...
To solve the described problem of the black-box network structure, the packet headers can be labeled: the priority required during the packet processing is set in a header field and is kept over the whole path. In this case, all intermediate devices can put the incoming data in a queue according to the field values in which the priority is indicated. This require requires the development of standard protocols and the implementation of these protocols by the equipment manufacturers.
...
The processing priority of a packet can be set using the service fields of various network protocols. This article describes the use of the Ethernet and of the IPv4 protocol headers.
...