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Accept-language: en Keep-Alive:300
Connection:keep-alive
Questions:
a. What is the URL of the document requested by the browser?
由第一行可得URL字段为www.ynu.edu.cn/info/2012-10-18/0-1-411.html b. What version of HTTP is the browser running?
由第一行的http/1.1可得知版本为HTTP1.1
c. Does the browser request a non-persistent or a persistent connection?
由Connection:keep-alive可得知请求的为持久连接 d. What is domain name of the server?
由Host: www.ynu.edu.cn可得知domain name 为www.ynu.edu.cn
(2)The text below shows the reply sent from the server in response to the HTTP GET
message in the question above (1). Please explain the meaning and function of the line denoted by numbers from ① to ⑤.(5 points)
HTTP/1.1 200 OK ① Content-Length: 19572 ② Content-Type: text/html
Last-Modified: Sun, 30 Sep 2012 07:23:15 GMT ③ Accept-Ranges: bytes Server: Microsoft-IIS/6.0 X-Powered-By: ASP.NET
Date: Thu, 25 Oct 2012 21:26:48 GMT ④ Connection: Keep-Alive ⑤
Answer:
1：指明服务器使用的协议是HTTP1.1并且一切正常 ○
2：表明被发送对象的字节数 ○
3：指示对象创建或者最后被修改的时间 ○
4：指示服务器产生并发送该响应报文的日期和时间 ○
5：通知客户机收到报文后仍然保持连接 ○
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一、Choose the best answer(10 problems, 1 points each, total 10)
1. A user enters http://www.cisco.com/web1.htm in the address line of a browser. Which
statement is true ( A )
A. The \B. The \C. The retrieved web page will be displayed in URL code.
D. The \
2. MIME is the SMTP’s extension to transmit ( C )
A. text data B.picture data C. non ASCII data D.video data
3. In the following presentation about router ( A b ) is wrong.
A. separate collision domains B. work in data link layer
C. separate broadcast domains D. provide best path to destination
4. ( A ) is a dynamic mapping protocol in which physical address is found for a
given IP address. A. ARP B. RARP C. ICMP D. None of the above
5. Below are four TCP packets captured on the network at different time, Which ones
belong to the same TCP connection? ( C )
(1)
(3)
src-address: 34.198.10.3 dest-address: 101.124.102.32 src-port: 3241 dest-port: 5432 seq-num: 2000 ack-num: 0 . . . . . .
(2)
(4)
src-address: 34.198.10.3 dest-address: 101.124.132.200 src-port: 3241 dest-port: 5432 seq-num: 1 ack-num: 2001 . . . . . . src-address: 34.198.10.3 dest-address: 101.124.102.32 src-port: 5432 dest-port: 3241 seq-num:2001 ack-num:0 . . . . . .
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src-address: 101.124.132.200 dest-address: 34.198.10.3 src-port: 5432 dest-port: 3241 seq-num:2000 ack-num:1 . . . . . . A. (1)and (2) B. (2)and(3) C. (3)and(4) D. (4)and (1)
6. The protocols below, which is no collision ( d )
A. CSMA B. ALOHA C. CSMA/CD D. TOKEN RING
7. For Ethernet, if an adapter determines that destination MAC address of a frame it has
just received is not equal to the adapter’s MAC address ( )
A. it discards the frame without sending an error message to the network layer B. it sends a NACK (not acknowledged frame) to the sending host
C. it delivers the frame to the network layer, and lets the network layer decide what to do
D. it discards the frame and sends an error message to the network layer
8. In CSMA/CD, after the 3rd collision, what is the maximum value of K? ( D )
A. 8 B. 15 C. 9 D. 7
9. In the TCP/IP protocol suite, we refer to a link layer’s PDU as a ( A )
A. A. frame B. datagram C. message D. segment
10. 10BaseT Ethernet has ( A )
A. a star topology B. a ring topology C. a bus topology D. none of the above
二、Fill in blanks (10 blanks, 1 points each blank, total 10)
1. 电线 , 光纤 , 卫星 and radio are the physical media that network can run over. 2. TCP/IP is the protocol suite for the current Internet.
3. 经内存交换 , 经一根总线交换 and 经一个互联网络交换 are three types of switching fabrics.
4. The Minimum length of Ethernet Frame is 64 Byte.
5. A router forwards a packet based on the packet’s dest IP address. A link-layer switch forwards a packet based on the packet’s dest MAC address.
三、True or false（1 points for each item, total 5 points）
1.
Routing is about moving a packet from a router’s input link to the appropriate output
link. Forwarding is about determining the shortest path between sources and destinations. ( f )(说反了) 2.
Switches are plug-and-play devices , according to store a frame’s destination address
(zixuexi)
and incoming interface to establish a switch table entry.
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( f ) 3.
The slotted Aloha protocol makes more efficient use of the network bandwidth than
( t )
the pure Aloha protocol. 4.
The value of the RcvWindow field of TCP segment is used to indicate the number of
bytes that a sender is willing to send. ( f ) 5.
The OSPF is a route protocol related to link state routing algorithm. ( t )
四、Please answer following question( total 40 points）
1. List three network-application user agents that you use on a daily basis.
IE、Netscape Navigator 、Outlook Express、Foxmail
2. What is the difference between nonpersistent HTTP and persistent HTTP without pipelining?
为每一个对象建立TCP连接 为每一个页面建立一个tcp连接
要点：前者是一个TCP连接传输多个页面；后者是一个TCP连接传输一个页面。
3. Briefly describe the principle of GBN.
发送端不需要在接收到上一个数据包的ACK后才发送下一个数据包，而是可以连续发送数据包。在发送端发送数据包的过程中，如果接收到对应已发送的某个数据包的ACK，则发送端将ACK对应的某个数据包进行重发，然后再将该数据包之后的数据包依次进行重发。
发送方：有发送窗口，窗口内的包按序发送，收到的包若是base包则窗口向右滑动一个包的位置，若在（base,base+N-1]中的第m个包则滑动到窗口内则滑动到（m，base+N-1]中的第一个未确认的包的位置（累计确认），若超时事件发生则重发窗口内未被确认的包。
接收方：没有缓冲区，若包按序到达，则发送该包的确认包，否则，重新发送最后一次按序到达包的确认包
4. Briefly describe the algorithm used by TCP to control the size of its congestion window.（简述TCP控制拥塞窗口尺寸的算法）
1.加性增、乘性减：如果没有检测到拥塞，TCP将缓慢地增加其拥塞窗口的长度，即每次收到一个确认后就把CongWin增大一点，当丢包时间发生时立即将当前CongWin的值减为一般，重复循环该过程已到达控制拥塞窗口尺寸的目的； 2.慢启动：当TCP开始建立链接时，CongWin的初始值置为一个MSS,此时不是线性的增加窗口尺寸而是，而是指数增加,及每过一个RTT将CongWin的值翻倍，知道出现拥塞是开始按照上述加性增、减性乘的方式控制窗口尺寸；
Initially, Congwin set to 1 MSS
Slow-start phase — When Congwin is below Threshold, Congwin grows exponentially. Congestion-avoidance phase — When Congwin is above Threshold, Congwin grows linearly.
Fast recovery phase — When 3 duplicate ACK occurs, Threshold set to Congwin/2 and Congwin set to Threshold；When timeout event occurs, Threshold set to Congwin/2 and Congwin is set to 1 MSS.
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