Subject: [MMUSIC] RFC 4756: Grouping for FEC

From: "Ali Begen (abegen)" <[email-masked]>
Date: Sun, 27 Jan 2008 00:20:58 -0800

Hi everyone,

I have a question regarding RFC 4756. 

Section 4.1 of RFC 4756 states that it is possible to have one payload
stream protected by more than one FEC stream , or multiple payload
streams sharing one FEC stream.

Section 5 of RFC 3388 states that an "m" line identified by its "mid"
attribute MUST NOT appear in more than one "a=group" line using the same
semantics.

Scenario under consideration:

Suppose I have two source flows. I generate a repair flow using FEC
Scheme #1 from source #1. I then combine sources #1 and #2 and apply FEC
Scheme #2 to generate repair flows #2 and #3.  

Source #1(mid:1) --> FEC Scheme #1 --> Repair flow 1(mid:10)
Source #1(mid:1) + Source #2(mid:2) --> FEC Scheme #2 --> Repair flow
#2(mid:11) and repair flow #3(mid:12)

Intuitively, I would write as below, but RFC 3388 does not allow this.
a=group:FEC 1 10
a=group:FEC 1 2 11 12

So, could/should I write the SDP as follows?
a=group:FEC 1 2 10 11 12

I suspect this is OK since "group:FEC" only describes the association of
source and repair flows, and does not say anything about whether a
particular repair flow is associated with a particular source flow.

Is that correct?

Thanks,
-acbegen

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From: Adam Li <[email-masked]>
Date: Sun, 27 Jan 2008 10:44:22 -0800

Ali,

You are right. One should put them in one line as you mentioned in order 
to comply with the restriction put forth by Section 5 of RFC 3388.

In the scenario you mentioned, it is considered as a "grand" protection 
scheme that protects two payload streams with three FEC streams. As 
mentioned in Section 4.1 of RFC 4756, the detail of the scheme are 
conveyed by other mechanism.

Hope it helps.

Adam