Subject: [payload] Updates on FLEX FEC (ver -19)

From: Giridhar Mandyam <[email-masked]>
Date: Thu, 28 Mar 2019 18:18:06 +0000

Dear Payload WG,

After offline discussion with the editors and reviewers (special thanks to Bernard Aboba), we have decided that the spec needed to be slightly modified.  Version -19 contains the following changes:

a) Recommendation against negotiation of RTP RTX when FLEX FEC is used.  A new section 1.1.7 on the use of FLEX FEC retransmissions has been added to indicate as such.

b) Elimination of all optional SDP parameters (e.g. ToP, L and D fields).  This means that the sender can send any FEC configuration (as indicated by the FEC header) as long as it stays consistent with the mandatory SDP parameters (rate and repair window).  Sec. 1.1.6, 1.1.7, 4.2.2, and 5 are the primary impacted sections.  We believe this will greatly simplify SDP O/A and while retaining the "flexibility" of FLEX FEC to adapt FEC repair data on a packet-by-packet basis. 
	- Note that L and D cannot be larger than 255, respectively.  As we gain implementation experience, if we decide that 255 is not sufficient then it could be possible to extend the corresponding header fields in the future. Currently there are header values that are reserved (R=F=1 and  L=D=0) that could be used to indicate the use of an extended L and D field, as an example of one way to implement extended fields for L and D.

c) Slight adjustment of Sec. 6.3.1.2 to correct for sequence numbering errors in the abstract procedure for grouping of source packets using their RTP sequence numbers.

d) Accounting for the remaining nit's in Benjamin Kaduk's last review.

e) Explicitly defining "FLEX FEC" in the abstract.

Please provide any feedback within 7 days of the sending of this notification.

Thank you,

-The Editors of FLEX FEC