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Every TCP packet contains a 32 bit sequence number and a 32 bit
acknowledgment number. The sequence number identifies the bytes in
each packet so they may be ordered into a reliable data stream. The
acknowledgment number is used by the receiving host to indicate which
bytes it has received, and indirectly, which it has not. When
in-sequence data is received, the receiver sends an acknowledgment specifying
the next sequence number that it expects and implicitly acknowledging
all sequence numbers preceding it. Since packets may be
lost, or reordered in flight, the acknowledgment number is only
incremented in response to the arrival of an in-sequence packet.
Consequently, out-of-order or lost packets will cause a receiver to
issue duplicate acknowledgments for the packet it was expecting.
Figure:
Data seeding phase of basic loss deduction algorithm.
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Figure:
Hole filling phase of basic loss deduction algorithm.
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Stefan Savage
8/31/1999