RFC 1549 (rfc1549) - Page 2 of 18

PPP in HDLC Framing

RFC 1549                      HDLC Framing                Decvember 1993


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.  PPP uses HDLC as a basis for
   the framing.

   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.

1.1 Specification of Requirements

   In this document, several words are used to signify the requirements
   of the specification.  These words are often capitalized.

    MUST

      This word, or the adjective "required", means that the definition
      is an absolute requirement of the specification.

    MUST NOT

      This phrase means that the definition is an absolute prohibition
      of the specification.

    SHOULD

      This word, or the adjective "recommended", means that there may
      exist valid reasons in particular circumstances to ignore this
      item, but the full implications must be understood and carefully
      weighed before choosing a different course.

    MAY

      This word, or the adjective "optional", means that this item is
      one of an allowed set of alternatives.  An implementation which
      does not include this option MUST be prepared to interoperate with
      another implementation which does include the option.




Simpson