This memo defines a Time Sliding Window Three Colour Marker (TSWTCM), which can be used as a component in a Diff-Serv traffic conditioner [RFC2475, RFC2474]. The marker is intended to mark packets that will be treated by the Assured Forwarding (AF) Per Hop Behaviour (PHB) [AFPHB] in downstream routers. The TSWTCM meters a traffic stream and marks packets to be either green, yellow or red based on the measured throughput relative to two specified rates: Committed Target Rate (CTR) and Peak Target Rate (PTR).

Much industry attention has recently been focused on providing differentiated levels of service to users on IP networks. One such proposal is the RIO scheme proposed by Clark [4]. RIO is an extension of the RED algorithm that relies on a differentiated drop treatment during congestion to cause different levels of service. The end result of differentiated dropping of packets during congestion is differentiated throughput rates for end-users. The IETF's Diffserv Working Group is currently discussing a similar differentiated drop mechanism called the AF (Assured Forwarding) PHB (Per Hop Behaviour).This paper raises issues with providing bandwidth assurance for TCP flows in a RIO-enabled Differentiated Services network. The main contribution is a detailed experimental study of five different factors that impact throughput assurances for TCP and UDP flows in such a network. Our study demonstrates that these factors can cause different throughput rates for end-users in spite of having contracted identical service agreements.

Issues related to bandwidth assurance in Assured Forwarding based Differentiated Services (Diffserv) networks have been discussed in recent research papers [7][8][11]. Some of the factors that can bias bandwidth assurance are Round Trip Time (RTT), UDP/TCP interaction and different target rates. The bias due to these factors needs to be mitigated before bandwidth assurance for a paying customer can be articulated in Service Level Agreements (SLAs). This paper proposes intelligent traffic conditioning approaches at the edge of the network to mitigate the effect of Round Trip Time, UDP/TCP interactions, and different target rates. The simulation results show a significant improvement in bandwidth assurance with intelligent traffic conditioning. The limitation of the proposed solutions is that they require communication between edge devices. In addition, these solutions are not applicable for a one-to-any network topology.

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