Demystifying and Mitigating TCP Capping

Today's Internet user experience greatly depends on some user-perceived network metrics, such as throughput and latency. To improve these metrics, many Internet content providers build the content delivery network (CDN) to provide their services. Generally, CDNs adopt TCP as their transport protocol. A recent line of work improves TCP by proposing novel congestion control algorithms. However, we measure TCP performance in the production CDN and identify an interesting phenomenon termed TCP capping. When the flows experience TCP capping, the fixed-size receive window (rwnd) restricts these flows from fully utilizing network bandwidth. Through in-depth analysis, we demystify that the root cause of TCP capping is an inappropriate constraint on rwnd due to not considering the receiver's processing capability. To mitigate it, this paper proposes a server-side scheme Apollo and a client-side scheme Artemis for Internet content providers and users, respectively. Apollo probes the receiver's processing capability and assists the sender in packet sending. And Artemis adjusts the receive buffer in light of the receiver's processing capability. In our evaluation, compared to vanilla TCP, TCP (w/ Apollo) and TCP (w/ Artemis) shorten flow completion time by up to 91.8% and 94.9%, respectively.