3rd Edition: Chapter 2

3rd Edition: Chapter 2

Chapter 2 Application Layer A note on the use of these ppt slides: Were making these slides freely available to all (faculty, students, readers). Theyre in PowerPoint form so you see the animations; and can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following: If you use these slides (e.g., in a class) that you mention their source (after all, wed like people to use our book!) If you post any slides on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material. Thanks and enjoy! JFK/KWR Computer Networking: A Top Down Approach 6th edition Jim Kurose, Keith Ross Addison-Wesley March 2012 All material copyright 1996-2012 J.F Kurose and K.W. Ross, All Rights Reserved Application Layer 2-1 Chapter 2: outline 2.1 principles of network applications 2.2 Web and HTTP

2.3 FTP 2.4 electronic mail 2.6 P2P applications 2.7 socket programming with UDP and TCP SMTP, POP3, IMAP 2.5 DNS Application Layer 2-2 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 creating network applications socket API Application Layer 2-3 Some network apps e-mail web text messaging remote login P2P file sharing multi-user network games

streaming stored video (YouTube, Hulu, Netflix) voice over IP (e.g., Skype) real-time video conferencing social networking search Application Layer 2-4 Creating a network app write programs that: run on (different) end systems communicate over network e.g., web server software communicates with browser software 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 Application Layer 2-5 Application architectures possible structure of applications: client-server peer-to-peer (P2P) Application Layer 2-6 Client-server architecture

server: always-on host permanent IP address data centers 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 P2P architecture

no always-on server arbitrary end systems directly communicate peers request service from other peers, provide service in return to other peers self scalability new peers bring new service capacity, as well as new service demands peers are intermittently connected and change IP addresses complex management peer-peer Application Layer 2-8 Processes communicating process: program clients, servers running within a client process: process that initiates host

within same host, two processes communicate using inter-process communication (defined by OS) processes in different hosts communicate by exchanging messages communication server process: process that waits to be contacted aside: applications with P2P architectures have client processes & server processes Application Layer 2-9 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 applicatio n process socket applicatio n process transport transport network network link physical Internet link controlled by app developer controlled by OS physical Application Layer 2-10

Addressing processes to receive messages, process must have identifier host device has unique 32-bit IP address Q: does IP address A: of host which no,onmany process runs can be processes suffice foron same running identifying the host process?

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 2-11 App-layer protocol defines types of messages exchanged,

e.g., request, response message syntax: what fields in messages & how fields are delineated message semantics meaning of information in fields rules for when and how processes send & respond to messages open protocols: defined in RFCs allows for interoperability e.g., HTTP, SMTP proprietary protocols: e.g., Skype Application Layer 2-12 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 security throughput encryption,they data get integrity, Application Layer 2-13 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 2-14

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 2-15 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 2-16 Securing TCP TCP & UDP no encryption cleartext passwds sent into socket traverse Internet in cleartext SSL provides encrypted TCP connection data integrity end-point authentication SSL is at app layer Apps use SSL libraries, which

talk to TCP SSL socket API cleartext passwds sent into socket traverse Internet encrypted See Chapter 7 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 and 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 HTMLfile which includes several referenced objects each object is addressable by a www.someschool.edu/someDept/pic.gif URL, 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 required multiple connections persistent HTTP multiple objects can be sent over single TCP connection between client, server Application Layer 2-22 Non-persistent 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 time someDepartment/home.i

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 3. client HTTP server receives request message, forms response message containing requested object, and sends message into its socket Application Layer 2-23 Non-persistent 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! Wireshark to look at captured HTTP request/respons Application Layer 2-33 User-server state: cookies many Web sites use cookies four components: 1) cookie header line of HTTP response message

2) cookie header line in next HTTP request message 3) cookie file kept on users host, managed by users browser 4) back-end database at Web site example: Susan always access Internet from PC visits specific ecommerce site for first time when initial HTTP requests arrives at site, site creates: unique ID entry in backend database for ID 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:

aside cookies and privacy: authorization cookies permit shopping carts sites to learn a recommendations lot about you user session state you may supply (Web e-mail) 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 server user sets browser: Web accesses via cache

browser sends all HTTP requests to cache object in cache: cache returns object else cache requests object from origin server, then returns object to client 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: 100K bits

avg request rate from browsers to origin servers:15/sec avg data rate to browsers: 1.50 Mbps RTT from institutional router to any origin server: 2 sec problem! access link rate: 1.54 Mbps origin servers public Internet 1.54 Mbps access link institutional network 1 Gbps LAN consequences: LAN utilization: 15% access link utilization = 99% total delay = Internet Application Layer 2-39 Caching example: fatter

access link assumptions: avg object size: 100K bits avg request rate from browsers to origin servers:15/sec avg data rate to browsers: 1.50 Mbps 154 RTT from institutional router to any origin Mbps server: 2 sec access link rate: 1.54 9.9% Mbps public Internet origin servers 1.54 Mbps 154 Mbps access link institutional network

1 Gbps LAN consequences: LAN utilization: 15% msecs access link utilization = Cost: 99%increased access link speed (not cheap!) total delay = Internet Application Layer 2-40 Caching example: install local cache assumptions: avg object size: 100K bits avg request rate from browsers to origin servers:15/sec avg data rate to browsers: 1.50 Mbps RTT from institutional router to any origin server: 2 sec

access link rate: 1.54 ? Mbps ? How to compute link consequences: utilization, LAN utilization: delay? 15% origin servers public Internet 1.54 Mbps access link institutional network 1 Gbps LAN local web cache access link utilization = Cost: web cache (cheap!) 100% total delay = Internet

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 public Internet 40% requests satisfied at cache, 60% requests satisfied at origin access link utilization: 1.54 Mbps access link 60% of requests use access link data rate to browsers over link = 0.6*1.50 access total

delay Mbps = .9 Mbps = 0.6 * (delay from origin utilization = 0.9/1.54 +0.4 * (delay when =servers) .58 satisfied at cache) = 0.6 (2.01) + 0.4 (~msecs) = ~ 1.2 secs less than with 154 Mbps link (and cheaper too!) institutional network 1 Gbps LAN local web cache Application Layer 2-42 Conditional GET server client Goal: dont send object if cache has up-to-date cached

version no object transmission delay lower link utilization cache: specify date of cached copy in HTTP request If-modified-since: server: response contains no object if cached copy is up-to-date: HTTP/1.0 304 Not Modified HTTP request msg If-modified-since: HTTP response HTTP/1.0 304 Not Modified HTTP request msg If-modified-since: HTTP response HTTP/1.0 200 OK object

not modified before 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 and 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 local file system FTP server 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 server opens commands over control connection another TCP data connection to when server receives file transfer command, transfer another server opens 2nd TCP data file connection (for file) to control connection:

client out of band after transferring one file, server closes data FTP server connection maintains state: Application Layer 2-46 current directory, 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 and 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 mailbox user agent 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 7bit 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 reread e-mail if he changes

client POP3 download-andkeep: 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 and TCP SMTP, POP3, IMAP 2.5 DNS Application Layer 2-60 DNS: domain name system people: many identifiers: 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 ? Domain Name System:

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 replicated Web servers: many IP addresses correspond to one name why not centralize DNS? single point of failure traffic volume distant centralized database maintenance A: doesnt scale! 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-toaddress translation pairs (but may be out of date!) acts as proxy, forwards query into hierarchy

Application Layer 2-66 DNS name resolution example host at cis.poly.edu wants IP address for gaia.cs.umass.ed iterated query: u contacted server replies with name of server to contact I dont know this name, but ask this server root DNS server 2 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 TLD DNS server puts burden of

name resolution on contacted name server heavy load at upper levels of hierarchy? 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 type=CNAME name is hostname value is IP name is alias name for name is domain

(e.g., foo.com) value is hostname of authoritative name server for this domain address type=NS 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 # questions # answer RRs # authority RRs # additional RRs bit # 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 Attacking DNS DDoS attacks Bombard root servers with traffic Not successful to date Traffic Filtering Local DNS servers cache IPs of TLD servers, allowing root server bypass Bombard TLD servers Potentially more dangerous

Redirect attacks Man-in-middle Intercept queries DNS poisoning Send bogus relies to DNS server, which caches Exploit DNS for DDoS Send queries with spoofed source address: target IP Requires amplification Application Layer 2-74 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 and

TCP SMTP, POP3, IMAP 2.5 DNS Application Layer 2-75 Pure P2P architecture no always-on server arbitrary end systems directly communicate peers are intermittently connected and change IP addresses examples: file distribution (BitTorrent) Streaming (KanKan) VoIP (Skype) Application Layer 2-76 File distribution: clientserver 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 2-77 File distribution time: client-server server transmission: must sequentially send (upload) N file copies:

F us time to send one copy: F/us time to send N copies: NF/ di network u ui s client: each client must download file copy d min = min client download rate min client time to download distribute F to min N clients using time: F/d Dc-s client-server approach

> max{NF/us,,F/dmin} increases linearly in N Application Layer 2-78 File distribution time: P2P server F transmission: must u upload at least one copy client: each client time to send one must download file copy: F/us copy min client download clients: as aggregate must time: F/d min bits download NF s di network ui max upload rate (limting max download is usF+ ui

timerate) to distribute to N clients using DP2P > max{F/us,,F/dmin,,NF/(us P2P approach + ui)} increases linearly in N but so does this, as each peer brings service capacity Application Layer 2-79 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 2-80 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 2-81 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) Application Layer 2-82 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: titfor-tat 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 Application Layer 2-83 BitTorrent: tit-fortat (1) Alice optimistically unchokes Bob lice becomes one of Bobs top-four providers; Bob reciprocat ) Bob becomes one of Alices top-four providers higher upload rate: find better trading partners, get file faster ! Application Layer 2-84

Distributed Hash Table (DHT) Hash table DHT paradigm Circular DHT and overlay networks Peer churn Simple Database Simple database with(key, value) pairs: key: value: social Key human name; Value John Washington 132-54-3570 security # Diana Louise Jones 761-55-3791 Xiaoming Liu

385-41-0902 Rakesh Gopal 441-89-1956 Linda Cohen 217-66-5609 . Lisa Kobayashi 177-23-0199 key: movie title; value: IP address Hash Table More convenient to store and search on numerical representation of key Original Key Key Value key = hash(original key) John Washington

8962458 132-54-3570 Diana Louise Jones 7800356 761-55-3791 Xiaoming Liu 1567109 385-41-0902 Rakesh Gopal 2360012 441-89-1956 Linda Cohen 5430938 217-66-5609 . Lisa Kobayashi 9290124

177-23-0199 Distributed Hash Table (DHT) Distribute (key, value) pairs over millions of peers pairs are evenly distributed over peers Any peer can query database with a key database returns value for the key To resolve query, small number of messages exchanged among peers Each peer only knows about a small number of other peers Robust to peers coming and going (churn) Assign key-value pairs to peers rule: assign key-value pair to

the peer that has the closest ID. convention: closest is the immediate successor of the key. e.g., ID space {0,1,2,3,,63} suppose 8 peers: 1,12,13,25,32,40,48,60 If key = 51, then assigned to peer 60 If key = 60, then assigned to peer 60 If key = 61, then assigned to peer 1 Circular DHT each peer only aware of immediate successor and predecessor. 1 12 60 13 48 25 40 32 overlay network Resolving a query What is the value associated with key 53 ? 1

value 12 60 13 48 (N) messages n avgerage to resolve 40 uery, when there re N peers 25 32 Circular DHT with shortcuts 1 What is the value for value key 53 12 60 13 48 25 40

32 each peer keeps track of IP addresses of predecessor, successor, short cuts. reduced from 6 to 3 messages. possible to design shortcuts with O(log N) neighbors, O(log N) messages in query Peer churn 1 3 handling peer churn: peers may come and go (churn) each peer knows 4 address of its two successors 12 each peer 5 periodically pings its 10 two successors to check 8 aliveness example: peer 5 abruptly leaves if immediate successor leaves, choose next successor as new immediate

successor 15 Peer churn 1 3 handling peer churn: peers may come and go (churn) each peer knows 4 address of its two successors 12 each peer periodically pings its 10 two successors to check 8 aliveness example: peer 5 abruptly leaves if peer 4 detects peer 5s immediate departure; makes 8 its immediate successor successor leaves, choosesuccessor next successor 4 asks 8 who its immediate is;

as new its immediate makes 8s immediate successor second successor successor. 15 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 and TCP SMTP, POP3, IMAP 2.5 DNS Application Layer 2-95 Socket programming goal: learn how to build client/server

applications that communicate using sockets socket: door between application process and end-end-transport protocol applicatio n process socket applicatio n process transport transport network network link physical Internet link controlled by app developer controlled by OS physical

Application Layer 2-96 Socket programming Two socket types for two transport services: UDP: unreliable datagram TCP: reliable, byte stream-oriented Application Example: 1. Client reads a line of characters (data) from its keyboard and sends the data to the server. 2. The server receives the data and converts characters to uppercase. 3. The server sends the modified data to the client. 4. The client receives the modified Application Layer 2-97 Socket programming with UDP UDP: no connection between client & server no handshaking before sending data sender explicitly attaches IP destination address and port # to each packet rcvr extracts sender IP address and port# from received packet UDP: transmitted data may be lost or received out-of-order

Application viewpoint: UDP provides unreliable transfer of groups of bytes (datagrams) between client and server Application Layer 2-98 Client/server socket interaction: UDP server (running on serverIP) create socket, port= x: serverSocket = socket(AF_INET,SOCK_DGRAM) read datagram from serverSocket write reply to serverSocket specifying client address, port number client create socket: clientSocket = socket(AF_INET,SOCK_DGRAM) Create datagram with server IP and port=x; send datagram via clientSocket read datagram from clientSocket

close clientSocket Application 2-99 Example app: UDP client Python UDPClient include Pythons socket library from socket import * serverName = hostname serverPort = 12000 create UDP socket for server get user keyboard input Attach server name, port to message; send into socket clientSocket = socket(socket.AF_INET, socket.SOCK_DGRAM) message = raw_input(Input lowercase sentence:) clientSocket.sendto(message,(serverName, serverPort)) read reply characters from socket into string modifiedMessage, serverAddress = print out received string and close socket print modifiedMessage clientSocket.recvfrom(2048)

clientSocket.close() Application Layer 2-100 Example app: UDP server Python UDPServer from socket import * serverPort = 12000 create UDP socket serverSocket = socket(AF_INET, SOCK_DGRAM) bind socket to local port number 12000 serverSocket.bind(('', serverPort)) print The server is ready to receive loop forever Read from UDP socket into message, getting clients address (client IP and port) send upper case string back to this client while 1: message, clientAddress = serverSocket.recvfrom(2048) modifiedMessage = message.upper() serverSocket.sendto(modifiedMessage, clientAddress) Application Layer 2-101 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: when contacted by client, server TCP creates new socket for server process to communicate with that particular client allows server to talk with multiple clients source port numbers used to distinguish clients (more in Chap 3) Creating TCP socket, specifying IP address, port number of server process application viewpoint: when client creates socket: client TCP

TCP provides reliable, in-ord establishes connection to byte-stream transfer (pipe) server TCP between client and server Application Layer 2-102 Client/server socket interaction: TCP server (running on hostid) client create socket, port=x, for incoming request: serverSocket = socket() wait for incoming TCP connection request connectionSocket = connection serverSocket.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 2-103 Example app: TCP client Python TCPClient from socket import * serverName = servername create TCP socket for server, remote port 12000 serverPort = 12000 clientSocket = socket(AF_INET, SOCK_STREAM) clientSocket.connect((serverName,serverPort)) sentence = raw_input(Input lowercase sentence:) No need to attach server name, port clientSocket.send(sentence) modifiedSentence = clientSocket.recv(1024) print From Server:, modifiedSentence clientSocket.close() Application Layer 2-104

Example app: TCP server Python TCPServer create TCP welcoming socket server begins listening for incoming TCP requests loop forever server waits on accept() for incoming requests, new socket created on return read bytes from socket (but not address as in UDP) close connection to this client (but not welcoming socket) from socket import * serverPort = 12000 serverSocket = socket(AF_INET,SOCK_STREAM) serverSocket.bind((,serverPort)) serverSocket.listen(1) print The server is ready to receive while 1: connectionSocket, addr = serverSocket.accept() sentence = connectionSocket.recv(1024) capitalizedSentence = sentence.upper() connectionSocket.send(capitalizedSentence) connectionSocket.close() Application Layer 2-105 Chapter 2: summary our study of network apps now complete!

application architectures client-server P2P 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 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-ofband centralized vs. decentralized stateless vs. stateful reliable vs. unreliable msg transfer Application Layer 2-107 Chapter 1 Additional Slides Introduction 1-108

application (www browser, packet analyzer email client) application OS packet capture (pcap) Transport (TCP/UDP) copy of all Ethernet frames sent/receive d Network (IP) Link (Ethernet) Physical

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