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CHAPTER TEN

CONNECTING NETWORKS

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We have looked at several different network topologies

Why do we have different types of networks?

Why do we create different, distinct LAN arrangements?

How do we connect them, so LANs can communicatewith other LANs and WANs?

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How we connect networks depends, to some degree, uponwhat we are trying to accomplish?

Are we connecting segments of a LAN, similar LANs, dissimilar LANs,or wide area networks?

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Contrary to what the author says, repeaters do not connect LANs, they connectsegments of a LAN. The figure shown below would, logically, behave asa single Local Area Network.

All devices, on any of these so-called LANs lie in the same collisiondomain, and thus, on the same LAN.

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As we discussed when examining the Ethernet Topology, Ethernet

Networks today commonly use hubs and their star configuration,

nevertheless, they constitute a single LAN

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We would, more likely, wish to use layer two connections, why?For this purpose, we might use a bridge rather than a repeater.

To use a bridge, all of these LANs must be of the same type.A bridge does not alter the frames it transmits, all networks wouldbe using the same MAC protocol. It is not technically correct to referto a device connecting two dissimilar LANs as a bridge, it is more of a router. It does alter frames and must have the MAC protocol of bothtypes of networks.

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How does a bridge know when, or where, to forward a particularframe, or whether it should be forwarded at all?

Next slide….

Bridges use routing tables, these can be installed on the bridgeby the systems manager or it can be a transparent bridge that will“learn” its routing table based on the traffic.

following slide…

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Routing tables for the network in previous figure.

CSC 31118. 10 pts. Indicate the contents of the bridge forwarding table given the following traffic. Indicate after each message, what action the bridge would take and the current contents of the routing table.

1. A sends a packet to C 2 . B sends a packet to E3. E sends a packet to C4. C sends a packet to E5. C sends a packet to D6. G sends a packet to A7. D sends a packet to F

A B C

D E

F

G

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You must, however, be careful if you are using transparent bridges,

certain configurations can cause difficulty.

What would the two routing tables look like if B sent a packet to A?

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Often there may be multiple paths with different costs associated asshown in the figure below.

In the absence of some learning protocol, this network would beflooded with copies of frames

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To solve this problem, we use a spanning tree algorithm.

From your data structures: A spanning tree corresponds to a minimalsubset of edges taken from a connected graph that connects the graph’svertices.

We must assign a cost associated with each bridge to LAN connection, we can then produce the following graph.

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We won’t go into the details of how to use the spanning tree algorithm.We would produce the following spanning tree.

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This would produce the following topology for our network,the bridges shown connected by dotted lines are “blocking” bridges,they will not forward packets.

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We have discussed two routing strategies for bridges, namely:

Routing tables loaded into each bridge by the system administrator FIXED ROUTING

Transparent bridges that “learn” the routes ADAPTIVE ROUTING, ROUTE LEARNING, ETC.

A third method is SOURCE ROUTING:The source specifies the route to the destination, routing infois included in the packet.

CSC 311Switches and Switched Ethernet

Since Ethernet is the dominant LAN, we will assume, hereafter, thatall of our LANs are Ethernets.

Switches perform the same function as bridges.While bridges typically connect a couple of LANs, switches mayhave a couple of dozen ports which provides many more connections.

Note, that thereis only one switchin this drawing,the remainingdevices arehubs, with noswitching logic

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A more common installation, one that is much more efficient, is aFULLY SWITCHED ETHERNETNote: there are no collisions in this topology so CSMA/CD is not needed.

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Virtual Lans

Use the “intelligence” of the switch to create Virtual LANs that arereally part of a single physical LAN. You might use this approachto isolate different workgroups working on different projectswithout having to physically separate them on separate LANs.

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Layer Three Connections

LANs and the layer 1 and 2 connections typically coversmall geographic areas

Wide Area Networks (WAN) span the globe and need moresophisticated techniques

Much like the highway system, internets or the Internet providemultiple paths from one point in the Internet to another. Weneed a technique to select the best among these manypossibilities.

In the highway example, what do you do if a bridge is out?Find an alternate route.Networks are frequently faced with the same problem when oneof the nodes in the network fails or becomes congested.

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Generalized Network Topology …. many routes

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In order to determine the best route through a network, we need someway to evaluate those routes. We generally associate a cost with eachlink in the route… such as shown below.

Routing tables can be developed using this information to specifythe optimum route between any two points in the network.

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Routing Tables may be used. These tables do not usually containthe entire route to a destination, rather, the next node in the routeto that ultimate destination

This is a partial routing table for nodes A, B, and E

Who defines these routing tables and how?

The process by which a routing table is defined is called a routing algorithm

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Routing Strategies

•Centralized

•Distributed

•Static

•Adaptive

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Centralized Routing

All interconnection information is generated and maintained ata centralized location.

That location then broadcasts this information to all network nodesso that each may define its own routing tables.

One way… routing matrix

Routing Matrix for Network in Figure 10.19

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Distributed Routing:

No centralized control.

Each node determines and maintains its routing information

Nodes exchange control information

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Static Routing:

Once a node determines its routing table, the node does not change it.

Adaptive Routing:

Nodes respond to changes in the network and update their routingtables accordingly.

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DIJKSTRA’S ALGORITHM

Used to determine path and cost for shortest possible pathbetween any two nodes in the graph.

The example that we will work will produce the path and thecost of the shortest possible path between one node andevery other node in the graph.

How doesit work?

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Graph for Dijkstra example:

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Skip Bellman-Ford

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Additional Routing Methods:

Link state routing: • Each node communicates what it knows to its neighbors

1. bit rate2. delay3. queue length4. reliability

• a node builds a link state packet for each link• nodes receiving link state packets forward them to neighbors• as these packets are exchanged , nodes learns about the network

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Hierarchical Routing

•Nodes are divided into groups, called domains

•routes between two nodes in the domain are determine bythan networks protocols

•each domain has one or more designated routers

•a large domain may have multiple subdomains.

The “Internet” uses such a hierarchical addressing scheme.

CSC 311Large networks, such the Internet, can be viewed as a collection ofdomains.

In such networks, we define two main categories of routing strategies:

interior and exterior

Interior routing protocols control routing among routers within anautonomous system or domain

Exterior routing protocols control routers among routers fromdifferent autonomous systems.

To send a packet from X toA, we woulduse an interiorprotocol------------------------------To send a packet from Zto X, Z sends to its routerW, which sends acrosssubdomains to C, whichsends to A, which sendsto X

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This domain concept can be represented using an hierarchical structure

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IPv4 Internet Addressing

Internet addresses are 32 bit numbers, usually shown in dotted decimalnotation

143.200.128.3 is an example, each of the decimal numbers can be represented by 8 bits.

The Internet uses a class address system, each such address can beinterpreted as having two parts: an Internet Protocol (IP) address whichis assigned to the network and a local device address.

The address above is an example of a class B address, the first 16 bits(143.200), is the IP network address and the other 16 bits (128.3)represents the address of the particular device on that network.

A class B address provides addresses for over 65,000 possible users, so it is quite common to further subdivide these users into subnets.

Using subnet addressing, the local management might designate part ofthe lower 16 bits to represent a subnet address.

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One example of an interior protocol:

Routing Information Protocol RIP•Use hop count

Hop count from N1 to N2 is 1Hop count from N1 to N4 is 2

The routers exchange information packets to obtain this information

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Another interior routing protocol:Open Shortest Path First (OSPF)

In this case, the “open” means the algorithm is not proprietarycan also use other factors in determining best route such as delay, bit rates,actual dollar cost.

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Exterior Protocols

Border Gateway Protocol:

current version is BGP-4

used in the Internet to establish paths among routers in differentautonomous systems.

Often concerned with just getting to destination, not necessarily the cheapestroute. There may be other considerations, such as avoiding a particularcountry in international traffic.

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Within AS3, an interior routing protocol would be used. In leaving AS3, an exteriorrouting protocol would be used. The gateway routers (A,B,C,F) , would shareinformation about their subnetworks.

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Congestion and Deadlock:

We have already discussed congestion and flow control. The routingstrategies must deal with the problem of congestion.

As congestion increases the efficiency of the network declines: How to deal with it?

•Packet elimination•Flow control•Buffer allocation•Choke packets

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In the worst case, Deadlock can occur.

Deadlock: congestion is so severe that nothing moves.

Problem described is “store and forward deadlock”

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Situation illustrated above is reassembly deadlock:

Buffers are full, packet 0 from both A and B cannot be accepted.

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Deadlock prevention:

Reassembly deadlock can be prevented by establishing and reserving

sufficient buffer space to handle the window size for a particular

communication

Store and forward is handled by allocating sufficient buffer space,

but… how much buffer space is enough?

One approach to deadlock is to allow it to occur and then deal with it.

It might be more cost effective to either prevent deadlock or take stepsto reduce the probability of occurrence.

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Another deadlock prevention technique uses hop count.

Nodes divide their buffer space into groups based on number of hops,hop count on a packet is incremented after each hop, a packet must waiton available space in its hop count group at the previous node, so a packet isalways waiting on a higher numbered group.

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