RFC 3153 (rfc3153) - Page 2 of 9
PPP Multiplexing
Alternative Format: Original Text Document
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