This paper presents XCP, a novel congestion control protocol that outperforms TCP in both conventional and high bandwidth-delay environments. XCP generalizes the Explicit Congestion Notification (ECN) proposal and introduces the concept of decoupling utilization control from fairness control. This allows for more flexible and analytically tractable protocol design and opens new avenues for service differentiation. XCP uses a control theory framework to model and demonstrate its stability and efficiency regardless of link capacity, round trip delay, or number of sources. Extensive packet-level simulations show that XCP achieves high utilization, small queue sizes, and near-zero packet drops, with both steady and highly varying traffic. XCP does not maintain any per-flow state in routers and requires few CPU cycles per packet, making it implementable in high-speed routers. XCP's design has several additional advantages, including the ability to distinguish error losses from congestion losses, facilitating the detection of misbehaving sources, and providing an incentive for both end users and network providers to deploy the protocol. XCP's performance is robust to estimation errors and is less aggressive as the round trip delay increases. The protocol's parameters are constant and independent of the number of sources, delay, and capacity of the bottleneck. XCP's stability analysis shows that the system is stable independently of delay, capacity, and number of sources when the parameters α and β satisfy certain conditions. XCP's performance is demonstrated through extensive simulations showing that it outperforms TCP in both conventional and high bandwidth-delay environments. XCP is robust to highly varying traffic demands and high variance in flows' round trip times. XCP is also robust to sudden increases or decreases in traffic demands and maintains high utilization even when flows are suddenly stopped. XCP provides a flexible framework for designing a variety of bandwidth allocation schemes, including weighted fairness, proportional fairness, and priority.This paper presents XCP, a novel congestion control protocol that outperforms TCP in both conventional and high bandwidth-delay environments. XCP generalizes the Explicit Congestion Notification (ECN) proposal and introduces the concept of decoupling utilization control from fairness control. This allows for more flexible and analytically tractable protocol design and opens new avenues for service differentiation. XCP uses a control theory framework to model and demonstrate its stability and efficiency regardless of link capacity, round trip delay, or number of sources. Extensive packet-level simulations show that XCP achieves high utilization, small queue sizes, and near-zero packet drops, with both steady and highly varying traffic. XCP does not maintain any per-flow state in routers and requires few CPU cycles per packet, making it implementable in high-speed routers. XCP's design has several additional advantages, including the ability to distinguish error losses from congestion losses, facilitating the detection of misbehaving sources, and providing an incentive for both end users and network providers to deploy the protocol. XCP's performance is robust to estimation errors and is less aggressive as the round trip delay increases. The protocol's parameters are constant and independent of the number of sources, delay, and capacity of the bottleneck. XCP's stability analysis shows that the system is stable independently of delay, capacity, and number of sources when the parameters α and β satisfy certain conditions. XCP's performance is demonstrated through extensive simulations showing that it outperforms TCP in both conventional and high bandwidth-delay environments. XCP is robust to highly varying traffic demands and high variance in flows' round trip times. XCP is also robust to sudden increases or decreases in traffic demands and maintains high utilization even when flows are suddenly stopped. XCP provides a flexible framework for designing a variety of bandwidth allocation schemes, including weighted fairness, proportional fairness, and priority.