Chapter 3: Basic Architecture
SDN is an approach to building networks that favors programmable commodity hardware, with the intelligence that controls packet forwarding and other network operations implemented in software. Realizing such a design is independent of any particular protocol stack, but instead requires a set of open APIs and a new collection of software components that support those APIs. This chapter introduces the basic architecture of such an SDN software stack.
This chapter defines the general architecture of such a software stack, and while there are multiple options for the specific components and tools that can be plugged into this architecture, it also introduces an example set. We do this to make the discussion more concrete, but the particular components we describe have two important attributes. One, they are open source and freely available on GitHub. Two, they are designed to work together, providing a comprehensive solution; there are no gaps in our story. Both attributes make it possible for anyone to build the same end-to-end system that is running today in production networks.
3.1 Software Stack
An overview of the software stack is given in Figure 15, which includes a Bare-Metal Switch running a local Switch OS, controlled by a global Network OS hosting a collection of Control Applications. Figure 15 also calls out a corresponding set of exemplar open source components (SD-Fabric, ONOS, and Stratum) on the right, as well as a related P4 Toolchain on the left. This chapter introduces these components, with later chapters giving more detail.
Note the similarity between this diagram and Figure 2 in Chapter 1. Both figures include two open interfaces: one between the Control Apps and the Network OS, and a second between the Network OS and the underlying programmable switches. These two interfaces are depicted as “API shims” in Figure 15, and in the context of the exemplar components, correspond to a combination of gNMI, gNOI and FlowObjective in the first case, and a combination of gNMI, gNOI and either P4Runtime or OpenFlow in the second case. gRPC, an open source remote procedure call framework, is shown as the transport protocol for these APIs—an implementation choice, but one that we will generally assume from here on. (Note that OpenFlow, unlike the other protocols, does not run over gRPC.) We discuss all of these acronyms and interfaces in further detail below.
It is important to keep in mind that the software components listed in Figure 15 correspond to active open source projects, and as a consequence, they continue to evolve (as do their APIs). Specific versions of each component—and their associated APIs—have been integrated and deployed into both trial and production environments. For example, while the figure shows P4Runtime as a candidate control interface exported by the Switch OS, there are deployed solutions that use OpenFlow instead. (This includes the Comcast deployment.) Similarly, while the figure shows gNMI/gNOI as the config/ops interface to each switch, there are solutions that use NETCONF instead.
For the purpose of this book, we do not attempt to track all possible combinations of component versions and APIs, but opt instead to focus on the single consistent stack enumerated in Figure 15, since it represents our best judgement as to the “right” approach based on experience (so far) with earlier versions up-and-down the stack.
3.1.1 Switch vs Host Implementation
The top-to-bottom view of the software stack shown in Figure 15 is from the perspective of a single switch, but it is important to also keep the network perspective in mind. Figure 16 gives such a perspective by focusing on an end-to-end path through the network, connecting Virtual Machines (VMs).
This perspective highlights two important aspects of the system. The first re-enforces the point we’ve been making: that the Network OS (e.g., ONOS) is network-wide, while the Switch OS (e.g., Stratum) is per-switch.
The second is that part of the SDN software stack runs on the end hosts. In particular, there is a Virtual Switch (vSwitch)—typically implemented in software as part of the hypervisor running on the server—that is responsible for forwarding packets to and from the VMs. (Of course, not every end-host runs VMs, but a similar architecture applies to containers hosts or bare-metal servers.) Just like a physical switch, the vSwitch forwards packets from input port to output port, but these are virtual ports connected to VMs (or containers) rather than physical ports connected to physical machines.
Fortunately, we can view a vSwitch as behaving just like a physical switch, including the APIs it supports. That a vSwitch is implemented in software on a general-purpose processor rather than in an ASIC is an implementation detail. While this is a true statement, being a software switch dramatically lowers the barrier to introducing additional features, so the feature set is both richer and more dynamic. For example, Open vSwitch (OVS) is a widely-used open source vSwitch that supports OpenFlow as a northbound API. It formed the data plane for the original Nicira network virtualization platform. OVS has been integrated with an assortment of complementary tools, such as DPDK (Data Plane Development Kit), another open source component that optimizes packet forwarding operations on x86 processors. Although it’s an important topic, this book does not explore the full range of possibilities for a vSwitch like OVS or other end-host optimizations, but instead treats vSwitches just like any other switch along the end-to-end path.
Another implementation detail shown in Figure 16 is that the host may have a Smart Network Interface Card (SmartNIC) that assists (or possibly even replaces) the vSwitch. Vendors have a long history of off-loading kernel functionality onto NICs (e.g., everything from computing TCP/IP checksums to supporting VMs), but in the SDN context, the interesting possibility is to replicate the forwarding pipeline found on the network switches. Again, there are a range of possible implementation choices, including both FPGA and ASIC, as well as whether the NIC is fixed-function or programmable (using P4). For our purposes, we will treat such Smart NICs as yet another switching element along the end-to-end path.
3.2 Bare-Metal Switch
Starting at the bottom and working our way up the stack shown in Figures 15 and 16, the network data plane is implemented by an interconnected set of bare-metal switches. Our focus for now is on a single switch, where the overall network topology is dictated by the Control Applications running at the top of the software stack. For example, we describe a Control Application that manages a leaf-spine topology in a later section.
The architecture is agnostic as to the switch vendor, but the full software stack outlined in this chapter runs on switches built using Tofino and Tomahawk switching chips manufactured by Barefoot Networks (now an Intel company) and Broadcom, respectively. The Tofino chip implements a programmable forwarding pipeline based on PISA, while the Tomahawk chip implements a fixed-function pipeline.
In the case of both chips, a pair of P4 programs defines the
forwarding pipeline. The first (
forward.p4) specifies the
forwarding behavior. The second (
arch.p4) specifies the logical
architecture of the target forwarding chip. The P4 compiler
generates target files that are loaded into both the Network OS and
the switch. These target files are not named in Figure 15 (we will return to the details in Chapters 4 and 5), but
both components need to know about the output because one implements
the forwarding behavior (the switch), and the other controls the
forwarding behavior (the Network OS).
We return to the details of the compiler toolchain in Chapter 4. For
now, we will just address the question of why we need a P4 program in
the case of a fixed-function switching chip (since we are not using P4
to modify its fixed behavior). The quick summary is that a formal
specification of the forwarding pipeline is required to generate the
API to the data plane. P4 programs are written to an abstract model
of the forwarding pipeline, and whether the chip’s actual hardware
pipeline is fixed or programmable, we still need to know how to map
the abstract pipeline onto the physical pipeline. This is where
arch.p4 plays a role. As for the role of
program actually prescribes the pipeline in the case of a
programmable chip, whereas for the fixed-function chip,
merely describes the pipeline. But we still need
both cases because the toolchain uses it, along with
generate the API that sits between the control and data planes.
3.3 Switch OS
Moving up from the base hardware, each switch runs a local Switch OS. Not to be confused with the Network OS that manages a network of switches, this Switch OS runs on a commodity processor internal to the switch (not shown in Figure 15). It is responsible for handling API calls issued to the switch, for example from the Network OS. This includes taking the appropriate action on the switch’s internal resources, which sometimes affects the switching chip.
Multiple open source Switch OSes are available (including SONiC, originally developed at Microsoft Azure), but we use a combination of Stratum and Open Network Linux (ONL) as our primary example. ONL is a switch-ready distribution of Linux (originally prepared by Big Switch Networks), while Stratum (originally developed at Google) is primarily responsible for translating between the external-facing API and the internal switch resources. For this reason, we sometimes refer to Stratum as a Thin Switch OS.
Stratum mediates all interactions between the switch and the outside world. This includes loading the target files generated by the P4 compiler, which defines a contract between the data plane and the control plane. This contract effectively replaces OpenFlow’s flow rule abstraction with an auto-generated specification. The rest of the Stratum-managed API is defined as follows:
P4Runtime: An interface for controlling forwarding behavior at runtime. It is the key for populating forwarding tables and manipulating forwarding state. The P4Runtime is independent of any particular P4 program and agnostic to the underlying hardware. This contrasts to OpenFlow which is rather prescriptive about the forwarding model and how the control plane interacts with it. (For completeness, Figure 15 also lists OpenFlow as an alternative control interface.)
gNMI (gRPC Network Management Interface): Used to set and retrieve configuration state. gNMI is usually paired with OpenConfig YANG models that define the structure of the configuration and state tree.
gNOI (gRPC Network Operations Interfaces): Used to set and retrieve operational state, for example supporting certificates management, device testing, software upgrades, and networking troubleshooting.
If you recall the distinction between Control and Configuration introduced in Chapter 1, then you will recognize P4Runtime as the Control API and the gNMI/gNOI combination as a modern version of a switch’s traditional Configuration API. This latter API has historically been called the OAM interface (for “Operations, Administration, and Maintenance”), and it has most often been implemented as a command-line interface (which is of course not really an API).
3.4 Network OS
The Network OS is a platform for configuring and controlling a network of switches. It runs off-switch as a logically centralized SDN controller, and manages a set of switches on a network-wide basis. Central to this role is responsibility for monitoring the state of those switches (e.g., detecting port and link failures), maintaining a global view of the topology that reflects the current state of the network, and making that view available to any interested Control Apps. Those Control Apps, in turn, “instruct” the Network OS to control packet flows through the underlying switches according to whatever service they are providing. The way these “control instructions” are expressed is a key aspect of the Network OS’s API.
Going beyond this conceptual description requires a specific Network OS, and we use ONOS (Open Network Operating System) as our exemplar. ONOS is best-of-breed in terms of performance, scalability, and availability. At a high-level, ONOS takes responsibility for three things:
Managing Topology: Tracks inventory of network infrastructure devices and their interconnection to provide a shared view of the network environment for the rest of the platform and applications.
Managing Configuration: Facilitates issuing, tracking, rolling back, and validating atomic configuration operations on multiple network devices. This effectively mirrors the per-switch configuration and operation interfaces (also using gNMI and gNOI), but does so at the network level rather than the device level.
Controlling Switches: Controls the data plane packet processing pipelines of the network switches and provides subsequent control of flow rules, groups, meters and other building blocks within those pipelines.
With respect to this last role, ONOS exports a northbound FlowObjectives abstraction, which generalizes Flow Rules in a pipeline-independent way.1 This interface, which Chapter 6 describes in more detail, is not standardized in the same way as the control interface exported by individual switches. As with a conventional server OS, applications written to the ONOS API do not easily port to another Network OS. The requirement is that this interface be open and well-defined; not that there be just one such interface. If over time there is consensus about the Network OS interface, then applications will be more easily portable. But just as with server operating systems, the higher one goes up the software stack, the more difficult it becomes to reach such a consensus.
We make no claim that FlowObjectives are an ideal interface for controlling a switch. The API evolved out of necessity, allowing developers to deal with different pipelines. Defining a general interface is the subject of ongoing research.
Finally, although Figure 15 does not show any details about the internals of ONOS, to better appreciate the role it plays in the larger scheme of things, we note that the most critical subsystem in any Network OS is a Scalable Key/Value Store. Because ONOS provides a logically centralized view of the network, the key to its performance, scalability, and availability is how it stores that state. In the case of ONOS, this store is provided by a companion open source project, called Atomix, which implements the RAFT consensus algorithm. Storage services like Atomix are the cornerstone of nearly all horizontally scalable cloud services today, as Chapter 6 describes in more detail.
3.5 Leaf-Spine Fabric
Because we use ONOS as the Network OS, we are limited to ONOS-hosted SDN Control Applications. For illustrative purposes, we use SD-Fabric as that Control App—it implements a leaf-spine fabric on a network of programmable switches. This means SD-Fabric dictates a particular network topology, in particular, the leaf-spine topology common to datacenter clusters. As outlined in Section 2.3, this topology includes a set of leaf switches, each of which serves as a Top-of-Rack switch (i.e., it connects all the servers in a single rack), where the leaf switches are, in turn, interconnected by a set of spine switches.
At a high level, SD-Fabric plays three roles. First, it provides a switching fabric that interconnects servers—and the VMs running on those servers—in a multi-rack cluster. Second, it connects the cluster as a whole upstream to peer networks, including the Internet, using BGP (i.e., it behaves much like a router). Third, it connects the cluster as a whole to downstream access networks (i.e., it terminates access network technologies like PON and RAN). In other words, instead of thinking about SD-Fabric as a conventional leaf-spine fabric that’s locked away in some datacenter, SD-Fabric is best viewed an interconnect running at the network edge, helping to bridge access-specific edge clouds to IP-based datacenter clouds.
In terms of implementation, SD-Fabric actually corresponds to a suite of Control Apps running on ONOS, as opposed to a single app. This suite supports several control plane features, including:
VLANs and L2 bridging
IPv4 and IPv6 unicast and multicast routing
DHCP L3 relay
Dual-homing of servers and upstream routers
For each of these features, the corresponding Control App interacts with ONOS—by observing changes in the network topology and issuing Flow Objectives—rather than by using any of the standard protocol implementations found in legacy routers and switches. The only time a legacy protocol is involved is when SD-Fabric needs to communicate with the outside world (e.g., upstream metro/core routers), in which case it uses standard BGP (as implemented by the open source Quagga server). This is actually a common feature of SDN environments: they avoid traditional routing protocols internally, or in a greenfield, but interaction with the outside world still requires them.
Finally, SD-Fabric is sometimes deployed at a single site with multiple RAN base stations connected via SD-Fabric leaf-switches. But SD-Fabric can also be extended to multiple sites deeper into the network using multiple stages of spines, as shown in Figure 17. Chapter 7 describes all of this in more detail.