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Versions: (draft-fair-ipdvb-ule) 00 01 02 03 04 05 06 RFC 4326

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

   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
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/1id-abstracts.html
   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   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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