An Elixir library designed to write Session Initiation Protocol middleware.

Introduction

SIP is a very flexible protocol that has great depth. It was designed to be a general-purpose way to set up real-time multimedia sessions between groups of participants. It is a text-based protocol modeled on the request/response model used in HTTP. This makes it easy to debug because the messages are relatively easy to construct and easy to see.

Sippet is designed as a simple SIP middleware library, aiming the developer to write any kind of function required to register users, get their availability, check capabilities, setup and manage sessions. On the other hand, Sippet does not intend to provide any feature available in a fully functional SIP UAC/UAS, proxy server, B2BUA, SBC or application; instead, it has only the essential building blocks to build any kind of SIP middleware.

One of the most central parts of Sippet is the Sippet.Message. Instead of many headers that you end up having to parse by yourself, there's an internal parser written in C++ (an Erlang NIF) that does all the hard work for you. This way, the Sippet.Message.headers is a key-value simple Map where the key is the header name, and the value varies accordingly the header type. For instance, the header :cseq has the form {sequence :: integer, method} where the method is an atom with the method name (like :invite).

Other than the Sippet.Message, you will find the Sippet.Transport and the Sippet.Transaction modules, which implement the two standard SIP layers. Message routing is performed just manipulating Sippet.Message headers; everything else is performed by these layers in a very standard way. That means you may not be able to build some non-standard behaviors, like routing the message to a given host that wasn't correctly added to the topmost Via header.

As Sippet is a simple SIP library, the developer has to understand the protocol very well before writing a middleware. This design decision came up because all attempts to hide any inherent SIP complexity by other frameworks have failed.

There is no support for plugins or hooks, these case be implemented easily with Elixir behaviors and macros, and the developer may custom as he likes. Incoming messages and transport errors are directed to a Sippet.Core module behavior.

Finally, there is no support for many different transport protocols; a simple Sippet.Transport.UDP (but still performatic) implementation is provided, which is enough for several SIP middleware apps. Transport protocols can be implemented quite easily using the Sippet.Transport.Plug behavior. In order to optimize the message processing, there's a Sippet.Transport.Queue which receives datagrams, case the transport protocol is datagram-based, or a Sippet.Message.t message, generally performed by stream-based protocols.

Installation

The package can be installed from Hex as:

1. Add sippet to your list of dependencies in mix.exs:

```elixir
def deps do
  [{:sippet, "~> 0.1.8"}]
end
```

1. Ensure sippet is started before your application:

```elixir
def application do
  [applications: [:sippet, :logger, :gen_state_machine, :socket, :poolboy]]
end
```