Application Layer-Computer Networking-Lecture Slides, Slides of Computer Networks

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Chapter 2 Application Layer SUM @Covenyaluiccly Networking 2: Application Lay és csity.com 2: Application Layer 2 Chapter 2: Application layer  2.1 Principles of network applications  2.2 Web and HTTP  2.3 FTP  2.4 Electronic Mail  SMTP, POP3, IMAP  2.5 DNS  2.6 P2P applications  2.7 Socket programming with TCP  2.8 Socket programming with UDP Docsity.com 2: Application Layer 5 Creating a network app write programs that  run on (different) end systems  communicate over network  e.g., web server software communicates with browser software No need to write software for network-core devices  Network-core devices do not run user applications  applications on end systems allows for rapid app development, propagation application transport network data link physical application transport network data link physical application transport network data link physical Docsity.com 2: Application Layer 6 Chapter 2: Application layer  2.1 Principles of network applications  2.2 Web and HTTP  2.3 FTP  2.4 Electronic Mail  SMTP, POP3, IMAP  2.5 DNS  2.6 P2P applications  2.7 Socket programming with TCP  2.8 Socket programming with UDP  2.9 Building a Web server Docsity.com 2: Application Layer 7 Application architectures  Client-server  Peer-to-peer (P2P) Hybrid of client-server and P2P Docsity.com 2: Application Layer 10 Hybrid of client-server and P2P Skype  voice-over-IP P2P application  centralized server: finding address of remote party:  client-client connection: direct (not through server) Instant messaging  chatting between two users is P2P  centralized service: client presence detection/location • user registers its IP address with central server when it comes online • user contacts central server to find IP addresses of buddies Docsity.com 2: Application Layer 11 Processes communicating Process: program running within a host.  within same host, two processes communicate using inter-process communication (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  Note: applications with P2P architectures have client processes & server processes Docsity.com 2: Application Layer 12 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 which brings message to socket at receiving process process TCP with buffers, variables socket host or server process TCP with buffers, variables socket host or server Internet controlled by OS controlled by app developer  API: (1) choice of transport protocol; (2) ability to fix a few parameters (lots more on this later) Docsity.com 2: Application Layer 15 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 Public-domain protocols:  defined in RFCs  allows for interoperability  e.g., HTTP, SMTP Proprietary protocols:  e.g., Skype Docsity.com 2: Application Layer 16 What transport service does an app need? Data loss  some apps (e.g., audio) can tolerate some loss  other apps (e.g., file transfer, telnet) require 100% reliable data transfer Timing  some apps (e.g., Internet telephony, interactive games) require low delay to be “effective” Throughput  some apps (e.g., multimedia) require minimum amount of throughput to be “effective”  other apps (“elastic apps”) make use of whatever throughput they get Security  Encryption, data integrity, … Docsity.com 2: Application Layer 17 Transport service requirements of common apps Application file transfer e-mail Web documents real-time audio/video stored audio/video interactive games instant messaging Data loss no loss no loss no loss loss-tolerant loss-tolerant loss-tolerant no loss Throughput elastic elastic elastic audio: 5kbps-1Mbps video:10kbps-5Mbps same as above few kbps up elastic Time Sensitive no no no yes, 100’s msec yes, few secs yes, 100’s msec yes and no Docsity.com 2: Application Layer 20 Chapter 2: Application layer  2.1 Principles of network applications  app architectures  app requirements  2.2 Web and HTTP  2.4 Electronic Mail  SMTP, POP3, IMAP  2.5 DNS  2.6 P2P applications  2.7 Socket programming with TCP  2.8 Socket programming with UDP Docsity.com 2: Application Layer 21 Web and HTTP First some jargon  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  Example URL: www.someschool.edu/someDept/pic.gif host name path name Docsity.com 2: Application Layer 22 HTTP overview HTTP: hypertext transfer protocol  Web’s application layer protocol  client/server model  client: browser that requests, receives, “displays” Web objects  server: Web server sends objects in response to requests PC running Explorer Server running Apache Web server Mac running Navigator Docsity.com 2: Application Layer 25 Nonpersistent HTTP Suppose user enters URL www.someSchool.edu/someDepartment/home.index 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) into TCP 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) Docsity.com 2: Application Layer 26 Nonpersistent 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 closes TCP connection. time Docsity.com 2: Application Layer 27 Non-Persistent HTTP: Response time Definition of RTT: time for a small packet to travel from client to server and back. Response time:  one RTT to initiate TCP connection  one RTT for HTTP request and first few bytes of HTTP response to return  file transmission time total = 2RTT+transmit time time to transmit file initiate TCP connection RTT request file RTT file received time time Docsity.com 2: Application Layer 30 HTTP request message: general format Docsity.com 2: Application Layer 31 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 Docsity.com 2: Application Layer 32 Method types HTTP/1.0  GET  POST  HEAD  asks server to leave requested object out of response HTTP/1.1  GET, POST, HEAD  PUT  uploads file in entity body to path specified in URL field  DELETE  deletes file specified in the URL field Docsity.com 2: Application Layer 35 Trying out HTTP (client side) for yourself 1. Telnet to your favorite Web server: 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 telnet cis.poly.edu 80 2. Type in a GET HTTP request: GET /~ross/ HTTP/1.1 Host: cis.poly.edu By typing this in (hit carriage return twice), you send this minimal (but complete) GET request to HTTP server 3. Look at response message sent by HTTP server! Docsity.com 2: Application Layer 36 User-server state: cookies Many major Web sites use cookies Four components: 1) cookie header line of HTTP response message 2) cookie header line in HTTP request message 3) cookie file kept on user’s host, managed by user’s browser 4) back-end database at Web site Example:  Susan always access Internet always from PC  visits specific e- commerce site for first time  when initial HTTP requests arrives at site, site creates:  unique ID  entry in backend database for ID Docsity.com 2: Application Layer 37 Cookies: keeping “state” (cont.) client server 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 Set-cookie: 1678 ebay 8734 amazon 1678 usual http request msg cookie: 1678 cookie- spectific action access ebay 8734 amazon 1678 backend database Docsity.com 2: Application Layer 40 More about Web caching  cache acts as both client and server  typically cache is installed by ISP (university, company, residential ISP) Why Web caching?  reduce response time for client request  reduce traffic on an institution’s access link.  Internet dense with caches: enables “poor” content providers to effectively deliver content (but so does P2P file sharing) Docsity.com 2: Application Layer 41 Caching example Assumptions  average object size = 100,000 bits  avg. request rate from institution’s browsers to origin servers = 15/sec  delay from institutional router to any origin server and back to router = 2 sec Consequences  utilization on LAN = 15%  utilization on access link = 100%  total delay = Internet delay + access delay + LAN delay = 2 sec + minutes + milliseconds origin servers public Internet institutional network 10 Mbps LAN 1.5 Mbps access link institutional cache Docsity.com 2: Application Layer 42 Caching example (cont) possible solution  increase bandwidth of access link to, say, 10 Mbps consequence  utilization on LAN = 15%  utilization on access link = 15%  Total delay = Internet delay + access delay + LAN delay = 2 sec + msecs + msecs  often a costly upgrade origin servers public Internet institutional network 10 Mbps LAN 10 Mbps access link institutional cache Docsity.com 2: Application Layer 45 Chapter 2: Application layer  2.1 Principles of network applications  2.2 Web and HTTP  2.3 FTP  2.4 Electronic Mail  SMTP, POP3, IMAP  2.5 DNS  2.6 P2P applications  2.7 Socket programming with TCP  2.8 Socket programming with UDP  2.9 Building a Web server Docsity.com 2: Application Layer 46 FTP: the file transfer protocol  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 file transfer FTP server FTP user interface FTP client local file system remote file system user at host Docsity.com 2: Application Layer 47 FTP: separate control, data connections  FTP client contacts FTP server at port 21, TCP is transport protocol  client authorized over control connection  client browses remote directory by sending commands over control connection.  when server receives file transfer command, server opens 2nd TCP connection (for file) to client  after transferring one file, server closes data connection. FTP client FTP server TCP control connection port 21 TCP data connection port 20  server opens another TCP data connection to transfer another file.  control connection: “out of band”  FTP server maintains “state”: current directory, earlier authentication Docsity.com 2: Application Layer 50 Electronic Mail Three major components:  user agents  mail servers  simple mail transfer protocol: SMTP User Agent  a.k.a. “mail reader”  composing, editing, reading mail messages  e.g., Eudora, Outlook, elm, Mozilla Thunderbird  outgoing, incoming messages stored on server user mailbox outgoing message queue mail server user agent user agent user agent mail server user agent user agent mail server user agent SMTP SMTP SMTP Docsity.com 2: Application Layer 51 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  client: sending mail server  “server”: receiving mail server mail server user agent user agent user agent mail server user agent user agent mail server user agent SMTP SMTP SMTP Docsity.com 2: Application Layer 52 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  commands: ASCII text  response: status code and phrase  messages must be in 7-bit ASCII Docsity.com 2: Application Layer 55 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) Docsity.com 2: Application Layer 56 SMTP: final words  SMTP uses persistent connections  SMTP requires message (header & body) to be in 7- bit ASCII  SMTP server uses CRLF.CRLF to determine end of message Comparison with HTTP:  HTTP: pull  SMTP: push  both have ASCII command/response interaction, status codes  HTTP: each object encapsulated in its own response msg  SMTP: multiple objects sent in multipart msg Docsity.com 2: Application Layer 57 Mail message format SMTP: protocol for exchanging email msgs RFC 822: standard for text message format:  header lines, e.g.,  To:  From:  Subject: different from SMTP commands!  body  the “message”, ASCII characters only header body blank line Docsity.com 2: Application Layer 60 POP3 protocol authorization phase  client commands:  user: declare username  pass: password  server responses  +OK  -ERR transaction phase, client:  list: list message numbers  retr: retrieve message by number  dele: delete  quit 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 Docsity.com 2: Application Layer 61 POP3 (more) and IMAP More about POP3  Previous example uses “download and delete” mode.  Bob cannot re-read e- mail if he changes client  “Download-and-keep”: copies of messages on different clients  POP3 is stateless across sessions IMAP  Keep all messages in one place: the server  Allows user to organize messages in folders  IMAP keeps user state across sessions:  names of folders and mappings between message IDs and folder name Docsity.com 2: Application Layer 62 Chapter 2: Application layer  2.1 Principles of network applications  2.2 Web and HTTP  2.3 FTP  2.4 Electronic Mail  SMTP, POP3, IMAP  2.5 DNS  2.6 P2P applications  2.7 Socket programming with TCP  2.8 Socket programming with UDP  2.9 Building a Web server Docsity.com 2: Application Layer 65 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 Distributed, Hierarchical Database Client wants IP for www.amazon.com; 1st approx:  client queries a 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 Docsity.com 2: Application Layer 66 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 13 root name servers worldwide b USC-ISI Marina del Rey, CA l ICANN Los Angeles, CA e NASA Mt View, CA f Internet Software C. Palo Alto, CA (and 36 other locations) i Autonomica, Stockholm (plus 28 other locations) k RIPE London (also 16 other locations) m WIDE Tokyo (also Seoul, Paris, SF) a Verisign, Dulles, VA c Cogent, Herndon, VA (also LA) d U Maryland College Park, MD g US DoD Vienna, VA h ARL Aberdeen, MD j Verisign, ( 21 locations) Docsity.com 2: Application Layer 67 TLD and Authoritative Servers  Top-level domain (TLD) servers:  responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp.  Network Solutions maintains servers for com TLD  Educause for edu TLD Authoritative DNS servers:  organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers (e.g., Web, mail).  can be maintained by organization or service provider Docsity.com 2: Application Layer 70 requesting host cis.poly.edu gaia.cs.umass.edu root DNS server local DNS server dns.poly.edu 1 2 4 5 6 authoritative DNS server dns.cs.umass.edu 7 8 TLD DNS server 3 recursive query:  puts burden of name resolution on contacted name server  heavy load? DNS name resolution example Docsity.com 2: Application Layer 71 DNS: caching and updating records  once (any) name server learns mapping, it caches mapping  cache entries timeout (disappear) after some time  TLD servers typically cached in local name servers • Thus root name servers not often visited  update/notify mechanisms under design by IETF  RFC 2136  http://www.ietf.org/html.charters/dnsind-charter.html Docsity.com 2: Application Layer 72 DNS records DNS: distributed db storing resource records (RR)  Type=NS  name is domain (e.g. foo.com)  value is hostname of authoritative name server for this domain RR format: (name, value, type, ttl)  Type=A  name is hostname  value is IP address  Type=CNAME  name is alias name for some “canonical” (the real) name www.ibm.com is really servereast.backup2.ibm.com  value is canonical name  Type=MX  value is name of mailserver associated with name Docsity.com 2: Application Layer 75 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  How do people get IP address of your Web site? Docsity.com 2: Application Layer 76 Chapter 2: Application layer  2.1 Principles of network applications  app architectures  app requirements  2.2 Web and HTTP  2.4 Electronic Mail  SMTP, POP3, IMAP  2.5 DNS  2.6 P2P applications  2.7 Socket programming with TCP  2.8 Socket programming with UDP Docsity.com 2: Application Layer 77 Pure P2P architecture  no always-on server  arbitrary end systems directly communicate  peers are intermittently connected and change IP addresses  Three topics:  File distribution  Searching for information  Case Study: Skype peer-peer Docsity.com 2: Application Layer 80 File distribution time: P2P us u2 d1 d2 u1 uN dN Server Network (with abundant bandwidth) F  server must send one copy: F/us time  client i takes F/di time to download  NF bits must be downloaded (aggregate)  fastest possible upload rate: us + Sui dP2P = max { F/us, F/min(di) , NF/(us + Sui) } i Docsity.com 2: Application Layer 81 0 0.5 1 1.5 2 2.5 3 3.5 0 5 10 15 20 25 30 35 N M in im u m D is tr ib u ti o n T im e P2P Client-Server Server-client vs. P2P: example Client upload rate = u, F/u = 1 hour, us = 10u, dmin ≥ us Docsity.com 2: Application Layer 82 File distribution: BitTorrent tracker: tracks peers participating in torrent torrent: group of peers exchanging chunks of a file obtain list of peers trading chunks peer  P2P file distribution Docsity.com 2: Application Layer 85 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 With higher upload rate, can find better trading partners & get file faster! Docsity.com 2: Application Layer 86 P2P: searching for information File sharing (eg e-mule)  Index dynamically tracks the locations of files that peers share.  Peers need to tell index what they have.  Peers search index to determine where files can be found Instant messaging  Index maps user names to locations.  When user starts IM application, it needs to inform index of its location  Peers search index to determine IP address of user. Index in P2P system: maps information to peer location (location = IP address & port number) . Docsity.com 2: Application Layer 87 P2P: centralized index original “Napster” design 1) when peer connects, it informs central server:  IP address  content 2) Alice queries for “Hey Jude” 3) Alice requests file from Bob centralized directory server peers Alice Bob 1 1 1 1 2 3 Docsity.com 2: Application Layer 90 Query flooding Query QueryHit Query QueryHit File transfer: HTTP  Query message sent over existing TCP connections  peers forward Query message  QueryHit sent over reverse path Scalability: limited scope flooding Docsity.com 2: Application Layer 91 Gnutella: Peer joining 1. joining peer Alice must find another peer in Gnutella network: use list of candidate peers 2. Alice sequentially attempts TCP connections with candidate peers until connection setup with Bob 3. Flooding: Alice sends Ping message to Bob; Bob forwards Ping message to his overlay neighbors (who then forward to their neighbors….)  peers receiving Ping message respond to Alice with Pong message 4. Alice receives many Pong messages, and can then setup additional TCP connections Peer leaving: see homework problem! Docsity.com 2: Application Layer 92 Hierarchical Overlay  between centralized index, query flooding approaches  each peer is either a super node or assigned to a super node  TCP connection between peer and its super node.  TCP connections between some pairs of super nodes.  Super node tracks content in its children ordinary peer group-leader peer neighoring relationships in overlay network Docsity.com 2: Application Layer 95 Chapter 2: Application layer  2.1 Principles of network applications  2.2 Web and HTTP  2.3 FTP  2.4 Electronic Mail  SMTP, POP3, IMAP  2.5 DNS  2.6 P2P applications  2.7 Socket programming with TCP  2.8 Socket programming with UDP Docsity.com 2: Application Layer 96 Socket programming Socket API  introduced in BSD4.1 UNIX, 1981  explicitly created, used, released by apps  client/server paradigm  two types of transport service via socket API:  unreliable datagram  reliable, byte stream- oriented a host-local, application-created, OS-controlled interface (a “door”) into which application process can both send and receive messages to/from another application process socket Goal: learn how to build client/server application that communicate using sockets Docsity.com 2: Application Layer 97 Socket-programming using TCP Socket: a door between application process and end- end-transport protocol (UCP or TCP) TCP service: reliable transfer of bytes from one process to another process TCP with buffers, variables socket controlled by application developer controlled by operating system host or server process TCP with buffers, variables socket controlled by application developer controlled by operating system host or server internet Docsity.com