6.3.2.3 TCP Port Publishing

TCP port publishing is the default protocol used by docker run -p, appropriate for the large majority of typical network services — web servers, databases, APIs — that communicate over TCP rather than UDP.

Publishing a TCP Port

Without specifying a protocol explicitly, a published port defaults to TCP, which is correct for most common services.

docker run -d -p 8080:80 nginx:alpine

This is equivalent to explicitly specifying TCP, since it is the implicit default protocol.

Explicitly Specifying TCP

The protocol can also be stated explicitly, which can improve clarity even though it matches the default behavior.

docker run -d -p 8080:80/tcp nginx:alpine

Why Most Services Use TCP

TCP's connection-oriented, reliable delivery model suits most application protocols — HTTP, database wire protocols, most custom application protocols — which generally require reliable, ordered delivery of data, making TCP the natural default choice for port publishing.

docker run -d -p 5432:5432 postgres:16

PostgreSQL's wire protocol, like most database protocols, relies on TCP for reliable communication between client and server.

Verifying a TCP Port Mapping Works

Testing connectivity to a published TCP port confirms the mapping is functioning correctly.

curl http://localhost:8080

A successful response confirms the TCP port mapping correctly connects to the application listening inside the container.

Why TCP Port Publishing Matters

Because the overwhelming majority of containerized services communicate over TCP, understanding TCP port publishing (even as the unstated, implicit default) covers the vast majority of everyday port publishing needs, with UDP publishing reserved for the smaller set of services that specifically require it.