contains an 8-bit CRC computed over the entire
   feedback payload, without the packet type and code octet, but
   including any CID fields, using the polynomial of section 5.9.1.  If
   the CID is given with an Add-CID octet, the Add-CID octet immediately
   precedes the FEEDBACK-1 or FEEDBACK-2 format.  For purposes of
   computing the CRC, the CRC fields of all CRC options are zero.
=========================================================

It says above that the CRC does not cover the packet type and code octet.
But does it cover the "size" octet if present?
The "size" is effectively an optional extension of the "code" octet.
On that basis, it would seem reasonable that the CRC does not cover it.
But the RFC does not say this. It seems to deny this.
Therefore it is ambiguous in a way which strongly affects interoperability.
Is the answer written somewhere? 

Cheers,
Alan Kennington.

--------------------------------------------------------------------
    name: Dr. Alan Kennington
    city: Adelaide, South Australia
  coords: 138.59 E, 34.88 S
 pgp-key: http://www.topology.org/ak/key_ak2.asc
    note: My e-mail address often changes due to spam abuse.
    note: If your e-mail bounces, please visit topology.org/extra/email.php

_______________________________________________
Rohc mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/rohc