RFC 1662 (rfc1662) - Page 1 of 25

PPP in HDLC-like Framing

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Network Working Group                                 W. Simpson, Editor
Request for Comments: 1662                                    Daydreamer
STD: 51                                                        July 1994
Obsoletes: 1549      
Category: Standards Track


                        PPP in HDLC-like Framing


Status of this Memo

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


Abstract

   The Point-to-Point Protocol (PPP) [1] provides a standard method for
   transporting multi-protocol datagrams over point-to-point links.

   This document describes the use of HDLC-like framing for PPP
   encapsulated packets.


Table of Contents


     1.     Introduction ..........................................    1
        1.1       Specification of Requirements ...................    2
        1.2       Terminology .....................................    2

     2.     Physical Layer Requirements ...........................    3

     3.     The Data Link Layer ...................................    4
        3.1       Frame Format ....................................    5
        3.2       Modification of the Basic Frame .................    7

     4.     Octet-stuffed framing .................................    8
        4.1       Flag Sequence ...................................    8
        4.2       Transparency ....................................    8
        4.3       Invalid Frames ..................................    9
        4.4       Time Fill .......................................    9
           4.4.1  Octet-synchronous ...............................    9
           4.4.2  Asynchronous ....................................    9
        4.5       Transmission Considerations .....................   10
           4.5.1  Octet-synchronous ...............................   10
           4.5.2  Asynchronous ....................................   10


Simpson                                                         [Page i]
RFC 1662                   HDLC-like Framing                   July 1994


     5.     Bit-stuffed framing ...................................   11
        5.1       Flag Sequence ...................................   11
        5.2       Transparency ....................................   11
        5.3       Invalid Frames ..................................   11
        5.4       Time Fill .......................................   11
        5.5       Transmission Considerations .....................   12

     6.     Asynchronous to Synchronous Conversion ................   13

     7.     Additional LCP Configuration Options ..................   14
        7.1       Async-Control-Character-Map (ACCM) ..............   14

     APPENDICES ...................................................   17
     A.     Recommended LCP Options ...............................   17
     B.     Automatic Recognition of PPP Frames ...................   17
     C.     Fast Frame Check Sequence (FCS) Implementation ........   18
        C.1       FCS table generator .............................   18
        C.2       16-bit FCS Computation Method ...................   19
        C.3       32-bit FCS Computation Method ...................   21

     SECURITY CONSIDERATIONS ......................................   24
     REFERENCES ...................................................   24
     ACKNOWLEDGEMENTS .............................................   25
     CHAIR'S ADDRESS ..............................................   25
     EDITOR'S ADDRESS .............................................   25




1.  Introduction

   This specification provides for framing over both bit-oriented and
   octet-oriented synchronous links, and asynchronous links with 8 bits
   of data and no parity.  These links MUST be full-duplex, but MAY be
   either dedicated or circuit-switched.

   An escape mechanism is specified to allow control data such as
   XON/XOFF to be transmitted transparently over the link, and to remove
   spurious control data which may be injected into the link by
   intervening hardware and software.

   Some protocols expect error free transmission, and either provide
   error detection only on a conditional basis, or do not provide it at
   all.  PPP uses the HDLC Frame Check Sequence for error detection.
   This is commonly available in hardware implementations, and a
   software implementation is provided.






Simpson
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