Browse Prior Art Database

Pre-emption (RFC0794)

IP.com Disclosure Number: IPCOM000003843D
Original Publication Date: 1981-Sep-01
Included in the Prior Art Database: 2000-Sep-13
Document File: 2 page(s) / 6K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

V.G. Cerf: AUTHOR

Abstract

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.

This text was extracted from a ASCII document.
This is the abbreviated version, containing approximately 47% of the total text.

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...