rfc1976.txt

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RFC 1976                        PPP DCE                      August 1996


   The following CCP Configuration Option MUST be supported:

      Compression-Type                      23 (LZS-DCP)

   The following default LZS-DCP Configuration Options MUST be
   supported:

      Check-Mode                    3 (sequence + LCB)
      History-Count                 1 (single history)
      Process-Mode                  0 (None)

   The default SDTP/SDCP Configuration Options MUST be supported.  They
   are:

      Packet-Format:                Header-Last
      Header-Type:                  H-Only
      Multiple-Packets:             Off
      Multi-Port:                   Off
      Transport-Mode:               HDLC-Synchronous
      Maximum-Frame-Size:           10,000 bytes
      Allow-Odd-Frames:             Off
      FCS-Type:                     Transparent-Transport
      Flow-Expiration-Time:         100

5.  Mode-1 Requirements

   DSUs that use only Mode-1 (non-negotiate mode) use only a
   predetermined set of PPP packets.  In addition to a fixed data packet
   format, two fixed formats are used to differentiate between Mode-1
   devices and Mode-0 (transparent) devices.  Mode-1 devices are
   designed to operate using compression if a peer has the same
   capability, or revert to transparent operation (Mode-0) if the peer
   does not support standard compression.

   Mode-1 devices use LZS-DCP Compression Packets as specified in [4].
   These packets include the capabilities of DCP:  Reset-Request and
   Acknowledge, Compressed/Transparent, etc.  Since the packets include
   signalling, these packets can be sent with an empty data field to
   signal a reset request if no data packets are ready for piggy-backed
   signalling.

   Exactly one LZS-DCP packet is encapsulated in the PPP Information
   field, where the PPP Protocol field indicates type 00FD (Compression
   Protocol).  Exactly one SDTP packet is transported by each LZS-DCP
   data packet.






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RFC 1976                        PPP DCE                      August 1996


   Operation in Mode-1 implies a set of predetermined values for LCP,
   LZS-DCP, and SDTP configuration options and parameters, using the
   values listed in the preceding section.

   The following PPP packets are permitted and recognized:

      LCP Configure-Request with DCE Mode-1 Configuration Option
      LCP Configure-Ack with DCE Mode-1 Configuration Option
      LZS-DCP Packet with the data field containing an SDTP packet
      LZS-DCP Packet with an empty data field

   Protocol-Field-Compression and Address-and-Control-Field-Compression
   is used on all packets except the handshake packets (LCP packets).

   Any Mode-1 or Mode-2 DCE that receives a Mode-1 request MUST
   Acknowledge the request.

5.1.  Detailed Mode-1 Example

   Detailed Example when using Mode-1 on a point-to-point leased or
   circuit switched link (using PPP in HDLC-like Framing [6]) (data
   shown is after flags and inserted 0s are removed; lower case letters
   and numbers represent actual values, uppercase represent data fields
   whose values may vary from packet to packet; parentheses surrounding
   a field indicate that the field may not be present in all packets of
   that type):

      LCP Configure-Request:
                                               Config. Opt.
      Addr. Ctl.  PID    Code ID   Length Type Lngth Mode
      +----+----+-------+----+----+-------+----+----+----+-----+
      | ff | 03 | c0 21 | 01 | 00 | 00 07 | 21 | 03 | 01 | FCS |
      +----+----+-------+----+----+-------+----+----+----+-----+

      LCP Configure-Ack:

                                               Config. Opt.
      Addr. Ctl.  PID    Code ID   Length Type Lngth Mode
      +----+----+-------+----+----+-------+----+----+----+-----+
      | ff | 03 | c0 21 | 02 | 00 | 00 07 | 21 | 03 | 01 | FCS |
      +----+----+-------+----+----+-------+----+----+----+-----+

      LZS-DCP Packet:

       PID  DCP
      +----+----+------+------ -+-------+-----+
      | fd | HD | (SQ) | (DATA) | (LCB) | FCS |
      +----+----+------+--------+-------+-----+



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RFC 1976                        PPP DCE                      August 1996


      The DATA field contains a compressed or uncompressed SDTP-PDU.
      The LCB field is only present on a packet containing compressed
      data.  The Sequence Number and Data fields are only present on
      packets that contain data.

                        +----+------+----+
            SDTP-PDU:   | 49 | DATA | HD |
                        +----+------+----+

6.  Initial Handshake Operation

   When a unit is powered up, or when the lower layer signals that the
   peer has gone out of service and returned, the handshake procedure is
   initiated.  The handshake procedure for Mode-1 and Mode-2 devices is
   described below.

   Mode-1:

      When starting Mode-1, each DCE sends out an LCP Configure-Request
      packet containing only the DCE-Identifier LCP Configuration Option
      described in Section 3, with the with the Mode Field set to a
      value of 1.  When a DCE device receives such a packet, it must
      answer with an LCP Configure-Ack packet.  In each of these
      packets, the identifier field is set to 0.  If the originator of
      the Configure-Request packet does not receive a Configure-Ack
      response within a user configurable time T1, the unit MUST revert
      to transparent (Mode-0) operation.

   Mode-2:

      A Mode-2 device will first try to operate in Mode-2 by starting
      PPP normally, following the state machine described in [1].  The
      LCP Configure-Request MUST include the DCE-Identifier
      Configuration Option with the Mode Field set to 2.  If the unit
      receives a Configure-Reject Packet Containing the DCE-Identifier,
      the unit MUST revert immediately to transparent (Mode-0)
      operation.  If the LCP state machine times out because a response
      was not received in user configurable time T2, or if a Mode-1
      Configuration-Request packet is received, the unit attempts to
      operate in Mode-1 by following the procedure listed above,
      ultimately reverting to Mode-0 operation if the Mode-1 procedure
      times out.

   In either case, the unit is not prohibited from sending multiple
   Configuration-Request packets before the applicable timer (T1, T2)
   expires.  A unit may also initiate the handshake procedure at any
   time.




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RFC 1976                        PPP DCE                      August 1996


7.  Security Considerations

   Security issues are not discussed in this memo.

8.  References

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

   [2]    Schneider, K., and S. Venters, "PPP Serial Data Transport
          Protocol (PPP-SDTP)", RFC 1963, August 1996.

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

   [4]    Lutz, R., "PPP LZS-DCP Compression Protocol", RFC 1967
          August 1996.

   [5]    CCITT Recommendation V.120, "Support by an ISDN of Data
          Terminal Equipment with V-Series Type Interfaces with
          Provision for Statistical Multiplexing (revised 1992)", ITU-T,
          1993.

   [6]    Simpson, W., "PPP in HDLC-like Framing", STD 51, RFC 1662,
          January 1994.

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

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: [e-mail]













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RFC 1976                        PPP DCE                      August 1996


Authors' Addresses

   Questions about this memo can also be directed to:

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

   Phone: [phone]
   EMail:  [e-mail]


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

   Phone: [phone]
   EMail: [e-mail]































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