- What is Label Switching?
- Why Use Label Switching?
- The ZIP Code Analogy
- Why A Label IS Not an Address
- How Label Switching is Implemented and How it Came About
- Clarification of Terms
- The Need for a QOS-based Internet
- Label Switching's Legacy: X.25 and Virtual Circuits
- MPLS: Status and Concepts
- Examples of Label and QOS Relationships
- Determination of the Physical Path Through the Network: The Label Switched Path (LSP)
Clarification of Terms
Later chapters explain the major responsibilities of internetworking units, such as label switching routers, but it is important to pause here and clarify some terms. Two protocols are employed by routers to successfully relay the user traffic to its receiver: (a) one protocol (say, protocol 1) relays packets from a source user to a destination user, and (b) the other protocol (say, protocol 2) finds a route for the packets to travel from the source to the destination.
Unfortunately, several terms are used to describe these two types of protocols, and the terms themselves are not models of clarity. Nonetheless, we must deal with them at the onset of our journey through label switching networks; otherwise, many parts of this book will be quite confusing. Figure 16 is used to explain these terms.
Figure 16 Terms and concepts.
The older term to describe protocol 1 is routing, and the older terms to describe protocol 2 are route advertising or route discovery. These two terms are still used in the industry in the context just described.
Today, as Figure 16 shows, the term routing describes protocol 2, and the terms forwarding and switching describe protocol 1. In keeping with current industry practice, the new terms are used in this book. I use route advertising, route discovery, and routing synonymously. However, I continue to use the term routing table or label table to describe the table of addresses or labels used to forward packets through the network.
To summarize, two protocols are involved in the internetworking process:
Forwarding/Switching: Using a routing table or a label table to make a forwarding decision.
Routing: Using route advertisements to acquire the knowledge to create the routing/label table that the forwarding protocol uses. For label switching networks, this advertising may entail the advertising of an address and its associated label.