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Application Protocols

Application Protocols. Outline. FTP Email DNS. FTP server. local file system. FTP: the File Transfer Protocol. file transfer. FTP user agent. user at host. remote file system. ftp: RFC 959 ftp client contacts ftp server, who listens at port 21

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Application Protocols

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  1. Application Protocols Computer Networks

  2. Outline • FTP • Email • DNS

  3. FTP server local file system FTP: the File Transfer Protocol file transfer FTP user agent user at host remote file system • ftp: RFC 959 • ftp client contacts ftp server, who listens at port 21 • two parallel TCP connections opened: • control port 21: exchange commands, responses • data port 20: file data to/from server

  4. 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

  5. What does this bi-direction mean for firewall filters? Can I disable all incoming connections? What if incoming connections are all denied, can you download files? Internet Intranet Impacts of the FTP dual connections: for system administrators • Firewall ACL rules • Permit all connections initiated from inside • Disable all connections initiated from outside FTP client FTP server TCP control connection port 21 TCP data connection port 20 Firewall Computer Networks

  6. Passive (PASV) mode Client initiates control connection by connecting to port 21 on server Client enables “Passive” mode Server responds with PORT command giving client the IP address and port to use for subsequent data connection (usually port 20, but can be bypassed) Client initiates data connection by connecting to specified port on server Most web browsers do PASV-mode ftp TCP control connection port 21 Client connect to some server port FTP client FTP server FTP Passive Mode

  7. Outline • FTP • Email • DNS

  8. 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., Eudora, Outlook, elm, Netscape Messenger 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

  9. 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

  10. Try SMTP interaction 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)

  11. Sample SMTP interaction 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

  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: SMTP (cont.) • 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

  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 What if I want to send data encoded other than ASCII? Mail message format header blank line body

  14. MIME: multimedia 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

  15. Text example subtypes: plain, html Image example subtypes: jpeg, gif Audio exampe 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

  16. SMTP: delivery/storage to receiver’s server Mail access protocol: retrieval from server 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 msgs on server HTTP: Hotmail , Yahoo! Mail, etc. user agent user agent sender’s mail server SMTP Mail access protocols SMTP access protocol receiver’s mail server

  17. 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 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

  18. 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 POP3 (more) and IMAP

  19. Outline • FTP • Email • DNS

  20. Function Machine like to use fix length number e.g., IP address (32 bit) - used for address Human like names, e.g., church.cse.ogi.edu - used by humans Map between names (e.g. www.cs.uga.edu) and IP addresses (e.g. 128.192.251.7) Domain Name System: distributed database implemented in hierarchy of many name servers application-layer protocol host, routers, name servers to communicate to resolvenames (address/name translation) note: core Internet function, implemented as application-layer protocol complexity at network’s “edge” DNS: Domain Name System

  21. Original Name to Address Mapping • Flat namespace • /etc/hosts • SRI kept main copy • Downloaded regularly • Problems • Count of hosts was increasing: • machine per domain  machine per user • Many more downloads • Many more updates

  22. no server has all name-to-IP address mappings local name servers: each ISP, company has local (default) name server /etc/resolv.conf 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 Why not centralize DNS? single point of failure traffic volume distant centralized database maintenance doesn’t scale! DNS name servers

  23. DNS: Domain Name System • Basic scheme: a hierarchical name space implemented by a distributed database called a zone

  24. 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

  25. Host atlas.cs.uga.edu wants IP address of gaia.cs.umass.edu 1. contacts its local DNS server, dns1.uga.edu 2.dns1 contacts root name server, if necessary 3. root name server contacts authoritative name server, dns.umass.edu, if necessary local name server dns1.uga.edu Simple DNS example root name server 2 4 3 5 authorititive name server dns.umass.edu 1 6 requesting host atlas.cs.uga.edu gaia.cs.umass.edu

  26. 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 dns1.uga.edu 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 atlas.cs.uga.edu gaia.cs.umass.edu

  27. iterated query 2 3 4 7 local name server dns1.uga.edu intermediate name server dns.umass.edu 5 6 1 8 DNS: Two Types of Queries root name server Recursive query: • puts burden of name resolution on contacted name server Iterated query: • contacted server replies with name of server to contact • “I don’t know this name, but ask this server” authoritative name server dns.cs.umass.edu requesting host atlas.cs.uga.edu gaia.cs.umass.edu

  28. once (any) name server learns mapping, it caches mapping cache entries timeout (disappear) after some time update/notify mechanisms under design by IETF RFC 2136 http://www.ietf.org/html.charters/dnsind-charter.html DNS: caching and updating records

  29. DNS: distributed db storing 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 “cannonical” (the real) name www.ibm.com is really servereast.backup2.ibm.com • value is cannonical name • Type=MX • value is name of mailserver associated with name

  30. DNS protocol :queryand reply messages, both with same message format DNS protocol, messages msg header • identification: 16 bit # for query, reply to query uses same # • flags: • query or reply • recursion desired • recursion available • reply is authoritative

  31. DNS protocol, messages Name, type fields for a query RRs in reponse to query records for authoritative servers additional “helpful” info that may be used

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