One of the consequential new features of emerging datacenter networks is losslessness, achieved by means of Priority Flow Control (PFC). Despite PFC's key role in the datacenter and its increasing availability - supported by virtually all Converged Enhanced Ethernet (CEE) products - its impact remains largely unknown. This has motivated us to evaluate the sensitivity of three widespread TCP versions to PFC, as well as to the more involved Quantized Congestion Notification (QCN) congestion management mechanism. As datacenter workloads we have adopted several representative commercial and scientific applications. For evaluation we employ an accurate Layer 2 CEE network simulator coupled with a TCP implementation extracted from FreeBSD v9. A somewhat unexpected outcome of this investigation is that PFC significantly improves TCP performance across all tested configurations and workloads, hence our recommendation to enable PFC whenever possible. In contrast, QCN can help or harm depending on its parameter settings, which are currently neither adaptive nor universal for datacenters. To the best of our knowledge this is the first performance evaluation of TCP performance in lossless CEE networks. © 2011 IEEE.