RFC 1598 (rfc1598) - Page 3 of 7

PPP in X

RFC 1598                      PPP in X.25                     March 1994


3.1.  Frame Format

   Since both "PPP in HDLC Framing" [3] and X.25 use ISO 3309 as a basis
   for framing, the X.25 header is easily substituted for the smaller
   HDLC header.  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
   +-+-+-+-+-+-+-+-+
   |  Flag (0x7e)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Address    |    Control    |D|Q| SVC# (hi) |   SVC# (lo)   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |p(r) |M|p(s) |0|         PPP Protocol          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The PPP Protocol field and the following Information and Padding
   fields are described in the Point-to-Point Protocol Encapsulation
   [1].



3.2.  Modification of the Basic Frame

   The Link Control Protocol can negotiate modifications to the basic
   frame structure.  However, modified frames will always be clearly
   distinguishable from standard frames.

   Address-and-Control-Field-Compression

      Because the Address and Control field values are not constant, and
      are modified as the frame is transported by the network switching
      fabric, Address-and-Control-Field-Compression MUST NOT be
      negotiated.

   Protocol-Field-Compression

      Note that unlike the HDLC framing, the X.25 framing does not align
      the Information field on a 32-bit boundary.  Alignment to a 16-bit
      boundary occurs when the Protocol field is compressed to a single
      octet.  When this improves throughput, Protocol-Field-Compression
      SHOULD be negotiated.









Simpson