On the effects of the IEEE 802.3x flow control in full-duplex Ethernet LANs

This paper presents ongoing work in the field of the IEEE 802.3x flow control scheme within a full-duplex Ethernet LAN. Our simulation-based investigations compare the throughput of TCP streams in selected network topologies when either the Ethernet flow control is disabled or enabled. In general, Ethernet's hop-by-hop flow control leads to good performance of TCP in homogenous networks where all clients are connected to a congested switch with the same link speed. However, as soon as a network consists of clients connected with slow (e.g. 10 Mb/s) and fast (e.g. 100 Mb/s) links to the switch, the effect known as 'head of line blocking' shows up as congestion occurs and flow control is triggered. As a result of this, the throughput of data streams destined to slow clients increases because packets are no longer discarded but the throughput of streams destined to fast clients is reduced considerably.