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10.1 | |
TCP Operation | |
| 10.1.5 | |
Sequencing numbers |
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TCP breaks data into segments. The data
segments are then transported from sender to receiver, following the
synchronization process and the negotiation of a window size that
dictates the number of bytes that can be transmitted at any one time. The data segments being transmitted must be reassembled once all the
data is received. There is no guarantee that the data will arrive in
the order it was transmitted. TCP applies sequence numbers to the data
segments it is transmitting so that the receiver will be able to
properly reassemble the bytes in their original order. If TCP segments
arrive out of order, the segments may be reassembled incorrectly. Sequencing numbers indicate to the destination device the
correct order in which to put the bytes when they are received.
These sequencing numbers also act as
reference numbers so that the receiver will know if it has received
all of the data. They also identify the missing data pieces to the
sender so it can retransmit the missing data.
Each TCP segment is numbered before
transmission.
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This
offers increased efficiency since the sender only needs to re-transmit the
missing segments instead of the entire set of data.
Notice
that following the destination port in the segment format is the
sequence number portion. At the receiving station, TCP uses the
sequence numbers to reassemble the segments into a complete message.
If a sequence number is missing in the series, that segment is
re-transmitted.