Re: Subject: I-D ACTION:draft-clausen-ipdvb-enc-00.txt

[Patrick Cipiere <Patrick.Cipiere@udcast.com>]

I am pretty much OK with what is said about the packing, but today,
the mpeg2 transport packets I am dealing with, do not use the
adaptation field, but just use the first byte of the transport packet
payload as an offset if PUSI bit is 1.
Of course doing this, breaks the 32 bits alignment.
If we think we need to respect this alignment, having this offset == 3
might do the job.
I am not sure we have to use the 32 bits AFC in all cases.
If saving bits is the goal, let's keep a single byte offset.

I have a lot of concerns about the lack of layer 2 adresses in the new
design.
I think there are a lot of situations where we need layer 2 MAC
addresses in the datagram:
Without layer 2 destination address, the filtering has to be done at
layer 3.
Without layer 2 destination address, what will be the behaviour of a
router receiving a layer 3 packet.
Without layer 2 source address, how do we recognize our own datagrams,
when we get them back on the link several ms after sending them.
...

If we are really convinced that in some cases either the destination
and/or source MAC address are uneeded, and if the goal is to save some
bits not having them, we could use 2 bits out of the length field
The length field would then be 14 bits ([0,16383])
MAC_FLAG

00 : no dst MAC, no src MAC
01 : dst MAC, no src MAC
10 : src MAC, no dst MAC
11 : dst MAC, src MAC

MAC is IEEE 48 bits
no dst MAC is equivalent to a broadcast: ff:ff:ff:ff:ff:ff

+--+--------------+----------------+----------
|00|    length    |      type      |Payload
+--+--------------+----------------+----------

+--+--------------+----------------+-----------  --+----------
|01|    length    |      type      |48 bits dst ~  | Paylaod
+--+--------------+----------------+-----------  --+----------

+--+--------------+----------------+-----------  --+----------
|10|    length    |      type      |48 bits src  ~ | Paylaod
+--+--------------+----------------+-----------  --+----------

+--+--------------+----------------+-----------  --+-----------  --+---------
|11|    length    |      type      |48 bits dst ~  |48 bits src ~  |Payload
+--+--------------+----------------+-----------  --+-----------  --+---------


Patrick.
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