One document matched: draft-yang-avt-rtp-synced-playback-00.txt
Audio/Video Transport WG P. Yang
Internet Draft Y.-K. Wang
Intended status: Standards track Huawei Technologies
Expires: January 2010
July 6, 2009
Synchronized Playback in Rapid Acquisition of Multicast Sessions
draft-yang-avt-rtp-synced-playback-00.txt
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Abstract
When watching the same IPTV channel, different TV sets may not
render the same picture and the associated audio at the same moment.
This variation of end-to-end delay between users is referred to as
inter-user playback delay. Unicast based rapid acquisition of
multicast RTP sessions (RAMS) as specified in [I-D.ietf-avt-rapid-
acquisition-for-rtp] is an important technique in achieving fast
channel switching in IPTV applications. In addition, RAMS also
significantly relaxes the requirement of relatively short random
access point period in encoding of video streams in multicast
applications, thus allowing significantly improved compression
efficiency. However, on the other hand, the use of RAMS increases
inter-user playback delay. This document specifies a mechanism to
help reduce inter-user playback delay in RAMS.
Table of Contents
1. Introduction..................................................3
2. Conventions...................................................4
3. Definitions...................................................5
4. Inter-User Playback Delay Reduction...........................5
5. Message Extensions............................................5
5.1. Extension to RAMS-R......................................6
5.2. Extension to RAMS-I......................................6
6. Security Considerations.......................................6
7. IANA Considerations...........................................6
8. Acknowledgements..............................................6
9. References....................................................6
10. Authors' Addresses...........................................7
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1. Introduction
The Internet-Draft "Unicast-Based Rapid Acquisition of Multicast
RTP Sessions" in [I-D.ietf-avt-rapid-acquisition-for-rtp] presents
a method based on unicast burst stream for rapid acquisition of
multicast RTP sessions (RAMS), thus to reduce the waiting time for
the so-called Reference Information (RI). This method is effective
in reducing channel switching and tune-in delay in multicast
applications, such as IPTV. The RI typically starts at an access
unit that is a random access point. In RAMS, RTP receivers start
playback from the random access point from which the RI starts when
they switch from one multicast session to another. As the unicast
burst stream starts from the RI and is transmitted as fast as
possible and faster than the media rate, on average the receiver
can start processing the unicast burst stream almost immediately,
and does not need to wait for the next random access point if it
directly joined the multicast group.
Another important benefit brought by RAMS is that significantly
improved coding efficiency for video streams is possible. In
conventional multicast applications, video streams must be encoded
with frequent random access points, e.g. 0.5 to 1 second, to allow
new receivers to tune-in or existing users to switching from
another multicast session. Random access points typically contain
intra-coded pictures, for which the compression efficiency is
significantly, e.g., several to ten times, lower than inter-coded
pictures. Therefore, the less the random access points, the higher
the coding efficiency. When RAMS is in use, random access period
length is less critical for the tune-in or channel switching delay.
This means that the video random access point frequency can be
significantly reduced, leading to significantly improved
compression efficiency.
In multicast applications, receivers receiving the same multicast
session can not playback the same content in an absolutely
synchronized manner, due to the variation in various delays
including end-to-end transmission delay, receiver buffering delay,
decoding buffering delay, and output buffering delay. Moreover,
events like session transfer or rebroadcast can further increase
inter-user playback delay. Thus, different users may watch
different pictures from different TV sets when watching the same
IPTV content. In this document, this playback delay variation
between different users is referred to as inter-user playback delay.
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In some application scenarios, e.g., remote education or a
discussion room for an ongoing TV program in a social network,
different users may be discussing the same content received through
multicast. In this case, an obvious playback synchronization loss
due to excessive inter-user playback delay can generate bad user
experience.
A disadvantage of RAMS is that the use of the technique increases
inter-user playback delay. Regardless of when the receiver starts
the RAMS request (i.e. joins the program), the playback will start
from the previous random access point. Thus, the later the
receiver joins the program in between two random access points, the
longer the display delay will the user have compared to other users.
The longest inter-user playback delay due to the use of RAMS is
close to the interval between the two random access points, when
one user joins the program right after the first random access
point while another user joins the program right before second
random access point, as depicted in Figure 1 below. In Figure 1,
the two I frames are two consecutive random access points.
IBBPBBPBBPBBPBBPBBPBBPBBPBBPBBPBBPBBPBBPBBPBBPBBPBBP...BBPBBPI...
^ ^
| |
|<--The longest inter-user playback delay due to RAMS use-->|
Figure 1 Inter-user playback delay due to the use of RAMS
As can be seen from the above analysis, the issue of inter-user
playback delay also constrains the use of long random access period
length for improved compression efficiency, which has been another
important benefit of RAMS.
In this document, we describe a mechanism to reduce inter-user
playback delay and to allow the use of long random access period
length for improved compression efficiency when RAMS is in use.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC 2119
[RFC2119].
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3. Definitions
This document uses the following acronyms and definitions:
Inter-User Playback Delay: The playback delay between different
users for the same content on the same multicast session.
4. Inter-User Playback Delay Reduction
The mechanism involves the following changes to the RAMS method:
1) When the RTP Receiver (RR) sends a rapid acquisition request for
the new multicast RTP session, the request MAY contain
additional information indicating whether RR supports inter-user
playback delay reduction.
2) When the Retransmission Sever (RS) receives the RAMS-R message
and decides to accept it, RS MAY include the following
additional information in the RAMS-I message to RR:
a) N, the playback delay reduction target in number of frame
durations; and
b) V, recommended interval, in frames, between two continuous
events for skipping of one frame.
When the RAMS-R message indicates that RR supports inter-user
playback delay reduction, RS SHOULD include the above
information in the RAMS-I message.
3) When RR receives an RAMS-I message containing the above
information, it SHOULD speed up media rendering during playback
taking into account the information as follows. During the
speedup playback, after each V frames, one frame is skipped as
if it was not present, and the presentation time of each
remaining frame is shifted earlier by one frame duration, until
totally N frames have been skipped. Receivers will playback the
media content with its original speed after totally N frames
have been skipped. Note that decoding remains the same as if
speedup playback was not in use.
5. Message Extensions
This section defines the extensions to RAMS-R and RAMS-I messages
for inter-user playback delay reduction.
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5.1. Extension to RAMS-R
TBD.
5.2. Extension to RAMS-I
TBD.
6. Security Considerations
TBD.
7. IANA Considerations
TBD.
8. Acknowledgements
TBD.
This document was prepared using 2-Word-v2.0.template.dot.
9. References
[I-D.ietf-avt-rapid-acquisition-for-rtp]
Steeg, B., Begen, A., Caenegem, T., and Z. Vax, "Unicast-
Based Rapid Acquisition of Multicast RTP Sessions",
draft-ietf-avt-rapid-acquisition-for-rtp-01 (work in
progress), June 2009.
[I-D.ietf-avt-rtcp-guidelines]
Ott, J. and C. Perkins, "Guidelines for Extending the RTP
Control Protocol (RTCP)", draft-ietf-avt-rtcp-guidelines-
01 (work in progress), March 2009.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
July 2006.
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[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
July 2006.
10. Authors' Addresses
Peilin Yang
Huawei Technologies Co., Ltd.
No.91,Baixia Road, Nanjing 210001
P. R. China
Phone: +86-25-84565881
EMail: yangpeilin@huawei.com
Ye-Kui Wang
Huawei Technologies Co., Ltd.
400 Somerset Corp Blvd, Suite 602
Bridgewater, NJ 08807
USA
Phone: +1-908-541-3518
EMail: yekuiwang@huawei.com
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