 or bandwidth sensitive. The primary goal was to in all of them). Table 3.2-7 illustrates the five layers of establish connectivity across a wide variety of systems the TCP/IP stack and the positioning of the key QoS and devices using a robust, inexpensive network. The mechanisms. philosophy was to assume that the network would do its Note that some of these mechanisms are inherent in best to deliver all messages that were submitted but with the protocols (e.g., priority based on physical ports, de- no guarantees of delivery or sequencing. This "best ef- cisions based on Layer 4 port numbers) rather than being forts" approach is quite satisfactory for basic data com- explicitly added on for QoS control. munications (and hence the popularity of the Internet) Most companies have become dependent on their but has proved less suitable for networks that must also networks. Who, for example, would be able to prosper in accommodate voice and video traffic (i.e., multiservice Network infrastructure CHAPTER 3.2 networking). The introduction of IP-based QoS mecha- administrator, and represents a tradeoff between audio nisms and the extension of network management sys- reproduction quality, audio path delay (latency), audio tems is an essential step in the transition to the new, loss, and network resource consumption. The network converged network infrastructure. Multiservice traffic is administrator is able to effect this tradeoff by means of difficult to handle efficiently by very nature, especially in audio codec selection and invocation of network priori- resource-limited environments. tization through the Diff-Serv capability [RFC1889 The goal of the QoS strategy is to provide tools and RFC1890], and/or the IEEE 802.1p/C MAC-layer pri- mechanisms that allow the network administrator the oritization and segregation. Use of Diff-Serv and/or means to optimize the performance of the network. This 802.1p/Q priorities applies to either or both ends of often involves tradeoffs between competing factors. For a voice path: The 