RFC 3153 (rfc3153) - Page 2 of 9

PPP Multiplexing

RFC 3153                    PPP Multiplexing                 August 2001


   During the NCP negotiation phase of PPP, a receiver can offer to
   receive multiplexed frames using the PPP Mux Control Protocol
   (PPPMuxCP), as described in Section 2.  Once PPPMuxCP has been
   negotiated, the transmitter may choose which PPP frames to multiplex.
   Frames should not be re-ordered by either the transmitter or receiver
   regardless of whether they arrive as part of the PPP multiplexed
   frame or by themselves.

   The scheme proposed is similar to the concatenated framing option
   [2].  The key differences are that PPP multiplexing is more efficient
   and that it allows concatenation of variable sized frames.  This is
   unlike concatenated framing which restricts all frames to be of fixed
   length.

   As with any concatenation scheme, the implementer has to consider the
   tradeoff between increased delay for multiplexing/demultiplexing and
   reduced packet overhead as the length of the multiplexed frame
   increases.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [7].

1.1. Payload Format

   The format of the complete PPP frame along with multiple subframes
   for PPP in HDLC-like framing [3] is shown in Figure 1.  Note that
   regardless of the order in which individual bits are transmitted,
   i.e., LSB first or MSB first, the PFF bit will be seen to be the MSB
   of a byte that contains both the PFF and the subframe length field.

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       +P|L|     +       +     +   +P|L|     +       +     +     |
   |  PPP/ +F|X|Len1 +  PPP  +     +   +F|X|LenN +  PPP  +     +     |
   |  HDLC +F|T|     + Prot. +Info1+ ~ +F|T|     + Prot. +InfoN+ CRC |
   | Header+ | |     + Field1+     +   + | |     +FieldN +     +     |
   | (2-5) +  (1-2 ) + (0-2) +     +   +  (1-2)  + (0-2) +     + (2) |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 1. Multiplexing subframes in a PPP frame.

   PPP Header:
        The PPP header contains the PPP Protocol Field for a PPP
        Multiplexed Frame (0x0059).  The PPP header compression
        options (ACFC and PFC) may be negotiated during LCP and
        could thus affect the format of this header.





Pazhyannur, et al.          Standards Track