Understanding Network Characteristics and Protocols
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1. The performance of a client-server system is strongly influenced by two major network characteristics: the bandwidth of the network (that is, how many bits/sec it can transport) and the latency (that is, how many seconds it takes for the first bit to get from the client to the server). Give an example of a network that exhibits i) high bandwidth but also high latency, and ii) then give an example of one that has both low bandwidth and low latency. i) High bandwidth but also high latency: A transcontinental fiber link might have many gigabits/sec of bandwidth, but the latency will also be high due to the speed of light propagation over thousands of kilometers. Similarly, a satellite link may run at megabits/sec but have a high latency to send a signal into orbit and back. II) Low bandwidth and low latency: A 56-kbps modem calling a computer in the same building has low bandwidth but low latency.
2. A client-server system uses a satellite network, with the satellite at a height of 40,000 km. What is the best-case delay in response to a request? The request has to go up and down, and the response has to go up and down. The total path length traversed is thus (4 × 40,000 km) or, 160,000 km. The speed of light in air and/or vacuum is 300,000 km/sec, so the propagation delay alone is 160,000/300,000 sec or, about 533 msec.
3. How long is a bit in the original 802.3 standards in meters? Use a transmission speed of 10 Mbps and assume the propagation speed in coax is 2/3 the speed of light in a vacuum (which is about 300,000 km/sec). The speed of light in coax is about 200,000 km/sec, which is 200 meters/μ sec. At 10 Mbps, it takes 0.1 μ sec to transmit a bit. Thus, the bit lasts 0.1 μ sec in time, during which it propagates 20 meters. Thus, a bit is 20 meters long here
4. A system has an n-layer protocol hierarchy. Applications generate messages of length M bytes. At each of the layers, an h-byte header is added. What fraction of the network bandwidth is filled with headers? With n layers and h bytes added per layer, the total number of header bytes per message is hn, so the space wasted on headers is hn. The total message size is M + hn, so the fraction of bandwidth wasted on headers is (hn/M+hn) Note: This estimate does not take into account fragmentation (one higher layer message is sent as multiple lower layer messages) or aggregation (multiple higher-layer messages are carried as one lower layer message) that may be present. If fragmentation is used, it will raise the overhead. If aggregation is used, it will lower the overhead.
5. What does ‘‘negotiation’’ mean when discussing network protocols? Give an example. Negotiation has to do with getting both sides to agree on some parameters or values to be used during the communication. Maximum packet size is one example, but there are many others.
6. Two networks each provide reliable connection-oriented service. One of them offers a reliable byte stream and the other offers a reliable message stream. Are these identical? If so, why is the distinction made? Message and byte streams are different. In a message stream, the network keeps track of message boundaries. In a byte stream, it does not. For example, suppose a process writes 1024 bytes to a connection and then a little later writes another 1024 bytes. The receiver then does a read for 2048 bytes. With a message stream, the receiver will get two messages, of 1024 bytes each. With a byte stream, the message boundaries do not count and the receiver will get the full 2048 bytes as a single unit. The fact that there were originally two distinct messages is lost.
7. Suppose the algorithms used to implement the operations at layer k is changed. How does this impact operations at layers k − 1 and k + 1? This has no impact on the operations at layers k-1 or k+1.
8. Suppose there is a change in the service (set of operations) provided by layer k. How does this impact services at layers k-1 and k+1? There is no impact at layer k-1, but operations in k+1 have to be reimplemented
9. What is the main difference between TCP and UDP? TCP is connection oriented, whereas UDP is a connectionless service. Alternatively, TCP provides a reliable service, whereas UDP provides an unreliable service.
10. When a file is transferred between two computers, two acknowledgement strategies are possible. In the first one, the file is chopped up into packets, which are individually acknowledged by the receiver, but the file transfer as a whole is not acknowledged. In the second one, the packets are not acknowledged individually, but the entire file is acknowledged when it arrives. If the network tends to lose packets, it is better to acknowledge each one separately, so the lost packets can be retransmitted. On the other hand, if the network is highly reliable, sending one acknowledgement at the end of the entire transfer saves bandwidth in the normal case (but requires the entire file to be retransmitted if even a single packet is lost).
11. (a) What are two reasons for using layered protocols? (b) What are the two possible disadvantages of using layered protocols? (a) Among other reasons for using layered protocols, using them leads to, (1) breaking up the design problem into smaller, more manageable pieces, and (2) layering means that protocols can be changed without affecting higher or lower ones. (b) Disadvantage of using layered protocols: (1) More overhead, due to added information or operation done in each layer. (2) Information for one layer which could be useful for other layer (say, application might care whether it is running over wired or wireless) could be hidden or, not available, or, may require added computational overhead for retrieval.