HFT (Hierarchical Flow Tables)

Hierarchical Flow Tables (HFT) is a data structure used in software-defined networking (SDN) to efficiently manage network traffic flows. HFT is a hierarchical form of flow tables that allows network administrators to organize and manage traffic flows in a more scalable and flexible way compared to traditional flat flow tables. In this article, we will explore the concept of HFT, its advantages over traditional flat flow tables, and how it works in SDN environments.

Background:

Traditionally, switches and routers used fixed-function hardware to manage network traffic. The hardware contained pre-defined rules that determined how traffic was forwarded through the network. This approach made it difficult to manage network traffic at scale and made it challenging to add new services or features. As networks grew larger and more complex, there was a need for a more flexible and programmable way to manage network traffic.

Software-defined networking (SDN) is an approach to networking that separates the control plane (which determines how traffic is forwarded) from the data plane (which forwards traffic). This separation allows for more flexible control over network traffic and enables network administrators to program the network behavior using software rather than fixed-function hardware.

What is HFT?

HFT is a hierarchical form of flow tables that is used to manage network traffic flows in SDN environments. HFT is organized into multiple levels, with each level containing a set of rules that determine how traffic is forwarded through the network.

Each level in the HFT hierarchy can be thought of as a separate flow table. The top-level flow table contains the most general rules that apply to all traffic, while the lower-level flow tables contain more specific rules that apply to subsets of traffic. For example, the top-level flow table may contain rules that apply to all traffic, while the next level down may contain rules that apply to traffic from a specific source or to traffic of a specific type.

The rules in each flow table are organized into entries that specify a match condition and an associated action. When a packet arrives at a switch, the switch checks the packet against the rules in the top-level flow table. If a matching rule is found, the associated action is taken. If no matching rule is found, the packet is forwarded to the next level down, and the process is repeated until a matching rule is found.

Advantages of HFT:

There are several advantages to using HFT over traditional flat flow tables.

1. Scalability: HFT allows for more efficient management of network traffic flows at scale. With HFT, rules can be organized into multiple levels, making it easier to manage large numbers of rules and traffic flows. This makes it easier to add new services or features to the network without disrupting existing traffic flows.
2. Flexibility: HFT provides greater flexibility in managing traffic flows compared to traditional flat flow tables. With HFT, rules can be organized into multiple levels, with each level containing more specific rules that apply to subsets of traffic. This allows for more granular control over network traffic and enables network administrators to create more complex traffic policies.
3. Efficiency: HFT is more efficient than traditional flat flow tables. With HFT, only the top-level flow table needs to be checked for each packet, reducing the number of rules that need to be checked for each packet. This makes the switch processing more efficient and reduces the latency of the network.

How HFT works:

HFT works by organizing flow table rules into multiple levels. Each level contains a set of rules that apply to a subset of traffic flows. When a packet arrives at a switch, the switch checks the packet against the rules in the top-level flow table. If a matching rule is found, the associated action is taken. If no matching rule is found, the packet is forwarded to the next level down, and the process is repeated until a matching rule is found.

The HFT architecture allows for the efficient use of resources by utilizing different types of flow tables at different levels in the hierarchy. For example, the top-level flow table may use a larger, more general purpose flow table that can handle a large number of rules. However, as the packet moves down the hierarchy, the flow tables can become more specialized and optimized for handling specific types of traffic.

In addition to using different types of flow tables, HFT also allows for the use of wildcard rules. Wildcard rules are rules that match multiple traffic flows based on a pattern match. For example, a wildcard rule may match all traffic flows with a specific source IP address or a specific type of traffic. The use of wildcard rules can significantly reduce the number of rules that need to be stored in the flow table, further improving the efficiency of the switch.

HFT also allows for the use of flow table groups. Flow table groups are groups of flow tables that are used to manage specific types of traffic flows. For example, a flow table group may be used to manage traffic flows for a specific application or service. By grouping flow tables together, network administrators can more easily manage complex traffic policies and ensure that traffic flows are properly managed across the network.

HFT and SDN:

HFT is a key component of SDN architectures. In SDN environments, the control plane is decoupled from the data plane, allowing for greater flexibility and programmability. HFT is used to manage traffic flows in the data plane by providing a scalable, flexible, and efficient way to manage flow tables.

SDN environments can benefit from HFT in several ways. For example, HFT can be used to manage traffic flows across multiple switches, enabling network administrators to create policies that span the entire network. HFT can also be used to manage traffic flows across multiple data centers, making it easier to manage large, distributed networks.

HFT can also be used to manage traffic flows in virtualized environments. In virtualized environments, virtual switches can use HFT to manage traffic flows within a single hypervisor or across multiple hypervisors. This allows for more efficient use of resources and better management of traffic flows in virtualized environments.

Conclusion:

Hierarchical Flow Tables (HFT) is a data structure used in software-defined networking (SDN) to efficiently manage network traffic flows. HFT is organized into multiple levels, with each level containing a set of rules that determine how traffic is forwarded through the network. The HFT architecture allows for the efficient use of resources by utilizing different types of flow tables at different levels in the hierarchy. HFT provides several advantages over traditional flat flow tables, including scalability, flexibility, and efficiency. HFT is a key component of SDN architectures and is used to manage traffic flows in the data plane.