Isolation on Digi Hunch

Virtualization 1 of 4 – Hypervisor

Mon, 27 Jul 2020 22:52:00 -0400

In broad terms, virtualization of computing resource is about isolation of resources, at different levels. There are five levels of virtualization:

Application level, such as JVM, .NET CLR
Library (user-level API) level
Operating system level, such as LXC, Docker, OpenVZ
Hardware abstraction layer (HAL) level, such as VMware, Xen, etc
Instruction set architecture (ISA) level

In my context I deal mostly with OS level and HAL (hardware abstraction layer) level of virtualization. In loose terms, the word containerization refers to OS level virtualization, while the word virtualization is exclusively reserved for HAL level virtualization, also referred to as hypervisor-based virtualization. This post will just focus on this family of technology and loosely refers to it as virtualization.

Virtualization technology evolved from on-premise data centre environment and now is the backbone of cloud computing. The challenges of IT operation in the era of virtualization involves managing VM sprawling, investigating performance issues, planning capacity and addressing storage I/O block. The idea of virtualization is sharing (thus isolating) resources for better utilization, leading to better return on investment. This posting is to cover only the very basics of virtualization.

Hypervisor

Hypervisor is the software layer which provides the capability to run multiple virtual machines on the same physical host. It is broken down into two categories:

Type I hypervisor (aka bare metal hypervisor): directly run on physical hardware. They control the hardware as well as manage the virtual machines. For example, Linux KVM, VMware ESXi, Xen and Microsoft Hyper-V
Type II hypervisor: runs as an application or service on top of the host operating system, which is installed on the bare metal. Guest operating system calls need to traverse via the host operating system stack to reach hardware resource. For example, Oracle Virtual Box, VMware Fusion and Linux Containers (LXC)

Hypervisor Types

Virtualization Techniques

The most primitive form of technology that can be arguably categorized under virtualization is hardware emulation, where a piece of (more accessible) hardware imitates another (less accessible). The architecture limits itself in functional testing only, and is not built for performance or production at all.

The original virtualization technology deals with CPU and memory virtualization. In this well-written whitepaper fromVMware, there are three CPU virtualization techniques introduced for x86 architecture.

The x86 architecture offers four levels of privilege known as Ring 0,1,2 and 3 to operating system and applications to manage access to the computer hardware. User-level applications typically run in Ring 3, the OS must execute its privileged instructions in Ring 0 since it needs to have direct access to memory and hardware. The two main challenges with virtualizing x86 architecture are:

A virtualization layer between hardware operating system who expects Ring 0 privilege;
Some instructions with different semantics when not executed in Ring 0 cannot be virtualized effectively. They need to be translated at runtime.

These challenges makes true virtualization of x86 architecture impossible and thus VMware developed three alternative technologies.

Full virtualization (using binary translation): virtual machine presents a complete simulation of the actual hardware environment so that an unmodified guest OS can run in isolation. The Guest OS is not aware that the underlying environment it is running on is virtualized, and issues hardware calls to communicate with (what it thinks as) hardware. The virtual processors have to understand guest CPU instruction, and reproduce the equivalent CPU instructions of the host machine. VMware’s technology to address this is called Binary Translation. This overhead makes true full virtualization difficult to achieve. In real life, a virtual environment that provides “enough representation of the underlying hardware” can be considered to provide full virtualization as long as it allows guest OS to run without modification. Full virtualization comes with a performance penalty.

Paravirtualization (aka OS assisted virtualization): refers to communication between the guest OS and the hypervisor to improve performance and efficiency. In this technology, guest OS is modified with an interface to host hardware to be able to communicate and operate seamlessly. Since the guest OS is modified, the VM does not need to be a complete simulation of the hardware. The modified guest OS knows it is running on a virtualized environment, and (vm driver) makes API calls (known as ‘hyper calls’) to the hypervisor. This allows para-virtualization technology to achieve performance closer to non-virtualized environment. However, since paravirtualization cannot support unmodified operating systems, its compatibility and portability is poor.

Hardware-Assisted Virtualization: hardware vendors such as Intel and AMD both have developed extensions (new features) to simplify virtualization techniques, for example, the introduction of privileged instructions with new CPU execution mode feature to allow hypervisor to run in a new root mode below ring 0. This removed the need for full virtualization and paravirtualization. With VMware originally as a promoter of full virtualization and Xen for paravirtualization, most virtualization technologies today utilizes hardware-assisted virtualization feature, for example, Linux KVM, VMware workstation, VMware fusion, Xen, VirtualBox, etc. Intel’s virtualization extension is VT-x. AMD’s counterpart is AMD-V technology.

To virtualize memory, another level of memory virtualization is required (similar to the virtual memory support in Linux). Hypervisor is responsible for mapping guest physical memory to the actual machine memory, and it uses shadow page tables to accelerate the mappings, usually at a performance cost.

Popular hypervisors

On the market there are a few popular hypervisor technologies. They are all type 1 hypervisors:

Xen is an open-source hypervisor project originally developed in Cambridge University, licensed under GPLv2. . Based on that, Citrix developed its commercial product XenServer, a bare-metal virtualization platform with enterprise-grade features for x86 and AMD environments. Oracle VM is another commercial implementation of Xen. The Xen project also supports many cloud platforms such as Openstack, Cloudstac, etc. Xen project supports paravirtualization (Xen-PV) as well as hardware-assisted virtualization (Xen-HVM) for virtualization of X86, IA64, ARM and other CPU architectures. The earlier versions does not support memory overcommit (aka “dynamic memory optimization”, “memory ballooning“, or as Citrix calls it “dynamic memory control, DMC”). This delivers better performance but also has higher budgetary requirement on hardware since there isn’t room for over-subscription.

Hyper-V is a Microsoft product. It executes in high CPU privilege (Microsoft calls it ring -1 which is equivalent to root mode as Intel calls it). On the guest VM, OS kernel and drivers run in ring 0, application rin in ring 3. This eliminates the need for binary translation. Hyper-V does not support memory overcommit either. Hyper-V is well integrated with Windows platform. It supports Linux as well although with some performance penalty.

Linux KVM (Kernel-based Virtual Machine) is a full open-source virtualization solution for GNU/Linux. What makes KVM a special hypervisor is that it uses a loadable kernel module kvm.ko that turns itself into a hypervisor and provides VMs with direct access to the hardware. So it is a type 1 hypervisor despite of the presence of Linux OS. KVM also contains a processor specific module, kvm-intel.ko or kvm-amd.ko. KVM leverages qemu to access devices. Because KVM runs as a process inside of Linux OS, KVM can use many existing feature in Linux kernel. Redhat has an enterprise solution based on KVM.

Xen vs KVM

VMware ESXi is VMware’s premium hypervisor product (not open-source) and is available for ~~free download~~, although the advanced features are not free. (Update no free download link anymore.) VMware vSphere is virtualization platform built on top of ESXi, including a whole family of virtualization products.

Market segments and players

Virtualization involves many market segments such as virtual desktop infrastructure (VDI, for desktop virtualization), server virtualization is the predominant domain in the virtualization of data centre environment. This effort led to Hyper-Converged Infrastructure (HCI) where almost all the traditional hardware resources are software-defined through the virtualization layer. The management of infrastructure is abstracted away from the physical hardware management. The three most fundamental areas in HCI are:

Server (compute) virtualization: the previous section covers the virtualization of memory and x86 CPU, which are the main focus on computing resource virtualization. Additionally, graphics computing resources can be virtualized today. Example products include: VMware vShpere (compute virtualization based on ESXi hypervisor).

Storage Virtualization: the technology to abstract physical data storage resource to make them appear as if they were a centralized resource. Storage virtualization takes place at three levels depending on the use case: block-level, file-level and object level. Example products include: VMWare vSAN (vSphere-native storage), HPE 3PAR (Tier-1 storage), EMC VxRail, PureStorage Flash Array (Tier 1), etc. Storage Virtualization enables Software-Defined Storage (SDS), the provisioning and management of data storage independent of the underlying hardware.

Network Virtualization: the technology to abstract network resources that were traditionally delivered in hardware to software. Network virtualization decouples network services from the underlying hardware management and allows virtual provisioning of an entire network. VLAN is a classic example of network virtualization. There are also various overlay technologies such as VXLAN, which provides an industry framework for overlaying virtualized layer 2 network over layer 3 network (used in Docker network) using an encapsulation mechanism and a control plane. Example products include: VMware NSX Data Center (L2-L7 network and security virtualization platform), Cisco ACI, Palo Alto Panorama. Network Virtualization enables Software-Defined Network (SDN), an approach to network management that enables dynamic, programmatically efficient network configuration in order to improve network performance and monitoring, making it more like cloud computing than traditional network management.

Delivery model

Virtualization allows managed service providers (MSPs) to deliver IT service in the following three models:

Iaas (Infrastructure as a Service): MSP delivers VM to customers.
PaaS (Platform as a Service): MSP delivers environments to customers (e.g. Database as a Service, managed RabbitMQ service, etc).
SaaS (Software as a Service): MSP delivers entire application for the customer.

IT service delivery models enabled by virtualization technology

Since virtualization is the backbone of cloud computing. This model is also referred to as cloud computing delivery model.

Virtualization and Containerization

These two concepts are similar and could be confusing to beginners. Both provide a mechanism to isolate computing resource for different applications, for the purpose of higher utilization of resource. The difference lies in how and where the isolation is made. Virtualization requires a guest operating system per VM (OS level isolation), whereas the container technology isolates application processes along with its runtime into a container (dependency level isolation), using some new Linux kernel features such as namespaces and cgroups. All containers make their system calls to the container engine on the host operating system. So they share a kernel on the same host. In this sense, container engine running on OS could be considered as type 2 hypervisor.

From VMs to containers

VMware is a major player in enterprise data centre virtualization, which is facing fierce competition from public and private cloud vendors. VMware also has its own private cloud services. Docker is the most popular container technology that conforms to the specifications of Open Container Initiative (OCI), a governance structure for industry standards around container formats and runtimes.

Virtualization and Cloud

Among public cloud vendors, AWS EC2 used Xen PV and Xen HVM in its earlier implementations. It has transitioned to AWS bare metal. The history is well summarized here. Microsoft Azure runs Azure Hypervisor as the native hypervisor in Azure Cloud Services platform. It is a customized version of Microsoft Hyper-V specifically for Azure platform. With GCP, Google Compute Engine (GCE) instance runs VMs on KVM as hypervisor. It can also enable nested virtualization.

The scope of cloud computing is evolving overtime. It originally only refers to a business model of offering IT services (in one of the three delivery models outlined above) based on virtualization technology. Therefore I cannot make comparison between a technology and a business model. Today, with public cloud vendor extending their offerings (with various managed services and platforms) and people’s misuse of the terms, the buzz-word “cloud” seems to suggest anything that is offered in public cloud service. The essence still remain the same where managed services and managed platforms are built on top of virtualized compute unit under the hood, which are driven by virtualization technologies.

Docker network in different modes

Wed, 01 Jul 2020 20:19:00 -0400

Reading notes of “Docker DeepDive”.

Docker networking is backed by libnetwork, which is an implementation of Container Network Model (CNM), an open-source pluggable architecture designed to provide networking to containers. Libnetwork also provides native service discovery and basic container load balancing solution. Docker networking also involves some drivers that extend the CNM model with specific network topology implementation.

Sandbox – an isolated network stack, including Ethernet interfaces, ports, routing tables, and DNS config, usually implemented through Linux namespace.
Endpoints – behave like regular network adapters, and can only be connected to a single network at a time. It connects sandbox to network. Endpoint is implemented in veth pair in Linux.
Networks – software implementation of an 802.1 bridge (aka switch). They group together, and isolate, a collection of endpoints that need to communicate.

Docker company separates network project out from its container project, as a plugin called libnetwork, which is developed in Golang and compliant to CNM. Libnetwork is the official implementation of CNM.

Libnetwork supports the following network modes:

network mode	mechanism	use case
null	no network is provided to containers	quarantined environment for security
bridge	containers communicate with each other through bridge	containers needs to communicate with each other or with host service
host	process in container has access to host network stack and use host port	container needs to use host network stack (e.g. licence by mac address)
container	place containers in a single net namespace so they can communicate with each other as localhost	proxy, kubernetes

Linux veth comes in pairs to connect virtual network devices. For example, connect two net namespaces to allow intercommunication. Linux bridge is a virtual device, to connect two net namespaces.

Dockers ships with several built-in drivers, known as native drivers or local drivers, such as bridge, overlay and macvlan on Linux. There are also 3rd-party network drivers for docker (aka remote drivers).

Host network

In this mode libnetwork will not create network and net namespace for container. Container process shares the network configuration of the host, and therefore uses the ports on host. Other than network sharing, other aspects (e.g. process, file system, hostname, etc) are separated from host.

Bridge networks

This type of network only exist on a single Docker host and can only connect containers that are on the same host. The word bridge refers to 802.1d bridge (layer 2 switch), which is used to connect multiple network interfaces.

Every Docker host gets a default single-host network, called bridge on Linux. This is the network that all new containers will attach to by default.
Docker networks built with the bridge driver on Linux hosts are based on the linux bridge technology that has existed in the Linux kernel for a while. They’re high performance and extremely stable. Linux brctl tool can inspect the linux bridge.

Bridge networks allows container on the same host to communicate with each other. Port mapping allows network connectivity between container and host. Traffic hitting host port will be redirected to container port.

Multi-host overlays

Cross-host networking usually uses an overlay network, which builds a mesh between host and employs a large block of IP addresses within that mesh. A mesh network is a local network topology in which the infrastructure nodes connect directly, dynamically and non-hierarchically to as many other nodes as possible and cooperate with one another to efficiently route data from/to clients.

You can attach a service to overlay network, which spans across multiple Docker hosts so that containers on different hosts can communicate at layer 2. They are much better alternatives than bridge network for container-to-container communication. Overlay networking is very common due to its scalability.

The trick is basically the layer 2 frame of the overlay network is encapsulated into layer 3 datagram transmitted across underlay network, at layer 3. This is achieved through VXLAN tunnels, which allows you to create a virtual Layer 2 network on top of an existing Layer 3 infrastructure. VXLAN is an encapsulation technology that existing routers and network infrastructure just see as regular IP/UDP packets without issue.

To create the virtual Layer 2 overlay network, a VXLAN tunnel is created through the underlying Layer 3 IP infrastructure (aka underlay network). Each end of the VXLAN tunnel is terminated by a VXLAN Tunnel Endpoint (VTEP). It’s this VTEP that performs the encapsulation/de-encapsulation.

VXLAN networking

To accomplish overlay network across multiple hosts, a new network sandbox was created on each host. A sandbox is like a container, but instead of running an application, it runs an isolated network stack – one that’s sandboxed from the network stack of the host itself.

A virtual switch (aka virtual bridge) called Br0 is created inside the sandbox. A VTEP is also created with one end plumbed into the Br0 virtual switch, and the other end plumbed into the host network stack (VTEP). The end in the host network gets an IP address on the underlay network the host is connected to and is bound to a UDP socket on port 4789. The two VTEPs on each host create the overlay via a VXLAN tunnel.

Each container then gets its own virtual Ethernet (veth) adapter that is also plumbed into the local Br0 virtual switch.

Let’s go over an example in the following diagram, where container C1 with an overlay IP needs to communicate to another container C2, with a different overlay IP, sitting on a different node (Docker host). Each node has its own underlay IP.

IP communication details:

C1 creates the IP datagram with destination IP (C2) and sends it over its veth interface, which is connected to the Br0 virtual switch on the host node.
The virtual switch doesn’t know where to send the datagram, as it doesn’t have an entry in its ARP table that corresponds to the destination IP address. As a result, it floods the packet to all ports. The VTEP interface connected to Br0 knows how to forward the frame, so responds with its own MAC address.
This is a proxy APR reply and results in the Br0 switch learning how to forward the packet. So it updates its ARP mapping the destination IP address to the MAC address of the local VTEP.
The VTEP knows about C2 because all newly started containers have their network details propagated to the other nodes in the Swarm using the network’s built-in gossip protocol. When the packet arrives at node2
The VTEP encapsulates the frame so it can be sent over the underlay transport infrastructure, by adding a VXLAN header to the Ethernet frame. The VXLAN header contains the VXLAN network ID (VNID) which is used to map frames from VLANs to VXLANs and vice versa.
Each VLAN gets mapped to VNID, so that the packet can be de-encapsulated on the receiving end and forwarded to the correct VLAN. This is how network isolation is maintained. The encapsulation also wraps the frame in a UDP packet with the IP address of the remote VTEP on node2 in the destination IP field, and the UDP port 4789 socket information. The underlying network does not know that it is transporting data frames for the overlay network.
When the packet arrives at node2, the kernel sees that it’s addressed to UDP port 4789. The kernel also knows that it has a VTEP interface bound to this socket. As a result, it sends the packet to the VTEP, which reads the VNID, de-encapsulates the packet, and sends it on to its own local Br0 switch on the VLAN that corresponds the VNID. From there it is delivered to container C2

Docker also supports Layer 3 routing within the same overlay network. For example, you can create an overlay network with two subnets, and Docker will take care of routing between them. Two subnets will require two virtual switches, Br0 and Br1, being created inside the sandbox, and routing happens by default.

Plugging into existing vLANs

The built-in MACVLAN driver was created for onnect containerized apps to external physical network. A good example is partially containerized app, in which the containerized parts will need a way to communicate with the non-containerized parts still running on existing physical networks.

To connect the container interface through the host interface to an external network, the host NIC needs to be in promiscuous mode. For public cloud, this is most likely prohibited. For data centers, this depends on the network policy.

Docker MACVLAN driver is built on top of Linux kernel driver with the same name. As such, it supports VLAN trunking. This means we can create multiple MACVLAN networks and connect containers on the same Docker host to them.

For connectivity issues between containers, it’s worth checking both the daemon logs (on host) and container logs.

Service discovery

allows all containers and Swarm services to locate each other by name, as long as they are on the same network. This leverages Docker’s embedded DNS server as well as a DNS resolver in each container.
Each Swarm Service and standalone container started with the –name flag will register its name and IP address with the Docker DNS service.
This name resolution, however, only works within the same network.
It is also possible to configure Swarm services and standalone containers with customized DNS options in case embedded Docker DNS server cannot resolve a query (/etc/resolv.conf)

Ingress load balancing

Services published via ingress mode (by default, as opposed to host mode) can be accessed from any node in the Swarm, even nodes not running a service replica. Ingress mode uses a layer 4 routing mesh called the Service Mesh or the Swarm Mode Service Mesh.

Updates:

The most common network modes that I use are host and bridge. With host network mode, container exposes ports on the interface of the host machine. Containers talk to each other via that interface. With bridge network, containers have their own namespace of networking separate from the one from the interface of the hosts, with a bridge getting the two networks connected.

Reference

Docker Deep dive