RFC 1549 (rfc1549) - Page 3 of 18

PPP in HDLC Framing

RFC 1549                      HDLC Framing                Decvember 1993


1.2 Terminology

   This document frequently uses the following terms:

    datagram

      The unit of transmission in the network layer (such as IP).  A
      datagram may be encapsulated in one or more packets passed to the
      data link layer.

    frame

      The unit of transmission at the data link layer.  A frame may
      include a header and/or a trailer, along with some number of units
      of data.

    packet

      The basic unit of encapsulation, which is passed across the
      interface between the network layer and the data link layer.  A
      packet is usually mapped to a frame; the exceptions are when data
      link layer fragmentation is being performed, or when multiple
      packets are incorporated into a single frame.

    peer

      The other end of the point-to-point link.

    silently discard

      This means the implementation discards the packet without further
      processing.  The implementation SHOULD provide the capability of
      logging the error, including the contents of the silently
      discarded packet, and SHOULD record the event in a statistics
      counter.

2. Physical Layer Requirements

   PPP is capable of operating across most DTE/DCE interfaces (such as,
   EIA RS-232-C, EIA RS-422, EIA RS-423 and CCITT V.35).  The only
   absolute requirement imposed by PPP is the provision of a full-duplex
   circuit, either dedicated or circuit-switched, which can operate in
   either an asynchronous (start/stop), bit-synchronous, or octet-
   synchronous mode, transparent to PPP Data Link Layer frames.

    Interface Format

      PPP presents an octet interface to the physical layer.  There is



Simpson