QoS at the IP layer, Part 1
 
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Last week, we introduced IEEE 802.1p and 802.1Q, which provide quality of service mechanisms for Ethernet. This time, we'd like to begin a discussion about different ways IP can provide QoS at Layer 3, focusing first on the type-of-service byte.
Every IP packet has an 8-bit field called the type-of-service (ToS) byte. The ToS byte has two subfields. The first subfield uses 3 bits called a precedence field. By using these 3 bits, the protocol can sustain eight levels of priority - coincidentally, the same number of priority levels offered by 802.1p and 802.1Q. By cross-mapping, IP can match one-for-one the priority values supported in Ethernet, enabling the network to continue the same level of service between Ethernet at Layer 2 and IP at Layer 3.
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The second ToS byte subfield has 4 bits, each of which contains additional information that can be used by the network to determine level of service. Each bit corresponds to one of four network characteristics: delay, throughput, routing and cost. Which bit is turned on determines how the network responds to the packet. For example:
* Delay - an active delay bit tells the router to choose a high speed to minimize delay.
* Throughput - an active throughput bit specifies that high capacity links should be used.
* Routing - an active routing bit directs routing protocols and network management applications to select fault-tolerant paths.
* Cost - an active cost bit is for low-priority applications and tells the network to choose the lowest cost path.
If no bit is activated in this subfield, then the IP packet is prioritized as " best effort. "
The ToS byte's eighth bit is unused. One more very important note: Unless ToS is supported by a service provider and ToS/QoS is backed by a service level agreement, then ToS doesn't really help much because the service provider will treat the IP packet just like any other best-effort transmission.
Next time, we'll take a look at another IP QoS technique called DiffServ.
RELATED LINKS
Steve Taylor is President of Distributed Networking Associates and Publisher/Editor-in-Chief of Webtorials.Com. For more detailed information on most of the topics discussed in this newsletter, connect to Webtorials.Com, the first Web site dedicated exclusively to market studies and technology tutorials in the Broadband Packet areas of Frame Relay, ATM, and IP.
Larry Hettick is an independent consultant, with 19 years of experience in telecommunications and data communications marketing and product management for service providers and equipment vendors. He can be reached at larry@larryhettick.com
You can reach the authors at taylor@webtorials.com or larry@larryhettick.com.
Convergence archive
Past newsletters.
Internet Engineering Task Force
RFC 1349 - Type of Service in the Internet Protocol Suite
Internet Engineering Task Force
Bear Stearns to monitor all-IP network
Network World, 11/26/01
