rfc1963.txt

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            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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RFC 1963                        PPP SDTP                     August 1996


            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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RFC 1963                        PPP SDTP                     August 1996


            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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RFC 1963                        PPP SDTP                     August 1996


         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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RFC 1963                        PPP SDTP                     August 1996


       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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RFC 1963                        PPP SDTP                     August 1996


   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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RFC 1963                        PPP SDTP                     August 1996


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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RFC 1963                        PPP SDTP                     August 1996


   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.