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西北工业大学软件学院计算机网络实验课作业（详细步骤）（全）.doc

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    实验 1：HTTP 一、 实验内容。 1) Is your browser running HTTP version 1.0 or 1.1? What version of HTTP is the server running? 浏览 http 版本 http 1.1 服务器 http 版本 http1.1 2) What languages (if any) does your browser indicate that it can accept to the server? zh-CN 3) What is the IP address of your computer? Of the gaia.cs.umass.edu server? 计算机的 IP 地址：10.26.12.11 服务器的 IP 地址：128.119.245.12 4) What is the status code returned from the server to your browser? 返回的状态码是200 5) When was the HTML file that you are retrieving last modified at the server? Fri,25 May 2018 05:59:01 GMT 6) How many bytes of content are being returned to your browser? 128 7) By inspecting the raw data in the packet content window, do you see any headers within the data that are not displayed in the packet-listing window? 没有 If so, name one.
    8) 9) Inspect the contents of the first HTTP GET request from your browser to the server. Do you see an “IF-MODIFIED-SINCE” line in the HTTP GET? 没有看到 Inspect the contents of the server response. Did the server explicitly return the contents of the file? 可以看到 How can you tell? 10) Now inspect the contents of the second HTTP GET request from your browser to the server. Do you see an “IF-MODIFIED-SINCE:” line in the HTTP GET? If so, what information follows the “IF-MODIFIED-SINCE:” header? 可以看到 If-Modified-Since:Fri,25 May 2018 05:59:01 GTM 11) What is the HTTP status code and phrase returned from the server in response to this second HTTP GET? Did the server explicitly return the contents of the file? Explain. 返回的状态码和短语 304 Not Modified 服务器没有明确返回文件内容
    12) How many HTTP GET request messages did your browser send? Which packet number in the trace contains the GET message for the Bill or Rights? 发送了一次HTTP请求 包号是8 13) Which packet number in the trace contains the status code and phrase associated with the response to the HTTP GET request? 响应报文号是16 14) What is the status code and phrase in the response? 状态码是200 状是OK 15) How many data-containing TCP segments were needed to carry the single HTTP response and the text of the Bill of Rights? 4个TCP，分别是编号为3044，3045，3046，3047的package 16) How many HTTP GET request messages did your browser send? To which Internet addresses were these GET requests sent? 三个GET请求 128.119.245.12 128.119.245.12 128.119.240.90 17) Can you tell whether your browser downloaded the two images serially, or whether they were downloaded from the two web sites in parallel? Explain. 两张图片的GET请求的tcp端口号分别是32232和32236，所以可以看出，两张图片是被连 续下载的。
    二、 实验操作步骤及结果。 三、实验中存在问题及分析。 实验 2：TCP 班级 学号 姓名 三、 实验内容。
    1. What is the IP address and TCP port number used by the client computer (source) that is transferring the file to gaia.cs.umass.edu? To answer this question, it’s probably easiest to select an HTTP message and explore the details of the TCP packet used to carry this HTTP message, using the “details of the selected packet header window” (refer to Figure 2 in the “Getting Started with Wireshark” Lab if you’re uncertain about the Wireshark windows. 源 ip 地址 10.26.12.11 源端口号：2139 2. What is the IP address of gaia.cs.umass.edu? On what port number is it sending and receiving TCP segments for this connection? IP：128.119.245.12 端口号：80 3. What is the IP address and TCP port number used by your client computer (source) to transfer the file to gaia.cs.umass.edu? IP：10.26.12.11 端口号：2139 4. What is the sequence number of the TCP SYN segment that is used to initiate the TCP connection between the client computer and gaia.cs.umass.edu? What is it in the segment that identifies the segment as a SYN segment? sequence number是0 SYN 是 1 ，当 SYN 标记位为 1 时表示这是条请求建立的 tcp 报文段 5. What is the sequence number of the SYNACK segment sent by gaia.cs.umass.edu to the client computer in reply to the SYN? What is the value of the Acknowledgement field in the SYNACK segment? How did gaia.cs.umass.edu determine that value? What is it in the segment that identifies the segment as a SYNACK segment? sequence number是0 Acknowledgement field是1 服务器可以决定是否响应ACK。 SYN和ACK表示服务器对客户端链接请求响应。报文通过设置SYN字段和 ACKnowledgment number字段为1，来标识这是一个SYNACK报文。
    6. What is the sequence number of the TCP segment containing the HTTP POST command? Note that in order to find the POST command, you’ll need to dig into the packet content field at the bottom of the Wireshark window, looking for a segment with a “POST” within its DATA field. sequence number是1 7. Consider the TCP segment containing the HTTP POST as the first segment in the TCP connection. What are the sequence numbers of the first six segments in the TCP connection (including the segment containing the HTTP POST)? At what time was each segment sent? When was the ACK for each segment received? Given the difference between when each TCP segment was sent, and when its acknowledgement was received, what is the RTT value for each of the six segments? What is the EstimatedRTT value (see Section 3.5.3, page 239 in text) after the receipt of each ACK? Assume that the value of the EstimatedRTT is equal to the measured RTT for the first segment, and then is computed using the EstimatedRTT equation on page 239 for all subsequent segments.
    报文段序列号： Segments1 :1 Segments2 :645 Segments3 :2105 Segments4:3565 Segments5 :5025 Segments6 :6485 报文段的发送时间分别为 Segments1 :0.026477 Segments2 :0.041737 Segments3 :0.054026 Segments4:0.054690 Segments5 :0.077405 Segments6 :0.078157 Ack收到的时间是 Segments1 :0.053937 Segments4:0.169118 Segments5 :0.217299 Segments6 :0.267802 RTT分别为（单位为秒）： Segments1 :0.02746 Segments2 :0.035557 Segments3 :0.070059 Segments4:0.11443 Segments5 :0.13989 EstimatedRTT分别为（单位为秒）： 公式：EstimatedRTT=0.875* EstimatedRTT+0.125*SampleRTT Segments2 :0.077294 Segments3 :0.124085 Segments6 :0.18964 8. What is the length of each of the first six TCP segments? Segments1 :644 字节 Segments2 :1460 字节 Segments3 :1460 字节 Segments4:1460 字节 Segments5 :1460 字节 Segments6 :1460 字节 9. What is the minimum amount of available buffer space advertised at the received for the entire trace? Does the lack of receiver buffer space ever throttle the sender? 接收方通知给发送方的最低窗口大小为 5840 字节，不会阻碍，接收方的窗口在不断的 变大，始终大于发送的量。 10. Are there any retransmitted segments in the trace file? What did you check for (in the trace) in order to answer this question? 、没有重传的现象，通过检查数据包的sequence number。 11. How much data does the receiver typically acknowledge in an ACK? Can you identify cases where the receiver is ACKing every other received segment (see Table 3.2 on page 247 in the text). 一般的MSS是1460，而这个是由于MTU决定的，所以一般是1460 ACK1：ack：651 Data：650 ACK2：ack：2111 Data：1460 12. What is the throughput (bytes transferred per unit time) for the TCP connection? Explain how you calculated this value. 最后一个 TCP 段的序列号，减去第一个 TCP 段的序列号，除以第一个 ACk 的时
    间和最后的 ACK 的时间的差值。（146091-1）/（5.455830-0.026477 ）=30.222 KB/S。 13. Use the Time-Sequence-Graph(Stevens) plotting tool to view the sequence number versus time plot of segments being sent from the client to the gaia.cs.umass.edu server. Can you identify where TCP’s slowstart phase begins and ends, and where congestion avoidance takes over? Comment on ways in which the measured data differs from the idealized behavior of text. TCP studied the that we’ve in 慢启动是当 cwnd（拥塞窗口，发送速率约束）的值设置为 1 MSS（最大段大小，可以 抓取并放置到段中的最大数据量）的增加的阶段＆增加每个段确认后，通过 1 MSS。这 种探测机制直到发生丢失事件，收到三个重复的 ACK 或者如果 ssthresh（慢启动阈值， cwnd 的一半）等于 cwnd。在第三种情况下，网络将进入拥塞避免 14. Answer each of two questions above for the trace that you have gathered when you transferred a file from your computer to gaia.cs.umass.edu 四、 实验操作步骤及结果。
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