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Network engineers and administrators are at a crossroad. On one hand, protocols like BGP, IS-IS and MPLS still...
play a critical role in networks, so it's vital we maintain their base knowledge of traditional networking. On the other hand, software-defined networking is here to stay, so new skills like network programmability present a slew of new technologies to conquer. This article is designed to help network professionals by diving into a potentially cryptic component of SDN and network programmability: application programming interfaces (APIs).
A network engineer's view of APIs
Simply put, an API is an interface presented by software (such as a network operating system) that provides the capability to collect information from or make a change to an underlying set of resources. The inner workings of software may seem alien, but the concept is quite familiar to us networking pros. Let's use the Simple Network Management Protocol (SNMP) as an analogy. SNMP provides the means to request or retrieve data like interface statistics from forwarding elements. SNMP also allows applications to write configuration to networking devices. Although that's not a common use case for SNMP, it's helpful to keep in mind because APIs provide this same functionality for a wider array of software applications.
APIs in the context of SDN
The definition of an API may not, on its own, be overly useful for a network engineer, so let's take that definition and examine it in the context of SDN. In an open SDN model, a common interface discussed is the northbound interface (NBI). The NBI is the interface between software applications, such as operational support systems, and a centralized SDN controller.
One of the common API technologies used at the northbound interface is the Representational State Transfer (REST) API. REST APIs use the HTTP/HTTPS protocol to execute common operations on resources represented by Uniform Resource Identifier (URI) strings. To provide some context, let's go back to the SNMP analogy. In SNMP, to request interface statistics from a particular resource, an application may send an SNMP GET message via a network device's management IP address, with an SNMP payload containing the object identifier (OID) for the relevant interface and statistic. Similarly, an application may use REST APIs to send an HTTP/HTTPS GET message via an SDN controller's IP address. That message would contain a URI string referencing the relevant network device and comprising an HTTP payload with a JSON header that has the proper parameters for a particular interface and statistic.
Getting hands-on with SDN and REST APIs
Since many engineers best learn new technologies by getting direct experience with them, let's look at some next steps to get our hands on some REST APIs. The following three steps outline how to get up and running.
First, if you have no previous programming experience, acquire a tool to generate REST API calls. The Chrome browser, for example, has multiple plug-ins to generate REST API messages. These include Postman and the Advanced REST Client. Firefox has the RESTClient add-on for the same functionality. For those more comfortable with a command-line interface, the curl utility may also be used.
Second, get access to an SDN controller or a controller-like platform that supports REST APIs. Ryu and ONOS are open source options that fit the bill. For those wanting to align to a particular vendor, there are options such as NEC's ProgrammableFlow Controller or Juniper's OpenContrail.
Lastly, dig up the relevant REST API documentation for the SDN controller or platform, such as those for the Ryu controller or for OpenContrail. Although the formatting of the documentation varies, look for the following items: URI string for the requested, HTTP method (e.g., GET, POST, PUT, DELETE) and JSON/XML payload and/or parameters. Both the Ryu and OpenContrail documentation provide examples illustrating how to send a valid REST API message.
To sum it up, we have to demystify APIs in the context of software-defined networks. Why? Because SDN presents new technologies and novel ways for networking professionals to think about and solve networking challenges.
Five open SDN controllers you need to know about
Northbound APIs: A primer
So what's an SDN API, anyway?
SDN-WAN market -- too many cooks?