This ticket concerns connections incoming to a mongos or mongod – we call the incoming connection path the ingress layer. Before the ingress layer accepts a connection, this connection sits in the accept queue on the TCP layer.

The accept queue has a fixed size, and when the queue is full, the OS will start dropping incoming connections at the pre-mongo level. An overflowing queue means that either a mongo process is not accepting connections fast enough, or that the kernel is throttling connection establishment.

Of particular note are generic TCP metrics TcpExtListenOverflows and TcpExtListenDrops.