Introduction of Segment Routing MPLS
Definition
Segment routing (SR) is a protocol designed to forward data packets on a network based on source routes. Segment Routing MPLS is segment routing based on the MPLS forwarding plane, which is segment routing for short hereafter. Segment routing divides a network path into several segments and assigns a segment ID to each segment and network forwarding node. The segments and nodes are sequentially arranged (segment list) to form a forwarding path.
Segment routing encodes the segment list identifying a forwarding path into a data packet header. The segment ID is transmitted along with the packet. After receiving the data packet, the receive end parses the segment list. If the top segment ID in the segment list identifies the local node, the node removes the segment ID and proceeds with the follow-up procedure. If the top segment ID does not identify the local node, the node uses the Equal Cost Multiple Path (ECMP) algorithm to forward the packet to a next node.
Purpose
With the progress of the times, more and more types of services pose a variety of network requirements. For example, real-time UC&C applications prefer to paths of low delay and low jitter, and big data applications prefer to high bandwidth tunnels with a low packet loss rate. In this situation, the rule helping the network adapt to service growth cannot catch up with the rapid service development and even makes network deployment more complex and difficult to maintain.
- Extends existing protocols such as IGP to allow for better smooth evolution of live networks.
- The SR supports both the controller's centralized control mode and forwarder's distributed control mode, providing a balance between centralized control and the distributed control.
- Uses the source routing technique to provide capabilities of rapid interaction between networks and upper-layer applications.
Benefits
The control plane of MPLS network is simplified.
A controller or an IGP is used to uniformly compute paths and distribute labels, without using RSVP-TE or LDP. Segment Routing can be directly applied to the MPLS architecture without any change in the forwarding plane.
Provides efficient topology independent-loop-free alternate (TI-LFA) FRR protection for fast path failure recovery.
Based on the Segment Routing technology, combined with the RLFA (Remote Loop-free Alternate) FRR algorithm, an efficient TI-LFA FRR algorithm is formed. TI-LFA FRR supports node and link protection of any topology and overcomes drawbacks in conventional tunnel protection.
Provides the higher network capacity expansion capability.
MPLS TE is a connection-oriented technique. To maintain connections, nodes need to send and process a large number of Keepalive packets, posing heavy burdens on the control plane. Segment routing controls any service paths by merely operating labels on the ingress, and transit node do not have to maintain path information, which reduces the burdens on the control plane.
In addition, segment routing labels equal to the sum of the number of network-wide nodes and the number of local adjacencies. The label quantity is related only to the network scale, not to the number of tunnels or the service volume.
Better smooth evolution to SDN network.
Segment routing is designed based on the source routing concept. Using the source node alone can control forwarding paths over which packets are transmitted across a network. The segment routing technique and the centralized path computing module are used together to flexibly and conveniently control and adjust paths.
Segment Routing supports both traditional networks and SDN networks. It is compatible with existing equipment and ensures smooth evolution of existing networks to SDN networks instead of subverting existing networks.
