September 20, 2005
VoIP QoS II
Along with intserv, another service developed by IETF to fulfill the QoS requirements is diffserv. intserv is not really a built-in service as it requires per hop signaling and reservation of resources along the route that the data is going to take. Also, intserv does not support scalability of a network that is becoming increasingly complex. Differentiated Services or diffserv do not reserve bandwidth but accomplish the transfer of information by using a field in the IP header.
The operations of diffserv are detailed in RFC 2474, RFC 2475, and RFC 3260. Essentially, diffserv is used to distinguish and prioritize between Internet services. This can be done in a variety of ways such as priority levels based on price levels, application requirements, etc; network attributes such as jitter, latency, etc. Network services are provided based on the definitions given by the diffserv architecture. This enables the provision of the available bandwidth to the traffic streams. The packets are classed into specific types for routing purposes.
The packets move from the source to the destination based upon the markings on them. diffserv utilizes the Differentiated Services (DS) field in IPv4 and the Traffic Class field in the IPv6. diffserv uses six of the eight fields that are present in the DS field. The six bits are collectively referred to as the Differentiated Services Code Point (DSCP). DSCP allows for 64 Internet service distinctions. According to IANA, IETF will standardize 32 codepoints and the remaining codepoints will be used equally for local and experimental use and for probable standardized assignments as the need occurs.
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