Network Working Group R. Pazhyannur
Request for Comments: 3153 I. Ali
Category: Standards Track Motorola
C. Fox
Cisco Systems
August 2001

PPP Multiplexing

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2001). All Rights Reserved.

Abstract

This document describes a method to redUCe the PPP (Point-to-Point
Protocol) framing overhead used to transport small packets over slow
links.

1. Description

The method, PPP Multiplexing, sends multiple PPP encapsulated packets
in a single PPP frame. As a result, the PPP overhead per packet is
reduced. PPP encapsulation (for example with PPP in HDLC framing)
adds several bytes of overhead: a HDLC flag (at least one to separate
adjacent packets), the Address (0xFF) and Control (0x03) field bytes,
a two byte PPP Protocol ID, and the two byte CRC field. Even with
the Address and Control Fields negotiated off and the PPP Protocol ID
compressed, each PPP encapsulated frame will include four bytes of
overhead. When PPP frames are tunneled, as in L2TP [1], the L2TP
overhead per PPP frame is significant.

The key idea is to concatenate multiple PPP encapsulated frames into
a single PPP multiplexed frame by inserting a delimiter before the
beginning of each frame. The description of the delimiters is
provided in Subsection 1.1. The delimiters are used by the
demultiplexor to separate the PPP frames within the multiplexed
frame. Each PPP encapsulated frame within the multiplexed frame is
called a PPP subframe.

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 RFC2119 [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.

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+PL + + + +PL + + +
PPP/ +FXLen1 + PPP + + +FXLenN + PPP + +
HDLC +FT + Prot. +Info1+ ~ +FT + 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.

Length Field:

The length field consists of three subfields:

1. Protocol Field Flag (PFF):

The PFF refers to the most significant bit of the first byte of
each subframe. This one bit field indicates whether the PPP
Protocol ID of the subframe follows the subframe length field.
For the first subframe, the PFF bit could be set to zero if the
PPP protocol ID of the first subframe is equal to the default
PID value negotiated in PPPMuxCP. PFF = 1 indicates that the
protocol field is present (and follows the length field) for
this subframe. PFF = 0 indicates that the protocol field is
absent for this subframe. If PFF = 0 then the PPP Protocol ID
is the same as that of the preceding subframe with PFF = 1, or
it is equal to default PID value of the PPPMuxCP Option for the
first subframe. The transmitter is not obligated to remove the
PPP Protocol ID for any subframe.

2. Length Extension (LXT)

This one bit field indicates whether the length field is one
byte or two bytes long. If the LXT bit is set, then the length
field is two bytes long (a PFF bit, a length extension bit, and
14 bits of sub-frame length). If the LXT bit is cleared, then
the length field is one byte long (a PFF bit, a length
extension bit, and 6 bits of sub-frame length).

3. Sub-frame Length (LEN):

This is the length of the subframe in bytes not including the
length field. However, it does include the PPP Protocol ID if
present (i.e., if PFF = 1). If the length of the subframe is
less than 64 bytes (less than or equal to 63 bytes), LXT is set
to zero and the last six bits of the length field is the
subframe length. If the length of the subframe is greater than
63 bytes, LXT is set to one and the last 14 bits of the length
field is the length of the subframe. The maximum length of a
subframe is 16,383 bytes. PPP packets larger than 16,383 bytes
will need to be sent in their own PPP frame. A transmitter is
not required to multiplex all frames smaller than 16,383 bytes.
It may chose to only multiplex frames smaller than a
configurable size into a PPP multiplexed frame.

Protocol Field:

This field contains the Protocol Field value for the subframe.
This field is optional. If PFF = 1 for a subframe, the protocol
field is present in the subframe, otherwise it is inferred at the
receiver.

The receiver MUST support Protocol-Field-Compression (PFC) one or
two bytes long. The transmitter SHOULD compress PPP Protocol IDs
in this field that have an upper byte of zero (i.e., Protocol IDs
from 0x21 thru 0xFD). This Protocol Field Compression in each PPP
subframe is not related to the negotiation of PFC during LCP
negotiation which affects the length of PPP Multiplexed Frame
Protocol ID.

Information Field:

This field contains the actual packet being encapsulated. Any
frame may be included here with the exception of LCP Configure
Request, ACK, NAK and Reject frames and PPP Multiplexed frames.
If LCP is renegotiated then PPP Multiplexing MUST be disabled
until the PPP Mux Control Protocol is negotiated.

1.2 Transmitter procedure

A simple implementation of the transmitter is provided. During the
transmission of a multiplexed PPP frame, the transmitter has a state
variable, Last_PID, which is used to hold the most recent value of
protocol field in a subframe with PFF=1. At the start of the
multiplexing process, Last_PID is set equal to the default PID value
negotiated in PPPMuxCP. Also, a user configurable parameter, maximum
subframe length (MAX_SF_LEN), is used to determine the maximum size
of a PPP frame which can be multiplexed. The value of MAX_SF_LEN
should be less or equal to the minimum of MRU-2(maximum size of
length field) and 16,383 (14 bits).

After transmitting a PPP frame (multiplexed or not) on the channel,
the PPP multiplexing logic looks at the buffers that hold the PPP
frames to be transmitted. In case there are multiple frames, the PPP
multiplexing logic checks if the length of the first frame in the
buffer is less than or equal to MAX_SF_LEN bytes. If so, the
transmitter starts compiling a multiplexed PPP frame with the
protocol field value corresponding to PPP Multiplexed Frame (0x59).
For each subframe, the test for deciding to prepend the protocol
field to a subframe is to compare the protocol field value of the
subframe to Last_PID. If they are equal, PFF is set to 0 and the
protocol field is deleted. If not, PFF is set to 1, the protocol
field is included, after PFC, in the subframe and Last_PID is set to

the protocol field value of the current subframe. The stopping
criteria in the concatenation process are (i) when the length of the
next subframe is greater than MAX_SF_LEN bytes or (ii) the length of
the entire PPP frame by including the new subframe exceeds the
maximum receive unit (MRU) parameter negotiated during LCP [4], or
(iii) there are no more subframes to concatenate.

Implementers may choose additionally to implement using timers. In
such a case a timeout in addition to the conditions stated above is
used as a stopping criteria of the multiplexing process. Moreover,
it may be desirable to limit the maximum size of a multiplexed packet
to be considerably smaller than MRU for reasons of multiplexing
latency and packet error considerations.

1.3 Receiver procedure

If a multiplexed frame, i.e., a frame with Protocol field value equal
to PPP Multiplexed Frame (0x0059), is received, the frame is
demultiplexed in order using the following input demultiplexing
logic. Similar to a transmitter, the receiver has a state variable
called Last_rcvd_PID, which is the value of the protocol field in the
most recently demultiplexed subframe with PFF=1. Last_rcvd_PID is
initialized to default PID value negotiated by PPPMuxCP. If PFF=0
for a subframe, Last_rcvd_PID is appended to the beginning of the
subframe before handing the subframe, as determined by the length
field, to the PPP logic. If PFF=1 for a subframe, Last_rcvd_PID is
set to this value and the subframe, as determined by the length
field, is passed to PPP logic. The remainder of the frame is
returned to the demultiplexor. Each succeeding subframe is processed
similarly. This processing is complete when the remainder of the
frame is empty, or when the size field of a subframe exceeds the
amount of data remaining in a packet. In the latter case, there is
an error either in the length field of the last subframe or in the
length field of one of the previous subframes. In either case the
last subframe must be dropped by the demultiplexing logic.

It is illegal to put a multiplexed frame within a multiplexed frame.

2. PPP Network Control Protocol for PPP Multiplexing (PPPMuxCP)

A receiver will offer its ability to received multiplexed frames by
negotiating NCP for PPP multiplexing, PPPMuxCP. The protocol field
value for a PPPMuxCP frames is 0x8059. PPPMuxCP is similar to other
NCPs such as IPCP [6]. A transmitter may not send a multiplexed
frame unless the peer has offered to receive multiplexed frames.
Support of multiplexed frame reception is negotiated in each
direction independently. Successful negotiation of PPPMuxCP does not
obligate a peer to transmit multiplexed frames.

As part of the PPPMuxCP negotiation, a 'default PID' option is always
negotiated. This enables the transmitter to transmit the first
subframe of a PPP multiplexed frame without a PID (PFF=0), thus
resulting in a saving of one or two bytes. Note that the negotiation
of default PID does not require the transmitter to send the first
subframe with PFF=0 even if doing so would optimize the transmission.
And, as always, the option (and thus the default PID) is negotiated
by the receiver, i.e., the receiver will interpret a received PPPmux
packet using the default PID it offered.

LCP frames MUST NOT be sent in Multiplexed frames. The only option in
PPPMuxCP is the negotiation of Default PID and is shown below

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type = 1 Length = 4 Default PID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2. Default PID option for PPPMuxCP

3. Interaction with PPP Multilink (MP) Protocol

PPP multiplexed frame option is negotiated by an NCP. LCP is
negotiated over each member link of a multilink bundle and not on the
bundle itself [5]. Thus in case of MP, PPPmux cannot be negotiated
for individual links, but only for the bundle.

Hence, on the transmitter side PPP multiplexing always occurs before
multilink PPP encapsulation. On a link, an MP header (if present)
MUST be outside of a PPPmux header (if present). Multilink frames
must not be sent in Multiplexed frames.

4. Interaction with CCP and ECP

PPP multiplexing must be performed below (after) any bundle-level CCP
and/or ECP, and above (before) MP and any per-link CCP and/or ECP.
Thus, to negotiate the hypothetical transmit path sequence CCP ->
PPPMux -> ECP, the bundle-level version of CCP (80fd) and the per-
link version of ECP (8055) are negotiated along with the PPPMux
Option.

An implementation that cannot perform PPPMux above CCP or ECP MUST
issue Protocol-Reject for the per-link forms of CCP and ECP if PPPMux
has been negotiated.

5. Security Considerations

This document does not impose additional security considerations
beyond those that apply to PPP and header-compression schemes over
PPP.

6. Acknowledgements

The authors would like to thank contributors on the PPPext mailing
list, especially James Carlson, for valuable inputs to this document.

7. References

[1] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G. and B.
Palter, "Layer Two Tunneling Protocol "L2TP"", RFC2661, August
1999.

[2] Simpson, W., Ed., "PPP LCP extensions", RFC1570, January, 1994.

[3] Simpson, W., Ed., "PPP in HDLC-like Framing", STD 51, RFC1662,
July 1994.

[4] Simpson, W., Ed., "The Point-To-Point Protocol (PPP)", STD 51,
RFC1661, July 1994.

[5] Sklower, K., Lloyd, B., McGregor, G., Carr, D., and T. Coradetti,
"The PPP Multilink Protocol (MP)", RFC1990, August 1996.

[6] McGregor, G., "The PPP Internet Protocol Control Protocol
(IPCP)", RFC1332, May 1992.

[7] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.

8. Author's Addresses

Rajesh Pazhyannur
Motorola, Network Solutions Sector
1501, W. Shure Drive
Arlington Heights, IL 60004

Phone: (847) 632-4524
EMail: pazhynnr@cig.mot.com

Irfan Ali
Motorola, Network Solutions Sector
1501, W. Shure Drive
Arlington Heights, IL 60004

Phone: (847) 632-3281
EMail: fia225@email.mot.com

Craig Fox
Cisco Systems
170 W. Tasman Street
San Jose, CA 95134

Phone: (408) 526-6296
EMail: fox@cisco.com

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