Computer networks module 5 content covered in this ppt
- 2. Application Layer 2-2 Outline ❑ Principles of Network Applications ❑ The Web and HTTP ❑ Electronic mail in the Internet o SMTP, POP3, IMAP ❑ DNS-The Internet Directory • Text book 2: Computer Networking: A Top down Approach- James F Kurose 7th Edition
- 3. Principles of Network Applications ➢Network-applications are the driving forces for the explosive development of the internet. ➢Examples:
- 4. Creating a network app Write Programs That: • run on (different) end systems • communicate over network • e.g., web server software communicates with browser software. Possible Structure of Applications: • client-server • peer-to-peer (P2P) application transport network data link physical application transport network data link physical application transport network data link physical
- 5. Client-Server Architecture Server: ➢ always-on host ➢ permanent IP address ➢ data centers for scaling Clients: ➢ Randomly on ➢ communicate with server ➢ may be intermittently connected ➢ do not communicate directly with each other. ➢ Examples: FTP, Web, e-mail client/server
- 6. Data Center ➢ A data-center is used to create a powerful virtual server. ➢ In date center, hundreds of servers must be powered and maintained. ➢ example: Google has around 50 data-centers distributed around the world. These 50 data-centers handle : search, YouTube, Gmail etc.
- 7. P2P architecture ➢ No dedicated server ➢ Pairs of hosts are called peers. ➢ The peers communicate directly with each other. ➢ The peers are not owned by the service- provider, instead these are laptops controlled by users. ➢ Ex file sharing (BitTorrent), Internet telephone (Skype) ➢ self-scalability ,Cost effective ➢ Challenges: ISP friendly : P2P traffic does not conform to traditional ISP traffic policies. ➢ Security: Openness & distribution ➢ Incentives: convincing users to volunteer bandwidth & resources to the applications. peer-peer
- 8. Processes Communicating Process: program running within a host. • within same host, two processes communicate using IPC (defined by OS). • processes in different hosts communicate by exchanging messages. Client process: process that initiates communication Server process: process that waits to be contacted
- 9. Application Layer 2-9 App-layer protocol defines • types of messages exchanged, – e.g., request, response • message syntax: – what fields in messages & how fields are delineated • message semantics – meaning of information in fields • rules for when and how processes send & respond to messages open protocols: • defined in RFCs • allows for interoperability • e.g., HTTP, SMTP proprietary protocols: • e.g., Skype
- 10. Transport Services Available to Applications 1. Reliable Data Transfer some apps (e.g., file transfer, web transactions) require 100% reliable data transfer • other apps (e.g., audio) can tolerate some loss 3. Timing • some apps (e.g., Internet telephony, interactive games) require low delay to be “effective” 2.Throughput ❖ BW sensitive app: Guaranteed throughput (e.g., multimedia) ❖ Elastic app: May not need guarantee throughput file transfer, email 4. Security ❖ encryption, data integrity, …
- 12. 2-12 Internet Transport Protocols Services TCP service: • reliable transport between sending and receiving process • flow control: sender won’t overwhelm receiver • congestion control: throttle sender when network overloaded • connection-oriented: setup required between client and server processes UDP service: • unreliable data transfer between sending and receiving process • does not provide: reliability, flow control, congestion control, timing, throughput guarantee, security, or connection setup,
- 13. Web and HTTP ➢web page consists of objects(HTML) ➢object is a file - HTML file, JPEG image, audio file,… ➢web page consists of HTML-file & several referenced objects. ➢object is addressable by a URL, www.someschool.edu/someDept/pic.gif host name path name
- 14. HTTP overview HTTP: hypertext transfer protocol( port 80) • client/server model – client: browser that requests, receives, and “displays” Web objects – server: Web server sends objects in response to requests PC running Firefox browser server running Apache Web server iphone running Safari browser
- 15. 2-15 HTTP overview (continued) uses TCP: • client initiates TCP connection (creates socket) to server, port 80 • server accepts TCP connection from client • HTTP messages exchanged between browser (HTTP client) and Web server (HTTP server) • TCP connection closed HTTP is “stateless” • server maintains no information about past client requests
- 16. HTTP connections non-persistent HTTP • at most one object sent over TCP connection – connection then closed • downloading multiple objects required multiple connections persistent HTTP(default) • multiple objects can be sent over single TCP connection between client, server
- 17. Non-persistent HTTP: 1Req->1Resp suppose user enters URL: 1a. HTTP client initiates TCP connection to HTTP server (process) at www.someSchool.edu on port 80 2. HTTP client sends HTTP request message (containing URL) intoTCP connection socket. Message indicates that client wants object someDepartment/home.index 1b. HTTP server at host www.someSchool.edu waiting for TCP connection at port 80. “accepts” connection, notifying client 3. HTTP server receives request message, forms response message containing requested object, and sends message into its socket time (contains text, references to 10 jpeg images) www.someSchool.edu/someDepartment/home.index
- 18. Non-persistent HTTP (cont.) 5. HTTP client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects 6. Steps 1-5 repeated for each of 10 jpeg objects 4. HTTP server closesTCP connection. time
- 19. Non-persistent HTTP: response time RTT: Time for a small packet to travel from client to server and back HTTP response time: • one RTT: To initiate TCP connection • one RTT: HTTP request and first few bytes of HTTP response to return • file transmission time • non-persistent HTTP response time = 2RTT+ file transmission time time to transmit file initiate TCP connection RTT request file RTT file received time time
- 20. 2-20 Persistent HTTP non-persistent HTTP issues: • requires 2 RTTs per object • OS overhead for each TCP connection • browsers often open parallel TCP connections to fetch referenced objects persistent HTTP: • server leaves connection open after sending response • subsequent HTTP messages between same 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
- 21. HTTP message: general format Two types of HTTP messages: request, response
- 22. HTTP request message GET: Browser requests an object from the server. POST: User fills out a form & sends to the server. PUT: Upload objects to servers. DELETE: Allows application to delete object on a server. request line (GET, POST, ,PUT,DELETE) header lines carriage return, line feed at start of line indicates end of header lines GET /index.html HTTP/1.1rn Host: www-net.cs.umass.edurn→ obj residing User-Agent: Firefox/3.6.10rn →browser Accept: text/html,application/xhtml+xmlrn Accept-Language: en-us,en;q=0.5rn Accept-Encoding: gzip,deflatern Accept-Charset: ISO-8859-1,utf-8;q=0.7rn Keep-Alive: 115rn Connection: keep-alivern rn carriage return character line-feed character
- 24. Application Layer 2-24 HTTP response message status line (protocol status code status phrase) header lines data, e.g., requested HTML file HTTP/1.1 200 OKrn 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 Accept-Ranges: bytesrn Content-Length: 2652rn Keep-Alive: timeout=10, max=100rn Connection: Keep-Alivern Content-Type: text/html; charset=ISO-8859-1rn rn data data data data data ...
- 25. User-server Interaction: cookies Websites to keep track of user Small text file created by website, stored in user computer(temporarily/perman ent) four components: 1) cookie header line of HTTP response message 2) cookie header line in next HTTP request message 3) cookie file kept on user’s host, managed by user’s browser 4) back-end DB at Web site example: • Susan always access Internet from PC • visits specific e-commerce site for first time • when initial HTTP requests arrives at site, site creates: – unique ID – entry in backend DB for ID
- 26. Cookies: keeping “state” (cont.) Client Host Server Host usual http response msg usual http response msg cookie file one week later: usual http request msg cookie: 1678 cookie- specific action access ebay 8734 usual http request msg Amazon server creates ID 1678 for user create entry usual http response Header set-cookie: 1678 ebay 8734 amazon 1678 usual http request msg cookie: 1678 cookie- specific action access ebay 8734 amazon 1678 backend database Browser tores ID into cookie file Browser put ID in http request
- 27. Web caching (Proxy Server) • user sets browser: Web accesses via cache • browser sends all HTTP requests to cache – object in cache: cache returns object – else cache requests object from origin server, then returns object to client – Advantages 1) To reduce response-time for client-request. 2) To reduce traffic on an institution’s access-link to the Internet. 3) To reduce Web-traffic in the Internet. Goal: satisfy client request without involving origin server client Web Cache/ proxy server client origin server origin server
- 28. The Conditional GET • It is a mechanism that allows a cache to verify that the objects are up to date. • cache: specify date of cached copy in HTTP request If-modified-since: <date> • server: response contains no object if cached copy is up- to-date: HTTP/1.0 304 Not Modified HTTP request msg If-modified-since: <date> HTTP response HTTP/1.0 304 Not Modified object not modified before <date> HTTP request msg If-modified-since: <date> HTTP response HTTP/1.0 200 OK <data> object modified after <date> client server
- 29. The Conditional GET ❖It is a mechanism that allows a cache to verify that the objects are up to date. ❖An HTTP request-message is called conditional GET if 1) Request-message uses the GET method and 2) Response-message includes an If-Modified-Since: header-line.
- 30. File Transfer: FTP • Used by local host to transfer files to or from a remote-host over the network. • FTP uses client-server architecture • FTP uses 2 parallel TCP connections 1) Control Connection : User ID, password ,commands to change directory , put & get files. 2) Data Connection: To transfer files. FTP commands & replies: USER,PASS,LIST(all files), RETR, STOR
- 31. FTP commands, responses sample commands: • USER username • PASS password • LIST return list of file in current directory • RETR filename retrieves (gets) file • STOR filename stores (puts) file onto remote host sample return codes • 331 Username OK, password required • 125 data connection already open; transfer starting • 425 Can’t open data connection • 452 Error writing file
- 32. Electronic mail Three major components: • User agents(UA) • Mail servers(MS) • Simple mail transfer protocol: SMTP User Agent • Read, reply, forward save compose messages • e.g., Outlook, Thunderbird, iPhone mail client user mailbox outgoing message queue mail server mail server mail server SMTP SMTP SMTP user agent user agent user agent user agent user agent user agent
- 33. 2-33 Electronic mail: mail servers Mail servers: • mailbox contains incoming messages for user • message queue of outgoing (to be sent) mail messages SMTP protocol: • Between mail servers to send email messages , TCP , port - 25 mail server mail server mail server SMTP SMTP SMTP user agent user agent user agent user agent user agent user agent user mailbox outgoing Message Q
- 34. user agent Scenario: Alice sends message to Bob 1) Alice uses UA to compose message “to” bob@someschool.edu 2) Alice’s UA sends message to her mail server; message placed in message queue 3) client side of SMTP opens TCP connection with Bob’s mail server 4) SMTP client sends Alice’s message over the TCP connection 5) Bob’s mail server places the message in Bob’s mailbox 6) Bob invokes his UA to read message mail server mail server 1 2 3 4 5 6 Alice’s mail server Bob’s mail server user agent SMTP
- 35. Mail Message Format SMTP: protocol for exchanging email msgs standard for text message format: • header lines, e.g., – To: – From: – Subject: • Body: the “message” – ASCII characters only header body blank line
- 36. Mail Access Protocols • SMTP: delivery/storage to receiver’s server • Mail access protocol: retrieval from server – POP: Post Office Protocol : authorization, download – IMAP: Internet Mail Access Protocol : manipulation of stored msgs on server, allows user to organize messages in folders – HTTP: gmail, Hotmail, Yahoo! Mail, etc. sender’s mail server SMTP SMTP mail access protocol receiver’s mail server (e.g., POP, IMAP) user agent user agent
- 37. POP3 protocol-port 110 1. Authorization phase • client commands: – user: username – pass: password • server responses – +OK – -ERR 2. Transaction phase, client: • list: list message numbers • retr: retrieve message by number • dele: delete • Quit 3. Update C: list S: 1 498 S: 2 912 S: . C: retr 1 S: <message 1 contents> S: . C: dele 1 C: retr 2 S: <message 1 contents> S: . C: dele 2 C: quit S: +OK POP3 server signing off S: +OK POP3 server ready C: user bob S: +OK C: pass hungry S: +OK user successfully logged on
- 38. DNS -The Internet’s Directory Service(port-53) ➢ Translates domain names → IP addresses. ➢ ex: domain-name(Host name) “www.google.com” →IP address “198.105.232.4”. ➢ Because domain-names are alphabetic, easier to remember for human being.
- 39. DNS provides following services 1) Host Aliasing : A host with a complicated hostname can have one or more alias names. 2) Mail Server Aliasing: For obvious reasons, it is highly desirable that e-mail addresses be mnemonic. 3) Load Distribution : It perform load distribution among replicated servers.
- 40. Overview of How DNS Works Distributed database design is more preferred over centralized : 1) A Single Point of Failure : If the DNS server crashes then the entire Internet will not stop. 2) Traffic Volume : A Single DNS Server cannot handle the huge global DNS traffic. 3) Distant Centralized DB: A single DNS server cannot be “close to” all the querying clients. ➢ If we put the single DNS server in Mysore, then all queries from USA must travel to the other side of the globe, leads delay. 4) Maintenance: The single DNS server would have to keep records for all Internet hosts.
- 41. Root DNS Servers com DNS servers org DNS servers edu DNS servers poly.edu DNS servers umass.edu DNS servers yahoo.com DNS servers amazon.com DNS servers pbs.org DNS servers DNS: a distributed, hierarchical database client wants IP for www.amazon.com: • 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 … … Authoritative
- 42. requesting host cis.poly.edu gaia.cs.umass.edu root DNS server local DNS server dns.poly.edu 1 2 3 4 5 6 authoritative DNS server dns.cs.umass.edu 7 8 TLD DNS server DNS name resolution example • host at cis.poly.edu wants IP address for gaia.cs.umass.edu iterative query: ❖ contacted server replies with name of server to contact ❖ “I don’t know this name, but ask this server”
- 43. 4 5 6 3 recursive query: ❖ puts burden of name resolution on contacted name server ❖ heavy load at upper levels of hierarchy? requesting host cis.poly.edu gaia.cs.umass.edu root DNS server local DNS server dns.poly.edu 1 2 7 authoritative DNS server dns.cs.umass.edu 8 DNS name resolution example TLD DNS server
- 44. DNS Record: Instructions in authoritative DNS servers, provide info( Domain+ IP address ) DNS: distributed database storing resource records (RR) type=NS(Name Server) – name is domain – value is hostname of authoritative name server for this domain RR format: (name, value, type, ttl) type=A(Address) ▪ name is hostname ▪ value is IP address type=CNAME ▪ name is alias name for some “canonical” (the real) name ▪ value is canonical name type=MX(mail exchange) ▪ value is name of mailserver associated with name when a resource should be removed from a cache
- 45. DNS : Messages • Query(Request) and reply messages, both with same message format msg header ❖ identification: 16 bit # for query, reply to query uses same # ❖ flags: ▪ 1.query(0) or reply(1) ▪ 2. Authoritative: set in reply msg when DNS is authoritative server ▪ 3. Recursion Desired: Set when client desires DNS performs recursion identification flags # questions questions (variable # of questions) # additional RRs # authority RRs # answer RRs answers (variable # of RRs) authority (variable # of RRs) additional info (variable # of RRs) 2 bytes 2 bytes Header Section (12bytes Question Section
- 46. Info about query: name, type fields for a query RRs in response to query records for authoritative servers additional “helpful” info that may be used identification flags # questions questions (variable # of questions) # additional RRs # authority RRs # answer RRs answers (variable # of RRs) authority (variable # of RRs) additional info (variable # of RRs) DNS Protocol, Messages 2 bytes 2 bytes domain-name question being asked about the domain-name