Digital Communication in the Modern World Application Layer cont. DNS, SMTP

Transcription

1 Digital Communication in the Modern World Application Layer cont. DNS, Some of the slides have been borrowed from: Computer Networking: A Top Down Approach Featuring the Internet, 2 nd edition. Jim Kurose, Keith Ross Addison-Wesley, July Electronic Mail Three major components: s (clients) s simple transfer protocol: User Agent a.k.a. reader composing, editing, reading messages e.g., Eudora, Outlook, elm, Netscape Messenger, PINE outgoing, incoming messages stored on outgoing message queue box Application Layer 2 Electronic Mail: s Electronic Mail: [RFC 282] Mail Servers box contains incoming messages for message queue of outgoing (to be sent) messages protocol between s to send e messages client: sending : receiving uses TCP to reliably transfer e message from client to, port 25 direct transfer: sending to receiving 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 Application Layer 3 Application Layer 4

2 Scenario: Alice sends message to Bob ) Alice uses UA to compose message and to bob@someschool.edu 2) Alice s UA sends message to her ; message placed in message queue 3) Client side of opens TCP connection with Bob s ) client sends Alice s message over the TCP connection 5) Bob s places the message in Bob s box 6) Bob invokes his to read message 5 6 Sample interaction S: 220.cs.huji.ac.il C: HELO.cs.huji.ac.il S: 250 Hello.cs.ac.il, pleased to meet you C: MAIL FROM: <falafel@cs.ac.il> S: 250 falafel@cs.ac.il... Sender ok C: RCPT TO: <sabih@pita.com> S: 250 sabih@pita.co... Recipient ok C: DATA S: 354 Enter, end with "." on a line by itself C: Do you want with hilbe? C: How about amba? C:. S: 250 Message accepted for delivery C: QUIT S: 22.cs.huji.ac.il closing connection Application Layer 5 Application Layer 6 Try interaction for yourself: : final words telnet name 25 see 220 reply from enter HELO, MAIL FROM, RCPT TO, DATA, QUIT commands above lets you send e without using e client (reader) uses persistent connections requires message (header & body) to be in 7- bit ASCII uses CRLF.CRLF to determine end of message Comparison with HTTP: HTTP: pull : push both have ASCII command/response interaction, status codes HTTP: each object encapsulated in its own response msg : multiple objects sent in multipart msg Application Layer 7 Application Layer 8

3 Mail message format Message format: multimedia extensions : protocol for exchanging e msgs RFC 822: standard for text message format: header lines, e.g., To: From: Subject: different from commands! body the message, ASCII characters only header body blank line MIME: multimedia extension, RFC 2045, 2056 additional lines in msg header declare MIME content type MIME version method used to encode data multimedia data type, subtype, parameter declaration encoded data From: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. MIME-Version:.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data base64 encoded data Application Layer 9 Application Layer 0 MIME types Content-Type: type/subtype; parameters 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 Multipart Type From: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. MIME-Version:.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? Application Layer Application Layer 2

4 Mail access protocols access protocol sender s receiver s : delivery/storage to receiver s Mail access protocol: retrieval from POP: Post Office Protocol [RFC 939] authorization ( <-->) and download IMAP: Internet Mail Access Protocol [RFC 730] more features (more complex) manipulation of stored msgs on HTTP: Hot, Yahoo! Mail, G, etc. POP3 protocol authorization phase client commands: : declare name pass: password responses +OK -ERR transaction phase, client: list: list message numbers retr: retrieve message by number dele: delete quit S: +OK POP3 ready C: bob S: +OK C: pass hungry S: +OK successfully logged on C: list S: 498 S: 2 92 S:. C: retr S: <message contents> S:. C: dele C: retr 2 S: <message contents> S:. C: dele 2 C: quit S: +OK POP3 signing off Application Layer 3 Application Layer 4 POP3 (more) and IMAP DNS: Domain Name System More about POP3 Previous example uses download and delete mode. Bob cannot re-read e- 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 Allows to organize messages in folders IMAP keeps state across sessions: names of folders and mappings between message IDs and folder name People: many identifiers: SSN, name, passport # Internet hosts, routers: IP address (32 bit) - used for addressing datagrams name, e.g., gaia.cs.umass.edu - used by humans Q: map between IP addresses and name? Domain Name System: distributed database implemented in hierarchy of many name s application-layer protocol host, routers, name s to communicate to resolve names (address/name translation) note: core Internet function, implemented as application-layer protocol complexity at network s edge Application Layer 5 Application Layer 6

5 DNS name s Why not centralize DNS? single point of failure traffic volume distant centralized database maintenance doesn t scale! no has all nameto-ip address mappings local name s: each ISP, company has local (default) name host DNS query first goes to local name authoritative name : for a host: stores that host s IP address, name can perform name/address translation for that host s name DNS: Root name s contacted by local name that can not resolve name root name : contacts authoritative name if name mapping not known gets mapping returns mapping to local name 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 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 k RIPE London i NORDUnet Stockholm m WIDE Tokyo 3 root name s worldwide Application Layer 7 Application Layer 8 Simple DNS example root name DNS example root name host surf.eurecom.fr wants IP address of gaia.cs.umass.edu. contacts its local DNS, dns.eurecom.fr 2. dns.eurecom.fr contacts root name, if necessary 3. root name contacts authoritative name, dns.umass.edu, if necessary local name dns.eurecom.fr 2 6 requesting host surf.eurecom.fr authorititive name dns.umass.edu gaia.cs.umass.edu Root name : may not know authoritative name may know intermediate name : who to contact to find authoritative name local name dns.eurecom.fr 2 8 requesting host surf.eurecom.fr intermediate name dns.umass.edu 4 5 authoritative name dns.cs.umass.edu gaia.cs.umass.edu Application Layer 9 Application Layer 20

6 DNS: iterated queries root name DNS: caching and updating records recursive query: puts burden of name resolution on contacted name heavy load? iterated query: contacted replies with name of to contact I don t know this name, but ask this local name dns.eurecom.fr 2 8 requesting host surf.eurecom.fr iterated query intermediate name dns.umass.edu 5 6 authoritative name dns.cs.umass.edu gaia.cs.umass.edu once (any) name learns mapping, it caches mapping cache entries timeout (disappear) after some time update/notify mechanisms under design by IETF RFC Application Layer 2 Application Layer 22 DNS records DNS protocol, messages DNS: distributed db storing resource records (RR) Type=A name is hostname Type=NS RR format: (name, value, type,ttl) value is IP address name is domain (e.g. foo.com) value is IP address of authoritative name for this domain Type=CNAME name is alias name for some cannonical (the real) name is really east.backup2.ibm.com value is cannonical name Type=MX value is name of associated with name DNS protocol : query and reply messages, both with same message format msg header identification: 6 bit # for query, reply to query uses same # flags: query or reply recursion desired recursion available reply is authoritative Application Layer 23 Application Layer 24

7 DNS protocol, messages Name, type fields for a query RRs in reponse to query records for authoritative s additional helpful info that may be used Web caches (proxy ) Goal: satisfy client request without involving sets browser: Web accesses via cache browser sends all HTTP requests to cache object in cache: cache returns object else cache requests object from, then returns object to client client client HTTP request HTTP response HTTP request HTTP response Proxy HTTP request HTTP response Application Layer 25 Application Layer 26 More about Web caching Cache acts as both client and Cache can do up-to-date check using If-modifiedsince HTTP header Issue: should cache take risk and deliver cached object without checking? Heuristics are used. Typically cache is installed by ISP (university, company, residential ISP) Why Web caching? Reduce response time for client request. Reduce traffic on an institution s access link. Internet dense with caches enables poor content providers to effectively deliver content Application Layer 27 Caching example () Assumptions average object size = 00,000 bits avg. request rate from institution s browser to serves = 5/sec delay from router to any and back to router = 2 sec Consequences utilization on LAN = 5% utilization on access link = 00% total delay = Internet delay + access delay + LAN delay = 2 sec + minutes + milliseconds network public Internet.5 Mbps access link 0 Mbps LAN s cache Application Layer 28

8 Caching example (2) Possible solution increase bandwidth of access link to, say, 0 Mbps Consequences utilization on LAN = 5% utilization on access link = 5% Total delay = Internet delay + access delay + LAN delay = 2 sec + msecs + msecs often a costly upgrade network public Internet 0 Mbps access link 0 Mbps LAN s cache Caching example (3) Install cache suppose hit rate is.4 Consequence 40% requests will be satisfied almost immediately 60% requests satisfied by utilization of access link reduced to 60%, resulting in negligible delays (say 0 msec) total delay = Internet delay + access delay + LAN delay =.6*2 sec +.6*.0 secs + milliseconds <.3 secs network public Internet.5 Mbps access link 0 Mbps LAN s cache Application Layer 29 Application Layer 30 Content distribution networks (CDNs) CDN example HTTP request for The content providers are the CDN customers. Content replication CDN company installs hundreds of CDN s throughout Internet in lower-tier ISPs, close to s CDN replicates its customers content in CDN s. When provider updates content, CDN updates s CDN in S. America in North America CDN distribution node CDN in Europe CDN in Asia Application Layer 3 distributes HTML Replaces: 2 3 Origin CDNs authoritative DNS Nearby CDN DNS query for with HTTP request for CDN company cdn.com distributes gif files uses its authoritative DNS to route redirect requests Application Layer 32

9 More about CDNs routing requests CDN creates a map, indicating distances from leaf ISPs and CDN nodes when query arrives at authoritative DNS : determines ISP from which query ates uses map to determine best CDN not just Web pages streaming stored audio/video streaming real-time audio/video CDN nodes create application-layer overlay network Application Layer 33