Segment Routing MPLS traffic engineering (SR-MPLS TE) is a new TE tunneling technology that uses SR as a control protocol. The controller calculates a path for an SR-MPLS TE tunnel and forwards a computed label stack to the ingress configured on a forwarder. The ingress uses the label stack to generate an LSP in the SR-MPLS TE tunnel. Therefore, the label stack is used to control the path along which packets are transmitted on a network.
SR-MPLS TE tunnels are capable of meeting the requirements for rapid development of software-defined networking (SDN), which Resource Reservation Protocol-TE (RSVP-TE) tunnels are unable to meet. Table 1 describes the comparison between SR-MPLS TE and RSVP-TE.
Item |
SR-MPLS TE |
RSVP-TE |
|---|---|---|
Label allocation |
Labels are allocated and propagated using IGP extensions. Each link is assigned only a single label, and all LSPs share the label, which reduces resource consumption and maintenance workload of label forwarding tables. |
MPLS allocates and distributes labels. Each LSP is assigned a label, which consumes a great number of labels resources and results in heavy workloads maintaining label forwarding tables. |
Control plane |
IGP extensions are used for signaling control, without requiring any MPLS control protocol. This reduces the number of required protocols. |
RSVP-TE is used, and the control plane is complex. |
Scalability |
High scalability. As transit nodes are unaware of tunnels and use packets to carry tunnel information, they only need to maintain forwarding entries instead of tunnel state information, enhancing scalability. |
Poor scalability. Both tunnel state information and forwarding entries need to be maintained. |
Path adjustment and control |
A service path can be controlled by operating a label only on the ingress. Configurations do not need to be delivered to each node, which improves programmability. When a node in the path fails, the controller recalculates the path and updates the label stack of the ingress node to complete the path adjustment. |
Whether it is a normal service adjustment or a passive path adjustment of a fault scenario, the configurations must be delivered to each node. |
Label Stack
A label stack is an ordered set of labels used to identify a complete LSP. Each adjacency label in the label stack identifies a specific adjacency, and the entire label stack identifies all adjacencies along the LSP. During packet forwarding, a node searches for the corresponding adjacency according to each adjacency label in the label stack, removes the label, and then forwards the packet. After all the adjacency labels in the label stack are removed, the packet traverses the entire LSP and reaches the destination node of the involved SR-MPLS TE tunnel.
Stitching Label and Stitching Node
If the depth of a label stack is greater than the maximum depth supported by forwarders, the label stack is unable to carry the adjacency labels of the entire LSP. In this case, the controller needs to allocate multiple label stacks to the forwarders and a special label to an appropriate node to stitch these label stacks, thereby implementing segment-by-segment forwarding. The special label is called a stitching label, and the appropriate node is called a stitching node.
The controller allocates a stitching label to the stitching node and places the label at the bottom of the label stack. After a packet is forwarded to the stitching node, according to the association between the stitching label and label stack, the node replaces the stitching label with a new label stack to further guide forwarding.