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The transport layer communication is from application to application defined by an IP and port combination.

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Figure: The Transport Layer

At each computer is a port manager. Each application is associated with a port. Communication from one application on a host is defined by the IP and port combination.

When two applications want to talk, they establish a session. All packet in the session are sequenced and the order of delivery is maintained. This is accomplished with the aid of the sequence number in each packet.

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Figure: A TCP Packet

Since one computer may have different performance characteristics than the other computer in the session, the two computers enforce flow control. This is accomplished with the aid of the maximum packet count field in each packet.

Each packet has a checksum on the data and every packet is acknowledged by the receiver.

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Figure: A inetd Table

The inetd deamon manages the ports on each machine. Some of these ports are pre allocated and established by convention. For example, email is on port 25.

Ronald LeRoi Burback
Wed Jul 30 15:24:07 PDT 1997