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Versions: 00 01 02 03 04 RFC 4259
Internet Engineering Task Force M.J. Montpetit ed.
Internet Draft MJMontpetit.com
Document: draft-ietf-ipdvb-arch-04.txt Gorry Fairhurst
University of Aberdeen
Horst D. Clausen
TIC Systems
Bernhard Collini-Nocker
Hilmar Linder
University of Salzburg
Category: Informational May 2005
A Framework for transmission of IP datagrams over MPEG-2 Networks
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
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The list of current Internet-Drafts can be accessed at
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Copyright (C) The Internet Society (2004), All Rights Reserved
Abstract
This document describes an architecture for the transport of IP
Datagrams over ISO MPEG-2 Transport Streams (TS). The MPEG-2 TS has
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.
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Table of Contents
1. Introduction
1.1 Salient Features of the Architecture
2. Conventions Used in This Document
3. Architecture
3.1 MPEG-2 Transmission Networks
3.2 TS Logical Channels
3.3 Multiplexing and Re-Multiplexing
3.4 IP Datagram Transmission
3.5 Motivation
4. Encapsulation Protocol Requirements
4.1 Payload_Unit Delimitation
4.2 Length Indicator
4.3 Next Level Protocol Type
4.4 L2 Subnet Addressing
4.5 Integrity Check
4.6 Identification of Scope
4.7 Extension Headers
4.8 Summary of Requirements for Encapsulation
5. Address Resolution Functions
5.1 Address Resolution for MPEG-2
5.2 Scenarios for MPEG AR
5.2.1 Table-based AR over MPEG-2
5.2.2 Table-based AR over IP
5.2.3. Query/Response AR over IP
5.3 Unicast Address Scoping
5.4 AR Authentication
5.5 Requirements for Unicast AR over MPEG-2
6. Multicast Support
6.1 Multicast AR Functions
6.2 Multicast Address Scoping
6.3 Requirements for Multicast over MPEG-2
7. Summary
8. Security Considerations
8.1 Link Encryption
9. IANA Considerations
10. Acknowledgments
11. References
11.1 Normative References
11.2 Informative References
12. Authors' Addresses
13. IPR Notices
14. Copyright Statements
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[***RFC Editor Note: Remove following text prior to publication***]
Change Notice:
- v00 Original ipdvb WG Version
Document has been shortened and focused.
Some annexe material has been removed.
Restructured to describe the full framework.
New text describing the various scenarios.
Text added on various issues including compatibility
with services on DVB and ATSC (and different physical links).
- v01 Revised version following IETF-60 discussions
Removed redundant AR info and clarify AR reqs.
Multicast address scoping moved to section on
multicast AR.
Removed examples in AR appendix.
Added a small description of "e2e" management requirements.
Updated reference list.
Updated terminology to agree with that in ULE Spec.
Review by all authors to fix last known inconsistencies.
- v02 Revised version following WGLC discussions
Updated figure 1.
Fixed author's affiliation.
Fixed remaining typos and inconsistencies in page numbering.
Added DVB-S2, Open Cable and MHP references.
Removed a controversial paragraph in the Appendix.
- v03 Revised definitions following WGLC of ULE
- v04 Revised Security Considerations following IESG Review
Punctuation: added commas in section 1.
Reordered IANA, Author's Addresses sections.
Double space removed between words.
[***RFC Editor Note: End of text to be removed***]
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1. Introduction
This document identifies requirements and an architecture for the
transport of IP Datagrams over ISO MPEG-2 Transport Streams [MPEG2].
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] and 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, and are, also used to build IP-only networks.
Standard system components offer advantages of improved
interoperability and larger deployment. However, often, 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.
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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
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.
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2. Conventions Used In This Document
A2. 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.
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-DVB]. 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.
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.
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PDU: Protocol Data Unit. Examples of a PDU include Ethernet frames,
IPv4 or IPv6 datagrams, and other network packets.
PES: Packetized Elementary Steam [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 LogicalChannels transmitted using different
TS Multiplexes.
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).
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Receiver: An 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: Subnetwork Data Unit [RFC3819]. 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) [ID-ipdvb-arch], 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 synchronise 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.
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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 [ID-
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 centred 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 uni-directional file delivery or bi-directional 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) Uni-directional Star IP Scenario
The Uni-directional Star IP Scenario utilises 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.
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D) Datacast Overlay
The Datacast Overlay scenario employs MPEG-2 physical and link
layers to provide additional connectivity such as uni-directional
multicast to supplement an existing IP-based Internet service.
Examples of such a network includes IP Datacast to mobile wireless
receivers [ID-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 bi-directional 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 uni-directional 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
uni-directional multicast, or a bi-directional service in which some
complementary link technology (e.g. Modem, LMDS, GPRS, ...) is used
to provide the return path from Receivers to the Internet. Routing
in this case could be provided using Uni-Directional 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
bi-directional protocols above and below the IP layer.
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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 centralised 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 and, thus the number of available channels ranges from
0 to 8191 decimal or 0x1FFF in hexadecimal, some of which are
reserved for transmission of SI tables. Non-reserved TS Logical
Channels may be used to carry audio [ISO-AUD], video [ISO-VID], IP
packets [ISO-DSMCC; ETSI-DAT; ATSC-DAT],or other data [ISO-DSMCC;
ETSI-DAT; ATSC-DAT]. The value 8191 decimal(0x1FFF) indicates a null
packet, used to maintain the physical bearer bit rate when there are
no other MPEG-2 TS packets to be sent.
TS-LC-A-1 /---\--------------------/---\
\ / \ / \
\ | | | |
TS-LC-A-2 ----------- | | -------------
-------------------- | | -------------
| | | |
/-------- / | -------------
/ \----/-------------------\----/
TS-LC-A-3/ MPEG-2 TS MUX A
/
TS-LC /
------------X
\ TS-LC-B-3 /---\------------------------/---\
\ / \ / \
\ | | | |
TS-LC-B-2 \----------- | | ---------
-------------------- | | ---------
| | | |
/-------- / | ---------
/ \----/-----------------------\----/
/ MPEG-2 TS MUX B
TS-LC-B-1
Figure 2: Example showing MPEG-2 TS Logical Channels carried
Over 2 MPEG-2 TS Multiplexes.
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TS Logical Channels are independently numbered on each MPEG-2 TS
Multiplex (MUX). In most cases the data sent over the TS Logical
Channels will differ for different multiplexes. Figure 2
shows a set of TS Logical Channels sent using two MPEG-2 TS
Multiplexes (A and B).
There are cases where the same data may be distributed over two or
more multiplexes (e.g., some SI tables; multicast content which
needs to be received by Receivers tuned to either MPEG-2 TS; unicast
data were the Receiver may be in either/both two potentially
overlapping MPEG-2 transmission cells). In figure 2, each multiplex
carries 3 MPEG-2 TS Logical Channels. These TS Logical Channels may
differ (TS-LC-A-1, TS-LC-A-2, TS-LC-B-2, TS-LC-B-1), or may be
common to both MPEG-2 TS Multiplexes (i.e. TS-LC-A-3 and TS-LC-B-3
carry identical content).
As can been seen, there are similarities between the way PIDs
are used and the operation of virtual channels in ATM. However,
unlike ATM, a PID defines a unidirectional broadcast channel and not
a point-to-point link. Contrary to ATM, there is, as yet, no
specified standard interface for MPEG-2 connection setup, or for
signaling mappings of IP flows to PIDs, or to set the Quality of
Service, QoS, assigned to a TS Logical Channel.
3.3 Multiplexing and Re-Multiplexing
In a simple example, one or more TS are processed by a MPEG-2
multiplexor resulting in a TS Multiplex. The TS Multiplex is
forwarded over a physical bearer towards one or more Receivers
(figure 3).
In a more complex example, the same TS may be fed to multiple MPEG-2
multiplexors and these may, in turn, feed other MPEG-2 multiplexors
(remultiplexing). Remultiplexing may occur in several places (and is
common in Scenarios A and B of section 3.1). One example is a
satellite that provides on-board processing of the TS packets,
multiplexing the TS Logical Channels received from one or more
uplink physical bearers (TS Multiplex) to one (or more in the case
of broadcast/multicast) down-link physical bearer (TS Multiplex). As
part of the remultiplexing process, a remultiplexor may renumber the
PID values associated with one or more TS Logical Channels to
prevent clashes between input TS Logical Channels with the same PID
carried on different input multiplexes. It may also modify and/or
insert new SI data into the control plane.
In all cases, the final result is a "TS Multiplex" which is
transmitted over the physical bearer towards the Receiver.
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+------------+ +------------+
| IP | | IP |
| End Host | | End Host |
+-----+------+ +------------+
| ^
+------------>+---------------+ |
+ IP | |
+-------------+ Encapsulator | |
SI-Data | +------+--------+ |
+-------+-------+ |MPEG-2 TS Logical Channel |
| MPEG-2 | | |
| SI Tables | | |
+-------+-------+ ->+------+--------+ |
| -->| MPEG-2 | . . .
+------------>+ Multiplexor | |
MPEG-2 TS +------+--------+ |
Logical Channel |MPEG-2 TS Mux |
| |
Other ->+------+--------+ |
MPEG-2 -->+ MPEG-2 | |
TS --->+ Multiplexor | |
---->+------+--------+ |
|MPEG-2 TS Mux |
| |
+------+--------+ +------+-----+
|Physical Layer | | MPEG-2 |
|Modulator +---------->+ Receiver |
+---------------+ MPEG-2 +------------+
TS Mux
Figure 3: An example configuration for a uni-directional
Service for IP transport over MPEG-2
3.4 IP Datagram Transmission
Packet data for transmission over an MPEG-2 Transport Multiplex is
passed to an Encapsulator, sometimes known as a Gateway. This
receives Protocol Data Units, PDUs, such as Ethernet frames or IP
packets, and formats each into a Sub-Network Data Unit, SNDU, by
adding an encapsulation header and trailer (see section 4). The
SNDUs are subsequently fragmented into a series of TS Packets.
To receive IP packets over a MPEG-2 TS Multiplex, a Receiver
needs to identify the specific TS Multiplex (physical link) and also
the TS Logical Channel (the PID value of a logical link). It is
common for a number of MPEG-2 TS Logical Channels to carry SNDUs,
and a Receiver must therefore filter (accept) IP packets sent with a
number of PID values, and must independently reassemble each SNDU.
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A Receiver that simultaneously receives from several TS Logical
Channels, must filter other unwanted TS Logical Channels by
employing for example specific hardware support. Packets for one IP
flow (i.e. a specific combination of IP source and destination
addresses) must be sent using the same PID. It should not be assumed
that all IP packets are carried on a single PID, as in some cable
modem implementations, and multiple PIDs must be allowed in the
architecture. Many current hardware filters limit the maximum number
of active PIDs (e.g. 32), although if needed, future systems may
reasonably be expected to support more.
In some cases, Receivers may need to select TS Logical Channels from
a number of simultaneously active TS Multiplexes. To do this, they
need multiple physical receive interfaces (e.g., RF front-ends and
demodulators). Some applications also envisage the concurrent
reception of IP Packets over other media that may not necessarily
use MPEG-2 transmission.
Bi-directional (duplex) transmission can be provided using a MPEG-2
Transmission Network by using one of a number of alternate return
channel schemes [DVB-RC]. Duplex IP paths may also be supported
using non-MPEG-2 return links (e.g. in Scenarios B-D of section
3.1). One example of such an application is that of Uni-Directional
Link Routing, UDLR [RFC3077].
3.5 Motivation
The network layer protocols to be supported by this architecture
include:
(i) IPv4 Unicast packets, destined for a single end host
(ii) IPv4 Broadcast packets, sent to all end systems in an IP
network
(iii) IPv4 Multicast packets
(iv) IPv6 Unicast packets, destined for a single end host
(v) IPv6 Multicast packets
(vi) Packets with compressed IPv4 / IPv6 packet headers (e.g.
[RFC2507; RFC3095])
(vii) Bridged Ethernet frames
(viii) Other network protocol packets (MPLS, potential new
protocols)
The architecture will provide:
(i) Guidance on which MPEG-2 features are pre-requisites for the IP
service, and identification of any optional fields that impact
performance/correct operation.
(ii) Standards to provide an efficient and flexible encapsulation
scheme that may be easily implemented in an Encapsulator or
Receiver. The payload encapsulation requires a type field for
the SNDU to indicate the type of packet and a mechanism to
signal which encapsulation is used on a certain PID.
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(iii) Standards to associate a particular IP address with a Network
Point of Attachment (NPA) that could or may not be a MAC
Address. This process resembles the IPv4 Address Resolution
Protocol, ARP, or IPv6 Neighbour Discovery, ND, protocol [AR-
DRAFT]. In addition, the standard will be compatible with IPv6
autoconfiguration.
(iv) Standards to associate a MPEG-2 TS interface with one or more
specific TS Logical Channels (PID, TS Multiplex). Bindings are
required for both unicast transmission, and multicast
reception. In the case of IPv4, this must also support network
broadcast. To make the schemes robust to loss and state changes
within the MPEG-2 transmission network, a soft-state approach
may prove desirable.
(v) Standards to associate the capabilities of a MPEG-2 TS Logical
Channel with IP flows. This includes mapping of QoS functions,
such as IP QoS/DSCP and RSVP, to underlying MPEG-2 TS QoS,
multi-homing and mobility. This capability could be associated
by the AR standard proposed above.
(vi) Guidance on Security for IP transmission over MPEG-2. The
framework must permit use of IPsec and clearly identify any
security issues concerning the specified protocols. The
security issues need to consider two cases: unidirectional
transfer (in which communication is only from the sending IP
end host to the receiving IP end host) and bi-directional
transfer. Consideration should also be given to security of the
TS Multiplex: the need for closed user groups and the use of
MPEG-2 TS encryption.
(vii) Management of the IP transmission, including standardised SNMP
MIBs and error reporting procedures. The need for and scope of
this is to be determined.
The specified architecture and techniques should be suited to a
range of systems employing the MPEG-2 TS, and may also suit other
(sub)networks offering similar transfer capabilities.
The following section, 4, describes encapsulation issues.
Sections 6 and 7 describe address resolution issues for unicast and
multicast respectively.
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4. Encapsulation Protocol Requirements
This section identifies requirements and provides examples of
mechanisms that may be used to perform the encapsulation of IPv4/v6
unicast and multicast packets over MPEG-2 Transmission Networks.
A network device, known as an Encapsulator receives PDUs (e.g. IP
Packets or Ethernet frames) and formats these into Subnetwork Data
Units,SNDUs. An encapsulation (or convergence) protocol transports
each SNDU over the MPEG-2 TS service and provides the appropriate
mechanisms to deliver the encapsulated PDU to the Receiver IP
interface.
In forming a SNDU, the encapsulation protocol typically adds
header fields that carry protocol control information, such
as the length of SNDU, Receiver address, multiplexing information,
payload type, sequence numbers etc. The SNDU payload is typically
followed by a trailer, which carries an Integrity Check (e.g.,
Cyclic Redundancy Check, CRC). Some protocols also add additional
control information and/or padding to or after the trailer
(figure 4).
+--------+-------------------------+-----------------+
| Header | PDU | Integrity Check |
+--------+-------------------------+-----------------+
<--------------------- SNDU ------------------------->
Figure 4: Encapsulation of a subnetwork PDU (e.g. IPv4 or IPv6
packet) to form an MPEG-2 Payload Unit.
Examples of existing encapsulation/convergence protocols include
ATM AAL5 [ITU-AAL5], and MPEG-2 MPE [ETSI-DAT].
When required, a SNDU may be fragmented across a number of TS
Packets (figure 5).
+-----------------------------------------+
|Encap Header|SubNetwork Data Unit (SNDU) |
+-----------------------------------------+
/ / \ \
/ / \ \
/ / \ \
+------+----------+ +------+----------+ +------+----------+
|MPEG-2| MPEG-2 |..|MPEG-2| MPEG-2 |...|MPEG-2| MPEG-2 |
|Header| Payload | |Header| Payload | |Header| Payload |
+------+----------+ +------+----------+ +------+----------+
Figure 5: Encapsulation of an PDU (e.g., IP packet) into a
Series of MPEG-2 TS Packets. Each TS Packet carries a header
with a common Packet ID (PID) value denoting the MPEG-2 TS
Logical Channel.
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The DVB family of standards currently defines a mechanism for
transporting an IP packet, or Ethernet frame using the
Multi-Protocol Encapsulation (MPE) [ETSI-DAT]. An equivalent scheme
is also supported in ATSC [ATSC-DAT; ATSC-DATG]. It allows
transmission of IP packets or (by using LLC) Ethernet frames by
encapsulation within a Table Section (with the format used by the
control plane associated with the MPEG-2 transmission). The MPE
specification includes a set of optional header components and
requires decoding of the control headers. This processing is
suboptimal for Internet traffic, since it incurs significant
receiver processing overhead and some extra link overhead [CLC99].
The existing standards carry heritage from legacy implementations.
These have reflected the limitations of technology at the time of
their deployment (v.g. design decisions driven by audio/video
considerations rather than IP networking requirements). IPv6, MPLS,
and other network-layer protocols are not natively supported.
Together, these justify the development of a new encapsulation
that will be truly IP-centric. Carrying IP packets over a TS
Logical Channel involves several convergence protocol functions.
This section briefly describes these functions and highlights the
requirements for a new encapsulation.
4.1 Payload_Unit Delimitation
MPEG-2 indicates the start of a Payload Unit (PU) in a new TS Packet
with a "start_of_payload_unit_indicator" (PUSI) [ISO-MPEG] carried
in the 4B TS Packet header. The PUSI is a 1 bit flag that has
normative meaning [ISO_MPEG] for TS Packets that carry PES Packets
or PSI/SI data.
When the payload of a TS Packet contains PES data, a PUSI value of
'1' indicates the TS Packet payload starts with the first byte of a
PES Packet. A value of '0' indicates that no PES Packet starts in
the TS Packet. If the PUSI is set to '1', then one, and only one,
PES Packet starts in the TS Packet.
When the payload of the TS Packet contains PSI data, a PUSI value of
'1' indicates the first byte of the TS Packet payload carries a
Payload Pointer (PP) that indicates the position of the first byte
of the Payload Unit (Table Section) being carried; if the TS Packet
does not carry the first byte of a Table Section, the PUSI is set to
'0', indicating that no Payload Pointer is present.
Using this PUSI bit, the start of the first Payload Unit in a TS
Packet is exactly known by the Receiver, unless that TS Packet has
been corrupted or lost in the transmission. In which case, the
payload is discarded until the next TS Packet is received with a
PUSI value of '1'.
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The encapsulation should allow packing of more than one SNDU into
the same TS Packet and should not limit the number of SNDUs that can
be sent in a TS Packet. In addition, it should allow an IP
Encapsulator to insert padding when there is an incomplete TS Packet
payload. A mechanism needs to be identified to differentiate this
padding from the case where another encapsulated SNDU follows.
A combination of the PUSI and a Length Indicator (see below) allows
an efficient MPEG-2 convergence protocol to receive accurate
delineation of packed SNDUs. The MPEG-2 standard [ISO_MPEG] does
not specify how private data should use the PUSI bit.
4.2 Length Indicator
Most services using MPEG-2 include a length field in the Payload
Unit header to allow the Receiver to identify the end of a Payload
Unit (e.g. PES Packet, Section, or an SNDU).
When parts of more than two Payload Units are carried in the same TS
Packet, only the start of the first is indicated by the Payload
Pointer. Placement of a Length Indicator in the encapsulation header
allows a Receiver to determine the number of bytes until the start
of the next encapsulated SNDU. This placement also provides the
opportunity for the Receiver to pre-allocate an appropriate-sized
memory buffer to receive the reassembled SNDU.
A Length Indicator is required, and should be carried in the
encapsulation header. This should support SNDUs of at least the MTU
size offered by Ethernet (currently 1500 bytes). Although the IPv4
and IPv6 packet format permits an IP packet of size up to 64 KB,
such packets are seldom seen on the current Internet. Since high
speed links are often limited by the packet forwarding rate of
routers, there has been a tendency for Internet core routers to
support MTU values larger than 1500 bytes. A value of 16 KB is not
uncommon in the core of the current Internet. This would seem a
suitable maximum size for an MPEG-2 transmission network.
4.3 Next Level Protocol Type
A key feature of the required encapsulation is to identify the
payload type being transported (e.g. to differentiate IPv4, IPv6,
etc). Most protocols use a type field to identify a specific
process at the next higher layer that is the originator or the
recipient of the payload (SNDU). This method is used by IPv4,
IPv6, and also by the original Ethernet protocol (DIX). OSI
uses the concept of a 'Selector' for this, (e.g. in the IEEE
802/ISO 8802 standards for CSMA/CD [LLC], although in this
case a SNAP (subnetwork access protocol) header is also
required for IP packets.
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A Next Level Protocol Type field is also required if compression
(e.g. Robust Header Compression [RFCROHC]) is supported. No
compression method has currently been defined that is directly
applicable to this architecture, however the ROHC framework
defines a number of header compression techniques that may yield
considerable improvement in throughput on links which have a limited
capacity. Since many MPEG-2 Transmission Networks are wireless,
the ROHC framework will be directly applicable for many
applications. The benefit of ROHC is greatest for smaller SNDUs
but does imply the need for additional processing at the Receiver
to expand the received compressed packets. The selected type
field should contain sufficient code points to support this
technique.
It is thus a requirement to include a Next Level Protocol Type field
in the encapsulation header. Such a field should specify values for
at least IPv4, IPv6, and must allow for other values (e.g., MAC-
level bridging).
4.4 L2 Subnet Addressing
In MPEG-2, the PID carried in the TS Packet header is used to
identify individual services with the help of SI tables. This was
primarily intended as a unidirectional (simplex) broadcast system.
A TS Packet stream carries either tables or one PES Packet stream
(i.e., compressed video or audio). Individual Receivers are not
addressable at this level.
IPv4 and IPv6 allocate addresses to end hosts and intermediate
systems (routers). Each system (or interface) is identified by a
globally assigned address. ISO uses the concept of a hierarchically
structured Network Service Access Point (NSAP) address to identify
an end host user process in an Internet environment.
Within a local network a completely different set of addresses for
the Network Point of Attachment (NPA) is used; frequently these NPA
addresses are referred to as Medium Access Control, MAC-level
addresses. In the Internet they are also called hardware addresses.
Whereas network layer addresses are used for routing, NPA addresses
are primarily used for Receiver identification.
Receivers may use the NPA of a received SNDU to reject unwanted
unicast packets within the (software) interface driver at the
Receiver, but must also perform forwarding checks based on the IP
address. IP multicast and broadcast may also filter using the
NPA, but Receivers must also filter unwanted packets at the network
layer based on source and destination IP addresses. This does not
imply that each IP address must map to a unique NPA (more than one
IP address may map to the same NPA). If a separate NPA address is
not required, the IP address is sufficient for both functions.
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If it is required to address an individual Receiver in an MPEG-2
transport system, this can be achieved either at the network level
(IP address) or via a hardware-level NPA address (MAC-address). If
both addresses used, they must be mapped either in a static or a
dynamic way (e.g., by an address resolution process). A similar
requirement may also exist to identify the PID and TS multiplex on
which services are carried.
Using an NPA address in a MPEG-2 TS may enhance security, in that
a particular PDU may be targeted for a particular Receiver by
specifying the corresponding Receiver NPA address. This is
however only a weak form of security, since the NPA filtering is
often reconfigurable (frequently performed in software), and may be
modified by a user to allow reception of specified (or all) packets,
similar to promiscuous mode operation in Ethernet. If security is
required, it should be applied at another place (e.g. link
encryption, authentication of address resolution, IPsec, transport
level security and/or application level security).
There are also cases where the use of a NPA is required (e.g. where
a system operates as a router) and if present this should be carried
in an encapsulation header where it may be used by Receivers as a
pre-filter to discard unwanted SNDUs. The addresses allocated do not
need to conform to the IEEE MAC address format. There are many cases
where a NPA is not required, and network layer filtering may be
used. A new encapsulation protocol should therefore support an
optional NPA.
4.5 Integrity Check
For the IP service, the probability of undetected packet error
should be small (or negligible) [RFC3819]. There is therefore
a need for a strong integrity check (e.g. Cyclic Redundancy Check
or CRC) to verify correctness of a received PDU [RFC3819].
Such checks should be sufficient to detect incorrect operation of
the encapsulator and Receiver (including reassembly errors
following loss/corruption of TS Packets), in addition to
protecting from loss and/or corruption by the transmission
network (e.g., multiplexors and links).
Mechanisms exist in MPEG-2 Transmission Networks that may assist in
detecting loss (e.g. the 4-bit continuity counter included in the
MPEG-2 TS Packet header).
An encapsulation must provide a strong integrity check for each
IP packet. The requirements for usage of a link CRC are provided
in [RFC3819]. To ease hardware implementation, this check should
be carried in a trailer following the SNDU. A CRC-32 is sufficient
for operation with up to a 12 KB payload, and may still provide
adequate protection for larger payloads.
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4.6 Identification of Scope.
The MPE section header contains information that could be used by
The Receiver to identify the scope of the (MAC) address carried as a
NPA, and prevent TS Packets intended for one scope being received by
another. Similar functionality may be achieved by ensuring that only
IP packets that do not have overlapping scope are sent on the same
TS Logical Channel. In some cases, this may imply the use of
multiple TS Logical Channels.
4.7 Extension Headers
The evolution of the Internet service may in future require
additional functions. A flexible protocol should therefore provide a
way to introduce new features when required, without having to
provide additional out-of-band configuration.
IPv6 introduced the concept of extension headers that carry extra
information necessary/desirable for certain subnetworks. The DOCSIS
cable specification also allows a MAC header to carry extension
headers to build operator-specific services. It is thus a
requirement for the new encapsulation to allow extension headers.
4.8 Summary of Requirements for Encapsulation
The main requirements for an IP-centric encapsulation include:
- support of IPv4 and IPv6 packets
- support for Ethernet encapsulated packets
- flexibility to support other IP formats and protocols (e.g.
ROHC, MPLS)
- easy implementation using either hardware or software
processing
- low overhead/managed overhead
- a fully specified algorithm that allows a sender to pack
multiple packets per SNDU and to easily locate packet
fragments
- extensibility
- compatibility with legacy deployments
- ability to allow link encryption, when required
- capability to support a full network architecture including
data, control and management planes
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5. Address Resolution Functions
Address Resolution (AR) provides a mechanism that associates L2
information with the IP address of a system. Many L2 technologies
employ unicast AR at the sender: an IP system wishing to send an IP
packet encapsulates it and places it into a L2 frame. It then
identifies the appropriate L3 adjacency (e.g. next hop router, end
host) and determines the appropriate L2 adjacency (e.g. MAC address
in Ethernet) to which the frame should be sent so that the packet
gets across the L2 link.
The L2 addresses discovered using AR are normally recorded in a data
structure known as the arp/neighbor cache. The results of previous
AR requests are usually cached. Further AR protocol exchanges may be
required as communication proceeds to update or re-initialise the
client cache state contents (i.e. purge/refresh the contents [ND]).
For stability, and to allow network topology changes and client
faults, the cache contents are normally "soft state", that is, they
are aged with respect to time and old entries removed.
In some cases (e.g. ATM, FR, X.25, MPEG-2 and many more), AR
involves finding other information than the MAC address. This
includes identifying other parameters required for L2 transmission,
such as channel IDs (VCs in X.25, VCIs in ATM, or PIDs in MPEG-2
TS).
Address resolution has different purposes for unicast and multicast.
Multicast address resolution is not required for many L2 networks,
but is required where MPEG-2 transmission networks carry IP
multicast packets using more than one TS Logical Channel.
5.1 Address Resolution for MPEG-2
There are three elements to the L2 information required to perform
AR before an IP packet is sent over a MPEG-2 TS. These are:
(i) A Receiver ID (e.g. a 6B MAC/NPA address).
(ii) A PID or index to find a PID.
(iii) Tuner information (e.g. Transmit Frequency of the
physical layer of a satellite/broadcast link
Elements (ii) and (iii) need to be de-referenced via indexes to the
Service Information (i.e. the Program Map Table, PMT) when the
MPEG-2 Transmission Network includes remultiplexors that renumber
the PID values of the TS Logical Channels that they process. (Note
that PIDs are not intended to be end-to-end identifiers). However,
although remultiplexing is common in broadcast TV networks
(scenarios A and B), many private networks do not need to employ
multiplexors that renumber PIDs (see section 3.2).
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The third element (iii) allows an AR client to resolve to a
different MPEG TS Multiplex. This is used when there are several
channels that may be used for communication (i.e. multiple
outbound/inbound links). In a mesh system, this could be used to
determine connectivity. This AR information is used in two ways at a
Receiver:
(i) AR resolves an IP unicast or IPv4 broadcast address to the (MPEG
TS Multiplex, PID, MAC/NPA address). This allows the Receiver to set
L2 filters to let traffic pass to the IP layer. This is used for
unicast, and IPv4 subnet broadcast.
(ii) AR resolves an IP multicast address to the (MPEG TS Multiplex,
PID, MAC/NPA address), and allows the Receiver to set L2 filters
enabling traffic to pass to the IP layer. A Receiver in a MPEG-2 TS
Transmission Network needs to resolve the PID value and the tuning
(if present)associated with a TS Logical Channel and (at least for
unicast) the destination Receiver NPA address.
A star topology MPEG-2 TS transmission network is illustrated below,
with two Receivers receiving a forward broadcast channel sent by a
Hub. (A mesh system has some additional cases.) The forward
broadcast channel consists of a "TS Multiplex" (a single physical
bearer) allowing communication with the terminals. These communicate
using a set of return channels.
Forward broadcast
MPEG-2 TS \
----------------X /-----\
/ / \
| Receiver|
/----------+ A |
/ \ /
/-----\ / \-----/
/ \ /
| Hub |/
| +\ /-----\
\ / \ / \
\-----/ \ | Receiver|
\-----------+ B |
\ /
\-----/
Figure 6: MPEG-2 Transmission Network with 2 Receivers
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There are three possibilities for unicast AR:
(1) A system at a Receiver, A, needs to resolve an address of a
system that is at the Hub;
(2) A system at a Receiver, A, needs to resolve an address that is
at another Receiver, B;
(3) A host at the Hub needs to resolve an address that is at a
Receiver. The sender (encapsulation gateway), uses AR to provide the
the MPEG TS Multiplex, PID, MAC/NPA address for sending unicast,
IPv4 subnet broadcast and multicast packets.
If the Hub is an IP router, then case (1) and (2) are the same:
The host at the Receiver does not know the difference. In these
cases, the address to be determined is the L2 address of the device
at the Hub to which the IP packet should be forwarded, and which
then relays the IP packet back to the forward (broadcast) MPEG-2
channel after AR (case 3).
If the Hub is a L2 bridge, then case 2 still has to relay the IP
packet back to the outbound MPEG-2 channel. The AR protocol needs to
resolve the specific Receiver L2 MAC address of B, but needs to send
this on a L2 channel to the Hub. This requires Receivers to be
informed of the L2 address of other Receivers.
An end host connected to the Hub needs to use the AR protocol to
resolve the Receiver terminal MAC/NPA address. This requires the AR
server at the Hub to be informed of the L2 addresses of other
Receivers.
5.2 Scenarios for MPEG AR
An AR protocol may transmit AR information in three distinct ways:
(i) An MPEG-2 signalling table transmitted at the MPEG-2 level
(e.g. within the control plane using a Table;
(ii) An MPEG-2 signalling table transmitted at the IP level
(no implementations of this are known);
(iii) An address resolution protocol transported over IP
(as in ND for IPv6)
There are three distinct cases in which AR may be used:
(i) Multiple TS-Mux and the use of re-multiplexors; e.g. Digital
Terrestrial, Satellite TV broadcast multiplexes. Many such systems
employ remultiplexors that modify the PID values associated with TS
Logical Channels as they pass through the MPEG-2 transmission
network (as in Scenario A of Section 3.1).
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(ii) Tuner configuration(s) that are fixed or controlled by some
other process. In these systems, the PID value associated with a TS
Logical Channel may be known by the Sender.
(iii) A service run over one TS Mux (i.e., uses only one PID, for
example DOCSIS and some current DVB-RCS multicast systems). In these
systems, the PID value of a TS Logical Channel may be known by the
Sender.
5.2.1 Table-based AR over MPEG-2
In current deployments of MPEG-2 networks, information about the set
of MPEG-2 TS Logical Channels carried over a TS Multiplex is usually
distributed via tables (service information, SI) sent using channels
assigned a specific (well-known) set of PIDs. This was originally
designed for audio/video distribution to STBs. This design requires
access to control plane by processing the SI table information
(carried in MPEG-2 section format [ISO_DSMCC]). The scheme
reflects the complexity of delivering and co-ordinating the various
TS Logical Channels associated with a multimedia TV programme.
One possible requirement to provide TS multiplex and PID information
for IP services is to integrate additional information into the
existing MPEG-2 tables, or to define additional tables specific to
the IP service. The DVB INT and the A/92 Specification from ATSC
[ATSC-A92] are examples of the realization of such a requirement.
5.2.2 Table-based AR over IP
AR information could be carried over a TS data channel, (e.g. using
an IP protocol similar to the Service Announcement Protocol, SAP).
Implementing this independently of the SI tables, would ease
implementation, by allowing it to operate on systems where IP
processing is performed in a software driver. It may also allow the
technique to be more easily adapted to other similar delivery
networks. It also is advantageous for networks which use the MPEG-2
TS, but do not necessarily support audio/video services and
therefore do not need to provide interoperability with TV
equipment (e.g. links used solely for connecting IP (sub)networks).
5.2.3. Query/Response AR over IP
A query/response protocol may be used at the IP level (similar to,
or based on IPv6 Neighbor Advertisements of the ND protocol). The AR
protocol may operate over an MPEG-2 TS Logical Channel using a
previously agreed PID (e.g. configured, or communicated using a SI
table). In this case, the AR could be performed by the target system
itself (as in ARP and ND). This has good soft-state properties, and
is very tolerant to failures. To find an address, a system sends a
"query" to the network, and the "target" (or its proxy) replies.
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5.3 Unicast Address Scoping
In some case, an MPEG-2 Transmission Network may support multiple IP
networks. If this is the case, it is important to recognise the
context (scope) within which an address is resolved, to prevent
packets from one addressed scope leaking into other scopes.
An examples of overlapping IP address assignments is the use of
private unicast addresses (e.g. in IPv4, 10/8 prefix;
172.16/12 prefix; 192.168/16 prefix). These should be confined to
the area to which they are addressed.
There is also a requirement for multicast address scoping
(section 6.2).
IP packets with these addresses must not be allowed to travel
outside their intended scope, and may cause unexpected behaviour if
allowed to do so. In addition, overlapping address assignments can
arise using level 2 NPA addresses:
(i) The NPA address must be unique within the TS Logical Channel.
Universal IEEE MAC addresses used in Ethernet LANs are
globally unique. If the NPA addresses are not globally
unique, the same NPA address may be re-used by Receivers
in different addressed areas.
(ii) The NPA broadcast address (all 1s MAC address). Traffic with
this address should be confined to one addressed area.
Reception of unicast packets destined for another addressed area may
lead to an increase in the rate of received packets by systems
connected via the network. IP end hosts normally filter received
unicast IP packets based on their assigned IP address. Reception of
the additional network traffic may contribute to processing load but
should not lead to unexpected protocol behaviour. It does however
introduce a potential Denial of Service (DoS) opportunity.
When the Receiver acts as an IP router, the receipt of such an IP
packet may lead to unexpected protocol behaviour. This also provides
a security vulnerability since arbitrary packets may be passed to
the IP layer.
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5.4 AR Authentication
In many AR designs authentication has been overlooked, because of
the wired nature of most existing IP networks, which makes it easy
to control hosts physically connected [RFC3819]. With wireless
connections, this is changing: unauthorised hosts actually can
claim L2 resources. The address resolution client (i.e. Receiver)
may also need to verify the integrity and authenticity of the
AR information that it receives. There are trust relationships
both ways: clients need to know they have a valid server and
that the resolution is valid. Servers should perform authorisation
before they allow a L2 address to be used.
The MPEG-2 Transmission Network may also require access control to
prevent unauthorised use of the TS Multiplex, however, this is
an orthogonal issue to address resolution.
5.5 Requirements for Unicast AR over MPEG-2
The requirement for AR over MPEG-2 networks include:
(i) Use of a table-based approach to promote AR scaling. This
requires definition of the frequency of update and volume of
AR traffic generated.
(ii) Mechanisms to install AR information at the server
(unsolicited registration).
(iii) Mechanisms to verify AR information held at the server
(solicited responses). Appropriate timer values need to be
defined.
(iv) An ability to purge client AR information (after IP network
renumbering, etc.).
(v) Support of IP subnetwork scoping.
(vi) Appropriate security associations to authenticate the sender.
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6. Multicast Support
This section addresses specific issues concerning IPv4 and IPv6
multicast [RFC1112] over MPEG-2 Transmission Networks. The primary
goal of multicast support will be efficient filtering of IP
multicast packets by the Receiver, and the mapping of IPv4 and
IPv6 multicast addresses [RFC3171] to the associated PID value
and TS Multiplex.
The design should permit a large number of active multicast groups,
and should minimise the processing load at the Receiver when
filtering and forwarding IP multicast packets. For example, schemes
that may be easily implemented in hardware would be beneficial,
since these may relieve drivers and operating systems from
discarding unwanted multicast traffic [RFC3819].
Multicast mechanisms are used at more than one protocol level. The
upstream router feeding the MPEG-2 Encapsulator may forward
multicast traffic on the MPEG-2 TS Multiplex using a static or
dynamic set of groups. When static forwarding is used, the set
of IP multicast groups may also be configured or set using SNMP,
Telnet, etc. A Receiver normally uses either an IP group management
protocol (IGMP [RFC 3376] for IPv4 or MLD [RFC2710][RFC3810] for
IPv6) or a multicast routing protocol to establish tables that it
uses to dynamically enable local forwarding of received groups. In
a dynamic case, this group membership information is fed-back to the
sender to enable it to start sending new groups and (if required) to
update the tables that it produces for multicast AR.
Appropriate procedures need to identify the correct action when the
same multicast group is available on more than one TS Logical
Channel. This could arise when different end hosts act as senders
to contribute IP packets with the same IP group destination address.
The correct behaviour for SSM [RFC3569] addresses must also be
considered. It may also arise when a sender duplicates the same IP
group over several TS Logical Channels (or even different TS
Multiplexes), and in this case a Receiver may potentially receive
more than one copy of the same packet. At the IP level, the
host/router may be unaware of this duplication.
6.1 Multicast AR Functions
The functions required for multicast AR may be summarised as:
(i) The Sender needs to know L2 mapping of a multicast group.
(ii) The Receiver needs to know L2 mapping of a multicast group.
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In the Internet, multicast AR is normally a mapping function rather
than a one-to-one association using a protocol. In Ethernet, the
sender maps an IP address to a L2 MAC address, and the Receiver uses
the same mapping to determine the L2 address to set a L2
hardware/software filter entry.
A typical sequence of actions for the dynamic case is:
L3) Populate the IP L3 membership tables at the Receiver.
L3) Receivers send/forward IP L3 membership tables to the Hub
L3) Dynamic/static forwarding at hub/upstream router of IP L3
groups
L2) Populate the IP AR tables at the encapsulator gateway
(i.e. Map IP L3 mcast groups to L2 PIDs)
L2) Distribute the AR information to Receivers
L2) Set Receiver L2 multicast filters for IP groups in the
membership table.
To be flexible AR must associate a TS Logical Channel (PID) not only
with a group address, but possibly also a QoS class, and other
appropriate MPEG-2 TS attributes. Explicit per group AR to
individual L2 addresses is to be avoided.
\
|
+---+----+ +---------+
| Tuner |---+TS Table | . . . .
+---+----+ +---------+ .
| - .
+--------+ +---------+ .
| deMux |---+PID Table|........
+---+----+ +---------+ :
| - :
+--------+ +---------+ +------------+
|MPE/ULE |---+AR Cache-|---+ L2 Table |
+---+----+ +---------+ +------------+
| | |
+---+----+ +---+-----+ +---+----+
| IP | | AR | |IGMP/MLD|
+---+----+ +---+-----+ +---+----+
| | |
*------------+------------+
Figure 7: Receiver Processing Architecture
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6.2 Multicast Address Scoping
As in unicast, it is important to recognise the context (scope)
within which a multicast IP address is resolved, to prevent
packets from one addressed scope leaking into other scopes.
Examples of overlapping IP multicast address assignments, include:
(i) Some multicast addresses, (e.g., scoped multicast addresses
[RFC2365] that may be used in private networks). These are
only valid within the addressed area (examples for IPv4
include: 239/8; 224.0.0/24; 224.0.1/24). Similar cases
exist for some IPv6 multicast addresses [RFC2375].
(ii) Scoped multicast addresses. Forwarding of these addresses
is controlled by the scope associated with the address.
(iii) Other non-IP protocols may also view sets of MAC multicast
addresses as link-local, and may produce unexpected results
if distributed across several private networks.
IP packets with these addresses must not be allowed to travel
outside their intended scope (see section 5.3). Performing multicast
AR at the IP level can enable providers to offer independently
scoped addresses and would need to use multiple Multicast AR
servers, one per multicast domain.
6.3 Requirements for Multicast over MPEG-2
The requirements for supporting multicast include, but are not
restricted to:
(i) Encapsulating multicast packets for transmission using a
MPEG-2 TS.
(ii) Mapping IP multicast groups to the underlying MPEG-2 TS
Logical Channel (PID) and the MPEG-2 TS Multiplex.
(iii) Provide AR information to allow a Receiver to locate an
IP multicast flow within an MPEG-2 TS Multiplex.
(iv) Error Reporting.
7. Summary
This document describes the architecture for a set of protocols to
perform efficient and flexible support for IP network services over
networks built upon the MPEG-2 Transport Stream (TS). It also
describes existing approaches. The focus is on IP networking, the
mechanisms that are used and their applicability to supporting IP
unicast and multicast services.
The requirements for a new encapsulation of IPv4 and IPv6 packets is
described, outlining the limitations of current methods and the need
for a streamlined IP-centric approach.
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The architecture also describes MPEG-2 Address Resolution (AR)
procedures to allow dynamic configuration of the sender and Receiver
using an MPEG-2 transmission link/network. These support IPv4 and
IPv6 services in both the unicast and multicast modes. Resolution
protocols will support IP packet transmission using both the
Multiprotocol Encapsulation (MPE), which is currently
widely deployed, and also any IETF defined ULE encapsulation
[ID-IPDVB-ULE].
8. Security Considerations
When the MPEG-2 transmission network is not using a wireline
network, the normal security issues relating to the use of wireless
links for transport of Internet traffic should be considered
[RFC3819].
End-to-end security (including confidentiality, authentication,
integrity and access control) is closely associated with the end
user assets that are protected. This close association cannot be
ensured when providing security mechanisms only within a subnetwork
(e.g. an MPEG-2 Transmission Network). Several security mechanisms
that can be used end-to-end have already been deployed in the
general Internet and are enjoying increasing use. Important examples
include:
- Transport Layer Security (TLS), which is primarily used to
protect web commerce;
- Pretty Good Privacy (PGP) and S/MIME, primarily used to protect
and authenticate email and software distributions;
- Secure Shell (SSH), used to secure remote access and file
transfer;
- IPsec, a general purpose encryption and authentication mechanism
above IP that can be used by any IP application.
However, subnetwork security is also important [RFC3819] and
should be encouraged, on the principle that it is better than the
default situation, which all too often is no security at all.
Users of especially vulnerable subnets (such as radio/broadcast
networks and/or shared media Internet access) often have control
over at most one endpoint - usually a client - and therefore
cannot enforce the use of end-to-end mechanisms.
A related role for subnetwork security is to protect users against
traffic analysis, i.e., identifying the communicating parties (by IP
or MAC address) and determining their communication patterns, even
when their actual contents are protected by strong end-to-end
security mechanisms (this is important for networks such as
broadcast/radio, where eaves-dropping is easy).
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Encryption performed at the Transport Stream (encrypting the
payload of all TS-Packets with the same PID) encrypts/scrambles
all parts of the SNDU, including the Layer 2 MAC/NPA address.
Encryption at the section level in MPE may also optionally
encrypt the Layer 2 MAC/NPA address in addition to the PDU data
[ETSI-DAT]. In both cases, encryption of the MAC/NPA address,
requires a Receiver to decrypt all encrypted data, before it can
then filter the PDUs with the set of MAC/NPA addresses that it
wishes to receive. This method also has the drawback that all
Receivers must share a common encryption key. Encryption of the
MPE MAC address is therefore not permitted in some systems
(e.g. [ETSI-DVBRCS]).
Where it is possible for an attacker to inject traffic into the
subnetwork control plane, subnetwork security can additionally
protect the subnetwork assets. This threat must specifically be
considered for the protocols used for subnetwork control functions
(e.g. address resolution, management, configuration). Possible
threats include theft of service and denial of service; shared media
subnets tend to be especially vulnerable to such attacks [RFC3819].
Appropriate security functions must therefore be provided for ipdvb
control protocols [RFC3819], particularly when the control functions
are provided above the IP-layer using IP-based protocols. Internet
level security mechanisms (e.g.IPsec) can mitigate such threats.
In general, End-to-End security is recommended for users of any
communication path, especially when it includes a wireless/radio
or broadcast link, where a range of security techniques already
exist. Specification of security mechanisms at the application
layer, or within the MPEG-2 transmission network are the concerns of
organisations beyond the IETF. The complexity of any such security
mechanisms should be considered carefully so that it will not unduly
impact IP operations.
8.1 Link Encryption
Link level encryption of IP traffic is commonly used in
broadcast/radio links to supplement End-to-End security (e.g.
provided by TLS, SSH, Open PGP, S/MIME, IPsec). The encryption
and key exchange methods vary significantly, depending on the
intended application.For example, DVB-S/DVB-RCS operated by
Access Network Operators may wish to provide their customers
(Internet Service Providers, ISP) with security services. Common
security services are: terminal authentication and data
confidentiality (for unicast and multicast) between an encapsulation
gateway and Receivers. A common objective is to provide the same
level of privacy as terrestrial links. An ISP may also wish to
provide end-to-end security services to the end-users (based on the
well known mechanisms such as IPsec).
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Therefore it is important to understand that both security solutions
(Access Network Operators to ISP and ISP to end users) may co-exist.
MPE supports optional link encryption [ETSI-DAT]. A pair of bits
within the MPE protocol header indicate whether encryption
(scrambling) is used. For encrypted PDUs the header bits
indicate which of a pair of previously selected encryption keys
is to be used.
It is recommended that any new encapsulation defined by the
IETF allows Transport Stream encryption and also supports optional
link level encryption/authentication of the SNDU payload. In ULE
[ID-IPDVB-ULE], this may be provided in a flexible way using
Extension Headers. This requires definition of a mandatory
header extension, but has the advantage that it decouples
specification of the security functions from the encapsulation
functions. This method also supports encryption of the NPA/MAC
addresses.
9. IANA Considerations
A set of protocols which meet these requirements will require the
IANA to make assignments. This document in itself, however, does not
require any IANA involvement.
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10. Acknowledgments
The authors wish to thank Isabel Amonou, Torsten Jaekel, Pierre
Loyer, Luoma Juha-Pekka and and Rod Walsh for their detailed inputs.
We also wish to acknowledge the input provided by the members of
the IETF ipdvb WG.
11. References
11.1 Normative References
[ISO-MPEG] ISO/IEC DIS 13818-1:2000 "Information Technology; Generic
Coding of Moving Pictures and Associated Audio Information Systems",
International Standards Organisation (ISO).
[ETSI-DAT] EN 301 192 Specifications for Data Broadcasting, European
Telecommunications Standards Institute (ETSI).
11.2 Informative References
[ATSC] A/53C, "ATSC Digital Television Standard", Advanced
Television Systems Committee (ATSC), Doc. A/53C, 2004.
[ATSC-DAT] A/90, "ATSC Data Broadcast Standard", Advanced Television
Systems Committee (ATSC), Doc. A/090, 2000.
[ATSC-DATG] A/91, "Recommended Practice: Implementation Guidelines
for the ATSC Data Broadcast Standard", Advanced Television Systems
Committee (ATSC), Doc. A/91, 2001.
[ATSC-A92] A/92 "Delivery of IP Multicast Sessions over ATSC Data
Broadcast", Advanced Television Systems Committee (ATSC), Doc. A/92,
2002.
[ATSC-G] A/54A, "Guide to the use of the ATSC Digital Television
Standard", Advanced Television Systems Committee (ATSC), Doc. A/54A,
2003.
[ATSC-PSIP-TC] A/65B, "Program and System Information Protocol for
Terrestrial Broadcast and Cable", Advanced Television Systems
Committee (ATSC), Doc. A/65B, 2003.
[ATSC-S] A/80, "Modulation and Coding Requirements for Digital TV
(DTV) Applications over Satellite", Advanced Television Systems
Committee (ATSC), Doc. A/80, 1999.
[CLC99] Clausen, H., Linder, H., and Collini-Nocker, B., "Internet
over Broadcast Satellites", IEEE Commun. Mag. 1999, pp.146-151.
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[ETSI-DAT] EN 301 192, "Specifications for Data Broadcasting",
European Telecommunications Standards Institute (ETSI).
[ETSI-DVBC] EN 300 800, "Digital Video Broadcasting (DVB); DVB
interaction channel for Cable TV distribution systems (CATV)",
European Telecommunications Standards Institute (ETSI).
[ETSI-DVBRCS] EN 301 790, "Digital Video Broadcasting (DVB);
Interaction channel for satellite distribution systems", European
Telecommunications Standards Institute (ETSI).
[ETSI-DVBS] EN 301 421,"Digital Video Broadcasting (DVB);
Modulation and Coding for DBS satellite systems at 11/12 GHz,
European Telecommunications Standards Institute (ETSI).
[ETSI-DVBS2] DCB, "Second generation framing structure, channel
coding and modulation systems for Broadcasting, Interactive
Services,News Gathering and Other Broadband Satellite Applications",
European Telecommunications Standards Institute (ETSI).
[ETSI-DVBT] EN 300 744, "Digital Video Broadcasting (DVB); Framing
structure, channel coding and modulation for digital terrestrial
television (DVB-T)", European Telecommunications Standards
Institute (ETSI).
[ETSI-MHP] ETSI TS 101 812, "Digital Video Broadcasting (DVB);
Multimedia Home Platform (MHP) Specification", v1.2.1, European
Telecommunications Standards Institute (ETSI), June 2002.
[ETSI-IPDC] "IP Datacast Specification", DVB Interim Specification
CNMS 1026 v1.0.0,(Work in Progress), April 2004.
[ID-IPDVB-ULE] Fairhurst, G., B. Collini-Nocker, "Ultra Lightweight
Encapsulation for transmission of IP datagrams over MPEG-2/DVB
networks", Internet Draft, draft-ipdvb-ule-01.txt, Work in Progress,
IPDVB WG.
[ID-IPDVB-AR] Fairhurst, G., M-J. Montpetit, "Address Resolution for
IP datagrams over MPEG-2 networks", Internet Draft,
draft-fair-ipdvb-ar-01.txt, Work in Progress, IP-DVB WG.
[ID-MMUSIC-IMG] Y. Nomura, R. Walsh, J-P. Luoma, J. Ott, H.
Schulzrinne, "Protocol Requirements for Internet Media Guides",
Internet Draft, draft-ietf-mmusic-img-req.txt, Work in Progress,
MMUSIC WG.
[ISO-AUD] ISO/IEC 13818-3:1995 "Information technology; Generic
coding of moving pictures and associated audio information; Part
3: Audio", International Standards Organisation (ISO).
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[ISO-DSMCC] ISO/IEC IS 13818-6 "Information technology; Generic
coding of moving pictures and associated audio information; Part
6: Extensions for DSM-CC", International Standards Organisation
(ISO).
[ISO-VID] ISO/IEC DIS 13818-2:1998 "Information technology;
Generic coding of moving pictures and associated audio information;
Video", International Standards Organisation (ISO).
[Ken87] Kent C.A., and J. C. Mogul, "Fragmentation Considered
Harmful", Proc. ACM SIGCOMM, USA, CCR Vol.17, No.5, 1988, pp.390-
401.
[OPEN-CABLE] "Open Cable Application Platform Specification;
OCAP 2.0 Profile", OC-SP-OCAP2.0-I01-020419, Cable Labs, April
2002.
[RFC793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC1112] Deering, S., "Host extensions for IP multicasting",
STD 5, RFC 1112, August 1989.
[RFC1122] B. Braden, ed., "Requirements for Internet Hosts -
Communication Layers", RFC 1122, October 1989.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[RFC2365] Meyer, D., "Administratively Scoped IP Multicast",
BCP 23, RFC 2365, July 1998.
[RFC2375] Hinden, R. and S. Deering, "IPv6 Multicast Address
Assignments", RFC 2375, July 1998.
[RFC2710] Deering, S., Fenner, W., and B. Haberman, "Multicast
Listener Discovery (MLD) for IPv6", RFC 2710, October 1999.
[RFC2507] Degermark, M., Nordgren, B., and S. Pink, "IP Header
Compression", RFC 2507, February 1999.
[RFC3077] Duros, E., Dabbous, W., Izumiyama, H., Fujii, N.,
and Y. Zhang, "A Link-Layer Tunneling Mechanism for
Unidirectional Links", RFC 3077, March 2001.
[RFC3095] Bormann, C., Burmeister, C., Degermark, M., Fukushima,
H., Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le, K.,
Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K., Wiebke, T.,
Yoshimura, T., and H. Zheng, "RObust Header Compression (ROHC):
Framework and four profiles: RTP, UDP, ESP, and uncompressed",
RFC 3095, July 2001.
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[RFC3171] Albanna, Z., Almeroth, K., Meyer, D., and M. Schipper,
"IANA Guidelines for IPv4 Multicast Address Assignments", BCP 51,
RFC 3171, August 2001.
[RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B.,
and A. Thyagarajan, "Internet Group Management Protocol,
Version 3", RFC 3376, October 2002.
RFC3569] Bhattacharyya, S., "An Overview of Source-Specific
Multicast (SSM)", RFC 3569, July 2003.
[RFC3810] Vida, R. and L. Costa, "Multicast Listener Discovery
Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.
[RFC3819] Phil Karn, C. Borman, G. Fairhurst, D. Grossman, R.
Ludwig,J. Mahdavi, G. Montenegro, J. Touch, L. Wood, "Advice for
Internet Subnetwork Designers", RFC 3819, BCP 89.
12. Authors' Addresses
Marie J. Montpetit
MJMontpetit.com
Email: marie@mjmontpetit.com
Godred Fairhurst
Department of Engineering
University of Aberdeen
Aberdeen, AB24 3UE
UK
Email: gorry@erg.abdn.ac.uk
Web: http://www.erg.abdn.ac.uk/users/gorry
Horst D. Clausen
TIC Systems
Lawrence, Kansas
Email: h.d.clausen@ieee.org
Bernhard Collini-Nocker
Department of Scientific Computing
University of Salzburg
Jakob Haringer Str. 2
5020 Salzburg
Austria
Email: bnocker@cosy.sbg.ac.at
Web: http://www.network-research.org
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Hilmar Linder
Department of Scientific Computing
University of Salzburg
Jakob Haringer Str. 2
5020 Salzburg
Austria
Email: hlinder@cosy.sbg.ac.at
Web: http://www.network-research.org
13. IPR Notices
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed
to pertain to the implementation or use of the technology described
in this document or the extent to which any license under such
rights might or might not be available; nor does it represent that
it has made any independent effort to identify any such rights.
Information on the procedures with respect to rights in RFC
documents can be found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use
of such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository
at http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
14. Copyright Statement
Copyright (C) The Internet Society (2004). This document is
subject to the rights, licenses and restrictions contained in
BCP 78, and except as set forth therein, the authors retain all
their rights.
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Appendix A: MPEG-2 Encapsulation Mechanisms
To transmit packet data over an MPEG-2 transmission network requires
that individual PDUs (e.g. IPv4, IPv6 packets, or bridged Ethernet
Frames) are encapsulated using a convergence protocol. The following
encapsulations are currently standardised for MPEG-2 transmission
networks:
(i) Multi-Protocol Encapsulation (MPE).
The Multi-Protocol Encapsulation, MPE, specification of DVB
[ETSI-DAT] uses private Sections for the transport of IP
packets and uses encapsulation that is similar to the IEEE
LAN/MAN standards [LLC]. Data packets are encapsulated in
datagram sections that are compliant with the DSMCC section
format for private data. Some Receivers may exploit section
processing hardware to perform a first-level filter of the
packets that arrive at the Receiver.
This encapsulation makes use of a MAC-level Network Point of
Attachment address. The address format conforms to the
ISO/IEEE standards for LAN/MAN [LLC]. The 48-bit MAC address
field contains the MAC address of the destination; it is
distributed over six 8-bit fields, labelled MAC_address_1 to
MAC_address_6. The MAC_address_1 field contains the most
significant byte of the MAC address, while MAC_address_6
contains the least significant byte. How many of these
bytes are significant is optional and defined by the value
of the broadcast descriptor table [SI-DAT] sent separately
over another MPEG-2 TS within the TS multiplex.
MPE is currently a widely deployed scheme. Due to
Investments in existing systems, usage is likely to continue
in current and future MPEG-2 Transmission Networks. ATSC
provides a scheme similar to MPE [ATSC-DAT] with some small
differences.
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(ii) Data Piping.
The Data Piping profile [ETSI-DAT] is a minimum overhead,
simple and flexible profile that makes no assumptions
concerning the format of the data being sent. In this
profile, the Receiver is intended to provide PID filtering,
packet reassembly according to [DVB-SIDAT-368], error
detection and optional Conditional Access (link encryption).
The specification allows the user data stream to be
unstructured or organized into packets. The specific
structure is transparent to the Receiver. It may conform to
any protocol, e.g., IP, Ethernet, NFS, FDDI, MPEG-2 PES,
etc.
(iii) Data Streaming.
The data broadcast specification profile [ETSI-DAT] for PES
tunnels (Data Streaming) supports unicast and multicast data
services that require a stream-oriented delivery of data
packets. This encapsulation maps an IP packet into a single
PES Packet payload.
Two different types of PES headers can are selected via the
stream_id values [ISO-MPEG]. The private_stream_2 value
permits the use of the short PES header with limited
overhead, while the private_stream_1 value makes available
the scrambling control and the timing and clock reference
features of the PES layer.
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