In circuit-switching systems, once a user has acquired a circuit, the communication bandwidth of that circuit is dedicated, even if it is not used. When the system saturates, additional circuit set-up requests are blocked. To allow high precedence users to gain access to circuit resources, systems such as AUTOVON associate a precedence with each telephone instrument. Those instruments with high precedence can pre-empt circuit resources, causing lower precedence users to be cut off.

In message switching systems such as AUTODIN I, incoming traffic is
stored on disks  (or drums or tape) and processed in order of
precedence.  If a high precedence message is entered into the system, it
is processed and forwarded as quickly as possible.  When the high
precedence message arrives at the destination message switch, it may
pre-empt the use of the output devices on the switch, interrupting the
printing of a lower precedence message.

In packet switching systems, there is little or no storage in the transport system so that precedence has little impact on delay for processing a packet. However, when a packet switching system reaches saturation, it rejects offered traffic. Precedence can be used in saturated packet switched systems to sort traffic queued for entry into the system.

In general, precedence is a tool for deciding how to allocate resources when systems are saturated. In circuit switched systems, the resource is circuits; in message switched systems the resource is the message switch processor; and in packet switching the resource is the packet switching system itself.

This capability can be realized in AUTODIN II without adding any new mechanisms to TCP (except to make precedence of incoming connection requests visible to the processes which use TCP). To allow pre-emptive access to a particular terminal, the software (i.e., THP) which supports terminal access to the TAC can be configured so as to always have a LISTEN posted for that terminal, even if the terminal has a connection in operation. For example in the ARPANET TENEX systems, the user TELNET permits a user to have many connections open at one time - the user can switch among them at will. To the extent that this can be done without violating security requirements, one could imagine a multi-connection THP which always leaves a LISTEN pending for incoming connection requests. If a connection is established, the THP can decide, based on its precedence, whether to pre-empt any existing connection and to switch the user to the high precedence one.

If the user is working with several connections of different precedence at the same time, the THP would close or abort the lowest precedence

                                                          September 1981
Pre-Emption

connection in favor of the higher precedence pre-empting one. Then the THP would do a new LISTEN on that terminal's port in case a higher precedence connection is attempted.

One of the reasons for suggesting this model is that processes are the users of TCP (in general) and that TCP itself cannot cause processes to be created on behalf of an incoming connection request. Implementations could be realized in which TCPs accept incoming connection requests and, based on the destination port number, create appropriate server processes. In terms of pre-empting access to a remote terminal, however, it seems more sensible to let the process which interfaces the terminal to the system mediate the pre-emption. If the terminal is not connected or is turned off, there is no point in creating a process to serve the incoming high precedence connection request.

For example, suppose a routine FTP is in operation between Host X and Host Y. Host Z decides to do a flash-override FTP to Host X. It opens a high precedence connection via its TCP and the "SYN" goes out to the FTP port on Host X.

FTP always leaves one LISTEN pending to pre-empt lower precedence remote users if it cannot serve one more user (and still keep a LISTEN pending). In this way, the FTP is naturally in a state permitting the high precedence connection request to be properly served, and the FTP can initiate any cleaning up that is needed to deal with the pre-emption.

In general, this strategy permits the processes using TCP to accommodate pre-emption in the context of the applications they support.

A non-pre-emptable process is one that does not have a LISTEN pending while it is serving one (or more) users.

The actions taken to deal with pre-emption of TCP connections will be application-process specific and this strategy of a second (or N+1st) LISTEN is well suited to the situation.

Pre-emption may also be necessary at the site initiating a high precedence connection request. Suppose there is a high precedence user who wants to open an FTP connection request from Host Z to Host X. But all FTP and/or TCP resources are saturated when this user tries to start the user FTP process. In this case, the operating system would have to know about the precedence of the user and would have to locally pre-empt resources on his behalf (e.g., by logging out lower precedence users). This is a system issue, not specific only to TCP. Implementation of pre-emption at the source could vary greatly. Precedence may be associated with a user or with a terminal. The TCP implementation may locally pre-empt resources to serve high precedence users. The operating system may make all pre-emption decisions.