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INFORMATIONAL

Network Working Group                                       K. Schneider
Request for Comments: 1963                                    S. Venters
Category: Informational                                     ADTRAN, Inc.
                                                             August 1996


               PPP Serial Data Transport Protocol (SDTP)

Status of This Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Abstract

   The Point-to-Point Protocol (PPP) [1] provides a standard method for
   transporting multi-protocol datagrams over point-to-point links.  PPP
   defines an extensible Link Control Protocol, and proposes a family of
   Network Control Protocols for establishing and configuring different
   network-layer protocols.

   This document describes a new Network level protocol (from the PPP
   point of view), PPP Serial Data Transport Protocol, that provides
   encapsulation and an associated control protocol for transporting
   serial data streams over a PPP link.  This protocol was developed for
   the purpose of using PPP's many features to provide a standard method
   for synchronous data compression.  The encapsulation uses a header
   structure based on that of the ITU-T Recommendation V.120 [2].

Table of Contents

     1.     Introduction ..........................................    2
     2.     SDTP Packets ..........................................    3
        2.1       Padding .........................................    4
        2.2       Packet Formats ..................................    4
     3.     Serial Data Control Protocol ..........................   11
     4.     SDCP Configuration Option Format ......................   12
        4.1       Packet-Format ...................................   13
        4.2       Header-Type .....................................   13
        4.3       Length-Field-Present ............................   14
        4.4       Multi-Port ......................................   14
        4.5       Transport-Mode ..................................   15
        4.6       Maximum-Frame-Size ..............................   16
        4.7       Allow-Odd-Frames ................................   16
        4.8       FCS-Type ........................................   17
        4.9       Flow-Expiration-Time ............................   18
     SECURITY CONSIDERATIONS ......................................   19



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     REFERENCES ...................................................   19
     CHAIR'S ADDRESS ..............................................   20
     AUTHORS' ADDRESSES ...........................................   20

1.  Introduction

   This document is a product of the TR30.1 ad hoc committee on
   compression of synchronous data.  It represents a component of a
   proposal to use PPP to provide compression of synchronous data in
   DSU/CSUs.

   In addition to providing support for multi-protocol datagrams, the
   Point-to-Point Protocol (PPP) [1] has defined an effective and robust
   negotiating mechanism that can be used on point to point links.  When
   used in conjunction with the PPP Compression Control Protocol [3] and
   one of the PPP Compression Protocols [4-10], PPP provides an
   interoperable method of employing data compression on a point-to-
   point link.

   This document provides a PPP encapsulation for serial data,
   specifying a transport protocol, PPP Serial Data Transport Protocol
   (PPP-SDTP), and an associated control protocol, PPP Serial Data
   Control Protocol (PPP-SDCP).  When these protocols are added to above
   mentioned PPP protocols, PPP can be used to provide compression of
   serial data on a point-to-point link.

   This first edition of PPP-SDTP/SDCP covers HDLC-like synchronous
   serial data and asynchronous serial data.  It does this by using a
   terminal adaption header based on that of ITU-T Recommendation V.120
   [2].  Support may be added in the future for other synchronous
   protocols as the marketplace demands.

   The V.120 terminal adaption header allows transported data frames to
   be split over several packets, supports the transport of DTE port
   idle and error information, and optionally supports the transport of
   DTE control state information.

   In addition to the V.120 Header, fields can be added to the packet
   format through negotiation to provide support for features not
   included in the V.120 header.  The extra fields are: a Length Field,
   which is used to distinguish packets in compound frames, and a Port
   field, which is used to provide multi-port multiplexing capability.
   The protocol also allows reserved bits in the V.120 header to be used
   to transport non-octet aligned frames and to provide a flow control
   mechanism.






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   To provide these features, PPP-SDTP permits a single frame format to
   be selected from several possible formats by using PPP-SDCP
   negotiation.  The terminal adaption header can be either fixed length
   or variable length, to allow either simplicity or flexibility.

   The default frame format places the terminal adaption header at the
   end of the packet.  This permits optimal transmitter timelines when
   user frames are segmented and compression is also used in conjunction
   with this protocol.

2.  SDTP Packets

   Before any SDTP packets may be communicated, PPP must reach the
   Network-Layer Protocol phase, and the SDTP Control Protocol must
   reach the Opened state.

   By default, exactly one SDTP packet is encapsulated in the PPP
   Information field, where the PPP Protocol field indicates type hex
   0049 (PPP-SDTP).  If the Length-Field-Present Configuration Option
   and the LCP Compound-Frames Configuration Option are successfully
   negotiated, multiple SDTP packets may be placed in the PPP
   Information field, and they are distinguished by the presence of
   Length fields in each packet.

   The maximum length of the SDTP datagram transmitted over a PPP link
   is limited only by the negotiated Maximum-Frame-Size and the maximum
   length of the Information field of a PPP encapsulated packet.  Note
   that if compression is used on the PPP link, this the maximum length
   of the SDTP datagram  may be larger or smaller than the maximum
   length of the Information field of a PPP encapsulated packet,
   depending on the particular compression algorithm and protocol used.

   ITU-T Recommendation V.120 [2] defines an adaption header that is
   used with its asynchronous and synchronous modes of operation.  SDTP
   packets include this header as a Header field to provide the protocol
   adaption function.  Using negotiation, additional fields can be added
   to the packet to provide sequencing and multiplexing capability
   within SDTP. SDTP also has an option of using the reserved bits of
   the header to provide a flow control mechanism and support for
   transporting non-octet aligned data frames.

   The default SDTP packet format is designed to allow the efficient use
   of the protocol's segmentation feature when combined with a PPP
   Compression Protocol [4-10].  This format is a little different from
   other PPP NCP's in that data is read from both ends of the packet.
   The Header field is placed at the end of the SDTP packet, with the
   order of the octets reversed.  This somewhat unique format has been
   selected to allow optimal transmitter timelines when compression is



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   used and transported data frames are split into multiple SDTP
   packets.  In such a situation, the Header field contains the
   information about whether the data is split into multiple packets or
   not, so if it is located at the end of a packet, the decision can be
   made after observing the compressed size of the packet.  The Header
   field can then simply be run through the compressor after the
   decision has been made.

   When the Header field is placed before the data, as in the optional
   packet format, the transmitter must make the decision about whether
   to split a frame over multiple packets without knowing about the
   compressibility of the frame.  Therefore the optional format is
   designed to be used when transported frames are not split into
   multiple SDTP packets or where SDTP is not coupled with compression.
   It is believed that this format may be useful for some hardware
   implementations.

2.1.  Padding

   If padding is used, SDTP packets require the use of the Length Field
   or the previous negotiation of the LCP Self-Describing-Padding
   Configuration Option [11].

2.2.  Packet Formats

   The default SDTP packet format is shown below. The fields are
   transmitted from left to right.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        PPP Protocol ID        |    Transported Data ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Header -  H   |
   +-+-+-+-+-+-+-+-+

   The two complete frame formats are shown below:  Header-Last and
   Header-First.  Header-Last is the default packet format. The
   additional fields provided support for:  Control State Information
   (CS), multiple packets and multi-port multiplexing.  Again, the
   fields are transmitted from left to right.  Descriptions of the
   fields follow the packet formats.









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   Header-Last

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        PPP Protocol ID        |          (Length)             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    (Port)     |  Transported Data / (Odd-Pad) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Header - (CS) :       H       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Header-First

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        PPP Protocol ID        |          (Length)             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    (Port)     | Header  -  H  :     (CS)      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Transported Data / (Odd-Pad) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   PPP Protocol ID

      The PPP Protocol ID field is described in the Point-to-Point
      Protocol Encapsulation [1].

      When the SDTP Protocol is successfully negotiated by the SDTP
      Control Protocol (SDCP), the value is 0049 hex.  This value may be
      compressed to one octet when Protocol-Field-Compression is
      negotiated, or if one of the PPP compression protocols [4-10] is
      used.

   Length

      The optional Length field is present in every SDTP packet upon
      successful negotiation of the Length-Field-Present Configuration
      Option.

      The value of the Length field is the combined lengths of the
      Length, Port (if present), Header, Transmitted Data, and Odd-Pad
      (if present) fields in octets.

      The length of the Length field defaults to one octet.  Valid
      lengths are from 2 to 255 octets, since each packet must include



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      at least a one octet Header field.

      If desired, the length field can be negotiated to be two octets in
      length.  In that case, valid lengths are from 2 to 65535 octets,
      and the field is transmitted most significant octet first.

      In either case, a length of 0 means that the combined length is
      the same as the length of the remainder of the PPP Information
      Field.

   Port

      The optional Port field is present in every SDTP packet upon
      successful negotiation of the Multi-Port Option.

      The length of the Port field is one octet. Valid Port numbers are
      0 to 254.  Port number 255 is reserved for control purposes (see
      section on flow control).

   Header

      The Header field is the terminal adaption header from ITU-T
      Recommendation V.120.  As specified in that document, it contains
      up to two octets: The terminal adaption header octet (H), and the
      optional header extension for control state information (CS).
      SDTP only supports the protocol sensitive operation of V.120; bit
      transparent operation is not supported.  The descriptions of the
      header bits provided below are derived from the descriptions
      provided in Recommendation V.120.  In addition to the bit
      definitions of V.120, SDTP optionally permits the use of reserved
      bits to be used for flow control and to provide support for non-
      octet aligned frames.

      The length of the Header field is either one or two octets, and is
      determined by the value of the E bit in the first octet.  By
      default, the E-bit must be set in the H octet and the CS octet is
      not present.  A Configuration Option may be negotiated to allow
      the use of the CS octet, or even to require its presence in every
      packet.












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      H (V.120 Terminal Adaption Header)

         The format of the first octet of the Header field is shown
         below:

            0     1     2     3     4     5     6     7
         +-----+-----+-----+-----+-----+-----+-----+-----+
         |  E  | BR  | Res | FC  | C2  | C1  |  B  |  F  |
         +-----+-----+-----+-----+-----+-----+-----+-----+

         E - Extension Bit

            The E bit is the extension bit.  If set to 0, it indicates
            that the Control-2 field is present.

         BR - Break / HDLC Idle Bit

            In asynchronous mode, the BR bit indicates the invocation of
            the BREAK function by the DTE.  A value of 1 indicates
            BREAK.

            In synchronous HDLC mode, the BR bit is used to indicate
            that DTE port is receiving HDLC idle condition.  A value of
            1 indicates this idle condition.

         Res - Reserved


            This bit is reserved and MUST be set to 0.  (This is a
            reserved bit in V.120.)


         FC - Flow Control

            This bit can be used for flow control of SDTP traffic on the
            network, for applications which require it.  When SDTP is
            used in conjunction with data compression, flow control may
            be needed.  Reasons for this could be that the DTE port uses
            an X.21 interface (and therefore does not have independent
            control of DTE transmit and receive clocks), or simply that
            the underlying link layer (such as PPP in HDLC-like Framing)
            does not include a mechanism for network flow control, so
            some flow control mechanism is needed.

            This bit set to a value of 0 indicates that the receiver is
            ready to receive data (Flow-On). A value of 1 indicates that
            the receiver does not wish to receive data and the
            transmitting peer should stop sending it (Flow-Off).  Flow



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            control operates on a per port basis.  Flow control messages
            on Port 255 affect all ports.

            To ensure that a missed Flow-On message cannot cause a
            hangup condition, a Flow-Off is defined to expire after a
            time of T1 seconds.  If a unit desires to keep its peer in
            the Flow-Off state for more than T1 seconds, it MUST
            transmit another Flow-Off message after every period of T1
            seconds.  A unit that receives a Flow-Off message may resume
            transmitting T1 seconds after the last Flow-Off was
            received.  The value of T1 is controlled by the Flow-
            Expiration-Time Configuration Option.  The default value is
            10 seconds.  There is not a separate value for T1 for each
            port; all ports use the same T1 value.

            (This bit is a reserved bit in V.120, which requires the bit
            to be set to a value of zero.  The above definition of flow
            control provides compatibility with this definition when
            flow control is not used.)


         C1, C2 - Error Control Bits

            The C1 and C2 bits are used for DTE port Error detection and
            transmission.  Their meaning is defined in the following
            table:

            +----+----+--------------+--------------+
            |         |           Meaning           |
            +----+----+--------------+--------------+
            | C1 | C2 | Synchronous  | Asynchronous |
            +----+----+--------------+--------------+
            |  0 |  0 | No Error     | No Error     |
            |    |    |     Detected |     Detected |
            +----+----+--------------+--------------+
            |  0 |  1 | FCS Error    | Stop-bit     |
            |    |    |      (DTE)   |     Error    |
            +----+----+--------------+--------------+
            |  1 |  0 | Abort        | Parity Error |
            |    |    |              | on the Last  |
            |    |    |              | Character in |
            |    |    |              | Frame        |
            +----+----+--------------+--------------+
            |  1 |  1 | DTE Overrun* | Stop-bit and |
            |    |    |              | Parity Error |
            +----+----+--------------+--------------+





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            Appropriate responses to these bits are provided in Sections
            2.2.1 and 2.2.2 of the V.120 standard (where R reference
            point is translated to mean DTE port.)


         B, F - Segmentation Bits

            The B and F bits are used for segmenting and reassembly of
            the transported frames in synchronous HDLC mode.  Setting
            the B bit to 1 indicates that the packet contains the
            beginning of a transported frame or a Begin Frame.  Setting
            the F bit indicates that the packet contains the final
            portion of a transported frame, or a Final Frame. A packet
            that contains neither the beginning of a frame nor the end
            is said to contain a Middle Frame.  For asynchronous mode
            and bit transparent mode operation both bits MUST be set to
            1.  The following table summarizes the use of these bits:

            +---+---+--------------+----------------+
            |       |         Application           |
            +---+---+--------------+----------------+
            | B | F | Synchronous  | Asynchronous   |
            +---+---+--------------+----------------+
            | 1 | 0 | Begin Frame  | Not Applicable |
            +---+---+--------------+----------------+
            | 0 | 0 | Middle Frame | Not Applicable |
            +---+---+--------------+----------------+
            | 1 | 0 | Final Frame  | Not Applicable |
            +---+---+--------------+----------------+
            | 1 | 1 | Single Frame | Required       |
            +---+---+--------------+----------------+


      CS (V.120 optional Header Extension for Control State Information)

         The format of the second Header octet (CS) is shown below:
            0     1     2     3     4     5     6     7
         +-----+-----+-----+-----+-----+-----+-----+-----+
         |  E  | DR  | SR  | RR  | Res |(Odd-Pad Length) |
         +-----+-----+-----+-----+-----+-----+-----+-----+

         E - Extension Bit

            The E bit is the extension bit, and allows further extension
            of the Header field.  It is set to 1, to indicate no further
            extension of the Header field.





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         DR - Data Ready

            This bit set to 1 indicates that the DTE port is activated.

         SR - Send Ready

            This bit set to 1 indicates that the DTE is ready to send
            data.

         RR - Receive Ready

            This bit set to 1 indicates that the DTE is ready to receive
            data.  It can be used for DTE flow control in half-duplex
            transmissions.

         Res - Reserved

            This bit is reserved and set to 0. (This is a V.120 reserved
            bit.)

         Odd-Pad Length (Optional)

            The Odd-Pad Length field is used when non-octet aligned HDLC
            frames are allowed.  It is a 3-bit field, that can take on
            the values of 0 through 7.  Its value is the length of the
            Odd-Pad field in bits.  This value is determined as the
            number of bits necessary to have the combined length of the
            Transported Data Field and the Odd-Pad Field be aligned with
            an octet boundary.

            If non-octet aligned frames are not allowed, this field is
            not used and all bits are set to the value of 0.  (These
            bits are reserved in V.120.)

   Transported Data

      The transported data field contains the transported serial data.

      When the serial data type has been negotiated to be HDLC-like
      synchronous, this field will contain all or part of a transported
      HDLC-like frame.

      A sample transported HDLC frame is shown below.  The figure does
      not show bits inserted for transparency.







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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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Flag:01111110 | (Address, Control and Information Fields) ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             (FCS)                                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - - - - - - - - -+
      | Flag:01111110 |
      +-+-+-+-+-+-+-+-+

      Only the data between the flags is transported.  The flags are not
      transported.  The FCS is tranported unless the FCS-Mode
      Configuration Option has been successfully negotiated otherwise.

   Odd-Pad

      The optional Odd-Pad (Odd Frame Pad) field is used when the
      transported data frame is non-octet aligned, and the Allow-Odd-
      Frames Option has been successfully negotiated.  It contains the
      bits that are required to pad the Transported Data field out to an
      octet boundary.  The Odd-Pad field is in the high order bits of
      the last octet of the Transported Data field.  The values of these
      bits are all zero.

3.  Serial Data Control Protocol

   The Serial Data Control Protocol (SDCP) is responsible for
   configuring, enabling and disabling the SDTP modules on both ends of
   the point-to-point link.  SDCP uses the same packet exchange
   mechanism and state machine as the Link Control Protocol.  SDCP
   packets may not be exchanged until PPP has reached the Network-Layer
   Protocol phase.  SDCP packets received before this phase is reached
   SHOULD be silently discarded.

   The Serial Data Control Protocol is exactly the same as the Link
   Control Protocol [1] with the following exceptions:

   Frame Modifications

      The packet may utilize any modifications to the basic frame format
      which have been negotiated during the Link Establishment phase.

   Data Link Layer Protocol Field

      Exactly one SDCP packet is encapsulated in the PPP Information
      field, where the PPP Protocol field indicates type hex 8049 (PPP-
      SDCP).




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   Code Field

      Only Codes 1 through 7 (Configure-Request, Configure-Ack,
      Configure-Nak, Configure-Reject, Terminate-Request, Terminate-Ack,
      and Code-Reject) are used.  other Codes SHOULD be treated as
      unrecognized and SHOULD result in Code-Rejects.

   Timeouts

      SDCP packets may not be exchanged until PPP has reached the
      Network-Layer Protocol phase.  An implementation SHOULD be
      prepared to wait for Authentication and Link Quality Determination
      to finish before timing out waiting for a Configure-Ack or other
      response.  It is suggested that an implementation give up only
      after user intervention or a configurable amount of time.

   Configuration Option Types

      SDCP has a distinct set of Configuration Options which are defined
      in this document.

4.  SDCP Configuration Option Format

   SDCP Configuration Options allow modifications to the default SDCP
   characteristics to be negotiated.  If a Configuration Option is not
   included in a Configure-Request packet, the default value for that
   Configuration Option is assumed.

   SDCP uses the same Configuration Option format defined in LCP [1],
   with a separate set of Options.

   The Option Types are:

      1   Packet-Format
      2   Header-Type
      3   Length-Field-Present
      4   Multi-Port
      5   Transport-Mode
      6   Maximum-Frame-Size
      7   Allow-Odd-Frames
      8   FCS-Type
      9   Flow-Expiration-Time

   Note that Option Types 5-8 are specific to a single port and require
   port numbers in their format.  Option Types 6-8 are specific to the
   HDLC-Synchronous Transport-Mode.





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4.1.  Packet-Format

   This option selects whether the Header field precedes or follows the
   data field.  When the Header field follows the data field, the order
   of its octets are reversed.

    0                   1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |     Format    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      1

   Length

      3

   Format

      0   Header-Last   (default)
      1   Header-First

4.2.  Header-Type

   This option selects the type of the Header field.  The Header-Type of
   H-and-CS means that the CS octet will be present if indicated by the
   E-bit in the H-octet.  The Header-Type of H-and-CS-Always signifies
   that both the H and CS octets are present in every packet.

    0                   1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |  Header-Type  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      2

   Length

      3






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   Header-Type

      0   H-Only (default)
      1   H-and-CS
      2   H-and-CS-Always

4.3.  Length-Field-Present

   By default, a PPP Information Field contains only a single SDTP
   packet, and an SDTP Packet does not contain a length field.
   Successful negotiation of this option causes all SDTP packets to
   contain the length field, and allows SDTP packets to be contained in
   compound frames (see LCP Compound-Frames Configuration Option [11]).

   This option is required if the LCP Length-Field-Present Configuration
   option has been negotiated.

   The size of the Length field is negotiated via the Length-Size
   parameter.

    0                   1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |  Length-Size  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      3

   Length

      3

   Length-Size

      0   No Length Field (default)
      1   Length field of 1 octet
      2   Length field of 2 octets

4.4.  Multi-Port

   By default, packets do not contain a port number and all packets are
   sent to the default port, Port 0.  The Successful negotiation of the
   Multi-Port configuration option means that every packet will contain
   a port number.  The maximum port number, and hence the number of
   ports, is negotiated by using the Max-Port-Num field.  A value of 0
   specifies that a single port is to be used and no port field will be



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   present in an SDTP packet.  (This is the same as not negotiating or
   rejecting this option.) Port numbers begin with 0 and range to 254.
   Port number 255 is reserved for control purposes (see section on flow
   control).

   Protocol Specific negotiations which are on a per port basis, require
   the port number to be specified as part of the configuration
   negotiation.

    0                   1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     | Max-Port-Num  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      4

   Length

      3

   Max-Port-Num

      The maximum port number that can be used.  The number of ports
      present is Max-Port-Num + 1.  The value can range from 0 to 254.

4.5.  Transport-Mode

   This parameter selects the mode of transport for the specified port.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |      Port     |     Mode      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      5

   Length

      4






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   Port

      The port for which this option applies.

   Mode

      The transport mode to be used for this port.

         0   HDLC Synchronous (default)
         1   Asynchronous

4.6.  Maximum-Frame-Size

   This parameter specifies the maximum number of octets allowed in a
   transported data frame.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |      Port     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Maximum-Frame-Size                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      6

   Length

      7

   Port

      The port for which this option applies.

   Maximum-Frame-Size

      The maximum allowed length of a transported data frame in octets.
      Default is 10,000.  Negotiable range is 1 to 2**31 - 1. The value
      0 is reserved to mean no limit.  This field is transmitted most
      significant octet first.

4.7.  Allow-Odd-Frames

   By default, only octet-aligned data frames are allowed for transport.
   Successful negotiation of this option allows the transport of non-
   octet aligned frames.  The size of the padding required to align the



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RFC 1963                        PPP SDTP                     August 1996


   frames is carried in the CS Header octet.

   Use of Header-Type H-Only is not permitted in conjunction with this
   option.

    0                   1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |      Port     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      7

   Length

      3

   Port

      The port for which this option applies.

4.8.  FCS-Type

   By default, the transported data frame FCS is transported.  This
   option allows the FCS to be removed by the transmitter and
   regenerated by the receiver.

   It is important that implementations not use regeneration unless they
   are using PPP Reliable Transmission [12] or operating over some other
   layer that will provide reliable notification of a dropped packet.
   Implementations are not permitted to send a incomplete or bad frame
   to the user with a good (regenerated) FCS.

   This option also selects the type of user FCS that will be
   regenerated.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |      Port     |    FCS-Type   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      8




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RFC 1963                        PPP SDTP                     August 1996


   Length

      4

   Port

      The port for which this option applies.

   FCS-Type

         0   Transparent-Transport (Default)
         1   16-bit ITU-T CRC
         2   32-bit ITU-T CRC

4.9.  Flow-Expiration-Time

   As described in section 2.2, Flow-Off messages expire after T1
   seconds.  By default, T1 is 10 seconds.  This configuration option
   allows the value of T1 to be changed.

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Flow-Expiration-Time      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      9

   Length

      5

   Flow-Expiration-Time

      The Flow-Expiration-Time field contains a 16 bit unsigned integer
      which is used to specify the value to be assigned to T1 as
      follows: T1 = Flow-Expiration-Time / 10 seconds.  Therefore this
      value is in units of 1/10 of a second, with allowable values from
      1 to 2^16-1 (0.1 to 6553.5 seconds).  It is transmitted most
      significant octet first.  The default value is 100 (10 seconds),
      which all must support.






Schneider & Venters          Informational                     [Page 18]

RFC 1963                        PPP SDTP                     August 1996


Security Considerations

   Security issues are not discussed in this memo.

References

   [1]    Simpson, W., ed., "The Point-to-Point Protocol (PPP)", STD
          51, RFC 1661, July 1994.

   [2]    CCITT Recommendation V.120 (09/92), "Support by an ISDN of
          Data Terminal Equipment with V-Series Type Interfaces with
          Provision for Statistical Multiplexing", 1993.

   [3]    Rand, D., "The PPP Compression Control Protocol (CCP)", RFC
          1962, June 1996.

   [4]    Friend, R., and W. Simpson, "PPP Stac LZS Compression
          Protocol", RFC 1974, August 1996.

   [5]    Rand, D., "PPP Predictor Compression Protocol", RFC 1978,
          August 1996.

   [6]    Petty, J., "PPP Hewlett-Packard Packet-by-Packet Compression
          (HP PPC) Protocol", Work in Progress.

   [7]    Carr, D., "PPP Gandalf FZA Compression Protocol", Work in
          Progress.

   [8]    Schryver, V., "PPP BSD Compression Protocol", RFC 1977,
          August 1996.

   [9]    Schremp, et. al., "PPP Magnalink Variable Resource
          Compression", RFC 1975, August 1996.

   [10]   Schneider, K., "PPP Stacker LZS Compression Protocol using a
          DCP Header (LZS-DCP)", RFC 1967, August 1996.

   [11]   Simpson, W.A., "PPP LCP Extensions", RFC 1570, January 1994.

   [12]   Rand, D., "PPP Reliable Transmission", RFC 1663, July 1994.











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RFC 1963                        PPP SDTP                     August 1996


Chair's Address

   The working group can be contacted via the current chair:

   Karl Fox
   Ascend Communications
   3518 Riverside Drive, Suite 101
   Columbus, Ohio 43221

   EMail: karl@ascend.com

Authors' Addresses

   Questions about this memo should be directed to:

   Kevin Schneider
   Adtran, Inc.
   901 Explorer Blvd.
   Huntsville, AL 35806-2807

   Phone: (205) 971-8000
   EMail:  kevin@adtran.com


   Stuart Venters
   Adtran, Inc.
   901 Explorer Blvd.
   Huntsville, AL 35806-2807

   Phone: (205) 971-8000
   EMail: sventers@adtran.com




















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