From a high level viewpoint, the TCP/IP stack can be seen as a black box that takes incoming packets, and demultiplexes them between the currently active connections. Before the data is delivered to the application, TCP sorts the packets so that they appear in the order they were sent. The TCP/IP stack will also send acknowledgments for the received packets.
Figure 1 shows how packets come from the network device, pass through the TCP/IP stack, and are delivered to the actual applications. In this example there are five active connections, three that are handled by a web server application, one that is handled by the e-mail sender application, and one that is handled by a data logger application.
A high level view of the output processing can be seen in Figure 2. The TCP/IP stack collects the data sent by the applications before it is actually sent onto the network. TCP has mechanisms for limiting the amount of data that is sent over the network, and each connection has a queue on which the data is held while waiting to be transmitted. The data is not removed from the queue until the receiver has acknowledged the reception of the data. If no acknowledgment is received within a specific time, the data is retransmitted.
Data arrives asynchronously from both the network and the application, and the TCP/IP stack maintains queues in which packets are kept waiting for service. Because packets might be dropped or reordered by the network, incoming packets may arrive out of order. Such packets have to be queued by the TCP/IP stack until a packet that fills the gap arrives. Furthermore, because TCP limits the rate at which data that can be transmitted over each TCP connection, application data might not be immediately sent out onto the network.
The full TCP/IP suite consists of numerous protocols, ranging from low level protocols such as ARP which translates IP addresses to MAC addresses, to application level protocols such as SMTP that is used to transfer e-mail. We have concentrated our work on the TCP and IP protocols and will refer to upper layer protocols as ``the application''. Lower layer protocols are often implemented in hardware or firmware and will be referred to as ``the network device'' that are controlled by the network device driver.
TCP provides a reliable byte stream to the upper layer protocols. It breaks the byte stream into appropriately sized segments and each segment is sent in its own IP packet. The IP packets are sent out on the network by the network device driver. If the destination is not on the physically connected network, the IP packet is forwarded onto another network by a router that is situated between the two networks. If the maximum packet size of the other network is smaller than the size of the IP packet, the packet is fragmented into smaller packets by the router. If possible, the size of the TCP segments are chosen so that fragmentation is minimized. The final recipient of the packet will have to reassemble any fragmented IP packets before they can be passed to higher layers.
