Audio/Video Transport P. Yang
Internet-Draft F. Xia
Intended status: Standards Track X. Wu
Expires: June 13, 2010 Huawei Technologies Co., Ltd.
December 10, 2009
Preamble Acquisition of MPEG2-TS Multicast Sessions
draft-xia-avt-mpeg2ts-preamble-01.txt
Abstract
The ITU-T Rec. H.222.0 | ISO/IEC 13818-1 Transport Stream (MPEG2-TS)
addresses the combining of one or more elementary streams of video
and audio, as well as other data, into single or multiple streams
which are suitable for storage or transmission. The necessary and
sufficient information contained in the Program Specific
Information(PSI) tables to demultiplex and present programs must be
acquired before a RTP receiver can process any data received in
MPEG2-TS. In this document, a Retransmission Server is specified to
deliver MPEG2-TS preamble prior to unicast burst RTP packets. The
Retransmission Server caches raw RTP packets with MPEG2-TS preamble
information, and sends them to the RTP receiver which initiates rapid
acquisition of MPEG2-TS multicast sessions.
Status of this Memo
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This Internet-Draft will expire on June 13, 2010.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Abbreviation . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Overview of MPEG-2 Transport Streams . . . . . . . . . . . . . 5
5. Preamble Acquisition of MPEG2-TS Multicast Sessions . . . . . 7
5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.2. MPEG2-TS Preamble Structure . . . . . . . . . . . . . . . 11
5.3. The Implementation of MPEG2-TS Preamble . . . . . . . . . 12
5.4. Message Flow . . . . . . . . . . . . . . . . . . . . . . . 14
6. The Processing of the Receiver . . . . . . . . . . . . . . . . 16
7. Security Considerations . . . . . . . . . . . . . . . . . . . 16
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
9.1. Normative References . . . . . . . . . . . . . . . . . . . 17
9.2. Informational References . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
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1. Introduction
Real-time multimedia multicast flows usually carry streams of inter-
related data. Certain information must first be acquired by the
receivers to start processing multimedia data sent in the multicast
session. [I-D.ietf-avt-rapid-acquisition-for-rtp] refers to this
information as Reference Information. Part of the Reference
Information is conventionally sent periodically in the multicast
session and usually consists of items such as a description of the
schema for the rest of the data, references to which data to process,
encryption information including keys, as well as any other
information required to process the data in the primary multicast
stream.
The ITU-T Rec. H.222.0 | ISO/IEC 13818-1 Transport Stream[MPEG2-TS]
is a stream definition which is tailored for communicating or storing
one or more programs of coded data according to ITU-T Rec. H.262 |
ISO/IEC 13818-2 and ISO/IEC 13818-3 and other data in environments.
The MPEG2-TS is already widespread in the digital broadcasting and
IPTV services over both terrestrial and satellite networks, not only
in Europe but also in Asia and North America. When the primary
multicast stream is using an encapsulation method of MPEG2-TS over
RTP that multiplexes video and audio content, together with ancillary
metadata, and produces a synchronized multiplexed stream, the
receiver must first acquire the necessary and sufficient information
before demultiplexing and decoding an incoming MPEG2-TS. However,
these necessary and sufficient information of MPEG2 Transport Stream
does not reside in MPEG2-TS contiguously and is usually dispersed
over a large period of time. When the receivers starts receiving the
Reference Information from the random access point as detailed in
[I-D.ietf-avt-rapid-acquisition-for-rtp], the Reference Information
of MPEG2-TS cannot be completely received particularly the MPEG2-TS
Program Specific Information (PSI) tables so that the receivers
cannot demultiplex and decode correctly. In order to demultiplex and
decode correctly for the receivers, parts of the Reference
Information of MPEG2-TS, extracted from MPEG2 transport stream
packets prior to the random access point(RAP) over a long period(
even several previous Reference Information away), must be acquired
before demultiplexing and decoding. In this document, we refers to
this information as MPEG2-TS preamble.
This document describes how to convey this preamble information to
the receiver before the unicast burst stream defined in RAMS
[I-D.ietf-avt-rapid-acquisition-for-rtp].
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2. Terminology
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].
3. Abbreviation
The following abbreviations are used in this document:
BAT: Bouquet Association Table
CAT: Conditional Access Table
DIT: Discontinuity Information Table
ECM: Entitlement Control Message
EIT: Event Information Table
EMM: Entitlement Management Message
GoP: Group of Picture
ICIT: IPMP Control Information Table
IDR: Instantaneous Decoding Refresh
IPMP: Intelligent Property Management and Protection
MP2T: MPEG2 Transport Stream
MPEG2-TS: MPEG2 Transport Stream
NIT: Network Information Table
OPCR: Original Program Clock Reference
PAT: Program Association Table
PCR: Program Clock Reference
PES: Packetized Elementary Stream
PMT: Program Map Table
PS: Private Section
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PSI: Program Specific Information
RAP: Random Access Point
RST: Running Status Table
SDT: Service Description Table
SIT: Selection Information Table
ST: Stuffing Table
TDT: Time and Date Table
TSDT: Transport Stream Description Table
TOT: Time Offset Table
4. Overview of MPEG-2 Transport Streams
ITU-T Rec. H.222.0 | ISO/IEC 13818-1 Transport Stream addresses the
combining of one or more elementary streams of video and audio, as
well as other data, into single or multiple streams which are
suitable for storage or transmission. Systems coding follows the
syntactical and semantic rules imposed by this Specification and
provides information to enable synchronized decoding of decoder
buffers over a wide range of retrieval or receipt conditions.
The basic multiplexing approach for single video and audio elementary
streams is illustrated in Figure 1. The video and audio data is
encoded as described in ITU-T Rec. H.262 | ISO/IEC 13818-2 and ISO/
IEC 13818-3. The resulting compressed elementary streams are
packetized to produce Packetized Elementary Streams(PES) packets.
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video +-------------+ +----------+ Video PES +---------+
input--->|Video Encoder|--->|Packetizer|---------->| |
+-------------+ +----------+ | |
| |
Audio +-------------+ +----------+ Audio PES |MPEG-2 |
input--->|Audio Encoder|--->|Packetizer|---------->| | Network
+-------------+ +----------+ |Transport|---->
| |
Optional Application Data(e.g subtitle)---> |Stream |
| |
Program Specific Information---> |Mux |
| |
... ...---> | |
+---------+
Figure 1: The overview of MPEG-2 Transport Streams
The MPEG2 Transport Stream coding layer allows one or more programs
to be combined into a single stream. Data from each elementary
stream are multiplexed together with information that allows
synchronized presentation of the elementary streams within a program.
A Transport Stream consists of one or more programs. Audio and video
elementary streams consist of access units(A coded representation of
a presentation unit).
Elementary Stream data is carried in PES packets. A PES packet
consists of a PES packet header followed by packet data. PES packets
are inserted into Transport Stream packets. The first byte of each
PES packet header is located at the first available payload location
of a Transport Stream packet.
Transport Stream packets begin with a 4-byte prefix, which contains a
13-bit Packet ID (PID). The PID identifies, via the Program Specific
Information (PSI) tables, the contents of the data contained in the
Transport Stream packet. Transport Stream packets of one PID value
carry data of one and only one elementary stream. The PSI tables are
carried in the Transport Stream. There are Six PSI tables shown in
table 2.28 of h222.0(2006) (There were only the first four PSI tables
in the ITU-T Rec. H.222.0(2000 E) | ISO/IEC 13818-1:2000 E):
o Program Association Table;
o Program Map Table;
o Conditional Access Table;
o Network Information Table;
o Transport Stream Description Table;
o IPMP Control Information Table.
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These tables contain the necessary and sufficient information to
demultiplex and present programs. Program Association Table contains
the PIDs of NIT and PMT, what's more, the private section in
Transport Stream packets with a PID value is designated as a Program
Map Table PID in the Program Association Table. The Program Map
Table provides among other information, which PIDs, and therefore
which elementary streams are associated to form each program. This
table also indicates the PID of the Transport Stream packets which
carry the PCR for each program. The Conditional Access Table shall
be present if scrambling is employed. The Network Information Table
is optional and its contents are not specified by the ITU-T Rec.
H.222.0 | ISO/IEC 13818-1. Transport Stream Description Table(TSDT)
has 256(an 8-bit field) transport stream description elements shown
in Table 2-45-Program and program element descriptors of
H.222.0(2006). The IPMP Control Information Table shall be present
if IPMP as described in ISO/IEC 13818-11 is used by any of the
components in the ITU-T Rec. H.222.0 | ISO/IEC 13818-1 stream.
Except for these PSI tables, ETSI DVB EN 300 468, defines nine PSI
tables which are Bouquet Association Table (BAT), Service Description
Table (SDT), Event Information Table (EIT), Running Status Table
(RST), Time and Date Table (TDT), Time Offset Table (TOT), Stuffing
Table (ST), Selection Information Table (SIT), Discontinuity
Information Table (DIT).There are other PSI tables defined by
organizations and operators.
In this document, terminologies PSI and MPEG2-TS preamble are
interchangeable. Media including video, audio and text in an
MPEG2-TS is self describing, and the receiver must parse certain
control information in the PAT, CAT and PMT tables (i.e., PSI)
contained in the transport stream in order to know how to parse the
rest of the stream (i.e., to find the audio and video elementary
streams, private data and the encryption information for a given
program). This document specifies a mechanism to acquire PSI rapidly
when a receiver joins in a MPEG2-TS multicast session.
5. Preamble Acquisition of MPEG2-TS Multicast Sessions
5.1. Overview
In video coding, a group of picture (GOP) specifies the order in
which intra-frame and inter-frames are arranged. The GOP is a group
of successive pictures within a coded video stream. Each coded video
stream consists of successive GOPs. A GOP always begins with a
Random Access Point (RAP)- an intra-frame(e.g. IDR-frame).
Afterwards several inter-frames(e.g. P-frame) follow. The Intra-
frame is a reference frame which contains the full image which
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represents a fixed image and which is independent of other picture
types and do not require any additional information to reconstruct
it. The inter-frame is a prediction frame which contains motion-
compensated difference information from the preceding Intra-frame or
other inter-frames in a video compression stream which is expressed
in terms of one or more neighboring frames.
When a receiver joins a primary multicast session, it does not have
control over what point in the flow is currently being transmitted,
it needs to wait until the next RAP and other necessary and
sufficient information for demultiplexing and decoding (which we
refer to as the Reference Information in the [I-D.ietf-avt-rapid-
acquisition-for-rtp]) shows up in the multicast stream before it can
start decoding. In order to reduce the acquisition delay when the
RTP receiver joins a multicast session at a random point in time,
RAMS in the [I-D.ietf-avt-rapid-acquisition-for-rtp] introduces the
method as Unicast- based Rapid Acquisition of Multicast RTP Sessions
illustrated in Figure 2 where an auxiliary unicast RTP session
carrying the Reference Information to the receiver precedes/
accompanies the primary multicast stream. This unicast RTP flow is
transmitted at a faster than natural rate to further accelerate the
acquisition.
In MPEG2 transport stream case, the Reference Information is often
not contiguous in the flow but dispersed over a large period, it even
spans several GOPs. It is often distributed in different RTP packets
with other MPEG2-TS packets which are not part of the reference
information. Figure 2 illustrates an RTP stream where PAT, PMT and
ECM information distribute in the three different RTP packets.
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...
RTP SN(28255) PAT(TS1)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|RTP(H)|TS1(188)|TS2(188)|TS3(188)|TS4(188)|TS5(188)|TS6(188)|TS7(188)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
RTP SN(28266) PMT(TS6)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|RTP(H)|TS1(188)|TS2(188)|TS3(188)|TS4(188)|TS5(188)|TS6(188)|TS7(188)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
RTP SN(28291) ECM(TS4)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|RTP(H)|TS1(188)|TS2(188)|TS3(188)|TS4(188)|TS5(188)|TS6(188)|TS7(188)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
RTP SN(28301)(Random Access point: IDR-frame(TS6)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|RTP(H)|TS1(188)|TS2(188)|TS3(188)|TS4(188)|TS5(188)|TS6(188)|TS7(188)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2 Original RTP sequence only with PAT, PMT and ECM
In Figure 2 the TS1 packet of RTP packet with sequence number 28255
is a PAT packet. The TS6 packet of RTP packet with sequence number
28266 is a PMT packet. The TS4 packet of RTP packet with sequence
number 28291 is an ECM packet. TS6 of RTP packet with sequence
number 28301 is a beginning of a Random Access point of IDR-frame.
The Reference Information for this RTP sequence starting at RTP
sequence number 28301 needs the PAT information in the RTP SN 28255,
the PMT information of in the RTP SN 28266 and the ECM information of
in the RTP SN 28291. The process is now to extracts the PSI
information and to build a new RTP packet illustrated in Figure 3
where PAT is in TS1, PMT in TS2 and ECM in TS3.
RTP SN(xxx) PAT(TS1),PMT(TS2),ECM(TS3)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|RTP(H)|TS1(188)|TS2(188)|TS3(188)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RTP(RFC4588) SN(28301)(Random Access point: IDR-frame(TS6)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|RTP(H)|TS1(188)|TS2(188)|TS3(188)|TS4(188)|TS5(188)|TS6(188)|TS7(188)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
Figure 3 Preamble MPEG2-TS RTP sequence
In MPEG2-TS PSI information, PAT, PMT, and ECM(ECM shall be present
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if scrambling is employed) are always present. Other reference
information enabling automatic configuration of the receiver to
demultiplex and decode the various streams of programs within the
multiplex are optionally provisioned depending on different policies
and demand of demultiplexing and decoding. For example, the
Conditional Access Table shall be present if scrambling is employed.
The IPMP Control Information Table shall be present if IPMP as
described in ISO/IEC 13818-11 is used by any of the components in the
ITU-T Rec. H.222.0 | ISO/IEC 13818-1 stream. The Reference
information (Event Information Table(EIT), Service Description Table
(SDT),etc.) in the ETSI DVB EN 300 468 shall be optionally present to
provide identification of services and events for the user. The
following RTP sequence illustrated in Figure 4 includes more PSI
table elements.
...
RTP SN(10739) CAT(TS2),EIT(TS7)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|RTP(H)|TS1(188)|TS2(188)|TS3(188)|TS4(188)|TS5(188)|TS6(188)|TS7(188)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
RTP SN(10758) PAT(TS3)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|RTP(H)|TS1(188)|TS2(188)|TS3(188)|TS4(188)|TS5(188)|TS6(188)|TS7(188)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
RTP SN(10796) PMT(TS3)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|RTP(H)|TS1(188)|TS2(188)|TS3(188)|TS4(188)|TS5(188)|TS6(188)|TS7(188)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
RTP SN(10829) ECM(TS2)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|RTP(H)|TS1(188)|TS2(188)|TS3(188)|TS4(188)|TS5(188)|TS6(188)|TS7(188)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
RTP SN(10834) PAT(TS4)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|RTP(H)|TS1(188)|TS2(188)|TS3(188)|TS4(188)|TS5(188)|TS6(188)|TS7(188)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
RTP SN(10851) NIT(TS5)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|RTP(H)|TS1(188)|TS2(188)|TS3(188)|TS4(188)|TS5(188)|TS6(188)|TS7(188)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
RTP SN(10872) PMT(TS3)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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|RTP(H)|TS1(188)|TS2(188)|TS3(188)|TS4(188)|TS5(188)|TS6(188)|TS7(188)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
RTP SN(10910) PAT(TS2)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|RTP(H)|TS1(188)|TS2(188)|TS3(188)|TS4(188)|TS5(188)|TS6(188)|TS7(188)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
RTP SN(10912) SDT(TS2),EIT(TS7)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|RTP(H)|TS1(188)|TS2(188)|TS3(188)|TS4(188)|TS5(188)|TS6(188)|TS7(188)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RTP SN(10913)(Random Access point: IDR-frame(TS1),ECM(TS2)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|RTP(H)|TS1(188)|TS2(188)|TS3(188)|TS4(188)|TS5(188)|TS6(188)|TS7(188)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4 Original RTP sequence with PAT, PMT, ECM, CAT, SDT...
Figure 5 shows an example of MPEG2-TS preamble providing the RTP
sequence with PAT, PMT ECM, NIT, CAT, SDT and EIT.
SN(xxx) PAT(TS1),PMT(TS2),ECM(TS3),NIT(TS4),CAT(TS5),SDT(TS6),EIT(TS7)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|RTP(H)|TS1(188)|TS2(188)|TS3(188)|TS4(188)|TS5(188)|TS6(188)|TS7(188)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RTP(RFC4588) SN(10913)(Random Access point: IDR-frame(TS1),ECM(TS2)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|RTP(H)|TS1(188)|TS2(188)|TS3(188)|TS4(188)|TS5(188)|TS6(188)|TS7(188)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5 Preamble MPEG2-TS RTP sequence
5.2. MPEG2-TS Preamble Structure
The order of PSI information need to be considered when rebuilding
RTP packets. Except for some fixed PIDs(0x0000 for PAT, 0x0001 for
CAT, 0x0002 for Transport Stream Description Table, 0x0003 for IPMP
Control Information Table), other PIDs are inherited from those fixed
PIDs. For example the PID of PMT is inherited from PAT. The PID of
ECM is inherited from CAT, or PMT, or Transport Stream Description
Table. The PIDs of Private_Section are designated as a Program Map
Table PID in the Program Association Table. Figure 6 shows the
inheritance of PAT and CAT.
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+-+-+ +-+-+
|PAT| PID(0x0000) |CAT| PID(0x0001)
+-+-+ +-+-+
| |
-------------------- ------------
| | | | |
+-+-+ +-+-+ +-+-+ +-+-+ +-+-+
|NIT| |PMT| | PS| Private Section |ECM| |EMM|
+-+-+ +-+-+ +-+-+ +-+-+ +-+-+
|
---------
| |
+-+-+ +-+-+
|ECM| |EMM|
+-+-+ +-+-+
Figure 6 The inheritance of PSI PIDs
5.3. The Implementation of MPEG2-TS Preamble
There are dozens of PSI tables of MPEG2-TS containing hundreds of the
Reference information, which include PAT, PMT, PCR, CAT, NIT, ECM/
EMM, Transport Stream Description Elements of the 8-bit field, IPMP
Control Information Table, Private Section, other extended reference
information, etc. Yet all these MPEG2-TS preambles are not present
simultaneously, they simply exist a bit partially except PAT, PMT and
ECM (ECM is present if scrambling is employed),and distribute in
different RTP packets over a large period.
Most of them have unique PIDs. Yet there are some information
elements with the same PID, For example, 256 transport stream
description elements (Transport Stream Description Table) which are
distributed in different locations have the same PID for 0x0002.
SDT, BAT and ST in Table 1: PID allocation for SI of the ETSI DVB EN
300 468 have the same PID for 0x0011. EIT, ST and CIT have the same
PID of 0x0012. TDT, TOT and SI have the same PID of 0x0014.
In addition, it is possible having the PSI information with different
PIDs. For instance, The Conditional Access Table (CAT) may give the
association between one or more CA systems with different ECM/EMM
PIDs.
In order to simplify these complicated factors and the receiver's
processing, efficiently transmitting these MPEG2-TS preamble
information, the flexible and effective method is to reconstruct news
RTP packets which duplicate the raw MPEG2-TS packets of the current
Reference information from the original MPEG2-TS packets sequence.
For example, as is shown in Figure 7. When duplicating the raw
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MPEG2-TS packets with the Reference information, it is necessary to
obey MPEG2-TS Preamble Order described in the above section.
+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+
...|RTP(H)|...TS2(ECM)...| ...|RTP(H)|...TS5(PAT)...|...
+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-|
| |
+------ + |
| +-----------------------------+
| | +-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+
| |...|RTP(H)|...TS3(PMT)...| ...|RTP(H)|...TS5(PCR)...|...
| | +-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+
| | | |
| | +---+ +----+
+----|------------|------------+ |
| | | |
SN(xxx) v v v v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|RTP (H)|TS1-PAT(188)|TS2-PMT(188)|TS3-ECM(188)|TS4-PCR(188)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7 RTP Reconstruction Process of MPEG2-TS Preamble
Except for the above Reference information, there are two elements of
the Reference Information which shall also be considered. One is the
information of PES Header, the other is the information of Adaptation
Field of TS Header.
Commonly the beginning MPEG2-TS packet of a Random Access Point
(RAP)-intra-frame contains a PES Header, followed by a video frame
encoding data. Its format is shown in Figure 8.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|TS Header| PES Header | Beginning of video frame encoding data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8 The beginning of RAP with PES Header
Yet in some cases, its format is as follows and doesn't contain PES
header shown in Figure 9.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|TS Header| Beginning of video frame encoding data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9 The beginning of RAP without PES Header
The PES Header is contained in an early video MPEG2-TS TS packets.
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In this case, the PES Header can be an MPEG2-TS preamble information
transmitted before RAMS.
Sometimes, the information of Adaptation field of TS header shall be
transmitted before RAMS. Adaptation field of TS header in Table 2-6
- Transport Stream adaptation field of H.222.0 contains dozens of
information elements which are important to the demultiplex and
decode. PCR/OPCR information probably contains in this field. If
PCR isn't present as a PCR TS packet with a PCR PID, the information
of Adaptation field of TS header shall be an MPEG2-TS preamble
information transmitted before RAMS.
As can be seen there are multiple PSI elements types and they can be
part of the preamble information or the RTP_Rx can receive them later
from the multicast stream. The important elements are the PAT, PMT
and ECM and the application can decide which elements to include in
the Preamble stream.
5.4. Message Flow
Figure 10 depicts message flows for PSI acquisition. A RTP receiver
sends a rapid acquisition request for a new multicast RTP session to
the feedback target address of that session. This RTCP feedback
message is defined as the RAMS-Request (RAMS-R) message in [I-D.ietf-
avt-rapid-acquisition-for-rtp] and MAY also contain parameters, such
as the bandwidth limit, buffering capacity available at the RTP
receiver, and so on.
A retransmission server receives the RAMS-R message and sends an
RAMS-Information (RAMS-I) message to the RTP receiver. The first
RAMS-I message MAY precede the unicast burst or it MAY be sent during
the burst. The retransmission server then starts sending unicast PSI
RTP packets prior to delivering Unicast RTP Burst.
In this document, a "unicast PSI RTP" delivering process is added for
transporting necessary MPEG2-TS preamble information. The unicast
PSI RTP stream shall be transmitted before the Unicast RTP burst, as
shown in Figure 10. The information is carried as RFC 2250 [RFC2250]
MP2T payload type RTP packets.
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+-----------+ +----------------+ +----------+ +------------+
| Multicast | | Retransmission | | | | RTP |
| Source | | Server | | Router | | Receiver |
| | | (RS) | | | | (RR) |
+-----------+ +----------------+ +----------+ +------------+
| | | |
|-- RTP Multicast ------------------->| |
| | | |
|-- RTP Multicast ->| | |
| | | |
| |<'''''''''''''''''' RTCP RAMS-R ''|
| | | |
| | | |
| |'' (RTCP RAMS-I) ''''''''''''''''>|
| | | |
| |....... Unicast PSI RTP .........>|
| | | |
| |....... Unicast RTP Burst .......>|
| | | |
| |<''''''''''''''''''(RTCP RAMS-R)''|
| | | |
| | | |
| |'' (RTCP RAMS-I) ''''''''''''''''>|
| | | |
| | |<~ SFGMP Join ~~|
| | | |
|-- RTP Multicast ------------------------------------>|
| | | |
~ ~ ~ ~
| | | |
'''> Unicast RTCP Messages
~~~> SFGMP Messages
...> Unicast RTP Flow
---> Multicast RTP Flow
Figure 10: Message flows for Preamble Acquisition
The SDP[RFC4566] description of the MPEG2-TS Preamble information is
conveyed in an RAMS session description as defined in [I-D.ietf-avt-
rapid-acquisition- for-rtp]. Figure 11 shows an SDP example which
multiplexes the MPEG2-TS Preamble information and an RAMS unicast
retransmission session based on Section 8.2 of [I-D.ietf-avt-rapid-
acquisition-for-rtp].
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v=0
o=S1 1122334455 1122334466 IN IP4 rams.example.com
s=Rapid Acquisition Example with MPEG2-TS Preamble Data
t=0 0
a=group:FID 3 4
a=rtcp-unicast:rsi
m=video 41000 RTP/AVPF 98
i=Primary Multicast Stream
c=IN IP4 233.252.0.2/255
a=source-filter: incl IN IP4 233.252.0.2 192.0.2.2
a=rtpmap:98 MP2T/90000
a=rtcp:41001 IN IP4 192.0.2.1
a=rtcp-fb:98 nack
a=rtcp-fb:98 nack ssli
a=ssrc:123321 cname:iptv-ch32@rams.example.com
a=mid:3
m=video 41002 RTP/AVPF 99 100
i=Unicast Retransmission Stream + Preamble Data
c=IN IP4 192.0.2.1
a=rtpmap:99 rtx/90000
a=rtcp:41003
a=fmtp:99 apt=98; rtx-time=5000
a=rtpmap:100 MP2T/90000
a=mid:4
Figure 11: The SDP description of the MPEG2-TS Preamble
information conveying in an RAMS session
6. The Processing of the Receiver
Once received, it is very simple for RTP receivers to process
MPEG2-TS Preamble information because it is packetized using raw
MPEG2-TS packets. The RTP receiver only removes the RTP header
before the demultiplexing and decoding, and does the same as for the
RAMS unicast RTP burst retransmission packets.
It is also very flexible for the RTP receiver to support any MPEG2-TS
Reference Information without any software Update. The information
is received as an MPEG-TS raw data packets using MP2T payload format
and therefore is transparent for RTP sessions and RTP receivers who
forward it to the demux/decoder.
7. Security Considerations
Comparing to [I-D.ietf-avt-rapid-acquisition-for-rtp], this document
specifies delivering PSI information to a RTP receiver prior to
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unicast RTP burst packets. PSI is key information for a decoder to
parse MPEG2-TS streaming packets, and any tampered PSI results in
denial of service of the RTP receiver. Security considerations in
[I-D.ietf-avt-rapid-acquisition-for-rtp] also applies.
8. Acknowledgements
TBD.
9. References
9.1. Normative References
[H.222.0] ITU-T H.222.0(2000 E), "Information technology - Generic
coding of moving pictures and associated audio information
- Part 1: Systems".
[H.222.0(2006)]
ITU-T H.222.0(2006), "Information technology - Generic
coding of moving pictures and associated audio information
- Part 1: Systems", May 2006.
[H.262] ITU-T H.262(2000), "Information technology - Generic
coding of moving pictures and associated audio information
- Part 2: Video".
[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-05 (work in
progress), November 2009.
[ISO13818-1]
ISO/IEC 13818-1:2000(E), "Information technology - Generic
coding of moving pictures and associated audio
information: Systems".
[ISO13818-11]
ISO/IEC 13818-11:2004, "Information technology - Generic
coding of moving pictures and associated audio information
- Part 11: IPMP on MPEG-2 systems".
[ISO13818-1:2007]
ISO/IEC 13818-1:2007, "Information technology - Generic
coding of moving pictures and associated audio
information: Systems".
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[ISO13818-2]
ISO/IEC 13818-2:2000, "Information technology - Generic
coding of moving pictures and associated audio information
- Part 2: Video".
[ISO13818-3]
ISO/IEC 13818-3:1998, "Information technology - Generic
coding of moving pictures and associated audio information
- Part 3: Audio".
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2250] Hoffman, D., Fernando, G., Goyal, V., and M. Civanlar,
"RTP Payload Format for MPEG1/MPEG2 Video", RFC 2250,
January 1998.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[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.
9.2. Informational References
[ETSI EN 300 468]
ETSI EN 300 468, "Digital Video Broadcasting (DVB);
Specification for Service Information (SI) in DVB
systems".
[ETSI TS 103 197]
ETSI TS 103 197, "Digital Video Broadcasting (DVB); Head-
end implementation of DVB SimulCrypt".
[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),
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March 2009.
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
Frank Xia
Huawei Technologies Co., Ltd.
1700 Alma Dr. Suite 500
Plano, TX 75075
Phone: +1 972-509-5599
Email: xiayangsong@huawei.com
Xingfen Wu
Huawei Technologies Co., Ltd.
No.91 Baixia Road
Nanjing 210001
P.R.China
Phone: +86 25 84565870
Email: wuxingfen@huawei.com
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