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CS3102 Networks > 2: Application Layer > Flashcards

2: Application Layer Flashcards

1
Q

What are end systems?

A

Devices connected to a network that are used by end users.

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2
Q

What are network-core devices?

A

Hosts in a network that are part of the core of the network, a subset of a network containing connections between routing devices, both of high bandwidth, that act to connect systems towards the network’s edge.

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3
Q

Describe the client-server architectrure.

A

A network topology that involves one or more always-on central hosts serving multiple clients that intermittently communicate with the server (only).

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4
Q

Describe the P2P architecture.

A

A network topology with no always-on servers in which peers intermittently communicate with each other, requesting services from each other and providing services to others in return. P2P networks are self-scalable since new peers add serving capability as well as requests.

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5
Q

What is a process?

A

A computer program running on a host. Server processes wait to be contacted, and client processes initiate communications.

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6
Q

What is inter-process communication?

A

OS-defined functionality allowing two processes running on the same host to communicate.

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7
Q

What is a socket?

A

The endpoint of a communication link, identified by IP address and port number (identifier) that provides an interface between a process on a client and an end-end transport protocol.

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8
Q

What is an identifier? What does it consist of?

A

A way of distinguishing processes for network communications that is compromised of the IP address and socket number of the end device.

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9
Q

What does an application-layer protocol handle?

A

When and how messages are exchanged, as well as the structure and meaning of the metadata in the messages.

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10
Q

What does an application-layer protocol require?

A

Systems that facilitate the exchange of data between two endpoints on a network, providing data integrity, timing, throughput, and security.

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11
Q

Compare TCP and UDP.

A

TCP provides reliable transport, flow control, and congestion control, and is connection-oriented. UDP does not provide any of these features. Neither provides security, timing, or a guarantee of a minimum throughput.

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12
Q

What is SSL?

A

Secure Sockets Layer is an application-layer security technology used to encrypt data to provide security.

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13
Q

What is the worldwide web?

A

An information system for accessing resources on the Internet.

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14
Q

What is HTTP?

A

Hypertext Transfer Protocol is the primary application-layer protocol used by the web. Clients send HTTP requests to servers that send a response before the resources requested are interpreted and displayed or otherwise used on the client device.

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15
Q

What is a client?

A

An intermittently connected end device that requests resources from servers on a network.

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16
Q

What is a server?

A

An always-on computer that provides network resources to client devices.

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17
Q

What is TCP?

A

Transmission Control Protocol is a transport-layer protocol facilitating the reliable transmission of data between two endpoints on a computer network.

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18
Q

What is IP?

A

Internet Protocol is a network-layer protocol that handles the routing and addressing of packets between nodes on a computer network.

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19
Q

Why is HTTP stateless?

A

Because the server maintains no information about past client requests, so each request is treated atomically.

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20
Q

What is persistent and non-persistent HTTP?

A

HTTP persistence is whether HTTP connections stay open after an object has been transmitted. In persistent HTTP, multiple objects can be sent over the same connection because it will stay open after the first is sent. In non-persistent HTTP, a connection closes after an object has been sent over it.

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21
Q

What is RTT?

A

Round trip time is the time for a small testing packet to travel from a client to a server and then back again.

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22
Q

What are the 2 types of HTTP messages?

A

request and response messages

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23
Q

What are cookies?

A

A small piece of data used to maintain the user-server state of a web browser across separate connections.

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24
Q

How can you maintain state?

A

You can use protocols that consider and store state or store data that maintains state, such as cookies or web storage.

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25
Q

What are web caches?

A

Computer systems that temporarily store web resources to bypass server lag in contacting the origin server when they are needed again.

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26
Q

What is link utilisation?

A

The proportion of the capacity of a link which it is actively being used with traffic. Link utilisation = current data rate on link ÷ link capacity.

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27
Q

How do you find access link utilisation?

A

Link utilisation = current data rate on link ÷ link capacity.

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28
Q

What is cache delay?

A

The delay between a request being made by a client and a cache returning the result of the request (rather than the origin server).

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29
Q

How do you find total delay when using a cache?

A

Where p = the probability a resource will be cached = cache hit rate,

total delay = p * delay to get resource from the origin server + (1 - p) * delay to get resource from the cache

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30
Q

What is a conditional GET?

A

Allows you to only get a resource if a condition is true, usually if it has been modified since a chosen date. This can be used with caches to only retrieve resources if the versions of them currently cached are out of date

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31
Q

What is email?

A

A group of technologies that can exchange written messages between computers on a network.

32
Q

What is a user agent?

A

Software that acts to communicate with mail servers to allow end users to communicate over email.

33
Q

What is SMTP?

A

Simple Mail Transfer Protocol is an email protocol that handles communications between mail servers to facilitate the transmission of email messages.

34
Q

What are mail servers?

A

A specialised server that handles the storing, distribution, and transfer of email messages on a network.

35
Q

What are the 3 major components of email?

A

User agents, mail servers, and SMTP.

36
Q

What is a mailbox?

A

A set of incoming email messages for a certain user.

37
Q

What is a message queue?

A

A set of outgoing email messages to be sent on behalf of a certain user.

38
Q

How does SMTP work?

A

The mail servers have a handshake, then the message sends, then the connection closes.

39
Q

What is POP?

A

Post Office Protocol is a protocol used to allow email clients to retrieve email messages from mail servers. Once downloaded, the messages are deleted from the server.

40
Q

What is IMAP?

A

Internet Message Access Protocol is a protocol used to allow email clients to retrieve email messages from mail servers. Emails aren’t deleted on the server when they’re retrieved, and the server is the single place messages are kept.

41
Q

How do SMTP, POP, and IMAP differ?

A

SMTP is server-server, POP and IMAP are server-client. POP deletes messages when retrieved from a server, IMAP doesn’t.

42
Q

What is DNS?

A

Domain Name System is a group of technologies allowing for the resolution of domain names on the Internet. It is a distributed, hierarchical database and an application-layer protocol.

43
Q

Why are name servers distributed?

A

A single server wouldn’t scale (to the whole Internet) and would mean there is a single point of failure.

44
Q

What is the heirarchy of DNS?

A

There are root, TLD, authoritative, and local DNS servers that handle iterative queries that are passed up through the DNS hierarchy to reduce strain.

45
Q

What are TLD servers?

A

Top-level domain servers are responsible for resolving the addresses under one particular domain, e.g. .com.

46
Q

What are authoritative DNS servers?

A

Domain name servers that provide name server mappings for the hosts of an organisation.

47
Q

What are local DNS name servers?

A

The default DNS server of an ISP that caches recent queries and their results.

48
Q

How do DNS queries get processed? Are they iterated? Are they recursive? Why?

A

They move up and then down the hierarchy (to find a path to the nameserver needed) until a server can answer them, then are passed back through the path, being cached at each hop back. They iterate to and from the nameserver needed, and the path through multiple name servers could be viewed as being recursion.

49
Q

How does DNS caching work?

A

Any time a DNS server learns a mapping from a euery it caches the result, with a set time to live.

50
Q

Wht are the types of DNS records?

A

A = mapping, NS = authoritative name server for a domain, CNAME = mapping for real name to alias, MX = mail server associated with a name.

51
Q

What is a DNS registrar?

A

An entity that handles the registration of domain names to IP addresses, saving them on TLD name servers.

52
Q

What are DNS RRs?

A

Resource records are the individual data entries in name servers.

53
Q

What are DNS query and reply messages?

A

Query messages are sent to name servers, they respond with reply messages.

54
Q

What is P2P architecture?

A

A network topology in which each node, or peer, both requests and provides services. It has no central always-on server, as with the client-server architecture.

55
Q

How do you find the time to distribute files in client-server networks?

A

A computer program running on a host. Server processes wait to be contacted, and client processes initiate communications.

56
Q

How do you find the time to distribute files in P2P networks?

A

time to distribute a file of size F to N clients using P2P approach, DP2P > max{F/us, F/dmin, NF/(us + Σui)}

F = file size
us = server upload capacity
dmin = min client download rate
ui = download capacity of the ith peer

F/us = time to send 1 copy
F/dmin = minimum client download time
us + Σui = max upload rate

57
Q

How does file distribution work on BitTorrent?

A

Files are divided into 256Kb chunks stored on peers and exchanged among groups called torrents. Trackers track peers participating in torrents and provide them to peers new to the network.

58
Q

What is a torrent? What is a torrent tracker?

A

A torrent is a group of peers that exchange the chunks of a file. A torrent tracker is a computer maintinging the list of peers storing different chunks in the torrent of a file.

59
Q

What is churning?

A

The idea that peers may come and go into a P2P network and a torrent.

60
Q

How does BitTorrent chunk requesting and sending work?

A

Peers wanting a file periodically ask each peer which chunks they have and request the rarest first from each. Chunks are sent to the 4 peers sending you chunks at the greatest rate: tit-for-tat. The list of the top 4 peers is reevaluated every 30 seconds, randomly unchoking a new peer which may join the top 4.

61
Q

Why is BitTorrent said to be tit-for-tat?

A

You only send chunks to the top 4 peers sending chunks to you at the fastest rate, choking all others. You reevaluate the list every 30 seconds, optimistically unchoking a peer which may join the top 4.

62
Q

Why is distributed, application-level infrastructure needed for video streaming services?

A

The massive number of users and variety in their capabilities.

63
Q

What is video coding? What is spatial coding? What is temporal coding?

A

Using redundancy within and between images to decrease the number of bits to encode an image, usually spatial and temporal. Spatial will be combining pixels of the same value, temporal only sending changes between frames in video rather than the whole frame each time.

64
Q

What is a CBR encoding?

A

Constant bit rate = a video of a fixed encoding rate.

65
Q

What is a VBR encoding?

A

Variable bit rate = a video of an encoding rate that changes as spatial and temporal codings change

66
Q

What is DASH?

A

Dynamic, Adaptive Streaming over HTTP is a system for sending videos over the Internet in such a way that the bandwidth is used optimally to provide the best quality possible. Video files are divided into multiple chunks that are stored at different encoding rates, whose URLs are stored in manifest files

67
Q

What are CDNs?

A

Geographically distributed private networks run by an organisation that connects its data centres to the Internet. They are aka content provider networks.

68
Q

Why are CDNs needed?

A

A single server or collection in one place would not scale to millions of simultaneous diverse users with different capabilities.

69
Q

What are the variants of CDNs used?

A

Enter deep: close to users in many access networks

Bring home: a smaller number of larger clusters of servers in POPs near to access networks - but not in them.

70
Q

What are Over-the-top media services? What are their challenges?

A

Internet0based streaming services that bypass the traditional media of TV and telecommunications, such as Netflix.

71
Q

What are the protocols for transport-layer networking?

A

TCP and UDP.

72
Q

What is UDP?

A

User Datagram Protocol is a transport-layer protocol that is a barebones alternative to TCP, not providing reliable transmission but facilitating faster transmission that is useful with some applications.

73
Q

How is UDP different from TCP?

A

TCP provides reliable transport, flow control, and congestion control, and is connection-oriented. UDP does not provide any of these features. Neither provides security, timing, or a guarantee of a minimum throughput.

74
Q

How does socket programming work with UDP?

A

Connectionless, unreliable datagrams are sent and may or may not be received. methods of ensuring reliability must be built on top of UDP. But might be faster than TCP with less overhead for some uses.

75
Q

How does socket programming work with TCP?

A

A connection is created with a handshake, providing a reliable, in order “pipe” across which data may be exchanged.

76
Q

What are the method types in HTTP/1.0?

A

GET, POST, and HEAD.

77
Q

What are the method types in HTTP/1.1?

A

GET, POST, HEAD, PUT, and, DELETE.

CS3102 Networks (10 decks)

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