CSCE 515 : Computer Network Programming

CSCE 515:Computer Network Programming Chin-Tser Huang huangct@cse.sc.edu University of South Carolina

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

TCP/IP E-mail Exchange Architecture user agent queue of mail to be sent message transfer agent user at a terminal client sender user agent user mailboxes message transfer agent user at a terminal server receiver

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

SMTP Commands • Basic implementation includes eight commands • HELO • MAIL • RCPT • DATA • QUIT • RSET • VRFY • NOOP • Additional commands include EXPN and TURN

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

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

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

Improvements on SMTP • Envelope changes: Extended SMTP • Header changes: Non-ASCII characters • Body changes: Multipurpose Internet Mail Extensions (MIME)

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

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

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

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)

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

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

SNMP Messages SNMP agent SNMP manager get-request UDP port 161 get-response get-next-request UDP port 161 get-response set-request UDP port 161 get-response trap UDP port 162

Structure of Management Info • Data types used by SNMP • INTEGER • OCTET STRING • DisplayString • OBJECT IDENTIFIER • NULL • IpAddress • PhysAddress • Counter • Gauge • TimeTicks • SEQUENCE • SEQUENCE OF

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

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

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

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

CSCE 515 : Computer Network Programming