Telnet logout option

　　NWG/RFC# 727 MRC 26-APR-77 18:24 40025

　　Telnet Logout Option

　　Network Working Group Mark Crispin

　　Request for Comments 727 MIT-AI

　　NIC 40025 27 April 1977

　　TELNET Logout Option

　　1. Command name and code.

　　LOGOUT 18

　　2. Command meanings.

　　IAC WILL LOGOUT

　　The sender of this command REQUESTS permission to, or confirms　that it will, forcibly log off the user process at its end.

　　IAC WON'T LOGOUT

　　The sender of this command REFUSES to forcibly log off the user　process at its end.

　　IAC DO LOGOUT

　　The sender of this command REQUESTS that the receiver forcibly log　off the user process at the receiver's end, or confirms that the　receiver has its permission to do so.

　　IAC DON'T LOGOUT

　　The sender of this command DEMANDS that the receiver not forcibly　log off the user process at the receiver's end.

　　3. Default.

　　WON'T LOGOUT

　　DON'T LOGOUT

　　i.e., no forcible logging off of the server's user process.

　　4. Motivation for the option.

　　Often, a runaway user process could be hung in such a state that it　cannot be interrupted by normal means. Conversely, the system itself　could be bottlenecked so that response delays are intolerable. A　user (human or otherwise) eventually will time out out of frustration　and take the drastic means of closing the connection to free itself　from the hung process. In some situations, even the simple operation　of logging out can take a long time.

　　Some systems treat a close to mean that it should log out its user　process under it. However, many hosts merely "detach" the process so　that an accidental close due to a user or temporary hardware error　will not cause all work done on that job to be lost; when the　connection is re-established, the user may "attach" back to its　process. While this protection is often valuable, if the user is　giving up completely on the host, it can cause this hung job to　continue to load the system.

　　This option allows a process to instruct the server that the user　process at the server's end should be forcibly logged out instead of　detached. A secondary usage of this option might be for a server to　warn of impending auto-logout of its user process due to inactivity.

　　5. Description of the option.

　　When a user decides that it no longer wants its process on the server　host and decides that it does not want to wait until the host's　normal log out protocol has been gone through, it sends IAC DO　LOGOUT. The receiver of the command may respond with IAC WILL　LOGOUT, in which case it will then forcibly log off the user process　at its end. If it responds with IAC WON'T LOGOUT, then it indicates　that it has not logged off the user process at its end, and if the　connection is broken, the process very possibly will be detached.

　　A truly impatient user that feels that it must break away from the　server immediately could even send IAC DO LOGOUT and then close. At　the worst, the server would only ignore the request and detach the　user process. A server that implements the LOGOUT option should know　to log out the user process despite the sudden close and even an　inability to confirm the LOGOUT request!

　　6. A sample implementation of the option.

　　The server implements the LOGOUT option both for accepting LOGOUT　requests and for auto-logout warning.

　　Case 1:

　　The user connects to the server, and starts interacting with the　server. For some reason, the user wishes to terminate interaction　with the server, and is reluctant to go through the normal log out　procedure, or perhaps the user is unable to go through the normal　log out procedure. It does not want the process at the server any　more, so it sends IAC DO LOGOUT. The server verifies the request　with IAC WILL LOGOUT, and then forcibly logs off the user process　(perhaps by using a system call that causes another process to be　logged out). It does not have to close the connection unless the　user closes or it wants to close. Neither does it wait until the　user has received its confirmation--it starts the log out　immediately so if the user has in the mean time closed the　connection without waiting for confirmation, its logout request　still is performed.

　　Case 2:

　　The user connects to the server, and after logging in, is idle for　a while, long enough to approach the server's autologout time.

　　The server shortly before the autologout sends IAC WILL LOGOUT;　the user sees this and sends IAC DON'T LOGOUT, and continues work　on the host. Nothing prevents the server from logging out the　user process if inactivity continues; this can be used to prevent　a malicious user from locking up a process on the server host by　the simple expedient of sending IAC DON'T LOGOUT every time it　sees IAC WILL LOGOUT but doing nothing else.