Internet Engineering Task Force Gorry Fairhurst
Internet Draft University of Aberdeen
Document: draft-ietf-ipdvb-ule-06.txt Bernhard Collini-Nocker
University of Salzburg
ipdvb WG
Category: Draft, Intended Standards Track June 2005
Unidirectional Lightweight Encapsulation (ULE) for transmission of
IP datagrams over an MPEG-2 Transport Stream
Status of this Draft
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Abstract
The MPEG-2 Transport Stream (TS) has been widely accepted not only
for providing digital TV services, but also as a subnetwork
technology for building IP networks.
This document describes a Unidirectional Lightweight Encapsulation
(ULE) mechanism for the transport of IPv4 and IPv6 Datagrams and
other network protocol packets directly over the ISO MPEG-2
Transport Stream as TS Private Data. ULE specifies a base
encapsulation format and supports an extension format that allows it
to carry additional header information to assist in network/Receiver
processing.
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Table of Contents
1. Introduction
2. Conventions used in this document
3. Description of method
4. SNDU Format
4.1 Destination Address Absent (D) Field
4.2 Length Field
4.3 End Indicator
4.4 Type Field
4.4.1 Type 1: Next-Header Type Fields
4.4.2 Type 2: EtherType Compatible Type Fields
4.5 SNDU Destination Address Field
4.6 SNDU Trailer CRC
4.7 Description of SNDU Formats
4.7.1 End Indicator
4.7.2 IPv4 SNDU Encapsulation
4.7.3 IPv6 SNDU Encapsulation
5. Extension Headers
5.1 Test SNDU
5.2 Bridged Frame SNDU Encapsulation
5.3 Extension-Padding Optional Extension Header
6.Processing at the Encapsulator
6.1 SNDU Encapsulation
6.2 Procedure for Padding and Packing
7. Receiver Processing
7.1 Idle State
7.1.1 Idle State Payload Pointer Checking
7.2 Processing of a Received SNDU
7.2.1 Reassembly Payload Pointer Checking
7.3 Other Error Conditions
8. Summary
9. Acknowledgments
10. Security Considerations
11. References
11.1 Normative References
11.2 Informative References
12. Authors' Addresses
13. IPR Notices
13.1 Intellectual Property Statement
13.2 Disclaimer of Validity
14. Copyright Statement
14.1 Intellectual Property Statement
14.2 Disclaimer of Validity
15. IANA Considerations
15.1 IANA Guidelines
ANNEX A: Informative Appendix - SNDU Packing Examples
ANNEX B: Informative Appendix - SNDU Encapsulation
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1. Introduction
This document describes an encapsulation for the transport of IP
datagrams, or other network layer packets, over ISO MPEG-2 Transport
Streams [ISO-MPEG2; RFCXARCHX]. The encapsulation satisfies the
requirement for a lightweight encapsulation defined in section 4 of
[RFCXARCHX]. The basic header provides the required set of protocol
fields. Extension headers may also be defined. This header structure
is significantly simpler to parse and process [SOOR05] than current
alternative methods (e.g. MPE [ETSI-DAT] that builds upon the DSM-CC
Table Section syntax [ISO-DSMCC]).
The encapsulation is suited to 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 provide
unidirectional (simplex) physical and link layer standards. Support
has been defined for a wide range of physical media (e.g.
Terrestrial TV [ETSI-DVBT; ATSC-PSIP-TC], Satellite TV [ETSI-DVBS;
ATSC-S], Cable Transmission [ETSI-DVBC; ATSC-PSIP-TC]).
Bi-directional (duplex) links may also be established using these
standards (e.g., DVB defines a range of return channel technologies,
including the use of two-way satellite links [ETSI-RCS] and dial-up
modem links [RFC3077]).
Protocol Data Units, PDUs, (Ethernet Frames, IP datagrams or other
network layer packets) for transmission over an MPEG-2 Transport
Multiplex are passed to an Encapsulator. This formats each PDU into
a SubNetwork Data Unit (SNDU) by adding an encapsulation header and
an integrity check trailer. The SNDU is fragmented into a series of
one or more MPEG-2 Transport Stream (TS) Packets that are sent over
a single TS Logical Channel.
The MPEG-2 specification [ISO-MPEG2] requires conformant TS
Multiplexes to provide Program Specific Information (PSI) for
each stream in the TS Multiplex. Other MPEG-2 based transmission
standards may also define Service Information (SI).
This information may allow Receivers and Re-multiplexors
[RFCXARCHX] to locate a specific ULE Stream (i.e., the PID value of
the TS Logical Channel that carries a ULE Stream). The conditions
under which this information is required, and the format in which it
is to be provided is beyond the scope of this document. Addressing
and mapping issues for ULE over MPEG-2 are also described in
[ID-ipdvb-ar].
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2. Conventions used in this document
The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
Other terms used in this document are defined below:
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-MPEG2].
AFC: Adaptation Field Control [ISO-MPEG2]. A pair of bits carried in
the TS Packet header that signal the presence of the Adaptation
Field and/or TS Packet payload.
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.
b: bit. For example, one byte consists of 8b.
B: Byte. Groups of bytes are represented in Internet byte order.
DSM-CC: Digital Storage Media Command and Control [ISO-DSMCC]. A
format for transmission of data and control information in an MPEG-2
Private Section, defined by the ISO MPEG-2 standard.
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.
End Indicator: A value that indicates to the Receiver that there are
no further SNDUs present within the current TS Packet.
LLC: Logical Link Control [ISO-8802-2, IEEE-802.2]]. A link layer
protocol defined by the IEEE 802 standard, which follows the
Ethernet MAC Header.
MAC: Medium Access Control [IEEE-802.3]. A link layer protocol
defined by the IEEE 802.3 standard (or by Ethernet v2 [DIX]).
MAC Header: The link layer header of the IEEE 802.3 standard
[IEEE-802.3] or Ethernet v2 [DIX]. It consists of a 6B destination
address, 6B source address, and 2B type field (see also NPA, LLC).
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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/IEC 13818-1) [ISO-MPEG2], and ITU-T (in H.222
[ITU-H222]).
Next-Header: A Type value indicating an Extension Header.
NPA: Network Point of Attachment. In this document, refers to a 6
byte destination address (resembling an IEEE MAC address) within the
MPEG-2 transmission network that is used to identify individual
Receivers or groups of Receivers.
Packing Threshold: A period of time an Encapsulator is willing to
defer transmission of a partially filled TS-Packet to accumulate
more SNDUs, rather than use Padding. After the Packet Threshold
period, the Encapsulator uses Padding to send the partially filled
TS-Packet.
"Padding: A method that fills the remaining unused bytes in a TS
Packet payload using the specific pattern of 0xFF."
Payload Unit, PU. A sequence of bytes sent using a TS. Examples of
Payload Units include: an MPEG-2 Table Section or a ULE SNDU.
PDU: Protocol Data Unit. Examples of a PDU include Ethernet frames,
IPv4 or IPv6 datagrams, and other network packets.
PES: Packetized Elementary Steam [ISO-MPEG2]. A format of MPEG-2 TS
packet payload usually used for video or audio information.
PID: Packet Identifier [ISO-MPEG2]. 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-MPEG2]. The TS Packets
forming the parts of a Table Section, PES, or other Payload Unit
must all carry the same PID value. The all zeros PID 0x0000 as well
as other PID values are reserved for specific PSI/SI Tables [ISO-
MPEG2]. The all ones PID value 0x1FFF 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.
PP: Payload Pointer [ISO-MPEG2]. An optional one byte pointer that
directly follows the 4 byte TS Packet header. It contains the number
of bytes that follow the Payload Pointer, up to the start of the
first Payload Unit (counted from the first byte of the TS Packet
payload field, and excluding the PP field itself). 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, Table
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Sections, and ULE. 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-MPEG2]. The structure may be used to
identify private information (i.e. not defined by [ISO-MPEG2])
relating to one or more elementary streams, or a specific MPEG-2
program, or the entire Transport Stream. 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-MPEG2]. Tables used to convey
information about the service carried in a TS Multiplex. The
information is carried in one of four specifically identified Table
Sections defined by MPEG-2 [ISO-MPEG2]. See also SI Table.
PU: Payload Unit.
PUSI: Payload_Unit_Start_Indicator [ISO-MPEG2]. 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: 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-MPEG2]. In this document,
this term describes a table that is defined by another standards
body to convey information about the services carried in a TS
Multiplex. 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-MPEG2].
SNDU: Subnetwork Data Unit. An encapsulated PDU sent as an MPEG-2
Payload Unit.
Table Section: A Payload Unit carrying all or a part of an SI or PSI
Table [ISO-MPEG2].
TS: Transport Stream [ISO-MPEG2], a method of transmission at the
MPEG-2 level using TS Packets; it represents layer 2 of the ISO/OSI
reference model. See also TS Logical Channel and TS Multiplex.
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TS Header: The 4 byte header of a TS Packet [ISO-MPEG2]. Each 188B
TS Packet incorporates a 4B header with the following fields (those
referenced within this document are marked with *):
Field Length Name/Purpose
(in bits)
8b Synchronisation pattern equal 0x47
*1b Transport Error Indicator
*1b Payload Unit Start Indicator (PUSI)
1b Transport Priority
*13b Packet IDentifier (PID)
2b Transport scrambling control
*2b Adaptation Field Control (AFC)
*4b Continuity Counter (CC)
TS Logical Channel: Transport Stream Logical Channel. In this
document, this term identifies a channel at the MPEG-2 level
[ISO-MPEG2]. 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). The term
"Stream" is defined in MPEG-2 [ISO-MPEG2]. This describes the
content carried by a specific TS Logical Channel (see, ULE Stream).
Some PID values are reserved (by MPEG-2) for specific signalling.
Other standards (e.g., ATSC, DVB) also reserve specific PID values.
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)
[RFCXARCHX], 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-MPEG2]. 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.
ULE Stream: An MPEG-2 TS Logical Channel that carries only ULE
encapsulated PDUs. ULE Streams may be identified by definition of a
stream_type in SI/PSI [ISO-MPEG2].
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3. Description of the Method
PDUs (IP packets, Ethernet frames or packets from other network
protocols) are encapsulated to form a Subnetwork Data Unit (SNDU).
The SNDU is transmitted over an MPEG-2 transmission network by
placing it either in the payload of a single TS Packet, or if
required, an SNDU may be fragmented into a series of TS Packets.
Where there is sufficient space, the method permits a single TS
Packet to carry more than one SNDU (or part there of), sometimes
known as Packing. All TS Packets comprising an SNDU MUST be assigned
the same PID, and therefore form a part of the same TS Logical
Channel.
The ULE encapsulation is limited to TS private streams only. The
header of each TS Packet carries a one bit Payload Unit Start
Indicator (PUSI) field. A PUSI field with a value of 1 indicates the
start of at least one Payload Unit (SNDU) within the TS Packet
payload. The semantics of the PUSI bit are defined for PES and PSI
packets [ISO-MPEG2]; for private data, its use is not defined in the
MPEG-2 Standard. In ULE, although being private data, the operation
follows that of PSI packets. Hence, the following PUSI values are
defined:
0: The TS Packet does NOT contain the start of an SNDU, but
contains the continuation, or end of an SNDU;
1: The TS Packet contains the start of an SNDU, and a one byte
Payload Pointer follows the last byte of the TS Packet header.
If a Payload Unit (SNDU) finishes before the end of a TS Packet
payload, but it is not intended to start another Payload Unit, a
stuffing procedure fills the remainder of the TS Packet payload with
bytes with a value 0xFF [ISO-MPEG2], known as Padding.
A Receiver processing MPEG-2 Table Sections that receives a value of
0xFF in place of the table_id field, interprets this as
Padding/Stuffing and silently discards the remainder of the TS
Packet payload. The payload of the next TS Packet for the same TS
Logical Channel will begin with a Payload Pointer of value 0x00,
indicating that the next Payload Unit immediately follows the TS
Packet header. The ULE protocol resembles this, but differs in the
exact procedure (see the following sections).
The TS Packet Header also carries a two bit Adaptation Field Control
(AFC) value. This adaptation field may extend the TS Packet Header
to carry timing and synchronisation information and may also be used
to include stuffing bytes before a TS Packet payload. Adaptation
Field stuffing is NOT used in this encapsulation method, and TS
Packets from a ULE Encapsulator MUST be sent with an AFC value of
'01'. For TS Logical Channels supporting ULE, Receivers MUST discard
TS Packets that carry other AFC values.
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4. SNDU Format
PDUs are encapsulated using ULE to form an SNDU. (Each SNDU is an
MPEG-2 Payload Unit.) The encapsulation format to be used for PDUs
is illustrated below:
< ----------------------------- SNDU ----------------------------- >
+-+-------------------------------------------------------+--------+
|D| Length | Type | Dest Address* | PDU | CRC-32 |
+-+-------------------------------------------------------+--------+
Figure 1: SNDU Encapsulation (* optional Destination Address)
All multi-byte values in ULE (including the Length/End Indicator
(4.2,4.3), Type (4.4), Destination Address (4.5), and Extension
Headers (5)) are transmitted in network byte order (most significant
byte first). The most significant bit of each byte is placed in the
left-most position of the 8-bit field. Appendix A provides
informative examples of usage.
4.1 Destination Address Absent (D) Field
The most significant bit of the Length Field carries the value of
the Destination Address Absent Field, the D-bit. A value of 0
indicates the presence of the Destination Address Field (see section
4.5). A value of 1 indicates that a Destination Address Field is not
present.
An End Indicator (4.3) MUST be sent with a D-bit value of 1. Other
SNDUs SHOULD be sent with a D-bit value of 0 (see 4.5).
4.2 Length Field
A 15-bit value that indicates the length, in bytes, of the SNDU
counted from the byte following the Type field, up to and including
the CRC. Note the special case described in 4.3.
4.3 End Indicator
When the first two bytes following an SNDU have the value 0xFFFF,
this denotes an End Indicator (i.e., all ones length combined with a
D-bit value of 1). This indicates to the Receiver that there are no
further SNDUs present within the current TS Packet (see section 6),
and that no Destination Address Field is present. The value 0xFF has
specific semantics in MPEG-2 framing, where it is used to indicate
the presence of Padding. This use resembles [ISO-DSMCC].
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4.4 Type Field
The 16-bit Type field indicates the type of payload carried in an
SNDU, or the presence of a Next-Header. The set of values that may
be assigned to this field is divided into two parts, similar to the
allocations for Ethernet.
EtherTypes were originally specified by Xerox under the Ethernet v2
Specification [DIX]. After specification of IEEE 802.3 [IEEE-802.3;
ISO-8802-2], the set of EtherTypes less than 1536 (0x0600), assumed
the role of a length indicator. Ethernet receivers use this feature
to discriminate LLC format frames. Hence any IEEE EtherType < 1536
indicates an LLC frame, and the actual value indicates the length of
the LLC frame.
There is a potential ambiguous case when a Receiver receives a PDU
with two length fields: The Receiver would need to validate the
actual length and the Length field and ensure that inconsistent
values are not propagated by the network. Specification of two
independent length fields is therefore undesirable. In the ULE
header, this is avoided in the SNDU header by including only one
length value, but bridging of LLC frames re-introduces this
consideration (section 5.2).
The Ethernet LLC mode of identification is not required in ULE,
since the SNDU format always carries an explicit Length Field, and
therefore the procedure in ULE is modified, as below:
The first set of ULE Type field values comprise the set of values
less than 1536 in decimal. These Type field values are IANA
assigned (see 4.4.1), and indicate the Next-Header.
The second set of ULE Type field values comprise the set of values
greater than or equal to 1536 in decimal. In ULE, this value is
identical to the corresponding type codes specified by the IEEE/DIX
type assignments for Ethernet and recorded in the IANA EtherType
registry.
4.4.1 Type 1: Next-Header Type Fields
The first part of the Type space corresponds to the values 0 to 1535
Decimal. These values may be used to identify link-specific
protocols and/or to indicate the presence of Extension Headers that
carry additional optional protocol fields (e.g. a bridging
encapsulation). Use of these values is co-ordinated by an IANA
registry. The following types are defined in this document:
0x0000: Test SNDU (see 5.1)
0x0001: Bridged Frame (see 5.2)
0x0100: Extension-Padding (see 5.3)
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The remaining values within the first part of the Type space are
reserved for Next-Header values allocated by the IANA.
4.4.2 Type 2: EtherType Compatible Type Fields
The second part of the Type space corresponds to the values between
0x600 (1536 decimal) and 0xFFFF. This set of type assignments
follow DIX/IEEE assignments (but exclude use of this field as a
frame length indicator). All assignments in this space MUST use the
values defined for IANA EtherType, the following two Type values are
used as examples (taken from the IANA EtherTypes registry):
0x0800: IPv4 Payload (see 4.7.2)
0x86DD: IPv6 Payload (see 4.7.3)
4.5 SNDU Destination Address Field
The SNDU Destination Address Field is optional (see 4.1). This field
MUST be carried (i.e. D=0) for IP unicast packets destined to
routers that are sent using shared links (i.e., where the same link
connects multiple Receivers). A sender MAY omit this field (D=1) for
an IP unicast packet and/or multicast packets delivered to Receivers
that are able to utilise a discriminator field (e.g. the IPv4/IPv6
destination address, or a bridged MAC destination address), which in
combination with the PID value, could be interpreted as a Link-Level
address.
When the SNDU header indicates the presence of an SNDU Destination
Address field (i.e. D=0), a Network Point of Attachment, NPA, field
directly follows the fourth byte of the SNDU header. NPA destination
addresses are 6 Byte numbers, normally expressed in hexadecimal,
used to identify the Receiver(s) in a MPEG-2 transmission network
that should process a received SNDU. The value 0x00:00:00:00:00:00,
MUST NOT be used as a destination address in an SNDU. The least
significant bit of the first byte of the address is set to 1 for
multicast frames, and the remaining bytes specify the link layer
multicast address. The specific value 0xFF:FF:FF:FF:FF:FF is the
link broadcast address, indicating this SNDU is to be delivered to
all Receivers.
IPv4 packets carrying an IPv4 subnetwork broadcast address need to
be delivered to all systems with the same network prefix. When a
SNDU Destination Address is present (D=0) the value MUST be set to
the NPA link broadcast address (0xFF:FF:FF:FF:FF:FF).
When the PDU is an IP multicast packet and an SNDU Destination
Address is present (D=0), the IP group destination address of the
multicast packet MUST be mapped to the multicast SNDU Destination
Address (following the method used to generate a destination MAC
address in Ethernet). The method for mapping IPv4 multicast
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addresses is specified in [RFC1112]. The method for mapping IPv6
multicast addresses is specified in [RFC2464].
4.6 SNDU Trailer CRC
Each SNDU MUST carry a 32-bit CRC field in the last four bytes of
the SNDU. This position eases CRC computation by hardware. The CRC-
32 polynomial is to be used. Examples where this polynomial is also
employed include Ethernet, DSM-CC section syntax [ISO-DSMCC] and
AAL5 [ITU-3563]. This is a 32 bit value calculated according to the
generator polynomial represented 0x104C11DB7 in hexadecimal:
x^32+x^26+x^23+x^22+x^16+x^12+x^11+x^10+x^8+x^7+x^5+x^4+x^2+x^1+x^0.
The Encapsulator initialises the CRC-32 accumulator register to the
value 0xFFFF FFFF. It then accumulates a transmit value for the
CRC32 that includes all bytes from the start of the SNDU header to
the end of the SNDU (excluding the 32-bit trailer holding the CRC-
32), and places this in the CRC Field. In ULE, the bytes are
processed in order of increasing position within the SNDU, the order
of processing bits is NOT reversed. This use resembles, but is
different to that in SCTP [RFC3309].
The Receiver performs an integrity check by independently
calculating the same CRC value and comparing this with the
transmitted value in the SNDU trailer. SNDUs that do not have a
valid CRC, are discarded, causing the Receiver to enter the Idle
State.
This description may be suited for hardware implementation, but this
document does not imply any specific implementation. Software-based
table-lookup or hardware-assisted software-based implementations are
also possible. Annexe B provides an example of an Encapsulated PDU
that includes the computed CRC-32 value.
The primary purpose of this CRC is to protect the SNDU (header, and
payload) from undetected reassembly errors and errors introduced by
unexpected software / hardware operation while the SNDU is in
transit across the MPEG-2 subnetwork and during processing at the
encapsulation gateway and/or the Receiver. It may also detect the
presence of uncorrected errors from the physical link (however,
these may also be detected by other means, e.g. section 7.3).
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4.7 Description of SNDU Formats
The format of an SNDU is determined by the combination of the
Destination Address Absent bit (D) and the SNDU Type Field. The
simplest encapsulation places a PDU directly into an SNDU payload.
Some Type 1 encapsulations may require additional header fields.
These are inserted in the SNDU following the NPA destination address
and directly preceding the PDU.
The following SNDU Formats are defined here:
End Indicator: The Receiver should enter the Idle State (4.7.1).
IPv4 SNDU: The payload is a complete IPv4 datagram (4.7.2)
IPv6 SNDU: The payload is a complete IPv6 datagram (4.7.3).
Test SNDU: The payload will be discarded by the Receiver (5.1).
Bridged SNDU: The payload carries a bridged MAC frame (5.2).
Other formats may be defined through relevant assignments in the
IEEE and IANA registries.
4.7.1 End Indicator
The format of the End Indicator is shown in figure 2. This format
MUST carry a D-bit value of 1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| 0x7FFF | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
= A sequence of zero or more bytes with a value 0xFF filling =
| the remainder of the TS Packet Payload |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Format for a ULE End Indicator.
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4.7.2 IPv4 SNDU
IPv4 datagrams are directly transported using one of the two
standard SNDU structures, in which the PDU is placed directly in the
SNDU payload. The two encapsulations are shown in figures 3 and 4.
(Note that in this, and the following figures, the IP datagram
payload is of variable size, and is directly followed by the CRC-
32).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| Length (15b) | Type = 0x0800 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receiver Destination NPA Address (6B) |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
= IPv4 datagram =
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (CRC-32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: SNDU Format for an IPv4 Datagram using L2 filtering (D=0).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| Length (15b) | Type = 0x0800 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
= IPv4 datagram =
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (CRC-32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: SNDU Format for an IPv4 Datagram using L3 filtering (D=1).
4.7.3 IPv6 SNDU Encapsulation
IPv6 datagrams are directly transported using one of the two
standard SNDU structures, in which the PDU is placed directly in the
SNDU payload. The two encapsulations are shown in figures 5 and 6.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| Length (15b) | Type = 0x86DD |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receiver Destination NPA Address (6B) |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
= IPv6 datagram =
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (CRC-32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: SNDU Format for an IPv6 Datagram using L2 filtering (D=0).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| Length (15b) | Type = 0x86DD |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
= IPv6 datagram =
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (CRC-32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: SNDU Format for an IPv6 Datagram using L3 filtering (D=1)
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5. Extension Headers
This section describes an extension format for the ULE
encapsulation. In ULE, a Type field value less than 1536 Decimal
indicates an Extension Header. These values are assigned from a
separate IANA registry defined for ULE.
The use of a single Type/Next-Header field simplifies processing and
eliminates the need to maintain multiple IANA registries. The cost
is that each Extension Header requires at least 2 bytes. This is
justified, on the basis of simplified processing and maintaining a
simple lightweight header for the common case when no extensions are
present.
A ULE Extension Header is identified by a 16-bit value in the Type
field. This field is organised as a 5-bit zero prefix, a 3-bit H-LEN
field and an 8-bit H-Type field, as follows:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0|H-LEN| H-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Structure of ULE Next-Header Field.
The H-LEN Assignment is described below:
0 Indicates a Mandatory Extension Header
1 Indicates an Optional Extension Header of length 2B
2 Indicates an Optional Extension Header of length 4B
3 Indicates an Optional Extension Header of length 6B
4 Indicates an Optional Extension Header of length 8B
5 Indicates an Optional Extension Header of length 10B
>=6 the combined H-LEN and H-TYPE values indicate the EtherType
of a PDU that directly follows this Type field.
The H-LEN value indicates the total number of bytes in an Optional
Extension Header (including the 2B Type field).
An H-LEN value of zero indicates a Mandatory Extension Header. Each
Mandatory Extension Header has a pre-defined length that is not
communicated in the H-LEN field. No additional limit is placed on
the maximum length of a Mandatory Extension Header. A Mandatory
Extension Header MAY modify the format or encoding of the enclosed
PDU (e.g. to perform encryption and/or compression).
The H-Type is a one byte field that is either one of 256 Mandatory
Header Extensions or one of 256 Optional Header Extensions. The set
of currently permitted values for both types of Extension Headers
are defined by an IANA Registry (section 15). Registry values for
Optional Extensions are specified in the form H=1 (i.e. a decimal
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number in the range 256-511), but may be used with an H-Length value
in the range 1-5 (see example in 5.3).
Two examples of Extension Headers are the Test SNDU and the use of
Extension-Padding. The Test SNDU Mandatory Extension Header results
in the entire PDU being discarded. The Extension-Padding Optional
Extension Header results in the following (if any) option header
being ignored (i.e. a total of H-LEN 16-bit words).
The general format for an SNDU with Extension Headers is:
< -------------------------- SNDU ------------------------- >
+---+--------------------------------------------------+--------+
|D=0| Length | T1 | NPA Address | H1 | T2 | PDU | CRC-32 |
+---+--------------------------------------------------+--------+
< ULE base header > < ext 1 >
Figure 8: SNDU Encapsulation with one Extension Header (for D=0).
Where:
D is the ULE D_bit (in this example D=0, however NPA addresses may
also be omitted when using Extension Headers).
T1 is the base header Type field. In this case, specifying a
Next-Header value.
H1 is a set of fields defined for header type T1. There may be 0
or more bytes of information for a specific ULE Extension Header.
T2 is the Type field of the next header, or an EtherType > 1535 B
indicating the type of the PDU being carried.
< -------------------------- SNDU ------------------------- >
+---+---------------------------------------------------+--------+
|D=1| Length | T1 | H1 | T2 | H2 | T3 | PDU | CRC-32 |
+---+---------------------------------------------------+--------+
< ULE base header >< ext 1 >< ext 2 >
Figure 9: SNDU Encapsulation with two Extension Headers (D=1).
Using this method, several Extension Headers MAY be chained in
series. Figure 12 shows an SNDU including two Extension Headers. In
the example, the values of T1 and T2 are both less than 1536
Decimal. Each indicates the presence of an Extension Header, rather
than a directly following PDU. T3 has a value > 1535 indicating the
EtherType of the PDU being carried. Although an SNDU may contain an
arbitrary number of consecutive Extension Headers, it is not
expected that SNDUs will generally carry a large number of
extensions.
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5.1 Test SNDU
A Test SNDU (figure 10) is a Mandatory Extension Header of Type 1.
This header must be the final (or only) extension header specified
in the header chain of a SNDU. The structure of the Data portion of
this SNDU is not defined by this document. All Receivers MAY record
reception in a log file, but MUST then discard any Test SNDUs. The
D-bit MAY be set in a TEST SNDU.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|D| Length (15b) | Type = 0x0000 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
= Data (not forwarded by a Receiver) =
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (CRC-32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: SNDU Format for a Test SNDU
5.2 Bridged Frame SNDU Encapsulation
A bridged SNDU is a Mandatory Extension Header of Type 1. It MUST be
the final (or only) extension header specified in the header chain
of a SNDU. The payload includes MAC address and EtherType [DIX] or
LLC Length [ISO-8802-2] fields together with the contents of a
bridged MAC frame. The SNDU has the format shown in figures 11 and
12.
When an NPA address is specified (D=0), Receivers MUST discard all
SNDUs that carry an NPA destination address that does NOT match
their own NPA address (or a broadcast/multicast address), the
payload of the remaining SNDUs are processed by the bridging rules
that follow. An SNDU without an NPA address (D=1) results in a
Receiver performing bridging processing on the payload of all
received SNDUs.
A Gateway MAY also use this encapsulation format to directly
communicate network protocol packets that require the LLC
encapsulation [IEEE-802.2; ISO-8802-2]. To do this, it constructs an
SNDU with a Bridge Extension Header containing the intended
destination MAC address, the MAC source address of the Gateway, and
the LLC-Length. The PDU comprises an LLC header followed by the
required payload. The Gateway MAY choose to suppress the NPA address
(see 4.5).
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| Length (15b) | Type = 0x0001 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receiver Destination NPA Address (6B) |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| MAC Destination Address (6B) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC Source Address (6B) |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | EtherType/LLC-Length (2B) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
= (Contents of bridged MAC frame) =
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (CRC-32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: SNDU Format for a Bridged Payload (D=0)
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| Length (15b) | Type = 0x0001 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAC Destination Address (6B) |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| MAC Source Address (6B) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| EtherType/LLC-Length (2B) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
= (Contents of bridged MAC frame) =
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (CRC-32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: SNDU Format for a Bridged Payload (D=1)
The EtherType/LLC-Length field of a frame is defined according to
IEEE 802.3 [IEEE-802.2] (see section 5).
In this special case, the Mandatory Extension Header format may be
interpreted as either an EtherType [DIX] or an LLC Length field,
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specified by IEEE 802 [IEEE-802.3] rather a value assigned in the
ULE Next-Header Registry maintained by the IANA.
The MAC addresses in the frame being bridged SHOULD be assigned
according to the rules specified by the IEEE and denote unknown,
unicast, broadcast, and multicast link addresses. These MAC
addresses denote the intended recipient in the destination LAN, and
therefore have a different function to the NPA addresses carried in
the SNDU header.
A frame Type < 1536 for a bridged frame, introduces a LLC Length
field. The Receiver MUST check this length and discard any frame
with a length greater than permitted by the SNDU payload size.
In normal operation, it is expected that any padding appended to the
Ethernet frame SHOULD be removed prior to forwarding. This requires
the sender to be aware of such Ethernet padding
(e.g. [DIX; IEEE-802.3]).
Ethernet frames received at the Encapsulator for onward transmission
over ULE carry a Local Area Network Frame Check sequence, LAN FCS,
field (e.g. CRC-32 for Ethernet [DIX; IEEE-802.3]). The Encapsulator
MUST check the LAN-FCS value of all frames received, prior to
further processing. Frames received with an invalid LAN FCS MUST be
discarded. After checking, the LAN FCS is then removed (i.e., it is
NOT forwarded in the bridged SNDU). As in other ULE frames, the
Encapsulator appends a CRC-32 to the transmitted SNDU. At the
Receiver, an appropriate LAN-FCS field will be appended to the
bridged frame prior to onward transmission on the Ethernet
interface.
This design is readily implemented using existing network interface
cards, and does not introduce an efficiency cost by transmitting two
integrity check fields for bridged frames. However, it also
introduces the possibility that a frame corrupted within the
processing performed at an Encapsulator and/or Receiver may not be
detected by the final recipient(s) (i.e. such corruption would not
normally result in an invalid LAN FCS).
5.3 Extension-Padding Optional Extension Header
The Extension-Padding Optional Extension Header is specified by an
IANA assigned H-Type value of 0x100. As in other Optional
Extensions, the total length of the extension is indicated by the H-
LEN field (specified in 16-bit words). The extension field is formed
of a group of one to five 16-bit fields.
For this specific option, only the last 16-bit word has an assigned
value, the sender SHOULD set the remaining values to 0x0000. The
last 16-bit field forms the Next-Header Type field. A Receiver MUST
interpret the Type field, but MUST ignore any other fields of this
Extension Header.
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6. Processing at the Encapsulator
The Encapsulator forms the PDUs queued for transmission into SNDUs
by adding a header and trailer to each PDU (section 4). It then
segments the SNDU into a series of TS Packet payloads (figure 9).
These are transmitted using a single TS Logical Channel over a TS
Multiplex. The TS Multiplex may be processed by a number of MPEG-2
(re)multiplexors before it is finally delivered to a Receiver
[RFCXARCHX].
+------+--------------------------------+------+
| ULE | Protocol Data Unit | ULE |
|Header| |CRC-32|
+------+--------------------------------+------+
/ / \ \
/ / \ \
/ / \ \
+--------+---------+ +--------+---------+ +--------+---------+
|MPEG-2TS| MPEG-2 |...|MPEG-2TS| MPEG-2 |...|MPEG-2TS| MPEG-2 |
| Header | Payload | | Header | Payload | | Header | Payload |
+--------+---------+ +--------+---------+ +--------+---------+
Figure 13: Encapsulation of an SNDU into a series of TS Packets
6.1 SNDU Encapsulation
When an Encapsulator has not previously sent a TS Packet for a
specific TS Logical Channel, or after an Idle period, it starts to
send an SNDU in the first available TS Packet. This first TS Packet
generated MUST carry a PUSI value of 1. It MUST also carry a Payload
Pointer value of zero indicating that the SNDU starts immediately
after the Payload Pointer in the TS Packet payload.
The Encapsulation MUST ensure that all TS Packets set the MPEG-2
Continuity Counter carried in the TS Packet header, according to
[ISO-MPEG2]. This value MUST be incremented by one (modulo 16) for
each successive fragment/complete SNDU sent using a TS Logical
Channel.
An Encapsulator MAY decide not to immediately send another SNDU,
even if space is available in a partially filled TS Packet. This
procedure is known as Padding (figure 11). The End Indicator informs
the Receiver that there are no more SNDUs in this TS Packet payload.
The End Indicator is followed by zero or more unused bytes until the
end of the TS Packet payload. All unused bytes MUST be set to the
value of 0xFF, following current practice in MPEG-2 [ISO-DSMCC]. The
Padding procedure trades decreased efficiency against improved
latency.
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+-/------------+
| SubNetwork |
| DU 1 |
+-/------------+
\ \
\ \
\ \
+--------+--------+--------+----------+
|MPEG-2TS| End of | 0xFFFF | Unused |
| Header | SNDU 1 | | Bytes |
+--------+--------+--------+----------+
PUSI=0 ULE
End
Indicator
Figure 14: A TS Packet carrying the end of SNDU 1, followed by an
End Indicator.
Alternatively, when more packets are waiting at an Encapsulator, and
a TS Packet has sufficient space remaining in the payload, the
Encapsulator can follow a previously encapsulated SNDU with another
SNDU using the next available byte of the TS Packet payload (see
6.2). This is called Packing (figure 15).
+-/----------------+ +----------------/-+
| Subnetwork | | Subnetwork |
| DU 2 | | DU 3 |
+-/----------------+ +----------------/-+
\ \ / /\
\ \ / / \
\ \ / / \. . .
+--------+--------+--------+----------+
|MPEG-2TS| Payload| end of | start of |
| Header | Pointer| SNDU 2 | SNDU 3 |
+--------+--------+--------+----------+
PUSI=1 | ^
| |
+--------------+
Figure 15: A TS Packet with the end of SNDU 2, followed by SNDU 3.
6.2 Procedure for Padding and Packing
Five possible actions may occur when an Encapsulator has completed
encapsulation of an SNDU:
(i) If the TS Packet has no remaining space, the Encapsulator
transmits this TS Packet. It starts transmission of the next SNDU in
a new TS Packet. (The standard rules [ISO-MPEG2] require the header
of this new TS Packet to carry a PUSI value of 1, and a Payload
Pointer value of 0x00.)
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(ii) If the TS Packet carrying the final part of an SNDU has one
byte of unused payload, the Encapsulator MUST place the value 0xFF
in this final byte, and transmit the TS Packet. This rule provides a
simple mechanism to resolve the complex behaviour that may arise
when the TS Packet has no PUSI set. To send another SNDU in the
current TS Packet, would otherwise require the addition of a Payload
Pointer that would consume the last remaining byte of TS Packet
payload. The behaviour follows similar practice for other MPEG-2
payload types [ISO-DSMCC]. The Encapsulator MUST start transmission
of the next SNDU in a new TS Packet. (The standard rules require the
header of this new TS Packet to carry a PUSI value of 1 and a
Payload Pointer value of 0x00.)
(iii) If the TS Packet carrying the final part of an SNDU has
exactly two bytes of unused payload, and the PUSI was NOT already
set, the Encapsulator MUST place the value 0xFFFF in this final two
bytes, providing an End Indicator (section 4.3), and transmit the TS
Packet. This rule prevents fragmentation of the SNDU Length Field
over two TS Packets. The Encapsulator MUST start transmission of the
next SNDU in a new TS Packet. (The standard rules require the header
of this new TS Packet to carry a PUSI value of 1 and a Payload
Pointer value of 0x00.)
(iv) If the TS Packet has more than two bytes of unused payload, the
Encapsulator MAY transmit this partially full TS Packet but MUST
first place the value 0xFF in all remaining unused bytes (i.e.
setting an End Indicator followed by Padding). The Encapsulator MUST
then start transmission of the next SNDU in a new TS Packet. (The
standard rules [ISO-MPEG2] require the header of this new TS Packet
to carry a PUSI value of 1 and a Payload Pointer value of 0x00.)
(v) If at least two bytes are available for SNDU data in the TS
Packet payload (i.e. three bytes if the PUSI was NOT previously set,
and two bytes if it was previously set), the Encapsulator MAY
encapsulate further queued PDUs, by starting the next SNDU in the
next available byte of the current TS Packet payload. When the
Encapsulator packs further SNDUs into a TS Packet where the PUSI has
NOT already been set, the PUSI MUST be updated (set to 1) and an 8-
bit Payload Pointer MUST be inserted in the first byte directly
following the TS Packet header. The value of the Payload Pointer
MUST be set to the position of the byte following the end of the
first SNDU in the TS Packet payload. If no further PDUs are
available, an Encapsulator MAY wait for additional PDUs to fill the
incomplete TS Packet. The maximum period of time an Encapsulator can
wait, known as the Packing Threshold, MUST be bounded and SHOULD be
configurable in the Encapsulator. If sufficient additional PDUs are
NOT received to complete the TS Packet within the Packing Threshold,
the Encapsulator MUST insert an End Indicator (using rule iv).
Use of the Packing method (v) by an Encapsulator is optional, and
may be determined on a per-session, per-packet, or per-SNDU basis.
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When an SNDU is less than the size of a TS Packet payload, a TS
Packet may be formed that carries a PUSI value of one and also an
End Indicator (using rule iv).
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7. Receiver Processing
A Receiver tunes to a specific TS Multiplex and sets a receive
filter to accept all TS Packets with a specific PID. These TS
Packets are associated with a specific TS Logical Channel and are
reassembled to form a stream of SNDUs. A single Receiver may be
able to receive multiple TS Logical Channels, possibly using a range
of TS Multiplexes. In each case, reassembly MUST be performed
independently for each TS Logical Channel. To perform this
reassembly, the Receiver may use a buffer to hold the partially
assembled SNDU, referred to here as the Current SNDU buffer. Other
implementations may choose to use other data structures, but MUST
provide equivalent operations.
Receipt of a TS Packet with a PUSI value of 1 indicates that the TS
Packet contains the start of a new SNDU. It also indicates the
presence of the Payload Pointer (indicating the number of bytes to
the start of the first SNDU in the TS-Packet currently being
reassembled). It is illegal to receive a Payload Pointer value
greater than 181, and this MUST cause the SNDU reassembly to be
aborted and the Receiver to enter the Idle State. This event SHOULD
be recorded as a payload pointer error.
A Receiver MUST support the use of both the Packing and Padding
method for any received SNDU, and MUST support reception of SNDUs
with or without a Destination Address Field (i.e. D=0 and D=1).
7.1 Idle State
After initialisation, errors, or on receipt of an End Indicator, the
Receiver enters the Idle State. In this state, the Receiver discards
all TS Packets until it discovers the start of a new SNDU, upon
which it then enters the Reassembly State. Figure 16 outlines these
state transitions:
+-------+
| START |
+---+---+
|
\/
+----------+
\| Idle |/
+-------/| State |\-------+
Insufficient | +----+-----+ |
unused space | | PUSI set | MPEG-2 TS Error
or | \/ | or
End Indicator| +----------+ | SNDU Error
| |Reassembly| |
+--------| State |--------+
+----------+
Figure 16: Receiver state transitions
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7.1.1 Idle State Payload Pointer Checking
A Receiver in the Idle State MUST check the PUSI value in the header
of all received TS Packets. A PUSI value of 1 indicates the presence
of a Payload Pointer. Following a loss of synchronisation, values
between 0 and 181 are permitted, in which case the Receiver MUST
discard the number of bytes indicated by the Payload Pointer
(counted from the first byte of the TS Packet payload field, and
excluding the PP field itself), before leaving the Idle State. It
then enters the Reassembly State, and starts reassembly of a new
SNDU at this point.
7.2 Processing of a Received SNDU
When in the Reassembly State, the Receiver reads a 2 byte SNDU
Length Field from the TS Packet payload. If the value is less than
or equal to 4, or equal to 0xFFFF, the Receiver discards the Current
SNDU and the remaining TS Packet payload and returns to the Idle
State. Receipt of an invalid Length Field is an error event and
SHOULD be recorded as an SNDU length error.
If the Length of the Current SNDU is greater than 4, the Receiver
accepts bytes from the TS Packet payload to the Current SNDU buffer
until either Length bytes in total are received, or the end of the
TS Packet is reached (see also 7.2.1). When Current SNDU length
equals the value of the Length Field, the Receiver MUST calculate
and verify the CRC value (see 4.6). SNDUs that contain an invalid
CRC value MUST be discarded. Mismatch of the CRC is an error event
and SHOULD be recorded as a CRC error. The under-lying physical-
layer processing (e.g. forward error correction coding) often
results in patterns of errors, rather than single bit errors, so the
Receiver needs to be robust to arbitrary patterns of corruption to
the TS Packet and payload, including potential corruption of the
PUSI, PP, and SNDU Length fields. Therefore, a Receiver SHOULD
discard the remaining TS Packet payload (if any) following a CRC
mismatch and return to the Idle State.
When the Destination Address is present (D=0), the Receiver accepts
SNDUs that match one of a set of addresses specified by the Receiver
(this includes the NPA address of the Receiver, the NPA broadcast
address and any required multicast NPA addresses). The Receiver MUST
silently discard an SNDU with an unmatched address.
After receiving a valid SNDU, the Receiver MUST check the Type Field
(and process any Type 1 Extension Headers). The SNDU payload is then
passed to the next protocol layer specified. An SNDU with an unknown
Type value < 1536 MUST be discarded. This error event SHOULD be
recorded as an SNDU type error.
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The Receiver then starts reassembly of the next SNDU. This MAY
directly follow the previously reassembled SNDU within the TS Packet
payload.
(i) If the Current SNDU finishes at the end of a TS Packet payload,
the Receiver MUST enter the Idle State.
(ii) If only one byte remains unprocessed in the TS Packet payload
after completion of the Current SNDU, the Receiver MUST discard this
final byte of TS Packet payload. It then enters the Idle State. It
MUST NOT record an error when the value of the remaining byte is
identical to 0xFF.
(iii) If two or more bytes of TS Packet payload data remain after
completion of the Current SNDU, the Receiver accepts the next 2
bytes and examines if this is an End Indicator. When an End
Indicator is received, a Receiver MUST silently discard the
remainder of the TS Packet payload and transition to the Idle State.
Otherwise this is the start of the next Packed SNDU and the Receiver
continues by processing this SNDU (provided that the TS Packet has a
PUSI value of 1, see 7.2.1, otherwise the Receiver has detected a
delimiting error and MUST discard all remaining bytes in the TS
Packet payload and transitions to the Idle State).
7.2.1 Reassembly Payload Pointer Checking
A Receiver that has partially received an SNDU (in the Current SNDU
buffer) MUST check the PUSI value in the header of all subsequent TS
Packets with the same PID (i.e. same TS Logical Channel). If it
receives a TS Packet with a PUSI value of 1, it MUST then verify the
Payload Pointer. If the Payload Pointer does NOT equal the number of
bytes remaining to complete the Current SNDU, i.e., the difference
between the SNDU Length field and the number of reassembled bytes,
the Receiver has detected a delimiting error.
Following a delimiting error, the Receiver MUST discard the
partially assembled SNDU (in the Current SNDU buffer), and SHOULD
record a reassembly error. It MUST then re-enter the Idle State.
7.3 Other Error Conditions
The Receiver SHOULD check the MPEG-2 Transport Error Indicator
carried in the TS Packet header [ISO-MPEG2]. This flag indicates a
transmission error for a TS Logical Channel. If the flag is set to a
value of one, a transmission error event SHOULD be recorded. Any
partially received SNDU MUST be discarded. The Receiver then enters
the Idle State.
The Receiver MUST check the MPEG-2 Continuity Counter carried in the
TS Packet header [ISO-MPEG2]. If two (or more) successive TS Packets
within the same TS Logical Channel carry the same Continuity Counter
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value, the duplicate TS Packets MUST be silently discarded. If the
received value is NOT identical to that in the previous TS Packet,
and it does NOT increment by one for successive TS Packets (modulo
16), the Receiver has detected a continuity error. Any partially
received SNDU MUST be discarded. A continuity counter error event
SHOULD be recorded. The Receiver then enters the Idle State.
Note that an MPEG2-2 Transmission network is permitted to carry
duplicate TS Packets [ISO-MPEG2], which are normally detected by the
MPEG-2 Continuity Counter. A Receiver that does not perform the
above Continuity Counter check, would accept duplicate copies of TS
Packets to the reassembly procedure. In most cases, the SNDU CRC-32
integrity check will result in discard of these SNDUs, leading to
unexpected PDU loss, however in some cases, duplicate PDUs (fitting
into one TS Packet) could pass undetected to the next layer
protocol.
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8. Summary
This document defines a Unidirectional Lightweight Encapsulation
(ULE) that performs efficient and flexible support for IPv4 and IPv6
network services over networks built upon the MPEG-2 Transport
Stream (TS). The encapsulation is also suited to transport of other
protocol packets and bridged Ethernet frames.
ULE also provides an Extension Header format and defines an
associated IANA registry for efficient and flexible support of both
mandatory and optional SNDU headers. This allows for future
extension of the protocol, while providing backwards compatibility
with existing implementations. In particular, Optional Extension
Headers may safely be ignored by Receiver drivers that do not
implement them, or choose not to process them.
9. Acknowledgments
This draft is based on a previous draft authored by: Horst D.
Clausen, Bernhard Collini-Nocker, Hilmar Linder, and Gorry
Fairhurst. The authors wish to thank the members of the ip-dvb
mailing list for their input provided. In particular, the many
comments received from Art Allison, Carstsen Borman, Patrick
Cipiere, Wolgang Fritsche, Hilmar Linder, Alain Ritoux, and William
Stanislaus. Alain also provided the original examples of usage.
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10. Security Considerations
The security considerations for ULE resemble those that arise when
the existing Multi-Protocol Encapsulation (MPE) is used. ULE does
not add specific new threats that will impact the security of the
general Internet.
There is a known security issue with un-initialised stuffing bytes.
In ULE, these bytes are set to 0xFF (normal practice in MPEG-2).
There are known integrity issues with the removal of the LAN FCS in
a bridged networking environment. The removal for bridged frames
exposes the traffic to potentially undetected corruption while being
processed by the Encapsulator and/or Receiver.
There is a potential security issue when a Receiver receives a PDU
with two length fields: The Receiver would need to validate the
actual length and the Length Field and ensure that inconsistent
values are not propagated by the network. In direct encapsulation of
IPv4/IPv6 in ULE, this is avoided by including only one SNDU Length
Field. However, this issue still arises in bridged LLC frames, and
frames with a LLC Length greater than the SNDU payload size MUST be
discarded, and an SNDU payload length error SHOULD be recorded.
A ULE Mandatory Extension Header may in future be used to define a
method to perform link encryption of the SNDU payload. This is as an
additional security mechanism to IP, transport or application layer
security - not a replacement [RFCXARCHX]. The approach is generic
and decouples the encapsulation from future security extensions. The
operation provides functions that resemble those currently used with
the MPE encapsulation.
Additional security control fields may be provided as a part of this
link encryption Extension Header, e.g. to associate an SNDU with one
of a set of Security Association (SA) parameters. As a part of the
encryption process, it may also be desirable to authenticate
some/all of the SNDU headers. The method of encryption and the way
in which keys are exchanged is beyond the scope of this
specification, as also are the definition of the SA format and that
of the related encryption keys.
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11. References
11.1 Normative References
[ISO-MPEG2] ISO/IEC IS 13818-1 "Information technology -- Generic
coding of moving pictures and associated audio information -- Part
1: Systems", International Standards Organisation (ISO), 2000.
[RFC2119] Bradner, S., "Key Words for Use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, 1997.
[RFC1112] Deering, S., "Host extensions for IP multicasting", STD 5,
RFC 1112, August 1989.
[RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet
Networks", RFC 2464, December 1998.
11.2 Informative References
[ID-ipdvb-ar] Fairhurst, G., M-J. Montpetit, "Address Resolution for
IP datagrams over MPEG-2 Networks", Internet Draft, Work in
Progress.
[ATSC] A/53, "ATSC Digital Television Standard", Advanced Television
Systems Committee (ATSC), Doc. A/53 Rev.C, 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-G] A/54, "Guide to the use of the ATSC Digital Television
Standard", Advanced Television Systems Committee (ATSC), Doc. A/54,
1995.
[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.
[DIX] Digital Equipment Corp, Intel Corp, Xerox Corp, "Ethernet
Local Area Network Specification" Version 2.0, November 1982.
[ETSI-DAT] EN 301 192 "Specifications for Data Broadcasting",
European Telecommunications Standards Institute (ETSI), 2004.
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[ETSI-DVBC] EN 300 800 "Digital Video Broadcasting (DVB); DVB
interaction channel for Cable TV distribution systems (CATV)",
European Telecommunications Standards Institute (ETSI), 1998.
[ETSI-DVBS] EN 300 421 "Digital Video Broadcasting (DVB); Modulation
and Coding for DBS satellite systems at 11/12 GHz", European
Telecommunications Standards Institute (ETSI), 1997.
[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), 2004.
[ETSI-RCS] ETSI 301 790 "Digital Video Broadcasting (DVB);
Interaction Channel for Satellite Distribution Systems", European
Telecommunications Standards Institute (ETSI), 2005.
[IEEE-802.2] IEEE 802.2, "Local and metropolitan area networks-
Specific requirements Part 2: Logical Link Control", IEEE Computer
Society, (also ISO/IEC 8802-2), 1998.
[IEEE-802.3] IEEE 802.3 "Local and metropolitan area networks-
Specific requirements Part 3: Carrier sense multiple access with
collission detection (CSMA/CD) access method and physical layer
specifications", IEEE Computer Society, (also ISO/IEC 8802-3), 2002.
[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), 1998.
[ITU-H222] H.222.0 "Information technology - Generic coding of
moving pictures and associated audio information: Systems",
International Telecommunication Union, (ITU-T), 1995.
[ITU-3563] I.363.5 "B-ISDN ATM Adaptation Layer specification: Type
5 AAL", International Telecommunication Union, (ITU-T), 1996.
[ISO-8802-2] ISO/IEC 8802.2 "Logical Link Control", International
Standards Organisation (ISO), 1998.
[RFC3077] E. Duros, W. Dabbous, H. Izumiyama, Y. Zhang, "A Link
Layer Tunneling Mechanism for Unidirectional Links", RFC3077,
Proposed Standard, 2001.
[RFC3309] Stone, J., R. Stewart, D. Otis. "Stream Control
Transmission Protocol (SCTP) Checksum Change". RFC3095, Proposed
Standard, 2001.
XXX RFC Editor - please replace the next reference and all citations
with the appropriate RFC number. The I-D referenced is currently
ahead in the RFC-ED queue.
XXX
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[RFCXARCHX] M.J. Montpetit, H. D. Clausen, B. Collini-Nocker, H.
Linder "A Framework for transmission of IP datagrams over MPEG-2
Networks", RFCXARCHX, 2005.
[SOOR05] M. Sooriyabandara, G. Fairhurst, A. Ang, B. Collini-Nocker,
H. Linder, W. Stering "A Lightweight Encapsulation Protocol for IP
over MPEG-2 Networks: Design, Implementation and Analysis", Computer
Networks 48 p5-19, 2005.
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12. Authors' Addresses
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
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.scicomp.sbg.ac.at/
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13. IPR Notices
13.1 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.
13.2 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 (2005).
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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15. IANA Considerations
This document will require IANA involvement. The ULE Next-Header
type field defined in this document requires creation of a registry:
ULE Next-Header registry
This registry allocates Next-Header values within the range 0-511
(decimal). For each allocated value, it also specifies the set of
allowed H-LEN values (see section 5). In combination, these define a
set of allowed values in the range 0-1535 for the first part of the
ULE Type space (see section 4.1).
15.1 IANA Guidelines
The following contains the IANA guidelines for management of the ULE
Next-Header registry. This registry allocates values 0-511 decimal
(0x0000-0x01FF, hexadecimal). It MUST NOT allocate values greater
than 0x01FF (decimal).
It subdivides the Next-Header registry in the following way:
1) 0-255 (decimal) IANA assigned values, indicating Mandatory
Extension Headers (or link-dependent type fields) for ULE,
requiring expert review leading to prior issue of an IETF RFC.
This specification MUST define the value, and the name associated
with the Extension Header, together with the procedure for
processing the Extension Header. It MUST also define the need for
the Mandatory Extension and the intended use. The size of the
Extension Header MUST be specified.
Assignments made in this document:
Type Name Reference
0: Test-SNDU Section 4.7.4.
1: Bridged-SNDU Section 4.7.5.
2) 256-511 (decimal) IANA assigned values, indicating Optional
Extension Headers for ULE, requiring expert review leading to
prior issue of an IETF RFC. This specification MUST define the
value, and the name associated with the Extension Header, together
with the procedure for processing the Extension Header. The entry
MUST specify the range of allowable H-LEN values that are
permitted (in the range 1-5). It MUST also define the need for the
Optional Extension and the intended use.
Assignments made in this document:
Type Name H-LEN Reference
256: Extension-Padding 1-5 Section 5.
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ANNEX A: Informative Appendix - SNDU Packing Examples
This appendix provides some examples of use. The appendix is
informative. It does not provide a description of the protocol. The
examples provide the complete TS Packet sequence for some sample
encapsulated IP packets.
The specification of the TS Packet header operation and field values
is provided in [ISO-MPEG2]. The specification of ULE is provided in
the body of this document.
The key below is provided for the following examples.
HDR 4B TS Packet Header
PUSI Payload Unit Start Indicator
PP Payload Pointer
*** TS Packet Payload Pointer (PP)
Example A.1: Two 186B PDUs.
SNDU A is 200 bytes (including the ULE destination NPA address)
SNDU B is 200 bytes (including the ULE destination NPA address)
The sequence comprises 3 TS Packets:
SNDU
PP=0 Length
+-----+------+------+------+- -+------+
| HDR | 0x00 | 0x00 | 0xC4 | ... | A182 |
+-----+----*-+-*----+------+- -+------+
PUSI=1 * *
*****
SNDU
PP=17 CRC for A Length
+-----+------+------+- -+--- --+------+------+- -+------+
| HDR | 0x11 | A183 | ... | A199 | 0x00 | 0xC4 | ... | B165 |
+-----+----*-+------+- -+------+-*----+------+- -+------+
PUSI=1 * *
*************************
End Stuffing
CRC for A Indicator Bytes
+-----+------+- -+------+----+----+- -+----+
| HDR | B166 | ... | B199 |0xFF|0xFF| ... |0xFF|
+-----+------+- -+------+----+----+- -+----+
PUSI=0
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Example A.2: Usage of last byte in a TS-Packet
SNDU A is 183 bytes
SNDU B is 182 bytes
SNDU C is 181 bytes
SNDU D is 185 bytes
The sequence comprises 4 TS Packets:
SNDU
PP=0 Length CRC for A
+-----+------+------+------+- -+------+
| HDR | 0x00 | 0x00 | 0x63 | ... | A182 |
+-----+----*-+-*----+------+- -+------+
PUSI=1 * *
*****
SNDU Unused
PP=0 Length CRC for B byte
+-----+------+------+------+- -+------+------+
| HDR | 0x00 | 0x00 | 0x62 | ... | B181 | 0xFF |
+-----+---*--+-*----+------+- -+------+------+
PUSI=1 * *
******
SNDU SNDU
PP=0 Length CRC for C Length
+-----+------+------+------+- -+------+------+------+
| HDR | 0x00 | 0x00 | 0x61 | ... | C180 | 0x00 | 0x65 |
+-----+---*--+-*----+------+- -+------+------+------+
PUSI=1 * *
****** Unused
byte
+-----+------+- -+------+------+
| HDR | D002 | ... | D184 | 0xFF |
+-----+------+- -+------+------+
PUSI=0
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Example A.3: Large SNDUs
SNDU A is 732 bytes
SNDU B is 284 bytes
The sequence comprises 6 TS Packets:
SNDU
PP=0 Length
+-----+------+------+------+- -+------+
| HDR | 0x00 | 0x02 | 0xD8 | ... | A182 |
+-----+---*--+-*----+------+- -+------+
PUSI=1 * *
******
+-----+------+- -+------+
| HDR | A183 | ... | A366 |
+-----+------+- -+------+
PUSI=0
+-----+------+- -+------+
| HDR | A367 | ... | A550 |
+-----+------+- -+------+
PUSI=0
SNDU
PP=181 CRC for A Length
+-----+------+------+- -+------+------+------+
| HDR | 0xB5 | A551 | ... | A731 | 0x01 | 0x18 |
+-----+---*--+------+- -+------+*-----+------+
PUSI=1 * *
*************************
+-----+------+- -+------+
| HDR | B002 | ... | B185 |
+-----+------+- -+------+
PUSI=0
End Stuffing
Indicator Bytes
+-----+------+- -+------+------+------+- -+------+
| HDR | B186 | ... | B283 | 0xFF | 0xFF | ... | 0xFF |
+-----+------+- -+------+------+------+- -+------+
PUSI=0
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Example A.4: Packing of SNDUs
SNDU A is 200 bytes
SNDU B is 60 bytes
SNDU C is 60 bytes
The sequence comprises two TS Packets:
SNDU
PP=0 Length
+-----+------+------+------+- -+------+
| HDR | 0x00 | 0x00 | 0xC4 | ... | A182 |
+-----+----*-+-*----+------+- -+------+
PUSI=1 * * + +
***** ++++++++
+
+++++++++++++++++
+ SNDU
PP=17 CRC for A + Length
+-----+------+------+- -+------+-+----+------+-
| HDR | 0x11 | A183 | ... | A199 | 0x00 | 0x38 | ...
+-----+----*-+------+- -+------+*-----+------+-
PUSI=1 * * + +
************************ +++++++++
+
+++++++++++++++++++++++++++++++++++++++
+
+ SNDU End Stuffing
+ Length Indicator bytes
+ -+------+------+------+ -+------+------+------+- -+------+
+ ... | B59 | 0x00 | 0x38 |...| C59 | 0xFF | 0xFF |...| 0xFF |
+ -+------+-+----+------+ -+------+-+----+------+- -+------+
+ + + + +
+ + ++++++++ +
+ + + +
++++++++++++++++ ++++++++++++++++++++++
*** TS Packet Payload Pointer (PP)
+++ ULE Length Indicator
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Example A.5: Three 44B PDUs.
SNDU A is 52 bytes (no ULE destination NPA address)
SNDU B is 52 bytes (no ULE destination NPA address)
SNDU C is 52 bytes (no ULE destination NPA address)
The sequence comprises 1 TS Packet:
SNDU
PP=0 Length
+-----+------+------+------+- -+-----+------+-----+- -+-----+-
| HDR | 0x00 | 0x80 | 0x34 | ... | A51 |0x80 | 0x34 | ... | B51 | ..
+-----+----*-+-*----+------+- -+-----+-*----+-----+- -+-----+-
PUSI=1 * *
*****
End Stuffing
Indicator bytes
-----+------+- -+-----+---------+- -+------+
... 0x80 | 0x34 | ... | C51 |0xFF|0xFF| | 0xFF |
-*---+------+- -+-----+---------+- -+------+
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ANNEX B: Informative Appendix - SNDU Encapsulation
An example of ULE encapsulation carrying an ICMPv6 packet generated
by ping6.
ULE SNDU Length : 63 decimal
D-bit value : 0 (NPA destination address present)
ULE Protocol Type : 0x86dd (IPv6)
Destination ULE NPA Address : 00:01:02:03:04:05
ULE CRC32 : 0x7c171763
Source IPv6 : 2001:DB8:3008:1965::1
Destination IPv6 : 2001:DB8:2509:1962::2
SNDU contents (including CRC-32):
0000: 00 3f 86 dd 00 01 02 03 04 05 60 00 00 00 00 0d
0016: 3a 40 20 01 0d b8 30 08 19 65 00 00 00 00 00 00
0032: 00 01 20 01 0d b8 25 09 19 62 00 00 00 00 00 00
0048: 00 02 80 00 9d 8c 06 38 00 04 00 00 00 00 00 7c
0064: 17 17 63
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[RFC EDITOR NOTE:
This section must be deleted prior to publication]
DOCUMENT HISTORY
Draft 00
This draft is intended as a study item for proposed future work by
the IETF in this area. Comments relating to this document will be
gratefully received by the author(s) and the ip-dvb mailing list at:
ip-dvb@erg.abdn.ac.uk
--------------------------------------------------------------------
DRAFT 01 (Protocol update)
* Padding sequence modified to 0xFFFF, this change aligns with other
usage by MPEG-2 streams. Treatment remains the same as specified for
ULE.
* SDNU Format updated to include R-bit (reserved).
* Updated procedure for TS Packet carrying the final part of an SNDU
with either less than two bytes of unused payload updated.
* A Receiver MUST silently discard the remainder of a TS Packet
payload when two or less bytes remain unprocessed following the end
of an SNDU, irrespective of the PUSI value in the received TS
Packet. It MUST NOT record an error when the value of the remaining
byte(s) is identical to 0xFF or 0xFFFF. The Receiver MUST then wait
for a TS Packet with a PUSI value set to 1.
* Payload Pointer description updated.
* CRC Calculation added.
* Decapsulator processing revised.
* Type field split into two.
* References updated.
* Security considerations added (first draft).
* Appendix added with examples.
--------------------------------------------------------------------
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DRAFT - 02 (Improvement of clarity)
* Corrected CRC-32 to follow standard practice in DSM-CC.
* Removed LLC frame type, now redundant by Bridge-Type (==1)
* Defined D-bit to use the reserved bit field (R ) - Gorry, Alain,
Bernhard
* Changes to description of minimum payload length. Gorry
* MPEG-2 Error Indicator SHOULD be used.Hilmar & Gorry
* MPEG-2 CC MAY be used (since CRC-32 is strong anyway). Hilmar &
Gorry
* Corrected CRC-32 to now follow standard practice in DSM-CC. Gorry,
Hilmar, Alain.
* Changed description of Encapsulator action for Packing. Gorry &
Hilmar.
* Changed description of Receiver to clarify packing. Gorry & Alain.
* Stuff/Pad of unused bytes MUST be 0xFF, to align with MPEG.
Hilmar/Bernhard.
* Recommend removal of section on Flushing bit stream. Gorry
* Updated SNDU figures to reflect D-bit and correct a mistake in the
bridged type field. Alain
* Reorganised section 5 to form sections 5 and 6, separating
encapsulation and receiver processing. Gorry, Hilmar, Alain.
* Added concept of Idle State and Reassembly State to the Receiver.
Renumbered sections 5,6 and following. Gorry.
* Nits from Alain, Hilmar and Gorry.
Moved security issue on the design of the protocol to appropriate
sections, since this is not a concern for deployment: Length field
usage and padding initialisation.
* Changed wording: All multi-byte values in ULE (including Length,
Type, and Destination fields) are transmitted in network byte order
(most significant byte first). old NiT from Alain, now fixed.
* Frame byte size in diagrams now updated to -standard- format, and
D bit action corrected, as requested by Alain.
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* Frame format diagrams, redrawn to 32-bit format below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
* Additional diagram requested by Alain for D=0 bridging (added, and
subsequent figures renumbered).
* Diagrams of encapsulation process, redrawn for clarity (no change
to meaning). Gorry.
* Reworded last para of CRC description.
* Clarification to the statements in the CRC coverage - to make it
clear that it is the entire SNDU (header AND payload) that is
checksummed. (Fritsche@iabg.de, hlinder@cosy.sbg.ac.at).
* References added for RCS (spotted by Alain) and AAL5 (provided by
Anthony Ang).
* Removed informative reference to MPEG part 1.Alain.
Spelling correction -> Allain to Alain.
* Added description of Receiver processing of the address
field.Gorry
* Added caution on LLC Length in bridged Packets thanks.
Gorry/wolfgang
* Removed Authors notes from text after their discussion on the list
Gorry
* Corrected text to now say maximum value of PP = 182 in ULE. Gorry
* Tidied diagrams at end (again) - Gorry,
Revision with following changes:
* Re issue as working group draft (filename change)
* Refinement of the text on CRC generation to be unambiguous.
* Revised CC processing at Encapsulator (B C-N/GF/A.Allison)
* Revised CC processing at Receiver (from List: A.Allison; et al )
* Corrections to length/PP field in Examples (M Sooriyabandara,
Alain)
* Corrections to pointer in Example 3 SNDU C (M Jose-Montpetit)
* Section 4.5 only SHARED routed links require D=0
* Packing Threshold defined
* Next-Layer-Header defined (Now called Next-Header)
* Addition of Appendix B (to aide verification of SNDFU format)
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Working Group ID rev 01
Issues addressed:
* Typographical
* Types > 1500 should be passed to the next higher protocol (Hilmar)
* The second part of the Type space corresponds to the values 1500
COMMENT: ~Range should be 1536 Decimal Decimal to 0xFFFF.
* IANA has already defined IP and IPv6 types - corrected text!
Added more security considerations (-01d).
* Should we allow an Adaptation Field within ULE (request for DVB-
RCS compatibility)? Requirement to be clarified! Implementation
impact to be evaluated!
Current Recommendation: The current spec does not preclude use of
AF, it simply says that this is not the standard for ULE. The use
case and requirement for this mode are not currently clear, based on
this there is no current intention to add this to ULE - text for
requirements would be welcome.
* Verify the minimum value allocated to DIX Ethernet Header Types.
Draft updated to align with IEEE Registry assignments.
--------------------------------------------------------------------
Working Group ID rev 02
Revised IPR disclosure
Revised copyright notice
Section 5 added to ULE to define optional Extension Headers (see
xule)
Correction of figure numbering.
Correction to capitalisation in Transport Stream definition of fields
Inserted space character after 1536 in line 2 of 4.4.2
Replaced } with ] after ISO-DSMCC
Replace reference to section 6.3 with section 7.3 at end of section
4.6.
Reference in 4.7.4 was changed to refer to figure 7 (not 6).
Note added after figure 9.
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Working Group ID rev 03
Changes with this revision of the document:
(i) The worked hexadecimal example in the annexe was reworked to
include a valid MAC address for an IPv6 unicast packet. -
(BCN)
(ii) The IANA procedures revised, based on inputs from IANA to
improve consistency of the term Next-Header and to add the
HLEN field to the IANA registry record for OPTIONAL headers.
(GF)
(iii) 7.2 Change to revert wording in the second para to MUST enter
IDLE after CRC failure of SNDU check.
(iv) In normal operation, it is expected that any padding appended
to a bridged Ethernet frame SHOULD be removed prior to
forwarding. This requires the sender to be aware of such
Ethernet padding (e.g. LLC). (Made this a SHOULD). (GF)
NiTS:
(v) Format of page Breaks was updated. (GF)
(vi) Check for <- -> sequences of characters. (GF)
(vii) Update refs to add RFC3667 / 3668. (GF)
(viii) Changed text defining M in DSMCC definition to the word Media
(ix) 7.1.1 Range of PP values corrected to 0-181.
(x) Definition of END INDICATOR corrected in section 2 - this is
not a TYPE value, but a LENGTH value.
(xi) Next-Header used throughout the document to replace
next-layer-header, and various other forms of wording.
(xii) In section 7.2, added a ref the section on PP checking
Working Group ID rev 04
This rev followed WGLC comments, which are defined in the ipdvb
mailing list. Important changes included:
(i) This text was moved to an appendix
(ii) ToC was updated and section headers made consistent
(iii) Revised definition text
(iv) Improved clarity with respect to terms defined in ISO 13818-1
(v) Bridging and Extension-Padding formats move to section 5
(vi) Clarification of the NPA in packet headers
(vii) Clarification of placement of NPA address with extension
headers.
Issues address in rev-05:
These revisions were made following a second WGLC and invited cross-
area IETF review of the Spec.
NiTS corrected:
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Abstract shortened.
Added separate references to Ethernet v2; LLC; and 802.3
Added RFC2119 Boilerplate for definitions of capitilised words.
Corrected English and 63 typos
Specified explicitly that Test & Bridge Extension Headers must be
the last in the extension chain (no other headers may follow)
7.1.1. para 1 - changed PP processing description to specify where
to count the number of bytes that were pointed to
Corrected the range 0-512 in the IANA Guidelines (should be 0-511).
Fixed NPA to consistently refer to the ULE destination address.
Specific Issues:
1) The reviewer suggested the title was confusing. A proposed new
Title is:
Ultra Lightweight Encapsulation (ULE) for transmission of
IP datagrams over an MPEG-2 Transport Stream
2) The reviewer suggested that the name of the D field was changed,
to make the meaning more obvious. The new name is Destination
Address Absent field, rather than the Destination Address Present
field. The semantics of the D-bit do not change.
3) The reviewer asked for a description of how to send an LLC frame
- in Section 4. This was added to the section on bridging.
4) The reviewer mentioned that we had NOT defined the values needed
for mapping addresses... I'm not sure this was an over-sight, but
This was an oversight, the new text was added to the end of the
description in section 4.5. Also added references to [RFC1112]
[RFC2464].
5) Added text on the need for data descriptors.
6) Removed reference to RFC3819 which was either ambiguous in the
definition of SNDU.
7) In final clause of 7.2 (Receiver processing) the last sentence
was extended by a bracketed clause to deal with the case when there
was excess data and no PUSI set).
(iii) If two or more bytes of TS Packet payload data remain after
completion of the Current SNDU, the Receiver accepts the next 2
bytes and examines if this is an End Indicator. When an End
Indicator is received, a Receiver MUST silently discard the
remainder of the TS Packet payload and transition to the Idle State.
Otherwise this is the start of the next Packed SNDU and the Receiver
continues by processing this SNDU (provided that the TS Packet has a
PUSI value of 1, see 7.2.1, otherwise the Receiver has detected a
delimiting error and MUST discard all remaining bytes in the TS
Packet payload and transitions to the Idle State).
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8) Revised IANA procedures to REQUIRE a definition of the PROCEDURE
when defining an extension header.
IESG Review Rev -06.
This rev was generated in response to issues raised during AD and
IESG review. The changes provide clarifications and corrections, but
do not modify the protocol behaviour.
Comments from Brian Carpenter; Margaret Wasserman; GenART review.
Figure 2 was also updated to reflect 16 bit alignment of the first
word.
In this review a change to the title was proposed by the IESG and was
accepted by the authors:
Ultra Lightweight Encapsulation (ULE)
-> Unidirectional Lightweight Encapsulation (ULE)
[END of RFC EDITOR NOTE]
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