transfer::connect - Connection setup
package require Tcl 8.4
package require snit ?1.0?
package require transfer::connect ?0.2?
?arg arg ...
objectName connect command
This package provides objects holding enough information to enable them to
either actively connect to a counterpart, or to passively wait for a
connection from said counterpart. I.e. any object created by this packages is
always in one of two complementary modes, called active
initiates the connection) and passive
(the object receives the
Of the two objects in a connecting pair one has to be configured for
mode, and the other then has to be configured for passive
mode. This establishes which of the two partners connects to whom (the
to the other), or, who is waiting on whom (the passive
the other). Note that this is completely independent of the direction of any
data transmission using the connection after it has been established. An
active object can, after establishing the connection, either transmit or
receive data. Equivalently the passive object can do the same after the
waiting for its partner has ended.
- transfer::connect objectName ?options...?
- This command creates a new connection object with an associated Tcl
command whose name is objectName. This object command is
explained in full detail in the sections Object command and
Object methods. The set of supported options is explained in
The object command will be created under the current namespace if the
objectName is not fully qualified, and in the specified namespace
otherwise. The fully qualified name of the object command is returned as
the result of the command.
All objects created by the ::transfer::connect
command have the following
- objectName method ?arg arg ...?
- The method method and its arg'uments determine the exact
behavior of the command. See section Object methods for the
- objectName destroy
- This method destroys the object. This is safe to do for an active
object when a connection has been started, as the completion callback is
synchronous. For a passive object currently waiting for its partner
to establish the connection however this is not safe and will cause errors
later on, when the connection setup completes and tries to access the now
missing data structures of the destroyed object.
- objectName connect command
- This method starts the connection setup per the configuration of the
object. When the connection is established the callback command
will be invoked with one additional argument, the channel handle of the
socket over which data can be transfered.
The detailed behaviour of the method depends on the configured mode.
- The connection setup is done synchronously. The object waits until the
connection is established. The method returns the empty string as its
- The connection setup is done asynchronously. The method returns
immediately after a listening socket has been set up. The connection will
be established in the background. The method returns the port number of
the listening socket, for use by the caller. One important use is the
transfer of this information to the counterpart so that it knows where it
has to connect to.
This is necessary as the object might have been configured for port
0, allowing the operating system to choose the actual port it will
The listening port is closed immediately when the connection was established
by the partner, to keep the time interval small within which a third party
can connect to the port too. Even so it is recommended to use additional
measures in the protocol outside of the connect and transfer object to
ensure that a connection is not used with an unidentified/unauthorized
partner One possibility for this is the use of SSL/TLS. See the option
-socketcmd and section Secure connections for information on
how to do this.
Connection objects support the set of options listed below.
- -mode mode
- This option specifies the mode the object is in. It is optional and
defaults to active mode. The two possible modes are:
- In this mode the two options -host and -port are relevant
and specify the host and TCP port the object has to connect to. The host
is given by either name or IP address.
- In this mode the option -host has no relevance and is ignored
should it be configured. The only option the object needs is -port,
and it specifies the TCP port on which the listening socket is opened to
await the connection from the partner.
- -host hostname-or-ipaddr
- This option specifies the host to connect to in active mode, either
by name or ip-address. An object configured for passive mode
ignores this option.
- -port int
- For active mode this option specifies the port the object is
expected to connect to. For passive mode however it is the port
where the object creates the listening socket waiting for a connection. It
defaults to 0, which allows the OS to choose the actual port to
- -socketcmd command
- This option allows the user to specify which command to use to open a
socket. The default is to use the builtin ::socket. Any compatible
with that command is allowed.
The envisioned main use is the specfication of tls::socket. I.e. this
option allows the creation of secure transfer channels, without making
this package explicitly dependent on the tls package.
See also section Secure connections.
- -encoding encodingname
- -eofchar eofspec
- -translation transspec
- These options are the same as are recognized by the builtin command
fconfigure. They provide the configuration to be set for the
channel between the two partners after it has been established, but before
the callback is invoked (See method connect).
One way to secure connections made by objects of this package is to require the
and then configure the option -socketcmd
the use of command tls::socket
to open the socket.
# Load and initialize tls
package require tls
tls::init -cafile /path/to/ca/cert -keyfile ...
# Create a connector with secure socket setup,
transfer::connect C -socketcmd tls::socket ...
BUGS, IDEAS, FEEDBACK¶
This document, and the package it describes, will undoubtedly contain bugs and
other problems. Please report such in the category transfer
report any ideas for enhancements you may have for either package and/or
active, channel, connection, passive, secure, ssl, tls, transfer
Copyright (c) 2006-2009 Andreas Kupries <email@example.com>