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rfc:rfc1662

Network Working Group W. Simpson, Editor Request for Comments: 1662 Daydreamer STD: 51 July 1994 Obsoletes: 1549 Category: Standards Track

                      PPP in HDLC-like 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-like framing for PPP
 encapsulated packets.

Table of Contents

   1.     Introduction ..........................................    1
      1.1       Specification of Requirements ...................    2
      1.2       Terminology .....................................    2
   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.     Octet-stuffed framing .................................    8
      4.1       Flag Sequence ...................................    8
      4.2       Transparency ....................................    8
      4.3       Invalid Frames ..................................    9
      4.4       Time Fill .......................................    9
         4.4.1  Octet-synchronous ...............................    9
         4.4.2  Asynchronous ....................................    9
      4.5       Transmission Considerations .....................   10
         4.5.1  Octet-synchronous ...............................   10
         4.5.2  Asynchronous ....................................   10

Simpson [Page i] RFC 1662 HDLC-like Framing July 1994

   5.     Bit-stuffed framing ...................................   11
      5.1       Flag Sequence ...................................   11
      5.2       Transparency ....................................   11
      5.3       Invalid Frames ..................................   11
      5.4       Time Fill .......................................   11
      5.5       Transmission Considerations .....................   12
   6.     Asynchronous to Synchronous Conversion ................   13
   7.     Additional LCP Configuration Options ..................   14
      7.1       Async-Control-Character-Map (ACCM) ..............   14
   APPENDICES ...................................................   17
   A.     Recommended LCP Options ...............................   17
   B.     Automatic Recognition of PPP Frames ...................   17
   C.     Fast Frame Check Sequence (FCS) Implementation ........   18
      C.1       FCS table generator .............................   18
      C.2       16-bit FCS Computation Method ...................   19
      C.3       32-bit FCS Computation Method ...................   21
   SECURITY CONSIDERATIONS ......................................   24
   REFERENCES ...................................................   24
   ACKNOWLEDGEMENTS .............................................   25
   CHAIR'S ADDRESS ..............................................   25
   EDITOR'S ADDRESS .............................................   25

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.
 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.

Simpson [Page 1] RFC 1662 HDLC-like Framing July 1994

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.

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
           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 [Page 2] RFC 1662 HDLC-like Framing July 1994

2. Physical Layer Requirements

 PPP is capable of operating across most DTE/DCE interfaces (such as,
 EIA RS-232-E, EIA RS-422, 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.
 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

Simpson [Page 3] RFC 1662 HDLC-like Framing July 1994

    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 described in ISO 3309-1979 HDLC frame
 structure, most recently the fourth edition 3309:1991 [2], which
 specifies modifications to allow HDLC use in asynchronous
 environments.
 The PPP control procedures use the Control field encodings described
 in ISO 4335-1979 HDLC elements of procedures, most recently the
 fourth edition 4335:1991 [4].
    This should not be construed to indicate that every feature of the
    above recommendations are included in PPP.  Each feature included
    is explicitly described in the following sections.
 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 [Page 4] RFC 1662 HDLC-like Framing July 1994

3.1. Frame Format

 A summary of the PPP HDLC-like frame structure is shown below.  This
 figure does not include bits inserted for synchronization (such as
 start and 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 |
         | 8/16 bits|      *      |    *    |
         +----------+-------------+---------+
         +----------+----------+-----------------
         |   FCS    |   Flag   | Inter-frame Fill
         |16/32 bits| 01111110 | or next Address
         +----------+----------+-----------------
 The Protocol, Information and Padding fields are described in the
 Point-to-Point Protocol Encapsulation [1].
 Flag Sequence
    Each frame begins and ends with a Flag Sequence, which is the
    binary sequence 01111110 (hexadecimal 0x7e).  All implementations
    continuously check for this flag, which is used for frame
    synchronization.
    Only one Flag Sequence is required between two frames.  Two
    consecutive Flag Sequences constitute an empty frame, which is
    silently discarded, and not counted as a FCS error.
 Address Field
    The Address field is a single octet, which contains the binary
    sequence 11111111 (hexadecimal 0xff), the All-Stations address.
    Individual station addresses are not assigned.  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.

Simpson [Page 5] RFC 1662 HDLC-like Framing July 1994

 Control Field
    The Control field is a single octet, which contains the binary
    sequence 00000011 (hexadecimal 0x03), the Unnumbered Information
    (UI) command with the Poll/Final (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 defaults to 16 bits (two octets).
    The FCS is transmitted least significant octet first, which
    contains the coefficient of the highest term.
    A 32-bit (four octet) FCS is also defined.  Its use may be
    negotiated as described in "PPP LCP Extensions" [5].
    The use of other FCS lengths may be defined at a later time, or by
    prior agreement.
    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.
       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 the
    Appendices.
 The end of the Information and Padding fields is found by locating
 the closing Flag Sequence and removing the Frame Check Sequence
 field.

Simpson [Page 6] RFC 1662 HDLC-like Framing July 1994

3.2. Modification of the Basic Frame

 The Link Control Protocol can negotiate modifications to the standard
 HDLC-like frame structure.  However, modified frames will always be
 clearly distinguishable from standard frames.
 Address-and-Control-Field-Compression
    When using the standard HDLC-like framing, the Address and Control
    fields contain the hexadecimal values 0xff and 0x03 respectively.
    When other Address or Control field values are in use, Address-
    and-Control-Field-Compression MUST NOT be negotiated.
    On transmission, compressed Address and Control fields are simply
    omitted.
    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.

Simpson [Page 7] RFC 1662 HDLC-like Framing July 1994

4. Octet-stuffed framing

 This chapter summarizes the use of HDLC-like framing with 8-bit
 asynchronous and octet-synchronous links.

4.1. 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).

4.2. Transparency

 An octet stuffing procedure is used.  The Control Escape octet is
 defined as binary 01111101 (hexadecimal 0x7d), most significant bit
 first.
 As a minimum, sending implementations MUST escape the Flag Sequence
 and Control Escape octets.
 After FCS computation, the transmitter examines the entire frame
 between the two Flag Sequences.  Each Flag Sequence, Control Escape
 octet, and any octet which is flagged in the sending Async-Control-
 Character-Map (ACCM), is replaced by a two octet sequence consisting
 of the Control Escape octet followed by the original octet
 exclusive-or'd with hexadecimal 0x20.
    This is bit 5 complemented, where the bit positions are numbered
    76543210 (the 6th bit as used in ISO numbered 87654321 -- BEWARE
    when comparing documents).
 Receiving implementations MUST correctly process all Control Escape
 sequences.
 On reception, prior to FCS computation, each octet with value less
 than hexadecimal 0x20 is checked.  If it is flagged in the receiving
 ACCM, it is simply removed (it may have been inserted by intervening
 data communications equipment).  Each Control Escape octet is also
 removed, and the following octet is exclusive-or'd with hexadecimal
 0x20, unless it is the Flag Sequence (which aborts a frame).
 A few examples may make this more clear.  Escaped data is transmitted
 on the link as follows:

Simpson [Page 8] RFC 1662 HDLC-like Framing July 1994

    0x7e is encoded as 0x7d, 0x5e.    (Flag Sequence)
    0x7d is encoded as 0x7d, 0x5d.    (Control Escape)
    0x03 is encoded as 0x7d, 0x23.    (ETX)
 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.    (XON)
    0x13 is encoded as 0x7d, 0x33.    (XOFF)
    0x91 is encoded as 0x7d, 0xb1.    (XON with parity set)
    0x93 is encoded as 0x7d, 0xb3.    (XOFF with parity set)

4.3. Invalid Frames

 Frames which are too short (less than 4 octets when using the 16-bit
 FCS), or which end with a Control Escape octet followed immediately
 by a closing Flag Sequence, or in which octet-framing is violated (by
 transmitting a "0" stop bit where a "1" bit is expected), are
 silently discarded, and not counted as a FCS error.

4.4. Time Fill

4.4.1. Octet-synchronous

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

4.4.2. Asynchronous

 Inter-octet 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 end and a frame begin.  After having received
 any frame, an idle receiver will always be in a frame begin state.

Simpson [Page 9] RFC 1662 HDLC-like Framing July 1994

 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, about
 1 second.

4.5. Transmission Considerations

4.5.1. Octet-synchronous

 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.

4.5.2. Asynchronous

 All octets are transmitted least significant bit first, with one
 start bit, eight bits of data, and one stop bit.  There is no
 provision for seven bit asynchronous links.

Simpson [Page 10] RFC 1662 HDLC-like Framing July 1994

5. Bit-stuffed framing

 This chapter summarizes the use of HDLC-like framing with bit-
 synchronous links.

5.1. 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 bit-synchronous to asynchronous and bit-
 synchronous to octet-synchronous converters.

5.2. Transparency

 After FCS computation, 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.
 On reception, prior to FCS computation, any "0" bit that directly
 follows five contiguous "1" bits is discarded.

5.3. Invalid Frames

 Frames which are too short (less than 4 octets when using the 16-bit
 FCS), or which end with a sequence of more than six "1" bits, are
 silently discarded, and not counted as a FCS error.

5.4. Time Fill

 There is no provision for inter-octet time fill.
 The Flag Sequence SHOULD be transmitted during inter-frame time fill.
 However, certain types of circuit-switched links require the use of

Simpson [Page 11] RFC 1662 HDLC-like Framing July 1994

 mark idle (continuous ones), 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.
    This differs from practice in ISO 3309, which allows 7 to 14 bit
    mark idle.

5.5. Transmission Considerations

 All octets are transmitted least significant bit first.
 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
 exacerbates the missing x1 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 includes the x1
 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 12] RFC 1662 HDLC-like Framing July 1994

6. Asynchronous to Synchronous Conversion

 There may be some use of asynchronous-to-synchronous converters (some
 built into modems and cellular interfaces), resulting in an
 asynchronous PPP implementation on one end of a link and a
 synchronous implementation on the other.  It is the responsibility of
 the converter to do all stuffing conversions during operation.
 To enable this functionality, synchronous PPP implementations MUST
 always respond to the Async-Control-Character-Map Configuration
 Option with the LCP Configure-Ack.  However, acceptance of the
 Configuration Option does not imply that the synchronous
 implementation will do any ACCM mapping.  Instead, all such octet
 mapping will be performed by the asynchronous-to-synchronous
 converter.

Simpson [Page 13] RFC 1662 HDLC-like Framing July 1994

7. Additional LCP Configuration Options

 The Configuration Option format and basic options are already defined
 for LCP [1].
 Up-to-date values of the LCP Option Type field are specified in the
 most recent "Assigned Numbers" RFC [10].  This document concerns the
 following values:
    2       Async-Control-Character-Map

7.1. Async-Control-Character-Map (ACCM)

 Description
    This Configuration Option provides a method to negotiate the use
    of control character transparency on asynchronous links.
    Each end of the asynchronous 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.
    For asynchronous links, 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 flagged (by prior configuration) as likely
    to be intercepted.
    For other types of links, the default value is 0, since there is
    no need for mapping.
       The default inclusion of all octets less than hexadecimal 0x20
       allows all ASCII control characters [6] excluding DEL (Delete)
       to be transparently communicated through all known data
       communications equipment.
    The transmitter MAY also send octets with values 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.

Simpson [Page 14] RFC 1662 HDLC-like Framing July 1994

       Note that this specification differs in detail from later
       amendments, such as 3309:1991/Amendment 2 [3].  However, such
       "extended transparency" is applied only by "prior agreement".
       Use of the transparency methods in this specification
       constitute a prior agreement with respect to PPP.
       For compatibility with 3309:1991/Amendment 2, the transmitter
       MAY escape DEL and ACCM equivalents with the 8th (most
       significant) bit set.  No change is required in the receiving
       algorithm.
       Following ACCM negotiation, the transmitter SHOULD cease
       escaping DEL.
    However, it is rarely necessary to map all control characters, and
    often it is unnecessary to map any control characters.  The
    Configuration Option is used to inform the peer which control
    characters MUST remain mapped when the peer sends them.
    The peer MAY still send any other octets in mapped format, if it
    is necessary because of constraints known to the peer.  The peer
    SHOULD Configure-Nak with the logical union of the sets of mapped
    octets, so that when such octets are spuriously introduced they
    can be ignored on receipt.
 A summary of the Async-Control-Character-Map Configuration Option
 format is shown below.  The fields are transmitted from left to
 right.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Type      |    Length     |               ACCM
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           ACCM (cont)           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type
    2
 Length
    6

Simpson [Page 15] RFC 1662 HDLC-like Framing July 1994

 ACCM
    The ACCM field is four octets, and indicates the set of control
    characters to be mapped.  The map is sent most significant octet
    first.
    Each numbered bit corresponds to the octet of the same value.  If
    the bit is cleared to zero, then that octet need not be mapped.
    If the bit is set to one, then that octet MUST remain mapped.  For
    example, if bit 19 is set to zero, then the ASCII control
    character 19 (DC3, Control-S) MAY be sent in the clear.
       Note: The least significant bit of the least significant octet
       (the final octet transmitted) is numbered bit 0, and would map
       to the ASCII control character NUL.

Simpson [Page 16] RFC 1662 HDLC-like Framing July 1994

A. Recommended LCP Options

 The following Configurations Options are recommended:
 High Speed links
    Magic Number
    Link Quality Monitoring
    No Address and Control Field Compression
    No Protocol Field Compression
 Low Speed or Asynchronous links
    Async Control Character Map
    Magic Number
    Address and Control Field Compression
    Protocol Field Compression

B. Automatic Recognition of PPP Frames

 It is sometimes desirable to detect PPP frames, for example during a
 login sequence.  The following octet sequences all begin valid PPP
 LCP frames:
    7e ff 03 c0 21
    7e ff 7d 23 c0 21
    7e 7d df 7d 23 c0 21
 Note that the first two forms are not a valid username for Unix.
 However, only the third form generates a correctly checksummed PPP
 frame, whenever 03 and ff are taken as the control characters ETX and
 DEL without regard to parity (they are correct for an even parity
 link) and discarded.
 Many implementations deal with this by putting the interface into
 packet mode when one of the above username patterns are detected
 during login, without examining the initial PPP checksum.  The
 initial incoming PPP frame is discarded, but a Configure-Request is
 sent immediately.

Simpson [Page 17] RFC 1662 HDLC-like Framing July 1994

C. 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.

C.1. FCS table generator

 The following code creates the lookup table used to calculate the
 FCS-16.
 /*
  * Generate a FCS-16 table.
  *
  * Drew D. Perkins at Carnegie Mellon University.
  *
  * Code liberally borrowed from Mohsen Banan and D. Hugh Redelmeier.
  */
 /*
  * The FCS-16 generator polynomial: x**0 + x**5 + x**12 + x**16.
  */
 #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--; )

Simpson [Page 18] RFC 1662 HDLC-like Framing July 1994

             v = v & 1 ? (v >> 1) ^ P : v >> 1;
         printf("\t0x%04x", v & 0xFFFF);
         if (++b == 256)
             break;
         printf(",");
     }
     printf("\n};\n");
 }

C.2. 16-bit 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 [7], [8], and [9].
 /*
  * 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.
  */
 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,

Simpson [Page 19] RFC 1662 HDLC-like Framing July 1994

    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 */
     trialfcs = pppfcs16( PPPINITFCS16, cp, len );
     trialfcs ^= 0xffff;                 /* complement */
     cp[len] = (trialfcs & 0x00ff);      /* least significant byte first */
     cp[len+1] = ((trialfcs >> 8) & 0x00ff);

Simpson [Page 20] RFC 1662 HDLC-like Framing July 1994

     /* check on input */
     trialfcs = pppfcs16( PPPINITFCS16, cp, len + 2 );
     if ( trialfcs == PPPGOODFCS16 )
         printf("Good FCS\n");
 }

C.3. 32-bit FCS Computation Method

 The following code provides a table lookup computation for
 calculating the 32-bit Frame Check Sequence as data arrives at the
 interface.
 /*
  * The FCS-32 generator polynomial: x**0 + x**1 + x**2 + x**4 + x**5
  *                      + x**7 + x**8 + x**10 + x**11 + x**12 + x**16
  *                      + x**22 + x**23 + x**26 + x**32.
  */
 /*
  * u32 represents an unsigned 32-bit number.  Adjust the typedef for
  * your hardware.
  */
 typedef unsigned long u32;
 static u32 fcstab_32[256] =
    {
    0x00000000, 0x77073096, 0xee0e612c, 0x990951ba,
    0x076dc419, 0x706af48f, 0xe963a535, 0x9e6495a3,
    0x0edb8832, 0x79dcb8a4, 0xe0d5e91e, 0x97d2d988,
    0x09b64c2b, 0x7eb17cbd, 0xe7b82d07, 0x90bf1d91,
    0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de,
    0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7,
    0x136c9856, 0x646ba8c0, 0xfd62f97a, 0x8a65c9ec,
    0x14015c4f, 0x63066cd9, 0xfa0f3d63, 0x8d080df5,
    0x3b6e20c8, 0x4c69105e, 0xd56041e4, 0xa2677172,
    0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b,
    0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940,
    0x32d86ce3, 0x45df5c75, 0xdcd60dcf, 0xabd13d59,
    0x26d930ac, 0x51de003a, 0xc8d75180, 0xbfd06116,
    0x21b4f4b5, 0x56b3c423, 0xcfba9599, 0xb8bda50f,
    0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924,
    0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d,
    0x76dc4190, 0x01db7106, 0x98d220bc, 0xefd5102a,
    0x71b18589, 0x06b6b51f, 0x9fbfe4a5, 0xe8b8d433,
    0x7807c9a2, 0x0f00f934, 0x9609a88e, 0xe10e9818,
    0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01,

Simpson [Page 21] RFC 1662 HDLC-like Framing July 1994

    0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e,
    0x6c0695ed, 0x1b01a57b, 0x8208f4c1, 0xf50fc457,
    0x65b0d9c6, 0x12b7e950, 0x8bbeb8ea, 0xfcb9887c,
    0x62dd1ddf, 0x15da2d49, 0x8cd37cf3, 0xfbd44c65,
    0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2,
    0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb,
    0x4369e96a, 0x346ed9fc, 0xad678846, 0xda60b8d0,
    0x44042d73, 0x33031de5, 0xaa0a4c5f, 0xdd0d7cc9,
    0x5005713c, 0x270241aa, 0xbe0b1010, 0xc90c2086,
    0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
    0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4,
    0x59b33d17, 0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad,
    0xedb88320, 0x9abfb3b6, 0x03b6e20c, 0x74b1d29a,
    0xead54739, 0x9dd277af, 0x04db2615, 0x73dc1683,
    0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8,
    0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1,
    0xf00f9344, 0x8708a3d2, 0x1e01f268, 0x6906c2fe,
    0xf762575d, 0x806567cb, 0x196c3671, 0x6e6b06e7,
    0xfed41b76, 0x89d32be0, 0x10da7a5a, 0x67dd4acc,
    0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5,
    0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252,
    0xd1bb67f1, 0xa6bc5767, 0x3fb506dd, 0x48b2364b,
    0xd80d2bda, 0xaf0a1b4c, 0x36034af6, 0x41047a60,
    0xdf60efc3, 0xa867df55, 0x316e8eef, 0x4669be79,
    0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236,
    0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f,
    0xc5ba3bbe, 0xb2bd0b28, 0x2bb45a92, 0x5cb36a04,
    0xc2d7ffa7, 0xb5d0cf31, 0x2cd99e8b, 0x5bdeae1d,
    0x9b64c2b0, 0xec63f226, 0x756aa39c, 0x026d930a,
    0x9c0906a9, 0xeb0e363f, 0x72076785, 0x05005713,
    0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38,
    0x92d28e9b, 0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21,
    0x86d3d2d4, 0xf1d4e242, 0x68ddb3f8, 0x1fda836e,
    0x81be16cd, 0xf6b9265b, 0x6fb077e1, 0x18b74777,
    0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c,
    0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45,
    0xa00ae278, 0xd70dd2ee, 0x4e048354, 0x3903b3c2,
    0xa7672661, 0xd06016f7, 0x4969474d, 0x3e6e77db,
    0xaed16a4a, 0xd9d65adc, 0x40df0b66, 0x37d83bf0,
    0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
    0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6,
    0xbad03605, 0xcdd70693, 0x54de5729, 0x23d967bf,
    0xb3667a2e, 0xc4614ab8, 0x5d681b02, 0x2a6f2b94,
    0xb40bbe37, 0xc30c8ea1, 0x5a05df1b, 0x2d02ef8d
    };
 #define PPPINITFCS32  0xffffffff   /* Initial FCS value */
 #define PPPGOODFCS32  0xdebb20e3   /* Good final FCS value */

Simpson [Page 22] RFC 1662 HDLC-like Framing July 1994

 /*
  * Calculate a new FCS given the current FCS and the new data.
  */
 u32 pppfcs32(fcs, cp, len)
     register u32 fcs;
     register unsigned char *cp;
     register int len;
     {
     ASSERT(sizeof (u32) == 4);
     ASSERT(((u32) -1) > 0);
     while (len--)
         fcs = (((fcs) >> 8) ^ fcstab_32[((fcs) ^ (*cp++)) & 0xff]);
     return (fcs);
     }
 /*
  * How to use the fcs
  */
 tryfcs32(cp, len)
     register unsigned char *cp;
     register int len;
 {
     u32 trialfcs;
     /* add on output */
     trialfcs = pppfcs32( PPPINITFCS32, cp, len );
     trialfcs ^= 0xffffffff;             /* complement */
     cp[len] = (trialfcs & 0x00ff);      /* least significant byte first */
     cp[len+1] = ((trialfcs >>= 8) & 0x00ff);
     cp[len+2] = ((trialfcs >>= 8) & 0x00ff);
     cp[len+3] = ((trialfcs >> 8) & 0x00ff);
     /* check on input */
     trialfcs = pppfcs32( PPPINITFCS32, cp, len + 4 );
     if ( trialfcs == PPPGOODFCS32 )
         printf("Good FCS\n");
 }

Simpson [Page 23] RFC 1662 HDLC-like Framing July 1994

Security Considerations

 As noted in the Physical Layer Requirements section, the link layer
 might not be informed when the connected state of the physical layer
 has 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.

References

 [1]   Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", 
       STD 50, RFC 1661, Daydreamer, July 1994.
 [2]   ISO/IEC 3309:1991(E), "Information Technology -
       Telecommunications and information exchange between systems -
       High-level data link control (HDLC) procedures - Frame
       structure", International Organization For Standardization,
       Fourth edition 1991-06-01.
 [3]   ISO/IEC 3309:1991/Amd.2:1992(E), "Information Technology -
       Telecommunications and information exchange between systems -
       High-level data link control (HDLC) procedures - Frame
       structure - Amendment 2: Extended transparency options for
       start/stop transmission", International Organization For
       Standardization, 1992-01-15.
 [4]   ISO/IEC 4335:1991(E), "Information Technology -
       Telecommunications and information exchange between systems -
       High-level data link control (HDLC) procedures - Elements of
       procedures", International Organization For Standardization,
       Fourth edition 1991-09-15.
 [5]   Simpson, W., Editor, "PPP LCP Extensions", RFC 1570, 
       Daydreamer, January 1994.
 [6]   ANSI X3.4-1977, "American National Standard Code for
       Information Interchange", American National Standards
       Institute, 1977.
 [7]   Perez, "Byte-wise CRC Calculations", IEEE Micro, June 1983.
 [8]   Morse, G., "Calculating CRC's by Bits and Bytes", Byte,
       September 1986.

Simpson [Page 24] RFC 1662 HDLC-like Framing July 1994

 [9]   LeVan, J., "A Fast CRC", Byte, November 1987.
 [10]  Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
       1340, USC/Information Sciences Institute, July 1992.

Acknowledgements

 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@merit.edu mailing list.
 This specification is based on previous RFCs, where many
 contributions have been acknowleged.
 The 32-bit FCS example code was provided by Karl Fox (Morning Star
 Technologies).
 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  93117
    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
    Bill.Simpson@um.cc.umich.edu
        bsimpson@MorningStar.com

Simpson [Page 25]

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