clausen-ipdvb-enc : Some questions

[alain.ritoux@6wind.com]

Hi all,

I have some question about the draft "Simple Encapsulation for
transport of IP over MPEG-2/DVB networks" (draft-clausen-ipdvb-enc-01.txt)
As a newbie in satellite stuff, some of these questions/precisions
may be irrelevant, thanks for indulgence ;-)


1) SNDU format
Type field : 2 bytes.
Tha's sure, getting the ether_type is coll way of next header
value, and can be esaily kept for other protocols, but do we really
need 64K possible protocols to be transported over DVB
May be it can be reduce to a single byte, wich would let a free
byte (because of alignement), but I'm rather sure, people would think
of plenty possible usage of those free bits ;-)

2) Usage of PUSI/AFC
I don't really get the usage of the AFC. where does the pointer points
in the two cases ? I think I see, but tell me if I'm wrong :

1st case
                                     +-------------------+
                                     |Encap | Subnetwork |
                                     |Header|  PU        |
                                     +-------------------+
                                      \                 /
                                       \               /
                                        \             /
         +------*--------*--------------+------------+
         |MPEG-2| Adapt. |   Stuffing   |     SNDU   |
         |Header| Header |     bytes    |            |
         +------+--------+--------------+^-----------+
                        |                |
                        +----------------+

2nd case
           +------------------+       +------------------+
           |   Subnetwork     |       |   Subnetwork     |
           |      DU 1        |       |      DU 2        |
           +------------------+       +------------------+
                      \        \     /          /
                       \        \   /          /
                        \        \ /          /
         +------*--------*--------+----------+
         |MPEG-2| Adapt. | end of | start of |
         |Header| Header | SNDU 1 | SNDU 2   |
         +------+--------+--------+^---------+
                        |          |
                        +----------+
and in the 2nd case, is it possible to stuff bytes between end-SNDU1
and SNDU2 (for alignement purpose) ?

In a more general way, wouldn't it be simpler (and cost less in
overhead) to avoid any AFC header, and to impose SNDU start on 32-bits
boundaries, because as SNDU will include length indicator, the padding
will be automatically detected at the receiver side. Surely I miss
something, and I feel it is around p14, but I don't see where this
value 0x47 come from  ?

3) Fluhsing the bit-stream
Indeed, the  fact that the encapsulator doesn't wait for another SNDU
to come to pack, is enough to avoid latency. OK, it resolves the pending
of an IP packet at the encaspultor level when no other IP packet comes.

BUT what must be done at TS-muliplex level ? This should be specified by
the encpaulsation method : if the push packet is not added, what is to
be done ?

Thanks again for having read so far ...
Regards.
Alain.
--
Alain RITOUX
Tel +33-1-39-30-92-32
Fax +33-1-39-30-92-11
visit our web http://www.6wind.com