What is OpenStack Networking?

OpenStack Networking is a pluggable, scalable, and API-driven system to manage networks in an OpenStack-based cloud. Like other core OpenStack components, OpenStack Networking can be used by administrators and users to increase the value and maximize the utilization of existing data center resources.

Neutron, the project name for the OpenStack Networking service, complements other core OpenStack services such as Compute (Nova), Image (Glance), Identity (Keystone), Block (Cinder), Object (Swift), and Dashboard (Horizon) to provide a complete cloud solution.

OpenStack Networking exposes an application programmable interface (API) to users and passes requests to the configured network plugins for additional processing. Users are able to define network connectivity in the cloud, and cloud operators are allowed to leverage different networking technologies to enhance and power the cloud.

OpenStack Networking services can be split between multiple hosts to provide resiliency and redundancy, or they can be configured to operate on a single node. Like many other OpenStack services, Neutron requires access to a database for persistent storage of the network configuration. A simplified example of the architecture can be seen here:

Figure 1.1

In figure 1.1, the Neutron server connects to a database where the logical network configuration persists. The Neutron server can take API requests from users and services and communicate with agents via a message queue. In a typical environment, network agents will be scattered across controller and compute nodes and perform duties on their respective node.

Features of OpenStack Networking

OpenStack Networking includes many technologies you would find in the data center, including switching, routing, load balancing, firewalling, and virtual private networks.

These features can be configured to leverage open source or commercial software and provide a cloud operator with all the tools necessary to build a functional and self-contained cloud networking stack. OpenStack Networking also provides a framework for third-party vendors to build on and enhance the capabilities of the cloud.

Switching

A virtual switch is defined as a software application or service that connects virtual machines to virtual networks at the data link layer of the OSI model, also known as layer 2. Neutron supports multiple virtual switching platforms, including Linux bridges provided by the bridge kernel module and Open vSwitch. Open vSwitch, also known as OVS, is an open source virtual switch that supports standard management interfaces and protocols, including NetFlow, SPAN, RSPAN, LACP, and 802.1q VLAN tagging. However, many of these features are not exposed to the user through the OpenStack API. In addition to VLAN tagging, users can build overlay networks in software using L2-in-L3 tunneling protocols, such as GRE or VXLAN. Virtual switches can be used to facilitate communication between instances and devices outside the control of OpenStack, which include hardware switches, network firewalls, storage devices, bare-metal servers, and more.

Additional information on the use of Linux bridges and Open vSwitch as switching platforms for OpenStack can be found in Chapter 4, Virtual Network Infrastructure Using Linux Bridges, and Chapter 5, Building a Virtual Switching Infrastructure Using Open vSwitch, respectively.

Routing

OpenStack Networking provides routing and NAT capabilities through the use of IP forwarding, iptables, and network namespaces. Each network namespace has its own routing table, interfaces, and iptables processes that provide filtering and network address translation. By leveraging network namespaces to separate networks, there is no need to worry about overlapping subnets between networks created by users. Configuring a router within Neutron enables instances to interact and communicate with outside networks or other networks in the cloud.

More information on routing within OpenStack can be found in Chapter 10, Creating Standalone Routers with Neutron, Chapter 11, Router Redundancy Using VRRP, and Chapter 12, Distributed Virtual Routers.

Load balancing

First introduced in the Grizzly release of OpenStack, Load Balancing as a Service (LBaaS v2) provides users with the ability to distribute client requests across multiple instances or servers. Users can create monitors, set connection limits, and apply persistence profiles to traffic traversing a virtual load balancer. OpenStack Networking is equipped with a plugin for LBaaS v2 that utilizes HAProxy in the open source reference implementation, but plugins are available that manage virtual and physical load-balancing appliances from third-party network vendors.

More information on the use of load balancers within Neutron can be found in Chapter 13, Load Balancing Traffic to Instances.

Firewalling

OpenStack Networking provides two API-driven methods of securing network traffic to instances: security groups and Firewall as a Service (FWaaS). Security groups find their roots in nova-network, the original networking stack for OpenStack built in to the Compute service, and are based on Amazon's EC2 security groups. When using security groups in OpenStack, instances are placed into groups that share common functionality and rule sets. In a reference implementation, security group rules are implemented at the instance port level using drivers that leverage iptables or OpenFlow. Security policies built using FWaaS are also implemented at the port level, but can be applied to ports of routers as well as instances. The original FWaaS v1 API implemented firewall rules inside Neutron router namespaces, but that behavior has been removed in the v2 API.

More information on securing instance traffic can be found in Chapter 8, Managing Security Groups. The use of FWaaS is outside the scope of this book.

Virtual private networks

A virtual private network (VPN) extends a private network across a public network such as the internet. A VPN enables a computer to send and receive data across public networks as if it were directly connected to the private network. Neutron provides a set of APIs to allow users to create IPSec-based VPN tunnels from Neutron routers to remote gateways when using the open source reference implementation. The use of VPN as a Service is outside the scope of this book.

Network functions virtualization

Network functions virtualization (NFV) is a network architecture concept that proposes virtualizing network appliances used for various network functions. These functions include intrusion detection, caching, gateways, WAN accelerators, firewalls, and more. Using SR-IOV, instances are no longer required to use para-virtualized drivers or to be connected to virtual bridges within the host. Instead, the instance is attached to a Neutron port that is associated with a virtual function (VF) in the NIC, allowing the instance to access the NIC hardware directly. Configuring and implementing SR-IOV with Neutron is outside the scope of this book.

OpenStack Networking resources

OpenStack gives users the ability to create and configure networks and subnets and instruct other services, such as Compute, to attach virtual devices to ports on these networks. The Identity service gives cloud operators the ability to segregate users into projects. OpenStack Networking supports project-owned resources, including each project having multiple private networks and routers. Projects can be left to choose their own IP addressing scheme, even if those addresses overlap with other project networks, or administrators can place limits on the size of subnets and addresses available for allocation.

There are two types of networks that can be expressed in OpenStack:

• Project/tenant network: A virtual network created by a project or administrator on behalf of a project. The physical details of the network are not exposed to the project.
• Provider network: A virtual network created to map to a physical network. Provider networks are typically created to enable access to physical network resources outside of the cloud, such as network gateways and other services, and usually map to VLANs. Projects can be given access to provider networks.

The terms project and tenant are used interchangeably within the OpenStack community, with the former being the newer and preferred nomenclature.

A project network provides connectivity to resources in a project. Users can create, modify, and delete project networks. Each project network is isolated from other project networks by a boundary such as a VLAN or other segmentation ID. A provider network, on the other hand, provides connectivity to networks outside of the cloud and is typically created and managed by a cloud administrator.

The primary differences between project and provider networks can be seen during the network provisioning process. Provider networks are created by administrators on behalf of projects and can be dedicated to a particular project, shared by a subset of projects, or shared by all projects. Project networks are created by projects for use by their instances and cannot be shared with all projects, though sharing with certain projects may be accomplished using role-based access control (RBAC) policies. When a provider network is created, the administrator can provide specific details that aren't available to ordinary users, including the network type, the physical network interface, and the network segmentation identifier, such as a VLAN ID or VXLAN VNI. Project networks have these same attributes, but users cannot specify them. Instead, they are automatically determined by Neutron.

There are other foundational network resources that will be covered in further detail later in this book, but are summarized in the following table for your convenience:

Resource	Description
Subnet	A block of IP addresses used to allocate ports created on the network.
Port	A connection point for attaching a single device, such as the virtual network interface card (vNIC) of a virtual instance, to a virtual network. Port attributes include the MAC address and the fixed IP address on the subnet.
Router	A virtual device that provides routing between self-service networks and provider networks.
Security group	A set of virtual firewall rules that control ingress and egress traffic at the port level.
DHCP	An agent that manages IP addresses for instances on provider and self-service networks.
Metadata	A service that provides data to instances during boot.

Virtual network interfaces

OpenStack deployments are most often configured to use the libvirt KVM/QEMU driver to provide platform virtualization. When an instance is booted for the first time, OpenStack creates a port for each network interface attached to the instance. A virtual network interface called a tap interface is created on the compute node hosting the instance. The tap interface corresponds directly to a network interface within the guest instance and has the properties of the port created in Neutron, including the MAC and IP address. Through the use of a bridge, the host can expose the guest instance to the physical network. Neutron allows users to specify alternatives to the standard tap interface, such as Macvtap and SR-IOV, by defining special attributes on ports and attaching them to instances.

Virtual network switches

OpenStack Networking supports many types of virtual and physical switches, and includes built-in support for Linux bridges and Open vSwitch virtual switches. This book will cover both technologies and their respective drivers and agents.

The terms bridge and switch are often used interchangeably in the context of OpenStack Networking, and may be used in the same way throughout this book.

Overlay networks

Neutron supports overlay networking technologies that provide network isolation at scale with little to no modification of the underlying physical infrastructure. To accomplish this, Neutron leverages L2-in-L3 overlay networking technologies such as GRE, VXLAN, and GENEVE. When configured accordingly, Neutron builds point-to-point tunnels between all network and compute nodes in the cloud using a predefined interface. These point-to-point tunnels create what is called a mesh network, where every host is connected to every other host. A cloud consisting of one combined controller and network node, and three compute nodes, would have a fully meshed overlay network that resembles figure 1.2:

Figure 1.2

Using the overlay network pictured in figure 1.2, traffic between instances or other virtual devices on any given host will travel between layer 3 endpoints on each of the underlying hosts without regard for the layer 2 network beneath them. Due to encapsulation, Neutron routers may be needed to facilitate communication between different project networks as well as networks outside of the cloud.

Virtual Extensible Local Area Network (VXLAN)

This book focuses primarily on VXLAN, an overlay technology that helps address scalability issues with VLANs. VXLAN encapsulates layer 2 Ethernet frames inside layer 4 UDP packets that can be forwarded or routed between hosts. This means that a virtual network can be transparently extended across a large network without any changes to the end hosts. In the case of OpenStack Networking, however, a VXLAN mesh network is commonly constructed only between nodes that exist in the same cloud.

Rather than use VLAN IDs to differentiate between networks, VXLAN uses a VXLAN Network Identifier (VNI) to serve as the unique identifier on a link that potentially carries traffic for tens of thousands of networks, or more. An 802.1q VLAN header supports up to 4,096 unique IDs, whereas a VXLAN header supports approximately 16 million unique IDs. Within an OpenStack cloud, virtual machine instances are unaware that VXLAN is used to forward traffic between hosts. The VXLAN Tunnel Endpoint (VTEP) on the physical node handles the encapsulation and decapsulation of traffic without the instance ever knowing.

Because VXLAN network traffic is encapsulated, many network devices cannot participate in these networks without additional configuration, if at all. As a result, VXLAN networks are effectively isolated from other networks in the cloud and require the use of a Neutron router to provide access to connected instances. More information on creating Neutron routers begins in Chapter 10, Creating Standalone Routers with Neutron.

While not as performant as VLAN or flat networks on some hardware, the use of VXLAN is becoming more popular in cloud network architectures where scalability and self-service are major drivers. Newer networking hardware that offers VXLAN offloading capabilities should be leveraged if you are considering implementing VXLAN-based overlay networks in your cloud.

More information on how VXLAN encapsulation works is described in RFC 7348, available at the following URL: https://tools.ietf.org/html/rfc7348

Generic Router Encapsulation (GRE)

A GRE network is similar to a VXLAN network in that traffic from one instance to another is encapsulated and sent over a layer 3 network. A unique segmentation ID is used to differentiate traffic from other GRE networks. Rather than use UDP as the transport mechanism, GRE uses IP protocol 47. For various reasons, the use of GRE for encapsulating tenant network traffic has fallen out of favor now that VXLAN is supported by both Open vSwitch and Linux Bridge network agents.

More information on how GRE encapsulation works is described in RFC 2784 available at the following URL: https://tools.ietf.org/html/rfc2784

As of the Pike release of OpenStack, the Open vSwitch mechanism driver is the only commonly used driver that supports GRE.

Generic Network Virtualization Encapsulation (GENEVE)

GENEVE is an emerging overlay technology that resembles VXLAN and GRE, in that packets between hosts are designed to be transmitted using standard networking equipment without having to modify the client or host applications. Like VXLAN, GENEVE encapsulates packets with a unique header and uses UDP as its transport mechanism. GENEVE leverages the benefits of multiple overlay technologies such as VXLAN, NVGRE, and STT, and may supplant those technologies over time. The Open Virtual Networking (OVN) mechanism driver relies on GENEVE as its overlay technology, which may speed up the adoption of GENEVE in later releases of OpenStack.