An Investigation into Transport Protocols and Data Transport ...

4.2. Protocol Description 55
As a response to out-of-order segments, the TCP receiver sends duplicate
acknowledgments, dupacks, that carry the same acknowledgment number as
the previous ack it had sent - hence this does not change snd_una.
A sequential receipt of a number of dupacks signifies that a data segment
has potentially been lost in the network, and therefore the sender should
send the data segment (the left most side of the sliding window) again.
In combination with retransmission timeouts (RTO) (See Section 4.5.1)
that will resend segments should no acks that advance snd_una be received
within this RTO, acks provide reliable data delivery [Bra89] by informing
the sender of when data has (and implicitly, has not) been received by the
receiver. Similarly, when new data is sent into the network, the right most
pointer, snd_nxt, advances to the right. Therefore, the window ‘slides’ across
the range of sequence numbers as the transfer takes place. Through the evolution
of the data transfer, this window will slide across the entire byte range
of the relevant data to transfer and hence is known as a ‘sliding window’.
The macroscopic result of this is that for each and every TCP connection,
this window has to be maintained as part of the connection state in memory
in order to be able to keep track of the number of packets in-flight. The
Bandwidth Delay Product (BDP) defines the relation between the throughput
b, the round-trip latency T , and the size of the sliding window w required.
b = w T
(4.1)
The relation between w and the number of packets is defined by:
w = n × s (4.2)