Internet Engineering Task Force PPPext Working Group
Internet Draft R. Pazhyannur,
I. Ali
Motorola
Craig Fox
Cisco Systems
Document: <draft-ietf-pppext-pppmux-00.txt>
Expires: June 5, 2000 January 5, 2000
PPP Multiplexed Frame Option
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
1. Abstract
This draft describes a method to reduce the PPP framing overhead
used to transport small packets over slow links. The method, PPP
Multiplexing, sends multiple PPP encapsulated packets in a single
PPP frame. As a result, the PPP overhead per packet is reduced.
2. Description
At a minimum, PPP encapsulating a packet adds several bytes of
overhead, including an HDLC flag character (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
if the Address and Control Fields are negotiated off and the PPP
Protocol ID is compressed, each PPP encapsulated frame will include
four bytes of overhead. This overhead can be reduced to one or two
bytes.
The key idea is to concatenate multiple PPP encapsulated frames into
a single PPP multiplexed frame by inserting a length field before
the beginning of each frame. Each PPP encapsulated frame is called a
PPP subframe. Removing the PPP framing characters can save several
bytes per packet, reducing overhead. The PPP Protocol ID field can
R. Pazhyannur, I. Ali, C. Fox [Page 1]
PPP Mux January, 2000
also be removed for those subframes which have the same PPP Protocol
ID as the preceding subframe.
For the sake of efficiency, the maximum size of a subframe is 127
bytes. This size includes the PPP Protocol ID, unless it is removed
during the multiplexing process. This size limitation should not be
significant since the impact of the PPP overhead is reduced as the
packet size is increased.
During the LCP negotiation phase of PPP, a receiver can offer to
receive multiplexed frames using an LCP Option, described in Section
4. Once LCP 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.
As with any concatenation scheme, the implementor has to consider
the the tradeoff between increased delay for
multiplexing/demultiplexing and reduced packet overhead as the
length of the multiplexed frame increases.
2.1. Protocol Field Elimination
The PPP Protocol ID field of a subframe can be removed if the PPP
Protocol ID of that subframe is the same as that for the preceding
subframe. A Protocol Field Flag (PFF) bit is defined part of the
length field (thus reducing the length field from an 8-bit to a 7-
bit field). The PFF bit is set if the PPP Protocol ID is included in
the subframe. The PFF bit is cleared if the PPP Protocol ID has been
removed from the subframe. The PFF bit MUST be set for the first
subframe in a PPP multiplexed Frame. The transmitter is not
obligated to remove the PPP Protocol ID for any subframe.
2.2. Payload Format
The format of the complete PPP frame along with multiple subframes
is shown in Figure 2. 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| + + + +P| + + + |
| PPP +F| Len1 + PPP + + +F| LenN + PPP + + |
|Header +F| + Prot. +Info1+ ~ +F| + Prot. +InfoN+ CRC |
| + |(7bits)+ Field1+ + + |(7bits)+FieldN + + |
| (2-5) +(1 byte )+ (0-2) + + +(1 byte) + (0-2) + + (2) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2. Multiplexing subframes in a PPP frame.
R. Pazhyannur, I. Ali, C. Fox [Page 2]
PPP Mux January, 2000
PPP Header:
The PPP header contains the PPP Protocol Field for a PPP
Multiplexed Frame (0x59). The PPP header compression
options (ACFC and PFC) may be negotiated during LCP and
could thus affect the format of this header.
Protocol Field Flag (PFF):
This one bit field indicates whether the PPP Protocol ID of the
subframe follows the subframe length field. PFF = 1 indicates
that the protocol field is present for this subframe. PFF = 0
indicates that the protocol field is absent for this subframe.
The first subframe of each PPP multiplexed frame MUST have
PFF = 1. If PFF = 0 then the PPP Protocol ID is the same as
that of the preceding subframe with PFF = 1.
Length Field:
Each subframe has a seven bit subframe length field. This
length does not include the byte containing the PFF and length
field but does include the PPP Protocol ID if present (i.e. if
PFF = 1). The maximum length of a subframe is 127 bytes. PPP
packets larger than 127 bytes will need to be sent in their
own PPP 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)
[2] for PPP Protocol IDs in this field. Thus the
field may be 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 is not related to the negotiation
of PFC during LCP negotiation.
Information Field:
This field contains the actual packet being encapsulated.
The maximum length of this field is 127 bytes, if the Protocol
Field is eliminated from the subframe. 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.
2.2 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
R. Pazhyannur, I. Ali, C. Fox [Page 3]
PPP Mux January, 2000
protocol field in a subframe with PFF=1. This variable is only valid
during the transmission of a multiplexed frame and no history is
maintained from the transmission of one multiplexed PPP frame to the
next one. This is because the first subframe in every multiplexed
subframe MUST contain the protocol field.
After transmitting a PPP frame (multiplexed or not) on the channel,
the PPP multiplexing logic looks at the buffers which hold the PPP
frames to be transmitted. In case there are multiple frames, the PPP
multiplexing logic checks if either the length of the first frame or
the second frame in the buffer is greater than 127 bytes. If either
of these two conditions are met, the first and second frame are
transmitted as non-multiplexed frames. The above logic ensures that
small frames separated by large frames will not be transmitted as
multiplexed frames with only one subframe. If both conditions are
not true, i.e., the length of the first PPP frame and that of the
second PPP frame are less than or equal to 127 bytes, the
transmitter starts compiling a multiplexed PPP frame with the
protocol field value corresponding to PPP_Multiplexed_Frame (0x59).
It sets PFF=1 and then prepends the length of the first PPP
subframe, after compressing the Protocol field. The protocol field
is included in the first subframe. For subsequent subframes, 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 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 127
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 [2], or (iii) there are no more subframes to concatenate.
2.3 Receiver procedure
If a multiplexed frame, i.e. a frame with Protocol field value equal
to PPP_Multiplexed_Frame (0x59), 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_ID, which is the value of the protocol field in the
most recently demultiplexed subframe with PFF=1. No history is
preserved in going from one multiplexed PPP frame to the next. From
the first subframe the protocol field value is copied to
Last_rcvd_PID. The length of the first subframe is determined, and
the subframe is passed to be processed as a PPP frame. The remainder
of the frame is returned to the demultiplexor.
Each succeeding subframe is processed similarly. If PFF=0 for a
subframe, Last_rcvd_PID is appended to the beginning of the subframe
before handing the subframe to the PPP logic. If PFF=1 for a
subframe, Last_rcvd_PID is set to this value and the subframe is
passed to PPP logic. This processing is complete when the remainder
of the frame is empty, or when the size field of a subframe exceeds
R. Pazhyannur, I. Ali, C. Fox [Page 4]
PPP Mux January, 2000
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.
3. PPP Link Control Protocol Extension
A receiver will offer its ability to received multiplexed frames by
negotiating LCP Option 30. A transmitter may not send a multiplexed
unless the peer has offered to receive multiplexed frames. Support
of multiplexed frame reception is negotiated in each direction
independently. Acknowledgement of the multiplexed Frame LCP Option
(30) does not obligate a peer to transmit Multiplexed frames.
LCP frames MUST NOT be sent in Multiplexed frames.
A summary of the PPP multiplexed frame option is shown below
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 30 | Length = 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3. PPP Multiplexed Frame LCP Option
The size of a multiplexed PPP frame MUST NOT exceed the maximum
receive unit (MRU) size negotiated during LCP [2].
4. Security Considerations
This draft does not impose additional security considerations beyond
those that apply to PPP and header-compression schemes over PPP.
5. References
[1] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
[2] Simpson, W., Ed., "The Point-To-Point Protocol (PPP)", STD 51,
RFC 1661, July 1994.
11. Author's Addresses
Rajesh Pazhyannur
Motorola, Network Solutions Sector
1501, W. Shure Drive
Arlington Heights, IL 60004
Phone: (847) 632-4524
R. Pazhyannur, I. Ali, C. Fox [Page 5]
PPP Mux January, 2000
Email: pazhynnr@cig.mot.com
Irfan Ali
Motorola, Network Solutions Sector
2A8, 1421 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
E-mail: fox@cisco.com
R. Pazhyannur, I. Ali, C. Fox [Page 6]
PPP Mux January, 2000
Full Copyright Statement
"Copyright (C) The Internet Society (date). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph
are included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into
R. Pazhyannur, I. Ali, C. Fox [Page 7]
