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Lecture 2 - Weeks 2 3.pptx
Lecture_2_-_Weeks_2___3.pptx
Showing 58-66 out of 70
Lecture 2 - Weeks 2 3.pptx-Data Communication an...
Lecture_2_-_Weeks_2___3.pptx-Data Communication and Computer Networks 2.
Lecture 2 - Weeks 2 3.pptx-Data C...
Lecture_2_-_Weeks_2___3.pptx-Data Communication and Computer Networks 2.
Page 58
Application Layer A 2-58
Try SMTP interaction for yourself:
❖
telnet servername 25
❖
see 220 reply from server
❖
enter HELO, MAIL FROM, RCPT TO, DATA, QUIT
commands
These above steps lets you send email without
using email client (reader)
Click here for another example
Click here to view SMTP codes
Page 59
Application Layer A 2-59
SMTP vs. HTTP
SMTP
HTTP
File transfer:
mail server to mail server
File transfer:
from a web server to a web
client
Persistent TCP connection
Either persistent
or non-
persistent TCP connection
Push
protocol
Pull
protocol
Messages (headers & body)
must be in ASCII
i.e.
special French characters,
images, etc. must be encoded
into ASCII
No such restriction
All objects (i.e. text and images)
are placed in one message
(multi-parts of the message)
Each object has its own HTTP
response message
Page 60
Application Layer A 2-60
Mail message format
SMTP: protocol for
exchanging email
msgs
RFC 822: standard for
text message
format:
❖
header lines, e.g.,
▪
To
:
▪
From
:
▪
Subject:
different
from
SMTP
MAIL
FROM
, RCPT
TO
:
commands!
❖
Body: the “message”
▪
ASCII characters only
header
body
blan
k
line
Page 61
Application Layer A 2-61
Mail access protocols
❖
Q:
Why the two-step procedure?
sender’s mail
server
receiver’s mail
server
user
agent
user
agent
Page 62
Application Layer A 2-62
Mail access protocols
❖
Q:
Why have a receiver server?
Just install it in the receiver’s local PC,
where receiver executes his e-mail agent
anyways!
sender’s mail
server
SMT
P
SMT
P
user
agent
user
agent
Page 63
Application Layer A 2-63
Mail access protocols
❖
Q:
Why have a sender server?
Just let the sender’s user agent place the
message directly on the receiver mail server
receiver’s mail
server
SMT
P
user
agent
user
agent
Page 64
Application Layer A 2-64
Mail access protocols
❖
OK:
We need both of them, but will it work?
sender’s mail
server
receiver’s mail
server
user
agent
user
agent
SMT
P
SMT
P
SMT
P
Page 65
Application Layer A 2-65
Mail access protocols
❖
SMTP:
delivery/storage to receiver’s server
❖
mail access protocol: retrieval from server
▪
POP:
Post Office Protocol [RFC 1939]:
authorization, download
▪
IMAP:
Internet Mail Access Protocol [RFC
1730]: more features, including manipulation
of stored msgs on server
▪
HTTP:
gmail, Hotmail, Yahoo! Mail, etc.
sender’s mail
server
SMT
P
SMT
P
mail
access
protocol
receiver’s mail
server
(e.g.,
POP,
IMAP
)
user
agent
user
agent
Page 66
Application Layer A 2-66
POP3 protocol
❖
extremely simple mail access protocol, but
❖
limited !
▪
User agent opens a TCP connection with the
mail server on port 110
▪
User progress through 3 phases:
•
authorization
–
Sends username/password
•
transaction, and
–
retrieves & manipulate message (i.e. mark message for deletion,
obtain statistics, …)
•
update
–
after clients “quits”, server updates what is needed (i.e. delete
messages that were marked for deletion).
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