Materi Perkuliahan Jaringan Komputer Teknik Informatika Chapter 2


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Materi Perkuliahan Jaringan Komputer Teknik Informatika Chapter 2

  1. 1. Chapter 2Application LayerA note on the use of these ppt slides:We’re making these slides freely available to all (faculty, students, readers). ComputerThey’re in PowerPoint form so you see the animations; and can add, modify,and delete slides (including this one) and slide content to suit your needs. Networking: A TopThey obviously represent a lot of work on our part. In return for use, we onlyask the following: Down Approach If you use these slides (e.g., in a class) that you mention their source (after all, we’d like people to use our book!) 6th edition If you post any slides on a www site, that you note that they are adapted Jim Kurose, Keith Ross from (or perhaps identical to) our slides, and note our copyright of this material. Addison-Wesley March 2012Thanks and enjoy! JFK/KWR All material copyright 1996-2012 J.F Kurose and K.W. Ross, All Rights Reserved Application Layer 2-1
  2. 2. Chapter 2: outline2.1 principles of network 2.6 P2P applications applications 2.7 socket programming2.2 Web and HTTP with UDP and TCP2.3 FTP2.4 electronic mail  SMTP, POP3, IMAP2.5 DNS Application Layer 2-2
  3. 3. Chapter 2: application layerour goals:  learn about protocols by conceptual, examining popular implementation aspects application-level of network application protocols protocols  HTTP  transport-layer  FTP service models  SMTP / POP3 / IMAP  client-server  DNS paradigm  creating network  peer-to-peer applications paradigm  socket API Application Layer 2-3
  4. 4. Some network apps e-mail  voice over IP (e.g., Skype) web  real-time video text messaging conferencing remote login  social networking P2P file sharing  search multi-user network games  … streaming stored video  … (YouTube, Hulu, Netflix) Application Layer 2-4
  5. 5. Creating a network app application transport network data link write programs that: physical  run on (different) end systems  communicate over network  e.g., web server software communicates with browser software no need to write software for application network-core devices transport network application data link  network-core devices do not physical transport network run user applications data link physical  applications on end systems allows for rapid app development, propagation Application Layer 2-5
  6. 6. Application architecturespossible structure of applications: client-server peer-to-peer (P2P) Application Layer 2-6
  7. 7. Client-server architecture server:  always-on host  permanent IP address  data centers for scaling clients:  communicate with serverclient/server  may be intermittently connected  may have dynamic IP addresses  do not communicate directly with each other Application Layer 2-7
  8. 8. P2P architecture no always-on server peer-peer arbitrary end systems directly communicate peers request service from other peers, provide service in return to other peers  self scalability – new peers bring new service capacity, as well as new service demands peers are intermittently connected and change IP addresses  complex management Application Layer 2-8
  9. 9. Processes communicatingprocess: program running clients, servers within a host client process: process that within same host, two initiates communication processes communicate server process: process that using inter-process waits to be contacted communication (defined by OS) processes in different hosts communicate by exchanging  aside: applications with P2P messages architectures have client processes & server processes Application Layer 2-9
  10. 10. Sockets process sends/receives messages to/from its socket socket analogous to door  sending process shoves message out door  sending process relies on transport infrastructure on other side of door to deliver message to socket at receiving process application application socket controlled by process process app developer transport transport network network controlled link by OS link Internet physical physical Application Layer 2-10
  11. 11. Addressing processes  to receive messages,  identifier includes both IP process must have identifier address and port numbers  host device has unique 32- associated with process on bit IP address host.  Q: does IP address of host  example port numbers: on which process runs  HTTP server: 80 suffice for identifying the  mail server: 25 process?  to send HTTP message to  A: no, many processes web can be running on same server: host  IP address:  port number: 80  more shortly… Application Layer 2-11
  12. 12. App-layer protocol defines types of messages open protocols: exchanged,  defined in RFCs  e.g., request, response  allows for interoperability message syntax:  e.g., HTTP, SMTP  what fields in messages proprietary protocols: & how fields are  e.g., Skype delineated message semantics  meaning of information in fields rules for when and how processes send & respond to messages Application Layer 2-12
  13. 13. What transport service does an app need?data integrity throughput some apps (e.g., file transfer,  some apps (e.g., web transactions) require multimedia) require 100% reliable data transfer minimum amount of other apps (e.g., audio) can throughput to be tolerate some loss “effective”  other apps (“elastic apps”)timing make use of whatever some apps (e.g., Internet throughput they get telephony, interactive security games) require low delay to be “effective”  encryption, data integrity, … Application Layer 2-13
  14. 14. Transport service requirements: common apps application data loss throughput time sensitive file transfer no loss elastic no e-mail no loss elastic no Web documents no loss elastic noreal-time audio/video loss-tolerant audio: 5kbps-1Mbps yes, 100’s msec video:10kbps-5Mbps stored audio/video loss-tolerant same as above yes, few secs interactive games loss-tolerant few kbps up yes, 100’s msec text messaging no loss elastic yes and no Application Layer 2-14
  15. 15. Internet transport protocols services TCP service: UDP service:  reliable transport between  unreliable data transfer sending and receiving between sending and process receiving process  flow control: sender won’t  does not provide: overwhelm receiver reliability, flow control,  congestion control: throttle congestion control, sender when network overloaded timing, throughput  does not provide: timing, guarantee, security, minimum throughput orconnection setup, guarantee, security  connection-oriented: setup Q: why bother? Why is required between client and there a UDP? server processes Application Layer 2-15
  16. 16. Internet apps: application, transport protocols application underlying application layer protocol transport protocol e-mail SMTP [RFC 2821] TCPremote terminal access Telnet [RFC 854] TCP Web HTTP [RFC 2616] TCP file transfer FTP [RFC 959] TCP streaming multimedia HTTP (e.g., YouTube), TCP or UDP RTP [RFC 1889] Internet telephony SIP, RTP, proprietary (e.g., Skype) TCP or UDP Application Layer 2-16
  17. 17. Securing TCPTCP & UDP SSL is at app layer no encryption  Apps use SSL libraries, cleartext passwds sent which “talk” to TCP into socket traverse SSL socket API Internet in cleartext  cleartext passwds sentSSL into socket traverse provides encrypted Internet encrypted TCP connection  See Chapter 7 data integrity end-point authentication Application Layer 2-17
  18. 18. Chapter 2: outline2.1 principles of network 2.6 P2P applications applications 2.7 socket programming  app architectures with UDP and TCP  app requirements2.2 Web and HTTP2.3 FTP2.4 electronic mail  SMTP, POP3, IMAP2.5 DNS Application Layer 2-18
  19. 19. Web and HTTPFirst, a review… web page consists of objects object can be HTML file, JPEG image, Java applet, audio file,… web page consists of base HTML-file which includes several referenced objects each object is addressable by a URL, e.g., host name path name Application Layer 2-19
  20. 20. HTTP overviewHTTP: hypertext transfer protocol HT Web’s application layer TP req ues protocol PC running HT t Firefox browser TP client/server model res pon se  client: browser that requests, receives, st (using HTTP protocol) req ue e server and “displays” Web HT TP ons running sp objects Pr e Apache Web TT  server: Web server H server sends (using HTTP protocol) objects in iphone running response to requests Safari browser Application Layer 2-20
  21. 21. HTTP overview (continued)uses TCP: HTTP is “stateless” client initiates TCP  server maintains no connection (creates information about socket) to server, port 80 past client requests server accepts TCP connection from client aside protocols that maintain HTTP messages “state” are complex! (application-layer protocol  past history (state) must be messages) exchanged maintained between browser (HTTP  if server/client crashes, their client) and Web server views of “state” may be (HTTP server) inconsistent, must be TCP connection closed reconciled Application Layer 2-21
  22. 22. HTTP connectionsnon-persistent HTTP persistent HTTP at most one object  multiple objects can sent over TCP be sent over single connection TCP connection  connection then between client, server closed downloading multiple objects required multiple connections Application Layer 2-22
  23. 23. Non-persistent HTTP suppose user enters URL: (contains text, references to 10 jpeg images) 1a. HTTP client initiates TCP connection to HTTP server (process) at 1b. HTTP server at host on port waiting 80 for TCP connection at port 80. “accepts” connection, notifying 2. HTTP client sends HTTP request client message (containing URL) into TCP connection socket. 3. HTTP server receives request Message indicates that client message, forms response wants object message containing requested someDepartment/home.index object, and sends message into its sockettime Application Layer 2-23
  24. 24. Non-persistent HTTP (cont.) 4. HTTP server closes TCP connection. 5. HTTP client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objectstime 6. Steps 1-5 repeated for each of 10 jpeg objects Application Layer 2-24
  25. 25. Non-persistent HTTP: response time RTT (definition): time for a small packet to travel from client to server and back HTTP response time: initiate TCP  one RTT to initiate TCP connection connection RTT  one RTT for HTTP request request file and first few bytes of HTTP RTT time to response to return transmit file  file transmission time file received  non-persistent HTTP response time = time time 2RTT+ file transmission time Application Layer 2-25
  26. 26. Persistent HTTPnon-persistent HTTP issues: persistent HTTP: requires 2 RTTs per object  server leaves connection OS overhead for each TCP open after sending connection response browsers often open  subsequent HTTP parallel TCP connections messages between same to fetch referenced objects client/server sent over open connection  client sends requests as soon as it encounters a referenced object  as little as one RTT for all the referenced objects Application Layer 2-26
  27. 27. HTTP request message  two types of HTTP messages: request, response  HTTP request message:  ASCII (human-readable format) carriage return character line-feed characterrequest line(GET, POST, GET /index.html HTTP/1.1rnHEAD commands) Host: www-net.cs.umass.edurn User-Agent: Firefox/3.6.10rn Accept: text/html,application/xhtml+xmlrn header Accept-Language: en-us,en;q=0.5rn lines Accept-Encoding: gzip,deflatern Accept-Charset: ISO-8859-1,utf-8;q=0.7rncarriage return, Keep-Alive: 115rnline feed at start Connection: keep-alivern rnof line indicatesend of header lines Application Layer 2-27
  28. 28. HTTP request message: general format method sp URL sp version cr lf request line header field name value cr lf header~~ ~ ~ lines header field name value cr lf cr lf~~ entity body ~ ~ body Application Layer 2-28
  29. 29. Uploading form input POST method:  web page often includes form input  input is uploaded to server in entity body URL method:  uses GET method  input is uploaded in URL field of request line: Application Layer 2-29
  30. 30. Method typesHTTP/1.0: HTTP/1.1: GET  GET, POST, HEAD POST  PUT HEAD  uploads file in entity  asks server to leave body to path specified requested object out in URL field of response  DELETE  deletes file specified in the URL field Application Layer 2-30
  31. 31. HTTP response messagestatus line(protocolstatus code HTTP/1.1 200 OKrnstatus phrase) Date: Sun, 26 Sep 2010 20:09:20 GMTrn Server: Apache/2.0.52 (CentOS)rn Last-Modified: Tue, 30 Oct 2007 17:00:02 GMTrn header ETag: "17dc6-a5c-bf716880"rn Accept-Ranges: bytesrn lines Content-Length: 2652rn Keep-Alive: timeout=10, max=100rn Connection: Keep-Alivern Content-Type: text/html; charset=ISO-8859- 1rn rn data, e.g., data data data data data ... requested HTML file Application Layer 2-31
  32. 32. HTTP response status codes status code appears in 1st line in server-to- client response message. some sample codes: 200 OK  request succeeded, requested object later in this msg 301 Moved Permanently  requested object moved, new location specified later in this msg (Location:) 400 Bad Request  request msg not understood by server 404 Not Found  requested document not found on this server 505 HTTP Version Not Supported Application Layer 2-32
  33. 33. Trying out HTTP (client side) for yourself 1. Telnet to your favorite Web server: telnet 80 opens TCP connection to port 80 (default HTTP server port) at anything typed in sent to port 80 at 2. type in a GET HTTP request: GET /~ross/ HTTP/1.1 by typing this in (hit carriage Host: return twice), you send this minimal (but complete) GET request to HTTP server 3. look at response message sent by HTTP server!(or use Wireshark to look at captured HTTP request/response) Application Layer 2-33
  34. 34. User-server state: cookies example:many Web sites use cookies  Susan always access Internetfour components: from PC 1) cookie header line of  visits specific e-commerce HTTP response site for first time message  when initial HTTP requests 2) cookie header line in arrives at site, site creates: next HTTP request  unique ID message  entry in backend 3) cookie file kept on database for ID user’s host, managed by user’s browser 4) back-end database at Web site Application Layer 2-34
  35. 35. Cookies: keeping “state” (cont.) client server ebay 8734 usual http request msg Amazon server cookie file creates ID usual http response 1678 for user create backend ebay 8734 set-cookie: 1678 entry database amazon 1678 usual http request msg cookie: 1678 cookie- access specific usual http response msg actionone week later: access ebay 8734 usual http request msg amazon 1678 cookie: 1678 cookie- specific usual http response msg action Application Layer 2-35
  36. 36. Cookies (continued) asidewhat cookies can be used cookies and privacy: for:  cookies permit sites to authorization learn a lot about you shopping carts  you may supply name and recommendations e-mail to sites user session state (Web e-mail)how to keep “state”: protocol endpoints: maintain state at sender/receiver over multiple transactions cookies: http messages carry state Application Layer 2-36
  37. 37. Web caches (proxy server)goal: satisfy client request without involving origin server user sets browser: Web accesses via cache browser sends all HTTP proxy HT requests to cache TP req server ues t eq  object in cache: cache HT ues T Pr e client TP t HT ons origin res esp returns object pon se TPr server HT  else cache requests st ue eq Pr se object from origin TT p on H res server, then returns HTT P object to client client origin server Application Layer 2-37
  38. 38. More about Web caching cache acts as both why Web caching? client and server  reduce response time  server for original requesting client for client request  client to origin server  reduce traffic on an typically cache is institution’s access link installed by ISP  Internet dense with (university, company, caches: enables “poor” residential ISP) content providers to effectively deliver content (so too does P2P file sharing) Application Layer 2-38
  39. 39. Caching example:assumptions: avg object size: 100K bits origin avg request rate from browsers to servers origin servers:15/sec public avg data rate to browsers: 1.50 Mbps Internet RTT from institutional router to any origin server: 2 sec access link rate: 1.54 Mbps 1.54 Mbps access linkconsequences: LAN utilization: 15% problem! institutional network access link utilization = 99% 1 Gbps LAN total delay = Internet delay + access delay + LAN delay = 2 sec + minutes + usecs Application Layer 2-39
  40. 40. Caching example: fatter access link assumptions:  avg object size: 100K bits origin  avg request rate from browsers to servers origin servers:15/sec public  avg data rate to browsers: 1.50 Mbps Internet  RTT from institutional router to any origin server: 2 sec  access link rate: 1.54 Mbps 154 Mbps 1.54 Mbps 154 Mbps access link consequences:  LAN utilization: 15% institutional network 9.9%  access link utilization = 99% 1 Gbps LAN  total delay = Internet delay + access delay + LAN delay = 2 sec + minutes + usecs msecsCost: increased access link speed (not cheap!) Application Layer 2-40
  41. 41. Caching example: install local cacheassumptions: avg object size: 100K bits origin avg request rate from browsers to servers origin servers:15/sec public avg data rate to browsers: 1.50 Mbps Internet RTT from institutional router to any origin server: 2 sec access link rate: 1.54 Mbps 1.54 Mbps access linkconsequences: LAN utilization: 15% institutional access link utilization? 100% network = 1 Gbps LAN total delay ? Internet delay + access = delay + LAN delay local web How to compute link cache = 2 sec + minutes + usecs utilization, delay?Cost: web cache (cheap!) Application Layer 2-41
  42. 42. Caching example: install local cacheCalculating access link utilization, delay with cache: origin suppose cache hit rate is 0.4 servers  40% requests satisfied at cache, public Internet 60% requests satisfied at originaccess link utilization:  60% of requests use access link data rate to browsers over access link 1.54 Mbps = 0.6*1.50 Mbps = .9 Mbps access link  utilization = 0.9/1.54 = .58 institutionaltotal delay network 1 Gbps LAN  = 0.6 * (delay from origin servers) +0.4 * (delay when satisfied at cache) local web  = 0.6 (2.01) + 0.4 (~msecs) cache  = ~ 1.2 secs  less than with 154 Mbps link (and cheaper too!) Application Layer 2-42
  43. 43. Conditional GET client server Goal: don’t send object if cache has up-to-date cached version HTTP request msg object If-modified-since: <date>  no object transmission not delay modified  lower link utilization HTTP response before HTTP/1.0 cache: specify date of 304 Not Modified <date> cached copy in HTTP request If-modified-since: <date> HTTP request msg If-modified-since: <date> object server: response contains modified no object if cached copy after HTTP response is up-to-date: HTTP/1.0 200 OK <date> HTTP/1.0 304 Not <data> Modified Application Layer 2-43
  44. 44. Chapter 2: outline2.1 principles of network 2.6 P2P applications applications 2.7 socket programming  app architectures with UDP and TCP  app requirements2.2 Web and HTTP2.3 FTP2.4 electronic mail  SMTP, POP3, IMAP2.5 DNS Application Layer 2-44
  45. 45. FTP: the file transfer protocol file transfer FTP FTP FTP user client server interface user at host remote file local file system 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 Application Layer 2-45
  46. 46. FTP: separate control, data connections TCP control connection, FTP client contacts FTP server port 21 server at port 21, using TCP client authorized over control TCP data connection, connection FTP server port 20 FTP client server client browses remote directory, sends commands  server opens another TCP over control connection data connection to transfer when server receives file another file transfer command, server opens 2nd TCP data connection  control connection: “out of (for file) to client band”  FTP server maintains after transferring one file, server closes data connection “state”: current directory, earlier authentication Application Layer 2-46
  47. 47. FTP commands, responsessample commands: sample return codes sent as ASCII text over  status code and phrase (as control channel in HTTP) USER username  331 Username OK, PASS password password required LIST return list of file in  125 data current directory connection already open; RETR filename transfer starting retrieves (gets) file  425 Can’t open STOR filename stores data connection (puts) file onto remote  452 Error writing host file Application Layer 2-47
  48. 48. Chapter 2: outline2.1 principles of network 2.6 P2P applications applications 2.7 socket programming  app architectures with UDP and TCP  app requirements2.2 Web and HTTP2.3 FTP2.4 electronic mail  SMTP, POP3, IMAP2.5 DNS Application Layer 2-48
  49. 49. Electronic mail outgoing message queue user mailboxThree major components: user agent user agents mail servers mail user server agent simple mail transfer protocol: SMTP SMTP mail user server agentUser Agent SMTP a.k.a. “mail reader” SMTP user agent mail composing, editing, reading server mail messages user agent e.g., Outlook, Thunderbird, iPhone mail client user agent outgoing, incoming messages stored on server Application Layer 2-49
  50. 50. Electronic mail: mail serversmail servers: user agent mailbox contains incoming messages for user mail user server agent message queue of outgoing (to be sent) mail messages SMTP mail user SMTP protocol between server agent mail servers to send email SMTP messages user  client: sending mail SMTP agent mail server server user  “server”: receiving mail agent server user agent Application Layer 2-50
  51. 51. Electronic Mail: SMTP [RFC 2821] uses TCP to reliably transfer email message from client to server, port 25 direct transfer: sending server to receiving server three phases of transfer  handshaking (greeting)  transfer of messages  closure command/response interaction (like HTTP, FTP)  commands: ASCII text  response: status code and phrase messages must be in 7-bit ASCI Application Layer 2-51
  52. 52. Scenario: Alice sends message to Bob1) Alice uses UA to compose 4) SMTP client sends Alice’s message “to” message over the TCP connection2) Alice’s UA sends message to 5) Bob’s mail server places the her mail server; message message in Bob’s mailbox placed in message queue 6) Bob invokes his user agent3) client side of SMTP opens to read message TCP connection with Bob’s mail server 1 user mail user mail agent agent server server 2 3 6 4 5 Alice’s mail server Bob’s mail server Application Layer 2-52
  53. 53. Sample SMTP interaction S: 220 C: HELO S: 250 Hello, pleased to meet you C: MAIL FROM: <> S: 250 Sender ok C: RCPT TO: <> S: 250 ... 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 closing connection Application Layer 2-53
  54. 54. Try SMTP interaction for yourself:  telnet servername 25  see 220 reply from server  enter HELO, MAIL FROM, RCPT TO, DATA, QUIT commands above lets you send email without using email client (reader) Application Layer 2-54
  55. 55. SMTP: final words SMTP uses persistent comparison with HTTP: connections  HTTP: pull SMTP requires message (header & body) to be in  SMTP: push 7-bit ASCII  both have ASCII SMTP server uses command/response CRLF.CRLF to interaction, status codes determine end of message  HTTP: each object encapsulated in its own response msg  SMTP: multiple objects sent in multipart msg Application Layer 2-55
  56. 56. Mail message formatSMTP: protocol for exchanging email msgs header blankRFC 822: standard for text line message format: header lines, e.g.,  To: body  From:  Subject: different from SMTP MAIL FROM, RCPT TO: commands! Body: the “message”  ASCII characters only Application Layer 2-56
  57. 57. Mail access protocols user mail access user SMTP SMTP protocol agent agent (e.g., POP, IMAP) sender’s mail receiver’s mail server server SMTP: delivery/storage to receiver’s server mail access protocol: retrieval from server  POP: Post Office Protocol [RFC 1939]: authorization, download  IMAP: Internet Mail Access Protocol [RFC 1730]: more features, including manipulation of stored msgs on server  HTTP: gmail, Hotmail, Yahoo! Mail, etc. Application Layer 2-57
  58. 58. POP3 protocol S: +OK POP3 server ready C: user bobauthorization phase S: +OK C: pass hungry client commands: S: +OK user successfully logged on  user: declare username  pass: password C: list S: 1 498 server responses S: 2 912  +OK S: .  -ERR C: retr 1transaction phase, client: S: S: <message 1 contents> . list: list message numbers C: dele 1 retr: retrieve message by C: retr 2 number S: <message 1 contents> dele: delete S: . quit C: dele 2 C: quit S: +OK POP3 server signing off Application Layer 2-58
  59. 59. POP3 (more) and IMAPmore about POP3 IMAP previous example uses  keeps all messages in one POP3 “download and place: at server delete” mode  allows user to organize  Bob cannot re-read e- messages in folders mail if he changes  keeps user state across client sessions: POP3 “download-and-  names of folders and keep”: copies of messages mappings between on different clients message IDs and folder POP3 is stateless across name sessions Application Layer 2-59
  60. 60. Chapter 2: outline2.1 principles of network 2.6 P2P applications applications 2.7 socket programming  app architectures with UDP and TCP  app requirements2.2 Web and HTTP2.3 FTP2.4 electronic mail  SMTP, POP3, IMAP2.5 DNS Application Layer 2-60
  61. 61. DNS: domain name systempeople: many identifiers: Domain Name System:  SSN, name, passport #  distributed databaseInternet hosts, routers: implemented in hierarchy of  IP address (32 bit) - many name servers used for addressing  application-layer protocol: hosts, datagrams name servers communicate to  “name”, e.g., resolve names (address/name - translation) used by humans  note: core Internet function, implemented as application-Q: how to map between IP layer protocol address and name, and vice versa ?  complexity at network’s “edge” Application Layer 2-61
  62. 62. DNS: services, structureDNS services why not centralize DNS? hostname to IP address  single point of failure translation  traffic volume host aliasing  distant centralized database  canonical, alias names  maintenance mail server aliasing load distribution A: doesn’t scale!  replicated Web servers: many IP addresses correspond to one name Application Layer 2-62
  63. 63. DNS: a distributed, hierarchical database Root DNS Servers … … com DNS servers org DNS servers edu DNS servers DNS servers DNS serversDNS serversDNS servers DNS serversclient wants IP for; 1st approx: client queries root server to find com DNS server client queries .com DNS server to get DNS server client queries DNS server to get IP address for Application Layer 2-63
  64. 64. 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 c. Cogent, Herndon, VA (5 other sites) d. U Maryland College Park, MD k. RIPE London (17 other sites) h. ARL Aberdeen, MD j. Verisign, Dulles VA (69 other sites ) i. Netnod, Stockholm (37 other sites)e. NASA Mt View, CA m. WIDE Tokyof. Internet Software C. (5 other sites)Palo Alto, CA (and 48 othersites)a. Verisign, Los Angeles CA 13 root name (5 other sites)b. USC-ISI Marina del Rey, CA “servers”l. ICANN Los Angeles, CA worldwide (41 other sites) g. US DoD Columbus, OH (5 other sites) Application Layer 2-64
  65. 65. TLD, authoritative serverstop-level domain (TLD) servers:  responsible for com, org, net, edu, aero, jobs, museums, and all top-level country domains, e.g.: uk, fr, ca, jp  Network Solutions maintains servers for .com TLD  Educause for .edu TLDauthoritative DNS servers:  organization’s own DNS server(s), providing authoritative hostname to IP mappings for organization’s named hosts  can be maintained by organization or service provider Application Layer 2-65
  66. 66. Local DNS name server does not strictly belong to hierarchy each ISP (residential ISP, company, university) has one  also called “default name server” when host makes DNS query, query is sent to its local DNS server  has local cache of recent name-to-address translation pairs (but may be out of date!)  acts as proxy, forwards query into hierarchy Application Layer 2-66
  67. 67. DNS name root DNS serverresolution example 2 host at 3 TLD DNS server wants IP address for 4 5 local DNS serveriterated query: contacted server 7 6 1 8 replies with name of server to contact authoritative DNS server “I don’t know this name, but ask this requesting host server” Application Layer 2-67
  68. 68. DNS name root DNS serverresolution example 2 3recursive query: 7 6 puts burden of name TLD DNS server resolution on contacted name local DNS server server 5 4 heavy load at upper 1 8 levels of hierarchy? authoritative DNS server requesting host Application Layer 2-68
  69. 69. DNS: caching, updating records once (any) name server learns mapping, it caches mapping  cache entries timeout (disappear) after some time (TTL)  TLD servers typically cached in local name servers • thus root name servers not often visited cached entries may be out-of-date (best effort name-to-address translation!)  if name host changes IP address, may not be known Internet-wide until all TTLs expire update/notify mechanisms proposed IETF standard  RFC 2136 Application Layer 2-69
  70. 70. DNS recordsDNS: distributed db storing resource records (RR) RR format: (name, value, type, ttl)type=A type=CNAME  name is hostname  name is alias name for some  value is IP address “canonical” (the real) nametype=NS  is really  name is domain (e.g.,  value is canonical name  value is hostname of authoritative name type=MX server for this domain  value is name of mailserver associated with name Application Layer 2-70
  71. 71. DNS protocol, messages query and reply messages, both with same message format 2 bytes 2 bytesmsg header identification flags identification: 16 bit # for # questions # answer RRs query, reply to query uses # authority RRs # additional RRs same # flags: questions (variable # of questions)  query or reply  recursion desired answers (variable # of RRs)  recursion available  reply is authoritative authority (variable # of RRs) additional info (variable # of RRs) Application Layer 2-71
  72. 72. DNS protocol, messages 2 bytes 2 bytes identification flags # questions # answer RRs # authority RRs # additional RRs name, type fields questions (variable # of questions) for a query RRs in response answers (variable # of RRs) to query records for authority (variable # of RRs) authoritative servers additional “helpful” additional info (variable # of RRs) info that may be used Application Layer 2-72
  73. 73. Inserting records into DNS example: new startup “Network Utopia” register name at DNS registrar (e.g., Network Solutions)  provide names, IP addresses of authoritative name server (primary and secondary)  registrar inserts two RRs into .com TLD server: (,, NS) (,, A) create authoritative server type A record for; type MX record for Application Layer 2-73
  74. 74. Attacking DNSDDoS attacks Redirect attacks Bombard root servers  Man-in-middle with traffic  Intercept queries  Not successful to date  DNS poisoning  Traffic Filtering  Send bogus relies to  Local DNS servers DNS server, which cache IPs of TLD caches servers, allowing root Exploit DNS for DDoS server bypass  Send queries with Bombard TLD servers  Potentially more spoofed source dangerous address: target IP  Requires amplification Application Layer 2-74
  75. 75. Chapter 2: outline2.1 principles of network 2.6 P2P applications applications 2.7 socket programming  app architectures with UDP and TCP  app requirements2.2 Web and HTTP2.3 FTP2.4 electronic mail  SMTP, POP3, IMAP2.5 DNS Application Layer 2-75
  76. 76. Pure P2P architecture no always-on server arbitrary end systems directly communicate peers are intermittently connected and change IP addressesexamples:  file distribution (BitTorrent)  Streaming (KanKan)  VoIP (Skype) Application Layer 2-76
  77. 77. File distribution: client-server vs P2PQuestion: how much time to distribute file (size F) from one server to N peers?  peer upload/download capacity is limited resource us: server upload capacity di: peer i downloadfile, size F u1 d1 us u2 d2 capacity server di uN network (with abundant bandwidth) ui dN ui: peer i upload capacity Application Layer 2-77
  78. 78. File distribution time: client-server server transmission: must sequentially send (upload) N F us file copies: di  time to send one copy: F/us network  time to send N copies: NF/us ui client: each client must download file copy  dmin = min client download rate  min client download time: F/dmin time to distribute F to N clients using Dc-s > max{NF/us,,F/dmin} client-server approach increases linearly in N Application Layer 2-78
  79. 79. File distribution time: P2P server transmission: must upload at least one copy F us  time to send one copy: F/us di client: each client must network download file copy ui  min client download time: F/dmin clients: as aggregate must download NF bits  max upload rate (limting max download rate) is u s + Σui time to distribute F DP2P to N clients using > max{F/us,,F/dmin,,NF/(us + Σui)} P2P approach increases linearly in N … … but so does this, as each peer brings service capacity Application Layer 2-79
  80. 80. Client-server vs. P2P: exampleclient upload rate = u, F/u = 1 hour, us = 10u, dmin ≥ us 3.5 P2P Minimum Distribution Time 3 Client-Server 2.5 2 1.5 1 0.5 0 0 5 10 15 20 25 30 35 N Application Layer 2-80
  81. 81. P2P file distribution: BitTorrent file divided into 256Kb chunks peers in torrent send/receive file chunkstracker: tracks peers torrent: group of peersparticipating in torrent exchanging chunks of a file Alice arrives … … obtains list of peers from tracker … and begins exchanging file chunks with peers in torrent Application Layer 2-81
  82. 82. P2P file distribution: BitTorrent peer joining torrent:  has no chunks, but will accumulate them over time from other peers  registers with tracker to get list of peers, connects to subset of peers (“neighbors”) while downloading, peer uploads chunks to other peers peer may change peers with whom it exchanges chunks churn: peers may come and go once peer has entire file, it may (selfishly) leave or (altruistically) remain in torrent Application Layer 2-82
  83. 83. BitTorrent: requesting, sending file chunksrequesting chunks: sending chunks: tit-for-tat at any given time, different  Alice sends chunks to those peers have different subsets four peers currently sending of file chunks her chunks at highest rate periodically, Alice asks each  other peers are choked by Alice peer for list of chunks that (do not receive chunks from her)  re-evaluate top 4 every10 secs they have  every 30 secs: randomly select Alice requests missing chunks from peers, rarest another peer, starts sending first chunks  “optimistically unchoke” this peer  newly chosen peer may join top 4 Application Layer 2-83
  84. 84. BitTorrent: tit-for-tat (1) Alice “optimistically unchokes” Bob (2) Alice becomes one of Bob’s top-four providers; Bob reciprocates (3) Bob becomes one of Alice’s top-four providers higher upload rate: find better trading partners, get file faster ! Application Layer 2-84
  85. 85. Distributed Hash Table (DHT) DHT: a distributed P2P database database has (key, value) pairs; examples:  key: ss number; value: human name  key: movie title; value: IP address Distribute the (key, value) pairs over the (millions of peers) a peer queries DHT with key  DHT returns values that match the key peers can also insert (key, value) pairs Application 2-85
  86. 86. Q: how to assign keys to peers? central issue:  assigning (key, value) pairs to peers. basic idea:  convert each key to an integer  Assign integer to each peer  put (key,value) pair in the peer that is closest to the key Application 2-86
  87. 87. DHT identifiers assign integer identifier to each peer in range [0,2n-1] for some n.  each identifier represented by n bits. require each key to be an integer in same range to get integer key, hash original key  e.g., key = hash(“Led Zeppelin IV”)  this is why its is referred to as a distributed “hash” table Application 2-87
  88. 88. Assign keys to peers rule: assign key to the peer that has the closest ID. convention in lecture: closest is the immediate successor of the key. e.g., n=4; peers: 1,3,4,5,8,10,12,14;  key = 13, then successor peer = 14  key = 15, then successor peer = 1 Application 2-88
  89. 89. Circular DHT (1) 1 3 15 4 12 5 10 8 each peer only aware of immediate successor and predecessor. “overlay network” Application 2-89
  90. 90. Circular DHT (1)O(N) messageson avgerage to resolve 0001 Who’s responsiblequery, when there for key 1110 ?are N peers I am 0011 1111 1110 1110 0100 1110 1100 1110 1110 0101Define closest 1110as closest 1010successor 1000 Application 2-90
  91. 91. Circular DHT with shortcuts 1 Who’s responsible for key 1110? 3 15 4 12 5 10 8  each peer keeps track of IP addresses of predecessor, successor, short cuts.  reduced from 6 to 2 messages.  possible to design shortcuts so O(log N) neighbors, O(log N) messages in query Application 2-91
  92. 92. Peer churn handling peer churn: 1 peers may come and go (churn) each peer knows address of its 3 two successors15 each peer periodically pings its 4 two successors to check aliveness if immediate successor leaves,12 5 choose next successor as new immediate successor 10 8 example: peer 5 abruptly leaves peer 4 detects peer 5 departure; makes 8 its immediate successor; asks 8 who its immediate successor is; makes 8’s immediate successor its second successor. what if peer 13 wants to join? Application 2-92
  93. 93. Chapter 2: outline2.1 principles of network 2.6 P2P applications applications 2.7 socket programming  app architectures with UDP and TCP  app requirements2.2 Web and HTTP2.3 FTP2.4 electronic mail  SMTP, POP3, IMAP2.5 DNS Application Layer 2-93
  94. 94. Socket programminggoal: learn how to build client/server applications that communicate using socketssocket: door between application process and end- end-transport protocol application application socket controlled by process process app developer transport transport network network controlled link by OS link Internet physical physical Application Layer 2-94
  95. 95. Socket programming Two socket types for two transport services:  UDP: unreliable datagram  TCP: reliable, byte stream-orientedApplication Example:1. Client reads a line of characters (data) from its keyboard and sends the data to the server.2. The server receives the data and converts characters to uppercase.3. The server sends the modified data to the client.4. The client receives the modified data and displays the line on its screen. Application Layer 2-95
  96. 96. Socket programming with UDPUDP: no “connection” between client & server no handshaking before sending data sender explicitly attaches IP destination address and port # to each packet rcvr extracts sender IP address and port# from received packetUDP: transmitted data may be lost or received out-of-orderApplication viewpoint: UDP provides unreliable transfer of groups of bytes (“datagrams”) between client and server Application Layer 2-96
  97. 97. Client/server socket interaction: UDPserver (running on serverIP) client create socket: create socket, port= x: clientSocket = serverSocket = socket(AF_INET,SOCK_DGRAM) socket(AF_INET,SOCK_DGRAM) Create datagram with server IP and port=x; send datagram via read datagram from clientSocket serverSocket write reply to serverSocket read datagram from specifying clientSocket client address, port number close clientSocket Application 2-97
  98. 98. Example app: UDP client Python UDPClientinclude Python’s socketlibrary from socket import * serverName = ‘hostname’ serverPort = 12000create UDP socket forserver clientSocket = socket(socket.AF_INET,get user keyboard socket.SOCK_DGRAM)input message = raw_input(’Input lowercase sentence:’)Attach server name, port tomessage; send into socket clientSocket.sendto(message,(serverName, serverPort))read reply characters from modifiedMessage, serverAddress =socket into string clientSocket.recvfrom(2048)print out received string print modifiedMessageand close socket clientSocket.close() Application Layer 2-98
  99. 99. Example app: UDP server Python UDPServer from socket import * serverPort = 12000create UDP socket serverSocket = socket(AF_INET, SOCK_DGRAM)bind socket to local portnumber 12000 serverSocket.bind((, serverPort)) print “The server is ready to receive”loop forever while 1:Read from UDP socketinto message, getting message, clientAddress = serverSocket.recvfrom(2048)client’s address (client IP modifiedMessage = message.upper()and port) send upper case string serverSocket.sendto(modifiedMessage, clientAddress) back to this client Application Layer 2-99
  100. 100. Socket programming with TCPclient must contact server  when contacted by client, server process must first be server TCP creates new socket running for server process to server must have created communicate with that socket (door) that welcomes particular client client’s contact  allows server to talk with multiple clientsclient contacts server by:  source port numbers used Creating TCP socket, to distinguish clients specifying IP address, port (more in Chap 3) number of server process when client creates socket: application viewpoint: client TCP establishes TCP provides reliable, in-order connection to server TCP byte-stream transfer (“pipe”) between client and server Application Layer 2-100
  101. 101. Client/server socket interaction: TCP server (running on hostid) client create socket, port=x, for incoming request: serverSocket = socket() wait for incoming create socket, connection request TCP connect to hostid, port=x connectionSocket = connection setup clientSocket = socket() serverSocket.accept() send request using read request from clientSocket connectionSocket write reply to connectionSocket read reply from clientSocket close connectionSocket close clientSocket Application Layer 2-101
  102. 102. Example app: TCP client Python TCPClient from socket import * serverName = ’servername’create TCP socket for serverPort = 12000server, remote port 12000 clientSocket = socket(AF_INET, SOCK_STREAM) clientSocket.connect((serverName,serverPort)) sentence = raw_input(‘Input lowercase sentence:’)No need to attach servername, port clientSocket.send(sentence) modifiedSentence = clientSocket.recv(1024) print ‘From Server:’, modifiedSentence clientSocket.close() Application Layer 2-102
  103. 103. Example app: TCP server Python TCPServer from socket import *create TCP welcoming serverPort = 12000socket serverSocket = socket(AF_INET,SOCK_STREAM) serverSocket.bind((‘’,serverPort))server begins listening forincoming TCP requests serverSocket.listen(1) print ‘The server is ready to receive’ loop forever while 1:server waits on accept()for incoming requests, new connectionSocket, addr = serverSocket.accept()socket created on return read bytes from socket (but sentence = connectionSocket.recv(1024) not address as in UDP) capitalizedSentence = sentence.upper()close connection to this connectionSocket.send(capitalizedSentence)client (but not welcomingsocket) connectionSocket.close() Application Layer 2-103
  104. 104. Chapter 2: summaryour study of network apps now complete! application architectures  specific protocols:  client-server  HTTP  P2P  FTP application service  SMTP, POP, IMAP requirements:  reliability, bandwidth, delay  DNS Internet transport service  P2P: BitTorrent, DHT model  socket programming: TCP,  connection-oriented, UDP sockets reliable: TCP  unreliable, datagrams: UDP Application Layer 2-104
  105. 105. Chapter 2: summarymost importantly: learned about protocols! typical request/reply important themes: message exchange:  control vs. data msgs  client requests info or service  in-band, out-of-band  server responds with  centralized vs. decentralized data, status code  stateless vs. stateful message formats:  reliable vs. unreliable msg  headers: fields giving info about data transfer  data: info being  “complexity at network communicated edge” Application Layer 2-105