Computer Networks Final
When a user clicks on a hyperlink in a webpage, the browser carries out a series of steps in order to fetch the page pointed to. Write the 9 steps.
1. Browser determines the URL 2. Browser asks DNS for the IP address of the server 3. DNS replies 4. The browser makes a TCP connections 5. Sends HTTP request for the page 6. Server sends the page as HTTP response 7. Browser fetches other URLs as needed 8. The browser displays the page 9. The TCP connections are released
As a collision‐free protocol, this protocol consists of exact N slots. If station 0 has a frame to send, it transmits a 1 bit during the slot 0. No other station is allowed to transmit during this slot. Regardless of what station 0 does, station 1 gets the opportunity to transmit a 1 bit during slot 1, but only if it has a frame queued. In general, station j may announce that it has a frame to send by inserting a 1 bit into slot j. After all N slots have passed by, each station has complete knowledge of which stations wish to transmit. At that point, they begin transmitting frames in numerical order. What is this protocol called?
Basic bit-map
Hosts need to send messages to many or all other hosts. For example, a service distributing weather reports, stock market updates, or live radio programs might work best by sending to all machines and letting those that are interested read the data. Sending a packet to all destinations simultaneously iscalled
Broadcasting
The main assumption in the Internet that DTNs seek to relax is that an end to end path between a source and a destination exists for the entire duration of a communication session. When this is not the case, the normal Internet protocols fail. DTNs get around the lack of end‐to‐end connectivity with an architecture that is based on message switching. What is this message called?
Bundle
802.11 tries to avoid collisions with channel sensing before sending and exponential backoff after collisions. What protocols are used for 802.11?
CSMA/CA
________________ is the basis of the classic Ethernet LAN, so it is worth devoting some time to looking at it in detail. It is important to realize that collision detection is an analog process. The station's hardware must listen to the channel while it is transmitting. If the signal it reads back is different from the signal it is putting out, it knows that a collision is occurring
CSMA/CD
What is NOT an error‐correcting code?
Checksums
A _________________ consists of a stiff copper wire as the core, surrounded by an insulating material. The insulator is encased by a cylindrical conductor, often as a closely woven braided mesh.
Coaxial Cables
_________________ forms the basis of 3G mobile phone networks and is also used in GPS (Global Positioning System). Even without different codes, direct sequence spread spectrum, like frequency hopping spread spectrum, can tolerate narrowband interference and multipath fading because only a fraction of the desired signal is lost.
Code Division Multiple Access
HTTP 1.1 supports persistent connections. With them, it is possible to establish a TCP connection, send a request and get a response, and then send additional requests and get additional responses. What is this strategy called?
Connection Reuse
To talk to someone, you pick up the phone, dial the number, talk, and then hang up. What service is appropriate to do that?
Connection‐oriented service
Systems in which multiple users share a common channel in a way that can lead to conflicts are known as __________
Contention System
There are five optional fields for the IP version 4 protocol. What is NOT an optional field?
Control
What is the interval between regular data frames in IEEE 802.11 called
DIFS
Because packets can take different paths through the network, they can arrive out of order. This will trigger duplicate acknowledgements even though no packets have been lost. However, this is uncommon in the Internet much of the time. When there is reordering across multiple paths, the received packets are usually not reordered too much. Thus, TCP somewhat arbitrarily assumes that three duplicate acknowledgements imply that a packet has been lost. The identity of the lost packet can be inferred from the acknowledgement number as well. It is the very next packet in sequence. This packet can then be retransmitted right away, before the retransmission timeout fires. What is this mechanism called?
Fast retransmission
What cable is used for 10GBase-SR?
Fiber optic cables are used for 10GBase-SR.
Now let us look at a different situation: station B transmitting to station A at the same time that station C wants to transmit to station D. If C senses the medium, it will hear a transmission and falsely conclude that it may not send to D. In fact, such a transmission would cause bad reception only in the zone between B and C, where neither of the intended receivers is located. What is this problem called?
Exposed terminal problem
____________ has two key components: tags and readers. RFID tags are small, inexpensive devices that have a unique 96-bit EPC identifier and a small amount of memory that can be read and written by the RFID reader.
It is EPC Gen 2 RFID Network.
_______________ is the physical layer protocol that is most commonly used over the wide‐area optical fiber links that make up the backbone of communications networks, including the telephone system.
It is SONET.
What modulation scheme is used to transmit 4 bits per symbol in quadrature amplitude modulation?
It is called QAM-16 (Quadrature Amplitude Modulation)
_________________ is a technique for regulating the average rate and burstiness of a flow of data that enters the network. The goal is to allow applications to transmit a wide variety of traffic that suits their needs, including some bursts, yet have a simple and useful way to describe the possible traffic patterns to the network.
It is called traffic shaping.
When this protocol is used, a bad frame that is received is discarded, but any good frames received after it are accepted and buffered. When the sender times out, only the oldest unacknowledged frame is retransmitted. If that frame arrives correctly, the receiver can deliver to the network layer, in sequence, all the frames it has buffered. What is this protocol called?
It is selective repeat.
If there were no noise, how many signal levels would be needed to achieve the channel capacity of 8 Mbps? (Use the Nyquist formula)
It would be 16 levels.
What is NOT a difficulty for mobile phones browsing the web?
Javascript is not supported.
Once the media information reaches the receiver, it must be played out at the right time. In general, this will not be the time at which the RTP packet arrived at the receiver because packets will take slightly different amounts of time to transit the network. Even if the packets are injected with exactly the right intervals between them at the sender, they will reach the receiver with different relative times. What is this variation in delay called?
Jitter
The__________________ standard for compressing continuous‐tone still pictures (e.g., photographs) was developed by photographic experts working under the joint auspices of ITU, ISO, and IEC, another standards body. It is widely used (look for files with the extension jpg) and often provides compression ratios of 10:1 or better for natural images.
Joint Photographic Experts Group
ariants of ______________ called IS-IS and OSPF are the routing algorithms that are most widely used inside large networks and the Internet today. The idea is fairly simple and can be stated as five parts. 1). Discover neighbors, learn network addresses. 2). Set distance/cost metric to each neighbor. 3). Construct packet telling all learned. 4). Send packet to, receive packets from other routers. 5). Compute shortest path to every other router.
Link state routing
What is a basic unit of a Bluetooth system?
Piconet
When an application (e.g., a user) process wishes to set up a connection to a remote application process, it must specify which one to connect to. The method normally used is to define transport addresses to which processes can listen for connection requests. In the Internet, these endpoints are called _______.
Ports
The data link layer takes the packets it gets from the network layer and encapsulates them into frames for transmission. Each frame contains three fields. What is NOT a field contained by each frame?
Preamble
Since the prefix length cannot be inferred from the IP address alone, routing protocols must carry the prefixes to routers. Sometimes prefixes are simply described by their length, as in a ''/16'' which is pronounced ''slash 16.'' The length of the prefix corresponds to a binary mask of 1s in the network portion. What is this mask called?
Subnet mask
This strategy is for the receiver simply to discard all subsequent frames, sending no acknowledgements for the discarded frames. It corresponds to a receive window of size 1. In other words, the data link layer refuses to accept any frame except the next one it must give to the network layer. If the sender's window fills up before the timer runs out, the pipeline will begin to empty. Eventually, the sender will time out and retransmit all unacknowledged frames in order, starting with the damaged or lost one. What is this strategy called?
go‐back‐n
A ____________ is a physical layer device that receives, amplifies (i.e., regenerates), and retransmits signals in both directions. As far as the software is concerned, a series of cable segments connected by repeaters is no different from a single cable.
repeater
Handling the general case of making two different networks interwork is exceedingly difficult. However, there is a common special case that is manageable even for different network protocols. This case is where the source and destination hosts are on the same type of network, but there is a different network in between. What technique is used to handle a connection between different networks?
tunneling
Write all elements of transport protocols.
• Addressing • Connection Establishment • Connection Release • Error Control and Flow Control • Multiplexing • Crash Recovery
At every clock tick, the counter in the head entry is decremented. When it hits zero, its event is processed and the next item on the list becomes the head. Its counter does not have to be changed. This way, inserting and deleting timers are expensive operations, with execution times proportional to the length of the list. A much more efficient approach can be used if the maximum timer interval is bounded and known in advance. To implement this, an array is used. What is this array called?
Timing wheel
What is a specific endpoint in the transport layer called?
Transport Service Access Point
What is the oldest and still most common transmission media?
Twisted pair
___________ sends a series of rapid pulses, varying their positions to communicate information. The rapid transitions lead to a signal that is spread thinly over a very wide frequency band.
Ultra‐Wide Band
Point‐to-point links connect individual pairs of machines. What is "Point‐to-point transmission with exactly one sender and exactly one receiver" called?
Unicasting
What is NOT a goal of routing algorithms?
Uniqueness
The Internet protocol suite supports a connectionless transport protocol called __________.
User Datagram Protocol
In wireless networks, a collision cannot be detected. What is used to infer collisions?
Acknowledgement
_____________ is a collection of computers that are associated so that they can directly send frames to each other on wireless networks.
Ad hoc network
Each network or link imposes some maximum size on its packets. These limits have various causes among many issues. What is NOT an issue?
Addressing
When a station has data to send, it first listens to the channel to see if anyone else is transmitting at that moment. If the channel is idle, the stations sends its data. Otherwise, if the channel is busy, the station just waits until it becomes idle. Then the station transmits a frame. If a collision occurs, the station waits a random amount of time and starts all over again. What is this protocol called?
1‐persistent CSMA
What is the channel capacity if the bandwidth is 1000 Hz and there are 8 signal levels? (Use the Nyquist formula)
6000 bps
How many active slaves can be handled by a master equipped with Bluetooth?
7 active slaves can be handled by a master equipped with Bluetooth.
Explain a hidden terminal problem.
A hidden terminal problem is where a station is not able to detect a potential competitor for the medium because the competitor is too far away.
What is a difference between slotted ALOHA and pure ALOHA?
A slotted ALOHA system turns the AHOLA system into a timed one. Users can only transmit data in slots to avoid damaging frames. In a pure ALOHA system users can transmit data whenever, there will be damaged frames.
When the packet arrives at the server, the operating system sees that the packet is requesting a connection. It checks to see if there is a listener, and if so it unblocks the listener. The server process can then establish the connection with the _____________ call.
ACCEPT
In this routing protocol, routes to a destination are discovered on demand, that is, only when a somebody wants to send a packet to that destination. This saves much work that would otherwise be wasted when the topology changes before the route is used. At any instant, the topology of an ad hoc network can be described by a graph of connected nodes. What is this routing protocol called?
AODV
The data link layer uses the services of the physical layer to send and receive bits over communication channels. What is NOT a service on the data link layer?
Assigning an identifier for each frame.
Compelling Web applications need responsive user interfaces and seamless access to data stored on remote Web servers. Scripting on the client (e.g., with JavaScript) and the server (e.g., with PHP) are basic technologies that provide pieces of the solution. These technologies are commonly used with several other key technologies in a combination called ______________.
Asynchronous Javascript and Xml
In the network layer, the Internet can be viewed as a collection of networks or _______________ that are interconnected. There is no real structure, but several major backbones exist. These are constructed from high-bandwidth lines and fast routers. The biggest of these backbones, to which everyone else connects to reach the rest of the Internet. (Do NOT write an acronym.)
Autonomous Systems
What is NOT a primitive for transport services?
BIND
the Internet is made up of a large number of independent networks that are operated by different organizations, usually a company, university, or ISP. Inside of its own network, an organization can use its own algorithm for internal routing, or ________________.
Intradomain Routing
Each router that is connected to two or more areas is called a(n) ___________. It must also be part of the backbone.
Area Border Router
If connectionless service is offered, packets are injected into the network individually and routed independently of each other. No advance setup is needed. In this context, the packets are frequently called __________.
Datagrams
What is NOT an assumption for dynamic channel allocation?
Dependent Traffic
______________________ uses a code sequence to spread the data signal over a wider frequency band. It is widely used commercially as a spectrally efficient way to let multiple signals share the same frequency band.
Direct Sequence Spread Spectrum
A ________________ operates by having each router maintain table giving the best known distance to each destination and which link to use to get there. These tables are updated by exchanging information with the neighbors. Eventually, every router knows the best link to reach each destination.
Distance vector routing algorithm
Every domain, whether it is a single host or a top‐level domain, can have a set of resource records associated with it. These records are the DNS database. Such a resource record contains a five‐tuple. Although they are encoded in binary for efficiency, in most expositions resource records are presented as ASCII text, one line per resource record. Write the format of five‐tuple.
Domain name Time to live Class Type Value
What is NOT an error‐detecting code?
Hamming codes
For the Internet, the top of the naming hierarchy is managed by an organization called __________.
Internet Corporation for Assigned Names and Numbers
What are two kinds of subsystems in email systems?
The two kinds of subsystems in email systems are user agents and message transfer agents.
Write how to manage windows in the TCP sliding window protocol.
Managing windows in TCP sliding window protocol decouples the issues of acknowledgement of the correct receipt of segments and receiver buffer allocation. For example, suppose the receiver has a 4096-byte buffer. If the sender transmits a 2048-byte segment that is correctly received, the receiver will acknowledge the segment. However, since it now has only 2048 bytes of buffer space, it will advertise a window of 2048 starting at the next byte expected. Now the sender transmits another 2048 bytes, which are acknowledge, but the advertised window is of size 0. The sender stops until the application process on the receiving host has removed some data from the buffer, at which time TCP can advertise a larger window and more data can be sent.
What is NOT a class of the services for 802.16 MAC Sublayer protocols?
Moduation service
To tackle the problem of serving a single request at a time, one strategy is to make the server ______________. In one design, the server consists of a front-end module that accepts all incoming requests and k processing modules.
Multithreaded
The problem of running out of IP addresses is not a theoretical one that might occur at some point in the distant future. It is happening right here and right now. The long‐term solution is for the whole Internet to migrate to IPv6, which has 128‐bit addresses. This transition is slowly occurring, but it will be years before the process is complete. To get by in the meantime, a quick fix was needed. The quick fix that is widely used today came in the form of ______________.
Network Address Translation
The problem of running out of IP addresses is not a theoretical one that might occur at some point in the distant future. It is happening right here and right now. The long-term solution is for the whole Internet to migrate to IPv6, which has 128-bit addresses. This transition is slowly occurring, but it will be years before the process is complete. To get by in the meantime, a quick fix was needed. The quick fix that is widely used today came in the form of ______________.
Network Address Translation
To reduce ambiguities about which station is sending, 802.11 defines channel sensing to consist of both physical sensing and virtual sensing. Physical sensing simply checks the medium to see if there is a valid signal. With virtual sensing, each station keeps a logical record of when the channel is in use by tracking _______________.
Network Allocation Vector
The physical layer process and some of the data link layer process run on dedicated hardware. What is this hardware called?
Network Interface Card
There are five goals of Mobile IP. Write one of the goals.
No overhead for mobile host at home.
The most straightforward form of digital modulation is to use a positive voltage to represent a 1 and a negative voltage to represent a 0. For an optical fiber, the presence of light might represent a 1 and the absence of light might represent a 0. What is this modulation called?
Non‐Return to Zero
What is NOT a category of QoS?
Packet bit rate
Why each packet is routed independently on datagram networks?
Packets are injected into the network individually and routed independently of each other. No advance setup is needed.
Each IP packet is sent with its header bits set to indicate that no fragmentation is allowed to be performed. If a router receives a packet that is too large, it generates an error packet, returns it to the source, and drops the packet. What is this strategy called?
Path MTU Discovery
When a process on machine 1 calls a procedure on machine 2, the calling process on 1 is suspended and execution of the called procedure takes place on 2. Information can be transported from the caller to the callee in the parameters and can come back in the procedure result. No message passing is visible to the application programmer. What is this technique called?
Remote Procedure Call
What is a unit of messages sent form transport entity to transport entity.
Segment
In the simplest of terms, one can imagine a company's information system as consisting of one or more databases with company information and some number of employees who need to access them remotely. What is a computer having the data in this model?
Server
Within the Internet, email is delivered by having the sending computer establish a TCP connection to port 25 of the receiving computer. Listening to this port is a mail server that speaks _______________.
Simple Mail Transfer Protocol
When the sender gets an acknowledgement, it increases the congestion window by one and immediately sends two packets into the network. (One packet is the increase by one; the other packet is a replacement for the packet that has been acknowledged and left the network. At all times, the number of unacknowledged packets is given by the congestion window.) However, these two packets will not necessarily arrive at the receiver as closely spaced as when they were sent. What is this protocol called?
Slow Start
What is a purpose of Slow‐Start?
Slow-Start is used for congestion control in TCP. It is designed to avoid sending more data than the network is capable of transmitting.
Write steps in making a remote procedure call.
Step 1 is the client calling the client stub. Step 2 is the client stub packing the parameters into a message and making a system call to send the message. (Packing the parameter is called marshaling). Step 3 is the operating system sending the message from the client machine to the server machine. Step 4 is the operating system passing the incoming packet to the server stub. Step 5 is the server stub calling the server procedure with the unmarshaled parameters.
What are difference between OSI and TCP/IP models?
The OSI model has seven layers and the TCP/IP model has four. The OSI model supports both connectionless and connection oriented communication in the network layer, but only connection-oriented communication in the transport layer. The TCP/IP model supports only one mode in the network layer but both in the transport layer. Also, the OSI reference model was devised before the corresponding protocols were invented. With TCP/IP the protocols were first, and the model was a description of existing protocols.
What method is used to transmit and receive independent signals in a fixed time slot?
Time Division Multiplexing
802.11 transmission is complicated by wireless conditions that vary with even small changes in the environment. At the frequencies used for 802.11, radio signals can be reflected off solid objects so that multiple echoes of a transmission may reach a receiver along different paths. The echoes can cancel or reinforce each other, causing the received signal to fluctuate greatly. What is this phenomenon called?
The phenomenon is called multipath fading.
Since wireless is inherently a broadcast medium, 802.11 radios also have to deal with the problem that multiple transmissions that are sent at the same time will collide, which may interfere with reception. What scheme is used to handle this problem?
The scheme used to handle this is Carrier Sense Multiply Access (CSMA).
Explain how a sliding window protocol works.
The sender maintains a set of sequence numbers corresponding to frames it is permitted to send. These frames are said to fall within the sending window. The receiver also maintains a receiving window corresponding to the set of frames it is permitted to accept.
List three kinds of resources that can potentially be reserved for different flows.
The three kinds of resources that can be reserved for different flows are bandwidth, buffer space, CPU Cycles
In response to customer requests for more flexibility, network vendors began working on a way to rewire buildings entirely in software. The resulting concept is called a ____________.
Virtual LAN
A _________________ spans a large geographical area, often a country or continent.
WAN (Wide Area Network)
Why do we regulate the sending rate to obtain a desirable bandwidth allocation?
We regulate the sending rate to obtain a desirable bandwidth allocation in order to avoid losing data. This is achieved in two ways: The first is flow control, in the case that there is insufficient buffering at the receiver. The second is congestion, in the case that there is insufficient capacity in the network.
The sender's data link layer inserts a special escape byte (ESC) just before each accidental flag byte in the data. A framing flag byte can be distinguished from one in the data by the absence or presence of an escape byte before it. The data link layer on the receiving end removes the escape bytes before giving the data to the network layer. What is this technique called?
byte stuffing
The "http://csit.udc.edu/index.html" URL consists of three parts. What is the host name of this URL?
csit.udc.edu
Multiplexing can also be useful in the transport layer for another reason. Suppose, for example, that a host has multiple network paths that it can use. If a user needs more bandwidth or more reliability than one of the network paths can provide, a way out is to have a connection that distributes the traffic among multiple network paths on a round‐robin basis. What is this modus operandi called?
inverse multiplexing
An allocation is ____________ if the bandwidth given to one flow cannot be increased without decreasing the bandwidth given to another flow with an allocation that is no larger.
max‐min fair
The ____________ layer controls the operation of the subnet. A key design issue is determining how packets are routed from source to destination.
network
Which layer of the OSI model does TCP (Transmission Control Protocol) belong to?
network
