计算机网络自顶向下方法：实验记录

WebServer

1. 实验目的

1. 了解Python中的TCP套接字编程基础，包括创建套接字，将套接字保定到指定的地址与接口，以及收发包
2. 了解一些 HTTP 首部行格式

2. 实验内容

1. 开发一个网页服务器，单线程地处理HTTP请求
2. 此服务器应该能完成以下工作
   • 接收并分析 HTTP 请求
   • 从本地文件系统中查找被请求地文件
   • 创建包含首部行和请求文件内容的响应报文
   • 将响应报文发送给客户端
   • 当文件不存在时，应发送给客户端“404 Not Found”

3. 程序代码

#import socket module
from socket import *
import sys # In order to terminate the program

# 1. Create a socket for TCP connection
serverSocket = socket(AF_INET, SOCK_STREAM)	# the parameters indicate this is a socket based on Ipv4 and TCP
#   Prepare a server socket
serverPort = 6789
serverSocket.bind(('', serverPort))
serverSocket.listen(1)      #the parameter specifies the number of unaccepted connections that the system will allow before refusing new connections

while True:
    #2. Establish the connection
    print('Ready to serve...')
    connectionSocket, addr = serverSocket.accept()
    try:
        # 3. recieve a HTTP query
        message = connectionSocket.recv(2048).decode()  
       	print(message)
        
        # 4. search the file in local system
        filename = message.split()[1]
        f = open(filename[1:])
        outputdata = f.read()
        
        # 5. create and send the response message
        # Send one HTTP header line into socket
        header = 'HTTP/1.1 200 OK\n'
        header += 'Connection: close\n'
        header += 'Content-Type: text/html\n'
        header += 'Content-Length: %d\n\n' % (len(outputdata))
        connectionSocket.send(header.encode())
        # Send the content of the requested file to the client
        for i in range(0, len(outputdata)):
            connectionSocket.send(outputdata[i].encode()) 
        #send the end message
        connectionSocket.send("\r\n".encode())	
    except IOError:
        #6. Send response message for file not found
        connectionSocket.send('HTTP/1.1 404 Not Found'.encode())
        connectionSocket.send("\r\n".encode())

    connectionSocket.close()
serverSocket.close()

sys.exit()#Terminate the program after sending the corresponding data

4. 实验过程

1. 在主机上运行网页服务器代码 WebServer.py
2. 通过浏览器访问服务器中的文件
   • http://localhost:6789/HelloWorld.html
   • http://localhost:6789/NotExisted.html

5. 实验结果

1. 使用http://localhost:6789/HelloWorld.html成功地请求并加载了本地的页面

2. 使用http://localhost:6789/HelloWorld.html得到404的提示

3. 网页服务器接收的请求报文部分内容如下

   • GET /HelloWorld.html HTTP/1.1
     Host: localhost:6789
     Connection: keep-alive
     Cache-Control: max-age=0
     Upgrade-Insecure-Requests: 1
     User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/83.0.4103.116 Safari/537.36
     Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
     Accept-Encoding: gzip, deflate, br
     Accept-Language: zh-CN,zh;q=0.9,en;q=0.8
     Cookie: _ga=GA1.1.873531135.1587267004; _gid=GA1.1.970751890.1594543167
     

6. 实验小结

1. 掌握了如何使用Python创建基于TCP连接的服务器端套接字

   • 对象 socket
   • 方法 .bind() .listen() .accept() .send() .recv()

2. 实际查看了 HTTP 的请求报文中的首行部信息，并在响应报文中编辑了相应的首部行

   • header = 'HTTP/1.1 200 OK\n'
     header += 'Connection: close\n'
     header += 'Content-Type: text/html\n'
     header += 'Content-Length: %d\n\n' % (len(outputdata))
     

UDP Pinger

1. 实验目的

1. 了解Python中的UDP套接字编程，包括收发数据包、设置套接字超时
2. 熟悉Ping的相关应用，如计算丢包率

2. 实验内容

1. 学习一个简单的互联网Ping服务器的Python代码，然后完成对应的客户端代码
   • 使用了简单的 UDP，而不是 ICMP(Internet Control Message Protocol)

3. 程序代码

# UDPPingerServer.py
# We will need the following module to generate randomized lost packets
import random
from socket import *
# Create a UDP socket
# Notice the use of SOCK_DGRAM for UDP packets
serverSocket = socket(AF_INET, SOCK_DGRAM)
# Assign IP address and port number to socket
serverSocket.bind(('', 12000))
while True:
    # Generate random number in the range of 0 to 10
    rand = random.randint(0, 10)
    # Receive the client packet along with the address it is coming from
    message, address = serverSocket.recvfrom(1024)
    # Capitalize the message from the client
    message = message.upper()
    # If rand is less is than 4, we consider the packet lost and do not respond
    if rand < 4:
        continue
    # Otherwise, the server responds
    serverSocket.sendto(message, address)

# UDPPingerClient.py
# We will need the following module to calculate the round-trip time
import time
from socket import *
# 1. Create a UDP socket
clientSocket = socket(AF_INET, SOCK_DGRAM)  # IPv4 + UDP
# set timeout as 1s
clientSocket.settimeout(1)

# 2. set the address and port of server
# serverName = '***.***.***.***'     #server IP
serverName = '127.0.0.1'            #localhost, for test
serverPort = 12000

# 3. Ping: send ping package and wait to recieve. The RTT is stored in the array RTTs.
RTTs = []
for i in range(10):  
    sendTime = time.time()
    sentence = 'Ping %d %s' % ((i+1),(sendTime)) 
    try:
        clientSocket.sendto(sentence.encode(), (serverName, serverPort))
        modifiedMessage, hostAddress = clientSocket.recvfrom(2048)
        RTT = time.time() - sendTime
        RTTs.append(RTT)
        print('Sequence %d: Reply from %s, RTT = %.3fs.' % ((i+1), hostAddress, RTT))
        print(modifiedMessage.decode())
    except Exception as e:
        print('Sequence %d: Request time out.' % (i+1))

# 4. Post processing: calculate some statistical information, then display them
maxRTT = max(RTTs)
minRTT = min(RTTs)
meanRTT = sum(RTTs)/len(RTTs)

print('**********************************************************************')
print(('The max RTT is: %.3fs. \nThe min RTT is: %.3fs.\nThe average RTT is : %.3fs')  
        % (maxRTT, minRTT, meanRTT)) 

4. 实验过程

1. 分别在两台主机上运行 UDPPingerServer.py 和 UDPPingerClient.py，或者在同一台主机上运行两个程序，注意此时IP地址要使用 localhost
2. 观察客户端的输出

5. 实验结果

Sequence 1: Reply from ('127.0.0.1', 12000), RTT = 0.001s.
PING 1 1594798645.6439474
Sequence 2: Reply from ('127.0.0.1', 12000), RTT = 0.001s.
PING 2 1594798645.6459286
Sequence 3: Reply from ('127.0.0.1', 12000), RTT = 0.000s.
PING 3 1594798645.6493983
Sequence 4: Reply from ('127.0.0.1', 12000), RTT = 0.001s.
PING 4 1594798645.6518812
Sequence 5: Reply from ('127.0.0.1', 12000), RTT = 0.000s.
PING 5 1594798645.6535127
Sequence 6: Request time out.
Sequence 7: Reply from ('127.0.0.1', 12000), RTT = 0.000s.
PING 7 1594798646.6565206
Sequence 8: Reply from ('127.0.0.1', 12000), RTT = 0.000s.
PING 8 1594798646.6580553
Sequence 9: Request time out.
Sequence 10: Reply from ('127.0.0.1', 12000), RTT = 0.000s.
PING 10 1594798647.6608508
**********************************************************************
The max RTT is: 0.001s.
The min RTT is: 0.000s.
The average RTT is : 0.001s

6. 实验小结

1. 学习了Python中UDP套接字相关的方法
   • .recv() 的返回是一个元组，由两个元素组成，分别是报文和发送方的地址
   • .sendto(message, address)
   • .settimeout()
2. Python可以方便的进行字符串格式化输出
   • sentence = 'Ping %d %s' % ((i+1),(sendTime))

STMP

1. 实验目的

1. 通过实践进一步了解 STMP

2. 实验内容

1. 通过 STMP 连接邮件服务器，与之对话（dialogue），并最终发送邮件报文

3. 程序代码

from socket import *
import base64

# message content
msg = "\r\n I love computer networks!"
subject = 'Computer networks'
contentType = 'text/plain'
endMsg = "\r\n.\r\n"

# Choose a mail server (e.g. Google mail server) and call it mailServer
mailServer = 'pop.qq.com'

# Create socket called clientSocket and establish a TCP connection with mailServer
# Sender and reciever
fromAddress = "********@qq.com"	#your email eddress
toAddress = "*******@qq.com"	#reciever address, can be the same to fromaddress

# Auth information (Encode with base64, which is suitable for mail to transmit data safely.)
username = base64.b64encode(fromAddress.encode()).decode()
password = base64.b64encode("************".encode()).decode()	#not your password, but Authorization code!!! see https://service.mail.qq.com/cgi-bin/help?subtype=1&&no=1001256&&id=28

# Create clientSocket
print('connect to mailServer:')
clientSocket = socket(AF_INET, SOCK_STREAM) 
clientSocket.connect((mailServer, 25))
recv = clientSocket.recv(1024).decode()
print(recv)
if recv[:3] != '220':
    print('220 reply not received from server.')

# Send HELO command and print server response.
print('send HELO command:')
heloCommand = 'HELO Alice\r\n'
clientSocket.send(heloCommand.encode())
recv1 = clientSocket.recv(1024).decode()
print(recv1)
if recv1[:3] != '250':
    print('250 reply not received from server.')

#Authorization
print('send AUTH LOGIN:')
clientSocket.sendall('AUTH LOGIN\r\n'.encode())
recv = clientSocket.recv(1024).decode()
print(recv)
if (recv[:3] != '334'):
	print('334 reply not received from server')

print('Send username:')
clientSocket.sendall((username + '\r\n').encode())
recv = clientSocket.recv(1024).decode()
print(recv)
if (recv[:3] != '334'):
	print('334 reply not received from server')

print('Send password:')
clientSocket.sendall((password + '\r\n').encode())
recv = clientSocket.recv(1024).decode()
print(recv)
if (recv[:3] != '235'):
	print('235 reply not received from server')

# Send MAIL FROM command and print server response.
print('Send FROM command:')
fromCommand = 'MAIL FROM: <' + fromAddress + '>\r\n' #must be the same of fromAddress
clientSocket.send(fromCommand.encode())
recv = clientSocket.recv(1024).decode()
print(recv)
if (recv[:3] != '250'):
	print('250 reply not received from server')

# Send RCPT TO command and print server response.
print('Send RCPT command:')
rcptCommand = 'RCPT TO: <' + toAddress + '>\r\n'
clientSocket.send(rcptCommand.encode())
recv = clientSocket.recv(1024).decode()
print(recv)
if (recv[:3] != '250'):
	print('250 reply not received from server')

# Send DATA command and print server response.
print('Send DATA command:')
clientSocket.send('DATA\r\n'.encode())
recv = clientSocket.recv(1024).decode()
print(recv)
if (recv[:3] != '354'):
	print('354 reply not received from server')

# Send message data.
message = 'from:' + fromAddress + '\r\n'
message += 'to:' + toAddress + '\r\n'
message += 'subject:' + subject + '\r\n'
message += 'Content-Type:' + contentType + '\t\n'
message += '\r\n' + msg
clientSocket.sendall(message.encode())

# Message ends with a single period.
print('Send endMsg:')
clientSocket.send(endMsg.encode())
recv = clientSocket.recv(1024).decode()
print(recv)
if (recv[:3] != '250'):
	print('250 reply not received from server')

# Send QUIT command and get server response.
print('Send QIUT command')
clientSocket.send('QUIT\r\n'.encode())
recv = clientSocket.recv(1024).decode()
print(recv)
if (recv[:3] != '221'):
	print('reply not received from server')
# Close connection
clientSocket.close()

4. 实验过程

1. 运行代码，观察输出，并在邮箱中查看邮件。

5. 实验结果

• #output:
  # connect to mailServer:
  # 220 newxmesmtplogicsvrszb5.qq.com XMail Esmtp QQ Mail Server.
  
  # send HELO command:
  # 250-newxmesmtplogicsvrszb5.qq.com-100.66.14.230-51224702
  # 250-SIZE 73400320
  # 250 OK
  
  # send AUTH LOGIN:
  # 334 VXNlcm5hbWU6
  
  # Send username:
  # 334 UGFzc3dvcmQ6
  
  # Send password:
  # 235 Authentication successful
  
  # Send FROM command:
  # 250 OK.
  
  # Send RCPT command:
  # 250 OK
  
  # Send DATA command:
  # 354 End data with ..
  
  # Send endMsg:
  # 250 OK: queued as.
  
  # Send QIUT command
  # 221 Bye.
  

• 在收件箱中成功查看邮件

6. 实验小结

1. 从代码中可以看出，STMP依靠TCP实现可靠数据传输，而非UDP
2. STMP使用端口 25
3. 使用 base64 来进行邮件报文编码
4. 与邮件服务器的对话包含一下几个基本环节
   1. HELO
   2. AUTH LOGIN
   3. FROM
   4. RCPT
   5. DATA + message + endMsg
   6. QUIT

ProxyServer

1. 实验目的

1. 实际观察代理服务器的工作过程与缓存功能

2. 实验内容

1. 开发一个自带缓存功能的代理服务器。为使其简单，该服务器只响应 GET 请求，但对象范围不仅包括网页，还有图像

   

3. 程序代码

# ProxyServer.py
from socket import *

# 1. Create a server socket, bind it to a port and start listening
port = 8888
tcpSerSock = socket(AF_INET, SOCK_STREAM)
tcpSerSock.bind(('', port))
tcpSerSock.listen(1)

while True:
    # 2. Strat receiving data from the client
    print('Ready to serve...')
    tcpCliSock, addr = tcpSerSock.accept()
    print('Received a connection from:', addr)
    message = tcpCliSock.recv(2048).decode()
    print(message)
    
    # 3. Extract the filename from the given message
    print(message.split()[1])
    filename = message.split()[1].partition("/")[2]
    print(filename)
    fileExist = "false"
    fileToUse = "/" + filename
    print(fileToUse)
    try:
        # 4. Check wether the file exist in the cache
        f = open(fileToUse[1:], "r")
        outputdata = f.readlines()
        fileExist = "true"
        # 5. ProxyServer finds a cache hit and generates a response message
        tcpCliSock.send("HTTP/1.0 200 OK\r\n")
        tcpCliSock.send("Content-Type:text/html\r\n")
        tcpCliSock.send("Content-Length:" + len(outputdata) + "\r\n")
        for i in range(0, len(outputdata)):
            tcpCliSock.send(outputdata[i].encode())
        print('Read from cache')
    # 5. Error handling for file not found in cache
    except IOError:
        if fileExist == "false":
            # Create a socket on the proxyserver
            c = socket(AF_INET, SOCK_STREAM)
            hostn = filename.replace("www.","",1)
            print("Host name:" + hostn)
            try:
                # Connect to the socket to port 80
                c.connect((hostn, 80))
                print('Socket connected to port 80 of the host')
                newMessage = message.replace('localhost:' + port, hostn)
                print('newMessage:')
                print(newMessage)
                c.sendall(newMessage.encode())
               	# 6. Create a new file in the cache for the requested file.
                # Also send the response in the buffer to client socket
                # and the corresponding file in the cache
                
                # Read the response into buffer
                buff = c.recv(4096)
                
                tmpFile = open("./" + filename,"w")
                tmpFile.writelines(buff.decode().replace('\r\n','\n'))
                tcpCliSock.sendall(buff)
                tmpFile.close()
            except Exception as e:
                print(e)
                print("Illegal request")
        else:
            # HTTP response message for file not found
            print('File Not Found')
    # 7. Close the client and the server sockets
    tcpCliSock.close()
tcpSerSock.close()

4. 实验步骤

1. 在主机上运行代码 ProxyServer.py
2. 首次通过浏览器访问 http://localhost:8888/www.baidu.com
3. 再次通过浏览器访问http://localhost:8888/www.baidu.com

5. 实验结果

• 首次运行代码后，在代码的根目录下，创建了一个新的文件 www.baidu.com