Internetworking
Organizational Communications and
Technologies
Prithvi Rao
H. John Heinz III School of Public
Policy and Management
Carnegie Mellon University
Objectives

To investigate use of bridges and routers

Define an internetwork

Recognize the importance of internetworking

List examples of relevant internetworking equiopment

Relate equipment back to protocols they use and the
OSI layer in which they reside
Readings
Chapters 14 and 15
Business Data Communications
William Stallings and Richard Van Slyke
Internetworking

Distributed organizations
distributed set of networks
LAN islands
cross functional work
teams across traditional org. boundaries

Any interconnected set of diverse or
homogeneous networks is called an INTERNET.

Each constituent network referred to as a
Subnetwork or Subnet.

The Role of Architectures

User’s perception is that there is a single network
Devices that make this possible are repeaters,
bridges, routers, and gateways

Internetworking requires an understanding of
architectures and their associated protocols

Architectures: Some
Examples

OSI

TCP/IP

SNA

DNA
Recall the OSI Architecture
Application
Presentation
Users of
transport
service
Session
Transport
Network
Data link
Physical
Users liaison
Network
service
A Simplified Architecture for File
Transfer
Computer 2
File transfer
application
Communications
service module
Network access
module
Computer 1
Files and file transfer commands
Communications-related messages
Network
interface
logic
File transfer
application
Communications
service module
Network access
module
Communications
network
Communications Architectures and
Networks
Service access point address
Processes
1 2 3
( )-( )-( )
Transport
Network
access
Host B
Communication
network
Processes
1 2
()()
Transport
Network
access
Host C
Peer to Peer Communications
Architecture
Application
A-send (Dest.
host; Dest.
SAP; Record)
Transport
Record
Record
TPDU
TPDU
DSAP
Record
Application
DSAP
Record
Transport
T-Send(DESt. Add, PDU)
Network
access
Computer X
Packet
DHost
DSAP
Record
Network
access
Computer Y
Internetworking devices
Repeaters

at the physical layer

Bridges

at the MAC layer

Routers

at the network layer

Gateways

at the network or higher layer

Repeater

End
system
Allows multiple LAN segments to be interconnected
at the physical layer. Extends physical coverage.
7
6
5
4
3
2
1
7
6
5
4
3
2
1
Repeater
1
Subnetwork
Subnetwork
End
system
Bridges
A device that operates at Layer 2 of the OSI
stack used to segment LAN’s

Acts as an address filter
maps MAC layer addresses to segments
picks up packets on one LAN addressed to a
destination on another LAN and passes those
packets on.

Variants on this theme exist in the marketplace
(e.g., brouters)

Bridge
End
system
7
6
5
4
3
2
1
End
system
7
6
5
4
3
2
1
Bridge
2
1
1
Subnetwork
Subnetwork
Routers
Devices that operate at Layer 3 of OSI
Stack

Used to connect networks that may or
may not be similar

Routers are a key component of
enterprise networks and the Internet

Router
End
system
7
6
5
4
3
2
1
End
system
7
6
5
4
3
2
1
Router
3
2 2
1
1
Subnetwork
Subnetwork
A router operates at layer 3 of OSI model

Router Operation
Host X
Router 1
TCP
IP
LLC
MAC
Physical
a
Router 2
IP
LLC
MAC
Physical
LAN A
Ethernet
a
X.25-3
X.25-2
X.25-1
X.25-3
X.25-2
X.25-1
b
Host Y
WAN C
X.25 Subnet
c
TCP
IP
LLC
MAC
Physical
IP
LLC
MAC
Physical
d
LAN B
Token Ring
d
Router Capabilities
Addressing schemes
differences between LAN addressing schemes
and X.25 schemes
Maximum packet sizes


Ethernet maximum is 1500 bytes while X.25
maximum is 1000 bytes
Interfaces


implement interfaces to the networks over which
data is routed
Gateways
End
system
7
6
5
4
3
2
1
End
system
Gateway
application
7
6
5
4
3
2
1
Subnetwork
Subnetwork
Good way to accommodate OSI and non OSI
protocols
Gateways use all seven layers of the OSI stack

Interconnecting diverse networks

Suppose there are N protocols to interconnect


Option 1: Have N(N-1) mappings
Option 2: Have one common protocol. Now we
require 2N mappings

IP uses this approach.
TCP/IP
Core of the Internet: DOD developed


Popular, mature protocol stack with large, market share
The DoD approach stems from extensive
experimentation with the ARPANET.

ARPANET started in the late 1960s', and has
grown to hundreds of nodes today.

TCP/IP

Fundamental Principle of the DoD architecture


Communication between local and remote processes is
achieved by first identifying the remote host and then
locating the remote process within the remote host.
The network now needs to route data between hosts,
without bothering about the remote process.



Hierarchical layering, with four layers: Application,
Transport, Internet, and Network.
Higher layers may bypass adjacent layers and directly
access a lower layer (Efficient!)
A Comparison of the OSI and
TCP/IP Communications
Architectures
Application
Process
Presentation
Session
Transport
Network
Data link
Physical
OSI
Host-to-host
Internet
Network
access
TCP/IP protocol
suite
Communication Using the TCP/IP
End
system
End
system
App
App
TCP
TCP
IP
IP
Router
NAP1
NAP2
IP
Subnet 1
NAP 1
NAP 2
Subnet2
Protocol Data Units in TCP/IP Application
User data
TCP
header
IP
header
Network
header
byte stream
TCP
segment
IP
datagram
Network-level
packet
IP Basics

Connectionless Datagram Network Protocol.


Core IP Functions




Designed with Internetworking in mind.
Support Fragmentation and Reassembly
Routing
Error Reporting
Error checking covers only the IP header.
IP Helper Protocols

Internet Control & Management Protocol
(ICMP)

Address Resolution Protocol (ARP)

Domain Name Service (DNS)

Routing Protocols


Interior: R(outing)IP, OSPF (Open Shortest Path
First)
Exterior: BGP (Border Gateway Protocol)
IP Addressing

Hierarchical addressing scheme for scalability.

Logic for addressing scheme

Class based addressing



There would be a few very large networks (class A).
There would be a large number of small networks (class
C).
32-bit addresses


Network and host parts
Length of network and host parts depends on the
class of network.
IP Addressing

Dotted decimal notation

128.102.16.10 (NS.NSAS.GOV)

Class B addresses. Start with 10

NS.NASA.GOV has a globally unique address
128.102.16.10
netid is 128.102, assigned by Network Information
Center (NIC)
subnetid is 16, assigned by NASA (in this case)
hostid is 10, assigned by NASA Lab(I n this case)

Broadcast Address: Hostid is all 1's.



IP Routing

Classless Inter-Domain Routing (CIDR)



introduced in the early 90s
to improve address space usage
IPv6 Changes



128-bit address length
class less routing
hooks for QoS
Location of hosts on the Internet

How do hosts find each other on the internet?






Need Physical Address.
Relationship between Physical Addresses and IP
Addresses.
Ethernet addresses are 48-bits.
IP addresses are 32-bits. Address Mapping is done
by the network.
Each machine has an associated (IP,NPA) address
pair.
Broadcast Address Resolution Packet using
the Address Resolution Protocol (ARP)
Controlling the Internet

Internet Control Message Protocol (ICMP)

ICMP Functions





Communicate errors back to host: destination
unreachable, datagram errors, excessively long routes
detected, other failures.
Testing destination reachability and status.
Datagram flow control.
Route change requests (redirect).
Obtain information such as NPA and subnet mask.
Transport Layer Functions




Provides an interface between higher layers and the
underlying network.
End-to-End Reliable Connectivity between hosts.
Connectivity is between ports on hosts.
The port addresses are only locally unique.

In TCP, some standard ports are defined for telnet, ftp, mail.

End-to-end error checking may be provided.

Common transport protocols:



User Datagram Protocol (UDP)
Transmission Control Protocol (TCP)
ISO Transport Protocol
User Datagram Protocol


Connectionless datagram protocol with low
overhead
Limited error checking or recovery. Assumes a
reliable network layer.



UDP checksum is optional and need not be used.
Used commonly on LANs. UDP is used with
SUN's network file system (NFS).
Port Number: Transport Service Access Point
(TSAP) in OSI
Transport Control Protocol


Reliable Transport Protocol
Assumes very little about the underlying
network, and can be used with a variety of
networks.






Dial-up telephone lines
Internet IP datagram service
LANs
High speed fiber optics network
Low speed long haul network
Wireless links
TCP Features

TCP is Connection-oriented.

TCP provides end-to-end error checking.

TCP provides end-to-end flow control (sliding
window flow control).

Full duplex connection.

Higher overhead.
Internet Administrative Bodies
Internet Society (ISOC)
 non-governmental international society
Technology management
 standards, RFC process

Internet Architecture Board (IAB: www.iab.org)
Internet Engineering Steering Group (IESG)
Internet Engineering Task Force (IETF: www.ietf.org)
Internet Assigned Number Authority (IANA: www.iana.org)
RFC Editor


InterNIC
 domain name registry and IP network number
assignment
Internet Administrative Bodies
Internet Corporation for Assigned Names and
Numbers (ICANN)
non-govermental group
Responsible for assigning names and
numbers for the Domain Name System (DNS)
Arose in an environment of controversy

Internet Services

TCP/IP based application layer protocols
SMTP (email)
 HTTP (WWW)
 SNMP (network management)
 FTP (file transfer)
 telnet (terminal emulation)

Ubiquity of this standards compliant platform has
profound implications


intra-organizational systems


intranets
inter-organizational systems

business to business commerce, business to consumer
Internet-enabled Applications

Pull technology


www, ftp
Push Technology

Pointcast
Using the WWW to increase reach
Customer access to internal systems

tracking of packages


fedex, ups,...
Mutual fund information


vanguard, fidelity
Frequent flier miles


American, Delta
Electronic Marketplaces

Amazon
bookstore on the web
over 1 million titles
low prices


Export Administration Regulations
developed and administered by NTIS
$21/month for access to regulation database

Push technologies
Pointcast

information bundled with advertising
brings newspapers, CNN etc. as per user interests
to the desktop
can be used as a screen saver
updates itself on predetermined schedule or on
demand
available at www.pointcast.com

Summary



Internetwork is a network of networks which must be
capable of connecting networks together.
Internetwork consists of a number of computer
platforms, operating systems and network interfaces.
Goal of open internetworking is to overcome these
differences.
Repeaters, bridges, routers and gateways required
for accomplishing communication outside single LAN.