Design
======

The vision and goals of libnetwork are highlighted in [roadmap](../ROADMAP.md).
This document describes how libnetwork has been designed in order to achieve this.
Requirements for individual releases can be found on the [Project Page](https://github.com/docker/libnetwork/wiki).

Many of the design decisions are inspired by the learnings from the Docker networking design as of Docker v1.6.
Please refer to this [Docker v1.6 Design](legacy.md) document for more information on networking design as of Docker v1.6.

## Goal

libnetwork project will follow Docker and Linux philosophy of developing small, highly modular and composable tools that work well independently.
Libnetwork aims to satisfy that composable need for Networking in Containers.

## The Container Network Model

Libnetwork implements Container Network Model (CNM) which formalizes the steps required to provide networking for containers while providing an abstraction that can be used to support multiple network drivers. The CNM is built on 3 main components (shown below)

![](/docs/cnm-model.jpg?raw=true)

**Sandbox**

A Sandbox contains the configuration of a container's network stack.
This includes management of the container's interfaces, routing table and DNS settings.
An implementation of a Sandbox could be a Linux Network Namespace, a FreeBSD Jail or other similar concept.
A Sandbox may contain *many* endpoints from *multiple* networks.

**Endpoint**

An Endpoint joins a Sandbox to a Network.
An implementation of an Endpoint could be a `veth` pair, an Open vSwitch internal port or similar.
An Endpoint can belong to *only one* network but may only belong to *one* Sandbox.

**Network**

A Network is a group of Endpoints that are able to communicate with each-other directly.
An implementation of a Network could be a Linux bridge, a VLAN, etc.
Networks consist of *many* endpoints.

## CNM Objects

**NetworkController**
`NetworkController` object provides the entry-point into libnetwork that exposes simple APIs for the users (such as Docker Engine) to allocate and manage Networks. libnetwork supports multiple active drivers (both inbuilt and remote). `NetworkController` allows user to bind a particular driver to a given network.

**Driver**
`Driver` is not a user visible object, but drivers provides the actual implementation that makes network work. `NetworkController` however provides an API to configure any specific driver with driver-specific options/labels that is transparent to libnetwork, but can be handled by the drivers directly. Drivers can be both inbuilt (such as Bridge, Host, None & overlay) and remote (from plugin providers) to satisfy various usecases & deployment scenarios. At this point, the Driver owns a network and is responsible for managing the network (including IPAM, etc.). This can be improved in the future by having multiple drivers participating in handling various network management functionalities.

**Network**
`Network` object is an implementation of the `CNM : Network` as defined above. `NetworkController` provides APIs to create and manage `Network` object. Whenever a `Network` is created or updated, the corresponding `Driver` will be notified of the event. LibNetwork treats `Network` object at an abstract level to provide connectivity between a group of end-points that belong to the same network and isolate from the rest. The Driver performs the actual work of providing the required connectivity and isolation. The connectivity can be within the same host or across multiple-hosts. Hence `Network` has a global scope within a cluster.

**Endpoint**
`Endpoint` represents a Service Endpoint. It provides the connectivity for services exposed by a container in a network with other services provided by other containers in the network. `Network` object provides APIs to create and manage endpoint. An endpoint can be attached to only one network. `Endpoint` creation calls are made to the corresponding `Driver` which is responsible for allocating resources for the corresponding `Sandbox`. Since Endpoint represents a Service and not necessarily a particular container, `Endpoint` has a global scope within a cluster as well.

**Sandbox**
`Sandbox` object represents container's network configuration such as ip-address, mac-address, routes, DNS entries. A `Sandbox` object is created when the user requests to create an endpoint on a network. The `Driver` that handles the `Network` is responsible to allocate the required network resources (such as ip-address) and pass the info called `SandboxInfo` back to libnetwork. libnetwork will make use of OS specific constructs (example: netns for Linux) to populate the network configuration into the containers that is represented by the `Sandbox`. A `Sandbox` can have multiple endpoints attached to different networks. Since `Sandbox` is associated with a particular container in a given host, it has a local scope that represents the Host that the Container belong to.

**CNM Attributes**

***Options***
`Options` provides a generic and flexible mechanism to pass `Driver` specific configuration option from the user to the `Driver` directly. `Options` are just key-value pairs of data with `key` represented by a string and `value` represented by a generic object (such as golang `interface{}`). Libnetwork will operate on the `Options` ONLY if the  `key` matches any of the well-known `Label` defined in the `net-labels` package. `Options` also encompasses `Labels` as explained below. `Options` are generally NOT end-user visible (in UI), while `Labels` are.

***Labels***
`Labels` are very similar to `Options` & in fact they are just a subset of `Options`. `Labels` are typically end-user visible and are represented in the UI explicitly using the `--labels` option. They are passed from the UI to the `Driver` so that `Driver` can make use of it and perform any `Driver` specific operation (such as a subnet to allocate IP-Addresses from in a Network).

## CNM Lifecycle

Consumers of the CNM, like Docker for example, interact through the CNM Objects and its APIs to network the containers that they manage.

1. `Drivers` registers with `NetworkController`. Build-in drivers registers inside of LibNetwork, while remote Drivers registers with LibNetwork via Plugin mechanism. (*plugin-mechanism is WIP*). Each `driver` handles a particular `networkType`.

2. `NetworkController` object is created using `libnetwork.New()` API to manage the allocation of Networks and optionally configure a `Driver` with driver specific `Options`.

3. `Network` is created using the controller's `NewNetwork()` API by providing a `name` and `networkType`. `networkType` parameter helps to choose a corresponding `Driver` and binds the created `Network` to that `Driver`. From this point, any operation on `Network` will be handled by that `Driver`.

4. `controller.NewNetwork()` API also takes in optional `options` parameter which carries Driver-specific options and `Labels`, which the Drivers can make use of for its purpose.

5. `network.CreateEndpoint()` can be called to create a new Endpoint in a given network. This API also accepts optional `options` parameter which drivers can make use of. These 'options' carry both well-known labels and driver-specific labels. Drivers will in turn be called with `driver.CreateEndpoint` and it can choose to reserve IPv4/IPv6 addresses when an `Endpoint` is created in a `Network`. The `Driver` will assign these addresses using `InterfaceInfo` interface defined in the `driverapi`. The IP/IPv6 are needed to complete the endpoint as service definition along with the ports the endpoint exposes since essentially a service endpoint is nothing but a network address and the port number that the application container is listening on.

6. `endpoint.Join()` can be used to attach a container to an `Endpoint`. The Join operation will create a `Sandbox` if it doesn't exist already for that container. The Drivers can make use of the Sandbox Key to identify multiple endpoints attached to a same container. This API also accepts optional `options` parameter which drivers can make use of.
  * Though it is not a direct design issue of LibNetwork, it is highly encouraged to have users like `Docker` to call the endpoint.Join() during Container's `Start()` lifecycle that is invoked *before* the container is made operational. As part of Docker integration, this will be taken care of.
  * One of a FAQ on endpoint join() API is that, why do we need an API to create an Endpoint and another to join the endpoint.
    - The answer is based on the fact that Endpoint represents a Service which may or may not be backed by a Container. When an Endpoint is created, it will have its resources reserved so that any container can get attached to the endpoint later and get a consistent networking behaviour.

7. `endpoint.Leave()` can be invoked when a container is stopped. The `Driver` can cleanup the states that it allocated during the `Join()` call. LibNetwork will delete the `Sandbox` when the last referencing endpoint leaves the network. But LibNetwork keeps hold of the IP addresses as long as the endpoint is still present and will be reused when the container(or any container) joins again. This ensures that the container's resources are reused when they are Stopped and Started again.

8. `endpoint.Delete()` is used to delete an endpoint from a network. This results in deleting an endpoint and cleaning up the cached `sandbox.Info`.

9. `network.Delete()` is used to delete a network. LibNetwork will not allow the delete to proceed if there are any existing endpoints attached to the Network.


## Implementation Details

### Networks & Endpoints

LibNetwork's Network and Endpoint APIs are primarily for managing the corresponding Objects and book-keeping them to provide a level of abstraction as required by the CNM. It delegates the actual implementation to the drivers which realize the functionality as promised in the CNM. For more information on these details, please see [the drivers section](#drivers)

### Sandbox

Libnetwork provides a framework to implement of a Sandbox in multiple operating systems. Currently we have implemented Sandbox for Linux using `namespace_linux.go` and `configure_linux.go` in `sandbox` package.
This creates a Network Namespace for each sandbox which is uniquely identified by a path on the host filesystem.
Netlink calls are used to move interfaces from the global namespace to the Sandbox namespace.
Netlink is also used to manage the routing table in the namespace.

## Drivers

## API

Drivers are essentially an extension of libnetwork and provide the actual implementation for all of the LibNetwork APIs defined above. Hence there is an 1-1 correspondence for all the `Network` and `Endpoint` APIs, which includes :
* `driver.Config`
* `driver.CreateNetwork`
* `driver.DeleteNetwork`
* `driver.CreateEndpoint`
* `driver.DeleteEndpoint`
* `driver.Join`
* `driver.Leave`

These Driver facing APIs make use of unique identifiers (`networkid`,`endpointid`,...) instead of names (as seen in user-facing APIs).

The APIs are still work in progress and there can be changes to these based on the driver requirements especially when it comes to Multi-host networking.

### Driver semantics

 * `Driver.CreateEndpoint`

This method is passed an interface `EndpointInfo`, with methods `Interface` and `AddInterface`.

If the value returned by `Interface` is non-nil, the driver is expected to make use of the interface information therein (e.g., treating the address or addresses as statically supplied), and must return an error if it cannot. If the value is `nil`, the driver should allocate exactly one _fresh_ interface, and use `AddInterface` to record them; or return an error if it cannot.

It is forbidden to use `AddInterface` if `Interface` is non-nil.

## Implementations

Libnetwork includes the following driver packages:

- null
- bridge
- overlay
- remote

### Null

The null driver is a `noop` implementation of the driver API, used only in cases where no networking is desired. This is to provide backward compatibility to the Docker's `--net=none` option.

### Bridge

The `bridge` driver provides a Linux-specific bridging implementation based on the Linux Bridge.
For more details, please [see the Bridge Driver documentation](bridge.md).

### Overlay

The `overlay` driver implements networking that can span multiple hosts using overlay network encapsulations such as VXLAN.
For more details on its design, please see the [Overlay Driver Design](overlay.md).

### Remote

The `remote` package does not provide a driver, but provides a means of supporting drivers over a remote transport.
This allows a driver to be written in a language of your choice.
For further details, please see the [Remote Driver Design](remote.md).