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rfc:rfc1963

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

Schneider & Venters Informational [Page 1] RFC 1963 PPP SDTP August 1996

   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.

Schneider & Venters Informational [Page 2] RFC 1963 PPP SDTP August 1996

 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

Schneider & Venters Informational [Page 3] RFC 1963 PPP SDTP August 1996

 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.

Schneider & Venters Informational [Page 4] RFC 1963 PPP SDTP August 1996

 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

Schneider & Venters Informational [Page 5] RFC 1963 PPP SDTP August 1996

    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.

Schneider & Venters Informational [Page 6] RFC 1963 PPP SDTP August 1996

    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

Schneider & Venters Informational [Page 7] 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 |
          +----+----+--------------+--------------+

Schneider & Venters Informational [Page 8] 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.

Schneider & Venters Informational [Page 9] 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.

Schneider & Venters Informational [Page 10] 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).

Schneider & Venters Informational [Page 11] 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.

Schneider & Venters Informational [Page 12] 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

Schneider & Venters Informational [Page 13] 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.
 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

Schneider & Venters Informational [Page 14] RFC 1963 PPP SDTP August 1996

 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

Schneider & Venters Informational [Page 15] RFC 1963 PPP SDTP August 1996

 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

Schneider & Venters Informational [Page 16] 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

Schneider & Venters Informational [Page 17] 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.

Schneider & Venters Informational [Page 19] 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

Schneider & Venters Informational [Page 20]

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