Since Envoy is fundamentally written as a L3/L4 server, basic L3/L4 proxy is easily implemented. The TCP proxy filter performs basic 1:1 network connection proxy between downstream clients and upstream clusters. It can be used by itself as an stunnel replacement, or in conjunction with other filters such as the MongoDB filter or the rate limit filter.
The TCP proxy filter will respect the connection limits imposed by each upstream cluster’s global resource manager. The TCP proxy filter checks with the upstream cluster’s resource manager if it can create a connection without going over that cluster’s maximum number of connections, if it can’t the TCP proxy will not make the connection.
TCP proxy filter configuration reference.