RFC 794 (rfc794) - Page 1 of 2


Pre-emption



Alternative Format: Original Text Document



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


Cerf