ECE453 – Introduction to
Computer Networks
Lecture
25
–
Application
Layer (
II)
Lecture
25
–
Application
Layer (
II)
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Application Layer in Computer Networks: Electronic Mail and Web, Study notes of Computer Systems Networking and Telecommunications
An in-depth look into the application layer of computer networks, focusing on electronic mail and the web. Topics include the components of email systems, mime, smtp, pop3, imap, and http. Understand the protocols and technologies that enable communication and data transfer between users and servers.
Typology: Study notes
Pre 2010
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ECE453 – Introduction toComputer Networks Lecture
Application
Layer (
II)
Lecture
Application
Layer (
II)
Electronic Mail Three major components: ^
user agents
mail servers
simple mail transfer protocol:smtp
user mailbox
outgoing
message queue mail server
useragent
user
mail server
useragent^ SMTP
SMTP
User Agent
“mail reader”
composing, editing, reading mailmessages
e.g., Eudora, Outlook,thunderbird, Netscape Messenger
outgoing, incoming messagesstored on server
server
user agent useragent
mail server
useragent
useragent
SMTP
SMTP
MIME : multipurpose internet mail extensions
MIME: the protocol to deal with email contents. Why MIME? RFC822 specified the headers but left the content entirelyup to the users. For world wide internet, this is not enough. MIME continue to use RFC822 but add structure to the message body. Five new message headers:Five new message headers:
MIME types
Mail Access Protocol – Final Delivery
SMTP: delivery/storage to receiver’s server
Mail access protocol: retrieval from server
useragent
sender’s mail server
useragent
SMTP
SMTP
POP3,
IMAP, or
HTTP
receiver’s mail server
http
Mail access protocol: retrieval from server^ –
POP: Post Office Protocol [RFC 1939] (port 110)
o authorization (agent <-->server) and download o Does not maintain state across POP sessions o Cannot manipulate emails at the server side
IMAP: Internet Mail Access Protocol [RFC 1730]
o more features (more complex) o manipulation of stored msgs on server o Maintain state for the user
HTTP: Hotmail , Yahoo! Mail, etc.
Slow
Web: Terminology
Web page^
Consists of “objects”
^
Addressed by “url”(universal resource locator)
Most of web page
One base web page
Web client^
Firefox
^
Mozilla
^
Microsoft IE browser
Web server
^
One base web page
^
Several referenced“objects”
Web server^
Apache
^
Microsoft InternetInformation Server (IIS)
Web: http Operation Flow
HTTP utilizes TCPtransport services HTTP client initiates TCPconnection (create socket) to server, at port 80
HTTP is
stateless
Server doesn’tmaintain the state of past requests
to server, at port 80 Server accepts thisconnection from client HTTP messages (definedby HTTP protocol) areexchanged between httpclient and http server TCP connection closed
past requests
‘back’?
HTTP Example^ Suppose user enters URL
www.someSchool.edu/someDepartment/home.index
1a
.^
http client initiates TCPconnection to http server (process) at
1b.
http server at host
(contains text,references to 10jpeg images)
(process) at www.someSchool.edu.
Port 80
is default for http server.
http client sends http
request
message
(containing URL) into TCP
connection socket
(plus another acknowledgemessage)
www.someSchool.edu
waiting for
TCP connection at port 80. “accepts”connection, notifying client 3.
http server receives request message, forms
response message
containing requested object(someDepartment/home.index
),
sends message into socket