Reliable link layer protocols
RFC - Unknown
(January 1985; No errata)
||RFC Editor Note
RFC 935 (Unknown)
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Network Working Group J. Robinson
Request for Comments: 935 BBN
RELIABLE LINK LAYER PROTOCOLS
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X3.28-1976 (which superseded the original version of 1971), and in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
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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.
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 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
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