[payload] draft-ietf-payload-rtp-ancillary-07: Comments on header layout

[Kjetil Oftedal <[e-mail]>]

Hi,

I have earlier commented upon the size of the line number/offset fields and
their counterparts in RFC4175, and with the latest changes to the ANC RTP
layout an new opportunity to address this has arrived.

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C|   Line_Number       |   Horizontal_Offset   |    reserved   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Moving some of the reserved bits and aligning to the 16-bit boundary will both
make it easier for software to build the header, and will
allow these Line_Number/Horizontal_Offset fields to easily be expanded in the
future if the need for ANC transport with higher resolution images should
arise:

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C| rsvd  |     Line_Number     | rsvd  |   Horizontal_Offset   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The only issue is handling the special value cases such as 0xFFF/0xFFE,
Expanding the fields to 15-bits at this time would maybe just be easier,
as there is no bandwidth penalty of doing so.

Best regards,
Kjetil Oftedal