宁波大学计算机网络协议题.doc

1.The quantity La/R is useful in gaining an intuitive understanding of the extent of the queuing delay. In particular, if traffic intensity is close to zero, then packet arrivals are few and far between and it is unlikely that an arriving packet will find another packet in the queue. Hence, the average queuing delay will be close to zero. On the other hand, when the traffic intensity is close to 1, there will be intervals of time when the arrival rate exceeds the transmission capacity (due to the burstiness of arrivals), and a queue will form. As the traffic intensity approaches 1, the average queue length gets larger and larger. The qualitative dependence of average queuing delay on the traffic intensity is shown in Figure 1.20. Figure 1.20: Dependence of average queuing delay on traffic densityOne important aspect of Figure 1.20 is the fact that as the traffic intensity approaches 1, the average queuing delay increases rapidly. A small percentage increase in the intensity will result in a much larger percentage-wise increase in delay. 根据文献中的论述，用中文分析和回答以下问题：1. 什么条件下节点的排队延时非常小？（2分）2. 什么条件下节点的排队延时不断加大？（2分）3. 什么条件下节点的排队延时变得非常大？（2分）4. 用你生活中的例子解释“A small percentage increase in the intensity will result in a much larger percentage-wise increase in delay.” （4分）2.The general format for a HTTP request message, as shown in Figure 2.7.Figure 2.7: General format of a request messageWe see that the general format of a request message closely follows our earlier example. You may have noticed, however, that after the header lines (and the additional carriage return and line feed) there is an entity body. The entity body is not used with the GET method, but is used with the POST method. The HTTP client uses the POST method when the user fills out a form-for example, when a user gives search words to a search engine such as Google. With a POST message, the user is still requesting a Web page from the server, but the specific contents of the Web page depend on what the user entered into the form fields. If the value of the method field is POST, then the entity body contains what the user entered into the form fields. The HEAD method is similar to the GET method. When a server receives a request with the HEAD method, it responds with an HTTP message but it leaves out the requested object. The HEAD method is often used by HTTP server developers for debugging. 根据文献中的论述，用中文分析和回答以下问题：1. HTTP发出哪种请求时格式中需要在entity body.字段填写内容？（2分）2. 在GOOGLE上递交多个搜索关键字是在图中哪个字段中传输的？（2分）3. HEAD方法经常用来实现什么功能？（2分）4. 在GET和HEAD请求的方法发送时有何差别？（2分）3. The send side of rdt2.0 has two states. In one state, the send-side protocol is waiting for data to be passed down from the upper layer. In the other state, the sender protocol is waiting for an ACK or a NAK packet from the receiver. If an ACK packet is received (the notation rdt_rcv(rcvpkt) & isACK(rcvpkt) corresponds to this event), the sender knows the most recently transmitted packet has been received correctly and thus the protocol returns to the state of waiting for data from the upper layer. If a NAK is received, the protocol retransmits the last packet and waits for an ACK or NAK to be returned by the receiver in response to the retransmitted data packet. It is important to note that when the receiver is in the wait-for-ACK-or-NAK state, it can not get more data from the upper layer; that will only happen after the sender receives an ACK and leaves this state. Thus, the sender will not send a new piece of data until it is sure that the receiver has correctly received the current packet. Because of this behavior, protocols such as rdt2.0 are known as stop-and-wait protocols.1. 根据文献中提到的要点，分析为何rdt2.0被叫做stop-and-wait protocols？2. 用中文表达带下划线文字的意思。4.Protocol rdt2.1 uses both positive and negative acknowledgements from the receiver to the sender. A negative acknowledgement is sent whenever a corrupted packet, or an out of order packet, is received. We can accomplish the same effect as a NAK if instead of sending a NAK, we instead send an ACK for the last correctly received packet. A sender that receives two ACKs for the same packet (i.e., receives duplicate ACKs) knows that the recevier did not correctly receive the packet following the packet that is being ACKed twice. Many TCP implementations use the receipt of so-called triple duplicate ACKs (three ACK packets all ACKing the same pac