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Application LayerApplication Layer
CE-143CE-143
CNE-0802CNE-0802
Applications and application-layer Applications and application-layer protocolsprotocols
Application: communicating, Application: communicating, distributed processesdistributed processes e.g., e-mail, Web, P2P file e.g., e-mail, Web, P2P file
sharing, instant messaging sharing, instant messaging running in end systems running in end systems
(hosts) (hosts) exchange messages to exchange messages to
implement applicationimplement application
Application-layer protocolsApplication-layer protocols one “piece” of an appone “piece” of an app define messages exchanged define messages exchanged
by apps and actions takenby apps and actions taken use communication services use communication services
provided by lower layer provided by lower layer protocols (TCP, UDP)protocols (TCP, UDP)
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
App-layer protocol App-layer protocol definesdefines
Types of messages Types of messages exchangedexchanged, eg, request , eg, request & response messages& response messages
Syntax of messageSyntax of message types: what fields in types: what fields in messages & how fields messages & how fields are delineatedare delineated
Semantics of the Semantics of the fieldsfields, ie, meaning of , ie, meaning of information in fieldsinformation in fields
RulesRules for when and how for when and how processes send & processes send & respond to messagesrespond to messages
Public-domain protocols:Public-domain protocols: defined in RFCsdefined in RFCs allows for allows for
interoperabilityinteroperability eg, HTTP, SMTPeg, HTTP, SMTP
Proprietary protocols:Proprietary protocols: eg, KaZaAeg, KaZaA
Client-server paradigmClient-server paradigmTypical network app has two Typical network app has two
pieces: pieces: clientclient and and serverserverapplicatio
ntransportnetworkdata linkphysical
application
transportnetworkdata linkphysical
ClientClient:: initiates contact with server initiates contact with server
(“speaks first”)(“speaks first”) typically requests service from typically requests service from
server, server, Web: client implemented in Web: client implemented in
browser; e-mail: in mail readerbrowser; e-mail: in mail reader
request
reply
ServerServer:: provides requested service to clientprovides requested service to client e.g., Web server sends requested Web e.g., Web server sends requested Web
page, mail server delivers e-mailpage, mail server delivers e-mail
What transport service does an What transport service does an app need?app need?
Data lossData loss some apps (e.g., audio) some apps (e.g., audio)
can tolerate some losscan tolerate some loss other apps (e.g., file other apps (e.g., file
transfer, telnet) transfer, telnet) require 100% reliable require 100% reliable data transferdata transfer
TimingTiming some apps (e.g., some apps (e.g.,
Internet telephony, Internet telephony, interactive games) interactive games) require low delay to require low delay to be “effective”be “effective”
BandwidthBandwidth some apps (e.g., some apps (e.g.,
multimedia) multimedia) require minimum require minimum amount of amount of bandwidth to be bandwidth to be “effective”“effective”
other apps (“elastic other apps (“elastic apps”) make use of apps”) make use of whatever whatever bandwidth they get bandwidth they get
Aplikasi InternetAplikasi InternetDNS, WEB, DNS, WEB,
EMAILEMAIL
Domain Name Domain Name SystemSystem(DNS)(DNS)
DNS: Domain Name SystemDNS: Domain Name System
Internet hosts:Internet hosts: IP address (32 bit) - used for addressing IP address (32 bit) - used for addressing
datagramsdatagrams ““name”, e.g., ww.yahoo.com - used by name”, e.g., ww.yahoo.com - used by
humanshumans
DNS:DNS: provides translation between host provides translation between host name and IP addressname and IP address distributed databasedistributed database implemented in implemented in
hierarchy of many hierarchy of many name serversname servers Distributed for scalability & reliabilityDistributed for scalability & reliability
DNS Services DNS Services
Hostname to IP address translationHostname to IP address translation Host aliasingHost aliasing
Canonical and alias namesCanonical and alias names Mail server aliasingMail server aliasing Load distributionLoad distribution
Replicated Web servers: set of IP Replicated Web servers: set of IP addresses for one canonical nameaddresses for one canonical name
requesting hostCe1.poltek.ac.id
www.google.com
root DNS server
local DNS serverdns.poltek.ac.id
1
23
4
5
6
authoritative DNS serverNs1.google.com
78
TLD DNS server
DNS DNS InfrastructureInfrastructure Host at Host at
cis.poly.edu wants cis.poly.edu wants IP address for IP address for gaia.cs.umass.edugaia.cs.umass.edu
Infrastructure:Infrastructure: Client resolverClient resolver Local DNS serverLocal DNS server Authoritative DNS Authoritative DNS
ServerServer Root DNS ServerRoot DNS Server Top-Level Domain Top-Level Domain
DNS ServerDNS Server
Distributed, Hierarchical Distributed, Hierarchical DatabaseDatabase
Root servers and TLD servers typically do Root servers and TLD servers typically do not contain hostname to IP mappings; not contain hostname to IP mappings; they contain mappings for locating they contain mappings for locating authoritative servers. authoritative servers.
Root DNS Servers
com DNS servers ca DNS servers id DNS servers
co.idDNS servers
ac.idDNS servers
yahoo.comDNS servers
amazon.comDNS servers
ucalgary.caDNS servers
TLDServers
usask.caDNS servers
requesting hostCe1.poltek.ac.id
www.google.com
root DNS server
local DNS serverNs1.poltek.ac.id
1
2
45
6
authoritative DNS serverNs1.google.com
7
8
TLD DNS server
3
Recursive Recursive queriesqueriesrecursive queryrecursive query::
puts burden of puts burden of name resolution name resolution on contacted on contacted name servername server
heavy load?heavy load?
iterated queryiterated query:: contacted server contacted server
replies with name replies with name of server to of server to contactcontact
““I don’t know this I don’t know this name, but ask name, but ask this server”this server”
DNS recordsDNS recordsDNSDNS:: distributed db storing resource records distributed db storing resource records (RR)(RR)
Type=NSType=NS namename is domain (e.g. is domain (e.g.
foo.com)foo.com) valuevalue is IP address of is IP address of
authoritative name server authoritative name server for this domainfor this domain
RR format: (name, value, type, ttl)
Type=AType=A namename is hostname is hostname valuevalue is IP address is IP address
Type=CNAMEType=CNAME namename is alias name for some is alias name for some
“cannonical” (the real) name“cannonical” (the real) name
www.ibm.com www.ibm.com is reallyis really servereast.backup2.ibm.comservereast.backup2.ibm.com
valuevalue is cannonical name is cannonical name
Type=MXType=MX valuevalue is name of mailserver is name of mailserver
associated with associated with namename
DNS protocol, messagesDNS protocol, messagesDNS protocolDNS protocol : : queryquery and and replyreply messages, both with same messages, both with same message formatmessage format
msg headermsg header identificationidentification:: 16 bit 16 bit
# for query, reply to # for query, reply to query uses same #query uses same #
flagsflags:: query or replyquery or reply recursion desired recursion desired recursion availablerecursion available reply is reply is
authoritativeauthoritative
DNS protocol, messagesDNS protocol, messages
Name, type fields for a query
RRs in reponseto query
records forauthoritative servers
additional “helpful”info that may be used
DNS Server Configuration DNS Server Configuration (Sample)(Sample)
; zone file for example.com $TTL 2d ; 172800 secs ; zone file for example.com $TTL 2d ; 172800 secs default TTL for zone @ IN SOA ns1.example.com. default TTL for zone @ IN SOA ns1.example.com. hostmaster.example.com. ( hostmaster.example.com. (
2003080800 ; 2003080800 ;
se = serial number 12h ; se = serial number 12h ;
ref = refresh 15m ; ref = refresh 15m ;
ret = update retry 3w ; ret = update retry 3w ;
ex = expiry 3h ; ex = expiry 3h ;
min = minimum ) min = minimum )
IN NS ns1.example.com. IN NS ns1.example.com.
IN MX 10 mail.example.net. IN MX 10 mail.example.net.
joe IN A 192.168.254.3 joe IN A 192.168.254.3
www IN CNAME joe www IN CNAME joe
Security Issue on DNSSecurity Issue on DNS
DNS is plain text DNS is plain text easy to capture easy to capture No security mechanism to secure No security mechanism to secure
requestrequest Even easy to do Name Server Even easy to do Name Server
HijackingHijacking It is vulnerable for Buffer Overflow It is vulnerable for Buffer Overflow
AttackAttack
World Wide WebWorld Wide Web
WWW BackgroundWWW Background
1989-1990 – Tim Berners-Lee invents the 1989-1990 – Tim Berners-Lee invents the World Wide Web at CERNWorld Wide Web at CERN Means for transferring text and graphics Means for transferring text and graphics
simultaneouslysimultaneously Client/Server data transfer protocolClient/Server data transfer protocol
Communication via application level protocolCommunication via application level protocol System ran on top of standard networking infrastructureSystem ran on top of standard networking infrastructure
Text mark up languageText mark up language Not invented by Bernes-LeeNot invented by Bernes-Lee Simple and easy to useSimple and easy to use Requires a client application to render text/graphicsRequires a client application to render text/graphics
WWW ComponentsWWW Components
Structural ComponentsStructural Components Clients/browsers – to dominant implementationsClients/browsers – to dominant implementations Servers – run on sophisticated hardwareServers – run on sophisticated hardware Caches – many interesting implementationsCaches – many interesting implementations Internet – the global infrastructure which Internet – the global infrastructure which
facilitates data transferfacilitates data transfer Semantic ComponentsSemantic Components
Hyper Text Transfer Protocol (HTTP)Hyper Text Transfer Protocol (HTTP) Hyper Text Markup Language (HTML)Hyper Text Markup Language (HTML)
eXtensible Markup Language (XML)eXtensible Markup Language (XML) Uniform Resource Identifiers (URI)Uniform Resource Identifiers (URI) Uniform Resource Locator (URL)Uniform Resource Locator (URL)
Uniform Resource Uniform Resource IdentifiersIdentifiers
Web resources need names/identifiers – Web resources need names/identifiers – Uniform Resource Identifiers (URIs)Uniform Resource Identifiers (URIs) Resource can reside anywhere on the InternetResource can reside anywhere on the Internet
URIs are a somewhat abstract notionURIs are a somewhat abstract notion A pointer to a resource to which request methods A pointer to a resource to which request methods
can be applied to generate potentially different can be applied to generate potentially different responsesresponses
A request method is eg. fetching or changing the objectA request method is eg. fetching or changing the object
Instance: Instance: http://www.foo.com/index.htmlhttp://www.foo.com/index.html Protocol, server, resourceProtocol, server, resource
Most popular form of a URI is the Uniform Most popular form of a URI is the Uniform Resource Locator (URL)Resource Locator (URL) FIND THE DIFFERENCE BETWEEN URL AND FIND THE DIFFERENCE BETWEEN URL AND
URI!!!URI!!!
HTTP BasicsHTTP Basics Protocol for client/server communicationProtocol for client/server communication
The heart of the WebThe heart of the Web Very simple request/response protocolVery simple request/response protocol
Client sends request message, server replies with response Client sends request message, server replies with response messagemessage
StatelessStateless Relies on URI naming mechanismRelies on URI naming mechanism
Three versions have been usedThree versions have been used 09/1.0 – very close to Berners-Lee’s original 09/1.0 – very close to Berners-Lee’s original
RFC 1945 (original RFC is now expired)RFC 1945 (original RFC is now expired) 1.1 – developed to enhance performance, caching, 1.1 – developed to enhance performance, caching,
compressioncompression RFC 2068RFC 2068
1.0 dominates today but 1.1 is catching up1.0 dominates today but 1.1 is catching up
HTTP Request MessagesHTTP Request Messages GET – retrieve document specified by URLGET – retrieve document specified by URL PUT – store specified document under given URLPUT – store specified document under given URL HEAD – retrieve info. about document specified by HEAD – retrieve info. about document specified by
URLURL OPTIONS – retrieve information about available OPTIONS – retrieve information about available
optionsoptions POST – give information (eg. annotation) to the POST – give information (eg. annotation) to the
serverserver DELETE – remove document specified by URLDELETE – remove document specified by URL TRACE – loopback request messageTRACE – loopback request message CONNECT – for use by cachesCONNECT – for use by caches
HTTP Request FormatHTTP Request Format
First type of HTTP message: First type of HTTP message: requestsrequests Client browsers construct and send messageClient browsers construct and send message
Typical HTTP request:Typical HTTP request: GET GET http://www.cs.wisc.edu/index.htmlhttp://www.cs.wisc.edu/index.html
HTTP/1.0HTTP/1.0
request-line ( request request-URI HTTP-version)headers (0 or more)<blank line>body (only for POST request)
HTTP Response FormatHTTP Response Format
Second type of HTTP message: Second type of HTTP message: responseresponse Web servers construct and send response messagesWeb servers construct and send response messages
Typical HTTP response:Typical HTTP response: HTTP/1.0 301 Moved PermanentlyHTTP/1.0 301 Moved Permanently
Location: http://www.wisc.edu/cs/index.htmlLocation: http://www.wisc.edu/cs/index.html
status-line (HTTP-version response-code response-phrase)headers (0 or more)<blank line>body
HTTP Response CodesHTTP Response Codes 1xx – Informational – request received, 1xx – Informational – request received,
processingprocessing 2xx – Success – action received, 2xx – Success – action received,
understood, acceptedunderstood, accepted 3xx – Redirection – further action 3xx – Redirection – further action
necessarynecessary 4xx – Client Error – bad syntax or 4xx – Client Error – bad syntax or
cannot be fulfilledcannot be fulfilled 5xx – Server Error – server failed5xx – Server Error – server failed
HTTP HeadersHTTP Headers Both requests and responses can contain a Both requests and responses can contain a
variable number of header fieldsvariable number of header fields Consists of field name, colon, space, field valueConsists of field name, colon, space, field value 17 possible header types divided into three 17 possible header types divided into three
categoriescategories RequestRequest ResponseResponse BodyBody
Example: Date: Friday, 27-Apr-01 13:30:01 Example: Date: Friday, 27-Apr-01 13:30:01 GMTGMT
Example: Content-length: 3001Example: Content-length: 3001
HTTP/1.0 Network HTTP/1.0 Network InteractionInteraction
Clients make requests to port 80 on serversClients make requests to port 80 on servers Uses DNS to resolve server nameUses DNS to resolve server name
Clients make separate TCP connection for each URLClients make separate TCP connection for each URL Some browsers open multiple TCP connectionsSome browsers open multiple TCP connections
Netscape default = 4, WATCH OUT ON PIPELININGNetscape default = 4, WATCH OUT ON PIPELINING
Server returns HTML pageServer returns HTML page Many types of servers with a variety of implementationsMany types of servers with a variety of implementations Apache is the most widely usedApache is the most widely used
Freely available in source formFreely available in source form
Client parses pageClient parses page Requests embedded objectsRequests embedded objects
HTTP/1.1 Performance HTTP/1.1 Performance EnhancementsEnhancements
HTTP/1.0 is a “stop and wait” protocolHTTP/1.0 is a “stop and wait” protocol Separate TCP connection for each fileSeparate TCP connection for each file
Connect setup and tear down is incurred for each fileConnect setup and tear down is incurred for each file Inefficient use of packetsInefficient use of packets Server must maintain many connections in TIME_WAITServer must maintain many connections in TIME_WAIT
ISSUE on HTTP/1.0ISSUE on HTTP/1.0 Resulted in HTTP/1.1 specification focused on Resulted in HTTP/1.1 specification focused on
performance enhancementsperformance enhancements Persistent connectionsPersistent connections PipeliningPipelining Enhanced caching optionsEnhanced caching options Support for compressionSupport for compression
Persistent Connections Persistent Connections and Pipeliningand Pipelining
Persistent connectionsPersistent connections Use the same TCP connection(s) for transfer of multiple filesUse the same TCP connection(s) for transfer of multiple files Reduces packet traffic significantlyReduces packet traffic significantly May or may not increase performance from client May or may not increase performance from client
perspectiveperspective Load on server increasesLoad on server increases
PipeliningPipelining Pack as much data into a packet as possiblePack as much data into a packet as possible Requires length field(s) within headerRequires length field(s) within header May or may not reduce packet traffic or increase May or may not reduce packet traffic or increase
performanceperformance Page structure is criticalPage structure is critical
HTML BasicsHTML Basics
Hyper-Text Markup LanguageHyper-Text Markup Language A subset of Standardized General Markup Language (SGML)A subset of Standardized General Markup Language (SGML) Facilitates a hyper-media environmentFacilitates a hyper-media environment
Embedded links to other documents Embedded links to other documents and applicationsand applications Documents use elements to “mark up” or identify Documents use elements to “mark up” or identify
sections of text for different purposes or display sections of text for different purposes or display characteristicscharacteristics
Mark up elements are not seen by the user when page Mark up elements are not seen by the user when page is displayedis displayed
Documents are rendered by browsersDocuments are rendered by browsers NOTE: Not all documents in the Web are HTML!NOTE: Not all documents in the Web are HTML! Most people use WYSIWYG editors (MS Word) to Most people use WYSIWYG editors (MS Word) to
generate HTMLgenerate HTML
HTML ExampleHTML Example
<HTML><HEAD><TITLE> PB’s HomePage </TITLE></HEAD><BODY><CENTER><IMG SRC = “bad_picture.gif” ALT = “ “><BR></CENTER><P><CENTER><H1>UW Computer Science Department</H1></CENTER>Welcome to my goofy HomePage!…<A HREF = http://www.cs.wisc.edu/~pb/mydogs_page.html> Spot’s Page </A></BODY></HTML>
Security Issue on WebSecurity Issue on Web
By UserBy User DefacingDefacing SQL or Shell InjectSQL or Shell Inject DoSDoS
By OwnerBy Owner Phissing (try this: http://www.kilkbca.com)Phissing (try this: http://www.kilkbca.com) Malware (Nice app to be downloded, Malware (Nice app to be downloded,
contain malware)contain malware) Trojan/Virus/Worm (via sec’ hole in the Trojan/Virus/Worm (via sec’ hole in the
Browser)Browser)
EMAILEMAIL
EMail protocolsEMail protocols
SMTP: delivery/storage to receiver’s serverSMTP: delivery/storage to receiver’s server Mail access protocol: retrieval from serverMail access protocol: retrieval from server
POP: Post Office Protocol [RFC 1939]POP: Post Office Protocol [RFC 1939] authorization (agent <-->server) and download authorization (agent <-->server) and download
IMAP: Internet Mail Access Protocol [RFC 1730]IMAP: Internet Mail Access Protocol [RFC 1730] more features (secure, database intragration, broad authentication, etcmore features (secure, database intragration, broad authentication, etc
HTTP: Hotmail , Yahoo! Mail, etc.HTTP: Hotmail , Yahoo! Mail, etc.
useragent
sender’s mail server
useragent
SMTP SMTP POP3/IMAP/
WebMailreceiver’s mail
server
Electronic MailElectronic MailThree major componentsThree major components:: user agents user agents mail servers mail servers simple mail transfer simple mail transfer
protocol: SMTPprotocol: SMTP
User Agent (User Agent (mail reader)mail reader) composing, editing, composing, editing,
reading mail messagesreading mail messages e.g., Eudora, Outlook, elm, e.g., Eudora, Outlook, elm,
Netscape MessengerNetscape Messenger outgoing, incoming outgoing, incoming
messages stored on servermessages stored on server
user mailbox
outgoing message queue
mailserver
useragent
useragent
useragent
mailserver
useragent
useragent
mailserver
useragent
SMTP
SMTP
SMTP
Electronic Mail: mail Electronic Mail: mail serversservers
Mail ServersMail Servers mailboxmailbox contains incoming contains incoming
messages for usermessages for user messagemessage queuequeue of of
outgoing (to be sent) mail outgoing (to be sent) mail messagesmessages
SMTP protocolSMTP protocol between between mail servers to send email mail servers to send email messagesmessages client: sending mail client: sending mail
serverserver ““server”: receiving mail server”: receiving mail
serverserver
mailserver
useragent
useragent
useragent
mailserver
useragent
useragent
mailserver
useragent
SMTP
SMTP
SMTP
Electronic Mail: SMTP Electronic Mail: SMTP [RFC [RFC 2821]2821]
uses uses TCPTCP to reliably transfer email message from to reliably transfer email message from client to server, port 25client to server, port 25
direct transfer: sending server to receiving serverdirect transfer: sending server to receiving server three phases of transferthree phases of transfer
handshaking (greeting)handshaking (greeting) transfer of messagestransfer of messages closureclosure
command/response interactioncommand/response interaction commands:commands: ASCII text ASCII text response:response: status code and phrase status code and phrase
messages must be in 7-bit ASCIImessages must be in 7-bit ASCII
Scenario: Alice sends Scenario: Alice sends message to Bobmessage to Bob
1) 1) Alice uses UA to Alice uses UA to compose message and compose message and “to” “to” [email protected]@someschool.edu
2) Alice’s UA sends 2) Alice’s UA sends message to her mail message to her mail server; message placed server; message placed in message queuein message queue
3) Client side of SMTP 3) Client side of SMTP opens TCP connection opens TCP connection with Bob’s mail serverwith Bob’s mail server
4) 4) SMTP client sends SMTP client sends Alice’s message over Alice’s message over the TCP connectionthe TCP connection
5) Bob’s mail server 5) Bob’s mail server places the message in places the message in Bob’s mailboxBob’s mailbox
6) Bob invokes his user 6) Bob invokes his user agent to read messageagent to read message
useragent
mailserver
mailserver user
agent
1
2 3 4 56
Sample SMTP interactionSample 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
Try SMTP interaction for Try SMTP interaction for yourself:yourself:
telnet servername 25telnet servername 25 see 220 reply from serversee 220 reply from server enter HELO, MAIL FROM, RCPT TO, enter HELO, MAIL FROM, RCPT TO,
DATA, QUIT commandsDATA, QUIT commands above lets you send email without using above lets you send email without using
email client (reader)email client (reader)
SMTP: final wordsSMTP: final words SMTP uses persistent SMTP uses persistent
connectionsconnections SMTP requires SMTP requires
message (header & message (header & body) to be in 7-bit body) to be in 7-bit ASCIIASCII
SMTP server uses SMTP server uses CRLF.CRLFCRLF.CRLF to to determine end of determine end of messagemessage
Comparison with Comparison with HTTPHTTP::
HTTP: pullHTTP: pull SMTP: pushSMTP: push
both have ASCII both have ASCII command/response command/response interaction, status interaction, status codescodes
HTTP: each object HTTP: each object encapsulated in its own encapsulated in its own response msgresponse msg
SMTP: multiple objects SMTP: multiple objects sent in multipart msgsent in multipart msg
Mail message formatMail message format
SMTP: protocol for SMTP: protocol for exchanging email msgsexchanging email msgs
RFC 822: standard for text RFC 822: standard for text message format:message format:
header lines, e.g.,header lines, e.g., To:To: From:From: Subject:Subject:
differentdifferent from SMTP from SMTP commandscommands!!
bodybody the “message”, ASCII the “message”, ASCII
characters onlycharacters only
header
body
blankline
Message format: multimedia Message format: multimedia extensionsextensions
MIME: multimedia mail extension, RFC MIME: multimedia mail extension, RFC 2045, 20562045, 2056
additional lines in msg header declare additional lines in msg header declare MIME content typeMIME 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
MIME typesMIME typesContent-Type: type/subtype; Content-Type: type/subtype;
parametersparametersTextText example subtypes: example subtypes: plain, plain,
htmlhtml
ImageImage example subtypes: example subtypes: jpeg, jpeg,
gifgif
AudioAudio exampe subtypes: exampe subtypes: basicbasic
(8-bit mu-law encoded), (8-bit mu-law encoded), 32kadpcm 32kadpcm (32 kbps coding)(32 kbps coding)
VideoVideo example subtypes: example subtypes: mpeg, mpeg,
quicktimequicktime
ApplicationApplication other data that must be other data that must be
processed by reader processed by reader before “viewable”before “viewable”
example subtypes: example subtypes: msword, octet-stream msword, octet-stream
Multipart TypeMultipart TypeFrom: [email protected] To: [email protected] Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Type: multipart/mixed; boundary=StartOfNextPart --StartOfNextPartDear Bob, Please find a picture of a crepe.--StartOfNextPartContent-Transfer-Encoding: base64Content-Type: image/jpegbase64 encoded data ..... ......................... ......base64 encoded data --StartOfNextPartDo you want the reciple?
POP3 protocolPOP3 protocolauthorization phaseauthorization phase client commands: client commands:
user:user: declare username declare username pass:pass: password password
server responsesserver responses +OK+OK -ERR-ERR
transaction phase, transaction phase, client:client: list:list: list message list message
numbersnumbers retr:retr: retrieve message by retrieve message by
numbernumber dele:dele: delete delete quitquit
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 bob S: +OK C: pass hungry S: +OK user successfully logged on
POP3 (more) and IMAPPOP3 (more) and IMAPMore about POP3More about POP3 Previous example Previous example
uses “download and uses “download and delete” mode.delete” mode.
Bob cannot re-read e-Bob cannot re-read e-mail if he changes mail if he changes clientclient
““Download-and-keep”: Download-and-keep”: copies of messages on copies of messages on different clientsdifferent clients
POP3 is stateless POP3 is stateless across sessionsacross sessions
IMAPIMAP Keep all messages in Keep all messages in
one place: the serverone place: the server Allows user to Allows user to
organize messages in organize messages in foldersfolders
IMAP keeps user IMAP keeps user state across sessions:state across sessions: names of folders and names of folders and
mappings between mappings between message IDs and message IDs and folder namefolder name
