CSE331:Introduction to Networksand Security

Lecture 12

Fall 2002

CSE331 Fall 2002 2

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

CSE331 Fall 2002 3

Recap

• Remote Procedure Call (RPC)– Java’s remote method invocation (RMI)

CSE331 Fall 2002 4

Today

• RMI Demo• Application-level Protocols

– SMTP: Simple Mail Transfer Protocol– HTTP: HyperText Transfer Protocol– SNMP: Simple Network Management Protocol

CSE331 Fall 2002 5

Java RMI Permissions

• Socket permissions in a policy file

// policy.polgrant { permission java.net.SocketPermission "<IP>:1024-65535", "connect,accept,listen,resolve"; permission java.net.SocketPermission "*:1099", "connect,resolve";};

CSE331 Fall 2002 6

Running Java programs with policies

• java –Djava.security.policy=policy Program

CSE331 Fall 2002 7

Protocol Stack Revisited

Application

Presentation

Session

Transport

Network

Data Link

Physical

SMTP, HTTP, SNMP

So far…

CSE331 Fall 2002 8

Common Features

• SMTP, HTTP, SNMP – Request/Reply protocols built on TCP or UDP– Designed to handle a fixed set of messages– Companion data format– Many applications

Protocol Data Format ProgramsSMTP RFC 822 and MIME Pine, NSMail, Eudora,…HTTP HTML Explorer, Netscape, OperaSNMP MIB snmpget, snmpset,…

CSE331 Fall 2002 9

SMTP: Simple Mail Transfer Protocol

• Data format RFC822– Adopted around 1982, extended 1993, 1996– http://www.faqs.org/rfcs/rfc822.html– ASCII text– Header and Body

CSE331 Fall 2002 10

RFC822 Headers

• <CRLF>-terminated lines containing pairs of form type: value

• Many valid Header types• Some headers filled out by client

– To: stevez@cis.upenn.edu– Subject: CSE331

• Others filled out by mail delivery system– Date:– Received:– From:

CSE331 Fall 2002 11

MIME: Multipurpose Internet Mail Extensions

• Consists of three parts1. Additional RFC822 Headers

• MIME-Version:• Content-Type:• Content-Transfer-Encoding:

2. Content types (and subtypes)• text/plain• text/rich• image/gif• application/postscript• Multipart/mixed; boundary=“…”

CSE331 Fall 2002 12

MIME Encoding

• Mail systems assume ASCII– Only 64 valid characters A-Z, a-z, 0-9, +, /

• Some datatypes include arbitrary binary data (e.g. JPEG)

• Base64 encoding– 3 bytes of data map to 4 ASCII Characters– A=0,B=1,…

• Also 7bit ASCII encoding– Human readable

From: Steve Zdancewic <stevez@cis.upenn.edu>MIME-Version: 1.0To: stevez@cis.upenn.eduSubject: Example MailContent-Type: multipart/mixed; boundary="------------020307000708030506070607"

This is a multi-part message in MIME format.--------------020307000708030506070607Content-Type: text/plain; charset=us-ascii; format=flowedContent-Transfer-Encoding: 7bit

This is the body.

--------------020307000708030506070607Content-Type: text/plain; name="example.txt"Content-Transfer-Encoding: 7bitContent-Disposition: inline; filename="example.txt"

Hello

--------------020307000708030506070607Content-Type: image/jpeg; name="doc.jpg"Content-Transfer-Encoding: base64Content-Disposition: inline; filename="doc.jpg"

/9j/4AAQSkZJRgABAQEASABIAAD//gAXQ3JlYXRlZCB3aXRoIFRoZSBHSU1Q/9sAQwAIBgYHBgUIBwcHCQkICgwUDQwLCwwZEhMPFB0aHx4dGhwcICQuJyAiLCMcHCg3KSwwMTQ0NB8nOT04…

CSE331 Fall 2002 14

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

CSE331 Fall 2002 15

SMTP Mail Gateways

• 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

Mail reader

Mail daemon

Mail daemon

Mail reader

Mail daemon

Mail gateway

SMTP/TCP SMTP/TCP

CSE331 Fall 2002 16

SMTP Dialogs

• Client posts commands– HELO, MAIL, RCPT, DATA, QUIT

• Server responds with code and human-readable explanation

17

Example SMTP DialogHELO seas.upenn.edu250 Hello daemon@smtpauth.seas.upenn.edu [158.130.12.180]

MAIL FROM:<stevez@seas.upenn.edu>250 OK

RCPT TO:<billg@microsoft.com>250 OK

RCPT TO:<ted@microsoft.com>550 No such user here

DATA354 Start mail input; end with <CRLF>.<CRLF>Blah blah blah…<CRLF>.<CRLF>250 OK

QUIT221 Closing Connection

CSE331 Fall 2002 18

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>

CSE331 Fall 2002 19

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/1.1Host: www.cis.upenn.edu

CSE331 Fall 2002 20

HTTP Request Operations

Operation DescriptionOPTIONS get info about server optionsGET retrieve document specified by URLHEAD retrieve metainfo document by URLPOST give info to the serverPUT store document under specified URLDELETE delete specified URLTRACE loopback request messageCONNECT for use by proxy servers

CSE331 Fall 2002 21

HTTP Response Messages

• START_LINE Version Code Reason– HTTP/1.1 202 Accepted– HTTP/1.1 404 Not Found

Code Type Example Reasons1xx Information request received2xx Success action successfully accepted3xx Redirection further action must be taken4xx Client Error request contains bad syntax5xx Server Error server didn’t fulfill valid request

CSE331 Fall 2002 22

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

CSE331 Fall 2002 23

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

CSE331 Fall 2002 24

MIP-II Variables• System

– Where located, Uptime, Name

• Interfaces– Physical addresses, # packets sent/received

• ARP– Contents of address translation table

• IP– Routing table– # packets forwarded– Reassembly statistics– Drop counts

• TCP– # of passive/active opens– # timeouts, default timeout settings