Tackling the p's and q's of LAN traffic

Thanks to the IEEE's recent ratification of 802.1p as part of a newly revised 802.1D bridging standard, LAN switching has taken a step forward in the move toward supporting the convergence of voice, video and data.

802.1p is a specification for giving Layer 2 switches the ability to prioritize traffic and perform dynamic multicast filtering. Both are valuable for improving LAN performance, but prioritization is expected to have a more immediate impact.

The prioritization specification works at the media access control (MAC) framing layer of the Open Systems Interconnect model. To be compliant with 802.1p, Layer 2 switches must be capable of grouping incoming LAN packets into separate traffic classes.

Eight classes are defined by 802.1p. Although network managers must determine actual mappings, IEEE has made broad recommendations. The highest priority is seven, which might go to network-critical traffic such as Routing Information Protocol and Open Shortest Path First table updates.

Values five and six might be for delay-sensitive applications such as interactive video and voice. Data classes four through one range from controlled-load applications such as streaming multimedia and business-critical traffic - carrying SAP data, for instance - down to "loss eligible" traffic. The zero value is used as a best-effort default, invoked automatically when no other value has been set.

In operation, 802.1p calls for the use of priority fields within the packet to signal the switch of the priority-handling requirements.

Unlike other 802-defined protocols such as FDDI and token ring, Ethernet does not carry defined priority fields. To address this, 802.1p is able to reference the priority fields contained in a tag header being developed by the IEEE's 802.1Q working group, which is focusing on virtual LAN identification.

The 802.1Q specification defines a tag header consisting of 32 bits, all inserted after the packet header's normal Destination and Source addresses. Of these, three priority bits are used for signaling 802.1p switches, a single bit identifies optional token-ring encapsulation, a 12-bit virtual LAN ID applies to virtual LAN membership, and 16 bits are used to modify the "Ether Type" frame.

The 802.1Q priority bits may be set by desktop systems, servers, routers or Layer 3 switches. In desktops and servers, 802.1p-compliant network interface cards set the bits by monitoring port and socket IDs to determine traffic priorities.

Routers and Layer 3 switches are able to differentiate among traffic priorities by inspecting packet payload information such as port number and service address. They use this information, coupled with network administrator-defined policies to classify any untagged packets.

As for Layer 2 switches, which can't "see" above the MAC layer, the 802.1p specification will give them the ability to recognize the packet priorities.

In order to act on traffic priorities, however, newer routers and switches will have to be capable of employing multiple buffer queues for each output port.

In a conventional single-buffer switch, when congestion occurs, all packets must wait their turn to move on. By contrast, a switch with multiqueue hardware can give higher priority packets fast handling inside the switch; some will actually overtake lower priority packets in the few milliseconds required to move through the switch.

The 802.1p specification anticipates multiqueue hardware by recommending how the eight traffic classes should be assigned in systems with two, three, four or more queues per port. Setting priority bits and implementing class-based forwarding are significant steps toward multimedia convergence and policy-enabled networking.

What's needed next will be directory-enabled networks that can provide for automated network configurations based on network manager-defined policies. For some, a valuable next step may also involve extending packet priorities through the WAN.

This may be performed through some interaction with the quality-of-service capabilities of ATM, or, in the case of IP packet networks, the Type of Service (TOS) subfields. TOS subfields were defined for IPv4 networks by the Internet Engineering Task Force in 1981 and are now undergoing a thorough redesign by the IETF Differentiated Services working group called Diff-Serv.

But for now, these technologies are in the earliest stages of proposal. What is real is the gathering momentum toward convergence in the LAN, strengthened considerably by the recent ratification of 802.1p.

Sherman is program manager in 3Com's technical marketing group. He can be reached at Sherman@nww.com.