rfc909.txt
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to usurp inactive connections. A connection with no activity
could be declared 'dead', but would not be usurped until the
connection resource was needed. However, this would still
require the transport layer to support two connection channels:
one to receive connection requests, and another to use for an
active connection.
3.3 Command Sequencing
Each command sent from the host to the target has a sequence
number. The sequence number is used by the target to refer to
the command in normal replies and error replies. To save space,
these numbers are not actually included in host commands.
Instead, each command sent from the host is assigned an implicit
sequence number. The sequence number starts at zero at the
beginning of the LDP session and increases by one for each
command sent. The host and target each keep track of the current
number. The SYNCH <sequence number> command may be used by the
host to synchronize the sequence number.
3.4 Data Packing and Transmission
The convention for the order of data packing was chosen for
its simplicity: data are packed most significant bit first, in
order of increasing target address, into eight-bit octets. The
octets of packed data are transmitted in sequential order.
Page 10
LDP Specification Protocol Operation
Data are always packed according to the address format of
the target machine. For example, in an LDP session between a
20-bit host and a 16-bit target, 16-bit words (packed into
octets) are transmitted in both directions. For ease of
discussion, targets are treated here as if they have uniform
address spaces. In practice, the size of address units may vary
within a target -- 16-bit macromemory, 32-bit micromemory, 10-bit
dispatch memory, etc. Data packing between host and target is
tailored to the units of the current target address space.
Figures showing the packing of data for targets with various
address unit sizes are given below. The order of transmission
with respect to the diagrams is top to bottom. Bit numbering in
the following diagrams refers to significance in the octet: bit
zero is the least significant bit in an octet. For an
explanation of the bit numbering convention that applies in the
rest of this document, please see Appendix A.
The packing of data for targets with word lengths that are
multiples of 8 is straightforward. The following diagram
illustrates 16-bit packing:
7 0
---------------------------------
Octet 0 | WORD 0 bits 15-08 |
---------------------------------
Octet 1 | WORD 0 bits 07-00 |
---------------------------------
Octet 2 | WORD 1 bits 15-08 |
---------------------------------
Octet 3 | WORD 1 bits 07-00 |
---------------------------------
*
*
*
---------------------------------
Octet 2n-1 | WORD n bits 07-00 |
---------------------------------
Packing of 16-bit Words
Figure 3
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RFC-909 July 1984
Packing for targets with peculiar word lengths is more
complicated. For 20-bit machines, 2 words of data are packed
into 5 octets. When an odd number of 20-bit words are
transmitted, the partially used octet is included in the length
of the command, and the octet is padded to the right with zeroes.
7 0
---------------------------------
Octet 0 | WORD 0 bits 19-12 |
---------------------------------
Octet 1 | WORD 0 bits 11-04 |
---------------------------------
Octet 2 | WORD 0 03-00 | WORD 1 19-16 |
---------------------------------
Octet 3 | WORD 1 bits 15-08 |
---------------------------------
Octet 4 | WORD 1 bits 07-00 |
---------------------------------
Packing of 20-bit Words
Figure 4
3.5 Implementations
A subset of LDP commands may be implemented in targets where
machine resources are limited and the full capabilities of LDP
are not needed. There are three basic levels of target
implementations: LOADER_DUMPER, BASIC_DEBUGGER and
FULL_DEBUGGER. The target communicates its LDP implementation
level to the host during session initiation. The implementation
levels are described below:
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LDP Specification Protocol Operation
LOADER_DUMPER
Used for loading/dumping of the target machine.
Includes all protocol class commands and replies; data
transfer commands READ, WRITE, MOVE and their responses;
control command START and control reply EXCEPTION.
Understands at least PHYS_MACRO and HOST addressing modes;
others if desired.
BASIC_DEBUGGER
Implements LOADER_DUMPER commands, all control commands,
all addressing modes appropriate to the target machine, but
does not have finite state machine (FSM) breakpoints or
watchpoints. Default breakpoints are implemented. The
target understands long addressing mode.
FULL_DEBUGGER
Implements all commands and addressing modes appropriate to
the target machine, and includes breakpoint commands,
conditional commands and BREAKPOINT_DATA. Watchpoints are
optional.
Page 13
RFC-909 July 1984
Page 14
LDP Specification Commands and Formats
CHAPTER 4
Commands and Formats
4.1 Packet Format
LDP commands are enclosed in RDP transport messages. An RDP
message may contain more than one command, but each command must
fit entirely within a single message. Network packets containing
LDP commands have the format shown in Figure 5.
+----------------+
| Local Network |
| Header(s) |
+----------------+
| IP Header |
+----------------+
| RDP Header |
+----------------+ +-+
| LDP Command | |
| Header | |
+----------------+ |
| Optional | |
. LDP . | LDP Command
. Data . | Format
| | |
+----------------+ |
| LDP Padding | |
+----------------+ +-+
| Additional |
. LDP .
. Commands .
. .
+----------------+
Network Packet Format
Figure 5
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RFC-909 July 1984
4.2 Command Format
LDP commands consist of a standard two-word header followed
optionally by additional data. To facilitate parsing of multi-
command messages, all commands contain an even number of octets.
Commands that contain an odd number of data octets must be padded
with a null octet.
The commands defined by the LDP specification are intended
to be of universal application to provide a common basis for all
implementations. Command class and type codes from 0 to 63. are
reserved by the protocol. Codes above 63. are available for the
implementation of target-specific commands.
4.2.1 Command Header
LDP commands begin with a fixed length header. The header
specifies the type of command and its length in octets.
0 0 0 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+---------------+---------------+
0 | Command Length (octets) |
+---------------+---------------+
1 | Command Class | Command Type |
+---------------+---------------+
LDP Command Header Format
Figure 6
HEADER FIELDS:
Command Length
The command length gives the total number of octets in the
command, including the length field and data, and excluding
padding.
Command Class
Command Type
Page 16
LDP Specification Commands and Formats
The command class and type together specify a particular
command. The class selects one of six command categories,
and the type gives the command within that category. All
codes are decimal. The symbols given in Figures 7 and 8 for
command classes and types are used in the remainder of this
document for reference.
The command classes that have been defined are:
Command Class | Symbol
----------------+-----------
1 | PROTOCOL
2 | DATA_TRANSFER
3 | CONTROL
4 | MANAGEMENT
5 | BREAKPOINT
6 | CONDITION
7 - 63 | <reserved>
Command Classes
Figure 7
Command type codes are assigned in order of expected
frequency of use. Commands and their responses/replies are
numbered sequentially. The command types, ordered by
command class, are:
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