WAN Technologies and Routing

Computer Networks and Internets (Comer)

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Networks Networks are grouped into (fuzzy)

categories based on the distances they span and the number of nodes HAN: Home Area Network LAN: Local Area Network CAN: Campus Area Network MAN: Metropolitan Area Network WAN: Wide Area Network

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Scalability As the number of nodes increases, issues of scalability arise. For a network to “scale” One should be able to add nodes easily.

The network’s functionality should remain relatively constant when nodes are added.

The physical problems (attenuation & interference) are more readily overcome than the sharing problem (many nodes trying to transmit on a shared connection).

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Bridges help

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Bridges Bridges work at Layer 2

Bridges help reduce traffic because a bridge “learns” whether it filters or forwards a frame.

Unicast messages do not have to be forwarded to all segments but only to those connecting the source and destination.

This allows different segments to transmit different signals simultaneously (in parallel) giving the network scalability.

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Router A router serves a very similar purpose

It separates traffic into local (network) traffic and (internet) traffic that must be sent on.

It sends the latter in the right general direction.

Routers and bridges work in different layers: A bridge operates at the Data Link Layer using

MAC addresses (hardware addresses).

A router operates at the Network Layer using network addresses (software addresses, e.g. IP addresses).

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That’s Illogical Bridges use the MAC address.

The problem with MAC addresses is that they are essentially distributed at random over the network. Two nearby computers can have very different

MAC addresses.

Recall that first part of the MAC address is associated with the manufacturer.

Computers with consecutive MAC addresses could be many thousands of miles apart.

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MAC addressing (random)

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705

533

579

760

014

774

301 814

709

289

045

414

Network addressing (logical)

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104

103

102

201

202

203

204 304

303

101

302

301

Hierarchical addressing

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Desi Donnelly

12 Park Range

Victoria Park

Manchester M14 5HQ

UNITED KINGDOM

Name

Address

Neighborhood

City

Country

Hierarchical addressing (Cont.) The postal service reads the previous

address starting from the bottom.

There’s no need to read past United Kingdom until the letter reaches the UK.

There’s no need to read past Manchester until the letter reaches Manchester.

And so on.

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Hierarchy of IP An IP address has a somewhat similar

hierarchy. An IP address is divided into two parts:

The first part specifies a particular network (a neighborhood of nearby computers).

The second part specifies a particular node.

There’s no sense looking at the second part (the node) until you have a match on the first part (the network).

The subnet mask provides the dividing line between the network and node portions of the address.

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ipconfig /all

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Subnet mask 255.255.0.0

Another Subnet mask

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Subnet mask 255.255.248.0

25511111111

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25511111111

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24811111000

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24811111000

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Subnet masks 255.255.0.0

11111111.11111111.00000000.00000000

255.255.248.0 11111111.11111111.11111000.00000000

Subnet masks usually have the property that in binary they are a string of all 1’s followed by a string of all 0’s.

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The subnet-mask division The part of the IP address (expressed in

binary) that corresponds to the 1’s portion of the subnet mask is the network portion. 1’s network

The part of the IP address (expressed in binary) that corresponds to the 0’s portion of the subnet mask is the node portion. 0’s node

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Example IP: 139.84.33.18

10001011.01010100. 00100001. 00010010

Subnet mask: 255.255.248.0 11111111.11111111.11111000.00000000

Network part:

10001011. 01010100. 00100000. 00000000

Node part:

00000000. 00000000. 00000001. 00010010

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Comparison IP addressing is simpler because it is

systematic.

A MAC address consists of 48 bits and all of the bits must be looked at by each and every bridge.

An IP(v4) address consists of 32 bits and only the first part must be looked at by a router until a match is found and after that only the second part must be considered.

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Router A router connects two or more networks to form

an internet. Sometimes the router connects two or more sub-

networks to form a network.

A router may be a specific device or it may be software on a computer.

A router determines where to forward “internet” traffic (messages whose source is on one network and destination is on another network).

It also filters out local traffic.

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Designing a WAN Similar to the ideas of dividing a network into

“bridged” segments, an internet can be divided into “routed”

networks.

Or a network can be divided into “routed” subnetworks.

Ideally one should study the usage (traffic patterns) and place routers in such a way that filtering is efficient.

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Store and Forward The network is designed so that many signals are being

transmitted simultaneously (in parallel), thus several packets may reach a router in very rapid succession.

This feature of packet switching is handled by the STORE and FORWARD paradigm.

Packets are queued up as they arrive (“stored”) and then sent on their way (“forwarded”) when the processor catches up.

If the queue is full, the packet is dropped and the router may or may not send a message indicating as much.

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Hop A hop is the transmission of a packet from one

router (or other intermediate point) to another.

In TCP/IP protocol, the number of hops a packet is allowed to make is kept in the packet header (in the TTL time-to-live field). Each router reduces the number by one. When the TTL reaches 0, the packet is discarded. This is another difference between a bridge and a

router: a router can make (small) changes to the packet.

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Next-Hop Forwarding A router has information about the next

place (hop) to forward the packet so that it will eventually reach its destination.

The next-hop information is put into a table (called a routing table). It tells the packet which interface to use

as it passes through the router.

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Fig. 13-6 (Comer)CSIT 320 (Blum)28

Fig. 13-6 (Comer)

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Packets don’t look back Next-hop switching has “source

independence” and more generally “history independence.” The next-hop only depends on the

packet’s destination, not on where it started or where it’s been.

It makes the forwarding mechanism more compact and efficient.

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Routing All destination addresses with the same first

part will be forwarded (routed) to the same packet switch. That’s not exactly true. A given network

destination (as opposed to node destination) may be listed on the table a few times.

This allows the router to “re-route” packets if previous packets have not gotten through, in case a connection goes down or is jammed with traffic.

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Routing Using only this first part of the address

(the network portion) to route the packet results in A shorter table: an entry per network as

opposed to an entry per computer A shorter computation time for routing as

the table is shorter and better organized think of searching a sorted array versus

searching an unsorted array

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Interior/Exterior Some routers do not connect to a network

(group of end-user computers) but only to other routers or gateways. They are said to be “interior.”

Those connected to computers are called “exterior.”

Analogy: interior gateways are like two major highways meeting without any cities nearby

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The best of all possible routes Routing tables should have an entry for

each possible destination, this is called Universal Routing.

Routing tables should have the “shortest” path for the destinations, this is called Optimal Routing.

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Default Routes One way to deal with the competing needs

of having a hop for each destination and having small tables is to use default routes.

It’s analogous to the default case in a switch statement, any case not explicitly found elsewhere in the table follows the default hop.

If one hop is very popular, making it the default shortens the table.

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Routing Table Computation Static Routing

A program computes and installs a packet switch when it boots and the routes do not change.

Routing is simplistic and low network overhead, but the routing is inflexible.

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Routing Table Computation Dynamic

A program builds an initial routing table when router boots and alters the table as the conditions in the network change.

The network can handle problems automatically.

Used by most large networks. They are designed with redundant hardware to handle the occasional failures. The dynamic computation allows the network to recover easily.

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Bridges/Routers Recall that bridges could not be used

simultaneously if there was a loop.

Routers can have loops and remain active; they are more “intelligent” and can avoid the “infinite loop” problem.

A connection does not have to be down for a router to reroute packets, heavy traffic may be enough.

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Dijkstra’s algorithm

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Building a routing table Each router sends packets to the routers it

connects to (its “nearest neighbors”). From the responses to these packets, the

router gathers information on the “cost” to transmit to each neighbor. Cost is based on bandwidth, traffic, etc.

This information is compiled into the “link state packet” (LSP).

The LSP is transmitted to the neighbors and beyond.

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Building a routing table (Cont.) The router now has information on

all of the nodes in the networks

the links connecting them

the cost thereof

The router wants to find the least costly path to all of the other nodes.

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Math terminology Abstractly, the network is represented

by what mathematicians call a graph. A graph is made up of two sets

The vertices (our nodes)

The edges (our transmission lines) which connect two vertices

A graph is said to be “connected” if there is at least one path along the edges connecting every pair of vertices.

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Math terminology (Cont.) A graph is called “simply connected” if

for each pair of vertices, there is only one path connecting them. A simply connected graph has no loops. A tree is a simply connected graph.

Networks are typically not simply connected (they are not trees), so there is often more than one path between nodes.

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Dijkstra’s algorithm Dijkstra’s algorithm is a way for a router

to select the least costly path to another node (router).

One constructs a “tree” that spans the graph.

The root of the tree is the router for which we are building the table.

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Dijkstra Demo

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Dijkstra Demo (the root)

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Nearest Neighbors Next one includes the branches to the

root’s nearest neighbors.

One selects the least costly one of these to continue the procedure.

This is shown in red in the demo screen captures.

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Dijkstra Demo

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Dijkstra Demo

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Ikeda is the cheapest at this stage.

Competing paths Examine the nearest neighbors of that

least costly path. The cost of a path is the sum of the

costs of the links connecting the source (root) to a node.

If a path takes us to a node which already has been reached, then we compare the costs of the two paths and keep the lower.

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Dijkstra DemoCSIT 320 (Blum)51

Ikeda connects to Takamatsu but the cost would be 131 = 74+ 57 but we can already get to Takamatsu for cheaper than that.

No turning back Once a node has been analyzed as the

least costly path, we are through with it. That’s not to say it won’t be used as part

of the path of many other paths.

It only means we are through analyzing it.

These nodes are shown in blue in the demo screen captures.

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Dijkstra Demo

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Branch with lowest cost at this stage

Dijkstra Demo

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Takamatsu connects to Kawanoe for a a cost of 149=76+73 but we can already get there for a cost of 100.

Dijkstra Demo

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The cheapest active node is now Kawanoe.

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Kawanoe connects to Kochi for a cost of 187 = 100+87 but we can already reach there for 156.

Dijkstra Demo

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The cheapest active node is now Muroto.

Dijkstra Demo

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Muroto connects to Kochi for a cost of 211=128+83 but we can already reach there for a cost of 156.

Dijkstra Demo

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Kochi is now the cheapest active link.

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Kochi connects to Matsuyama for a cost of 283=156+127 but Matsuyama can already be reached for a cost of 195.

Dijkstra Demo

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Matsuyama is now the cheapest active node.

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Matsuyama connects to Uwajima for a cost of 287=195+92 beating the previous cost of 304, so we throw out the old path and keep the new.

Dijkstra Demo

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Nakamura is now the cheapest active node.

Dijkstra Demo

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Nakamura connects to Uwajima for a cost of 371=284+87 but Uwajima can already be reached for a cost of 287.

Dijkstra Demo

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Uwajima is now the cheapest active node.

Dijkstra Demo

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There are no more active nodes.

Other References http://www-b2.is.tokushima-u.ac.jp/

~ikeda/suuri/dijkstra/Dijkstra.shtml

http://www.webopedia.com

http://www.whatis.com

http://www.networkcomputing.com/netdesign/1122ipr.html (IP Routing Primer)

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