Computer Networks - TDC 463 September 13, 1999 Homework Set #1 Due September 20, 1999 by the start of class 1) Use an analogy (like the postal service example we used in class) which demonstrates the use of the OSI Reference Model and the services it describes. Try to use all seven layers if possible. You can start by showing the communication of two diplomats at the highest layer from two different foreign countries (similar to the Tanenbaum example on p19). Answer: L7. Birthday card is created and read by grandmother in China L6. Message is written in English, but read by a translator in Chinese L5. Message is encrypted L4. Certified mail is used to be sure receiver signs for it L3. ZIP code and street address provide world-wide addressing L2. Secretary delivers envlope to post office L1. Postal service mail bin and airplane moves message from USA to China 2) Visit the IETF web page at http://www.ietf.org. Read the overview and become familar with the organization. a. What are the eight areas of the IETF? b. Approximately how long has the IETF been in operation? c. Find a RFC from one of the working groups that you think is important. In a short paragraph, describe what it is and why you think it's crucial to the operation of the Internet. Don't forget to tell me the RFC number. Answer: a. Applications, General, Internet, Operations and Management, Routing, Security, Transport, User Services. b. Approximately 14 years (1st meeting in 1/1986) c. RFC 791 - Internet Protocol This RFC defines the backbone of the Internet, namely the glue that holds most everything else together. IP provides addressing, routing and fragmentation necessary for data packets to traverse the global Internet. 3) Numbering systems are an important concept for computer operation and communication. Complete the following calculations. a. What is the binary equivalent of decimal number 408? b. The binary number 11010110 is what in hexadecimal? c. What is the sum of the binary numbers 10010010 and 01011001 (in binary)? Answer: a. 1 1001 1000 b. 0xd6 c. 1110 1011 4) Answer Tanenbaum 1.7. Answer: Connection-oriented communication has three phases. In the establishment phase a request is made to setup a connection. Only after this phase has been sucessfully completed can the data transfer phase be started and data transported. Then comes the release phase. Connectionless communication does not have these phases. It just sends the data. 5) How long does it take to transmit x KB over a y Mb/s link? Give your answer as a ratio of x and y. Answer: (8192x)/1,000,000y 6) If the data link layer can handle error and flow control, why are these functions often performed also in the transport layer? Answer: The data link only handles these functions on physical, data link networks. When messages traverse multiple data links, end-to-end flow control and error control need to performed at the network layer or higher. 7) From a computer, use the PING utility to test the reachability to www.yahoo.com. Now use traceroute (tracert on Windows machines) to www.yahoo.com. Show the results and comment on what you find. Are these tests bandwidth sensitive or delay sensitive? How many "hops" or intermediate systems do you go through to get to www.yahoo.com? Answer: PING output: ------------ $ ping www.yahoo.com PING www.yahoo.com (204.71.200.68): 56 data bytes 64 bytes from 204.71.200.68: icmp_seq=0 ttl=242 time=133.7 ms 64 bytes from 204.71.200.68: icmp_seq=1 ttl=242 time=108.3 ms 64 bytes from 204.71.200.68: icmp_seq=2 ttl=242 time=106.3 ms 64 bytes from 204.71.200.68: icmp_seq=3 ttl=242 time=119.0 ms --- www.yahoo.com ping statistics --- 4 packets transmitted, 4 packets received, 0% packet loss round-trip min/avg/max = 106.3/116.8/133.7 ms ------------------ TRACEROUTE output: ------------------ traceroute: Warning: www.yahoo.com has multiple addresses; using 204.71.200.68 traceroute to www.yahoo.com (204.71.200.68), 30 hops max, 40 byte packets 1 brtr (140.192.91.120) 1.943 ms 0.721 ms 0.697 ms 2 border-iag.netequip.depaul.edu (140.192.228.1) 2.924 ms 2.375 ms 2.144 ms 3 serial3-5-gw.csr1.or.il.verio.net (131.103.26.129) 2.707 ms 2.505 ms 2.244 ms 4 qualnet-0.ord0-0.verio.net (129.250.16.77) 4.730 ms 9.193 ms 3.348 ms 5 ord0.sea2.verio.net (129.250.2.221) 65.409 ms 57.210 ms 62.510 ms 6 sea2.sea0.verio.net (129.250.3.49) 53.338 ms 52.388 ms 70.409 ms 7 s8-1-0.br1.SEA1.globalcenter.net (206.132.254.53) 61.035 ms 72.639 ms 58.676 ms 8 pos2-2-155M.cr2.SEA1.globalcenter.net (206.132.41.101) 77.752 ms 54.439 ms 63.373 ms 9 pos3-2-155M.cr2.DEN1.globalcenter.net (206.132.41.86) 97.194 ms 79.940 ms 83.534 ms 10 pos0-0-622M.cr1.DEN1.globalcenter.net (206.132.117.201) 94.713 ms 98.254 ms 80.221 ms 11 pos3-0-622M.wr2.SFO1.globalcenter.net (206.132.110.113) 109.350 ms 88.643ms 95.921 ms 12 pos4-0-622M.cr2.SNV2.globalcenter.net (206.132.254.58) 100.043 ms 92.639 ms 89.635 ms 13 pos7-0-622M.cr2.SNV.globalcenter.net (206.132.151.22) 105.120 ms 91.099 ms 95.067 ms 14 pos1-0-2488M.hr8.SNV.globalcenter.net (206.132.254.41) 95.990 ms 101.485 ms 90.818 ms 15 bas2r-ge2-0-hr8.snv.yahoo.com (208.178.103.58) 96.809 ms 94.146 ms 106.841 ms 16 www3.yahoo.com (204.71.200.68) 88.604 ms 91.179 ms 96.089 ms ------------------- These tests are not bandwidth sensitive since the messages are just small connectivity messages. They are however delay sensitive since they are measuring response time. There are 16 "hops" including www.yahoo.com (www3.yahoo.com in thise case). 8) Answer Tanenbaum 1.27. Answer: One of the advantages is that if everyone uses the standard, everyone can talk to everyone. Another advantage is that widespread use of any standard will give it economies of scale, such as VLSI chips. A disadvantage is that the political comprimises necessary to achieve standardization frequently lead to poor standards. Another disadvantage is that once a standard has been widely adopted, it is very difficult to change it, even if new and better techniques or methods are discovered. 9) Broadcast, multicast and unicast (point to point) are various ways of sending data from a source to a destination. From the list of applications below, which transmission method would be most appropriate? Explain. a. File download from a web site to a PC b. Video conference session for this class c. Intenet radio broadcast of a NFL playoff game d. Distribution of network layer routing info across a internetwork e. Sending a print job to a network printer f. A network printer notifying the data link network of it's availability g. Sending e-mail Answer: a. unicast b. broadcast or multicast depending on the scope of the data link c. multicast preferred, lack of multicast support means unicast today. Broadcasting is bad and impractical in global Internet d. multicast preferred, broadcast OK for small networks e. unicast f. broadcast preferred g. unicast 10) Answer Tanenbaum 2.20 (note: assume voice bandwidth is 64Kb/s). Answer: Ignoring speech compression, a digital PCM telephones needs 64Kb/s. If we divide 10Gb/s by 64Kb/s we get 156,250 houses per cable. Current systems have hundreds of houses per cable.