  State-of-the-art congestion control algorithms for data centers alone do not
cope well with transient congestion and high traffic bursts. To help with
these, we revisit the concept of direct \emph{backward} feedback from switches
and propose Back-to-Sender (BTS) signaling to many concurrent incast senders.
Combining it with our novel approach to in-network caching, we achieve
near-source sub-RTT congestion signaling. Source Flow Control (SFC) combines
these two simple signaling mechanisms to instantly pause traffic sources, hence
avoiding the head-of-line blocking problem of conventional hop-by-hop flow
control. Our prototype system and scale simulations demonstrate that
near-source signaling can significantly reduce the message completion time of
various workloads in the presence of incast, complementing existing congestion
control algorithms. Our results show that SFC can reduce the
$99^{th}$-percentile flow completion times by $1.2-6\times$ and the peak switch
buffer usage by $2-3\times$ compared to the recent incast solutions.
