<?xml version='1.0'?>

<reference anchor='RFC0794'>

<front>
<title>Pre-emption</title>
<author initials='V.' surname='Cerf' fullname='V. Cerf'>
<organization>Advanced Research Projects Agency (ARPA)</organization></author>
<date month='September' day='1' year='1981' />
<abstract>
<t>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.</t>
<t>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.</t></abstract></front>

<seriesInfo name='RFC' value='794' />
</reference>
