This section specifies requirements and guidelines for appropriate
processing of the IP Precedence field in routers. Precedence is a
scheme for allocating resources in the network based on the
relative importance of different traffic flows. The IP
specification defines specific values to be used in this field for
various types of traffic.
The basic mechanisms for precedence processing in a router are
preferential resource allocation, including both precedence-
ordered queue service and precedence-based congestion control, and
selection of Link Layer priority features. The router also
selects the IP precedence for routing, management and control
traffic it originates. For a more extensive discussion of IP
Precedence and its implementation see [FORWARD:6].
Precedence-ordered queue service, as discussed in this section,
includes but is not limited to the queue for the forwarding
process and queues for outgoing links. It is intended that a
router supporting precedence should also use the precedence
indication at whatever points in its processing are concerned with
allocation of finite resources, such as packet buffers or Link
Layer connections. The set of such points is implementation-
dependent.
DISCUSSION
Although the Precedence field was originally provided for use in
DOD systems where large traffic surges or major damage to the
network are viewed as inherent threats, it has useful applications
for many non-military IP networks. Although the traffic handling
capacity of networks has grown greatly in recent years, the
traffic generating ability of the users has also grown, and
network overload conditions still occur at times. Since IP-based
routing and management protocols have become more critical to the
successful operation of the Internet, overloads present two
additional risks to the network:
High delays may result in routing protocol packets being lost.
This may cause the routing protocol to falsely deduce a
topology change and propagate this false information to other
routers. Not only can this cause routes to oscillate, but an
extra processing burden may be placed on other routers.
High delays may interfere with the use of network management
tools to analyze and perhaps correct or relieve the problem
in the network that caused the overload condition to occur.
Implementation and appropriate use of the Precedence mechanism
alleviates both of these problems.
