Reliable link layer protocols
RFC 935

Document Type RFC - Unknown (January 1985; No errata)
Last updated 2013-03-02
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Network Working Group                                        J. Robinson
Request for Comments: 935                                            BBN
                                                            January 1985


Status of This Memo

   This RFC discusses protocols proposed recently in RFCs 914 and 916,
   and suggests a proposed protocol that could meet the same needs
   addressed in those memos.  The stated need is reliable communication
   between two programs over a full-duplex, point-to-point communication
   link, and in particular the RFCs address the need for such
   communication over an asynchronous link at relatively low speeds.
   The suggested protocol uses the methods of existing national and
   international data link layer standards.  This RFC suggests a
   proposed protocol for the ARPA-Internet community, and requests
   discussion and suggestions for improvements.  Distribution of this
   memo is unlimited.


   This RFC is motivated by recent RFCs 914 and 916, which propose new
   standards for protocols that transfer serial data reliably over
   asynchronous communication lines.  In this note, I summarize
   widely-used standards that have been in existence for some time that
   might be appropriate for this environment.  I hope that the existing
   standards will be found to meet the needs the new proposals seek to

   In both the US and international standards areas, there are two major
   categories of serial data communication standards for the link layer.
   These categories are character-oriented and bit-oriented.  The first
   is the older class; it is standardized in the US standard ANSI
   X3.28-1976 (which superseded the original version of 1971), and in
   the ISO standards IS 1745, IS 2111, IS 2628 and IS 2629.  Although
   frequently used in synchronous environments, wherein the name binary
   synchronous (or bisynch) is used, these standards use the term "basic
   mode" to describe their procedures.  The latter class is standardized
   in the US standard ADCCP (Advanced Data Communication Control
   Procedures), ANSI X3.66- 1979, and in the ISO standard HDLC
   (High-level Data Link Control procedures), in IS 3309, IS 4335 and IS

   Other international standards, draft standards and vendor standards
   follow the ADCCP/HDLC procedures.  Among these are SDLC (IBM), X.25
   LAPB (CCITT), IEEE 802.2/ISO 8802.2 LLC (LAN Logical Link Control)
   and ISDN LAPD (CCITT).  Many vendors have built equipment which meets

Robinson                                                        [Page 1]

RFC 935                                                     January 1985
Reliable Link Layer Protocols

   the character-oriented standards, in both synchronous and
   asynchronous environments, including all the major US mainframe

   The only other serial link layer protocol known to the author in as
   wide use as these is DEC's DDCMP (Digital Data Communications Message
   Protocol).  This protocol uses a character count instead of framing
   characters, but is in other respects a character-oriented protocol.

   The next sections of this note will compare the three protocols above
   on several bases, paying particular attention to the characteristics
   that make particular aspects of the protocol appropriate to the
   low-speed, asynchronous, serial environment.

Frame Structure

   All serial protocols divide the data to be transmitted into units
   known as frames.  A frame is typically one to several hundred
   characters in length.  The frame structure is the major difference
   used above to divide the protocols into three classes.

Character-Oriented Framing

   Character-oriented protocols use two techniques for defining a frame.
   First, it is necessary to determine where characters start and stop.
   The technique used for this purpose is to transmit a number of unique
   characters prior to the start of a frame.  The character generally
   used for this is the SYN character.

   Note that this is not required when using asynchronous transmission.
   Since each character is itself framed by start and stop bits, there
   is never a question of where characters begin and end.

   The main technique for structuring a frame is the use of special
   framing characters to delineate the start and end of a frame, and to
   delineate portions of the frame (such as header and text).  Some uses
   of character-oriented protocols require that these characters never
   appear in the header or text of the frame, while others allow
   "transparent" transmission.  Transparency is obtained by preceding
   each framing character by a unique control character, typically DLE.
   In this way, all characters may be sent as header or text, except for
   DLE.  In order to allow DLE to be sent in the header or text, the DLE
   is doubled.

Robinson                                                        [Page 2]
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