PUTTING IT ALL TOGETHER
To put it all together, envision this. A web server is in the middle of sending a file containing HTML text
to a web browser. The web server has just read the next line of the file, so it writes the line to the TCP socket
the server opened for communication with the browser. To the web server, this write operation is very much like
writing to a file.
When the TCP software receives the buffer of text, it builds a header for a TCP packet that, among other
details, includes the source and destination IP addresses and port numbers of the two computers, and the
sequence number information. The TCP software also computes the checksum for the header and data of the
TCP packet, and writes the checksum into the header. When the header is complete, the TCP software passes
the original line from the file, prefixed with the TCP header, as a single buffer (a buffer is a series of bytes) to
the IP protocol.
The IP software may separate the full TCP buffer into several smaller series of bytes. For each IP packet,
the IP software builds an IP header that includes the source and destination IP addresses, and a variety of other
details. The IP software calculates a checksum for the IP header, and inserts that number in the checksum field
of the IP header.
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