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Lecture 2 - Weeks 2 3.pptx
Lecture_2_-_Weeks_2___3.pptx
Showing 52-57 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 52
Application Layer A 2-52
FTP commands, responses
sample commands:
❖
sent as ASCII text
over control channel
❖
USER
username
❖
PASS
password
❖
LIST
return list of file
in current directory
❖
RETR filename
retrieves (gets) file
❖
STOR filename
stores (puts) file onto
remote host
sample return
codes
❖
status code and
phrase (as in HTTP)
❖
331 Username OK,
password required
❖
125 data
connection
already open;
transfer starting
❖
425 Can’t open
data connection
❖
452 Error writing
file
Page 53
Application Layer A 2-53
Electronic mail
Three major
components:
❖
user agents
❖
mail servers
❖
simple mail transfer
protocol: SMTP
User Agent
❖
a.k.a. “mail reader”
❖
composing, editing,
reading, forwarding,
replying mail
messages
❖
e.g., Outlook,
Thunderbird, iPhone
mail client
❖
outgoing, incoming
user mailbox
outgoing
message queue
mail
server
SMT
P
SMT
P
SMTP
mail
server
user
agent
user
agent
user
agent
user
agent
mail
server
user
agent
user
agent
Page 54
Application Layer A 2-54
Electronic mail: mail servers
mail servers:
❖
mailbox
contains
incoming messages
for user
❖
message queue
of
outgoing (to be sent)
mail messages
❖
SMTP protocol
between mail servers
to send email
messages
▪
client: sending mail
server
▪
“server”: receiving
mail server
user mailbox
outgoing
message queue
mail
server
SMT
P
SMT
P
SMTP
mail
server
user
agent
user
agent
user
agent
user
agent
mail
server
user
agent
user
agent
Page 55
Application Layer A 2-55
Simple Mail Transfer Protocol: SMTP [RFC
5321]
❖
the principle application-layer protocol
for e-mail
❖
uses TCP to reliably transfer email
message from client to server, port 25
❖
direct transfer
: sending server to
receiving server
❖
three phases of transfer
▪
handshaking (greeting)
▪
transfer of messages
▪
closure
❖
command/response interaction (like
HTTP, FTP
)
▪
commands:
ASCII text
▪
response:
status code and phrase
❖
messages must be in
7-bit ASCI
Page 56
Application Layer A 2-56
user
agent
Scenario: Alice sends message
to Bob
1) Alice uses UA to
compose message “to”
bob@someschool.edu
2) Alice’s UA sends
message to her mail
server; message
placed in message
queue
3) client side of SMTP
opens TCP connection
with Bob’s mail server
4) SMTP client sends
Alice’s message over
the TCP connection
5) Bob’s mail server
places the message in
Bob’s mailbox
6) Bob invokes his user
agent to read message
mail
server
mail
server
1
2
3
4
5
6
Alice’s mail server
Bob’s mail server
user
agent
Page 57
Application Layer A 2-57
Sample SMTP interaction
S: 220 sunset.edu
C: HELO jupiter.fr
S: 250
Hello jupiter.fr, pleased to meet you
C: MAIL FROM: <alice@jupiter.fr>
S: 250 alice@jupiter.fr... Sender ok
C: RCPT TO: <bob@sunset.edu>
S: 250 bob@sunset.edu ... Recipient ok
C: DATA
S:
354 Enter mail, end with "." on a line by itself
C: Hi Bob,
C: How is it in beautiful Paris?
C: .
S: 250 Message accepted for delivery
C: QUIT
S: 221 sunset.edu closing connection
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