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Networks Flashcards

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1
Q

Consider a 1 Gbps LAN “a.com”, with a local HTTP cache and a local DNS server, connected to the Internet through an access (bottleneck) link of 1 Mbps. Suppose that there are two user machines, m1.a.com and m2.a.com, in the network a.com. Suppose the user at m1.a.com types in the URL www.b.com/bigfile.htm into a browser to retrieve a large file of 1 G bits (1000 M bits) from another 1 Gbps LAN “b.com” on the Internet (via a 1 Mbps router).

(a) List the sequence of DNS and HTTP messages sent/received from/by m1.a.com, and any that leave/enter the a.com network.
(b) how long would it take assuming the propagation time from a.com to anywhere on the internet is 500 msec?

A 
(a)
1. m1.a.com -> Local HTTP Cache
2. Cache is empty, therefore go to DNS server:
    Local HTTP Cache -> Local DNS Server
3. Local DNS Server -> Root DNS Server
4. Root DNS Server -> Local DNS Server
5. Local DNS Server -> .com DNS Server
6. .com DNS Server -> Local DNS Server
7. Local DNS Server -> b.com DNS Server
8. b.com DNS Server -> Local DNS Server
9. Local DNS Server -> Local HTTP Cache
10. Cache attempt to obtain information:
      Local HTTP Cache -> b.com
11. b.com -> Local HTTP Cache
12. Local HTTP Cache -> m1.a.com

(b)
1005 sec

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2
Q

Minimum Distribution time for Client-Server and P2P Architectures

A 
Where
F       = file size
u(s)   = upload rate of the server
u(i)    = upload rate at user i
N      = number of peers
dmin = minimum download time of any peer

Client-Server:
Min Dist = max( F * N / u(s), F / dmin)

P2P Dist:
Min Dist = max( (F / u(s), f/dmin ), (N * F / u(s) ) + N * u(i) )

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3
Q

Utilization

A

U = (L / R) / (RTT + L / R)

Where L = packet size
R = rate
RTT = round-trip time

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4
Q

OSI (Open Systems Interconnection) Model

A 
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Link Layer
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5
Q

TCP/IP Model

A

Application Layer
Transport Layer
Internet Layer
Link Layer

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6
Q

Why use Layered Protocols?

A

(1) Provides an abstraction for upper layers, so that the upper layers can use the services without worrying about how they operate.
(2) Different types of upper layers can use the same lower layer.

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7
Q

What is the difference between connection-oriented and connection-less?

A

Connection-oriented: Establish a connection before any data is transferred. It’s reliable and in-order, needed when we want to make sure data from a particular layer is transferred correctly. Think TCP

Connection-less: No connection is established beforehand, and data is not guaranteed to be reliable or in-order. Used when minimal over is preferred and reliability isn’t critical. Think UDP

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8
Q

FDM (Frequency Division Multiplexing)

A

Each circuit continually gets a fraction of the bandwidth.

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9
Q

STDM (Synchronous Time Division Multiplexing)

A

Each circuit gets all of the bandwidth periodically during brief intervals of time.

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10
Q

A cable TV system has 100 commercial channels; each of them is alternating between programs and advertising. Is this more like TDM (Time Division
Multiplexing) or like FDM (Frequency Division Multiplexing)?

A

The TV System is like FDM insofar as each channel is getting a fraction of the bandwidth at any given time, and the alternating between programs and advertising is like TDM, since each program / advertisement is getting the full bandwidth when it airs.

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11
Q

With the help of a diagram, describe the different components and the operation of an electronic mail application.

A

Client 1 -> over SMTP -> Client 1 Server -> Client 2 Server -> PoP3/IMAP/HTTP -> Client 2

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12
Q

We have seen a number of application-layer protocols. Give examples and explain the difference between (i) stateless and stateful protocols, (ii) push and pull protocols, and (iii) persistent and non-persistent protocols.

A

(i)
Stateless: HTTP 1.0 (without cookies), PoP3 (logging onto a different client will not be able to see messages that have been read since they’ve been deleted)
Stateful: IMAP
(Preserving state vs. not)

(ii)
Push: SMTP
Pull: HTTP, IMAP, PoP3

(iii)
Persistent: HTTP 1.1
Non-persistent: HTTP 1.0

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13
Q

Mention at least three techniques to scale client-server application architectures (i.e. reduce their load on the server and/or network for a large number of clients)?

A

(1) Caching
(2) Multiple Servers
(3) TCP Congestion Control
(4) Increase the bottleneck of the network

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14
Q

(a) A client sends a 128-byte request to a server located 100 km away over a 1-Gbps optical fiber, and then waits for a reply to get back. What is the utilization of the link (i.e. percentage of time the link is busy transmitting) during this transaction? Take the speed of light in fiber optics to be 200 km/msec. (b) Consider the situation of part (a) again. Compute the minimum possible response time both for the 1-Gbps link and for a 1-Mbps link. What conclusion can you draw?

A

(a)
U = L / R / (RTT + L / R)

L / R = (128 * 8 bits) / 1,000,000,000 bps
= 0.001 msec
RTT = 1 msec

Therefore U = 0.001

(b)
1-Mbps: 1.001 msec
1-Gbps: 1.000001 msec
Conclusion: Negligible.

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15
Q

Bandwidth Delay Product

A

= capacity (bits / s) * RTT

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16
Q

How many bits you need for a sequence number

A

Max N = (RTT + Tx) / Tx
(Tx = transmission time of a packet)
Then decide how many bits are needed to obtain N

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17
Q

Connection Establishment in TCP

A

1) Sender sends a SYN with sequence # x
2) Receiver sends a SYN with sequence # y
3) Receiver sends an ACK with sequence # x + 1
4) Sender sends an ACK with sequence # y+1

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18
Q

What is the role of the network layer?

A

To move packets from a sending host to a receiving host.

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19
Q

What is a virtual-circuit network?

A

A network that provides only connection service at the network layer (as opposed to connection-less). This is implemented in the routers in the network core as well as in the end systems.

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20
Q

What is split-horizon route advertisement?

A

A method of preventing routing loops in distance-vector routing protocols. It stops a router from advertising a route back onto the interface from which it was learned. So: if A reaches node C via B, it won’t tell B its path to C.

21
Q

What is DHCP?

A

Dynamic Host Configuration Protocol:

It allocates IP addresses to computers that connect to a network dynamically & automatically.

22
Q

What is a MAC address?

A

A Media Access Control address is a unique identifier assigned to network interfaces for communications on the physical network segment. They’re usually stored in the network interface controller (hardware of a computer). Same as ethernet/physical addresses.

23
Q

What is CDMA?

A

Code division multiple access, a channel partitioning protocol (the other two main ones being TDM, time-division multiplexing, and FDM, frequency-division multiplexing). CDMA assigns a different code to each node, which then uses that code to encode the data bits it sends.If all of the codes are indeed unique, then the different nodes can transmit simultaneously without worrying about interference from other nodes!

24
Q

What is WiFi?

A

The IEEE 802.11 wireless LAN

25
Q

What is the BSS?

A

The Basic Service Set. It is the fundamental building block of the 802.11 architecture. It contains one or more wireless stations and a central base station, known as an access point.

26
Q

What is SSID?

A

A one or two-word Service Set Identifier. It’s an ID for an access point.

27
Q

What does it mean to associate with an access point?

A

The wireless station creates a virtual wire between itself and the access point; in other words, only the associated AP will send data frames to your wireless station if you associate with it, and your wireless station will send data frames into the Internet only through that associated AP.

28
Q

How does association occur?

A

Passive Scanning: Access points periodically send out beacon frames that contain their SSID and MAC address. Your wireless station, knowing that APs send out beacon frames, scans the 11 channels of 802.11 (between 2.4 GHz and 2.485 GHz), seeking out any beacon frames. After finding one, your wireless station can select it for association.

Alternatively, a wireless station can perform Active Scanning by braodcasting a probe frame that will be received by all APs within the wireless host’s range. The APs will then reply with a probe response frame, allowing the wireless station to choose between them.

After associating, the host will want to join the subnet to which the AP belongs, so it will send a DHCP discovery message into the subnet via the AP in order to obtain an IP address on the subnet. Once the address is obtained, the rest of the world then views that host simply as another host with an IP address in that subnet.

29
Q

What are the 3 classes of multiple access protocols?

A

1) Channel partitioning (including CDMA)
2) Random Access
3) Taking turns

30
Q

What sort of multiple access protocol does 802.11 use for handling datagrams from multiple wireless stations?

A

Random access protocol, specifically Carrier Sense Multiple Access with collision avoidance, or CSMA/CA. It’s the same sort of thing as Ethernet uses. Each station senses the channel before transmitting and refrains from transmitting when the channel is sensed busy.

31
Q

What are some differences between the Ethernet and 802.11 CSMA MAC protocols?

A

1) Ethernet uses collision detection techniques while 802.11 uses collision-avoidance techniques.
2) 802.11 uses a link-layer ACK/retransmission (ARQ) scheme (Ethernet doesn’t need to because it’s fairly reliable, while wireless channels have high bit error rates).

32
Q

Why doesn’t 802.11 CSMA implement collision detection?

A

1) The ability to detect collisions requires the ability to send and receive at the same time (needs to determine if another station is also transmitting). Because the strength of the received signal is typically very small compared to the strength of the transmitted signal at the 802.11 adapter, it is costly to build hardware that can detect a collision.
2) Even if the adapter could transmit and listen at the same time, the adapter would still not be able to detect all collisions due to the hidden terminal problem and fading.

33
Q

How does CSMA/CA work?

A

1) If the station senses the channel idle, it transmits its frame after a short period of time known as the Distributed Inter-frame Space (DIFS)
2) If the channel is busy, it chooses a random backoff value and counts it down whenever the channel is sensed idle.
3) When the counter reaches 0, the station transmits its entire frame and then waits for an ACK.
4) If an ACK is received, the transmitting station knows that its frame has been correctly received at the destination station. If it has more frames to send, it begins again at step 2. Otherwise, it enters the backoff phase in step 2.

34
Q

What is RTS and CTS?

A

Control frames to reserve access to the channel of an Access Point. RTS = Request to Send, CTS = Clear to Send. It’s to solve the problem of hidden terminals (which means that a wireless station might not see another because it’s too far away, and so will think the channel on the access point is idle when it’s not).

When a sender wants to send a DATA frame, it can first send an RTS frame to the AP, indicating the total time required to transmit the DATA frame and the ACK frame. When the AP receives the RTS frame, it responds by broadcasting a CTS frame. This CTS gives the sender explicit permission to send and also instructs the other stations not to send for the reserved duration.

35
Q

True or False: It is always good to use RTS and CTS when wirelessly transmitting data frames.

A

False: While it can reduce collisions, it also introduces delay and consumes channel resources. It should only be used for transmitting very long DATA frames.

36
Q

What is ARP?

A

Address Resolution Protocol. It’s used to convert IP addresses to physical addresses such as Ethernet addresses.

37
Q

What is the CRC field in an 802.11 or Ethernet frame?

A

A 32-bit Cyclic Redundancy Check so that the receiver can detect bit errors in the received frame.

38
Q

What are the 4 6-byte MAC addresses in an 802.11 frame?

A

Address 1: The MAC address of the wireless station that is to receive the frame.

Address 2: The MAC address of the station that transmits the frame.

Address 3: The MAC address of the router interface that connects this BSS’s subnet to other subnets.

Address 4: Used when Access Points forward frames to each other in ad hoc mode.

39
Q

What is Bluetooth?

A

A low-power, short-range, low-rate wireless network, aka a WPAN (wireless personal area network). These networks are 802.15.1 and are ad hoc, and therefore there is no access point; 802.15.1 devices thus organize themselves. The devices use a master-slave relationship, with one master and multiple slaves.

40
Q

What is a BTS?

A

Base Transceiver Station - transmits signals to and receives signals from the mobile stations in its cell (each cell usually contains a BTS, unless a few very close cells share one). These cells are geographic areas that make up a cellular network.

41
Q

If host H1 is sending an 802.11 frame through an AP to the internet via router R1, what will the frame’s MAC addresses look like?

A

Address 1: AP MAC Address
Address 2: H1 MAC Address
Address 3: R1 MAC Address

After going through the Access point, the frame will be an 802.3 frame with:

Dest. Address: R1 MAC Address
Src. Address: H1 MAC Address

42
Q

What services can be provided by the Link Layer?

A

1) Framing (encapsulate each network-layer datagram within a link-layer frame before transmitting. This consists of a data field (that contains the network-layer datagram) and several header fields.
2) Link Access (uses a medium access control (MAC) protocol)
3) Reliable delivery (some links can guarantee delivery without error using ACK’s and retransmissions, similar to the transport-layer’s TCP)
4) Error detection and correction

43
Q

Where is the link layer usually implemented?

A

A network adapter, i.e. a network interface card (NIC)

44
Q

What is the binary exponential backoff algorithm?

A

When transmitting a frame that has already experienced n collisions, a node chooses a value K at random from the set {0, 1, 2, … 2^n - 1} and waits that amount of time before retransmitting. For Ethernet, the actual amount is K*512, or K * the amount of time needed to send 512 bits into the Ethernet.

45
Q

What is the efficiency of CSMA/CD?

A

The long-run fraction of time during which frames are being transmitted on the channel without collisions when there is a large number of active nodes, with each node having a large number of frames to send.

Efficiency = 1 / ( 1 + 5*dprop / dtrans )

Where:
dprop = the max time it takes signal energy to propagate between any two adapters
dtrans = the time to transmit a max-size frame

46
Q

How does ARP work?

A

1) The sender constructs a special packet called an ARP query packet and sends it to its network adapter. (ARP query and response packets have the same format, including fields such as the sending and receiving IP and MAC addresses).
2) The adapter encapsulates the ARP packet in a link-layer frame, uses the broadcast address (FF-FF-FF-FF-FF-FF) for the frame’s destination address, and transmits the frame into the subnet.
3) All other adapters on the subnet receive the broadcast packet and pass it to their ARP modules.
4) The ARP modules check to see if their IP address matches the destination IP address in the ARP packet.
5) If they match, it sends an ARP response packet with the desired mapping to the querying host, which then updates its ARP table and can send its IP datagram encapsulated in a link-layer frame whose destination MAC is that of the host or router responding to the earlier ARP query.
6) If they don’t, it simply discards the ARP packet.

47
Q

What is a load balancer?

A

Distributes network connections among a pool of similar servers.

48
Q

FTP vs. NFS

A

FTP = File Transfer Protocol

  • Used on the internet for exchanging files
  • Uses TCP/IP

NFS = Network File System

  • Client/server application that allows network users to access shared files stored on computers of different types.
  • Uses a Virtual File System (VFS) that runs on top of TCP/IP
  • Users can either operate as clients or servers, depending on if they’re accessing remote files or providing access to local ones.

Computer Science (13 decks)

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