Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Distributed firewall wikipedia , lookup
Server Message Block wikipedia , lookup
Cracking of wireless networks wikipedia , lookup
Dynamic Host Configuration Protocol wikipedia , lookup
Deep packet inspection wikipedia , lookup
Internet protocol suite wikipedia , lookup
Remote Desktop Services wikipedia , lookup
Recursive InterNetwork Architecture (RINA) wikipedia , lookup
1DT066 Distributed Information Systems Application Layer 1 2: Application Layer Chapter 2 2: Application Layer Q: What’s your Most Favorite Internet Application? 2 SOME NETWORK APPS voice over IP real-time video conferencing 2: Application Layer e-mail web instant messaging remote login P2P file sharing multi-user network games streaming stored video clips 3 CHAPTER 2: APPLICATION LAYER 2.1 Principles of network applications Application architecture Application requirements 2: Application Layer 2.2 Web and HTTP 2.3 DNS 2.4 P2P applications 4 APPLICATION ARCHITECTURES Client-server Peer-to-peer (P2P) Hybrid of client-server and P2P 2: Application Layer 5 CLIENT-SERVER ARCHITECTURE server: always-on host permanent IP address server farms for scaling 2: Application Layer clients: client/server communicate with server may be intermittently connected may have dynamic IP addresses do not communicate directly with each other 6 PURE P2P ARCHITECTURE no always-on server arbitrary end systems directly communicate peer-peer peers are intermittently connected and change IP addresses 2: Application Layer Highly scalable but difficult to manage 7 HYBRID OF CLIENT-SERVER AND P2P Instant messaging chatting between two users is P2P centralized service: client presence detection/location 2: Application Layer user registers its IP address with central server when it comes online user contacts central server to find IP addresses of buddies 8 PROCESSES COMMUNICATING process controlled by app developer 2: Application Layer process sends/receives messages to/from its socket API: (1) choice of transport protocol; (2) ability to fix a few parameters (lots more on this later) host or server host or server process socket socket TCP with buffers, variables Internet TCP with buffers, variables controlled by OS 9 ADDRESSING PROCESSES to receive messages, process must have identifier host device has unique 32bit IP address Q: does IP address of host suffice for identifying the process? 2: Application Layer 10 ADDRESSING PROCESSES identifier includes both IP address and port numbers associated with process on host. Example port numbers: 2: Application Layer to receive messages, process must have identifier host device has unique 32-bit IP address Q: does IP address of host on which process runs suffice for identifying the process? HTTP server: 80 Mail server: 25 A: No, many processes can be running on same host 11 WHAT TRANSPORT SERVICE DOES AN APP NEED? 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 12 integrity, … 2: Application Layer 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” TRANSPORT SERVICE REQUIREMENTS OF COMMON APPS Application stored audio/video interactive games instant messaging no loss Time Sensitive (yes / no) no elastic elastic elastic audio: 5kbps-1Mbps video:10kbps-5Mbps same as above few kbps up elastic 2: Application Layer file transfer e-mail Web documents real-time audio/video Data loss (no loss / loss-tolerant) Throughput 13 TRANSPORT SERVICE REQUIREMENTS OF COMMON APPS Throughput Time Sensitive file transfer e-mail Web documents real-time audio/video no loss no loss no loss loss-tolerant no no no yes, 100’s msec stored audio/video interactive games instant messaging loss-tolerant loss-tolerant no loss elastic elastic elastic audio: 5kbps-1Mbps video:10kbps-5Mbps same as above few kbps up elastic 2: Application Layer Data loss Application yes, few secs yes, 100’s msec yes and no 14 TRANSPORT LAYER PROTOCOLS 2: Application Layer TCP VS. UDP ? 15 INTERNET TRANSPORT PROTOCOLS SERVICES TCP service: unreliable data transfer between sending and receiving process does not provide: connection setup, reliability, flow control, congestion control, timing, throughput guarantee, or security 2: Application Layer connection-oriented: setup required between client and server processes reliable transport between sending and receiving process flow control: sender won’t overwhelm receiver congestion control: throttle sender when network overloaded does not provide: timing, minimum throughput guarantees, security UDP service: Q: why bother? Why is there a UDP? 16 INTERNET APPS: APPLICATION, TRANSPORT PROTOCOLS Application Transport protocol (TCP / UDP) 2: Application Layer e-mail remote terminal access Web file transfer streaming multimedia Application layer protocol Internet telephony 17 INTERNET APPS: APPLICATION, TRANSPORT PROTOCOLS Application Internet telephony Underlying transport protocol SMTP [RFC 2821] Telnet [RFC 854] HTTP [RFC 2616] FTP [RFC 959] HTTP (eg Youtube), RTP [RFC 1889] SIP, RTP, proprietary (e.g., Skype) TCP TCP TCP TCP TCP or UDP 2: Application Layer e-mail remote terminal access Web file transfer streaming multimedia Application layer protocol typically UDP 18 CHAPTER 2: APPLICATION LAYER 2.1 Principles of network applications app architectures app requirements 2.2 Web and HTTP 2.3 DNS 2.4 P2P applications 2: Application Layer 19 WEB AND HTTP host name path name 2: Application Layer 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 20 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 2: Application Layer Server running Apache Web server Mac running Navigator 21 USER-SERVER STATE: COOKIES HTTP is “stateless” server maintains no information about past client requests 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 2: Application Layer Many major Web sites use cookies Four components: 22 COOKIES: KEEPING “STATE” (CONT.) client ebay 8734 ebay 8734 amazon 1678 usual http request msg usual http response Set-cookie: 1678 usual http request msg cookie: 1678 one week later: ebay 8734 amazon 1678 usual http response msg usual http request msg cookie: 1678 usual http response msg Amazon server creates ID 1678 for user create entry cookiespecific action access access cookiespectific action 2: Application Layer cookie file server backend database 23 WEB CACHES (PROXY SERVER) Goal: satisfy client request without involving origin server origin server object in cache: cache returns object else cache requests object from origin server, then returns object to client client client Proxy server 2: Application Layer user sets browser: Web accesses via cache browser sends all HTTP requests to cache origin server 24 MORE ABOUT WEB CACHING Why Web caching? 2: Application Layer cache acts as both client and server typically cache is installed by ISP (university, company, residential ISP) 25 MORE ABOUT WEB CACHING 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) 2: Application Layer cache acts as both client and server typically cache is installed by ISP (university, company, residential ISP) 26 CHAPTER 2: APPLICATION LAYER 2.1 Principles of network applications 2.2 Web and HTTP 2.3 DNS 2.4 P2P applications 2: Application Layer 27 DNS: DOMAIN NAME SYSTEM People: many identifiers: SSN, name, passport # Internet hosts, routers: IP address (32 bit) used for addressing datagrams “name”, e.g., ww.yahoo.com - used by humans distributed database implemented in hierarchy of many name servers application-layer protocol host, routers, name servers to communicate to resolve names (address/name translation) 2: Application Layer Domain Name System: 28 DNS Canonical, alias names mail server aliasing load distribution replicated Web servers: set of IP addresses for one canonical name Why not centralize DNS? single point of failure traffic volume distant centralized database maintenance 2: Application Layer DNS services hostname to IP address translation host aliasing doesn’t scale! 29 DISTRIBUTED, HIERARCHICAL DATABASE Root DNS Servers yahoo.com amazon.com DNS servers DNS servers org DNS servers pbs.org DNS servers edu DNS servers 2: Application Layer com DNS servers poly.edu umass.edu DNS serversDNS servers 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 30 DNS NAME root DNS server RESOLUTION EXAMPLE 2 Host at cis.poly.edu wants IP address for gaia.cs.umass.edu contacted server replies with name of server to contact “I don’t know this name, but ask this server” TLD DNS server 4 5 local DNS server dns.poly.edu 1 8 requesting host 7 6 2: Application Layer iterated query: 3 authoritative DNS server dns.cs.umass.edu cis.poly.edu gaia.cs.umass.edu 32 DNS NAME root DNS server RESOLUTION EXAMPLE recursive query: 2 resolution on contacted name server heavy load? 7 6 2: Application Layer puts burden of name 3 TLD DNS server local DNS server dns.poly.edu 1 5 4 8 requesting host authoritative DNS server dns.cs.umass.edu cis.poly.edu gaia.cs.umass.edu 33 CHAPTER 2: APPLICATION LAYER 2.1 Principles of network applications app architectures app requirements 2.2 Web and HTTP 2.4 Electronic Mail 2.6 P2P applications 2: Application Layer SMTP, POP3, IMAP 2.5 DNS 34 PURE P2P ARCHITECTURE no always-on server arbitrary end systems directly communicate peer-peer peers are intermittently connected and change IP addresses 2: Application Layer Three topics: File distribution Searching for information Case Study: Skype 35 FILE DISTRIBUTION 2: Application Layer Server-Client vs P2P ? 36 FILE DISTRIBUTION: SERVER-CLIENT VS P2P Question : How much time to distribute file from one server to N peers? Server us File, size F dN uN u1 d1 u2 2: Application Layer us: server upload bandwidth ui: peer i upload bandwidth d2 di: peer i download bandwidth Network (with abundant bandwidth) 37 FILE DISTRIBUTION TIME: SERVER-CLIENT Server server sequentially sends N copies: NF/us time client i takes F/di time to download us dN u1 d1 u2 d2 2: Application Layer F Network (with abundant bandwidth) uN Time to distribute F to N clients using = dcs = max { NF/us, F/min(di) } i client/server approach increases linearly in N (for large N) 38 FILE DISTRIBUTION TIME: P2P Server server must send one F u1 d1 u2 d2 copy: F/us time us client i takes F/di time to Network (with dN download abundant bandwidth) uN NF bits must be downloaded (aggregate) fastest possible upload rate: us + Sui i 2: Application Layer dP2P = max { F/us, F/min(di) , NF/(us + Sui) } 39 Server-client vs. P2P: example Client upload rate = u, F/u = 1 hour, us = 10u, dmin ≥ us 40 P2P Client-Server 3 2: Application Layer Minimum Distribution Time 3.5 2.5 2 1.5 1 0.5 0 0 5 10 15 20 N 25 30 35 CHAPTER 2: SUMMARY our study of network apps now complete! application architectures application service requirements: client-server P2P hybrid reliability, bandwidth, delay Internet transport service model 2: Application Layer specific protocols: HTTP FTP SMTP, POP, IMAP DNS P2P: BitTorrent, Skype connection-oriented, reliable: TCP unreliable, datagrams: UDP 45