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

FTP: the File Transfer Protocol user interface client file transfer FTP server user at host local file system remote file system 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 2: Application Layer

FTP: separate control, data connections client server TCP control connection port 21 TCP data connection port 20 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. Server opens a second TCP data connection to transfer another file. Control connection: “out of band” FTP server maintains “state”: current directory, earlier authentication Because FTP uses a second connection to send control info, FTP is said to send control info out of band. 2: Application Layer

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

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 2: Application Layer

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

FTP, SFTP FTP is not secure – nothing is encrypted! SFTP uses SSH, and should be used instead of FTP when possible. SFTP i.e. SSH with FTP uses public key encryption. SSH2 uses the Diffie-Hellman key exchange (generate a secret key and then use symmetric key encryption). Each message is individually encrpyted. FTPS or FPT with SSL – uses certificates, public key cryptography and symmetric key – secure connection between client/server. SSH – UNIX SSL – Windows SSL – privacy and trust SSH – privacy and authentication 2: Application Layer

Electronic Mail Three major components: SMTP SMTP SMTP user agents user mailbox outgoing message queue user agent 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 mail server user agent SMTP mail server user agent SMTP mail server SMTP user agent user agent user agent 2: Application Layer

Electronic Mail: mail servers user agent 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 mail server user agent SMTP mail server user agent SMTP mail server SMTP user agent Note that the client and server are both mail-servers. user agent user agent 2: Application Layer

Scenario: Alice sends message to Bob 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 mail server mail server 1 user agent user agent 2 3 6 4 5 2: Application Layer

Electronic Mail: SMTP [RFC 2821] 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 2: Application Layer

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

SMTP: final words Comparison with HTTP: 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 2: Application Layer

Mail message format header body 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 header blank line body 2: Application Layer

Message format: multimedia extensions 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 MIME version method used to encode data multimedia data type, subtype, parameter declaration encoded data 2: Application Layer

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 other data that must be processed by reader before “viewable” example subtypes: msword, octet-stream 2: Application Layer

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. Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data ..... ......................... ......base64 encoded data Do you want the recipe? 2: Application Layer

Mail access protocols SMTP SMTP access protocol user agent user agent sender’s mail server receiver’s mail server 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. 2: Application Layer

POP3 protocol authorization phase C: list transaction phase, client: S: +OK POP3 server ready C: user bob S: +OK C: pass hungry S: +OK user successfully logged on 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. C: list S: 1 498 S: 2 912 S: . C: retr 1 S: <message 1 contents> C: dele 1 C: retr 2 C: dele 2 C: quit S: +OK POP3 server signing off 2: Application Layer

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

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

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

DNS DNS services Why not centralize DNS? 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 Why not centralize DNS? single point of failure traffic volume distant centralized database maintenance doesn’t scale!

DNS name servers no server has all name-to-IP address mappings DNS is a distributed database implemented in hierarchy of many name servers 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 2: Application Layer

Distributed, Hierarchical Database Root DNS Servers com DNS servers org DNS servers edu DNS servers poly.edu DNS servers umass.edu yahoo.com amazon.com pbs.org 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 Com DNS servers  Top-level domain (TLD) severs Amazon DNS servers  authoritative DNS servers

DNS: Root name servers 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 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

authoritative name server Simple DNS example root name server 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 2 4 3 5 local name server dns.eurecom.fr authoritative name server dns.cs.umass.edu 1 6 requesting host surf.eurecom.fr gaia.cs.umass.edu 2: Application Layer

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

DNS: iterated queries recursive query: iterated query: root name server 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” iterated query 2 3 4 7 local name server dns.eurecom.fr intermediate name server dns.umass.edu 5 6 1 8 authoritative name server dns.cs.umass.edu requesting host surf.eurecom.fr gaia.cs.umass.edu 2: Application Layer

DNS: caching and updating records 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. 2: Application Layer

RR format: (name, value, type, ttl) DNS records DNS: distributed db stores resource records (RR) RR format: (name, value, type, ttl) 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=NS name is domain (e.g. foo.com) value is IP address of authoritative name server for this domain Type=MX value is name of mailserver associated with name Use “nslookup” and “dig” to see RRs. 2: Application Layer