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+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]