Request for Comments: 4259 Motorola Connected Home Solutions
Category: Informational G. Fairhurst
University of Aberdeen
H. Clausen
TIC Systems
B. Collini-Nocker
H. Linder
University of Salzburg
November 2005
A Framework for Transmission of IP Datagrams over MPEG-2 Networks
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document describes an architecture for the transport of IP
Datagrams over ISO MPEG-2 Transport Streams (TS). The MPEG-2 TS has
been widely accepted not only for providing digital TV services but
also as a subnetwork technology for building IP networks. Examples
of systems using MPEG-2 include the Digital Video Broadcast (DVB) and
Advanced Television Systems Committee (ATSC) Standards for Digital
Television.
The document identifies the need for a set of Internet standards
defining the interface between the MPEG-2 Transport Stream and an IP
subnetwork. It suggests a new encapsulation method for IP datagrams
and proposes protocols to perform IPv6/IPv4 address resolution, to
associate IP packets with the properties of the Logical Channels
provided by an MPEG-2 TS.
Table of Contents
1. Introduction ....................................................3
1.1. Salient Features of the Architecture .......................4
2. Conventions Used in This Document ...............................4
3. Architecture ....................................................8
3.1. MPEG-2 Transmission Networks ...............................8
3.2. TS Logical Channels .......................................10
3.3. Multiplexing and Re-Multiplexing ..........................12
3.4. IP Datagram Transmission ..................................13
3.5. Motivation ................................................14
4. Encapsulation Protocol Requirements ............................16
4.1. Payload Unit Delimitation .................................17
4.2. Length Indicator ..........................................18
4.3. Next Level Protocol Type ..................................19
4.4. L2 Subnet Addressing ......................................19
4.5. Integrity Check ...........................................21
4.6. Identification of Scope. ..................................21
4.7. Extension Headers .........................................21
4.8. Summary of Requirements for Encapsulation .................22
5. Address Resolution Functions ...................................22
5.1. Address Resolution for MPEG-2 .............................23
5.2. Scenarios for MPEG AR .....................................25
5.2.1. Table-Based AR over MPEG-2 .........................25
5.2.2. Table-Based AR over IP .............................26
5.2.3. Query/Response AR over IP ..........................26
5.3. Unicast Address Scoping ...................................26
5.4. AR Authentication .........................................27
5.5. Requirements for Unicast AR over MPEG-2 ...................28
6. Multicast Support ..............................................28
6.1. Multicast AR Functions ....................................29
6.2. Multicast Address Scoping .................................30
6.3. Requirements for Multicast over MPEG-2 ....................31
7. Summary ........................................................31
8. Security Considerations ........................................32
8.1. Link Encryption ...........................................33
9. IANA Considerations ............................................34
10. Acknowledgements ..............................................34
11. References ....................................................34
11.1. Normative References .....................................34
11.2. Informative References ...................................34
Appendix A ........................................................39
1. Introduction
This document identifies requirements and an architecture for the
transport of IP Datagrams over ISO MPEG-2 Transport Streams
[ISO-MPEG]. The prime focus is the efficient and flexible delivery
of IP services over those subnetworks that use the MPEG-2 Transport
Stream (TS).
The architecture is designed to be compatible with services based on
MPEG-2, for example the Digital Video Broadcast (DVB) architecture,
the Advanced Television Systems Committee (ATSC) system [ATSC,
ATSC-G], and other similar MPEG-2-based transmission systems. Such
systems typically provide unidirectional (simplex) physical and link
layer standards, and have been defined for a wide range of physical
media (e.g., Terrestrial TV [ETSI-DVBT, ATSC-PSIP-TC], Satellite TV
[ETSI-DVBS, ETSI-DVBS2, ATSC-S], Cable Transmission [ETSI-DVBC,
ATSC-PSIP-TC, OPEN-CABLE], and data transmission over MPEG-2
[ETSI-MHP].
+-+-+-+-+------+------------+---+--+--+---------+
|T|V|A|O| O | | O |S |O | |
|e|i|u|t| t | | t |I |t | |
|l|d|d|h| h | IP | h | |h | Other |
|e|e|i|e| e | | e |T |e |protocols|
|t|o|o|r| r | | r |a |r | native |
|e| | | | | | |b | | over |
|x| | | | | +---+----+-+ |l | |MPEG-2 TS|
|t| | | | | | | MPE | |e | | |
| | | | | +--+---+ +------+ | | | |
| | | | | | AAL5 |ULE|Priv. | | | | |
+-+-+-+-+---+------+ | +-+--+--+ |
| PES | ATM | |Sect. |Section| |
+-------+----------+---+------+-------+---------+
| MPEG-2 TS |
+---------+-------+----------------+------------+
|Satellite| Cable | Terrestrial TV | Other PHY |
+---------+-------+----------------+------------+
Figure 1: Overview of the MPEG-2 protocol stack
Although many MPEG-2 systems carry a mixture of data types, MPEG-2
components may be, and are, also used to build IP-only networks.
Standard system components offer advantages of improved
interoperability and larger deployment. However, some MPEG-2
networks do not implement all parts of a DVB / ATSC system, and may,
for instance, support minimal, or no, signalling of Service
Information (SI) tables.
1.1. Salient Features of the Architecture
The architecture defined in this document describes a set of
protocols that support transmission of IP packets over the MPEG-2 TS.
Key characteristics of these networks are that they may provide
link-level broadcast capability, and that many supported applications
require access to a very large number of subnetwork nodes.
Some, or all, of these protocols may also be applicable to other
subnetworks, e.g., other MPEG-2 transmission networks, regenerative
satellite links [ETSI-BSM], and some types of broadcast wireless
links. The key goals of the architecture are to reduce complexity
when using the system, while improving performance, increasing
flexibility for IP services, and providing opportunities for better
integration with IP services.
Since a majority of MPEG-2 transmission networks are bandwidth-
limited, encapsulation protocols must therefore add minimal overhead
to ensure good link efficiency while providing adequate network
services. They also need to be simple to minimize processing, robust
to errors and security threats, and extensible to a wide range of
services.
In MPEG-2 systems, TS Logical Channels, are identified by their PID
and provide multiplexing, addressing, and error reporting. The TS
Logical Channel may also be used to provide Quality of Service (QoS).
Mapping functions are required to relate TS Logical Channels to IP
addresses, to map TS Logical Channels to IP-level QoS, and to
associate IP flows with specific subnetwork capabilities. An
important feature of the architecture is that these functions may be
provided in a dynamic way, allowing transparent integration with
other IP-layer protocols. Collectively, these will form an MPEG-2 TS
Address Resolution (AR) protocol suite [IPDVB-AR].
2. Conventions Used in This Document
Adaptation Field: An optional variable-length extension field of the
fixed-length TS Packet header, intended to convey clock references
and timing and synchronization information as well as stuffing over
an MPEG-2 Multiplex [ISO-MPEG].
ATSC: Advanced Television Systems Committee [ATSC]. A framework and
a set of associated standards for the transmission of video, audio,
and data using the ISO MPEG-2 standard [ISO-MPEG].
DSM-CC: Digital Storage Media Command and Control [ISO-DSMCC]. A
format for transmission of data and control information defined by
the ISO MPEG-2 standard that is carried in an MPEG-2 Private Section.
DVB: Digital Video Broadcast [ETSI-DVBC, ETSI-DVBRCS, ETSI-DVBS]. A
framework and set of associated standards published by the European
Telecommunications Standards Institute (ETSI) for the transmission of
video, audio, and data, using the ISO MPEG-2 Standard [ISO-MPEG].
Encapsulator: A network device that receives PDUs and formats these
into Payload Units (known here as SNDUs) for output as a stream of TS
Packets.
Forward Direction: The dominant direction of data transfer over a
network path. Data transfer in the forward direction is called
"forward transfer". Packets travelling in the forward direction
follow the forward path through the IP network.
MAC: Medium Access and Control. The link layer header of the
Ethernet IEEE 802 standard of protocols, consisting of a 6B
destination address, 6B source address, and 2B type field (see also
NPA).
MPE: Multiprotocol Encapsulation [ETSI-DAT, ATSC-DAT, ATSC-DATG]. A
scheme that encapsulates PDUs, forming a DSM-CC Table Section. Each
Section is sent in a series of TS Packets using a single TS Logical
Channel.
MPEG-2: A set of standards specified by the Motion Picture Experts
Group (MPEG), and standardized by the International Standards
Organisation (ISO) [ISO-MPEG].
NPA: Network Point of Attachment. Addresses primarily used for
station (Receiver) identification within a local network (e.g., IEEE
MAC address). An address may identify individual Receivers or groups
of Receivers.
PAT: Program Association Table [ISO-MPEG]. An MPEG-2 PSI control
table that associates program numbers with the PID value used to send
the corresponding PMT. The PAT is sent using the well-known PID
value of zero.
PDU: Protocol Data Unit. Examples of a PDU include Ethernet frames,
IPv4 or IPv6 datagrams, and other network packets.
PES: Packetized Elementary Stream [ISO-MPEG]. A format of MPEG-2 TS
packet payload usually used for video or audio information.
PID: Packet Identifier [ISO-MPEG]. A 13 bit field carried in the
header of TS Packets. This is used to identify the TS Logical
Channel to which a TS Packet belongs [ISO-MPEG]. The TS Packets
forming the parts of a Table Section, PES, or other Payload Unit must
all carry the same PID value. The all 1s PID value indicates a Null
TS Packet introduced to maintain a constant bit rate of a TS
Multiplex. There is no required relationship between the PID values
used for TS Logical Channels transmitted using different TS
Multiplexes.
PMT: Program Map Table. An MPEG-2 PSI control table that associates
the PID values used by the set of TS Logical Channels/Streams that
comprise a program [ISO-MPEG]. The PID value which is used to send
the PMT for a specific program is defined by an entry in the PAT.
PP: Payload Pointer [ISO-MPEG]. An optional one byte pointer that
directly follows the TS Packet header. It contains the number of
bytes between the end of the TS Packet header and the start of a
Payload Unit. The presence of the Payload Pointer is indicated by
the value of the PUSI bit in the TS Packet header. The Payload
Pointer is present in DSM-CC and Table Sections; it is not present in
TS Logical Channels that use the PES-format.
Private Section: A syntactic structure constructed in accordance with
Table 2-30 of [ISO-MPEG]. The structure may be used to identify
private information (i.e., not defined by [ISO-MPEG]) relating to one
or more elementary streams, or a specific MPEG-2 program, or the
entire TS. Other Standards bodies (e.g., ETSI, ATSC) have defined
sets of table structures using the private_section structure. A
Private Section is transmitted as a sequence of TS Packets using a TS
Logical Channel. A TS Logical Channel may carry sections from more
than one set of tables.
PSI: Program Specific Information [ISO-MPEG]. PSI is used to convey
information about services carried in a TS Multiplex. It is carried
in one of four specifically identified table section constructs
[ISO-MPEG], see also SI Table.
PU: Payload Unit. A sequence of bytes sent using a TS. Examples of
Payload Units include: an MPEG-2 Table Section or a ULE SNDU.
PUSI: Payload_Unit_Start_Indicator [ISO-MPEG]. A single bit flag
carried in the TS Packet header. A PUSI value of zero indicates that
the TS Packet does not carry the start of a new Payload Unit. A PUSI
value of one indicates that the TS Packet does carry the start of a
new Payload Unit. In ULE, a PUSI bit set to 1 also indicates the
presence of a one byte Payload Pointer (PP).
Receiver: A piece of equipment that processes the signal from a TS
Multiplex and performs filtering and forwarding of encapsulated PDUs
to the network-layer service (or bridging module when operating at
the link layer).
SI Table: Service Information Table [ISO-MPEG]. In this document,
this term describes a table that is used to convey information about
the services carried in a TS Multiplex, that has been defined by
another standards body. A Table may consist of one or more Table
Sections, however all sections of a particular SI Table must be
carried over a single TS Logical Channel [ISO-MPEG].
SNDU: Sub-Network Data Unit. An encapsulated PDU sent as an MPEG-2
Payload Unit.
STB: Set-Top Box. A consumer equipment (Receiver) for reception of
digital TV services.
Table Section: A Payload Unit carrying all or a part of an SI or PSI
Table [ISO-MPEG].
TS: Transport Stream [ISO-MPEG], a method of transmission at the
MPEG-2 level using TS Packets; it represents level 2 of the ISO/OSI
reference model. See also TS Logical Channel and TS Multiplex.
TS Header: The 4-byte header of a TS Packet [ISO-MPEG].
TS Logical Channel: Transport Stream Logical Channel. In this
document, this term identifies a channel at the MPEG-2 level
[ISO-MPEG]. It exists at level 2 of the ISO/OSI reference model.
All packets sent over a TS Logical Channel carry the same PID value
(this value is unique within a specific TS Multiplex). According to
MPEG-2, some TS Logical Channels are reserved for specific
signalling. Other standards (e.g., ATSC, DVB) also reserve specific
TS Logical Channels.
TS Multiplex: In this document, this term defines a set of MPEG-2 TS
Logical Channels sent over a single lower layer connection. This may
be a common physical link (i.e., a transmission at a specified symbol
rate, FEC setting, and transmission frequency) or an encapsulation
provided by another protocol layer (e.g., Ethernet, or RTP over IP).
The same TS Logical Channel may be repeated over more than one TS
Multiplex (possibly associated with a different PID value), for
example to redistribute the same multicast content to two terrestrial
TV transmission cells.
TS Packet: A fixed-length 188B unit of data sent over a TS Multiplex
[ISO-MPEG]. Each TS Packet carries a 4B header, plus optional
overhead including an Adaptation Field, encryption details and time
stamp information to synchronize a set of related TS Logical
Channels. It is also referred to as a TS_cell. Each TS Packet
carries a PID value to associate it with a single TS Logical Channel.
ULE: Unidirectional Lightweight Encapsulation (ULE) [IPDVB-ULE]. A
scheme that encapsulates PDUs, into SNDUs that are sent in a series
of TS Packets using a single TS Logical Channel.
3. Architecture
The following sections introduce the components of the MPEG-2
Transmission Network and relate these to a networking framework.
3.1. MPEG-2 Transmission Networks
There are many possible topologies for MPEG-2 Transmission Networks.
A number of example scenarios are briefly described below, and the
following text relates specific functions to this set of scenarios.
A) Broadcast TV and Radio Delivery
The principal service in the Broadcast TV and Radio Delivery scenario
is Digital TV and/or Radio and their associated data [MMUSIC-IMG,
ETSI-IPDC]. Such networks typically contain two components: the
contribution feed and the broadcast part. Contribution feeds provide
communication from a typically small number of individual sites
(usually at high quality) to the Hub of a broadcast network. The
traffic carried on contribution feeds is typically encrypted, and is
usually processed prior to being resent on the Broadcast part of the
network. The Broadcast part uses a star topology centered on the Hub
to reach a typically large number of down-stream Receivers. Although
such networks may provide IP transmission, they do not necessarily
provide access to the public Internet.
B) Broadcast Networks used as an ISP
Another scenario resembles that above, but includes the provision of
IP services providing access to the public Internet. The IP traffic
in this scenario is typically not related to the digital TV/Radio
content, and the service may be operated by an independent operator
such as unidirectional file delivery or bidirectional ISP access.
The IP service must adhere to the full system specification used for
the broadcast transmission, including allocation of PIDs and
generation of appropriate MPEG-2 control information (e.g., DVB and
ATSC SI tables).
C) Unidirectional Star IP Scenario
The Unidirectional Star IP Scenario utilizes a Hub station to provide
a data network delivering a common bit stream to typically medium-
sized groups of Receivers. MPEG-2 transmission technology provides
the forward direction physical and link layers for this transmission;
the return link (if required) is provided by other means. IP
services typically form the main proportion of the transmission
traffic. Such networks do not necessarily implement the MPEG-2
control plane, i.e., PSI/SI tables.
D) Datacast Overlay
The Datacast Overlay scenario employs MPEG-2 physical and link layers
to provide additional connectivity such as unidirectional multicast
to supplement an existing IP-based Internet service. Examples of
such a network includes IP Datacast to mobile wireless receivers
[MMUSIC-IMG].
E) Point-to-Point Links
Point-to-Point connectivity may be provided using a pair of transmit
and receive interfaces supporting the MPEG-2 physical and link
layers. Typically, the transmission from a sender is received by
only one or a small number of Receivers. Examples include the use of
transmit/receive DVB-S terminals to provide satellite links between
ISPs utilising BGP routing.
F) Two-Way IP Networks
Two-Way IP networks are typically satellite-based and star-based
utilising a Hub station to deliver a common bit stream to medium-
sized groups of receivers. A bidirectional service is provided over
a common air-interface. The transmission technology in the forward
direction at the physical and link layers is MPEG-2, which may also
be used in the return direction. Such systems also usually include a
control plane element to manage the (shared) return link capacity. A
concrete example is the DVB-RCS system [ETSI-DVBRCS]. IP services
typically form the main proportion of the transmission traffic.
Scenarios A-D employ unidirectional MPEG-2 Transmission Networks.
For satellite-based networks, these typically have a star topology,
with a central Hub providing service to large numbers of down-stream
Receivers. Terrestrial networks may employ several transmission
Hubs, each serving a particular coverage cell with a community of
Receivers.
From an IP viewpoint, the service is typically either unidirectional
multicast, or a bidirectional service in which some complementary
link technology (e.g., modem, Local Multipoint Distribution Service
(LMDS), General Packet Radio Service (GPRS)) is used to provide the
return path from Receivers to the Internet. In this case, routing
could be provided using UniDirectional Link Routing (UDLR) [RFC3077].
Note that only Scenarios A-B actually carry MPEG-2 video and audio
(intended for reception by digital Set Top Boxes (STBs)) as the
primary traffic. The other scenarios are IP-based data networks and
need not necessarily implement the MPEG-2 control plane.
Scenarios E-F provide two-way connectivity using the MPEG-2
Transmission Network. Such networks provide direct support for
bidirectional protocols above and below the IP layer.
The complete MPEG-2 transmission network may be managed by a
transmission service operator. In such cases, the assignment of
addresses and TS Logical Channels at Receivers are usually under the
control of the service operator. Examples include a TV operator
(Scenario A), or an ISP (Scenarios B-F). MPEG-2 transmission
networks are also used for private networks. These typically involve
a smaller number of Receivers and do not require the same level of
centralized control. Examples include companies wishing to connect
DVB-capable routers to form links within the Internet (Scenario B).
3.2. TS Logical Channels
An MPEG-2 Transport Multiplex offers a number of parallel channels,
which are known here as TS Logical Channels. Each TS Logical Channel
is uniquely identified by the Packet ID (PID) value that is carried
in the header of each MPEG-2 TS Packet. The PID value is a 13 bit
field; thus, the number of available channels ranges from 0 to 8191