Network Layer Functions and Concepts
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Questions and Answers

What are the two primary functions of the network layer?

The two primary functions are forwarding, which moves packets from the router's input to the appropriate output, and routing, which determines the path packets take from source to destination.

Explain the difference between forwarding and routing.

Forwarding is the process of moving packets to the right output port in a router, while routing is the planning process that determines the best path for the packets to travel to reach their destination.

What role does a router play in the network layer?

A router examines the header fields of incoming IP datagrams and directs them to the appropriate outbound link based on routing information.

What are the different types of network layer service models mentioned?

<p>The network layer service models include virtual circuit networks and datagram networks.</p> Signup and view all the answers

What is the main purpose of IPv4 addressing within the network layer?

<p>IPv4 addressing serves to uniquely identify devices on a network and facilitates the routing of datagrams between these devices.</p> Signup and view all the answers

What is the primary characteristic that differentiates virtual-circuit networks from datagram networks?

<p>Virtual-circuit networks provide a connection-oriented service with a dedicated path, while datagram networks offer a connectionless service.</p> Signup and view all the answers

Explain how ATM ABR (Available Bit Rate) service behaves concerning congestion management.

<p>ATM ABR provides guaranteed minimum bandwidth but does not guarantee a constant rate and utilizes feedback for congestion control.</p> Signup and view all the answers

In the context of routing algorithms, what is the difference between link-state and distance vector algorithms?

<p>Link-state algorithms maintain a complete view of the network's topology, while distance vector algorithms rely on neighbors to share routing information.</p> Signup and view all the answers

Describe the role of a VC identifier in a virtual circuit network.

<p>The VC identifier is included in each packet to indicate the specific connection path, allowing routers to maintain state information for that connection.</p> Signup and view all the answers

What advantages does a virtual-circuit network have regarding resource allocation compared to a datagram network?

<p>Virtual-circuit networks can allocate dedicated resources like bandwidth and buffers to each connection, providing predictable performance.</p> Signup and view all the answers

What are the two key functions of a router?

<p>Run routing algorithms/protocols and forward datagrams from incoming to outgoing links.</p> Signup and view all the answers

Explain the difference between 'dumb' and 'smart' end systems.

<p>'Dumb' end systems, like telephones, have limited capabilities, while 'smart' end systems, such as computers, can adapt, perform control, and handle error recovery.</p> Signup and view all the answers

What is the purpose of forwarding tables in routers?

<p>Forwarding tables are used to determine the appropriate output port for incoming datagrams based on their destination.</p> Signup and view all the answers

What is the significance of switching fabrics in networking?

<p>Switching fabrics transfer packets from input buffers to output buffers and determine the switching rate of packets.</p> Signup and view all the answers

What does a Virtual Circuit (VC) number represent in the context of packet forwarding?

<p>A VC number uniquely identifies the packet along the path in the network, as opposed to using a destination address.</p> Signup and view all the answers

Describe the limitations of switching via memory in first-generation routers.

<p>The speed of switching via memory is limited by memory bandwidth, as it involves duplicating datagrams onto system memory requiring multiple bus crossings.</p> Signup and view all the answers

How is a VC number determined at each router in a VC network?

<p>The VC number is determined by looking up the incoming VC number in the router's forwarding table and using the corresponding outgoing VC number.</p> Signup and view all the answers

How does a bus-switching method work in routers?

<p>In bus-switching, datagrams are transferred from the input port memory to output port memory via a shared bus, but contention limits switching speed.</p> Signup and view all the answers

What are signaling protocols used for in Virtual Circuits?

<p>Signaling protocols are used to establish, maintain, and tear down Virtual Circuits.</p> Signup and view all the answers

What strategies are used to overcome limitations of bus bandwidth in switching?

<p>Advanced designs like banyan networks and crossbar switches are employed to enhance switching efficiency and reduce bandwidth limitations.</p> Signup and view all the answers

What is the primary difference between packet forwarding in Virtual Circuit networks and Datagram networks?

<p>In a Virtual Circuit network, packets are forwarded based on VC numbers, whereas in a Datagram network, packets are forwarded using destination host addresses without a concept of connection.</p> Signup and view all the answers

What is stored in the forwarding table of a VC router?

<p>The forwarding table of a VC router contains mappings of incoming VC numbers to outgoing VC numbers along with their corresponding incoming and outgoing interfaces.</p> Signup and view all the answers

Why are Virtual Circuits not commonly used in today's Internet?

<p>Virtual Circuits require stateful connections and call setups, which are less flexible compared to the connectionless nature of Datagram networks utilized in the Internet.</p> Signup and view all the answers

What is an advantage of a forwarding table in maintaining connection state information for VC routers?

<p>The forwarding table allows VC routers to efficiently manage the routing of packets in a connection-oriented manner by holding state information.</p> Signup and view all the answers

What role does the physical layer play in the Virtual Circuit setup process?

<p>The physical layer is responsible for the actual transmission of the signaling and data packets that establish and maintain the Virtual Circuits.</p> Signup and view all the answers

Study Notes

Network Layer

  • The network layer is responsible for transporting segments from the sending to the receiving host.
  • It encapsulates segments into datagrams.
  • On the receiving side, it delivers segments to the transport layer.

Network Layer Protocols

  • Network layer protocols are present in every host and router.
  • Routers examine the header fields in all IP datagrams passing through them.

Two Key Network Layer Functions

  • Forwarding: Moving packets from a router's input to the appropriate output.
  • Routing: Determining the path taken by packets from source to destination. This involves routing algorithms.

Interplay Between Routing and Forwarding

  • The routing algorithm determines the end-to-end path through the network.
  • The forwarding table determines the local forwarding actions at each router.
  • The value in the arriving packet's header determines the output link.

Connection Setup

  • In some network architectures (e.g., ATM, frame relay, X.25), before datagrams flow, two end hosts and intervening routers establish a virtual connection.
  • This involves routers getting involved.
  • Network layer connection service is between two hosts.
  • Transport layer connection service is between two processes.

Network Service Models

  • The network service model describes how datagrams are transported from sender to receiver.
  • Service models for individual datagrams include guaranteed delivery and guaranteed delivery with less than 40msec delay.
  • Models for datagram flows include in-order delivery and guaranteed bandwidth.

Chapter 4 Outline

  • The chapter covers introduction, virtual circuits and datagram networks, what's inside a router, IP protocol, datagram format, IPv4 addressing, ICMP, IPv6, routing algorithms, routing in the Internet, broadcast, and multicast routing.

Virtual Circuits

  • A virtual circuit behaves much like a telephone circuit in terms of its performance.
  • Each packet carries a VC identifier (not destination host address).
  • Every router on the path maintains state information for each connection.
  • Link resources (bandwidth, buffers) can be allocated to a VC.

VC Implementation

  • A VC consists of a path from source to destination, VC numbers for each link, and entries in the forwarding tables in routers along the path.
  • Packets belonging to a VC carry a VC number(rather than destination address).
  • The VC number can change on each link.
  • The new VC number comes from the forwarding table.

Virtual Circuits: Signaling Protocols

  • Used for setting up, maintaining, and tearing down virtual circuits in ATM, frame relay, and X.25.
  • Not used in today's internet.

Datagram Networks

  • No call setup at the network layer for end-to-end connections.
  • No network-level concept of connection.
  • Packets are forwarded based on the destination host address.

Datagram Forwarding Table

  • The forwarding table lists ranges of destination addresses (aggregate table entries) and the corresponding output link.
  • It uses longest prefix matching for finding the forwarding entry.

Routing Algorithms

  • Link state: Routing algorithms that use all the link state information from all routers to compute the shortest path.
  • Distance vector: Routing algorithms that use information from directly connected neighbors to calculate the shortest path.
  • Hierarchical routing: A routing strategy that divides the network into smaller regions or hierarchical levels.

Routing in the Internet

  • Protocols like RIP, OSPF, and BGP are used for routing in the internet.

Broadcast and Multicast Routing

  • Methods for routing broadcast and multicast traffic within a network.

Router Architecture Overview

  • The two functions are protocols(RIP, OSPF, BGP), and forwarding.
  • A routing processor runs routing algorithms and the high-speed switching fabric handles packet forwarding.

Input Port Functions

  • Input ports perform bit-level reception, data link layer receive function, and decentralized switching operations in routers.
  • Given a datagram's destination, it looks up the output port using the forwarding table.
  • Queuing is done if datagrams arrive faster than the forwarding rate.

Switching Fabrics

  • Switching fabrics transfer packets from input to output buffers.
  • Switching rate is the rate at which packets are transferred from inputs to outputs.
  • Common fabrics include memory, bus, and crossbar.

Switching via Memory

  • In the first-generation routers, packet copying to system memory limits speed by memory bandwidth.

Switching via a Bus

  • Datagrams transfer between input port memory and output port memory via a shared bus.
  • Bus contention limits switching speed by bus bandwidth

Switching via Interconnection Network

  • It overcomes bus speed limitations by using a banyan network, crossbar, or other interconnection networks to connect processors in a multiprocessor.
  • Datagrams are fragmented into fixed-length cells and switch cells through the fabrics.

Output Ports

  • Output ports perform buffering and scheduling datagrams if the arrival rate from the fabric exceeds the output line speed.
  • Datagram loss can occur due to congestion.

How Much Buffering

  • RFC 3439 suggests average buffering should equal the RTT multiplied by link capacity.
  • Recent recommendations suggest buffering equal to (RTT * C) / N, where N is the number of flows.

Input Port Queueing

  • If the fabric is slower than the combined input ports, queueing occurs at input queues.
  • Head-of-the-Line blocking occurs when a queued datagram prevents others in the queue from progressing.
  • Output port contention can cause queueing (delay) and loss due to overflow
  • Only one red datagram can be transferred at a time, the lower red packet is blocked.

ICMP: Internet Control Message Protocol

  • Used by hosts and routers to communicate network-level information.
  • Includes error reporting, unreachable host, network, port, protocol, echo request/reply (ping), and network-layer messages (e.g. source quench messages).

IPv6: Motivation

  • The initial motivation was the impending exhaustion of 32-bit address space.
  • Additional improvements include faster processing/forwarding and header modifications for QoS support.
  • IPv6 uses a fixed-length header that allows no fragmentation.

IPv6 Datagram Format

  • Includes IPv6 header fields (version, priority, payload length, flow label, next header, hop limit, source address, destination address), and data.

Other Changes From IPv4

  • The checksum is removed to reduce processing time.
  • Options are allowed but are outside the header, indicated by "Next Header."
  • ICMPv6 includes new message types like "Packet Too Big," and multicast group management functions.

Transition From IPv4 to IPv6

  • Tunneling is a method for carrying IPv6 datagrams as payload in IPv4 datagrams among IPv4 routers.
  • Routers can carry different types of headers concurrently.

Tunneling

  • IPv6 datagrams are carried as payload in IPv4 datagrams for transition between IPv4 routers.
  • This allows IPv6 traffic to traverse networks running IPv4 infrastructure.

IPv6 Adoption

  • The US National Institutes of Standards (NIST) have estimated the adoption rate of IPv6 in industry and government.
  • Deployment has been gradual, with application-level changes taking place in 20 years, starting with WWW and Facebook.

IP Addressing: Introduction

  • IP address is a 32-bit identifier for hosts and routers along with interfaces.
  • An interface connects a host or router to a physical link.
  • Routers and typically have multiple interfaces.

IP Addressing: CIDR

  • Classless Inter-Domain Routing (CIDR) is a method for allocating IP addresses that uses variable length subnet masks. It defines an address format (a.b.c.d/x).
  • X identifies how many 'bits' are used for the subnet portion of the address.

IP Addresses: How to Get One?

  • Hosts can get IP addresses through several methods, such as hardcoded by system admins or using DHCP.
  • DHCP is used for dynamic assignment of IP addresses via broadcast. It allows devices to obtain their IP addresses, and additional data, from a DHCP server on the network.

DHCP: More Than IP Addresses

  • DHCP can return more than just an IP address, such as the address of the first-hop router and the DNS server's address.
  • It also provides network mask information.

DHCP: Client-Server Scenario

  • The DHCP consists of client-server interactions where a client requests and obtains an IP address from a DHCP server.
  • A series of messages (DHCP discover, DHCP offer, DHCP request, DHCP ack) are exchanged.

DHCP: Example

  • The DHCP server encapsulates its responses in UDP, which further gets encapsulated in IP, and that encapsulated in an Ethernet frame and broadcast onto the LAN.

IP Addresses: How to Get One?

  • ISPs receive blocks of IP addresses allocated by ICANN (Internet Corporation for Assigned Names and Numbers).
  • Organizations receive portions of address space for their networks.

NAT: Network Address Translation

  • NAT allows a local network to use a single IP address to the outside world.
  • NAT translates local addresses to a global address.

NAT: Network Address Translation (Implementation)

  • NAT routers translate source IP addresses and port numbers in outgoing datagrams.
  • They also translate destination fields in incoming datagrams.
  • Using a NAT translation table, the NAT router maintains source and destination pairings.

NAT Traversal Problem

  • The NAT traversal problem happens when a client wants to connect to a server, but the server's address is local to the LAN.
  • A solution is to statically configure NAT to forward incoming requests to the server at a given port.

NAT: Motivation

  • Local networks use one IP address for the outside world.
  • This saves the need for address space from an ISP.
  • It allows changes of devices and ISP without notifying the outside world.
  • Devices inside a local network do not need to be explicitly addressable to the outside world.

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Chapter 4 Network Layer PDF

Description

This quiz explores the essential functions of the network layer in networking, focusing on key concepts such as forwarding vs. routing, various service models, and IPv4 addressing. It also covers the characteristics of virtual-circuit and datagram networks, along with routing algorithms and congestion management in ATM networks.

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