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05 - FTP, Email, and DNS

Learn about the File Transfer Protocol (FTP), Simple Mail Transfer Protocol (SMTP), and Domain Name System (DNS) in the application layer. Understand how to transfer files, send emails, and manage domain names.

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05 - FTP, Email, and DNS

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  1. 05 - FTP, Email, and DNS 2: Application Layer

  2. transfer file to/from remote host client/server model client: side that initiates transfer (either to/from remote) server: remote host ftp: RFC 959 ftp server: port 21 FTP user interface FTP client FTP server local file system FTP: the File Transfer Protocol file transfer user at host remote file system 2: Application Layer

  3. FTP client contacts FTP server at port 21, specifying TCP as transport protocol Client obtains authorization over control connection Client browses remote directory by sending commands over control connection. When server receives a command for a file transfer, the server opens a TCP data connection to client After transferring one file, server closes connection. TCP control connection port 21 TCP data connection port 20 FTP client FTP server FTP: separate control, data connections • Server opens a second TCP data connection to transfer another file. • Control connection: “out of band” • FTP server maintains “state”: current directory, earlier authentication 2: Application Layer

  4. Sample commands • sent as ASCII text over control channel • Authentication • USER: specify the user name to log in as • PASS: specify the user’s password • Exploring the files • LIST: list the files for the given file specification • CWD: change to the given directory • Downloading and uploading files • TYPE: set type to ASCII (A) or binary image (I) • RETR: retrieve the given file • STOR: upload the given file • Closing the connection • QUIT: close the FTP connection 2: Application Layer

  5. Why two connections? • Avoids need to mark the end of the data transfer • Data transfer ends by closing of data connection • Yet, the control connection stays up • Aborting a data transfer • Can abort a transfer without killing the control connection • … which avoids requiring the user to log in again • Done with an ABOR on the control connection • Third-party file transfer between two hosts • Data connection could go to a different host • … by sending a different client IP address to the server • E.g., user coordinates transfer between two servers • But: this is rarely needed, and presents security issues 2: Application Layer

  6. FTP, SFTP • FTP is not secure – nothing is encrypted! • SFTP uses SSH, and should be used instead of FTP when possible. 2: Application Layer

  7. Three major components: user agents mail servers simple mail transfer protocol: SMTP User Agent a.k.a. “mail reader” composing, editing, reading mail messages e.g., Apple Mail, Outlook, elm outgoing, incoming messages stored on server user agent user agent user agent user agent user agent user agent SMTP SMTP SMTP mail server mail server mail server outgoing message queue user mailbox Electronic Mail 2: Application Layer

  8. Mail Servers mailbox contains incoming messages for user messagequeue 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 agent user agent user agent user agent user agent user agent SMTP SMTP SMTP mail server mail server mail server Electronic Mail: mail servers 2: Application Layer

  9. 1) Alice uses UA to compose message and “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 user agent user agent mail server mail server Scenario: Alice sends message to Bob 1 2 6 3 4 5 2: Application Layer

  10. uses TCP to reliably transfer email message from client to server, port 25 direct transfer: sending server (client) to receiving server (server) three phases of transfer handshaking (greeting) transfer of messages closure command/response interaction commands: ASCII text response: status code and phrase messages must be in 7-bit ASCII Electronic Mail: SMTP [RFC 2821] 2: Application Layer

  11. Sample SMTP interaction >telnet hamburger.edu 25 S: 220 hamburger.edu C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: <alice@crepes.fr> S: 250 alice@crepes.fr... Sender ok C: RCPT TO: <bob@hamburger.edu> S: 250 bob@hamburger.edu ... 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 Handshake 2: Application Layer

  12. SMTP uses persistent connections 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 interaction, status codes HTTP: each object encapsulated in its own response msg SMTP: multiple objects sent in multipart msg SMTP: final words 2: Application Layer

  13. SMTP: protocol for exchanging email msgs RFC 822: standard for text message format: header lines, e.g., To: From: Subject: differentfrom SMTP commands! body the “message”, ASCII characters only Mail message format header blank line body 2: Application Layer

  14. MIME: Multipurpose Internet Mail Extension, RFC 2045, 2056 additional lines in msg header declare MIME content type From: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data ..... ......................... ......base64 encoded data Message format: multimedia extensions MIME version method used to encode data multimedia data type, subtype, parameter declaration encoded data 2: Application Layer

  15. Text example subtypes: plain, html Image example subtypes: jpeg, gif Audio example subtypes: basic (8-bit mu-law encoded), 32kadpcm (32 kbps coding) Video example subtypes: mpeg, quicktime Application other data that must be processed by reader before “viewable” example subtypes: msword, octet-stream MIME typesContent-Type: type/subtype; parameters 2: Application Layer

  16. Multipart Type From: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Type: multipart/mixed; boundary=StartOfNextPart --StartOfNextPart Dear Bob, Please find a picture of a crepe. --StartOfNextPart Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data ..... ......................... ......base64 encoded data --StartOfNextPart Do you want the recipe? 2: Application Layer

  17. SMTP: delivery/storage to receiver’s server Mail access protocol: retrieval from server POP: Post Office Protocol [RFC 1939] TCP, port 110 authorization (agent <-->server) and download IMAP: Internet Mail Access Protocol [RFC 1730] more features (more complex) manipulation of stored msgs on server HTTP: gmail, Hotmail , Yahoo! Mail, etc. user agent user agent sender’s mail server SMTP Mail access protocols SMTP access protocol receiver’s mail server 2: Application Layer

  18. 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 Update phase- server deletes files. POP3 protocol S: +OK POP3 server ready C: user bob S: +OK C: pass hungry S: +OK user successfully logged on 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 2: Application Layer

  19. 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: names of folders and mappings between message IDs and folder name Can also download only portions of a message e.g. headers POP3 (more) and IMAP 2: Application Layer

  20. Web-based E-mail • User agent: browser • Use HTTP to send e-mail to server and receive e-mail from server. • SMTP between servers. 2: Application Layer

  21. DNS: Domain Name System people: many identifiers: • SSN, name, passport # Internet hosts, routers: • IP address (32 bit) - used for addressing datagrams • “name”, e.g., www.yahoo.com - used by humans Q: map between IP address and name, and vice versa ? www.rose-hulman.edu  DNS  137.112.18.43

  22. Why not centralize DNS? single point of failure traffic volume distant centralized database maintenance doesn’t scale! DNS services hostname to IP address translation host aliasing Canonical, alias names mail server aliasing Core Internet function implemented as application layer protocol load distribution replicated Web servers: set of IP addresses for one canonical name DNS

  23. no server has all name-to-IP address mappings local name servers: each ISP, company has local (default) name server host DNS query first goes to local name server authoritative name server: for a host: stores that host’s IP address, name can perform name/address translation for that host’s name DNS is a distributed database implemented in hierarchy of many name servers DNS name servers 2: Application Layer

  24. Root DNS Servers org DNS servers edu DNS servers com DNS servers poly.edu DNS servers umass.edu DNS servers pbs.org DNS servers yahoo.com DNS servers amazon.com DNS servers Distributed, Hierarchical Database client wants IP for www.amazon.com; 1st approx: • client queries a root server to find com DNS server • client queries com DNS server to get amazon.com DNS server • client queries amazon.com DNS server to get IP address for www.amazon.com

  25. contacted by local name server that can not resolve name root name server: contacts authoritative name server if name mapping not known gets mapping returns mapping to local name server a NSI Herndon, VA c PSInet Herndon, VA d U Maryland College Park, MD g DISA Vienna, VA h ARL Aberdeen, MD j NSI (TBD) Herndon, VA k RIPE London i NORDUnet Stockholm m WIDE Tokyo e NASA Mt View, CA f Internet Software C. Palo Alto, CA b USC-ISI Marina del Rey, CA l ICANN Marina del Rey, CA DNS: Root name servers 13 root name servers worldwide (actually > 80 using anycasting) 2: Application Layer Zonefile stored at a root server

  26. host surf.eurecom.fr wants IP address of gaia.cs.umass.edu 1. contacts its local DNS server, dns.eurecom.fr 2.dns.eurecom.fr contacts root name server, if necessary 3. root name server contacts authoritative name server, dns.umass.edu, if necessary local name server dns.eurecom.fr Simple DNS example root name server 2 4 3 5 authoritative name server dns.cs.umass.edu 1 6 requesting host surf.eurecom.fr gaia.cs.umass.edu 2: Application Layer

  27. Root name server: may not know authoritative name server may know intermediate name server: who to contact to find authoritative name server local name server dns.eurecom.fr intermediate name server dns.umass.edu DNS example root name server 6 2 3 7 5 4 1 8 authoritative name server dns.cs.umass.edu requesting host surf.eurecom.fr gaia.cs.umass.edu 2: Application Layer

  28. recursive query: puts burden of name resolution on contacted name server heavy load? iterated query: contacted server replies with name of server to contact “I don’t know this name, but ask this server” local name server dns.eurecom.fr intermediate name server dns.umass.edu DNS: iterated queries root name server iterated query 2 3 4 7 5 6 1 8 authoritative name server dns.cs.umass.edu requesting host surf.eurecom.fr gaia.cs.umass.edu 2: Application Layer

  29. once (any) name server learns mapping, it caches mapping cache entries timeout (disappear) after some time If the TLD servers and intermediate DNS servers perform their functions correctly, the root servers will rarely be contacted. DNS: caching and updating records 2: Application Layer

  30. DNS: distributed db stores resource records (RR) Type=NS name is domain (e.g. foo.com) value is IP address of authoritative name server for this domain RR format: (name, value, type, ttl) DNS records • Type=A • name is hostname • value is IP address • Type=CNAME • name is alias name for some “canonical” (the real) name • www.ibm.com is really • servereast.backup2.ibm.com • value is canonical name • Type=MX • value is name of mailserver associated with name Use “nslookup” and “dig” to see RRs. 2: Application Layer

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