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Computer Networks

Application layer (Part 2)

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Application layer protocols

• Last class– Application layer functions

– Application protocols• DNS

• ftp

• This class– More application protocols

• http

• smtp

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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

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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

PC runningExplorer

Server running

NCSA Webserver

Mac runningNavigator

http request

http re

quest

http response

http re

sponse

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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

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

aside

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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.

2. http client sends http request message (containing URL) into TCP connection socket

1b. http server at host www.someSchool.edu waiting for TCP connection at port 80. “accepts” connection, notifying client

3. http server receives request message, forms response message containing requested object (someDepartment/home.index), sends message into socket

time

(contains text, references to 10

jpeg images)

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AL: http example (cont.)

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

4. http server closes TCP connection.

time

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AL: http message format: request

• two types of http messages: request, response

• http request message:– ASCII (human-readable format)

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)

request line(GET, POST,

HEAD commands)

header lines

Carriage return, line feed

indicates end of message

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AL: http request message: general format

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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

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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

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AL: http request

• Blank-line– Separate request headers from POST information

– End of request

• Body– If POST, send POST information

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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

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AL: http message format: response

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 ...

status line(protocol

status codestatus phrase)

header lines

data, e.g., requestedhtml file

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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

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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:

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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

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AL: http response

• Blank-line– Separate headers from data

• Body– Data being returned to client

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AL: http response example

HTTP/1.1 200 OKDate: Tue, 27 Mar 2001 03:49:38 GMTServer: 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/1.24Last-Modified: Mon, 29 Jan 2001 17:54:18 GMTETag: "7a11f-10ed-3a75ae4a"Accept-Ranges: bytesContent-Length: 4333Keep-Alive: timeout=15, max=100Connection: Keep-AliveContent-Type: text/html…..

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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.eduAnything typed in sent to port 80 at www.cse.ogi.edu

telnet www.cse.ogi.edu 80

2. Type in a GET http request:

GET /~wuchang/index.html HTTP/1.0 By typing this in (hit carriagereturn twice), you sendthis minimal (but complete) GET request to http server

3. Look at response message sent by http server!

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AL: More HTTP examples

• http://www.cse.ogi.edu/class/cse524/http.txt• http://www.cse.ogi.edu/class/cse524/http_post.txt

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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

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AL: Some HTTP headers by function

• User, server tracking– Client

• Cookie, Referer, From, User-agent

– Server• Set-cookie, Server

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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, sendsWWW authenticate:

header line in response• http://www.sandbox.com/clipboard/p

ub-doc/home.jsp

client server

usual http request msg401: authorization req.

WWW authenticate:

usual http request msg

+ Authorization:lineusual http response

msg

usual http request msg

+ Authorization:lineusual http response

msg

time

Browser caches name & password sothat user does not have to repeatedly enter it.

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AL: User-server interaction: cookies

• server sends “cookie” to client in response mstSet-cookie: 1678453

• client presents cookie in later requestscookie: 1678453

• server matches presented-cookie with server-stored info

– authentication

– remembering user preferences, previous choices, shopping carts

client server

usual http request msgusual http response

+Set-cookie: #

usual http request msg

cookie: #usual http response

msg

usual http request msg

cookie: #usual http response msg

cookie-spectificaction

cookie-spectificaction

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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 requestIf-modified-since: <date>

• server: response contains no object if cached copy up-to-date: HTTP/1.0 304 Not Modified

client server

http request msgIf-modified-since:

<date>

http responseHTTP/1.0

304 Not Modified

object not

modified

http request msgIf-modified-since:

<date>

http responseHTTP/1.1 200 OK

…

<data>

object modified

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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

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AL: Example Cache Check Request

GET / HTTP/1.1Accept: */*Accept-Language: en-usAccept-Encoding: gzip, deflateIf-Modified-Since: Mon, 29 Jan 2001 17:54:18 GMTIf-None-Match: "7a11f-10ed-3a75ae4a"User-Agent: Mozilla/4.0 (compatible; MSIE 5.5;

Windows NT 5.0)Host: www.cse.ogi.eduConnection: Keep-Alive

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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"

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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

client

Proxyserver

client

http request

http re

quest

http response

http re

sponse

http re

quest

http re

sponse

http requesthttp response

origin server

origin server

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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

originservers

public Internet

institutionalnetwork 10 Mbps LAN

1.5 Mbps access link

institutionalcache

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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 useparallel TCP connections.

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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

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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?

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AL: Single Transfer Example

Client ServerSYN

SYN

SYN

SYN

ACK

ACK

ACK

ACK

ACK

DAT

DAT

DAT

DAT

FIN

ACK

0 RTT

1 RTT

2 RTT

3 RTT

4 RTT

Server reads from disk

FIN

Server reads from disk

Client opens TCP connection

Client sends HTTP request for HTML

Client parses HTMLClient opens TCP connection

Client sends HTTP request for image

Image begins to arrive

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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

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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

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AL: Persistent Connection Example

Client Server

ACK

ACK

DAT

DAT

ACK

0 RTT

1 RTT

2 RTT

Server reads from diskClient sends HTTP request for HTML

Client parses HTMLClient sends HTTP request for image

Image begins to arrive

DAT

Server reads from disk

DAT

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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

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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 mailbox

outgoing message queue

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP

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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

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP

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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

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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

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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)

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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

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AL: Mail message format

smtp: protocol for exchanging email 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

blankline

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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

multimedia datatype, subtype,

parameter declaration

method usedto encode data

MIME version

encoded data

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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

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AL: Multipart TypeFrom: alice@crepes.fr To: bob@hamburger.edu Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Type: multipart/mixed; boundary=98766789 --98766789Content-Transfer-Encoding: quoted-printableContent-Type: text/plain

Dear Bob, Please find a picture of a crepe.--98766789Content-Transfer-Encoding: base64Content-Type: image/jpeg

base64 encoded data ..... ......................... ......base64 encoded data --98766789--

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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.

useragent

sender’s mail server

useragent

SMTP SMTP POP3 orIMAP

receiver’s mail server

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AL: POP3 protocol

authorization phase• client commands:

– user: declare username– pass: password

• server responses– +OK– -ERR

transaction phase, client:• list: list message numbers• top: get head of message• retr: retrieve message by

number• dele: delete• quit

C: list S: 1 498 S: 2 912 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 server signing off

S: +OK POP3 server ready C: user alice S: +OK C: pass hungry S: +OK user successfully logged on