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Computer Networks

Computer Networks. Application layer (Part 2). Application layer protocols. Last class Application layer functions Application protocols DNS ftp This class More application protocols http smtp. AL: WWW basics. Web page: consists of “objects” addressed by a URL

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Computer Networks

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  1. Computer Networks Application layer (Part 2)

  2. Application layer protocols • Last class • Application layer functions • Application protocols • DNS • ftp • This class • More application protocols • http • smtp

  3. AL: WWW basics Web page: consists of “objects” addressed by a URL Most Web pages consist of: base HTML page, and several referenced objects. URL has two components: host name and path name: User agent for Web is called a browser: MS Internet Explorer Netscape Communicator Server for Web is called Web server: Apache (public domain) MS Internet Information Server www.someSchool.edu/someDept/pic.gif

  4. AL: HTTP basics http: hypertext transfer protocol Web’s application layer protocol client/server model client: browser that requests, receives, “displays” Web objects server: Web server sends objects in response to requests HTTP/1.0: RFC 1945 http://www.rfc-editor.org/rfc/rfc1945.txt HTTP/1.1: RFC 2068 http://www.rfc-editor.org/rfc/rfc2068.txt HTTP state management (cookies): RFC 2109 http://www.rfc-editor.org/rfc/rfc2109.txt http request PC running Explorer http response http request Server running NCSA Web server http response Mac running Navigator

  5. AL: http http: TCP transport service: client initiates bi-directional TCP connection (via socket) to server, port 80 server accepts TCP connection http protocol messages exchanged between client and server Requests/responses encoded in text Client sends request to server, followed by response from server to client How to mark end of message? Size of message Content-Length Must know size of transfer in advance Server closes connection TCP connection closed (only server can do this) 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

  6. AL: http example Suppose user enters URL www.someSchool.edu/someDepartment/home.index 1a. http client initiates TCP connection to http server (process) at www.someSchool.edu. Port 80 is default for http server. (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 2.http client sends http request message (containing URL) into TCP connection socket 3.http server receives request message, forms response message containing requested object (someDepartment/home.index), sends message into socket time

  7. AL: http example (cont.) 5. http client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects 4.http server closes TCP connection. 6.Steps 1-5 repeated for each of 10 jpeg objects time

  8. AL: http message format: request • two types of http messages: request, response • http request message: • ASCII (human-readable format) request line (GET, POST, HEAD commands) GET /somedir/page.html HTTP/1.0 User-agent: Mozilla/4.0 Accept: text/html, image/gif,image/jpeg Accept-language:fr (extra carriage return, line feed) header lines Carriage return, line feed indicates end of message

  9. AL: http request message: general format

  10. AL: http request • Request line • Method • GET – return object sepecified by URI • HEAD – return headers only of GET response • POST – send data to the server (forms, etc.) • Other methods: OPTIONS, PUT, DELETE, TRACE, CONNECT • URI • E.g. http://www.cse.ogi.edu/index.html with a proxy • E.g. /index.html if no proxy • HTTP version

  11. AL: http request • Header lines (HTTP request headers) • Authorization • Authentication info • Accept • Acceptable document types, encodings, languages, character sets • From • User email (when privacy is disabled) • If-Modified-Since • For use with caching • Referer • URL which caused this page to be requested • User-Agent • Client software • Host • For multiple web sites hosted on same server • Connection • Keep connection alive for subsequent request or close connection

  12. AL: http request • Blank-line • Separate request headers from POST information • End of request • Body • If POST, send POST information

  13. AL: http request example GET / HTTP/1.1 Accept: */* Accept-Language: en-us Accept-Encoding: gzip, deflate User-Agent: Mozilla/4.0 (compatible; MSIE 5.5; Windows NT 5.0) Host: www.cse.ogi.edu Connection: Keep-Alive

  14. AL: http message format: response status line (protocol status code status phrase) HTTP/1.0 200 OK Date: Thu, 06 Aug 1998 12:00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 1998 …... Content-Length: 6821 Content-Type: text/html data data data data data ... header lines data, e.g., requested html file

  15. AL: http response • Status-line • HTTP version • 3 digit response code • 1XX – informational • 2XX – success • 3XX – redirection • 4XX – client error • 5XX – server error • Reason phrase

  16. AL: http response 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 server 404 Not Found requested document not found on this server 505 HTTP Version Not Supported In first line in server->client response message. A few sample codes:

  17. AL: http response • Header lines • Location • redirection • Server • server software • WWW-Authenticate • request for authentication • Allow • list of methods supported (get, head, etc) • Content-Encoding • x-gzip • Content-Length • Content-Type • Expires • Last-Modified • ETag

  18. AL: http response • Blank-line • Separate headers from data • Body • Data being returned to client

  19. AL: http response example HTTP/1.1 200 OK Date: Tue, 27 Mar 2001 03:49:38 GMT Server: Apache/1.3.14 (Unix) (Red-Hat/Linux) mod_ssl/2.7.1 OpenSSL/0.9.5a DAV/1.0.2 PHP/4.0.1pl2 mod_perl/1.24 Last-Modified: Mon, 29 Jan 2001 17:54:18 GMT ETag: "7a11f-10ed-3a75ae4a" Accept-Ranges: bytes Content-Length: 4333 Keep-Alive: timeout=15, max=100 Connection: Keep-Alive Content-Type: text/html …..

  20. AL: Trying out http (client side) for yourself 1. Telnet to your favorite Web server: Opens TCP connection to port 80 (default http server port) at www.cse.ogi.edu Anything typed in sent to port 80 at www.cse.ogi.edu telnet www.cse.ogi.edu 80 2. Type in a GET http request: By typing this in (hit carriage return twice), you send this minimal (but complete) GET request to http server GET /~wuchang/index.html HTTP/1.0 3. Look at response message sent by http server!

  21. AL: More HTTP examples • http://www.cse.ogi.edu/class/cse524/http.txt • http://www.cse.ogi.edu/class/cse524/http_post.txt

  22. AL: Some HTTP headers by function • Authentication • Client • Authorization, Proxy-Authorization • Server • WWW-authenticate, Proxy-Authenticate • Caching • General • Cache-control, Pragma • Client • If-Modified-Since, If-Unmodified-Since, If-Match • Server • Last-Modified, Expires, ETag, Age

  23. AL: Some HTTP headers by function • User, server tracking • Client • Cookie, Referer, From, User-agent • Server • Set-cookie, Server

  24. AL: User-server interaction: authentication Authentication goal: control access to server documents stateless: client must present authorization in each request authorization: typically name, password authorization: header line in request if no authorization presented, server refuses access, sends WWW authenticate: header line in response http://www.sandbox.com/clipboard/pub-doc/home.jsp usual http request msg + Authorization:line usual http request msg + Authorization:line usual http response msg usual http response msg time server client usual http request msg 401: authorization req. WWW authenticate: Browser caches name & password so that user does not have to repeatedly enter it.

  25. AL: User-server interaction: cookies server sends “cookie” to client in response mst Set-cookie: 1678453 client presents cookie in later requests cookie: 1678453 server matches presented-cookie with server-stored info authentication remembering user preferences, previous choices, shopping carts usual http request msg cookie: # usual http request msg cookie: # usual http response msg usual http response msg server client usual http request msg usual http response + Set-cookie: # cookie- spectific action cookie- spectific action

  26. AL: User-server interaction: caching 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> server: response contains no object if cached copy up-to-date: HTTP/1.0 304 Not Modified http response HTTP/1.0 304 Not Modified server client http request msg If-modified-since: <date> object not modified http request msg If-modified-since: <date> object modified http response HTTP/1.1 200 OK … <data>

  27. AL: HTTP caching • Additional caching methods • If-Modified-Since requests to check • HTTP 0.9/1.0 used just date • ETag and If-Match • HTTP 1.1 has file signature as well • When/how often should the original be checked for changes? • Check every time? • Check each session? Day? Etc? • Use Expires header • If no Expires, often use Last-Modified as estimate

  28. AL: Example Cache Check Request GET / HTTP/1.1 Accept: */* Accept-Language: en-us Accept-Encoding: gzip, deflate If-Modified-Since: Mon, 29 Jan 2001 17:54:18 GMT If-None-Match: "7a11f-10ed-3a75ae4a" User-Agent: Mozilla/4.0 (compatible; MSIE 5.5; Windows NT 5.0) Host: www.cse.ogi.edu Connection: Keep-Alive

  29. AL: Example Cache Check Response HTTP/1.1 304 Not Modified Date: Tue, 27 Mar 2001 03:50:51 GMT Server: Apache/1.3.14 (Unix) (Red-Hat/Linux) mod_ssl/2.7.1 OpenSSL/0.9.5a DAV/1.0.2 PHP/4.0.1pl2 mod_perl/1.24 Connection: Keep-Alive Keep-Alive: timeout=15, max=100 ETag: "7a11f-10ed-3a75ae4a"

  30. AL: Web Caches (proxy server) user 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 server, then returns http response to client Goal: satisfy client request without involving origin server origin server Proxy server http request http request client http response http response http request http request http response http response client origin server

  31. AL: Why Web Caching? Assume: cache is “close” to client (e.g., in same network) smaller response time: cache “closer” to client decrease traffic to distant servers link out of institutional/local ISP network often bottleneck Further info on web caching http://www.ircache.net/ http://www.squid.org ICP http://www.rfc-editor.org/rfc/rfc2186.txt http://www.rfc-editor.org/rfc/rfc2187.txt origin servers public Internet 1.5 Mbps access link institutional network 10 Mbps LAN institutional cache

  32. AL: Non-persistent and persistent connections Non-persistent HTTP/1.0 server parses request, responds, and closes TCP connection Each object transfer suffers from slow start Persistent default for HTTP/1.1 on same TCP connection: server, parses request, responds, parses new request,.. Client sends requests for all referenced objects as soon as it receives base HTML. But most 1.0 browsers use parallel TCP connections.

  33. AL: HTTP 0.9/1.0 • One request/response per TCP connection • Simple to implement • Disadvantages • Multiple connection setups  three-way handshake each time • Several extra round trips added to transfer (if done serially) • Multiple slow starts

  34. AL: Non-persistent connections • Short transfers are hard on TCP • Stuck in slow start • Loss recovery is poor when windows are small • Lots of extra connections • Increases server state/processing • Server also forced to keep TIME_WAIT connection state • Why must server keep these? • Tends to be an order of magnitude greater than #of active connections, why?

  35. AL: Single Transfer Example Client Server 0 RTT SYN Client opens TCP connection SYN 1 RTT ACK DAT Client sends HTTP request for HTML ACK Server reads from disk DAT FIN 2 RTT ACK Client parses HTML Client opens TCP connection FIN ACK SYN SYN 3 RTT ACK Client sends HTTP request for image DAT Server reads from disk ACK 4 RTT DAT Image begins to arrive

  36. AL: Netscape Solution • Use multiple concurrent connections to improve response time • Different parts of Web page arrive independently • Can grab more of the network bandwidth than other users • Doesn’t necessarily improve response time • TCP loss recovery ends up being timeout dominated because windows are small

  37. AL: Persistent Connection Solution • Multiplex multiple transfers onto one TCP connection • Serialize transfers  client makes next request only after previous response • Benefits greatest for small objects • Up to 2x improvement in response time • Server resource utilization reduced due to fewer connection establishments and fewer active connections • TCP behavior improved • Longer connections help adaptation to available bandwidth • Larger congestion window improves loss recovery • HTTP/1.1 vs. HTTP/1.0 example • Multiple requests to www.cse.ogi.edu

  38. AL: Persistent Connection Example Client Server 0 RTT DAT Client sends HTTP request for HTML ACK Server reads from disk DAT 1 RTT ACK Client parses HTML Client sends HTTP request for image DAT Server reads from disk ACK DAT 2 RTT Image begins to arrive

  39. AL: Persistent Connection Solution • Pipelining requests • Getall – request HTML document and all embeds • Requires server to parse HTML files • Embeds returned serially • Doesn’t consider client cached documents • Getlist – request a set of documents • Implemented as a simple set of GETs • Problems with pipelined serialized requests • Stall in one object prevents delivery of others • Much of the useful information in first few bytes (layout info) • Multiple connections allow incremental rendering of images as they come in • Need application-level demux to emulate multiple connections • HTTP-NG, HTTP/2.0, HTTP range requests • Application specific solution to transport protocol problems • SCTP

  40. AL: Electronic Mail 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., Eudora, Outlook, elm, Netscape Messenger outgoing, incoming messages stored on server user agent user agent user agent user agent user agent user agent SMTP SMTP SMTP mail server mail server mail server outgoing message queue user mailbox

  41. AL: Electronic Mail: mail servers Mail Servers mailbox contains incoming messages (yet to be read) 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 user agent user agent user agent user agent user agent SMTP SMTP SMTP mail server mail server mail server

  42. AL: Electronic Mail: smtp [RFC 821] uses tcp to reliably transfer email msg from client to server, port 25 direct transfer: sending server to receiving server three phases of transfer handshaking (greeting) transfer of messages closure command/response interaction commands: ASCII text response: status code and phrase messages must be in 7-bit ASCII

  43. AL: Sample smtp interaction S: 220 hamburger.edu C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: <alice@crepes.fr> S: 250 alice@crepes.fr... Sender ok C: RCPT TO: <bob@hamburger.edu> S: 250 bob@hamburger.edu ... 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

  44. AL: 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)

  45. AL: 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 server uses CRLF.CRLF to determine end of message Comparison with http http: pull email: 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

  46. AL: Mail message format smtp: protocol for exchanging email msgs RFC 822: standard for text message format: header lines, e.g., To: From: Subject: differentfrom smtp commands! body the “message”, ASCII characters only header blank line body

  47. AL: Message format: multimedia extensions MIME: multimedia mail extension, RFC 2045, 2056 additional lines in msg header declare MIME content type From: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data ..... ......................... ......base64 encoded data MIME version method used to encode data multimedia data type, subtype, parameter declaration encoded data

  48. AL: MIME typesContent-Type: type/subtype; parameters Text example subtypes: plain, html Image example subtypes: jpeg, gif Audio exampe subtypes: basic (8-bit mu-law encoded), 32kadpcm (32 kbps coding) Video example subtypes: mpeg, quicktime Application other data that must be processed by reader before “viewable” example subtypes: msword, octet-stream

  49. AL: Multipart Type From: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Type: multipart/mixed; boundary=98766789 --98766789 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain Dear Bob, Please find a picture of a crepe. --98766789 Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data ..... ......................... ......base64 encoded data --98766789--

  50. AL: Mail access protocols SMTP: delivery/storage to receiver’s server Mail access protocol: retrieval from server POP: Post Office Protocol [RFC 1939] authorization (agent <-->server) and download IMAP: Internet Mail Access Protocol [RFC 1730] more features (more complex) manipulation of stored msgs on server HTTP: Hotmail , Yahoo! Mail, etc. user agent user agent sender’s mail server SMTP SMTP POP3 or IMAP receiver’s mail server

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