CSCE 515:Computer Network

Programming

Chin-Tser Huanghuangct@cse.sc.edu

University of South Carolina

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Simple Mail Transfer Protocol (SMTP) Email is one of most popular

applications In 1991 about ½ TCP connections are for

SMTP Mail exchange is performed by a

message transfer agent (MTA), for example Sendmail

Users don’t interact with MTA but can choose their user agent

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TCP/IP E-mail Exchange Architecture

queue of mailto be sent

useragent

clientuser at aterminal

messagetransferagent

serveruser at aterminal

usermailboxes

useragent

messagetransferagent

sender

receiver

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SMTP Protocol Specified in RFC 821 Use NVT ASCII for communication

between two MTAs Client sends commands to server Server responds with numeric reply

codes with optional strings

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SMTP Commands Basic implementation includes eight commands

HELO MAIL RCPT DATA QUIT RSET VRFY NOOP

Additional commands include EXPN and TURN

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Envelopes, Headers, and Body Envelopes are used by MTAs for delivery

Specified by MAIL and RCPT commands Headers are used by user agents

RFC 822 specifies header format Each header field contains a name, a colon, and field

value Body is content of message from sending user

to receiving user When transferred using DATA command, headers are

sent first, followed by a blank line, followed by body

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Transmission of a Mail User agent takes body, adds some

headers, and passes to MTA MTA adds some headers, adds

envelope, and sends to another MTA

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Relay Agents All individual systems send their mail to a

relay system Two reasons of using relay system

Simplify configuration of all MTAs other than relay system’s MTA

Allow one system at an organization to act as mail hub

Most organizations use relay system for Internet mail

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Improvements on SMTP Envelope changes: Extended SMTP Header changes: Non-ASCII

characters Body changes: Multipurpose

Internet Mail Extensions (MIME)

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Extended SMTP Framework for adding extensions to

SMTP Client wishing to use new features can

use EHLO, instead of HELO, to initiate session

Server lists extended commands in the 250 reply to EHLO command

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Non-ASCII Characters Allow additional characters in sender

and receiver names and in subject Header can contain encoded words

=? charset ? encoding ? encoded-text ?= Charset can be us-ascii or iso-8859-1 Encoding can be quoted-printable or base-

64 Using base-64, three consecutive bytes are

encoded as four 6-bit values

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Multipurpose Internet Mail Extensions Allow structures in body Five new header fields

Mime-Version:Content-Type:Content-Transfer-Encoding:Content-ID:Content-Description:

Seven content types: text, multipart, message, application, image, audio, video

Five encoding formats: 7bit, quoted-printable, base64, 8bit, binary

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Simple Network Management Protocol Need to manage systems within a coherent

framework Number of networks within organization grows Heterogeneity of systems in network

Network management consists of communication between managers and agents

Managers are network management stations Agents are management software running on

network elements, e.g. hosts, routers, X terminals Current version is SNMPv3 (RFC 2570)

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Components of Network Management Three components of SNMP

Management Information Base (MIB): specifies what variables network elements maintain

Structure of Management Information (SMI): a set of common structures and an identification scheme used to reference variables in MIB

Simple Network Management Protocol (SNMP): protocol between manager and element

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SNMP Protocol Use UDP to send messages Seven types of messages are exchanged between managers

and agents get-request allows manager to fetch value of variables get-next-request allows manager to fetch next variable after

specified variables set-request allows manager to set value of variables get-response allows agent to return value of variables in response

to get-request, get-next-request, and set-request trap allows agent to notify manager when something happens on

agent get-bulk-request allows manager to retrieve large blocks of data inform-request allows one manager to send info to another

manager

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SNMP MessagesSNMP

agentSNMP manager

UDP port 161

UDP port 161

UDP port 161

UDP port 162

get-request

get-response

get-next-request

get-response

set-request

get-response

trap

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Structure of Management Info Data types used by SNMP

INTEGER OCTET STRING DisplayString OBJECT IDENTIFIER NULL IpAddress PhysAddress Counter Gauge TimeTicks SEQUENCE SEQUENCE OF

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Object Identifiers A sequence of integers separated by

decimal points These integers traverse a tree structure,

similar to DNS Each node in the tree also has a textual

name All variables in MIB start with the object

identifier 1.3.6.1.2.1

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Instance Identification Every variable in MIB must be identified Simple variables are referenced by

appending “.0” to variable’s object identifier

Table entries are identified by combination of variables in table

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Traps Agent can send traps to manager to indicate

that something has happened on agent Traps are sent to UDP port 162 on manager Seven types of traps

coldStart warmStart linkDown linkUp authenticationFailure egpNeighborLoss enterpriseSpecific

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Next Class Peer-to-peer applications Read JNP Sec. 24.4