Assignments for CSC 460

• Assignment 1
• Assignment 2
• Assignment 3
• Assignment 4
• Assignment 5
• Assignment 6
• Assignment 7

• Page 64, Problem 7: In this problem we consider sending voice from Host A to Host B over a packet-switched network (for example, Internet phone). Host A converts analog voice to a digital 64 kbps bit stream on the fly. Host A then groups the bits into 48-byte packets. There is one link between Host A and B; its transmission rate is 1 Mbps and its propagation delay is 2 msec. As soon as Host A gathers a packet, it sends it to Host B. As soon as Host B receives an entire packet, it converts the packet's bits to an analog signal. How much time elapses from the time a bit is created (from the original analog signal as Host A) until the bit is decoded (as part of the analog signal at Host B)?
• Page 66, Problem 18: Suppose there is a 10 Mbps microwave link between a geostationary satellite and its base station on Earth. Every minute the satellite takes a digital photo and sends it to the base station. Assume a propagation speed of 2.4 * 10⁸ meters/sec.
  1. What is the propagation delay of the link?
  2. What is the bandwidth-delay product, R * t_prop?
  3. Let x denote the size of the photo. What is the minimum value of x for the microwave link to be continuously transmitting?
• Page 172, Problem 2: Read RFC 959 for FTP. List all of the client commands that are supported by the RFC.
• Page 172, Problem 6: Suppose within your Web browser you click on a link to obtain a Web page. The IP address for the associated URL is not cached in your local host, so a DNS look-up is necessary to obtain the IP address. Suppose that n DNS servers are visited before your host receives the IP address from DNS; the successive visits incur a RTT of RTT₁, ..., RTT_n. Further suppose that the Web page associated with the link contains exactly one object, consisting of a small amount of HTML text. Let RTT₀ denote the RTT time between the local host and the server containing the object. Assuming zero transmission time of the object, how much time elapses from when the client clicks on the link until the client receives the object?
• Page 174, Problem 14: Suppose you are downloading MP3s using some P2P file-sharing system. The bottleneck in the Internet is your residential access link, which is a 128 kbps full-duplex link. While you are downloading MP3s, all of a sudden 10 other users start uploading MP3s from your computer. Assuming that your computer is very powerful, and all of these downloads and uploads are not putting any strain on your computer (CPU, disk I/O, and so on), will the simultaneous uploads - which are also passing through your bottleneck link - slow down your downloads? Why or why not? Also answer the same question for when you have 128kpbs upstream and 512kbps downstream as part of an ADSL connection.
• Page 174, Problem 16: In our coverage of Gnutella in Section 2.6, we described in some detail how a new peer joins the Gnutella network. In this problem we want to explore what happens when a peer leaves the Gnutella network. Suppose every participating node maintains TCP connections to at least four distinct peers at all times. Suppose Peer X, which has five TCP connections to other peers, wants to leave.
1. First consider the case of a graceful departure, that is, Peer X explicitly closes his application, thereby gracefully closing its five TCP connections. What actions would each of the five formerly connected peers take?
2. Now suppose that X abruptly disconnects from the Internet without notifying its five neighbors that it is closing the TCP connections. What would happen?

• Page 287, Problem 7: Give a trace of the operation of protocol rdt3.0 when data packets and acknowledgement packets are garbled. Your trace should be similar to that used in Figure 3.16
• Page 289, Problem 15: Consider a scenario in which a Host, A, wants to simultaneously send messages to Hosts B and C. A is connected to B and C via a broadcast channel - a packet sent by A is carried by the channel to both B and C. Suppose that the broadcast channel connecting A, B, and C can independently lose and corrupt messages (and so, for example, a message sent from A might be correctly received by B, but not by C). Design a stop-and-wait-like error-control protocol for reliably transferring a packet from A to B and C, such that A will not get new data from the upper layer until it knows that both B and C have correctly received the current packet. Give FSM descriptions of A and C. (Hint: The FSM for B should be essentially the same as for C.) Also, give a description of the packet format(s) used.
• Page 291, Problem 25: In Section 3.5.4, we saw that TCP waits until it has received three duplicate ACKs before performing a fast retransmit. Why do you think the TCP designers chose not to perform a fast retransmit after the first duplicate ACK for a segment is received?
• Page 291, Problem 27: Consider the following plot of TCP window size as a function of time.
  Image on page 291 not shown
  Assuming TCP Reno is the protocol experiencing the behavior shown above, answer the following questions. In all cases, you should provide a short discussion justifying your answer
  1. Identify the intervals of time when TCP slow start is operating
  2. Identify the intervals of time when TCP congestion avoidance is operating
  3. After the 16th transmission round, is segment loss detected by a triple-duplicate ACK or by a timeout?
  4. After the 22nd transmission round, is segment loss detected by a triple-duplicate ACK or by a timeout?
  5. What is the initial value of Threshold at the first transmission round?
  6. What is the value of Threshold at the 18th transmission round?
  7. What is the value of Threshold at the 24th transmission round?
  8. During what transmission round is the 70th segment sent?
  9. Assuming a packet loss is detected after the 26th round by the receipt of a triple duplicate ACK, what will be the values of the congestion-window size and of Threshold?
• Page 293, Problem 34: Consider sending an object of size O = 100 Kbytes from server to client. Let S = 536 bytes and RTT = 100 msec. suppose the transport protocol uses static windows with window size W. (See Section 3.7.2)
  1. For a transmission rate of 28 kbps, determine the minimum possible latency. Determine the minimum window size that achieves this latency.
  2. Repeat (a) for 100 kbps.
  3. Repeat (a) for 1 Mbps.
  4. Repeat (a) for 10 Mbps.

• Page 404, Problem 4: Consider a VC network with a 2-bit field for the VC number. Suppose that the network wants to set up a virtual circuit over four links: link A, link B, link C, and link D. Suppose that each of these links is currently carrying two other virtual circuits, and the VC numbers of these other VCs are as follows:

  Link A	Link B	Link C	Link D
  00 01	01 10	10 11	11 00

  In answering the following questions, keep in mind that each of the existing VCs may only traverse one of the four links.
  1. If each VC is required to use the same VC number on all links along its path, what VC number could be assigned to the new VC?
  2. If each VC is permitted to have different VC numbers in the different links along its path (so that the forwarding table must perform VC number translation), how many different combinations of four VC numbers (one for each of the four links) could be used?
• Page 405, Problem 7: Consider a datagram network using 32-bit host addresses. Suppose a router has four links, numbered 0 through 3, and packets are to be forwarded to the link interfaces as follows:

  Destination Address Range	Link Interface
  11100000 00000000 00000000 00000000 through 11100000 11111111 11111111 11111111	0
  11100001 00000000 00000000 00000000 through 11100001 00000000 11111111 11111111	1
  11100001 00000001 00000000 00000000 through 11100001 11111111 11111111 11111111	0
  otherwise	0

  1. Provide a forwarding table that has four entries, uses longest-prefix matching, and forwards packets to the correct link interfaces
  2. Describe how your forwarding table determines the appropriate link interface for datagrams with destination addresses:
     11001000 10010001 01010001 01010101
     11100001 00000000 11000011 00111100
     11100001 10000000 00010001 01110111
• Page 407, Problem 15: Consider sending a 3000-byte datagram into a link that has a MTU of 500 bytes. Suppose the original datagram is stamped with the identification number 422. How many fragments are generated? What are their characteristics?
• Page 407, Problem 19: Following up on the previous problem, now suppose that both Arnold and Bernard are behind NATs. Try to devise a technique that will allow Arnold to establish a TCP connection with Bernard without application-specific NAT configuration. If you have difficulty devising such a technique, discuss why.
• Page 408, Problem 21: Consider the following network (figure on page 408). With the indicated link costs, use Dijkstra's shortest-path algorithm to compute the shortest path from x to all network nodes. Show how the algorithm works by computing a table similar to Table 4.3.
• Page 411, Problem 33: Consider the topology shown in Figure 4.41. Suppose that all links have unit cost and that node E is the broadcast source. Using arrows like those shown in Figure 4.41, indicate links over which packets will be forwarded using RPF, and links over which packets will not be forwarded, given that node E is the source.

• Page 494, Problem 4: Consider the 4-bit generator, G, shown in Figure 5.8, and suppose the D has the value 10101010. What is the value of R?
• Page 496, Problem 11: Suppose nodes A and B are on the same 10Mbps Ethernet segment, and the propagation delay between the two nodes is 225 bit times. Suppose node A begins transmitting a frame and, before it finishes, node B begins transmitting a frame. Can A finish transmitting before it detects that B has transmitted? Why or why not? If the answer is yes, then A incorrectly believes that its frame was successfully transmitted without a collision. Hint: Suppose at time t=0 bit times, A begins transmitting a frame. In the worst case, A transmits a minimum-sized frame of 512+64 bit times. So A would finish transmitting the frame at t=512+64 bit times. Thus, the answer is no, if B's signal reaches A before bit time t=512+64. In the worst case, when does B's signal reach A?
• Page 497, Problem 164: Recall that ATM uses 53-byte packets consisting of 5 header bytes and 48 payload bytes. Fifty-three bytes is unusually small for fixed-length packets; most networking protocols (IP, Ethernet, Frame Relay, and so forth) use packets that are, on average, significantly larger. One of the drawbacks of a small packet size is that a large fraction of link bandwidth is consumed by overhead bytes; in the case of ATM, almost 10 percent of the bandwidth is "wasted" by the ATM header. In this problem we investigate why such a small packet size was chosen. To this end, suppose that the ATM cell consists of L bytes (possibly different from 48) and 5 bytes of header.
  1. Consider sending a digitally encoded voice source directly over ATM. Suppose the source is encoded at a constant rate of 64 kbps. Assume each cell is entirely filled before the source sends the cell into the network. The time required to fill a cell is the packetization delay. In terms of L, determine the packetization delay in milliseconds
  2. Packetization delays greater than 20 msec can cause a noticeable and unpleasant echo. Determine the packetization delay for L=1,500 bytes (roughly corresponding to a maximum-sized Ethernet packet) and for L=48 (corresponding to an ATM cell)
  3. Calculate the store-and-forward delay at a single ATM switch for a link rate of R=155 Mbps (a popular link speed for ATM) for L=1,500 bytes, and for L=48 bytes.
  4. Comment on the advantages of using a small cell size.
• Page 560, Problem 3 Suppose that the receiver in Figure 6.5 wanted to receive the data being sent by sender 2. Show (by calculation) that the receiver is indeed able to recover sender 2's data from the aggregate channel signal by using sender 2's code.
• Page 561, Problem 11 Consider two mobile nodes in a foreign network having a foreign agent. It is possible for the two mobile nodes to use the same care-of address in mobile IP? Explain your answer.

You are to create a simple POP3 client. You may choose any language you want (it does not have to be in Java, although I expect many of them will be). When starting, it should ask the user for their account name and password (you may be lazy and allow the password to show on the screen). When you connect to the POP3 server, you should list the numbers assigned to the available messages. You should then respond to four commands:

Create a Perl/CGI script to handle a pizza buisness. The CGI script should display a form for ordering a pizza. The user should have a choice between Large, Medium, and Small pizza, and have a choice of at least four toppings, as well as a place to enter their name and address. When the user submits the form, you should display the requested pizza, the cost of the pizza, and the user name and address. You may choose for yourself the cost of each pizza size and toppings. If you wish, you may use either one file or separate files for ordering the pizza and processing the order.

• Page 723, Problem 1: Using the monoalphabetic cipher in Fig 8.3, encode the message "this is an easy problem." Decode the message "rmij'u uamu xyj."
• Using the Vigenère cipher discussed in class, encode the message "I am a computer science student." using the keyword "binary".
• Page 724, Problem 7: The Internet BGP routing protocol uses the MD5 digest rather than public key encryption to sign BGP messages. Why do you think MD5 was chosen over public key encryption?
• Page 724, Problem 9: In the protocol and discussion of Figure 8.19, why doesn't Alice have to authenticate Bob explicitly?
• Page 724, Problem 12: Consider the following variation of the packet-filtering firewall discussed in Section 8.6. Suppose Alice wants to disallow access to her network 222.22.0.0/16 from the public Internet (rule R3 in the first table below). Alice is again collaborating with Bob and his colleagues, who are at an university, and so Alice wants to let users from Bob's university (whose network address is 111.11.0.0/16) access a specific subnet, 222.22.22/24, within her company's network (rule R1 below). Alice knows that Trudy, a well-known hacker, is in Bob's university and that Trudy's subnet, 111.11.11/24, is an insecure hacker haven. So Alice doesn't want any traffic from 111.11.11/24 entering anywhere into her network (rule R2 below) except into the special subnet 222.22.22/24 (allowed under rule R1; this exception is the important difference from our example in Section 8.6). Alice's packet filtering rules are summarized in the table below.
  

  Rule	Source Address	Destination Address	Action	Comments
  R1	111.11/16	222.22.22/24	permit	Let datagrams from Bob's university network (including Trudy's hacker subnet) into a restricted subnet
  R2	111.11.11/24	222.22/16	deny	Don't let traffic from Trudy's subnet into Alice's network; but see R1.
  R3	0.0.0.0/0	0.0.0.0/0	deny	don't let traffic into Alice's network

  • Fill in the table below for the actions taken for this scenario under ordering R1, R2, and R3, and under ordering R2, R1, R3.
  • For packets P1, P2, P3, and P4, what would be the result of removing rule R2?

  Datagram Number	Source IP Address	Destination IP Address	Desired Action	Action Under R2, R1, R3	Action Under R1, R2, R3
  P1	111.11.11.1 (hacker subnet)	222.22.6.6 (corp net)	deny
  P2	111.11.11.1 (hacker subnet)	222.22.22.2 (special subnet)	permit
  P3	111.11.6.6 (univ. net, not the hacker subnet)	222.22.22.2 (special subnet)	permit
  P4	111.11.6.6 (univ. net, not the hacker subnet)	222.22.6.6 (corp net)	deny