import socket import struct import time # Fill in start ICMP$\_$ECHO$\_$REQUEST $=8$ # Echo request type ICMP$\_$ECHO$\_$REPLY $=0$ # Echo reply type # Fill in end def checksum$($source$\_$string$)$: $"""$ A helper function that calculates the checksum of the ICMP packet $"""$ sum $=0$ countTo $=($len$($source$\_$string$)/2)*2$ count $=0$ while count $<$ countTo: thisVal $=$ ord$($source$\_$string$[$count $+1])*256+$ ord$($source$\_$string$[$count$])$ sum $=$ sum $+$ thisVal sum $=$ sum & $0$xffffffff count $=$ count $+2$ if countTo $<$ len$($source$\_$string$)$: sum $=$ sum $+$ ord$($source$\_$string$[$len$($source$\_$string$)-1])$ sum $=$ sum & $0$xffffffff sum $=($sum $>>16)+($sum & $0$xffff$)$ sum $=$ sum $+($sum $>>16)$ answer $=$ ~sum answer $=$ answer & $0$xffff answer $=$ answer $>>8|($answer $<<8$ & $0$xff$00)$ return answer def sendOnePing$($mySocket$,$ destAddr, ID$)$: $"""$ Send one ping to the destAddr. $"""$ destAddr $=$ socket.gethostbyname$($destAddr$)$ # Header is type $(8),$ code $(8),$ checksum $(16),$ id $(16),$ sequence $(16)$ myChecksum $=0$ # Make a dummy header with a $0$ checksum 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$)$ & $0$xffff else: myChecksum $=$ htons$($myChecksum$)$ header $=$ struct.pack$("$bbHHh$",$ ICMP$\_$ECHO$\_$REQUEST, $0,$ myChecksum, ID$,1)$ packet $=$ header $+$ data mySocket.sendto$($packet$,($destAddr$,1))$ def receiveOnePing$($mySocket$,$ ID$,$ timeout, destAddr$)$: $"""$ Receive the ping from the socket. $"""$ timeLeft $=$ timeout while True: startedSelect $=$ time.time$()$ whatReady $=$ select.select$([$mySocket$],[],[],$ timeLeft$)$ howLongInSelect $=($time$.$time$()-$ startedSelect$)$ if whatReady$[0]==[]$: # Timeout return "Request timed out." timeReceived $=$ time.time$()$ recPacket, addr $=$ mySocket.recvfrom$(1024)$ icmpHeader $=$ recPacket$[20$:$28]$ type, code, checksum, packetID, sequence $=$

return answer def createPacket$($id$)$: # Header is type $(8),$ code $(8),$ checksum $(16),$ id $(16),$ sequence $(16)$ header $=$ struct.pack$("$bbHHh$",$ ICMP$\_$ECHO$\_$REQUEST, $0,0,$ id$,1)$ data $=$ struct.pack$("$d$",$ time.time$())$ # Calculate the checksum on the data and the header my$\_$checksum $=$ checksum$($header $+$ data$)$ # Put the checksum back into the header header $=$ struct.pack$("$bbHHh$",$ ICMP$\_$ECHO$\_$REQUEST, $0,$ socket.htons$($my$\_$checksum$),$ id$,1)$ return header $+$ data def sendPing$($my$\_$socket, dest$\_$addr, id$)$: dest$\_$addr $=$ gethostbyname$($dest$\_$addr$)$ # Create the packet to send packet $=$ createPacket$($id$)$ my$\_$socket.sendto$($packet$,($dest$\_$addr, $1))$ def receivePing$($my$\_$socket, id$,$ timeout$)$: timeLeft $=$ timeout while True: startedSelect $=$ time.time$()$ whatReady $=$ select.select$([$my$\_$socket$],[],[],$ timeLeft$)$ howLongInSelect $=($time$.$time$()-$ startedSelect$)$ if whatReady$[0]==[]$: # Timeout return timeReceived $=$ time.time$()$ recPacket, addr $=$ my$\_$socket.recvfrom$(1024)$ icmpHeader $=$ recPacket$[20$:$28]$ type, code, checksum, packetID, sequence $=$ struct.unpack$("$bbHHh$",$ icmpHeader$)$ if packetID $==$ id: bytesInDouble $=$ struct.calcsize$("$d$")$ timeSent $=$ struct.unpack$("$d$",$ recPacket$[28$:$28+$ bytesInDouble$])[0]$ return timeReceived $-$ timeSent timeLeft $=$ timeLeft $-$ howLongInSelect if timeLeft $<=0$: return def ping$($dest$\_$addr, timeout$=1)$: my$\_$socket $=$ socket$($AF$\_$INET, SOCK$\_$RAW, IPPROTO$\_$ICMP$)$ my$\_$id $=$ os$.$getpid$()$ & $0$xFFFF sendPing$($my$\_$socket, dest$\_$addr, my$\_$id$)$ delay $=$ receivePing$($my$\_$socket, my$\_$id$,$ timeout$)$ my$\_$socket.close$()$ return delay