[AVTCORE] Length in draft-ietf-avtext-framemarking

Currently, draft-ietf-avtext-framemarking defines RTP header extensions of
lengths 1, 2 or 3:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  ID=? |  L=2  |S|E|I|D|B| TID |   LID         |    TL0PICIDX  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              or
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  ID=? |  L=1  |S|E|I|D|B| TID |   LID         | (TL0PICIDX omitted)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              or
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  ID=? |  L=0  |S|E|I|D|B| TID | (LID and TL0PICIDX omitted)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


Is an implementation required to be able to send and receive any of
these formats?


Within Section 3.2.1 we have LID mappings for various codecs, all
using the L=2 format.

But if the LID was not necessary (temporal scalability only), could
the L=0 format be used instead?


Currently, the URI that is defined has no extension attributes:

      a=extmap:3 urn:ietf:params:rtp-hdrext:framemarking


Let us say that there is an SFU that only supports temporal
scalability, and therefore would prefer the L=0 format.

Is it possible for the SFU to indicate to the client that it would
prefer it use the L=0 header extension?

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