CS447/CS642 Laboratory Exercise

The purpose of this exercise is to show how protocols transport each other's frames. You will observe how:

• Ethernet 802.3 frames carry IP packets between MAC addresses.
• IP packets carry TCP packets between IP addresses
• TCP packets carry data between port numbers using a sliding window protocol with piggybacked acknowledgements.
• IP addresses are translated to MAC addresses
• Netscape uses a TCP connection for each page
• IP Packets travel over numerous intervening networks to a destination

0. Starting MS Windows

Find an NCR desktop computer (NOT an UNH Pentium computer). If it is off, turn it on and wait for it to display the C:\> prompt. At the prompt, type win to start Windows 3.1, then, in the box for the INTERNET group, you should see icons labeled: PING, TRUMPHOP, EWAN (or TERMINAL) and NETSCAPE.

1. Adjusting TCPMAN

Double click the PING icon. A blue and yellow TCPMAN icon should now be present in the lower left corner of the screen.

(You may need to minimize the main window to see the TCPMAN icon. To minimize a window, place the mouse's arrow in the box at the window's top left corner. Then hold down the left mouse button, drag the arrow to the MINIMIZE menu item, and release the mouse button. The window should become an icon in the lower left corner. It can be restored by double clicking that icon.)

Double click the TCPMAN icon and adjust it so that the entire window is visible. Select TRACE from its menu, and check the EXTRA DETAIL and ETHERNET items.

2. Pinging

Return to the PING window by clicking it, and enter charger for its host address. Then enter 1000 for the period and click START. Observe the activity in the TCPMAN window. It should display Ethernet MAC addresses of source and destination, once per second.

Next, check the TCPMAN TRACE IP menu item and observe the exchange of IP packets between your station, 204.60.72.2xx and charger at 204.60.72.11.

Uncheck the TCPMAN TRACE ETHERNET item and close PING.

3. Telnet Terminal Communication

While still in the TCPMAN window, check the TCPMAN TRACE TCP menu item. Next, back in the INTERNET group, double click the EWAN (or TERMINAL) icon, select charger from the TERMINAL FILE OPEN menu, and click OK. The telnet service uses the connection-oriented TCP transport protocol. Each connection is uniquely identified by a (source-port, destination-port) pair. The telnet server always uses port 23. Observe that:

• Each IP packet traveling between your station, 204.60.72.2xx and charger at 204.60.72.11, contains prot 6, which is the protocol number which IP uses when carrying TCP data.
  
• Packets sent to 204.60.72.11 (charger) use destination port 23, the "well-known" port number for telnet service; while the source port number at your station's telnet client has been temporarily assigned just for this telnet session.
  
• Your station's client passes through state = syn_sent to state = established during the exchange of telnet handshaking information.
  
• A sliding window protocol is used, where sequence numbers count bytes of data transmitted. Thus, for example, when a packet with (hexadecimal) seq 00000004 and (decimal) data 21 is sent to port 23, then next packet sent to port 23 has (hexadecimal) seq 00000019 (= decimal 25 = 4+21).
  
• When a packet arrives outside the (huge) window (i.e. a duplicate packet), it is rejected with the remark Unacceptable segment.
  
• There is a negotiation process for telnet options, which are exchanged in 3-byte units beginning with a FF byte and ending with FF FB 01 from charger which means that charger agrees (FB) to echo (01) characters it receives. Thereafter, 512-byte packets are received from charger, containing its welcoming banner.

Type a few characters to the login: prompt (or log in if you have a charger account). Observe that:

• The characters you type are sent in packets of data 1 or data 2 bytes each, and are echoed back by charger (as promised in the telnet negotiation).
  
• The Password: prompt arrives in a data 10 byte packet.

Now return to the TCPMAN window and uncheck the TRACE TCP item.

4. Netscape HTTP HyperText Transfer Protocol

Netscape's HyperText Transfer Protoocol actually uses a short duration TCP connection to transfer each page. You can observe this by starting (double clicking) Netscape in the INTERNET group, and then entering the URL for a simple page like:   zeus/course/cs642/pix, BUT DON'T HIT RETURN YET. First, return to the TCPMAN window to check TRACE TCP. Next, click the mouse in the NETSCAPE window to re-activate it, and now hit return. Observe the conversation in the TCPMAN window which passes through TCP states syn_sent and established using IP protocol 6 (TCP). You might also be able to see your Netscape browser (User-Agent: Mozilla) exchange information with zeus's HTTP server (Server: Apache/1.1.0) identifying each other. After the page arrives, TCP returns to state closed.

Can you find the IP number of zeus? Can you tell the number of the "well-known" port used for HTTP service?

5. ARP Address Resolution Protocol

Uncheck (for the last time) the TCPMAN TRACE TCP option, and return to the TERMINAL window. If you are still logged in to charger, then just hit RETURN a couple of times to make sure the connection is still alive. If not, you can login again by selecting the SESSION CONNECT menu item.

If you have no charger account, you can use the visitor account on cs. Its password is password. Select FILE OPEN cs from the TERMINAL menu, and click OK. Log in to cs.

Next, type /sbin/arp on charger, (but if you are on cs, just type arp). The table that is displayed contains the mapping between IP numbers and MAC (Ethernet) addresses. If you can find your station's IP address in the TCPMAN window, you should now be able to find its matching MAC address from the table you just displayed. This is the hardware address of the network card in your PC.

You should now type control-c control-d to exit from charger (or cs), and then close the TERMINAL window.

6. Internet Hop Utility

Return to the INTERNET group and select (double click) the HOP icon. The HOP program can display the route taken by an IP packet on the way to its destination. The program sets the RECORD ROUTE IP option which requests that gateway computers through which the packet passes, return a packet to the source, your station. Enter yale.edu in the HOP Host field and hit RETURN. Observe that:

• While the route is being displayed in the HOP window, the TCPMAN window displays each IP packet as it returns.
• A gateway computer that fails to return a packet to the source, is identified as *** in the HOP window.

The process ends with the remark Trace route finished - Port unreachable. Note that if you attempt again to trace the hops to the same destination, the second attempt may yield a route different from the first route. This is because network traffic conditions are in constant flux, and therefore routing tables may have changed in the interval between attempts.

Do not close the HOP window yet.

7. Domain Name Service

Return to the TCPMAN window and check the TRACE DNS menu item. This option displays your station's interaction with the huge distributed data base of names and IP numbers which is called Domain Name Service. You are about to ask cs.newhaven.edu at IP address 204.60.72.200 for the IP address of charger.newhaven.edu). When a domain name server can not answer a question, it forwards the question to its parent domain name server in the tree of domain name servers.

Highlight (click) the HOST field in the HOP window and type charger.newhaven.edu. For a second time, click the START button in the HOP window, to cause another route trace, but this time, observe the intial display in the TCPMAN window. Observe that:

• IP packets use prot 17, which is UDP, a connectionless datagram protocol in which no connections are established (unlike TCP).
  
• Your station first attempts to find the requested name in the local domain, newhaven.edu, by posing the Question: charger.newhaven.edu.newhaven.edu. There is no answer.
  
• Your station next poses a different Question to its domain name server. It poses the question: charger.newhaven.edu, and receives Answer charger.newhaven.edu ... 204.60.72.11, followed by a list of (2) domain name servers which have information about the newhaven.edu domain.

This interaction with a domain name server occurs whenever an internet user types a domain name, for example, in a URL, or as the result of clicking a hypertext link.

8. Cleaning up

Before exiting, please return to the TCPMAN window to UNCHECK ALL THE TRACE OPTIONS. Then exit all the other windows.

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