2: Application Layer 2 Chapter 2 Application Layer Computer Networking: A Top Down Approach Featuring the Internet, 3 rd edition. Jim Kurose, Keith Ross Addison-Wesley, July 2004. A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following: If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!) If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material. Thanks and enjoy! JFK/KWR All material copyright 1996-2004 J.F Kurose and K.W. Ross, All Rights Reserved
2: Application Layer 3 FTP: the File Transfer Protocol r transfer file to/from remote host r client/server model m client: side that initiates transfer (either to/from remote) m server: remote host r ftp: RFC 959 r ftp server: port 21 file transfer FTP server FTP user interface FTP client local file system remote file system user at host
2: Application Layer 4 FTP: separate control, data connections r FTP client contacts FTP server at port 21, specifying TCP as transport protocol r Client obtains authorization over control connection r Client browses remote directory by sending commands over control connection. r When server receives a command for a file transfer, the server opens a TCP data connection to client r After transferring one file, server closes connection. FTP client FTP server TCP control connection port 21 TCP data connection port 20 r Server opens a second TCP data connection to transfer another file. r Control connection: “out of band” r FTP server maintains “state”: current directory, earlier authentication
2: Application Layer 5 Sample commands r sent as ASCII text over control channel r Authentication m USER: specify the user name to log in as m PASS: specify the user’s password r Exploring the files m LIST: list the files for the given file specification m CWD: change to the given directory r Downloading and uploading files m TYPE: set type to ASCII (A) or binary image (I) m RETR: retrieve the given file m STOR: upload the given file r Closing the connection m QUIT: close the FTP connection
2: Application Layer 6 Sample return codes r status code and phrase (as in HTTP) r 331 Username OK, password required r 125 data connection already open; transfer starting r 425 Can’t open data connection r 452 Error writing file
2: Application Layer 7 Why two connections? r Avoids need to mark the end of the data transfer m Data transfer ends by closing of data connection m Yet, the control connection stays up r Aborting a data transfer m Can abort a transfer without killing the control connection m … which avoids requiring the user to log in again m Done with an ABOR on the control connection r Third-party file transfer between two hosts m Data connection could go to a different host m … by sending a different client IP address to the server m E.g., user coordinates transfer between two servers m But: this is rarely needed, and presents security issues
2: Application Layer 8 FTP, SFTP r FTP is not secure – nothing is encrypted! r SFTP uses SSH, and should be used instead of FTP when possible.
2: Application Layer 9 Electronic Mail Three major components: r user agents r mail servers r simple mail transfer protocol: SMTP User Agent r a.k.a. “mail reader” r composing, editing, reading mail messages r e.g., Eudora, Outlook, elm, Netscape Messenger r outgoing, incoming messages stored on server user mailbox outgoing message queue mail server user agent user agent user agent mail server user agent user agent mail server user agent SMTP
2: Application Layer 10 Electronic Mail: mail servers Mail Servers r mailbox contains incoming messages for user r message queue of outgoing (to be sent) mail messages r SMTP protocol between mail servers to send email messages m client: sending mail server m “server”: receiving mail server mail server user agent user agent user agent mail server user agent user agent mail server user agent SMTP
2: Application Layer 11 Scenario: Alice sends message to Bob 1) Alice uses UA to compose message and “to” email@example.com 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 mail server mail server user agent 1 2 3 4 5 6
2: Application Layer 12 Electronic Mail: SMTP [RFC 2821] r uses TCP to reliably transfer email message from client to server, port 25 r direct transfer: sending server to receiving server r three phases of transfer m handshaking (greeting) m transfer of messages m closure r command/response interaction m commands: ASCII text m response: status code and phrase r messages must be in 7-bit ASCII
2: Application Layer 13 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: S: 250 firstname.lastname@example.org... Sender ok C: RCPT TO: S: 250 email@example.com... 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 14 SMTP: final words r SMTP uses persistent connections r SMTP requires message (header & body) to be in 7- bit ASCII SMTP server uses CRLF.CRLF to determine end of message Comparison with HTTP: r HTTP: pull r SMTP: push r both have ASCII command/response interaction, status codes r HTTP: each object encapsulated in its own response msg r SMTP: multiple objects sent in multipart msg
2: Application Layer 15 Mail message format SMTP: protocol for exchanging email msgs RFC 822: standard for text message format: r header lines, e.g., m To: m From: m Subject: different from SMTP commands! r body m the “message”, ASCII characters only header body blank line
2: Application Layer 18 Message format: multimedia extensions r MIME: Multipurpose Internet Mail Extension, RFC 2045, 2056 r additional lines in msg header declare MIME content type From: firstname.lastname@example.org To: email@example.com Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data....................................base64 encoded data multimedia data type, subtype, parameter declaration method used to encode data MIME version encoded data
2: Application Layer 19 MIME types Content-Type: type/subtype; parameters 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 r other data that must be processed by reader before “viewable” example subtypes: msword, octet-stream
2: Application Layer 20 Multipart Type From: firstname.lastname@example.org To: email@example.com 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 21 Mail access protocols r SMTP: delivery/storage to receiver’s server r Mail access protocol: retrieval from server m POP: Post Office Protocol [RFC 1939] TCP, port 110 authorization (agent server) and download m IMAP: Internet Mail Access Protocol [RFC 1730] more features (more complex) manipulation of stored msgs on server m HTTP: Hotmail, Yahoo! Mail, etc. user agent sender’s mail server user agent SMTP access protocol receiver’s mail server
2: Application Layer 22 POP3 protocol authorization phase r client commands: user: declare username pass: password r server responses m +OK -ERR transaction phase, client: list: list message numbers retr: retrieve message by number dele: delete r Quit Update phase- server deletes files. C: list S: 1 498 S: 2 912 S:. C: retr 1 S: S:. C: dele 1 C: retr 2 S: 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 on
2: Application Layer 23 POP3 (more) and IMAP More about POP3 r Previous example uses “download and delete” mode. r Bob cannot re-read e- mail if he changes client r “Download-and-keep”: copies of messages on different clients r POP3 is stateless across sessions IMAP r Keep all messages in one place: the server r Allows user to organize messages in folders r IMAP keeps user state across sessions: m names of folders and mappings between message IDs and folder name r Can also download only portions of a message e.g. headers
2: Application Layer 24 Web-based E-mail r User agent: browser r Use HTTP to send e-mail to server and receive e-mail from server. r SMTP between servers.
2: Application Layer 25 DNS: Domain Name System Domain Name System: r application-layer protocol host, routers, name servers to communicate to resolve names (address/name translation) m note: core Internet function, implemented as application-layer protocol m complexity at network’s “edge” www.rose-hulman.edu (name) (DNS) 22.214.171.124 (IP address)
2: Application Layer 26 DNS name servers r no server has all name- to-IP address mappings r local name servers: m each ISP, company has local (default) name server m host DNS query first goes to local name server r authoritative name server: m for a host: stores that host’s IP address, name m can perform name/address translation for that host’s name Centralized DNS? single point of failure traffic volume distant centralized database maintenance “doesn’t scale!” DNS is a distributed database implemented in hierarchy of many name servers
2: Application Layer 27 Top level domain servers (TLD) Authoritative name servers
2: Application Layer 28 DNS: Root name servers r contacted by local name server that can not resolve name r root name server: m contacts authoritative name server if name mapping not known m gets mapping m returns mapping to local name server b USC-ISI Marina del Rey, CA l ICANN Marina del Rey, CA e NASA Mt View, CA f Internet Software C. Palo Alto, CA i NORDUnet Stockholm k RIPE London m WIDE Tokyo 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 13 root name servers worldwide (actually > 80 using anycasting) Zonefile stored at a root server
2: Application Layer 29 Simple DNS example 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 requesting host surf.eurecom.fr gaia.cs.umass.edu root name server authoritative name server dns.cs.umass.edu local name server dns.eurecom.fr 1 2 3 4 5 6
2: Application Layer 30 DNS example Root name server: r may not know authoritative name server r may know intermediate name server: who to contact to find authoritative name server requesting host surf.eurecom.fr gaia.cs.umass.edu root name server local name server dns.eurecom.fr 1 2 3 4 5 6 authoritative name server dns.cs.umass.edu intermediate name server dns.umass.edu 7 8
2: Application Layer 31 DNS: iterated queries recursive query: r puts burden of name resolution on contacted name server r heavy load? iterated query: r contacted server replies with name of server to contact r “I don’t know this name, but ask this server” requesting host surf.eurecom.fr gaia.cs.umass.edu root name server local name server dns.eurecom.fr 1 2 3 4 5 6 authoritative name server dns.cs.umass.edu intermediate name server dns.umass.edu 7 8 iterated query
2: Application Layer 32 DNS: caching and updating records r once (any) name server learns mapping, it caches mapping m cache entries timeout (disappear) after some time r If the TLD servers and intermediate DNS servers perform their functions correctly, the root servers will rarely be contacted.
2: Application Layer 33 DNS records DNS: distributed db stores resource records (RR) r 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) r Type=A name is hostname value is IP address r 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 r Type=MX value is name of mailserver associated with name Use “nslookup” and “dig” to see RRs.