3rd Edition: Chapter 2 - UMass Amherst

3rd Edition: Chapter 2 - UMass Amherst

Chapter 2: outline 2.1 principles of network applications 2.2 Web and HTTP 2.3 FTP 2.4 electronic mail SMTP, POP3, IMAP 2.6 P2P applications 2.7 socket programming with UDP 2.8 socket programming with TCP 2.5 DNS Application Layer 2-1 Chapter 2: application layer our goals: conceptual, implementation aspects of network application protocols transport-layer service models client-server paradigm peer-to-peer paradigm

learn about protocols by examining popular application-level protocols HTTP FTP SMTP / POP3 / IMAP DNS programming network applications socket API Application Layer 2-2 Some network apps

P2P file sharing multi-user network games streaming stored video (YouTube, Hulu, Netflix) e-mail web text messaging remote login voice over IP (e.g., Skype) real-time video conferencing cloud computing Application Layer 2-3 Creating a network app application transport

network data link physical 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 Layer 2-4 Chapter 2: outline 2.1 principles of

network applications app architectures app requirements 2.2 Web and HTTP 2.3 FTP 2.4 electronic mail 2.6 P2P applications 2.7 socket programming with UDP 2.8 socket programming with TCP SMTP, POP3, IMAP 2.5 DNS Application Layer 2-5 Application architectures possible structure of applications: client-server peer-to-peer (P2P) hybrid of client-server and P2P Application Layer 2-6 Client-server architecture server:

always-on host permanent IP address server farms for scaling clients: client/server communicate with server may be intermittently connected may have dynamic IP addresses do not communicate directly with each other Application Layer 2-7 Pure P2P architecture no always-on server arbitrary end systems directly communicate peers request service from other peers,

provide service in return to other peers peer-peer highly scalable 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 Hybrid client-server/P2P skype voice-over-IP P2P application centralized server: finding address of remote party client-client connection: direct (not through server) text 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 Application Layer 2-9 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 clients, servers client process: process that initiates communication server process: process that waits to be contacted aside: applications with P2P architectures have client processes & server processes Application Layer 2-10

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 process socket application process transport transport network network link physical Internet link controlled by

app developer controlled by OS physical Application Layer 2-11 Addressing processes to receive messages, process must have identifier host device has unique 32-bit IP address Q: does IP address of host on which process A:suffice runs for no, many identifying thecan be processes process? running on same host

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: 128.119.245.12 port number: 80 more shortly Application Layer 2-12 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 Application Layer 2-13 What transport service does an app need? data integrity some apps (e.g., file transfer, web transactions) require 100% reliable data transfer other apps (e.g., audio) can tolerate some loss 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, Application Layer 2-14 Transport service requirements: common apps application data loss throughput file transfer e-mail Web documents real-time audio/video no loss no loss no loss

loss-tolerant stored audio/video interactive games text messaging loss-tolerant loss-tolerant no loss elastic no elastic no elastic no audio: 5kbps-1Mbps yes, 100s msec video:10kbps-5Mbps same as above yes, few secs few kbps up yes, 100s msec elastic yes and no time sensitive Application Layer 2-15 Internet transport protocols services TCP service: UDP service:

reliable transport between sending and receiving process flow control: sender wont overwhelm receiver congestion control: throttle sender when network overloaded does not provide: timing, minimum throughput guarantee, security connection-oriented: setup required between client and server processes unreliable data transfer between sending and receiving process does not provide: reliability, flow control, congestion control, timing, throughput

guarantee, security, orconnection setup, Q: why bother? Why is there a UDP? Application Layer 2-16 Internet apps: application, transport protocols application e-mail remote terminal access Web file transfer streaming multimedia Internet telephony application layer protocol underlying transport protocol SMTP [RFC 2821] Telnet [RFC 854] HTTP [RFC 2616] FTP [RFC 959] HTTP (e.g., YouTube), RTP [RFC 1889] SIP, RTP, proprietary (e.g., Skype) TCP TCP TCP

TCP TCP or UDP TCP or UDP Application Layer 2-17 Chapter 2: outline 2.1 principles of network applications app architectures app requirements 2.2 Web and HTTP 2.3 FTP 2.4 electronic mail 2.6 P2P applications 2.7 socket programming with UDP 2.8 socket programming with TCP 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, www.someschool.edu/someDept/pic.gif e.g., host name path name Application Layer 2-19 HTTP overview HTTP: hypertext transfer protocol Webs application layer protocol client/server model client: browser that requests, receives, (using HTTP protocol) and displays Web objects server: Web server sends (using HTTP

protocol) objects in response to requests HT TP PC running Firefox browser req ues t HT TP res pon se t es u eq server r P se T n running po HT s e r Apache Web

TP T server H iphone running Safari browser Application Layer 2-20 HTTP overview (continued) uses TCP: client initiates TCP connection (creates socket) to server, port 80 server accepts TCP connection from client HTTP messages (application-layer 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 aside protocols that maintain state are complex! past history (state) must be maintained if server/client crashes, their views of state may be inconsistent, must be reconciled Application Layer 2-21 HTTP connections non-persistent HTTP at most one object sent over TCP connection connection then closed downloading multiple objects requires multiple connections persistent HTTP multiple objects can be sent over single

TCP connection between client, server Application Layer 2-22 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.i time ndex (contains text, references to 10 jpeg images) 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 Application Layer 2-23 Nonpersistent HTTP (cont.) 5. HTTP client receives time 4. HTTP server closes TCP connection. 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 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: one RTT to initiate TCP connection one RTT for HTTP request and first few bytes of

HTTP response to return file transmission time non-persistent HTTP response time = 2RTT+ file transmission time initiate TCP connection RTT request file time to transmit file RTT file received time time Application Layer 2-25 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 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 headerAccept-Language: en-us,en;q=0.5\r\n linesAccept-Encoding: gzip,deflate\r\n Accept-Charset: ISO-8859-1,utf-8;q=0.7\r\n Keep-Alive: 115\r\n carriage return, Connection: keep-alive\r\n line feed at start \r\n of line indicates end of header lines Application Layer 2-27 HTTP request message: general format method sp

URL header field name sp value version cr lf header field name cr value cr lf request line header lines ~ ~ ~ ~ ~ ~ cr

lf 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: 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 Application Layer 2-30 HTTP response message status line (protocol status code

status phrase) header lines data, e.g., requested HTML file HTTP/1.1 200 OK\r\n 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 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-8859-1\ r\n \r\n data data data data data ... 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 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! use Wireshark to look at captured HTTP request/response) Application Layer 2-33

User-server state: cookies example: many Web sites use Susan always access cookies Internet from PC four components: visits specific e1) cookie header line of commerce site for first HTTP response time message when initial HTTP 2) cookie header line requests arrives at in next HTTP request site, site creates: message unique ID 3) cookie file kept on users host, entry in backend managed by users database for ID browser 4) back-end database at Web site Application Layer 2-34 Cookies: keeping state (cont.) client ebay 8734 cookie file ebay 8734

amazon 1678 server usual http request msg usual http response set-cookie: 1678 usual http request msg cookie: 1678 usual http response msg Amazon server creates ID 1678 for user create backend entry database cookiespecific action one week later: ebay 8734 amazon 1678 access access usual http request msg cookie: 1678 usual http response msg cookiespecific action

Application Layer 2-35 Cookies (continued) what cookies can be used for: authorization shopping carts recommendations user session state (Web e-mail) aside cookies and privacy: cookies permit sites to learn a lot about you you may supply name and e-mail to sites 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 user sets server 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 proxy HT st TP e u req server req ues HT P se client TP TT n t o H p origin

res res pon P T server se HT t es u req e P ns T o T p H es r TP T H client origin server Application Layer 2-37 More about Web caching cache acts as

both client and server server for original requesting client client to origin server typically cache is installed by ISP (university, company, residential ISP) why Web caching? reduce response time for client request reduce traffic on an institutions access link Internet dense with caches: enables poor content providers to effectively deliver content (so too does P2P file sharing) Application Layer 2-38 Caching example: assumptions: avg object size: 1Mbits avg request rate from browsers to origin servers:15/ sec avg data rate to browsers:

150 Mbps RTT from institutional router to any origin server: 2 sec access link rate: 15 Mbps problem! consequences: LAN utilization: 15% access link utilization = 99% total delay = Internet delay + origin servers public Internet 15 Mbps access link institutional network 100 Mbps LAN access delay + LAN delay = 2 sec + minutes + usecs Application Layer 2-39 Caching example: fatter access link assumptions: avg object size: 1Mbits avg request rate from

browsers to origin servers:15/ sec avg data rate to browsers: 150 Mbps RTT from institutional router to any origin server: 2100 sec Mbps access link rate: 15 Mbps consequences: 15% public Internet origin servers 15 Mbps 100 Mbps access link institutional network LAN utilization: 15% access link utilization = 99% total delay = Internet delay + 1 Gbps LAN access delaymsecs + LAN delay = 2 sec + minutes + usecs

Cost: increased access link speed (not cheap!) Application Layer 2-40 Caching example: install local cache assumptions: avg object size: 1Mbits avg request rate from browsers to origin servers:15/ sec avg data rate to browsers: 150 Mbps RTT from institutional router to any origin server: 2 sec access link rate: 15 Mbps consequences:? LAN utilization: 15% ? access link utilization = 100% How to compute link total delay = Internet delay + utilization, delay? access delay + LAN delay = 2 sec + minutes + usecs origin servers

public Internet 15 Mbps access link institutional network 1 Gbps LAN local web cache Cost: web cache (cheap!) Application Layer 2-41 Caching example: install local cache Calculating access link utilization, delay with cache: suppose origin servers cache hit rate is 0.4 40% requests satisfied at cache, 60% requests satisfied at origin public Internet access link utilization: 60% of requests use access link

data rate to browsers over access link = 0.6*15 Mbps = 9 Mbps utilization = 9/15 = .6 total delay = 0.6 * (delay from origin servers) +0.4 * (delay when satisfied at cache) = 0.6 (2.01) + 0.4 (~msecs) = ~ 1.2 secs less than with 100 Mbps link (and cheaper too!) 15 Mbps access link institutional network 1 Gbps LAN local web cache Application Layer 2-42 Conditional GET server client

Goal: dont send object if cache has upto-date cached version HTTP request msg no object transmission delay lower link utilization If-modified-since: cache: specify date of cached copy in HTTP request HTTP/1.0 304 Not Modified HTTP response object not modified before If-modified-since: server: response contains no object if cached copy is up-todate: HTTP/1.0 304 Not

Modified HTTP request msg If-modified-since: HTTP response HTTP/1.0 200 OK object modified after Application Layer 2-43 Chapter 2: outline 2.1 principles of network applications app architectures app requirements 2.2 Web and HTTP 2.3 FTP 2.4 electronic mail 2.6 P2P applications 2.7 socket programming with UDP 2.8 socket programming with TCP SMTP, POP3, IMAP

2.5 DNS Application Layer 2-44 FTP: the file transfer protocol FTP user interface user at host file transfer FTP client FTP server local file system remote file 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 TCP control connection, FTP client contacts FTP server port 21 server at port 21, using TCP TCP data connection, client authorized over FTP FTP server port 20 control connection client server client browses remote directory, sends commands server opens another over control connection TCP data connection when server receives file to transfer another file transfer command, server control connection: opens 2nd TCP data out of band connection (for file) to

client FTP server maintains after transferring one file, state: current server closes data directory, earlier connection authentication Application Layer 2-46 FTP commands, responses sample commands: sent as ASCII text over control channel 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

status code and phrase (as in HTTP) 331 Username OK, password required 125 data connection already open; transfer starting 425 Cant open data connection 452 Error writing file Application Layer 2-47 Chapter 2: outline 2.1 principles of network applications app architectures app requirements 2.2 Web and HTTP 2.3 FTP 2.4 electronic mail 2.6 P2P applications 2.7 socket

programming with UDP 2.8 socket programming with TCP SMTP, POP3, IMAP 2.5 DNS Application Layer 2-48 Electronic mail outgoing message queue 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., Outlook, Thunderbird, iPhone mail client outgoing, incoming messages stored on server user agent user mailbox mail server user agent SMTP mail server user agent SMTP SMTP mail server user agent user

agent user agent Application Layer 2-49 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 user agent mail server user agent

SMTP mail server user agent SMTP SMTP mail server user agent user agent 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 4) SMTP client sends Alices message over the TCP connection 5) Bobs mail server places the message in Bobs mailbox 6) Bob invokes his user agent to read message 1) Alice uses UA to compose message to [email protected] 2) Alices UA sends message to her mail server; message placed in message queue

3) client side of SMTP opens TCP connection with Bobs mail server 1 user agent 2 mail server 3 Alices mail server user agent mail server 4 6 5 Bobs mail server Application Layer 2-52 Sample SMTP interaction S: C: S: C: S: C: S: C: S:

C: C: C: S: C: S: 220 hamburger.edu HELO crepes.fr 250 Hello crepes.fr, pleased to meet you MAIL FROM: 250 [email protected] Sender ok RCPT TO: 250 [email protected] ... Recipient ok DATA 354 Enter mail, end with "." on a line by itself Do you like ketchup? How about pickles? . 250 Message accepted for delivery QUIT 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 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 Application Layer 2-55 Mail message format SMTP: protocol for exchanging email msgs RFC 822: standard for text message format: header lines, e.g., To: From: Subject: header blank line body different from SMTP MAIL FROM, RCPT TO: commands!

Body: the message ASCII characters only Application Layer 2-56 Mail access protocols user agent SMTP mail access protocol (e.g., POP, SMTP user agent IMAP) senders mail server receivers mail server SMTP: delivery/storage to receivers 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 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 S: C: S:

C: S: +OK POP3 server ready user bob +OK pass hungry +OK user successfully logged C: S: S: S: C: S: S: C: C: S: S: C: C: S: list 1 498 2 912 . retr 1 . dele 1 retr 2 . dele 2

quit +OK POP3 server signing off on Application Layer 2-58 POP3 (more) and IMAP more about POP3 previous example uses POP3 download and delete mode Bob cannot re-read email if he changes client POP3 download-and- keep: copies of messages on different clients POP3 is stateless across sessions IMAP

keeps all messages in one place: at server allows user to organize messages in folders keeps user state across sessions: names of folders and mappings between message IDs and folder name Application Layer 2-59 Chapter 2: outline 2.1 principles of network applications app architectures app requirements 2.2 Web and HTTP 2.3 FTP 2.4 electronic mail 2.6 P2P applications 2.7 socket programming with UDP 2.8 socket programming with TCP SMTP, POP3, IMAP

2.5 DNS Application Layer 2-60 DNS: domain name system people: many identifiers: Domain Name System: SSN, name, passport # Internet hosts, routers: IP address (32 bit) used for addressing datagrams name, e.g., www.yahoo.com used by humans Q: how to map between IP address and name, and vice versa ? distributed database implemented in hierarchy of many name servers application-layer protocol: hosts, name servers communicate to resolve names (address/name translation) note: core Internet function, implemented as application-layer protocol

complexity at networks edge Application Layer 2-61 DNS: services, structure DNS services hostname to IP address translation host aliasing canonical, alias names mail server aliasing load distribution why not centralize DNS? single point of failure traffic volume distant centralized database maintenance A: doesnt 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 yahoo.com amazon.com DNS servers DNS servers org DNS servers pbs.org DNS servers edu DNS servers poly.edu umass.edu DNS serversDNS 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 c. Cogent, Herndon, VA (5 other sites) mapping to local name server d. U Maryland College Park, MD h. ARL Aberdeen, MD j. Verisign, Dulles VA (69 other sites ) e. NASA Mt View, CA f. Internet Software C. Palo Alto, CA (and 48 other sites) a. Verisign, Los Angeles CA (5 other sites) b. USC-ISI Marina del Rey, CA l. ICANN Los Angeles, CA (41 other sites) g. US DoD Columbus,

OH (5 other sites) k. RIPE London (17 other sites) i. Netnod, Stockholm (37 other sites) m. WIDE Tokyo (5 other sites) 13 root name servers worldwide 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: organizations own DNS server(s), providing authoritative hostname to IP mappings for organizations 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 resolution example root DNS server 2 host at cis.poly.edu wants IP address for gaia.cs.umass.edu iterated query: contacted server replies with name of server to contact I dont know this name, but ask this

server 3 4 TLD DNS server 5 local DNS server dns.poly.edu 1 8 requesting host 7 6 authoritative DNS server dns.cs.umass.edu cis.poly.edu gaia.cs.umass.edu Application Layer 2-67 DNS name resolution example root DNS server 2

recursive query: 3 7 6 puts burden of name resolution on contacted name server heavy load at upper levels of hierarchy? 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

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 name is hostname value is IP address

type=NS name is domain (e.g., foo.com) value is hostname of authoritative name server for this domain 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 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 # # questions # answer RRs # authority RRs # additional RRs for query, reply to query uses same # flags: query or reply recursion desired recursion available reply is authoritative questions (variable # of questions) answers (variable # of RRs) authority (variable # of RRs) additional info (variable # of RRs) Application Layer 2-71 DNS protocol, messages name, type fields for a query RRs in response to query records for authoritative servers

additional helpful info that may be used 2 bytes 2 bytes identification flags # questions # answer RRs # authority RRs # additional RRs questions (variable # of questions) answers (variable # of RRs) authority (variable # of RRs) additional info (variable # of RRs) 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 Chapter 2: outline 2.1 principles of network applications app architectures app requirements 2.2 Web and HTTP 2.3 FTP 2.4 electronic mail 2.6 P2P applications 2.7 socket programming with UDP 2.8 socket programming with TCP SMTP, POP3, IMAP 2.5 DNS Application Layer 2-74

Pure P2P architecture no always-on server arbitrary end systems directly communicate peers are intermittently connected and change IP addresses examples: file distribution (BitTorrent) pplive (streaming) Skype Application Layer 2-75 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 file, size F server uN dN

us u1 d1 u2 di: peer i download capacity d2 network (with abundant bandwidth) di ui ui: peer i upload capacity Application Layer 2-76 File distribution time: client-server server transmission: must sequentially send (upload) N file copies: F us di time to send one copy: F/us

time to send N copies: NF/us network ui client: each client must download file copy d min = min client download rate min client download time: F/dmin time to distribute F to N clients using Dc-s > client-server approach max{NF/us,,F/dmin} increases linearly in N Application Layer 2-77 File distribution time: P2P server transmission: must upload at least one copy time each client: to sendclient one copy: must F/us download file copy

F us di network ui min client download time: F/dmin as aggregate must download NF clients: bits max upload rate (limting max download is us + timerate) to distribute Fui to N clients using DP2P P2P approach > max{F/us,,F/dmin,,NF/(us + ui)} increases linearly in N but so does this, as each peer brings service capacity Application Layer 2-78 Client-server vs. P2P: example client upload rate = u, F/u = 1 hour, us = 10u, dmin us

Minimum Distribution Time 3.5 P2P Client-Server 3 2.5 2 1.5 1 0.5 0 0 5 10 15 20 25 30 35 N Application Layer 2-79 P2P file distribution: BitTorrent file divided into 256Kb chunks peers in torrent send/receive file chunks

tracker: tracks peers participating in torrent torrent: group of peers exchanging chunks of a file Alice arrives obtains list of peers from tracker and begins exchanging file chunks with peers in torrent Application Layer 2-80 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 2-81 BitTorrent: requesting, sending file chunks requesting chunks: at any given time, different peers have different subsets of file chunks periodically, Alice asks each peer for list of chunks that they have Alice requests missing chunks from peers, rarest first sending chunks: tit-fortat Alice sends chunks to those four peers currently sending her chunks at highest rate other peers are choked by Alice (do not receive chunks from her) re-evaluate top 4 every10 secs every 30 secs: randomly

select another peer, starts sending chunks optimistically unchoke this peer Application Layer 2-82 BitTorrent: tit-for-tat (1) Alice optimistically unchokes Bob (2) Alice becomes one of Bobs top-four providers; Bob reciprocates (3) Bob becomes one of Alices top-four providers higher upload rate: find better trading partners, get file faster ! Application Layer 2-83 P2P voice-over-IP: skype encrypted msgs Skype clients (SC) proprietary application-layer protocol (inferred via reverse engineering) components: clients: skype

Skype login server peers connect directly to each other for VoIP call super nodes (SN): skype peers with special functions overlay network: among SNs to locate login SCs server supernode (SN) supernode overlay network Application Layer 2-84 P2P voice-over-IP: skype skype client operation: 1. joins skype network by contacting SN (IP address cached) TCP 2.using logs-in (usename, password) to centralized skype server 3.login obtains

IP address for callee from SN, SN overlay Skype login server or client buddy list 4. initiate call directly to callee Application Layer 2-85 Skype: peers as relays problem: both Alice, Bob are behind NATs NAT prevents outside peer from initiating connection to insider peer inside peer can initiate relay solution: Alice, Bob connection to outside maintain open connection to their SNs Alice signals her SN to

connect to Bob Alices SN connects to Bob s SN Bobs SN connects to Bob over open connection Bob initially initiated to his SN Application Layer 2-86 Chapter 2: outline 2.1 principles of network applications app architectures app requirements 2.2 Web and HTTP 2.3 FTP 2.4 electronic mail 2.6 P2P applications 2.7 socket programming with TCP 2.8 socket programming with UDP SMTP, POP3, IMAP 2.5 DNS Application Layer 2-87 Socket programming

goal: learn how to build client/server application that communicate using sockets socket API socket a host-local, 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 streamoriented application-created, OS-controlled interface (a door) into which application process can both send and receive messages to/from another application process Application Layer 2-88 Socket-programming using

TCP Socket: door between application process and end-end-transport protocol (UDP or TCP) TCP service: reliable, in-order transfer of bytes from one process to another application process socket application process transport transport network network link physical Internet link controlled by app developer controlled by OS physical

Application Layer 2-89 Socket programming with TCP client must contact server server process must first be running server must have created socket (door) that welcomes clients contact client contacts server by: creating client-local TCP socket specifying IP address, port number of server process when client creates socket: client TCP establishes connection to server TCP when contacted by client, server TCP creates new socket for server process

to communicate with client allows server to talk with multiple clients source port numbers used to distinguish clients (more in Chap 3) application viewpoint: TCP provides reliable, in-order byte-stream transfer (pipe) between client and server Application Layer 2-90 Client/server socket interaction: TCP server (running on hostid) client create socket, port=x, for incoming request: welcomeSocket = ServerSocket() wait for incoming TCP connection request connectionSocket = connection welcomeSocket.accept() read request from connectionSocket write reply to connectionSocket close connectionSocket

setup create socket, connect to hostid, port=x clientSocket = Socket() send request using clientSocket read reply from clientSocket close clientSocket Application Layer 2-91 input stream client Process process output stream monitor inFromUser stream: sequence of characters that

flow into or out of a process input stream: attached to some input source for process, e.g., keyboard or socket output stream: attached to output source, e.g., monitor or socket outToServer keyboard inFromServer Stream jargon input stream client TCP clientSocket socket to network TCP socket from network

Application Layer 2-92 Example TCP client/server application server (running on hostid) server reads line from socket server converts line to uppercase, sends back to client client client reads line from standard input (inFromUser stream) sends to server via socket (outToServer stream) client reads, prints modified line from socket (inFromServer stream) Application Layer 2-93 Example: Java client (TCP) import java.io.*; import java.net.*; class TCPClient { create input stream create clientSocket object of type Socket, connect to server create

output stream attached to socket this package defines Socket() and ServerSocket() classes public static void main(String argv[]) throws Exception { server name, String sentence; e.g., www.umass.edu String modifiedSentence; server port # BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in)); Socket clientSocket = new Socket("hostname", 30000); DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream()); Application Layer 2-94 Example: Java client (TCP) BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream())); create input stream attached to socket sentence = inFromUser.readLine(); send line to server outToServer.writeBytes(sentence + '\n');

read line from server modifiedSentence = inFromServer.readLine(); System.out.println("FROM SERVER: " + modifiedSentence); close socket clientSocket.close(); (clean up behind yourself!) } } Application Layer 2-95 Example: Java server (TCP) import java.io.*; import java.net.*; class TCPServer { public static void main(String argv[]) throws Exception { String clientSentence; String capitalizedSentence; ServerSocket welcomeSocket = new ServerSocket(30000); wait, on welcoming socket accept() method for client contact create, new socket on return create input stream, attached to socket while(true) { Socket connectionSocket = welcomeSocket.accept();

BufferedReader inFromClient = new BufferedReader(new InputStreamReader(connectionSocket.getInputStream())); Application Layer 2-96 Example: Java server (TCP) create output stream, attached to socket DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream()); read in line from socket clientSentence = inFromClient.readLine(); capitalizedSentence = clientSentence.toUpperCase() + '\n'; write out line to socket outToClient.writeBytes(capitalizedSentence); } } } end of while loop, loop back and wait for another client connection Application Layer 2-97 Chapter 2: outline

2.1 principles of network applications app architectures app requirements 2.2 Web and HTTP 2.3 FTP 2.4 electronic mail 2.6 P2P applications 2.7 socket programming with TCP 2.8 socket programming with UDP SMTP, POP3, IMAP 2.5 DNS Application Layer 2-98 Socket programming with UDP UDP: no connection between client, server no handshaking sender explicitly

attaches IP destination address, port # to each application viewpoint: UDP provides unreliable packet transfer server must extract of groups of bytes sender IP address, port# (datagrams) from received packet UDP: transmitted data may be lost or received out-of-order between client and server Application Layer 2-99 Client/server socket interaction: UDP server (running on hostid) create socket, port= x. serverSocket = DatagramSocket() read datagram from serverSocket write reply to serverSocket specifying client address,

port number client create socket, clientSocket = DatagramSocket() Create datagram with server IP and port=x; send datagram via clientSocket read datagram from clientSocket close clientSocket Application Layer 2-100 Application 2-100 Example: Java client (UDP) input stream client process monitor inFromUser keyboard Process

Input: receives packet (recall thatTCP received byte stream) UDP packet receivePacket packet (recall that TCP sent byte stream) sendPacket Output: sends client UDP clientSocket socket to network UDP packet UDP socket from network Application Layer 2-101 Example: Java client (UDP)

import java.io.*; import java.net.*; class UDPClient { public static void main(String args[]) throws Exception { create input stream create client socket translate hostname to IP addr using DNS BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in)); DatagramSocket clientSocket = new DatagramSocket(); InetAddress IPAddress = InetAddress.getByName("hostname"); byte[] sendData = new byte[1024]; byte[] receiveData = new byte[1024]; String sentence = inFromUser.readLine(); sendData = sentence.getBytes(); Application Layer 2-102 Example: Java client (UDP) create datagram with data-to-send, length, IP addr, port DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9876); send datagram to server

clientSocket.send(sendPacket); DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length); read datagram from server clientSocket.receive(receivePacket); String modifiedSentence = new String(receivePacket.getData()); System.out.println("FROM SERVER:" + modifiedSentence); clientSocket.close(); } } Application Layer 2-103 Example: Java server (UDP) import java.io.*; import java.net.*; create datagram socket at port 9876 class UDPServer { public static void main(String args[]) throws Exception { DatagramSocket serverSocket = new DatagramSocket(9876); byte[] receiveData = new byte[1024]; byte[] sendData = new byte[1024]; while(true) { create space for

received datagram receive datagram DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length); serverSocket.receive(receivePacket); Application Layer 2-104 Example: Java server (UDP) String sentence = new String(receivePacket.getData()); get IP addr port #, of sender InetAddress IPAddress = receivePacket.getAddress(); int port = receivePacket.getPort(); String capitalizedSentence = sentence.toUpperCase(); sendData = capitalizedSentence.getBytes(); create datagram to send to client DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, port); write out datagram to socket serverSocket.send(sendPacket); } }

} end of while loop, loop back and wait for another datagram Application Layer 2-105 Chapter 2: summary our study of network apps now complete! application architectures client-server P2P hybrid application service requirements: reliability, bandwidth, delay Internet transport service model connection-oriented, reliable: TCP unreliable, datagrams: UDP specific protocols: HTTP FTP SMTP, POP, IMAP

DNS P2P: BitTorrent, DHT, Skype socket programming: TCP, UDP sockets Application Layer 2-106 Chapter 2: summary most importantly: learned about protocols! typical request/reply important themes: message exchange: client requests info or service server responds with data, status code message formats: headers: fields giving info about data data: info being communicated control vs. data msgs in-band, out-of-band centralized vs. decentralized

stateless vs. stateful reliable vs. unreliable msg transfer complexity at network edge Application Layer 2-107

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