IP Precedence

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

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 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:

(1) 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.

(2) 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.

5.3.3.1 Precedence-Ordered Queue Service

Routers SHOULD implement precedence-ordered queue service.

Precedence-ordered queue service means that when a packet is selected for output on a (logical) link, the packet of highest precedence that has been queued for that link is sent. Routers that implement precedence-ordered queue service MUST also have a configuration option to suppress precedence-ordered queue service in the Internet Layer.

Any router MAY implement other policy-based throughput management procedures that result in other than strict

precedence ordering, but it MUST be configurable to suppress them (i.e., use strict ordering).

As detailed in Section [5.3.6], routers that implement

precedence-ordered queue service discard low precedence packets before discarding high precedence packets for congestion

control purposes.

Preemption (interruption of processing or transmission of a packet) is not envisioned as a function of the Internet Layer.

Some protocols at other layers may provide preemption features.

5.3.3.2 Lower Layer Precedence Mappings

Routers that implement precedence-ordered queueing MUST IMPLEMENT, and other routers SHOULD IMPLEMENT, Lower Layer Precedence Mapping.

A router which implements Lower Layer Precedence Mapping:

o MUST be able to map IP Precedence to Link Layer priority mechanisms for link layers that have such a feature defined.

o MUST have a configuration option to select the Link Layer’s default priority treatment for all IP traffic

o SHOULD be able to configure specific nonstandard mappings of IP precedence values to Link Layer priority values for each interface.

DISCUSSION:

Some research questions the workability of the priority features of some Link Layer protocols, and some networks may have faulty implementations of the link layer priority

mechanism. It seems prudent to provide an escape mechanism in case such problems show up in a network.

On the other hand, there are proposals to use novel queueing strategies to implement special services such as low-delay service. Special services and queueing strategies to support them need further research and experimentation before they are put into widespread use in the Internet.

Since these requirements are intended to encourage (but not force) the use of precedence features in the hope of

providing better Internet service to all users, routers supporting precedence-ordered queue service should default to maintaining strict precedence ordering regardless of the type of service requested.

Implementors may wish to consider that correct link layer mapping of IP precedence is required by DOD policy for TCP/IP systems used on DOD networks.

5.3.3.3 Precedence Handling For All Routers

A router (whether or not it employs precedence-ordered queue service):

(1) MUST accept and process incoming traffic of all precedence levels normally, unless it has been administratively

configured to do otherwise.

(2) MAY implement a validation filter to administratively restrict the use of precedence levels by particular

traffic sources. If provided, this filter MUST NOT filter out or cut off the following sorts of ICMP error messages:

Destination Unreachable, Redirect, Time Exceeded, and Parameter Problem. If this filter is provided, the

procedures required for packet filtering by addresses are

required for this filter also.

DISCUSSION:

Precedence filtering should be applicable to specific source/destination IP Address pairs, specific

protocols, specific ports, and so on.

An ICMP Destination Unreachable message with code 14 SHOULD be sent when a packet is dropped by the validation filter, unless this has been suppressed by configuration choice.

(3) MAY implement a cutoff function which allows the router to be set to refuse or drop traffic with precedence below a specified level. This function may be activated by management actions or by some implementation dependent heuristics, but there MUST be a configuration option to disable any heuristic mechanism that operates without human intervention. An ICMP Destination Unreachable message with code 15 SHOULD be sent when a packet is dropped by the cutoff function, unless this has been suppressed by configuration choice.

A router MUST NOT refuse to forward datagrams with IP precedence of 6 (Internetwork Control) or 7 (Network Control) solely due to precedence cutoff. However, other criteria may be used in conjunction with precedence cutoff to filter high precedence traffic.

DISCUSSION:

Unrestricted precedence cutoff could result in an unintentional cutoff of routing and control traffic.

In general, host traffic should be restricted to a value of 5 (CRITIC/ECP) or below although this is not a requirement and may not be valid in certain systems.

(4) MUST NOT change precedence settings on packets it did not originate.

(5) SHOULD be able to configure distinct precedence values to be used for each routing or management protocol supported (except for those protocols, such as OSPF, which specify which precedence value must be used).

(6) MAY be able to configure routing or management traffic precedence values independently for each peer address.

(7) MUST respond appropriately to Link Layer