Queuing algorithm to provide type-of-service for IP links
RFC 1046

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Network Working Group                                            W. Prue
Request for Comments:  1046                                    J. Postel
                                                                     ISI
                                                           February 1988

      A Queuing Algorithm to Provide Type-of-Service for IP Links

Status of this Memo

   This memo is intended to explore how Type-of-Service might be
   implemented in the Internet.  The proposal describes a method of
   queuing which can provide the different classes of service.  The
   technique also prohibits one class of service from consuming
   excessive resources or excluding other classes of service.  This is
   an "idea paper" and discussion is strongly encouraged.  Distribution
   of this memo is unlimited.

Introduction

   The Type-of-Service (TOS) field in IP headers allows one to chose
   from none to all the following service types; low delay, high
   throughput, and high reliability.  It also has a portion allowing a
   priority selection from 0-7.  To date, there is nothing describing
   what should be done with these parameters.  This discussion proposes
   an approach to providing the different classes of service and
   priorities requestable in the TOS field.

Desired Attributes

   We should first consider how we want these services to perform.  We
   must first assume that there is a demand for service that exceeds
   current capabilities.  If not, significant queues do not form and
   queuing algorithms become superfluous.

   The low delay class of service should have the ability to pass data
   through the net faster than regular data.  If a request is for low
   delay class of service only, not high throughput or high reliability,
   the Internet should provide low delay for relatively less throughput,
   with less than high reliability.  The requester is more concerned
   with promptness of delivery than guaranteed delivery.  The Internet
   should provide a Maximum Guaranteed Delay (MGD) per node, or better,
   if the datagram successfully traverses the Internet.  In the worst
   case, a datagram's arrival will be MGD times the number of nodes
   traversed.  A node is any packet switching element, including IP
   gateways and ARPANET IMP's.  The MGD bound will not be affected by
   the amount of traffic in the net.  During non-busy hours, the delay
   provided should be better than the guarantee.  If the delay a

Prue & Postel                                                   [Page 1]
RFC 1046                Type-of-Service Queuing            February 1988

   satellite link introduces is less than the MGD, that link should be
   considered in the route.  If however, the MGD is less than the
   satellite link can provide, it should not be used.  For this
   discussion it is assumed that delay for individual links are low
   enough that a sending node can provide the MGD service.

   Low delay class of service is not the same as low Round Trip Time
   (RTT).  Class of service is unidirectional.  The datagrams responding
   to low delay traffic (i.e., Acking the data) might be sent with a
   high reliability class of service, but not low delay.

   The performance of TCP might be significantly improved with an
   accurate estimate of the round trip time and the retransmission
   timeout.  The TCP retransmission timeout could be set to the maximum
   delay for the current route (if the current route could be
   determined).  The timeout value would have to be redetermined when
   the number of hops in the route changes.

   High throughput class of service should get a large volume of data
   through the Internet.  Requesters of this class are less concerned
   with the delay the datagrams have crossing the Internet and the
   reliability of their delivery.  This type of traffic might be served
   well by a satellite link, especially if the bandwidth is high.
   Another attribute this class might have is consistent one way
   traversal time for a given burst of datagrams.  This class of service
   will have its traversal times affected by the amount of Internet
   load.  As the Internet load goes up, the throughput for each source
   will go down.

   High reliability class of service should see most of its datagrams
   delivered if the Internet is not too heavily loaded.  Source Quenches
   (SQ) should not be sent only when datagrams are discarded.  SQs
   should be sent well before the queues become full, to advise the
   sender of the rate that can be currently supported.

   Priority service should allow data that has a higher priority to be
   queued ahead of other lower priority data.  It is important to limit
   the amount of priority data.  The amount of preemption a lower
   priority datagram suffers must also be limited.

   It is assumed that a queuing algorithm provides these classes of
   service.  For one facility to be used over another, that is, making
   different routing decisions based upon the TOS, requires a more