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The Network Layer : 

The Network Layer

Slide 2: 

Position of network layer

Slide 3: 

Network layer duties

Network Layer Design Issues : 

Network Layer Design Issues Store-and-Forward Packet Switching Services Provided to the Transport Layer Implementation of Connectionless Service Implementation of Connection-Oriented Service Comparison of Virtual-Circuit and Datagram Subnets

Store-and-Forward Packet Switching : 

Store-and-Forward Packet Switching The environment of the network layer protocols.

Implementation of Connectionless Service : 

Routing within a diagram subnet. Implementation of Connectionless Service

Implementation of Connection-Oriented Service : 

Implementation of Connection-Oriented Service Routing within a virtual-circuit subnet.

Comparison of Virtual-Circuit and Datagram Subnets : 

Comparison of Virtual-Circuit and Datagram Subnets 5-4

Routing Algorithms : 

Routing Algorithms n/w layer s/w responsible for deciding which output line an incoming packet should be transmitted on. Datagram – new for each data packet VC- when new circuit to be established Also called session routing 2 process in routing Handles each packet as it arrives – forwarding Responsible for filling and updating routing tables

Propertities of Algorithms : 

Propertities of Algorithms Correctnes Simplicity Robustness Stability Fairness optimality Types of Algorithms Adaptive NonAdaptive

Routing Algorithms : 

Routing Algorithms Conflict between fairness and optimality.

Routing Algorithms : 

Routing Algorithms The Optimality Principle Shortest Path Routing Flooding Distance Vector Routing Link State Routing Hierarchical Routing Broadcast Routing Multicast Routing Routing for Mobile Hosts Routing in Ad Hoc Networks

The Optimality Principle : 

The Optimality Principle (a) A subnet. (b) A sink tree for router B.

Shortest Path Routing : 

Shortest Path Routing The first 5 steps used in computing the shortest path from A to D. The arrows indicate the working node.

Flooding : 

Flooding Static algorithm Every incoming packet is sent on every outgoing line except the one it came on Vast number of duplicate packets Measures Hop count  decrement at each hop Keep track of which packets have been flooded  can be done with the help of seq.nos Selective Flooding Send only in the right direction

Slide 16: 

Applications Military Distributed databases necessary to update all the databases concurrently Wireless networks All messages transmitted by a station received by all other stations within the radio range Can used to calculate the metric

Distance Vector Routing : 

Distance Vector Routing (a) A subnet. (b) Input from A, I, H, K, and the new routing table for J.

Distance Vector Routing : 

Distance Vector Routing The count-to-infinity problem.

Link State Routing : 

Link State Routing Each router must do the following: Discover its neighbors, learn their network address. Measure the delay or cost to each of its neighbors. Construct a packet telling all it has just learned. Send this packet to all other routers. Compute the shortest path to every other router.

Learning about the Neighbors : 

Learning about the Neighbors (a) Nine routers and a LAN. (b)A graph model of (a).

Measuring Line Cost : 

Measuring Line Cost A subnet in which the East and West parts are connected by two lines.

Building Link State Packets : 

Building Link State Packets (a) A subnet. (b) The link state packets for this subnet.

Distributing the Link State Packets : 

Distributing the Link State Packets The packet buffer for router B in the previous slide

Hierarchical Routing : 

Hierarchical Routing Hierarchical routing.

Broadcast Routing : 

Broadcast Routing Reverse path forwarding. (a) A subnet. (b) a Sink tree. (c) The tree built by reverse path forwarding.

Multicast Routing : 

Multicast Routing (a) A network. (b) A spanning tree for the leftmost router. (c) A multicast tree for group 1. (d) A multicast tree for group 2.

Internetworking Terms : 

Internetworking Terms Communications Network Facility that provides data transfer service An internet Collection of communications networks interconnected by bridges and/or routers The Internet - note upper case I The global collection of thousands of individual machines and networks Intranet Corporate internet operating within the organization Uses Internet (TCP/IP and http)technology to deliver documents and resources

Internetworking Terms : 

Internetworking Terms End System (ES) Device attached to one of the networks of an internet Supports end-user applications or services Intermediate System (IS) Device used to connect two networks Permits communication between end systems attached to different networks

Internetworking Terms (3) : 

Internetworking Terms (3) Bridge IS used to connect two LANs using similar LAN protocols Address filter passing on packets to the required network only OSI layer 2 (Data Link) Router Connects two (possibly dissimilar) networks Uses internet protocol present in each router and end system OSI Layer 3 (Network)

Internetworking Protocols : 

Internetworking Protocols

Internetworking : 


Slide 32: 

Links in an internetwork

Slide 33: 

Network layer in an internetwork

Slide 34: 

Network layer at the source

Slide 35: 

Network layer at a router

Slide 36: 

Network layer at the destination

Slide 37: 

Datagram approach

Internetworking : 

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

Connecting Networks : 

Connecting Networks A collection of interconnected networks.

How Networks Differ : 

How Networks Differ 5-43

How Networks Can Be Connected : 

How Networks Can Be Connected (a) Two Ethernets connected by a switch. (b) Two Ethernets connected by routers.

Concatenated Virtual Circuits : 

Concatenated Virtual Circuits Internetworking using concatenated virtual circuits.

Connectionless Internetworking : 

Connectionless Internetworking A connectionless internet

Connectionless Operation : 

Connectionless Operation Corresponds to datagram mechanism in packet switched network Each NPDU treated separately Network layer protocol common to all DTEs and routers Known generically as the internet protocol Internet Protocol One such internet protocol developed for ARPANET RFC 791 (Get it and study it) Lower layer protocol needed to access particular network

Connectionless Internetworking : 

Connectionless Internetworking Advantages Flexibility Robust No unnecessary overhead Unreliable Not guaranteed delivery Not guaranteed order of delivery Packets can take different routes Reliability is responsibility of next layer up (e.g. TCP)

Tunneling : 

Tunneling Tunneling a packet from Paris to London.

Tunneling : 

Tunneling Tunneling a car from France to England.

Internetwork Routing : 

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

Design Issues : 

Design Issues Routing Datagram lifetime Fragmentation and re-assembly Error control Flow control

Routing : 

Routing End systems and routers maintain routing tables Indicate next router to which datagram should be sent Static May contain alternative routes Dynamic Flexible response to congestion and errors Source routing Source specifies route as sequential list of routers to be followed Security Priority Route recording

Datagram Lifetime : 

Datagram Lifetime Datagrams could loop indefinitely Consumes resources Transport protocol may need upper bound on datagram life Datagram marked with lifetime Time To Live field in IP Once lifetime expires, datagram discarded (not forwarded) Hop count Decrement time to live on passing through a each router Time count Need to know how long since last router

Fragmentation and Re-assembly : 

Different packet sizes When to re-assemble At destination Results in packets getting smaller as data traverses internet Intermediate re-assembly Need large buffers at routers Buffers may fill with fragments All fragments must go through same router Inhibits dynamic routing Fragmentation and Re-assembly

IP Fragmentation : 

IP Fragmentation IP re-assembles at destination only Uses fields in header Data Unit Identifier (ID) Identifies end system originated datagram Source and destination address Protocol layer generating data (e.g. TCP) Identification supplied by that layer Data length Length of user data in octets

IP Fragmentation : 

IP Fragmentation Offset Position of fragment of user data in original datagram In multiples of 64 bits (8 octets) More flag Indicates that this is not the last fragment

Fragmentation Example : 

Fragmentation Example

Fragmentation : 

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

Fragmentation : 

Fragmentation 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.

Dealing with Failure : 

Dealing with Failure Re-assembly may fail if some fragments get lost Need to detect failure Re-assembly time out Assigned to first fragment to arrive If timeout expires before all fragments arrive, discard partial data Use packet lifetime (time to live in IP) If time to live runs out, kill partial data

Error Control : 

Error Control Not guaranteed delivery Router should attempt to inform source if packet discarded e.g. for time to live expiring Source may modify transmission strategy May inform high layer protocol Datagram identification needed (Look up ICMP)

Flow Control : 

Flow Control Allows routers and/or stations to limit rate of incoming data Limited in connectionless systems Send flow control packets Requesting reduced flow e.g. ICMP

The Network Layer in the Internet : 

The Network Layer in the Internet The IP Protocol IP Addresses Internet Control Protocols OSPF – The Interior Gateway Routing Protocol BGP – The Exterior Gateway Routing Protocol Internet Multicasting Mobile IP IPv6

Design Principles for Internet : 

Design Principles for Internet Make sure it works. Keep it simple. Make clear choices. Exploit modularity. Expect heterogeneity. Avoid static options and parameters.

Slide 63: 

Design Principles for Internet 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 : 

Collection of Subnetworks The Internet is an interconnected collection of many networks.

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