Fuchsia Networking API Evolution

Fuchsia Networking offers a broad range of APIs with sometimes overlapping functionality. This document is the basis of ongoing API evolution.


At the time of writing, Fuchsia Networking APIs are the result of rapid evolution and often-unintentional stabilization, resulting in a large surface with out-of-tree consumers. Many of these APIs evolved before newer FIDL features were available, making in-place migration even more difficult.

More than other system areas, Networking must be mindful of its APIs becoming prematurely load bearing.

Design Principles

Deliberate Stabilization

Consider this excerpt from the FIDL API Rubric:

How you decompose these definitions into libraries has a large effect on the consumers of these definitions because a FIDL library is the unit of dependency and distribution for your protocols.

Be mindful of this when creating new protocols. Undue grouping of protocols, even when semantically logical, can produce unwanted distribution coupling. In other words, consider minting new libraries for new protocols to avoid premature inclusion in the SDK.

Consider fuchsia.net.Connectivity as a motivating example: by having been placed in fuchsia.net, its inclusion in the SDK was entangled with that library; it was de-facto stabilized without a formal decision.

Note that fuchsia.net.Connectivity was removed in 5500eec6aa48c65516eae4d5ef4.

Role Separation

The FIDL API Rubric advises:

To decide whether to decompose a library into smaller libraries, consider the following questions:

  • Do the customers for the library break down into separate roles that would want to use a subset of the functionality or declarations in the library? If so, consider breaking the library into separate libraries that target each role.

We prefer a different framing in Networking: separate your API into different roles by breaking it into multiple protocols; separate your API into different stabilization units by breaking into multiple libraries.

Consider fuchsia.netstack/Netstack and fuchsia.net.stack/Stack as motivating examples: neither encode access roles, requiring that any agent that wishes to observe the system state is also granted access to modify it. In practice, this needlessly grants consumers such as Chromium write-access to networking internals.


Avoid introducing APIs that are non-orthogonal to existing APIs. Providing multiple entry points for the same purpose can lead to confusion for consumers and subtle behaviour differences.

Consider fuchsia.netstack/Netstack.GetInterfaces and fuchsia.netstack/Netstack.OnInterfacesChanged as motivating examples: OnInterfacesChanged provides a superset of the functionality of GetInterfaces by emitting a synthetic event upon binding, yet consumers misunderstood and misused the API.

Prior Art and Compatibility

Consider compatibility with existing software when designing APIs. As Fuchsia's product requirements grow in scope, so will the burden of porting software to platform-specific APIs; consider how your API might be translated to an existing API that is widely used in the industry. Furthermore, existing APIs have evolved to provide solutions to problems we may not have considered, it behooves us to learn from their efforts.

The obvious motivating example here is POSIX sockets; fuchsia.posix.socket evolved directly to address this need, complete with the over-fitting described above. At the time of writing, the API transports C structures on the wire directly, creating portability problems and resulting in an ABI that is not fully defined in FIDL.

A less-obvious motivating example is netlink; networking management APIs are not standardized in POSIX, yet netlink is widely used in existing software to interact with Linux's networking subsystems. Both fuchsia.netstack/Netstack and fuchsia.net.stack/Stack were designed without considering equivalent functionality provided by netlink.

Time Pressure

It will sometimes be necessary to provide partners with SDK-exposed APIs on a time scale that does not permit a reasoned general-purpose solution to be designed. Prefer the narrowest possible design that provides for the partner's needs to avoid acquiring unintended consumers. Provide your API in a new protocol to ease future deprecation.

This approach allows us to respond quickly without compromising our design principles in the long term.

Site Survey and Ongoing Work

At present, these are the libraries considered in this document:

  • fuchsia.hardware.ethernet
    • being replaced with fuchsia.hardware.network
    • neither is planned for SDK inclusion
  • fuchsia.net
    • included in SDK
    • contains only common types, no protocols
  • fuchsia.net.dhcp
  • not planned for SDK inclusion
  • fuchsia.net.dhcpv6
    • not planned for SDK inclusion
  • fuchsia.net.filter
    • not planned for SDK inclusion
    • contains only protocol Filter
    • needs rework in consideration of NETLINK_NETFILTER (undocumented?)
  • fuchsia.net.http
    • out of scope of this document, not maintained by networking
  • fuchsia.net.mdns
    • out of scope of this document. do we need to bring it in scope?
  • fuchsia.net.name
    • not included in SDK
    • protocol LookupAdmin is the role-separated pair of Lookup
  • fuchsia.net.neighbor
    • not planned for SDK inclusion
  • fuchsia.net.routes
    • included in SDK
    • narrow route+MAC resolution API in service of partner needs
  • fuchsia.net.stack
    • not included in SDK
    • broad general-purpose protocol Stack does not employ role separation, should be decomposed into several protocols after consulting industry standards
    • protocol Log to be replaced by inspect in https://fxbug.dev/42432
  • fuchsia.net.tun
    • not included in SDK
    • ownership-based
  • fuchsia.netstack
    • not included in SDK
    • removal tracked in https://fxbug.dev/21222
    • contains few remaining methods, most have been moved to fuchsia.net.stack
  • fuchsia.posix.socket
    • not included in SDK
    • used via fdio which is included in SDK