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Versions: 00 01 02 03 rfc3153                                           
PPPext Working Group                               R. Pazhyannur,
Internet Engineering Task Force                            I. Ali
Internet Draft                                           Motorola
                                                           C. Fox
                                                    Cisco Systems

Expires: April 2, 2000                            October 2, 2000

                            PPP Multiplexing

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

   The list of Internet-Draft Shadow Directories can be accessed at

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

   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 is 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 2.1. The delimiters are used by the

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                               PPP Mux                  October, 2000

   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 3. 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

2.1. 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 รป2 ) + (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:

R. Pazhyannur, I. Ali, C. Fox                                 [Page 2]

                               PPP Mux                  October, 2000

        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)
        [4] for subframe PPP Protocol IDs. 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

R. Pazhyannur, I. Ali, C. Fox                                 [Page 3]

                               PPP Mux                  October, 2000

        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.

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
   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 maximum packet
   length for multiplexing, MAX_SF_LEN, could be set which is less than
   MRU and 16,384.

   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 either the length of the first frame or
   the second frame in the buffer is greater than MAX_SF_LEN 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 MAX_SF_LEN bytes, 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 alternately to implement using timers. In
   such a case a timeout in addition to the conditions stated above is
   used for the stopping criteria of the multiplexing process.

R. Pazhyannur, I. Ali, C. Fox                                 [Page 4]

                               PPP Mux                  October, 2000

2.3 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.

3. 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 like 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.

R. Pazhyannur, I. Ali, C. Fox                                 [Page 5]

                               PPP Mux                  October, 2000

   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

4. 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.

5. Interaction with CCP and ECP

   Though PPP multiplexing in itself does not place any requirements on
   the sequence in which it is performed with respect to compression
   and encryption, in order to simplify implementation and ensure
   interoperability, PPPmux header MUST always be outside CCP and ECP

6. Security Considerations

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

7. Acknowledgements

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

8. References

   [1] Townsley, M., et al, "Layer Two Tunneling Protocol "L2TP"", RFC
   2661, August 1999.

R. Pazhyannur, I. Ali, C. Fox                                 [Page 6]

                               PPP Mux                  October, 2000

   [2] Simpson, W., Ed., "PPP LCP extensions", RFC 1570, January, 1994.

   [3] Simpson, W., Ed., "PPP in HDLC-like Framing", STD 51, RFC 1662,
   July 1994.

   [4] Simpson, W., Ed., "The Point-To-Point Protocol (PPP)", STD 51,
   RFC 1661, July 1994.

   [5] Sklower, K., et al, "The PPP Multilink Protocol (MP)", RFC 1990,
   August 1996.

   [6] McGregor, G., "The PPP Internet Protocol Control Protocol
   (IPCP)", RFC 1332, May 1992.

7. 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
   E-mail: fox@cisco.com

R. Pazhyannur, I. Ali, C. Fox                                 [Page 7]

                               PPP Mux                  October, 2000

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R. Pazhyannur, I. Ali, C. Fox                                 [Page 8]