Transcript EE579T-Class 2A

EE579T / CS525T
Network Security
2: Introduction to Networking
Prof. Richard A. Stanley
Spring 2002
© 2000-2002, Richard A. Stanley
WPI
EE579T/2 #1
Overview of Tonight’s Class
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Class list issues
Review of last week’s class
Network security in the news
An overview of networking
Introduction to symmetric cryptography
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WPI
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Last Week in Review
• Computer security is a real need in real
systems
• Without computer security, network security
is a pipedream
• Network security is an even more difficult
problem than computer security, for a
number of reasons
• Absolute security does not exist
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Network Security in the News
• “The rate of growth of our vulnerabilities is
exceeding the rate of improvements in
security measures.” Michael Vatis, former director, NIPC
• CERT reports for 2001:
– 52,658 security breaches
– 2,437 computer vulnerabilities
– more than a 100% increase over 2000
Source: “Net still seen as a dangerous place,” The Boston Globe, January 21, 2002, p. C4
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Convenience vs. Security
• Universal Plug and Play (Win 9x, ME, XP)
– extends local recognition of devices to network
– “...just by booting onto a network your client
can discover and install network printers, link
to Internet gateways or connect to a wide range
of network attached devices or services.”
– listens on TCP port 5000, UDP port 1900
– opens opportunity for illicit entry into system
Source: SEARCHSECURITY.COM, Security Tech Tip, January 22, 2002
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Networks
• A network is an interconnected group of
communicating devices.
• Two primary network types
– Circuit-switched (connection oriented)
– Packet-switched (connectionless)
• Span
– WAN, MAN, LAN
– So what?
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Data Networks
• Almost exclusively packet switched
– Higher efficiency than circuit-switched
– Computationally intensive to provide
– Packet loss rate is very high
• Largely due to collisions rather than circuit faults
– Require extensive protocols to operate
• X.25
• TCP
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Network Topology
• The topology of a network is a view of its
interconnections, as they would be seen by an
observer looking down from great height
• Topology is important because it has implications
for security
• Three major topologies:
– star
– buss
– ring
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Star Topology
The orange lines depict one
star -- this slide actually shows
a star-star architecture.
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Buss Topology
Buss
In a buss topology, all signals pass by all terminals
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Ring Topology
A ring is simply a buss with
the ends connected to one another.
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How To Get There?
• Every destination on the network must have
an address, just as every postal destination
must have an address
– Addresses must be unique
– Network must know how to recognize address
– Various addressing schema, e.g.
• Ethernet
• IP
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Two Network Technologies
• Token ring
– Users remain silent until they receive token
– Pioneered by IBM, not widely used
• Ethernet
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Carrier-sense, multiple access/collision detect
Binary exponential backoff on collision sense
This is a radio network!  Another vulnerability
Most widely used architecture today, largely because it
is less expensive than token ring
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Other Network Technologies
• Fiber-Distributed Data Interconnect (FDDI)
– Self-healing, 100 Mbps dual ring
• Frame relay
– Packet data service, built on X.25
• Synchronous Optical Network (SONET)
• Asynchronous Transfer Mode (ATM)
– Can operate at gigabit speeds
• 53 byte packets; 5 of the bytes are overhead
These are of interest in networking, but not security per se;
they will not be discussed further in this course
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Topology Misconceptions
• The physical interconnection of network
elements does not necessarily reflect the
logical network topology
– Ethernet is logically a buss architecture
– Ethernet, connected using hubs, uses a physical
star interconnection
– Ethernet, connected using coaxial cable, uses a
physical buss interconnection
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WPI
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Some Network Security Issues
• Users not necessarily registered at the node they
are accessing
– How to authenticate users?
– What is basis for access control decisions?
• Some options:
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User ID
User address
Service being invoked
Cryptographic-based solutions
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WPI
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Ethernet Misconceptions
• IEEE 802.3 = Ethernet
– Nope! Pure Ethernet is 802.2
• All Ethernets are created equal
– Vendor implementation issues
• The faster the network speed, the faster I
can work
– Signaling speed  data throughput
• Ethernet maps to the internet
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CSMA/CD Throughput
Signaling speed
~40%
Throughput
Users
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Ethernet Addresses
• 48 bits long
• Address space managed by the IEEE
• Usually fixed in hardware at time of
manufacture, but increasingly in EEPROM
• Hardware must recognize at least it’s own
physical address and the network multicast
address, and possibly alternate addresses
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Ethernet Frame
NOTE: The proper term in this context for groups of 8 bits is an octet, not a byte.
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Network Size
• Networks cannot grow to be arbitrarily large
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Address space
Physical interconnection limitations
Increasing collisions as users increase
Protocol/OS/machine incompatibilities
• So, how to extend the ability to interconnect
an arbitrarily large number of computers?
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The ARPANET
• Father of the Internet; first elements in 1969
• Began as an attempt to conduct and share research
to ensure continuity of communications after
nuclear war, so
– Connectionless
– Assured delivery
– Self-reconfiguring (sort of)
• Demonstrated feasibility of internetworking
disparate computer networks and machines
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Internetworking
• Internetworking is the interconnection of
networks
• The Internet is an internetwork; all
internetworks are not the Internet
• Very few modern networks exist in
isolation; most are internetworked
• This has important security and legal
implications
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Internetworking Concepts
• Networks are interconnected by routers or
gateways
– More about this later in the course
• Routers route a packet using the destination
network address, not the destination host
address
– Analogous to the world postal system and how
letters are routed
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Internetwork Architecture
Net 1
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Net 2
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Extended Internetworking
Net 1
Clearly, this can be
extended ad infinitum,
to form very large
internetworks.
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Net 2
R
Net 3
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Some Terms
• TCP = transmission control protocol
• IP = internet protocol
• These protocols have become widely used
outside the formally-defined Internet
• They have some serious flaws, but they
work
– They were not planned to have/need security
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IP Addressing
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Class Discrimination
• Address space is 32 bits long (IPv4)
– Therefore, at most 232 possible addresses (or
4,294,967,296 in decimal notation)
• Easy to extract netid from address
• There is not a one-to-one correspondence
between IP addresses and physical devices
– Consider the router
• Address with hostid=0 refers to network
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WPI
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IP Addressing Weaknesses
• If a host moves to another network, its IP
address must change
• If a network grows beyond its class size (B
or C), it must get a new address of the next
larger size
• Because routing is by IP address, the path
taken by packets to a multiple-addressed
host depends on the address used
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IP Address Presentation
• Usually done in dotted decimal, e.g.,
10000000 00001010 00000010 00011110
is usually written as
128.10.2.30
• What class of network address is this?
• As you see, each notation has its uses
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Address Limits
Class
A
B
C
D
E
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Lowest Address
0.1.0.0
128.0.0.0
192.0.1.0
224.0.0.0
240.0.0.0
WPI
Highest Address
126.0.0.0
191.255.0.0
223.255.255.0
239.255.255.255
247.255.255.255
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Special Purpose Addresses
• 0.0.0.0
• 255.255.255.255
Addresses current host
Addresses hosts on
current network
• Host bits zero
Identifies a network
• Host bits one
Addresses hosts on
addressed network
• Network bits zero Addresses specific host
on current network
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Reserved Addresses
• First Quad=127 is used for loopback
– Traffic doesn’t leave the computer
– Routed to the IP input queue
– Usually see 127.0.0.1
• Unregistered addresses
– Class A
– Class B
– Class C
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10.0.0.0 thru 10.255.255.255
172.16.0.0 thru 172.31.255.255
192.168.0.0 thru 198.168.255.255
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The Future of IP
• IPv4 has shortcomings that are becoming
important for modern networking
• The IETF’s solution is a new version of IP,
Version 6, written as IPv6
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Increased address space (128 vs. 32 bits)
Support for network autoconfiguration
Better support for routing
Better security support
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WPI
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IPv6 Issues
• It is not backwards compatible with IPv4
– Given the change in address space alone, how could it
be?
– Requires translator to go v4v6, vice versa
• Huge investment in installed IPv4 mitigates
against rapid changeover
• Network address translation (NAT) helps reduce
need for new address space
• Some services, like IPSec, now available for IPv4
• Bottom line: changeover not likely to be quick
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Ports and Sockets
• Ports are associated with services, e.g.,
– Port 53 is usually the domain name service
(DNS)
– Port 80 is usually the hypertext transfer
protocol service
• A socket is the combination of an IP address
and a port, e.g. 192.168.2.45:80
• Sockets enable multiple simultaneous
services to run on a single address
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WPI
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Address Registration
• Internet Corporation for Assigned Names and
Numbers (ICANN) handles:
– IP address space allocation
– protocol parameter assignment
– domain name system management
– root server system management functions
• Only essential to register addresses that appear on
the global network, but registration is preferred
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Routing
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Protocols
• A protocol is simply an agreed-upon
exchange of information required to
perform a given task
– IP is a protocol
– So is TCP
• Networks utilize protocols to accomplish all
the important tasks they perform
• Layered protocols are common
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ISO Protocol Model
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Protocol Layering
• Refers to a protocol running on top of
another protocol
• Layered protocols are designed so that layer
n at the destination receives exactly the
same object sent by layer n at the source
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TCP/IP Layering Model
Application
Application-specific
messages/streams
Transport
TCP Packets
Internet
IP Datagrams
Network Interface
Ethernet/Token Ring
Hardware
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Some Common Protocols
• ARP maps IP addresses to physical addresses
• RARP determines IP address at startup
• IP provides for assured connectionless datagram
delivery
• ICMP handles error and control messages
• UDP defines user datagrams (no assurance of
delivery)
• IKE handles crypto key management functions
• TCP provides reliable stream transport
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How Protocol Layering Works
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Protocol Layering & Internet
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Important Boundaries
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TCP
• Assumes little about underlying network
• Reliable delivery characteristics:
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Stream orientation
Virtual circuit connection
Buffered transfer
Unstructured stream
Full duplex connection
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Positive Acknowledgement
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Positive Acknowledgement
With Lost Packet
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Sliding Window
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Positive ACK With Sliding
Window
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TCP
• A communications protocol, NOT a piece of
software
• Provides
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Data format
Data acknowledgement for reliable transfer
How to distinguish multiple destinations
How to set up and break down a session
• Very complex
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WPI
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Conceptual TCP Layering
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Internet Round Trip Delays
This data is old, but
still meaningful if you
ignore the absolute values
of the delays.
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WPI
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Delays
• Cannot be avoided or predicted (except
statistically)
– Packet delivery times will vary
– Many packets will simply be lost
• So, as a network designer...
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How long do you wait to assume nondelivery?
How do you slide the window?
How do you back off on collision detect?
How do you respond to congestion?
…etc.
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WPI
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Establishing a TCP Session
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Ending a TCP Session
This implies that a TCP session could be left “half open.” That is true.
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WPI
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TCP State Machine
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Other Network Protocols
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NetBIOS
NetBUI
IPX
X.25
ATM
Message: TCP/IP is not the only show in
town BUT...it is the most popular show in town
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WPI
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Network Facts
• Most computers today are connected to a
network (consider the Internet), at least for
part of the time they are in operation
• Most local networks are internetworked
• How to provide authenticity, integrity,
confidentiality, availability?
• Cryptography can help provide all the
security services except availability
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WPI
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So....
Next, we study cryptography
in some detail.
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WPI
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Summary
• Networks and internetworks have become
ubiquitous
• Networking allows interconnection of computers
without much concern for the local OS or machine
architecture
• Networking raises many serious security issues,
which must be solved
• The pace of network security problem
development is exceeding the pace of their
solution
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WPI
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Assignment for Next Class
• Re-read course text, Chapter 2
– Pay particular attention to Feistel rounds
• Review your notes on symmetric cryptography
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WPI
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Homework - 1
1. What is the single greatest advantage of
having the IP checksum cover only the
datagram header and not the data? What is
the disadvantage?
2. Exactly how many class A, B, and C
networks can exist? How many hosts can a
network in each class have?
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WPI
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Homework - 2
3. How many IP addresses would be needed
to assign a unique network number to every
home in the U.S.A.? Is the address space
sufficient?
4. What is the chief difference between the IP
addressing scheme and the North American
Numbering Plan used for telephone
numbers?
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WPI
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Homework - 3
5. Complete routing tables for all routers
shown on slide 35.
6. Can you think of any security issues,
hardware or software, that arise from what
you have studied so far?
Spring 2002
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WPI
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