QoS at the IP layer, Part 2
 
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Last time, we introduced the type-of-service byte as a way IP can support quality of service. Because ToS has limitations, especially when multiple networks are involved, the IETF has created another QoS mechanism called Differentiated Services, or DiffServ. DiffServ, which offers up to 64 service classes, reinterprets the values offered in the ToS byte.
The DiffServ model was developed to differentiate IP traffic so that the traffic's relative priority could be determined on a per-hop basis. By using DiffServ, traffic is classified based on priority. Then the traffic is forwarded using one of three IETF-defined per-hop behavior (PHB) mechanisms. This approach allows traffic with similar service characteristics to be passed with similar traffic guarantees across multiple networks, even if the multiple networks don't provide the same service the same way. This is an important feature because, as our readers know, the Internet is really a network of multiple service provider networks.
DiffServ replaces the first bits in the ToS byte with a differentiated services code point (DSCP). The DSCP is then mapped to the PHB. This technique allows service providers to control how the DSCP codepoints are mapped to PHBs, and each time a packet enters a network domain it may be re-marked.
DiffServ, as described in RFC 2475, allows for five categories of service differentiation. They are: Relative Priority Marking; Service Marking; Label Switching; Integrated Services/Resource Reservation Protocol; and Static per-hop Classification.
We'll continue our discussion of DiffServ and PHBs next week.
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.
IETF.org
RFC 2475 - An Architecture for Differentiated Services
IETF.org
