Transcript CSE331-12

CSE331:
Introduction to Networks
and Security
Lecture 12
Fall 2002
Announcements
• Reminder:
– Project 1 due on Monday, Oct. 7th
– In-class midterm Wednesday, Oct. 9th
• Monday’s Class
– Further Topics in Networking
– Review / Question & Answer
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Recap
• Remote Procedure Call (RPC)
– Java’s remote method invocation (RMI)
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Today
• RMI Demo
• Application-level Protocols
– SMTP: Simple Mail Transfer Protocol
– HTTP: HyperText Transfer Protocol
– SNMP: Simple Network Management Protocol
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Java RMI Permissions
• Socket permissions in a policy file
// policy.pol
grant {
permission java.net.SocketPermission
"<IP>:1024-65535",
"connect,accept,listen,resolve";
permission java.net.SocketPermission
"*:1099", "connect,resolve";
};
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Running Java programs with policies
• java –Djava.security.policy=policy Program
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Protocol Stack Revisited
Application
SMTP, HTTP, SNMP
Presentation
Session
Transport
Network
So far…
Data Link
Physical
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Common Features
• SMTP, HTTP, SNMP
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Request/Reply protocols built on TCP or UDP
Designed to handle a fixed set of messages
Companion data format
Many applications
Protocol
SMTP
HTTP
SNMP
Data Format
RFC 822 and MIME
HTML
MIB
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Programs
Pine, NSMail, Eudora,…
Explorer, Netscape, Opera
snmpget, snmpset,…
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SMTP: Simple Mail Transfer Protocol
• Data format RFC822
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Adopted around 1982, extended 1993, 1996
http://www.faqs.org/rfcs/rfc822.html
ASCII text
Header and Body
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RFC822 Headers
• <CRLF>-terminated lines containing pairs of
form type: value
• Many valid Header types
• Some headers filled out by client
– To: [email protected]
– Subject: CSE331
• Others filled out by mail delivery system
– Date:
– Received:
– From:
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MIME: Multipurpose Internet Mail Extensions
•
Consists of three parts
1. Additional RFC822 Headers
• MIME-Version:
• Content-Type:
• Content-Transfer-Encoding:
2. Content types (and subtypes)
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text/plain
text/rich
image/gif
application/postscript
Multipart/mixed; boundary=“…”
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MIME Encoding
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Mail systems assume ASCII
– Only 64 valid characters A-Z, a-z, 0-9, +, /
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Some datatypes include arbitrary binary
data (e.g. JPEG)
Base64 encoding
– 3 bytes of data map to 4 ASCII Characters
– A=0,B=1,…
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Also 7bit ASCII encoding
– Human readable
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From: Steve Zdancewic <[email protected]>
MIME-Version: 1.0
To: [email protected]
Subject: Example Mail
Content-Type: multipart/mixed; boundary="------------020307000708030506070607"
This is a multi-part message in MIME format.
--------------020307000708030506070607
Content-Type: text/plain; charset=us-ascii; format=flowed
Content-Transfer-Encoding: 7bit
This is the body.
--------------020307000708030506070607
Content-Type: text/plain; name="example.txt"
Content-Transfer-Encoding: 7bit
Content-Disposition: inline; filename="example.txt"
Hello
--------------020307000708030506070607
Content-Type: image/jpeg; name="doc.jpg"
Content-Transfer-Encoding: base64
Content-Disposition: inline; filename="doc.jpg"
/9j/4AAQSkZJRgABAQEASABIAAD//gAXQ3JlYXRlZCB3aXRoIFRoZSBHSU1Q/9sAQwAIBgYH
BgUIBwcHCQkICgwUDQwLCwwZEhMPFB0aHx4dGhwcICQuJyAiLCMcHCg3KSwwMTQ0NB8n
OT04…
SMTP
• Mail Reader
– User edits/reads/search e-mail
• Mail Daemon
– Process running on each host (port 27)
– Uses SMTP/TCP to transmit mail to daemons on
other machines
– Most daemons based on Berkley’s sendmail
• Mail Gateways
– Store and forward e-mail (much like IP router)
– Buffers on disk
– Attempts to resend
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SMTP Mail Gateways
Mail
reader
Mail
daemon
SMTP/TCP
Mail gateway
Mail
daemon
Mail
reader
Mail
daemon
SMTP/TCP
• No need for explicit host in e-mail address
– User can receive mail at different machines
• Recipient’s machine may not be up
– Mail gateway can hold message for later
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SMTP Dialogs
• Client posts commands
– HELO, MAIL, RCPT, DATA, QUIT
• Server responds with code and humanreadable explanation
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Example SMTP Dialog
HELO seas.upenn.edu
250 Hello [email protected] [158.130.12.180]
MAIL FROM:<[email protected]>
250 OK
RCPT TO:<[email protected]>
250 OK
RCPT TO:<[email protected]>
550 No such user here
DATA
354 Start mail input; end with <CRLF>.<CRLF>
Blah blah blah…
<CRLF>.<CRLF>
250 OK
QUIT
221 Closing Connection
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HTTP: HyperText Transfer Protocol
• Text-oriented protocol on top of TCP
• http://www.w3.org/Protocols/
• Messages have the form
START_LINE <CRLF>
MESSAGE_HEADER <CRLF>
<CRLF>
MESSAGE_BODY <CRLF>
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HTTP Request Messages
• START_LINE: Operation URL Version
– Absolute URL:
GET http://www.cis.upenn.edu/ HTTP/1.1
• Using the MESSAGE_HEADER
– Relative URL:
GET / HTTP/1.1
Host: www.cis.upenn.edu
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HTTP Request Operations
Operation
OPTIONS
GET
HEAD
POST
PUT
DELETE
TRACE
CONNECT
Description
get info about server options
retrieve document specified by URL
retrieve metainfo document by URL
give info to the server
store document under specified URL
delete specified URL
loopback request message
for use by proxy servers
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HTTP Response Messages
• START_LINE Version Code Reason
– HTTP/1.1 202 Accepted
– HTTP/1.1 404 Not Found
Code
1xx
2xx
3xx
4xx
5xx
Type
Information
Success
Redirection
Client Error
Server Error
Example Reasons
request received
action successfully accepted
further action must be taken
request contains bad syntax
server didn’t fulfill valid request
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HTTP Caching
• Store a copy of the data closer to the client
– Browser caches web pages
– ISPs can cache pages
– Sites can cache pages at proxies
• How to cache changing web pages?
– Server assigns expiration date using Expires
header
– Set of cache directives that specify whether
documents can be cached, for how long, etc.
– Not easy to cache dynamic content
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SNMP:Simple Network Management Protocol
• Specialized request/reply protocol
– Two basic operations: GET and SET
– Runs on UDP
• Companion data: Management Info.
Database (MIB)
– Stores information about various protocols running
on nodes in the network
• Example uses:
– Measure # packets dropped/received
– Watch for faulty hardware/software
– Determine # IP datagram reassemblies that are
aborted
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MIP-II Variables
• System
– Where located, Uptime, Name
• Interfaces
– Physical addresses, # packets sent/received
• ARP
– Contents of address translation table
• IP
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Routing table
# packets forwarded
Reassembly statistics
Drop counts
• TCP
– # of passive/active opens
– # timeouts, default timeout settings
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