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Network Computing > Application Layer > Flashcards

Application Layer Flashcards

1
Q

What is application architecture?

A

The application architecture is designed by the application developer and dictates how the application is structured over the various end systems.

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2
Q

What is the client-server architecture?

A

In a client-server architecture there is a server which services requests from clients. Examples: Web, FTP, Telnet, email.

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3
Q

What is a P2P architecture?

A

In a P2P architecture there is minimal or no reliance on dedicated servers in data centers. Instead the application exploits direct communication between pairs of intermittently connect hosts called peers. Examples are BitTorrent, and Skype.

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4
Q

What is a hybrid architecture?

A

A hybrid architecture combines both client-server and P2P elements. For example, an instant messaging app will sent messages P2P, but first must look up the IP address of their contact before sending the message.

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5
Q

What is a compelling feature of P2P architecture?

A

One of the most compelling features of P2P architectures is their self-scalability. Each peer both generates workload by requesting files, and adds to service capacity by distributing files to other systems.

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6
Q

What are 3 major challenges facing P2P applications?

A

1- ISP friendly. Most ISPs have been dimensioned for asymmetrical bandwidth usage.
2- Security. Because of their highly distributed and open nature, P2P applications can be a challenge to secure.
3- Incentives. The success of future P2P applications also depends on convincing users to volunteer bandwidth, storage and computation resources to the applications.

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7
Q

What is a socket?

A

A socket is an interface through which a process sends and receives messages. A process is analogous to a house and its socket is analogous to its door.

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8
Q

What do we need to identify the receiving process?

A

1 - the address of the host

2 - an identified that specifies the receiving process in the destination host

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9
Q

How is a host identified in the Internet?

A

The host is identified by its IP address.

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10
Q

What is a bandwidth-sensitive application?

A

An application that has a throughput requirement (such as a multimedia application: Netflix)

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11
Q

What is an elastic application?

A

An application that can make use of as much or as little throughput as happens to be available: Email

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12
Q

What is an application layer protocol?

A

An application layer protocol defines how an applications processes running on different end systems pass messages to each other. In particular:

  • Type of messages
  • Syntax of messages
  • Semantics of fields
  • Rules
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13
Q

Which transport layer protocol does HTTP use?

A

HTTP uses TCP

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14
Q

Why is HTTP said to be a stateless protocol?

A

HTTP is said to be a stateless protocol because the server maintains no information about the clients

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15
Q

What is the difference between non-persistent and persistent connections?

A

A non-persistent connection makes each request/response pair over a separate TCP connection. A persistent connection makes each request/response pair on the same TCP connection.

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16
Q

What is round-trip time (RTT)

A

Round trip time is the time it takes for a small packet to travel from client to server and then back to the client

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17
Q

What are the parts of an HTTP response message?

A

Status line, header lines and entity body

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18
Q

How does HTTP use cookies?

A

HTTP uses cookies to identify the client (to the server)

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19
Q

What are the four components of a cookie?

A

header line in the response, header line in the request, cookie files on the users end system, and a back end database at the website

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20
Q

What is a web cache?

A

Also known as a proxy server, a web cache is a network entity that satisfies HTTP requests on behalf of an origin web server. The web cache has its own disk storage and keeps copies of recently requested objects in this storage.

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21
Q

Why use caching?

A

Caching can substantially reduce the response time for a client request, particularly if the bottleneck bandwidth between the client and the origin server is much less than the bottleneck bandwidth between the client and cache. Web caches can significantly reduce traffic on an institutions link to the Internet.

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22
Q

What is a CDN?

A

CDN stands for Content Distributing Networks. A CDN company installs caches throughout the Internet to localize traffic and improve performance

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23
Q

What is FTP?

A

FTP stands for File Transfer Protocol. It is used when a user wants to transfer files to a remote host.

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24
Q

What is the striking difference between FTP and HTTP?

A

FTP uses two parallel TCP connections to transfer a file: a control connection (for authentication) and a data connection

25
Q

What are the three major components of internet mail?

A

user ages, mail servers and Simple Mail Transfer Protocol

26
Q

What is a user agent?

A

An email client like Outlook or Apple Mail

27
Q

What does a mail server do?

A

Stores an outgoing email in an outgoing message queue, and incoming emails in incoming message queues

28
Q

What does SMTP do?

A

It uses the reliable data transfer service of TCP to transfer mail from the sender mail server to the recipients mail server.

29
Q

What is are the three important differences between HTTP and SMTP?

A

HTTP is a pull protocol (requests things). SMTP is a push protocol (sends things).
SMTP requires 7-bit ASCII encoding and HTTP does not.
HTTP encapsulates messages in a response message, SMTP sends messages as a single object

30
Q

What are POP3 and IMAP?

A

mail access protocols to retrieve inbox messages from a mail server.

31
Q

What is the main goal of the Domain Name System?

A

The DNS is a distributed database implemented in hierarchy of DNS servers, and an application layer that allows host to query the distributed database

32
Q

What happens (DNS) when a user types in a web address in a browser and hits enter?

A
  • Browser extracts the hostname and passes it to the client side of the DNS
  • The DNS client sends a query containing the hostname to the DNS server
  • The DNS client eventually receives a reply which is the IP address for the hostname
  • When the browser receives the IP address it can initiate a TCP connection
33
Q

What is a canonical hostname?

A

A hostname that has more mnemonic aliases

34
Q

How does the DNS implement load distribution?

A

A busy site may use several servers to service requests from clients. DNS will return a batch of IP addresses and rotate the order of the addresses (round robin style) in the list to ensure no one server is being used more than any other

35
Q

What would be the problem with a single centralized DNS database?

A
  • a single point of failure
  • traffic volume
  • distant centralized database
  • maintenance
36
Q

What are the types of DNS servers?

A

Root, Top-Level, Authoritative, Local

37
Q

What is the typical journey of a message from sender to receiver?

A

The message is sent by the senders user agent to the senders mail server outgoing mail queue. It then travels to the receivers mail server where it is deposited in the receivers mailbox and retrieved by the receivers user agent.

38
Q

How does a mail server deal with delivery failure?

A

The senders message is held in an outgoing message queue, if the message can not be delivered it will attempt to resend the message at regular intervals until it gives up. The sender will then receive an email notification of that failure.

39
Q

What is a major downfall of SMTP?

A

SMTP requires that messages be encoded in 7 bit ASCII

40
Q

How does SMTP transfer mail from a sender’s mail server to a receiver’s mail server?

A
  • SMTP creates a TCP connection between the sender and receiver mail servers. It should be noted that this is a direct connection and mail will never end up on an intermediate server.
  • There are some handshakes performed between the sender and receiver servers (identifying the sender and receivers email address)
  • If the handshake is successful the message is sent.
  • If there are multiple messages the rest of the messages will be sent, otherwise the TCP connection is closed.
41
Q

What type of connection does SMTP use?

A

Persistent connections

42
Q

Describe POP3

A

POP3 stands for Post Office Protocol version 3. It is a simple mail protocol.
POP3 happens in 3 steps: authorization, transaction and update.
A user agent will gain authorization by submitting a username and password to the mail server. During transaction the user agent can get new messages and mark old messages for deletion. In the third phase after the connection has been closed the mail server will delete the messages that had been marked.

43
Q

What is the problem with POP3?

A

POP3 makes it hard to remotely access emails, and impossible to manage them by storing them in remote folders

44
Q

What is a main difference between IMAP and POP3

A

IMAP associates each message with a folder. All messages are first associated with the INBOX folder, but can then be moved into user designated folders. IMAP also allows users to obtain components of messages in the event that they don’t want to get the whole thing (for instance, if there was a large video attachment)

45
Q

What protocol is used to send web-based mail?

A

HTTP

46
Q

What is a DNS root server?

A

A root server contains the IP addresses of TLD (top level domain) registry organizations.

47
Q

What is a TLD server?

A

A TLD (top level domain) server is responsible for all major domains such as .com, .gov, .org and all country domains such as .ca

48
Q

What are Authoritative DNS servers?

A

Every organization with publicly accessible hosts must provide publicly accessible records that map the names of those hosts to IP addresses.

49
Q

What is a local DNS server?

A

A local DNS server does not strictly belong to the hierarchy of servers but is nevertheless central to DNS architecture. Each ISP has a local DNS server.

50
Q

What are resource records (RR)?

A

Resource records are hostname to IP address mappings.

51
Q

What is a registrar?

A

A registrar is a commercial entity that verifies the uniqueness of a domain name and enters the name into the DNS database then charges you a fee for its services.

52
Q

What is distribution time?

A

Distribution time is the time it takes to get a copy of the file to all N peers.

53
Q

How do we calculate distribution rate?

A

Distribution rate = max (F/u, F/dmin, NF/utotal)
Where N is the number of peers, F is the number of bits in the file, u is the upload speed, and dmin is the lower download rate of any peer.

54
Q

Why is P2P architecture considered to be self-scaling?

A

The scalability of P2P architectures is a direct consequence of peers being redistributors as well as consumers of bits.

55
Q

What is a torrent?

A

The collection of all peers participating in the distribution of a particular file.

56
Q

What is a tracker?

A

A tracker is an infrastructure that registers each peer in the torrent, and periodically verifies that they are still there.

57
Q

What is the technique rarest first?

A

Rarest first is a technique used to decide which chunk to request first from a list of peers and their respective chunks. The idea is to determine from among the chunks the peer does not have which are the rarest from among the neighbours and request those first.

58
Q

What is a DHT?

A

A distributed hash table is a database of key, value pairs which is stored across various peers rather than in a single server.

59
Q

What is an overlay network?

A

In an overlay network the peers form an abstract logical network which resides above the computer network consisting of physical links.

Network Computing (4 decks)

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