Defined in RFC3153
Purpose : To reduce the PPP header overhead by putting multiple ppp frames in one PPP frame
Put multiple PPP frames in one PPP frame. In this case the inner ppp frame is called as sub-frame.
Normally the PPP header overhead is maximum 7bytes
If the compression scheme defined in PPP, this is reduced to 4bytes
For the N ppp frame and N bytes data payload, the max and min overhead ratio are
N * 7 ----------- N*(7+M)
If the the M is small, the overhead ratio will be not tolerable.
PPPmux is aimed to reduce this PPP overhead by putting one once for the commonly used field through the multiple PPP frame.s
PPPmuxed frames are
Normal PPP header (2+5) +
(PPPmux header(1 or 2) + PPP protocol(0 or 2) + Payload) * N
The PPPmux header is 1 or 2 byte long which depends on the length. If the payload lengh is small than 2^6(64) the 1 byte PPPmux header can be used. Otherwise if the length if larger than this, the PPPmux header is extended to 2byte length
PPPmux header is consisted of ProtocolFieldFlag(1b) + Length(1b) + 6bit or 14bit(Length).
Protocol Field length means the protcol for this subframe is same to the default(or the previous one which has an explicit protocol field for that subframe) or not. If so, the PPP subframe does not have to include PPP protocol field.
The length flag indicates length field is 6bit or 14bit long.
As a result, the best and worst case PPP overhead of the PPPmuxed frames
2 + 1 *N + 2 --------------------- (2+ N + 2) + (M * N ) 5 + 4*N -------------------------- (5 + 4*N + 2) + (M * N)
As you can find, the PPPmux give just a little advantage in bandwidth efficiency. If the each sub-frame has a different protocol field and no PPP header compressions are negotiated and the packet is size larger than 64bytes, it just has CRC oeverhead reduction.
Practically the Protocol fields will be same to almost every PPP sub-frame(like IP) this 2 byte and 2bytes(CRC) reduction is worthwhile.
If the PPP is used for wireless system which has short packet size, the PPPmux could give good bandwidth utilization.