B) Consider the scenario in Figure 5 where the sender and the receiver are direc

Question

B) Consider the scenario in Figure 5 where the sender and the receiver are
directly connected by a duplex physical link, and they employ the Selective Repeat protocol
for reliable data transfer across the link. Assume that the sequence numbers are represented
with 3 bits, and a window size of W = 4 is used in the Selective Repeat protocol. Figure 5
gives a snapshot of the data transfer, where the sender's and receiver's windows at the mo-
ment are shown. The sender sends the 4 packets as allowed by the sender's window. The
receiver receives three of them, with packet no. 4 lost during the transmission. Describe
the action at the receiver. In particular, specify what the receiver will do with packet no.
3, what acknowledgments will be sent by the receiver and what the receiver's window looks
like afterward.

C)What will the sender's window look like after it has received the three ac-
knowledgments as indicated in Figure 5? How many new packets can it transmit afterward?

Sender Receiver currend window state at receiver's side current window state at sender's side 4507 Data ast receiver's window ACK ACK ACK Figure 5: Figure for Questions 4.b and 4.c

Explanation / Answer

A simple handshaking protocol might only involve the receiver sending a message meaning "I received your last message and I am ready for you to send me another one." A more complex handshaking protocol might allow the sender to ask the receiver if it is ready to receive or for the receiver to reply with a negative acknowledgement meaning "I did not receive your last message correctly, please resend it" (e.g., if the data was corrupted en route)

Packet loss is the failure of one or more transmitted packets to arrive at their destination. This event can cause noticeable effects in all types of digital communications

The Transmission Control Protocol (TCP) detects packet loss and performs retransmissions to ensure reliable messaging. Packet loss in a TCP connection is also used to avoid congestion and thus produces an intentionally reduced throughput for the connection

Packet loss is detected by application protocols such as TCP, but when a person such as a network administrator needs to detect and diagnose packet loss, they typically use a purpose-built tool. Many routers have status pages or logs, where the owner can find the number or percentage of packets dropped over a particular period. For remote detection and diagnosis, the Internet Control Message Protocol provides an "echo" functionality, where a special packet is transmitted that always produces a reply after a certain number of network hops, from whichever node has just received it. Tools such as ping, traceroute, and MTR use this protocol to provide a visual representation of the path packets are taking, and to measure packet loss at each hop.

Packet recovery for reliable delivery: The Internet Protocol leaves responsibility for any retransmission of dropped packets, or "packet recovery" to the endpoints - the computers sending and receiving the data. They are in the best position to decide whether retransmission is necessary, because the application sending the data should know whether speed is more important than reliability, whether a message should be re-attempted in whole or in part, whether or not the need to send the message has passed, and how to vary the amount of bandwidth consumed to account for any congestion

Protocols such as User Datagram Protocol (UDP) provide no recovery for lost packets. Applications that use UDP are expected to define their own mechanisms for handling packet loss

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