One document matched: draft-yang-avt-rtp-synced-playback-03.txt
Differences from draft-yang-avt-rtp-synced-playback-02.txt
Audio/Video Transport P. Yang
Internet-Draft Ye-K. Wang
Intended status: Standards Track Huawei Technologies Co., Ltd.
Expires: August 31, 2010 February 27, 2010
Synchronized Playback in Rapid Acquisition of Multicast Sessions
draft-yang-avt-rtp-synced-playback-03.txt
Abstract
When watching the same IPTV channel, different TV sets may not render
the same picture and the associated audio at the same moment. The
variation in end-to-end delay that resulted in such asynchronous
playback 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 as
well as other multicast 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, which makes users receiving the same
multicast session playback the same content asynchronously. This
document specifies a mechanism to help reduce inter-user playback
delay caused by the use of RAMS.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
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and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
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The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
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This Internet-Draft will expire on August 31, 2010.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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the Trust Legal Provisions and are provided without warranty as
described in the BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Reducing Inter-User Playback Delay due to RAMS . . . . . . . . 5
5. Message Extensions . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Extension to RAMS-R . . . . . . . . . . . . . . . . . . . 7
5.2. Extension to RAMS-I . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
9. Normative References . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
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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 faster, because it does not
need to wait for the next random access point, as it does 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 not the decision factor 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 a video communication system, an end-to-end delay is unavoidable.
Typical end-to-end delay components are transmission delay, receiver
buffering delay (which also handles transmission jitter), decoding
buffering delay, output buffering delay, and other processing delays.
Use of RAMS causes additional much longer end-to-end delay, the
amount of which is equal to the time difference between the latest
RTP timestamp of primary multicast packets buffered in the RS when
the unicast burst starts and the RTP timestamp of the starting point
of the unicast burst.
In multicast applications, receivers receiving the same multicast
session can have different end-to-end delays and thus the playback of
the same content among the receivers is not synchronized. In this
document, a delay in playback time of the same content between any
two users is referred to as inter-user playback delay (IUPD). This
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document further refers to the typical end-to-end delay (when RAMS is
not in use) as Common End-to-end Delay (CED). Among the delay
components of CED, jitter buffer delay is the main part. Typical
set-top box de-jitter buffers can store 100-500 ms (of SDTV) video,
so network jitter must be within these limits and delay variation
beyond these limits will manifest itself as loss [TR-126]. Compared
to jitter buffer delay under the level of several hundreds of
milliseconds, the acquisition of Reference Information by RAMS may
result in longer and more serious delay which depends on the period
of random access points, which sometimes are, vaguely, referred to as
Group of Pictures (GOP) length (distance between key or I-frames).
This document refers to this additional longer and more serious end-
to-end delay caused by the use of RAMS as Rapid Acquisition Playback
Delay (RAPD), and is specific to eliminate the RAPD to reduce the
IUPD. The methods employed to solve CED is out of scope for this
document.
Due to IUPD, different users may watch different pictures from
different TV sets when watching the same IPTV content. 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.
As mentioned above, a disadvantage of RAMS is that the use of the
technique causes RAPD for each user. Receivers are not synchronized
in sending RAMS requests. Regardless of when the receiver starts the
RAMS request (i.e. joins the program), the playback will start from a
previous random access point. Given the starting point, the closer
to the next random access point the receiver joins the program, the
longer the RAP will be. Thus, the use of RAMS increases IUPD.
When obvious IUPD affects user experiences in above application
scenarios, some actions must be taken. One way is to constrain the
use of long random access period length, which significantly hurts
video compression efficiency. In this document, we describe some
mechanisms to eliminate the RAPD to reduce IUPD due to the use of
RAMS and to allow the use of long random access period length for
improved compression efficiency at the same time.
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 [RFC2119].
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3. Definitions
This document uses the following acronyms and definitions frequently:
Inter-User Playback Delay (IUPD): The playback delay between
different users, for the same content transmitted on the same
multicast session, due to variations in end-to-end delays.
Common End-to-end Delay (CED): The end-to-end delay when RAMS is not
in use, which consists of transmission delay, receiver buffering
delay (which also handles transmission jitter), decoding buffering
delay, output buffering delay, and other processing delays.
Rapid Acquisition Playback Delay (RAPD): The additional end-to-end
delay introduced by the use of RAMS. The value is equal to the time
difference between the latest RTP timestamp of primary multicast
packets buffered in the RS when the unicast burst starts and the RTP
timestamp of the starting point of the unicast burst.
4. Reducing Inter-User Playback Delay due to RAMS
Inter-User Playback Delay (IUPD) is causes by the variation in end-
to-end delay, which includes Common End-to-End Delay (CED) and Rapid
Acquisition Playback Delay (RAPD). CED is primarily determined by
jitter buffering delay which is in the range from 100 ms to 500 ms
[TR-126]. In practice, IUPD caused by CED, and primarily by jitter
buffer delay variations, should be under the level of tens of
milliseconds. In contrast to this small range of IUPD values caused
by CED, the IUPD caused by RAPD can be much higher, up to a few
seconds or even to ten seconds, as long period of random access
points can be used for significantly improved video compression
efficiency when RAMS is in us.
To lower IUPD the receiver can speed up video playback until it
catches up with the primary multicast stream. The procedures are
outlined bellow.
The playback synchronization of receivers uses the Primary Multicast
Stream as a reference point to address RAPD due to the use of RAMS.
Each receiver keeps the synchronization with the primary multicast
stream so that all receivers can keep an approximate synchronization
in playback. The advantage is that it does not need every receiver
to get playback synchronization information of other receivers, and
thus scalability is not an issue as the procedure does not depend on
the number of receivers.
The mechanism for speeding up video frames uses a method of skipping
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video frame. In other words, one frame per an interval of some video
frames is skipped until the number of extra video frames has been
skipped. This way, the additional end-to-end delay, RAPD, can be
compensated, and at the same time a smooth playback is ensured.
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.
Retransmission server (RS) can precisely determine the value of
N by detecting the time of RAMS-R, as the amount of additional
end-to-end delay introduced by the use of RAMS, i.e. RAPD, is
known to RS. The value is equal to the time difference between
the latest RTP timestamp of primary multicast packets buffered
in the RS when the unicast burst starts and the RTP timestamp of
the starting point of the unicast burst. The value V is a
recommended skip frame interval and the value must be chosen
such that there is no noticeable audio distortion. For a video
frame rate of 30 frames per second, typically when V is greater
than 15 there is no noticeable audio distortion.
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.
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Besides the above mechanism, RS can use selective transmission of
packets in the beginning of the unicast burst, by taking advantage of
the temporal scalability of video bitstreams.
Video bitstreams are typically temporally scalable, in the sense that
a portion of the bitstream can be extracted and decoded with a lower
frame rate. Conventionally video coding standards like MPEG- 2 can
realize temporal scalability using different types of frames, i.e. I
frame, P frame, and B frame, wherein I frames only consist of the
lowest temporal layer (corresponding to the lowest frame rate), P
frames only consist of the middle temporal layer, and B frames only
consist of the highest temporal layer. Decoding of one temporal
layer requires the presence of the temporal layer itself and all the
lower temporal layers, but not the presence of any higher layer.
H.264/AVC and its scalable extension SVC (Scalable Video Coding)
support advanced temporal scalability thanks to the flexible inter
prediction scheme and flexible reference frame management scheme. It
should be noted that in H.264/AVC and its extensions, B frames
themselves may be reference frames, which may be used by other frames
for inter prediction, therefore the discarding of which may lead to
problems in decoding of the rest of the bitstream.
The RS MAY transmit the unicast burst as follows. In the beginning
of the unicast burst, the RS discards one or more of the highest
temporal layers and transmits only the remaining lower temporal
layers. After a certain point, all temporal layers are transmitted.
This would speed up the acquisition of the multicast session under
the same unicsat burst rate, at the cost of lower initial frame rate.
At the same time, if the playback of the initial stream with lower
frame rate is sped up, inter-user playback delay can be reduced.
5. Message Extensions
This section defines the extensions to RAMS-R and RAMS-I messages for
inter-user playback delay reduction.
5.1. Extension to RAMS-R
The RAMS Request message is identified by SFMT=1. This message is
used by RTP_Rx to request rapid acquisition for a primary multicast
RTP session, or one or more primary multicast streams belonging to
the same primary multicast RTP session. The FCI field MUST contain
only one RAMS Request. The FCI field has the structure depicted in
Figure 1.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SFMT=1 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Optional TLV-encoded Fields (and Padding, if needed) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: FCI field syntax for the RAMS Request message
o Request for Inter-user Playback Delay Reduction (0 bits): TLV element
that indicates that RTP_Rx is only requesting the inter-user playback
delay reduction for the desired primary multicast stream(s). If this
TLV element exists in the RAMS-R message, RS SHALL send the value N -
the playback delay reduction target in number of frame durations and
the value V - the recommended interval, in frames, between two
continuous events for skipping of one frame. Note that this TLV
element does not carry a Value field. Thus,the Length field MUST be
set to zero.
Type: 6
5.2. Extension to RAMS-I
The RAMS Information message is identified by SFMT=2. This message
is used to describe the unicast burst that will be sent for rapid
acquisition. It also includes other useful information for RTP_Rx as
described below. The FCI field MUST contain only one RAMS
Information. The FCI field has the structure depicted in Figure 2.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SFMT=2 | MSN | Response |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Optional TLV-encoded Fields (and Padding, if needed) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: FCI field syntax for the RAMS Information message
o The value N(16 bits) - the playback delay reduction target in number
of frame durations: TLV element that denotes the amount of frame
delay which is equal to the frame difference between the latest RTP
timestamp of primary multicast packets buffered in the RS when the
unicast burst starts and the RTP timestamp of the starting point of
the unicast burst.
If the request for inter-user playback delay reduction has been
accepted, RS MUST send this field at least once, so that RTP_Rx
knows to speed up media rendering during playback. Otherwise, RS
ignore the request for inter-user playback delay reduction.
Type: 36
o The value V(8 bits) - recommended interval, in frames, between two
continuous events for skipping of one frame: Optional TLV element
that denotes the recommended interval for skipping of one frame.
During the speedup playback of RTP_Rx, 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.
Type: 37
6. Security Considerations
TBD.
7. IANA Considerations
TBD.
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8. Acknowledgements
The authors thank the following individuals for their contributions,
comments and suggestions to this document: Roni Even, Colin Perkins,
Ali C. Begen (abegen) and Bill Ver Steeg (versteb).
9. Normative 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-07 (work in
progress), February 2010.
[I-D.ietf-avt-rtcp-guidelines]
Ott, J. and C. Perkins, "Guidelines for Extending the RTP
Control Protocol (RTCP)",
draft-ietf-avt-rtcp-guidelines-03 (work in progress),
February 2010.
[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.
[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
July 2006.
[TR-126] DSL Forum TR-126, "Triple-play Services Quality of
Experience (QoE) Requirements", December 2006.
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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 Corporate Blvd, Suite 602
Bridgewater, NJ 08807
USA
Phone: +1-908-541-3518
Email: yekuiwang@huawei.com
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