3rd Edition: Chapter 2

Chapter 2 Application Layer 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 Intrudction to Application Layer 2.2 Web and HTTP 2.3 FTP 2.4 electronic mail SMTP, POP3, IMAP 2.5 DNS Application Layer 2-2 Chapter 2: application layer Lecture Objectives: 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 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 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 2-9 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 2-10 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 2-11 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 2-12 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 2-13 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 2-14 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 2-15 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 2-16 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 2-17

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 2-18 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 2-19 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 2-20 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 to when server receives file transfer another file transfer command, server control connection: out opens 2nd TCP data of band connection (for file) to FTP server maintains client state: current after transferring one file, directory, earlier server closes data connection authentication Application Layer 2-21 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 2-22 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 2-23 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 2-24 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 2-25 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 2-26 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 2-27 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 2-28 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 2-29 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 2-30 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 2-31 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 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 2-32 DNS: services, structure DNS services hostname to IP address translation

host aliasing canonical, alias names why not centralize DNS? single point of failure traffic volume distant centralized database maintenance mail server aliasing load distribution replicated Web servers: many IP addresses correspond to one name Application Layer 2-33 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 2-34 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 k. RIPE London (17 other sites) 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) i. Netnod, Stockholm (37 other sites) m. WIDE Tokyo (5 other sites) 13 root name servers worldwide Application Layer 2-35 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 2-36 DNS name resolution example 2 recursive query: puts burden of name resolution on contacted name server heavy load at upper levels of hierarchy? root DNS server 3 7 6

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 2-37 DNS protocol, messages 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 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 2-38 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 2-39 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 2-40 Chapter 2: summary most importantly: learned about protocols! typical request/reply

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 important themes: 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 2-41

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