[AVT] Proposed change to Enhanced CRTP

[Tmima Koren <tmima@cisco.com>]

We'd like to propose a change to the Enhanced CRTP draft,
draft-ietf-avt-crtp-enhance-07.txt
http://www.ietf.org/internet-drafts/draft-ietf-avt-crtp-enhance-07.txt

One of the enhancements is to add a Headers Checksum (HDRCKSUM) when UDP
checksum is not present (see section 2.2).
The UDP checksum is used by the decompressor to periodically validate the
reconstructed packets. When not present, 
a HDRCKSUM may be inserted by the compressor replacing the missing UDP
checksum. The decompressor removes the HDRCKSUM after validating the
reconstructed packet.

Currently the HDRCKSUM covers the pseudo-IP header, the UDP header and
the fixed part of the RTP header (first 12 bytes).
Those are the only parts of the reconstructed packet that can be
validated by the HDRCKSUM.
The CSRC list is not included in the HDRCKSUM. 

We propose to include the CSRC list in the HDRCKSUM since the CSRC list
is part of the header compression context.

The CSRC list is stored at the decompressor context and inserted to the
restored packets. If the HDRCKSUM does not cover the CSRC list and the
CSRC list at the decompressor is errored, a bad CSRC list will be
restored into multiple packets until the CSRC
list changes and is resent by the compressor as an update.
If the HDRCKSUM covers the CSRC list, an error in the CSRC list will be
caught early and corrected.

When there is a change in the CSRC list, the CSRC list is sent as an
update to the context. 
The CSRC list should be treated in the same way as the other context
updating changes and be included in the HDRCKSUM.

On the other hand, adding the CSRC list to the HDRCKSUM requires more
processing calculating the HDRCKSUM. Also if
the accuracy of the CSRC list is not crucial and the CSRC list is
included in the HDRCKSUM, then an error in the CSRC list
will result in dropping a few packets while the contexts are getting
synchronized.


The proposed change to the text is as follows:

Old:

   The HDRCKSUM is calculated
in the same way as a UDP checksum except
   that it does not cover all of the UDP data.  That is,
the HDRCKSUM is
   the 16-bit one's complement of the one's complement sum of
the
   pseudo-IP header (as defined for UDP), the UDP header, and
the first
   12 bytes of the UDP data which are assumed to hold the fixed
part of
   an RTP header.  The extended part of the RTP header and
the RTP data
   will not be included in the HDRCKSUM.

New:

   The HDRCKSUM is calculated
in the same way as a UDP checksum except
   that it does not cover all of the UDP data.  That is,
the HDRCKSUM is
   the 16-bit one's complement of the one's complement sum of
the
   pseudo-IP header (as defined for UDP), the UDP
header,     the first
                                                         
^^^^
   12 bytes of the UDP data which are assumed to hold the fixed
part of
   an RTP header, and the CSRC list.
               
^^^^^^^^^^^^^^^^^^^
   The extended part of the RTP header beyond the CSRC list and
the RTP data
                                      
^^^^^^^^^^^^^^^^^^^^ 
   will not be included in the HDRCKSUM.