Application

After Mobile IP registration succeeds, the mobile will attempt to contact some sort of application server or proxy. The details of your particular application protocol are beyond the scope of this manual. Many applications are based on TCP, so that is used as an example here.

A typical TCP connection starts with a SYN (synchronize) segment from the mobile. The server then replies with a segment with both SYN and ACK (acknowledge) flags set, then each subsequent segment has the ACK flag set. The connection continues until each end sends a FIN (finish) segment and receives an ACK for it. In the example shown, the server starts the connection close; this is typical of protocols such as HTTP.

Either end can signal a connection error by sending a RST (reset) segment. This will typically cause the other end to start some sort of error handling. The error handling depends on when the RST is received.

Irregularities in the connection opening (no reply at all from the server or a RST segment) generaly indicate that the server process is not running on the remote system (perhaps it's the wrong system) or there is a firewall in the way. In addition, some network configuration problems such as incorrect gateway address or netmask can prevent any reply from appearing.