Upgrade to remove ads
Upgrade to remove ads
Unformatted text preview:

1 Computer)Networks)FTP)and)email)1 FTP:)the)file)transfer)protocol)• transfer)file)to/from)remote)host)• client/server)model)– client:)side)that)ini=ates)transfer)(either)to/from)remote))– server:)remote)host)• @p:)RFC)959)• @p)server:)port)21)2 file transfer FTP server FTP user interface FTP client local file system remote file system user at host2 FTP:)separate)control,)data)connec=ons)• FTP)client)contacts)FTP)server)at)port)21,)TCP)is)transport)protocol)• client)authorized)over)control)connec=on)• client)browses)remote)directory)by)sending)commands)over)control)connec=on.)• when)server)receives))file)transfer)command,)server)opens)2nd-TCP)connec=on)(for)file))to)client)• [email protected])transferring)one)file,)server)closes)data)connec=on.)3 FTP)client)FTP)server)TCP)control)connec=on)port)21)TCP)data)connec=on)port)20)• server)opens)another)TCP)data)connec=on)to)transfer)another)file.)• control)connec=on:)“out)of)band”)• FTP)server)maintains)“state”:)current)directory,)earlier)authen=ca=on)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)43 Electronic)Mail)Three)major)components:))• user)agents))• mail)servers))• simple)mail)transfer)protocol:)SMTP)User)Agent)• a.k.a.)“mail)reader”)• composing,)edi=ng,)reading)mail)messages)• e.g.,)Eudora,)Outlook,)elm,)Mozilla)Thunderbird)• outgoing,)incoming)messages)stored)on)server)5 user mailbox outgoing message queue mail server user agent user agent user agent mail server user agent user agent mail server user agent SMTP SMTP SMTP 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)6 mail server user agent user agent user agent mail server user agent user agent mail server user agent SMTP SMTP SMTP4 Electronic)Mail:)SMTP)[RFC)2821])• 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)(gree=ng))– transfer)of)messages)– closure)• command/response)interac=on)– commands:)ASCII)text)– response:)status)code)and)phrase)• messages)must)be)in)7^bit)ASCII)7 Scenario:)Alice)sends)message)to)Bob)1))Alice)uses)UA)to)compose)message)and)“to”)[email protected] 2))Alice’s)UA)sends)message)to)her)mail)server;)message)placed)in)message)queue)3))Client)side)of)SMTP)opens)TCP)connec=on)with)Bob’s)mail)server)4))SMTP)client)sends)Alice’s)message)over)the)TCP)connec=on)5))Bob’s)mail)server)places)the)message)in)Bob’s)mailbox)6))Bob)invokes)his)user)agent)to)read)message)8 user agent mail server mail server user agent 1 2 3 4 5 65 Sample)SMTP)interac=on)9 S: 220 hamburger.edu C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: <[email protected]> S: 250 [email protected] Sender ok C: RCPT TO: <[email protected]> S: 250 [email protected] ... Recipient ok C: DATA S: 354 Enter mail, end with "." on a line by itself C: Do you like ketchup? C: How about pickles? C: . S: 250 Message accepted for delivery C: QUIT S: 221 hamburger.edu closing connection Try)SMTP)interac=on)for)yourself:)• telnet servername 25)• see)220)reply)from)server)• enter)HELO,)MAIL)FROM,)RCPT)TO,)DATA,)QUIT)commands))above)lets)you)send)email)without)using)email)client)(reader))106 SMTP:)final)words)• SMTP)uses)persistent)connec=ons)• SMTP)requires)message)(header)&)body))to)be)in)7^bit)ASCII)• SMTP)server)uses)CRLF.CRLF)to)determine)end)of)message)Comparison)with)HTTP:)• HTTP:)pull)• SMTP:)push)• both)have)ASCII)command/response)interac=on,)status)codes)• HTTP:)each)object)encapsulated)in)its)own)response)msg)• SMTP:)mul=ple)objects)sent)in)mul=part)msg)11 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-commands!)• body)– the)“message”,)ASCII)characters)only)12 header body blank line7 Mail)access)protocols)• SMTP:)delivery/storage)to)receiver’s)server)• Mail)access)protocol:)retrieval)from)server)– POP:)Post)Office)Protocol)[RFC)1939])• authoriza=on)(agent)<^^>server))and)download))– IMAP:)Internet)Mail)Access)Protocol)[RFC)1730])• more)features)(more)complex))• manipula=on)of)stored)msgs)on)server)– HTTP:)gmail,)Hotmail,)Yahoo!)Mail,)etc.)13 user agent sender’s mail server user agent SMTP SMTP access protocol receiver’s mail server POP3)protocol)authoriza=on)phase)• client)commands:))– user:)declare)username)– pass:)password)• server)responses)– +OK – -ERR)transac=on)phase,)client:)• list:)list)message)numbers)• retr:)retrieve)message)by)number)• dele:)delete)• quit)14 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 bob S: +OK C: pass hungry S: +OK user successfully logged on8 POP3)(more))and)IMAP)More)about)POP3)• Previous)example)uses)“download)and)delete”)mode.)• Bob)cannot)re^read)e^mail)if)he)changes)client)• “Download^and^keep”:)copies)of)messages)on)different)clients)• POP3)is)stateless)across)sessions)IMAP)• Keep)all)messages)in)one)place:)the)server)• Allows)user)to)organize)messages)in)folders)• IMAP)keeps)user)state)across)sessions:)–


View Full Document
Download Computer Networks - FTP and Email
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view Computer Networks - FTP and Email and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Computer Networks - FTP and Email 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?