The carrier-class transport network has experienced various network technologies such as SDH, PTN and IP RAN. With the development trend of all-IP mobile base stations and large bandwidth, the IP/MPLS technology has been widely used. However, cloud-based 5G 2C and 2B services require the network to provide flexible links, accurate and controllable paths, and precise awareness, and the great development of SDN requires an open, programmable, and end-to-end collaborative network to enable differentiated services and agile OAM. These requirements are difficult to meet with the traditional MPLS and segment routing MPLS (SR-MPLS). Therefore, SRv6 comes into being to meet new business requirements and emerge as the best technology choice for a new-generation carrier-class bearer.

Carrier-Class PTN Review
Since the birth of TCP/IP in the 1980s, the development of packet transport network (PTN) technology can be divided into two stages:
—The first stage is the IP/ETH era that promotes the Internet revolution. The network technology is based on IP+Ethernet to serve best-effort services, with poor transport quality, low reliability and limited network scale. It mainly serves public services, and its base stations generally adopt SDH transport mode.
—The second stage is the IP/MPLS VPN era. IP/MPLS provides connection-oriented services that can ensure carrier-class QoS and reliability. With the innovation of all-IP mobile technologies in the 3G era, the basic mobile backhaul network technologies have also changed from SDH to PTN. Key technologies such as PTN E2E services, millisecond-level OAM inspection and reliability, graphical NM, strong QoS and 1588v2+Sync-E have been widely recognized by the industry.
However, IP/MPLS LSP relies on distributed protocol route calculation or manual static planning. VPN supports various service protocols, but it is not suitable for the development of SDN due to its layered services, complex deployment and inconsistent technology.

SR-MPLS Shortcomings 
SR-MPLS can satisfy the partial centralized route calculation of cloud networks and overcome deficiencies of the traditional LSP technology. However, SR-MPLS is essentially the MPLS forwarding plane and has shortcomings in the cloud-network era.
—SR-MPLS cannot scale for it uses 4-byte MPLS labels to identify the paths.
—SR-MPLS cannot extend LSP to the cloud, which is not good for cloud-network synergy or for access of tunnels to the cloud.
—SR-MPLS cannot adapt to non-MPLS networks as is belongs to the MPLS family.
The next-generation carrier-class transport network needs to provide precise service and network awareness. Moreover, IPv4 addresses are running out and new-generation service terminals, RAN and core networks are transforming to IPv6, so carrier-class transport networks also need to evolve to IPv6.

SRv6 Benefits
SRv6 comes out to overcome the limitations of SR/MPLS. It uses IPv6 addresses as path node data, and its path list data is placed in the IPv6 header, compatible with traditional IPv6 forwarding. In addition to identifying node/link data, SRv6 header can also support customized extension data to meet future new needs such as in-band measurement. SRv6 takes IP addresses as its protocol stack to adapt to end-to-end orchestration of future cloud-network synergy services.
SRv6 supports traffic engineering (TE), robust scalability, and compatibility with IPv6, providing a technical foundation for future fixed-mobile convergence (FMC) and unified IP forwarding. As instu-OAM in SRv6 has been drafted in the IETF, SRv6 can offer millisecond-level service awareness by multi-functional network nodes, fit the development of intelligent routing, and guarantee carrier-class in multiple dimensions. Driven by 5G and cloud services, a new-generation carrier-class transport network has entered the SRv6 era.

Transition to SRv6
SRv6 adapts to future network development, but it also has shortcomings. On the one hand, the SR SID list adopts the 16-byte IP address for identification, which has low forwarding efficiency and high requirements for forwarding chips. Only a few chips in the industry can achieve 10+ layers of label encapsulation. Now the ITEF is studying the SRv6 label compression solution that can greatly solve the problem of label byte depth.
On the other hand, unlike wireless networks that use one-generation technologies for one-generation networks, one-generation carrier-class transport networks carry multi-generation services. Some devices in the transport network support SRv6, but some do not. Therefore, it is necessary to provide a transition solution for the coexistence of MPLS and SRv6, and also to further study its transport approach. The mainstream solutions include MPLS and SRv6 splicing solution, dual-plane solution, and overlay solution.

Building a Carrier-Class SRv6 Transport Network
The basic SRv6 protocol is mature and ready for commercial deployment. With the development of 5G and cloud services, building a new-generation carrier-class transport network based on SRv6 (Fig. 1) for new-generation business development has become a hot topic in the industry. 

Commercial Trials of ZTE SRv6 
ZTE has been studying SRv6 since 2017, and its SRv6 has been deployed commercially on a large scale. The company has participated in several SRv6 commercial trials:
—ZTE developed the SRv6 lab test for Japan's Softbank in 2018, and participated in China Telecom's centralized procurement test in 2019 where SRv6 functions were ready.
—ZTE completed China Telecom's STN SRv6 interoperability test in the lab in 2020, and carried out a large scale upgrade of the existing networks. This helped China Telecom build the world's largest SRv6 commercial network, deploy 2B/2C slices, introduce MPLS/SRv6 splicing and dual-plane technologies, and achieve the integrated commercial deployment of IP RAN1.0 and STN.
—ZTE participated in China Telecom's STN-based new MAN trial in 2020. The basic SRv6 protocol was used to achieve FMC transport. ZTE took the lead in implementing unified access and transport of fixed-line and 5G services through STN in Wuxi, Jiangsu Province.

Conclusion
SRv6 promotes the unification of FMC and cloud-network technologies in the future. It is a key technology to achieve service differentiation, real-time network awareness, and service isolation and meets the needs of future service development. SRv6 has become the core technology of the next-generation carrier-class intelligent transport network. ZTE's 5G IP RAN 9000E has been widely deployed by China Telecom and upgraded to support SRv6. In China Telecom's new MAN trial oriented to FMC, the 5G IP RAN 9000E can be interconnected with the SRv6 of other vendors to carry integrated FMC services. ZTE can also support SRv6 at the access layer, allowing existing network products to smoothly upgrade to SRv6.