DECnet Protocols

Digital Equipment Corporation (DEC) developed the DECnet protocol to allow high-speed communication between DEC minicomputers across local and wide area networks.

The DECnet protocol suite is illustrated here in relation to the OSI model:
Click the protocols on the map to see more details.

RP

The Routing Protocol (RP) distributes routing information among DECnet hosts. It defines routing classes into two levels: level 1, which handles routing within a single DECnet routing area; and level 2, which handles routing between areas.

Frames

RP frames can be one of the following types:

Frame Parameter

All RP frames have the following parameter:

Node address
Node address. The DECnet area and node given in the decimal dot form: Area.Node (where Area can extend from 1-63 and Node can extend from 1-1023).

Hello Parameters

Hello frames have the following parameters:

Routing priority
Routing priority on a scale of 100 (not used for [endnode hello] frames).

Hello period
Period between routing update hello messages.

Version
The version in use.

Multicast status
Y indicates that this protocol supports multicast traffic on the link; N indicates it does not support multicast traffic.

Maximum
Maximum frame size supported on the link (1500 for Ethernet).

Router Parameters

RP router frames have the following parameters:

Source node
The ID of the sending node.

Number of IDs
Number of IDs contained in the routing table for this level 1 routing message.

Number of areas
Number of areas contained in the routing table for this level 2 message.

Data Parameters

RP [routed data] frames have the following parameters:

Request return
When set to 1, sender is requesting that the other party return the frame. When set to 0, sender is suggesting that the other party discard the frame.

Return path
When set to 1, frame is on return path; when set to 0, frame is on outbound path.

Intra-Ethernet
When set to 1, frame is from a directly connected Ethernet segment; when set to 0, the system forwarded the frame from another segment.

Version
Must be 0.

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MOP

The Maintenance Operation Protocol (MOP) is used for utility services such as uploading and downloading system software, remote testing and problem diagnosis.

Frames

MOP frames can be one of the following commands:

Memory Dump and Memory Load Frames

MOP memory dump and memory load frames use the following parameters:

Load number
Data segment sequence number of the current memory data segment.

Load address
Memory load address for storage of the memory data.

Transfer address
Starting memory address of the current segment.

Memory address
Starting physical memory address for the dump.

Count
Number of memory locations to dump.

Version
Protocol format version, currently 1.

Memory size
Size of physical machine memory.

Bits
Generally set to 2 for compatibility reasons.

Buffer size
Local buffer size in bytes.

Communication device
Device type of the requesting system. The following device codes can be used:

Parameter Load Frames

MOP parameter load frames have the following fields:

Load number
The data segment sequence number of the current data segment.

Target name
ASCII system name for target system.

Target address
Hex address of target system.

Host system name
ASCII system name of host.

Host system address
Hex address of host system.

Host system time
Current time for host system.

Request Program Frames

MOP [request program] frames have the following fields:

Communication device
Refer to the values given for dump/load frames above.

Version
Version currently in use.

Type
Type of program that the system requested:

Software ID
Software type that the system requested:

Processor
System processor type:

Boot Request Frames

Boot request frames have the following fields:

Verification number
A verification code that must match before this protocol can honor a boot request.

Boot server
Boot system device type:

Software ID
The software type as given for the [request program] frame above.

Remote Console Frames
MOP remote console frames have the following fields:

Receipt number
Used to identify a particular request.

Command status
Console command data status given as OK if received, or Lost if not received.

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NSP

The Network Services Protocol (NSP) provides reliable virtual connection services with flow control to the network layer Routing Protocol (RP).

Frames

NSP frames can be one of the following commands:

Parameters

NSP frames can contain the following fields:

Destination link address
Destination port of the link.

Source link address
Source port of the logical link.

Acknowledge number
The segment number of the last message received successfully or, if followed by {NAK}, the segment number of the message for which the system requests a retransmission.

Acknowledge other
Same as acknowledge number, but used to acknowledge other data.

Segment number
Number of the current data frame.

Flow control
Can indicate the following services:

Flow control information
The data segment messages can include flow control messages ({send} or {stop}) to indicate the desired action of the receiving system.

BOM/EOM
Beginning of message/end of message. Indicates the start or end of a data segment message.

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SCP

The Session Control Protocol (SCP) manages logical links for DECnet connections.

Frames

SCP frames can be one of the following commands:

Connect Data Parameters

SCP [connect data] frames contain the destination name (Dest) and source name (Src) parameters that can consist of the following fields:

Object type
One of the following object types:

Group
Group code identifier.

User
User code identifier.

Descriptor
A user-defined string of data.

Version
The SCP version, as in {SCP 1.0}.

Requestor ID
User name for access verification.

Password
Password for user verification.

Account
Link or service account data.

User data
End user connect data.

Disconnect/Reject Parameters

The decoding for SCP [disconnect] and [reject data] frames lists the reason for disconnection, as follows:

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DAP

The Data Access Protocol (DAP) provides remote file access to systems supporting the DECnet architecture.

Frames

DAP frames can be one of the following commands:

Frame Parameters

DAP commands can contain the following parameters:

Allocation size
The number of blocks allocated to a file.

Attribute
File attribute represented as follows:

Bit count
Indicates the number of unused bits in the last byte of the data message.

Bits per byte
Number of bits in each byte.

Block size
Physical media block size in bytes.

Bucket size
Bucket size used to access relative, hashed and indexed files.

Checksum
The 16-bit file checksum.

Data type
Type of file data:

Device type
Code which indicates the type of device that DAP associates with the file.

File access mode
The open mode for file access which is specified as one of the following:

FilSys
File system, represented as one of the following:

Maximum buffer size
The maximum buffer size the sending system can receive.

Operating system type
Operating system type can be of the following:

Password
Password required for file access.

Record attributes
Record attributes code.

Record format
Represented as follows:

Record size
File record size in bytes.

Record number
The record used when accessing file data.

Shared access mode
The open mode for sharing file access, specified as follows:

Stream ID
The ID code used to multiplex data streams on one file.

System capabilities
System capabilities code.

System specific information
Information specific to homogeneous systems.

Ver
Version. The DAP version number, the DAP software version number, followed by the user modification number in parenthesis.

Continue Parameters

DAP [continue] frames indicate the recovery action as one of the following:

Status Parameters

DAP [status] frames can report the following status information:

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CTERM

The Command Terminal (CTERM) protocol is the terminal emulation protocol of the Digital Network Architecture. CTERM uses DECnet to provide a command terminal connection between DEC terminals and DEC operating systems such as VMS and RSTS/E.

Frames

CTERM frames can be one of the following commands:

CTERM Parameters

The following are possible CTERM parameters:

Buffer size
Size of the terminal character input buffer.

Character count
Number of characters in the input buffer.

End
Ending character position. Current position of the last character displayed.

EOP
End of prompt. Character position of the first character after the prompt.

Horizontal position
Current horizontal position of the displayed output.

Horizontal position change
Horizontal position change. Horizontal position change since the last read.

Low water mark
Position of the last character not modified.

Maximum receive size
Length of the input character buffer.

Maximum transmit
Maximum transmit buffer size.

Character
Out-of-band character. The out-of-band character received.

Postfix
Postfix new line count. New line postfix count for the current write.

Prefix
Prefix new line count. New line prefix count for the current write.

Software revision
Software revision currently in use.

Start of display
Position of the first character to display.

Termination
Termination set bitmask. The 256-bit termination set for the read.

Time out
Amount time in seconds before a read request aborts.

Version
Protocol version currently in use.

Vertical position
Current vertical position of the displayed output.

Vertical position change
Change in vertical position since the last read.

Messages

For certain CTERM frames various messages can be displayed as detailed below:

[input state] frames display the current input status as
{more characters} or {no more characters}.

[write status] frames can display the write status as
{some output lost} or {no output lost}.

[discard stat] frames can display the discard status as
{discard} or {no discard}.

[abort read] frames display the abort request as
{unconditional} or {if no more input}.

[out-of-band] frames can contain the disposition of the data as [discard].

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LAT

The Local Area Transport (LAT) protocol is designed to handle multiplexed terminal traffic to/from timesharing hosts.

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STP

The Spanning Tree Protocol (STP) prevents the formation of logical looping in the network. It is implemented by the 802.1d MAC Bridge Management Protocol, to provide information on bridge topology.

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LAVC

The Local Area VAX Cluster (LAVC) protocol communicates between DEC VAX computers in a cluster.

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