CSE 524: Lecture 16

1
CSE 524 Lecture 16

• 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
• Webs 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/someDep
  artment/home.index

(contains text, references to 10 jpeg images)

• 1a. http client initiates TCP connection to http
  server (process) at www.someSchool.edu. Port 80
  is default for http server.

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.)
4. http server 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
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-languagefr (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 120015
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
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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

16
AL http response codes
In first line in server-gtclient 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 server
• 404 Not Found
• requested document not found on this server
• 505 HTTP Version Not Supported

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

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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 OK
• Date Tue, 27 Mar 2001 034938 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 175418 GMT
• ETag "7a11f-10ed-3a75ae4a"
• Accept-Ranges bytes
• Content-Length 4333
• Keep-Alive timeout15, max100
• 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

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.
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AL User-server interaction cookies
server
client

• 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
usual http response Set-cookie
cookie- spectific action
cookie- spectific action
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AL User-server interaction caching
server
client

• Goal dont send object if client has up-to-date
  stored (cached) version
• client specify date of cached copy in http
  request
• If-modified-since ltdategt
• server response contains no object if cached
  copy up-to-date
• HTTP/1.0 304 Not Modified

http request msg If-modified-since ltdategt
object not modified
http request msg If-modified-since ltdategt
object modified
http response HTTP/1.1 200 OK ltdatagt
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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/1.1
• Accept /
• Accept-Language en-us
• Accept-Encoding gzip, deflate
• If-Modified-Since Mon, 29 Jan 2001 175418 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

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AL Example Cache Check Response

• HTTP/1.1 304 Not Modified
• Date Tue, 27 Mar 2001 035051 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 timeout15, max100
• ETag "7a11f-10ed-3a75ae4a"

30
AL Web Caches (proxy server)
Goal satisfy client request without involving
origin 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

origin server
Proxy server
http request
http request
client
http response
http response
http request
http request
http response
http response
client
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

origin servers
public Internet
1.5 Mbps access link
institutional network
10 Mbps LAN
institutional cache
32
AL Non-persistent and persistent connections

• 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.

• Non-persistent
• HTTP/1.0
• server parses request, responds, and closes TCP
  connection
• Each object transfer suffers from slow start

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
SYN
0 RTT
SYN
Client opens TCP connection
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
DAT
Client sends HTTP request for image
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
• Doesnt 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
• Doesnt 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

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

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 ltalice_at_crepes.frgt
S 250 alice_at_crepes.fr... Sender ok C RCPT
TO ltbob_at_hamburger.edugt S 250
bob_at_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
• different from 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

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_at_crepes.fr To bob_at_hamburger.edu
Subject Picture of yummy crepe. MIME-Version
1.0 Content-Type multipart/mixed
boundary98766789 --98766789 Content-Transfer-En
coding 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
POP3 or IMAP
receivers mail server

• SMTP delivery/storage to receivers server
• Mail access protocol retrieval from server
• POP Post Office Protocol RFC 1939
• authorization (agent lt--gtserver) and download
• IMAP Internet Mail Access Protocol RFC 1730
• more features (more complex)
• manipulation of stored msgs on server
• HTTP Hotmail , Yahoo! Mail, etc.

51
AL POP3 protocol
S OK POP3 server ready C user alice S OK
C pass hungry S OK user successfully logged
on

• 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 ltmessage 1
contentsgt S . C dele 1 C retr
2 S ltmessage 1 contentsgt S .
C dele 2 C quit S OK POP3 server
signing off