Application Layer. voice over IP real-time video conferencing cloud computing
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1 Application Layer web instant messaging remote login P2P file sharing multi-user network games streaming stored video (YouTube) voice over IP real-time video conferencing cloud computing Application 2-1
2 Creating a network app Write programs that run on (different) end systems communicate over network e.g., web server - browser application transport network data link physical No need to write software for network-core devices application transport network data link physical application transport network data link physical Application 2-2
3 Client-server architecture client/server server: always-on permanent IP address server farms for scaling clients: communicate with server not always connected dynamic IP addresses do not communicate directly with each other Application 2-3
4 Pure P2P architecture no always-on server end systems directly communicate peers are not always connected and change IP addresses peer-peer Application 2-4
5 Hybrid of client-server and P2P Skype voice-over-ip P2P application server: finding address of remote party client-client speech: direct Application 2-5
6 Processes communicating process: program running within a host. processes communicate by exchanging messages client process: process that initiates communication server process: process that waits to be contacted Application 2-6
7 Sockets process sends/receives messages to/from its socket socket analogous to door host or server process controlled by app developer host or server process socket socket TCP with buffers, variables Internet TCP with buffers, variables controlled by OS Application 2-7
8 Addressing processes a process must have an identifier host device has unique IP address identifier includes both IP address and port numbers associated with process on host. example port numbers: HTTP server: 80 Mail server: 25 to send HTTP message to gaia.cs.umass.edu web server: IP address: Port number: 80 Application 2-8
9 App-layer protocol defines types of messages e.g., request, response message syntax: what fields in messages message semantics meaning of information in fields rules for when and how processes send & respond to messages public-domain protocols: defined in RFCs e.g., HTTP, SMTP proprietary protocols: e.g., Skype Application 2-9
10 What transport service does an app need? Data loss some apps (e.g., audio) can tolerate some loss other apps (e.g., file transfer) require 100% reliable data transfer Timing some apps (e.g., Internet telephony, interactive games) require low delay 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 Application 2-10
11 Internet transport protocols services TCP service: connection-oriented: setup required between client and server processes reliable transport flow control congestion control does not provide: timing, minimum throughput guarantees UDP service: unreliable data transfer between sending and receiving process Application 2-11
12 Web and HTTP web page consists of objects object can be HTML file, JPEG image, audio file, web page consists of base HTML-file which includes several referenced objects each object is addressable by a URL example URL: host name path name Application 2-12
13 HTTP overview HTTP: hypertext transfer protocol Web s application layer protocol client/server model client: browser server: Web server PC running Explorer Server running Apache Web server Mac running Chrome Application 2-13
14 HTTP overview Uses TCP: client initiates TCP connection (creates socket) to server, port 80 server accepts TCP connection from client HTTP messages (applicationlayer protocol 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 protocols that maintain state are complex! past history (state) must be maintained if server/client crashes, their views of state may be inconsistent Application 2-14
15 HTTP connections non-persistent HTTP at most one object sent over TCP connection. persistent HTTP multiple objects can be sent over single TCP connection between client, server. Application 2-15
16 HTTP request message two types of HTTP messages: request, response HTTP request message: ASCII (human-readable format) request line (GET, POST, HEAD commands) header lines carriage return, line feed at start of line indicates end of header lines carriage return character line-feed character GET /index.html HTTP/1.1\r\n Host: www-net.cs.umass.edu\r\n User-Agent: Firefox/3.6.10\r\n Accept: text/html,application/xhtml+xml\r\n Accept-Language: en-us,en;q=0.5\r\n Accept-Encoding: gzip,deflate\r\n Accept-Charset: ISO ,utf-8;q=0.7\r\n Keep-Alive: 115\r\n Connection: keep-alive\r\n \r\n Application 2-16
17 HTTP response message status line (protocol status code status phrase) header lines data, e.g., requested HTML file HTTP/ OK\r\n Date: Sun, 26 Sep :09:20 GMT\r\n Server: Apache/ (CentOS)\r\n Last-Modified: Tue, 30 Oct :00:02 GMT\r\n ETag: "17dc6-a5c-bf716880"\r\n Accept-Ranges: bytes\r\n Content-Length: 2652\r\n Keep-Alive: timeout=10, max=100\r\n Connection: Keep-Alive\r\n Content-Type: text/html; charset=iso \r\n \r\n data data data data data... Application 2-17
18 HTTP response status codes status code appears in 1st line in server->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 2-18
19 User-server state: cookies many 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 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 Application 2-19
20 Cookies: keeping state (cont.) client ebay 8734 cookie file ebay 8734 amazon 1678 one week later: ebay 8734 amazon 1678 usual http request msg usual http response Set-cookie: 1678 usual http request msg cookie: 1678 usual http response msg usual http request msg cookie: 1678 usual http response msg server Amazon server creates ID 1678 for user create entry cookiespecific action cookiespecific action access access backend database Application 2-20
21 Web caches (proxy server) Goal: satisfy client request without involving origin server user sets browser: Web accesses via cache browser sends all HTTP requests to cache client Proxy server origin server object in cache: cache returns object else cache requests object from origin server, then returns object to client client origin server Application 2-21
22 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 Application 2-22
23 Conditional GET Goal: don t send object if cache has up-to-date cached version cache: specify date of cached copy in HTTP request server: response contains no object if cached copy is up-to-date: cache HTTP request msg If-modified-since: <date> HTTP response HTTP/ Not Modified HTTP request msg If-modified-since: <date> HTTP response HTTP/ OK <data> server object not modified before <date> object modified after <date> Application 2-23
24 outgoing message queue Three major components: user agents mail servers simple mail transfer protocol: SMTP User Agent mail reader composing, editing, reading mail messages e.g., Outlook, elm, Mozilla Thunderbird, iphone mail client outgoing, incoming messages stored on server mail server SMTP mail server user agent user agent SMTP SMTP user agent mail server user mailbox user agent user agent user agent Application 2-24
25 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 messages client: sending mail server mail server SMTP mail server user agent SMTP SMTP mail server user agent user agent user agent server : receiving mail server user agent user agent Application 2-25
26 SMTP uses TCP to reliably transfer 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 Application 2-26
27 Mail message format SMTP: protocol for exchanging msgs RFC 822: standard for text message format: header lines, e.g., To: From: Subject: body the message header body blank line Application 2-27
28 Mail access protocols user agent SMTP SMTP access protocol user agent sender s mail server receiver s mail server SMTP: delivery/storage to receiver s server mail access protocol: retrieval from server POP: Post Office Protocol authorization (agent <-->server) and download IMAP: Internet Mail Access Protocol more features (more complex) manipulation of stored msgs on server HTTP: gmail, Hotmail, Yahoo! Mail, etc. Application 2-28
29 DNS DNS services hostname to IP address translation 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 doesn t scale! Application 2-29
30 Distributed, Hierarchical Database Root DNS Servers com DNS servers org DNS servers edu DNS servers yahoo.com DNS servers amazon.com DNS servers pbs.org DNS servers poly.edu umass.edu DNS serversdns servers client wants IP for 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 Application 2-30
31 DNS: Root name servers e NASA Mt View, CA f Internet Software C. Palo Alto, CA (and 36 other locations) 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) k RIPE London (also 16 other locations) i Autonomica, Stockholm (plus 28 other locations) m WIDE Tokyo (also Seoul, Paris, SF) b USC-ISI Marina del Rey, CA l ICANN Los Angeles, CA 13 root name servers worldwide Application 2-31
32 TLD and 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 Authoritative DNS servers: organization s DNS servers, hostname to IP mappings for organization s servers can be maintained by organization or service provider Application 2-32
33 DNS name resolution example root DNS server host at cis.poly.edu wants IP address for gaia.cs.umass.edu TLD DNS server iterated query: contacted server replies with name of server to contact I don t know this name, but ask this server local DNS server dns.poly.edu 1 8 requesting host cis.poly.edu 7 6 authoritative DNS server dns.cs.umass.edu gaia.cs.umass.edu Application 2-33
34 DNS name resolution example root DNS server recursive query: 2 3 puts burden of name resolution on contacted name server 7 6 TLD DNS server local DNS server dns.poly.edu requesting host cis.poly.edu authoritative DNS server dns.cs.umass.edu gaia.cs.umass.edu Application 2-34
35 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 Application 2-35
36 DNS protocol, messages DNS protocol : query and reply messages, both with same message format msg header identification: 16 bit # for query, reply to query uses same # flags: query or reply recursion desired recursion available reply is authoritative Application 2-36
37 Pure P2P architecture no always-on server arbitrary end systems directly communicate peers are intermittently connected and change IP addresses peer-peer Application 2-37
38 File Distribution: Server-Client vs P2P Question : How much time to distribute file from one server to N peers? File, size F Server u 1 d 1 u 2 u d 2 s u s : server upload bandwidth u i : peer i upload bandwidth d i : peer i download bandwidth d N u N Network (with abundant bandwidth) Application 2-38
39 Server-client vs. P2P: example Minimum Distribution Time P2P Client-Server N Application 2-39
40 File distribution: BitTorrent P2P file distribution tracker: tracks peers participating in torrent torrent: group of peers exchanging chunks of a file obtain list of peers trading chunks peer Application 2-40
41 BitTorrent file divided into 256KB chunks. peer joining torrent: has no chunks, but will accumulate them over time registers with tracker to get list of peers, connects to subset of peers ( neighbors ) while downloading, peer uploads chunks to other peers. peers may come and go once peer has entire file, it may (selfishly) leave or (altruistically) remain Application 2-41
42 BitTorrent Pulling Chunks at any given time, different peers have different subsets of file chunks periodically, a peer (Alice) asks each neighbor for list of chunks that they have. Alice sends requests for her missing chunks rarest first Sending Chunks: tit-for-tat Alice sends chunks to four neighbors currently sending her chunks at the highest rate re-evaluate top 4 every 10 secs every 30 secs: randomly select another peer, starts sending chunks newly chosen peer may join top 4 Application 2-42
43 P2P example: Skype pairs of users communicate proprietary application-layer protocol hierarchical overlay with SNs Index maps usernames to IP addresses; distributed over SNs Skype login server Skype clients (SC) Supernode (SN) Application 2-43
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