PQ ensures that important traffic gets the fastest handling at each point where it is used. It was designed to give strict priority to important traffic. Priority queuing can flexibly prioritize according to network protocol (for example IP, IPX, or AppleTalk), incoming interface, packet size, source/destination address, and so on. In PQ, each packet is placed in one of four queues-high, medium, normal, or low-based on an assigned priority. Packets that are not classified by this priority list mechanism fall into the normal queue (see Figure: Priority Queuing Places Data into Four Levels of Queues: High, Medium, Normal, and Low ). During transmission, the algorithm gives higher-priority queues absolute preferential treatment over low-priority queues.
As of 2005 [update] , six volumes of the second generation proof have been published (Gorenstein, Lyons & Solomon 1994 , 1996 , 1998 , 1999 , 2002 , 2005 ). In 2012 Solomon estimated that the project would need another 5 volumes, but said that progress on them was slow. It is estimated that the new proof will eventually fill approximately 5,000 pages. (This length stems in part from second generation proof being written in a more relaxed style.) Aschbacher and Smith wrote their two volumes devoted to the quasithin case in such a way that those volumes can be part of the second generation proof.