.. _arch_overview_tcp_proxy: TCP proxy ========= 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 :ref:`MongoDB filter ` or the :ref:`rate limit ` filter. The TCP proxy filter will respect the :ref:`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 :ref:`configuration reference `.