Original Publication Date: 1981-Sep-01
Included in the Prior Art Database: 2019-Feb-14
Internet Society Requests For Comment (RFCs)
Network Working Group V. Cerf Request for Comments: 794 ARPA Replaces: IEN 125 September 1981 PRE-EMPTION
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
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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 att...