This paper says, if we require a network to be not oversubscribed, the cost is high. For example, if we use fat-tree topology, the networking cost is 2-3x of a tree topology. Using VL2 is even 4-5x. However, it is not necessary to prevent oversubscription because in most of the time, the network is not widely congested.

The paper shows some data collected in their lab (Microsoft) using map-reduce machine learning computation. It finds that,

  • largest demand (source-destination pair) contributes around 0.5% of total traffic
  • demand size is mostly within ±80% of the mean
  • 55% of traffic by ToR switch is to only 10 other ToR switch
  • top 10 ToR switches account for 95% of total traffic

therefore, it is reasonable to make by-pass between hot ToR switches to offload traffic from the main network. In this case the performance will be comparable to that of non-oversubscribed network without the high cost.

The paper proposes two methods to make flyways: Use 60 GHz wireless channel, which allows small antenna (1 inch) and high capacity but short range (within 10m); or use wired channel such as a gigabit switch connection some of the ToRs, but this lacks flexibility.

Bibliographic data

   title = "Flyways To De-Congest Data Center Networks",
   author = "Srikanth Kandula and Jitendra Padhye and Paramvir Bahl",
   booktitle = "Proc Hot Nets VIII",
   year = "2009",