Service chaining is a networking technique that involves the sequential connection of multiple network services to form a chain. Rather than sending data packets directly from the source to the destination, they are routed through a series of services, each performing a specific function. This enables the customization and optimization of network traffic, leading to improved performance, security, and scalability.

Service chaining is required to move traffic to these virtualized services. Therefore, its role is to help automate traffic flow between services in a virtual network. It also optimizes network resources to improve application performance using the best routing path. An example sequence is passing through the firewall, encryption, and software-defined WAN.

Network Service Chaining

Service chains are policy constructs that can steer application traffic through a series of service nodes. These nodes may include firewalls, load balancers, intrusion detection devices, and virtual email security agents.

For example, we want to add a stateful packet engine to an application flow. In a classic case, we usually implement a physical or virtual firewall as the default gateway. All traffic leaving the host will follow its default gateway, and traffic gets inspected. 

This design is a typical topology-dependent service chain. What if you need to go one step further and add several service devices to the chain, such as an IPS or load balancer? This will soon become a complicated design, and complexity comes at a cost in troubleshooting and maintenance. 

The lack of end-to-end service visibility

Service chaining is static and bound to the insertion and policy selection topology. One major drawback is that network service deployments are tightly coupled to the network topology. This limits network agility, especially in a virtual environment. They are typically built through manual configuration and are prone to human error. Policy-based routing (PBR) and VLAN stitching are existing technologies used for service chaining. They lack end-to-end service visibility, and troubleshooting is complicated.

**Policy-Based Routing**

PBR is configured per box, per flow, and autonomous routing protocols do not understand it. PBR breaks routing. You usually build that chain statically if you have to run traffic through some network service. Still, in a data center that uses a lot of multi-tenancy and is highly segmented, you need to route traffic in a much more flexible way.

Implementing network services and security policies into an application network has traditionally been complex. Implementing service nodes into an application path, independent of location, has challenged many data centers and cloud providers.

Example Policy Based Routing:

### Introduction to Policy-Based Routing

In the ever-evolving landscape of digital networks, ensuring the efficient flow of data is more crucial than ever. Policy-Based Routing (PBR) emerges as an innovative solution to manage and direct data traffic based on predefined policies. Unlike traditional routing, which relies solely on destination addresses, PBR allows network administrators to dictate routing decisions based on various criteria, including source address, protocol type, and application. This approach not only enhances traffic management but also ensures optimal resource utilization.

### The Mechanics of Policy-Based Routing

Understanding the mechanics of PBR is essential for implementing it effectively. At its core, PBR involves creating routing policies that act as rules for directing traffic. These policies can be configured based on multiple parameters, such as the type of service or the time of day, offering a level of customization that traditional routing lacks. By applying these policies at the network’s edge, administrators can influence the path that data packets take through the network, optimizing performance and security.

### Benefits of Implementing Policy-Based Routing

The advantages of PBR extend beyond mere traffic management. One of its primary benefits is the ability to prioritize critical applications, ensuring that essential services receive the bandwidth they need. This is particularly valuable in environments with limited resources, where congestion can lead to performance bottlenecks. Additionally, PBR enhances security by allowing administrators to route sensitive data through secure paths, mitigating the risk of interception or unauthorized access.

Example: Service chaining and the virtual switch

The Nexus 1000V virtual switch initially introduced the concept of service chaining. It implements a service-chaining technology called vPath, which provides traffic interception and reroutes to the required service node. However, it initially lacked because it could only service chain one service at a time and for one type of device: the Virtual Security Gateway (VSG).

It was later expanded to service multiple workloads between multiple service hops. While vPath was a success, it could only work with virtual nodes. A solution was needed to enable physical and virtual nodes to be in the virtual chaining path.

Network Service Header (NSH)

Cisco has developed the Network Service Header (NSH). It creates a dedicated service plane independent of the underlying transport networks. A node inserts it into encapsulated packets or frames, usually at ingress, and describes a series of service nodes to which a packet should be routed. It also adds additional metadata about the packet. The packets are then encapsulated in an outer header for transport.

Service Function Forwarder (SFF)

The traffic is sent via an overlay to the Service Function Forwarder (SFF), which looks at the service path header and tells it what service needs to be applied at the particular chain. NSH requires NSH-aware nodes, i.e., front-end service nodes, but it doesn’t require any change to the transport network. The SFF is an NSH-aware forwarder in front of the service node. 

The SFF only needs to know how to do a simple lookup and ask for a location. The locator can be delivered via SDN controller ODL, LISP, and BGP. Because the control and data plane are decoupled, it is simplified. The abstraction between the control and data plane allows you to build more complicated (scale and topology) service chains with NSH rather than using flows. 

Closing Points on Service Chaining 

Service chaining is crucial in today’s complex network environments where data packets need to traverse through various services such as firewalls, intrusion detection systems, and load balancers. Traditional network setups often require manual configuration and management of these services, leading to increased complexity and potential for errors. By implementing service chaining, organizations can automate the flow of data across these services, ensuring a more efficient and error-free network operation.

The process of service chaining involves a series of steps that automatically direct network traffic through the required services. This is typically achieved through the use of network functions virtualization (NFV) and software-defined networking (SDN) technologies. NFV allows for the virtualization of network services, while SDN provides a centralized control mechanism to manage the flow of traffic. Together, they enable the dynamic creation of service chains that can be easily modified and optimized to meet the specific needs of the network.

There are several benefits to implementing service chaining in a network environment. One of the most significant advantages is the increased agility it provides. Organizations can quickly adapt to changing network demands by reconfiguring service chains on the fly. Additionally, service chaining enhances network security by ensuring that data passes through all necessary security services before reaching its destination. This reduces the risk of data breaches and ensures compliance with security policies.

While service chaining offers numerous benefits, there are also challenges to consider. One of the primary challenges is the complexity involved in designing and managing service chains. Organizations need to have a clear understanding of their network architecture and the specific requirements of each service within the chain. Additionally, there may be compatibility issues between different network services, which can complicate the implementation process.