With 5G, network operators are evolving to a cloud-based network model that will reduce costs, scale rapidly and speed the development of new services. However, as we move cloud technologies out of the data center and into the wide area network, traffic engineering becomes essential. The mechanics of moving packets around a WAN in a deterministic way requires an end-to-end smart network fabric that scales efficiently across multiple network domains. The signaling protocols currently used to reserve IP network resources, LDP and RSVP-TE, are simply not scalable for the 5G era. Hence, in order to operationalize 5G, the industry needs to move to segment routing. The transition, which has already begun, will dramatically accelerate in 2019.
#1: SEGMENT ROUTING
Simply, segment routing provides tunnels for services such as VPRN or VPWS across multiple networks without the need for other protocols such as LDP or RSVP. A segment routing path is a sequential list of sub-paths or segments encoded as a stack of one or more MPLS labels or IPv6 addresses. It is easy to implement as it is based on extensions to IGP routing protocols such as IS-IS or OSPF in common use today. In a fully realized 5G implementation, segment routing uses a centralized SDN path control element (PCE) to determine paths through the network. This allows operators to dynamically engineer forwarding paths with much more granular policy parameters such as physical diversity, link state, available bandwidth, accumulative latency and maximum number of forwarding hops.
Although it has been around for a while, industry adoption has been slow - about 10 percent of the industry to date*. So, what is holding operators back? In the majority of cases, it simply requires a software upgrade to a router to support it, along with minimal training for personnel to move from RSVP/LDP. In part, the slow rate of adoption is because the pressing need for segment routing lies somewhere in the future. Why do today what you can put off until tomorrow? Other reasons include uneven support from vendors and a lack of incumbent reference cases.
Putting off today what you can do tomorrow notwithstanding, the industry is beginning to make the move to segment routing. In a survey of network operators done in 2018, 60 percent were expected to have begun implementing it by the end of 2019, with a further 25 percent saying they were committed to it, but not yet willing to commit to a date*.
Besides greater simplicity and reduced operations costs, the real force driving adoption of segment routing is the coming of 5G. New 5G services will have various requirements for bandwidth, latency, reliability and security that can be served by placing computing engines and data in the appropriate private or public cloud, whether that is at the edge, in the core or somewhere in between. Network slices are engineered to provide connectivity with the desired level of service between the end user and these resources across multiple network domains. This could be for a specific application, such as an IoT sensor network for a highway, or for a specific enterprise customer.
The logical abstraction of the network slice will be a powerful tool for simplifying everything from security to quality of service guarantees embedded in SLAs. But it can only work as a simplifying service abstraction if the underlying network fabric can be intelligently engineered to deliver those stringent requirements without adding operational overhead.
LDP and RSVP-TE use control plane messaging to set up paths. With growing traffic and application diversity on 5G networks, this signaling will increase. Also, path state information is stored in each intermediate router, consuming resources and limiting future scalability. With segment routing, only the ingress provider-edge (PE) router holds state. For transit or egress routers, the required state information is contained in the segment list appended to each packet. No control plane signaling is required to determine paths through the network. Instead, a centralized SDN PCE determines the route using IGP protocols with extensions to route packets through the network. This solution solves the scalability problem, providing the room for the growth needed to accommodate 5G-era service-level traffic-flow volume.
Besides scalability, other benefits from segment routing will accrue. Centralized traffic engineering in the path control element allows for path engineering across multiple network domains and has been proven to improve network utilization. Techniques such as topology independent loop free alternatives (TI LFA) are available to provide fast reroute in less than 50 msec. With segment routing, there may be multiple forwarding paths that are able to enforce traffic engineering policies, and, when this is the case, equal-cost multipath forwarding (ECMF) is able to load-balance the traffic. Relaxed requirements for memory and CPU for intermediary routers mean that white box or merchant silicon platforms can be used in some places, saving on CAPEX. Finally, because it runs over a variety of data planes, including MPLS, MPLS over UDP and IPv6, services can be seamlessly delivered across data centers and the WAN.
Migration to segment routing is relatively easy. In the beginning, it can be deployed in some parts of the network with paths stitched through from one network to another, then expanded gradually across multiple networks. It can be encapsulated to travel across networks that do not support segment routing. Many operators start using it with shortest path first (SPF) routing and then add a centralized PCE later.
In tomorrow’s network there will be hundreds of times more users and devices that need to access network functions. Services and data will be located in a variety of smart central offices and distributed data centers. Not only do we need more paths through the network, we need many more different types of paths. Engineering the traffic traversing the resulting network fabric with today’s tools will overwhelm tomorrow’s network operations teams. Segment routing is one of a handful of key technologies that will not only make 5G networks work better, it will make them manageable and affordable to run.
*IHS Markit, Routing NFV and Packet-Optical Strategies, Service Provider Survey, 22 Jan 2018