Post on 11-Dec-2020
transcript
APPLICATION LAYER AND
ELECTRONIC MAIL
Outline à Application layer à Mail delivery architecture à Mail transfer protocol à Mail access protocols
APPLICATION LAYER
In the Protocol Architecture
RPC
XDR
NFS
RTP SNMP
Telnet FTP SMTP HTTP
IP
Data link
Network
Trasport
Session
Presenta:on
Applica:on
ICMP IGMP
TCP UDP
ARP
Rou:ng protocols
Client-Server Paradigm
à Server always in execution
à Waiting for requests à Client initiates communication
à Known IP address (name) and port à Static port à Standard port
à Traditional model on the Internet à FTP, WWW, e-mail
Peer-to-peer (P2P) Paradigm
à No pre-defined role à Each host can contact and be contacted à I.e., running both client and server
à Server (or super peer) might be needed to know others
à Newer model à VoIP, emule, Skype
Text-based Protocols
à Inefficient encoding à Easy to troubleshoot à No need for protocol analyzer support
MAIL DELIVERY ARCHITECTURE
Mail Server
Mail Server
SMTP
SMTP
POP IMAP HTTP
MAIL TRANSFER PROTOCOL
SMTP: Simple Mail Transfer Protocol
à Text based à Client-server à TCP - port 25
à Opened by client à Command-response
à Status code
SMTP Session
HELO zeit.polito.it 250 …
220 polito.it
MAIL FROM: baldi@polito.it
250 baldi@polito.it sender OK
RCPT TO: box@baldi.info
250 box@baldi.info recipient OK
Handshaking
354 Enter mail, end with “.” by itself
This is an e-‐mail message. It contains mul:ple lines. .
250 message accepted QUIT
221 polito.it closing connec:on
SMTP Session Data transfer
Termina:on
HELO baldi.info 250 …
220 polito.it
MAIL FROM: box@baldi.info
250 box@baldi.info sender OK
RCPT TO: baldi@gmail.com
571 baldi@gmail.com prohibited. We do not relay
An:-‐spamming Measures
An:-‐spamming Measures
HELO baldi.info 250 …
220 polito.it
250 baldi@polito.it recipient OK
MAIL FROM: box@baldi.info
250 box@baldi.info sender OK
RCPT TO: baldi@polito.it
An:-‐spamming Measures
HELO zeit.polito.it 250 …
220 polito.it
MAIL FROM: baldi@polito.it
250 baldi@polito.it sender OK
RCPT TO: baldi@gmail.com
250 baldi@gmail.com recipient OK
An:-‐spamming, An:-‐spoofing Measures
HELO zeit.polito.it 250 …
220 polito.it
MAIL FROM: baldi@polito.it
250 baldi@polito.it sender OK
RCPT TO: baldi@gmail.com 473 baldi@gmail.com relaying prohibited. You should authen:cate first
Check Client IP
Message Format à ASCII character sequence à Possibly limited length lines
Body
From: <sender> To: <addresses> CC: <carbon copied> Subject: <subject line> …
How about images?
à Can be encoded as character sequence
à E.g. base64 à Recipient must know
Multipurpose Internet Mail Extensions: MIME
à
Additional headers
MIME-‐Version: 1.0 Content-‐Type: image/png; name="image001.png" Content-‐Descrip:on: image001.png Content-‐Transfer-‐Encoding: base64
iVBORw0KGgoAAAANSUhEUgAAAKgAAABDCA xAAADsQBlSsOGwAAABl0RVh0U29mdHdhcm EEQXBBxR19URQF3OEzRo0rKvoeCYm4xJen
Content-Type à text
à plain, html à image
à jpeg, gif, png à audio à video
Important for “rendering”
MAIL ACCESS PROTOCOLS
Webmail
à Web server running on mail server host
à Providing access to messages through web interface
à Messages remain on server
Pros and Cons
à Ideal when not using own PC
à Available from everywhere à Available only with Internet connection
Post Office Protocol: POP
à Mono-PC users à Messages moved to client
à Available for off-line access
à Character-based à TCP on port 11O
POP Session
USER baldi +OK
+OK POP3 serv ready
PASS whatever_it_is
+OK Congratula:ons!
Authoriza:on
+OK 2 messages 1 1495 2 1948
RETR 1 +OK 1495 octets Received: from …
QUIT +OK Bye!
DELE 2 +OK Message 2 deleted
SMTP Session Transac:on
Termina:on
Internet Message Access Protocol: IMAP
à Multi-PC users à E.g. 1 PC at work, 1 PC at home
à Character-based à TCP on port 143
Best of both worlds
à Available for off-line access
à Messages remain on server à In folder hierarchy
à Synchronize with local copy