Como Funciona El Ftp

FTP Transfer (FTP Session Sequence Diagram) Internet DNS FTP Server Port 53 Port 20 Port 21 1030

FTP Client PC FTP Client 1175 5001

EventHelix.com/EventStudio 2.5

5002

Console User

18-Mar-05 06:45 (Page 1)

This sequence diagram was generated with EventStudio 2.5 (http://www.EventHelix.com/EventStudio). Here we explore the sequence of interactions in a typical FTP (File TransferProtocol) session. The example here illustrates the use of multiple TCP connections by FTP. We will cover how FTP establishes a telnet TCP connection (TCP Port 21) to control the overall flow of the FTP transfer. Then we examine the use of TCP Port 21 for establishing TCP connections for directory transfer and file retrieval. The complete sequence diagram can be divided into the following steps: -DNS Query to obtain the IP address for the FTP Server - FTP Telnet connection setup and login. (USER and PASS commands) - Obtaining a directory listing (PORT and LIST command) - Changing directory (CWD command) - Downloading a file using FTP get (PORT and RETR command) Copyright © 2005 EventHelix.com Inc. All Rights Reserved. DNS Query to obtain the IP address for the FTP Server.
Invoke FTPclient with ftp.any-domain.com

DNS Query
recdesired = 1, queries = 1, name = ftp.any-domain.com, type = A, class = IN

DNS Response
recdesired = 1, answers = 1, IP Address

The FTP client sends a DNS query for domain name ftp.any-domain.com The DNS responds back with the IP address.

FTP Telnet connection setup and login. (USER and PASS commands). TCP SYN
srcport = 1175, dstport = 21, syn= 1

TCP SYN+ACK
srcport = 21, dstport = 1175, syn = 1, ack = 1

FTP client initiates establishment of the telnet session TCP connection by sending a SYN to TCP port 21 The FTP server responds with SYN+ACK The client machine responds with an ACK, this completes the TCP three way handshake. The FTP server then sends a 220 response to indicate that the FTP server is ready to accept a login.Display welcome message The client machine acknowledges the FTP 220 TCP packet. Enter login information User's login name is transported in a TCP segment. In this example an anonymous FTP is being initiated with the user name "ftp"

TCP ACK
srcport = 1175, dstport = 21, ack = 1

FTP 220
ftp.response.code = 220, ftp.response.arg = ANY-DOMAIN.COM FTP Service

TCP ACK
srcport = 1175, dstport =21, ack = 1

USER ftp

TCP ACK
srcport = 21, dstport = 1175, ack = 1

FTP 331
code = 331, arg = Anonymous access allowed, send identity (e-mail name) as password.

FTP server indicates that anonymous FTP is allowed. Password prompt

TCP ACK
srcport = 1175, dstport = 21, ack = 1

Password information

FTP Transfer (FTP Session Sequence Diagram) Internet DNS FTP Server Port 53 Port20 Port 21 1030

FTP Client PC FTP Client 1175 5001

EventHelix.com/EventStudio 2.5

5002

Console User

18-Mar-05 06:45 (Page 2)

PASS abc@any-domain.com

User enters his or her e-mail address as the password. This password is being transported by this TCP segment.

TCP ACK
srcport = 21, dstport = 1175, ack = 1

FTP 230
code = 230, arg = Welcome to ftp.any-domain.comWelcome message after login. User login notification. Login successful prompt

FTP 230
code = 230, arg = Anonymous user logged in.

TCP ACK
srcport = 1175, dstport = 21, ack = 1

Obtaining a directory listing (PORT and LIST command). dir command PORT 192.168.0.2 Port 5001 TCP ACK
srcport = 21, dstport = 1175, ack = 1

FTP client provides port number information.

FTP 200
code = 200, arg =PORT command successful.

FTP server positively acknowledges the PORT command. User requests a directory listing.

LIST seq-251
srcport = 21, dstport = 1175, ack = 1

FTP 150
ftp.response.code = 150, ftp.response.arg = Opening ASCII mode data connection for /bin/ls.

TCP SYN
srcport = 20, dstport = 5001, syn = 1

FTP server notifies the client that it is about to transfer the...