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Application Layer


Application Layer Goals: conceptual + implementation aspects of network application protocols client server paradigm service models learn about protocols by examining ... – PowerPoint PPT presentation

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Title: Application Layer

Application Layer
  • Goals
  • conceptual implementation aspects of network
    application protocols
  • client server paradigm
  • service models
  • learn about protocols by examining popular
    application-level protocols
  • More goals
  • specific protocols
  • http our focus
  • ftp
  • smtp
  • pop
  • dns done!
  • programming network applications
  • socket programming
  • Done!

Applications and application-layer protocols
  • Application communicating, distributed processes
  • running in network hosts in user space
  • exchange messages to implement app
  • e.g., email, file transfer, the Web
  • Application-layer protocols
  • one piece of an app
  • define messages exchanged by apps and actions
  • user services provided by lower layer protocols

Network applications some jargon
  • A process is a program that is running within a
  • Within the same host, two processes communicate
    with interprocess communication defined by the
  • Processes running in different hosts communicate
    with an application-layer protocol
  • A user agent is an interface between the user and
    the network application.
  • Webbrowser
  • E-mail mail reader
  • streaming audio/video media player

Client-server paradigm
  • Typical network app has two pieces client and
  • Client
  • initiates contact with server (speaks first)
  • typically requests service from server,
  • for Web, client is implemented in browser for
    e-mail, in mail reader, e.g., outlook
  • Server
  • provides requested service to client
  • e.g., Web server sends requested Web page, mail
    server delivers e-mail

Application-layer protocols (cont).
  • 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
  • Q 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

refer to socket prorgamming
What transport service does an app need?
  • Data loss
  • some apps (e.g., audio) can tolerate some loss
  • other apps (e.g., file transfer, telnet) require
    100 reliable data transfer
  • Timing
  • some apps (e.g., Internet telephony, interactive
    games) require low delay to be effective
  • Bandwidth
  • some apps (e.g., multimedia) require minimum
    amount of bandwidth to be effective
  • other apps (elastic apps) make use of whatever
    bandwidth they get

Transport service requirements of common apps
Time Sensitive no no no yes, 100s msec yes,
few secs yes, 100s msec yes and no
Application file transfer e-mail Web
documents real-time audio/video stored
audio/video interactive games financial apps
Data loss no loss no loss loss-tolerant loss-tole
rant loss-tolerant loss-tolerant no loss
Bandwidth elastic elastic elastic audio
5Kb-1Mb video10Kb-5Mb same as above few Kbps
up elastic
Services provided by Internet transport protocols
  • UDP service
  • unreliable data transfer between sending and
    receiving process
  • does not provide connection setup, reliability,
    flow control, congestion control, timing, or
    bandwidth guarantee
  • Q why bother? Why is there a UDP?
  • TCP service
  • connection-oriented setup required between
    client, server
  • reliable transport between sending and receiving
  • flow control sender wont overwhelm receiver
  • congestion control throttle sender when network
  • does not providing timing, minimum bandwidth

Internet apps their protocols and transport
Application layer protocol smtp RFC 821 telnet
RFC 854 http RFC 2068 ftp RFC
959 proprietary (e.g. RealNetworks) NSF proprieta
ry (e.g., Vocaltec)
Underlying transport protocol TCP TCP TCP TCP TCP
or UDP TCP or UDP typically UDP
Application e-mail remote terminal access Web
file transfer streaming multimedia remote file
server Internet telephony
The Web some jargon
  • User agent for Web is called a browser
  • MS Internet Explorer
  • Netscape Communicator
  • Firefox
  • Server for Web is called Web server
  • Apache (public domain)
  • MS Internet Information Server
  • 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
The Web the http protocol
  • 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
  • http1.0 RFC 1945
  • http1.1 RFC 2068

http request
PC running Explorer
http response
http request
Server running NCSA Web server
http response
Mac running Navigator
The http protocol more
  • http is stateless
  • server maintains no information about past client
  • http TCP transport service
  • 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

  • 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

HTTP Usage
  • HTTP is the protocol that supports communication
    between web browsers and web servers.
  • A Web Server is a HTTP server
  • Most clients/servers today speak version 1.1, but
    1.0 is also in use.

From the RFC
  • HTTP is an application-level protocol with the
    lightness and speed necessary for distributed,
    hypermedia information systems.

http 1.0 example
  • Suppose user enters URL

(contains text, references to 10 jpeg images)
  • 1a. http client initiates TCP connection to http
    server (process) at Port 80
    is default for http server.

1b. http server at host
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
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
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.
  • Fewer RTTs and less slow start.
  • Non-persistent
  • HTTP/1.0
  • server parses request, responds, and closes TCP
  • 2 RTTs to fetch each object
  • Each object transfer suffers from slow start

But most 1.0 browsers use parallel TCP
A Typical HTTP Session
  • User types into a
  • Browser translates into an IP
    address and tries to open a TCP connection with
    port 80 of that address
  • Once a connection is established, the browser
    sends the following byte stream
  • GET /dcs/index.html HTTP/1.1
  • HOST (plus an empty line
  • The host responds with
  • a set of headers indicating which protocol is
    actually being used, whether or not the file
    requested was found, how many bytes are contained
    in that file, and what kind of information is
    contained in the file ("MIME" type)
  • a blank line to indicate the end of the headers
  • the contents of the page

A Typical HTTP Session
  • If the browser finds images embedded in the page,
    it starts a separate request for each image
  • The TCP connection is kept alive a bit longer,
    then closes if no further requests are received
    from the browser

Request - Response
  • HTTP has a simple structure
  • client sends a request
  • server returns a reply.
  • HTTP can support multiple request-reply exchanges
    over a single TCP connection.

Well Known Address
  • The well known TCP port for HTTP servers is
    port 80.
  • Other ports can be used as well...

HTTP Versions
  • The original version now goes by the name HTTP
    Version 0.9
  • HTTP 0.9 was used for many years.
  • Starting with HTTP 1.0 the version number is part
    of every request.
  • tells the server what version the client can talk
    (what options are supported, etc).

HTTP 1.0 Request
Headers . . .
  • Lines of text (ASCII).
  • Lines end with CRLF \r\n
  • First line is called Request-Line

blank line
Request Line
  • Method URI HTTP-Version\r\n
  • The request line contains 3 tokens (words).
  • space characters separate the tokens.
  • Newline (\n) seems to work by itself (but the
    protocol requires CRLF)

HTTP Request
  • Format
  • Method URI HttpVersion

Method Description
OPTIONS capabilities of resource/server
GET retrieve resource
HEAD retrieve headers for resource, metadata
POST submit data to server retrieve result
PUT replace/insert resource on server
DELETE remove resource from server
TRACE trace request route through Web
Common Usage
  • GET, HEAD and POST are supported everywhere .
  • HTTP 1.1 servers often support PUT, DELETE,

URI Universal Resource Identifier
  • URIs defined in RFC 2396.
  • Absolute URI scheme//hostnameport/path
  • http//www.cs.rpi.edu80/blah/foo
  • Relative URI /path
  • /blah/foo

No server mentioned
URI Usage
  • When dealing with a HTTP 1.1 server, only a path
    is used (no scheme or hostname).
  • HTTP 1.1 servers are required to be capable of
    handling an absolute URI, but there are still
    some out there that wont
  • When dealing with a proxy HTTP server, an
    absolute URI is used.
  • client has to tell the proxy where to get the
  • more on proxy servers in a bit.

HTTP Version Number
  • HTTP/1.0 or HTTP/1.1
  • HTTP 0.9 did not include a version number in a
    request line.
  • If a server gets a request line with no HTTP
    version number, it assumes 0.9

The Header Lines
  • After the Request-Line come a number (possibly
    zero) of HTTP header lines.
  • Each header line contains an attribute name
    followed by a followed by a space and the
    attribute value.

The Name and Value are just text.
  • Request Headers provide information to the server
    about the client
  • what kind of client
  • what kind of content will be accepted
  • who is making the request
  • There can be 0 headers (HTTP 1.0)
  • HTTP 1.1 requires a Host header

HTTP Headers
  • Accept Indicates which data formats are
  • Accept text/html, text/plain
  • Content-Language Language of the content
  • Content-Language en
  • Content-Length Size of message body
  • Content-Length 1234
  • Content-Type MIME type of content body
  • Content-Type text/html
  • Date Date of request/response
  • Date Tue, 15 Nov 1994 081231 GMT
  • Expires When content is no longer valid
  • Expires Tue, 15 Nov 1994 081231 GMT
  • Host Machine that request is directed to
  • Host
  • Location Redirection to a different resource
  • Location http//
  • Retry-After Indicates that client must try again
    in future
  • Retry-After 120

Example HTTP Headers
  • Accept text/html
  • Host
  • From
  • User-Agent Mozilla/4.0
  • Referer http//

End of the Headers
  • Each header ends with a CRLF ( \r\n )
  • The end of the header section is marked with a
    blank line.
  • just CRLF
  • For GET and HEAD requests, the end of the headers
    is the end of the request!

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
http request message general format
Entity body is empty for GET, but not for POST
  • A POST request includes some content (some data)
    after the headers (after the blank line).
  • There is no format for the data (just raw bytes).
  • A POST request must include a Content-Length line
    in the headers
  • Content-length 267

Example GET Request
  • GET /hollingd/testanswers.html HTTP/1.1
  • Accept /
  • Host
  • User-Agent Internet Explorer
  • From
  • Referer http//

There is a blank line here!
Example POST Request
POST /hollingd/changegrade.cgi HTTP/1.1 Accept
/ Host User-Agent SecretAgent
V2.3 Content-Length 35 Referer
http// stuid6660182722ite
Typical Method Usage
  • GET used to retrieve an HTML document.
  • HEAD used to find out if a document has changed.
  • POST used to submit a form.

http message format respone
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
HTTP Response
  • Format
  • HTTP Version Status Code Reason

Status Reason Description
200 OK Successful request
206 Partial Content Successful request for partial content
301 Moved Permanently Resource has been relocated
304 Not Modified Conditional GET but resource has not changed
400 Bad Request Request not understood
403 Forbidden Access to resource not allowed
404 Not Found URI/resource not found on server
500 Internal Server Error Unexpected error
http response status codes
In first line in server-gtclient response
message. A few sample codes
  • 200 OK
  • request succeeded, requested object later in this
  • 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

Response Headers
  • Provide the client with information about the
    returned entity (document).
  • what kind of document
  • how big the document is
  • how the document is encoded
  • when the document was last modified
  • Response headers end with blank line

Response Header Examples
  • Date Wed, 30 Jan 2002 124817 EST
  • Server Apache/1.17
  • Content-Type text/html
  • Content-Length 1756
  • Content-Encoding gzip

  • Content can be anything (sequence of raw bytes).
  • Content-Length header is required for any
    response that includes content.
  • Content-Type header also required.

Single Request/Reply
  • The client sends a complete request.
  • The server sends back the entire reply.
  • The server closes its socket.
  • If the client needs another document it must open
    a new connection.

This was the default for HTTP 1.0
Persistent Connections
  • HTTP 1.1 supports persistent connections (this is
    the default).
  • Multiple requests can be handled over a single
    TCP connection.
  • The Connection header is used to exchange
    information about persistence (HTTP/1.1)
  • 1.0 Clients used a Keep-alive header

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 Anything typed in
sent to port 80 at
telnet 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 /index.html HTTP/1.0
3. Look at response message sent by http server!
User-server interaction authentication
  • Authentication goal control access to server
  • 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

usual http request msg
401 authorization req. WWW authenticate
Browser caches name password so that user does
not have to repeatedly enter it.
User-server interaction cookies
Sometimes it is deirable for servers to identify
users!!! We need states!!!
  • 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 (Database)
  • authentication
  • remembering user preferences, previous choices

usual http request msg
usual http response Set-cookie
cookie- spectific action
cookie- spectific action
User-server interaction conditional GET
Handle cached objects stale or fresh?
  • Goal dont send object if client has up-to-date
    stored (cached) version
  • client specify date of cached copy in http
  • 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
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
http response
http response
http request
http request
http response
http response
origin server
Why Web Caching?
origin servers
  • Assume cache is close to client (e.g., in same
  • smaller response time cache closer to client
  • decrease traffic to distant servers
  • link out of institutional/local ISP network often

public Internet
1.5 Mbps access link
institutional network
10 Mbps LAN
institutional cache
  • Why do we need web proxy or web caching?
  • If we use http/1.0 to request an object from a
    Web server, how many RTTs would it take?
  • How about http/1.1?

  • How do we implement a web proxy?

How do we implement a Web proxy?
  • Based on TCP, using HTTP 1.0/1.1
  • Refer to http1.0 RFC 1945 and http1.1 RFC 2068
    for HTTP protocols
  • Two basic functionalities
  • Relay requests and responses for clients and web
  • Web caching
  • Cache hit, check the freshness of objects
  • Cache miss, get it from a server and cache it
  • Support http 1.0 and 1.1 ?

ftp the file transfer protocol
file transfer
user at host
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

ftp separate control, data connections
  • ftp client contacts ftp server at port 21,
    specifying TCP as transport protocol
  • two parallel TCP connections opened
  • control exchange commands, responses between
    client, server.
  • out of band control
  • data file data to/from server
  • ftp server maintains state current directory,
    earlier authentication

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
  • 425 Cant open data connection
  • 452 Error writing file

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

Electronic Mail mail servers
  • Mail Servers
  • mailbox contains incoming messages (yet to be
    read) for user
  • message queue of outgoing (to be sent) mail
  • smtp protocol between mail servers to send email
  • client sending mail server
  • server receiving mail server

Electronic Mail smtp RFC 821
  • uses tcp to reliably transfer email msg from
    client to server, port 25
  • direct transfer sending server 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
  • Binary -gt ascii -gt binary painful!

Sample smtp interaction
S 220 C HELO
S 250 Hello, pleased to meet
you C MAIL FROM ltalice_at_crepes.frgt
S 250 Sender ok C RCPT
TO ltbob_at_hamburger.edugt 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 closing connection
try smtp interaction for yourself
  • telnet servername 25
  • see 220 reply from server
  • above lets you send email without using email
    client (reader)

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
  • smtp server uses CRLF.CRLF to determine end of
  • 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

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

blank line
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
encoded data
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

Multipart Type
From To
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--
Mail access protocols
receivers mail server
  • SMTP delivery/storage to receivers server
  • Mail access protocol retrieval from server
  • POP Post Office Protocol RFC 1939 POP3 version
  • 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.

POP3 protocol
S OK POP3 server ready C user alice S OK
C pass hungry S OK user successfully logged
  • authorization phase
  • client commands
  • user declare username
  • pass password
  • server responses
  • OK
  • -ERR
  • transaction phase, client
  • list list message numbers
  • 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
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