MPLS requires a set of procedures to enhance network layer packets with label stacks, which turns them into labeled packets. Routers that support MPLS are known as Label Switching Routers (LSRs). To transmit a labeled packet on a particular data link, an LSR must support the encoding technique which, when a label stack and a network layer packet are added, produces a labeled packet.
In MPLS, packets can carry not just one label, but a set of labels in a stack. An LSR can swap the label at the top of the stack, pop the stack, or swap the label and push one or more labels into the stack. The processing of a labeled packet is completely independent of the level of hierarchy. The processing is always based on the top label, without regard for the possibility that some number of other labels may have been above it in the past, or that some number of other labels may be below it at present.
As described in RFC 3032, MPLS Label Stack Encoding, the label stack is represented as a sequence of label stack entries. Each label stack entry is represented by 4 octets. Figure 1 displays the label placement in a packet.
Field |
Description |
|---|---|
Label |
This 20-bit field carries the actual value (unstructured) of the label. |
Exp |
This 3-bit field is reserved for experimental use. It is currently used for Class of Service (CoS). |
S |
This bit is set to 1 for the last entry (bottom) in the label stack, and 0 for all other label stack entries. |
TTL |
This 8-bit field is used to encode a TTL value. |
A stack can carry several labels, organized in a last in/first out order. The top of the label stack appears first in the packet and the bottom of the stack appears last, as shown in Figure 2.
The label value at the top of the stack is looked up when a labeled packet is received. A successful lookup reveals:
The next hop where the packet is to be forwarded.
The operation to be performed on the label stack before forwarding.
In addition, the lookup may reveal outgoing data link encapsulation and other information needed to properly forward the packet.
An empty label stack can be thought of as an unlabeled packet. An empty label stack has zero (0) depth. The label at the bottom of the stack is referred to as the Level 1 label. The label above it (if it exists) is the Level 2 label, and so on. The label at the top of the stack is referred to as the Level m label.
Labeled packet processing is independent of the level of hierarchy. Processing is always based on the top label in the stack which includes information about the operations to perform on the packet's label stack.