CS2005 - Lecture 3 - Application Layer II

Question 1

What does HTTP stand for?

Answer 1

Hypertext Transfer Protocol.

Question 2

What port does HTTP use?

Answer 2

Port 80.

Question 3

Which RFCs define HTTP 1.0 and HTTP 1.1?

Answer 3

RFC 1945 (HTTP/1.0) and RFC 2616 (HTTP/1.1).

Question 4

What are some typical objects found in a web page?

Answer 4

HTML, JavaScript, Java Applets, and audio files.

Question 5

How are web page objects referenced?

Answer 5

By a URL.

Question 6

What type of protocol is HTTP?

Answer 6

A client/server protocol.

Question 7

What is the role of a browser in HTTP communication?

Answer 7

Acts as the client to request, receive, and display web objects.

Question 8

What does it mean that an HTTP server is stateless?

Answer 8

It does not retain information about past client requests.

Question 9

Which transport protocol does HTTP use?

Answer 9

TCP.

Question 10

How does the HTTP client initiate communication?

Answer 10

By initiating a TCP connection to the server.

Question 11

What messages does an HTTP client send and receive?

Answer 11

HTTP request and response messages.

Question 12

What are the two types of HTTP connections?

Answer 12

Non-persistent and persistent HTTP.

Question 13

What characterizes non-persistent HTTP?

Answer 13

Only one object is sent over a TCP connection before it is closed.

Question 14

What is required to download multiple objects with non-persistent HTTP?

Answer 14

Multiple TCP connections.

Question 15

What characterizes persistent HTTP?

Answer 15

Multiple objects can be sent over a single TCP connection.

Question 16

In non-persistent HTTP, what is the sequence to transfer a web page with 10 JPEGs?

Answer 16

Client requests HTML → Server sends HTML & closes TCP → Client finds 10 references → Repeats for each JPEG.

Question 17

What does RTT stand for in HTTP?

Answer 17

Round Trip Time.

Question 18

What factors contribute to RTT?

Answer 18

Packet propagation, queuing, and processing delays.

Question 19

What is the response time for non-persistent HTTP?

Answer 19

2 RTTs + file transmission time.

Question 20

How does persistent HTTP improve performance?

Answer 20

Uses a single TCP connection for multiple requests, reducing the number of RTTs.

Question 21

What mechanism maintains user state in HTTP?

Answer 21

Cookies.

Question 22

What components make up an HTTP cookie?

Answer 22

Cookie header in HTTP response, Cookie header in HTTP request, Cookie file on client system.

Question 23

Why are cookies useful?

Answer 23

They help identify users and restrict access or personalize content.

Question 24

What is the purpose of a web cache (proxy server)?

Answer 24

To satisfy HTTP requests on behalf of the origin server, reducing response time and network traffic.

Question 25

How is stale data avoided in web caching?

Answer 25

Using conditional GET with headers like ‘If-Modified-Since’ or ‘Last-Modified’.

Question 26

What are the three main components of an email system?

Answer 26

User agents, Mail servers, SMTP protocol.

Question 27

What functions do user agents provide?

Answer 27

Compose, edit, read, and store emails.

Question 28

What does SMTP stand for and what does it do?

Answer 28

Simple Mail Transfer Protocol; sends email between mail servers.

Question 29

What port does SMTP use?

Answer 29

Port 25.

Question 30

What are the three phases of SMTP communication?

Answer 30

Handshaking, Message transfer, Closure.

Question 31

What format must SMTP messages follow?

Answer 31

7-bit ASCII.

Question 32

What are the components of a mail message format?

Answer 32

Headers (to, from, subject) Body separated by blank line (CRLF).

Question 33

Why can’t SMTP be used to retrieve emails?

Answer 33

It is a push protocol; message retrieval is a pull operation.

Question 34

What protocols are used to access mail?

Answer 34

IMAP, POP3, HTTP.

Question 35

What are the three phases of POP3?

Answer 35

Authorization (username + password) Transaction (retrieve and manage messages) Update (apply deletions).

Question 36

What operations can be performed during the POP3 transaction phase?

Answer 36

Retrieve messages, Mark/unmark messages for deletion, Obtain mail statistics.

Question 37

How does HTTP differ from SMTP in terms of request handling?

Answer 37

HTTP is pull-based; SMTP is push-based.

Question 38

What protocol do web clients use to retrieve emails?

Answer 38

IMAP, POP3, or HTTP.