CHAPTER 20

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1. You can obtain the details of Interplanetary Internet at http://www.ipnsig.org.

2. The TCP/IP protocol stack does not perform well on a satellite network because of the large propagation delay. There will be timeouts before the acknowledgement is received, so packets are retransmitted by the sender though the packets are received at the other end. This causes unnecessary traffic on the network. To overcome these problems, spoofing and link accelerators are used.

3. When the communication is one-way only, the TCP protocol cannot be used at the transport layer because acknowledgements cannot be sent in the reverse direction. In such a case, the connectionless transport protocol UDP has to be used. To transfer a file, the file has to be divided into UDP datagrams and sent over the communication link. At the receiving end, the datagrams have to be assembled by the application layer protocol. It is possible that some of the datagrams are received with errors, but retransmission cannot be done because of lack of the reverse link. The receiver has to check every datagram for errors, and if there is an error even in a single packet, the whole file is discarded. The sender may send the file multiple times so that at least once all the datagrams are received without error.

4. If the transmission medium is very reliable, the packets will be received correctly. Also, if there is no congestion, the packets are likely to be received without variable delay and in sequence. Hence, UDP provides a fast data transfer, and the transmission medium is utilized effectively.

5. Stop-and-wait protocol is very inefficient because the communication channel bandwidth is not utilized well. After the first packet is sent, an acknowledgement has to be received, and then only the second packet can be sent. On the other hand, in sliding window protocol, a number of packets can be sent without waiting for acknowledgements. Hence, channel utilization is better if sliding window protocol is used.

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