IPv6 Network Administration

Quality of Service

Quality of service (QoS) refers to the ability to give guarantees that the network traffic you send gets there on time (there's a longer discussion of QoS in the section Section 3.10). One of the earliest approaches to guaranteeing a certain level of QoS in IPv4 networks was a field called "Type of Service" in the IP header. This field is included in the RFC 791 definition of IP, and clarified further in RFC 1349. In this model, the IPv4 header itself contains fields that are set to particular values depending on what kind of treatment the packets "want" from routers; the idea being that packets proclaiming themselves to be worthy of immediate forwarding will be plucked out of queues by routers and preferentially dealt with. Unfortunately this was a rather crude approach, not widely implemented, and in its first revision died a death. Perhaps the two biggest problems with it were that it provided no mechanism for authenticating the request for a particular QoS, and that there was no flexible way to assign priorities within a particular set of flows, such that certain ones could be designated lower priority. In essence, we have the rather contradictory result that a mechanism introduced to allow for more appropriate and fairer treatment of packets leads to unfairness!

There have been various efforts to retrofit more complete QoS features to IPv4, especially now that some people consider IP networks `mission critical' and others want to run their telephones over ...