Network Working Group W A Simpson Internet Draft Daydreamer expires in six months July 1993 PPP HDLC Framing Status of this Memo This document is the product of the Point-to-Point Protocol Working Group of the Internet Engineering Task Force (IETF). Comments should be submitted to the ietf-ppp@ucdavis.edu mailing list. Distribution of this memo is unlimited. This document is an Internet Draft. 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. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a ``working draft'' or ``work in progress.'' Please check the 1id-abstracts.txt listing contained in the internet-drafts Shadow Directories on nic.ddn.mil, nnsc.nsf.net, nic.nordu.net, ftp.nisc.sri.com, or munnari.oz.au to learn the current status of any Internet Draft. Abstract The Point-to-Point Protocol (PPP) [1] provides a method for transmitting multi-protocol datagrams over point-to-point links. This document describes the use of HDLC for framing PPP encapsulated datagrams. Simpson expires in six months [Page i] DRAFT PPP Framing July 1993 1. Introduction This specification provides for framing over both bit-oriented and octet-oriented synchronous links, and asynchronous links with 8 bits of data and no parity. These links MUST be full-duplex, but MAY be either dedicated or circuit-switched. PPP uses HDLC as a basis for the framing. An escape mechanism is specified to allow control data such as XON/XOFF to be transmitted transparently over the link, and to remove spurious control data which may be injected into the link by intervening hardware and software. Some protocols expect error free transmission, and either provide error detection only on a conditional basis, or do not provide it at all. PPP uses the HDLC Frame Check Sequence for error detection. This is commonly available in hardware implementations, and a software implementation is provided. 1.1. Specification of Requirements In this document, several words are used to signify the requirements of the specification. These words are often capitalized. MUST This word, or the adjective "required", means that the definition is an absolute requirement of the specification. MUST NOT This phrase means that the definition is an absolute prohibition of the specification. SHOULD This word, or the adjective "recommended", means that there may exist valid reasons in particular circumstances to ignore this item, but the full implications should be understood and carefully weighed before choosing a different course. MAY This word, or the adjective "optional", means that this item is one of an allowed set of alternatives. An implementation which does not include this option MUST be prepared to interoperate with another implementation which does include the option. 1.2. Terminology This document frequently uses the following terms: Simpson expires in six months [Page 1] DRAFT PPP Framing July 1993 peer The other end of the point-to-point link. silently discard This means the implementation discards the packet without further processing. The implementation SHOULD provide the capability of logging the error, including the contents of the silently discarded packet, and SHOULD record the event in a statistics counter. Simpson expires in six months [Page 2] DRAFT PPP Framing July 1993 2. Physical Layer Requirements PPP is capable of operating across most DTE/DCE interfaces (such as, EIA RS-232-C, EIA RS-422, EIA RS-423 and CCITT V.35). The only absolute requirement imposed by PPP is the provision of a full-duplex circuit, either dedicated or circuit-switched, which can operate in either an asynchronous (start/stop), bit-synchronous, or octet- synchronous mode, transparent to PPP Data Link Layer frames. Interface Format PPP presents an octet interface to the physical layer. There is no provision for sub-octets to be supplied or accepted. Transmission Rate PPP does not impose any restrictions regarding transmission rate, other than that of the particular DTE/DCE interface. Encoding PPP does not require any particular synchronous encoding, such as FM, NRZ, or NRZI. The use of various encodings and scrambling is the responsibility of the DTE/DCE equipment in use, and is outside the scope of this specification. While PPP will operate without regard to the underlying representation of the octet stream, lack of standards for transmission will hinder interoperability as surely as lack of data link standards. At speeds of up to 2.0 Mbps, NRZ is currently most widely available, and on that basis is recommended as a default. When some configuration of the encoding is allowed, NRZI is recommended as an alternative, because of its relative immunity to signal inversion configuration errors, and instances when it MAY allow connection without an expensive DSU/CSU. Unfortunately, NRZI encoding obviates the (1 + x) factor of the 16-bit FCS, so that one error in 2**15 goes undetected instead of one in 2**16, and triple errors are not detected. Therefore, when NRZI is in use, it is recommended that the 32-bit FCS be negotiated, which does not include the (1 + x) factor. At speeds of up to 45 Mbps, some implementors have chosen the ANSI High Speed Synchronous Interface as the underlying transport. [Anybody have any restrictions or references?] Simpson expires in six months [Page 3] DRAFT PPP Framing July 1993 Control Signals PPP does not require the use of modem control signals, such as Request To Send (RTS), Clear To Send (CTS), Data Carrier Detect (DCD), and Data Terminal Ready (DTR). When available, using such signals can allow greater functionality and performance. In particular, such signals SHOULD be used to signal the Up and Down events in the LCP Option Negotiation Automaton [1]. When such signals are not available, the implementation MUST signal the Up event to LCP upon initialization, and SHOULD NOT signal the Down event. Simpson expires in six months [Page 4] DRAFT PPP Framing July 1993 3. The Data Link Layer PPP uses the principles, terminology, and frame structure of the International Organization For Standardization's (ISO) High-level Data Link Control (HDLC) procedures (ISO 3309-1979 [2]), as modified by ISO 3309:1984/PDAD1 "Addendum 1: Start/stop transmission" [3]. ISO 3309-1979 specifies the HDLC frame structure for use in synchronous environments. ISO 3309:1984/PDAD1 specifies proposed modifications to ISO 3309-1979 to allow its use in asynchronous environments. The PPP control procedures use the definitions and Control field encodings standardized in ISO 4335-1979 [4] and ISO 4335- 1979/Addendum 1-1979 [5]. The PPP frame structure is also consistent with CCITT Recommendation X.25 LAPB [6], and CCITT Recommendation Q.922 [7], since those are also based on HDLC. The purpose of this specification is not to document what is already standardized in ISO 3309. It is assumed that the reader is already familiar with HDLC, or has access to a copy of [2] or [6]. Instead, this paper attempts to give a concise summary and point out specific options and features used by PPP. Since "Addendum 1: Start/stop transmission", is not yet standardized and widely available, it is summarized in a following section. To remain consistent with standard Internet practice, and avoid confusion for people used to reading RFCs, all binary numbers in the following descriptions are in Most Significant Bit to Least Significant Bit order, reading from left to right, unless otherwise indicated. Note that this is contrary to standard ISO and CCITT practice which orders bits as transmitted (network bit order). Keep this in mind when comparing this document with the international standards documents. Simpson expires in six months [Page 5] DRAFT PPP Framing July 1993 3.1. Frame Format A summary of the PPP HDLC frame structure is shown below. This figure does not include start/stop bits (for asynchronous links), nor any bits or octets inserted for transparency. The fields are transmitted from left to right. +----------+----------+----------+ | Flag | Address | Control | | 01111110 | 11111111 | 00000011 | +----------+----------+----------+ +----------+-------------+---------+ | Protocol | Information | Padding | | 16 bits | * | * | +----------+-------------+---------+ +----------+----------+----------------- | FCS | Flag | Inter-frame Fill | 16 bits | 01111110 | or next Address +----------+----------+----------------- The Protocol, Information and Padding fields are described in the Point-to-Point Protocol Encapsulation [1]. Flag Sequence The Flag Sequence indicates the beginning or end of a frame, and always consists of the binary sequence 01111110 (hexadecimal 0x7e). The Flag Sequence is a frame separator. Only one Flag Sequence is required between two frames. Two consecutive Flag Sequences constitute an empty frame, which is ignored, and not counted as a FCS error. Address Field The Address field is a single octet and contains the binary sequence 11111111 (hexadecimal 0xff), the All-Stations address. PPP does not assign individual station addresses. The All- Stations address MUST always be recognized and received. The use of other address lengths and values may be defined at a later time, or by prior agreement. Frames with unrecognized Addresses SHOULD be silently discarded, and reported through the normal network management facility. Control Field The Control field is a single octet and contains the binary Simpson expires in six months [Page 6] DRAFT PPP Framing July 1993 sequence 00000011 (hexadecimal 0x03), the Unnumbered Information (UI) command with the P/F bit set to zero. The use of other Control field values may be defined at a later time, or by prior agreement. Frames with unrecognized Control field values SHOULD be silently discarded. Frame Check Sequence (FCS) Field The Frame Check Sequence field is normally 16 bits (two octets). The use of other FCS lengths may be defined at a later time, or by prior agreement. The FCS is transmitted with the coefficient of the highest term first. The FCS field is calculated over all bits of the Address, Control, Protocol, Information and Padding fields, not including any start and stop bits (asynchronous) nor any bits (synchronous) or octets (asynchronous) inserted for transparency. This also does not include the Flag Sequences or the FCS field itself. Note: When octets are received which are flagged in the Async- Control-Character-Map, they are discarded before calculating the FCS. For more information on the specification of the FCS, see ISO 3309 [2] or CCITT X.25 [6]. The end of the Information and Padding fields is found by locating the closing Flag Sequence and removing the Frame Check Sequence field. Simpson expires in six months [Page 7] DRAFT PPP Framing July 1993 3.2. Modification of the Basic Frame The Link Control Protocol can negotiate modifications to the basic HDLC frame structure. However, modified frames will always be clearly distinguishable from standard frames. Address-and-Control-Field-Compression When using the default HDLC framing, the Address and Control fields contain the hexadecimal values 0xff and 0x03 respectively. On transmission, compressed Address and Control fields are formed by simply omitting them. On reception, the Address and Control fields are decompressed by examining the first two octets. If they contain the values 0xff and 0x03, they are assumed to be the Address and Control fields. If not, it is assumed that the fields were compressed and were not transmitted. By definition, the first octet of a two octet Protocol field will never be 0xff (since it is not even). The Protocol field value 0x00ff is not allowed (reserved) to avoid ambiguity when Protocol-Field-Compression is enabled and the first Information field octet is 0x03. When other Address or Control field values are in use, Address- and-Control-Field-Compression MUST NOT be negotiated. Simpson expires in six months [Page 8] DRAFT PPP Framing July 1993 4. Bit-synchronous HDLC This section summarizes the considerations for interoperability of ISO 3309-1979, as applied in the Point-to-Point Protocol to bit- synchronous links. Flag Sequence The Flag Sequence indicates the beginning or end of a frame, and is used for frame synchronization. The bit stream is examined on a bit-by-bit basis for the binary sequence 01111110 (hexadecimal 0x7e). The "shared zero mode" Flag Sequence "011111101111110" SHOULD NOT be used. When not avoidable, such an implementation MUST ensure that the first Flag Sequence detected (the end of the frame) is promptly communicated to the link layer. Use of the shared zero mode hinders interoperability with synchronous-to-asynchronous converters. Transparency The transmitter examines the entire frame between the two Flag Sequences. A "0" bit is inserted after all sequences of five contiguous "1" bits (including the last 5 bits of the FCS) to ensure that a Flag Sequence is not simulated. When receiving, any "0" bit that directly follows five contiguous "1" bits is discarded. There may be some use of synchronous-to-asynchronous converters (some built into modems) in point-to-point links resulting in a synchronous PPP implementation on one end of a link and an asynchronous implementation on the other. It is the responsibility of the converter to do all mapping conversions during operation. To enable this functionality, bit-synchronous PPP implementations MUST always respond to an LCP Configure- Request specifying the Async-Control-Character-Map Configuration Option with an LCP Configure-Ack. However, acceptance of the Configuration Option does not imply that the bit-synchronous implementation will do any character mapping, since bit- synchronous equipment uses bit-stuffing rather than character- stuffing. Instead, all such character mapping will be performed by the asynchronous-to-synchronous converter. Aborting a Transmission A sequence of more than six "1" bits indicates an invalid frame, Simpson expires in six months [Page 9] DRAFT PPP Framing July 1993 which is ignored, and not counted as a FCS error. Inter-frame Time Fill For bit-synchronous links, the Flag Sequence SHOULD be transmitted during inter-frame time fill. There is no provision for inter- octet time fill. Mark idle (continuous ones) SHOULD NOT be used for inter-frame time fill. However, certain types of circuit-switched links require the use of mark idle, particularly those that calculate accounting based on periods of bit activity. When mark idle is used on a bit-synchronous link, the implementation MUST ensure at least 15 consecutive "1" bits between Flags during the idle period, and that the Flag Sequence is always generated at the beginning of a frame after an idle period. Simpson expires in six months [Page 10] DRAFT PPP Framing July 1993 5. Asynchronous HDLC This section summarizes the modifications to ISO 3309-1979 proposed in ISO 3309:1984/PDAD1, as applied in the Point-to-Point Protocol. These modifications allow HDLC to be used with 8-bit asynchronous links. Transmission Considerations All octets are transmitted with one start bit, eight bits of data, and one stop bit. There is no provision in either PPP or ISO 3309:1984/PDAD1 for seven bit asynchronous links. Flag Sequence The Flag Sequence indicates the beginning or end of a frame. The octet stream is examined on an octet-by-octet basis for the value 01111110 (hexadecimal 0x7e). Transparency On asynchronous links, a character stuffing procedure is used. The Control Escape octet is defined as binary 01111101 (hexadecimal 0x7d) where the bit positions are numbered 87654321 (not 76543210, BEWARE). Each end of the link maintains two Async-Control-Character-Maps. The receiving ACCM is 32 bits, but the sending ACCM may be up to 256 bits. This results in four distinct ACCMs, two in each direction of the link. After FCS computation, the transmitter examines the entire frame between the two Flag Sequences. Each Flag Sequence, Control Escape octet, and octet with value less than hexadecimal 0x20 which is flagged in the sending Async-Control-Character-Map, is replaced by a two octet sequence consisting of the Control Escape octet and the original octet with bit 6 complemented (exclusive- or'd with hexadecimal 0x20). Prior to FCS computation, the receiver examines the entire frame between the two Flag Sequences. Each octet with value less than hexadecimal 0x20 is checked. If it is flagged in the receiving Async-Control-Character-Map, it is simply removed (it may have been inserted by intervening data communications equipment). For each Control Escape octet, that octet is also removed, but bit 6 of the following octet is complemented, unless it is the Flag Sequence. Simpson expires in six months [Page 11] DRAFT PPP Framing July 1993 Note: The inclusion of all octets less than hexadecimal 0x20 allows all ASCII control characters [8] excluding DEL (Delete) to be transparently communicated through almost all known data communications equipment. The transmitter may also send octets with value in the range 0x40 through 0xff (except 0x5e) in Control Escape format. Since these octet values are not negotiable, this does not solve the problem of receivers which cannot handle all non-control characters. Also, since the technique does not affect the 8th bit, this does not solve problems for communications links that can send only 7- bit characters. A few examples may make this more clear. Packet data is transmitted on the link as follows: 0x7e is encoded as 0x7d, 0x5e. 0x7d is encoded as 0x7d, 0x5d. 0x01 is encoded as 0x7d, 0x21. Some modems with software flow control may intercept outgoing DC1 and DC3 ignoring the 8th (parity) bit. This data would be transmitted on the link as follows: 0x11 is encoded as 0x7d, 0x31. 0x13 is encoded as 0x7d, 0x33. 0x91 is encoded as 0x7d, 0xb1. 0x93 is encoded as 0x7d, 0xb3. Aborting a Transmission On asynchronous links, frames may be aborted by transmitting a "0" stop bit where a "1" bit is expected (framing error) or by transmitting a Control Escape octet followed immediately by a closing Flag Sequence. Time Fill For asynchronous links, inter-octet and inter-frame time fill MUST be accomplished by transmitting continuous "1" bits (mark-hold state). Inter-frame time fill can be viewed as extended inter-octet time fill. Doing so can save one octet for every frame, decreasing delay and increasing bandwidth. This is possible since a Flag Sequence may serve as both a frame close and a frame begin. After Simpson expires in six months [Page 12] DRAFT PPP Framing July 1993 having received any frame, an idle receiver will always be in a frame begin state. Robust transmitters should avoid using this trick over-zealously since the price for decreased delay is decreased reliability. Noisy links may cause the receiver to receive garbage characters and interpret them as part of an incoming frame. If the transmitter does not transmit a new opening Flag Sequence before sending the next frame, then that frame will be appended to the noise characters causing an invalid frame (with high reliability). Transmitters should avoid this by transmitting an open Flag Sequence whenever "appreciable time" has elapsed since the prior closing Flag Sequence. It is suggested that implementations will achieve the best results by always sending an opening Flag Sequence if the new frame is not back-to-back with the last. The maximum value for "appreciable time" is likely to be no greater than the typing rate of a slow typist, say 1 second. Simpson expires in six months [Page 13] DRAFT PPP Framing July 1993 A. Fast Frame Check Sequence (FCS) Implementation The FCS was originally designed with hardware implementations in mind. A serial bit stream is transmitted on the wire, the FCS is calculated over the serial data as it goes out, and the complement of the resulting FCS is appended to the serial stream, followed by the Flag Sequence. The receiver has no way of determining that it has finished calculating the received FCS until it detects the Flag Sequence. Therefore, the FCS was designed so that a particular pattern results when the FCS operation passes over the complemented FCS. A good frame is indicated by this "good FCS" value. A.1. FCS Computation Method The following code provides a table lookup computation for calculating the Frame Check Sequence as data arrives at the interface. This implementation is based on [9], [10], and [11]. The table is created by the code in section B.2. /* * u16 represents an unsigned 16-bit number. Adjust the typedef for * your hardware. */ typedef unsigned short u16; /* * FCS lookup table as calculated by the table generator in section B.2. */ static u16 fcstab[256] = { 0x0000, 0x1189, 0x2312, 0x329b, 0x4624, 0x57ad, 0x6536, 0x74bf, 0x8c48, 0x9dc1, 0xaf5a, 0xbed3, 0xca6c, 0xdbe5, 0xe97e, 0xf8f7, 0x1081, 0x0108, 0x3393, 0x221a, 0x56a5, 0x472c, 0x75b7, 0x643e, 0x9cc9, 0x8d40, 0xbfdb, 0xae52, 0xdaed, 0xcb64, 0xf9ff, 0xe876, 0x2102, 0x308b, 0x0210, 0x1399, 0x6726, 0x76af, 0x4434, 0x55bd, 0xad4a, 0xbcc3, 0x8e58, 0x9fd1, 0xeb6e, 0xfae7, 0xc87c, 0xd9f5, 0x3183, 0x200a, 0x1291, 0x0318, 0x77a7, 0x662e, 0x54b5, 0x453c, 0xbdcb, 0xac42, 0x9ed9, 0x8f50, 0xfbef, 0xea66, 0xd8fd, 0xc974, 0x4204, 0x538d, 0x6116, 0x709f, 0x0420, 0x15a9, 0x2732, 0x36bb, 0xce4c, 0xdfc5, 0xed5e, 0xfcd7, 0x8868, 0x99e1, 0xab7a, 0xbaf3, 0x5285, 0x430c, 0x7197, 0x601e, 0x14a1, 0x0528, 0x37b3, 0x263a, 0xdecd, 0xcf44, 0xfddf, 0xec56, 0x98e9, 0x8960, 0xbbfb, 0xaa72, 0x6306, 0x728f, 0x4014, 0x519d, 0x2522, 0x34ab, 0x0630, 0x17b9, 0xef4e, 0xfec7, 0xcc5c, 0xddd5, 0xa96a, 0xb8e3, 0x8a78, 0x9bf1, 0x7387, 0x620e, 0x5095, 0x411c, 0x35a3, 0x242a, 0x16b1, 0x0738, Simpson expires in six months [Page 14] DRAFT PPP Framing July 1993 0xffcf, 0xee46, 0xdcdd, 0xcd54, 0xb9eb, 0xa862, 0x9af9, 0x8b70, 0x8408, 0x9581, 0xa71a, 0xb693, 0xc22c, 0xd3a5, 0xe13e, 0xf0b7, 0x0840, 0x19c9, 0x2b52, 0x3adb, 0x4e64, 0x5fed, 0x6d76, 0x7cff, 0x9489, 0x8500, 0xb79b, 0xa612, 0xd2ad, 0xc324, 0xf1bf, 0xe036, 0x18c1, 0x0948, 0x3bd3, 0x2a5a, 0x5ee5, 0x4f6c, 0x7df7, 0x6c7e, 0xa50a, 0xb483, 0x8618, 0x9791, 0xe32e, 0xf2a7, 0xc03c, 0xd1b5, 0x2942, 0x38cb, 0x0a50, 0x1bd9, 0x6f66, 0x7eef, 0x4c74, 0x5dfd, 0xb58b, 0xa402, 0x9699, 0x8710, 0xf3af, 0xe226, 0xd0bd, 0xc134, 0x39c3, 0x284a, 0x1ad1, 0x0b58, 0x7fe7, 0x6e6e, 0x5cf5, 0x4d7c, 0xc60c, 0xd785, 0xe51e, 0xf497, 0x8028, 0x91a1, 0xa33a, 0xb2b3, 0x4a44, 0x5bcd, 0x6956, 0x78df, 0x0c60, 0x1de9, 0x2f72, 0x3efb, 0xd68d, 0xc704, 0xf59f, 0xe416, 0x90a9, 0x8120, 0xb3bb, 0xa232, 0x5ac5, 0x4b4c, 0x79d7, 0x685e, 0x1ce1, 0x0d68, 0x3ff3, 0x2e7a, 0xe70e, 0xf687, 0xc41c, 0xd595, 0xa12a, 0xb0a3, 0x8238, 0x93b1, 0x6b46, 0x7acf, 0x4854, 0x59dd, 0x2d62, 0x3ceb, 0x0e70, 0x1ff9, 0xf78f, 0xe606, 0xd49d, 0xc514, 0xb1ab, 0xa022, 0x92b9, 0x8330, 0x7bc7, 0x6a4e, 0x58d5, 0x495c, 0x3de3, 0x2c6a, 0x1ef1, 0x0f78 }; #define PPPINITFCS16 0xffff /* Initial FCS value */ #define PPPGOODFCS16 0xf0b8 /* Good final FCS value */ /* * Calculate a new fcs given the current fcs and the new data. */ u16 pppfcs16(fcs, cp, len) register u16 fcs; register unsigned char *cp; register int len; { ASSERT(sizeof (u16) == 2); ASSERT(((u16) -1) > 0); while (len--) fcs = (fcs >> 8) ^ fcstab[(fcs ^ *cp++) & 0xff]; return (fcs); } /* * How to use the fcs */ tryfcs16(cp, len) register unsigned char *cp; register int len; { u16 trialfcs; /* add on output */ Simpson expires in six months [Page 15] DRAFT PPP Framing July 1993 trialfcs = pppfcs16( PPPINITFCS16, cp, len ); trialfcs ^= 0xffff; /* complement */ cp[len] = (trialfcs & 0x00ff); /* least significant byte first */ cp[len+1] = ((trialfcs >> 8) & 0x00ff); /* check on input */ trialfcs = pppfcs16( PPPINITFCS16, cp, len + 2 ); if ( trialfcs == PPPGOODFCS16 ) printf("Good FCS\n"); } A.2. Fast FCS table generator The following code creates the lookup table used to calculate the FCS. /* * Generate a FCS table for the HDLC FCS. * * Drew D. Perkins at Carnegie Mellon University. * * Code liberally borrowed from Mohsen Banan and D. Hugh Redelmeier. */ /* * The HDLC polynomial: x**0 + x**5 + x**12 + x**16 (0x8408). */ #define P 0x8408 main() { register unsigned int b, v; register int i; printf("typedef unsigned short u16;\n"); printf("static u16 fcstab[256] = {"); for (b = 0; ; ) { if (b % 8 == 0) printf("\n"); v = b; for (i = 8; i--; ) v = v & 1 ? (v >> 1) ^ P : v >> 1; printf("\t0x%04x", v & 0xFFFF); if (++b == 256) Simpson expires in six months [Page 16] DRAFT PPP Framing July 1993 break; printf(","); } printf("\n};\n"); } Simpson expires in six months [Page 17] DRAFT PPP Framing July 1993 Security Considerations Security issues are not discussed in this memo. References [1] Simpson, W. A., "The Point-to-Point Protocol (PPP)", work in progress. [2] International Organization For Standardization, ISO Standard 3309-1979, "Data communication - High-level data link control procedures - Frame structure", 1979. [3] International Organization For Standardization, Proposed Draft International Standard ISO 3309:1983/PDAD1, "Information processing systems - Data communication - High-level data link control procedures - Frame structure - Addendum 1: Start/stop transmission", 1984. [4] International Organization For Standardization, ISO Standard 4335-1979, "Data communication - High-level data link control procedures - Elements of procedures", 1979. [5] International Organization For Standardization, ISO Standard 4335-1979/Addendum 1, "Data communication - High-level data link control procedures - Elements of procedures - Addendum 1", 1979. [6] International Telecommunication Union, CCITT Recommendation X.25, "Interface Between Data Terminal Equipment (DTE) and Data Circuit Terminating Equipment (DCE) for Terminals Operating in the Packet Mode on Public Data Networks", CCITT Red Book, Volume VIII, Fascicle VIII.3, Rec. X.25., October 1984. [7] International Telegraph and Telephone Consultative Committee, CCITT Recommendation Q.922, "ISDN Data Link Layer Specification for Frame Mode Bearer Services", April 1991. [8] American National Standards Institute, ANSI X3.4-1977, "American National Standard Code for Information Interchange", 1977. [9] Perez, "Byte-wise CRC Calculations", IEEE Micro, June, 1983. [10] Morse, G., "Calculating CRC's by Bits and Bytes", Byte, September 1986. [11] LeVan, J., "A Fast CRC", Byte, November 1987. Simpson expires in six months [Page 18] DRAFT PPP Framing July 1993 Acknowledgments This specification is based on previous RFCs, where many contributions have been acknowleged. Additional implementation detail for this version was provided by Fred Baker (ACC), and Craig Fox (NSC). Chair's Address The working group can be contacted via the current chair: Fred Baker Advanced Computer Communications 315 Bollay Drive Santa Barbara, California, 93111 EMail: fbaker@acc.com Author's Address Questions about this memo can also be directed to: William Allen Simpson Daydreamer Computer Systems Consulting Services P O Box 6205 East Lansing, MI 48826-6205 EMail: Bill.Simpson@um.cc.umich.edu Simpson expires in six months [Page 19] DRAFT PPP Framing July 1993 Table of Contents 1. Introduction .......................................... 1 1.1 Specification of Requirements ................... 1 1.2 Terminology ..................................... 1 2. Physical Layer Requirements ........................... 3 3. The Data Link Layer ................................... 5 3.1 Frame Format .................................... 6 3.2 Modification of the Basic Frame ................. 8 4. Bit-synchronous HDLC .................................. 9 5. Asynchronous HDLC ..................................... 11 APPENDICES ................................................... 14 A. Fast Frame Check Sequence (FCS) Implementation ........ 14 A.1 FCS Computation Method .......................... 14 A.2 Fast FCS table generator ........................ 16 SECURITY CONSIDERATIONS ...................................... 18 REFERENCES ................................................... 18 ACKNOWLEDGEMENTS ............................................. 19 CHAIR'S ADDRESS .............................................. 19 AUTHOR'S ADDRESS ............................................. 19