《第三章课后习题及答案(共17页)》由会员分享，可在线阅读，更多相关《第三章课后习题及答案(共17页)（17页珍藏版）》请在金锄头文库上搜索。
1、第三章1. (Q1) Suppose the network layer provides the following service. The network layer in the source host accepts a segment of maximum size 1,200 bytes and a destination host address from the transport layer. The network layer then guarantees to deliver the segment to the transport layer at the dest
2、ination host. Suppose many network application processes can be running at the destination host.a. Design the simplest possible transport-layer protocol that will get application data to the desired process at the destination host. Assume the operating system in the destination host has assigned a 4
3、-byte port number to each running application process. b. Modify this protocol so that it provides a “return address” to the destination process. c. In your protocols, does the transport layer “have to do anything” in the core of the computer network. Answer: a. Call this protocol Simple Transport P
4、rotocol (STP). At the sender side, STP accepts from the sending process a chunk of data not exceeding 1196 bytes, a destination host address, and a destination port number. STP adds a four-byte header to each chunk and puts the port number of the destination process in this header. STP then gives th
5、e destination host address and the resulting segment to the network layer. The network layer delivers the segment to STP at the destination host. STP then examines the port number in the segment, extracts the data from the segment, and passes the data to the process identified by the port number. b.
6、 The segment now has two header fields: a source port field and destination port field. At the sender side, STP accepts a chunk of data not exceeding 1192 bytes, a destination host address, a source port number, and a destination port number. STP creates a segment which contains the application data
7、, source port number, and destination port number. It then gives the segment and the destination host address to the network layer. After receiving the segment, STP at the receiving host gives the application process the application data and the source port number. c. No, the transport layer does no
8、t have to do anything in the core; the transport layer “lives” in the end systems.2. (Q2) Consider a planet where everyone belongs to a family of six, every family lives in its own house, each house has a unique address, and each person in a given house has a unique name. Suppose this planet has a m
9、ail service that delivers letters form source house to destination house. The mail service requires that (i) the letter be in an envelope and that (ii) the address of the destination house (and nothing more ) be clearly written on the envelope. Suppose each family has a delegate family member who co
10、llects and distributes letters for the other family members. The letters do not necessarily provide any indication of the recipients of the letters.a. Using the solution to Problem Q1 above as inspiration, describe a protocol that the delegates can use to deliver letters from a sending family member
11、 to a receiving family member.b. In your protocol, does the mail service ever have to open the envelope and examine the letter in order to provide its service.Answer: a. For sending a letter, the family member is required to give the delegate the letter itself, the address of the destination house,
12、and the name of the recipient. The delegate clearly writes the recipients name on the top of the letter. The delegate then puts the letter in an envelope and writes the address of the destination house on the envelope. The delegate then gives the letter to the planets mail service. At the receiving
13、side, the delegate receives the letter from the mail service, takes the letter out of the envelope, and takes note of the recipient name written at the top of the letter. The delegate than gives the letter to the family member with this name.b. No, the mail service does not have to open the envelope
14、; it only examines the address on the envelope.3. (Q3) Describe why an application developer might choose to run an application over UDP rather than TCP.Answer: An application developer may not want its application to use TCPs congestion control, which can throttle the applications sending rate at t
15、imes of congestion. Often, designers of IP telephony and IP videoconference applications choose to run their applications over UDP because they want to avoid TCPs congestion control. Also, some applications do not need the reliable data transfer provided by TCP.4. (P1) Suppose Client A initiates a T
16、elnet session with Server S. At about the same time, Client B also initiates a Telnet session with Server S. Provide possible source and destination port numbers for a. The segment sent from A to B. b. The segment sent from B to S. c. The segment sent from S to A. d. The segment sent from S to B. e. If A and B are different hosts, is it possible that the source port number in the segment from A to S is the same as that from B to
