csci 6433 class 10
DESCRIPTION
Dave Roberts. CSCI 6433 Class 10. Agenda. DHCP SMTP. DHCPv4: How does a host get started?. What Addresses Are Needed?. Address of default router Network mask Addresses of mail server, DNS server MTU of local network TTL value to use for IP datagrams IP address of host. - PowerPoint PPT PresentationTRANSCRIPT
CSCI 6433CLASS 10Dave Roberts
2PRINCIPLES
Good Citizen Principle It’s about giving up resources when they are scarce It’s not about normal operation
Layering Isolates operations from each other Flexibility in configuration
End to End Operation High level functions performed by endpoints Superior reliability
Conserving router time No Internet if router job is too difficult Drives design of all Internet protocols
ELECTRONIC MAILSimple Mail Transport Protocol
SMTPPOPIMAPMIME 3
ELECTRONIC MAIL
Email allows users to send memos across the Internet.
Notes can be short or quite large Notes can have multiple attachments Must work when remote machine is
unreachable
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ACTORS IN THE EMAIL SYSTEM
Post Office SMTP servers move email between each other SMTP servers store email for delivery to end
users Users
POP, IMAP clients pick up email from SMTP servers
POP, IMAP clients hand outgoing mail to SMTP servers
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6COMPONENTS
DELAYED DELIVERY7Sender and receiver do not need to be
connected to the server at the same time
8AN SMTP TRANSACTION
MAIL command: establishes return address and bounce address
RCPT command: establishes recipient of this message
DATA signals beginning of the message text
9QUESTIONS
What protocol do SMTP servers use to deliver messages?
How does SMTP differ between v4 and v6?
SMTP TRANSACTION SEQUENCE
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11EXAMPLE
S: 220 smtp.example.com ESMTP Postfix C: HELO relay.example.org S: 250 Hello relay.example.org, I am glad to meet you C: MAIL FROM:<[email protected]> S: 250 Ok C: RCPT TO:<[email protected]> S: 250 Ok C: RCPT TO:<[email protected]> S: 250 Ok C: DATA S: 354 End data with <CR><LF>.<CR><LF> C: From: "Bob Example" <[email protected]> C: To: "Alice Example" <[email protected]> C: Cc: [email protected] C: Date: Tue, 15 January 2008 16:02:43 -0500 C: Subject: Test message C: C: Hello Alice. C: This is a test message with 5 header fields and 4 lines in
the message body. C: Your friend, C: Bob C: . S: 250 Ok: queued as 12345 C: QUIT S: 221 Bye {The server closes the connection}
ALIAS EXPANSION, MAIL FORWARDING
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INTERNETWORKING AND EMAIL
TCP/IP internet makes universal delivery service possible
Mail systems built on TCP/IP are inherently reliable because of end-to-end delivery
Alternatively, mail gateways are used Allow mail transfer between different systems When gateway has a message, sender
discards it
13Question: Is email through a gateway better or worse than a direct
SMTP transfer?
TCP/IP EMAIL STANDARDS
Format and computer interaction are specified separately
Format: header, blank line, body Body unspecified Header is key word, colon, value Some keywords required, others optional Header is readable
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EMAIL ADDRESSES
local-part @ domain-name
domain-name: name of mail destination local-part: address of a mailbox at destinationNote: when gateways are employed, mail
addresses are site-dependent
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SMTP
Simple Mail Transport Protocol Focuses on how mail delivery system passes
messages from one machine to a server on another machine
Does not specify anything about user interface Does not specify how mail is stored
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ASSURED DELIVERY
SMTP server forms TCP/IP connection with receiving server
Once receiving server has put message into safe store, then it acknowledges and sender discards message
If SMTP can’t transfer message on the first try, it keeps trying
After several days of failure, SMTP reports failure to deliver.
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SMTP HIGHLIGHTS
All communications is readable ASCII text Transcript of interactions is readable Each message is acknowledged separately Addresses of the form local-part@domain-
name
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MAIL RETRIEVAL AND MAILBOX MANIPULATION
POP3—Post Office Protocol POP3 client creates TCP connection to POP3
server on mailbox computer Mailbox computer runs two servers:
SMTP to place mail into user mailboxes POP3 server to allow user to extract messages
from user mailbox POP3 retrieves messages, deletes from user
mailbox Two servers coordinate use of the user
mailbox
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INTERNET MESSAGE ACCESS PROTOCOL
POP stores messages offline; not compatible with the use of multiple computers
IMAP allows message access, manipulation from multiple computers
Platform-independent access to mail
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Question: How does server resource usage of IMAP compare with POP?
MIME EXTENSION FOR NON-ASCII DATA
MIME—Multipurpose Internet Mail Extensions Defined to allow transmission of non-ASCII data
through mail MIME allows arbitrary data to be encoded in
ASCII, transmitted as standard email message MIME message tells recipient type of data, type
of encoding used Data type and subtype is specified MIME information is in 822 mail header
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MIME CONTENT TYPES22
MIME EXAMPLE
From: [email protected]: [email protected]: 1/0Content-Type: image/gifContent-Transfer Encoding: base64
….data for the image….
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MAIL RETRIEVAL, MAILBOX PROTOCOLS
Post Office Protocol—client login with userid/password
Client can then retrieve, delete messages Server computer must run two servers: POP
and SMTP POP and SMTP must coordinate use of mailbox
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SUMMARY
Email is a very important Internet service Separate standards are used for message
format and transfer SMTP—how a mail system on one machine
transfers to a server on another POP3—how a user can retrieve contents of a
mailbox IMAP—user protocol for use from multiple
computers MIME allows arbitrary data to be exchanged
using SMTP
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