Serval Mesh app for FirefoxOS

FirefoxOS is an attractive platform for Serval Mesh (app for Android) because it targets low-cost mobile devices and is likely to be adopted rapidly by vendors and carriers in developing countries who seek to avoid the compliance and commercial obligations imposed by Google, Apple and other platform suppliers.

A port of the Serval Mesh app to FirefoxOS would pose the following technical challenges:

  • rewrite the user interface in HTML5 using FirefoxOS API (on Android is written entirely in Java/XML using the Android API)
  • port the Serval DNA daemon to FirefoxOS (see below)
  • the following interactions between Serval Mesh and FirefoxOS would have to be made usable:
    • dialler screen for making and receiving calls
    • contact list for managing known Serval Identities
    • text message sending and receiving
    • app manager for auto-upgrade via Rhizome
    • gallery and camera for sending and receiving photos via Rhizome
    • file manager for sending and receiving files via Rhizome

Serval DNA on FirefoxOS

The Serval DNA component cannot be ported to FirefoxOS as a native Linux executable because FirefoxOS does not and will never provide a native code facility.

Port using Emscripten

It may be possible to translate the Serval DNA C source code to JavaScript using Emscripten. The following technical issues would have to be surmounted:

Network access

Serval DNA broadcasts its Mesh Datagram Protocol (MDP) packets over Wi-Fi by encapsulating them in UDP/IP packets. Without packet broadcast, the Serval Mesh cannot work. The socket code would have to be rewritten to use the proposed FirefoxOS UDP Datagram Socket API, which was in early planning stages as at September 2013.

Daemon process

Serval DNA uses Posix process control system calls like fork(2), _exit(2) and signal(2) to create and control a background daemon process. These are probably not available on FirefoxOS. Instead, the FirefoxOS Background Services API (only in proposal stage as at September 2013) would have to be used, necessitating a rewrite of the daemon control logic.

Persistent storage

Serval DNA uses Posix file system calls such as open(2) for persistent storage of the keyring file, configuration, logging, and temporary storage of Rhizome payloads. These would have to be rewritten to use the FirefoxOS Device Storage API.

Inter-process communication

Serval DNA uses local filesystem-bound sockets and named pipes for communication between client and daemon processes. These would have to be rewritten to use abstract local sockets (if supported) or local port numbers (if supported) or some other IPC mechanism not yet proposed (as at September 2013) in FirefoxOS.

SQLite

Serval DNA uses SQLite for persistent storage of Rhizome bundles. This presents several issues:

  • On Android, the SQLite library contributes signficantly to Serval Mesh's APK binary size and consumes a great deal of CPU and RAM overhead with its data indexing and consistency. The SQLite library may not scale well to low-end devices if compiled with Emscripten and executed as JavaScript.
  • SQLite may depend on certain Posix file operations (eg, open(2) O_EXCL and fcntl(2) advisory locking) that are not supported by the FirefoxOS Device Storage API.
Crypto

Cryptographic operations such as signature verification and Rhizome payload encryption/decryption are performed on Android using the native NaCl library which has been optimised for the ARM processor. This optimisation brings Serval's security features within reach of low-end devices. The Emscripten compiled code has known speed limitations, so how much CPU overhead would that introduce to crypographic operations? Would that make Serval Mesh for FirefoxOS unusable in practical terms due to slowness and battery drain?

Test suite

Any port of Serval DNA is controlled for quality using the automated test suite, which is a set of Bash scripts executed natively on the target platform. This presents an enormous challenge for porting to FirefoxOS, as the test suite relies on a close-to-standard GNU/Posix command-line environment, which may be impossible to achieve under FirefoxOS. The Emscripten JavaScript port of Serval DNA would have to be invokable via the normal Serval DNA command-line interface, from within an Emscripten port of Bash running the test framework script within a Posix environment that may include Emscripten ports of standard Posix utilities like grep, sed and awk.

The most likely solution would be to re-implement all or most of the test cases using a test framework built specifically for FirefoxOS.

Packet code

The Mesh Datagram Protocol (MDP) packet assembly/disassembly code functions correctly even when ported to different-endian architectures (ARM, Sparc, Intel 32-bit and 64-bit) but it may not work correctly when compiled by Emscripten. Extensive testing on a heterogeneous network would be needed, possibly followed by rewriting of the packet code for Emscripten/JavaScript.


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