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The transport-layer protocols used in the TCP/IP protocol suite. It starts with the simplest protocol and gradually adds more complexity. It covers topics such as Stop-and-Wait protocol, Go-Back-N protocol, and Selective-Repeat protocol. It also includes examples and calculations to better understand the concepts. useful for students studying computer networks and related fields.
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Figure 13.16 Simple protocol
Figure 13.17 FSMs for simple protocol
Figure 13.18 shows an example of communication using this protocol. It is very simple. The sender sends packets one after another without even thinking about the receiver. Example 13.
In the Stop-and-Wait protocol, we can use a 1-bit field to number the packets. The sequence numbers are based on modulo- arithmetic.
In the Stop-and-Wait protocol, the acknowledgment number always announces in modulo-2 arithmetic the sequence number of the next packet expected.
Figure 13.21 shows an example of Stop-and-Wait protocol. Packet 0 is sent and acknowledged. Packet 1 is lost and resent after the time-out. The resent packet 1 is acknowledged and the timer stops. Packet 0 is sent and acknowledged, but the acknowledgment is lost. The sender has no idea if the packet or the acknowledgment is lost, so after the time-out, it resends packet 0, which is acknowledged. Example 13.
What is the utilization percentage of the link in Example 13.5 if we have a protocol that can send up to 15 packets before stopping and worrying about the acknowledgments? Solution The bandwidth-delay product is still 20,000 bits. The system can send up to 15 packets or 15,000 bits during a round trip. This means the utilization is 15,000/20,000, or 75 percent. Of course, if there are damaged packets, the utilization percentage is much less because packets have to be resent. Example 13.
In the Go-Back-N Protocol, the sequence numbers are modulo 2 m , where m is the size of the sequence number field in bits.
Figure 13.23 Send window for Go-Back-N
The send window is an abstract concept defining an imaginary box of maximum size = 2 m − 1 with three variables: S f , S n , and S size .