Using MPLS to Improve Network Performance
- 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)
This chapter explains why label switching networks and Multiprotocol Label Switching (MPLS) have become key players in the emerging multiservice public Internet and private internets. It explains the problems associated with conventional IP routing procedures and introduces the concepts of the alternative: label switching. The chapter also introduces the idea of quality of service (QOS) and explains its importance, as well as the importance of label switching to QOS. The chapter concludes with an example of a label switching and QOS network operation at a label switching router (LSR).
What is Label Switching?
The basic concept of label switching is simple. To show why, let's assume a user's traffic (say, an email message) is relayed from the user's computer to the recipient's computer. In traditional internets (those that do not use label switching), the method to relay this email is similar to postal mail: a destination address is examined by the relaying entity (for our work, a router; for the postal service, a mail handler). This address determines how the router or mail handler forwards the data packet or the mail envelope to the final recipient.
Label switching is different. Instead of a destination address being used to make the routing decision, a number (a label) is associated with the packet. In the postal service analogy, a label value is placed on the envelope and is thereafter used in place of the postal address to route the mail to the recipient. In computer networks, a label is placed in a packet header and is used in place of an address (an IP address, usually), and the label is used to direct the traffic to its destination.