# Class Nettelnet_client.telnet_session

    class telnet_session : object .. end

A telnet session

## Overwiew

The `telnet_session` object has two queues, one for arriving data, one for data to send. Once the session object is attached to an event system, it connects to the remote peer, and processes the queues. Input is appended to the input queue; output found on the output queue is sent to the other side. If input arrives, a callback function is invoked. You may close the output side of the socket by putting `Telnet_eof` onto the output queue. Once the EOF marker has been received, a `Telnet_eof` is appended to the input queue, and the connection is closed (completely). The session object detaches from the event system automatically in this case.

Hints

Set an input handler as callback function in the session object. The input handler is called when new input data have been arrived. It should inspect the input queue, process the queue as much as possible, and it should remove the processed items from the queue. While processing, it may add new items to the output queue.

If you are not within the callback function and add items to the output queue, the session object will not detect that there are new items to send - unless you invoke the `update` method.

If you want option negotiation, it is the simplest way to use the special option negotiation methods. Configure the options as you want (invoking `enable`, `disable` etc), but do not forget to modify the way input is processed. Every `Telnet_will`, `_wont`, `_do`, and `_dont` command must be passed to `process_option_command`.

    method set_connection : telnet_connector -> unit

Sets the host name and the port of the remote server to contact.

    method set_event_system : Unixqueue.event_system -> unit

Sets the event system to use. By default, a private event system is used.

    method set_callback : (bool -> unit) -> unit

Sets the callback function. This function is called after new commands have been put onto the input queue. The argument passed to the callback function indicates whether a 'Synch' sequence was received from the remote side or not.

Note Synch: If the client sees a data mark command it will assume that it is actually a Synch sequence. The client automatically discards any `Telnet_data` commands from the input queue (but not `Telnet_data`s inside subnegotiations). The data mark command itself remains on the queue.

    method set_exception_handler : (exn -> unit) -> unit

Sets the exception handler. Every known error condition is caught and passed to the exception handler. The exception handler can do whatever it wants to do. If it raises again an exception, the new exception is always propagated up to the caller (whoever this is). Often the caller is the event system scheduler (i.e. `Unixqueue.run`); see the documention there.

If you do not set the exception handler, a default handler is active. It first resets the session (see method `reset`), and then wraps the exception into the `Telnet_protocol` exception, and raises this exception again.

    method output_queue : telnet_command Stdlib.Queue.t

The queue of commands to send to the remote side. If you add new commands to this queue, do not forget to invoke the `update` method which indicates to the event system that new data to send is available. After commands have been sent, they are removed from the queue.

    method input_queue : telnet_command Stdlib.Queue.t

The queue of commands received from the remote side. This class only adds commands to the queue (and invokes the callback function). The user of this class is responsible for removing commands from the queue which have been processed.

    method get_options : telnet_options

Get the configuration options.

    method set_options : telnet_options -> unit

Set the configuration options.

    method reset : unit -> unit

Closes the connection immediately and empties all queues. All negotiated options are reset, too.

## Telnet options

The following methods deal with Telnet protocol options. These are negotiated between local and remote side by the Will, Won't, Do and Don't commands.

The "local" options describe the modification of the behaviour of the local side; the "remote" options describe the modifications of the remote side. Both set of options are independent. This object may track the set of accepted and rejected options if the following methods are used; but this works only if the `Telnet_will`, `_wont`, `_do`, and `_dont` commands received from the remote side are processed by `process_option_command`. So you need to invoke this method for the mentioned commands in your command interpretation loop.

The idea is: If you enable an option, it is possible to switch it on. If the remote side requests the option to be enabled, the request will be acknowledged. If the remote side does not request the option, it remains off.

You can also actively demand an option (`offer_local_option`, `request_remote_option`); this is of course only possible if the option is already enabled. In this case the client tries actively to switch it on.

You can also disable an option. If the option is on, the client actively rejects the option; following the Telnet protocol this is always possible (rejections cannot be rejected).

The `reset` methods are somewhat dangerous. They simply reset the internal state of the client, but do not negotiate. This possibility was added to allow the Timing Mark option to send again timing marks even if the previous timing marks have already been accepted. After `reset`, the client thinks the option was never negotiated; but nothing is done to tell the remote side about this.

`option_negotiation_is_over`: true if no option negotiation is pending (i.e. nothing has still to be acknowledged).

    method enable_local_option : telnet_negotiated_option -> unit

    method enable_remote_option : telnet_negotiated_option -> unit

    method disable_local_option : telnet_negotiated_option -> unit

    method disable_remote_option : telnet_negotiated_option -> unit

    method offer_local_option : telnet_negotiated_option -> unit

    method request_remote_option : telnet_negotiated_option -> unit

    method reset_local_option : telnet_negotiated_option -> unit

    method reset_remote_option : telnet_negotiated_option -> unit

    method get_local_option : telnet_negotiated_option ->
           telnet_option_state

    method get_remote_option : telnet_negotiated_option ->
           telnet_option_state

    method option_negotiation_is_over : bool

    method process_option_command : telnet_command -> unit

    method fetch_subnegotiation : string option

This method should be called as follows: If you find a `Telnet_sb` at the beginning of the input queue, remove this command `Queue.take`, and invoke `fetch_subnegotiation`. This method scans the queue and looks for the associated `Telnet_se` command. If it does not find it, `None` is returned. If `Telnet_se` is found, the parameter enclosed by the two commands is returned as `Some s` where `s` is the parameter string. Furthermore, in the latter case the data items and the closing `Telnet_se` are removed from the queue.

## Running the session

    method attach : unit -> unit

Attach to the event system. After being attached, the client is ready to work.

    method run : unit -> unit

Run the event system

    method update : unit -> unit

If there are commands in the output queue, the event system is signaled that this client wants to do network I/O.

    method send_synch : telnet_command list -> unit

At the next output oppurtunity, a Synch sequence is sent to the remote peer. This means that the passed commands, extended by an additional Data Mark command, are sent to the peer as urgent data.

Sending a Synch sequence has higher priority than the output queue; processing of the output queue is deferred until the Synch sequence has been completely sent.

    method expect_input : bool -> unit

Set whether the timeout value is to be applied to the input side of the connection. This is `true` by default.

    method start_tls : (module Netsys_crypto_types.TLS_CONFIG) -> peer_name:string option -> unit

Switch to TLS. Note that this is not covered by the TELNET protocol standard. However, some applications of TELNET use this nevertheless (e.g. FTP control connections).

Switching to TLS can be done at any time, but should be done at a moment when the other side of the connection expects it. If `start_tls` is invoked before `attach`, TLS is enabled at the very beginning of the TELNET connection.

The arguments are the TLS configuration, and the peer name for validation (pass the host name here).

    method tls_session_props : Nettls_support.tls_session_props option

TLS session properties (if on)

    method tls_session_data : string option

TLS session data (if on)