Unit 5: The Network Layer
In this unit, we will learn how packets (groupings of data) travel on a network and how each machine can be addressed uniquely so that data transport between two nodes is reliable. We will learn that networks can run out of space, meaning that unique addresses for different machines are no longer available. In these situations, computer scientists must manage IP addressing using CIDR and subnetting – techniques we will learn about in this unit.
The network layer is responsible for the delivery of packets from any source to any destination through intermediate routers. Follow the links to explore in detail the IPv4, IPv6, RIP, OSPF, and BGP protocols used in today's Internet.
Completing this unit should take you approximately 10 hours.
Upon successful completion of this unit, you will be able to:
- explain the correct network layer protocol to perform packet forwarding using both IPv4 and IPv6;
- illustrate IP addressing on both IPv4 and IPv6 networks;
- compare Classless Interdomain Routing (CIDR) with subnetting activities within the network layer;
- use protocols like Dynamic Host Configuration Protocol (DHCP), Address Resolution Protocol (ARP), and Network Address Translation (NAT) to manage IP address assignment, re-assignment, and resolution;
- illustrate the use of Interior Routing Protocols based on shortest path, distance vector, and link state routing models;
- compare interior routing protocols like Routing Information Protocol (RIP) and Open Shortest Path First (OSPF) with exterior routing protocols like Border Gateway Protocol (BGP);
- use multicasting principles, including addressing schemes and associated protocols;
- use quality of service (QoS) principles and associated protocols like Multiprotocol Label Switching (MPLS); and
- use protocols like Internet Control Message Protocol (ICMP) to configure and troubleshoot a network in both IPv4 and IPv6.
- explain the correct network layer protocol to perform packet forwarding using both IPv4 and IPv6;
5.1: Principles of the Network Layer
The network layer includes the datagram and virtual circuit modes, the separation between the data plane and the control plane, and the algorithms used by routing protocols. As you read this section, it is important to understand the purpose of the router and the use of the packet in enabling you to send data and receive the response. How does the packet interact with the router?
What are the two internal organizations used in the network layer? When is each used? What is a datagram? This page explores how a datagram is used at the network level. Compare and contrast the datagram organization with the virtual circuit discussed in this section. The concept of hop-by-hop forwarding is in this section. What does this concept have to do with the routing table? What is the difference between the data and control planes?
This video explores the elements of the Network Layer, which is the OSI Model's third tier. The primary role of the Network Layer is to route datagrams across various subnetworks. What does it mean for the datagram transmission to be a connectionless message? Make sure you can distinguish between unicast and broadcast messaging. Ensure you know the function of routers when directing traffic between networks, the structure of datagrams (especially the header and the payload), which component carries the transmitted information, and the differences between the two prevalent Internet Protocol versions used today: IPv4 and IPv6.
5.2: Static Routing, Distance Vector Routing, and Link State Routing
The control plane maintains the routing table using three techniques: static routing, distance vector routing, and link state routing. As you explore each one in detail, note their advantages and disadvantages. How does each method deal with link and router failures?
As you read this section, ensure you understand what static routing is and its advantages and disadvantages.
What is distance vector routing? What are its advantages and disadvantages?
Read this section on link state routing and its advantages and disadvantages. How does link state routing handle link and routing failures?
Watch this video to learn about Dijkstra's algorithm, a fundamental concept in graph theory for finding the shortest path in graphs with non-negative weights. Think about how the algorithm determines the shortest path from a source to all vertices. It does this by updating the distances and by prioritizing the nearest vertex, and then it constructs a minimum path tree. The algorithm finds shortest paths but only when using non-negative weights. If negative weights are used, what are the consequences?
The Bellman-Ford algorithm is a powerful tool for finding the shortest paths in weighted graphs that can handle negative weight edges. A weighted graph can be represented by circles representing nodes and edges represented by lines between or connecting the nodes. Unlike Dijkstra's algorithm, Bellman-Ford accommodates graphs with negative weights by iterating N-1 times, where N is the number of vertices. Why can't the graph contain negative weight cycles? How does the algorithm compare with Dijkstra's in terms of speed and versatility? When might the slower speed of Bellman-Ford be a worthwhile trade-off for its broader applicability?
5.3: Internet Protocol
This section explores the Internet Protocol (IP) to discover how IP enables the applications running above the transport layer (UDP/TCP) to utilize any of the different datalink layers available.
This section discusses IP version 4 before exploring network addressing in more detail. What are two of the problems with IP version 4 that led to the development of the Classless Interdomain Routing (CIDR) architecture? How does CIDR improve the scalability of the IP routing system?
IPv4 addresses are notated using a dotted decimal format. The current IPv4 system lacks sufficient addresses for today's device volume. After you watch this video, can you explain the difference between public and private IPv4 address ranges and their allocation strategies? What are the functions of special addresses like 0.0.0.0, 127.0.0.1, and 255.255.255.255? Can you think of scenarios where IPv4 addresses are autoconfigured versus when they are leased? To better understand the autoconfiguration process, consider reviewing RFC 3927, which formalizes this protocol.
Read these pages for a deeper and more detailed understanding of the need and use of subnetting and supernetting. Make sure that you understand the exact subnet to which an address/mask combination belongs. Once you determine the exact subnet, make sure that you can name the first and last hosts on that subnet, as well as the network name and broadcast address. These are very important principles that need to be mastered.
Sometimes, a router or the destination host has to inform the sender of the packet of a problem that occurred while processing that packet. In the TCP/IP protocol suite, this reporting is done by the Internet Control Message Protocol (ICMP). How does the ICMP generate these messages?
As the popularity of the Internet grew exponentially, it became necessary for an expanded addressing architecture, IP version 6 (IPv6). This section discusses how IPv6 has resolved a number of routing issues while becoming the new standard.
ICMPv6 is the companion protocol for IPv6, just as ICMPv4 is the companion protocol for IPv4. ICMPv6 is used by routers and hosts to report problems when processing IPv6 packets. In addition, ICMPv6 is used when auto-configuring addresses. This section discusses messaging for IPv6.
Large corporations and government agencies prefer their networks to be private (that is, not seen on the Internet). This section explores the concept of private networks to learn how their need to communicate with the outside world and with specific machines under certain conditions has resulted in NAT: the mechanism that allows private networks to communicate openly with the outside world. How does NAT work?
5.4: Routing in IP Networks
Routing protocols are discussed in terms of two classifications: intradomain and interdomain. What are the differences between these classifications?
The initial group of routing protocols comprises intradomain routing protocols, also called interior gateway protocols or IGPs. These protocols are called intradomain because they facilitate information exchange within the same domain or network. Routers use these protocols to share information about reachable destinations within the domain. Intradomain routing protocols include the Routing Information Protocol (RIP), a distance vector protocol, and the Open Shortest Path First (OSPF) that uses link-state routing.
What is RIP? What are the features of the RIP protocol? How does RIP meet the objectives of intradomain routing? What are its weaknesses?
What is OSPF? What are the features of the OSPF protocol? How does OSPF meet the objectives of intradomain routing? What are its weaknesses? How is OSPF different from RIP?
What are the objectives of interdomain routing? What is the difference between transit and stub domains? What are some of the relationships you can expect to find in an interdomain routing policy?
How does BGP differ from the intradomain protocols RIP and OSPF you have studied? What messages might the BGP generate? What is router convergence? Why is router convergence necessary? How does the BGP handle router convergence?
Watch this video from 59:10 to 1:06:39 to learn about multicast networking and how it enables data to be sent to multiple specified recipients by using special IP addresses representing a group of hosts. This type of selective broadcasting is helpful for applications such as streaming media, where it is efficient for one source to transmit to many receivers, such as a live lecture being sent to online students. Why is multicast more efficient than unicast or broadcast in this scenario? What mechanisms allow a host to join or leave a multicast group, and how does the network keep track of the memberships? What is multicast's role in optimizing network traffic and managing bandwidth, particularly when the same data is sent to many users?
This video explores Quality of Service (QoS) and details strategies for network traffic prioritization. Flows are characterized by bandwidth, delay, jitter, and packet loss for traffic management. There are four primary QoS enhancement techniques and three scheduling methods that dictate packet transmission priorities. Traffic shaping and its two main algorithms are designed to regulate data flow and maintain network efficiency. Admission control and resource reservation are two crucial processes for managing network congestion and ensuring the availability of necessary resources.
5.5: Practice Exercises
These exercises expand the key principles in this unit. If you are a computer professional, you will enjoy the challenges and higher-level discussions in this section. If you are a novice, explore the presentations and spend more time on the topics that are meaningful to you.
This is a software tool that helps you perform networking experiments on your computer. If you'd like to explore it, download it and try it with a few simple examples.
If you'd like more practice, you can scan for network devices in your local area network using an app on your mobile device.
Unit 5 Assessment
- Receive a grade
Take this assessment to see how well you understood this unit.
- This assessment does not count towards your grade. It is just for practice!
- You will see the correct answers when you submit your answers. Use this to help you study for the final exam!
- You can take this assessment as many times as you want, whenever you want.