Question
When answer the questions, please provide screenshots of the captured packet to support your conclusion?50% penalty will be applied if no supported captures are provided.
When answer the questions, please provide screenshots of the captured packet to support your conclusion?50% penalty will be applied if no supported captures are provided. If one capture can be used in multiple questions, you do not have to repeated show it.
? Capturing packets from an execution of traceroute
In order to generate a trace of IP datagrams for this lab, well use the traceroute program to send datagrams of different sizes towards some destination, X. Traceroute operates by first sending one or more datagrams with the time-to-live (TTL) field in the IP header set to 1; it then sends a series of one or more datagrams towards the same destination with a TTL value of 2; it then sends a series of datagrams towards the same destination with a TTL value of 3; and so on. Recall that a router must decrement the TTL in each received datagram by 1 (actually, RFC 791 says that the router must decrement the TTL by at least one). If the TTL reaches 0, the router returns an ICMP message (type 11 TTL-exceeded) to the sending host. As a result of this behavior, a datagram with a TTL of 1 (sent by the host executing traceroute) will cause the router one hop away from the sender to send an ICMP TTL-exceeded message back to the sender; the datagram sent with a TTL of 2 will cause the router two hops away to send an ICMP message back to the sender; the datagram sent with a TTL of 3 will cause the router three hops away to send an ICMP message back to the sender; and so on. In this manner, the host executing traceroute can learn the identities of the routers between itself and destination X by looking at the source IP addresses in the datagrams containing the ICMP TTL-exceeded messages. Well want to run traceroute and have it send datagrams of various lengths.
? Windows. Download and install pingplotter(http://www.pingplotter.com.), and test it out by performing a few traceroutes to your favorite sites. The size of the ICMP echo request message can be explicitly set in pingplotter by selecting the menu item Edit-> Options->Packet Options and then filling in the Packet Size field. The default packet size is 56 bytes. Once pingplotter has sent a series of packets with the increasing TTL values, it restarts the sending process again with a TTL of 1, after waiting Trace Interval amount of time. The value of Trace Interval and the number of intervals can be explicitly set in pingplotter.
? Linux/Unix/MacOS. With the Unix/MacOS traceroute command, the size of the UDP datagram sent towards the destination can be explicitly set by indicating the number of bytes in the datagram; this value is entered in the traceroute command line immediately after the name or address of the destination. For example, to send traceroute datagrams of 2000 bytes towards gaia.cs.umass.edu, the command would be: traceroute gaia.cs.umass.edu 2000
1. Start up Wireshark and begin packet capture (Capture->Start) and then press OK on the Wireshark Packet Capture Options screen (well not need to select any options here).
2. If you are using a Windows platform, start up pingplotter. If you do not want to purchase a license for this software. It provides a 30 days trail. Be sure you try the PRO version in order to do this LAB. Enter the name of a target destination in the Target name. Set the time interval as 10 seconds and focus time as 60 seconds, so you dont gather too much data. Select the menu item Edit > Options>Default Settings->Engine and enter a value of 56 in the Packet Size field and then press OK. You should see a pingplotter window that looks something like this:
3. Stop Wireshark tracing. In the questions below, well assume you are using a Windows machine; Answer the following question: Select the first ICMP Echo Request message sent by your computer (filter it with ip.src== your IP), and expand the Internet Protocol part of the packet in the packet details window.
a) Within the IP packet header, what is the value in the upper layer protocol field?
b) How many bytes are in the IP header? How many bytes are in the payload of the IP datagram? Explain how you determined the number of payload. Next, sort the traced packets according to IP source address by clicking on the Source column header; a small downward pointing arrow should appear next to the word Source. If the arrow points up, click on the Source column header again. Select the first ICMP Echo Request message sent by your computer, and expand the Internet Protocol portion in the details of selected packet header window. In the listing of captured packets window, you should see all of the subsequent ICMP messages (perhaps with additional interspersed packets sent by other protocols running on your computer) below this first ICMP. Use the down arrow to move through the ICMP messages sent by your computer.
d) Which fields in the IP datagram always change from one datagram to the next within this series of ICMP messages sent by your computer? (Hint: the right answer are three fields from {header checksum, source IP address, header length, time to live, Identification number, protocol}.) Next (with the packets still sorted by source address) find the series of ICMP TTL-exceeded replies sent to your computer by the nearest (first hop) router. (Hint: you can look at your network configuration like what we did in class to obtain the IP address of your default gateway or use ipconfig in the command line to check)
e) What is the value in the Identification field and the TTL field?
? Fragmentation
4. Next, repeat the process and send a set of datagrams with a longer length, by selecting Edit > Options>Default Settings->Engine and enter a value of 2000 in the Packet Size field and then press OK.
f) Find the first ICMP Echo Request message that was sent by your computer after you changed the Packet Size in pingplotter to be 2000. Has that message been fragmented across more than one IP datagram? (Hint: You can simply start a new packet capture of pingplotter by setting the packet size to 2000 bytes and then sort the captured packets based on the source IP address).
g) Capturing the first fragment of the fragmented IP datagram. What information in the IP header indicates that the datagram been fragmented? What information in the IP header indicates whether this is the first fragment versus a latter fragment? How long is this IP datagram?
h) Capturing the second fragment of the fragmented IP datagram. What information in the IP header indicates that this is not the first datagram fragment? Are the more fragments? How can you tell? 5. Finally, start a new trace and send a set of datagrams with a longer length, by selecting Edit > Options>Default Settings->Engine and enter a value of 3500 in the Packet Size field and then press OK.
i) How many fragments were created from the original datagram?
j) What fields change in the IP header among the fragments?
? Remark: If you are using a Unix or Mac platform, enter three traceroute commands, one with a length of 56 bytes, one with a length of 2000 bytes, and one with a length of 3500 bytes.
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