1. Lecturer: Maxim Podlesny Sep 29 2008 CSE 473 File Transfer and Electronic Email in Internet

2. 2 File Transfer Overview Application protocol: FTP Main parts: client host and server host Parts of interaction: client authentication and file transfer Used transport protocol: TCP

3. 3 FTP Session Two TCP connections (out-of-band) –Control connection employing port 21 –Data connection employing port 20 Data transfer –One data connection per one file transfer Control information –One control connection per FTP session –User account –Commands to change directory on server side –Commands to put and get a file from server Keeping state information for each session

4. 4 Control Interaction Client commands: –USER –PASS –LIST –RETR –STOR Server reply: –331: username OK, password required –125: transfer starting –425: can’t open data connection –452: error writing file

5. 5 Email System Overview Application protocol: SMTP (Simple Mail Transfer Protocol) Main parts: –Sender side: agent, mail server –Receiver side: agent, mail server Used transport protocol: TCP

6. 6 SMTP Interaction between mail servers and sender-his mail server Text of a mail: 7-bit ASCII TCP connection uses 25 port Handshaking One connection for sending a bunch of messages between the servers

7. 7 Message Structure From: To: Subject: [MIME headers] Received: from by <> –Inserted to a message by a SMTP receiving server –May consist of several records due to forwarding

8. 8 Comparison with HTTP HTTP vs. SMTP –transferring files –persistent connections –pull vs. push protocol –any characters vs. 7-bit ASCII –one message per object vs. one message per all objects

9. 9 MIME Purpose: sending content different from ASCII text Placement: additional headers in the message Structure: –Content-Type header Indicates the type of non-ASCII data (video file, jpeg file, …) –Content-Transfer-Encoding header Type of encoding used for non-ASCII data (base64, …)

10. 10 Mail Access Protocol Purpose: reading email from receiver’s SMTP server –SMTP – pushing protocol –Getting email is a pulling operation Protocols: POP3, IM AP, HTTP

11. 11 POP3 TCP connection uses port 110 Phases: –Authentication Getting access to SMTP server –Transaction Retrieving messages Marking messages for deletion Obtaining mail statistics –Update Deleting messages Closing a session Download-and-keep, download-and-delete

12. 12 POP3 (cont.) Authentication –user –pass Transaction –list, retr, dele Update –quit Server response –+OK –-ERR No support of remote folders for messages

13. 13 IMAP Purpose: supporting remote folders Features: –Maintaining user state information across sessions –Retrieving parts of messages

14. 14 Web-Based Email Yahoo, Google, Gmail, … HTTP: –sending message to client mail server –retrieving message from recipient mail server SMTP: –interaction between mail servers

15. 15 DNS Purpose: translating hostname to IP address Architecture: distributed system Additional functions: –Host aliasing –Mail server aliasing –Load distribution Transport protocol: –UDP –port 53

16. 16 Structure Hierarchy: –Root DNS server Top-level domain (TLD) DNS server –Authoritative DNS server Local DNS server –Part of DNS –Interaction between end-host and the rest of DNS Intermediate DNS server –Interaction between TLD and authoritative server Accelerating performance through caching

17. 17 DNS Record DNS resource record: (Name, Value, Type, TTL) –Type: A, NS, CNAME, MX –Name: hostname, domain, alias hostname, alias name of a mail server –Value: IP address, hostname of authoritative DNS server, canonical hostname, canonical name of a mail server –TTL: time of keeping a record in a cache