CS 6250 Class 27 12'3'2002

1
CS 6250 Class 27 12.3.2002

• Some application-layer protocols
• SMTP, HTTP, SNMP

2
News

• Reminders
• Project deadline TODAY, 300pm
• Submit source code (no revised report)
• Project demos on Dec 4 and 5
• Dec 4 (W) 200-530 (5 groups)
• Dec 5 (R) 945-1145 (3 groups) 145-300 (2
  groups)
• All group members must attend demo
• Last class on Thursday
• Solve several homework problems from second part
  of the course

3
Application-layer protocols

• New Internet applications, that nobody had
  foreseen, keep emerging
• What makes IP-based networks so appropriate for
  supporting new applications?
• Application programs versus application protocols
• SMTP Simple Mail Transfer Protocol
• HTTP HyperText Transport Protocol
• SNMP Simple Network Management Protocol
• All based on client-server request-reply model
• Recently, peer-to-peer applications have become
  very popular (see Gnutella and related software)
• Distinguish between message exchange protocol and
  message format protocol

4
E-mail (SMTP, MIME)

• Message transfer protocol (SMTP) vs message
  format protocols (RFC 822, Multipurpose Internet
  Mail Extensions or MIME)
• RFC 822 message consists of header and body
• Header fields lttype value CRLFgt (e.g., subject
  hi!)
• Body ASCII only
• Body separated by header with a blank line
• Content-type/length/encoding/description..
• MIME extension (93 and 96) allows many different
  types of body types (audio, images, Word
  documents, etc)
• MIME also defines multipart type, allowing
  multiple data types in single message
• Base64 encoding of 8-bit binary words in 6-bit
  printable ASCII characters
• Maps every 3-byte group of binary message to 4
  6-bit characters
• MIME also supports 7-bit ASCII encoding of
  text-only messages

5

• From mike_at_cc.gatech.edu Fri Oct 25 153908 2002
• Received from sark.cc.gatech.edu
  (sark.cc.gatech.edu 130.207.7.23)
• by tokyo.cc.gatech.edu (8.11.6/8.11.6)
  with ESMTP id g9PJd7K29064
• for ltdovrolis_at_tokyo.cc.gatech.edugt Fri,
  25 Oct 2002 153907 -0400 (EDT)
• Received from boris.cc.gatech.edu
  (boris.cc.gatech.edu 130.207.6.141)
• by sark.cc.gatech.edu (8.11.6/8.11.6)
  with ESMTP id g9PJd5a08589
• for ltdovrolis_at_cc.gatech.edugt Fri, 25 Oct
  2002 153905 -0400 (EDT)
• Received from 130.207.6.84 (mccracken.cc.gatech
  .edu 130.207.6.84)
• by boris.cc.gatech.edu (8.11.6/8.11.6)
  with ESMTP id g9PJd5709928
• for ltdovrolis_at_cc.gatech.edugt Fri, 25 Oct
  2002 153905 -0400 (EDT)
• Mime-Version 1.0
• Content-Type text/plain charset"us-ascii"
• Message-Id ltv04020a23b9df4dcf85b5_at_130.207.6.84gt
  
• In-Reply-To ltPine.GSO.4.44.0210230920560.1561-100
  000_at_tokyo.cc.gatech.edugt
• Date Fri, 25 Oct 2002 154017 -0400
• To Constantine ltdovrolis_at_cc.gatech.edugt
• From Mike McCracken ltmike_at_cc.gatech.edugt
• Subject Re MS thesis/project
• Content-Length 1011

6

• From guesswho_at_cc.gatech.edu Mon Nov 18 164218
  2002
• Mime-Version 1.0
• Message-Id ltp05111a36b9ff07b97d8a_at_130.207.3.125
  gt
• X-Priority 2 (High)
• Date Mon, 18 Nov 2002 164321 -0500
• To Constantinos Dovrolis ltdovrolis_at_cc.gatech.edugt
  
• From Guess Who ltguesswho_at_cc.gatech.edugt
• Subject Hello World
• Content-Type multipart/mixed boundary"
  _-1174466688_"
• Content-Length 36892
• Status RO
• X-Status
• X-Keywords
• X-UID 7984
• --_-1174466688_
• Content-Type text/plain charset"us-ascii"
  format"flowed"

7
SMTP email message transfer
Mail
Mail gateway
Mail
reader
reader
Mail
Mail
Mail
daemon
daemon
daemon
SMTP/TCP
SMTP/TCP

• Mail readers, daemons, and gateways
• Network transfer takes place over TCP
• Mail gateways can buffer pending messages for
  days, are always up, and they can also be used
  for email preprocessing (e.g., spam detection)
• Each SMTP session is a sequence of text-based
  commands/responses between two sendmail daemons

8
SMTP example

• HELO cc.princeton.edu
• 250 Hello daemon_at_mail.cc.gatech.edu
  130.207.7.23
• MAIL FROMltdovrolis_at_cc.gatech.edugt
• 250 OK
• RCPT TOltAlice_at_cisco.comgt
• 250 OK
• RCPT TOltTom_at_cisco.comgt
• 550 No such user here
• DATA
• 354 Start mail input end with ltCRLFgt.ltCRLFgt
• Blah blah blah..
• ltCRLFgt.ltCRLFgt
• 250 OK
• QUIT

9
HTTP

• HyperText Transport Protocol (HTTP) the Web
• Like SMTP
• runs over TCP
• text-based
• request-response protocol
• HTTP message format (both requests replies)
• START_LINE ltCRLFgt
• MESSAGE_HEADER ltCRLFgt
• ltCRLFgt
• MESSAGE_BODY ltCRLFgt

10
HTTP request messages
GET http//www.cc.gatech.edu/index.html HTTP/1.1
11
HTTP response codes
Example HTTP/1.1 301 Moved Permanently
Location http//www.cc.gatech.edu/CS6250/index.ht
ml
12
Transport issues in HTTP

• Number of TCP connections
• HTTP 1.0
• separate connection for each embedded object of
  web page
• Multiple parallel connections
• HTTP 1.1
• Persistent connections and pipelining
• Trade-offs with HTTP 1.0 ?
• Web caching
• Browser caching vs proxy caching
• HTTP responses can be redirected to cache from
  server, or from intermediate router
• Servers assign EXPIRES header field to web
  pages, instructing caches for expiration date
• Or, caches/clients can use HEAD request, or
  conditional GET request

13
SNMP network management

• Request/reply protocol running over UDP
• Agents (servers) run at each router/switch
  providing info for their status and configuration
• Clients retrieve specific Management Information
  Base (MIB) variables
• SNMP commands GET, SET, GET-NEXT
• MIBs are organized in 10 tree-structured groups
• System variables
• Interface variables
• Address translation
• IP
• TCP
• UDP
• A few more..
• Both MIB variables and their values are
  represented in a standardized format called ASN.1
  (Abstract Syntax Notation One)
• Example the MIB variable IPInReceives is
  represented in ASN.1 as 1.3.6.1.2.1.4.3