- 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)
Examples of Label and QOS Relationships
We have learned that the label is used for forwarding operationsto determine how to relay the packet to the next node. We also learned that it can be used to determine the services that will be provided to the packet during its journey through the network. Thus, the label may be associated with the packet's QOS support. The words "may be" must be emphasized because some MPLS implementations use the label to make QOS decisions and some do not.
Figure 17 shows the way in which two packets are processed at an LSR and the relationships of QOS and label operations. The packets, identified with labels 30 and 70, are sent to the switch's interface from an upstream node (say, another router, not shown in this figure). The labels are then used to access a label switching table. The two table entries for labels 30 and 70 are shown at the bottom of the figure.
Figure 17 Labels and associated QOS operations.
Each table entry contains the label number and the associated ingress interface number: 30.a and 70.a for these two packets. The ingress interface is the communications link interface on the router. The table information is associated with the profile conformance entries and is used to monitor the flow associated with each of these packets. These profile examples are usually relevant to the first switch in the QOS domain; that is, at the user-to-network interface (UNI). They determine if the packet flow is conforming or nonconforming to the service SLA.5 The profile for flow 30 is a burst tolerance (BT) of 210 packets per second (210 p/s); the profile for flow 70 is a packet delay variation (PDV) of no greater than 1 microsecond.
The service-level entries in the tables also reveal how the packet is to be treated if the traffic for each flow adheres to its SLA. This example uses priorities to differentiate the treatments. Priority = C is for asynchronous traffic, and priority = A identifies synchronous, real-time traffic. The priorities in the example are relative; priority C is not as high as A.
Another entry in the table is the scope of service, shown as "Scope" in the figure. The service for these two packets is scoped to the switch's egress interfaces, the router's output link interfaces. If the services for these packets are end-to-end, perhaps through multiple QOS domains, the ingress and egress must be provisioned at each node that resides in the QOS region. Therefore, we can assume in this example that these egress ports are provisioned with an end-to-end path in mind. Notice that packet 70 is scoped to two egress interfaces, b and c. This approach enables the packet to be routed to the link that is experiencing better performance or to be routed around network failures.
Another entry in the table reflects the operations that are to be performed on the packet if its flow does not conform to the SLA conformance profile. Packet 30 will be marked (tagged) if it is nonconforming. "Mark only" means the packet is not to be discarded (unless the switch is in a precipitous situation). The tag will relegate the service on the packet flow to a lesser quality of service. Packet 70 belongs to a synchronous real-time flow, so if it is nonconforming, it is discarded.
The last entry in the tables is the label that is placed in the packet header for transmittal out of the egress interface to the next node. Label 30 is mapped to label 21, and label 70 is mapped to label 50. The term map is also called a swap. So, label swapping is the changing of the label value at the LSR. Label swapping is quite important in label switching networks and is explained in Chapters 2, 3, and 4.