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CMSC 332Computer NetworkingEmail and DNSProfessor SzajdaCMSC 332: Computer Networks

Review Last lecture we talked about design principles, and theapplication protocols HTTP and FTP‣Text commands sent over a port (recall telnet example)‣Difference in statefullness‣HTTP and FTP are primarily pull protocolsCMSC 332: Computer Networks2

Chapter 2: Application layer 2.1 Principles of network applications 2.2 Web and HTTP 2.3 FTP 2.4 Electronic Mail 2.5 DNS 2.6 P2P ApplicationsCMSC 332: Computer Networks3

Electronic MailThree major components: useragentuser agentsmail serverssimple mail transfer protocol:SMTPUser Agent a.k.a. “mail reader” e.g., Eudora, Outlook, elm, pine,Apple Mail, GMail outgoing, incoming messagesstored on servercomposing, editing, reading mailmessagesCMSC 332: Computer Networksoutgoing message queueuser eragentmailserveruseragentuseragentuseragent4

Electronic Mail: mail serversMail Servers mailbox contains incomingmessages for usermessage queue of outgoing (tobe sent) mail messagesSMTP protocol between mailservers to send email messages‣‣client: sending mail server“server”: receiving mail ragentCMSC 332: Computer gent5

Electronic Mail: SMTP [RFC 5321] uses TCP to reliably transfer email message from client to server, port 25direct transfer: sending server to receiving serverthree phases of transfer‣handshaking (greeting)‣transfer of messages‣closurecommand/response interaction‣commands: ASCII text‣response: status code and phrase messages must be in 7-bit ASCIICMSC 332: Computer Networks6

Scenario: Alice sends message to Bob4) SMTP client sends Alice’s messageover the TCP connection5) Bob’s mail server places themessage in Bob’s mailbox6) Bob invokes his user agent toread message1) Alice uses UA to composemessage and “to”bob@someschool.edu2) Alice’s UA sends message toher mail server; messageplaced in message queue3) Client side of SMTP opens TCPconnection with Bob’s mailserver1useragentCMSC 332: Computer Networks2mailserver3mailserver456useragent7

Sample SMTP interactionS:C:S:C:S:C:S:C:S:C:C:C:S:C:S:220 hamburger.eduHELO crepes.fr250 Hello crepes.fr, pleased to meet youMAIL FROM: alice@crepes.fr 250 alice@crepes.fr. Sender okRCPT TO: bob@hamburger.edu 250 bob@hamburger.edu . Recipient okDATA354 Enter mail, end with "." on a line by itselfDo you like ketchup?How about pickles?.250 Message accepted for deliveryQUIT221 hamburger.edu closing connectionCMSC 332: Computer Networks8

Try SMTP interaction for yourself: telnet servername 25 see 220 reply from server enter HELO, MAIL FROM, RCPT TO, DATA, QUITcommandsabove lets you send email without using email client (reader)CMSC 332: Computer Networks9

SMTP: final words SMTP uses persistentconnections‣Just like.? SMTP requires message (header& body) to be in 7-bit ASCII SMTP server uses CRLF.CRLFto determine end of messageCMSC 332: Computer NetworksComparison with HTTP: HTTP: pull SMTP: push both have ASCII command/response interaction, statuscodes HTTP: each object encapsulatedin its own response msg SMTP: multiple objects sent inmultipart msg10

Mail message formatSMTP: protocol for exchangingemail msgsRFC 2822: standard for textmessage format: headerheader lines, nt from SMTPcommands! body‣the “message”, ASCII charactersonlyCMSC 332: Computer Networks11

Message format: multimedia extensions MIME: multimedia mail extension, RFC 2045, 2056additional lines in msg header declare MIME content typeMIME versionmethod usedto encode datamultimedia datatype, subtype,parameter declarationFrom: alice@crepes.frTo: bob@hamburger.eduSubject: Picture of yummy crepe.MIME-Version: 1.0Content-Transfer-Encoding: base64Content-Type: image/jpegbase64 encoded data .base64 encoded dataencoded dataCMSC 332: Computer Networks12

Mail access protocolsuseragent SMTPSMTPsender’s mailserveraccessprotocoluseragentreceiver’s mailserverSMTP: delivery/storage to receiver’s serverMail access protocol: retrieval from server‣POP: Post Office Protocol [RFC 1939] authorization (agent -- server) and download‣IMAP: Internet Mail Access Protocol [RFC 3501] more features (more complex) manipulation of stored msgs on server‣HTTP: Gmail, Hotmail ,Yahoo! Mail, etc.CMSC 332: Computer Networks13

POP3 protocolauthorization phase client commands:‣user: declare username‣pass: passwordserver responses‣ OK‣-ERRtransaction phase, client: list: list message numbersretr: retrieve message by numberdele: deletequitCMSC 332: Computer NetworksS:C:S:C:S: OK POP3 server readyuser bob OKpass hungry OK user successfully loggedC:S:S:S:C:S:S:C:C:S:S:C:C:S:list1 4982 912.retr 1 message 1 contents .dele 1retr 2 message 1 contents .dele 2quit OK POP3 server signing offon14

POP3 (more) and IMAPMore about POP3IMAP Keep all messages in oneplace: the server Allows user to organizemessages in folders IMAP keeps user stateacross sessions:Previous example uses“download and delete”mode. Bob cannot re-read e-mail if he changes client “Download-and-keep”:copies of messages ondifferent clients‣names of folders andmappings between messageIDs and folder name POP3 is stateless acrosssessionsCMSC 332: Computer Networks15

Chapter 2: Application layer 2.1 Principles of networkapplications 2.2 Web and HTTP 2.3 FTP 2.4 Electronic Mail 2.5 DNS 2.6 P2P ApplicationsCMSC 332: Computer Networks16

DNS: Domain Name SystemPeople: many identifiers:‣SSN, name, passport #Internet hosts, routers:‣IP address (32 bit) - used foraddressing datagrams‣“name”, e.g., ww.yahoo.com used by humansQ: map between IPaddresses and name ?CMSC 332: Computer NetworksDomain Name System: distributed databaseimplemented in hierarchy ofmany name servers application-layer protocol host,routers, name servers tocommunicate to resolve names(address/name translation)‣note: core Internet function,implemented as application-layerprotocol‣complexity at network’s “edge”17

DNSDNS services Hostname to IP addresstranslation Host aliasing‣Canonical and alias names Mail server aliasing Load distribution‣Replicated Web servers: setof IP addresses for onecanonical nameCMSC 332: Computer Networks18

DNSDNS servicesWhy not centralize DNS? Hostname to IP addresstranslation Host aliasing single point of failure traffic volume distant centralized database maintenance‣Canonical and alias names Mail server aliasing Load distribution‣Replicated Web servers: setof IP addresses for onecanonical nameCMSC 332: Computer NetworksIn summary, it doesn’t scale!18

Distributed, Hierarchical DatabaseRoot DNS Serverscom DNS serversyahoo.comDNS serversamazon.comDNS serversorg DNS serverspbs.orgDNS serversedu DNS serverspoly.eduumass.eduDNS servers DNS serversClient wants IP for www.amazon.com; 1st approx: Client queries a root server to find com DNS serverqueries com DNS server to get amazon.com DNS Clientserverqueries amazon.com DNS server to get Clientfor www.amazon.comCMSC 332: Computer NetworksIP address19

DNS: Root name servers contacted by local name server that can not resolve nameroot name server:‣contacts authoritative name server if name mapping not known‣gets mapping‣returns mapping to local name servera Verisign, Dulles, VAc Cogent, Herndon, VA (also Los Angeles)d U Maryland College Park, MDk RIPE London (also Amsterdam, Frankfurt)g US DoD Vienna, VAh ARL Aberdeen, MDi Autonomica, Stockholm (plus 3 other locations)j Verisign, ( 11 locations)e NASA Mt View, CAf Internet Software C. Palo Alto,m WIDE TokyoCA (and 17 other locations)13 root name serversworldwideb USC-ISI Marina del Rey, CAl ICANN Los Angeles, CACMSC 332: Computer Networks20

TLD and Authoritative Serversdomain (TLD) servers: responsible for com, Top-levelorg, net, edu, etc, and all top-level country domains uk,fr, ca, jp.‣Network solutions maintains servers for com TLD‣Educause for edu TLDDNS servers: organization’s DNS servers, Authoritativeproviding authoritative hostname to IP mappings fororganization’s servers (e.g., Web andmail).‣Can be maintained by organization orservice providerCMSC 332: Computer Networks21

Local Name Server Does not strictly belong to hierarchy Each ISP (residential ISP, company, university) has one.‣Also called “default name server” When a host makes a DNS query, query is sent to itslocal DNS server‣Acts as a proxy, forwards query into hierarchy.CMSC 332: Computer Networks22

Example root DNS serverHost at cis.poly.eduwants IP address forgaia.cs.umass.eduTLD DNS serverlocal DNS serverdns.poly.edurequesting hostauthoritative DNS uCMSC 332: Computer Networks23

Example root DNS serverHost at cis.poly.eduwants IP address forgaia.cs.umass.eduTLD DNS serverlocal DNS serverdns.poly.edu1requesting hostauthoritative DNS uCMSC 332: Computer Networks23

Example root DNS serverHost at cis.poly.eduwants IP address forgaia.cs.umass.edu2TLD DNS serverlocal DNS serverdns.poly.edu1requesting hostauthoritative DNS uCMSC 332: Computer Networks23

Example root DNS serverHost at cis.poly.eduwants IP address forgaia.cs.umass.edu23TLD DNS serverlocal DNS serverdns.poly.edu1requesting hostauthoritative DNS uCMSC 332: Computer Networks23

Example root DNS serverHost at cis.poly.eduwants IP address forgaia.cs.umass.edu234TLD DNS serverlocal DNS serverdns.poly.edu1requesting hostauthoritative DNS uCMSC 332: Computer Networks23

Example root DNS serverHost at cis.poly.eduwants IP address forgaia.cs.umass.edu234TLD DNS server5local DNS serverdns.poly.edu1requesting hostauthoritative DNS uCMSC 332: Computer Networks23

Example root DNS serverHost at cis.poly.eduwants IP address forgaia.cs.umass.edu234TLD DNS server5local DNS serverdns.poly.edu1requesting host6authoritative DNS uCMSC 332: Computer Networks23

Example root DNS serverHost at cis.poly.eduwants IP address forgaia.cs.umass.edu234TLD DNS server5local DNS serverdns.poly.edu1requesting host76authoritative DNS uCMSC 332: Computer Networks23

Example root DNS serverHost at cis.poly.eduwants IP address forgaia.cs.umass.edu234TLD DNS server5local DNS serverdns.poly.edu18requesting host76authoritative DNS uCMSC 332: Computer Networks23

Recursive queriesroot DNS serverrecursive query: puts burden of nameresolution oncontacted name server heavy load?iterated query: contacted serverreplies with name ofserver to contact “I don’t know thisname, but ask thisserver”2376TLD DNS serverlocal DNS serverdns.poly.edu1548requesting hostauthoritative DNS uCMSC 332: Computer Networks24

DNS: caching and updating records once (any) name server learns mapping, it caches the mapping‣cache entries timeout (disappear) after some time‣TLD servers typically cached in local name servers Thus root name servers not often visited update/notify mechanisms‣RFC 2136, arter.htmlCMSC 332: Computer Networks25

DNS recordsDNS: distributed db storing resource records (RR)RR format: Type A‣name is hostname‣value is IP address Type NS‣name is domain (e.g.foo.com)‣value is hostname ofauthoritative nameserver for this domainCMSC 332: Computer Networks(name, value, type, ttl) Type CNAME‣name is alias name for some“canonical” (the real) namewww.ibm.com is reallyservereast.backup2.ibm.com‣value is canonical name Type MX‣value is name of mailserverassociated with name26

DNS protocol, messagesDNS protocol : query and reply messages, both with same message formatmsg header identification: 16 bit # forquery, reply to query usessame # flags:‣query or reply‣recursion desired‣recursion available‣reply is authoritativeCMSC 332: Computer Networks27

DNS protocol, messagesName, type fieldsfor a queryRRs in responseto queryrecords forauthoritative serversadditional “helpful”info that may be usedCMSC 332: Computer Networks28

Viewing DNS Queries Text recommends nslookupI use digCMSC 332: Computer Networks29