CSCE 515: Computer Network Programming

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CSCE 515Computer Network Programming

• Chin-Tser Huang
• huangct_at_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 dont interact with MTA but can choose
  their user agent

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