Each
datagram is numbered before transmission. At the receiving station, TCP reassembles the
segments into a complete message. If a sequence number is missing in the series, that
segment is retransmitted. If segments are not acknowledged within a given time period, that
Sender Receiver
Receive 1
Send ACK 2
Window Size = 1
Send 1
Receive ACK 2
Receive 2
Send ACK 3
Send 2
Receive ACK 3
Receive 3
Send ACK 4
Send 3
Receive ACK 4
Understanding TCP/IP??™s Transport and Application Layers 77
results in retransmission. Figure 1-52 illustrates the role that acknowledgment numbers
play when datagrams are transmitted.
Figure 1-52 Acknowledgment Numbers
Session Multiplexing
Session multiplexing is an activity by which a single computer, with a single IP address, is
able to have multiple sessions occur simultaneously. A session is created when a source
machine needs to send data to a destination machine. Most often, this involves a reply, but
a reply is not mandatory. The session is created and controlled within the IP network
application, which contains the functionality of OSI Layers 5 through 7.
A best-effort session is very simple. The session parameters are sent to UDP. A best-effort
session sends data to the indicated IP address using the port numbers provided.
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