Документ:Спецификация протокола DDCMP
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DECnet
DIGITAL Network Architecture
D D C M P
Functional Specification
Phase IV
Version 4.1
August, 1984
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ABSTRACT
The Digital Data Communications Message
Protocol (DDCMP) is a data link control
procedure that ensures a reliable data
communication path between communication
devices connected by data links. DDCMP
has been designed to operate over full-
and half-duplex synchronous and
asynchronous channels in both
point-to-point and multipoint modes. It
can be used in a variety of applications
such as distributed computer networking,
host/front-end processing, remote
terminal concentration, and remote job
entry-exit system operation.
This document describes the functions,
characteristics, capabilities, and
operation of DDCMP. It is primarily
intended to assist the individual
implementing DDCMP within a system. It
is structured, however, to also provide
general information describing the
protocol for others who may need this
level of information. It is not
intended to instruct those unfamiliar
with the principles of data
communications.
DIGITAL EQUIPMENT CORPORATION
MAYNARD, MASSACHUSETTS 01754
Copyright (C) 1980, 1982 Digital Equipment Corporation
This material may be copied, in whole or in part, provided that the
above copyright notice is included in each copy along with an
acknowledgment that the copy describes protocols, algorithms, and
structures developed by Digital Equipment Corporation.
This material may be changed without notice by Digital Equipment Corporation, and Digital Equipment Corporation is not responsible for any errors which may appear herein. � Page 3
CONTENTS
1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . 7
2 FUNCTIONAL DESCRIPTION . . . . . . . . . . . . . . . 7
2.1 RELATIONSHIP TO DECNET . . . . . . . . . . . . . . 8
2.2 FEATURES . . . . . . . . . . . . . . . . . . . . . 8
2.3 OPERATING REQUIREMENTS . . . . . . . . . . . . . . 9
2.3.1 Goals . . . . . . . . . . . . . . . . . . . . . 9
2.3.2 Restrictions . . . . . . . . . . . . . . . . . 10
2.4 DATA LINK FUNCTIONS . . . . . . . . . . . . . . 11
2.5 FUNCTIONAL ORGANIZATION . . . . . . . . . . . . 12
2.5.1 Framing Component . . . . . . . . . . . . . . 12
2.5.1.1 Bit Synchronization . . . . . . . . . . . . 13
2.5.1.2 Byte Synchronization . . . . . . . . . . . . 13
2.5.1.3 Message Synchronization . . . . . . . . . . 13
2.5.2 Link Management Component . . . . . . . . . . 13
2.5.3 Message Exchange Component . . . . . . . . . . 14
2.5.3.1 Negative Acknowledgement (NAK) . . . . . . . 15
2.5.3.2 Reply To Message Number (REP) . . . . . . . 16
2.5.3.3 Pipelining . . . . . . . . . . . . . . . . . 16
2.5.3.4 Piggybacking . . . . . . . . . . . . . . . . 16
2.5.3.5 ACK Implied In NAK . . . . . . . . . . . . . 17
2.5.3.6 Initialization . . . . . . . . . . . . . . . 17
3 INTERFACES . . . . . . . . . . . . . . . . . . . . 17
3.1 USER INTERFACE . . . . . . . . . . . . . . . . . 17
3.1.1 Commands To DDCMP . . . . . . . . . . . . . . 18
3.1.2 Responses From DDCMP . . . . . . . . . . . . . 19
3.2 DEVICE DRIVER INTERFACE . . . . . . . . . . . . 19
3.2.1 Commands To The Driver . . . . . . . . . . . . 20
3.2.2 Responses From The Driver . . . . . . . . . . 21
3.2.3 Initialization And Management Interface . . . 21
4 MESSAGE FORMATS . . . . . . . . . . . . . . . . . 23
4.1 NOTATION . . . . . . . . . . . . . . . . . . . . 24
4.2 DATA MESSAGES . . . . . . . . . . . . . . . . . 25
4.3 CONTROL MESSAGES . . . . . . . . . . . . . . . . 27
4.3.1 Acknowledge Message (ACK) . . . . . . . . . . 28
4.3.2 Negative Acknowledge Message (NAK) . . . . . . 29
4.3.3 Reply To Message Number (REP) . . . . . . . . 30
4.3.4 Start Message (STRT) . . . . . . . . . . . . . 31
4.3.5 Start Acknowledge Message (STACK) . . . . . . 32
4.4 MAINTENANCE MESSAGES . . . . . . . . . . . . . . 33
5 OPERATION . . . . . . . . . . . . . . . . . . . . 34
5.1 FRAMING . . . . . . . . . . . . . . . . . . . . 34
5.1.1 Byte Framing . . . . . . . . . . . . . . . . . 34
5.1.1.1 Asynchronous Links . . . . . . . . . . . . . 34
5.1.1.2 Synchronous Links . . . . . . . . . . . . . 35
5.1.1.3 Parallel Links . . . . . . . . . . . . . . . 36
5.1.2 Message Framing . . . . . . . . . . . . . . . 36
5.1.2.1 CRC Performance Option . . . . . . . . . . . 37
5.1.3 Synchronization . . . . . . . . . . . . . . . 37
5.1.3.1 Receiver Synchronization . . . . . . . . . . 37
5.1.3.2 Transmitter Synchronization . . . . . . . . 38
5.1.3.3 Synchronization Rules . . . . . . . . . . . 39
5.2 LINK MANAGEMENT . . . . . . . . . . . . . . . . 41
� Page 4
5.2.1 Link Management On Full-Duplex Point-To-Point
Links . . . . . . . . . . . . . . . . . . . . 42
5.2.2 Link Management On Half-Duplex Point-To-Point
Links . . . . . . . . . . . . . . . . . . . . 42
5.2.3 Link Management On Full-Duplex Multipoint Links 43
5.2.4 Link Management On Half-Duplex Multipoint Links 44
5.2.5 Link Management Summary Rules . . . . . . . . 45
5.3 MESSAGE EXCHANGE . . . . . . . . . . . . . . . . 47
5.3.1 Initialization . . . . . . . . . . . . . . . . 47
5.3.2 Reply Time-outs . . . . . . . . . . . . . . . 47
5.3.2.1 Reply Timer Interval . . . . . . . . . . . . 48
5.3.3 Valid Messages . . . . . . . . . . . . . . . . 49
5.3.4 Message Exchange Variables And States . . . . 49
5.3.4.1 Data Variables . . . . . . . . . . . . . . . 50
5.3.4.2 Control Flag Variables . . . . . . . . . . . 50
5.3.4.3 DDCMP States . . . . . . . . . . . . . . . . 51
5.3.5 Message Queueing, Reply Timers, And Buffering 52
5.3.6 Reply Timer Operation Rules . . . . . . . . . 53
5.3.6.1 Start Timer: . . . . . . . . . . . . . . . . 53
5.3.6.2 Stop Timer: . . . . . . . . . . . . . . . . 53
5.3.7 NAK Reasons . . . . . . . . . . . . . . . . . 54
5.3.8 Start-Up . . . . . . . . . . . . . . . . . . . 54
5.3.9 Data Transfer . . . . . . . . . . . . . . . . 55
5.4 MAINTENANCE MODE . . . . . . . . . . . . . . . . 61
6 ERROR RECORDING . . . . . . . . . . . . . . . . . 63
6.1 STRUCTURE OF THE COUNTERS . . . . . . . . . . . 64
6.2 DATA LINK COUNTERS . . . . . . . . . . . . . . . 64
6.2.1 Data Errors Outbound . . . . . . . . . . . . . 65
6.2.1.1 NAKs Received Header Block Check Error . . . 65
6.2.1.2 NAKs Received Data Field Block Check Error . 65
6.2.1.3 NAKs Received REP Response . . . . . . . . . 65
6.2.2 Data Errors Inbound . . . . . . . . . . . . . 65
6.2.2.1 Header Block Check Errors . . . . . . . . . 65
6.2.2.2 NAKs Sent Data Field Block Check Error . . . 65
6.2.2.3 NAKs Sent REP Response . . . . . . . . . . . 66
6.2.3 Local Reply Timeouts . . . . . . . . . . . . . 66
6.2.4 Remote Reply Timeouts . . . . . . . . . . . . 66
6.2.5 Local Buffer Errors . . . . . . . . . . . . . 66
6.2.5.1 NAKs Sent Buffer Temporarily Unavailable . . 66
6.2.5.2 NAKs Sent Buffer Too Small . . . . . . . . . 66
6.2.6 Remote Buffer Errors . . . . . . . . . . . . . 67
6.2.6.1 NAKs Received Buffer Temporarily Unavailable 67
6.2.6.2 NAKs Received Buffer Too Small . . . . . . . 67
6.2.7 Selection Timeouts . . . . . . . . . . . . . . 67
6.2.7.1 No Reply To Select . . . . . . . . . . . . . 67
6.2.7.2 Incomplete Reply To Select . . . . . . . . . 68
6.2.8 Data Messages Transmitted . . . . . . . . . . 68
6.2.9 Data Messages Received . . . . . . . . . . . . 68
6.2.10 Selection Intervals . . . . . . . . . . . . . 68
6.2.11 Data Bytes Transmitted . . . . . . . . . . . . 68
6.2.12 Data Bytes Received . . . . . . . . . . . . . 69
6.3 STATION COUNTERS . . . . . . . . . . . . . . . . 69
6.3.1 Remote Station Errors . . . . . . . . . . . . 69
6.3.1.1 NAKs Received Receive Overrun . . . . . . . 69
6.3.1.2 NAKs Sent Message Header Format Error . . . 69
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6.3.1.3 Selection Address Errors . . . . . . . . . . 69
6.3.1.4 Streaming Tributaries . . . . . . . . . . . 70
6.3.2 Local Station Errors . . . . . . . . . . . . . 70
6.3.2.1 NAKs Sent Receive Overrun . . . . . . . . . 70
6.3.2.2 Receive Overruns, NAK Not Sent . . . . . . . 70
6.3.2.3 Transmit Underruns . . . . . . . . . . . . . 70
6.3.2.4 NAKs Received Message Header Format Errors . 71
6.4 THRESHOLD ERROR COUNTERS . . . . . . . . . . . . 71
6.4.1 Transmit Threshold Errors . . . . . . . . . . 71
6.4.2 Receive Threshold Errors . . . . . . . . . . . 72
6.4.3 Selection Threshold Errors . . . . . . . . . . 72
6.5 NETWORK MANAGEMENT INTERFACE SUPPORTING THE DDCMP
COUNTERS . . . . . . . . . . . . . . . . . . . . 72
6.5.1 Read Data Link Counters (ADDR) . . . . . . . . 73
6.5.2 Read And Clear Data Link Counters (ADDR) . . . 73
6.5.3 Read Station Counters . . . . . . . . . . . . 73
6.5.4 Read And Clear Station Counters . . . . . . . 73
6.6 SUMMARY . . . . . . . . . . . . . . . . . . . . 74
7 EVENT LOGGING . . . . . . . . . . . . . . . . . . 75
7.1 LOCALLY-INITIATED STATE CHANGE . . . . . . . . . 75
7.2 REMOTELY INITIATED STATE CHANGE . . . . . . . . 76
7.3 STRT RECEIVED WHILE IN MAINTENANCE . . . . . . . 76
7.4 TRANSMIT ERROR THRESHOLD REACHED . . . . . . . . 77
7.5 RECEIVE ERROR THRESHOLD REACHED . . . . . . . . 77
7.6 SELECTION ERROR THRESHOLD REACHED . . . . . . . 77
7.7 MESSAGE HEADER FORMAT ERRORS . . . . . . . . . . 78
7.8 SELECTION ADDRESS ERRORS . . . . . . . . . . . . 78
7.9 STREAMING TRIBUTARIES . . . . . . . . . . . . . 78
7.10 NAKS SENT BUFFER TOO SMALL . . . . . . . . . . . 79
APPENDIX A GLOSSARY
APPENDIX B FORMAL SYNTAX DEFINITION
B.1 SYMBOLOGY . . . . . . . . . . . . . . . . . . . . B-1
B.2 MESSAGE SYNTAX . . . . . . . . . . . . . . . . . . B-1
APPENDIX C MESSAGE FORMAT SUMMARY
C.1 GENERAL MESSAGE FORMATS . . . . . . . . . . . . . C-1
C.2 UNNUMBERED MESSAGE SUMMARY . . . . . . . . . . . . C-1
APPENDIX D DDCMP BLOCK CHECK COMPUTATION
D.1 DDCMP ERROR DETECTION . . . . . . . . . . . . . . D-1
D.2 THE CRC-16 POLYNOMIAL . . . . . . . . . . . . . . D-1
D.3 CRC COMPUTATION . . . . . . . . . . . . . . . . . D-2
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APPENDIX E EXAMPLES
E.1 EXAMPLE 1 - START-UP SEQUENCE WITH NO ERRORS . . . E-1
E.2 EXAMPLE 2 - START-UP SEQUENCE WITH ERRORS . . . . E-2
E.3 EXAMPLE 3 - DATA TRANSFER WITH NO ERRORS . . . . . E-3
E.4 EXAMPLE 4 - DATA TRANSFER WITH CRC ERRORS AND
NAKING . . . . . . . . . . . . . . . . . . . . . . E-4
E.5 EXAMPLE 5 - DATA TRANSFER WITH ERRORS CAUSING
REPLY TIMEOUTS . . . . . . . . . . . . . . . . . . E-5
APPENDIX F REVISION HISTORY
TABLES
1 Summary of Synchronization Rules . . . . . . . . . 39
2 Link Management Summary . . . . . . . . . . . . . 45
3 Startup State Table . . . . . . . . . . . . . . . 56
4 RUNNING State Table . . . . . . . . . . . . . . . 59
5 Maintenance State Table . . . . . . . . . . . . . 63
6 DDCMP Counters . . . . . . . . . . . . . . . . . . 74
�Introduction Page 7
1 INTRODUCTION
In the design of computer communications networks, one of the basic considerations is the physical transmission of data from one computer to another over a physical data channel. In the absence of transmission errors, this task becomes relatively simple. Once errors are introduced, however, data sequencing and synchronization problems occur between the transmitter and receiver. The solution to these problems consists of a data link control procedure or communications protocol that ensures the correct sequencing and integrity of data transmitted between computers over a data link.
Digital Equipment Corporation recognized the need for such a communications protocol to establish reliable computer-to-computer communications. A standard protocol was designed to serve the needs of interprocessor communications. That protocol has been named the Digital Data Communications Message Protocol (DDCMP) and has been adopted as a Digital Equipment Corporation standard for intercomputer data communications.
2 FUNCTIONAL DESCRIPTION
The Digital Data Communications Message Protocol (DDCMP) is designed for use over communication channels to provide data integrity, message sequencing, and management of the physical channel. The protocol defines the structure, content, and sequencing procedures for the transmission of data between computers and the techniques used for error detection and recovery. DDCMP resides at a level above the communication medium (i.e., the physical transmission of bits over the communication channel). DDCMP is concerned with the logical transmission of data grouped into physical blocks known as data messages. The primary function of the protocol is to exchange these data messages while ensuring their correct sequencing and integrity when sent over communication channels.
Computers adhering to the protocol will be able to correctly exchange data (between their respective address spaces) over a link. It is the level above the protocol that is concerned with the meaning and understanding of this data once correctly exchanged. With remote entry stations and concentrators, this includes device addressing, device control, and data formatting. With computer networks, it includes problems of network routing, process synchronization, link multiplexing, flow control, and network management.
Programs wishing to communicate using DDCMP must agree on the syntax and semantics of the data transmitted within the DDCMP envelope. DDCMP may thus be viewed as a black box creating an error-free sequential communication path over which data may be transmitted. On multipoint links, DDCMP creates multiple sequential data paths between the control station and the multiple tributaries on the link. If the physical channel connecting two communicating computers were truly error-free, much of DDCMP would not be necessary. �Functional Description Page 8
2.1 RELATIONSHIP TO DECNET
DECnet is a family of hardware and software products that create distributed computer communication networks using Digital computers and their interconnecting data links. DECnet creates a general mechanism for sharing resources and providing interprogram communications within a distributed data processing environment. DECnet implementations adhere to a common network architecture that defines the structure and protocols each must use to communicate through the network. The Digital Network Architecture (DNA) defines this common structure.
The DNA structure is based on a hierarchy of communication layers. That is, each layer is built upon the layers beneath it. The lowest layer is the physical link or media over which bits are transmitted. The highest layer is the user service or application program. Intermediate layers perform such functions as error detection, error recovery, routing, sequencing, flow control, and connection management. More details on the DNA/DECnet structure can be found in current DECnet literature.
The DDCMP protocol creates and supports the functions of the Data Link Layer within the DECnet architecture. This layer provides control over the physical link operation and ensures both data integrity and the sequentiality of data transmitted over a single physical link. It should be noted that DECnet is not a part of the DDCMP standard specification and that DDCMP may be used, independently, in a wide variety of systems and environments where error-free communication is desired.
2.2 FEATURES
DDCMP includes the following features:
a. Error detection using the 16-bit, CRC-16, cyclic redundancy
check error detection polynomial.
b. Error correction by retransmission.
c. Message sequencing, with up to 255 outstanding messages for
pipelining.
d. Operation that is independent of the channel bit width
(serial or parallel) and transmission characteristics
(asynchronous and synchronous). DDCMP will operate with a
wide variety of communication hardware (e.g., character
interrupt and block transfer or DMA) and modems.
e. Common operation over full- and half-duplex, point-to-point
and multipoint channels.
f. A positive start-up procedure that synchronizes both ends of
the link.
�Functional Description Page 9
g. Simplicity and efficiency with only a few message formats.
h. A maintenance mode for diagnostic testing and bootstrapping
functions.
i. Data transparency of any bit sequence using a length field
framing technique.
2.3 OPERATING REQUIREMENTS
DDCMP is designed to serve the needs of interprocessor communications in a wide variety of applications and environments. DDCMP will provide:
a. High Performance - DDCMP provides high data throughput on
links capable of such and makes optimum use of link
characteristics.
b. Wide Applicability - DDCMP ensures operation that is
independent of channel type over a wide range of system
configurations.
c. Use of Available Hardware - DDCMP is able to operate with
most currently available communications equipment.
2.3.1 Goals - In addition to ensuring the error-free transmission of
data, DDCMP is designed to meet specific performance and compatibility
requirements. These goals are:
a. Create a protocol for the transmission of data over
communication links to provide for the correct sequencing and
integrity of the data transmitted (even when the link may
distort the information).
b. Operate over a wide variety of communications devices
available on micro, mini, and maxi computers in bit-serial
(asynchronous and synchronous) and bit-parallel modes.
c. Operate over point-to-point and multipoint circuits in both
full- and half-duplex modes using a common set of messages
and operating procedures.
d. Provide for the efficient transmission of binary
(transparent) data.
e. Ensure both high performance and simultaneous operation over
full-duplex channels where long circuit delays may be
encountered.
�Functional Description Page 10
f. Provide error recording and reporting features so that a
degraded link can be detected and repaired prior to link
failure.
g. Provide a positive indication (and synchronization) that the
protocol module on the other end of the link has
re-initialized or started.
h. Provide a basic operational mode for maintenance functions
such as bootstrapping and diagnostic testing.
i. Provide a rigid enough protocol so that all implementations
on the same channel type will operate together, independent
of implementation techniques.
j. Create a protocol to minimize the memory requirements and
execution time in the systems implementing the protocol.
k. Create a protocol that allows the physical characteristics of
the channel to become transparent to the user.
2.3.2 Restrictions - Even though DDCMP is a general purpose link
protocol offering high performance over a wide range of applications,
there are a number of situations in which it may not be optimal. Some
of these restrictions are:
a. DDCMP accepts data in blocks that are a multiple of 8-bit
bytes. Within a data block, a user can interpret the data in
any manner (e.g., 5-bit quantities), but the total block must
be a multiple of 8 bits.
b. DDCMP may not be optimal when operating on links with long
propagation delays and a high probability of error. Optimal
techniques in these cases might include forward error
correction and/or single message retransmission. When an
error occurs, DDCMP must go back to the last sequential
correct message, thereby losing any pipelining in effect on
the link.
c. DDCMP may not be suitable in some multipoint systems having
many tributaries with low utilization and fast response
requirements. Optimal techniques might include contention
selection and broadcast. DDCMP uses a polling selection
mechanism, which in some environments may result in long
response times.
d. On multipoint links, DDCMP supports only a single control
station. No messages can be exchanged directly between
tributaries. Within a given system, the control station can
not float among the tributaries.
�Functional Description Page 11
e. On multipoint links there is no broadcast or multiple
addressing facility.
f. There is no shut down capability within the protocol. It is
the user responsibility, at higher levels, to complete
communications and shut down the link.
2.4 DATA LINK FUNCTIONS
The DDCMP protocol is an extension of the data communications link, providing a number of functions to the user of the protocol. DDCMP may be viewed as a black box creating an error-free, sequential, managed data link. On the transmit side, messages are given to DDCMP, which delivers them over the link and notifies the user when the delivery has successfully occurred. On the receive side, the user provides buffers which are filled with correctly received messages by DDCMP. The term "user" refers to the process or program exchanging messages with the protocol. In DECnet, it will be the higher levels in the DNA structure. In other systems or structures it might be a service process or the end-user directly. DDCMP extends the capabilities of a physical data link to include the following features:
a. Creates an error-free data path. DDCMP transfers data
between protocol users over a physical link, while
maintaining data integrity within some very small undetected
error probability. If data integrity cannot be maintained,
no data will be transferred.
b. Transfers messages in proper sequence. Messages will be
delivered from one user to the other in the same order as
they are sent, even though DDCMP may require the use of
retransmission or other error recovery techniques.
c. Manages the characteristics of the channel. If the channel
requires receiver addressing and/or arbitration of
transmission requests, DDCMP is responsible for that
management.
d. Interfaces to modem control signals. DDCMP must interface
with signals necessary for the operation of the physical
channel, (e.g., modem control signals not handled by other
components in the system). It may do this directly, leave it
up to the hardware device driver, or let the user of the
modem control code control these signals through the protocol
interface.
e. Accesses data in blocks consisting of byte quantities. DDCMP
accepts data in blocks consisting of 8-bit bytes. All 256
8-bit combinations are transmittable, and transparent to
DDCMP. The protocol will allow blocks of up to 16,383 bytes
to be transmitted. However, the CRC-16 error detection
�Functional Description Page 12
polynomial used is most effective with blocks no longer than
4093 bytes.
f. Provides restart or initialization notification. If the
other end of the link resets or initializes, DDCMP will
notify the user.
g. Provides start and stop control. The user controls the
protocol and can start (or re-initialize), and stop (or halt)
the operation of DDCMP.
h. Provides notification of channel error. When a persistent
error is detected, the user is notified of such a condition.
Such errors might be:
1. too high a block error rate;
2. outages;
3. nonexistent communications;
4. modem failure.
i. Provides a maintenance mode. DDCMP creates a data envelope
with bit error-detection-only capability for use in
diagnostic testing and system bootstrapping functions.
2.5 FUNCTIONAL ORGANIZATION
From an operational viewpoint, DDCMP consists of three functional components:
1. Framing,
2. Link Management,
3. Message Exchange.
The following provide a generic model describing each of these components. This model is helpful in understanding DDCMP operation. It is provided as an aid in implementation design (by enabling an individual to understand the protocol, its operational intentions and motivations). It is not intended to describe specific operating details of DDCMP or subsets of DDCMP. For specific information on the actual protocol operation, refer to subhead 5.
2.5.1 Framing Component - Framing is the process of locating the beginning and end of a message, at the receiving end of a link. Synchronization is the process of locating some entity (e.g., a bit or byte) and then staying in step or operating at the same rate as that entity. Synchronization of data on a link must occur at the bit, byte, and message levels before framing can be accomplished. The following paragraphs describe how DDCMP provides synchronization at these levels: �Functional Description Page 13
2.5.1.1 Bit Synchronization - Locating a bit on the link. This
function is accomplished by the modems or interfaces on the link and
is not a part of DDCMP. This makes the protocol independent of the
physical characteristics of the data link.
2.5.1.2 Byte Synchronization - Grouping bits into 8-bit byte quantities. Byte synchronization is the process of locating the proper 8-bit window in the bit stream and then staying in step with it for every 8-bit byte grouping. On 8-bit asynchronous links, this process is inherent in the start/stop transmission technique on the link. Byte framing is established with each byte sent. On synchronous links, byte synchronization is established by searching for a unique 8-bit sequence called the sync byte, checking for two in sequence, and then counting every 8 bits as a byte. The unique pattern is such that any skewing to the right or left will not produce a sequence match. On 8-bit or 8-bit multiple parallel links, byte synchronization is inherent in the link. For other types of links, techniques must be designed to locate the proper 8-bit byte window. The technique is, thus, part of DDCMP but defined specific to each link type.
2.5.1.3 Message Synchronization - Locating a message or block. This involves finding the first and last bytes of a message. The first byte is found by searching for one of three special first starting bytes after achieving byte synchronization. The last byte is found via a count field in the message header which denotes the length of the message. Once the message is framed it is then available for processing by the remainder of the protocol. The starting byte defines the format type of the message and how the remaining bytes are to be interpreted.
The byte and message synchronization techniques were chosen to allow the greatest flexibility and independence from the actual data link characteristics. By using these techniques, DDCMP can operate on serial synchronous links with typical character interrupt or block transfer devices and on serial asynchronous links using 8-bit bytes. Byte synchronization is specified in DDCMP and is specific for each data link and its characteristics. Message synchronization is the same for all link types once byte synchronization has been established.
2.5.2 Link Management Component - The Link Management Component resolves the transmission and reception on links that are connected to two or more transmitters and/or receivers in a given direction. This is true of half-duplex and multipoint channels. Link management occurs for both the transmitting and receiving functions.
On half-duplex links, one station must be receiving while the other is transmitting. The switching between transmit and receive is via a �Functional Description Page 14
selection flag. The station that is transmitting ends transmission by
setting the flag in its last message. This signals the receiver to
complete reception of this message and then enter transmit mode.
For reception on multipoint links, the link appears as a party line, with one station designated the control station and the others tributaries. All messages contain a tributary address to identify them. Messages to a tributary are received by all tributaries and ignored by all except the one with the matching address. Messages from tributaries are ignored by other tributaries and received by the control station that verifies the tributary address to be the one selected. Message traffic is only between the control station and tributaries.
For transmission on multipoint links, the control station manages the link and assigns transmission ownership or selection to tributary stations via a selection flag. Tributary stations, once selected, may transmit and will terminate transmission by sending a selection flag to the control station in the last message of a transmission sequence.
Timers are used by half-duplex or control stations to handle the case of a lost flag (e.g., the message containing the flag is in error). A timer is started when waiting for the next message. If it expires it is assumed the selection flag was received in error and the station operates as if it received a valid selection flag.
2.5.3 Message Exchange Component - The message exchange component is the part of DDCMP that creates the sequential error-free link. This component transfers the data correctly and in sequence over a link that has some probability of introducing errors. Once framing is accomplished, this component operates at the message level, exchanging data and control messages. DDCMP is a positive acknowledgement retransmission protocol. For each data message correctly received and passed to the user, a positive acknowledgement is returned on the link notifying the transmitter of the correct receipt of the data message. If incorrectly received, the data message is not passed to the user and not acknowledged. Eventually, an error recovery mechanism will be invoked and the message will be retransmitted. DDCMP uses the CRC-16 cyclic redundancy check for error detection. This section describes the component parts of the message exchange mechanism along with their design characteristics and functions.
The basic positive acknowledgement message exchange component requires the following:
a. A data message with message number n;
b. A positive acknowledgement with message number n (ACK); and
c. A timer.
�Functional Description Page 15
It operates in the following manner:
1. The transmitter puts the next available message number n in
the data message, adds the CRC block check to the message,
puts it in the required framing envelope, and sends it. When
it has been transmitted on the link, a timer is started.
2. The receiver frames and receives the message, checks the CRC
for errors, and compares the message number with the next
expected. If the number is correct, the receiver returns a
positive acknowledgement (ACK) with that number, passes the
message to the receiving user, and increments the next
expected number to n + 1 (modulo 256). If the number is in
error, the message is ignored.
3. The transmitter follows one of two procedures;
a. It receives a positive acknowledgement and compares the
number received with the one expected. If it agrees, the
transmitter releases the message, notifies the
transmitting user of successful receipt, stops the timer,
and increments the next available message number to n + 1
(modulo 256). If the acknowledgement does not agree with
the expected number, it is ignored.
OR
b. It receives nothing and the timer expires. The
transmitter initiates error recovery. Various error
recovery options are available. The ones used in DDCMP
are presented below.
The timer in the message exchange component is keyed to the selection
of a tributary (multipoint) or other station (half-duplex). That is,
the controlling station must wait until a tributary or the other
station is selected and transmits before it determines that a message
or ACK was not properly received. This makes the timing independent
of the selection of stations. On full duplex point to point channels
the timer is a clock which allows enough time for the receiver to
respond with the ACK.
This mechanism is adequate for creating a message exchange component. The following additional messages and operational techniques of DDCMP are used to achieve higher performance (via pipelining) and faster error recovery (via error notification) but do not add to the basic integrity of the data transfer mechanisms.
2.5.3.1 Negative Acknowledgement (NAK) - The time-out value used to detect an error when an ACK is not returned must be long enough to account for delays such as propagation, line turnaround, local processing of the data message, and the generation of the ACK. Time-out values might be a few seconds while the actual delay may be �Functional Description Page 16
on the order of a few milliseconds. If the only way to determine an
error is to wait for a time-out, undue waste and inefficiency are
encountered. A negative acknowledgement provides a means for more
immediate notification of some error conditions. If the receiver does
not receive the message correctly but does recognize that a message
was sent, it sends a NAK, which triggers the retransmission long
before the timer expires.
In DDCMP, NAKs are sent in response to cyclic redundancy check errors, but not to wrong message numbers. If the receiver gets a message with the wrong number, the message is ignored and the time-out condition triggers the transmitter to initiate error recovery. If NAKs were sent in both cases, race conditions and long delays could occur under certain timing conditions.
2.5.3.2 Reply To Message Number (REP) - When the timer expires, it is unclear whether the message was received in error or the returned ACK was received in error. (ACKs also have CRC checks on them). In this case, rather than retransmit perhaps a long message, a REP is sent with the message number of the message previously sent. If the message with that number was received correctly, the response to the REP is an ACK; otherwise it is a NAK. The receipt of a NAK causes retransmission to occur. The REP forces the transmitter and receiver to synchronize their numbering and start retransmission if required. The transmission timer is restarted after sending a REP. If it expires again, the process is repeated. After some specified number of these time-outs, the transmitter will notify the DDCMP user, who may declare the link out-of-service.
2.5.3.3 Pipelining - The ability to send more than one message without waiting for ACKs to each successive message is called pipelining. Within DDCMP, messages are numbered from 0 to 255. This numbering is cyclic (modulo 256) in that after message number 255 the next message number is 0. ACKs not only confirm that the specified message number has been received correctly, but that all previous messages with numbers between the one acknowledged in the last ACK and the one acknowledged by the current ACK (modulo 256) have been received correctly. If an ACK message is in error, the information lost is automatically included in subsequent ACKs, eliminating the sending of REP messages if the ACKS are received prior to the expiration of the transmission timer. This technique is also used with the REP message, the number sent in the REP being the number of the last message transmitted.
2.5.3.4 Piggybacking - The purpose of an ACK is to convey the message number of the last successfully received data message. If data message traffic is being transmitted in both directions, the ACK number can be sent piggybacked on or within the frame of the message being transmitted in the reverse direction. This technique saves �Functional Description Page 17
separate framing overhead for the ACK.
2.5.3.5 ACK Implied In NAK - The number referenced in a NAK reply identifies the last successfully received message as well as noting a received error. So NAK implies that all messages prior to the one being negatively acknowledged were received correctly.
2.5.3.6 Initialization - The method of setting message numbers to initial values is called initialization. It is accomplished by STRT and STACK messages that reset message numbers to zero. It is used initially or after a failure to reset number values at both ends. It is designed so that one end cannot be initialized without the other.
3 INTERFACES
This section describes how DDCMP is viewed by a user of the protocol and how the physical interface device or driver is viewed by DDCMP. A generic description of the information that must be passed across the interfaces to the user and the device is presented as an aid for implementation design.
3.1 USER INTERFACE
The interface between DDCMP and the user consists of a number of commands to DDCMP and responses from DDCMP. In these commands and responses, the user exchanges data and control information with the protocol. The actual interface mechanism depends heavily on the features and capabilities within the operating systems running DDCMP. Mechanisms for exchanging this information might include shared tables, calls with parameter lists, I/O registers, and interrupt mechanisms.
Three kinds of information are exchanged in the command/response sequences:
a. Data,
b. Control information, and
c. Error information.
Data is the user information to be sent or received by the protocol. Its description usually consists of a starting buffer address and a length or character count, or a chain of addresses and counts. Control is information to start and stop the protocol and notify the user of protocol initialization. Error information is provided by the protocol for use in determining the physical condition of the link and �Interfaces Page 18
when maintenance is necessary. DDCMP is totally controlled by the
user of the protocol. It is only activated by a command request from
the user and continues to operate even when large numbers of data
errors occur on the physical link. It is started, stopped, and
reinitialized only upon commands from the user. On multipoint links,
independent command and response sequences are maintained between the
control station and each tributary on the link. The link appears
logically as multiple point-to-point links, one for each tributary
address.
The exact interface between DDCMP and the user depends on the system implementation and will vary in the manner in which errors are handled. In some systems, the error handling code may be totally at the user level, each error being reported to the user. In other systems, the error handling code may be part of the protocol module and only persistent errors will be reported to the user. Subhead 6 describes the error information recording techniques that may be employed within DDCMP.
3.1.1 Commands To DDCMP - The basic commands to DDCMP are:
a. Initialize Link - Initialize the protocol and start the data
link. See 3.2.3 below for more details on initialization and
management commands.
b. Stop Link - Halt the protocol. In some dial-up situations, a
method may be employed to force the modem to hang-up.
c. Transmit Message - Give a message to DDCMP for transmission.
As an option the user may specify that it wishes to send the
message within the maintenance mode, or the protocol
implementation may require a separate Maintenance Mode
Initialization command prior to a transmit request.
d. Receive Message - Give an empty buffer to DDCMP for reception
of the next sequential message. Alternatively, the user
might supply a pool of buffers to DDCMP initially, and have
the protocol select one. In this mode, there will be an
alternate command to Return Empty Buffers to the pool so they
may again be used by DDCMP.
e. Return Transmit Buffers - This optional command, which can be
employed after halting DDCMP, returns outstanding transmit
buffers to the user. The response to this command would
include whether they were already transmitted and
acknowledged, not yet acknowledged, or not yet transmitted.
f. Enter Maintenance Mode - This command is an option to first
change to the maintenance mode before transmitting or
receiving maintenance mode messages.
�Interfaces Page 19
3.1.2 Responses From DDCMP - The responses from DDCMP are:
a. Initialization on Other End - The other end has restarted or
initialized. This response will halt the protocol. The
command to restart the protocol on this end will be an
Initialize Link command.
b. Initialization Complete - Response to Initialize Link
command. This response is optional. If it is omitted, the
reply to a successfully received or transmitted message will
serve as initialization completion notification.
c. Message Transmitted - Response to the Transmit Message
command. The message was successfully received on the other
end (acknowledged).
d. Message Received - The next sequential message was
successfully received. Either the user buffer specified in
the Receive Message command will be used, or a buffer will be
taken from a pool, if such a buffering technique is employed.
Optionally, if the message was received in the maintenance
mode it may be so marked, or a separate response may be first
sent to the user to indicate that the other end is in
maintenance mode. At that point, the protocol will halt, and
the user will have to initialize the protocol into the
maintenance mode before receiving maintenance mode messages.
e. Transient Error - Threshold Counter Overflow - An error
threshold counter has overflowed. The protocol will continue
operation. It must be halted by the user if the user wishes
to cease operation. Refer to subhead 6, ERROR RECORDING.
f. Persistent Error - An error has occurred from which recovery
may not be possible. Some implementations of the protocol
may halt operation. Some errors that are classified as
persistent errors in one system, might be transient errors in
another. The various types of errors are described in
subhead 6.
3.2 DEVICE DRIVER INTERFACE
The interface between DDCMP and the line driver includes a number of commands and responses used to transmit and receive message blocks to and from the link, respectively. The actual interface depends heavily on the mechanisms and capabilities available in the I/O structure of the systems within which this interface operates. It also depends heavily on the split of protocol functions between DDCMP and the driver. The driver may be very protocol independent and rely on heavy interaction with DDCMP for message framing, CRC calculation, and the syntactic and semantic interpretation of message fields. Alternately, it may embody much of DDCMP including framing, CRC checking, link �Interfaces Page 20
management, and link turnaround. In this mode, there would be less
interaction with the semantic or message exchange portion of DDCMP.
Consequently the driver would handle many of the functions related to
link type and device characteristics. The choice of driver
capabilities and the split of functions depends on system
characteristics, device requirements, driver generality, and the
interface to other protocols. The interface described here lies
between these two extremes and is presented as an aid to understanding
what information must pass across this interface.
Message blocks are usually passed to the driver via a buffer address and length (or a chain of addresses and lengths to allow fragmented message blocks).
3.2.1 Commands To The Driver - The driver receives the following commands:
a. Link and Modem Control - These commands activate and connect
a physical link to DDCMP. They also control the modem
signals necessary for proper operation. These signals may be
implicit in enabling the link (i.e., turn Data Terminal Ready
(DTR) on) or explicit via modem control commands to allow
DDCMP to directly control the modem. Typical commands might
be:
o Enable link. This command connects the driver to DDCMP
and turns DTR on.
o Disable link. This command disconnects the driver from
DDCMP and turns DTR off.
b. Buffer Management - Received message blocks are passed to
DDCMP via buffers. The buffers may be (a) individually given
to the driver via Receive commands or (b) initially allocated
to the driver in a Set Buffers command or the Enable Link
command. In this second mode there must also be a command
for DDCMP to return the buffers to the driver. On
disconnection (Disable Link Command), the buffers must be
returned to DDCMP or to a buffer pool.
c. Transmit a Block - This command passes a block to the driver
for transmission. The request might include one of the
following options: (a) precede with a synchronization
sequence; (b) end with a pad; (c) calculate CRC; or (d)
shutdown the transmitter after the message. These options
depend on the precise division of functions between the
driver and protocol.
d. Receive a Block - This command passes buffers to the driver
if individual explicit buffers are used. Otherwise, the
driver might simply queue received blocks to DDCMP using
buffers from a previously obtained pool (as noted in
�Interfaces Page 21
paragraph b above). This command may also request the driver
to resynchronize or reframe the receiver or there may be a
separate Resync Receiver command.
e. Set Parameters - If the driver design is protocol
independent, this command might be included to set such
parameters as synchronization sequences and the CRC
polynomial.
3.2.2 Responses From The Driver - The driver issues the following
responses:
a. Modem Status - The driver returns modem signals, such as Data
Set Ready (DSR), if appropriate to the interface.
b. Received Block - The driver passes a received data block to
DDCMP. Depending on the functional split between the driver
and DDCMP, the driver may calculate CRC (either in the driver
or device itself) and pass this status with the block. When
DDCMP is finished with the buffer it returns it to the driver
via either (a) a Receive command or (b) a Return Buffer
command, depending on the buffering scheme used.
c. Transmit Complete - The driver will notify DDCMP when a
previous Transmit a Block command has been completed.
3.2.3 Initialization And Management Interface - The Initialize Link
command may be expanded to provided more control and monitoring of
DDCMP, especially in multipoint configurations.
This may included the following commands and responses:
1. Reset
This command resets the state of DDCMP, placing it into a
default mode and disabling any tributary addresses. All
tributary state information is cleared (to HALTED state).
2. Set mode (mode)
Where mode is one of the following:
inoperative
full duplex point-to-point
half duplex point-to-point
full duplex multipoint control
half duplex multipoint control
full duplex multipoint tributary
half duplex multipoint tributary
�Interfaces Page 22
This command is required only if more than one mode is
supported. The choice of default mode following Reset is
implementation dependent.
Some implementations may permit this command only immediately
following a Reset. Others may permit changing between full
and half duplex operation at other times.
3. Status mode
This command returns the current station mode. This command
should be implemented if Set Mode is implemented.
4. Enable Data Link (address)
This command is used by multipoint stations only. It is
required for multipoint control stations and optional for
multipoint tributary stations. (If not implemented by
tributaries, alternate means are required to define the
tributary address(es).)
Until an address has been enabled, a multipoint station will
not accept commands from the user interface for that data
link. That is, multipoint control station will not poll the
specified tributary, and a multipoint tributary will not
accept received messages for the specified address.
5. Disable Data Link (address)
This command is the inverse of the Enable and clears any
state information associated with the address.
6. Status Data Link (address)
This command returns the following states for the specified
address:
Disabled (if multipoint)
HALTED
ISTRT
ASTRT
RUNNING
MAINTENANCE
7. Set timer (value)
This command sets the timer used for the reply timer on
full-duplex point-to-point links and the select timer on
half-duplex point-to-point links and multipoint control
stations. This command is optional.
8. Status timer
This command returns the timer value. It is also optional.
�Interfaces Page 23
Commands and responses to read and clear error and maintenance
counters are described in heading 6.
4 MESSAGE FORMATS
This section describes the message formats of DDCMP. Data is exchanged over DDCMP links between the data source (master) and data sink (slave) within numbered data messages. Responses and control information are returned from the slave to the master within unnumbered control messages. Stations contain both a master and slave. For the purpose of exchanging data, the station plays the role of master or slave depending on whether it is transmitting or receiving the data. It is a distinction used for easy understanding and explanation of DDCMP. In reality, data is usually exchanged in both directions. In the following explanation only a single direction is described.
Each data message carries a number assuring correct message sequencing at the slave. The numbering begins with number one after initialization via the STRT/STACK control message sequence and is incremented by one (modulo 256) for each subsequent data message. The slave always acknowledges the correct receipt of data messages by returning the message number as a response either in the response field of numbered data messages being sent or, in an ACK unnumbered control message. For efficiency, an acknowledgement of the data message with number n implies an acknowledgement of all consecutive data messages sent up to and including data message number n. Retransmission is used to recover from errors. The error recovery mechanism uses time-outs and NAK and REP control messages to resynchronize and cause retransmission if required. All messages also include station addresses and link control flags for use on multipoint and half-duplex channels. �Message Formats Page 24
4.1 NOTATION
The following notation is used to describe the messages:
Field (length) : coding = description of field
Field = the name of the field being described
Length = the length of the field as:
a. A number meaning the number of 8-bit bytes
OR
b. A number followed by a B, meaning the number of bits.
Coding = the representation type used:
B = Binary
BM = Bit map (each bit has independent meaning)
C = Constant
Null = Interpretation data dependent
Fields in separate messages that have the identical name are the same
field and have identical meaning.
All numeric values in this document are shown in decimal representation unless otherwise noted.
All header fields and bytes of data are transmitted low-order or least-significant bit first on the data links unless otherwise specified. �Message Formats Page 25
4.2 DATA MESSAGES
Numbered data messages carry user data over DDCMP links. The format of a numbered message is:
__________________________________________________________________
| | | | | | | | | | | SOH | COUNT | FLAGS | RESP | NUM |ADDR | BLKCK1 | DATA | BLKCK2| | | | | | | | | | |
------------------------------------------------------------------
Where:
SOH(1) : C = The numbered data message identifier. It has a
value of 129 (octal - 201, hex - 81).
COUNT(14B) : B = The byte count field. It specifies the number
of 8-bit bytes in the DATA field. The value
zero is not allowed.
FLAGS(2B) : BM = The link flags. They are used to control link
ownership and message synchronization. These
flags are:
bit 0 = Quick sync flag (QSYNC flag), used to
notify the receiver that the next message
will not abut this message and resynchron-
ization should follow this message. The
quick sync flag reduces the length of sync
sequences on synchronous links.
bit 1 = Select flag (SELECT flag), used to control
transmission ownership on multipoint and
half-duplex links. Reverses link direction
on half-duplex links. Invites a tributary
to send and signals end of tributary
selection on multipoint links.
Note
COUNT and FLAGS form a 2-byte quantity. The
first byte contains the 8 low-order bits of the
COUNT. The second byte contains the 6
high-order bits of the COUNT, the SELECT flag
the highest order or most significant bit of the
byte, and the QSYNC flag the next bit in the
byte.
_______________
| | | |
| S | Q | COUNT |
| | | |
---------------
high order bit low order bit
transmitted last transmitted first
�Message Formats Page 26
RESP(1) : B = The response number. It is used to acknowledge
correctly received messages (the piggybacked
ACK). It is the number of the last consecutive
correctly received message received from the
addressed station by the station transmitting
this message. It implies that all
unacknowledged messages between the one
acknowledged in the last RESP field received and
the one acknowledged by this RESP field (modulo
256), have been received correctly.
NUM(1) : B = The transmit number. It is used to denote the
number of this data message.
ADDR(1) : B = The station address field. It is used to
designate the address of tributary stations on
multipoint links. Stations on point-to-point
links use the address value 1.
BLKCK1(2) : B = The block check on the numbered message header.
It is computed on SOH through ADDR using the
CRC-16 polynomial (X^16+X^15+X^2+1). BLKCK1 is
initialized to zero prior to computation and
transmitted X^15 bit first. On reception the
inclusion of BLKCK1 in the computation will
result in a zero remainder or CRC value if no
bit errors exist. See Appendix D for a
description of CRC computation.
DATA (COUNT) = The numbered message data field. This field is
totally transparent to the protocol and has no
restrictions on bit patterns, groupings, or
interpretations. The only requirement is that
it contain the number of 8-bit bytes specified
in the COUNT field.
BLKCK(2) : B = The block check on the data field. It is
computed on the DATA field only using the
polynomial and technique described above for
BLKCK1.
�Message Formats Page 27
4.3 CONTROL MESSAGES
Unnumbered control messages carry channel information, transmission status, and initialization notification between the protocol modules. The individual fields are specific for each type of control message. Control messages have the following general form:
____________________________________________________
| | | | | | | | |
|ENQ |TYPE |SUBTYPE |FLAGS |RCVR |SNDR |ADDR |BLKCK3 |
| | | | | | | | |
----------------------------------------------------
where:
ENQ(1) : C = The unnumbered control message identifier. It
has a value of 5 (octal - 005, hex - 05).
TYPE(1) : B = The control message type. This value denotes
the type of each control message.
SUBTYPE(6B) : B = The subtype or type modifier field. It provides
additional information for some message types.
Its use is specific for each message type.
FLAGS(2B) : BM = The link flags. They are the same as described
for numbered data messages (See subhead 4.2).
RCVR(1) : B = The control message receiver field. It is used
to pass information from the data message
receiver or slave station to the data message
sender or master station. Its use is specific
for each control message type.
SNDR(1) : B = The control message sender field. It is used to
pass information from the data message sender or
master to the data message receiver or slave.
Its use is specific for each control message
type.
ADDR(1) : B = The station address field. It is the same as
described for numbered data messages (See
subhead 4.2).
BLKCK3(2) : B = The block check on the control message. BLKCK3
is computed on fields ENQ through ADDR using the
polynomial and technique described for numbered
data message BLKCK1 (See subhead 4.2).
�Message Formats Page 28
Note
The common fields in data and control
messages are in the same position
relative to the beginning of the message.
The two types line up as follows:
______________________________________________________________
| | | | | | | | | | |SOH| C O U N T |FLAGS |RESP |NUM |ADDR | BLKCK1| DATA|BLKCK2 | | | | | | | | | | |
--------------------------------------------------------------
| | | | | | | | | |ENQ|TYPE|SUBTYPE|FLAGS |RCVR |SNDR|ADDR | BLKCK3| | | | | | | | | |
------------------------------------------------
4.3.1 Acknowledge Message (ACK) - The ACK message is used to acknowledge the correct receipt of numbered data messages. It conveys the same information as the RESP field in numbered messages and is used when acknowledgements are required, and when no numbered messages are to be sent in the reverse direction. The form of the ACK message is:
_____________________________________________________
| | | | | | | | | |ENQ | ACKTYPE|ACKSUB |FLAGS |RESP |FILL |ADDR |BLKCK3| | | | | | | | | |
-----------------------------------------------------
Where:
ENQ(1) : C = The control message identifier.
ACKTYPE(1) : C = The ACK message type with a value of 1.
ACKSUB(6B) : C = The ACK subtype with a value of 0.
FLAGS(2B) : BM = The link flags.
RESP(1) : B = The response number used to acknowledge correctly
received messages. It is the same as described
for numbered data messages (See subhead 4.2).
FILL(1) : C = A fill byte with value 0.
ADDR(1) : B = The station address field.
BLKCK3(2) : B = The control message block check. �Message Formats Page 29
4.3.2 Negative Acknowledge Message (NAK) - The NAK message is used to
pass error information from the slave (or data receiver) to the master
(or data sender). The error reason is included in the subtype field.
The NAK message also includes the same information as the ACK message,
thus serving two functions: acknowledging previously received
messages and notifying the master of some error condition. The form
of the NAK message is:
______________________________________________________
| | | | | | | | | |ENQ |NAKTYPE |REASON |FLAGS |RESP |FILL |ADDR |BLKCK3 | | | | | | | | | |
------------------------------------------------------
Where:
ENQ(1) : C = The control message identifier.
NAKTYPE(1) : C = The NAK message type with a value of 2.
REASON(6B) : B = The NAK error reason. Identifies the source and
reason for the NAK.
1. Error usually due to transmission medium:
Value and Reason
1 = header block check error (data message BLKCK1
or control message BLKCK3).
2 = data field block check error (data message
BLKCK2).
3 = REP response.
2. Error usually due to computer and/or interface:
Value and Reason
8 = buffer temporarily unavailable.
9 = receiver overrun.
16 = message too long.
17 = message header format error.
FLAGS(2B) : BM = The link flags.
RESP(1) : B = The response number used to acknowledge
correctly received messages. When used in a NAK
message usually implies some error in a message
with number RESP+1 (modulo 256) or beyond.
FILL(1) : C = A fill byte with a value of 0.
ADDR(1) : B = The station address field.
BLKCK3(2) : B = The control message block check. �Message Formats Page 30
4.3.3 Reply To Message Number (REP) - The REP message is used to
request received message status from the slave or data receiver. It
is usually sent when the master has transmitted data messages and has
not received a reply within a time-out period. The response to a REP
is either an ACK or NAK depending on whether the slave has or has not
received all messages previously sent by the master. The form of the
REP message is:
_____________________________________________________
| | | | | | | | | |ENQ |REPTYPE |REPSUB |FLAGS |FILL |NUM |ADDR |BLKCK3 | | | | | | | | | |
-----------------------------------------------------
Where:
ENQ(1) : C = The control message identifier.
REPTYPE(1) : C = The REP message type with a value of 3.
REPSUB(6B) : C = The REP subtype with a value of 0.
FLAGS(2B) : BM = The link flags.
FILL(1) : C = A fill byte with a value of 0.
NUM(1) : B = The number of the last sequential numbered
data message (not including retransmissions)
sent by the master. This is compared against
the number of the last sequential message
received by the slave and results in either an
ACK being returned if they agree or a NAK if
they do not. The NAK will contain the number
of the last sequential message that was
received.
ADDR(1) : B = The station address field.
BLKCK3(2) : B = The control message block check. �Message Formats Page 31
4.3.4 Start Message (STRT) - The STRT message is used to establish
initial contact and synchronization on a DDCMP link. It is used only
on link startup or reinitialization. It operates with the start
acknowledge message STACK described below. The start sequence resets
message numbering at the transmitter and addressed receiver. The form
of the STRT message is:
______________________________________________________
| | | | | | | | |ENQ |STRTTYPE |STRTSUB |FLAGS |FILL |FILL |ADDR|BLKCK3| | | | | | | | | |
------------------------------------------------------
where:
ENQ(1) : C = The control message identifier.
STRTTYPE(1) : C = The STRT message type with a value of 6.
STRTSUB(6B) : C = The STRT subtype with a value of 0.
FLAGS(2B) : C = The link flags. For STRT, both flags are ones
(flags value of 3).
FILL(1) : C = A fill byte with a value of 0.
FILL(1) : C = A fill byte with a value of 0.
ADDR(1) : B = The station address field.
BLKCK3(2) : B = The control message block check. �Message Formats Page 32
4.3.5 Start Acknowledge Message (STACK) - The STACK message is
returned in response to a STRT when the station has completed
initialization and has reset its message numbering. The form of the
STACK message is:
________________________________________________________
| | | | | | | | | |ENQ |STCKTYPE |STCKSUB |FLAGS |FILL |FILL |ADDR |BLKCK3 | | | | | | | | | |
--------------------------------------------------------
where:
ENQ(1) : C = The control message identifier.
STCKTYPE(1) : C = The STACK message type with a value of 7.
STCKSUB(6B) : C = The STACK subtype with a value of 0.
FLAGS(2B) : C = The link flags. For STACK, both flags are ones
(flags value of 3).
FILL(1) : C = A fill byte with a value of 0.
FILL(1) : C = A fill byte with a value of 0.
ADDR(1) : B = The station address field.
BLKCK3(2) : B = The control message block check. �Message Formats Page 33
4.4 MAINTENANCE MESSAGES
The DDCMP protocol operates in two basic modes:
a. On-line, or the normal running mode.
b. Off-line, or the maintenance mode.
The previous messages and operation describe the on-line mode. The off-line, or maintenance mode, may be used for basic diagnostic testing and simple operating procedures such as bootstrapping, down-line loading, or dumping. It provides a basic envelope compatible with DDCMP framing, link management, and the CRC check for bit errors, but does not include any error recovery, retransmission time-outs, or sequence checks. All these functions, if necessary, are handled by the user of this mode within the data field. The maintenance message is similar in format to the data message. The format of the maintenance message is:
______________________________________________________
| | | | | | | | | | |DLE|COUNT |FLAGS |FILL |FILL |ADDR|BLKCK1 |DATA|BLKCK2| | | | | | | | | | |
------------------------------------------------------
where:
DLE(1) : C = The maintenance message identifier, has the
value 144 (octal - 220, hex - 90).
COUNT(14B) : B = The byte count field, specifies the number of
8-bit bytes in the DATA field. The value zero
is not allowed.
FLAGS(2B) : C = The link flags. Both flags are ones for
maintenance messages (flags value of 3).
FILL(1) : C = A fill byte with a value of 0.
FILL(1) : C = A fill byte with a value of 0.
ADDR(1) : B = The station address field.
BLKCK1(2) : B = The header block check on fields DLE through
ADDR. Same as described for data messages
(See subhead 4.2).
DATA (COUNT) = The data field. It consists of COUNT 8-bit
bytes.
BLKCK2(2) : B = The block check on the DATA field only. Same
as described for numbered data messages (See
subhead 4.2).
�Operation Page 34
5 OPERATION
The DDCMP functions may be grouped into three areas: Framing, Link Management, and Message Exchange. These functional components are:
a. Framing - The process of locating the beginning and end of a
message. It may involve bit, byte, and message
synchronization. Once framing is accomplished the protocol
operates at the logical message level, both sending and
receiving message blocks.
b. Link Management - The process of controlling the transmission
and reception on links connected to two or more transmitters
and/or receivers on a common signal channel. This is true of
half-duplex and multipoint links. There must be an orderly
mechanism for the proper receiver to identify its data and
for only one transmitter on a common signal channel to be
active at a given time.
c. Message Exchange - The process of transferring user data over
the link sequentially and without bit errors. DDCMP is a
positive acknowledgement retransmission protocol, returning
an indication to the transmitter for each message that has
been successfully received.
5.1 FRAMING
The basic concepts of framing are presented in subhead 2.5. This section discusses the specific details of framing for each link type on which DDCMP operates. Framing occurs at the bit, byte, and message levels.
Bit framing is handled by the modems and interfaces, and is not a part of this standard.
5.1.1 Byte Framing - This process entails framing on the proper 8-bit byte sequence so that bits may be grouped into meaningful 8-bit bytes. Byte framing differs for each type of link. The byte framing procedures for asynchronous, synchronous, and parallel links are described in the following subheads:
5.1.1.1 Asynchronous Links - DDCMP operates on serial asynchronous links using 8-bit bytes. Byte framing is inherent in the asynchronous operation. An asynchronous link is a communications path that has no fixed time relationship between bytes. When bytes are to be sent, the link is activated. When there is no byte flow the link remains in a steady state. This steady state is called the mark state, 1 state, stop state, or Z condition and by convention is the binary 1 �Operation Page 35
condition. For sending a byte (or 8 bits) over the line, the framing
technique used is based on a start and a stop bit placed at each end
of the byte.
The presence of the start bit is recognized by the receiving computer as a change from the 1 to 0 state. This change (a) starts the receiver sampling the line at preset timing intervals and (b) tells the receiver that the next eight bits will be the data byte followed by a ninth bit, the stop bit. An important consideration is that there is no framing until the start bit is received for each byte.
A potential problem on asynchronous links is that framing may be lost during the transmission of multiple abutting bytes (i.e., where the next start bit immediately follows the preceding stop bit). If the receiver shifts in bit timing, it may time its search for a start bit at the exact interval when a data bit with the same value as the start bit is received. The receiver would then think that the next eight bits were data, look for the stop bit, and wait for the next start bit to reframe. The error will be caught by the block check for the current message but may lead to misframing and missing the next message if uncorrected.
The solution to synchronization on asynchronous links specifies that the transmitter must either send an all ones byte DEL (Value 255., 377 Octal, FF Hex) or idle the link for 10 bit times when resynchronization is required. This technique will guarantee proper byte framing for the next byte at the receiver. The receiver does not look for this DEL; it simply causes proper framing on the next byte. If the DEL is received, it is ignored when resynchronizing. On asynchronous links, bytes always logically abut due to their independence from each other in time. So resynchronization is required only for error recovery and is not required preceding messages where the link has just been idle for some time.
5.1.1.2 Synchronous Links - DDCMP operates on serial synchronous links using 8-bit bytes. Bit timing is provided by the modem or superimposed digitally on the data signal. On synchronous links, byte framing is established by searching for a unique 8-bit pattern or sequence in the bit stream. Once this is found every eight bits forms the next byte. This pattern is called the SYN byte (value 150., octal - 226, hex - 96). The receiver must locate and lock on to a sequence of two consecutive SYN bytes to achieve byte synchronization. The transmitter must send four or more SYN bytes to allow for the loss from missynchronization and hardware interface constraints. Additional consecutive SYN bytes are ignored on receive while searching for the first non-SYN byte. This sequence of four or more SYN bytes is the synchronous synchronization sequence.
Since timing between bytes is determinate, messages must either abut (i.e., the first byte of the next message immediately follows the last byte of the current message with no intervening time) or byte framing is assumed lost following the end of the current message. The next message must re-establish or resynchronize byte framing at the �Operation Page 36
receiver. This resynchronization requires the next message to be
preceded by a synchronization sequence. On some communication
interfaces (usually block-oriented or DMA type), received data may be
buffered within the device and when the software driver determines
that the next message does not abut, the device may have buffered many
bytes further ahead on the link. Therefore, on synchronous links, a
long synchronization sequence is required when messages do not abut to
account for the potential buffering in the interface. This value is
the number passed over or buffered by the device plus four more for
resynchronization. Current devices and programming techniques have
set the long sync sequence to eight or more SYN bytes. This allows
for four bytes of buffering in the device followed by the 4-byte SYN
sequence.
A feature has been included in DDCMP that can be used to reduce the length of this sequence and improve efficiency of the protocol. This is implemented via the QSYNC or Quick Sync flag present in all messages. If set in a message, the QSYNC flag notifies the receiver that the next message will not abut and the synchronization sequence preceding the next message may be the short sequence. When the transmitter knows the next message will not abut the current message, it may set the QSYNC flag. The receiver seeing this may set resynchronization on the device immediately following the current message without looking ahead into the next message (and possibly requiring a long synchronization sequence due to device buffering). This allows the receiver to synchronize on a short sequence. A long sequence may always be used; the additional SYN bytes are simply ignored.
5.1.1.3 Parallel Links - If the transmission bit grouping on the link is a multiple of eight bits, then byte framing is inherent on the link. If the transmission bit grouping is a multiple of some other number of bits, then other means must be sought to achieve byte framing. Such a mechanism is not currently specified by this standard.
5.1.2 Message Framing - Message framing is achieved by searching for one of the three starting message bytes SOH, ENQ, or DLE. One of these bytes must appear immediately after the byte framing sequence or immediately after the previous message's last byte (if abutting). If these bytes do not appear at the receiver in the proper location, then the next message does not abut and byte framing is assumed lost. Once one of these starting message bytes is found, the end of the message is determined by a single set of rules for all link types:
a. If the starting byte is SOH or DLE, then the next five bytes
will complete the message header, followed by two bytes of
header block check (CRC), followed by COUNT bytes of data
(where COUNT is the 14-bit field following SOH or DLE),
followed by two bytes of data block check.
�Operation Page 37
b. If the starting byte is ENQ, then the next five bytes will
complete the message, followed by two bytes of header block
check.
The data field is totally transparent. No pattern searching is done once the starting byte is found. If the header CRC is in error the framing stops and resynchronization will occur (see Subhead 5.3, Message Exchange). If the station is a multipoint tributary and the ADDR in the header does not match the station address, the tributary still tracks the message by framing it (see subhead 5.2, Link Management).
5.1.2.1 CRC Performance Option - In some cases, where errors caused by the loss of synchronization on synchronous links have resulted in one or more bits being either removed or added, the performance of the CRC block check is not as robust as in cases of simple bit changes. To increase the error detection capability in these cases, the following additions should be made to the techniques described above (synchronous only):
Transmission - Whenever idling the link, ensure that the transmission ends with eight more 1 bits (DEL bytes). This is a requirement on the end of a message where the next message does not abut, is not immediately followed by a sync sequence separating messages or before shutdown. Additional DEL bytes may be needed before shutdown to satisfy the requirements of specific modems.
Reception - (Optional to achieve better bit error detection capability). After receiving the end of a data message with a valid CRC, do not process the message until one more byte is received. Discard the message (treat as a data CRC error) if this byte is not either: DEL, SYN, SOH, ENQ, or DLE. The reception function is optional.
5.1.3 Synchronization - Synchronization is the process of establishing both byte and message framing.
5.1.3.1 Receiver Synchronization - Synchronization takes place at the receiver under the following conditions:
a. Initially on receiver start-up, or for half-duplex and
multipoint operation on link turnaround or selection (see
subhead 5.2, Link Management).
b. If messages do not abut (i.e., the next abutting byte is not
SOH, ENQ or DLE).
c. If the QSYNC flag is set in the current message,
resynchronize at the end of the message.
�Operation Page 38
d. If a block check (CRC) error or other error occurs that might
have caused synchronization to be lost (e.g., receiver
overrun).
The receiver should track the link as much as possible and only resynchronize when synchronization may have been lost. This will increase the efficiency in receiving abutting messages and reduce the chance of synchronizing on a false message inside a data message (the aliasing problem).
5.1.3.2 Transmitter Synchronization - The transmitter will send a synchronization sequence prior to transmitting messages under the following conditions:
a. Initially on transmitter start-up or following link
turnaround (half-duplex and multipoint).
b. If a multipoint control station when changing the ADDR in
messages to different tributaries.
c. If messages do not abut (synchronous mode only - they always
abut in the asynchronous mode).
d. If QSYNC is set in the current message, precede the next
message with the sync sequence.
e. In the next message sent after receiving a NAK.
f. Preceding all control (ENQ) messages except ACK. A
synchronization sequence will also precede an ACK if one of
the other conditions is satisfied.
g. Preceding all maintenance mode (MAINT) messages.
h. Preceding all messages with the SELECT flag on (see subhead
5.2, Link Management).
i. Preceding the next message if a hardware/driver error (e.g.,
overrun) occurs while transmitting the current message.
The transmitter should send a synchronization sequence whenever it believes synchronization may have been lost at the receiving end. Preceding every message with a synchronization sequence is legal in the protocol but will reduce the potential overall performance efficiency. �Operation Page 39
5.1.3.3 Synchronization Rules - Table 1 summarizes the DDCMP
synchronization rules.
Table 1 Summary of Synchronization Rules
______________________________________________________________________
| |
Type of Framing | Mode | Synchronization Rules
| |
| |
1. Byte Framing | |
| |
Asynchronous | Receiver | Inherent in transmission mode
| | (none for DDCMP)
| |
| Transmitter | Send an all ones byte or idle the
| | link for a 10-bit time interval.
| | Not required on initial message
| | or link turnarounds.
| |
Synchronous | Receiver | Search for two adjacent SYN
| | bytes. Strip any more abutting.
| |
| Transmitter | Send short sequence if:
| |
| | 1. Initial message (after
| | start-up) or link turnaround.
| |
| | 2. QSYNC set in previous message.
| |
| | Send long sequence if:
| |
| | 1. QSYNC not set in previous
| | message (not initial message).
| |
�Operation Page 40
Table 1. Summary of Synchronization Rules (Cont'd)
______________________________________________________________________
| |
Type of Framing | Mode | Synchronization Rules
| |
| |
2. Message Framing | |
| Receiver | Search for SOH, ENQ, DLE
| | immediately following byte
| | framing.
| |
| | After SOH or DLE:
| |
| | Header = 5 bytes
| | CRC = 2 bytes
| | Data = COUNT bytes
| | CRC = 2 bytes
| |
| | After ENQ:
| |
| | Header = 5 bytes
| | CRC = 2 bytes
| |
| Transmitter | Send message immediately
| | after byte framing
| | synchronization sequence.
Notes To Table 1
1. The recommended synchronous short synchronization sequence is
four or more SYN bytes.
2. The recommended synchronous long synchronization sequence is
eight or more SYN bytes.
3. In response to a NAK, to assure that a receiver is in the
reframing mode and has not already framed on an erroneous
message, the transmitter may optionally increase the
synchronization sequence to:
a. Eight or more all ones bytes (DEL bytes - 255.) for the
asynchronous mode.
b. 10 or more SYN bytes (150.) for the synchronous mode.
4. A simple implementation may precede all messages with a sync
sequence. It may also always send a long synchronization
sequence, at some cost in time and efficiency, since extra
synchronization bytes are ignored.
5. If modems and/or interfaces require PAD sequences to clear
them and to assure transmission of the last message byte
prior to transmitter shutdown they should use all ones bytes
(DEL bytes - 255.) for synchronous and asynchronous modes.
�Operation Page 41
5.2 LINK MANAGEMENT
Link management is the process of controlling the transmission and reception of data on links where there may be two or more transmitters and/or receivers actively connected to the same signal channels. This will be true of half-duplex point-to-point links, as well as full- and half-duplex multipoint links. On half-duplex links, only one transmitter may be active at a time; on full-duplex links, only one transmitter may be active in each direction on the link at a time.
A station on such a link may transmit when it has been selected or granted ownership of the link. This ownership is passed by use of the SELECT flag contained in all messages. A SELECT flag set in a received message allows the addressed station to transmit after completing reception of the message. The SELECT flag also means that the transmitter will cease transmitting after the message is sent, if this is necessary for the proper operation of the link type. The process of receiving a valid SELECT flag and transmitting is called selection. The interval between receiving the SELECT flag, transmitting, and terminating selection by sending a SELECT flag is called the selection interval. A SELECT flag may be sent and received in any DDCMP message. If there is no message to send and a SELECT flag needs to be transmitted, the ACK message is used for sending the flag. A selection timer is used to detect a lost SELECT flag. It is started when a station is selected and reset when valid messages are received. If it expires, no message was received for a timer interval and it is assumed the messages with the SELECT flag set, either those transmitted or received, were in error.
The length of a selection interval depends on the response and throughput considerations of a particular implementation or system. When a station is selected, it should limit the length of the selection interval to some maximum period, either: a) by using a timer to time the length of the interval, b) by using a counter to count the number of bytes transmitted, or c) by limiting the number of data messages transmitted during the selection interval. If no explicit limit is implemented for half duplex point-to-point and multipoint stations, transmissions will eventually cease due to unacknowledged data messages exhausting buffer capacity or sequence numbers. If no explicit limit is implemented for full-duplex multipoint stations, the selection interval might never terminate, since messages may be acknowledged as other messages are being transmitted. Therefore full-duplex multipoint stations must have an explicit limit to their selection interval.
When there are two or more receivers on a link (multipoint) the ADDR field is used to address the tributary stations. A multipoint configuration consists of a single control station and a number of tributary stations. In a point-to-point configuration both stations are considered control stations. Address "1" is used in the ADDR field on point-to-point links. Addresses 1-255 are used to address tributaries on multipoint links and appear in the ADDR field of all messages both to and from tributaries. Address "0" is reserved. A tributary will only accept messages that contain its address (after CRC checks) and will ignore all others. Only SELECT flags set (i.e. �Operation Page 42
on) in messages with the matching tributary address are accepted as
selecting the addressed station. The SELECT flag is always set in
STRT, STACK, and MAINTENANCE messages for all link types. The start
up procedure and maintenance mode operate in the half-duplex, single
message at-a-time mode for consistency on all link types.
5.2.1 Link Management On Full-Duplex Point-To-Point Links - There is effectively no link management required on these links. Both stations are control stations and always selected. The address value 1 is used in the ADDR field of all messages. The checking of this address is optional on reception. For all messages other than STRT, STACK, and MAINTENANCE, in which the SELECT flag is set, the SELECT flag is optional in the full-duplex point-to-point mode and is essentially ignored. That is, it may be optionally set on transmission but is not checked on reception.
5.2.2 Link Management On Half-Duplex Point-To-Point Links - As in the full-duplex mode, both stations are control stations and use the ADDR value 1. Checking of this on reception is optional. Stations transfer control back and forth by use of the SELECT flag. A station setting the SELECT flag in a message invites the other station to transmit and will shut down its transmitter at the end of the current message after sending a number of DEL pad bytes appropriate to the modem and circuit. A station receiving a valid message with the SELECT flag set will transmit after completion of reception of the current message. Depending on the characteristics of the communication channel and modem this may require waiting until any pad characters have been received and the carrier has been turned off. The station ends its selection interval with the SELECT flag set in its last message transferring control back to the other station.
When a station sends a SELECT it starts a selection timer. The selection timer is stopped and restarted when a valid data message, maintenance message, or control message is received; or when resynchronization occurs at the receiver. See the message exchange subhead 5.3 for the description of a valid message. The purpose of the selection timer is to detect a lost SELECT flag either to or from the other station. It also serves to detect a loss of data signal part way through a message that may not be detected by other components of the system (e.g. loss of signal on an asynchronous link after receiving part of the data field of a message), preventing deadlocking of the protocol due to a link failure. If no message is received when the timer expires, the station assumes the message sent with the SELECT flag was received in error or the message sent by the other station which contained the return SELECT was in error. The station assumes ownership of the link and transmits as if it had received a valid SELECT return. When a station is selected and transmitting, its selection timer is not running. The timer value should be different at both stations to avoid a deadlock race condition. The value of the timer must include such factors as maximum message length, sync sequence lengths, link speed, link �Operation Page 43
turnaround time, and processing delays.
The selection timer detects the loss of the SELECT flag by timing the interval it takes to receive the longest message from the other station. It is reset after every message is received. Typical values might be on the order of a few seconds. To avoid excessive overhead, (when there is no data traffic) of constantly turning the channel around and selecting the other station, a station can delay replying to a selection for a short period of time (typical value would be 10-20% of select timer value). The decision to do this and the value of the delay is based on such factors as: response time requirements, message queuing delays, turnaround time, and the duration of the selection timer.
5.2.3 Link Management On Full-Duplex Multipoint Links - DDCMP supports configurations with a single control station and up to 255 tributaries. Messages are only exchanged between the control station and the tributaries. There is no direct tributary to tributary traffic. The tributaries use address values 1-255. These addresses are used in the ADDR field in messages sent both to and from tributaries.
Transmission from the control station to any tributary can be simultaneous with transmission from a selected tributary to the control station. The control station must maintain separate error counters, starting sequence and state transition logic, and data message number sequences for each active tributary. The messages transmitted to a given tributary are independent of the messages sent to any other tributary. A tributary is only allowed to send after it receives a message addressed to it with the SELECT flag on from the control station. The control station is allowed to send data messages (with SELECT off) to either the tributary it has just selected or to any other tributary. The control station is allowed to transmit continuously. A message and associated SELECT flag sent to a tributary are valid only if the data message header CRC, maintenance message header CRC, or control message CRC checks and the ADDR field matches that of the tributary.
The control station uses a selection timer to recover from messages where the SELECT flag has been transmitted or received in error. It operates the same as in the half-duplex, point-to-point case. The tributaries have no timer and wait to be selected again after ending a selection interval.
Tributaries only accept messages with their own addresses. A tributary is selected by receiving a message with its address with the SELECT flag on. It may then transmit and it ends the selection interval by setting the SELECT flag in its last message. While transmitting, it may be simultaneously receiving messages addressed to it.
The order in which the control station selects tributaries is not defined in this protocol. The control station might select each �Operation Page 44
station in turn in round-robin fashion, or it might have one or more
lists that determine the order and frequency of selection. The lists
might be supplied by a higher level process or might be developed by
selecting all the possible stations to determine which ones are
on-line.
A single physical station is allowed to respond to several different addresses on a multipoint channel only if it acts as if it were multiple separate tributaries. That is, the control station does not know that several addresses are located at a single physical station.
Once the control station has selected a tributary, it must wait for either:
a. The addressed tributary returning a message with the SELECT
flag on.
OR
b. The selection timer expiring.
before selecting another tributary.
On data and maintenance messages received by the tributary, the SELECT flag on is valid if the header CRC and ADDR check even if the data CRC is in error. The tributary must wait, however, until the entire message is received before starting transmission.
Tributary stations track all messages on the link by framing them and accepting only those with matching addresses. They resynchronize only on non-abutting messages or messages with CRC errors.
5.2.4 Link Management On Half-Duplex Multipoint Links - These links operate in the same way as the full-duplex multipoint links except that the control station must cease transmitting when it selects a tributary. It regains control only when:
a. It receives a message with the SELECT flag on from the
addressed tributary.
OR
b. The selection timer expires.
The control station operates in the same manner as the half-duplex point-to-point control station does, except that the ADDR field addresses one specific tributary rather than the only other station on the link. The tributary stations operate as if they were full-duplex tributaries, except, they do not receive while transmitting. �Operation Page 45
5.2.5 Link Management Summary Rules - Table 2 summarizes the major
modes of operation.
Table 2 Link Management Summary
______________________________________________________________________
Mode | Summary
|
|
Full-Duplex | Always selected. Point-to-Point | Uses ADDR = 1. Control Station | Checks ADDR on receive (optional).
| SELECT flag set (on) in STRT, STACK, and
| MAINTENANCE messages.
| SELECT flag ignored in other messages (may
| be set but not checked on receive).
| No selection timer.
|
|
Half-Duplex | Selected alternately. Point-to-Point | Uses ADDR = 1. Control Station | Checks ADDR on receive (optional).
| SELECT flag turns link around and selects
| other station.
| SELECT flag set (on) in STRT, STACK and
| MAINTENANCE messages.
| Selection timer used by both stations.
|
| Selection timer rules:
|
| 1. Start when sending SELECT to select
| other station.
|
| 2. Stop and restart when a valid data
| message, maintenance message, or
| control message is received.
|
| 3. Stop and restart when resynchronization
| occurs at the receiver.
|
| 4. Stop timer when receiving a SELECT
| (being reselected).
|
| 5. If the timer expires, treat as if a
| valid SELECT had been received.
|
�Operation Page 46
Table 2. Link Management Summary (Cont'd)
______________________________________________________________________ Mode | Summary
|
|
Full-Duplex | Always selected. Multipoint | Uses tributary address in ADDR field (1-255). Control Station | Checks for proper tributary ADDR on receive
| SELECT flag set (on) in STRT, STACK, and
| MAINTENANCE messages.
| Start selection timer when selecting a
| tributary.
| Timer operates as indicated for Half-Duplex
| Point-to-Point mode.
| Select another tributary when SELECT is
| received or selection timer expires.
|
|
Full-Duplex | Selected on receipt of SELECT where CRC and Multipoint | ADDR checks. Tributary Station | Ends selection with SELECT set in last
| message.
| Uses its own address in ADDR field
| Accepts received messages with matching ADDR.
| SELECT flag set in STRT, STACK, and
| MAINTENANCE messages.
| No selection timer for tributaries.
|
|
Half-Duplex | Selected alternately with tributaries. Multipoint | Uses tributary address in ADDR field. Control Station | Checks for proper tributary ADDR on receive.
| SELECT flag set in STRT, STACK, and
| MAINTENANCE messages.
| Selection timer used when selecting a
| tributary (same as for full-duplex
| multipoint control stations).
|
|
Half-Duplex | Same as Full-Duplex Multipoint Tributary Multipoint | station except that it does not receive Tributary Station | while transmitting.
|
�Operation Page 47
5.3 MESSAGE EXCHANGE
Using the framing and link management mechanisms described above, messages are exchanged by the protocol modules to create a protocol that ensures the sequentiality and integrity of data sent via DDCMP. The basic concepts of this exchange were presented in subhead 2.5 Functional Organization. This section presents the details of that message exchange mechanism.
Before discussing the actual message exchanges, three concepts must first be presented:
a. Initialization
b. Reply Time-outs
c. Valid messages
5.3.1 Initialization - DDCMP can operate in two modes:
a. The on-line, or running, mode.
and
b. The off-line, or maintenance, mode.
The on-line mode creates the sequential error-free link. The off-line or maintenance mode provides a data integrity block check, DDCMP framing, and link management procedures. The on-line mode is entered via a DDCMP start sequence. This start-up or initialization resets the message numbering, clears any outstanding messages and prepares for the start of data message transfers. The start sequence is designed so that both stations must enter the start sequence before either station can complete the sequence and the message exchange can commence. Start-up or restart is initiated by the user of DDCMP when it first starts up, on a fatal error, usually on a persistent error, and wherever else restarting is appropriate (See subhead 6, Error Recording). DDCMP does not initiate start-up itself.
5.3.2 Reply Time-outs - A necessary component of DDCMP is the reply time-out. The replies to some transmitted messages are timed and if there is no response within a time-out interval the expiration of the timer triggers recovery action. The time-out is necessary to recover from outages and messages distorted by the link such that even framing may be lost. Time-outs prevent the protocol from being deadlocked.
The timer is keyed to the selection of a station. That is, a station is given a chance to respond during a selection interval and if no proper response is received before the end of the selection interval then error recovery is initiated. In full-duplex point-to-point �Operation Page 48
links, where each station is always selected, a clock is used as the
timer. On the other link configurations, the time-out interval is set
to be one selection interval and expires at the end of the next
selection of the selected station. A station is given one interval to
reply. A message with the SELECT flag set is processed before the
SELECT flag itself is processed to denote the beginning or ending of a
selection interval. A more detailed description of the selection
process is given in subhead 5.2, Link Management.
5.3.2.1 Reply Timer Interval - The reply timer controls the maximum period a station will wait between sending a message and receiving a proper response before taking error recovery action. The timer interval for each station type is as follows:
a. Point-to-Point
Full-Duplex: Uses a real clock. The clock period is the
same as that of the selection timer used in other modes. It
must include link delays, turnaround, processing and message
transmission times. A typical value is three seconds for
telephone type channels.
Half-Duplex: Next selection interval. The other station is
given a selection interval in which to respond. The other
station is selected, it transmits (if it received the
selection correctly), and selection is ended (by the other
station sending a select or by the selection timer expiring).
If the proper reply is not returned in that interval, the
reply timer is assumed to have expired.
b. Multipoint
Full-Duplex-Control Station: Next complete selection
interval. A tributary is given at least one complete
selection interval in which to reply. For messages
transmitted to a tributary that is not selected, operation is
as described for half-duplex point-to-point (see above). For
messages transmitted to a tributary that is selected, the
tributary is expected to reply prior to the ending of the
next selection interval rather than prior to the ending of
the current interval. That is, a reply timer started during
a selection interval does not expire until the end of the
next selection interval, not the current interval.
Full-Duplex-Tributary Station: Before the next selection.
The tributary station expects the control station to reply in
the interval starting with the tributary sending the message
requiring the reply, completing the current selection, and
ending with the tributary being selected again by the control
station. The message that selects the tributary again is
included in that interval and may contain the valid reply.
If there is no proper response within that interval between
selections the tributary assumes the timer has expired.
�Operation Page 49
Half-Duplex-Control Station: The next selection interval.
Same as described above for half-duplex point-to-point
stations.
Half-Duplex-Tributary Station: Before the next selection.
Same as described above for full duplex multipoint tributary
stations with the addition that the reply cannot be received
before ending the current selection due to the half-duplex
mode.
Only point-to-point full-duplex control stations use a real clock; all others key reply time-out to a selection interval as indicated. Half-duplex and multipoint control stations use a real clock to time the selection interval. Tributary stations have no clock and rely on the control station for timing intervals (selection).
5.3.3 Valid Messages - Only valid messages are processed by the protocol. A message that has been properly framed is checked for:
a. Good block checks (header and data).
b. Valid message TYPE and SUBTYPE - control messages.
c. Matching ADDR for multipoint stations.
Optionally, a station may check for:
a. FILL fields for being zero.
b. ADDR=1 for point-to-point stations.
Multipoint tributary stations must ignore messages with header block check errors, due to the uncertainty of the ADDR field, and rely on time-outs for recovery. Control stations (point-to-point and multipoint) may process messages with header block check errors (e.g. reply with NAK), as they would process messages with data block check errors.
Additional checks are specific to each message and are described in the state tables where appropriate.
5.3.4 Message Exchange Variables And States - The following states and variables are used in the message exchange state tables and descriptions. On multipoint links, there is a set of states and variables for each control station - tributary station pair. A multipoint link appears logically as multiple point-to-point links operating on a single physical channel. Arithmetic and comparisons between the variables listed below are always computed modulo 256. �Operation Page 50
5.3.4.1 Data Variables -
R = The number of the highest sequential data message received at this
station. Sent in the RESP field of data messages, ACK messages, and NAK messages as acknowledgement to the other station.
N = The number of the highest sequential data message transmitted by
this station. Sent in the NUM field of REP messages. N is the number assigned to the last user transmit request that has been transmitted (sent in the NUM field of that data message).
A = The number of the highest sequential data message that has been
acknowledged to this station. Received in the RESP field of data messages, ACK messages, and NAK messages.
T = The number of the next data message to be transmitted. When
sending new data messages T will have the value N+1. When retransmitting T will be set back to A+1 and will advance to N+1.
X = The number of the last data message that has completed
transmission. When a new data message has been completely transmitted, X will have the value N. When retransmitting and receiving acknowledgements asynchronously with respect to transmission, X will have some value less than or equal to N.
Other variables are as defined in the message format section and refer to fields in specific messages (e.g., ACK(RESP=0) refers to the RESP field of an ACK message with value = 0). ACKs refer to receiving the RESP field value in an ACK, NAK, or DATA message. Relationship of data variables (modulo 256. arithmetic):
A <= N The data message numbers from A+1 to N are the current
unacknowledged data messages. ACKs within this range
are valid and accepted. ACKs outside this range are
ignored.
A < T <= N+1 If new data messages are being sent T = N+1. If
retransmissions are being sent T will lie between A and
N+1. If an ACK is received that increments A, T is set
to A+1 to start retransmission, possibly skipping some
data messages in a retransmission sequence already in
progress.
X <= N The last data message transmitted is always less than
or equal to the highest sequential data message
transmitted (N) and may be less than the highest one
acknowledged (A) due to retransmissions and the NAKing
of control message CRC errors.
5.3.4.2 Control Flag Variables - These flags control the sending of
DDCMP control messages. They are indicators specifying which control
messages to send when the transmitter is idle.
�Operation Page 51
SACK - Send ACK. This flag is set when either R is incremented,
meaning a new sequential data message has been received which
requires an ACK reply, or a REP message is received which
requires an ACK reply. The SACK flag is cleared when sending
either a DATA message with the latest RESP field information,
an ACK with the latest RESP field information, or when the SNAK
flag is set.
SNAK - Send NAK. This flag is set when a receive error occurs that
requires a NAK reply. It is cleared when a NAK message is sent
with the latest RESP information, or when the SACK flag is set.
The SACK and SNAK flags are mutually exclusive. At most, one will be set at a given time. The events that set the SACK flag, R is incremented or a REP is received requiring an ACK, also clear the SNAK flag. Similarly, the events that set the SNAK flag, reasons for sending a NAK (see Subhead 5.3.7), also clear the SACK flag. Setting or clearing a flag that is already set or cleared respectively has no effect. For the SNAK flag, a reason variable (or field) is also maintained which is overwritten with the latest NAK error reason. Whenever the SNAK flag is set the NAK reason variable is set to the reason for the NAK.
SREP - Send REP. This flag is set when a reply timer expires in the
running state and a REP should be sent. It is independent of
the SACK and SNAK flags.
It is desirable for DDCMP implementations to implement the sending of control messages via the use of these control flags. Events that require control messages to be sent will overwrite previous events, for which the control messages have not yet been sent, and only the necessary current control messages will be sent. In general, this will increase the performance of DDCMP by eliminating the sending of unnecessary control messages which add to protocol overhead and may result in unnecessary retransmissions. It is only necessary to have a single ACK or NAK, and/or a single REP marked for transmission with the latest information. A station when selected to transmit must send all pending control messages (i.e., clear all pending control flags) before deselecting itself.
It is also possible to implement the sending of control messages via a queuing mechanism, which actually builds and adds control messages to the transmit queue. In this case, all events that would cause a control message to be sent must generate a message for the queue.
5.3.4.3 DDCMP States -
HALTED Protocol stopped and not running. The user can halt the
protocol anytime by giving it a stop command (see user
interface description subhead 3.1). Commands to start
cause a transition through halted first.
�Operation Page 52
STARTING An attempt is being made to initialize the protocol via
an exchange of STRT and STACK messages. This state has
two internal substates.
1. ISTRT (Initiate Start) sends the STRT message.
2. ASTRT (Acknowledging Start) sends the STACK message.
RUNNING Signifies DDCMP is in the on-line mode exchanging data
messages.
MAINTENANCE Signifies DDCMP is in the off-line mode sending and
receiving maintenance messages.
5.3.5 Message Queueing, Reply Timers, And Buffering - Messages are
passed from DDCMP to a device driver for actual transmission on the
link. They may be passed either one message at a time, waiting for a
transmit complete before passing the next one to the driver; or more
than one message may be passed to the driver, letting the driver
maintain a transmit queue. One message at a time is the most
efficient method from the message exchange viewpoint, since it defers
decisions on which message to transmit next to the latest possible
instant, making use of the latest available acknowledgement
information. This technique, however, may not be optimal for
implementations requiring high performance via queueing and pipelining
techniques. The state tables for DDCMP message exchange operation
assume single message at a time operation. If queueing techniques are
used in an implementation, the operation described in the state table
as "send" will mean "add to transmit queue" or "pass to device
driver".
The reply timer on full duplex point-to-point links uses a real clock. The description of the message exchange operation assumes the timer is started after the message has been completely transmitted. In this case, the timer value must include link delays and processing time, but is independent of the length of the transmitted message. However, if the timer is started when the message is passed to the driver it must also include the time to actually transmit the message. If many messages are being queued to the driver, then the timer must also include the time to transmit all the messages ahead on the queue. These increases in the timer value make it less precise and, therefore, it does not perform as well as a reply timer that is started after actual transmission. The state table describes the reply timer starting at two possible times: one when messages are actually transmitted and the other when they are passed to the driver. The operation affects the setting of the variable X and the starting of the reply timer. The choice of technique in a particular implementation depends on the split of functions between the driver and DDCMP, the sharing of the protocol data base, and the capabilities and environment presented by the operating system. All other factors being equal, the best technique uses queueing for high performance and �Operation Page 53
starts the timer after actual message transmission. On other than
full-duplex point-to-point links, the timer is keyed to station
selection. The driver transmit queue is always emptied before ending
selection, and, thus, has no effect on the reply timing.
A message must not be marked complete and returned to the user until the message is acknowledged. If the user interface does not separate the completion or acknowledgement of data messages from the returning of buffers to the user (e.g. transmit buffering is supplied by the user), then the message must not be returned to the user while it is still residing on the transmit queue. This is required to prevent buffers from being released prematurely and given back to the user while they are still being used for transmission. If the user interface separates the completion of data messages from the returning of buffers (e.g. transmit buffering is supplied by DDCMP, the message is copied to a local DDCMP buffer), then the message may be marked complete to the user as soon as it is acknowledged, even though the actual transmit buffer (local to DDCMP) may still be on the transmit queue (i.e., being retransmitted).
5.3.6 Reply Timer Operation Rules -
5.3.6.1 Start Timer: -
a. When timer is a clock - set clock to the interval value,
start it running. When interval expires, clock will issue a
signal and stop.
b. When timer is another event (e.g. ending selection interval)
- set flag marking timer as running. When event occurs and
flag is set, timer will issue a signal and turn flag off.
c. If timer is running when a Start Timer command is issued, it
is first stopped and then started.
5.3.6.2 Stop Timer: -
a. When timer is a clock - clock is halted, no signal is issued.
The timer can be set running via the Start Timer command.
b. When timer is another event - turn flag off.
�Operation Page 54
5.3.7 NAK Reasons - NAKs are returned in response to message, device,
and buffering errors in the RUNNING state.
Specifically the NAK reasons are:
NAK Error Reason Code NAK Reasons (NAK Message REASON Field)
---- --- ------- --- ------- ------ -----
1. Header Block Check Error - A data message header CRC or
a control message CRC is in error.
2. Data Field Block Check Error - A data message data
field CRC is in error.
3. REP Response - The NAK is sent in response to a
received REP message where the NUM field in the REP did
not equal R (the highest sequential message received).
8. Buffer Temporarily Unavailable - A buffer was not
available to store the incoming data message. Either
the user did not allocate one in time, or the buffer
pool was empty.
9. Receive Overrun - The receiving hardware and/or
software was not able to respond fast enough to
incoming bytes and an overrun occurred.
16. Message Too Long - A received data message (COUNT) was
too long for the allocated buffer.
17. Message Header Format Error - The header CRC checked
but one or more header fields was invalid. Possible
errors are:
a. Invalid SELECT flag,
b. Invalid ADDR value (check is optional for
point-to-point),
c. FILL fields not zero (optional check),
d. Invalid control TYPE or SUBTYPE value.
5.3.8 Start-Up - Start-up is the process of initializing the protocol
states and variables, and synchronizing both stations on a link.
Table 3 is the start-up state table.
Start-Up Notes:
1. Implementations may ignore messages in error (invalid
messages) or messages other than that expected and wait for
the reply timer to expire during the start sequence to
�Operation Page 55
initiate recovery action.
2. The SELECT flag is always set in STRT and STACK messages, the
starting sequence being an alternate (half-duplex) exchange,
one message at a time. For all stations, except multipoint
control stations, the receipt of a STRT or STACK will trigger
an immediate response due to selection by these start
sequence messages. Multipoint control stations need not
respond immediately to the starting tributaries, but may
select and send messages to other tributaries before
returning and completing start-up with tributaries waiting
for start sequence replies.
3. After start-up is complete the data variables have the
following values: R=0, N=0, A=0, T=1, X=0. The control flag
variables are all clear.
5.3.9 Data Transfer - This is the process of sending data messages,
waiting for positive (ACK) or negative (NAK) acknowledgement,
retransmitting on NAK, and sending REP on reply time-out to cause an
acknowledgement to be returned. Table 4 is the state table for the
RUNNING State. These events always leave the stations in the RUNNING
state. The entries that change from RUNNING to the other states are
listed in Table 3, Start-Up State Table.
Running State Notes:
1. All message numbering is modulo 256 (i.e., after message
number 255 is message 0, then 1 and so on). For example,
3>250 is correct where 3 follows messages: 255,0,1,2.
2. An ACK always sends RESP=R. A NAK always sends RESP=R and an
appropriate NAK reason. A REP always sends NUM=N. A data
message always sends RESP=R, NUM=assigned sequential number.
A retransmitted message always uses the same NUM value and
DATA field as the original message, but sends the latest
receive value, R, in the RESP field.
3. The maximum number of outstanding unacknowledged messages is
255. No more can be sent until some are acknowledged. It is
always required that (A + 255) > = N (modulo 256).
4. Positive acknowledgements can be sent in the RESP field of
data messages transmitted in the reverse direction, in ACK
messages, or in NAK messages. They are all equivalent on
receive to providing a positive acknowledgement for
outstanding messages. In the state table, ACK refers to any
of the above forms of acknowledgement.
�Operation Page 56
Table 3 Startup State Table
______________________________________________________________________
State | Event | New State | Action
| | |
| | |
Any State|User requests halt | HALTED |None-stop timer if
| | |running
| | |
| | |
HALTED |User requests startup | ISTRT |Send STRT, reset
| | |variables, start timer
| | |
| | |
|User requests |MAINTENANCE|See subhead 5.4,
|Maintenance Mode | |MAINTENANCE MODE
| | |
| | |
ISTRT |Receive STACK |RUNNING |Send ACK (RESP=0),
| | |stop timer if running
| | |
| | |
|Receive STRT |ASTRT |Send STACK, start timer
| | |
| | |
|Timer expires |ISTRT |Send STRT, start timer
| | |
| | |
|Receive MAINT message |MAINTENANCE|See subhead 5.4,
| | |MAINTENANCE MODE
| | |
| | |
|Message in error or |ISTRT |Either: Send STRT,
|other message received| |start timer; or
| | |ignore(do nothing)
| | |
�Operation Page 57
Table 3. Startup State Table (Cont'd)
______________________________________________________________________
State | Event | New State | Action
| | |
| | |
ASTRT |Receive ACK (RESP=0) |RUNNING |See Data Transfer;
|or Receive Data msg | |stop timer
|(RESP=0) | |
| | |
| | |
|Receive STACK |RUNNING |Send ACK (RESP=0),
| | |stop timer
| | |
| | |
|Receive STRT |ASTRT |Send STACK,
| | |start timer
| | |
| | |
|Timer expires |ASTRT |Send STACK,
| | |start timer
| | |
| | |
|Receive MAINT message |MAINTENANCE|See subhead 5.4,
| | |MAINTENANCE MODE
| | |
| | |
|Message in error or |ASTRT |Either: Send STACK,
|other message received| |start timer; or
| | |ignore (do nothing)
| | |
5. The order for transmitting messages in the running state is:
NAK, REP, DATA messages, ACK.
6. A data message contains four independent pieces of
information:
a. Data consisting of the NUM, COUNT, and DATA fields.
b. An ACK response consisting of the RESP field.
c. Link management information consisting of an ADDR and
SELECT flag.
d. Framing information consisting of the QSYNC flag.
These are independent of each other and treated separately.
Thus, a received data message is treated as both a received
data message and a received ACK. For example, even if the
DATA CRC is in error the RESP field in a good header is still
accepted and processed.
7. When a message that starts or ends a selection interval
(SELECT flag set) is received, the message exchange fields
(RESP, NUM, COUNT, DATA) are processed according to the
�Operation Page 58
message exchange rules (Table 4) prior to the starting or
ending of the selection interval. For example, a RESP field
which acknowledges one or more outstanding messages will
complete those messages and stop the reply timer prior to
processing the SELECT flag. If the SELECT flag were
processed first, the reply timer might erroneously expire.
8. If there is no message to send in the RUNNING state, and a
SELECT flag wants to be sent, use the ACK message for this
purpose. ACKs are legal at any time and may be used for time
fill and to select and/or terminate selection.
9. Messages must never be truncated on transmission. A station
should always finish transmitting the current message before
starting a new one, including retransmissions. This is a
requirement of the byte count framing technique.
10. The SELECT flag can be set in any message. All outstanding
control messages must be sent before a station ends a
selection interval by sending a SELECT flag. A selection
interval is ended when the message with the SELECT flag set
is added to the transmit queue (given to the driver for
transmission), even though the message has not yet been
transmitted on the physical link.
11. The transmitter is idle when it is permissible to pass a
message to the driver. It is busy when this is not
permissible. For non-queued transmission, passing a message
to the driver is permissible only when nothing is being
transmitted and the station is selected. For queued
transmission, passing a message to the driver is permissible
only when the driver will accept a message (queuing space
available) and the station is selected.
The message with the SELECT flag set is the last message
passed to the driver in a selection interval. The
transmitter will remain busy after passing this message to
the driver until another selection interval is started by
receiving a message with the SELECT flag set. This technique
is necessary to maintain the proper relationship between the
reply timer and station selection.
12. The notation "A <- B" is the notation for the assignment
statement which sets the value of the variable A to the value
of the variable B.
�Operation Page 59
Table 4 RUNNING State Table
______________________________________________________________________
State | Event | New State | Action
| | |
| | |
RUNNING |Receive STRT |HALTED |Notify user of
| | |STRT received
| | |
| | |
|Receive MAINT |MAINTENANCE|See Subhead 5.4,
| | |MAINTENANCE MODE
| | |
| | |
|Receive STACK |RUNNING |Send ACK (RESP=R)
| | |
| | |
|User requests halt |HALTED |None Stop timer if
| | |running
The following events and actions occur in the RUNNING state and the protocol remains in the RUNNING state, thus the columns for State and New State are omitted and are implied to be the RUNNING State.
______________________________________________________________________
Event | Action
|
|
Receive Data Message | If buffer available, R <- R+1, give (NUM = R+1) | message to user, set SACK flag; (Also see Receive ACK) | otherwise set SNAK flag, (reason
| 8. or 16.)
|
|
Receive Data Message | Ignore (NUM =/ R+1) | (Also see Receive ACK) |
|
|
Receive Message In Error | Set SNAK flag, see NAK reasons
| (subhead 5.3.7)
|
|
Receive REP | Set SACK flag (NUM = R) |
|
|
Receive REP | Set SNAK flag, (reason 3.) (NUM =/ R) |
|
�Operation Page 60
Table 4. RUNNING State Table (Cont'd)
______________________________________________________________________
Event | Action
|
|
Receive ACK, or Data Message | For all messages (A < NUM <= RESP), (A < RESP <= N) | complete message to user,
| A <- RESP
| If T <= A, then T <- A+1
| If A < X, start timer
| If A >= X, stop timer
|
|
Receive ACK or Data Message | Ignore (RESP <= A or RESP > N) |
|
|
Receive NAK | For all messages (A < NUM <= RESP), (A <= RESP <= N) | complete message to user,
| A <- RESP
| T <- A+1, stop timer
|
|
Receive NAK | Ignore (RESP < A or RESP > N) |
|
|
Reply timer expires | Set SREP flag
|
|
Transmitter is idle | Send NAK with reason value, and SNAK flag is set | clear SNAK flag
|
|
Transmitter is idle, | Send REP, clear SREP flag, SNAK flag is clear, | * start timer SREP flag is set |
|
|
Transmitter is idle | MSG(T) is retransmitted, SNAK and SREP flags are clear | * X <- T, T < N+1 | * if timer not running start timer
| T <- T+1, clear SACK flag
|
|
User requests message | User waits until: to be sent: T < N+1, | T = N+1, transmitter transmitter is busy, | is idle, SNAK flag is clear, SNAK flag is set, or | and SREP flag is clear SREP flag is set |
|
�Operation Page 61
Table 4. RUNNING State Table (Cont'd)
______________________________________________________________________
Event | Action
|
|
User requests message | NUM <- N+1 to be sent, T = N+1, | Send MSG(NUM) transmitter is idle, | N <- NUM SNAK and SREP flags | T <- N+1, clear SACK flag are clear | * X <- N,
| * if timer not running start timer
|
|
Transmitter is idle, | Send ACK, clear SACK flag SNAK and SREP flags | are clear, T = N+1, | no user requests waiting, | SACK flag is set |
|
- If messages are queued for transmission and the timer is started and
stopped after actual transmissions of messages are completed rather
than when messages are queued, then ignore the asterisk (*) actions
listed above for the events: Transmitter is idle (SREP set),
Transmitter is idle (T < N+1), and user requests message to be sent
(T = N+1) and add the following events and actions:
|
Data message (NUM) | X <- NUM transmission completed | If A < X and timer not running, on link | then start timer
| If A >= X, then stop timer
|
|
REP message transmission | start timer completed on link |
|
5.4 MAINTENANCE MODE
Maintenance mode operation is used where the requirements dictate minimum code and compatibility with the DDCMP message structure. This is true of bootstrap (load), dump, and test facilities where the code might reside in read-only memory (ROM). Compatibility is necessary for operation on multipoint channels where one station may be in the maintenance mode and the other stations in the on-line, or running, mode. By using the same structure, both functions can occur concurrently on the link using the same control station protocol.
The maintenance mode uses the framing and link management portions of DDCMP without having the complexity or the functionality of the message exchange facility. That is, it creates a frame or envelope for transmitting and receiving message blocks and providing a CRC-16 error detection check. In this mode, there is no message sequencing �Operation Page 62
or acknowledgement. Sequencing and acknowledgement, if required, must
be provided by the user of the maintenance mode.
Maintenance mode messages always have the SELECT flag set, so they operate in the alternate half-duplex mode on both point-to-point and multipoint channels. The link management function operates in the same manner as described for on-line mode except that only a single message is transferred before ending selection. Transmit complete is returned to the user immediately after the message is transmitted, since there is no ACK returned to guarantee successful delivery over the link.
The maintenance mode is entered via either a command from the user or a received maintenance message. To return to the on-line (or running mode) the protocol must first go through the halted and starting states. Both operating modes cannot be mixed within a single station-to-station pair. The exact details of how maintenance mode is entered is implementation specific and may vary in different systems. If a maintenance message is received while not in maintenance mode, some implementation may enter maintenance mode and present the message to the user. Other implementations may discard the message, enter the HALTED state, and provide an indication that a maintenance message was received. Similarly, a received STRT message may cause the protocol to enter the HALTED state and notify the user of the received STRT or the protocol may remain in the maintenance state and simply notify the user of the received STRT. The maintenance message header is similar in format to the data message header except the RESP and NUM fields are zero FILLS (since maintenance messages are neither numbered nor acknowledged). In the maintenance mode, the protocol adds the header and block check to transmitted messages and starts a select timer if necessary for proper operation of the channel (e.g., multipoint). On received messages, the protocol removes the header and checks the block check. If in error it may either discard the message or notify the user that a message was received with a block check error. The latter action improves responsiveness rather than waiting for the timer to expire. It also assists in maintaining the link as such errors can be recorded.
The maintenance mode is used for functions such as bootstrapping, dumping, and link testing. If sequencing or acknowledgement are necessary it must be done within the data field of the maintenance messages. In DECnet, the maintenance message protocol is the Maintenance Operation Protocol (MOP) and is not part of this standard. Maintenance mode messages are not guaranteed to be delivered or have the assurance that they will arrive in the proper sequence. If they are delivered, however, they will have been block checked within the error limits of the CRC-16 check. The STRT message is the only non-maintenance message recognized while in the maintenance state; all others are ignored.
�Operation Page 63
Table 5 Maintenance State Table
______________________________________________________________________
State | Event | New State | Action
| | |
| | |
MAINTENANCE|User requests halt |HALTED |None-stop timer if running
| | |
| | |
| | |
|Receive MAINT |MAINTENANCE|If buffer available
| | |give to user,
| | |otherwise ignore
| | |
|Message in error |MAINTENANCE|Ignore
|or other message | |
|received(not STRT) | |
| | |
| Receive STRT |MAINTENANCE|Notify user of STRT
| | |received
| | |
|User requests MAINT|MAINTENANCE|If transmitter idle:
|message to be sent | |send MAINT message,
| | |If transmitter busy:
| | |wait until idle
| | |
Note
Transmitter busy and idle conditions have
the same meaning as described in the
RUNNING State notes (subhead 5.3.9).
6 ERROR RECORDING
This section specifies how error recording may be performed within DDCMP implementations. An implementation may utilize all, a subset, or none of these techniques and statistics. The selection of techniques used and statistics maintained depends on the system environment and requirements of the application. It is recommended that error recording be employed to compile statistics on the condition of the link, determine overall error rates, and detect a malfunctioning link. The protocol has been designed so that even if one of the stations on the link cannot record errors, the other station can be used to record errors for all communications sent in both directions on the link. This is accomplished by use of the NAK reason field. For most errors, the error is recorded locally and a NAK is returned with the error reason. The other station, upon receiving the NAK, can record the error for the remote station. On occasion, a NAK will be lost, but this should not noticeably affect �Error Recording Page 64
the record being kept of a given link.
A station that has no means for reading out its counters, or whose controlling user cannot maintain such records, would not maintain them (e.g. a transaction terminal). It is strongly recommended, however, that at least one station on a link maintain error statistics. On multipoint links, the control station will maintain a separate set of counters for each control-tributary pair. In the maintenance mode, error counters and records need not be maintained.
The following defines the standard set of DDCMP counters. These counters have been standardized so that consistent error information can be provided across a network. In DECnet, the higher level Network Management layer enables these counters to be accessed locally or remotely, and captured periodically, maintaining a log of data link operation.
In selecting these counters and specifying their encoding, a compromise was reached between the conflicting goals of capturing all useful information and minimizing required memory space. A further goal was to structure these counters so that a subset of the counters relates directly to the cause of most failures. The complete set is available for isolating unusual and difficult failures.
6.1 STRUCTURE OF THE COUNTERS
Some counters are maintained for each tributary/control station pair on a physical link. These are called "Data-Link Counters" and "Threshold Counters". Others are maintained for the physical link as a whole. These are called "Station Counters". Unless otherwise specified, all counters count up to a maximum value and latch at that value until cleared. The maximum value is 2n - 1 where n is the number of bits in the counter.
Some occurrences are counted twice:
1. in a one-bit counter associated with the specific occurrence
2. in an 8-bit counter associated with a set of occurrences
having similar diagnostic significance.
Some occurrences are not counted at all. These are deemed to be of sufficient importance that they require special handling described in Section 7.
6.2 DATA LINK COUNTERS
A point-to-point station maintains a single set of data link counters. A multipoint control station maintains a separate set of data link counters for each tributary. A multipoint tributary station maintains a single set of data link counters unless it supports multiple �Error Recording Page 65
tributary addresses; in this case, it supports a separate set for each
supported address.
6.2.1 Data Errors Outbound - This 8-bit group counter records occurrences which normally result from data errors on the communications channel outbound from this station. The specific errors associated with this counter are those counted on the following 1-bit counters:
6.2.1.1 NAKs Received Header Block Check Error - This 1-bit counter records the receipt of NAKs with a reason code of 1.
6.2.1.2 NAKs Received Data Field Block Check Error - This 1-bit counter records the receipt of NAKs with a reason code of 2.
6.2.1.3 NAKs Received REP Response - This 1-bit counter records the receipt of NAKs with a reason code of 3.
6.2.2 Data Errors Inbound - This 8-bit group counter records occurrences that normally result from data errors on the communications channel inbound to this station. The specific errors associated with this counter are those counted on the following 1-bit counters:
6.2.2.1 Header Block Check Errors - This 1-bit counter records the receipt of messages with header block check errors. Point-to-point stations and multipoint control stations set the SNAK flag and associated NAK reason code of 1 in these circumstances. Multipoint control stations record this error for the selected tributary, regardless of the address field in the received message. Multipoint tributary stations record this error only if the address field matches their station address.
6.2.2.2 NAKs Sent Data Field Block Check Error - This 1-bit counter records the setting of the SNAK flag and associated NAK reason code of 2. �Error Recording Page 66
6.2.2.3 NAKs Sent REP Response - This 1-bit counter records the
setting of the SNAK flag and associated NAK reason code of 3.
6.2.3 Local Reply Timeouts - This 8-bit counter records occurrences which normally result from either:
1. the loss of communications between stations while this
station has data to transmit or,
2. the choice of an inappropriate value for this station's reply
timer.
This counter records REPs sent. Specifically, it records the setting of the SREP flag.
6.2.4 Remote Reply Timeouts - This 8-bit counter records occurrences which normally result from either:
1. the loss of communication between stations while the remote
station has data to transmit or,
2. the choice of an inappropriate value for the remote station's
reply timer.
This counter records ACKs sent in response to a REP. Specifically, it records the setting of the SACK flag when a REP is received with NUM = R.
6.2.5 Local Buffer Errors - This 8-bit group counter records occurrences which normally result from the failure of the user of DDCMP at this station to properly coordinate the supply of receive buffers at this station to data messages supplied by the remote user. The specific errors associated with this counter are those counted in the following 1-bit counters:
6.2.5.1 NAKs Sent Buffer Temporarily Unavailable - This 1-bit counter records the setting of the SNAK flag and associated NAK reason code of 8.
6.2.5.2 NAKs Sent Buffer Too Small - This 1-bit counter records the setting of the SNAK flag and the associated NAK reason code of 16. �Error Recording Page 67
6.2.6 Remote Buffer Errors - This 8-bit group counter records
occurrences which normally result from the failure of the user of
DDCMP at the remote station to properly coordinate the supply of
receive buffers at that station to data messages supplied by the user
of DDCMP at this station. The specific errors associated with this
counter are those counted in the following 1-bit counters:
6.2.6.1 NAKs Received Buffer Temporarily Unavailable - This 1-bit counter records the receipt of NAKs with a reason code of 8.
6.2.6.2 NAKs Received Buffer Too Small - This 1-bit counter records the receipt of NAKs with a reason code of 16.
6.2.7 Selection Timeouts - This 8-bit group counter records occurrences on a half-duplex or multipoint line which normally result from
1. loss of communication with a remote station,
2. data errors on the communications channel to or from that
station, or,
3. the choice of an inappropriate value for this station's
Select Timer.
This counter is maintained only by point-to-point half-duplex stations and multipoint control stations; it is not maintained by full-duplex point-to-point stations or by multipoint tributary stations. The specific errors associated with this counter are those counted in the following 1-bit counters:
6.2.7.1 No Reply To Select - This 1-bit counter is used by point-to-point half-duplex and multipoint control stations to record selection intervals in which no transmission was received from the tributary and in which no attempt to transmit could be detected. Specifically, it records expiration of the select timer without receipt of a valid control message or a valid header to a data or maintenance message, and without detection of an attempted transmission. Detection of an attempted transmission is optional and is implementation dependent. Typical means include:
1. presence of a carrier signal,
2. receipt of a DDCMP synchronization sequence, and
3. receipt of an SOH, ENQ, or DLE.
�Error Recording Page 68
6.2.7.2 Incomplete Reply To Select - This 1-bit counter is used by
point-to-point half-duplex and multipoint control stations to record
selection intervals which were not properly terminated by receipt of a
message header with SELECT flag on, during which a transmission was
received from the tributary or an attempt to transmit was detected.
Specifically, it records expiration of the select timer preceded by
1. receipt of a valid control message,
2. receipt of a valid header to a data or maintenance message,
or
3. detection of an attempted transmission. (Refer to "No Reply
to Select").
6.2.8 Data Messages Transmitted - This 32-bit counter records
messages transmitted by this station. It can be used as a statistical
base when evaluating the number of data errors outbound, local reply
time-outs, and remote buffer errors. This counter records new
messages sent. Specifically, it records the number of times N is
incremented.
6.2.9 Data Messages Received - This 32-bit counter records messages received by this station. It can be used as a statistical base when evaluating the number of data errors inbound, remote reply time-outs, and local buffer errors. This counter records new messages received. Specifically, it records the number of times R is incremented.
6.2.10 Selection Intervals - This 16-bit counter is used by half-duplex and multipoint control stations. It can be used as a statistical base when evaluating the number of selection time-outs.
This counter records the number of times this station selects the other station. Specifically, it records the number of messages transmitted with the SELECT flag on.
6.2.11 Data Bytes Transmitted - This 32-bit counter records data bytes transmitted by this station. It can be used, together with Data Messages Transmitted, to determine the traffic load outbound.
This counter records new bytes sent. Specifically, it is incremented by COUNT when N is incremented. �Error Recording Page 69
6.2.12 Data Bytes Received - This 32-bit counter records data bytes
received by this station. It can be used, together with Data Messages
Received, to determine the traffic load inbound.
This counter records new bytes received. Specifically, it is incremented by COUNT when R is incremented.
6.3 STATION COUNTERS
The following counters record unusual occurrences. They may be the result of
1. a hardware or software fault at this station,
2. a hardware or software fault at a remote station, or,
3. a data error on the communications channel undetected by the
header block check field. (Except as otherwise noted each
type of station maintains a single set of these counters.)
Because these errors are not related to normal channel faults, and thus are not normal occurrences, the memory required to count these on a per tributary basis is not justified. Consequently a single set of counters is used for all tributaries on a multipoint line.
6.3.1 Remote Station Errors - This 8-bit group counter records occurrences most often caused by a fault in a remote station or by an undetected data error on the channel inbound to this station. The specific errors associated with this counter are those counted in the following 1-bit counters:
6.3.1.1 NAKs Received Receive Overrun - This 1-bit counter records the receipt of NAKs with a NAK reason code of 9.
6.3.1.2 NAKs Sent Message Header Format Error - This 1-bit counter records the setting of the SNAK flag and associated NAK reason code of 17.
6.3.1.3 Selection Address Errors - This 1-bit counter records the receipt of a message by a multipoint control station containing an address field which does not match the address of the currently selected tributary. It is not used by other stations. �Error Recording Page 70
6.3.1.4 Streaming Tributaries - The 1-bit counter is only used by
point-to-point half-duplex and multipoint control stations. It
records the occurrence of a point-to-point half-duplex station or
multipoint tributary station either:
1. exceeding an implementation-dependent maximum transmission
interval without releasing the channel or
2. failing to release the channel following transmission of a
message with the SELECT flag on.
Specifically, it records the occurrence of any of the following after:
1. the maximum transmission interval has expired, or
2. a message with SELECT flag on is received:
a. Receipt of a valid control message or message header.
b. Detection of attempted transmission (see "No Reply to
Select").
6.3.2 Local Station Errors - This 8-bit group counter records occurrences most often caused by a fault in this station or by an undetected data error on the channel outbound from this station. The specific errors associated with this counter are those counted in the following 1-bit counters:
6.3.2.1 NAKs Sent Receive Overrun - This 1-bit counter records the setting of the SNAK flag and associated NAK reason code of 9.
6.3.2.2 Receive Overruns, NAK Not Sent - This 1-bit counter records the occurrence of a receive overrun when a NAK could not be sent. For all stations this can occur when the state is not RUNNING. For multipoint tributary stations this can occur if an overrun occurs while receiving a header.
6.3.2.3 Transmit Underruns - This 1-bit counter records the occurrence of transmit underrun errors. A transmit underrun occurs if the transmitting hardware and/or software is unable to generate outgoing bytes fast enough. �Error Recording Page 71
6.3.2.4 NAKs Received Message Header Format Errors - This 1-bit
counter records the receipt of NAKs with a NAK reason code of 17.
6.4 THRESHOLD ERROR COUNTERS
A point-to-point station maintains a single set of threshold counters.
A multipoint control station maintains a separate set for each tributary. A multipoint tributary maintains a single set unless it supports multiple tributary addresses; in this case it supports a separate set for each supported address.
Threshold error counters are used to determine when certain error conditions have occurred so many consecutive times that a persistent fault probably exists. Whenever a threshold counter reaches its maximum value, Network Management and the DDCMP user are informed. In the RUNNING state threshold counters are cleared when Network Management and the DDCMP user are informed, so that they can be continually informed of a persistent fault. In the ISTRT and ASTRT states, threshold counters are not cleared when Network Management and the DDCMP user are informed, so that they will not be continually informed of an inoperative remote station.
DDCMP continues to operate across threshold counter overflows. It clears the threshold counters and counts again, notifying the user on subsequent overflows. Protocol and link shutdown is left up to the user.
The specific threshold counters are as follows:
6.4.1 Transmit Threshold Errors - This 3-bit counter is incremented, if less than 7, in the following circumstances:
1. In the ISTRT state when a STRT is sent
2. In the ASTRT state when a STACK is sent
3. In the RUNNING state
a. upon receipt of a NAK with reason code not = 3 (REP
response)
b. upon setting the SREP flag
The counter is cleared in the following circumstances:
1. Upon entering the ISTRT, ASTRT, and RUNNING states
2. In the RUNNING state
�Error Recording Page 72
a. upon reporting a transmit threshold error
b. upon receiving a NAK, ACK, or DATA message acknowledging
a new message (A < RESP <= N).
c. upon receiving an NAK, ACK, or DATA message when no
messages are outstanding (A = RESP = N).
6.4.2 Receive Threshold Errors - This 3-bit counter is incremented, if less than 7, in the RUNNING state when setting the SNAK flag and the associated NAK reason code to one of the following values:
1. (header block check error)
2. (data field block check error)
3. (REP response)
8. (buffer temporarily unavailable)
17. (message header format error)
This counter is cleared in the following circumstances:
1. Upon entering the ISTRT, ASTRT, and RUNNING states.
2. Upon receipt of a DDCMP control message without a header
format error and having a correct header block check.
3. Upon receipt of a DDCMP data message without a header format
error and having correct header and data field block checks
4. In the RUNNING state, upon reporting a receive threshold
error.
6.4.3 Selection Threshold Errors - This 3-bit counter is only used by
multipoint control stations and half-duplex point-to-point stations.
It is incremented, if less than 7, upon an occurrence recorded by the
Selection Time-outs counter. It is cleared upon receipt of a message
with the select bit on from the other station on point-to-point lines
or from the selected station on multipoint lines. It is cleared in
the RUNNING state upon reporting a selection threshold error.
6.5 NETWORK MANAGEMENT INTERFACE SUPPORTING THE DDCMP COUNTERS
The following functions comprise an interface which allows access to �Error Recording Page 73
the DDCMP counters by higher level software. In DECnet this software
resides in the Network Management layer.
6.5.1 Read Data Link Counters (ADDR) - This function returns all the counters defined in subhead 6.2 for a data link. For point-to-point stations the ADDR argument is not used. For multipoint stations it specifies the tributary address.
6.5.2 Read And Clear Data Link Counters (ADDR) - This function operates exactly as Read Data Link Counters, except that after reading the data link counters it clears them.
6.5.3 Read Station Counters - This function returns all the counters defined in subhead 6.3.
6.5.4 Read And Clear Station Counters - This function operates exactly as Read Station Counters except that after reading the station counters it clears them. �Error Recording Page 74
6.6 SUMMARY
Table 6 summarizes the DDCMP counters.
Table 6 DDCMP Counters
Data Link Counters
8-, 16- & 32 bit Counters Associated 1-bit Counters
-------------------------
Data Errors Outbound NAKs Received Header Block Check Error
NAKs Received Data Field Block Check Error
NAKs Received REP Response
Data Errors Inbound Header Block Check Errors
NAKs Sent Data Field Block Check Error
NAKs Sent REP Response
Local Reply Time-outs Remote Reply Time-outs Local Buffer Errors NAKs Sent Buffer Temporarily Unavailable
NAKs Sent Buffer Too Small
Remote Buffer Errors NAKs Received Buffer Temporarily
Unavailable
NAKs Received Buffer Too Small
Selection Time-outs No Reply to Select
Incomplete Reply to Select
Data Messages Transmitted Data Messages Received Selection Intervals Data Bytes Transmitted Data Bytes Received
Station Counters
8-bit Counters Associated 1-bit Counters
-------------------------
Remote Station Errors NAKs Received Receive Overrun
NAKs Sent Message Header Format Error
Selection Address Errors
Streaming Tributaries
Local Station Errors NAKs Sent Receive Overrun
Receive Overruns, NAK not sent
Transmit Underruns
NAKs Received Message Header Format
Errors
Threshold Error Counters Receive Threshold Errors Transmit Threshold Errors Selection Threshold Errors
�Event Logging Page 75
7 EVENT LOGGING
Certain events in the operation of DDCMP may be of immediate interest to network operations and maintenance personnel. Other events, if logged, may facilitate detecting and isolating faults which prevent correct or efficient protocol operation. This specification defines a complete set of events and associated information. Standardization enables consistent capture, transmission, storage and interpretation of events across a network. The implementation of event logging is optional. Some implementations may include all, none, or a subset of these events.
In selecting these events, two criteria were used:
1. An event was included if it had potential real-time
significance to the operation of the network.
2. An event was included if it provided information facilitating
the detection and isolation of faults not provided by the
error counters. This is the case where the timing and
sequence of events might be significant, or where the use of
counters to capture details of infrequent occurrences is
impractical.
In DECnet, the Network Management layer has facilities for capturing events, selectively filtering based on operator control, time stamping events, transmitting events, and logging events. If DDCMP is used outside of DECnet, similar facilities will be required for event logging to be a useful tool.
Events are numbered for identification. It is assumed that the event logging mechanism can filter each numbered event separately. Several similar events are sometimes combined into a numbered class. In these cases, filtering is done on a class basis.
The data captured by some events includes optional fields. It is recommended that this information be provided: however, it is not required.
The events are as follows:
7.1 LOCALLY-INITIATED STATE CHANGE
Information captured:
o Event number = 1.
o Tributary Address (1 on point-to-point links)
o Old State
o New State
�Event Logging Page 76
This event occurs when the state of the data link changes as a result
of a command issued locally by the user of DDCMP. The possible
transitions are:
HALT --> ISTRT,
HALT --> MAINTENANCE,
ISTRT --> HALT,
RUNNING --> HALT,
MAINTENANCE --> HALT.
7.2 REMOTELY INITIATED STATE CHANGE
Information captured:
o Event number = 2.
o Tributary address
o Old State
o New State
This event occurs when the state of the data-link changes as a result of a command by the remote user of DDCMP. The possible state changes are:
ISTRT --> RUNNING,
ASTRT --> RUNNING,
RUNNING --> HALT,
RUNNING --> MAINTENANCE,
MAINTENANCE --> HALT,
ISTRT --> ASTRT (optional).
7.3 STRT RECEIVED WHILE IN MAINTENANCE
Information captured:
o Event number = 3.
�Event Logging Page 77
o Tributary address
This event occurs when a STRT message is received in MAINTENANCE state.
7.4 TRANSMIT ERROR THRESHOLD REACHED
Information captured:
o Event number = 4.
o Tributary address
o All Data Link Counters for the Tributary
o All Station Counters for the Line
This event occurs when the Transmit Threshold Error Counter reaches the value 7.
7.5 RECEIVE ERROR THRESHOLD REACHED
Information captured:
o Event number = 5.
o Tributary address
o All Data Link Counters for the Tributary
o All Station Counters for the line
This event occurs when the Receive Threshold Error Counter reaches the value 7.
7.6 SELECTION ERROR THRESHOLD REACHED
Information captured:
o Event number = 6.
o Tributary address
o All Data Link Counters for the Tributary
o All Station Counters for the line
This event occurs when the Selection Threshold Error Counter reaches the value 7. �Event Logging Page 78
7.7 MESSAGE HEADER FORMAT ERRORS
The following information is recorded:
o Event type = 7.
o Tributary address
o complete header, optionally
This event occurs when the SNAK flag is set with associated NAK reason code of 17. Recording the complete header is optional.
7.8 SELECTION ADDRESS ERRORS
The following information is recorded:
o Event type = 8.
o Tributary address in received message
o Tributary address selected
o Optionally, previously selected tributary address
This event occurs only for a multipoint control station. It occurs when the control station receives a message which does not match the address of the currently selected tributary.
7.9 STREAMING TRIBUTARIES
The following information is recorded:
o Event type = 9.
o Tributary address (of last selected tributary)
o Address in header (if available, otherwise 0)
o Event sub-type:
0 Streaming tributary
1 Continued transmission following time-out
2 Continued transmission after deselection
3 End of continued transmission
This event records the potential jamming of a multipoint line by a
�Event Logging Page 79
defective tributary or a point-to-point half-duplex line by a
defective station. It is recorded by multipoint control stations and
point-to-point half duplex stations. The specific circumstances are
indicated by an event sub-type.
A streaming tributary (sub-type 0) is a tributary which continues to transmit valid control messages, or message headers for data or maintenance messages, beyond the end of an implementation dependent time-out. It indicates a fault in the remote station, or inappropriate choice of timer in this station or the remote station. This event is only recorded for the first header which exceeds the time limit.
Some implementations may detect continued use of the channel inbound by a tributary when no good data is being received. There are two cases: (sub-type 1) where the tributary exceeds a time limit on maximum transmission, and (sub-type 2) where the tributary fails to release the channel after sending a message with the SELECT flag on. Either condition usually indicates a fault in the tributary hardware or software.
Events of sub-type 1 and 2 are reported only once for a continuous fault. When the fault clears, an event is generated (sub-type 3) and subsequent occurrences will then be reported.
7.10 NAKS SENT BUFFER TOO SMALL
The following information is recorded:
o Event type = 10.
o Tributary address
o count in received header
o buffer size available
This event is optional. It is recorded when the corresponding error is counted. �
APPENDIX A
GLOSSARY
Asynchronous Serial Data Transmission - A technique in which the
information required to determine when each byte (bit grouping) begins is sent along with the byte (the "start" and "stop" bits). The time interval between successive bytes is unspecified.
Channel - The data path joining two or more stations, including the
communications control capability of the associated stations.
Control Station - A station that has overall responsibility for
orderly operation of the channel.
Duplex Channel - A channel over which simultaneous (in time)
communications in both directions (to and from the station) is possible. Also called a full-duplex channel.
Half-Duplex Channel (Alternate Simplex) - A channel that permits
two-way communications, but in only one direction at any instant.
Master Station - A station that has control of a channel at a given instant, for the purpose of sending data messages to a slave station (whether or not it actually does). Also referred to as a "transmitting station".
Multipoint Channel - A channel connecting more than two stations.
Parallel Data Transmission - A data communication technique in which
more than one code element (i.e., bit) of each byte is sent or received simultaneously.
Point-to-Point Channel - A channel connecting only two stations.
Serial Data Transmission - A data communication technique in which the
code element (bits) of each encoded symbol of the data are sent and received one after the other (serially), rather than simultaneously.
Slave Station - A station to which a master station intends to or does
send a data message. Also referred to as a "receiving station".
Station - An independently controllable configuration of logical �GLOSSARY Page A-2
elements (e.g., a computer) to or from which messages are transmitted on a channel.
Synchronous Serial Data Transmission - A data communication technique
in which the information required to determine when each byte begins is sent at the beginning of a group of bytes (the "sync bytes"). The time interval between successive bytes in the group is zero. The time interval between successive groups of bytes is unspecified.
Tributary Station - A station on a channel that is not a control
station.
�
APPENDIX B
FORMAL SYNTAX DEFINITION
B.1 SYMBOLOGY
In the following formal definition of the protocol grammar, the symbols have the following meanings:
1. < > denotes a metalinguistic variable.
2. := means "is produced by" or "has the value of".
3. <a><b> means "a followed by b" or "b concatenated with a".
4. <a>!<b> means "a OR b" (exclusive OR).
5. quantities not surrounded by brackets < > are constants or
literal values.
6. (<a>*b) means "a repeated b times" (<a><a>---<a>).
7. <a>** means "a occurs from zero to infinity times in
succession".
8. null means "the empty set".
B.2 MESSAGE SYNTAX
The following definition of the protocol does not include the specific sync sequences and rules when they are used on each link type. Refer to the operation section for these specifics.
<protocol>:=<transmission><trailer>!<transmission><protocol>
<transmission>:=<msg>!<syncseq><msg>
Note that the form <syncseq><msg> is required in
numerous cases. See Operation, subhead 5.1.3.2.
�FORMAL SYNTAX DEFINITION Page B-2
<syncseq>:=<leader><sync>*m
Where m is a parameter determined by hardware and
interchange considerations. (m >= 1 for asynchronous
circuits, m >= 4 for synchronous circuits.)
Note that <syncseq> is used for inter-message padding
as well as for synchronizing.
<sync>:=<syn> for synchronous circuits
:=10-bit idle line !for asynchronous circuits :=null for parallel circuits
Where "10-bit idle line" means that the channel is held
continuously in the state of the stop element (i.e.
Mark, condition Z, 1) for a period not less than 10 bit
times.
<leader>:=(<one>*j)(<sync>*k) for synchronous circuits
Where j+8k >= 0 if qsync is set in the previous
message. This is a short sync sequence.
Where j+8k >= 32 if qsync is not set in the previous
message. This is a long sync sequence.
Either j=0 or j>=8
:= idle line !** for asynchronous circuits
Where "idle line" means that the channel is held
continuously in the state of the stop element (i.e.
Mark, condition Z, 1)
:=null for parallel circuits
<trailer>:=<one>*j for synchronous circuits,
where j>=8
:=<leader> for asynchronous circuits
:=null for parallel circuits
<one>:=1
<msg>:=<nummsg>!<unnummsg>!<maintmsg>
<nummsg>:=<soh><header><bcc><bcc>
<unnummsg>:=<enq><body><bcc>
<maintmsg>:=<dle><mhdr><bcc><bcc>
<header>:=<count><qsync><select><resp><num><addr>
:=(<byte> * value of <count>) �FORMAL SYNTAX DEFINITION Page B-3
<bcc>:=<byte><byte>
<body>:=<ackm>!<nakm>!<repm>!<strtm>!<stackm>
<mhdr>:=<count><qsync1><select1><fill><fill><addr>
<count>:=(<bit>*14)
<select>:=<bit>
<select1>:=1
<qsync>:=<bit>
<qsync1>:=1
<resp>:=<byte>
<num>:=<byte>
<addr>:=<byte>
<ackm>:=<ack><fill6><qsync><select><resp><fill><addr>
<nakm>:=<nak><rnak><qsync><select><resp><fill><addr>
<repm>:=<rep><fill6><qsync><select><fill><num><addr>
<strtm>:=<strt><fill6><qsync1><select><fill><fill><addr>
<stackm>:=<stack><fill6><qsync1><select1><fill><fill><addr>
<rnak>:=000001!000010!000011!001000!001001!010000!010001
<byte>:=00000000!00000001!...!11111111
<bit>:=0!1
<syn>:=10010110
<soh>:=10000001
<enq>:=00000101
:=11111111
<fill>:=00000000
<fill6>:=000000
<dle>:=10010000
<ack>:=00000001
<nak>:=00000010 �FORMAL SYNTAX DEFINITION Page B-4
<rep>:=00000011
<strt>:=00000110
<stack>:=00000111
�
APPENDIX C
MESSAGE FORMAT SUMMARY
C.1 GENERAL MESSAGE FORMATS
Numbered Unnumbered Maintenance Messages Messages Messages
<soh> <enq> <dle> <count> <type> <count>
<subtype>
<qsync> <qsync> <qsync1> <select> <select> <select1> <resp> <rcvr> <fill> <num> <sndr> <fill> <addr> <addr> <addr> <bcc1> <bcc1> <bcc1> <bcc2> <bcc2> <bcc2> <bcc3> <bcc3> <bcc4> <bcc4>
C.2 UNNUMBERED MESSAGE SUMMARY
message ACK NAK REP STRT STACK
<enq> <enq> <enq> <enq> <enq> <enq> <type> <ack> <nak> <rep> <strt> <stack> <subtype> <fill6> <rnak> <fill6> <fill6> <fill6> <qsync> <qsync> <qsync> <qsync> <qsync1> <qsync1> <select> <select> <select> <select> <select1> <select1> <rcvr> <resp> <resp> <fill> <fill> <fill> <sndr> <fill> <fill> <num> <fill> <fill> <addr> <addr> <addr> <addr> <addr> <addr> <bcc1> <bcc1> <bcc1> <bcc1> <bcc1> <bcc1> <bcc2> <bcc2> <bcc2> <bcc2> <bcc2> <bcc2> �
APPENDIX D
DDCMP BLOCK CHECK COMPUTATION
D.1 DDCMP ERROR DETECTION
Error detection is provided in this protocol by means of block check bytes after each of the message headers and message blocks. This block check consists of computing a 16-bit cyclic redundancy check using a polynomial known as the CRC-16 polynomial and appending the check bits computed to each block. This polynomial and scheme have been widely used in BiSync and other protocols.
D.2 THE CRC-16 POLYNOMIAL
The CRC-16 polynomial [X^16 + X^15 + X^2 + 1 = (X + 1) * (X^15 + X + 1)] [see section D3, step 3 below] has the following error detection properties:
1. It will detect all errors that change an odd number of bits
(i.e., 1, 3, 5, ... bit errors).
2. It will detect all errors that change two bits provided that
the block length is less than 32767 bits including the CRC
bits. Thus the maximum <count> (length of data field) should
be 4093.
3. It will detect all errors that consist of a single burst
error of 16 or fewer bits. A burst error is a group of bits
in which the first bit and the last bit are in error and the
intervening bits may or may not be in error. Thus a 16-bit
burst error might have as many as 16 bits in error. The
partitioning of bits in error into burst errors is not
unique.
4. It will detect all errors that consist of two occurrences of
two adjacent bits in error provided that the block length is
less than 32767 bits including the CRC bits.
5. It will detect all except the fraction 1/2^15 of errors that
consist of a single burst error of 17 bits.
�DDCMP BLOCK CHECK COMPUTATION Page D-2
6. It will detect all except the fraction 1/2^16 of errors that
consist of a single burst error of 18 or more bits.
7. It will not detect some errors that change four bits. For
example, it will not detect the error pattern that is
identical to the CRC polynomial. Thus the minimum Hamming
distance between two valid messages (including the CRC bits)
is four bits.
D.3 CRC COMPUTATION
The algorithm for computing the cyclic redundancy check is as follows:
1. Consider the header or data portion of the message as it
appears on a serial line (LSB of the first byte first, MSB of
the final byte last) and append 16 zeros after the header or
data.
2. Take the string of bits constructed in 1, and treat each bit
as the coefficient of a term of a polynomial with the LSB of
the first byte being the coefficient of the highest order
polynomial term. The highest order term is A * X^63 for a
header block and A * X^(8 * <count> + 15) for a data block
where A is the least significant bit of the first byte of the
header or data. The lowest order term is 0 * X^0 for both
cases.
3. Divide the polynomial constructed in 2, by the CRC-16
polynomial X^16 + X^15 + X^2 + 1 using synthetic division and
using modulo 2 arithmetic on the coefficients (i.e., addition
= subtraction = XOR. All carries and borrows are ignored)
obtaining a quotient that is discarded and a 16-bit
remainder.
Transmit the coefficients of the remainder as the block check
bytes following the original message bits, transmitting the
coefficients of the highest order term (X^15) first. Thus,
the first byte represents coefficients of the X^8 through
X^15 terms of the remainder (from left to right) and the
second byte represents coefficients of the X^0 through X^7
terms of the remainder (also from left to right).
4. On reception, perform the same algorithm and compare the
received block check bytes with the computed block check
bytes. If the bytes are not identical, an error has
occurred.
Note:
1. On a parallel circuit, the same algorithm is used and the
same block check bytes are computed although the bytes are
sent in parallel instead of serially. Notice that for the
�DDCMP BLOCK CHECK COMPUTATION Page D-3
purposes of the block check byte computation, the LSB of the
first byte is always treated as the highest order term (i.e.,
the term with the largest exponent) in the message
polynomial.
2. On reception, the message may be handled with the block check
bytes included (two bytes longer) and the algorithm computed
based on this longer message. If the remainder is not zero
an error has occurred.
�
APPENDIX E
EXAMPLES
MESSAGE EXCHANGE EXAMPLES - These examples are presented here to show the operation of the message exchange component of DDCMP in specific cases of states and occurring events. They do not show actual link timings, as these vary with link characteristics and station selection. The variables used are those presented in subhead 5.3 and in the message formats in subhead 4. The variables of importance are listed just above the message being sent or just below the message received. Not all variables, user requests, and timers are shown in the examples. They are only shown when needed to illustrate a specific operational detail of DDCMP. The diagram symbols have the following meaning:
symbol meaning
------> Send message, received with no errors
---/--> Send message, received with errors
--//--> Send message, not received
! Reply timer running
E.1 EXAMPLE 1 - START-UP SEQUENCE WITH NO ERRORS
User requests start-up R=0,N=0,A=0,T=1,X=0
------>
STRT Notify user of start-up at
other station
User requests start-up
R=0,N=0,A=0,T=1,X=0
<------
STRT
------->
STACK Enter running state
<------
Enter running state ACK (RESP=0)
�EXAMPLES Page E-2
E.2 EXAMPLE 2 - START-UP SEQUENCE WITH ERRORS
User requests start-up
--//-->
STRT
!
!
Time-out !
!
!
------>
STRT
Notify user, user requests
start-up
<------
STRT !
!
!
--//-->
STACK !
!
! Time-out
!
<------
STRT
------>
STACK Enter running mode
<--//--
ACK(RESP=0)
!
!
!
Time-out !
!
!
------>
STACK
<------
ACK(RESP=0)
Enter running state
�EXAMPLES Page E-3
E.3 EXAMPLE 3 - DATA TRANSFER WITH NO ERRORS
User requests transmit R=0,N=1,A=0,T=2,X=1
------>
DATA(NUM=1,RESP=0)
Message received and given
to user
R=1,N=0,A=0,T=1,X=0
<------
ACK(RESP=1)
User requests transmit R=0,N=2,A=1,T=3,X=2
------>
DATA(NUM=2,RESP=0)
Message received and user
requests transmit
R=2,N=1,A=0,T=2,X=1
<------
DATA(NUM=1,RESP=2)
Message received and user requests transmit R=1,N=3,A=2,T=4,X=3
------>
DATA(NUM=3,RESP=1)
Message received
R=3,N=1,A=1,T=2,X=1
User requests transmit R=1,N=4,A=2,T=5,X=4
------>
DATA(NUM=4,RESP=1)
Message received
R=4,N=1,A=1,T=2,X=1
User requests transmit
R=4,N=2,A=1,T=3,X=2
<------
DATA(NUM=2,RESP=4)
Message received R=2,N=4,A=4,T=5,X=4
------>
ACK(RESP=2)
ACK received
R=4,N=2,A=2,T=3,X=2
�EXAMPLES Page E-4
E.4 EXAMPLE 4 - DATA TRANSFER WITH CRC ERRORS AND NAKING
User requests transmit R=0,N=1,A=0,T=2,X=1
----/-->
DATA(NUM=1,RESP=0)
Received in error
R=0,N=0,A=0,T=1,X=0
<------
NAK(RESP=0)
Nak received R=0,N=1,A=0,T=1,X=1 Retransmit R=0,N=1,A=0,T=2,X=1
------>
DATA(NUM=1,RESP=0)
Message received and
user requests transmit
R=1,N=1,A=0,T=2,X=1
<------
DATA(NUM=1,RESP=1)
---/-->
ACK(RESP=1)
Queue NAK for R=1
User requests 3 transmits R=1,N=2,A=1,T=3,X=2
------>
DATA(NUM=2,RESP=1)
Message received
R=2,N=1,A=1,T=2,X=1
R=1,N=3,A=1,T=4,X=3
------>
DATA(NUM=3,RESP=1)
Message received
R=3,N=1,A=1,T=2,X=1
R=1,N=4,A=1,T=5,X=4
------>
DATA(NUM=4,RESP=1)
Message received
R=4,N=1,A=1,T=2,X=1
Queue ACK for R=4
<------
NAK(RESP=1) Queued NAK returned
NAK received R=1,N=4,A=1,T=2,X=4 Retransmit R=1,N=4,A=1,T=3,X=2
------>
DATA(NUM=2,RESP=1)
Message ignored
<------
ACK(RESP=4) Queued ACK returned
All messages complete R=1,N=4,A=4,T=5,X=2
�EXAMPLES Page E-5
E.5 EXAMPLE 5 - DATA TRANSFER WITH ERRORS CAUSING REPLY TIMEOUTS
User requests transmit R=0,N=1,A=0,T=2,X=1
------>
DATA(NUM=1,RESP=0)
!
! Message received
! R=1,N=0,A=0,T=1,X=0
!
<--//--
! ACK(RESP=1)
Time-out !
!------>
REP(NUM=1)
<------
ACK(RESP=1) ACK response to REP
R=1,N=0,A=0,T=1,X=0
ACK received R=0,N=1,A=1,T=2,X=1 User requests transmit R=0,N=2,A=1,T=3,X=2
!--//-->
! DATA(NUM=2,RESP=0)
!
!
!
Time-out !
!------>
REP(NUM=2)
<------
NAK(RESP=1) NAK response to REP
R=1,N=0,A=0,T=1,X=0
NAK received R=0,N=2,A=1,T=2,X=2 Retransmit R=0,N=2,A=1,T=3,X=2
------>
DATA<NUM=2,RESP=0)
Message received
R=2,N=0,A=0,T=1,X=0
<------
ACK(RESP=2)
ACK received R=0,N=2,A=2,T=3,X=2 �EXAMPLES Page E-6
User requests transmit
R=0,N=3,A=2,T=4,X=3
!------>
! DATA(NUM=3,RESP=0)
!
! Message received
! R=3,N=0,A=0,T=1,X=0
User requests transmit ! R=0,N=4,A=2,T=5,X=4 !
!--//-->
! DATA(NUM=4,RESP=0)
User requests transmit ! R=0,N=5,A=2,T=6,X=5 !
!------>
! DATA(NUM=5,RESP=0)
!
! Message ignored
! R=3,N=0,A=0,T=1,X=0
!
<--//-- ACK to received message
! ACK(RESP=3)
Time-out !
!------>
REP(NUM=5)
<------
NAK(RESP=3) NAK response to REP
R=3,N=0,A=0,T=1,X=0
NAK received R=0,N=5,A=3,T=4,X=5 Retransmit R=0,N=5,A=3,T=5,X=4
------>
DATA(NUM=4,RESP=0)
Message received
R=4,N=0,A=0,T=1,X=0
Retransmit R=0,N=5,A=3,T=6,X=5
------>
DATA(NUM=5,RESP=0)
Message received
R=5,N=0,A=0,T=1,X=0
<------
ACK(RESP=5) ACK to received messages
All messages complete R=0,N=5,A=5,T=6,X=5
�
APPENDIX F
REVISION HISTORY
This list of DDCMP changes from version 3.0 to the current version 4.1 is provided as a guide for those who have implemented and/or are familiar with version 3.0 or version 4.0 and are interested in the changes from that version to the present 4.1 level. Revision levels between 3.0 and 4.1 are shown for those who have such documentation. The composite list of changes are all the technical changes from version 3.0 to version 4.1. Only technical changes and major clarifications are listed. Changes in wording or documentation level are not included in this revision history.
Version 3.0
There were three printed versions of this level spec, May 1974, August 1974, and December 1974. The December 1974 document was the most readable, correct, and widely circulated of the three. This document is taken as the base 3.0 level and all changes are from this copy of version 3.0.
Changes 3.0 to 3.02:
1. Changed FINAL flag bit to QSYNC flag bit. The SELECT flag
bit is now used for both selecting and ending selection. The
QSYNC flag signals non-abutting messages allowing short sync
sequences between messages on synchronous links.
2. Removed the RESET and RESAK messages. The link is reset in
both directions at the same time by use of the STRT/STACK
start-up sequence.
3. Clarified the synchronous and asynchronous synchronization
and pad sequences. The sequences were made dependent on the
link transmission characteristics.
4. Removed RESP and NUM fields from the STRT and STACK messages,
replacing them with FILLs. After a start-up sequence message
numbering always starts with message number 1.
5. Changed ADDR field to always be tributary address in messages
both to and from tributaries. This change added a positive
�REVISION HISTORY Page F-2
identification to the messages being returned from a
tributary to the control station.
6. Framing requires first byte to be SOH, ENQ, or DLE. If not
one of these then no message is framed and, thus, no NAKs are
generated as in the previous version.
7. Start-up sequence revised to send an acknowledge to a
received STACK. The new sequence adds an ACK response to a
STACK, creating a positive three-way handshake.
8. Multipoint operation and sync sequences clarified. Many of
the operational details for multipoint were added to the
specification.
Version 3.02
This version was the result of the above listed changes made by the DDCMP committee. This and intermediate review edition 3.01 were dated July 1975 - August 1975. The document was also partially rewritten to add additional clarification to the specification.
Changes 3.02 to 3.03:
1. Require full duplex tributary to wait until the entire
message with the SELECT flag bit on is received before
transmitting. Previously it had to wait only for the header.
This change provides for common operation among the different
modes and link types.
2. Require that STRT, STACK, and MAINT messages have the SELECT
and QSYNC flag bits always set. These messages always
operate one message at a time alternately. This change makes
their use common in all modes and link types.
3. Multipoint control stations must precede a message with a
sync sequence if addressed to a tributary different from the
one addressed in the previous message. This change improves
receiver synchronization at the tributaries.
4. DATA and MAINT messages with zero length data fields are not
allowed.
5. The BOOT message is changed in name to the MAINTENANCE
message.
Version 3.03
This version resulted from the changes listed above by the DDCMP committee. Version 3.03 dated December 1975 was never totally reviewed by the DDCMP committee.
�REVISION HISTORY Page F-3
Changes 3.03 to 4.0:
1. Divided protocol functions into three semi-independent
components: framing, link management, and message exchange.
This is not a technical change but helps clarify and more
precisely specify the protocol.
2. Clarified the user and device interfaces to the protocol.
This is not a technical change but assists in the
understanding of the protocol.
3. Allowed the SELECT flag bit to be set in full duplex
point-to-point mode with no checking by the receiver. This
change makes for more compatible full and half duplex
implementations.
4. Modified the link idle signal and optional end-of-message
checking to increase CRC-16 error detection capability in
cases of synchronous clock loss.
5. Allowed an optional increase in the sync sequence length to
recover from framing errors when responding to a NAK.
Especially useful on asynchronous links.
6. Made checking of ADDR field optional in point-to-point mode
and checking of FILL fields optional in all modes.
7. Described the operation of the reply timer in cases of
retransmission. This is not a technical change but a precise
description of the timer operation during retransmission.
8. Changed error recording counters to be a recommendation not a
requirement. Error recording is now a recommended optional
part of a DDCMP implementation.
9. Clarified messages to be retransmitted when NAKs and ACKs are
received asynchronous to transmission. This is not a
technical change but a more precise definition of the
operation in these cases.
10. Removed the option of multipoint tributaries operating in
sheltered mode. The only valid mode is circumspect mode,
where the tributaries always track messages on the link.
This mode is now simply called multipoint tributary
operation.
11. Changed the operation of the select timer to be stopped and
restarted after every good received message or when
resynchronizing. This changes the timer to one that times a
lost select flag (message with flag in error), rather than
one that times the duration of a total selection interval.
�REVISION HISTORY Page F-4
Version 4.0
This version is a total rewrite of the DDCMP protocol specification. Three versions of 4.0 were used for review in limited circulation, dated June 1977, October 1977, and December 1977. It is an attempt to more clearly and precisely define the DDCMP protocol. It incorporates the changes listed above from the previous version.
Version 4.1
This version is an update of version 4.0. It includes many minor changes in the document for clarification only. It includes one change to the protocol in the maintenance state, and adds detailed specifications on maintainability functions: error recording and event logging. These changes do not affect the operation in the running state and are only a minor change to the maintenance state. A version 4.1 implementation will operate with version 4.0 implementations.
Changes 4.0 to 4.1:
1. Change maintenance mode state table to notify the user when a
STRT message is received.
2. Added a specification of standardized error counters and
event logging. Added a specification of a network management
interface to support error counters. Error counters and
event logging are optional. However, if implemented they are
required to conform to the specification.
3. Added an Initialization and Management Interface
specification to support implementations which operate in
more than one mode, to enable tributary addresses to be
specified, and to set parameters and timer values.