A simulation study of multi-color marking of TCP aggregates

Service Level Agreements (SLAs) are contracts signed between a provider and a customer to govern the amount of traffic that will be serviced. This work pinpoints an important problem faced by the Internet service provider (ISP) which is to be able to differentiate between the services given to aggregates of multiple TCP connections. The Metro-Ethernet access network, the Differentiated Services (DiffServ) architecture and the ATM reference model are three architectural models where edge routers perform traffic metering and coloring of aggregated flows according to the SLA. Finer color marking was suggested to improve differentiation quality. We observe that increasing the number of colors indeed provides a good differentiation between the aggregates according to the committed and the excess rates. We also show that the token bucket coloring policies, which are widely used for this purpose, prefer short packets and mark them with higher priority colors. The differentiation process is more difficult for the short TCP connections that remain in the slow start phase, than for the long connections that are usually in the congestion avoidance phase.