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Department of Computer and IT Engineering University of Kurdistan Computer Networks II Application Layer By: Dr. Alireza Abdollahpouri. Application Layer. - PowerPoint PPT Presentation
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Department of Computer and IT EngineeringDepartment of Computer and IT EngineeringUniversity of KurdistanUniversity of Kurdistan
Computer Networks IIApplication Layer
By: Dr. Alireza AbdollahpouriBy: Dr. Alireza Abdollahpouri
الیه کاربرد مستقیما با کاربر )برنامه ها یا اشخاص( در ارتباط است. این الیه از طریق پروتکلهاي مختلفی که در اختیار دارد،
خدمات مورد نیاز کاربران را فراهم میآورد. هر کدام از پروتکلهاي این الیه بسته به نوع و ماهیت آنها از یکی از
در الیه پایینتر استفاده میکنند. UDP یا TCP پروتکلهاي
Application Layer
2
3
مشهورترین پروتكلهاي اين اليه عبارتند از:
FTPپروتكلي براي انتقال فايل :HTTP پروتكلي براي دسترسي به :
صفحات وبDNS پروتكلي براي ترجمه نامهاي :
IPنمادين به آدرسهاي
Telnet پروتكلي براي ورود به :سيستم از راه دور
SMTP و POP3 پروتكلهايي براي :E-mailارسال و دريافت
Application-Layer Protocols
4
application
transportnetworkdata linkphysical
modem
modem
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
Application: communicating, distributed processes e.g., e-mail, Web, P2P file
sharing, instant messaging running in end systems (hosts) exchange messages to
implement application
Application-layer protocols one “piece” of an app define messages exchanged
by apps and actions taken use communication services
provided by lower layer protocols (TCP, UDP)
Applications and Application-Layer Protocols
5
modem
modem
Typical network app has two pieces: client and server
Client: initiates contact with server
(“speaks first”) typically requests service
from server, Web: client implemented in
browser; e-mail: in mail reader replyServer:
provides requested service to client e.g., Web server sends requested
Web page, mail server delivers e-mail
applicatio
transport
networkdata linkphysical
applicatin
transport
networkdata linkphysical
request
Client-Server Paradigm
6
process sends/receives messages to/from its socket
socket analogous to door sending process pushes
message out door sending process assumes
transport infrastructure on other side of door which brings message to socket at receiving process
process
TCP withbuffers,variables
socket
host orserver
process
TCP withbuffers,variables
socket
host orserver
Internet
controlledby OS
controlled byapp developer
Processes Communicating Across Network
7
Data loss some apps (e.g., audio)
can tolerate some loss other apps (e.g., file
transfer, telnet) require 100% reliable data transfer
Timing some apps (e.g.,
Internet telephony, interactive games) require low delay to be “effective”
Bandwidth some apps (e.g.,
multimedia) require minimum amount of bandwidth to be “effective”
other apps (“elastic apps”) make use of whatever bandwidth they get
What transport service does an app need?
8
Application Data loss
Bandwidth Time Sensitive
file transfer no loss elastic no
e-mail no loss elastic no
web documents
no loss elastic (few kbps) no
real-time audio/video
loss-tolerant
audio: few kbps-1Mbps
video:10kbps-5Mbps
yes, 100s of msec
stored audio/video
loss-tolerant
same as above yes, few sec
interactive games
loss-tolerant
few kbps-10kbps yes, 100s of msec
instant messaging
no loss elastic yes and no
Requirements of Selected Network Applications
9
TCP service: connection-oriented: setup
required between client and server processes
reliable transport between sending and receiving process
flow control: sender won’t overwhelm receiver
congestion control: throttle sender when network overloaded
no guarantee on: timing, minimum bandwidth
UDP service: unreliable data transfer
between sending and receiving process
does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee
Internet Transport Protocols Services
10
Internet apps: application, transport protocols
applications application-layer protocol
underlying transport protocol
e-mail SMTP [RFC 2821] TCP
remote terminal access
Telnet [RFC 854] TCP
web HTTP [RFC 2616] TCP
file transfer FTP [RFC 959] TCP
Name server DNS [ RFC 1034] UDP or TCP
streaming multimedia proprietary(e.g., youtube)
Typically UDP
11
Connectionless iterative server
UDP
12
Connection-oriented concurrent server
TCP
13
Sockets
ClientsocketClientsocket
ConnectionsocketConnectionsocket
WelcomingsocketWelcomingsocket
Three-way handsh
ake
Client processClient process Server processServer process
Client IP Address&
Port Number
Server IP Address&
Port Number2
Server IP Address&
Port Number1
bytes
sendread read write
14
Domain Name System(DNS)
15
16
راه حل: استفاده از یک پایگاه داده سلسله مراتبی توزیع شده
راه حل: استفاده از یک پایگاه داده سلسله مراتبی توزیع شده
در اولين سالهاي راه اندازي شبكه اينترنت، راه حل IP اي براي ترجمه نامهاي نمادين به آدرس بسيار ساده
IP وجود داشت و آن تعريف تمام نامها و آدرسهاي بود. اين فايل داراي hosts.txt معادل، در يك فايل بنام
دو ستون بود كه در يك طرف آدرس نمادين و در طرف معادل آن نوشته شده بود. به دليل اينكه IP ديگر آدرس
در آن تاريخ تعداد آدرسها زياد نبود، حجم چنين فايلي توانست شد و هر ماشين ميزبان مي چندان بزرگ نمي
يك نسخه از اين فايل را در اختيار داشته باشد و هر شب اين فايل را از روي فايل مرجع 24سـاعت
كرد تا هر گونه تغيير احتمالي و روز مي تازه سازي و بهتعريف آدرسهاي جديد اعمال شود. بديهي است كه
امروزه با حجم ميليوني آدرسها در اينترنت، داشتن يك فايل متمركز و قرار دادن تمام آدرسها و معادل
. در آن، امكان پذير نيستIPآدرس
در اولين سالهاي راه اندازي شبكه اينترنت، راه حل IP اي براي ترجمه نامهاي نمادين به آدرس بسيار ساده
IP وجود داشت و آن تعريف تمام نامها و آدرسهاي بود. اين فايل داراي hosts.txt معادل، در يك فايل بنام
دو ستون بود كه در يك طرف آدرس نمادين و در طرف معادل آن نوشته شده بود. به دليل اينكه IP ديگر آدرس
در آن تاريخ تعداد آدرسها زياد نبود، حجم چنين فايلي توانست شد و هر ماشين ميزبان مي چندان بزرگ نمي
يك نسخه از اين فايل را در اختيار داشته باشد و هر شب اين فايل را از روي فايل مرجع 24سـاعت
كرد تا هر گونه تغيير احتمالي و روز مي تازه سازي و بهتعريف آدرسهاي جديد اعمال شود. بديهي است كه
امروزه با حجم ميليوني آدرسها در اينترنت، داشتن يك فايل متمركز و قرار دادن تمام آدرسها و معادل
. در آن، امكان پذير نيستIPآدرس
Hierarchical Distributed Name Space
Root Server
عدد سرویس دهنده 13تعداد ریشه وجود دارد
عدد سرویس دهنده 13تعداد ریشه وجود دارد
17
18
DNS root Servers
DNS root Servers
19
Domains
20
DNS in the Internet
The inverse domain is used to map an IP address to a name
21
Generic domains
Description
com Commercial organizations
edu Educational institutions
gov Government institutions
int International organizations
mil Military groups
net Network support centers
org Nonprofit organizations
22
Country domains
های پژوهشگاه دانش در تهران مسئولیت بنیادی
و زیر دامنه ir کنترل دامنههای آن را به عهده دارد
های پژوهشگاه دانش در تهران مسئولیت بنیادی
و زیر دامنه ir کنترل دامنههای آن را به عهده دارد
http://www.nic.ir/http://www.nic.ir/
23
Recursive resolution
پرس و جوي تکراري
قسمت اعظم تالش براي تبدیل یک نام
سرویس سرویس برعهده دهنده محلیدهنده محلی
است
24
DNS Protocol: Forward Lookup Query
“ .”
org. edu. com. ir.
ac.ir.
uok.ac.ir.Local
Name Server
RootName Server
Client
www.icann.org.
Authorized nameserver for org. zone
Forward Lookup Query:What is IP Address of www.icann.org
IP Address of www.icann.org is 142.12.01.23
25
Iterative resolution
پرس و جوي بازگشتی
26
Query and response messages
27
DNS can use the services of UDP or TCP,
using the well-known port 53.
NoteNote::
28
HTTP and WWW
وب جهان گستر و پروتکل انتقال صفحات ابرمتن
29
World Wide Web
Distributed services
30
Hypertext
31
Browser architecture
Interpreter
32
Categories of Web documents
33
Static document
34
Dynamic document
35
Active document
36
Web page consists of objects Object can be HTML file, JPEG image, Java
applet, audio file,… Web page consists of base HTML-file which
includes several referenced objects Each object is addressable by a URL Example URL:
Web page consists of objects Object can be HTML file, JPEG image, Java
applet, audio file,… Web page consists of base HTML-file which
includes several referenced objects Each object is addressable by a URL Example URL:
eng.uok.ac.ir/abdollahpouri/index.html
host name path name
Web and HTTP
37
HTTP: hypertext transfer protocol
Web’s app layer protocol client/server model
client: browser that requests, receives, “displays” Web objects
server: Web server sends objects in response to requests
HTTP 1.0: RFC 1945 HTTP 1.1: RFC 2068
PC runningExplorer
Server running
Apache Webserver
Mac runningNavigator
HTTP request
HTTP request
HTTP response
HTTP resp
onseStorage
contents:base files, objects
HTTP Overview
38
HTTP uses the services of TCP on well-known port 80.
NoteNote::
39
Uses TCP: client initiates TCP
connection (creates socket) to server, port 80
server accepts TCP connection from client
HTTP messages (application-layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server)
TCP connection closed
Uses TCP: client initiates TCP
connection (creates socket) to server, port 80
server accepts TCP connection from client
HTTP messages (application-layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server)
TCP connection closed
HTTP is “stateless” server maintains no
information about past client requests
HTTP Overview (cont.)
40
Nonpersistent HTTP At most one object is
sent over a TCP connection.
HTTP/1.0 uses nonpersistent HTTP
Persistent HTTP Multiple objects can be
sent over single TCP connection between client and server.
HTTP/1.1 uses persistent connections in default mode
HTTP Connections
41
Suppose user enters URL www.uok.ac.ir/computer/home.html
Suppose user enters URL www.uok.ac.ir/computer/home.html
1. HTTP client initiates TCP connection to HTTP server )process( at www.uok.ac.ir on port 80
3. HTTP client sends HTTP request message )containing URL( into TCP connection socket. Message indicates that client wants object Computer/home.html
2. HTTP server at host www.uok.ac.ir waiting for TCP connection at port 80. “accepts” connection, notifying client
4. HTTP server receives request message, forms response message containing requested object, and sends message into its socket
tim
e
(contains text, references to 10
jpeg images)
(contains text, references to 10
jpeg images)
Non-persistent HTTP
42
5. HTTP client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects
7. Steps 1-6 repeated for each of 10 jpeg objects
6. HTTP server closes TCP connection.
time
4. …
Non-persistent HTTP (cont.)
43
Response time: one RTT to initiate TCP
connection one RTT for HTTP request
and first few bytes of HTTP response to return
file transmission time
total = 2RTT+transmit time
time to transmit file
initiate TCPconnection
RTT
requestfile
RTT
filereceived
time time
Response Time Modeling
44
Nonpersistent HTTP issues: requires 2 RTTs per object OS must work and allocate
host resources for each TCP connection
but browsers often open parallel TCP connections to fetch referenced objects
Persistent HTTP server leaves connection
open after sending response subsequent HTTP messages
between same client/server are sent over connection
Nonpersistent HTTP issues: requires 2 RTTs per object OS must work and allocate
host resources for each TCP connection
but browsers often open parallel TCP connections to fetch referenced objects
Persistent HTTP server leaves connection
open after sending response subsequent HTTP messages
between same client/server are sent over connection
Persistent without pipelining: client issues new request only
when previous response has been received
one RTT for each referenced object
Persistent with pipelining: default in HTTP/1.1 client sends requests as soon
as it encounters a referenced object
as little as one RTT for all the referenced objects
Persistent HTTP
45
46
HTTP request and response format
47
Status line
Request line
خط درخواست
خط وضعیت
هر سطر سرآیند به این فرمت است
درخواست دیگر :انواع
,OPTIONS, PATCH, COPY, MOVE, LINK, UNLINK, TRACE
درخواست دیگر :انواع
OPTIONS, PATCH, COPY, MOVE, LINK, UNLINK, TRACE,
48
HTTP request message: ASCII (human-readable format)
GET /somedir/page.html HTTP/1.1Host: www.someschool.edu User-agent: Mozilla/4.0Connection: close Accept-language:frIf-modified-since:Sat, 2 Nov 2002 13:45:12 (carriage return, line feed)
request line)GET, POST,
HEAD commands(
header lines
Carriage return, line feed
indicates end of message
HTTP Request Message - example
49
Example 1
This example retrieves a document. We use the GET method to retrieve an image with the path /usr/bin/image1. The request line shows the method (GET), the URL, and the HTTP version (1.1).
The header has two lines that show that the client can accept images in GIF and JPEG format. The request does not have a body.
The response message contains the status line and four lines of header. The header lines define the date, server, MIME version,
and length of the document. The body of the document follows the header (see next slide).
50
Example 1
51
Example 2
This example retrieves information about a document. We use the HEAD method to retrieve information about an HTML document.
The request line shows the method (HEAD), URL, and HTTP version (1.1). The header is one line showing that the client can accept the document in any format (wild card). The request does
not have a body. The response message contains the status line and five lines of header. The header lines define the date, server, MIME
version, type of document, and length of the document (see next slide). Note that the response message does not contain a body.
52
Example 2
53
فرض كنيد كاربر، آدرس زير را در مرورگر خود وارد مي كند:
http://www.w3.org/hyper/www/project.html
مرورگر با تحليل آدرس متوجه ميشود كه بايد تقاضاي به سمت سرويس دهنده HTTP فايلي را طبق پروتكل
بفرستد. مراحلي كه اتفاق مي افتند به شرح زير خواهند بود:
مرورگر آدرس را تحليل كرده و قسمتهاي پروتكل، (1آدرس نام حوزه، شاخه و نام فايل را از آن استخراج
ميكند. سرويس دهنده 53 با پورت UDP مرورگر يك اتصال (2
DNS برقرار نموده و تقاضاي ترجمه آدرس نام حوزه را ماشين سرويس دهنده IP به آن ارسال مي نمايد تا آدرس
بدست آيد. در اين مثال مرورگر تقاضاي ترجمه نامwww.w3.org را به DNS ارسال ميكند.
54
3) DNS در پاسخ، آدرس IP معادل با نام حوزه رارا IP آدرس DNS برميگرداند. فرض كنيد در اين مثال
برگردانده است. 128.30.52.31و 128.30.52.31با آدرس TCP مرورگر يك ارتباط(4
برقرار ميكند.80پورت پس از برقراري ارتباط، يك پيغام درخواست به صورت (5
زير به سمت سرويس دهنده ارسال ميشود:“GET/ hyper/www/project.html http/1.1”
سرويس دهنده اين رشته را دريافت و پس از پردازش (6استخراج كرده / hyper/www /را از شاخه project.htmlآن، فايل
و براي مرورگر ارسال مي نمايد. مرورگر فايل را دريافت كرده و پس از خاتمه دريافت (7
را قطع ميكند. TCPارتباط مرورگر فايل ابرمتني را تفسير كرده و آنرا روي (8
خروجي نمايش ميدهد. اگر فايل ابرمتني در جايي داراي صدا يا تصوير باشد به (9
را تكرار نموده و آنها 8 تا 1 ازاي تك تك آنها مراحل(persistent httpكند )با فرض را بترتيب دريافت مي
3) DNS در پاسخ، آدرس IP معادل با نام حوزه رارا IP آدرس DNS برميگرداند. فرض كنيد در اين مثال
برگردانده است. 128.30.52.31و 128.30.52.31با آدرس TCP مرورگر يك ارتباط(4
برقرار ميكند.80پورت پس از برقراري ارتباط، يك پيغام درخواست به صورت (5
زير به سمت سرويس دهنده ارسال ميشود:“GET /hyper/www/project.html http/1.1”
سرويس دهنده اين رشته را دريافت و پس از پردازش (6استخراج كرده / hyper/www /را از شاخه project.htmlآن، فايل
و براي مرورگر ارسال مي نمايد. مرورگر فايل را دريافت كرده و پس از خاتمه دريافت (7
را قطع ميكند. TCPارتباط مرورگر فايل ابرمتني را تفسير كرده و آنرا روي (8
خروجي نمايش ميدهد. اگر فايل ابرمتني در جايي داراي صدا يا تصوير باشد به (9
را تكرار نموده و آنها 8 تا 1 ازاي تك تك آنها مراحل(persistent httpكند )با فرض را بترتيب دريافت مي
55
SMTPand POP3
پروتکلهای انتقال و دریافت ایمیل
56
Electronic MailElectronic Mail
Sending/Receiving Mail
Addresses
User Agent
MIME
Mail Transfer Agent
Mail Access Protocols
Format of an email
Email address
User agent
Some examples of command-driven user agents are mail, pine, and elm.
NoteNote::
Some examples of GUI-based user agents are Eudora, Outlook, and
Netscape.
NoteNote::
1) User1 (sender) uses UA to compose message to user2@yahoo.com .
2) User1’s UA sends message to his mail server; message placed in message queue.
3) Client side of SMTP opens TCP connection with User2’s mail server.
4) SMTP client sends User1’s message over the TCP connection.
5) User2’s mail server places the message in User2’s mailbox.
6) User2 invokes his/her user agent to read message.
Scenario: User1 sends message to User2
useragent
mailserver
mailserver user
agent
1
23 4 5
6User1
User2
user2@yahoo.com
user1@webmail.uok.ac.ir
webmail.uok.ac.ir
yahoo.com
MIME
MIMEدهد كاراكترهاي غير يك پروتكل تكميلي است كه اجازه مي ASCII يك پروتكل MIME منتقل شوند. دقت كنيد كهSMTP نيز از طريق
باشد بلكه نيست و قادر به ارسال نامه نميSMTP جايگزين براي را به MIME شود. ميتوان محسوب ميSMTP اي براي پروتكل توسعه
و ASCII را به دادهASCII اي تصور نمود كه داده غير صورت برنامه بالعكس ترجمه ميكند
MIMEدهد كاراكترهاي غير يك پروتكل تكميلي است كه اجازه مي ASCII يك پروتكل MIME منتقل شوند. دقت كنيد كهSMTP نيز از طريق
باشد بلكه نيست و قادر به ارسال نامه نميSMTP جايگزين براي را به MIME شود. ميتوان محسوب ميSMTP اي براي پروتكل توسعه
و ASCII را به دادهASCII اي تصور نمود كه داده غير صورت برنامه بالعكس ترجمه ميكند
MIME header
Data types and subtypes in MIMEData types and subtypes in MIME
Type Subtype Description
Text Plain Unformatted text
Multiport
Mixed Body contains ordered parts of different data types
Parallel Same as above, but no order
Digest Similar to mixed, but the default is message/RFC822
Alternative Parts are different versions of the same message
Message
RFC822 Body is an encapsulated message
Partial Body is a fragment of a bigger message
Ext. Body Body is a reference to another message
ImageJPEG Image is in JPEG
GIF Video is in GIF format
Video MPEG Video is in MPEG format
Audio Basic Single-channel encoding of voice at 8 KHz
ApplicationPostScript Adobe PostScript
Octet-Stream General binary data (8-bit bytes)
Content-transfer encodingContent-transfer encoding
Category Description
Type ASCII characters and short lines
7bit Non-ASCII characters and short lines
8bit Non-ASCII characters with unlimited-length lines
Binary 6-bit blocks of data are encoded into 8-bit ASCII characters
Base64Non-ASCII characters are encoded as an equal sign followed by an ASCII code
Base64
Base64 encoding tableBase64 encoding table
Value Code Value Code Value Code Value Code Value Code Value Code
0 A 11 L 22 W 33 h 44 s 55 3
1 B 12 M 23 X 34 i 45 t 56 4
2 C 13 N 24 Y 35 j 46 u 57 5
3 D 14 O 25 Z 36 k 47 v 58 6
4 E 15 P 26 a 37 l 48 w 59 7
5 F 16 Q 27 b 38 m 49 x 60 8
6 G 17 R 28 c 39 n 50 y 61 9
7 H 18 S 29 d 40 o 51 z 62 +
8 I 19 T 30 e 41 p 52 0 63 /
9 J 20 U 31 f 42 q 53 1
10 K 21 V 32 g 43 r 54 2
MTA client and server
SMTP: delivery/storage to receiver’s server (PUSH) Mail access protocol: retrieval from server (PULL)
POP: Post Office Protocol [RFC 1939] authorization (agent <-->server) and download
IMAP: Internet Mail Access Protocol [RFC 1730] more features (more complex) manipulation of stored messages on server
HTTP: Hotmail , Yahoo! Mail, etc.
useragent
sender’s mail server
useragent
SMTP SMTP accessprotocol
receiver’s mail server
Sender Receiver
Mail Access Protocols
C:\> telnet www.uok.ac.ir 25Connecting to www.uok.ac.ir ...
اتصال ================== ================برقراري220 PARSDATA Mail Server (IMail 8.00 2586-5) NT-ESMTP Server
X1HELO PARSDATA250 hello PARSDATA Mail Serverنامه ===================== =================== پوششMAILFROM: Abdollahpour@uok.ac.ir250 okRCPT TO: Abdollahpour@uok.ac.ir250 ok deliver to alternateنامه =================== بدنه و سرآيند
==================DATA
354 ok, send it; end with <CRLF>.<CRLF>FROM: AbdollahpourTO: myselfHi this is a sample e-mail to show SMTP in action. .اتصال ============= ===============خاتمه 250 Message queuedQUIT221 GoodbyeConnection to host lost
authorization phase client commands:
user: declare username pass: password
server responses +OK -ERR
transaction phase, client: list: list message numbers retr: retrieve message by
number dele: delete quit
C: list S: 1 498 S: 2 912 S: . C: retr 1 S: <message 1 contents> S: . C: dele 1 C: retr 2 S: <message 1 contents> S: . C: dele 2 C: quit S: +OK POP3 server signing off
S: +OK POP3 server ready C: user USER1S: +OK C: pass zxcdvf S: +OK user successfully logged on
POP3 Protocol
File Transfer File Transfer
Connections
Communication
File Transfer
User Interface
Anonymous
FTP uses the services of TCP. It needs two TCP connections. The well-known
port 21 is used for the control connection, and the well-known port 20 is used for the data connection.
NoteNote::
FTP
control connection: “out of band”control connection: “out of band”
FTP client contacts FTP server at port 21, specifying TCP as transport protocol.
Client obtains authorization over control connection. Client browses remote directoryremote directory by sending commands
over control connection. When server receives a command for a file transfer,
server opens a TCP data conn. to client at port 20.
FTP clientFTP server
TCP control connectionport 21 )persistent(
TCP data connectionport
20)nonpersistent(
FTP: Separate Control, Data Connections1
Using the control connection
Using the data connection
File transfer
Example 1Example 1
Figure 26.16 (next slide) shows an example of how a file is stored.
1. The control connection is created, and several control commands and responses are exchanged.
2. Data are transferred record by record.
3. A few commands and responses are exchanged to close the connection.
Example 1
List of FTP commands in UNIXList of FTP commands in UNIX
Commands
!, $, account, append, ascii, bell, binary, bye, case, cd, cdup, close, cr, delete, debug, dir, discount, form, get, glob, hash, help, lcd, ls, macdef, mdelete, mdir, mget, mkdir, mls, mode, mput, nmap, ntrans, open, prompt, proxy, sendport, put, pwd, quit, quote, recv, remotehelp, rename, reset, rmdir, runique, send, status, struct, sunique, tenex, trace, type, user, verbose,?
Example 2Example 2
We show some of the user interface commands that accomplish the same task as in Example 1. The user input is shown in boldface. As shown below, some of the commands are provided automatically by the interface. The user receives a prompt and provides only the
arguments.$ ftp challenger.atc.fhda.eduConnected to challenger.atc.fhda.edu220 Server readyName: forouzanPassword: xxxxxxxftp > ls /usr/user/report200 OK150 Opening ASCII mode......................226 transfer completeftp > close221 Goodbyeftp > quit
Example 3Example 3
We show an example of using anonymous FTP. We connect to internic.net, where we assume there are some public data available.$ ftp internic.netConnected to internic.net220 Server readyName: anonymous331 Guest login OK, send "guest" as passwordPassword: guestftp > pwd257 '/' is current directoryftp > ls200 OK150 Opening ASCII modebin...ftp > close221 Goodbyeftp > quit
Remote Login:Remote Login:TelnetTelnet
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