1 Mail access protocols DNS: Domain Name System SMTP SMTP access - PDF document

FTP: separate control, data connections TCP control connection ❒ FTP client contacts FTP port 21 server at port 21, specifying TCP as transport protocol TCP data connection FTP, SMTP and DNS ❒ Client obtains authorization FTP FTP port 20 over control connection client server ❒ Client browses remote ❒ Server opens a second TCP directory by sending data connection to transfer commands over control connection. another file. ❒ When server receives a ❒ Control connection: “out of command for a file transfer, band” the server opens a TCP data ❒ FTP server maintains connection to client “state”: current directory, ❒ After transferring one file, earlier authentication server closes connection. 2: Application Layer 2: Application Layer 1 2 Electronic Mail Electronic Mail: mail servers outgoing message queue user mailbox Three major components: user user Mail Servers agent agent ❒ user agents mail ❒ mailbox contains incoming mail user ❒ mail servers user server agent messages for user server agent ❒ simple mail transfer SMTP SMTP ❒ message queue of outgoing mail protocol: SMTP mail server user (to be sent) mail messages server user SMTP agent SMTP agent User Agent ❒ SMTP protocol between SMTP mail servers to send email ❒ a.k.a. “mail reader” SMTP user user mail messages mail ❒ composing, editing, reading agent server agent server mail messages ❍ client: sending mail user ❒ e.g., Eudora, Outlook, elm, server user agent Netscape Messenger agent ❍ “server”: receiving mail user user ❒ outgoing, incoming messages agent server agent stored on server 2: Application Layer 2: Application Layer 3 4 Scenario: Alice sends message to Bob Electronic Mail: SMTP [RFC 2821] 1) Alice uses UA to compose 4) SMTP client sends Alice’s message and “to” message over the TCP ❒ uses TCP to reliably transfer email message from client connection bob@someschool.edu to server, port 25 2) Alice’s UA sends message 5) Bob’s mail server places ❒ direct transfer: sending server to receiving server to her mail server; the message in Bob’s ❒ three phases of transfer message placed in message mailbox ❍ handshaking (greeting) queue 6) Bob invokes his user agent ❍ transfer of messages 3) Client side of SMTP opens to read message ❍ closure TCP connection with Bob’s mail server ❒ command/response interaction 1 mail ❍ commands: ASCII text mail user user server server ❍ response: status code and phrase 2 agent agent 3 6 4 5 ❒ messages must be in 7-bit ASCII 2: Application Layer 5 2: Application Layer 6 1

Mail access protocols DNS: Domain Name System SMTP SMTP access Domain Name System: People: many identifiers: user user protocol agent ❒ distributed database agent ❍ SSN, name, passport # implemented in hierarchy of Internet hosts, routers: receiver’s mail many name servers sender’s mail server server ❒ application-layer protocol ❍ IP address (32 bit) - ❒ SMTP: delivery/storage to receiver’s server host, routers, name servers used for addressing to communicate to resolve ❒ Mail access protocol: retrieval from server datagrams names (address/name ❍ POP: Post Office Protocol [RFC 1939] ❍ “name”, e.g., translation) • authorization (agent <-->server) and download ww.yahoo.com - used by ❍ note: core Internet humans ❍ IMAP: Internet Mail Access Protocol [RFC 1730] function, implemented as application-layer protocol Q: map between IP • more features (more complex) ❍ complexity at network’s addresses and name ? • manipulation of stored msgs on server “edge” ❍ HTTP: Hotmail , Yahoo! Mail, etc. 2: Application Layer 2: Application Layer 7 8 DNS and Applications DNS ❒ Which applications use DNS? Why not centralize DNS? ❒ single point of failure ❒ HTTP ❒ traffic volume ❍ Browser extracts hostname ❒ distant centralized database ❍ Sends hostname to DNS ❒ maintenance ❍ DNS does lookup and returns IP address ❍ Browser sends HTTP GET to IP address doesn’t scale! 2: Application Layer 2: Application Layer 9 10 Distributed, Hierarchical Database DNS: Root name servers Root DNS Servers ❒ contacted by local name server that can not resolve name ❒ root name server: org DNS servers edu DNS servers com DNS servers ❍ contacts authoritative name server if name mapping not known ❍ gets mapping poly.edu umass.edu pbs.org yahoo.com amazon.com DNS servers DNS servers DNS servers ❍ returns mapping to local name server DNS servers DNS servers a Verisign, Dulles, VA c Cogent, Herndon, VA (also Los Client wants IP for www.amazon.com; 1 st approx: Angeles) k RIPE London (also Amsterdam, d U Maryland College Park, MD Frankfurt) i Autonomica, Stockholm g US DoD Vienna, VA ❒ Client queries a root server to find com DNS (plus 3 other locations) h ARL Aberdeen, MD j Verisign, ( 11 locations) m WIDE Tokyo server e NASA Mt View, CA f Internet Software C. Palo Alto, CA (and 17 other locations) ❒ Client queries com DNS server to get amazon.com 13 root name DNS server servers worldwide b USC-ISI Marina del Rey, CA ❒ Client queries amazon.com DNS server to get IP l ICANN Los Angeles, CA address for www.amazon.com 2: Application Layer 11 2: Application Layer 12 2

TLD and Authoritative Servers Local Name Server ❒ Top-level domain (TLD) servers: responsible ❒ Does not strictly belong to hierarchy for com, org, net, edu, etc, and all top-level ❒ Each ISP (residential ISP, company, country domains uk, fr, ca, jp. university) has one. ❍ Network solutions maintains servers for com TLD ❍ Also called “default name server” ❍ Educause for edu TLD ❒ When a host makes a DNS query, query is ❒ Authoritative DNS servers: organization’s sent to its local DNS server DNS servers, providing authoritative hostname to IP mappings for organization’s ❍ Acts as a proxy, forwards query into hierarchy. servers (e.g., Web and mail). ❍ Can be maintained by organization or service provider 2: Application Layer 2: Application Layer 13 14 Example Recursive queries root DNS server root DNS server 2 recursive query: ❒ Host at cis.poly.edu 3 2 3 TLD DNS server ❒ puts burden of name wants IP address 4 6 resolution on 7 for gaia.cs.umass.edu 5 TLD DNS server contacted name server local DNS server local DNS server dns.poly.edu ❒ heavy load? 4 6 5 7 dns.poly.edu 1 8 iterated query: 1 8 authoritative DNS server ❒ contacted server dns.cs.umass.edu authoritative DNS server replies with name of requesting host dns.cs.umass.edu server to contact requesting host cis.poly.edu cis.poly.edu ❒ “I don’t know this gaia.cs.umass.edu name, but ask this gaia.cs.umass.edu server” 2: Application Layer 2: Application Layer 15 16 DNS: caching and updating records DNS records DNS: distributed db storing resource records (RR) ❒ once (any) name server learns mapping, it caches mapping RR format: (name, value, type, ttl) ❍ cache entries timeout (disappear) after some ❒ Type=A ❒ Type=CNAME time ❍ name is hostname ❍ name is alias name for some ❍ TLD servers typically cached in local name “cannonical” (the real) name ❍ value is IP address servers ❒ Type=NS www.ibm.com is really • Thus root name servers not often visited ❍ name is domain (e.g. servereast.backup2.ibm.com ❒ update/notify mechanisms under design by IETF ❍ value is cannonical name foo.com) ❒ Type=MX ❍ RFC 2136 ❍ value is IP address of authoritative name ❍ value is name of mailserver ❍ http://www.ietf.org/html.charters/dnsind-charter.html server for this domain associated with name 2: Application Layer 17 2: Application Layer 18 3