





Network Working Group                                       G. Fairhurst
Request for Comments: 4326                        University of Aberdeen
Category: Standards Track                              B. Collini-Nocker
                                                  University of Salzburg
                                                           December 2005


           Unidirectional Lightweight Encapsulation (ULE) for
   Transmission of IP Datagrams over an MPEG-2 Transport Stream (TS)

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   The MPEG-2 Transport Stream (TS) has been widely accepted not only
   for providing digital TV services, but also as a subnetwork
   technology for building IP networks.

   This document describes a Unidirectional Lightweight Encapsulation
   (ULE) mechanism for the transport of IPv4 and IPv6 Datagrams and
   other network protocol packets directly over the ISO MPEG-2 Transport
   Stream as TS Private Data.  ULE specifies a base encapsulation format
   and supports an extension format that allows it to carry additional
   header information to assist in network/Receiver processing.

















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Table of Contents

   1. Introduction ....................................................3
   2. Conventions Used in This Document ...............................4
   3. Description of the Method .......................................8
   4. SNDU Format .....................................................9
      4.1. Destination Address Absent (D) Field ......................10
      4.2. Length Field ..............................................10
      4.3. End Indicator .............................................10
      4.4. Type Field ................................................10
           4.4.1. Type 1: Next-Header Type Fields ....................11
           4.4.2. Type 2: EtherType Compatible Type Fields ...........11
      4.5. SNDU Destination Address Field ............................12
      4.6. SNDU Trailer CRC ..........................................12
      4.7. Description of SNDU Formats ...............................13
           4.7.1. End Indicator ......................................14
           4.7.2. IPv4 SNDU Encapsulation ............................14
           4.7.3. IPv6 SNDU Encapsulation ............................15
   5. Extension Headers ..............................................16
      5.1. Test SNDU .................................................18
      5.2. Bridged Frame SNDU Encapsulation ..........................18
      5.3. Extension-Padding Optional Extension Header ...............21
   6. Processing at the Encapsulator .................................22
      6.1. SNDU Encapsulation ........................................22
      6.2. Procedure for Padding and Packing .........................24
   7. Receiver Processing ............................................25
      7.1. Idle State ................................................26
           7.1.1. Idle State Payload Pointer Checking ................26
      7.2. Processing of a Received SNDU .............................26
           7.2.1. Reassembly Payload Pointer Checking ................28
      7.3. Other Error Conditions ....................................28
   8. Summary ........................................................29
   9. Acknowledgements ...............................................29
   10. Security Considerations .......................................29
   11. IANA Considerations ...........................................30
      11.1. IANA Guidelines ..........................................30
   12. References ....................................................31
      12.1. Normative References .....................................31
      12.2. Informative References ...................................32
   Appendix A. SNDU Packing Examples .................................35
   Appendix B. SNDU Encapsulation ....................................40










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1.  Introduction

   This document describes an encapsulation for the transport of IP
   datagrams, or other network-layer packets, over ISO MPEG-2 Transport
   Streams [ISO-MPEG2, RFC4259].  The encapsulation satisfies the
   requirement for a lightweight encapsulation defined in section 4 of
   [RFC4259].  The basic header provides the required set of protocol
   fields.  Extension headers may also be defined.  This header
   structure is significantly simpler to parse and process [SOOR05] than
   current alternative methods (e.g., MPE [ETSI-DAT], which builds upon
   the DSM-CC Table Section syntax [ISO-DSMCC]).

   The encapsulation is suited to services based on MPEG-2; for example,
   the Digital Video Broadcast (DVB) architecture, the Advanced
   Television Systems Committee (ATSC) system [ATSC, ATSC-G], and other
   similar MPEG-2-based transmission systems.  Such systems provide
   unidirectional (simplex) physical and link-layer standards.  Support
   has been defined for a wide range of physical media (e.g.,
   Terrestrial TV [ETSI-DVBT, ATSC-PSIP-TC], Satellite TV [ETSI-DVBS,
   ATSC-S], and 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), such as Ethernet Frames, IP datagrams, or
   other network-layer packets, used for transmission over an MPEG-2
   Transport Multiplex are passed to an Encapsulator.  This formats each
   PDU into a SubNetwork Data Unit (SNDU) by adding an encapsulation
   header and an integrity check trailer.  The SNDU is fragmented into a
   series of one or more MPEG-2 Transport Stream (TS) Packets that are
   sent over a single TS Logical Channel.

   The MPEG-2 specification [ISO-MPEG2] requires that conformant TS
   Multiplexes provide Program Specific Information (PSI) for each
   stream in the TS Multiplex.  Other MPEG-2-based transmission
   standards may also define Service Information (SI).

   A format_identifier value has been registered for ULE [ULE1].  This
   32 bit number has a hexadecimal value of 0x554C4531.  Transport
   Streams that utilise the Programme Map Table (PMT) defined in ISO
   13818-1 [ISO-MPEG2] and that use the ULE format defined in this
   document, SHOULD insert a descriptor with this value in the PMT
   ES_info descriptor loop.  ULE Streams may also be identified by the
   stream_type value of 0x91 [ATSC-REG] in a SI/PSI Table [ISO_MPEG2].

   This information may allow Receivers and Re-multiplexors [RFC4259] to
   locate a specific ULE Stream (i.e., the PID value of the TS Logical



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   Channel that carries a ULE Stream).  The conditions under which this
   information is required and the format in which it is to be provided
   are beyond the scope of this document.  Addressing and mapping issues
   for ULE over MPEG-2 are also described in [IPDVB-AR].

2.  Conventions Used in This Document

   The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
   "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

   Other terms used in this document are defined below:

   Adaptation Field: An optional variable-length extension field of the
   fixed-length TS Packet header, intended to convey clock references
   and timing and synchronization information as well as stuffing over
   an MPEG-2 Multiplex [ISO-MPEG2].

   AFC: Adaptation Field Control [ISO-MPEG2].  A pair of bits carried in
   the TS Packet header that signal the presence of the Adaptation Field
   and/or TS Packet payload.

   ATSC: Advanced Television Systems Committee [ATSC].  A framework and
   a set of associated standards for the transmission of video, audio,
   and data using the ISO MPEG-2 standard.

   b: bit.  For example, one byte consists of 8b.

   B: Byte.  Groups of bytes are represented in Internet byte order.

   DSM-CC: Digital Storage Media Command and Control [ISO-DSMCC].  A
   format for transmission of data and control information in an MPEG-2
   Private Section, defined by the ISO MPEG-2 standard.

   DVB: Digital Video Broadcast.  A framework and set of associated
   standards published by the European Telecommunications Standards
   Institute (ETSI) (e.g., [ETSI-DVBC, ETSI-DVBS, ETSI-DVBT]) for the
   transmission of video, audio, and data using the ISO MPEG-2 Standard
   [ISO-MPEG2].

   Encapsulator: A network device that receives PDUs and formats these
   into Payload Units (known here as SNDUs) for output as a stream of TS
   Packets.

   End Indicator: A value that indicates to the Receiver that there are
   no further SNDUs present within the current TS Packet.




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   LLC: Logical Link Control [ISO-8802-2, IEEE-802.2].  A link-layer
   protocol defined by the IEEE 802 standard, which follows the Ethernet
   MAC Header.

   MAC: Medium Access Control [IEEE-802.3].  A link-layer protocol
   defined by the IEEE 802.3 standard (or by Ethernet v2 [DIX]).

   MAC Header: The link-layer header of the IEEE 802.3 standard
   [IEEE-802.3] or Ethernet v2 [DIX].  It consists of a 6B destination
   address, 6B source address, and 2B Type field (see also NPA, LLC).

   MPE: Multiprotocol Encapsulation [ETSI-DAT, ATSC-DAT, ATSC-DATG].  A
   scheme that encapsulates PDUs, forming a DSM-CC Table Section.  Each
   Section is sent in a series of TS Packets using a single TS Logical
   Channel.

   MPEG-2: A set of standards specified by the Motion Picture Experts
   Group (MPEG) and standardized by the International Standards
   Organisation (ISO/IEC 13818-1) [ISO-MPEG2], and ITU-T (in H.222
   [ITU-H222]).

   Next-Header: A Type value indicating an Extension Header.

   NPA: Network Point of Attachment.  In this document, refers to a
   6-byte destination address (resembling an IEEE MAC address) within
   the MPEG-2 transmission network that is used to identify individual
   Receivers or groups of Receivers.

   Packing Threshold: A period of time an Encapsulator is willing to
   defer transmission of a partially filled TS-Packet to accumulate more
   SNDUs, rather than use Padding.  After the Packet Threshold period,
   the Encapsulator uses Padding to send the partially filled TS-Packet.

   Padding: A method that fills the remaining unused bytes in a TS
   Packet payload using the specific pattern of 0xFF.

   Payload Unit, PU.  A sequence of bytes sent using a TS.  Examples of
   Payload Units include: an MPEG-2 Table Section or a ULE SNDU.

   PDU: Protocol Data Unit.  Examples of a PDU include Ethernet frames,
   IPv4 or IPv6 datagrams, and other network packets.

   PES: Packetized Elementary Steam [ISO-MPEG2].  A format of MPEG-2 TS
   packet payload usually used for video or audio information.

   PID: Packet Identifier  [ISO-MPEG2].  A 13-bit field carried in the
   header of TS Packets.  This is used to identify the TS Logical
   Channel to which a TS Packet belongs [ISO-MPEG2].  The TS Packets



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   forming the parts of a Table Section, PES, or other Payload Unit must
   all carry the same PID value.  The all-zeros PID 0x0000 as well as
   other PID values are reserved for specific PSI/SI Tables [ISO-MPEG2].
   The all-ones PID value 0x1FFF indicates a Null TS Packet introduced
   to maintain a constant bit rate of a TS Multiplex.  There is no
   required relationship between the PID values used for TS Logical
   Channels transmitted using different TS Multiplexes.

   PP: Payload Pointer [ISO-MPEG2].  An optional one-byte pointer that
   directly follows the 4-byte TS Packet header.  It contains the number
   of bytes that follow the Payload Pointer, up to the start of the
   first Payload Unit (counted from the first byte of the TS Packet
   payload field, and excluding the PP field itself).  The presence of
   the Payload Pointer is indicated by the value of the PUSI bit in the
   TS Packet header.  The Payload Pointer is present in DSM-CC, Table
   Sections, and ULE.  It is not present in TS Logical Channels that use
   the PES-format.

   Private Section: A syntactic structure constructed in accordance with
   Table 2-30 of [ISO-MPEG2].  The structure may be used to identify
   private information (i.e., not defined by [ISO-MPEG2]) relating to
   one or more elementary streams, or a specific MPEG-2 program, or the
   entire Transport Stream.  Other Standards bodies, e.g., ETSI, ATSC,
   have defined sets of table structures using the private_section
   structure.  A Private Section is transmitted as a sequence of TS
   Packets using a TS Logical Channel.  A TS Logical Channel may carry
   sections from more than one set of tables.

   PSI: Program Specific Information [ISO-MPEG2].  Tables used to convey
   information about the service carried in a TS Multiplex.  The
   information is carried in one of four specifically identified Table
   Sections defined by MPEG-2 [ISO-MPEG2].  See also SI Table.

   PU: Payload Unit.

   PUSI: Payload_Unit_Start_Indicator [ISO-MPEG2].  A single-bit flag
   carried in the TS Packet header.  A PUSI value of zero indicates that
   the TS Packet does not carry the start of a new Payload Unit.  A PUSI
   value of one indicates that the TS Packet does carry the start of a
   new Payload Unit.  In ULE, a PUSI bit set to 1 also indicates the
   presence of a one-byte Payload Pointer (PP).

   Receiver: Equipment that processes the signal from a TS Multiplex and
   performs filtering and forwarding of encapsulated PDUs to the
   network-layer service (or bridging module when operating at the link
   layer).





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   SI Table: Service Information Table [ISO-MPEG2].  In this document,
   this term describes a table that is defined by another standards body
   to convey information about the services carried in a TS Multiplex.
   A Table may consist of one or more Table Sections; however, all
   sections of a particular SI Table must be carried over a single TS
   Logical Channel [ISO-MPEG2].

   SNDU: SubNetwork Data Unit.  An encapsulated PDU sent as an MPEG-2
   Payload Unit.

   Table Section: A Payload Unit carrying all or part of an SI or PSI
   Table [ISO-MPEG2].

   TS: Transport Stream [ISO-MPEG2], a method of transmission at the
   MPEG-2 level using TS Packets; it represents layer 2 of the ISO/OSI
   reference model.  See also TS Logical Channel and TS Multiplex.

   TS Header: The 4-byte header of a TS Packet [ISO-MPEG2].  Each 188B
   TS Packet incorporates a 4B header with the following fields (those
   referenced within this document are marked with *):

        Field Length            Name/Purpose
         (in bits)

         8b             Synchronisation pattern equal to 0x47
        *1b             Transport Error Indicator
        *1b             Payload Unit Start Indicator (PUSI)
         1b             Transport Priority
        *13b            Packet IDentifier (PID)
         2b             Transport Scrambling Control
        *2b             Adaptation Field Control (AFC)
        *4b             Continuity Counter (CC)

   If the PUSI bit is set to a value of 1, there is one
   additional field following the TS packet header:

        *8b             Payload Pointer (PP)

   TS Logical Channel: Transport Stream Logical Channel.  In this
   document, this term identifies a channel at the MPEG-2 level
   [ISO-MPEG2].  It exists at level 2 of the ISO/OSI reference model.
   All packets sent over a TS Logical Channel carry the same PID value
   (this value is unique within a specific TS Multiplex).  The term
   "Stream" is defined in MPEG-2 [ISO-MPEG2] to describe the content
   carried by a specific TS Logical Channel (see ULE Stream).  Some PID
   values are reserved (by MPEG-2) for specific signalling.  Other
   standards (e.g., ATSC, DVB) also reserve specific PID values.




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   TS Multiplex: In this document, this term defines a set of MPEG-2 TS
   Logical Channels sent over a single lower-layer connection.  This may
   be a common physical link (i.e., a transmission at a specified symbol
   rate, FEC setting, and transmission frequency) or an encapsulation
   provided by another protocol layer (e.g., Ethernet, or RTP over IP).
   The same TS Logical Channel may be repeated over more than one TS
   Multiplex (possibly associated with a different PID value) [RFC4259];
   for example, to redistribute the same multicast content to two
   terrestrial TV transmission cells.

   TS Packet: A fixed-length 188B unit of data sent over a TS Multiplex
   [ISO-MPEG2].  Each TS Packet carries a 4B header, plus optional
   overhead including an Adaptation Field, encryption details, and time
   stamp information to synchronise a set of related TS Logical
   Channels.

   ULE Stream: An MPEG-2 TS Logical Channel that carries only ULE
   encapsulated PDUs.  ULE Streams may be identified by definition of a
   stream_type in SI/PSI [ISO-MPEG2].

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 either by
   being placed in the payload of a single TS Packet, or, if required,
   by being 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 thereof), a practice sometimes known as
   Packing.  All TS Packets comprising an SNDU MUST be assigned the same
   PID, and therefore form a part of the same TS Logical Channel.

   The ULE encapsulation is limited to TS private streams only.  The
   header of each TS Packet carries a one-bit Payload Unit Start
   Indicator (PUSI) field.  A PUSI field with a value of 1 indicates the
   start of at least one Payload Unit (SNDU) within the TS Packet
   payload.  The semantics of the PUSI bit are defined for PES and PSI
   packets [ISO-MPEG2]; for private data, its use is not defined in the
   MPEG-2 Standard.  Although ULE uses private data, the operation
   follows that of PSI packets.  Hence, the following PUSI values are
   defined:

        0: The TS Packet does NOT contain the start of an SNDU, but
        contains the continuation, or end, of an SNDU;

        1: The TS Packet contains the start of an SNDU, and a one byte
        Payload Pointer follows the last byte of the TS Packet header.




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   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 (known as Padding) fills the remainder of the TS
   Packet payload with bytes with a value 0xFF [ISO-MPEG2].

   A Receiver processing MPEG-2 Table Sections that receives a value of
   0xFF in the first byte of a Table Section (table_Id) interprets this
   as Padding/Stuffing and silently discards the remainder of the TS
   Packet payload.  The payload of the next TS Packet for the same TS
   Logical Channel will begin with a Payload Pointer of value 0x00,
   indicating that the next Payload Unit immediately follows the TS
   Packet header.  The ULE protocol resembles this, but differs in the
   exact procedure (see the following sections).

   The TS Packet Header also carries a two-bit Adaptation Field Control
   (AFC) value.  This adaptation field may extend the TS Packet Header
   to carry timing and synchronisation information and may also be used
   to include stuffing bytes before a TS Packet payload.  Adaptation
   Field stuffing is NOT used in this encapsulation method, and TS
   Packets from a ULE Encapsulator MUST be sent with an AFC value of
   '01'.  For TS Logical Channels supporting ULE, Receivers MUST discard
   TS Packets that carry other AFC values.

4.  SNDU Format

   PDUs are encapsulated using ULE to form an SNDU.  (Each SNDU is an
   MPEG-2 Payload Unit.) The encapsulation format to be used for PDUs is
   illustrated below:

   < ----------------------------- SNDU ----------------------------- >
   +-+-------------------------------------------------------+--------+
   |D| Length | Type | Dest Address* |           PDU         | CRC-32 |
   +-+-------------------------------------------------------+--------+

       Figure 1: SNDU Encapsulation (* optional Destination Address)

   All multi-byte values in ULE (including the Length/End Indicator
   (4.2,4.3), Type (4.4), Destination Address (4.5), and Extension
   Headers (5)) are transmitted in network byte order (most significant
   byte first).  The most significant bit of each byte is placed in the
   left-most position of the 8-bit field.  Appendix A provides
   informative examples of usage.









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4.1.  Destination Address Absent (D) Field

   The most significant bit of the Length field carries the value of the
   Destination Address Absent Field, the D-bit.  A value of 0 indicates
   the presence of the Destination Address Field (see section 4.5).  A
   value of 1 indicates that a Destination Address Field is not present.

   An End Indicator (4.3) MUST be sent with a D-bit value of 1.  Other
   SNDUs MAY be sent with a D-bit value of 0 or 1.  The default method
   SHOULD use a D-bit value of 0 (see section 4.5).

4.2.  Length Field

   A 15-bit value that indicates the length, in bytes, of the SNDU
   counted from the byte following the Type field of the SNDU base
   header (figure 9) up to and including the CRC.  This Length includes
   the size of any extension headers that may be present (section 5).
   Note the special case described in section 4.3.

4.3.  End Indicator

   When the first two bytes following an SNDU have the value 0xFFFF,
   this denotes an End Indicator (i.e., all ones length combined with a
   D-bit value of 1).  This indicates to the Receiver that there are no
   further SNDUs present within the current TS Packet (see section 6),
   and that no Destination Address Field is present.  The value 0xFF has
   specific semantics in MPEG-2 framing, where it is used to indicate
   the presence of Padding.  This use resembles [ISO-DSMCC].

4.4.  Type Field

   The 16-bit Type field indicates the type of payload carried in an
   SNDU, or the presence of a Next-Header.  The set of values that may
   be assigned to this field is divided into two parts, similar to the
   allocations for Ethernet.

   EtherTypes were originally specified by Xerox under the Ethernet v2
   Specification  [DIX].  After specification of IEEE 802.3 [IEEE-802.3,
   ISO-8802-2], the set of EtherTypes less than 1536 (0x0600) assumed
   the role of a length indicator.  Ethernet receivers use this feature
   to discriminate LLC format frames.  Hence, any IEEE EtherType < 1536
   indicates an LLC frame, and the actual value indicates the length of
   the LLC frame.

   There is a potential ambiguous case when a Receiver receives a PDU
   with two Length fields:  The Receiver would need to validate the
   actual length and the Length field and ensure that inconsistent
   values are not propagated by the network.  Specification of two



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   independent Length fields is therefore undesirable.  In the ULE
   header, this is avoided in the SNDU header by including only one
   length value, but bridging of LLC frames re-introduces this
   consideration (section 5.2).

   The Ethernet LLC mode of identification is not required in ULE, since
   the SNDU format always carries an explicit Length field, and
   therefore the procedure in ULE is modified, as below:

   The first set of ULE Type field values comprise the set of values
   less than 1536 in decimal.  These Type field values are IANA assigned
   (see section 4.4.1) and indicate the Next-Header.

   The second set of ULE Type field values comprise the set of values
   greater than or equal to 1536 in decimal.  In ULE, this value is
   identical to the corresponding type codes specified by the IEEE/DIX
   type assignments for Ethernet and recorded in the IANA EtherType
   registry.

4.4.1.  Type 1: Next-Header Type Fields

   The first part of the Type space corresponds to the values 0 to 1535
   decimal.  These values may be used to identify link-specific
   protocols and/or to indicate the presence of Extension Headers that
   carry additional optional protocol fields (e.g., a bridging
   encapsulation).  Use of these values is co-ordinated by an IANA
   registry.  The following types are defined in this document:

           0x0000: Test SNDU (see section 5.1)
           0x0001: Bridged Frame (see section 5.2)
           0x0100: Extension-Padding (see section 5.3)


   The remaining values within the first part of the Type space are
   reserved for Next-Header values allocated by the IANA.

4.4.2.  Type 2: EtherType Compatible Type Fields

   The second part of the Type space corresponds to the values between
   0x600 (1536 decimal) and 0xFFFF.  This set of type assignments
   follows DIX/IEEE assignments (but excludes use of this field as a
   frame length indicator).  All assignments in this space MUST use the
   values defined for IANA EtherType.  The following two Type values are
   used as examples (taken from the IANA EtherTypes registry):

           0x0800: IPv4 Payload (see section 4.7.2)
           0x86DD: IPv6 Payload (see section 4.7.3)




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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, or a bridged MAC destination address),
   which, in combination with the PID value, could be interpreted as a
   Link-Level address.

   When the SNDU header indicates the presence of an SNDU Destination
   Address field (i.e., D=0), a Network Point of Attachment (NPA) field
   directly follows the fourth byte of the SNDU header.  NPA destination
   addresses are 6 Byte numbers, normally expressed in hexadecimal, used
   to identify the Receiver(s) in a MPEG-2 transmission network that
   should process a received SNDU.  The value 0x00:00:00:00:00:00 MUST
   NOT be used as a destination address in an SNDU.  The least
   significant bit of the first byte of the address is set to 1 for
   multicast frames, and the remaining bytes specify the link-layer
   multicast address.  The specific value 0xFF:FF:FF:FF:FF:FF is the
   link broadcast address, indicating that this SNDU is to be delivered
   to all Receivers.

   IPv4 packets carrying an IPv4 subnetwork broadcast address need to be
   delivered to all systems with the same network prefix.  When a SNDU
   Destination Address is present (D=0), the value MUST be set to the
   NPA link broadcast address (0xFF:FF:FF:FF:FF:FF).

   When the PDU is an IP multicast packet and an SNDU Destination
   Address is present (D=0), the IP group destination address of the
   multicast packet MUST be mapped to the multicast SNDU Destination
   Address (following the method used to generate a destination MAC
   address in Ethernet).  The method for mapping IPv4 multicast
   addresses is specified in [RFC1112].  The method for mapping IPv6
   multicast addresses is specified in [RFC2464].

4.6.  SNDU Trailer CRC

   Each SNDU MUST carry a 32-bit CRC field in the last four bytes of the
   SNDU.  This position eases CRC computation by hardware.  The CRC-32
   polynomial is to be used.  Examples where this polynomial is also
   employed include Ethernet, DSM-CC section syntax [ISO-DSMCC], and
   AAL5 [ITU-3563].  This is a 32-bit value calculated according to the
   generator polynomial represented 0x104C11DB7 in hexadecimal:

   x^32+x^26+x^23+x^22+x^16+x^12+x^11+x^10+x^8+x^7+x^5+x^4+x^2+x^1+x^0.



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   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 from 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.  Appendix 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
   Encapsulator and/or the Receiver.  It may also detect the presence of
   uncorrected errors from the physical link (however, these may also be
   detected by other means, e.g., section 7.3).

4.7.  Description of SNDU Formats

   The format of an SNDU is determined by the combination of the
   Destination Address Absent bit (D) and the SNDU Type field.  The
   simplest encapsulation places a PDU directly into an SNDU payload.
   Some Type 1 encapsulations may require additional header fields.
   These are inserted in the SNDU following the NPA destination address
   and directly preceding the PDU.

   The following SNDU Formats are defined here:

   End Indicator: The Receiver should enter the Idle State (4.7.1).
   IPv4 SNDU: The payload is a complete IPv4 datagram (4.7.2).
   IPv6 SNDU: The payload is a complete IPv6 datagram (4.7.3).
   Test SNDU: The payload will be discarded by the Receiver (5.1).
   Bridged SNDU: The payload carries a bridged MAC frame (5.2).

   Other formats may be defined through relevant assignments in the IEEE
   and IANA registries.




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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           |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
      |                                                               |
      =   A sequence of zero or more bytes with a value 0xFF filling  =
      |           the remainder of the TS Packet Payload              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 2: Format for a ULE End Indicator

4.7.2.  IPv4 SNDU Encapsulation

   IPv4 datagrams are directly transported using one of the two standard
   SNDU structures, in which the PDU is placed directly in the SNDU
   payload.  The two encapsulations are shown in Figures 3 and 4.  (Note
   that in this, and the following figures, the IP datagram payload is
   of variable size and is directly followed by the CRC-32).

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|        Length  (15b)        |         Type = 0x0800         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Receiver Destination NPA Address  (6B)             |
      +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                               |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
      |                                                               |
      =                           IPv4 datagram                       =
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             (CRC-32)                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 3: SNDU Format for an IPv4 Datagram using L2 filtering (D=0)









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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 directly transported using one of the two standard
   SNDU structures, in which the PDU is placed directly in the SNDU
   payload.  The two encapsulations are shown in Figures 5 and 6.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|        Length  (15b)        |         Type = 0x86DD         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Receiver Destination NPA Address  (6B)             |
      +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                               |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
      |                                                               |
      =                           IPv6 datagram                       =
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             (CRC-32)                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 5: SNDU Format for an IPv6 Datagram using L2 filtering (D=0)














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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 = 0x86DD         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      =                           IPv6 datagram                       =
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             (CRC-32)                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 6: SNDU Format for an IPv6 Datagram using L3 filtering (D=1)

5.  Extension Headers

   This section describes an extension format for the ULE encapsulation.
   In ULE, a Type field value less than 1536 decimal indicates an
   Extension Header.  These values are assigned from a separate IANA
   registry defined for ULE.

   The use of a single Type/Next-Header field simplifies processing and
   eliminates the need to maintain multiple IANA registries.  The cost
   is that each Extension Header requires at least 2 bytes.  This is
   justified, on the basis of simplified processing and maintaining a
   simple lightweight header for the common case when no extensions are
   present.

   A ULE Extension Header is identified by a 16-bit value in the Type
   field.  This field is organised as a 5-bit zero prefix, a 3-bit H-LEN
   field, and an 8-bit H-Type field, as follows:

           0                   1
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |0 0 0 0 0|H-LEN|    H-Type     |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 7: Structure of ULE Next-Header Field

   The H-LEN Assignment is described below:

   0    Indicates a Mandatory Extension Header
   1    Indicates an Optional Extension Header of length 2B (Type only)
   2    Indicates an Optional Extension Header of length 4B (Type + 2B)
   3    Indicates an Optional Extension Header of length 6B (Type + 4B)
   4    Indicates an Optional Extension Header of length 8B (Type + 6B)
   5    Indicates an Optional Extension Header of length 10B (Type + 8B)



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   >=6  The combined H-LEN and H-TYPE values indicate the EtherType
        of a PDU that directly follows this Type field.

   The H-LEN value indicates the total number of bytes in an Optional
   Extension Header (including the 2B Type field).

   An H-LEN value of zero indicates a Mandatory Extension Header.  Each
   Mandatory Extension Header has a pre-defined length that is not
   communicated in the H-LEN field.  No additional limit is placed on
   the maximum length of a Mandatory Extension Header.  A Mandatory
   Extension Header MAY modify the format or encoding of the enclosed
   PDU (e.g., to perform encryption and/or compression).

   The H-Type is a one-byte field that is either one of 256 Mandatory
   Header Extensions or one of 256 Optional Header Extensions.  The set
   of currently permitted values for both types of Extension Headers are
   defined by an IANA Registry (section 15).  Registry values for
   Optional Extensions are specified in the form H=1 (i.e., a decimal
   number in the range 256-511), but may be used with an H-Length value
   in the range 1-5 (see example in section 5.3).

   Two examples of Extension Headers are the Test SNDU and the use of
   Extension-Padding.  The Test SNDU Mandatory Extension Header results
   in the entire PDU's being discarded.  The Extension-Padding Optional
   Extension Header results in the following (if any) option header
   being ignored (i.e., a total of H-LEN 16-bit words).

   The general format for an SNDU with Extension Headers is:

   < --------------------------   SNDU   ------------------------- >
   +---+--------------------------------------------------+--------+
   |D=0| Length | T1 | NPA Address | H1 | T2 |    PDU     | CRC-32 |
   +---+--------------------------------------------------+--------+
   < ULE base header >             <  ext 1  >

   Figure 8: SNDU Encapsulation with one Extension Header (for D=0)

   Where:
   D  is the ULE D_bit (in this example D=0; however, NPA addresses may
      also be omitted when using Extension Headers).
   T1 is the base header Type field.  In this case, specifying a
      Next-Header value.
   H1 is a set of fields defined for header type T1.  There may be 0
      or more bytes of information for a specific ULE Extension Header.
   T2 is the Type field of the next header, or an EtherType > 1535 B
      indicating the type of the PDU being carried.





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   < --------------------------   SNDU   ------------------------- >
   +---+---------------------------------------------------+--------+
   |D=1| Length | T1 | H1 | T2 | H2 | T3 |       PDU       | CRC-32 |
   +---+---------------------------------------------------+--------+
   < ULE base header >< ext 1  >< ext 2  >

   Figure 9: SNDU Encapsulation with two Extension Headers (D=1)

   Using this method, several Extension Headers MAY be chained in
   series.  Figure 12 shows an SNDU including two Extension Headers.  In
   the example, the values of T1 and T2 are both less than 1536 decimal.
   Each indicates the presence of an Extension Header, rather than a
   directly following PDU.  T3 has a value > 1535 indicating the
   EtherType of the PDU being carried.  Although an SNDU may contain an
   arbitrary number of consecutive Extension Headers, it is not expected
   that SNDUs will generally carry a large number of extensions.

5.1.  Test SNDU

   A Test SNDU (Figure 10) is a Mandatory Extension Header of Type 1.
   This header must be the final (or only) extension header specified in
   the header chain of an SNDU.  The structure of the Data portion of
   this SNDU is not defined by this document.  Receivers MAY record
   reception in a log file, but MUST then discard any Test SNDUs.  The
   D-bit MAY be set in a TEST SNDU.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |D|        Length  (15b)        |         Type = 0x0000         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      =               Data (not forwarded by a Receiver)              =
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             (CRC-32)                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 10: SNDU Format for a Test SNDU

5.2.  Bridged Frame SNDU Encapsulation

   A bridged SNDU is a Mandatory Extension Header of Type 1.  It MUST be
   the final (or only) extension header specified in the header chain of
   an SNDU.  The payload includes MAC address and EtherType [DIX] or LLC
   Length [ISO-8802-2] fields together with the contents of a bridged
   MAC frame.  The SNDU has the format shown in Figures 11 and 12.




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   When an NPA address is specified (D=0), Receivers MUST discard all
   SNDUs that carry an NPA destination address that does NOT match their
   own NPA address (or a broadcast/multicast address); the payload of
   the remaining SNDUs are processed by the bridging rules that follow.
   An SNDU without an NPA address (D=1) results in a Receiver performing
   bridging processing on the payload of all received SNDUs.

   An Encapsulator MAY also use this encapsulation format to directly
   communicate network protocol packets that require the LLC
   encapsulation [IEEE-802.2, ISO-8802-2].  To do this, it constructs an
   SNDU with a Bridge Extension Header containing the intended
   destination MAC address, the MAC source address of the Encapsulator,
   and the LLC-Length.  The PDU comprises an LLC header followed by the
   required payload.  The Encapsulator MAY choose to suppress the NPA
   address (see 4.5).

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0|        Length  (15b)        |         Type = 0x0001         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Receiver Destination NPA Address  (6B)             |
      +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                               |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
      |                MAC Destination Address  (6B)                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    MAC Source Address  (6B)                   |
      +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                               |   EtherType/LLC-Length (2B)   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      =                 (Contents of bridged MAC frame)               =
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             (CRC-32)                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 11: SNDU Format for a Bridged Payload (D=0)












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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/LLC-Length (2B)   |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
      |                                                               |
      =                 (Contents of bridged MAC frame)               =
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             (CRC-32)                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 12: SNDU Format for a Bridged Payload (D=1)

   The EtherType/LLC-Length field of a frame is defined according to
   IEEE 802.3 [IEEE-802.2] (see section 5).

   In this special case, the Mandatory Extension Header format may be
   interpreted as either an EtherType [DIX] or an LLC Length field,
   specified by IEEE 802 [IEEE-802.3] rather than as a value assigned in
   the ULE Next-Header Registry maintained by the IANA.

   The MAC addresses in the frame being bridged SHOULD be assigned
   according to the rules specified by the IEEE and denote unknown,
   unicast, broadcast, and multicast link addresses.  These MAC
   addresses denote the intended recipient in the destination LAN, and
   therefore have a different function from the NPA addresses carried in
   the SNDU header.

   A frame Type < 1536 for a bridged frame introduces a LLC Length
   field.  The Receiver MUST check this length and discard any frame
   with a length greater than permitted by the SNDU payload size.

   In normal operation, it is expected that any padding appended to the
   Ethernet frame SHOULD be removed prior to forwarding.  This requires
   the sender to be aware of such Ethernet padding (e.g., [DIX,
   IEEE-802.3]).

   Ethernet frames received at the Encapsulator for onward transmission
   over ULE carry a Local Area Network Frame Check sequence (LAN FCS)



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   field (e.g., CRC-32 for Ethernet [DIX, IEEE-802.3]).  The
   Encapsulator MUST check the LAN-FCS value of all frames received,
   prior to further processing.  Frames received with an invalid LAN FCS
   MUST be discarded.  After checking, the LAN FCS is then removed
   (i.e., it is NOT forwarded in the bridged SNDU).  As in other ULE
   frames, the Encapsulator appends a CRC-32 to the transmitted SNDU.
   At the Receiver, an appropriate LAN-FCS field will be appended to the
   bridged frame prior to onward transmission on the Ethernet interface.

   This design is readily implemented using existing network interface
   cards and does not introduce an efficiency cost by
   calculating/verifying two integrity check fields for bridged frames.
   However, it also introduces the possibility that a frame corrupted
   within the processing performed at an Encapsulator and/or Receiver
   may not be detected by the final recipient(s) (i.e., such corruption
   would not normally result in an invalid LAN FCS).

5.3.  Extension-Padding Optional Extension Header

   The Extension-Padding Optional Extension Header is specified by an
   IANA-assigned H-Type value of 0x100.  As in other Optional
   Extensions, the total length of the extension is indicated by the
   H-LEN field (specified in 16-bit words).  The extension field is
   formed of a group of one to five 16-bit fields.

   For this specific option, only the last 16-bit word has an assigned
   value; the sender SHOULD set the remaining values to 0x0000.  The
   last 16-bit field forms the Next-Header Type field.  A Receiver MUST
   interpret the Type field, but MUST ignore any other fields of this
   Extension Header.





















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6.  Processing at the Encapsulator

   The Encapsulator forms the PDUs queued for transmission into SNDUs by
   adding a header and trailer to each PDU (section 4).  It then
   segments the SNDU into a series of TS Packet payloads (Figure 13).
   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
   [RFC4259].

                +------+--------------------------------+------+
                | ULE  |        Protocol Data Unit      | ULE  |
                |Header|                                |CRC-32|
                +------+--------------------------------+------+
               /         /                             \       \
              /         /                               \       \
             /         /                                 \       \
   +--------+---------+   +--------+---------+   +--------+---------+
   |MPEG-2TS| MPEG-2  |...|MPEG-2TS| MPEG-2  |...|MPEG-2TS| MPEG-2  |
   | Header | Payload |   | Header | Payload |   | Header | Payload |
   +--------+---------+   +--------+---------+   +--------+---------+

   Figure 13: Encapsulation of an SNDU into a series of TS Packets

6.1.  SNDU Encapsulation

   When an Encapsulator has not previously sent a TS Packet for a
   specific TS Logical Channel, or after an Idle period, it starts to
   send an SNDU in the first available TS Packet.  This first TS Packet
   generated MUST carry a PUSI value of 1.  It MUST also carry a Payload
   Pointer value of zero, indicating that the SNDU starts immediately
   after the Payload Pointer in the TS Packet payload.

   The Encapsulation MUST ensure that all TS Packets set the MPEG-2
   Continuity Counter carried in the TS Packet header, according to
   [ISO-MPEG2].  This value MUST be incremented by one (modulo 16) for
   each successive TS Packet containing a fragment/complete SNDU sent
   using the same TS Logical Channel.

   An Encapsulator MAY decide not to send another SNDU immediately, even
   if space is available in a partially filled TS Packet.  This
   procedure is known as Padding (Figure 14).  The End Indicator informs
   the Receiver that there are no more SNDUs in this TS Packet payload.
   The End Indicator is followed by zero or more unused bytes until the
   end of the TS Packet payload.  All unused bytes MUST be set to the
   value of 0xFF, following current practice in MPEG-2 [ISO-DSMCC].  The
   Padding procedure trades decreased efficiency against improved
   latency.



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                 +-/------------+
                 |  SubNetwork  |
                 |     DU 1     |
                 +-/------------+
                        \        \
                         \        \
                          \        \
                 +--------+--------+--------+----------+
                 |MPEG-2TS| End of | 0xFFFF |  Unused  |
                 | Header | SNDU 1 |        |  Bytes   |
                 +--------+--------+--------+----------+
                   PUSI=0            ULE
                                     End
                                     Indicator

   Figure 14: A TS Packet carrying the end of SNDU 1, followed by an
              End Indicator

   Alternatively, when more packets are waiting at an Encapsulator, and
   a TS Packet has sufficient space remaining in the payload, the
   Encapsulator can follow a previously encapsulated SNDU with another
   SNDU using the next available byte of the TS Packet payload (see
   6.2).  This is called Packing (Figure 15).

              +-/----------------+       +----------------/-+
              |   Subnetwork     |       |   Subnetwork     |
              |      DU 2        |       |      DU 3        |
              +-/----------------+       +----------------/-+
                         \        \     /          /\
                          \        \   /          /  \
                           \        \ /          /    \. . .
          +--------+--------+--------+----------+
          |MPEG-2TS| Payload| end of | start of |
          | Header | Pointer| SNDU 2 | SNDU 3   |
          +--------+--------+--------+----------+
            PUSI=1     |              ^
                       |              |
                       +--------------+

   Figure 15: A TS Packet with the end of SNDU 2, followed by SNDU 3











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6.2.  Procedure for Padding and Packing

   Five possible actions may occur when an Encapsulator has completed
   encapsulation of an SNDU:

   (i) If the TS Packet has no remaining space, the Encapsulator
   transmits this TS Packet.  It starts transmission of the next SNDU in
   a new TS Packet.  (The standard rules [ISO-MPEG2] require that the
   header of this new TS Packet carry a PUSI value of 1 followed by a
   Payload Pointer value of 0x00.)

   (ii) If the TS Packet carrying the final part of an SNDU has one byte
   of unused payload, the Encapsulator MUST place the value 0xFF in this
   final byte and transmit the TS Packet.  This rule provides a simple
   mechanism to resolve the complex behaviour that may arise when the TS
   Packet has no PUSI set.  To send another SNDU in the current TS
   Packet would otherwise require the addition of a Payload Pointer that
   would consume the last remaining byte of TS Packet payload.  The
   behaviour follows similar practice for other MPEG-2 payload types
   [ISO-DSMCC].  The Encapsulator MUST start transmission of the next
   SNDU in a new TS Packet.  (The standard rules require the header of
   this new TS Packet to carry a PUSI value of 1 followed by a Payload
   Pointer value of 0x00.)

   (iii) If the TS Packet carrying the final part of an SNDU has exactly
   two bytes of unused payload, and the PUSI was NOT already set, the
   Encapsulator MUST place the value 0xFFFF in these final two bytes,
   providing an End Indicator (section 4.3), and transmit the TS Packet.
   This rule prevents fragmentation of the SNDU Length field over two TS
   Packets.  The Encapsulator MUST start transmission of the next SNDU
   in a new TS Packet.  (The standard rules require the header of this
   new TS Packet to carry a PUSI value of 1 followed by a Payload
   Pointer value of 0x00.)

   (iv) If the TS Packet has more than two bytes of unused payload, the
   Encapsulator MAY transmit this partially full TS Packet but MUST
   first place the value 0xFF in all remaining unused bytes (i.e.,
   setting an End Indicator followed by Padding).  The Encapsulator MUST
   then start transmission of the next SNDU in a new TS Packet.  (The
   standard rules [ISO-MPEG2] require that the header of this new TS
   Packet carry a PUSI value of 1 and a Payload Pointer value of 0x00.)

   (v) If at least two bytes are available for SNDU data in the TS
   Packet payload (i.e., three bytes if the PUSI was NOT previously set,
   and two bytes if it was previously set), the Encapsulator MAY
   encapsulate further queued PDUs, by starting the next SNDU in the
   next available byte of the current TS Packet payload.  When the
   Encapsulator packs further SNDUs into a TS Packet where the PUSI has



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   NOT already been set, the PUSI MUST be updated (set to 1), and an
   8-bit Payload Pointer MUST be inserted in the first byte directly
   following the TS Packet header.  (This reduces the size of the TS
   Packet payload field that is available for data by one byte.)  The
   value of the Payload Pointer MUST be set to the position of the byte
   following the end of the first SNDU in the TS Packet payload.  If no
   further PDUs are available, an Encapsulator MAY wait for additional
   PDUs to fill the incomplete TS Packet.  The maximum period of time an
   Encapsulator can wait, known as the Packing Threshold, MUST be
   bounded and SHOULD be configurable in the Encapsulator.  If
   sufficient additional PDUs are NOT received to complete the TS Packet
   within the Packing Threshold, the Encapsulator MUST insert an End
   Indicator (using rule iv).

   Use of the Packing method (v) by an Encapsulator is optional and may
   be determined on a per-session, per-packet, or per-SNDU basis.

   When an SNDU is less than the size of a TS Packet payload, a TS
   Packet may be formed that carries a PUSI value of one and also an End
   Indicator (using rule iv).

7.  Receiver Processing

   A Receiver tunes to a specific TS Multiplex carrying a ULE Stream 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).




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7.1.  Idle State

   After initialisation or errors, or on receipt of an End Indicator,
   the Receiver enters the Idle State.  In this state, the Receiver
   discards all TS Packets until it discovers the start of a new SNDU,
   upon which it then enters the Reassembly State.  Figure 16 outlines
   these state transitions:

                                +-------+
                                | START |
                                +---+---+
                                    |
                                   \/
                               +----------+
                              \|   Idle   |/
                      +-------/|   State  |\-------+
         Insufficient |        +----+-----+        |
         unused space |             | PUSI set     | MPEG-2 TS Error
         or           |            \/              | or
         End Indicator|        +----------+        | SNDU Error
                      |        |Reassembly|        |
                      +--------|  State   |--------+
                               +----------+

   Figure 16: Receiver state transitions

7.1.1.  Idle State Payload Pointer Checking

   A Receiver in the Idle State MUST check the PUSI value in the header
   of all received TS Packets.  A PUSI value of 1 indicates the presence
   of a Payload Pointer.  Following a loss of synchronisation, values
   between 0 and 181 are permitted, in which case the Receiver MUST
   discard the number of bytes indicated by the Payload Pointer (counted
   from the first byte of the TS Packet payload field, and excluding the
   PP field itself), before leaving the Idle State.  It then enters the
   Reassembly State, and starts reassembly of a new SNDU at this point.

7.2. Processing of a Received SNDU

   When in the Reassembly State, the Receiver reads a 2-byte SNDU Length
   field from the TS Packet payload.  If the value is less than or equal
   to 4, or equal to 0xFFFF, the Receiver discards the Current SNDU and
   the remaining TS Packet payload and returns to the Idle State.
   Receipt of an invalid Length field is an error event and SHOULD be
   recorded as an SNDU length error.






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   If the Length of the Current SNDU is greater than 4, the Receiver
   accepts bytes from the TS Packet payload to the Current SNDU buffer
   until either Length bytes in total are received, or the end of the TS
   Packet is reached (see also 7.2.1).  When the Current SNDU length
   equals the value of the Length field, the Receiver MUST calculate and
   verify the CRC value (see 4.6).  SNDUs that contain an invalid CRC
   value MUST be discarded.  Mismatch of the CRC is an error event and
   SHOULD be recorded as a CRC error.  The underlying physical-layer
   processing (e.g., forward error correction coding) often results in
   patterns of errors, rather than single bit errors, so the Receiver
   needs to be robust to arbitrary patterns of corruption to the TS
   Packet and payload, including potential corruption of the PUSI, PP,
   and SNDU Length fields.  Therefore, a Receiver SHOULD discard the
   remaining TS Packet payload (if any) following a CRC mismatch and
   return to the Idle State.

   When the Destination Address is present (D=0), the Receiver accepts
   SNDUs that match one of a set of addresses specified by the Receiver
   (this includes the NPA address of the Receiver, the NPA broadcast
   address, and any required multicast NPA addresses).  The Receiver
   MUST silently discard an SNDU with an unmatched address.

   After receiving a valid SNDU, the Receiver MUST check the Type field
   (and process any Type 1 Extension Headers).  The SNDU payload is then
   passed to the next protocol layer specified.  An SNDU with an unknown
   Type value < 1536 MUST be discarded.  This error event SHOULD be
   recorded as an SNDU type error.

   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 whether 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.  (This is provided that the TS Packet has a



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   PUSI value of 1, see 7.2.1; otherwise, the Receiver has detected a
   delimiting error and MUST discard all remaining bytes in the TS
   Packet payload and transitions to the Idle State.)

7.2.1.  Reassembly Payload Pointer Checking

   A Receiver that has partially received an SNDU (in the Current SNDU
   buffer) MUST check the PUSI value in the header of all subsequent TS
   Packets with the same PID (i.e., same TS Logical Channel).  If it
   receives a TS Packet with a PUSI value of 1, it MUST then verify the
   Payload Pointer.  If the Payload Pointer does NOT equal the number of
   bytes remaining to complete the Current SNDU (i.e., the difference
   between the SNDU Length field and the number of reassembled bytes),
   the Receiver has detected a delimiting error.

   Following a delimiting error, the Receiver MUST discard the partially
   assembled SNDU (in the Current SNDU buffer) and SHOULD record a
   reassembly error.  It MUST then re-enter the Idle State.

7.3.  Other Error Conditions

   The Receiver SHOULD check the MPEG-2 Transport Error Indicator
   carried in the TS Packet header [ISO-MPEG2].  This flag indicates a
   transmission error for a TS Logical Channel.  If the flag is set to a
   value of one, a transmission error event SHOULD be recorded.  Any
   partially received SNDU MUST be discarded.  The Receiver then enters
   the Idle State.

   The Receiver MUST check the MPEG-2 Continuity Counter carried in the
   TS Packet header [ISO-MPEG2].  If two (or more) successive TS Packets
   within the same TS Logical Channel carry the same Continuity Counter
   value, the duplicate TS Packets MUST be silently discarded.  If the
   received value is NOT identical to that in the previous TS Packet,
   and it does NOT increment by one for successive TS Packets (modulo
   16), the Receiver has detected a continuity error.  Any partially
   received SNDU MUST be discarded.  A continuity counter error event
   SHOULD be recorded.  The Receiver then enters the Idle State.

   Note that an MPEG2-2 Transmission network is permitted to carry
   duplicate TS Packets [ISO-MPEG2], which are normally detected by the
   MPEG-2 Continuity Counter.  A Receiver that does not perform the
   above Continuity Counter check would accept duplicate copies of TS
   Packets to the reassembly procedure.  In most cases, the SNDU CRC-32
   integrity check will result in discard of these SNDUs, leading to
   unexpected PDU loss; however, in some cases, duplicate PDUs (fitting
   into one TS Packet) could pass undetected to the next layer protocol.





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8.  Summary

   This document defines a Unidirectional Lightweight Encapsulation
   (ULE) that performs efficient and flexible support for IPv4 and IPv6
   network services over networks built upon the MPEG-2 Transport Stream
   (TS).  The encapsulation is also suited to transport of other
   protocol packets and bridged Ethernet frames.

   ULE also provides an Extension Header format and defines an
   associated IANA registry for efficient and flexible support of both
   mandatory and optional SNDU headers.  This allows for future
   extension of the protocol, while providing backwards compatibility
   with existing implementations.  In particular, Optional Extension
   Headers may safely be ignored by Receivers that do not implement
   them, or choose not to process them.

9.  Acknowledgements

   This document is based on a previous document 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; in particular, the many comments received from Art
   Allison, Carstsen Borman, Patrick Cipiere, Wolgang Fritsche, Hilmar
   Linder, Alain Ritoux, and William Stanislaus.  Alain also provided
   the original examples of usage.

10.  Security Considerations

   The security considerations for ULE resemble those that arise when
   the existing Multi-Protocol Encapsulation (MPE) is used.  ULE does
   not add specific new threats that will impact the security of the
   general Internet.

   There is a known security issue with un-initialised stuffing bytes.
   In ULE, these bytes are set to 0xFF (normal practice in MPEG-2).

   There are known integrity issues with the removal of the LAN FCS in a
   bridged networking environment.  The removal for bridged frames
   exposes the traffic to potentially undetected corruption while being
   processed by the Encapsulator and/or Receiver.

   There is a potential security issue when a Receiver receives a PDU
   with two Length fields:  The Receiver would need to validate the
   actual length and the Length field and ensure that inconsistent
   values are not propagated by the network.  In direct encapsulation of
   IPv4/IPv6 in ULE, this is avoided by including only one SNDU Length





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   Field.  However, this issue still arises in bridged LLC frames, and
   frames with a LLC Length greater than the SNDU payload size MUST be
   discarded, and an SNDU payload length error SHOULD be recorded.

   In the future, a ULE Mandatory Extension Header may be used to define
   a method to perform link encryption of the SNDU payload.  This is as
   an additional security mechanism to IP-, transport-, or application-
   layer security, not a replacement [RFC4259].  The approach is generic
   and decouples the encapsulation from future security extensions.  The
   operation provides functions that resemble those currently used with
   the MPE encapsulation.

   Additional security control fields may be provided as part of this
   link encryption Extension Header, e.g., to associate an SNDU with one
   of a set of Security Association (SA) parameters.  As a part of the
   encryption process, it may also be desirable to authenticate some or
   all of the SNDU headers.  The method of encryption and the way in
   which keys are exchanged is beyond the scope of this specification,
   as are the definition of the SA format and that of the related
   encryption keys.

11.  IANA Considerations

   The IANA has created the ULE Next-Header Type field registry as
   defined in this document.

   ULE Next-Header registry

      This registry allocates Next-Header values within the range 0-511
      (decimal).  For each allocated value, it also specifies the set of
      allowed H-LEN values (see section 5).  In combination, these
      define a set of allowed values in the range 0-1535 for the first
      part of the ULE Type space (see section 4.4.1).

11.1.  IANA Guidelines

   The following contains the IANA guidelines for management of the ULE
   Next-Header registry.  This registry allocates values 0-511 decimal
   (0x0000-0x01FF, hexadecimal).  It MUST NOT allocate values greater
   than 0x01FF (decimal).

   It subdivides the Next-Header registry in the following way:

   1) 0-255 (decimal) IANA-assigned values, indicating Mandatory
      Extension Headers (or link-dependent Type fields) for ULE,
      requiring expert review leading to prior issue of an IETF RFC.
      This specification MUST define the value and the name associated
      with the Extension Header, together with the procedure for



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      processing the Extension Header.  It MUST also define the need for
      the Mandatory Extension and the intended use.  The size of the
      Extension Header MUST be specified.

      Assignments have been made in this document, and registered by
      IANA:

      Type      Name                             Reference

      0:       Test-SNDU                        Section 5.1
      1:       Bridged-SNDU                     Section 5.2

   2) 256-511 (decimal) IANA-assigned values, indicating Optional
      Extension Headers for ULE, requiring expert review leading to
      prior issue of an IETF RFC.  This specification MUST define the
      value and the name associated with the Extension Header, together
      with the procedure for processing the Extension Header.  The entry
      MUST specify the range of allowable H-LEN values that are
      permitted (in the range 1-5).  It MUST also define the need for
      the Optional Extension and the intended use.

      Assignments have been made in this document, and registered by
      IANA:

      Type      Name                    H-LEN   Reference

      256:      Extension-Padding       1-5     Section 5.3

12. References

12.1.  Normative References

   [ISO-MPEG2]    IS 13818-1, "Information technology -- Generic coding
                  of moving pictures and associated audio information --
                  Part 1: Systems", International Standards Organisation
                  (ISO), 2000.

   [RFC2119]      Bradner, S., "Key Words for Use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119, 1997.

   [RFC1112]      Deering, S., "Host extensions for IP multicasting",
                  STD 5, RFC 1112, August 1989.

   [RFC2464]      Crawford, M., "Transmission of IPv6 Packets over
                  Ethernet Networks", RFC 2464, December 1998.






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   [ULE1]         Registration for format_identifier ULE1, SMPTE
                  Registration Authority, LLC,
                  http://www.smpte-ra.org/ule1.html.

12.2.  Informative References

   [IPDVB-AR]     Fairhurst, G. and M-J. Montpetit, "Address Resolution
                  for IP datagrams over MPEG-2 Networks", Work in
                  Progress, September 2005.

   [ATSC]         A/53, "ATSC Digital Television Standard", Advanced
                  Television Systems Committee (ATSC), Doc. A/53 Rev.C,
                  2004

   [ATSC-DAT]     A/90, "ATSC Data Broadcast Standard", Advanced
                  Television Systems Committee (ATSC), Doc. A/090, 2000.

   [ATSC-DATG]    A/91, "Recommended Practice: Implementation Guidelines
                  for the ATSC Data Broadcast Standard", Advanced
                  Television Systems Committee (ATSC), Doc. A/91, 2001.

   [ATSC-G]       A/54, "Guide to the use of the ATSC Digital Television
                  Standard", Advanced Television Systems Committee
                  (ATSC), Doc. A/54, 1995.

   [ATSC-PSIP-TC] A/65B, "Program and System Information Protocol for
                  Terrestrial Broadcast and Cable", Advanced Television
                  Systems Committee (ATSC), Doc. A/65B, 2003.

   [ATSC-REG]     ATSC "Code Point Registry"
                  www.atsc.org/standards/Code_Point_Registry.pdf.

   [ATSC-S]       A/80, "Modulation and Coding Requirements for Digital
                  TV (DTV) Applications over Satellite", Advanced
                  Television Systems Committee (ATSC), Doc. A/80, 1999.

   [DIX]          Digital Equipment Corp, Intel Corp, Xerox Corp,
                  "Ethernet Local Area Network Specification" Version
                  2.0, November 1982.

   [ETSI-DAT]     EN 301 192, "Specifications for Data Broadcasting",
                  European Telecommunications Standards Institute
                  (ETSI), 2004.

   [ETSI-DVBC]    EN 300 800, "Digital Video Broadcasting (DVB); DVB
                  interaction channel for Cable TV distribution systems
                  (CATV)", European Telecommunications Standards
                  Institute (ETSI), 1998.



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   [ETSI-DVBS]    EN 300 421, "Digital Video Broadcasting (DVB);
                  Modulation and Coding for DBS satellite systems at
                  11/12 GHz", European Telecommunications Standards
                  Institute (ETSI), 1997.

   [ETSI-DVBT]    EN 300 744, "Digital Video Broadcasting (DVB); Framing
                  structure, channel coding and modulation for digital
                  terrestrial television (DVB-T)", European
                  Telecommunications Standards Institute (ETSI), 2004.

   [ETSI-RCS]     ETSI 301 790, "Digital Video Broadcasting (DVB);
                  Interaction Channel for Satellite Distribution
                  Systems", European Telecommunications Standards
                  Institute (ETSI), 2005.

   [IEEE-802.2]   IEEE 802.2, "Local and metropolitan area networks-
                  Specific requirements Part 2: Logical Link Control",
                  IEEE Computer Society, (also ISO/IEC 8802-2), 1998.

   [IEEE-802.3]   IEEE 802.3, "Local and metropolitan area networks-
                  Specific requirements Part 3: Carrier sense multiple
                  access with collision detection (CSMA/CD) access
                  method and physical layer specifications", IEEE
                  Computer Society, (also ISO/IEC 8802-3), 2002.

   [ISO-DSMCC]    IS 13818-6, "Information technology -- Generic coding
                  of moving pictures and associated audio information --
                  Part 6: Extensions for DSM-CC", International
                  Standards Organisation (ISO), 1998.

   [ITU-H222]     H.222.0, "Information technology - Generic coding of
                  moving pictures and associated audio information:
                  Systems", International Telecommunication Union,
                  (ITU-T), 1995.

   [ITU-3563]     I.363.5, "B-ISDN ATM Adaptation Layer specification:
                  Type 5 AAL", International Telecommunication Union,
                  (ITU-T), 1996.

   [ISO-8802-2]   ISO/IEC 8802.2, "Logical Link Control", International
                  Standards Organisation (ISO), 1998.

   [RFC3077]      Duros, E., Dabbous, W., Izumiyama, H., Fujii, N., and
                  Y. Zhang, "A Link-Layer Tunneling Mechanism for
                  Unidirectional Links", RFC 3077, March 2001.






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   [RFC3309]      Stone, J., Stewart, R., and D. Otis, "Stream Control
                  Transmission Protocol (SCTP) Checksum Change", RFC
                  3309, September 2002.

   [RFC4259]      Montpetit, M.-J., Fairhurst, G., Clausen, H.,
                  Collini-Nocker, B., and H. Linder, "A Framework for
                  Transmission of IP Datagrams over MPEG-2 Networks",
                  RFC 4259, November 2005.

   [SOOR05]       M. Sooriyabandara, G. Fairhurst, A. Ang, B. Collini-
                  Nocker, H. Linder, W. Stering  "A Lightweight
                  Encapsulation Protocol for IP over MPEG-2 Networks:
                  Design, Implementation and Analysis", Computer
                  Networks 48 p5-19, 2005.





































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Appendix A: SNDU Packing Examples

   This appendix provides some examples of use.  The appendix is
   informative.  It does not provide a description of the protocol.  The
   examples provide the complete TS Packet sequence for some sample
   encapsulated IP packets.

   The specification of the TS Packet header operation and field values
   is provided in [ISO-MPEG2].  The specification of ULE is provided in
   the body of this document.

   The key below is provided for the following examples.

   HDR    4B TS Packet Header
   PUSI   Payload Unit Start Indicator
   PP     Payload Pointer
   ***    TS Packet Payload Pointer (PP)

   Example A.1: Two 186B PDUs.

     SNDU A is 200 bytes (including the ULE destination NPA address)
     SNDU B is 200 bytes (including the ULE destination NPA address)

   The sequence comprises 3 TS Packets:

                      SNDU
           PP=0      Length
   +-----+------+------+------+-   -+------+
   | HDR | 0x00 | 0x00 | 0xC4 | ... | A182 |
   +-----+----*-+-*----+------+-   -+------+
   PUSI=1     *   *
              *****
                                          SNDU
           PP=17           CRC for A     Length
   +-----+------+------+-   -+--- --+------+------+-   -+------+
   | HDR | 0x11 | A183 | ... | A199 | 0x00 | 0xC4 | ... | B165 |
   +-----+----*-+------+-   -+------+-*----+------+-   -+------+
   PUSI=1     *                       *
              *************************

                                 End     Stuffing
                    CRC for A Indicator   Bytes
   +-----+------+-   -+------+----+----+-   -+----+
   | HDR | B166 | ... | B199 |0xFF|0xFF| ... |0xFF|
   +-----+------+-   -+------+----+----+-   -+----+
   PUSI=0





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   Example A.2: Usage of last byte in a TS-Packet

     SNDU A is 183 bytes
     SNDU B is 182 bytes
     SNDU C is 181 bytes
     SNDU D is 185 bytes

   The sequence comprises 4 TS Packets:

                       SNDU
            PP=0      Length     CRC for A
    +-----+------+------+------+-   -+------+
    | HDR | 0x00 | 0x00 | 0xB3 | ... | A182 |
    +-----+----*-+-*----+------+-   -+------+
    PUSI=1     *   *
               *****
                       SNDU                  Unused
            PP=0      Length       CRC for B  byte
    +-----+------+------+------+-   -+------+------+
    | HDR | 0x00 | 0x00 | 0xB2 | ... | B181 | 0xFF |
    +-----+---*--+-*----+------+-   -+------+------+
    PUSI=1    *    *
              ******
                       SNDU                       SNDU
            PP=0      Length      CRC for C      Length
    +-----+------+------+------+-   -+------+------+------+
    | HDR | 0x00 | 0x00 | 0xB1 | ... | C180 | 0x00 | 0x65 |
    +-----+---*--+-*----+------+-   -+------+------+------+
    PUSI=1    *    *
              ******           Unused
                                byte
    +-----+------+-   -+------+------+
    | HDR | D002 | ... | D184 | 0xFF |
    +-----+------+-   -+------+------+
     PUSI=0

   Example A.3: Large SNDUs

   SNDU A is 732 bytes
   SNDU B is 284 bytes

   The sequence comprises 6 TS Packets:









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                       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: Illustration of SNDU Length field

     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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RFC 4326              ULE for IP over MPEG-2/DVB           December 2005


   Example A.5: Three 44B PDUs.

     SNDU A is 52 bytes (no ULE destination NPA address) SNDU B is 52
     bytes (no ULE destination NPA address) SNDU C is 52 bytes (no ULE
     destination NPA address)

   The sequence comprises 1 TS Packet:

                      SNDU
           PP=0      Length
   +-----+------+------+------+-   -+-----+------+------+-   -+-----+-
   | HDR | 0x00 | 0x80 | 0x30 | ... | A51 | 0x80 | 0x30 | ... | B51 | ..
   +-----+----*-+-*----+------+-   -+-----+------+------+-   -+-----+-
   PUSI=1     *   *
              *****

                                           End        Stuffing
                                         Indicator     bytes
                -----+------+-   -+-----+---------+- -+------+
            ... 0x80 | 0x30 | ... | C51 |0xFF|0xFF|   | 0xFF |
                -----+------+-   -+-----+---------+- -+------+






























Fairhurst & Collini-Nocker  Standards Track                    [Page 39]

RFC 4326              ULE for IP over MPEG-2/DVB           December 2005


Appendix B: SNDU Encapsulation

   An example of ULE encapsulation carrying an ICMPv6 packet generated
   by ping6.

   ULE SNDU Length  :            63 decimal
   D-bit value  :                0 (NPA destination address present)
   ULE Protocol Type :           0x86dd (IPv6)
   Destination ULE NPA Address : 00:01:02:03:04:05
   ULE CRC32 :                   0x7c171763

   Source IPv6 :                 2001:DB8:3008:1965::1
   Destination IPv6 :            2001:DB8:2509:1962::2

   SNDU contents (including CRC-32):

   0000: 00 3f 86 dd 00 01 02 03 04 05 60 00 00 00 00 0d
   0016: 3a 40 20 01 0d b8 30 08 19 65 00 00 00 00 00 00
   0032: 00 01 20 01 0d b8 25 09 19 62 00 00 00 00 00 00
   0048: 00 02 80 00 9d 8c 06 38 00 04 00 00 00 00 00 7c
   0064: 17 17 63






























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RFC 4326              ULE for IP over MPEG-2/DVB           December 2005


Authors' Addresses

   Godred Fairhurst
   Department of Engineering
   University of Aberdeen
   Aberdeen, AB24 3UE
   UK

   EMail: gorry@erg.abdn.ac.uk
   Web: http://www.erg.abdn.ac.uk/users/Gorry


   Bernhard Collini-Nocker
   Department of Scientific Computing
   University of Salzburg
   Jakob Haringer Str. 2
   5020 Salzburg
   Austria

   EMail: bnocker@cosy.sbg.ac.at
   Web: http://www.scicomp.sbg.ac.at/






























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RFC 4326              ULE for IP over MPEG-2/DVB           December 2005


Full Copyright Statement

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