Application Layer Chapter 2

Transcription

1 Application Layer Chapter 2 Silvia Giordano SUPSI CH-6928 Manno silvia.giordano@supsi.ch SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 1 Chapter goals: learn about protocols by examining popular applicationlevel protocols Http (web) Ftp SMTP ( ) Dns conceptual + implementation aspects of network application protocols client paradigm service model introduction to programming network applications socket programming SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 2

2 Applications and application-layer protocols C Application: communicating, distributed processes running in network hosts in space exchange messages to implement app e.g., , file transfer, the Web Application-layer protocols define messages exchanged by apps and actions taken one piece of an app services provided by lower layer protocols Typical application-layer has two pieces: client and application transport network data link physical application transport network data link physical application transport network data link physical SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 3 C S Some Definitions A process is a program that is running within a host. Within the same host, two processes communicate with interprocess communication defined by the OS. Processes running in different hosts communicate with an application-layer protocol A is an interface between the and the network application. Web:browser mail reader streaming audio/video: media player P1 Interprocess communication P2 Application Layer protocol P3 SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 4

3 Application-layer protocols API: application programming interface defines interface between application and transport layer socket: Internet API two processes communicate by sending data into socket, reading data out of socket controlled by application developer controlled by operating system process socket TCP with buffers, variables host or internet process socket TCP with buffers, variables host or controlled by application developer controlled by operating system How does a process identify the other process with which it wants to communicate? IP address of host running other process port number - allows receiving host to determine to which local process the message should be delivered SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 5 Internet apps: their protocols and transport protocols Application remote terminal access Web file transfer streaming multimedia remote file Internet telephony Application layer protocol smtp [RFC 821] telnet [RFC 854] http [RFC 2068] ftp [RFC 959] proprietary (e.g. RealNetworks) NSF proprietary (e.g., Vocaltec) Underlying transport protocol TCP TCP TCP TCP TCP or UDP TCP or UDP typically UDP SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 6

4 The Web: the http protocol http: hypertext transfer protocol Web s application layer protocol client/ model client: browser that requests, receives, displays Web objects : Web sends objects in response to requests PC running Explorer Mac running Navigator http request http response http request http response Server running NCSA Web SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 7 Web page: consists of objects addressed by a URL The Web: some jargon Most Web pages consist of: base HTML page, and several referenced objects. URL has two components: host name and path name: User for Web is called a browser: MS Internet Explorer Netscape Communicator Server for Web is called Web : Apache (public domain) MS Internet Information Server SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 8

5 The RFCs for the http protocol http1.0: RFC 1945? features implemented in most HTTP/1.0-5/96 implementations are usually consistent others http1.1: RFC /97 don t consider the effects of hierarchical proxies, caching, the need for persistent connections, and virtual hosts. more stringent requirements for ensuring reliable implementation of features PC running 1.0 XXX PC running 1.0 YYY http request http request? Server running 1.0 ZZZ SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 9 The http protocol: more http: TCP transport service: client initiates TCP connection (creates socket) to, port 80 accepts TCP connection from client http messages (applicationlayer protocol messages) exchanged between browser (http client) and Web (http ) TCP connection closed http is stateless maintains no information about past client requests aside Protocols that maintain state are complex! past history (state) must be maintained if /client crashes, their views of state may be inconsistent, must be reconciled SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 10

6 http example Suppose enters URL to a page with 10jpeg images 1a. http client initiates TCP connection to http (process) at Port 80 is default for http. 2. http client sends http request message (containing URL) into TCP connection socket 1b. http at host waiting for TCP connection at port 80. accepts connection, notifying client 3. http receives request message, forms response message containing requested object (somedep/home.index), sends message into socket time 4. http closes TCP connection. 5. http client receives 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 SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 11 Non-persistent and persistent connections Non-persistent HTTP/1.0 parses request, responds, and closes TCP connection 2 RTTs to fetch each object Each object transfer suffers from slow start But most 1.0 browsers use parallel TCP connections. SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 12

7 Non-persistent and persistent connections Persistent default for HTTP/1.1 on same TCP connection:, parses request, responds, parses new request,.. Client sends requests for all referenced objects as soon as it receives base HTML. Fewer RTTs and less slow start. Further improvement with pipelining SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 13 http message format: request two types of http messages: request, response http request message: ASCII (human-readable format) request line (GET, POST, HEAD commands) Carriage return, line feed indicates end of message header lines GET /somedir/page.html HTTP/1.0 User-: Mozilla/4.0 Accept: text/html, image/gif,image/jpeg Accept-language:fr (extra carriage return, line feed) SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 14

8 http request message: general format SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 15 http message format: response status line (protocol status code status phrase) header lines HTTP/ OK Date: Thu, 06 Aug :00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun Content-Length: 6821 Content-Type: text/html data, e.g., requested html file data data data data data... SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 16

9 http response status codes In first line in ->client response message. A few sample codes: 200 OK request succeeded, requested object later in this message 301 Moved Permanently requested object moved, new location specified later in this message (Location:) 400 Bad Request request message not understood by 404 Not Found requested document not found on this 505 HTTP Version Not Supported SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 17 Trying out http (client side) for yourself 1. Telnet to your favorite Web : telnet 80 Opens TCP connection to port 80 (default http port) at Anything typed in sent to port 80 at 2. Type in a GET http request: GET /~ross/index.html HTTP/1.0 By typing this in (hit carriage return twice), you send this minimal (but complete) GET request to http 3. Look at response message sent by http! SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 18

10 User- interaction: authentication client Authentication goal: control access to documents stateless: client must present authorization in each request authorization: typically name, password authorization: header line in request if no authorization presented, refuses access, sends WWW authenticate: header line in response usual http request msg 401: authorization req. WWW authenticate: usual http request msg + Authorization:line usual http response msg usual http request msg + Authorization:line Browser caches name & password so that does not have to repeatedly enter it. usual http response msg time SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 19 User- interaction: cookies client sends cookie to client in response mst Set-cookie: client presents cookie in later requests cookie: matches presented-cookie with -stored info authentication remembering preferences, previous choices usual http request msg usual http response + Set-cookie: # usual http request msg cookie: # usual http response msg usual http request msg cookie: # usual http response msg cookiespectific action cookiespectific action SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 20

11 User- interaction: conditional GET Goal: don t send object if client client has up-to-date stored (cached) version client: specify date of cached copy in http request If-modified-since: <date> http request msg If-modified-since: <date> http response HTTP/ OK <data> object modified SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 21 User- interaction: conditional GET Goal: don t send object if client has up-to-date stored (cached) version client: specify date of cached copy in http request If-modified-since: <date> : response contains no object if cached copy upto-date: HTTP/ Not Modified client http request msg If-modified-since: <date> http response HTTP/ Not Modified object not modified SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 22

12 Web Caches (proxy ) Goal: satisfy client request without involving origin sets browser: Web accesses via web cache client sends all http requests to web cache if object at web cache, web cache immediately returns object in http response else requests object from origin, then returns http response to client client client http request http response http request http response Proxy http request http response http request http response origin origin SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 23 Assume: cache is close to client (e.g., in same network) smaller response time: cache closer to client decrease traffic to distant s link out of institutional/local ISP network often bottleneck Why Web Caching? institutional network public Internet 1.5 Mbps access link 10 Mbps LAN origin s institutional cache SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 24

13 ftp: the file transfer protocol at host transfer file to/from remote host client/ model client: side that initiates transfer (either to/from remote) : remote host ftp: RFC 959 FTP interface ftp : port 21 FTP client local file system file transfer FTP remote file system SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 25 ftp: separate control, data connections ftp client contacts ftp at port 21, specifying TCP as transport protocol two parallel TCP connections opened: control: exchange commands, responses between client,. out of band control data: file data to/from ftp maintains state : current directory, earlier authentication FTP client TCP control connection port 21 TCP data connection port 20 FTP SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 26

14 ftp commands, responses Sample commands: sent as ASCII text over control channel USER name 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 Can t open data connection 452 Error writing file SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 27 Three major components: s mail s simple mail transfer protocol: smtp Electronic Mail mail SMTP mail SMTP SMTP outgoing message queue mail mailbox SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 28

15 Three major components: s mail s simple mail transfer protocol: smtp Electronic Mail User Agent SMTP a.k.a. mail reader composing, editing, reading mail messages e.g., Eudora, Outlook, elm, Netscape Messenger outgoing, incoming messages stored on mail mail SMTP SMTP outgoing message queue mail mailbox SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 29 Electronic Mail: mail s Mail Servers mailbox contains incoming messages (yet to be read) for message queue of outgoing (to be sent) mail messages smtp protocol between mail s to send messages client: sending mail : receiving mail mail SMTP mail SMTP SMTP mail SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 30

16 Electronic Mail: smtp [RFC 821] uses tcp to reliably transfer msg from client to, port 25 direct transfer: sending to receiving three phases of transfer handshaking (greeting) transfer of messages closure command/response interaction commands: ASCII text response: status code and phrase messages must be in 7-bit ASCII SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 31 Sample smtp interaction S: 220 hamburger.edu C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: S: 250 Sender ok C: RCPT TO: S: 250 Recipient ok C: DATA S: 354 Enter mail, end with "." on a line by itself C: Do you like ketchup? C: How about pickles? C:. S: 250 Message accepted for delivery C: QUIT S: 221 hamburger.edu closing connection SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 32

17 try smtp interaction for yourself telnet name 25 see 220 reply from enter HELO, MAIL FROM, RCPT TO, DATA, QUIT commands above lets you send without using client (reader) if port 25 is not closed! SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 33 Mail message format smtp: protocol for exchanging msgs RFC 822: standard for text message format: header lines, e.g., To: From: Subject: different from smtp commands! body the message, ASCII characters only header body blank line SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 34

18 Message format: multimedia extensions MIME: multimedia mail extension, RFC 2045, 2056 additional lines in msg header declare MIME content type MIME version method used to encode data multimedia data type, subtype, parameter declaration encoded data From: To: Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data base64 encoded data SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 35 Text MIME types Content-Type: type/subtype; parameters Video example subtypes: plain, html example subtypes: mpeg, quicktime Image example subtypes: jpeg, gif Audio exampe subtypes: basic (8-bit mu-law encoded), 32kadpcm (32 kbps coding) Application other data that must be processed by reader before viewable example subtypes: msword, octet-stream SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 36

19 Multipart Type From: To: Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Type: multipart/mixed; boundary= Content-Transfer-Encoding: quoted-printable Content-Type: text/plain Dear the receiver, Please find a picture of a crepe Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data base64 encoded data SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 37 Mail access protocols SMTP SMTP POP3 or IMAP sender s mail receiver s mail SMTP: delivery/storage to receiver s Mail access protocol: retrieval from POP: Post Office Protocol [RFC 1939] authorization ( <-->) and download IMAP: Internet Mail Access Protocol [RFC 1730] more features (more complex) manipulation of stored msgs on HTTP: Hotmail, Yahoo! Mail, etc. SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 38

20 authorization phase client commands: : declare name pass: password responses +OK -ERR transaction phase, client: list: list message numbers retr: retrieve message by number dele: delete Quit update POP3 protocol S: +OK POP3 ready C: alice S: +OK C: pass hungry S: +OK successfully logged on C: list S: S: S:. C: retr 1 S: <message 1 contents> S:. C: dele 1 C: retr 2 S: <message 1 contents> S:. C: dele 2 C: quit S: +OK POP3 signing off SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 39 smtp: final words smtp uses persistent connections smtp requires that message (header & body) be in 7-bit ascii certain character strings are not permitted in message (e.g., CRLF.CRLF). Thus message has to be encoded (usually into either base-64 or quoted printable) smtp uses CRLF.CRLF to determine end of message Comparison with http SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 40 http: pull push both have ASCII command/response interaction, status codes http: each object is encapsulated in its own response message smtp: multiple objects message sent in a multipart message

21 DNS: Domain Name System People: many identifiers: SSN, name, Passport # Internet: two way to identify: IP addresses and name IP address (32 bit) - used for addressing datagrams name, e.g., gaia.cs.umass.edu - used by humans Domain Name System: mapping between IP addresses and name SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 41 DNS: Domain Name System Domain Name System: distributed database implemented in hierarchy of many name s application-layer protocol host, routers, name s to communicate to resolve names (address/name translation) note: core Internet function implemented as application-layer protocol complexity at network s edge SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 42

22 DNS name s Why not centralize DNS? single point of failure traffic volume distant centralized database maintenance doesn t scale! no has all name-to-ip address mappings local name s: each ISP, company has local (default) name host DNS query first goes to local name authoritative name : for a host: stores that host s IP address, name can perform name/address translation for that host s name SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 43 uses port53 RFCs: 1034, 1035, 2136 client- paradigm located at the edges additional services: host aliasing (from complex names to alias names) mail aliasing (for a mnemonic mail address) load distribution (set of IP addresses associated with one canonical hostname for high load) DNS name s provides a core Internet function translating hostnames to IP addresses BUT is an application-layer protocol: runs between end systems relies on an underlying endto-end transport protocol to transfer data SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 44

23 DNS: Root name s contacted by local name that can not resolve name root name : contacts authoritative name if name mapping not known gets mapping returns mapping to local name ~ dozen root name s worldwide SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 45 host surf.eurecom.fr wants IP address of gaia.cs.umass.edu 1. Contacts its local DNS, dns.eurecom.fr 2. dns.eurecom.fr contacts root name, if necessary 3. root name contacts authoritative name, dns.umass.edu, if necessary Simple DNS example local name dns.eurecom.fr requesting host surf.eurecom.fr root name authorititive name dns.umass.edu gaia.cs.umass.edu SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer

24 DNS example root name Root name : may not know authoratiative name may know intermediate name : who to contact to find authoritative name local name dns.eurecom.fr requesting host surf.eurecom.fr intermediate name dns.umass.edu 4 5 authoritative name dns.cs.umass.edu gaia.cs.umass.edu SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 47 recursive query: puts burden of name resolution on contacted name heavy load? iterated query: contacted replies with name of to contact I don t know this name, but ask this Iterated queries local name dns.eurecom.fr requesting host surf.eurecom.fr root name iterated query gaia.cs.umass.edu SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer intermediate name dns.umass.edu 5 6 authoritative name dns.cs.umass.edu

25 DNS: caching and updating records once (any) name learns mapping, it caches mapping cache entries timeout (disappear) after some time update/notify mechanisms under design by IETF RFC SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 49 DNS records DNS: distributed db storing resource records (RR) Type=NS RR format: (name, value, type,ttl) Type=A name is hostname value is IP address name is domain (e.g. foo.com) value is IP address of authoritative name for this domain Type=CNAME name is an alias name for some cannonical (the real) name value is cannonical name Type=MX value is hostname of mail associated with name SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 50

26 DNS protocol, messages DNS protocol : query and repy messages, both with same message format msg header identification: 16 bit # for query, repy to query uses same # flags: query or reply recursion desired recursion available reply is authoritative SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 51 DNS protocol, messages Name, type fields for a query RRs in reponse to query records for authoritative s additional helpful info that may be used SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 52

27 Socket API introduced in BSD4.1 UNIX, 1981 explicitly created, used, released by apps allow programmers to treat network connections as streams data are written and read onto these streams Socket programming socket a host-local, applicationcreated/owned, OS-controlled interface (a door ) into which application process can both send and receive messages to/from another (remote or local) application process SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 53 Chapter 2: Summary Our study of standard network apps now complete! application service requirements: reliability, bandwidth, delay client- paradigm Internet transport service model connection-oriented, reliable: TCP unreliable, datagrams: UDP specific protocols: http ftp smtp dns SUPSI-DTI Silvia Giordano 10/06/2004 Application Layer 54