Re: [Fecframe] About the Generic Explicit Source FEC Payload ID

[Abhishek ANAND <[e-mail]>]

On 02/17/2011 04:50 PM, Ali C. Begen (abegen) wrote:

 > -----Original Message-----
 > From: [e-mail] [mailto:[e-mail]] On 
Behalf Of Abhishek ANAND
 > Sent: Thursday, February 17, 2011 6:04 AM
 > To: [e-mail]
 > Subject: Re: [Fecframe] About the Generic Explicit Source FEC Payload ID
 >
 > Hello everyone,
 >
 > I am writing to introduce another scenario where we may use multiple 
FEC Schemes to protect the same Source Packet flow.
 > The particular scenario is one in which we protect multiple Source 
Packet flows together (in any possible combination) using
 > multiple FEC Schemes.
 >
 > Example:
 > We have two flows F1 and F2 and two FEC Scheme instances FEC1 and 
FEC2 where FEC1 protects the Source Packets from F1
 > and FEC2 protects the Source packets from F1 as well as F2. This 
example can be extended to add additional flows and
 > schemes.

abegen: Like figure 1 in 
https://datatracker.ietf.org/doc/rfc5956/?include_text=1 ?

aanand: Yes like that, with LA-FEC being one such application. So in 
addition to DVB ALFEC we have other potential technologies which might 
use interdependent FECFrameworks.

 > One such scenario is using Layer Aware FEC for protecting layered 
streams such as H.264 SVC/MVC (
 > 
http://www.hhi.fraunhofer.de/en/departments/image-processing/image-communication/multimedia-transmission/layer-
 > aware-forward-error-correction-l-fec/ ). There could be other such 
use cases as well.
 >
 > In such cases even when the Source Packet flows are RTP flows we must 
add Source Packet Payload IDs. In addition we can
 > not use the Generic Explicit Source FEC Payload ID as it is described 
in Section 5.3.1 of the FEC Framework Draft because the
 > position of a Source Packet in the Source Block will not be unique 
across all FEC Schemes.

abegen: Sorry, I don't understand this. The place of a source packet is 
derived from the information put in the repair packets, not source packets.

anand: I was of the opinion that the Source Packet's place is derived 
from the information put in the Repair Packets only when they belong to 
a single sequenced flow. In this case the Repair FEC Payload ID carries 
the Initial Sequence Number which is used to place the Source Packet in 
its place. In other cases where the Source Packets are not sequenced or 
they belong to multiple arbitrary sequenced flows I believe the Source 
Packet's place is calculated using the information present in the Source 
Payload ID, such as Source Block Number and Encoding Symbol ID(ESI). I 
think that is how the Raptor Schemes for DVB AL-FEC are designed. Also 
"draft-roca-fecframe-simple-rs" defines one such FEC Scheme for Reed 
Solomon codec where a FEC Source Packet MUST contain the specified 
Source FEC payload ID.

abegen: It should be sufficient to have the source packets seqnum'ed. 
And RTP satisfies that requirement nicely.

aanand: When we mix two RTP flows we loose the unique sequence number 
sequences which would have been otherwise used by the FECFrameworks at 
the sender and the receiver to place Source Packets. This problem 
together with my opinion above makes it difficult to use the Generic 
Explicit Source FEC Payload ID which is based on sequence numbers that 
are unique across the FECFrameworks.
For example:
Lets continue the example with flows F1, F2 and frameworks FEC1, FEC2.
We add the following assumptions:
FEC1 uses Source Blocks of length 3 and FEC2 of length 6.
RTP sequence numbers for F1: 1,2,3
RTP sequence numbers for F2: 3,4,5.
I think RTP sequence numbers or any unique "consecutive/"/ sequence 
numbers across all FECFrameworks can not be used to place the Source 
Packets in the Source Block for FEC2, so we must add Source FEC Payload 
IDs.

Maybe I am missing some points, in that case I apologize for my ignorance.

 > Another observation regarding the Generic Explicit Source FEC Payload 
ID, since it does not contain the Source Block Number
 > and relies solely on consecutive sequence numbers it seems that the 
corresponding Repair FEC Payload IDs must also use the
 > Initial Sequence Number of each Source Block instead of the Source 
Block Number.

abegen: The generic field cannot carry the source block number as you 
said block sizes can and will vary among fecframe instances. And source 
block number is something used by the decoder but it is derived from the 
seqnum and base seqnum of a block (i.e., its value is not really used in 
an absolute fashion). I am not sure whether you saw something in the 
text that said otherwise.

aanand: It was just an observation because it means that some FEC 
Schemes which explicitly mention a Source FEC Payload ID, Repair FEC 
Payload ID combination not based on sequence numbers will not be usable 
in the use cases of interdependent FECFrameworks. And if my points above 
are correct we might not be able to use unique sequence numbers to place 
Source Packets across all the interdependent FECFrameworks.

-acbegen

 > Regards,
 > Abhishek
 >
 > On 02/11/2011 03:14 PM, [e-mail] wrote:
 >
 > > -----Original Message-----
 > > From: [e-mail] [mailto:[e-mail]] 
On Behalf Of Vincent Roca
 > > Sent: Friday, February 11, 2011 4:03 AM
 > > To: [e-mail]
 > > Subject: [Fecframe] About the Generic Explicit Source FEC Payload ID
 > >
 > > Hi everybody,
 > >
 > > While preparing the new O&M section proposal with Ali,
 > > we spotted two points we'd like to discuss. Here is the first
 > > one.
 > >
 > > Point 1: about the "Generic Explicit Source FEC Payload ID"
 > > -------------------------------------------------
 > >
 > > Section 5.3.1. of the FECFRAME Framework I-D says:
 > >
 > >    "In order to apply FEC protection using multiple FEC schemes to a
 > >     single source flow, all schemes have to use the same Explicit 
Source
 > >     FEC Payload ID format.  In order to enable this, it is RECOMMENDED
 > >     that FEC schemes support the Generic Explicit Source FEC Payload ID
 > >     format described below."
 > >
 > > Then it explains what is meant by "Generic source FEC payload ID",
 > > i.e. a two byte field, in network byte order, that contains a sequence
 > > number shared by all FEC schemes.
 > >
 > > When reading 5.3.1, it is clear to me that this field is to be carried
 > > in an **Explicit Source FEC Payload ID**, i.e. at the end of the 
packet.
 > > Section 5.3.1 does not suggest to reuse any sequence number that
 > > may exist in the application flow (like the RTP sequence number) to
 > > achieve this goal.
 > >
 > > We also notice that the sentence is rather strong in what FEC Schemes
 > > should define (it says "RECOMMENDED").
 > >
 > > It turns out that none of the current FEC Schemes define such a
 > > Generic Explicit FEC Payload ID.
 > > Additionally, the only use-case where an application flow is
 > > protected by two different AL-FEC codes and that is considered
 > > by IETF is the DVB-IPTV (draft-ietf-fecframe-dvb-al-fec-04). In
 > > that case the solution consists in reusing the RTP SN field, rather
 > > than defining a "Generic source FEC payload ID".
 > >
 > >
 > > So we see two options here:
 > > 1- Extend section 5.3.1 recommendations so that, if the
 > >      application flow already includes a sequence number that
 > >      fulfills the requirements of section 5.3.1, then this field can
 > >      be used to fulfill the needs specified here. But this is not
 > >      strictly speaking a special case of Explicit FEC Payload ID
 > >      (nothing is added to source packets).
 >
 >       Right. If there is a seqnum that can be used, it could be used 
and no chances to the source packets at all. If not, we
 > need to specify the format for the generic source fec payload ID 
field. And the format we define in the framework draft is
 > RECOMMENDED to be used when an fec scheme desires to support being 
able to be used with other FEC schemes jointly. In
 > other words, if an FEC scheme chooses a different format for the 
explicit source fec payload id, it won't be able to be used
 > with other FEC schemes that also need an explicit source fec payload 
id. This is not interoperability but for greater good, I
 > think we should RECOMMEND the generic format we will define.
 >
 >       -acbegen
 >
 > > 2- or weaken current 5.3.1 recommendation:
 > > Old text:
 > >         "In order to enable this, it is RECOMMENDED
 > >          that FEC schemes support the Generic Explicit Source FEC
 > > Payload ID
 > >          format described below."
 > > New text:
 > >         "When this feature is desired, one solution can be for the FEC
 > > schemes
 > >          to support the Generic Explicit Source FEC Payload ID format
 > > described
 > >          below."
 > >
 > > Opinions?
 > >
 > > Cheers,
 > >
 > >     Vincent and Ali.
 > > _______________________________________________
 > > Fecframe mailing list
 > > [e-mail]
 > > https://www.ietf.org/mailman/listinfo/fecframe
 >
 >
 >
 >
 > --
 > Abhishek Anand
 > Consultant
 > STMicroelectronics - Torino - ITALY
 > www.st.com
 >
 > Phone: [phone]
 > mailto: [e-mail]

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