Preparation for Data Center TCP (DCTCP) recitation

• Read Data Center TCP (DCTCP)
• Skim section 4 (Results)
• Closely observe figures 15 and 19

DCTCP customizes the TCP congestion control algorithm for data centers. It leverages the Explicit Congestion Notification (ECN) to obtain an early congestion feedback from routers/switches, before the queue drops packets. Further, DCTCP provides a smooth reaction to congestion, i.e., when congestion is limited, it reduces its congestion window by a small amount. In contrast, when congestion is severe, it reduces its congestion window by a large amount.

Read the first 3 sections of the paper and try to understand how DCTCP compares to TCP. Skim the Results section (i.e., section 4) to check that the empirical results match your expectations. In particular check Figures 15 and 19, which show the queue occupancy as a function of time, and number of sources.

The following slides summarize the paper and could be helpful in understanding the content: PPTx Version | PDF Version

Consider the following questions:

• How do the requirements of congestion control change as we move from the Internet to Data Centers?
• What is the incast problem?
• What particular features in the design of DCTCP cause the queues to be smaller than the case where the network uses TCP?
• What happens if we have two flows that share the same bottleneck but one of them uses TCP and the other uses DCTCP? In particular, which flow do you expect to have a higher throughput and why?

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