SR-MPLS

What Is SR-MPLS?

SR-MPLS uses the source routing model to forward data packets. Its core concept involves dividing a packet forwarding path into segments, assigning segment identifiers (SIDs) to these segments, and encapsulating segment information into packets at the path's ingress to direct packet forwarding.

What Basic Concepts does SR-MPLS Contain？

SID, Segment List, and SRGB are common concepts in SR-MPLS. The following will provide a detailed explanation of these concepts.

1. SID

A Segment Identifier (SID) is a unique identifier for a segment within the SR domain, which can be mapped to an MPLS label in the forwarding plane. SIDs are categorized as prefix, node, or adjacency SIDs.

• Prefix SID (or prefix label) is a label mapped to a destination IP address, equivalent to destination addresses in traditional IP forwarding.

• Node SID (or node label) is a label mapped to the IP address of a loopback interface on a device, serving as a special prefix SID and equivalent to destination addresses in traditional IP forwarding.

• Adjacency SID (or adjacency label) is advertised by a device to its interface neighbor to explicitly specify a link for packet forwarding in the designated direction, equivalent to outbound interfaces in traditional IP forwarding.

2. Segment List

A segment list is a sequentially ordered list of segments that defines the path a packet takes for forwarding. It can be likened to an MPLS label stack used in the forwarding plane.

3. SRGB

An SRGB (Segment Routing Global Block) is a collection of user-defined labels specifically reserved for SR-MPLS. It must be configured manually. Labels within the SRGB set are exclusively allocated to prefix and node SIDs, while adjacency SIDs are local SIDs outside the SRGB range.

Background of SR-MPLS

The global digitalization is accelerating the development of the Internet. Meanwhile, the emergence of new demands and services is bringing new opportunities and challenges to the Internet. In this context, the challenges faced by traditional MPLS technology, combined with the influence of SDN, have led to the emergence of SR-MPLS.

Challenges Faced by Traditional MPLS Technologies

MPLS, short for Multi-Protocol Label Switching, is a data forwarding technology that operates between Layer 2 and Layer 3. While MPLS's label switching concept is praised for its forwarding plane, the MPLS control plane is criticized for its complex protocols, limited extensibility, and challenging deployment and maintenance. The two main protocols used in the MPLS control plane, Label Distribution Protocol (LDP) and Resource Reservation Protocol-Traffic Engineering (RSVP-TE), have drawbacks.

LDP, which distributes labels based on IGP path computation results, has the following disadvantages. As LDP and IGP are independent protocols, they can become asynchronous, leading to traffic blackholing. At the same time, LDP only supports traffic forwarding along the shortest path and lacks support for traffic engineering or path selection.

While RSVP-TE enables traffic engineering, allowing flexible path selection based on service needs, it also has significant drawbacks. RSVP-TE involves complex configuration and maintenance, with complex protocol states and a high packet exchange rate to maintain tunnel states, making large-scale deployment challenging.RSVP-TE does not support load balancing, reducing network resource utilization.

SR-MPLS aims to address the challenges of traditional MPLS networks while preserving their advantages.

Influence of SDN

Traditional network architectures are inherently complex, with tightly integrated hardware, operating systems, and network applications. To address these issues, Professor Nick McKeown's team proposed a new network architecture - SDN, drawing on universal hardware, software-defined functions, and an open-source model from the computer field.

SDN is characterized by network openness and programmability, logical centralized control, and a clear separation between control and forwarding planes. Any network possessing these attributes can be classified as an SDN. OpenFlow, an early example of SDN, is a protocol used for communication between SDN control and forwarding planes. Its introduction necessitated the upgrade or replacement of all network hardware, fundamentally transforming traditional networks from distributed to centralized architectures. However, the practical implementation of this revolutionary concept faced challenges. In contrast, SR supports both traditional and SDN networks and is compatible with existing devices, facilitating a smooth transition to SDN networks.

Key Functions of SR-MPLS

SR-MPLS has emerged alongside the SDN trend, offering simplified networks and excellent extensibility, evident in several key aspects:

• Network path programming support: With the source routing mechanism, SR-MPLS enables ingress to control a service path by performing label operations on packets, relieving transit nodes from maintaining path information and reducing control plane pressure.

• Simplified device control plane: SR-MPLS reduces the number of required routing protocols and lowers O&M costs. The label forwarding table is simple, reducing device resource usage.

• Smooth evolution to SDN: As an SDN-oriented protocol, SR-MPLS integrates the advantages of independent forwarding and centralized programming control, supporting both traditional and SDN networks and ensuring compatibility with existing devices for a smooth transition.

• Simplified control plane: SR-MPLS eliminates the need for deploying LDP/RSVP-TE, requiring only IGP/BGP extensions for label distribution and synchronization. Alternatively, a controller can uniformly distribute SR labels to devices.

• Easy-to-extend forwarding plane: SR-MPLS reuses the existing MPLS forwarding plane, allowing network devices to support SR forwarding through simple upgrades or without modification. Segments are mapped to MPLS labels, and paths are represented as label stacks.

SR-MPLS has been widely accepted in the industry for its low complexity, high efficiency, and excellent extensibility. However, MPLS forwarding plane-based SR-MPLS may not meet the requirements of services needing to carry data through extension headers due to MPLS encapsulation's poor extensibility. IPv6 forwarding plane-based SRv6 inherits SR-MPLS's advantages while providing greater extensibility.