Matching differentiated services PHBs to ATM service categories-the AF to VBR case

An important component of the differentiated services concept being developed by the IETF is traffic conditioning which includes the shaping of traffic at the boundaries of differentiated services domains, and also at interior nodes in order to match the requirements of particular link layer technologies. However shaping alters the characteristics of a traffic flow and is in danger of degrading individual flow's QoS, particularly where no per flow connection admission control is used, unless the source is able to respond to the shaping operation. The IETF's Assured Forwarding Specification defines a multi precedence level, probabilistic packet dropping mechanism to give controlled sources such as TCP the ability to respond to congestion. In this paper we propose combining this with a suitable shaping mechanism to enable controlled sources to respond to shaping operations. While we believe this to be generally applicable, we develop the idea in the specific context of differentiated services over an ATM link where traffic shaping at the ATM level is required to exploit ATM's QoS capability, for example using VBR. An experimental implementation is described, based on commercial equipment, which is used to demonstrate the essential features of an Assured Forwarding PHB with three levels of drop precedence. In particular we show that sources of traffic with different precedence levels can, simultaneously, be in different modes of the TCP control algorithm.