Chapter_2_V6.3.pdf

Chapter 2 Application Layer A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). Computer They’re in PowerPoint form so you see the animations; and can add, modify, and delete slides (including this one) and slide content to suit y...

Chapter 2 Application Layer A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). Computer They’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 Top They obviously represent a lot of work on our part. In return for use, we only ask 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 2012 Thanks and enjoy! JFK/KWR All material copyright 1996-2012 J.F Kurose and K.W. Ross, All Rights Reserved Application Layer 2-1 Chapter 2: outline 2.1 principles of network 2.6 P2P applications applications 2.7 socket programming 2.2 Web and HTTP with UDP and TCP 2.3 FTP 2.4 electronic mail  SMTP, POP3, IMAP 2.5 DNS Application Layer 2-2 Chapter 2: application layer our 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 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 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 data link application  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 Application architectures possible structure of applications:  client-server  peer-to-peer (P2P) Application Layer 2-6  Client-server architecture server:  always-on host  permanent IP address  data centers for scaling clients:  communicate with server client/server  may be intermittently connected  may have dynamic IP addresses  do not communicate directly with each other Application Layer 2-7 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 Processes communicating process: 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 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 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 gaia.cs.umass.edu web can be running on same server: host  IP address: 128.119.245.12  port number: 80  more shortly… Application Layer 2-11 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 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 throughput they get  some apps (e.g., Internet telephony, interactive security games) require low delay to be “effective”  encryption, data integrity, … Application Layer 2-13  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 no real-time audio/video loss-tolerant audio: 5kbps-1Mbps yes, 100’s video:10kbps-5Mbps msec stored audio/video loss-tolerant same as above interactive games loss-tolerant few kbps up yes, few secs text messaging no loss elastic yes, 100’s msec yes and no Application Layer 2-14 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 Internet apps: application, transport protocols application underlying application layer protocol transport protocol e-mail SMTP [RFC 2821] TCP remote 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 Securing TCP TCP & 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 sent SSL into socket traverse  provides encrypted Internet encrypted TCP connection  See Chapter 7  data integrity  end-point authentication Application Layer 2-17 Chapter 2: outline 2.1 principles of network 2.6 P2P applications applications 2.7 socket programming  app architectures with UDP and TCP  app requirements 2.2 Web and HTTP 2.3 FTP 2.4 electronic mail  SMTP, POP3, IMAP 2.5 DNS Application Layer 2-18 Web and HTTP First, 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., www.someschool.edu/someDept/pic.gif host name path name Application Layer 2-19 HTTP overview HTTP: hypertext transfer protocol  Web’s application layer protocol PC running  client/server model Firefox browser  client: browser that requests, receives, (using HTTP protocol) server and “displays” Web running objects Apache Web  server: Web server server sends (using HTTP protocol) objects in iphone running response to requests Safari browser Application Layer 2-20 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 HTTP connections non-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  Non-persistent HTTP suppose user enters URL: (contains text, www.someSchool.edu/someDepartment/home.index references to 10 jpeg images) 1a. HTTP client initiates TCP connection to HTTP server (process) at 1b. HTTP server at host www.someSchool.edu on port www.someSchool.edu 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 socket time Application Layer 2-23  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 objects time 6. Steps 1-5 repeated for each of 10 jpeg objects Application Layer 2-24 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 Persistent HTTP non-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  HTTP request message  two types of HTTP messages: request, response  HTTP request message:  ASCII (human-readable format) carriage return character line-feed character request line (GET, POST, GET /index.html HTTP/1.1\r\n HEAD commands) Host: www-net.cs.umass.edu\r\n User-Agent: Firefox/3.6.10\r\n Accept: text/html,application/xhtml+xml\r\n header Accept-Language: en-us,en;q=0.5\r\n lines Accept-Encoding: gzip,deflate\r\n Accept-Charset: ISO-8859-1,utf-8;q=0.7\r\n carriage return, Keep-Alive: 115\r\n line feed at start Connection: keep-alive\r\n \r\n of line indicates end of header lines Application Layer 2-27 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 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: www.somesite.com/animalsearch?monkeys&banana Application Layer 2-29 Method types HTTP/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  HTTP response message status line (protocol status code HTTP/1.1 200 OK\r\n status phrase) Date: Sun, 26 Sep 2010 20:09:20 GMT\r\n Server: Apache/2.0.52 (CentOS)\r\n Last-Modified: Tue, 30 Oct 2007 17:00:02 GMT\r\n header ETag: "17dc6-a5c-bf716880"\r\n Accept-Ranges: bytes\r\n lines Content-Length: 2652\r\n Keep-Alive: timeout=10, max=100\r\n Connection: Keep-Alive\r\n Content-Type: text/html; charset=ISO-8859- 1\r\n \r\n data, e.g., data data data data data... requested HTML file Application Layer 2-31 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 Trying out HTTP (client side) for yourself 1. Telnet to your favorite Web server: telnet cis.poly.edu 80 opens TCP connection to port 80 (default HTTP server port) at cis.poly.edu. anything typed in sent to port 80 at cis.poly.edu 2. type in a GET HTTP request: GET /~ross/ HTTP/1.1 by typing this in (hit carriage Host: cis.poly.edu 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 User-server state: cookies example: many Web sites use cookies  Susan always access Internet four 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  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 action one week later: access ebay 8734 usual http request msg amazon 1678 cookie: 1678 cookie- specific usual http response msg action Application Layer 2-35 Cookies (continued) aside what 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 Web caches (proxy server) goal: satisfy client request without involving origin server  user sets browser: Web accesses via cache  browser sends all HTTP proxy requests to cache server  object in cache: cache client origin returns object server  else cache requests object from origin server, then returns object to client client origin server Application Layer 2-37 More about Web caching  cache acts as both why Web caching? client and server  reduce response time  server for original for client request requesting client  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 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 consequences: access link  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 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 consequences: access link  LAN utilization: 15% institutional  access link utilization = 99% 9.9% network 1 Gbps LAN  total delay = Internet delay + access delay + LAN delay = 2 sec + minutes + usecs msecs Cost: increased access link speed (not cheap!) Application Layer 2-40 Caching example: install local cache 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 consequences: access link  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 = 2 sec + minutes + usecs cache utilization, delay? Cost: web cache (cheap!) Application Layer 2-41 Caching example: install local cache Calculating 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 Conditional GET client server  Goal: don’t send object if cache has up-to-date cached version HTTP request msg object If-modified-since:  no object transmission not delay modified  lower link utilization HTTP response before HTTP/1.0  cache: specify date of 304 Not Modified cached copy in HTTP request If-modified-since: HTTP request msg  server: response contains If-modified-since: object modified no object if cached copy after HTTP response is up-to-date: HTTP/1.0 200 OK HTTP/1.0 304 Not Modified Application Layer 2-43 Chapter 2: outline 2.1 principles of network 2.6 P2P applications applications 2.7 socket programming  app architectures with UDP and TCP  app requirements 2.2 Web and HTTP 2.3 FTP 2.4 electronic mail  SMTP, POP3, IMAP 2.5 DNS Application Layer 2-44 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 FTP: separate control, data connections  FTP client contacts FTP server TCP control connection, server port 21 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 over control connection  server opens another TCP data connection to transfer  when server receives file another file transfer command, server opens 2nd TCP data  control connection: “out of connection (for file) to client band”  after transferring one file,  FTP server maintains server closes data connection “state”: current directory, earlier authentication Application Layer 2-46 FTP commands, responses sample 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 Chapter 2: outline 2.1 principles of network 2.6 P2P applications applications 2.7 socket programming  app architectures with UDP and TCP  app requirements 2.2 Web and HTTP 2.3 FTP 2.4 electronic mail  SMTP, POP3, IMAP 2.5 DNS Application Layer 2-48 Electronic mail outgoing message queue user mailbox Three major components: user agent  user agents  mail servers mail user server agent  simple mail transfer protocol: SMTP SMTP mail user server agent User Agent SMTP  a.k.a. “mail reader” SMTP user agent  composing, editing, reading mail server mail messages user  e.g., Outlook, Thunderbird, agent iPhone mail client user agent  outgoing, incoming messages stored on server Application Layer 2-49 Electronic mail: mail servers mail servers: user agent  mailbox contains incoming messages for user mail user server  message queue of outgoing agent (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  “server”: receiving mail user agent server user agent Application Layer 2-50 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 Scenario: Alice sends message to Bob 1) Alice uses UA to compose 4) SMTP client sends Alice’s message “to” message over the TCP [email protected] connection 2) Alice’s UA sends message 5) Bob’s mail server places the to her mail server; message message in Bob’s mailbox placed in message queue 6) Bob invokes his user agent 3) 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 Sample SMTP interaction S: 220 hamburger.edu C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: S: 250 [email protected]... Sender ok C: RCPT TO: S: 250 [email protected]... 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 Application Layer 2-53 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 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 Mail message format SMTP: protocol for exchanging email msgs header blank RFC 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 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 POP3 protocol S: +OK POP3 server ready C: user bob authorization phase S: C: +OK 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 1 transaction phase, client: S: S:.  list: list message numbers C: dele 1  retr: retrieve message by C: retr 2 number S:  dele: delete S:.  quit C: dele 2 C: quit S: +OK POP3 server signing off Application Layer 2-58 POP3 (more) and IMAP more 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 Chapter 2: outline 2.1 principles of network 2.6 P2P applications applications 2.7 socket programming  app architectures with UDP and TCP  app requirements 2.2 Web and HTTP 2.3 FTP 2.4 electronic mail  SMTP, POP3, IMAP 2.5 DNS Application Layer 2-60 DNS: domain name system people: many identifiers: Domain Name System:  SSN, name, passport #  distributed database Internet 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 www.yahoo.com - translation) used by humans  note: core Internet function, Q: how to map between IP implemented as application- layer protocol address and name, and vice versa ?  complexity at network’s “edge” Application Layer 2-61 DNS: services, structure DNS 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 DNS: a distributed, hierarchical database Root DNS Servers … … com DNS servers org DNS servers edu DNS servers pbs.org poly.edu umass.edu yahoo.com amazon.com DNS servers DNS serversDNS servers DNS servers DNS servers client wants IP for www.amazon.com; 1st approx:  client queries 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 Application Layer 2-63  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 Tokyo f. Internet Software C. (5 other sites) Palo Alto, CA (and 48 other sites) 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 TLD, authoritative servers top-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 TLD authoritative 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 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 DNS name root DNS server resolution example 2  host at cis.poly.edu 3 TLD DNS server wants IP address for 4 gaia.cs.umass.edu 5 iterated query: local DNS server dns.poly.edu  contacted server 7 6 1 8 replies with name of server to contact authoritative DNS server  “I don’t know this dns.cs.umass.edu name, but ask this requesting host server” cis.poly.edu gaia.cs.umass.edu Application Layer 2-67 DNS name root DNS server resolution example 2 3 recursive query: 7 6  puts burden of name TLD DNS server resolution on contacted name local DNS server server dns.poly.edu 5 4  heavy load at upper 1 8 levels of hierarchy? authoritative DNS server dns.cs.umass.edu requesting host cis.poly.edu gaia.cs.umass.edu Application Layer 2-68 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 DNS records DNS: 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) name type=NS  www.ibm.com is really  name is domain (e.g., servereast.backup2.ibm.com foo.com)  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 DNS protocol, messages  query and reply messages, both with same message format 2 bytes 2 bytes msg 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 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 Inserting records into DNS  example: new startup “Network Utopia”  register name networkuptopia.com 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: (networkutopia.com, dns1.networkutopia.com, NS) (dns1.networkutopia.com, 212.212.212.1, A)  create authoritative server type A record for www.networkuptopia.com; type MX record for networkutopia.com Application Layer 2-73 Attacking DNS DDoS 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 spoofed source  Potentially more dangerous address: target IP  Requires amplification Application Layer 2-74 Chapter 2: outline 2.1 principles of network 2.6 P2P applications applications 2.7 socket programming  app architectures with UDP and TCP  app requirements 2.2 Web and HTTP 2.3 FTP 2.4 electronic mail  SMTP, POP3, IMAP 2.5 DNS Application Layer 2-75 Pure P2P architecture  no always-on server  arbitrary end systems directly communicate  peers are intermittently connected and change IP addresses examples:  file distribution (BitTorrent)  Streaming (KanKan)  VoIP (Skype) Application Layer 2-76 File distribution: client-server vs P2P Question: 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 download file, size F u1 d1 capacity us u2 d2 server di uN network (with abundant bandwidth) ui dN ui: peer i upload capacity Application Layer 2-77 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 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 us + Sui time to distribute F to N clients using DP2P > max{F/us,,F/dmin,,NF/(us + Sui)} P2P approach increases linearly in N … … but so does this, as each peer brings service capacity Application Layer 2-79 Client-server vs. P2P: example client 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 P2P file distribution: BitTorrent  file divided into 256Kb chunks  peers in torrent send/receive file chunks tracker: tracks peers torrent: group of peers participating 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 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 BitTorrent: requesting, sending file chunks requesting chunks: sending chunks: tit-for-tat  at any given time, different  Alice sends chunks to those peers have different subsets four peers currently sending her of file chunks 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) they have  re-evaluate top 4 every10 secs  Alice requests missing  every 30 secs: randomly select 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 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 Distributed Hash Table (DHT)  Hash table  DHT paradigm  Circular DHT and overlay networks  Peer churn Simple Database Simple database with(key, value) pairs: key: human name; value: social security # Key Value John Washington 132-54-3570 Diana Louise Jones 761-55-3791 Xiaoming Liu 385-41-0902 Rakesh Gopal 441-89-1956 Linda Cohen 217-66-5609 ……. ……… Lisa Kobayashi 177-23-0199 key: movie title; value: IP address Hash Table More convenient to store and search on numerical representation of key key = hash(original key) Original Key Key Value John Washington 8962458 132-54-3570 Diana Louise Jones 7800356 761-55-3791 Xiaoming Liu 1567109 385-41-0902 Rakesh Gopal 2360012 441-89-1956 Linda Cohen 5430938 217-66-5609 ……. ……… Lisa Kobayashi 9290124 177-23-0199 Distributed Hash Table (DHT)  Distribute (key, value) pairs over millions of peers  pairs are evenly distributed over peers  Any peer can query database with a key  database returns value for the key  To resolve query, small number of messages exchanged among peers  Each peer only knows about a small number of other peers  Robust to peers coming and going (churn) Assign key-value pairs to peers  rule: assign key-value pair to the peer that has the closest ID.  convention: closest is the immediate successor of the key.  e.g., ID space {0,1,2,3,…,63}  suppose 8 peers: 1,12,13,25,32,40,48,60  If key = 51, then assigned to peer 60  If key = 60, then assigned to peer 60  If key = 61, then assigned to peer 1  Circular DHT each peer only aware of immediate successor and predecessor. 1 12 60 13 48 25 40 32 “overlay network”  Resolving a query 1 What is the value associated with key 53 ? value 12 60 13 48 O(N) messages 25 on avgerage to resolve query, when there 40 32 are N peers Circular DHT with shortcuts 1 What is the value for value key 53 12 60 13 48 25 40 32 each peer keeps track of IP addresses of predecessor, successor, short cuts. reduced from 6 to 3 messages. possible to design shortcuts with O(log N) neighbors, O(log N) messages in query Peer churn handling peer churn: 1 peers may come and go (churn) each peer knows address of its 3 two successors 15 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 churn handling peer churn: 1 peers may come and go (churn) each peer knows address of its 3 two successors 15 each peer periodically pings its 4 two successors to check aliveness if immediate successor leaves, 12 choose next successor as new immediate successor 10 8 example: peer 5 abruptly leaves peer 4 detects peer 5’s departure; makes 8 its immediate successor  4 asks 8 who its immediate successor is; makes 8’s immediate successor its second successor. Chapter 2: outline 2.1 principles of network 2.6 P2P applications applications 2.7 socket programming  app architectures with UDP and TCP  app requirements 2.2 Web and HTTP 2.3 FTP 2.4 electronic mail  SMTP, POP3, IMAP 2.5 DNS Application Layer 2-95 Socket programming goal: learn how to build client/server applications that communicate using sockets socket: 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 2-96  Socket programming Two socket types for two transport services:  UDP: unreliable datagram  TCP: reliable, byte stream-oriented Application 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 2-97 Socket programming with UDP UDP: 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 packet UDP: transmitted data may be lost or received out-of-order Application viewpoint:  UDP provides unreliable transfer of groups of bytes (“datagrams”) between client and server Application Layer 2-98 Client/server socket interaction: UDP server (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 2-99  Example app: UDP client Python UDPClient include Python’s socket library from socket import * serverName = ‘hostname’ serverPort = 12000 create UDP socket for clientSocket = socket(socket.AF_INET, server get user keyboard socket.SOCK_DGRAM) input message = raw_input(’Input lowercase sentence:’) Attach server name, port to message; 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 modifiedMessage and close socket clientSocket.close() Application Layer 2-100  Example app: UDP server Python UDPServer from socket import * serverPort = 12000 create UDP socket serverSocket = socket(AF_INET, SOCK_DGRAM) bind socket to local port number 12000 serverSocket.bind(('', serverPort)) print “The server is ready to receive” loop forever while 1: Read from UDP socket into message, clientAddress = serverSocket.recvfrom(2048) message, getting client’s address (client IP and port) modifiedMessage = message.upper() send upper case string serverSocket.sendto(modifiedMessage, clientAddress) back to this client Application Layer 2-101 Socket programming with TCP client 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 particular client welcomes client’s contact  allows server to talk with multiple clients client 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 2-102 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 2-103  Example app: TCP client Python TCPClient from socket import * serverName = ’servername’ create TCP socket for serverPort = 12000 server, remote port 12000 clientSocket = socket(AF_INET, SOCK_STREAM) clientSocket.connect((serverName,serverPort)) sentence = raw_input(‘Input lowercase sentence:’) No need to attach server clientSocket.send(sentence) name, port modifiedSentence = clientSocket.recv(1024) print ‘From Server:’, modifiedSentence clientSocket.close() Application Layer 2-104  Example app: TCP server Python TCPServer from socket import * create TCP welcoming serverPort = 12000 socket serverSocket = socket(AF_INET,SOCK_STREAM) serverSocket.bind((‘’,serverPort)) server begins listening for incoming 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 sentence = connectionSocket.recv(1024) read bytes from socket (but not address as in UDP) capitalizedSentence = sentence.upper() close connection to this connectionSocket.send(capitalizedSentence) client (but not welcoming socket) connectionSocket.close() Application Layer 2-105 Chapter 2: summary our study of network apps now complete!  application architectures  specific protocols:  client-server  HTTP  P2P  FTP  application service requirements:  SMTP, POP, IMAP  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 2-106 Chapter 2: summary most 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 2-107  Chapter 1 Additional Slides Introduction 1-108  application packet (www browser, analyzer email client) application OS packet Transport (TCP/UDP) copy of all Network (IP) capture Ethernet frames Link (Ethernet) (pcap) sent/receive d Physical