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Packet-switched technologies are well suited for data communications, which, at first, did not care as much about timely delivery or resource reservation. With packet-switched technologies, the faith of packets is entrusted entirely to the best efforts and good will of the network. Although not reassuring, the approach is more flexible and allowed these technologies to develop faster than the circuit-switched ones. Besides being capable of multiplexing the user traffic, they offer features that could easily enable new, revenue-generating services on existing networks. The success of packet switching led to an ever-expanding set of applications leveraging IP. It was not long before time-sensitive applications were IP enabled, and they required special handling of their packets, especially under congestion conditions.
Because not all applications have the same requirements, the IP infrastructure must be enabled to offer different levels of service for their traffic. Services can be grouped in several types depending on packet drop, delay, and delay-variation constraints. These constraints are driven by applications such as interactive voice communications, audio, and video that are sensitive to delay and delay variations (jitter), but they can afford to lose randomly a small percentage of the traffic. At the other end of the spectrum is the mission-critical application that needs reliable, no-loss data transfers while placing a lower emphasis on timing requirements. The service level is an end-to-end concept that qualifies a mode of handling traffic based on its various characteristics such as type, content, source, and destination. Deploying QoS in a network enables it to support various levels of service. For some, the well-being of their network and proper support of deployed applications depend on it. For others, QoS is a revenue source based on contractual agreements called service level agreements.
IPv4 QoS is implemented through two architectures. Integrated Services (IntServ, as in RFC 1633, was modeled based on the concept of circuit). In this case, the user, with the help of a signaling protocol called Resource Reservation Protocol (RSVP; RFC 2208), reserves the resources end to end before sending the data. Differentiated Services (DiffServ), described in RFC 2475 and RFC 3260, is the second architecture, and it relies on the information carried within each packet to make resource-allocation decisions at each network node. This approach is simpler and more dynamic; however, it requires a good understanding of the traffic profiles in the network and a consistent end-to-end implementation of policies. DiffServ is a widely adopted QoS deployment architecture. Although the QoS concepts are reviewed in Chapter 5 “Implementing QoS,” in-depth information on them and their deployment strategies can be found in various specialized books such as IP Quality of Service, a Cisco Press book by Srinivas Vegesna.
With the dramatic increase of traffic transported over packet networks and the demands of the various services and applications supported, it is difficult to understate the value of QoS in today’s IPv4 networks. The same holds true for IPv6. Some hoped for an improved QoS in the next generation of the IP protocol, and some still believe that IPv6 is better than IPv4 in this respect. The reality is that neither evolutionary nor revolutionary changes were made to IPv6 versus IPv4. Some claim radical QoS improvements in IPv6. These are but a myth at the time of this writing. The same concepts and architectures apply to the new protocol with few and insignificant differences that are discussed in Chapter 5 of this book. An additional packet header field (Flow Label) is believed to hold the potential for improving the QoS mechanisms, but the means to use it are still being evaluated.
Although there appears to be no significant differences between IPv4 and IPv6 QoS today, things might shape up differently going forward. IPv6′s aim to reestablish a peer-to-peer model for IP transport will most definitely change the current approaches on deploying QoS. IPv6 networks hold the potential to implement true end-to-end service level policies. But for now, IPv6 is content to follow in the footsteps of its predecessor.
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