Network Debugging Protocol (RFC0643)
Original Publication Date: 1974-Jul-01
Included in the Prior Art Database: 2019-Feb-14
Internet Society Requests For Comment (RFCs)
To be used in an implementation of a PDP-11 network bootstrap device and a cross-network debugger.
Network Debugging Protocol Eric Mader RFC: 643 July 1974 NIC #30873
This document describes a proposed protocol to be used in an implementation of a PDP-11 network Bootstrap service and a cross-network debugger. The protocol is designed for debugging processes running under an operating system which can perform some of the "lower level" debugging tasks, such as planting and removing breakpoints and single stepping. A subset of the protocol could be used if such a capability does not exist (a stand-alone program for example).
The protocol is a level 2 protocol, which bypasses the ARPANET HOST-HOST protocol. (This is implemented on TENEX using special privileged system calls which allow messages to be sent directly to and received directly from the IMP). Messages are sent between the PDP-11 and the remote debugger on one link (currently 377 octal). Each message from the remote debugger to the PDP-ll is a request that the PDP-11 perform some action, and each message from the PDP-11 to the remote debugger is either a reply to that request or an indication that a process has stopped running (i.e. has trapped, hit a breakpoint, etc). The exact format of the messages is shown in Figure 1 below.
Each command consists of an 8-bit op-code, and an 8-bit process-id, two 16-bit arguments, and an optional string of 8-bit bytes. The op-code field from the PDP-11 should be the same as that sent by the remote host to indicate successful completion of the request or be the same as that set by the remote host with the 200 bit set to indicate failure to complete the request. Op-codes from the PDP-11 which have the 100 bit set are asynchronous indication that a process has stopped for a reason other than a request from the remote host. (See description of asynchronous replies below). An op-code from the PDP-11 with both the 100 and 200 bit set is meaningless. Thus, the 8-bit op-code field sent by the PDP-11 can be thought of as a CAN’T Flag, an Asynchronous STOP Flag, and a 6-bit op-code.
In the description that follows the commands will be given as
NAME (Process-ID, Argument 1, Argument 2, BYTE STRING) with only as many of the fields present as are used.
Op-Code 0 - NOP_______ _ _ ___
This command is intended to be used to determine if the PDP-11 is operational. It has no effect on any process running in the PDP-11. The response is NOP.
Op-Code 1 - DEBUG (Process)_______ _ _ _____ _________
This command requests the ability to debug the given process. The PDP-11 should respond with
(op-code 1) if no other remote host is currently doing so, and
CAN’T DEBUG (process)
(op-code 201) if another remote host has been given permission to debug the process, or the process doesn’t exist.
Op-Code 2 - END DEBUG (Process)_______ _ _ ___ _____ _________
This command relinquishes the ability to debug a process. The PDP-11 should reply
END DEBUG (Process)
(op-code 2) unless the remote host isn’t debugging the process, in which case it should respond