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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: will be discussed later - PowerPoint PPT Presentation
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1
Application LayerGoals: 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: will be discussed
later
programming network applications socket programming
• Done!
2
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 WebApplication-layer protocols
one “piece” of an app define messages
exchanged by apps and actions taken
use services provided by lower layer protocols
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
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Network applications: some jargon
A process is a program that is running within a host.
Within the same host, two processes communicate with IPC defined by the OS.
Processes running in different hosts communicate with an application-layer protocol
A user agent is an interface between the user and the network application. Web:browser E-mail: mail reader streaming
audio/video: media player
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Client-server paradigm
Typical network app has two pieces: client and server
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
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
request
reply
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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
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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
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Transport service requirements of common apps
Application
file transfere-mail
Web documentsreal-time audio/video
stored audio/videointeractive games
financial apps
Data loss
no lossno lossloss-tolerantloss-tolerant
loss-tolerantloss-tolerantno loss
Bandwidth
elasticelasticelasticaudio: 5Kb-1Mbvideo:10Kb-5Mbsame as above few Kbps upelastic
Time Sensitive
nononoyes, 100’s msec
yes, few secsyes, 100’s msecyes and no
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Services provided by Internet transport protocols
TCP service: connection-oriented: setup
required between client, server
reliable transport between sending and receiving process
flow control: sender won’t overwhelm receiver
congestion control: throttle sender when network overloaded
does not providing: timing, minimum bandwidth guarantees
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?
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Internet apps: their protocols and transport protocols
Application
e-mailremote terminal access
Web file transfer
streaming multimedia
remote file serverInternet telephony
Applicationlayer protocol
smtp [RFC 821]telnet [RFC 854]http [RFC 2068]ftp [RFC 959]proprietary(e.g. RealNetworks)NSFproprietary(e.g., Vocaltec)
Underlyingtransport protocol
TCPTCPTCPTCPTCP or UDP
TCP or UDPtypically UDP
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The Web: some jargon
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 Firefox
Server for Web is called Web server: Apache (public
domain) MS Internet
Information Server
www.someSchool.edu/someDept/pic.gif
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The Web: the http protocol
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
http1.0: RFC 1945 http1.1: RFC 2068 (1997),
2616 (1999)
PC runningExplorer
Server runningWeb
server
Mac runningNavigator
http request
http re
quest
http response
http re
sponse
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The http protocol: more
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
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
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.
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From the RFC
“HTTP is an application-level protocol with the lightness and speed necessary for distributed, hypermedia information systems.”
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http 1.0 exampleSuppose 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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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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Non-persistent and persistent connections
Non-persistent HTTP/1.0 server parses
request, responds, and closes TCP connection
2 RTTs to fetch each object
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.
Fewer RTTs and less slow start.But most 1.0 browsers use
parallel TCP connections.
A Typical HTTP Session
User types “www.seattleu.edu/index.html” into a browser Browser translates www.seattleu.edu 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 /index.html HTTP/1.1 HOST: www.seattleu.edu” (plus an empty line below)
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
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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
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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.
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Well Known Address
The “well known” TCP port for HTTP servers is port 80.
Other ports can be used as well...
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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).
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HTTP 1.0+ Request
Lines of text (ASCII).
Lines end with CRLF “\r\n”
First line is called “Request-Line”
Request-LineHeaders...
Content...blank lineblank line
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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)
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HTTP Request Format:
Method URI HttpVersion
Method Description
OPTIONS capabilities of resource/server
GET retrieve resource
HEADretrieve 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 Web25
Common Usage
GET, HEAD and POST are supported everywhere .
HTTP 1.1 servers often support PUT, DELETE, OPTIONS & TRACE.
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URI: Universal Resource Identifier
URIs defined in RFC 2396.
Absolute URI: scheme://hostname[:port]/pathhttp://www.cs.rpi.edu:80/blah/foo
Relative URI: /path/blah/foo
No server mentioned27
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 won’t…
When dealing with a proxy HTTP server, an absolute URI is used. client has to tell the proxy where to get the
document! more on proxy servers in a bit….
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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
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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.
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Headers
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
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HTTP Headers
Accept: Indicates which data formats are acceptable. 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 08:12:31 GMT
Expires: When content is no longer valid Expires: Tue, 15 Nov 1994 08:12:31 GMT
Host: Machine that request is directed to Host: www.cs.uct.ac.za
Location: Redirection to a different resource Location: http://myserver.org/
Retry-After: Indicates that client must try again in future Retry-After: 120
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Example HTTP Headers
Accept: text/html
Host: www.seattleu.edu
From: [email protected]
User-Agent: Mozilla/4.0
Referer: http://foo.com/blah
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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!
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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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http request message: general format
Entity body is empty for “GET”, but not for “POST”
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
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Example GET Request
GET /testanswers.html HTTP/1.1
Accept: */*
Host: www.seattleu.edu
User-Agent: Internet Explorer
From: [email protected]
Referer: http://foo.com/There is a blank line here!
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Example POST RequestExample POST Request
POST /changegrade.cgi HTTP/1.1POST /changegrade.cgi HTTP/1.1
Accept: */*Accept: */*
Host: www.seattleu.eduHost: www.seattleu.edu
User-Agent: SecretAgent V2.3User-Agent: SecretAgent V2.3
Content-Length: 35Content-Length: 35
Referer: http://monte.seattleu.edu/blahReferer: http://monte.seattleu.edu/blah
stuid=6660182722&item=test1&grade=99stuid=6660182722&item=test1&grade=99
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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.
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http message format: respone
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
HTTP Response Format:
HTTP Version Status Code Reason
Status Reason Description
200 OK Successful request
206 Partial Content Successful request for partial content
301Moved Permanently
Resource has been relocated
304 Not ModifiedConditional 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
500Internal Server Error
Unexpected error42
43
http response status 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:
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
44
Response Header Examples
Date: Wed, 30 Jan 2002 12:48:17 EST
Server: Apache/1.17
Content-Type: text/html
Content-Length: 1756
Content-Encoding: gzip
45
Content
Content can be anything (sequence of raw bytes).
Content-Length header is required for any response that includes content.
Content-Type header also required.
46
Single Request/Reply
The client sends a complete request. The server sends back the entire reply. The server closes it’s socket.
If the client needs another document it must open a new connection.
This was the default for HTTP 1.0
47
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
48
49
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.amazon.comAnything typed in sent to port 80 at www.eurecom.fr
telnet www.google.com 80
2. Type in a GET http request:
GET /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!
50
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
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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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 (Database) authentication remembering user
preferences, previous choices
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
Sometimes it is deirable for servers to identify users!!! We need states!!!
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User-server 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 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
Handle cached objects: stale or fresh?
53
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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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
originservers
public Internet
institutionalnetwork 10 Mbps LAN
1.5 Mbps access link
institutionalcache
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Discussion
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?
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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 servers
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 ?
57
ftp: the file transfer protocol
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
file transfer FTPserver
FTPuser
interface
FTPclient
local filesystem
remote filesystem
user at host
58
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
FTPclient
FTPserver
TCP control connection
port 21
TCP data connectionport 20
59
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 starting
425 Can’t open data connection
452 Error writing file
60
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
61
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
62
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 Binary -> ascii -> binary: painful!
63
Sample smtp interaction S: 220 hamburger.edu C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: <[email protected]> S: 250 [email protected]... Sender ok C: RCPT TO: <[email protected]> S: 250 [email protected] ... 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
64
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)
65
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
66
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
67
Message format: multimedia extensions
MIME: multimedia mail extension, RFC 2045, 2056 additional lines in msg header declare MIME content
type
From: [email protected] To: [email protected] 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
68
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
69
Multipart TypeFrom: [email protected] To: [email protected] 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--
70
Mail access protocols
SMTP: delivery/storage to receiver’s server Mail access protocol: retrieval from server
POP: Post Office Protocol [RFC 1939] POP3 version 3 • 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
71
POP3 protocol
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: <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