lwIP - A Minimal TCP/IP implementation - Wikia

10 TCP PROCESSING 10.2 Data structures
Application
Application layer
tcp_write()
tcp_receive()
tcp_enqueue()
tcp_process()
Transport layer
tcp_output()
tcp_input()
ip_route()
ip_output_if()
ip_input()
Internetwork layer
netif->output()
Network interface layer
Figure 9. TCP processing
When an application wants to send TCP data, tcp write() is called. The function tcp write()
passes control to tcp enqueue() which will break the data into appropriate sized TCP segments
if necessary and put the segments on the transmission queue for the connection. The function
tcp output() will then check if it is possible to send the data, i.e., if there is enough space in the
receiver’s window and if the congestion window is large enough and if so, sends the data using
ip route() and ip output if().
Input processing begins when ip input() after verifying the IP header hands over a TCP
segment to tcp input(). In this function the initial sanity checks (i.e., checksumming and TCP
options parsing) are done as well as deciding to which TCP connection the segment belongs. The
segment is then processed by tcp process(), which implements the TCP state machine, and any
necessary state transitions are made. The function tcp receive() will be called if the connection
is in a state to accept data from the network. If so, tcp receive() will pass the segment up to
an application program. If the segment constitutes an ACK for unacknowledged (thus previously
buffered) data, the data is removed from the buffers and its memory is reclaimed. Also, if an ACK
for data was received the receiver might be willing to accept more data and therefore tcp output()
is called.
10.2 Data structures
The data structures used in the implementation of TCP are kept small due to the memory constraints
in the minimal system for which lwIP is intended. There is a tradeoff between the
complexity of the data structures and the complexity of the code that uses the data structures,
and in this case the code complexity has been sacrificed in order to keep the size of the data
structures small.
The TCP PCB is fairly large and is shown in Figure 10. Since TCP connections in the LISTEN
and TIME-WAIT needs to keep less state information than connections in other states, a smaller
PCB data structure is used for those connections. This data structure is overlaid with the full PCB
structure, and the ordering of the items in the PCB structure in Figure 10 is therefore somewhat
awkward.
The TCP PCBs are kept on a linked list, and the next pointer links the PCB list together.
The state variable contains the current TCP state of the connection. Next, the IP addresses and
port numbers which identify the connection are stored. The mss variable contains the maximum
segment size allowed for the connection.
The rcv nxt and rcv wnd fields are used when receiving data. The rcv nxt field contains
the next sequence number expected from the remote end and is thus used when sending ACKs
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