Transcript presentation source

Local Internets
Cabletron SmartSwitch 2100
1
Local Internets

Internet
 System
of subnets such that any station on any
subnet can communicate with any station on any
other subnet by placing the receiver’s address in a
message
 Subnets
are individual networks in an internet
2
Local Internets

Local Internets
 Links
LAN
LAN
LAN
LAN
multiple LANs at a single site
 Entirely
on customer premises
 Planned
and managed by the owner
Company has no limits
 Company has all the headaches

 High-speed
transmission (roughly LAN speeds)
3
Why a Local Internet?

Overcome distance limitations


Overcome congestion and latency


10Base-T networks span only 500 meters
Individual shared media networks running around 10
Mbps become saturated at 200-300 stations.
Connect dissimilar LANs

Link Ethernet and Token-Ring Network LANs
4
Local Internetting to Increase
Distance Spans
10Base-T LAN in
Headquarters Building
(500 m maximum distance)
Internetting
Device
HQ LAN
10Base-T LAN in
Factory Building
(500 m maximum distance)
Internetting
Device
Transmission Link
(no max distance)
Factory LAN
5
A Congested Shared Media LAN
Department 1:
150 Stations
A
Stations B
B transmits to A
Before: Single LAN
Department 2:
150 Stations
C
Stations D
All stations in Department 2
hear the message
Each station hears the traffic of 300 stations:
Heavily congested.
6
Internetting keep most traffic within LANs
Department 1:
150 Stations
A
Stations B
B transmits to A
Traffic of 150 stations:
Not Congested
After Resegmentation
Internetting
Device
Department 2:
150 Stations
C
Stations D
Internetting Device
Blocks the Transmission of this message
to Department 2
Traffic of 150 stations:
Not Congested
7
Internetting Devices: Bridges

Simple, automatic, inexpensive, fast

Usually only two ports

A fast, cost-effective choice for small internets

See CISCO whitepaper for more details
8
Multiple Bridges
LAN 2
LAN 1
X
LAN 3
LAN 4
No Loops Allowed
Problematic for large bridged internets
LAN 5
9
Multiple Bridges
Route Between
LANs 1, 5
LAN 2
X
LAN 3
LAN 1
No loops means only one path between LANs
No alternative routing if failures, congestion
No way to optimize routing for security, etc.
LAN 5
10
802.1 Spanning Tree Standard
Route Between
LANs 1, 5
LAN 2
LAN 3
LAN 1
Backup
Link
Allows backup links
Disabled during normal operation
If a failure occurs, automatically initiated
LAN 5
11
Bridging LANs with Different Physical and MAC
Layers
Bridge
Hub
802.3 10Base-T
Ethernet LAN
10Base-T
Connection
802.5
Token-Ring Network
802.5
Connection
12
Bridging LANs with Different Physical and MAC Layers
802.2
LLC Standard
LLC Layer (Same)
802.2
LLC Standard
802.1
Bridging Standard
Bridging Layer
(Same)
802.1
Bridging Standard
802.3 MAC Layer
(CSMA/CD)
MAC Layer
(Different)
802.5 MAC Layer
(Token-Passing)
10Base-T Connection
to Hub
Physical Layer
(Different)
802.5 Connection
to Access Unit
13
Problems of Bridges

Do Not Stop Broadcast Messages
 Servers
broadcast their existence about twice a
minute
 In
contrast to normal messages, which are
designed to go to single stations, broadcast
messages go to all stations.
 Goes
to all stations on the network; bridges pass
these messages on
 Problematic
in large bridged intranets
14
Problems of Bridges

Do Not Stop Any Client from Logging into
Any Server
 Poor
security. Only password protection on
servers
 Bad
if servers hold grades in a university
 Bad
for departmental servers holding key
personnel or financial data in a firm
15
Switches Solve Bridge Problems

Begin as Multiport Bridges
 Add
broadcast reduction, security
16
Simple Switched Internet
Connection 1
LAN A
Connection 1
No Waiting!
Switches can carry
messages between
several pairs of LANs
simultaneously.
LAN C
LAN B
Connection 2
Connection 2
LAN D
17
Switched Internet with Multiple
Switches
Switch A
Switch B
Switch C
Switch D
LAN 1
Switches are arranged in a hierarchy
Only one route between any two LANs
No routing around failure, congestion
No optimization of routes
LAN 2
Route: 1-B-A-C-2
18
Switch Hierarchy

Switches can be arranged hierarchically

Levels of Switches
 Desktop
switches (only a few MAC addresses
can be supported)
 Workgroup
switches (MAC addresses for
members of a department)
 Enterprise
switches (large number of MAC
addresses)
19
Virtual LANs Reduce Broadcasting

Stations are Divided into Groups



Called Virtual LANs (VLANs)
Server, other broadcasts limited to VLANs
Not to all stations on all ports
LAN A
LAN B
LAN C
LAN D
Server only broadcasts to its VLAN stations on LAN A, LAN C
20
VLANs Add Security

Only stations on the same VLAN as a server
can reach it to log in
On VLAN 7
LAN A
On VLAN 36
X
LAN B
LAN C
LAN D
Client can only reach server if they are on the same VLAN
21
Simple Local Internet Using
Ethernet Switching and 10Base-T
Ethernet Switch
10Base-T Hub
10Base-T Hub
In a switched Ethernet
internet:
Stations connect to hubs.
Hubs connect to switches.
10Base-T Hub
LAN
LAN
22
Switched Internets

The Move Toward Switched Networks

All-switched LANs with stations connected to switches
are still too expensive for most firms. Need a port for
each station.

Using switches as internetting devices is cost-effective
today. Only hubs connect to switches. Only need a
port for each hub

As switching costs fall, companies can later move
switching down to individual LANs by replacing hubs
by switches. See CISCO white paper for details.
23
Routers

Most sophisticated internetting devices
 Provide
 Used
services for linking thousands of subnets
in the worldwide Internet, also within firms
 Efficient
for long-distance transmission
 Provide
wide range of management services to
give relatively automatic operation
 By
far the most expensive internetting devices
24
Route

End-to-End Connection
1
LAN A
LAN B
2
3
4
LAN D
LAN A - 1 - 3 - 5 - LAN D
5
25
Alternative Routes

Multiple Ways to Get from LAN A to LAN D
1
LAN A
LAN B
2
A-1-3-5-D
A-1-3-4-D
A-2-5-D
Etc.
3
4
LAN D
5
26
Advantages of Alternative Routing

Routing Around Failures
 Failed

Routing Around Congestion
 More

switches, trunk lines connecting switches
common than outright failures
Route Optimization
 Least
cost route
 Most reliable route
 Most secure route, etc.
27
Mixing Switches and Routers
Site A
LAN
LAN
Site B
Switch
LAN
Router
Switch
Router
LAN
Site C
Router
28
Distributed Backbone Network
LAN 1
Router
FDDI Backbone Ring
Router
LAN 2
Router
LAN 3
29
Backbone Network

Network that Links Subnets
 Subnets

take the place of stations
Distributed Backbone
 Backbone
runs past all stations
 If
a single router (or other internetting device)
fails, only that station is disconnected
 FDDI
is popular because of its possible 200 km
circumference, 100 Mbps speeds
30
Local Internet Using Collapsed
Backbone
LAN A
LAN B
Routers
at LANs
LAN C
Routers
at LANs
Central Switch or Router
31
Collapsed Backbone

Single point of maintenance
 Easy

Single point of failure
 If

to maintain the network
the central device fails, serious problems
Types of central devices
 Switches
 Routers
32
OSI Layers

Layer 1 (Physical)



Electrical signaling over a physical link
Layer 2 (Data Link)

Data framing and administration of communication over a single data link

Point to point connection

Shared media LAN with only one possible path between two station
Layer 3 (Network)

Routing across an internet with multiple alternative routes

Or a subnet that offers alternative routes, but these rarely exist
33
Internetting Devices

Hubs
 Layer

1: merely reflect bits back out
Bridges, Switches
 Layer
2: Work with MAC addresses
 No alternative routing

Routers
 Layer
3: routing across internet
 Only device with alternative routing
34
TCP/IP Internetting

Subnet layer
TCP/IP
OSI
Application
Transport
Internet
Subnet

Links stations on same subnet

Often IEEE LAN standards

PPP for telephone connections

TCP/IP specifies almost any subnet standard

For LANs, etc., specifies OSI

OSI further subdivides into Physical, Data Link
Data Link
Physical
35
TCP/IP Internetting

Application
Transport
Internet
Subnet
Internet layer:
 Links
stations across internets
 Main
standard is the Internet Protocol (IP)
 Dominant
protocol for routers
36
TCP/IP Internetting


Transport layer:
Application
Transport
Internet
Subnet

Links computers, even if different platforms

Main standards are Transmission Control Protocol
(TCP) and User Datagram Protocol (UDP)
Application layer:

Links application programs even if from different
vendors

Many standards, because many applications

SMTP for e-mail; HTTP for the WWW, etc.
37
Universal Addressing

Each host has a unique IP Number

32-bit binary number

Goes in the IP header’s source and destination fields

10000000101010110001000100001101

Impossible to remember
IP Packet
Source
Destination
4 Bytes
4 Bytes
38
Subnet Mask

Problem: IP numbers do not include subnetting

Solution: Create a second number: a Subnet Mask

Define which bits of the IP address refer to subnets
vs. hosts on subnet

Subnet mask is 32 bits long, in dot quad format

See last meeting TCP/IP in NT for basic IP and
Subnet Mask concepts.
39
Routers

Routers also get IP addresses
 So
packets can be sent to them for routing
 Has network ID of the network on which it sits
 Must be assigned a host ID
 Example: 128.171.17.1
Default
Router
IP Packet
for Delivery
128.171.17.104
Another Router
128.171.17.1
40
Routers

Subnets can have Multiple Routers
 There
is usually a default router for packet
delivery
 Default router is used if no router is specified
 Routers are sometimes called gateways in TCP/IP
Default
Router
IP Packet
for Delivery
Other Router
41
Routing Protocols
Routing
Table
There are no “master” routers.
Each router works independently to do routing.
This requires each router to build a “routing table” that
contains information about the locations of other routers.
42
Routing Protocols
Routing
Table
Routing protocols allow
routers to exchange
information in their
routing tables.
43
Peer Control Among Routers

Routers Communicate Among Themselves



To coordinate their actions without central control
Share knowledge of network connectivity
Common standards are RIP, OSPF, BGP
Router
Coordination
Message
44
Routing Protocols

RIP - Router Information Protocol


OSPF - Open Shortest Path First


Optimizes routing, but complex
BGP - Border Gateway (Router) Protocol


High overhead, but simple and OK for small networks
Used in Internet Backbone Routers
Read Cisco whitepaper for more on routing
45
Autonomous Systems
RIP
or
OSPF
Autonomous
Router
Organization can select any
routing protocol to synchronize
its autonomous (internal)
routers. RIP and OSPF are
common.
Border routers that link
autonomous systems normally
use BPG.
Border Router
RIP
or
OSPF
BPG
Border Router
Autonomous System
46
Error Handling

TCP/IP a comprehensive set of error
handling processes

The Internet Control Message Protocol (ICMP) is used
to send error messages.

Hosts, Routers send ICMP messages to one another if a
problem occurs

“Host not found” is a common ICMP error message.
Host
ICMP Error Message
Router
47
Internet Control Message
Protocol (ICMP)
The Internet Control Message Protocol (ICMP)
is for delivering supervisory messages
among hosts and routers
48
Internet Control Message
Protocol (ICMP)
“Host Unreachable”
Error Messages
49
Internet Control Message
Protocol (ICMP)
Flow Control
“Source Quench” tells host
to reduce transmission rate.
Source
Quench
50
Internet Control Message
Protocol (ICMP)
“Echo
Request”
Source host can ask questions of
destination hosts.
“Echo Request” asks if the other host
is reachable.
“Echo
Response”
Destination host sends back
“Echo Response.”
Usually implemented with “Ping”
program.
51
Autoconfiguration


Autoconfiguration Server has a bank of addresses

When a PC “logs in,” it gets a temporary IP number.

Popular standards are DHCP (in Windows NT) and RARP
Large stations receive permanent addresses
DHCP Request for Address
DHCP Response:
Your Temporary Address is
127.171.17.35
DHCP
Server
52
Autoconfiguration Protocol
Source
Host
Autoconfiguration Request Message
AutoConfiguration
Host
Source host sends Autoconfigutation Request
Message to the autoconfiguration host
“My 48-bit MAC subnet address is X.
Please give me a 32-bit IP host address.”
53
Autoconfiguration Protocol
Source
Host
AutoConfiguration
Autoconfiguration Response Message
Host
Autoconfiguration host sends back a
Autoconfiguration response message.
“Computer at MAC Address X,
your 32-bit IP host number is ‘110100…’.”
54
Autoconfiguration Protocols

RARP: Reverse Address Resolution
Protocol
 Older

autoconfiguration protocol
Bootp
 Another

older protocol
DHCP
 Dynamic
 Built
Host Configuration Protocol
into Windows NT Server
55
Domain Name Service

Hosts also have IP host names
 Voyager.cba.hawaii.edu
 Like
nicknames

IP packets require formal IP numbers to put in
their source and destination fields

If tell your software the IP host name, it must
look up the IP number
56
Domain Name Service

Program knowing a host name sends request
to Domain Name Service (DNS) Server;
receives IP Number
DNS Request for
Voyager.cba.hawaii.edu
DNS
Server
DNS Response: 128.171.17.13
57
Domain Name System (DNS)
Source
Host
DNS Request Message
DNS
Host
Source host sends DNS Request Message to DNS host.
“I need the 32-bit IP host number for the host named
voyager.cba.hawaii.edu.”
58
Domain Name System (DNS)
Source
Host
DNS
Host
DNS Response Message
DNS host returns a DNS Reply Message.
“The 32-bit host number is 128.171.44.53”.
DNS
Host
59
Domain Name System (DNS)
Source
Host

Each network has a DNS host
 May also have a secondary DNS host
 Network DNS host may only know the
IP names and numbers of local hosts on
the network
 For other IP names, contacts another
DNS host, especially root DNS hosts,
which should have extensive information
DNS
Host
DNS
Host
60
Internet Protocol Packet
Version
IHL
Type of Service
Identifier
Time to Live
Total Length (in Bytes)
Flags
Protocol
Fragment Offset
Header Checksum
Source Address
Current version is
Version 4.
A new version,
Version 6,
is coming.
Destination Address
Options Plus Padding
Data
61
Internet Protocol Packet
Version
IHL
Type of Service
Identifier
Time to Live
Total Length (in Bytes)
Flags
Protocol
Fragment Offset
Header Checksum
There is only error checking for the header,
not for the entire packet.
If an error is detected in the header,
the packet is discarded
62
Internet Protocol Packet
Total Length (in Bytes)
Version 4 addresses only have 32 bits.
Not enough for the number of Internet hosts.
Will be raised to 128 bits in Version 6
Fragment Offset
Header Checksum
Source Address (32 bits)
Destination Address (32 bits)
Options Plus Padding
Data
63