Encspulation Methods : Some questions.

[alain.ritoux@6wind.com]

Hi all,

1) Precision about byte ordering
About the encapsulation methods (both), a precision should be
added in the SNDU format, about byte order telling that :
  - length is in network order
  - type is in network order
network order being MSB first, hence a 1000 bytes IPv4 packet
encapsulation would begin with :
  0x03 0xE8 0x08 0x00 0x45 ...

2) The length field itself
In description of IPv4/v6 SNDU it is said
"the payload shall be a complete IPv4 datagram"
My understanding (correct me if I'm wrong) is that it can be IPv4/v6
fragments, not necesseraly re-assembled IP datagrams, for it would
introduce :
 - latency
 - work on both sides (re-ass in the sending, and frag on the
   recv side before IP processing)

So, indeed IP packet may be up to 64Ko, BUT IP provides fragmentation
and it will be IP fragments that will flow through the DVB-link, so
what should be decided is what is the optimal MTU value for the DVB
link. Maybe something like 1500 (a-la ethernet), or 4096 ??

In such case Length field could be limited to 12 bits (leaving 4
more bits for any other stuff)


3) The CRC

Where is the exact CRC-32 defined ?

More over, is tehr some CR/whatever mechansim at TS(or below) level,
ensuring that TS frames are OK ? If yes, then is raly a CRC needed in
the encapsulation, for IP checksum will be check by end user, and so
maybe we don't need extr checks (and overhead), in the same p^hilosophy
that lead the IPv6 header to not have checksum anymore.


Thanks for all
Regards.

Alain.
--
Alain RITOUX
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Fax +33-1-39-30-92-11
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