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Routing Each node in a WAN is a router

that accepts an input packet, examines the destination address, and forwards the packet on to a particular telecommunications line.

How does a router decide which line to transmit on?

A router must select the one transmission line that will best provide a path to the destination and in an optimal manner.

Often many possible routes exist between sender and receiver.


Routing

The subnet with its nodes and telecommunication links is essentially a weighted network graph.

The edges, or telecommunication links, between nodes, have a cost associated with them.

The cost could be a delay cost, a queue size cost, a limiting speed, or simply a dollar amount for using that link.


Routing

The routing method, or algorithm, chosen to move packets through a network should be: Optimal, so the least cost can be found Fair, so all packets are treated equally Robust, in case link or node failures occur and the

network has to reroute traffic. Not too robust so that the chosen paths do not

oscillate too quickly between troubled spots.


Routing Algorithms

Conflict between fairness and optimality.


Least Cost Routing Algorithm

Dijkstras least cost algorithm finds all possible paths between two locations.

By identifying all possible paths, it also identifies the least cost path.

The algorithm can be applied to determine the least cost path between any pair of nodes.


Flooding Routing When a packet arrives at a node,

the node sends a copy of the packet out every link except the link the packet arrived on.

Traffic grows very quickly when every node floods the packet.

To limit uncontrolled growth, each packet has a hop count. Every time a packet hops, its hop count is incremented. When a packets hop count equals a global hop limit, the packet is discarded.

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Centralized Routing One routing table is kept at a central node. Whenever a node needs a routing decision, the central node is consulted. To survive central node failure, the routing table should be kept at a backup

location. The central node should be designed to support a high amount of traffic

consisting of routing requests.


Distributed Routing Each node maintains its own routing table. No central site holds a global table. Somehow each node has to share information with other nodes

so that the individual routing tables can be created. Possible problem with individual routing tables holding

inaccurate information.


Isolated Routing

Each node uses only local information to create its own routing table.

Advantage - routing information does not have to be passed around the network.

Disadvantage - a nodes individual routing information could be inaccurate, or out of date.


Adaptive Routing versus Static Routing

With adaptive routing, routing tables can change to reflect changes in the network

Static routing does not allow the routing tables to change.

Static routing is simpler but does not adapt to network congestion or failures.


Network Congestion When a network or a part of a

network becomes so saturated with data packets that packet transfer is noticeably impeded, network congestion occurs.

Preventive measure include providing backup nodes and links and preallocation of resources.

To handle network congestion, you can perform buffer preallocation, choke packets, or permit systems.

Forward and backward explicit congestion control also used

When too much traffic is offered, congestion sets in and

performance degrades sharply.


Congestion Prevention PoliciesPolicies that affect congestion.

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Quality of Service

Before making a connection, user requests how much bandwidth is needed, or if connection needs to be real-time

Network checks to see if it can satisfy user request If user request can be satisfied, connection is

established If a user does not need a high bandwidth or real-time,

a simpler, cheaper connection is created


Requirements

How stringent the quality-of-service requirements are.


Buffering

Smoothing the output stream by buffering packets.


Internetworking How Networks Differ How Networks Can Be Connected Concatenated Virtual Circuits Connectionless Internetworking Tunneling Internetwork Routing Fragmentation


Connecting Networks


How Networks DifferSome of the many ways networks can differ.

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How Networks Can Be Connected

(a) Two Ethernets connected by a switch.

(b) Two Ethernets connected by routers.


Concatenated Virtual Circuits

Internetworking using concatenated virtual circuits.


Connectionless Internetworking

A connectionless internet.


Tunneling

Tunneling a packet from Paris to London.


Tunneling (2)

Tunneling a car from France to England.


Internetwork Routing

(a) An internetwork. (b) A graph of the internetwork.

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Fragmentation

(a) Transparent fragmentation.(b) Nontransparent fragmentation.


Fragmentation (2)

Fragmentation when the elementary data size is 1 byte.(a) Original packet, containing 10 data bytes.(b) Fragments after passing through a network with maximum

packet size of 8 payload bytes plus header.(c) Fragments after passing through a size 5 gateway.


The Network Layer in the Internet

The IP Protocol IP Addresses Internet Control ProtocolsOSPF The Interior Gateway Routing Protocol BGP The Exterior Gateway Routing Protocol Internet MulticastingMobile IP IPv6


Design Principles for Internet

Make sure it works. Keep it simple. Make clear choices. Exploit modularity. Expect heterogeneity. Avoid static options and parameters. Look for a good design; it need not be perfect. Be strict when sending and tolerant when receiving. Think about scalability. Consider performance and cost.


Collection of Subnetworks

The Internet is an interconnected collection of many networks.


Communication in the Internet

The transport layer takes data streams and breaks them up into datagrams

Each datagram is transmitted through the Internet (possibly fragmented into smaller units)

When all the pieces get to the destination machines they are reassembled by the network layer to the original datagram which is then handed to the transport layer

The transport layer inserts the received datagram into the receiving process input stream.

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The Internet Protocol (IP)

IP prepares a packet for transmission across the Internet.

The IP header is encapsulated onto a transport data packet.

The IP packet is then passed to the next layer where further network information is encapsulated onto it.


Progression of a packet from one network to another


Using IP, a subnet router: Makes routing decision based on the destination

address. May have to fragment the datagram into smaller

datagrams (very rare) using Fragment Offset. May determine that the current datagram has been

hopping around the network too long and delete it (Time to Live).

The Internet Protocol (IP)


The IP Protocol

The IPv4 (Internet Protocol) header.


The IP Protocol (2)

Some of the IP options.

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IP Addresses

All devices connected to the Internet have a 32-bit IP address associated with it. Think of the IP address as a logical address (possibly

temporary), while the 48-bit address on every NIC is the physical, or permanent address.

Computers, networks and routers use the 32-bit binary address, but a more readable form is the dotted decimal notation. For example, the 32-bit binary address

10000000 10011100 00001110 00000111translates to 128.156.14.7

There are basically four types of IP addresses: Classes A, B, C and D.

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IP Addresses

IP address formats.


IP Addresses (2)

Special IP addresses.


Subnets

A campus network consisting of LANs for various departments.


Sometimes you have a large number of IP address to manage.

By using subnet masking, you can break the host ID portion of the address into a subnet ID and host ID.

Some examples: the subnet mask 255.255.255.0 applied to a class B address

will break the host ID (normally 16 bits) into an 8-bit subnet ID and an 8-bit host ID;

the subnet mask 255.255.252.0 applied to a class B address will break the host ID (normally 16 bits) into an 6-bit subnet ID and an 10-bit host ID.

IP Subnet Masking


Subnets (2)

Class B network subnetted into 64 subnets.


CIDR Classless InterDomainRouting

A set of IP address assignments.

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Network Address Translation (NAT)

NAT lets a router represent an entire local area network to the Internet as a single IP address.

All traffic leaving this LAN appears as originating from a global IP address.

All traffic coming into this LAN uses this global IP address.

This security feature allows a LAN to hide all the workstation IP addresses from the Internet.

Since the outside world cannot see into the LAN, you do not need to use registered IP addresses on the inside LAN.


Network Address Translation (NAT)

We can use the following blocks of addresses for private use: 10.0.0.0 10.255.255.255 172.16.0.0 172.31.255.255 192.168.0.0 192.168.255.255

When a user on inside sends a packet to the outside, the NAT interface changes the users inside address to the global IP address. This change is stored in a cache.

When the response comes back, the NAT looks in the cache and switches the addresses back.

No cache entry? The packet is dropped. Unless NAT has a service table of fixed IP address mappings. This service table allows packets to originate from the outside.


NAT Network Address Translation

Placement and operation of a NAT box.


Internet Control Message Protocol

The principal ICMP message types.

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ARP The Address Resolution Protocol

Three interconnected /24 networks: two Ethernets and an FDDI ring.


Dynamic Host Configuration Protocol

Operation of DHCP.

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The Main IPv6 HeaderThe IPv6 fixed header (required).


Extension Headers

IPv6 extension headers.


Extension Headers (2)

The hop-by-hop extension header for large datagrams (jumbograms).


Extension Headers (3)

The extension header for routing.

/ColorImageDict > /JPEG2000ColorACSImageDict > /JPEG2000ColorImageDict > /AntiAliasGrayImages false /DownsampleGrayImages true /GrayImageDownsampleType /Bicubic /GrayImageResolution 300 /GrayImageDepth -1 /GrayImageDownsampleThreshold 1.50000 /EncodeGrayImages true /GrayImageFilter /DCTEncode /AutoFilterGrayImages true /GrayImageAutoFilterStrategy /JPEG /GrayACSImageDict > /GrayImageDict > /JPEG2000GrayACSImageDict > /JPEG2000GrayImageDict > /AntiAliasMonoImages false /DownsampleMonoImages true /MonoImageDownsampleType /Bicubic /MonoImageResolution 1200 /MonoImageDepth -1 /MonoImageDownsampleThreshold 1.50000 /EncodeMonoImages true /MonoImageFilter /CCITTFaxEncode /MonoImageDict > /AllowPSXObjects false /PDFX1aCheck false /PDFX3Check false /PDFXCompliantPDFOnly false /PDFXNoTrimBoxError true /PDFXTrimBoxToMediaBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXSetBleedBoxToMediaBox true /PDFXBleedBoxToTrimBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXOutputIntentProfile (None) /PDFXOutputCondition () /PDFXRegistryName (http://www.color.org) /PDFXTrapped /Unknown

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