by n.gopinath ap/cse unit – ii introduction to network layers
TRANSCRIPT
ByN.Gopinath
AP/CSE
Unit – II
Introduction to network layers
Internetworking• An internetwork is a collection of individual networks, connected by
intermediate networking devices, that functions as a single large network.
• different kinds of network technologies that can be interconnected by routers and other networking devices to create an internetwork
Types• Local-area networks (LANs)enabled multiple users in a relatively small
geographical area to exchange files and messages, as well as access shared resources such as file servers and printers.
• Wide-area networks (WANs) interconnect LANs with geographically dispersed users to create connectivity.
• technologies used for connecting LANs include T1, T3, ATM, ISDN, ADSL, Frame Relay, radio links, and others.
ETH
IPV4 Packet HeaderVersion HLen TOS Length
Ident Flags Offset
TTL Protocol Checksum
SourceAddr
Destination Addr
Options(variable) Pad(variable)
Data
Datagram Delivery
Packet Format
IPV4 Packet header
Fragmentation and Reassembly
Fragmentation and Reassembly
Fragmentation and Reassembly
(RARP)Reverse Address Resolution Protocol
• (RARP) is a Link layer networking protocol• RARP is described in internet EngineeringTask ForceETF) publication
RFC 903• It has been rendered obsolete by the Bootstrap Protocol (BOOTP) and
the modern Dynamic Host Configuration Protocol(DHCP)• BOOTP configuration server assigns an IP address to each client from a
pool of addresses. • BOOTP uses the User Datagram Protocol (UDP)
RouterA router is a device that determines the next network point to which a packet should be forwarded toward its destination
Allow different networks to communicate with each other
A router creates and maintain a table of the available routes and their conditions and uses this information to determine the best route for a given packet.
A packet will travel through a number of network points with routers before arriving at its destination.
There can be multiple routes defined. The route with a lower weight/metric will be tried first.
Routing
Routing
Routing
Routing ProtocolsStatic Routing
Dynamic RoutingIGP (Interior Gateway Protocol): Route data within an Autonomous System
RIP (Routing Information Protocol)
RIP-2 (RIP Version 2)
OSPF (Open Shortest Path First)
IGRP (Interior Gateway Routing Protocol)
EIGRP (Enhanced Interior Gateway Routing Protocol)
IS-IS
EGP (Exterior Gateway Protocol): Route data between Autonomous Systems
BGP (Border Gateway Protocol)
Routing
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The Routing AlgorithmThe Routing Algorithm
the shortest path tree is contained in the routing table
Calculations are based on the Bellman-Ford algorithm
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The Centralized Version of the The Centralized Version of the AlgorithmAlgorithm
Cycle Node B C D E
Initial (., ) (., ) (., ) (., )
1 (1, 1) (2, 2) (3, 1) (4, 2)
A B C
D E
2
3 45
6
1 A B C
D E
2
3 4
1
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The Distributed Version
A B C
D E
1 2
3 4
5
6
Example of simple network with 5 nodes (routers) and 6 links (interfaces)The cost of all links is assumed to be 1
From A to Link Cost B 1 1 C 1 2 D 3 1 E 1 2
Routing table for A
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Advantages
• simple to implement
• low requirement in processing and memory at the nodes
• suitable for small networks
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Disadvantages
• Slow convergence• Bouncing effect• Counting to infinity problem
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Slow Convergence
A B C
D E
2
3 4
5
6
XXX
When a link breaks the routers are supposed to reestablish the routing tables
link 1 breaks
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The Bouncing Effect
link 2 breaks and A sends its routing table to B before B sends it to A
A B C
D E
3 4
5
6
XXX
1
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Counting to Infinity Problems
D E
3
A B C2
4
5
XXX
XXX
Links 1 and 6break.
A sends its old routing table before D sends the new routing table
Subnets
• Each organization assigns IP addresses to specific computers on its networks
• IP addresses are assigned so that all computers on the same LAN have similar addresses
• Each of these lans is known as a TCP/IP subnet• Any portion of the IP address can be
designated as a subnet using a subnet mask*
* Subnet masks tell computers what part of an IP address is to be used to determine whether a destination is on the same or a different subnet
Subnet Addressing
Figure 5-6
Subnet Addressing
Example 1
Suppose that the first two bytes are the subnet indicator with addresses of the form 131.156.x.x
Then, 131.156.29.156 and 131.156.34.215 would be on the same subnet.
The subnet mask would be 255.255.0.0, which corresponds to 11111111.11111111.00000000.00000000, where 1 indicates that the position is part of the subnet address and a 0 indicates that it is not.
Subnet Addressing
Example 2
Partial bytes can also be used as subnets.
For example, consider the subnet mask 255.255.255.128, which is 11111111.11111111.11111111.10000000.
Here, all computers with the same first three bytes and last byte from 128 to 254 would be on the same subnet.
Providing Addresses
• Providing addresses to networked computers
– Static addressing– Dynamic addressing
Static Addressing
• Each computer is given an address through a configuration file
• Stored on individual computers• Problems
– Moves, changes, adds and deletes– Individuals could change their own IP address– Network renumbered
• Companies do not have a good way of tracking the addresses
Dynamic Addressing
• Server supplies a network layer address automatically– Each time user logs in– For a specific lease period
• Two standards for dynamic addressing– Bootstrap protocol (bootp) - developed in 1985– Dynamic host control* protocol (DHCP), developed
in 1993
* some say C = configuration
Dynamic Addressing
• Bootp and DHCP– Software installed on the client instructs the
client to contact the server using data link layer addresses
– Message asks server to assign the client a unique network layer address
– Server runs corresponding software that sends the client its network address and subnet mask
Leasing
• Bootp or DHCP server can either:– Assign the same network layer address to the
client each time the client requests it (bootp)– Lease the network address from the next
available on a list of authorized addresses for as long as the client is connected or for a specified amount of time -- common with isps and dial-up users (DHCP)
Address Resolution
• The process of:– Translating an application layer address to a
network address (server name resolution)– Translating the server name address to a data
link layer address (data link layer address resolution)
Address Resolution
• Server name resolution– Accomplished by the use of domain name
service (DNS)– Computers called name servers provide these
DNS services• Address data base includes: server names and their
corresponding IP address
Client computer
DNS Servernetmgr.cso.niu.edu131.156.1.11
DNS Request
LAN
LAN
Internet
DNS Request
Root DNS Server for .EDU
domain
Weber State University
Northern Illinois University
DNS Request
DNS Response
DNS Response
DNS Response DNS Serversol.acs.unt.edu137.90.2.122
Figure 5-7
Address Resolution
• Data link layer address resolution– Broadcast message is sent to all computers in
its subnet • “if your IP address is xxx.Yyy.Zzz.Ttt, please send
your data link layer address”• Uses address resolution protocol (ARP)
Network Routing
• The process of determining the route a message will take through the network– Centralized– Decentralized
• Static routing• Dynamic routing• Broadcast or multicast routing
– Connectionless– Connection-oriented routing
Route and Route Table
Computer B Destination Route
A AC CD AE EF EG C
A
BC
G
E
D F
Internet Routes
UEN
WSU CanadaOther destinations
West Coast
Europe Asia
WSUDestination RouteUEN UtahOxford EuropeU of Toronto CanadaU of Singapore AsiaUC Stanford West CoastOther Other
Types of Routing• Centralized routing• Static routing (decentralized)• Dynamic routing (adaptive and
decentralized)– Distance vector– Link state
• Other types – Broadcast routing– Multicast routing
Centralized Routing• All routing decisions are made by one computer• Main routing for star and mesh topologies• Routing tables located on each computer
– Central computer sends updated tables as needed– Routing table tells the device where to send messages
• Simplicity - no wasted resources• Hardware failures or changing conditions cause
table to be out of sync
Decentralized Routing
• Each of the following types of routing fall under the heading of decentralized routing
• Each device makes its own routing decisions with the use of a formal routing protocol
• Routing protocols are self-adjusting– Can automatically adapt to changes in the network configuration
• Drawbacks– Slows down the network with status messages– Requires more processing by each computer
Static Routing
• Routing table developed by the network manager or some type of committee– Initial table sent to each computer which then
updates the routing table as needed– Reroutes as needed with down or removed
circuits– Updated when new devices announce their
presence– Used in relatively static networks that have few
routing options
Dynamic Routing (Adaptive)• Routing messages over the fastest route
– Used when there are multiple routes in the network
– Improves network performance by selecting the fastest route to avoid bottlenecks or busy circuits
– Initial table developed by network manager– Dynamically updated with changing conditions by
the devices themselves– Monitors message transmission time or each
device reports how busy it is to avoid bottlenecks• Disadvantages
– Requires more processing by each computer– “Wastes” network capacity
Dynamic Routing (Adaptive)• Distance vector dynamic routing
– The number of hops along a route – Exchange information with the neighboring computers every few
minutes
• Link state dynamic routing– The number of hops along a route– The speed of the circuits on the route– How busy the route is– Exchanges information with other routing devices every 15-30
minutes– Tries to determine the fastest route– Converges reliable routing information more quickly
Routing Protocols
• RIP, IGP, OSPF, EGP, BGP• Distance vector routing protocols (RIP,
Appletalk,IPX, IGRP)– Routers inform neighboring routers of table– Closest router is used to route packets
• Link State routing protocols (OSPF)– Routers have at least a partial map of the network– Changes are flooded throughout network– Routes are recomputed
Interior and Exterior Routing
• Interior routing is within an autonomous system (collection of routers under a single administrative control)—RIP, OSPF
• Exterior routing occurs between autonomous systems
• Network access protocols operate at Layer 2. – Transport of IP datagrams– IP over point-to-point connections is used by ISP
when you dial in
Routing Protocols• Border Gateway Protocol• Internet Control Message Protocol • Routing Information Protocol• Open Shortest Path First• Enhanced Interior Gateway Routing
Protocol
Routing Protocols• Internet protocols
– BGP (border gateway protocol)• Exchanges information between autonomous systems
about the condition of the internet• Complex, hard to administer, exterior routing protocol
– ICMP (internet control message protocol)• Simple, interior routing protocol used with the internet• Reports routing errors but is limited in the ability to
update – RIP (routing information protocol)
• Dynamic distance vector interior routing protocol• Counts the number of devices on each route• Selects the route with the least number of devices
Routing Protocols– OSPF (open shortest path first)
• Link state interior routing protocol used on the internet• Counts number of computers, network traffic, network error
rates to select the best route• Doesn’t broadcast to all devices just to routing devices• Preferred TCP/IP, but also used by IPX/SPX
– EIGRP (enhanced interior gateway routing protocol)• Link state interior routing protocol developed by CISCO• Uses route transmission capacity, delay, reliability and load
to select best route• Stores multiple routing tables
– SAP (service advertisement protocol)• Netware servers send SAP advertisements• Novell’s broadcast protocol
Broadcast Routing
• Sends the message to all computers on the network
• Only computer with correct address processes the message
• Used only in bus networks• Wastes network bandwidth
Multicasting– Similar to broadcasting – Only works within one LAN or subnet– Messages sent from one computer to another on the network
is called a unicast message– Messages sent to a group of computers is called a multicast
message– Targeting a specific work group – IGMP (internet group management protocol)
• Sends an IGMP multicast request to the routing computer• Assigned a special class D IP address to identify the group• The routing computer sets the data link layer address• All participating machines will process messages sent to this
address• Sends a IGMP message notifying of end of session
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