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Versions: 00 01 02 draft-ietf-ipdvb-ule

     Internet Engineering Task Force                         Gorry Fairhurst
     Internet Draft                             University of Aberdeen, U.K.
     Document: draft-fair-ipdvb-ule-02.txt           Bernhard Collini-Nocker
                                                   University of Salzburg, A
     
     
     
     
     Category: Draft Intended Standards Track                 November 2003
     
     
             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 - Hilamr & 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 -
     
        * Tidied diagrams at end (again) - Gorry.
          Authors last call unitl 22-nov-03 - no new comments.
     
        [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
            Appendix A.
     
     
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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].  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 typically
        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.
     
        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]
     
        NPA: Network Point of Attachment. In this document, refers to a 6 B
        destination address within the MPEG-2 transmission network used to
        identify individual Receivers or groups of Receivers.
     
        PDU: Protocol Data Unit. Examples of PDU include Ethernet frames,
        IPv4 or IPv6 datagrams, and other network packets
     
        PES: Programme Elementary Scheme of MPEG-2 [ISO-MPEG].
     
     
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        PID: Packet Identifier. A 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: Programme SI. An table 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.
     
        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 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
     
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        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]. Operation resembles that of cell in  an ATM network, and
        may also be referred to as a TS_Cell.  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.
     
     
     
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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. If required,
        a 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 differently for PES and PSI packets [ISO-
        MPEG]; for private data, its use is not defined in the MPEG-2
        Standard. In ULE, 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 convenient 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 or Stuffing.
     
        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
        carry 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 has the value 0xFFFF, this
        denotes an End Indicator (i.e., all 1√ěs length combined with a D-bit
        value of 1). It indicates to the Receiver that there are no further
        SNDU are 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].
     
     
        4.4 Type Field
     
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        The 16-bit Type field indicates the type of payload carried in a
        SNDU. 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 or equal to 1500 (0x05FC), assumed the
        role of a length indicator. Ethernet receivers use this feature to
        discriminate LLC format frames. Hence any IEEE Ethertype <= 1500
        indicates an LLC frame, and the actual value indicates the length of
        the LLC frame.
     
        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. Specification of two
        independent length fields is therefore undesirable.  In the ULE
        header, this 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 <=
        1500.  These Type Field values are IANA assigned (see 4.4.1).
     
        The second set of ULE Type Field values comprise the set of values >
        1500. 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: IANA Assigned Type Fields
     
        The first part of the Type space corresponds to the values 0x0000 to
        1500 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.
        0x0001: Bridged Ethernet Frame (i.e. MAC source address follows)
     
        [XXX END OF IANA ACTION REQUIRED XXX]
     
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        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 1500
        Decimal and 0xFFFF.  This set of type assignments follow DIX/IEEE
        assignments (but exclude use of this field as a frame length
        indicator) [LLC]. The following types are defined in this document
        for part 2:
     
        0x0800 : IPv4 Payload (according to IANA EtherTypes)
        0x86DD : IPv6 Payload (according to IANA EtherTypes)
     
        All other assignments in part two of this space should be
        coordinated with the values defined for IANA EtherType
        encapsulations.
     
     
        4.5 SNDU Destination Address Field
     
        The SNDU Destination Address Field is optional (see section 4.1).
        This field MUST be carried for IP unicast packets destined to
        routers(i.e. D=0). 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 B 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.
     
     
        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. This is a 32 bit value calculated
     
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        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.
     
        Examples where this polynomial is also employed include Ethernet,
        DSM-CC section syntax [ISO-DSMCC} and AAL5 [ITU3563]. The use
        resembles, but is different to that in SCTP [RFC3309].
     
        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), and places this
        in the CRC Field.  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.
     
        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, in
        some cases, these may also be detected by other means).
     
     
        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:
     
     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 of bytes >= 0 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 (ignored by Receivers)                 =
           |                                                               |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |                             (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)
     
        The MAC addresses are those specified in the frame being bridged and
        are 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 frame
        is defined according to Ethernet/LLC [LLC].
     
        A frame type <1500 for a bridged frames, 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 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
        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
     
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        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.  This value MUST
        be incremented by one (using modulo arithmetic) for each TS Packet
        sent using a TS Logical Channel [ISO-MPEG].
     
        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.
     
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                      +-/------------+
                      |  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.
     
     
        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
     
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        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
        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 MUST be bounded and SHOULD be configurable by the user. If no
        additional PDUs are received after this period of time, it MUST
        insert an End Indicator instead (using rule iv).
     
     
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        Use of the Packing method (v) by an Encapsulation Gateway 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.
     
     
     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 is 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 182, 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 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. For the first TS Packet received, the Payload
        Pointer will also have a value of 0.  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 SHOULD
        be recorded as an SNDU length error.
     
        If the Length of the Current SNDU is greater than 4, it then 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.
        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 ant Type 1 extensions specified). The SNDU payload is
        then passed to the next protocol layer specified. An SNDU with an
        unknown Type value 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 received TS
        Packets. 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
        one, a transmission error event SHOULD be recorded. Any partially
     
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        received SNDU MUST be discarded. The Receiver then enters the Idle
        State.
     
        The Receiver MAY also check the MPEG-2 Continuity Counter carried in
        the TS Packet header. If the Receiver does perform Continuity
        Counter checking and the received value 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.
     
     
     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
     
        There is a known security issue with un-initialised stuffing bytes.
        In ULE, these bytes are set to 0xFF.
     
        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 the ULE header, this
        avoided by including only one SNDU length value.  However, this
        issue still arises in bridged LLC frames, and frames with a LLC
        Length greater than the SNDU payload size MUST be discarded.
     
     
     
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     10. References
     
     
        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).
     
     
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        [ETSI-DVBS] EN 301 421 "Digital Video Broadcasting (DVB); Modulation
        and Coding for DBS satellite systems at 11/12 GHz", European
        Telecommunications Standards Institute (ETSI).
     
        [ETSI-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.
     
        [SI-DAT] SI-DAT Group, "Second Draft DVB Specification for Data
        Broadcasting", Geneva, 1997.
     
     
     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
        Institute of Computer Sciences
        University of Salzburg
        Jakob Haringer Str. 2
        5020 Salzburg
        Austria
     
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     INTERNET DRAFT  Encapsulation for IP over MPEG-2/DVB     November 2003
     
     
        Email: [bnocker]@cosy.sbg.ac.at
        Web: http://www.cosy.sbg.ac.at/cs/
     
     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 payload type field defined in this document must be aligned with
        an existing IANA registry or the following values need to be
        assigned by the IANA:
     
             Payload 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 | 0xC8 | ... | A182 |
         +-----+----*-+-*----+------+-   -+------+
         PUSI=1     *   *
                    *****
                                               SNDU
                 PP=16           CRC for A    Length
         +-----+------+------+-   -+--- --+------+------+-   -+------+
         | HDR | 0x10 | A183 | ... | A199 | 0x00 | 0xC0 | ... | 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 | A000 | A001 | ... | A182 |
          +-----+----*-+-*----+------+-   -+------+
          PUSI=1     *   *
                     *****
                             SNDU                  Unused
                  PP=0      Length       CRC for B  byte
          +-----+------+------+------+-   -+------+------+
          | HDR | 0x00 | B000 | B001 | ... | B181 | 0xFF |
          +-----+---*--+-*----+------+-   -+------+------+
          PUSI=1    *    *
                    ******
                             SNDU                      SNDU
                  PP=0      Length      CRC for C     Length
          +-----+------+------+------+-   -+------+------+------+
          | HDR | 0x00 | C000 | C001 | ... | C180 | D000 | D001 |
          +-----+---*--+-*----+------+-   -+------+------+------+
          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 | A000 | A000 | ... | A182 |
          +-----+---*--+-*----+------+-   -+------+
          PUSI=1    *    *
                    ******
     
          +-----+------+-   -+------+
          | HDR | A183 | ... | A366 |
          +-----+------+-   -+------+
          PUSI=0
     
     
          +-----+------+-   -+------+
          | HDR | A367 | ... | A550 |
          +-----+------+-   -+------+
          PUSI=0
     
                                                SNDU
                  PP=181         CRC for A     Length
          +-----+------+------+-   -+------+------+------+
          | HDR | 0xB5 | A551 | ... | A731 | B000 | B001 |
          +-----+---*--+------+-   -+------+*-----+------+
          PUSI=1    *                       *
                    *************************
     
          +-----+------+-   -+------+
          | HDR | B002 | ... | B186 |
          +-----+------+-   -+------+
          PUSI=0
     
                                          End          Stuffing
                                       Indicator        Bytes
          +-----+------+-   -+------+------+------+-   -+------+
          | HDR | B187 | ... | B283 | OxFF | 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 | A000 | A001 | ... | A182 |
          +-----+----*-+-*----+------+-   -+------+
          PUSI=1     *   *  +      +
                     *****  ++++++++
                             +
                             +++++++++++++++++
                                             +   SNDU
                 PP=17            CRC for A  +  Length
          +-----+------+------+-   -+------+-+----+------+-
          | HDR | 0x11 | A183 | ... | A199 | B000 | B001 | ...
          +-----+----*-+------+-   -+------+*-----+------+-
          PUSI=1     *                      *  +       +
                     ************************  +++++++++
                                                +
          +++++++++++++++++++++++++++++++++++++++
          +
          +                  SNDU                       End      Stuffing
          +                 Length                   Indicator     bytes
          +    -+------+------+------+  -+------+------+------+- -+------+
          + ... | B059 | C000 | C001 |...| C059 | 0xFF | 0xFF |...| 0xFF |
          +    -+------+-+----+------+  -+------+-+----+------+- -+------+
          +                +  +      +                +
          +                +  ++++++++                +
          +                +   +                      +
          ++++++++++++++++++   ++++++++++++++++++++++++
     
        *** TS Packet Payload Pointer (PP)
        +++ ULE Length Indicator
     
     
     
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