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Chapter 1 - Introduction to IP Multicasting

Cisco Multicast Routing & Switching
William R. Parkhurst
  Copyright © 1999 The McGraw-Hill Companies

Chapter 1: Introduction to IP Multicasting
Overview
Before we begin our exploration of IP multicasting and multicast routing protocols, we will examine the models of communication between two or more hosts in an intranet or over the Internet. Any book bearing resemblance to a networking book should include a review of the OSI layered communication model (see Figure 1-1). The communication protocols that exist at the various levels in the OSI layered model interoperate extremely well because of the adherence to a layered protocol model. The original model was developed by the OSI to provide a logical separation between the various functions of a network. This model allows for the interaction of software modules from different vendors to coexist and operate properly as long as the published standards are followed.
Figure 1-1: TCP/IP and OSI layered network models
The lowest layer of the OSI model is the physical layer. The physical layer deals with the electrical and mechanical specifications of a particular transport medium and associated interfaces. Physical layer examples are 10 and 100 Mbit ethernet, synchronous and asynchronous serial links, and ATM, to name a few. The physical layer is concerned with getting bits, in an electrical or optical form, from point A to point B. The physical layer does not care about the structure or format of the data that is being transmitted or received; it is only concerned with delivering ones and zeros from the source to the destination.
The next level in the OSI model above the physical layer is the data link layer. This layer is responsibile for creating frames that contain source and destination addresses, adding error detection and possibly correction fields to the frame, and, of course, incorporating a user’s data into the frame. Protocols at the data link layer are not routable, and examples of such layers are ethernet and token ring.
The layer where a network designer spends the most time is the network layer. This layer handles routing across the Internet and is the most important layer as far as multicasting is concerned. For a protocol to be routable, the addressing scheme must include a network and a host address. The last statement is true for “normal” IP traffic, but not for multicast traffic. As we will see, multicast addresses are not in the form of network/host but represent a group address. Although a network/host address pair is not present in a multicast address, multicast traffic is routable. Examples of routable protocols are IP, IPX, AppleTalk, and DECNet.
The transport layer is used to multiplex and demultiplex data streams between upper layer application processes as seen in Figure 1-2. The three upper layers of the OSI model, application, presentation, and session, have been combined in the application layer in the TCP/IP layered model. Typically, it is more difficult to determine where a particular upper layer application should be logically placed. Networks can be designed without knowing which applications the users are going to be employing. Therefore, the specific application is not important, just the protocol that the application will be using. In fact, we will only concern ourselves with the lower four layers of the OSI and TCP/IP models.
Figure 1-2: Multiplexing and demultiplexing in the TCP/IP model
When an application such as telnet wants to send data, the data is sent to the TCP module at the transport layer and TCP then assigns a number to the local and remote telnet session, allowing TCP to determine the session where the data is to be delivered. IP either receives or delivers data to the UDP or TCP module, depending on the type of application.
Finally, an ethernet frame contains an identifier that identifies the network layer protocol it received the data from or the network layer protocol to which it should deliver the data.
To illustrate the interaction between the different layers in the OSI model, we will follow the flow of data from one host to another (see Figure 1-3). Assume we are running a telnet session between two hosts. User data is generated at the application layer and is then passed down the protocol stack to the TCP module in the transport layer. The TCP layer uses an identifier for the session, which is contained in the TCP header, and passes the TCP segment to the IP module at the network layer. IP then tags the packet as a TCP or UDP packet. When the packet is received at the data link layer, an ethernet frame is constructed with an ethernet header and trailer. The header, among other things, contains a field tagging the frame as one that carries the IP data. Finally, the frame is passed to the physical layer for transmission onto the network media.
Figure 1-3: Data encapsulation
When the ethernet frame is received by the remote host, the data link ethernet module strips off the ethernet header and trailer after determining that this frame carries IP data and passes the data to the IP module in the network layer. IP determines if the packet is a TCP or UDP packet and passes it to the appropriate module at the transport layer. Finally, TCP extracts the user data and sends it to the proper user process.
Figure 1-4a: Resolution of IP to ethernet address mapping
Figure 1-4b: Resolution of IP to ethernet address mapping, step two
Figure 1-4c: Resolution of IP to ethernet address mapping, step three

 


 
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