COSC $4436$ COMPUTER NETWORKS

The assignment needs to be completed individually.

Total Marks: $20$ Marks

Due date: Friday, July $5$th$,2024$

Assignment $3$

In this assignment, we will investigate the IP protocol, focusing on the IPv$4.$ In the first part, we$$ll analyse

packets in a trace of IPv$4$ datagrams sent and received by the traceroute program. We will also explore

several aspects of the ICMP protocol.

Capturing packets from an execution of traceroute

To generate a trace of IPv$4$ datagrams for the first two parts of this lab, we$$ll use the traceroute

program to send datagrams of two different sizes to algomau.ca$.$ The 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 decrease the TTL in each received datagram by $1($actually$,$ RFC $791$ says that the router must

decrease the TTL by at least $1).$ If the TTL reaches $0,$ the router returns an ICMP message $($type $11$

TTL$-$exceeded$)$ to the sending host. As a result of this behaviour, 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 IP addresses of the routers between itself and the

destination by looking at the source IP addresses in the datagrams containing the ICMP TTL$-$exceeded

messages.

Let$$s run traceroute and have it send datagrams of two different sizes. The larger of the two

datagram lengths will require traceroute messages to be fragmented across multiple IPv$4$

datagrams.

$$ Linux$/$MacOS$.$ With the Linux$/$MacOS traceroute command, the size of the UDP datagram

sent towards the final 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 algomau.ca $2000$

$$ Windows. The tracert program provided with Windows does not allow one to change the

size of the ICMP message sent by tracert. So$,$ it won$$t be possible to use a Windows

machine to generate ICMP messages that are large enough to force IP fragmentation.

However, you can use tracert to generate small, fixed$-$length packets to perform this

assignment. At the DOS command prompt, enter:

$2$

$>$tracert algomau.ca

Do the following:

$$ Start up Wireshark and begin packet capture. $($Capture$->$Start or click on the blue shark fin

button in the top left of the Wireshark window$).$

$$ Enter two traceroute commands, using algomau.ca as the destination, the first with a length of

$56$ bytes. Once that command has finished executing, enter a second traceroute command for

the same destination, but with a length of $3000$ bytes.

$$ Stop Wireshark tracing.

I. Basic IPv$4$

In your trace, you should be able to see the series of UDP segments $($in the case of MacOS$/$Linux$)$ or

ICMP Echo Request messages $($Windows$)$ sent by traceroute on your computer, and the ICMP TTLexceeded messages returned to your computer by the intermediate routers. In the questions below,

we$$ll assume you$$re using a MacOS$/$Linux computer; the corresponding questions for the case of a

Windows machine should be clear. Your screen should look similar to the screenshot in Figure $1,$

where we have used the display filter $$udp$||$icmp$($see the light$-$green$-$filled display$-$filter field in Figure

$2)$ so that only UDP and$/$or ICMP protocol packets are displayed.