path: root/doc/rfc/rfc1549.txt

Network Working Group                                 W. Simpson, Editor
Request for Comments: 1549                                    Daydreamer
Category: Standards Track                                  December 1993


                          PPP in HDLC Framing

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.

Abstract

   The Point-to-Point Protocol (PPP) [1] provides a standard method for
   transporting multi-protocol datagrams over point-to-point links.

   This document describes the use of HDLC for framing PPP encapsulated
   packets. 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.

Table of Contents

   1.   Introduction ..................................................2
   1.1  Specification of Requirements .................................2
   1.2  Terminology ...................................................3
   2.   Physical Layer Requirements ...................................3
   3.   The Data Link Layer ...........................................4
   3.1  Frame Format ..................................................5
   3.2  Modification of the Basic Frame ...............................7
   4.   Asynchronous HDLC .............................................7
   5.   Bit-synchronous HDLC ..........................................5
   6.   Octet-synchronous HDLC ........................................12
   APPENDIX A. Fast Frame Check Sequence (FCS) Implementation .........13
   A.1  FCS Computation Method ........................................13
   A.2  Fast FCS table generator ......................................15
   SECURITY CONSIDERATIONS ............................................16
   REFERENCES .........................................................17
   ACKNOWLEDGEMENTS ...................................................17
   CHAIR'S ADDRESS ....................................................18
   EDITOR'S ADDRESS ...................................................18





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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 must 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.




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1.2 Terminology

   This document frequently uses the following terms:

    datagram

      The unit of transmission in the network layer (such as IP).  A
      datagram may be encapsulated in one or more packets passed to the
      data link layer.

    frame

      The unit of transmission at the data link layer.  A frame may
      include a header and/or a trailer, along with some number of units
      of data.

    packet

      The basic unit of encapsulation, which is passed across the
      interface between the network layer and the data link layer.  A
      packet is usually mapped to a frame; the exceptions are when data
      link layer fragmentation is being performed, or when multiple
      packets are incorporated into a single frame.

    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.

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



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      no provision for sub-octets to be supplied or accepted.


    PPP does not impose any restrictions regarding transmission rate,
      other than that of the particular DTE/DCE interface.

    Control Signals

      PPP does not require the use of 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.

      Because signalling is not required, the physical layer MAY be
      decoupled from the data link layer, hiding the transient details
      of the physical transport.  This has implications for mobility in
      cellular radio networks, and other rapidly switching links.

      When moving from cell to cell within the same zone, an
      implementation MAY choose to treat the entire zone as a single
      link, even though transmission is switched among several
      frequencies.  The link is considered to be with the central
      control unit for the zone, rather than the individual cell
      transceivers.  However, the link SHOULD re-establish its
      configuration whenever the link is switched to a different
      administration.

      Due to the bursty nature of data traffic, some implementations
      have choosen to disconnect the physical layer during periods of
      inactivity, and reconnect when traffic resumes, without informing
      the data link layer.  Robust implementations should avoid using
      this trick over-zealously, since the price for decreased setup
      latency is decreased security.  Implementations SHOULD signal the
      Down event whenever "significant time" has elapsed since the link
      was disconnected.  The value for "significant time" is a matter of
      considerable debate, and is based on the tariffs, call setup
      times, and security concerns of the installation.

3. The Data Link Layer

   PPP uses the principles, terminology, and frame structure of the
   International Organization For Standardization's (ISO) 3309-1979



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   High-level Data Link Control (HDLC) frame structure [2], as modified
   by "Addendum 1: Start/stop transmission" [3], which specifies
   modifications to allow HDLC 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].  PPP framing 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 document attempts to give a concise summary and point out
   specific options and features used by PPP.

   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.

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].




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    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.

    Control Field

      The Control field is a single octet and contains the binary
      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 or synchronous) inserted for transparency.  This
      also does not include the Flag Sequences nor 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



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         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.

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.

4.  Asynchronous HDLC

   This section summarizes the use of HDLC with 8-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).






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    Transparency

      An octet 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.

      The default receiving ACCM is 0xffffffff.  The default sending
      ACCM is 0xffffffff, plus the Control Escape and Flag Sequence
      characters themselves, plus whatever other outgoing characters are
      known to be intercepted.

      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.

         Note: The inclusion of all octets less than hexadecimal 0x20
         allows all ASCII control characters [8] excluding DEL (Delete)
         to be transparently communicated through 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:



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         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
      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 send 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).
      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.  Transmitters SHOULD send an open Flag
      Sequence whenever "appreciable time" has elapsed after the prior
      closing Flag Sequence.  The maximum value for "appreciable time"
      is likely to be no greater than the typing rate of a slow typist,
      say 1 second.

    Encoding

      All octets are transmitted with one start bit, eight bits of data,



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      and one stop bit.  There is no provision for seven bit
      asynchronous links.

5. Bit-synchronous HDLC

   This section summarizes the use of HDLC with 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.

      Since the Control Escape octet-stuffing method is not used, the
      default receiving and sending Async-Control-Character-Maps are 0.

      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 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 octet mapping.
      Instead, all such octet mapping will be performed by the
      asynchronous-to-synchronous converter.



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    Aborting a Transmission

      A sequence of more than six "1" bits indicates an invalid frame,
      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
      ill.  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.

    Encoding

      The definition 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 bit stream, lack of standards for
      transmission will hinder interoperability as surely as lack of
      data link standards.  At speeds of 56 Kbps through 2.0 Mbps, NRZ
      is currently most widely available, and on that basis is
      recommended as a default.

      When 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 higher speeds of up to 45 Mbps, some implementors have chosen
      the ANSI High Speed Synchronous Interface [HSSI].  While this
      experience is currently limited, implementors are encouraged to
      cooperate in choosing transmission encoding.




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6.  Octet-synchronous HDLC

   This section summarizes the use of HDLC with octet-synchronous links,
   such as SONET and optionally ISDN B or H channels.

   Although the bit rate is synchronous, there is no bit-stuffing.
   Instead, the octet-stuffing feature of 8-bit asynchronous HDLC is
   used.

    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

      An octet stuffing procedure is used.  The Control Escape octet is
      defined as binary 01111101 (hexadecimal 0x7d).

      The octet stuffing procedure is described in "Asynchronous HDLC"
      above.

      The sending and receiving implementations need escape only the
      Flag Sequence and Control Escape octets.

      Considerations concerning the use of converters are described in
      "Bit-synchronous HDLC" above.

    Aborting a Transmission

      Frames may be aborted by transmitting a Control Escape octet
      followed immediately by a closing Flag Sequence.  The preceding
      frame is ignored, and not counted as a FCS error.

    Inter-frame Time Fill

      The Flag Sequence MUST be transmitted during inter-frame time
      fill.  There is no provision for inter-octet time fill.

    Encoding

      The definition of various encodings and scrambling is the
      responsibility of the DTE/DCE equipment in use, and is outside the
      scope of this specification.






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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.































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/*
 * 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,
   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 */

/*



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 * 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 */
    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 FCS0);
}

A.2.  Fast FCS table generator

The following code creates the lookup table used to calculate the FCS.












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/*
 * 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;0);
    printf("static u16 fcstab[256] = {");
    for (b = 0; ; ) {
        if (b % 8 == 0)
            printf("0);

        v = b;
        for (i = 8; i--; )
            v = v & 1 ? (v >> 1) ^ P : v >> 1;

        printf("0x%04x", v & 0xFFFF);
        if (++b == 256)
            break;
        printf(",");
    }
    printf("0;0);
}

Security Considerations

   As noted in the Physical Layer Requirements section, the link layer
   might not be informed when the connected state of physical layer is
   changed.  This results in possible security lapses due to over-
   reliance on the integrity and security of switching systems and
   administrations.  An insertion attack might be undetected.  An
   attacker which is able to spoof the same calling identity might be
   able to avoid link authentication.





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References

   [1]  Simpson, W., Editor, "The Point-to-Point Protocol (PPP)",
        RFC 1548, December 1993

   [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 3309-1991/PDAD1, "Information
        processing systems - Data communication - High-level data link
        control procedures - Frame structure - Addendum 1: Start/stop
        transmission", 1991.

   [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.

Acknowledgments

   This specification is based on previous RFCs, where many



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   contributions have been acknowleged.

   Additional implementation detail for this version was provided by
   Fred Baker (ACC), Craig Fox (NSC), and Phil Karn (Qualcomm).

   Special thanks to Morning Star Technologies for providing computing
   resources and network access support for writing this specification.

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

Editor's Address

   Questions about this memo can also be directed to:

      William Allen Simpson
      Daydreamer
      Computer Systems Consulting Services
      1384 Fontaine
      Madison Heights, Michigan  48071

      EMail: Bill.Simpson@um.cc.umich.edu





















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