Subject: [AVT] Merge the two retransmission drafts

From: Magnus Westerlund <[email-masked]>
Date: Fri, 05 Jul 2002 13:34:15 +0200

Hi,

This mail is arguing for the merger of  the two retransmission drafts 
draft-ietf-avt-rtp-selret-05.txt and 
draft-ietf-avt-rtp-retransmission-02.txt. I do believe that there is 
significant advantages of merging the two proposals.

First, the two proposed solutions for retransmission are very similar. 
The only real difference is if the retransmitted packets are done in the 
original stream or in a new session. Otherwise there is a number of 
smaller issues regarding how the sever and client should behave. This 
would benefit from having a single document explaining how they should 
behave. If there are differences that are necessary for different use 
cases these could clearly be presented and signaling introduced. This 
would help future implementors from having to read two documents that 
are slightly contradicting and form there own idea.

Secondly, the selective retransmission draft is actually describing two 
mechanisms that can be used independently of each other. The draft 
describes how to do actual retransmissions with the help of the RTX 
format, this functionality does also exist in the retransmission draft. 
However the mechanism for giving the client prioritization information 
so that it can better utilize a narrow back-channel at some cost in the 
forward channel is not present. By merging the two draft the 
retransmission transport mechanism in Leon's draft can be used together 
with SEL payloads. This could be solved by splitting out the mechanism 
for selective retransmissions request, however I do believe that this 
would make an overview of the possible solutions harder.

Third, the two retransmission payload formats are very similar. Actually 
the selective retransmission drafts format can be seen as a superset of 
the other drafts format. So I would argue that the RTX payload formats 
should be common between the two solutions. This would be very simpler 
if the two solutions is contained in the same draft.

Fourth, the SDP signaling part for how to use retransmission are 
immature for both proposals. I strongly believe that this would be 
simpler to align and verify if merged into one document. There are a 
number of possible use cases of either of the solutions that needs to 
possible to distinguish. These may more simply be missed when having two 
documents.

Comments?

Regards

Magnus Westerlund 

Multimedia Technologies, Ericsson Research ERA/TVA/A
----------------------------------------------------------------------
Ericsson AB                | Phone +46 8 4048287
Torshamsgatan 23           | Fax   +46 8 7575550
S-164 80 Stockholm, Sweden | mailto: [email-masked]

_______________________________________________
Audio/Video Transport Working Group
[email-masked]
https://www1.ietf.org/mailman/listinfo/avt

---

From: Rahul Agarwal <[email-masked]>
Date: Fri, 05 Jul 2002 14:46:43 -0700

I completely agree that the two drafts should be merged. My working 
assumption all along has been that there will really be only one standard 
for RTP re-transmission and these two drafts will ultimately evolve into a 
single draft. I just do not see companies implementing support for multiple 
re-transmission schemes in RTP servers/clients when they essentially solve 
the same problem.

SDP signalling does need more work. The re-transmission feature 
selection/negotiation can be done via SDP as the SDP usage section matures, 
or by SETUP extensions in RTSP.

- Rahul

At 01:34 PM 7/5/2002 +0200, Magnus Westerlund wrote:
>First, the two proposed solutions for retransmission are very similar. The 
>only real difference is if the retransmitted packets are done in the 
>would help future implementors from having to read two documents that are 
>slightly contradicting and form there own idea.
>
>However the mechanism for giving the client prioritization information so 
>that it can better utilize a narrow back-channel at some cost in the 
>with SEL payloads. This could be solved by splitting out the mechanism for 
>selective retransmissions request, however I do believe that this would 
>make an overview of the possible solutions harder.
>
>should be common between the two solutions. This would be very simpler if 
>the two solutions is contained in the same draft.
>
>Fourth, the SDP signaling part for how to use retransmission are immature 
>for both proposals. I strongly believe that this would be simpler to align 
>and verify if merged into one document. There are a number of possible use 
>cases of either of the solutions that needs to possible to distinguish. 
>These may more simply be missed when having two documents.
>
>Comments?
>
>Regards
>
>Magnus Westerlund

---

From: Colin Perkins <[email-masked]>
Date: Mon, 08 Jul 2002 12:07:57 -0400

Magnus,

I agree with your analysis. I'd like to see the process start with some
agreement on exactly which problems are - and are not - being solved by
the two drafts. Much of the recent discussion seems to be due to feature
creep, and trying to apply these formats to slightly different problems. 

Colin