Question

Please don't copy and paste from the internet. Thank you

- What event will cause the sender to initiate fast retransmit when using TCP?

- Describe briefly the relationship between the round trip time (RTT) observed between a sender and a receiver and the retransmit timer used in TCP

- Describe briefly the basic difference in service provided by an email server using POP3 protocol compared to an email server using IMAP protocol.

Please don't copy and paste from the internet. Thank you

Answer 1

1- Fast retransmit is used to decrease  the time a sender waits before retransmitting a lost segment.

In TCP when a sender receives three or more duplicate acknowledgements, it can be reasonably confident that the segment carrying the data that followed the last in-order byte specified in the acknowledgment was lost. A sender with fast retransmit will then retransmit this packet immediately without waiting for its timeout.

2- Round Trip Time is the time which is taken by a pulse for reaching source to destinatation and again to source and the retransmit time is the maximum time limit that can be taken by RTT. So the relationship between RRT and Retransmit time is that the retransmit time is always greater then maximum RTT. Or we can say that to retransmit lost segments, TCP uses retransmission timeout (RTO) or retransmit timer . When TCP sends a segment the timer starts and stops when the acknowledgment is received. If the timer expires timeout occurs and the segment is retransmitted. Retransmit timeout is for 1 Round Trip Time.

3- The basic difference in service provided by an email server using POP3(Post Office Protocol version 3) protocol compared to an email server using IMAP(Internet Message Access Protocol) protocol is that the if we use POP3 it will connect and attempt to keep the mail located on the local device and  POP3 works better if we are only using one device, but have a very large number of emails. It is also better if we have a poor internet connection and need to access emails offline whereas in IMAP  the message does not remain on the local deviceand also IMAP is better if we are going to be accessing our email from multiple devices. but IMAP needs a stable internet connection for accessing emails.