- 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)
The ZIP Code Analogy
To understand more about the basic ideas of label switching, let's return to the postal system example. As depicted in Figure 12(a), a piece of mail is being forwarded though the postal system from one party to another. Notice that the actual address of the mail recipient is not used in the postal "network" to relay the envelope. Rather, the ZIP code 88888 is used as a label to identify where the mail is to go. After the envelope reaches its destination ZIP area (the end of the "postal path"), then the address (street number, etc.) is used to forward the mail to its intended reader.
Figure 12 ZIP codes and labels.
This idea holds for label switching. In Figure 12(b), an IP datagram (packet) is sent to a label switching router for delivery to a destination IP address. The router appends a label to the packet (something like a ZIP code). Thereafter, the label, not the IP address, is used in the network to forward the traffic. Once the traffic has reached the end of the "label path," the IP address is used to make the final delivery to the end user.
Thus in both networks, the cumbersome addresses are not processed. This common sense approach saves a great deal of time and substantially reduces the overhead of both the postal network and an internet. This reduction of time and overhead is the essence of label switching technology, an indispensable tool in today's internets. In later discussions, we show more detailed examples of IP forwarding and label switching operations and explain further why label switching is so effective.