Lecture 3 Flashcards
What is HTTP?
Hypertext Transfer Protocol (HTTP), web’s apllication layer protocol, defined in [v1.0-RFc 1945]
HTTP is a client/server protocol. The client, typically a browser requests, recivies and “displays” Web objects. The web server sends objects in response to requests. The HTTP server is stateless.
What does a web page consist of?
A web page consists of objects (e.g. HTML file, JPEG image, Java applet, audio file).
The base HTML-file includes several referenced objects , each object is addressable by a URL
(e.g. www.someschool.edu/someDept/pic.gif - host name / path name)
What does HTTP use as its underlying transport protocol?
HTTP uses TCP as its underlying transport protocol.
The HTTP client first initiatess a TCP connection with the server.
Once the connection is established, the browser and the server processes access TCP through their socket interfaces.
The client sends HTTP request messages and receives HTTP response messages. The server receives request messages and sends response messages.
Explain the two types of HTTP connections?
Persistent- where multiple objects can be sent over a single TCP connection.
Non-persistent - where at most one object sent over a TCP connection and then the connection is closed requiring multiple connections to downloading multiple objects
What is Round- trip time (RTT)
It is the time it takes for a small packet to travel from client to server and then back to the client (including delays packet-propagation delays, packet queueing delays intermediate routers and switches, and packet- processing delays).
What is the time needed to request and receive a HTML file?
The time needed to request and receive an HTML file is 2RTTs + file transmission time
Explain cookies
An HTTP server is stateless.But often it is useful that the server keeps session state information.
HTTP uses cookies for this purpose. An HTTP cookie has four components:
1) a cookie header line in the HTTP response message
2) a cookie header line in the HTTP request message
3) a cookie file kept on the user’s end system and managed by the user’s browser
4) a back-end database at the Web site
Explain Web cache (proxy server)
A web cache satisfies HTTP requests on behalf of and origin Web server. It is both a client (to the original server) and a server (to the client).
Web catching reduces response time and network traffic (access link to the internet)
A web cache stores a copy of a requested object in its local storage and sends a copy to the client browser
To avoid stale data , HTTP uses | conditional GET |
Transport services provided by the internet:
TCP
Reliable transport, flow control, congestion control
Does not provide: timing, minimum throughput guarantee, security
Connection-oriented: setup required between client and server processes
Transport services provided by the internet:
UDP
Unreliable data transfer between sending and receiving process.
Does not provide: reliability, flow control, congestion control, timing, throughput guarantee, security or connection setup
Three major components of email:
User agents
User agents compose, edit, read and save (outgoing, incoming messages stored on server) mail messages
Three major components of email:
Mail servers
Mail servers have a mailbox (incoming messages), a message queue (outgoing messages) and an SMTP implementation.
Three major components of email:
Simple Mail Transfer Protocol (SMTP)
Simple Mail Transfer Protocol (SMTP) protocol between mail servers to send email messages
Client: sending email messages
Server: receiving email messages
More information on SMTP
The SMTP is defined in [RFC 5321] and uses TCP for reliable email transfer and port 25
Normally the sending server connects directly to the receiving server
Command/ response interaction (Like HTTP)
command: ASCII text
response: status code and phrase
Messages must be in a 7- bit ASCII
Mail message format: Header lines (eg. To: From:Subject:) and the body of the message separated by a bank line (CRLF)
What are the three phases of SMTP transfer
Handshaking (greeting)
Transfer of messages
Closure
