Subject: [MMUSIC] I-D ACTION:draft-ietf-mmusic-media-loopback-03.txt

From: [email-masked]
Date: Mon, 26 Jun 2006 18:50:02 -0400

A New Internet-Draft is available from the on-line Internet-Drafts directories.
This draft is a work item of the Multiparty Multimedia Session Control Working Group of the IETF.

	Title		: An Extension to the Session Description Protocol (SDP) for Media Loopback
	Author(s)	: K. Hedayat, et al.
	Filename	: draft-ietf-mmusic-media-loopback-03.txt
	Pages		: 17
	Date		: 2006-6-26
	
The wide deployment of Voice over IP (VoIP), Real-time Text and 
Video over IP services has introduced new challenges in managing 
and maintaining voice/real-time Text/video quality, reliability, 
and overall performance.  In particular, media delivery is an area 
that needs attention.  One method of meeting these challenges is 
monitoring the media delivery performance by looping media back to 
the transmitter.  This is typically referred to as "active 
monitoring" of services.   Media loopback is especially popular in 
ensuring the quality of transport to the edge of a given VoIP, 
Real-time Text or Video over IP service.  Today in networks that 
deliver real-time media, short of running 'ping' and 'traceroute' 
to the edge, service providers are left without the necessary tools 
to actively monitor, manage, and diagnose quality issues with their 
service.  The extension defined herein adds new SDP media 
attributes which enables establishment of media sessions where the 
media is looped back to the transmitter. Such media sessions will 
serve as monitoring and troubleshooting tools by providing the 
means for measurement of more advanced VoIP, Real-time Text and 
Video Over IP performance metrics.

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From: Colin Perkins <[email-masked]>
Date: Tue, 27 Jun 2006 18:45:58 +0100

> 	Filename	: draft-ietf-mmusic-media-loopback-03.txt
...
> http://www.ietf.org/internet-drafts/draft-ietf-mmusic-media- 

I've reviewed this draft, and have some serious concerns about the  
rtp-pkt-loopback mode. Since RTP sequence numbers and timestamps are  
not regenerated, it is not possible to determine if loss/jitter  
occurs on the forward or reverse path, and difficult to generate  
appropriate RTCP reports. It would seem to me more appropriate to  
define an encapsulating payload format, and use that to loop back the  
media packets. The encapsulated loopback packets would have correct  
RTP sequence numbers and timestamps, representing the time at which  
they were looped back, so allowing the monitor to distinguish forward  
and reverse path loss. In addition, RTCP could be generated in the  
normal manner for the looped back stream, other RTP mechanisms (e.g.  
security, FEC, etc.) could be employed to protect the loopback  
stream, and the result would be fully compliant with the relevant RTP  
standards.

Section 6 recommends the loopback stream ignore congestion control,  
which I don't believe is acceptable in an standards track document.  
Use of an encapsulating payload format for looped back packets would  
allow thinning of the loopback stream as a means of congestion  
control, and allow the monitor to determine that thinning occurred on  
the reverse path (since the thinned stream would have missing RTP  
sequence numbers, separate to the sequence numbers of the  
encapsulated packets).

Finally, the ABNF in section 5.1 is unusual since rtp-start-loopback  
is included in both the loopback-type-choice and in loopback-type  
lines. This allows "a=loopback: rtp-start-loopback rtp-start-loopback  
rtp-start-loopback" which is probably not what was intended.

Colin