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# TCP transport
The {{i:*tcp* transport}}{{hi:*tcp*}} provides communication support between
sockets across a {{i:TCP/IP}} network.
Both IPv4 and {{i:IPv6}} are supported when the underlying platform also supports it.
## URI Format
This transport uses URIs using the scheme {{i:`tcp://`}}, followed by
an IP address or hostname, followed by a colon and finally a
TCP {{i:port number}}.
For example, to contact port 80 on the localhost either of the following URIs
could be used: `tcp://127.0.0.1:80` or `tcp://localhost:80`.
A URI may be restricted to IPv6 using the scheme `tcp6://`, and may
be restricted to IPv4 using the scheme `tcp4://`.
> [!NOTE]
> Specifying `tcp6://` may not prevent IPv4 hosts from being used with
> IPv4-in-IPv6 addresses, particularly when using a wildcard hostname with
> listeners.
> The details of this varies across operating systems.
> [!NOTE]
> Both `tcp6://` and `tcp4://` are specific to _NNG_, and might not
> be understood by other implementations.
> [!TIP]
> We recommend using either numeric IP addresses, or names that are
> specific to either IPv4 or IPv6 to prevent confusion and surprises.
When specifying IPv6 addresses, the address must be enclosed in
square brackets (`[]`) to avoid confusion with the final colon
separating the port.
For example, the same port 80 on the IPv6 loopback address (`::1`) would
be specified as `tcp://[::1]:80`.
The special value of 0 ({{i:`INADDR_ANY`}})
can be used for a listener to indicate that it should listen on all
interfaces on the host.
A short-hand for this form is to either omit the address, or specify
the asterisk (`*`) character.
For example, the following three URIs are all equivalent,
and could be used to listen to port 9999 on the host:
1. `tcp://0.0.0.0:9999`
2. `tcp://*:9999`
3. `tcp://:9999`
The entire URI must be less than `NNG_MAXADDRLEN` bytes long.
## Socket Address
When using an [`nng_sockaddr`][sockaddr] structure,
the actual structure is either of type
[`nng_sockaddr_in`][sockaddr_in] (for IPv4) or
[`nng_sockaddr_in6`][sockaddr_in6] (for IPv6).
## Transport Options
The following transport options are supported by this transport,
where supported by the underlying platform.
- [`NNG_OPT_LOCADDR`][NNG_OPT_LOCADDR]
- [`NNG_OPT_REMADDR`][NNG_OPT_REMADDR]
- [`NNG_OPT_TCP_KEEPALIVE`][NNG_OPT_TCP_KEEPALIVE]
- [`NNG_OPT_TCP_NODELAY`][NNG_OPT_TCP_NODELAY]
- [`NNG_OPT_URL`][NNG_OPT_URL]
{{#include ../refs.md}}
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