Subject: Question on RFC2733, packet-based FEC

From: Stephan Wenger <[email-masked]>
Date: Wed, 31 May 2000 09:41:54 +0200

Hi all,

I don't understand the reason for an (artificial?) limitation of
RFC2733.  RFC2733 says that, when calculating FEC over
several packets of different sizes, the FEC packet has to
protect all bytes of the biggest packet (and padding is to be
used for the other packets to bring them up to the size of
the biggest packet).  Is there some hidden secret about this
requirement?

The rest of the EMail describes scenarios where this limitation
is counter-productive.

In a recent ITU meeting, Adam Li of UCLA brought to our
attention an idea to implement unequal error protection using
packet based FEC.  What they want to do is to use data
partitioning to put important video data (such as headers and
motion vectors) to the front of a packet, and put the less
important coefficients to the end.  Then apply packet based
FEC only on the first (more important) part of the packets.
Resulting FEC packets are smaller -- lower bandwidth requirements.

Thinking about this idea, several more applications come
readily in mind.  When using FEC on RTP packets using audio
redundancy coding, you can protect only the original audio
but not the redundant copy.  This could, for example, make
sense in a multi-unicast environment with very different QoS
to the various endpoint (telephone conference with wired and
wireless terminals -- the conference bridge applies FEC only
for the wireless links).

So, why MUST a FEC packet protect all bytes of any source packet?

Stephan