P4-enabled Network-assisted Congestion Feedback: A Case for NACKs

There exists an extensive body of work, spanning more than two decades, on congestion control schemes and signaling mechanisms. The majority of prior work does not, however, entertain the notion of network-assisted feedback for congestion control. The scope of the remaining work has also been, unfortunately, rather narrow: Some efforts limit themselves to using weak signals (involving a few bits in the header) and relying on receivers to reflect such signals to the sender; few others maintain per-flow statistics or explicitly set the rates the senders should use. Virtually all suggested network-assisted congestion feedback mechanisms are ineffective, not scalable, or limited to data-center contexts. In this proposal, we exploit data-plane programmability of P4 switches as well as hardware-supported priority classes to present a novel network-assisted congestion feedback (NCF) mechanism. The feedback entails NACKs that are directly sent to the sender, and does not involve the receiver; it is, hence, quick and efficient. We propose sending such NACKs during periods of congestion to senders of elephant flows and outline a scalable approach to identify elephant flows. Unlike prior work, NCF is applicable to both data-centers as well as Internet-wide.