Since different pieces of a message may travel by different routes to the destination, it??™s possible for
component pieces of a message to arrive out of order. The sequence numbers on messages make it possible to
put the pieces into correct order before delivering the full message to the application (e.g., the web server or the
web browser).
Both the header and the data in a TCP packet are protected with a checksum. This allows the TCP protocol
to insure that data in the message have not been corrupted. As you can imagine by now, if the destination
computer detects corrupted data in a packet, it can simply ask the sender to resend data corresponding to
a particular sequence number.
We have simplified the discussion of sequence numbering somewhat in hopes of making it easier to understand
conceptually. In practice, every byte in a TCP message is sequenced. The message header in a TCP packet
has the starting sequence number, and the receiver will increment that number by the number of bytes in the
message. The receiver will expect the next packet to start with the incremented number plus one. The sequence
numbering scheme insures correct sequencing of bytes in the message, efficient resending of corrupted or lost
packets, and correct disposal of duplicate packets.
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