Sebastien Soudan, Romaric Guillier, Ludovic Hablot, Yuetsu Kodama,
Tomohiro Kudoh, Fumihiro Okazaki, Ryousei Takano and Pascale Primet
This work examines the interactions between layer 2 (Ethernet) switches and TCP in high bandwidth delay product networks. First, the behavior of a range of Ethernet switches when two long lived connections compete for the same output port is investigated. Then, we explored the impact of these behaviors on TCP protocol in long and fast networks (LFNs). Several conditions in which scheduling mechanisms introduce heavy unfair bandwidth sharing and loss burst which impact TCP performance are shown.
Packet scheduling algorithms for Ethernet equipments have been designed for heterogeneous traffics and highly multiplexed environments. Nowadays Ethernet switches are also used in situations where these assumptions can be incorrect such as grid environments. This work shows several conditions in which these scheduling mechanisms introduce heavy unfairness (or starvation) on large intervals (300 ms) and loss burst which impact TCP performance. These conditions correspond to situations where huge data movements occur simultaneously. It also shows that behaviors are different from switch to switch and not easily predictable. These observations offer some tracks to better understand layer interactions. They may explain some congestion collapse situations and why and how parallel transfers mixing packets of different connections take advantages over single stream transfers. We plan to pursue this investigation of layer two - layer four interaction and explore how to model it and better adapt control algorithms to fit the new applications requirements. We also plan to do the same precise measurements with flow control (802.3x) and sender-based software pacing enabled which both tend to avoid queue overflows.