CSE 3300: Programming Assignment 2 ICMP Pinger Lab and Raw Sockets
In this lab, you will gain a better understanding of Internet Control Message Protocol (ICMP). You will learn to implement a Ping application using ICMP request and reply messages using raw sockets.
Ping is a computer network application used to test whether a particular host is reachable across an IP network. It is also used to self-test the network interface card of the computer or as a latency test. It works by sending ICMP “echo reply” packets to the target host and listening for ICMP “echo reply” replies. The “echo reply” is sometimes called a pong. Ping measures the round-trip time, records packet loss, and prints a statistical summary of the echo reply packets received (the minimum, maximum, and the mean of the round-trip times and in some versions the standard deviation of the mean).
Your task is to develop your own Ping application in Python. Your application will use ICMP but, in order to keep it simple, will not exactly follow the official specification in RFC 1739. Note that you will only need to write the client side of the program, as the functionality needed on the server side is built into almost all operating systems.
You should complete the Ping application so that it sends ping requests to a specified host separated by approximately one second. Each message contains a payload of data that includes a timestamp. After sending each packet, the application waits up for one second to receive a reply. If one second goes by without a reply from the server, then the client assumes that either the ping packet or the pong packet was lost in the network (or that the server is down).
Code
Below you will find the skeleton code for the client. You are to complete the code. The places where you need to fill in code are marked with #Fill in start and #Fill in end.
Additional Notes
1. In “receiveOnePing” method, you need to receive the structure ICMP_ECHO_REPLY and fetch the information you need, such as checksum, sequence number, etc. Study the “sendOnePing” method before trying to complete the “receiveOnePing” method.
2. You do not need to be concerned about the checksum, as it is already given in the code.
3. This lab requires the use of raw sockets. In some operating systems, you may need
administrator/root privileges to be able to run your Pinger program.
4. See the end of this programming exercise for more information on ICMP.
5. For your reference, a few slides on ICMP and bitwise operation in Python are posted with the
programming assignment.
Testing the Pinger
First, test your client by sending packets to localhost, that is, 127.0.0.1.
Then, you should see how your Pinger application communicates across the network by pinging servers in different continents.
What to Hand in
When you hand in your programming assignment, you should include the following (please submit to HuskyCT):
• A program listing containing in-line documentation. Uncommented code will be heavily penalized.
• A separate (typed) document describing the overall program design, a verbal description of “how it works”, and design tradeoffs considered and made. Also describe possible improvements and extensions to your program (and sketch how they might be made). The format of the description file should be as follows (If your turn-in does not follow the above format, you’ll get 10 points off automatically):
Description: describe the overall program design and “how it works”
Tradeoffs: discuss the design tradeoffs you considered and made.
Extensions: describe possible improvements and extensions to your program, and
describe briefly how to achieve them.
Test cases: describe the test cases that you ran to convince yourself (and us) that it is indeed correct. Also describe any cases for which your program is known not to work correctly.
Grading policy
Program Listing
works correctly in-line documentation quality of design
Design Document
description
tradeoffs discussion extensions discussion
Thoroughness of test cases Total
50 points (shown by testing results) 10
10
5 5 5
15
100 points
Late Programs NOT allowed Notes:
• A full 10 points for quality of design will only be given to well-designed, thorough programs. A correctly working, documented and tested program will not necessarily receive these 10 points.
A Few Words About Borrowing Code…
As many of you probably already know, there is a wealth of sample sockets code out on the Web and in books that you can use in your projects. Often learning from sample codes is the best way to learn. I have no problem with your borrowing code as long as you follow some guidelines:
• You may not borrow code written by a fellow student for the same project.
• You must acknowledge the source of any borrowed code. This means providing a
reference to a book or URL where the code appears (you don’t need to provide reference
for the code that was given to you).
• In your design document, you must identify which portions of your code were borrowed
and which were written by yourself. This means explaining any modifications and extension you made to the code you borrowed. You should also use comments in your source code to distinguish borrowed code from code you wrote yourself.
• You should only use code that is freely available in the public domain.
Appendix: Skeleton Python Code for the ICMP Pinger
from socket import *
import os
import sys
import struct
import time
import select
import binascii
ICMP_ECHO_REQUEST = 8
def checksum(string):
csum = 0
countTo = (len(string) // 2) * 2
count = 0
while count < countTo: thisVal = ord(string[count+1]) * 256 + ord(string[count]) csum = csum + thisVal csum = csum & 0xffffffff count = count + 2 if countTo < len(string): csum = csum + ord(string[len(string) – 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 answerdef receiveOnePing(mySocket, ID, timeout, destAddr): timeLeft = timeout while 1: startedSelect = time.time() whatReady = select.select([mySocket], [], [], timeLeft) howLongInSelect = (time.time() – startedSelect) if whatReady[0] == []: # Timeoutreturn “Request timed out.” timeReceived = time.time() recPacket, addr = mySocket.recvfrom(1024)#Fill in start #Fetch the ICMP header from the IP packet#Fill in end timeLeft = timeLeft – howLongInSelect if timeLeft <= 0:return “Request timed out.”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(str(header + data)) # Get the right checksum, and put in the header if sys.platform == ‘darwin’: # Convert 16-bit integers from host to networkbyte order myChecksum = htons(myChecksum) & 0xffff 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: http://sock-raw.org/papers/sock_raw mySocket = socket(AF_INET, SOCK_RAW, icmp) myID = os.getpid() & 0xFFFF# Return the current process i sendOnePing(mySocket, destAddr, myID) delay = receiveOnePing(mySocket, myID, timeout, destAddr) mySocket.close() return delaydef ping(host, timeout=1): # timeout=1 means: If one second goes by without a reply from the server, # the client assumes that either the client’s ping or the server’s pong is lost dest = gethostbyname(host) print(“Pinging ” + dest + ” using Python:”) print(“”) # Send ping requests to a server separated by approximately one second while 1 : delay = doOnePing(dest, timeout) print(delay) time.sleep(1)# one second return delayping(“google.com”)Appendix: Internet Control Message Protocol (ICMP) ICMP HeaderThe ICMP header starts after bit 160 of the IP header (unless IP options are used).• Type – ICMP type.• Code – Subtype to the given ICMP type.• Checksum – Error checking data calculated from the ICMP header + data, with value 0for this field.• ID – An ID value, should be returned in the case of echo reply.• Sequence – A sequence value, should be returned in the case of echo reply.Echo RequestThe echo request is an ICMP message whose data is expected to be received back in an echo reply (“pong”). The host must respond to all echo requests with an echo reply containing the exact data received in the request message.• Type must be set to 8.• Code must be set to 0.• The Identifier and Sequence Number can be used by the client to match the reply with therequest that caused the reply. In practice, most Linux systems use a unique identifier for every ping process, and sequence number is an increasing number within that process. Windows uses a fixed identifier, which varies between Windows versions, and a sequence number that is only reset at boot time.• The data received by the echo request must be entirely included in the echo reply.Echo ReplyThe echo reply is an ICMP message generated in response to an echo request, and is mandatory for all hosts and routers.• Type and code must be set to 0.• The identifier and sequence number can be used by the client to determine which echorequests are associated with the echo replies.• The data received in the echo request must be entirely included in the echo reply.Bits160-167 168-175 176-183 184-191 160Type Code Checksum 192ID Sequence
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