Yet, while the drafts' advances provide prioritization services to the most popular Layer 2 topology, they do not necessarily provide end-to-end prioritization across your entire network. In particular, 802.1Q and 802.1p won't help you prioritize the Layer 3 IP traffic traveling across your low-speed WAN and ISP connections--the points in your overall infrastructure where bottlenecks usually occur.

To handle network congestion across your entire network, you must first provide for the prioritization of IP traffic. Doing this effectively raises a series of design questions. Does your internal network support IP-prioritization services? Does your WAN equipment? What about your Internet service provider? What about the infrastructure at the other end of the connection? If any device between two systems cannot provide IP-prioritization services, you won't be able to implement an end-to-end solution.

The Basics of IP Prioritization

Unlike Ethernet, IP has had prioritization capabilities ever since version 4 was first published in 1981. Every IP packet has an 8-bit field--called the ToS (Type of Service) byte--that consists of two subfields. These subfields consist of a 3-bit precedence field used for prioritization as well as a 4-bit field for the specific ToS desired by this packet (the remaining bit is unused).

By using 3 bits for precedence, IP has the same eight levels of prioritization--0 through 7--as 802.1p, 802.1Q and most of the other LAN topologies. By implementing a cross-mapping service, it becomes feasible to provide a one-to-one mapping between 802.1Q Ethernet frames and the IP precedence, letting you build a network that carries prioritization from one Ethernet LAN to another across an IP-based WAN or ISP connection. Many router vendors say they will be implementing this functionality in the coming months as they roll out their 802.1Q-compliant products.

The remaining 4 bits of ToS information let administrators implement per-packet routing based on the characteristics of the packet's data. Thus, an NNTP (Network News Transfer Protocol) packet that carries UseNet news traffic can be marked as a low-cost service, while a telnet packet can be marked as a low-latency service .

Originally, only three types of services were defined in RFC 791, the original basis of the IP standard. These services were identified with unique bits that were either on or off, depending on whether the specific ToS was desired. However, this setup was modified by RFC 1349, which added a fourth service to the mix; it also stated that the bits were to be numerically additive rather than distinct entities. Because they were additive, the four bits provided for a maximum of 16 possible values--0 through 15.

Network managers handling complex networks with multiple routing paths can use these ToS flags in conjunction with ToS-based routing protocols such as OSPF to provide specific routing services across their networks.