Ubuntu Manpage: comm_wire - The comm wire protocol

NAME

       comm_wire - The comm wire protocol

SYNOPSIS

       package require comm

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DESCRIPTION

       The  comm  command  provides an inter-interpreter remote execution facility much like Tk's
       send(3tk), except that it uses sockets rather than the  X  server  for  the  communication
       path.   As a result, comm works with multiple interpreters, works on Windows and Macintosh
       systems, and provides control over the remote execution path.

       This document contains a specification of the various versions of the wire  protocol  used
       by  comm  internally  for  the communication between its endpoints. It has no relevance to
       users of comm, only to developers who wish to  modify  the  package,  write  a  compatible
       facility in a different language, or some other facility based on the same protocol.

WIRE PROTOCOL VERSION 3

BASIC LAYER
The basic encoding for all data is UTF-8. Because of this binary data, including the NULL
character, can be sent over the wire as is, without the need for armoring it.

BASIC MESSAGE LAYER
On top of the Basic Layer we have a message oriented exchange of data. The totality of
all characters written to the channel is a Tcl list, with each element a separate message,
each itself a list. The messages in the overall list are separated by EOL. Note that EOL
characters can occur within the list as well. They can be distinguished from the message-
separating EOL by the fact that the data from the beginning up to their location is not a
valid Tcl list.

EOL is signaled through the linefeed character, i.e LF, or, hex 0x0a. This is following
the unix convention for line-endings.

As a list each message is composed of words. Their meaning depends on when the message was
sent in the overall exchange. This is described in the upcoming sections.

NEGOTIATION MESSAGES - INITIAL HANDSHAKE
The command protocol is defined like this:

• The first message send by a client to a server, when opening the connection,
contains two words. The first word is a list as well, and contains the versions of
the wire protocol the client is willing to accept, with the most preferred version
first. The second word is the TCP port the client is listening on for connections
to itself. The value 0 is used here to signal that the client will not listen for
connections, i.e. that it is purely for sending commands, and not receiving them.

• The first message sent by the server to the client, in response to the message
above contains only one word. This word is a list, containing the string vers as
its first element, and the version of the wire protocol the server has selected
from the offered versions as the second.

SCRIPT/COMMAND MESSAGES
All messages coming after the initial handshake consist of three words. These are an
instruction, a transaction id, and the payload. The valid instructions are shown below.
The transaction ids are used by the client to match any incoming replies to the command
messages it sent. This means that a server has to copy the transaction id from a command
message to the reply it sends for that message.

send

async

command
The payload is the Tcl script to execute on the server. It is actually a list
containing the script fragments. These fragment are concatenated together by the
server to form the full script to execute on the server side. This emulates the
Tcl "eval" semantics. In most cases it is best to have only one word in the list,
a list containing the exact command.

Examples:

(a) {send 1 {{array get tcl_platform}}}
(b) {send 1 {array get tcl_platform}}
(c) {send 1 {array {get tcl_platform}}}

are all valid representations of the same command. They are
generated via

(a') send {array get tcl_platform}
(b') send array get tcl_platform
(c') send array {get tcl_platform}

respectively

Note that (a), generated by (a'), is the usual form, if only single commands are sent by
the client. For example constructed using list, if the command contains variable
arguments. Like

send [list array get $the_variable]

These three instructions all invoke the script on the server side. Their difference is in
the treatment of result values, and thus determines if a reply is expected.

send A reply is expected. The sender is waiting for the result.

async No reply is expected, the sender has no interest in the result and is not
waiting for any.

command
A reply is expected, but the sender is not waiting for it. It has arranged
to get a process-internal notification when the result arrives.

reply Like the previous three command, however the tcl script in the payload is highly
restricted. It has to be a syntactically valid Tcl return command. This contains
result code, value, error code, and error info.

Examples:

{reply 1 {return -code 0 {}}}
{reply 1 {return -code 0 {osVersion 2.4.21-99-default byteOrder littleEndian machine i686 platform unix os Linux user andreask wordSize 4}}}

BUGS, IDEAS, FEEDBACK

       This document, and the package it describes,  will  undoubtedly  contain  bugs  and  other
       problems.    Please   report   such   in   the   category  comm  of  the  Tcllib  Trackers
       [http://core.tcl.tk/tcllib/reportlist].  Please also report any ideas for enhancements you
       may have for either package and/or documentation.

KEYWORDS

       comm, communication, ipc, message, remote communication, remote execution, rpc, socket

COPYRIGHT

       Copyright (c) 2005 Docs. Andreas Kupries <andreas_kupries@users.sourceforge.net>