What is the difference between a host and an end system? List the types of end systems. Is a web server an End system?

There is no difference. Throughout this text, the words “host” and “end system” are used interchangeably. End systems include PCs, workstations, Web servers, mail servers, Internet-connected PDAs, WebTVs, etc.

What is a client program? What is a server program? Does a server program request and receive services from a client program?

1.       Dial-up modem over telephone line: residential;

2.       DSL over telephone line:

1.       residential or small office;

2.       Cable to HFC: residential;

3.       100 Mbps switched

4.       Ethernet: company;

5.       Wireless LAN: mobile;

6.       Cellular mobile access (for example, WAP): mobile

Is HFC transmission rate dedicated or shared among users? Are collisions possible in a downstream HFC channel? Why?

HFC bandwidth is shared among the users. On the downstream channel, all packets emanate from a single source, namely, the head end. Thus, there are no collisions in the downstream channel.

List the available residential access technologies in your city. For each type of access provide the advertised downstream rate, upstream rate and monthly price.

1.       dial-up;

2.       DSL;

3.       cable modem;

4.       fiber-to-the-home.

What is the transmission rate of Ethernet LANs? For a given transmission rate can each user on the LAN continuously transmit at that rate?

Ethernet LANs have transmission rates of 10 Mbps, 100 Mbps, 1 Gbps and 10 Gbps. For an X Mbps Ethernet (where  X = 10, 100, 1,000 or 10,000), a user can continuously transmit at the rate X Mbps if that user is the only person sending data. If there are more than one active user, then each user cannot continuously transmit at X Mbps

What are some of the physical media and Ethernet can run over?

Ethernet most commonly runs over twisted-pair copper wire and “thin” coaxial cable. It also can run over fibers optic links and thick coaxial cable.

Dial-up modems, HFC, DSL, and FTTH are all user for redisential access. For each of these access technologies, provide a range of transmission rate and comment on whether the transmission rate is shared or dedicated.

Dial up modems: up to 56 Kbps, bandwidth is dedicated; ISDN: up to 128 kbps, bandwidth is dedicated; ADSL: downstream channel is .5-8 Mbps, upstream channel is up to 1 Mbps, bandwidth is dedicated; HFC, downstream channel is 10-30 Mbps and upstream channel is usually less than a few Mbps, bandwidth is shared. FTTH: 2-10Mbps upload; 10-20 Mbps download; bandwidth is not shared.

Describe the most popular wireless Internet access technologies today. Compare and contrast them.      

There are two most popular wireless Internet access technologies today:

a) Wireless LAN: In a wireless LAN, wireless users transmit/receive packets to/from a base station  (wireless access point) within a radius of few tens of meters. The base station is  typically connected to the wired Internet and thus serves to connect wireless users  to the wired network.

b) Wide-area wireless access network: In these systems, packets are transmitted over the same wireless infrastructure used for cellular telephony, with the base station thus being managed by a telecommunications provider. This provides wireless access to users within a  radius of tens of kilometers of the base station.

What advantage does a circuit-switched network have over a packet-switched network? What advantages does TDM have over FDM in a circuit-switched network?

A circuit-switched network can guarantee a certain amount of end-to-end bandwidth for the duration of a call.  Most packet-switched networks today (including the Internet) cannot make any end-to-end guarantees for bandwidth.

What is it said that packet switching employs statistical multiplexing? Contrast statistical multiplexing with the multiplexing that takes places in TDM.

In a packet switched network, the packets from different sources flowing on a link do not follow any fixed, pre-defined pattern. In TDM circuit switching, each host gets the same slot in a revolving TDM frame.  

Suppose there is exactly one packet switch between a sending host and a receiving host. The transmission rate between the sending host and the switch and between the switch and the receiving host are R1 and R2, respectively. Assuming that the switch uses store-and-forward packet switching, what is the end-to-end delay to send a Packet of length L?

At time t0 the sending host begins to transmit. At time t1 = L/R1, the sending host completes transmission and the entire packet is received at the router (no propagation delay). Because the router has the entire packet at time t1, it can begin to transmit the packet to the receiving host at time t1. At time t2 = t1 + L/R2, the router completes transmission and the entire packet is received at the receiving host (again, no propagation delay). Thus, the end-to-end delay is L/R1 + L/R2.

What is the key distinguishing difference between a tier-1 ISP and a tier-2 ISP?

A tier-1 ISP connects to all other tier-1 ISPs; a tier-2 ISP connects to only a few of the tier-1 ISPs. Also, a tier-2 ISP is a customer of one or more tier-1.

Suppose users share a 2 Mbps link. Also suppose each user transmits continuosly at 1 Mbps when transmitting, but each user transmits only 20 percent of the time.

a.Whe na cirvuit switching is used, how many users can be supported?

2 users can be supported because each user requires half of the link bandwidth

b.For the remainer of this problem suppose packet switching is used. Wher will there be essentially no queuing delay before the link if two or fewer users transmit at the same time? What will there be a queuing delay if three users transmit at the same time?

                Since each user requires 1Mbps when transmitting, if two or fewer users transmit  simultaneously, a maximum of 2Mbps will be required. Since the available  bandwidth of the shared link is 2Mbps, there will be no queuing delay before the  link. Whereas, if three users  transmit  simultaneously, the bandwidth required  will be 3Mbps which is more than the available bandwidth of the shared link. In  this case, there will be queuing delay before the link.

                c. Find the probability that a given user is transmitting

                Probability that a given user is transmitting = 0.2

                d. Suppose now there are three users. Find the probability that at any given time, all three users are transmitting simultaneously. Find the fraction of time during which the queue grows.

Probability that all three users are transmitting simultaneously =  (3/3)p^3(1-p)^(3-3) = (0.2)^3 = 0.008. Since the queue grows when all the users are transmitting, the  fraction of time during which the queue grows (which is equal to the probability that all three users are transmitting simultaneously) is 0.008.  

R16. Consider sending a packet from a source host to a destination host over a fixed route. List the delay components in the end-to-end delay. Which of these delays are constant and which are variable?

The delay components are processing delays, transmission delays, propagation delays, and queuing delays. All of these  delays are fixed, except for the queuing delays, which are variable.

Java Applet

R18. How long does it take a packet of length 1,000 bytes to propagate over a link of distance 2,500 km, propagation speed 2.5 · 108 m/s, and transmission rate 2 Mbps? More generally, how long does it take a packet of length L to propagate over a link of distance d, propagation speed s, and transmission rate R bps? Does this delay depend on packet length? Does this delay depend on transmission rate?

10msec; d/s; no; no

R19. Suppose Host A wants to send a large file to Host B. The path from Host A to Host B has three links, of rates R1 = 500 kbps, R2 = 2 Mbps, and R3 = 1 Mbps.

a.       Assuming no other traffic in the network, what is the throughput for the file transfer.

a.       Suppose the file is 4 million bytes. Dividing the file size by the through put, roughly how long will it take to transfer the file to Host B?

a.       Repeat (a) and (b), but now with R2 reduced to 100 kbps.

a.  500 kbps

b. 64 seconds

 c. 100 kbps; 320 seconds

Suppose end system A wants to send a large file to end system B. At a very high level, describe how end system A creates packets from the file. When one of these packets arrives to a packet switch, what information in the packet does the switch use to determine the link onto which the packet is forwarded? Why is packet switching in the Internet analogous to driving from one city to another and asking directions along the way?

End system A breaks the large file into chunks. To each chunk, it adds header generating multiple packets from the file. The header in each packet includes the address of the destination: end system B. The packet switch uses the destination address to determine the outgoing link. Asking which road to take is analogous to a packet asking which outgoing link it should be forwarded on, given the packet’s address.

R22. List five tasks that a layer can perform. Is it possible that one (or more) of these tasks could be performed by two (or more) layers?

Five generic tasks are error control, flow control, segmentation and reassembly, multiplexing, and connection setup. Yes, these tasks can be duplicated at different layers. For example, error control is often provided at more than one layer.

R23. What are the five layers in the Internet protocol stack? What are the principal responsibilities of each of these layers?

The five layers in the Internet protocol stack are – from top to bottom – the application layer, the transport layer, the network layer, the  link layer, and the physical layer. The principal responsibilities are outlined in Section 1.5.1.

R24. What is an application-layer message? A transport-layer segment? A network-layer datagram? A link-layer frame?

Application-layer message: data which an application wants to send and passed onto the transport layer; transport-layer segment: generated by the transport layer and encapsulates application-layer message with transport layer header; network-layer datagram: encapsulates transport-layer segment with a network-layer header; linklayer frame: encapsulates network layer datagram with a link-layer header.

R25. Which layers in the Internet protocol stack does a router process? Which layers does a link-layer switch process? Which layers does a host process?

Routers process layers 1 through 3. (This  is a little bit of a white lie, as modern routers sometimes act as firewalls or caching components, and process layer four as well.) Link layer switches process layers 1 through 2. Hosts process all five layers.

R26. What is the difference between a virus, a worm, and a Trojan horse?

a) Virus: Requires some form of human interaction to spread. Classic example: E-mail  viruses.

b) Worms: No user replication needed. Worm in infected host scans IP  addresses and port numbers, looking for vulnerable processes to infect.

c) Trojan horse: Hidden, devious part of some otherwise useful software

Creation of a botnet requires an attacker to find vulnerability in some application or system (e.g. exploiting the buffer overflow  vulnerability that might exist in an application). After finding the vulnerability, the attacker needs to scan for hosts that are vulnerable. The target is basically  to compromise a series of systems by exploiting that particular vulnerability.  Any system that is part of the botnet can automatically scan its environment and propagate by exploiting the vulnerability. An important property of such botnets is that the originator of the botnet can remotely control and issue commands to all the  nodes in the botnet. Hence, it becomes possible for the attacker to  issue a command to all the nodes, that target a single node (for example, all nodes in the botnet might be commanded by the attacker to send a TCP SYN message to the target, which might result in a TCP SYN flood attack at the target). 

R28. Suppose Alice and Bob are sending packets to each other over a computer network. Suppose Trudy positions herself in the network so that she can capture all the packets sent by Alice and send whatever she wants to Bob; she can also capture all the packets sent by Bob and send whatever she wants to Alice. List some of the malicious things Trudy can do from this position.

Trudy can pretend to be Bob to Alice (and vice-versa) and partially or completely modify the message(s) being sent from Bob to Alice. For example, she can easily change the phrase “Alice, I owe you  $1000” to “Alice, I owe you $10,000”. Furthermore, Trudy can even drop the packets that are being sent by Bob to Alice (and vise-versa), even if the packets from Bob to Alice are encrypted.