Transcript Network2

Internet Protocols
Each host computer on the network must
have a unique (logical) address
Scaled up very well to support more
sophisticated distributed applications
IP Defines
A format for assigning addresses
An (unreliable) mechanism for transferring
data (datagram)
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IP Protocol
 Provides an unreliable or best-effort delivery service
Only checksum is the header checksum.
 IP layer
Puts IP datagrams into network packets suitable for
transmission in the underlying network;
E.g., Ethernet.
When the datagram is longer than MTU of the underlying
network, it is broken into smaller segments and
reassembled at the destination.
Must insert “physical” network address of the message
destination if necessary;
Depends on the underlying network technology, i.e.,
Ethernet requires and Ethernet address for the host on the
local Ethernet.
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Mobility and IP
Dynamic Host Configuration Protocol (DHCP)
Designed to support the ability of a mobile device to
maintain simple access to services;
Assigns a temporary IP address to the device.
To provide permanent access by clients to a
mobile computer it must maintain a permanent
IP address.
Problem: IP routing is subnet-based.
Subnets are at fixed locations.
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MobileIP
A transparent solution based on tunnelling.
When a mobile computer is connected to the
Internet elsewhere, two agents take
responsibility for routing.
 Home agent (HA):
holds up-to-date knowledge of the mobile host’s
current location;
The IP address at which it can be reached.
The mobile host informs HA upon leaving home
HA acts as a proxy to the clients communicating to the
mobile host during this time.
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MobileIP (cont’d)
 Foreign agent (FA):
Allocates a temporary IP address to a mobile host upon its
arrival to a new site;
Contact HA and supplies it with the contact address for the
mobile host (FA’s address).
 HA encapsulates original IP packets and sends them
to FA.
 FA unpacks the packets and delivers them to the
mobile host.
 HA sends the contact address for the mobile host o
the original sender
If the sender is Mobile-enabled it communicates to the FA
directly from now on;
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If not, the HA continues to act as a proxy for it.
TCP and UDP
Provide communication capabilities to the
application programs.
IPv6 will support TCP/UDP as well as other
connection protocols (remember the Internet
Model).
Enable interprocess communication through the
use of ports attached to applications.
Port number is included in the header.
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UDP
Almost transport-level replica of IP.
Offers no guarantee of delivery.
The header is short, but includes an
optional checksum for the payload;
The packets that fail the check are dropped.
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TCP
Provides reliable delivery of arbitrary long
sequences of bytes via stream-based
programming abstraction.
Connection-oriented;
The sending and the receiving processes
establish a communication channel;
Use of ACK (acknowledgement) messages).
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TCP Reliability Mechanisms
 Sequencing: a sequence number is attached to every
TCP segment;
Used for message re-assembly at the destination.
 Flow control: overflow prevention;
The receiver send an ACK with the highest sequence
number in its input stream (no segments before that
one have been omitted) and a window size.
Window size specifies the amount of data the sender is
permitted to send.
ACK are attached to the backward flow if there is any.
 Burstiness of network traffic is smoothed through the
use of local buffering an a configurable time-out on it.
Naggle’s algorithm.
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TCP (cont’d)
Due to the unreliability of wireless networks
these mechanisms are not efficient.
Solutions: WAP and modified TCP.
Modified TCP for wireless networks.
Implement a TCP support component at the base
station (gateway between wired and wireless
networks).
The support component snoops on TCP packets to
and from the wireless network
re-transmitting segments that are not promptly
acknowledged.
Requesting re-transmission of inbound segments when gaps
in sequence numbers are noticed.
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WAP
Wireless Application Protocol
“An open, global specification that empowers
mobile users with wireless devices to easily access
and interact with information and services
instantly.”
- WAP Forum
“The de facto worldwide standard for providing
Internet communications and advanced telephony
services on digital mobile phones, pagers,
personal digital assistants and other wireless
terminals.”
- WAP Forum (www.wapforum.org)
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Why is WAP needed?
Traditional internet protocols (HTML, HTTP, TCP,
etc.) and their security mechanisms (TLS) are
inefficient over mobile networks.
Handheld devices tend to have less powerful
CPUs, less memory and more restrictions on
power consumption than desktops, so require
special considerations.
Handheld devices tend to use input devices
other than keyboards (e.g. voice, keypad).
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Bearer Limitations
Power consumption
 increased bandwidth requires increased power.
Cellular network economics
 Fixed bandwidth shared among many users, so
efficient bandwidth use required.
Latency
wide range of network latencies common (< 1
second to 10s of seconds).
Bandwidth
 Less bandwidth than found in wired environments.
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WAP Forum: www.wapforum.org
WAP Forum founded in December 1997 by Nokia,
Ericsson, Motorola and Phone.com (formerly
Unwired Planet)
Currently contains over 200 members;
Carriers with more than 100 million subscribers;
Infrastructure providers;
Software developers, and others.
Represent over 95% of the global handset market.
WAP Protocol development
Current WAP Version: 1.2
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How does WAP work?
Uses client-server model.
Phone incorporates a microbrowser, while
the intelligence is in the WAP gateways.
Services and applications reside on
servers.
Similar to Java – applications written for
WAP, which then run on multiple bearers
(e.g. GSM, SMS, USSD, etc.)
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What works with WAP?
Designed for use with:
All mobile phones;
Any service, e.g. SMS (Short Message Service), CSD
(Circuit Switched Data), USSD (Unstructured
Supplementary Services Data), GPRS (General Packet
Radio Service);
Any network, e.g. CDMA (Code Division Multiple
Access), GSM (Global System for Mobiles), UMTS
(Universal Mobile Telephone System);
Any input device, e.g. keyboard, stylus, touch screen,
keypad.
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WAP Protocol Model (Stack)
Application Layer
Session Layer
Transaction Layer
Security Layer
Transport Layer
Network Layer
Wireless Application
Environment (WAE)
Other Services and
Applications
Wireless Session
Protocol (WSP)
Wireless Transaction
Protocol (WTP)
Wireless Transport
Layer Security (WTLS)
Datagrams (UDP/IP)
Datagrams (WDP)
Wireless Bearers:
SMS USSD CSD IS-136 CDMA CDPD
* Source: the WAP White Paper, October 1999.
Etc…
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WAP Architecture
WAP Phone
WAP Gateway
Web Server
Internet
Client
Encoded request
Encoded response
Gateway
Web Server
Request
Response
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WDP Layer
Wireless Datagram Protocol.
Provides consistent service and common
interface to upper layers of the protocol.
Supports: SMS, USSD, CSD, CDPD,
IS-136 packet data, and GPRS.
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WTLS Layer
 Wireless Transport Layer Security (TLS).
 Implements options for authentication and
encryption.
 Optimized for mobile environment.
 Based on Transport Layer Security (TLS), which was
formerly Secure Sockets Layer (SSL).
 Optimized for use over narrow-band communication
channels.
 Ensures data integrity, privacy, authentication and
denial-of-service protection.
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WTP Layer
 Wireless Transaction Protocol
 Runs on top of datagram service.
 Works over both secure and non-secure wireless services.
 Features:
Three classes of transaction service
Class 0: for applications requiring an “unreliable push” service
Class 1: for applications requiring a “reliable push” service
Class 2: to provide the basic invoke/response transaction
service
Optional user-to-user reliability.
Asynchronous transactions.
PDU (protocol data unit) concatenation and delayed
acknowledgements to reduce number of messages sent.
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WSP Layer
Wireless Session Protocol
Provides consistent interface for both connectionoriented and connectionless services.
Provides the following functionality:
HTTP 1.1 compliance;
Long-lived session state;
Session suspend and resume;
Facility for data “push”.
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WAE
Wireless Application Environment
Interoperable environment for multiple
wireless platforms.
Consists of:
Wireless Markup Language (WML);
WMLScript;
Wireless Telephony Application (WTA);
Content Formats.
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WML
WAP Mark-up Language
WML is an XML application.
Also uses WMLScript, which is similar to
JavaScript.
Optimized for use with handheld devices.
Minimal use of CPU and memory.
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Benefits of WAP
 Reduces amount of data to be transmitted (by
translating HTTP headers from text into binary).
 Allows sessions to be suspended and resumed.
 Provides reliable datagram service without the
unnecessary overhead of TCP.
 TCP stack is not required on handheld device.
 WAP protocol stack requires less packets for interaction
than HTTP/TCP/IP.
 Support for “push” functionality built into protocol.
 WML developers can use standard web tools
(e.g. CGI, Perl, ASP, etc.).
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Drawbacks to WAP
Difficult to configure WAP phones for new WAP
services.
Not yet widely supported.
Current services (e.g. SMS, USSD) not optimized
for WAP.
Expected to be expensive.
WAP does not support cookies.
Premature encryption endpoint (gateway
decrypts data, then forwards via https – see
www.gsmworld.com/technology/wap_06.html).
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Caches and proxy servers
 Cache: a store of recently used data objects that is
closer than the objects themselves.
 When a new object is received it is placed in the cache
possibly evicting another object.
 When an object is requested, the cache is checked first
for an up-to-date copy;
If it’s not available, a fresh copy is fetched.
 A cache can be collocated with each client or located on
a proxy server.
 Proxy server: a machine/process performing tasks on
behalf of its clients.
A web proxy server maintains a cache of web resources for its
clients; all the requests go though it.
The actual client is transparent for outside servers.
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DNS
A name service design whose principal database
is used across the Internet to perform name
resolution for web resources.
A name is resolved when it is translated into
data about the named resource or object in
order to invoke an action upon it.
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The Internet Naming Scheme
 The Internet support a scheme for the use of
symbolic names for hosts and networks.
 The named entities are organized into a hierarchy.
 The named entities are called domains and the
symbolic names are called domain names.
 Domains are organized into a hierarchy that intends
to reflect organizational structure.
 Naming is entirely independent from the network
physical layout.
 Domain names must be translated into IP
Responsibility of DNS.
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DNS Operation
 Implemented as a server process that can run on
host computers anywhere on the Internet.
 There are at least 2 DNS servers in each domain.
 Servers in each domain hold a partial map of the
domain name tree below their domain.
 Requests for the translation of domain names outside
their portion of the domain tree are handled by
issuing requests to DNS servers in the relevant
domains;
Recursive procedure that follows from right to left
resolving the name in segments.
 The resulting translation is then cached at the server
handling the original request.
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DNS and caching
 Caching is a key to a name service performance;
Assists in maintaining availability and masking server
crashes.
 Caching is successful because naming data are
changed relatively rarely.
 The possibility exists of a name service returning
out-of-date attributes during resolution.
 DNS allows naming data to become inconsistent;
Stale data might be provided for periods in order of
days.
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Internet and Network
Security
Types of Attacks on Internet
Break-ins: Unauthorized attempts to gain access to a
secure system
Denial of service: A legitimate user is denied access
to a service (e.g. Flooding a WWW server with
requests)
Bombs: Large email messages or other large data
intended to overwhelm and possibly weaken a
system.
Eavesdropping - Listening in on an electronic
conversation. Perhaps with intent to gather
information for a future break-in.
Viruses.
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Firewall
 Monitors and controls all the traffic into and out of an
intranet.
 Firewall security policy
Service control: determine which services are available for
external access and reject all other requests;
Levels of filtering: IP, TCP.
Example: reject HTTP request unless they are directed to the
official website.
Behavioral control: prevent behavior that infringes
organization policies;
Levels of filtering: IP, TCP, application;
Example: filtering of ‘spam’ e-mail.
User control: discriminate between users’ privileges;
Example: management of dial-up provided for off-site users.
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Filtering levels
 IP packet filtering
Decisions made based on the destination and the
source IP addresses, the service type field in the IP
header, port numbers in TCP/UDP headers.
Example: prohibition of external access to NFS
servers.
Performed by a process within the operating system
kernel of a router.
 TCP Gateway
A TCP Gateway process checks TCP connection
requests and segment transmission for correctness.
Example: Denial-of-service attack prevention.
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Filtering levels (cont’d)
Application-level gateway
An application-level gateway process acts as a proxy
for an application process.
Example: a Telnet proxy. All telnet requests are
routed through the proxy process for approval.
A firewall is a combination of several processes
working at different protocol levels running on
more than one machine (for fault-tolerance).
 Two overall (mutually exclusive) policies:
Anything not explicitly denied is allowed.
Anything not explicitly allowed is denied.
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Basic Internet Firewalls
 A basic firewall is a router (a host with at least 2 network
interfaces).
One interface is connected to the Internet - the Host side.
The other(s) is(are) connected to the company's internal network.
Performs IP packet filtering.
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Advanced Internet Firewalls
 When TCP and application-level gateway processes are
required, they usually run on another computer: Bastion.
A host located inside the intranet and protected by an IP
router/filter, to which it is attached by a Stub LAN.
Stub LAN only has 1 or 2 hosts on it. Not connected to any other
company LANs.
A bastion host is connected to both the stub LAN and to the
company network
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Advanced Internet Firewalls
(cont’d)
Further protection can be insured by placing
another router/filter between the bastion and
the company intranet.
Note that for performance reasons company
web/ftp severs are placed on the Stub LAN.
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Virtual Private Networks
Suppose a company wants to connect the
intranets of its 5 offices.
One option is to lease a private line.
Another is to connect through the internet.
But then everything is open.
The solution is to use encryption schemes to
establish secure tunnels through the internet.
Such a set-up is called a virtual private
network.
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Directory and Discovery
Services
 Directory service: A service that stores collections of
bindings between names and attributes and that
looks up entries that match attribute-based
specifications.
Example: MS Active Directory Service, UNIX X.500, etc.
 Discovery service: a directory service that registers
the services in a spontaneous networking
environment.
Provides an interface for automatically registering and deregistering services (fax machines, printers, etc.).
Provides a lookup interface for mobile devices
Example: Jini
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Jini
A system designed for spontaneous networking.
Java-based: assumes that JVMs run on all of the
computers, allowing them to communicate
through RMI (remote method invocation, a
flavor of interprocess communication in an
object-oriented environment).
Provides facilities for service discovery,
transactions and shared data spaces called
JavaSpaces.
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Jini Directory-Related
Component
 Lookup service, Jini services and Jini clients.
 The lookup service implements what we have
termed a discovery service;
Jini uses discovery only for discovering the lookup
service itself.
Allows Jini services to register the services they offer and
Jini clients to request services that match their
requirements.
 A Jini service provides an object that provides the
service as well as the attributes of the service.
May be registered with several lookup services that store
the objects.
Example: printing service.
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Jini Directory-Related
Component (cont’d)
 Jini clients query lookup service to find Jini services
that match their requirements.
 If a match is found they download an object that
provides the service from the lookup service.
 Bootstrap connectivity: how to find the lookup
service upon entering a network.
 Solutions:
A priory knowledge of lookup services IP addresses.
Doesn’t scale up.
Use a multicast IP address that is known to all
instances of Jini software.
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Jini Directory-Related
Component (cont’d)
 When a Jini client or service starts up it sends a
request stamped with time-to-live value to a wellknown multicast address.
 Lookup services listen on a socket bound to this
address and replies to a unicast address from which
it received the request.
 The client can then perform RMI to query the
lookup service.
 Lookup services sometimes broadcast datagrams
announcing their existence to the same multicast
address, and client and services listen on it.
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