Tom Kelliher, CS 325
Jan. 28, 2011
Characteristics of packet-switched networks, and protocols.
USPS analogy. A data stream is split into discrete packets.
How do packets move about the network? Store and forward; routing.
What is a protocol?
Example SMTP protocol exchange:
bluebird:~/Class/Cs325/Lectures % Mail -v -s "Protocol illustration" email@example.com SMTP protocol demonstration. Cc: firstname.lastname@example.org... Connecting to [127.0.0.1] via relay... 220 bluebird.goucher.edu ESMTP Sendmail 8.13.8/8.13.8; Thu, 31 Jan 2008 18:36:28 -0500 >>> EHLO bluebird.goucher.edu 250-bluebird.goucher.edu Hello bluebird.goucher.edu [127.0.0.1], pleased to meet you 250-ENHANCEDSTATUSCODES 250-PIPELINING 250-8BITMIME 250-SIZE 250-DSN 250-ETRN 250-AUTH GSSAPI DIGEST-MD5 CRAM-MD5 250-DELIVERBY 250 HELP >>> MAIL From:<email@example.com> SIZE=94 AUTHfirstname.lastname@example.org 250 2.1.0 <email@example.com>... Sender ok >>> RCPT To:<firstname.lastname@example.org> >>> DATA 250 2.1.5 <email@example.com>... Recipient ok 354 Enter mail, end with "." on a line by itself >>> . 250 2.0.0 m0VNaSul003537 Message accepted for delivery firstname.lastname@example.org... Sent (m0VNaSul003537 Message accepted for delivery) Closing connection to [127.0.0.1] >>> QUIT 221 2.0.0 bluebird.goucher.edu closing connection``Doing it yourself'' via a telnet connection to SMTP port (25).
Protocols in TCP/IP:
How does peer-to-peer fit in?
Length limits bandwidth
Phone company is ISP. Shorter link length means higher speed. Still, there's only so much bandwidth available in twisted pair.
Coaxial cable has much higher bandwidth.
Much higher bandwidth.
A cable modem gives access approximately equal to a T1.
Goucher: four T1 lines in 2002. How many today? Why doesn't Goucher just use cable modems?
Network integrity, security issues.
Connections to a large number of tier 2 ISPs (clients).
Connections to a large number of tier 3 ISPs (clients).