Routes and interfaces

In order to communicate with a remote host through a socket, a protocol must establish:

• the IP address and port of the socket

• a route to the remote host

• a network interface by which to access that route

The actions to do this are performed when:

• a TCP socket is connected with RSocket::Connect()

• a UDP socket is written to with RSocket::Send(), RSocket::Write(), or RSocket::SendTo()

• a UDP socket is ‘connected’ with RSocket::Connect()

The standard steps are as follows:

1. The IP routing table is checked to see if a route to the destination exists. If there is no route (including no default route), then the protocol attempts to invoke its default dial-up interface.

2. If dial-up is successful, PPP is invoked to establish a connection to the remote peer.

3. In its set-up stage, PPP obtains a local IP address from the remote peer (if so configured) and is informed of the peer’s IP address.

4. When TCP/IP is informed that the interface is up, it adds routing entries for the interface. Normally this means that the remote peer becomes the default gateway.

5. If a local address has not been set by RSocket::Bind(), then the protocol sets the socket’s local address to that obtained in step 3 (which is recorded in the route entry created in step 4), and allocates an ephemeral port number. This process is called automatic binding.