Routers that implement precedence-ordered queuing MUST IMPLEMENT, and other routers SHOULD IMPLEMENT, Lower Layer Precedence Mapping.
A router that implements Lower Layer Precedence Mapping:
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 queuing strategies to implement special services such as multimedia bandwidth reservation or low-delay service. Special services and queuing strategies to support them are current research subjects and are in the process of standardization.
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. 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.