FleXoSIP — A comfortable High-Level SIP API to eXosip and osip

The goal of FleXoSIP — Fast Lane for eXosip is to make eXosip and osip easy to use for common tasks involving a single inbound or outbound voice channel.

Even though the name might imply flexibility, it does not add any flexibility, as eXosip and osip already provide it all. Instead, it limits the use cases, trying to make the interface easier. Unfortunately, I did not find anything pronouncible denoting comfort and ending in -exo.

It uses eXosip, oRTP, and libsndfile to provide basic single-channel SIP functionality for building your own simple devices and/or applications.

For your convenience, here are also links to the source code:

Installation: Preconditions and building

To create the library, just run make. However, installing the dependencies beforehand is — depending on the distribution you are running — more complicated:

Installation on Ubuntu 20.04

The eXoSIP library was dropped from the repository, so you need to download or git clone it yourself and then compile and install it yourself.

But first, we must install the dependent libraries:

sudo apt install libosip2-dev libc-ares-dev libortp-dev libsndfile1-dev libinih-dev automake

Building is complicated by the fact that libosip2 shipped in an OS released in 2020 is still version 4.1.0 from 2013, so we need to check out a compatible version as well…

git clone https://git.savannah.nongnu.org/git/exosip.git
cd exosip && git checkout 4.1.0 && ./configure && make && make install

Installation on Ubuntu 18.04/18.10/19.10

sudo apt install libexosip2-dev libc-ares-dev libortp-dev libsndfile1-dev libinih-dev

Installation on Raspbian Buster

sudo apt install libexosip2-dev libc-ares-dev libortp-dev libsndfile1-dev libinih-dev

Installation on Raspbian Stretch

libinih is not part of Debian Stretch (and thus, Raspbian Stretch). Having the source code in an inih/ subsubdirectory will build and link it as part of make in this directory.

sudo apt install libexosip2-dev libc-ares-dev libortp-dev libsndfile1-dev
git clone https://github.com/benhoyt/inih

Optional: If you also want to make inih available system-wide, you can run:

sudo apt install meson
cd inih && meson setup build -Ddistro_install=true && cd build && sudo ninja install

Why another SIP library?

I was looking for a simple, high-level SIP library for an ToT (Telephony of Things 😉) device, a motion detector who could be controlled by SIP calls and would call back on events. (The result is cowbell.) If you insist, you may also call it an IoT (Internet of Things) device…

Inspired by the ease of a solution based on Asterisk, I have been trying to use PJSIP (PJSUA2) with Python, but I never got beyond crashes. Nothing else seemed to be available in Python, so I looked for a plain C solution. I found eXosip, but couldn't make head or tails of the documentation and did not consider the API to be newbie-friendly. So I decided to start my own library, flexosip, following a simple API of

Register to the PBX
Make a call or accept a call (only one call at a time)
Send audio files
Receive DTMF tones
Hang up

This is all I needed and I guess it brings ToT devices a big step ahead (and also helps the beginner to softly make his feet wet with the complex world of SIP, SDP, and RDP).

API

The API is described in API.md

Bugs

Currently only handles IPv4 addresses (partly, because global c=/o= parameters do not allow multiple addresses)
DTMF digits are only sent/received in outdated INFO format, not in RTP telephone-event format
Does not yet do audio format negotiation (requires the caller to be able to receive PCMA/8000 aka A-Law 8 kHz audio)

MarcelWaldvogel / flexosip