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

本文件规定了互联网社区的互联网标准跟踪协议,并要求进行讨论和提出改进建议。有关本协议的标准化状态和状态,请参考当前版本的“互联网官方协议标准”(STD 1)。本备忘录的分发不受限制。

Copyright Notice


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




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
1. 描述

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.

在PPP的NCP协商阶段,接收机可以使用PPP Mux控制协议(PPPMuxCP)提供接收复用帧,如第2节所述。一旦协商了PPPMuxCP,发射机可以选择要复用的PPP帧。无论帧是作为PPP多路复用帧的一部分到达的,还是由帧自身到达的,发射机或接收机都不应重新排序帧。

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].

本文件中的关键词“必须”、“不得”、“要求”、“应”、“不应”、“应”、“不应”、“建议”、“可”和“可选”应按照RFC 2119[7]中所述进行解释。

1.1. Payload Format
1.1. 有效载荷格式

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) |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       +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.


Length Field:


The length field consists of three subfields:


1. Protocol Field Flag (PFF):

1. 协议字段标志(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)

2. 长度延伸(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):

3. 子帧长度(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.

此字段包含要封装的实际数据包。除了LCP配置请求、ACK、NAK和Reject帧以及PPP多路复用帧之外,这里可以包括任何帧。如果重新协商LCP,则必须禁用PPP多路复用,直到协商PPP Mux控制协议。

1.2 Transmitter procedure
1.2 变送器程序

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).

本发明提供了发射机的简单实现。在多路复用PPP帧的传输期间,发射机具有状态变量Last_PID,其用于保持PFF=1的子帧中协议字段的最新值。在多路复用过程开始时,Last_PID设置为PPPMuxCP中协商的默认PID值。此外,用户可配置参数maximum subframe length(MAX_SF_LEN)用于确定可复用的PPP帧的最大大小。MAX_SF_LEN的值应小于或等于MRU-2(长度字段的最大大小)和16383(14位)中的最小值。

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
1.3 接收程序

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.

如果接收到复用帧,即协议字段值等于PPP复用帧(0x0059)的帧,则使用以下输入解复用逻辑按顺序解复用该帧。与发射机类似,接收机具有称为Last_rcvd_PID的状态变量,它是PFF=1的最近解复用子帧中的协议字段的值。最后一个\u rcvd \u PID初始化为PPPMuxCP协商的默认PID值。如果子帧的PFF=0,则在将由长度字段确定的子帧传递到PPP逻辑之前,最后一个_rcvd _PID被附加到子帧的开头。如果子帧的PFF=1,则最后一个_rcvd _PID设置为该值,并且由长度字段确定的子帧被传递给PPP逻辑。帧的其余部分返回到解复用器。对每个后续子帧进行类似的处理。当帧的剩余部分为空时,或者当子帧的大小字段超过数据包中剩余的数据量时,该处理完成。在后一种情况下,在最后一个子帧的长度字段中或在前一个子帧之一的长度字段中存在错误。在任何一种情况下,解复用逻辑都必须丢弃最后一个子帧。

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


2. PPP Network Control Protocol for PPP Multiplexing (PPPMuxCP)
2. PPP多路复用的PPP网络控制协议(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.

接收机将通过协商用于PPP多路复用的NCP PPPMuxCP来提供接收多路复用帧的能力。PPPMuxCP帧的协议字段值为0x8059。PPPMuxCP与IPCP等其他NCP类似[6]。除非对等方已提供接收多路复用帧,否则发射机不能发送多路复用帧。多路复用帧接收的支持在每个方向上独立协商。PPPMuxCP的成功协商并不要求对等方传输多路复用帧。

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            |
    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
3. 与PPP多链路(MP)协议的交互

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
4. 与CCP和ECP的互动

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
5. 安全考虑

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


6. Acknowledgements
6. 致谢

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

作者要感谢PPPext邮件列表上的贡献者,特别是James Carlson,为本文件提供了宝贵的投入。

7. References
7. 工具书类

[1] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G. and B. Palter, "Layer Two Tunneling Protocol "L2TP"", RFC 2661, August 1999.

[1] 汤斯利,W.,巴伦西亚,A.,鲁本斯,A.,帕尔,G.,佐恩,G.和B.帕尔特,“第二层隧道协议“L2TP”,RFC 26611999年8月。

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

[2] 辛普森,W.,编辑,“PPP LCP扩展”,RFC 15701994年1月。

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

[3] 辛普森,W.,编辑,“HDLC类框架中的PPP”,STD 51,RFC 16621994年7月。

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

[4] 辛普森,W.,编辑,“点对点协议(PPP)”,STD 51,RFC 1661994年7月。

[5] Sklower, K., Lloyd, B., McGregor, G., Carr, D., and T. Coradetti, "The PPP Multilink Protocol (MP)", RFC 1990, August 1996.

[5] K.Sklower、Lloyd、B.McGregor、G.Carr、D.和T.Coradetti,“PPP多链路协议(MP)”,RFC 1990,1996年8月。

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

[6] McGregor,G.“PPP互联网协议控制协议(IPCP)”,RFC 1332,1992年5月。

[7] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.

[7] Bradner,S.,“RFC中用于表示需求水平的关键词”,BCP 14,RFC 2119,1997年3月。

8. Author's Addresses
8. 作者地址

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

Rajesh Pazhyannur摩托罗拉,伊利诺伊州阿灵顿高地舒尔大道西1501号网络解决方案区,邮编60004

Phone: (847) 632-4524 EMail:


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

伊利诺伊州阿灵顿高地舒尔大道西1501号网络解决方案区Irfan Ali Motorola 60004

Phone: (847) 632-3281 EMail:


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


Phone: (408) 526-6296 EMail:


Full Copyright Statement


Copyright (C) The Internet Society (2001). 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 languages other than English.


The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns.






Funding for the RFC Editor function is currently provided by the Internet Society.