Internet Core Protocols: The Definitive Guide

Rejected Connections

The most common failing that TCP has is related to the inability of a client to connect to a remote system. This can be caused by either the client specifying a destination port number that does not have a listening application associated with it (such as trying to connect to a nonexistent web server), or by the listening server refusing to accept a connection due to a configuration issue. Figure 7.44 shows what a session looks like when the specified destination does not have a listening application associated with it, while Figure 7.45 shows what a session looks like when the destination server refuses to accept the connection.

Figure 7.44. A connection being rejected because there’s no listener

As stated, the most likely cause for this error is that the destination port number specified in the active open is not active. In the example shown in Figure 7.44, the destination port number of 80 (the well-known port number for HTTP) was inactive, meaning that no web server was available on the destination host.

Notice that the destination system’s TCP provider simply rejects the connection request. In the first segment, the client attempts to establish a connection, and the server responds with a Reset segment. This tells the client to just go away. The circuit never even gets established, with no Synchronize segment being returned.

Figure 7.45 shows what happens ...