One document matched: draft-xia-avt-splicing-for-mpeg2ts-ps-00.txt
AVT Working Group J. Xia
Internet-Draft H. Xia
Intended status: Informational J. Zhang
Expires: September 1, 2010 Huawei
February 28, 2010
Splicing for MPEG2-TS Problem Statement in RTP Sessions
draft-xia-avt-splicing-for-mpeg2ts-ps-00
Abstract
The base splicing related protocols, i.e., Digital Program Insertion
Splicing API [SCTE30] and Digital Program Insertion Cueing Message
for Cable [SCTE35], only take into account how to insert the content
(e.g. spot advertisements of various lengths, program substitution,
public service announcements etc.) into any MPEG-2 Output Multiplex
in the Splicer and how to carry notification of upcoming Splicing and
other timing information in the transport stream, but not give enough
mechanisms to direct underlying transport protocol such as RTP how to
work compatibly with the spliced MPEG2-TS. This memo highlights the
issues of what's the impact on RTP layer when performing splicing
between the primary channel and the insertion channel.
Status of this Memo
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Copyright Notice
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Copyright (c) 2010 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Overlap between Insertion Channel and Primary Channel . . . 5
3.2. Gap between Insertion Channel and Primary Channel . . . . . 7
4. Potential Approach . . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
6. IANA considerations . . . . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 8
8. Normative References . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
MPEG2 Transport Stream (MPEG2-TS) [MPEG2TS] is an encapsulation
method and transport that multiplexes digital video and audio
content, together with ancillary metadata, and produces a
synchronized multiplexed stream that is tailored for transport over
packet or cell-oriented networks. MPEG2-TS are composed of 188 byte
TS Packets, whose payloads may contain program information as well as
Packetized Elementary Streams (PES), typically video and audio
streams. The RTP payload format for carrying TS packets in an RTP
stream is specified in [RFC2250].
With the expansive application of the MPEG2-TS, cable operators will
need solutions that can efficiently manage and process an increasing
number of ads. Solutions that can process and splice hundreds of ads
in a single device is Digital Program Insertion (DPI) which involve
the splicing of a single program transport stream and advertisements
in most cases, into a broadcast program. The splicer will seamlessly
insert valuable ads into primary programs. The details for
describing splicing configuration defined in [SCTE30] are shown in
Figure 1.
Primary Channel +-----------+
----------->| | Output Channel
| Splicer |---------->
---------->| |
| +-----------+
| ^
| Insertion Channel '
| '
+----------+ '
| | '
| Server | <''''''''''''''
| | Signaling Connect .
+----------+
----------> Data
''''''''''> Signaling
Figure 1: Single Splicing Configuration
[SCTE30] creates a standardized method for communication between
Server and Splicer for the insertion of content into any MPEG2 output
multiplex in the Splicer. Based on the cue message [SCTE35], Server
and Splicer know when and how long for a insertion channel to splice
into the specific primary channel.
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However SCET only takes into account the splicing process on
application layer for MPEG2-TS regardless of the underlying transport
protocol, such as RTP. From this viewpoint, this does not cause any
problem for CATV operators who directly carry MPEG2-TS over cable.
CATV operators concurrently output the ads MPEG2-TS instead of
primary program on Splicer and return to the primary program after
the Avail time. The whole procedure is processed only within the
MPEG2-TS layer and without any transport protocol limitation. But
IPTV operators who provide IP-based video services have to face the
issues raised by splicing.
In IPTV deployments, Splicer not only performs MPEG2-TS splicing but
also needs to coordinate underlying RTP layer to compatibly with
MPEG2-TS splicing. By deeply detecting the timing information
conveied in MPEG2-TS payload, Splicer can clearly know when to
perform splicing. However, during the certain Avail time, the number
of insertion RTP packets, which are inserted into primary program by
Splicer, is variable due to the different entropy coding. In such
case, if the splicing between Insertion channel and Primary channel
can not be appropriately handled by Splicer at the beginning and end
of Avail time, the RTP sequence number gap or overlay between
Insertion channel and Primary channel maybe result in the RTP
receiver to request unnecessary retransmission for inexistent packet
missing or to discard the useful packets due to the reduplicate
sequence number. The use cases where the problems exist are briefly
described in section Section Section 3.
2. Terminology
The keywords "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].
Throughout this document, the terms used have specific meanings.
Because some of the terms that are defined in SCTE30 and SCTE35 have
very specific technical meanings, the reader is referred to the
original source for their definition. For terms used in this
document, brief definitions are given below.
Digital Program Insertion (DPI)
the digital splicing of MPEG programs into other MPEG programs.
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Avail
Time space provided to cable operators by cable programming
services during a program for use by the CATV operator; the time
is usually sold to local advertisers or used for channel self
promotion.
Channel
Channel is a synonym for a "Service" in DVB terminology, or a
"Program" in MPEG terminology.
Primary Channel
The Primary Multiplex Channel carrying MPEG2-TS. It is spliced in
whole or in part by insertion channle.
Insertion Channel
The Insertion Multiplex Channel(s) that replace the Primary
Channel in whole or in part for the duration of a splicing.
Splicer
The device that splices the Insertion Channel(s) into the Primary
Channel(s). It may receive cue messages [SCTE35]. This device
also communicates with the Server about when and what to splice.
Output Channel
The Channel that is produced at the output of the Splicer
Server
The device that originates the Insertion Channel(s) to be spliced
into the Primary Channel(s). This device communicates with the
Splicer about when and what to splice.
3. Use Cases
In this section, two use cases are listed to illustrate the issues
resulting from this imperfect switch on RTP layer.
3.1. Overlap between Insertion Channel and Primary Channel
Figure 2 gives the overlay issue at the beginning and end of Avail
time.
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' '
Primary Channel '<-------Avail Time-------->'
' '
+-----+ +-----+ ' +-----+ +-----+ ' +-----+ +-----+
| SN1 |...| SN15| ' | SN16| | SN17| ' | SN18|...| SN25|
+-----+ +-----+ ' +-----+ +-----+ ' +-----+ +-----+
| | ' ' | |
| | ' ' | |
| | ' Insertion Channel ' | |
| | ' ' | |
| | ' +-----+ +-----+ +-----+ ' | |
| | ' | SN1 | | SN2 | | SN3 | ' | |
| | ' +-----+ +-----+ +-----+ ' | |
| | ' ' ' ' ' | |
| | ' ' ' ' ' | |
| | ' ' ' ' ' | |
v v ' v v v ' v v
+-----+ +-----+ ' +-----+ +-----+ +-----+ ' +-----+ +-----+
| SN1 |...| SN15| ' | SN16| | SN17| | SN18| ' | SN19|...| SN26|
+-----+ +-----+ ' +-----+ +-----+ +-----+ ' +-----+ +-----+
' '
Output Channel ' '
Figure 2: Overlap at the end of insertion
1. Before the splicing, the stream of Output Channel on Splicer is
from Primary Channel.
2. At the beginning of Avail time, The Splicer switches this
Insertion Channel stream to the Output Channel. In order to
achieve seamless switch-over, Splicer needs to coordinate the RTP
sequence number of Insertion Channel packets with pervious
Primary Channel packets prior to delivering them to Output
Channel. For example, in figure 2, the RTP sequence number of
last Primary Channel packet is SN15, the new RTP sequence number
of next insertion packet should be changed to SN16 from SN1.
3. At the end of Avail time, The Splicer returns to the Primary
Channel for direction to the Output Channel. If the Insertion
Channel packets delivered through Output channel are more than
the overridden Primary Channel packets during the Avail time,
Splicer needs to coordinate the RTP sequence number of Primary
Channel packets with pervious Insertion Channel packets again at
this time prior to delivering them to Output Channel. For
example, in figure 2, two Primary Channel packets are overridden
by three Insertion Channel packets during the Avail time, the new
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RTP sequence number of next Primary Channel packet should be
changed to SN19 from SN18.
3.2. Gap between Insertion Channel and Primary Channel
Figure 3 gives the gap issue at the beginning and end of Avail time.
' '
Primary Channel '<-------Avail Time-------->'
' '
+-----+ +-----+ ' +-----+ +-----+ +-----+ ' +-----+ +-----+
| SN1 |...| SN15| ' | SN16| | SN17| | SN18| ' | SN19|...| SN25|
+-----+ +-----+ ' +-----+ +-----+ +-----+ ' +-----+ +-----+
| | ' ' | |
| | ' ' | |
| | ' Insertion Channel ' | |
| | ' ' | |
| | ' +-----+ +-----+ ' | |
| | ' | SN1 | | SN2 | ' | |
| | ' +-----+ +-----+ ' | |
| | ' ' ' ' | |
| | ' ' ' ' | |
| | ' ' ' ' | |
v v ' v v ' v v
+-----+ +-----+ ' +-----+ +-----+ ' +-----+ +-----+
| SN1 |...| SN15| ' | SN16| | SN17| ' | SN18|...| SN24|
+-----+ +-----+ ' +-----+ +-----+ ' +-----+ +-----+
' '
Output Channel ' '
Figure 3: Gap at the end of insertion
1. First two steps are same as overlay issue specified in section
Section 3.1
2. At the end of Avail duration, The Splicer returns to the Primary
Channel for direction to the Output Channel. If the Insertion
Channel packets delivered through Output channel are less than
the overridden Primary Channel packets during the Avail time,
Splicer needs to coordinate the RTP sequence number of Primary
Channel packets with pervious Insertion Channel packets again at
this time prior to delivering them to Output Channel. For
example, in figure 3, three Primary Channel packets are
overridden by two Insertion Channel packets during the Avail
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time, the new RTP sequence number of next Primary Channel packet
should be changed to SN18 from SN19.
4. Potential Approach
Using a powerful Splicer for splicing is a possibility. Splicer
takes off the RTP header of Primary Channel packets and Insertion
Channel packet when receiving them and performs MPEG2-TS splicing
specified in [SCTE30], then recomposes RTP packets to carry the
compound Output Channel MPEG2-TS packets.
However, Splicer has to decode and encode RTP packets for primary
program even if no splicing occur. So the additional overhead is
introduced on Splicer when Splicer performs unnecessary decoding and
encoding. If we assume Splicer to decode and encode only after
splicing occurs, the rest RTP packets of primary program still need
to be recomposed even if the splicing is end, i.e., the overhead
issue still exists.
Furthermore, a Splicer may serve more than one Primary Channel
simultaneously, the heavy-laden Splicer may has unpredictable
overheads when performing splicing.
Based on the above observations, a more efficient splicing mechanism
on RTP layer can be justified that would be part of the RTP/RTCP base
protocol.
5. Security Considerations
6. IANA considerations
7. Acknowledgments
The authors would like to thank all colleagues for their review and
comments of this draft.
8. Normative References
[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.
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Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC2250] Hoffman, D., Fernando, G., Goyal, V., and M. Civanlar,
"RTP Payload Format for MPEG1/MPEG2 Video", RFC 2250,
January 1998.
[SCTE30] "Digital Program Insertion Splicing API", 2001.
[SCTE35] "Digital Program Insertion Cueing Message for Cable",
2004.
[MPEG2TS] "Generic Coding of Moving Pictures and Associated Audio
Information: Systems", 2006.
Authors' Addresses
Jinwei Xia
Huawei
Hui Hong Mansion
Nanjing, Baixia District 210001
China
Phone: +86-025-84565890
Email: xiajinwei@huawei.com
Hui Xia
Huawei
Site B,Floor 12F,Huihong Mansion, No.91 Baixia Rd.
Nanjing, Jiangsu 21001
China
Phone: +86-25-84565868
Jinhui Zhang
Huawei
Site B,Floor 12F,Huihong Mansion, No.91 Baixia Rd.
Nanjing, Jiangsu 21001
China
Phone: +86-25-84565866
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