GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc1549

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

Simpson [Page 1] RFC 1549 HDLC Framing Decvember 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 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.

Simpson [Page 2] RFC 1549 HDLC Framing Decvember 1993

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

Simpson [Page 3] RFC 1549 HDLC Framing Decvember 1993

    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

Simpson [Page 4] RFC 1549 HDLC Framing Decvember 1993

 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].

Simpson [Page 5] RFC 1549 HDLC Framing Decvember 1993

  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

Simpson [Page 6] RFC 1549 HDLC Framing Decvember 1993

       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).

Simpson [Page 7] RFC 1549 HDLC Framing Decvember 1993

  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:

Simpson [Page 8] RFC 1549 HDLC Framing Decvember 1993

       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,

Simpson [Page 9] RFC 1549 HDLC Framing Decvember 1993

    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.

Simpson [Page 10] RFC 1549 HDLC Framing Decvember 1993

  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.

Simpson [Page 11] RFC 1549 HDLC Framing Decvember 1993

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.

Simpson [Page 12] RFC 1549 HDLC Framing Decvember 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.

Simpson [Page 13] RFC 1549 HDLC Framing Decvember 1993

/* * 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 */

/*

Simpson [Page 14] RFC 1549 HDLC Framing Decvember 1993

* 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.

Simpson [Page 15] RFC 1549 HDLC Framing Decvember 1993

/* * 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: x0 + x5 + x12 + x16 (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.

Simpson [Page 16] RFC 1549 HDLC Framing Decvember 1993

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

Simpson [Page 17] RFC 1549 HDLC Framing Decvember 1993

 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

Simpson [Page 18]

/data/webs/external/dokuwiki/data/pages/rfc/rfc1549.txt · Last modified: 1993/12/04 01:10 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki