Re: [Fecframe] WGLC on draft-ietf-fecframe-dvb-al-fec-02

[Vincent Roca <vincent.roca at inrialpes.fr>]

Hello everybody,

After reading the above I-D, I don't have any major comments, only
a few clarification proposals (see below).

In particular once comment 1- is addressed, then I support this
document, taking into account the fact that I have no technical opinion
WRT the RTP discussion in section 3.1 (sorry).

----

1- Explicit reference to DVB-IPTV

Currently the first reference to DVB-IPTV and DVB-IPI appear in
section 3.1. Instead, the abstract, introduction and title only
refer to the "DVB consortium". This is a bit misleading since the
various DVB Technical Modules (i.e. working groups) have standardized
several FEC schemes for the erasure channel (i.e. several FEC codes
and several ways of using these FEC codes).
(NB: this is the DVB UL-FEC document discussion we had during July's
 meeting)

I suggest to:
- mention DVB-IPTV (and possibly DVB TM-IPI, up to you) in the
  abstract and introduction.
- change a little bit the long and short titles to clarify this
  as well. For instance: "DVB-IPTV Application-Level Hybrid FEC
  Protection" and "DVB-IPTV AL-FEC Protocol".


2- Introduction, p3, minor clarification proposal:
Current text mentions 1-D interleaved parity code in the introduction,
and then only uses the "parity encoder/decoder" (fig. 1, 2, 3) and
"parity repair packet" (bottom of p3).
I suggest you say once and for all that in the remaining of the
document, "parity code" (and derivatives, like parity encoder/decoder
or parity repair packet) will refer to the "1-D interleaved parity code"
(and derivatives).


3- Introduction, p3, clarification needed:
It is said:
   "The source can be any type of media
    such as audio, video, text or application.  However, in the AL-FEC
    protocol, the source stream can only be an MPEG-2 transport stream."
While I understand the idea, the juxtaposition of "can be anything",
"however it can only be..." puzzles me a little bit. Additionally, is
there any difference between "the source" (1st sentence) and "the
source stream" (2nd sentence)?


4- Introduction, p3, RTP stream definition:
It is said:
   "In DVB AL-FEC, the source packets are carried in the source RTP
    stream and the generated FEC repair packets at each layer are carried
    in separate streams."
For an RTP expert, it's probably sufficient. However, as far as I am
concerned, I'd like to see a definition of an "RTP stream" and how
the "separate streams" are distinguished (source or destination
port/address, or both, or...). I guess there are several possibilities.


5- Section 3.1., Base-Layer FEC, p5, minor clarification proposal:
It is said:
   "In a group of D x L source packets,
    the XOR operation is applied to the group of the source packets whose
    sequence numbers are L apart from each other to generate L repair
    packets."
Instead, I suggest:
   "In a group of D x L source packets,
    the XOR operation is applied to each group of D source packets whose
    sequence numbers are L apart from each other to generate a total of L repair
    packets."


Best regards,


    Vincent