Oh Flow, Are Thou Happy? TCP Sendbuffer Advertising for Make Benefit of Clouds and Tenants

Datacenter networks have evolved from simple trees to multi-rooted tree topologies such as FatTree or VL2 that provide many paths between any pair of servers to ensure high performance under all traffic patterns. The standard way to load balance traffic across these links is Equal Cost Multipathing that randomly places flows on paths. ECMP may wrongly place multiple flows on the same congested link, wasting as much as 60% of total capacity in a worst case scenarios for FatTree networks. These networks need information about the traffic they route to avoid collisions, by steering it towards idle paths, or by creating more capacity on the fly between groups of hot racks. Additionally, ECMP creates uncertainty about the path a given flow has taken, making network debugging difficult.