Internet Draft 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]