Bridges, Routers and Internet Woking

8/18/2019 Bridges, Routers and Internet Woking

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Basic’s of Networking

Instructor: Rocco Piro

Office: TSH 205

Phone: Ex. 24421

Email: [email protected]

CCE 570-891

Winter 2000


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Bridges, Routers and InternetworkingSession 10


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Internetworking

• Organization usually have more than one type of LAN to

satisfy a number of different applications and needs.

• The interconnection and joining of these LANS has come to

be Known as the internetworking


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Internetworking

• Communication Network: A facility that provides data

transfer services among stations attached to the network.• Internet: A collection of communication networks

interconnected by bridges and/or routers.

• Sub-network: Refers to a constituent network of the Internet.

• End System: Device attached to one of the sub-network of an

Internet that is used to support end users applications and

services.


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Internetworking

• Intermediate Systems(IS): a device used to connect two sub-networks and permit communication between end systems attached

to different sub-networks.

• Bridge: An IS used to connect two LAN's that use similar LAN protocols.

– The bridge acts as an address filter. It delivers packets from one

LAN intended for a destination on another LAN and passes themon. It operates in layer 2 of the OSI model.

• Router: A device used to connect two sub-networks that may ormay not be similar.

– Routers employ Internet protocols present in each router and ineach end system of the network. It operates in layer 3 of the OSImodel.


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Routers versus Bridges

• Bridges, in general, take action based upon the MAC addresses, anddo not worry about the Packet Type.

– This means that Bridge's operate at Layer 2 of the OSI model.

– The Bridge does not modify the contents of the packet. Bridgesusually operate between similar LANs.

• Routers, in general, take their action based upon informationcontained in the Datagram portion of the packet, which requiresthem to be aware of the specific protocols they transport.

– Thus Routers operate at Layer 3 of the OSI model.

– Routers can modify the contents of the packet, and can work between two different kinds of networks.

•


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Functions of a Bridge

• A bridge is the simplest of internetworking devices. It isimplemented between LAN's of identical protocols and

require minimal processing. Bridges perform the followingfunctions ( in the simplest terms).

1. Read all frames on LAN A, accept those addressed to LAN B.

2. Using MAC on B re-transmit to B.

3. Do the same for B to A traffic.


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The reasons for using bridges are:

• RELIABILITY: Dividing one system into smaller interconnectedsystems can increase the chance that any one device will not be partof a system that is disabled.

• PERFORMANCE: In general, performance will degrade as thenumber of devices increases.

• SECURITY: Keeping specific packets on specific physical medialimits both access and monitoring of traffic by devices that are notauthorized to do so.

• GEOGRAPHY: For distantly connected LAN's (i.e. separate physical buildings).


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Protocol Architecture

• Under the 802 architecture, each node on the network is identifiedat the MAC (Medium Access Control) level.

• At the LLC (Logical Link Control) Level, only the SAP (ServiceAccess Point) Addresses are used.

• Across two Bridged networks, the same MAC and LLC protocolsmust be employed.

• The Bridge need not have any LLC implemented since it is onlyconcerned with the MAC addresses.

• Bridges do not strip MAC frames they just relay them betweenLAN's.


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Bridge Connections


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Routing with Bridges

• In complex arrangements bridge's must also make some

routing decisions.


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• For a packet that is traveling from host 1 to host 6, the packet istransmitted on LAN A, all of the nodes and bridges ignore the packetexcept for the bridge linking LAN A and LAN B.

– This bridge re-transmits the packet onto LAN B, on which all

nodes and devices ignore the packet except for the bridge linkingLAN B and LAN C.

– This bridge re-transmits the packet on LAN C where it is received by host 6 the intended recipient.

• For a packet that is traveling between host 2 and host 8, there are twooptions.

– One option goes through one bridge, and the other goes throughtwo bridges.

– The packet should get directed down the shortest path (in terms ofnumber of HOPS), unless that path is down or congested.


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• There are three basic strategies for carrying out routing they

are:

1. Fixed Routing ( 802.3 Ethernet)

2. Transparent Bridging (802.3 Ethernet)

3. Source Routing ( 802.5 Token Ring)


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Routers

• It should be clear that as network complexity increases, the

basic Bridge does not have enough functionality or capability

to achieve all possible Internet Working requirements.

• These requirements can be summed up as:

– Link networks both Physically and with LC (Link control).

– Route and Delivery Data across networks.

– Track use of network resources and monitor their Status.

– Do the above without requiring any modification of the

nodes on the networks, while hiding all of the differences

between the networks.


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Routers

• Due to differences between networks, routers function takingthe following in account:

– Different Addressing Schemes

– Different Maximum Packet Sizes (using segmentation)

– Different Network Interfaces.

– Different Time-Outs

– Error Recovery

– Status reporting

– Routing Techniques

– Access Control


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Routing Protocol Architecture

• Routing depends on the Internet Protocol (IP) of TCP/IP which

operates at the OSI Layer 3.

• For two nodes to communicate across a Router, they must

share the same protocol above IP (in layer 4).

• The resulting Datagram represents a form of messageencapsulation of the original protocol.


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When ES 1 wants to send something to ES 2, the IP module of ES

1 constructs a Datagram with the global address of ES 2.

1. This Datagram is then sent.

2. The Datagram gets passed down to LLC and is sent to the router

using MAC level addressing.

3. The packet travels across the LAN to the Router.


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4. The router removes the MAC and LLC parts of the packet and

analyses the IP Header.

5. The router sends the datagram to the destination network or

router.

6. The datagram gets encapsulated into a packet to match the

MAC type that corresponds to the destination

LAN.

7. Segmentation may be needed to achieve step 6.


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• Routing is accomplished by maintaining tables in each end

system (ES) that gives, for each possible destination, the next

router in the Internet that the datagram should be sent to.

• The tables can be both static and dynamic.

• When routers go down neighbouring routers send status

reports to other routers on the internet allowing for updates of

these tables so that traffic can be diverted away from a

downed router.


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Datagram Lifetime:

• By imposing the number of hops ( 1 hop is one passingthrough a router) assures that the datagram won't circulate the

internet indefinitely if destinations are not found. This is an

actual field which is part of the datagram.

• See TCP/IP datagram Frame (next slide). This field gets

decremeted every time it crosses a router by that router.

• When the value of the field reaches zero the datagram is

discarded.


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Segmentation and Reassembly:

• To accommodate the different packet sizes allowed ondifferent networks - the datagram may have to go through a

process of segmentation and reassembly.• Once a packet is segmented by a router it will not get

reassembled until it reaches its destination.

• The frames are reassembled by using the following fields inthe datagram:

– Data unit identifier

– Data length offset

– More flag

(See text page 520 for detailed explanation)


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Internet Protocol

• The most widely used Internet protocol is the TCP/IP protocol suite.

• The protocol allows the interconnection of individual networks togive the appearance of a single unified internet in which all systemscan freely exchange data with each other as if directly connected to

one another.

• TCP/IP includes a number of alternative routing protocols that can be use to communicate with end systems and with other routers to perform routing functions and to relay user data through the internet.

• The routing protocols that are used depend on the size andcomplexity of the internet.

• Large TCP/IP Internet's are divided into what are called autonomoussystems. Some routing protocols are designed to route traffic within

single autonomous systems, while others are designed to route traffic between individual autonomous systems.


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Autonomous Systems

• A TCP/IP internet is made up of one or more autonomous

systems.

• An autonomous system consists of a set of computer systems

and data links, making up one or more physical networks(subnetworks), administered by one authority.

• Routers are frequently referred to as gateways in TCP/IP

literature.

• Routers within autonomous systems are called Interior

Gateway using Interior Gateway Protocols (IGP).

• Routers that connect one autonomous system to another arecalled Exterior Gateways using Exterior Gateway Protocol.


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Autonomous Systems


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Interior Gateway Protocols

• A common IGP used with routers within an autonomous

system is the Open Shortest Path First Protocol (OSPF).

• The OSPF protocol, as the name implies, computes a route

that incurs the least cost.

• Costs being based on delays, data rates and actual dollar costs

of other factors.


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• The protocol uses a link state algorithm in which each routerknows the complete topology of the internet in term ofexistence of other routers and the links between them.

• Each router broadcast information about the routers it isattached to and the status of the datalinks between them.

• A router takes this information and constructs a map of therelevant internet consisting of a graph with routers as systems

and links as edges.

• Routers can then calculate routes based on this graph usinganother algorithm known as the least-cost algorithm or Dijkstra shortest path algorithm ( see page 538 - 539 for

detailed explanation - if interested!).


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Exterior Gateway Protocol (EGP)

• In order for data to flow between two autonomous systems inlarge TCP/IP internets, a router in one system must

communicate with a router in the other system. A widely used

and standardized EGP is the Border Gateway Protocol (BGP).

• EGP packets are used to perform three basic functions:

– Exchange of routing information with another autonomous

system.

– Check to see if a router in another autonomous system is

still responding.

– Obtaining routing information from another autonomous

system.


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• BGP is based on a system of exchanging messages.These messagescan be summarized as follows:

– Open - used to open a neighbour relationship with anotherrouter.

– Update - Used exchange information about routes beingwithdrawn.

– Keepalive - Used to acknowledge messages and existingrelationship between routers.

– Notification- Used to for exchange of error messages.

• Routers are considered to be neighbours if they share the same

subnetwork.


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• The functional procedures that are carried out by the BGP are asfollows:

– Neighbour Acquisition - The process of agreeing to exchangeinformation with another router in an autonomous system. Onerouter sends an "open" message to the other router whichresponds with a keepalive.

– Neighbour Reachability - periodic exchange of "keepalive"messages for ongoing confirmation of the relationship.

– Network Reachability - Each router keeps a database or lookuptables of subnets and the routes for reaching a particular subnet.These entries are continually updated by the broadcasting of

"update" messages.

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Bridges, Routers and Internet Woking

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