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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, U.K.
                    Document: draft-ietf-ipdvb-ule-01.txt           Bernhard Collini-Nocker
                                                                  University of Salzburg, A
                    
                    
                    ipdvb WG
                    
                    Category: Draft Intended Standards Track                       May 2004
                    
                    
                            Ultra Lightweight Encapsulation (ULE) for transmission of
                                    IP datagrams over MPEG-2/DVB networks
                    
                    Status of this Draft
                    
                       This document is an Internet-Draft and is in full conformance with
                          all provisions of Section 10 of RFC2026.
                    
                       Internet-Drafts are working documents of the Internet Engineering
                       Task Force (IETF), its areas, and its working groups. Note that
                       other groups may also distribute working documents as Internet-
                       Drafts. Internet-Drafts are draft documents valid for a maximum of
                       six months and may be updated, replaced, or obsoleted by other
                       documents at any time. It is inappropriate to use Internet- Drafts
                       as reference material or to cite them other than as "work in
                       progress."
                    
                       The list of current Internet-Drafts can be accessed at
                       http://www.ietf.org/ietf/1id-abstracts.txt
                       The list of Internet-Draft Shadow Directories can be accessed at
                       http://www.ietf.org/shadow.html.
                    
                       Abstract
                    
                       The MPEG-2 TS has been widely accepted not only for providing
                       digital TV services, but also as a subnetwork technology for
                       building IP networks. This document describes an Ultra Lightweight
                       Encapsulation (ULE) mechanism for the transport of IPv4 and IPv6
                       Datagrams and other network protocol packets directly over ISO MPEG-
                       2 Transport Streams (TS) as TS Private Data.
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
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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).
                    
                       * Procedure for TS Packet carrying the final part of a 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 a 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
                       * 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.
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
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                       Issues pending working group consensus:
                    
                    
                       1) Query about the code point value for an Ethernet Bridging SNDU -
                       should the ULE type-field be 0x0001; 0x0007; or should the IEEE
                       Ethertype for bridging be used instead?
                       Author Note: This may depend on other assignments, to be
                       determined.
                    
                       2) Should we allow configuration of an optional non-default CC
                       processing???
                    
                       3) Consider amendment to ULE to define optional extension headers
                       (various proposals)
                       Author Note: Design trade-offs need to be considered.
                       Current Recommendation: Document to be produced with concrete
                       proposal.
                    
                       4) Should ULE support FEC?
                       Author Note 1: No concrete proposal yet, although this seems within
                       the scope of the use of extension headers.
                       Author Note 2: Text is required for the requirements ID so this may
                       first be updated to reflect the need for this option.
                       Current Recommendation: No change to ULE Spec, but extensions may
                       subsequently be defined to support this - text would be welcome to
                       requirements documents on why and what should be added.
                    
                       5) Should ULE support Encryption?
                       Author Note: In principle, this is just a code-point issue, since we
                       only defining an encapsulation here. This seems within the scope of
                       the use of type fields.
                       Author Note 2: Text is required for the requirements ID so this may
                       first be updated to reflect the need for this option (some inputs
                       from L. Claverotte/H. Cruickshank/et al).
                    
                    
                       6) Do we need to define OPTIONAL extension header fields to allow
                       Receivers backwards compatibility with unknown options?
                    
                    
                       [END of RFC EDITOR NOTE]
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
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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 Present Field
                         4.2 Length Field
                         4.3 End Indicator
                         4.4 Type Field
                           4.4.1 Type 1: IANA Assigned 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
                           4.7.4 Test SNDU
                       5. Processing at the Encapsulator
                         5.1 SNDU Encapsulation
                         5.2 Procedure for Padding and Packing
                       6. Receiver Processing
                         6.1 Idle State
                           6.1.1 Reassembly Payload Pointer Checking
                         6.2 Processing of a Received SNDU
                           6.2.1 Reassembly Payload Pointer Checking
                         6.3 Other Error Conditions
                       7. Summary
                       8. Acknowledgments
                       9. Security Considerations
                       10. References
                         10.1 Normative References
                         10.2 Informative References
                       11. Authors' Addresses
                       12. IANA Considerations
                    
                       ANNEXE A: Informative Appendix - SNDU Packing Examples
                       ANNEXE B: Informative Appendix - SNDU Encapsulation
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
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                    1. Introduction
                    
                       This document describes an encapsulation for transport of IP
                       datagrams, or other network layer packets, over ISO MPEG-2 Transport
                       Streams [ISO-MPEG; ID-ipdvb-arch].  It 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
                       TS Packets) that are sent over a single TS Logical Channel.
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
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                    2. Conventions used in this document
                    
                       ADAPTATION FIELD: An optional variable-length extension field of the
                       fixed-length TS Packet header, intended to convey clock references
                       and timing and synchronization information as well as stuffing over
                       an MPEG-2 Multiplex [ISO-MPEG].
                    
                       AFC: Adaptation Field Control, 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.
                    
                       DSM-CC: Digital Storage Management Command and Control [ISO-DSMCC].
                       A format for transmission of data and control information defined by
                       the ISO MPEG-2 standard that is carried in an MPEG-2 Private
                       Section.
                    
                       DVB: Digital Video Broadcast [ETSI-DVB]. A framework and set of
                       associated standards published by the European Telecommunications
                       Standards Institute (ETSI) for the transmission of video, audio, and
                       data, using the ISO MPEG-2 Standard.
                    
                       ENCAPSULATOR: A network device that receives PDUs and formats these
                       into Payload Units (known here as SNDUs) for output as a stream of
                       TS Packets.
                    
                       END INDICATOR: A Type value that indicates to the Receiver that
                       there are no further SNDUs present within the current TS Packet.
                    
                       MAC: Medium Access and Control. The link layer header of the
                       Ethernet IEEE 802 standard of protocols, consisting of a 6B
                       destination address, 6B source address, and 2B type field.
                    
                       MPE: Multiprotocol Encapsulation [ETSI-DAT; ATSC-DAT ; ATSC-DATG]. A
                       scheme that encapsulates PDUs, forming a DSM-CC Table Section. Each
                       Section is sent in a series of TS Packets using a single TS Logical
                       Channel.
                    
                       MPEG-2: A set of standards specified by the Motion Picture Experts
                       Group (MPEG), and standardized by the International Standards
                       Organisation (ISO) [ISO-MPEG].
                    
                       NEXT-HEADER: A Type value indicating an extension header.
                    
                       NPA: Network Point of Attachment. In this document, refers to a 6 B
                       destination address (similar to an Ethernet MAC address) within the
                       MPEG-2 transmission network used to identify individual Receivers or
                       groups of Receivers.
                    
                    
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                       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.
                    
                    
                       PDU: Protocol Data Unit. Examples of PDU include Ethernet frames,
                       IPv4 or IPv6 datagrams, and other network packets
                    
                       PES: Packetized Elementary Stream of MPEG-2 [ISO-MPEG].
                    
                       PID: Packet Identifier. A 13 bit field carried in the header of TS
                       Packets. This is used to identify the TS Logical Channel to which a
                       TS Packet belongs [ISO-MPEG]. The TS Packets forming the parts of a
                       Table Section, PES, or other payload unit must all carry the same
                       PID value.  The all 1s PID value indicates a Null TS Packet
                       introduced to maintain a constant bit rate of a TS Multiplex.
                    
                       PP: Payload Pointer. An optional one byte pointer that directly
                       follows the TS Packet header. It contains the number of bytes
                       between the end of the TS Packet header and the start of a Payload
                       Unit. The presence of the Payload Pointer is indicated by the value
                       of the PUSI bit in the TS Packet header. The Payload Pointer is
                       present in DSM-CC, and Table Sections, it is not present in TS
                       Logical Channels that use the PES-format.
                    
                       PU: Payload Unit. A sequence of bytes sent using a TS. Examples of
                       Payload Units include: an MPEG-2 Table Section or a ULE SNDU.
                    
                       PUSI: Payload_Unit_Start_Indicator of MPEG-2 [ISO-MPEG]. A single
                       bit flag carried in the TS Packet header. A PUSI value of zero
                       indicates that the TS Packet does not carry the start of a new
                       Payload Unit. A PUSI value of one indicates that the TS Packet does
                       carry the start of a new Payload Unit. In ULE, a PUSI bit set to 1
                       also indicates the presence of a one byte Payload Pointer (PP).
                    
                       PRIVATE SECTION: a syntactic structure used for mapping all service
                       information (e.g. an SI table) into TS Packets.  A Table may be
                       divided into a number of Table Sections, however all Table Sections
                       must be carried over a single TS Logical Channel.
                    
                       PSI: Program Specific Information. Tables used to convey information
                       about the service carried in a TS Multiplex. The set of PSI tables
                       is defined by [ISO-MPEG], see also SI Table.
                    
                       SI TABLE: Service Information Table. In this document, this term
                       describes any table used to convey information about the service
                       carried in a TS Multiplex. SI tables are carried in MPEG-2 private
                       sections.
                    
                    
                    
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                       SNDU: Subnetwork Data Unit. An encapsulated PDU sent as an MPEG-2
                       Payload Unit.
                    
                       TABLE SECTION: A Payload Unit carrying a part of a MPEG-2 SI Table.
                    
                       TS: Transport Stream [ISO-MPEG], a method of transmission at the
                       MPEG-2 level using TS Packets; it represents level 2 of the ISO/OSI
                       reference model. See also TS Logical Channel and TS Multiplex.
                    
                       TS HEADER: The 4 byte header of a TS Packet as illustrated in the
                       introduction.
                    
                       TS LOGICAL CHANNEL: Transport Stream Logical Channel, a channel
                       identified at the MPEG-2 level [ISO-MPEG]. It exists at level 2 of
                       the ISO/OSI reference model. All packets sent over a TS Logical
                       Channel carry the same PID value. According to MPEG-2, some TS
                       Logical Channels are reserved for specific signalling purposes.
                       Other standards (e.g., ATSC, DVB) also reserve specific TS Logical
                       Channels.
                    
                       TS MULTIPLEX: A set of MPEG-2 TS Logical Channels sent over a single
                       common physical link (i.e. a transmission at a specified symbol
                       rate, FEC setting, and transmission frequency). The same TS Logical
                       Channel may be repeated over more than one TS Multiplex, for example
                       to redistribute the same multicast content to two terrestrial TV
                       transmission cells.
                    
                       TS PACKET: A fixed-length 188B unit of data sent over a TS Multiplex
                       [ISO-MPEG]. Each TS Packet carries a 4B header, plus optional
                       overhead including an Adaptation Field, encryption details and time
                       stamp information to synchronise a set of related Transport Streams.
                       The 188B TS Packets incorporate a 4B header with the following
                       fields, those referenced within this document marked with “*”:
                    
                            Field Length   Name/Purpose
                    
                             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
                            *4b             continuity counter (CC)
                    
                    
                    
                    
                    
                    
                    
                    
                    
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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 a 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. The PUSI identifies the start of a payload
                       unit (SNDU) within the MPEG-2 TS Packet payload. The semantics of
                       the PUSI bit are defined for PES and PSI packets [ISO-MPEG]; 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 a SNDU, but
                            contains the continuation, or end of a SNDU;
                    
                            1: The TS Packet contains the start of a 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 is aware that when it
                       receives a table_id value of 0xFF, this indicates Padding/Stuffing
                       occurred 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. The purpose of the adaptation field is primarily to
                       extend the TS header for timing and synchronisation information and
                       may be used to also include stuffing bytes before a TS Packet
                       payload. Standard Receivers discard TS Packets with an
                       adaptation_field_control field value of '00'. 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'.
                       Receivers MUST discard TS Packets that carry other AFC values.
                    
                    
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                    4. SNDU Format
                    
                       PDUs (IP packets and bridged Ethernet frames) are encapsulated using
                       ULE to form a SNDU. Each SNDU is sent as an MPEG-2 Payload Unit. The
                       encapsulation format to be used for PDUs  is illustrated below:
                    
                       < ----------------------------- SNDU ----------------------------- >
                       +-+-------------------------------------------------------+--------+
                       |D| Length | Type |                 PDU                   | CRC-32 |
                       +-+-------------------------------------------------------+--------+
                    
                       Figure 1: SNDU Encapsulation
                    
                       All multi-byte values in ULE (including Length, Type, and
                       Destination fields) are transmitted in network byte order (most
                       significant byte first). Appendix A provides informative examples of
                       usage.
                    
                    
                       4.1 The Destination Address Present Field
                    
                       The most significant bit of the Length Field carries the value of
                       the Destination Address Present 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 (i.e. it is omitted).
                    
                       By default, the D-bit value MUST be set to a value of 0, except for
                       the transmission of an End Indicator (see 4.3), in which this bit
                       MUST be set to the value of 1.
                    
                    
                       4.2 Length Field
                    
                       A 15-bit value that indicates the length, in bytes, of the SNDU
                       (encapsulated Ethernet frame, IP datagram or other packet) 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 of a SNDU have the value 0xFFFF, this
                       denotes an End Indicator (i.e., all 1’s length combined with a D-bit
                       value of 1). It 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 a
                       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 DIX
                       framework for Ethernet. After specification of IEEE 802.3 [LLC], 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 4.7.5).
                    
                       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 <
                       1536.  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
                       >= 1536. In ULE, this indicates that the 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-Layer-Header
                    
                       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:
                    
                       [XXX IANA ACTION REQUIRED XXX]
                    
                       0x0000: Test SNDU, discarded by the Receiver.
                    
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                       0x0001: Bridged Ethernet Frame (i.e. MAC source address follows)
                    
                       [XXX END OF IANA ACTION REQUIRED XXX]
                    
                       The remaining values within the first part of the Type space are
                       reserved for allocation by the IANA.
                    
                       [Author NOTE: Type allocation and appropriate IANA Procedure to be
                       determined.]
                    
                    
                       4.4.2 Type 2: Ethertype compatible Type Fields
                    
                       The second part of the Type space corresponds to the values
                       1536Decimal (0x600) and 0xFFFF.  This set of type assignments follow
                       DIX/IEEE assignments (but exclude use of this field as a frame
                       length indicator) [LLC].  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
                       0x86DD : IPv6 Payload
                    
                    
                    
                       4.5 SNDU Destination Address Field
                    
                       The SNDU Destination Address Field is optional (see section 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), which in combination
                       with the PID value, could be interpreted as a Link-Level address.
                    
                       When the SNDU header indicates the presence of a SNDU Destination
                       Address field (i.e. D=0), a Network Point of Attachment, NPA, field
                       directly follows the SNDU Type Field.  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 a 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.
                    
                    
                    
                    
                    
                    
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                       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 [ITU3563]. This is a 32 bit value calculated according to the
                       generator polynomial represented 0x04C11DB7 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 6.3).
                    
                    
                       4.7 Description of SNDU Formats
                    
                       The format of a SNDU is determined by the combination of the
                       Destination Address Present bit (D) and the SNDU Type Field.  The
                       simplest encapsulation places a PDU directly into a SNDU payload.
                       Some Type 1 encapsulations may require additional header fields.
                       These are inserted in the SNDU directly preceding the PDU.
                    
                       The following SNDU Formats are defined here:
                    
                    
                    
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                       End Indicator: The Receiver should enter the Idle State.
                       IPv4 SNDU: The payload is a complete IPv4 datagram
                       IPv6 SNDU: The payload is a complete IPv6 datagram.
                       Test SNDU: The payload will be discarded by the Receiver.
                       Bridged SNDU: The payload carries a bridged MAC or LLC frame.
                    
                       All other formats are currently reserved.
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
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                       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                           |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          |                                                               |
                          =        Arbitrary number >= 0  of bytes with value 0xFF        =
                          |                                                               |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    
                         Figure 2: SNDU Format for an End Indicator.
                    
                    
                       4.7.2 IPv4 SNDU
                    
                       IPv4 datagrams are 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 Address  (6B)              |
                          +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          |                               |                               |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
                          |                                                               |
                          =                           IPv4 datagram                       =
                          |                                                               |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          |                             (CRC-32)                          |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    
                       Figure 3: SNDU Format for an IPv4 Datagram using L2 filtering (D=0).
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
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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
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          |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 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.
                    
                           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 = 0x086DD        |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          |               Receiver Destination 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 = 0x086DD        |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          |                                                               |
                          =                           IPv6 datagram                       =
                          |                                                               |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          |                             (CRC-32)                          |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    
                       Figure 6: SNDU Format for an IPv6 Datagram using L3 filtering (D=1).
                    
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                       4.7.4 Test SNDU
                    
                       A Test SNDU is of Type 1 (figure 6). 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 7: SNDU Format for a Test SNDU
                    
                    
                       4.7.5 Bridge Frame SNDU Encapsulation
                    
                       A bridged SNDU is of Type 1.  The payload includes a MAC source and
                       Ether-Type field together with the contents of a bridged MAC frame.
                       The SNDU has the format shown in figures 8 and 9.
                    
                           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 Address  (6B)               |
                          +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          |                               |                               |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
                          |                MAC Destination Address  (6B)                  |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          |                    MAC Source Address  (6B)                   |
                          +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          |                               |          EtherType (2B)       |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          |                                                               |
                          =                 (Contents of bridged MAC frame)               =
                          |                                                               |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          |                             (CRC-32)                          |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    
                       Figure 8: SNDU Format for a Bridged Payload (D=0)
                    
                    
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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
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          |1|        Length  (15b)        |         Type = 0x0001         |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          |                   MAC Destination Address  (6B)               |
                          +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          |                               |                               |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
                          |                     MAC Source Address  (6B)                  |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          |          EtherType (2B)       |                               |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
                          |                                                               |
                          =                 (Contents of bridged MAC frame)               =
                          |                                                               |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          |                             (CRC-32)                          |
                          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    
                       Figure 9: SNDU Format for a Bridged Payload (D=1)
                       When an NPA address is specified (D=0), Receivers MUST discard all
                       SNDUs that carry an NPA address that does NOT match their own NPA
                       address (or a broadcast/mcast 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.
                    
                       The MAC addresses in the frame being bridged SHOULD be assigned
                       according to the rules specified by the IEEE and may 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. The EtherType field of a frame is defined according
                       to Ethernet/LLC [LLC].
                    
                       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 will be removed prior to forwarding. This requires
                       the sender to be aware of such Ethernet padding.
                    
                       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). 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
                    
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                       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. 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.
                    
                                    +------+--------------------------------+------+
                                    | 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 10: Encapsulation of a SNDU into a series of TS Packets
                    
                    
                       5.1 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 a 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 the SNDU starts in the first
                       available byte of 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-MPEG].  This value MUST be incremented by one (modulo 16) for
                       each successive fragment/complete SNDU sent using a TS Logical
                       Channel.
                    
                    
                    
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                       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). It 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.
                    
                    
                                     +-/------------+
                                     |  SubNetwork  |
                                     |     DU 3     |
                                     +-/------------+
                                            \        \
                                             \        \
                                              \        \
                                     +--------+--------+--------+----------+
                                     |MPEG-2TS| End of | 0xFFFF |  Unused  |
                                     | Header | SNDU 3 |        |  Bytes   |
                                     +--------+--------+--------+----------+
                                       PUSI=0            ULE
                                                         End
                                                         Indicator
                    
                       Figure 11: A TS Packet carrying the end of SNDU 3, 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
                       5.2). This is called Packing (figure 12).
                    
                                  +-/----------------+       +----------------/-+
                                  |   Subnetwork     |       |   Subnetwork     |
                                  |      DU 1        |       |      DU 2        |
                                  +-/----------------+       +----------------/-+
                                             \        \     /          /\
                                              \        \   /          /  \
                                               \        \ /          /    \. . .
                              +--------+--------+--------+----------+
                              |MPEG-2TS| Payload| end of | start of |
                              | Header | Pointer| SNDU 1 | SNDU 2   |
                              +--------+--------+--------+----------+
                                PUSI=1     |              ^
                                           |              |
                                           +--------------+
                    
                       Figure 12: A TS Packet with the end of SNDU 1, followed by SNDU 2.
                    
                    
                    
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                       5.2 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 require the header of this new
                       TS Packet to carry a PUSI value of 1, and a Payload Pointer value of
                       0x00.)
                    
                       (ii) If the TS Packet carrying the final part of a 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 a 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 (4.7.1), 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
                       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.)
                    
                       (v) If at least two bytes are available for payload 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. The PUSI MUST
                       be set.  When the Encapsulator packs further SNDUs into a TS Packet
                       where the PUSI has NOT already been set, this requires the PUSI to
                       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
                       MUST be set to the position of the byte following the end of the
                    
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                       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.
                    
                       When a 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).
                    
                    
                    6. 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).
                    
                    
                       6.1 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, when it
                       then enters the Reassembly State. Figure 13 outlines these state
                       transitions:
                    
                    
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                                                    +-------+
                                                    | START |
                                                    +---+---+
                                                        |
                                                       \/
                                                   +----------+
                                                  \|   Idle   |/
                                          +-------/|   State  |\-------+
                             Insufficient |        +----+-----+        |
                             unused space |             | PUSI set     | MPEG-2 TS Error
                             or           |            \/              | or
                             End Indicator|        +----------+        | SNDU Error
                                          |        |Reassembly|        |
                                          +--------|  State   |--------+
                                                   +----------+
                    
                       Figure 13: Receiver state transitions
                    
                    
                       6.1.1 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 1 and 182 are permitted, in which case the Receiver MUST
                       discard the number of bytes indicated by the Payload Pointer from
                       the start of the TS Packet payload, before leaving the Idle State.
                       It then enters the Reassembly State, and starts reassembly of a new
                       SNDU at this point.
                    
                    
                       6.2 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. When Current SNDU length equals the value of
                       the Length Field, the Receiver MUST calculate and verify the CRC
                       value (section 4.6). SNDUs that contain an invalid CRC value MUST be
                       discarded, causing the Receiver to re-enter the Idle State.
                    
                       When the Destination Address is present, 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
                    
                    
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                       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 extensions specified). 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 a SNDU type error.
                    
                       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.
                    
                    
                       6.2.1 Reassembly Payload Pointer Checking
                    
                       A Receiver that has partially received a 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.
                    
                    
                       6.3 Other Error Conditions
                    
                       The Receiver SHOULD check the MPEG-2 Transport Error indicator
                       carried in the TS Packet header.  This flag indicates a transmission
                       error for a TS Logical Channel. If the flag is set to a value of
                    
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                       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-MPEG]. If two (or more) successive TS Packets
                       within the same TS Logical Channel carry the same Continuity Counter
                       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 the MPEG2-2 Transmission network is permitted to carry
                       duplicate TS Packets [ISO-MPEG], 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.
                    
                    
                    7. Summary
                    
                       This document defines an Ultra Lightweight Encapsulation (ULE) to
                       perform 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.
                    
                    
                    8. 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 Patrick Cipiere, Wolgang Fritsche, and Alain
                       Ritoux. Alain also provided the original examples of usage.
                    
                    
                    9. Security Considerations
                    
                       The security considerations for ULE resemble those that arise when
                       the exiting Multi-Protocol Encapsulation (MPE) is used.  ULE does
                       not add specific new threats that will impact the security of the
                       general Internet.
                    
                    
                    
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                       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 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 a SNDU payload
                       length error SHOULD be recorded.
                    
                       ULE supports optional link level encryption of the SNDU payload.
                       This is as an additional security mechanism to IP, transport or
                       application layer security - not a replacement [ID-ipdvb-arch].
                    
                    
                       XXX Authors Note: Text below to be revised when this requirement is
                       included in the Spec - depending on style of extension mechanism
                       that is used XXX
                    
                       Options that may be used include a next header extension type field
                       in the ULE header (16 bits) that may indicate that some part of the
                       ULE PDU is encrypted. Two potential solutions for the use of the
                       type field are:
                    
                            a. Define a range of types X-to-Y for security.  These types
                               will act as security key ID to enable the decryption of the
                               incoming ULE PDUs.
                            b. 2. A single type value may be defined for encryption (say X)
                               followed by any required Security Association (SA)
                               parameters.  The definition of this SA and the related
                               encryption keys are out of the scope for ULE draft.
                    
                       The second solution is more generic and decouples the encapsulation
                       from future security extensions. The former provides functions that
                       resemble those currently used for the MPE encapsulation.
                    
                    
                       Additional security control fields may be provided as a part of the
                       extension header. The method of encryption and the way in which keys
                       are exchanged is beyond the scope of this specification.  However,
                       the specification provides appropriate code points to allow such
                       encryption to be implemented at the link layer. As a part of the
                       encryption process, it may be desirable to authenticate some/all of
                       the SNDU headers.
                    10. References
                    
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                       10.1 Normative References
                    
                       [ISO-MPEG] ISO/IEC DIS 13818-1 "Information technology -- Generic
                       coding of moving pictures and associated audio information:
                       Systems", International Standards Organisation (ISO).
                    
                       [RFC2026] Bradner, S., "The Internet Standards Process - Revision
                       3", BCP 9, RFC 2026, BCP 9, 1996.
                    
                       [RFC2119] Bradner, S., "Key Words for Use in RFCs to Indicate
                       Requirement Levels", BCP 14, RFC 2119, 1997.
                    
                    
                       10.2 Informative References
                    
                       [ATSC] A/53, "ATSC Digital Television Standard", Advanced Television
                       Systems Committee (ATSC), Doc. A/53, 1995.
                    
                       [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/65A, "Program and System Information Protocol for
                       Terrestrial Broadcast and Cable", Advanced Television Systems
                       Committee (ATSC), Doc. A/65A, 23 Dec 1997, Rev. A, 2000.
                    
                       [ATSC-S] A/80, "Modulation and Coding Requirements for Digital TV
                       (DTV) Applications  over Satellite", Advanced Television Systems
                       Committee (ATSC), Doc. A/80, 1999.
                    
                       [CLC99] Clausen, H., Linder, H., and Collini-Nocker, B., "Internet
                       over Broadcast Satellites", IEEE Commun. Mag. 1999, pp.146-151.
                    
                       [ETSI-DAT] EN 301 192 "Specifications for Data Broadcasting",
                       European Telecommunications Standards Institute (ETSI).
                    
                       [ETSI-DVBC] EN 300 800 "Digital Video Broadcasting (DVB); DVB
                       interaction channel for Cable TV distribution systems (CATV)",
                       European Telecommunications Standards Institute (ETSI).
                    
                       [ETSI-DVBS] EN 301 421 "Digital Video Broadcasting (DVB); Modulation
                       and Coding for DBS satellite systems at 11/12 GHz", European
                       Telecommunications Standards Institute (ETSI).
                    
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                       [ETSI-DVBT] EN 300 744 "Digital Video Broadcasting (DVB); Framing
                       structure, channel coding and modulation for digital terrestrial
                       television (DVB-T)", European Telecommunications Standards Institute
                       (ETSI).
                    
                       [ETSI-RCS] ETSI 301 791 "Digital Video Broadcasting (DVB);
                       Interaction Channel for Satellite Distribution Systems", European
                       Telecommunications Standards Institute (ETSI).
                    
                       [ISO-DSMCC] ISO/IEC IS 13818-6 "Information technology -- Generic
                       coding of moving pictures and associated audio information -- Part
                       6: Extensions for DSM-CC is a full software implementation",
                       International Standards Organisation (ISO).
                    
                       [ITU-I363] ITU-T I.363.5 B-ISDN ATM Adaptation Layer Specification
                       Type AAL5, International Standards Organisation (ISO), 1996.
                    
                       [LLC] "IEEE Logical Link Control" (ANSI/IEEE Std 802.2/ ISO 8802.2),
                       1985.
                    
                       [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.
                    
                    
                    
                       [ID-ipdvb-arch] "Requirements for transmission of IP datagrams over
                       MPEG-2 networks", Internet Draft, Work in Progress.
                    
                    11. 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.cosy.sbg.ac.at/sc/
                    
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                    Full Copyright Statement
                    
                       "Copyright (C) The Internet Society (date). All Rights Reserved.
                       This document and translations of it may be copied and furnished to
                       others, and derivative works that comment on or otherwise explain it
                       or assist in its implementation may be prepared, copied, published
                       and distributed, in whole or in part, without restriction of any
                       kind, provided that the above copyright notice and this paragraph
                       are included on all such copies and derivative works. However, this
                       document itself may not be modified in any way, such as by removing
                       the copyright notice or references to the Internet Society or other
                       Internet organizations, except as needed for the purpose of
                       developing Internet standards in which case the procedures for
                       copyrights defined in the Internet Standards process must be
                       followed, or as required to translate it into languages other than
                       English.
                    
                       The limited permissions granted above are perpetual and will not be
                       revoked by the Internet Society or its successors or assigns.
                    
                    
                    12. IANA Considerations
                    
                       This document will require IANA involvement.
                    
                       The ULE type field defined in this document requires a registry.
                       This registry allocates values 0-1499 (decimal). It MUST NOT
                       allocate values greater than or equal to 1536 (decimal), since such
                       values overlap the assignments made in the IANA Ethertypes registry.
                    
                    
                       The following values need to be assigned by the IANA:
                    
                            ULE Type Field
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
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                       ANNEXE A: Informative Appendix
                    
                       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-MPEG].  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 destination MAC address)
                         SNDU B is 200 bytes (including destination MAC 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 destination MAC address)
                         SNDU B is 52 bytes (no destination MAC address)
                         SNDU C is 52 bytes (no destination MAC 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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                       ANNEXE 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 Present)
                       ULE Protocol Type :           0x86dd (IPv6)
                       Destination ULE NPA Address:  01:02:03:04:05:06
                       ULE CRC32 :                   0x784679a5
                    
                       Source IPv6:                  2001:660:3008:1789::5
                       Destination IPv6:             2001:660:3008:1789::6
                    
                       SNDU contents (including CRC-32):
                    
                       0000:  00 3f 86 dd 01 02 03 04 05 06 60 00 00 00 00 0d
                       0010:  3a 40 20 01 06 60 30 08 17 89 00 00 00 00 00 00
                       0020:  00 05 20 01 06 60 30 08 17 89 00 00 00 00 00 00
                       0030:  00 06 80 00 9d 8c 06 38 00 04 00 00 00 00 00 78
                       0040:  46 79 a5
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
                    
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