在看《计算机网络：自顶向下方法》第四章的时候，有一个编程作业是用python编写一个ping小程序，我在实现的时候加深了一写对python的了解和对IP报文头部和ICMP报文头部信息有了一定理解，故记录一下。

代码如下：

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from socket import *
import os
import sys
import struct
import time
import select
import binascii
ICMP_ECHO_REQUEST = 8
def checksum(strr):
	csum = 0
	countTo = (len(strr) / 2) * 2
	count = 0
	while count < countTo:
		thisVal = strr[count+1] * 256 + strr[count]
		csum = csum + thisVal
		csum = csum & 0xffffffff
		count = count + 2
	if countTo < len(strr):
		csum = csum + str[len(strr) - 1]
		csum = csum & 0xffffffff
	csum = (csum >> 16) + (csum & 0xffff)
	csum = csum + (csum >> 16)
	answer = ~csum
	answer = answer & 0xffff
	answer = answer >> 8 | (answer << 8 & 0xff00)
	return answer


def receiveOnePing(mySocket, ID, timeout, destAddr):
	timeLeft = timeout
	while 1:
		startedSelect = time.time()
		whatReady = select.select([mySocket], [], [], timeLeft)
		howLongInSelect = (time.time() - startedSelect)
		if whatReady[0] == []: # Timeout
			return "请求超时。"
		timeReceived = time.time()
		recPacket, addr = mySocket.recvfrom(1024)
		#Fill in start
		#Fetch the ICMP header from the IP packet
		t, c, checksum, recID, seq = struct.unpack("bbHHh", recPacket[20:28])
		ttl, = struct.unpack("b", recPacket[8:9])
		if t == 3:
			if c == 0:
				return "Destination Network Unreachable."
			elif c == 1:
				return "Destination Host Unreachable."
		if t != 0 or c != 0 or recID != ID or seq != 1:
			return "Recieve error."
		#Fill in end
		timeLeft = timeLeft - howLongInSelect
		if timeLeft <= 0:
			return "请求超时。"
		return [1-timeLeft, ttl, len(recPacket)]

def sendOnePing(mySocket, destAddr, ID):
	# Header is type (8), code (8), checksum (16), id (16), sequence (16)
	myChecksum = 0
	# Make a dummy header with a 0 checksum.
	# struct -- Interpret strings as packed binary data
	header = struct.pack("bbHHh", ICMP_ECHO_REQUEST, 0, myChecksum, ID, 1)
	data = struct.pack("d", time.time())
	# Calculate the checksum on the data and the dummy header.
	myChecksum = checksum(header + data)
	# Get the right checksum, and put in the header
	if sys.platform == 'darwin':
		myChecksum = htons(myChecksum) & 0xffff
		#Convert 16-bit integers from host to network byte order.
	else:
		myChecksum = htons(myChecksum)
	header = struct.pack("bbHHh", ICMP_ECHO_REQUEST, 0, myChecksum, ID, 1)
	packet = header + data
	mySocket.sendto(packet, (destAddr, 1)) # AF_INET address must be tuple, not str
	#Both LISTS and TUPLES consist of a number of objects
	#which can be referenced by their position number within the object


def doOnePing(destAddr, timeout):
	icmp = getprotobyname("icmp")
	#SOCK_RAW is a powerful socket type. For more details see: http://sock-raw.org/papers/sock_raw
	#Fill in start
	mySocket = socket(AF_INET, SOCK_RAW, icmp)
	#Fill in end
	myID = os.getpid() & 0xFFFF #Return the current process i
	sendOnePing(mySocket, destAddr, myID)
	delay = receiveOnePing(mySocket, myID, timeout, destAddr)
	mySocket.close()
	return delay
def ping(host, timeout=1):
	dest = gethostbyname(host)
	print("Pinging " + dest + " using Python:")
	print()
	pingTimes = 4
	lost = 0
	delayList = []
	#Send ping requests to a server separated by approximately one second
	for i in range(pingTimes):
		res = doOnePing(dest, timeout)
		if type(res) == str:
			lost+=1;
			print(res)
			continue
		delay, TTL, packetSize = res
		delay = int(delay * 1000)
		delayList.append(delay)
		print("来自",dest,"的回复: 字节=",packetSize," 时间=",delay,"ms TTL=",TTL)
		time.sleep(1)# one second
	
	print(dest + " 的 Ping 统计信息:")
	print("        数据包: 已发送 = ",pingTimes," ，已接收 = ",pingTimes-lost," ，丢失 = ",lost, " ( ",lost/pingTimes*100,"% 丢失)")
	if len(delayList) > 0:
		print("往返行程的估计时间(以毫秒为单位):")
		print("最短 = ",min(delayList),"ms，最长 = ",max(delayList),"ms，平均 = ",sum(delayList)/len(delayList),"ms")

if len(sys.argv) <= 1:
	print("请输入要ping的主机地址!")
	exit()
ping(sys.argv[1])

作业中主要填写两个空，一个在doOnePing函数中，一个在receiveOnePing函数中。

首先看doOnePing函数，根据上下文可以猜测出这里需要定义一个socket变量，然后我们在第二章中做的实验知道，创建TCP类型的socket的语句是：

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socket(AF_INET, SOCK_STREAM)

创建UDP类型的socket的语句是：

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socket(AF_INET, SOCK_DGRAM)

而我们要发送的是ICMP报文，要知道ICMP工作在网络层，所以不能用传输层的协议来传输，进一步根据注释知道了使用SOCK_RAW来创建一个原始套接字，通过查API知道创建包含ICMP协议的原始套接字的语句是：

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icmp = getprotobyname("icmp")
mySocket = socket(AF_INET, SOCK_RAW, icmp)

第一个空完成！

接下来做receiveOnePing函数中的空。

这个函数要结合着sendOnePing函数看，因为一个是从当前主机往目标主机发数据，一个是从目标主机往当前主机接收数据，所以用什么方法打包数据，就要用同样的方法反向解包。

可以看到sendOnePing函数是通过struct.pack()来将header和data打包起来发送的，所以在receiveOnePing函数使用struct.unpack()来进行解包。注意这里recPacket是IP报文数据，又因为一般的IP首部占20个字节，所以从第20个字节开始才是ICMP报文。所以 recPacket[20:28] 就是ICMP的报文头，然后用struct.unpack()来把数据解包。注释中 Header is type (8), code (8), checksum (16), id (16), sequence (16) 可以帮助理解ICMP的报文头部信息。根据ICMP报文头部的信息确定ICMP的类型，我们需要的是TYPE为0，CODE为0，该报文类型为Echo Reply——回显应答（Ping应答），到这里我们解析ICMP报文的任务就完成了。

为了做的和windows中自带的ping看起来更像一些，我适当的做了一些修改。

比如字节数和TTL，由于TTL是在IP报文头部的第八个字节中存储的，所以也单独的提取出来。