internet applications

Data and ComputerData and Computer
CommunicationsCommunications
Eighth EditionEighth Edition
by William Stallingsby William Stallings
Lecture slides by Lawrie BrownLecture slides by Lawrie Brown
Chapter 22 – Internet ApplicationsChapter 22 – Internet Applications
Electronic Mail andElectronic Mail and
Network ManagementNetwork Management

InternetInternet ApplicationsApplications
Electronic Mail andElectronic Mail and
One of the most exciting aspects of birds' lives is how
they interact with others during such social activities
as defending territories, courting mates, nesting,
raising young, and flocking. Birds' level of sociability
changes with the seasons; they may be gregarious at
certain times of year yet highly territorial at others.
Some of the most fascinating behavior occurs in spring
and summer when birds are engaged in breeding.
During a social interaction, an individual is
coordinating its activities with those of another. This
inevitably requires communication.
—Secret Lives of Common Birds, Marie Read

Electronic MailElectronic Mail
 most heavily used application on any networkmost heavily used application on any network
 Simple Mail Transfer Protocol (SMTP)Simple Mail Transfer Protocol (SMTP)

TCP/IPTCP/IP

delivery of simple text messagesdelivery of simple text messages
 Multi-purpose Internet Mail Extension (MIME)Multi-purpose Internet Mail Extension (MIME)

delivery of other types of datadelivery of other types of data

voice, images, video clipsvoice, images, video clips

SMTPSMTP
 RFC 821RFC 821
 not concerned with format of messages or datanot concerned with format of messages or data

covered in RFC 822 (see later)covered in RFC 822 (see later)
 SMTP uses info written on envelope of mailSMTP uses info written on envelope of mail

message headermessage header
 does not look at contentsdoes not look at contents

message bodymessage body
 except:except:

standardize message character set to 7 bit ASCIIstandardize message character set to 7 bit ASCII

add log info to start of messageadd log info to start of message

Basic OperationBasic Operation
 email message is created by user agentemail message is created by user agent
program (mail client), and consists of:program (mail client), and consists of:

header with recipient’s address and other infoheader with recipient’s address and other info

body containing user databody containing user data
 messages queued and sent as input tomessages queued and sent as input to
SMTP sender programSMTP sender program

yypically a server process (daemon on UNIX)yypically a server process (daemon on UNIX)

Mail Message ContentsMail Message Contents
 each queued message has two partseach queued message has two parts
 message textmessage text

RFC 822 header with envelope and list of recipientsRFC 822 header with envelope and list of recipients

message body, composed by usermessage body, composed by user
 list of mail destinationslist of mail destinations

derived by user agent from headerderived by user agent from header

may be listed in headermay be listed in header

may require expansion of mailing listsmay require expansion of mailing lists

may need replacement of mnemonic names withmay need replacement of mnemonic names with
mailbox namesmailbox names
 if BCCs indicated, user agent needs to prepareif BCCs indicated, user agent needs to prepare
correct message formatcorrect message format

SMTP SenderSMTP Sender
 takes message from queuetakes message from queue
 transmits to proper destination hosttransmits to proper destination host

via SMTP transactionvia SMTP transaction

over one or more TCP connections to port 25over one or more TCP connections to port 25
 host may have multiple senders activehost may have multiple senders active
 host must create receivers on demandhost must create receivers on demand
 when delivery complete, sender deleteswhen delivery complete, sender deletes
destination from list for that messagedestination from list for that message
 when all destinations processed, message iswhen all destinations processed, message is
deleteddeleted

Sending OptimizationsSending Optimizations
 if message destined for multiple users onif message destined for multiple users on
a given host, it is sent only oncea given host, it is sent only once

delivery to users handled at destination hostdelivery to users handled at destination host
 if multiple messages ready for given host,if multiple messages ready for given host,
a single TCP connection can be useda single TCP connection can be used

saves overhead of setting up and droppingsaves overhead of setting up and dropping
connectionconnection

Possible ErrorsPossible Errors
 host unreachablehost unreachable
 host out of operationhost out of operation
 TCP connection fail during transferTCP connection fail during transfer
 sender can re-queue mailsender can re-queue mail

give up after a periodgive up after a period
 faulty destination addressfaulty destination address

user erroruser error

target user changed addresstarget user changed address

redirect if possibleredirect if possible

inform user if notinform user if not

SMTP Protocol - ReliabilitySMTP Protocol - Reliability
 used to transfer messages from sender toused to transfer messages from sender to
receiver over TCP connectionreceiver over TCP connection
 attempts to provide reliable serviceattempts to provide reliable service
 no guarantee to recover lost messagesno guarantee to recover lost messages
 no end to end acknowledgement to originatorno end to end acknowledgement to originator
 error indication delivery not guaranteederror indication delivery not guaranteed
 generally considered reliablegenerally considered reliable

SMTP ReceiverSMTP Receiver
 accepts arriving messageaccepts arriving message
 places in user mailbox or copies to outgoingplaces in user mailbox or copies to outgoing
queue for forwardingqueue for forwarding
 receiver must:receiver must:

verify local mail destinationsverify local mail destinations

deal with errorsdeal with errors
 sender responsible for message until receiversender responsible for message until receiver
confirm complete transferconfirm complete transfer

indicates mail has arrived at host, not userindicates mail has arrived at host, not user

SMTP ForwardingSMTP Forwarding
 mostly direct transfer from sender host tomostly direct transfer from sender host to
receiver hostreceiver host
 may go through intermediate machine viamay go through intermediate machine via
forwarding capabilityforwarding capability

sender can specify routesender can specify route

target user may have movedtarget user may have moved

ConversationConversation
 SMTP limited to conversation betweenSMTP limited to conversation between
sender and receiversender and receiver
 main function is to transfer messagesmain function is to transfer messages
 rest of mail handling beyond scope ofrest of mail handling beyond scope of
SMTP and may differ between systemsSMTP and may differ between systems

SMTP System OverviewSMTP System Overview
 commands and responses exchangedcommands and responses exchanged
between sender and receiverbetween sender and receiver
 initiative with senderinitiative with sender

establishes TCP connectionestablishes TCP connection
 sender sends a command to receiversender sends a command to receiver

e.g. HELO<SP><domain><CRLF>e.g. HELO<SP><domain><CRLF>
 generates exactly one replygenerates exactly one reply

e.g. 250 requested mail action ok; completede.g. 250 requested mail action ok; completed

SMTP CommandsSMTP Commands
Name Description
HELO Send identification
MAIL Identifies originator of mail
RCPT Identifies recipient of mail
DATA Transfer message text
RSET Abort current mail transaction
NOOP No operation
QUIT Close TCP connection
SEND Send mail to terminal
SOML Send mail to terminal if possible; otherwise to mailbox
SAML Send mail to terminal and mailbox
VRFY Confirm user name
EXPN Return membership of mailing list
HELP Send system-specific documentation
TURN Reverse role of sender and receiver

SMTP RepliesSMTP Replies
 positive completion reply (2xx)positive completion reply (2xx)

e.g. 220e.g. 220 <domain> Service ready

e.g. 250e.g. 250 Requested mail action okay, completed
 positive intermediate reply (3xx)positive intermediate reply (3xx)

e.g. 354e.g. 354 Start mail input; end with <CRLF>.<CRLF>
 transient negative completion reply (4xx)transient negative completion reply (4xx)

e.g. 452e.g. 452 Requested action not taken: insufficient system
storage
 permanent negative completion reply (5xx)permanent negative completion reply (5xx)

e.g. 500e.g. 500 Syntax error, command unrecognized

e.g. 550e.g. 550 Requested action not taken: mailbox unavailable
(e.g., mailbox not found, no access)

Connection SetupConnection Setup
 sender opens TCP connection with receiversender opens TCP connection with receiver
 once connected, receiver identifies itselfonce connected, receiver identifies itself

220 <domain> service ready220 <domain> service ready
 sender identifies itselfsender identifies itself

HELOHELO
 receiver accepts sender’s identificationreceiver accepts sender’s identification

250 OK250 OK
 if mail service not available, step 2 returns:if mail service not available, step 2 returns:

421 service not available421 service not available

Mail TransferMail Transfer
 sender may then send one or more messagessender may then send one or more messages
 MAIL command identifies originatorMAIL command identifies originator

gives reverse path to used for error reportinggives reverse path to used for error reporting

receiver returns 250 OK or fail/error messagereceiver returns 250 OK or fail/error message
 one or more RCPT commands identifies recipients forone or more RCPT commands identifies recipients for
the messagethe message
 DATA command transfers message textDATA command transfers message text

end of message shown by line containing just period .end of message shown by line containing just period .

Example SMTP TransferExample SMTP Transfer
 S: MAIL FROM:<Smith@Alpha.ARPA>S: MAIL FROM:<Smith@Alpha.ARPA>
 R: 250 OKR: 250 OK
 S: RCPT TO:<Jones@Beta.ARPA>S: RCPT TO:<Jones@Beta.ARPA>
 R: 250 OKR: 250 OK
 S: RCPT TO:<Green@Beta.ARPA>S: RCPT TO:<Green@Beta.ARPA>
 R: 550 No such user hereR: 550 No such user here
 S: RCPT TO:<Brown@Beta.ARPA>S: RCPT TO:<Brown@Beta.ARPA>
 R: 250 OKR: 250 OK
 S: DATAS: DATA
 R: 354 Start mail input; end with <CRLF>.<CRLF>R: 354 Start mail input; end with <CRLF>.<CRLF>
 S: Blah blah blah...S: Blah blah blah...
 S: ...etc. etc. etc.S: ...etc. etc. etc.
 S: <CRLF>.<CRLF>S: <CRLF>.<CRLF>
 R: 250 OKR: 250 OK

Closing ConnectionClosing Connection
 two stepstwo steps
 sender sends QUIT and waits for replysender sends QUIT and waits for reply
 then initiate TCP close operationthen initiate TCP close operation
 receiver initiates TCP close after sendingreceiver initiates TCP close after sending
reply to QUITreply to QUIT

RFC 882 Email FormatRFC 882 Email Format
 message has envelope and contentsmessage has envelope and contents
 envelope contains information required toenvelope contains information required to
transmit and deliver messagetransmit and deliver message
 content defined by RFC822content defined by RFC822
 message is sequence of lines of textmessage is sequence of lines of text

uses general memo frameworkuses general memo framework
• multiple header lines, rigid format, then arbitrarymultiple header lines, rigid format, then arbitrary
text bodytext body

each header line has form:each header line has form:
• keyword: argumentskeyword: arguments

Example MessageExample Message
Date:Tue, 16 Jan 1996 10:37:17 (EST)Date:Tue, 16 Jan 1996 10:37:17 (EST)
From: “William Stallings” <ws@host.com>From: “William Stallings” <ws@host.com>
Subject:The syntax of RFC 822Subject:The syntax of RFC 822
To: Smith@otherhost.comTo: Smith@otherhost.com
Cc: Jones@Yet-another_host.comCc: Jones@Yet-another_host.com
This is the main text, delimited from the header byThis is the main text, delimited from the header by
a blank line.a blank line.

Multipurpose Internet MailMultipurpose Internet Mail
Extension (MIME)Extension (MIME)
 extension to RFC822 to resolve issues like:extension to RFC822 to resolve issues like:
 SMTP can not transmit executablesSMTP can not transmit executables
 can not transmit text including internationalcan not transmit text including international
characters (e.g. â, å, ä, è, é, ê, ë)characters (e.g. â, å, ä, è, é, ê, ë)
 servers may reject mail over certain sizeservers may reject mail over certain size
 ASCII to EBCDIC translation not standardASCII to EBCDIC translation not standard
 SMTP gateways to X.400 can not handle non-SMTP gateways to X.400 can not handle non-
text data in X.400 messagestext data in X.400 messages
 some SMTP implementations do not adhere tosome SMTP implementations do not adhere to
standardstandard

Overview of MIMEOverview of MIME
 five new message header fieldsfive new message header fields

MIME versionMIME version

Content typeContent type

Content transfer encodingContent transfer encoding

Content IdContent Id

Content DescriptionContent Description
 number of content formats definesnumber of content formats defines
 transfer encoding definedtransfer encoding defined

Content Type/SubtypeContent Type/Subtype
 Text body - in given character setText body - in given character set
 Multipart - body contains multiple partsMultipart - body contains multiple parts
 MessageMessage
 ImageImage
 VideoVideo
 AudioAudio
 ApplicationApplication

Multipart MIME ExampleMultipart MIME Example
From: John Smith <js@company.com>From: John Smith <js@company.com>
To: Ned Jones <ned@soft.com>To: Ned Jones <ned@soft.com>
Subject: Sample messageSubject: Sample message
MIME-Version: 1.0MIME-Version: 1.0
Content-type: multipart/mixed; boundary="simple boundary"Content-type: multipart/mixed; boundary="simple boundary"
This is the preamble. It is to be ignored, though it is a handy place for mail composersThis is the preamble. It is to be ignored, though it is a handy place for mail composers
to include an explanatory note to non-MIME conformant readers.to include an explanatory note to non-MIME conformant readers.
--simple boundary--simple boundary
This is implicitly typed plain ASCII text. It does NOT end with a linebreak.This is implicitly typed plain ASCII text. It does NOT end with a linebreak.
--simple boundary--simple boundary
Content-type: text/plain; charset=us-asciiContent-type: text/plain; charset=us-ascii
This is explicitly typed plain ASCII text. It DOES end with a linebreak.This is explicitly typed plain ASCII text. It DOES end with a linebreak.
--simple boundary----simple boundary--
This is the epilogue. It is also to be ignored.This is the epilogue. It is also to be ignored.

MIME Transfer EncodingsMIME Transfer Encodings
 reliable delivery over range of environmentsreliable delivery over range of environments
 uses content transfer encoding fielduses content transfer encoding field

3 specify bit interpretation, other 3 encodings3 specify bit interpretation, other 3 encodings
 Quoted-printableQuoted-printable

data largely printable ASCII charactersdata largely printable ASCII characters

non-printing characters represented by hex codenon-printing characters represented by hex code
 Base64Base64

maps arbitrary binary input onto printable outputmaps arbitrary binary input onto printable output
 X-tokenX-token

named nonstandard encodingnamed nonstandard encoding

 networks are becoming indispensablenetworks are becoming indispensable
 more complexity makes failure more likelymore complexity makes failure more likely
 require automatic network management toolsrequire automatic network management tools
 standards required to allow multi-vendor networksstandards required to allow multi-vendor networks
 covering:covering:

servicesservices

protocolsprotocols

Management information base (MIB)Management information base (MIB)

SystemsSystems
 collection of tools for network managementcollection of tools for network management
 single operator interfacesingle operator interface
 powerful, user friendly command setpowerful, user friendly command set
 performing most or all management tasksperforming most or all management tasks
 minimal amount of separate equipmentminimal amount of separate equipment

i.e. use existing equipmenti.e. use existing equipment
 view entire network as unified architectureview entire network as unified architecture
 active elements provide regular feedbackactive elements provide regular feedback

Simple Network ManagementSimple Network Management
Protocol (SNMP)Protocol (SNMP)
 SNMP v1 developed for managing TCP/IPSNMP v1 developed for managing TCP/IP
(inter) networks(inter) networks
 defines protocol, database, other conceptsdefines protocol, database, other concepts
 basic conceptsbasic concepts

management station or managermanagement station or manager

agentagent

management information basemanagement information base

network management protocolnetwork management protocol

Management StationManagement Station
 stand alone system or part of shared systemstand alone system or part of shared system
 interface for human network managerinterface for human network manager
 set of management applicationsset of management applications

data analysisdata analysis

fault recoveryfault recovery
 interface to monitor and control networkinterface to monitor and control network
 translate manager’s requirements intotranslate manager’s requirements into
monitoring and control of remote elementsmonitoring and control of remote elements
 data base of network management informationdata base of network management information
extracted from managed entitiesextracted from managed entities

ManagementManagement AgentAgent
 equip key platforms with agent softwareequip key platforms with agent software

e.g. hosts, bridges, hubs, routerse.g. hosts, bridges, hubs, routers
 allows their management by managementallows their management by management
stationstation
 respond to requests for informationrespond to requests for information
 respond to requests for actionrespond to requests for action
 asynchronously supply unsolicitedasynchronously supply unsolicited
informationinformation

Management InformationManagement Information
Base (MIB)Base (MIB)
 representation of network resources as objectsrepresentation of network resources as objects
 each object a variable representing one aspecteach object a variable representing one aspect
of managed objectof managed object
 MIB is collection of access points at agent forMIB is collection of access points at agent for
management of stationmanagement of station
 objects standardized across class of systemobjects standardized across class of system

bridge, router etc.bridge, router etc.
 management stationmanagement station

retrieves values of MIB objects to provide monitoringretrieves values of MIB objects to provide monitoring

sets MIB object values to change configurationsets MIB object values to change configuration

ProtocolProtocol
 link between management station & agentlink between management station & agent

TCP/IP uses SNMPTCP/IP uses SNMP

OSI uses Common Management InformationOSI uses Common Management Information
Protocol (CMIP)Protocol (CMIP)

SNMPv2 (enhanced SNMP) for OSI & TCP/IPSNMPv2 (enhanced SNMP) for OSI & TCP/IP
 key capabilitieskey capabilities

get - management station retrieves valueget - management station retrieves value

set - management station sets valueset - management station sets value

notify - agent sends event notificationnotify - agent sends event notification

Management LayoutManagement Layout
 may be centralized in simple networkmay be centralized in simple network
 may be distributed in large, complexmay be distributed in large, complex
networknetwork

multiple management serversmultiple management servers

each manages pool of agentseach manages pool of agents

management may be delegated tomanagement may be delegated to
intermediate managerintermediate manager

Distributed NetworkDistributed Network
Management ExampleManagement Example

SNMP ArchitectureSNMP Architecture

SNMP v1SNMP v1
 August 1988 SNMP specification issuedAugust 1988 SNMP specification issued
 stand alone management stations and bridges,stand alone management stations and bridges,
routers workstations etc supplied with agentsrouters workstations etc supplied with agents
 defines limited, easily implemented MIB of scalardefines limited, easily implemented MIB of scalar
variables and two dimensional tablesvariables and two dimensional tables
 streamlined protocolstreamlined protocol
 limited functionalitylimited functionality
 lack of securitylack of security
 SNMP v2 1993, revised 1996SNMP v2 1993, revised 1996

RFC 1901-1908RFC 1901-1908

FrameworkFramework
 have framework on which network managementhave framework on which network management
applications can be builtapplications can be built

e.g fault management, performance monitoring,e.g fault management, performance monitoring,
accountingaccounting
 management protocol is used to exchangemanagement protocol is used to exchange
management informationmanagement information

e.g. SNMP v1 & v2e.g. SNMP v1 & v2
 each player maintains local MIBeach player maintains local MIB
 at least one system responsible for managementat least one system responsible for management

houses management applicationshouses management applications

ExmpleExmple
SNMP v2SNMP v2
ManagedManaged
ConfigConfig

SNMP v2SNMP v2
 support central or distributed managementsupport central or distributed management
 in distributed system, some elementsin distributed system, some elements
operate as manager and agentoperate as manager and agent
 exchanges use SNMP v2 protocolexchanges use SNMP v2 protocol

simple request/response protocolsimple request/response protocol

typically uses UDPtypically uses UDP
• ongoing reliable connection not requiredongoing reliable connection not required
• reduces management overheadreduces management overhead

Structure ofStructure of
Management Information (SMI)Management Information (SMI)
 defines general framework with which MIBdefines general framework with which MIB
defined and constructeddefined and constructed

identifies data typesidentifies data types

how resources are represented and namedhow resources are represented and named
 encourages simplicity and extensibilityencourages simplicity and extensibility

has scalars and two dimensional arrays ofhas scalars and two dimensional arrays of
scalars (tables) onlyscalars (tables) only
 three key elements:three key elements:

data types, objects, object identifiersdata types, objects, object identifiers

SNMP v2 PDU FormatsSNMP v2 PDU Formats
PDU type request-id 0 0 variable-bindings
(a) GetRequest, GetNextRequest, SetRequest, SNMPv2-Trap, InformRequest
PDU type request-id error-status error-index variable-bindings
(b) Response
PDU type request-id non-repeaters max-repetitions variable-bindings
(c) GetBulkRequest
name1 value1 name2 value2 • • • namen valuen
(d) variable-bindings

SNMP v3SNMP v3
 addresses security issues of SNMP v1/2addresses security issues of SNMP v1/2
 RFC 2570-2575RFC 2570-2575
 proposed standard January 1998proposed standard January 1998
 defines overall architecture and securitydefines overall architecture and security
capabilitycapability
 to be used with SNMP v2to be used with SNMP v2
 defines three security servicesdefines three security services

authenticationauthentication

privacyprivacy

access controlaccess control

SNMP v3 ServicesSNMP v3 Services
 authentication assures that message is:authentication assures that message is:

from identified source, not altered, not delayed orfrom identified source, not altered, not delayed or
replayedreplayed

includes HMAC message authentication codeincludes HMAC message authentication code
 privacyprivacy

encrypts messages using DESencrypts messages using DES
 access controlaccess control

pre configure agents to provide a number of levels ofpre configure agents to provide a number of levels of
access to MIB for different managersaccess to MIB for different managers

restricting access to informationrestricting access to information

limit operationslimit operations

SummarySummary
 electronic mailelectronic mail

SMTP (RFC821) mail exchangeSMTP (RFC821) mail exchange

RFC822 & MIME mail content formatsRFC822 & MIME mail content formats
 network managementnetwork management

elements and operation of SNMP v1, 2 & 3elements and operation of SNMP v1, 2 & 3

internet applications

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Editor's Notes