Kamailio routing with rtjson and http_async_client

11 November 2019

Kamailio routing with RTJSON and HTTP async client

The problem

In the Wazo Platform C4 we are committed to delivering the most flexible, as well as easy to configure and set up, Softswitch in the market. The SIP router key component is provided by Kamailio, the leading open-source SIP server.

Our goal was while supporting high degrees of flexibility and ease to configure, to avoid any compromise regarding performances: we aim to support a high number of concurrent calls, high call rates with predictable and linear degradation of performances in case of overload.

The solution

To provide flexible, API-driven routing features in the Wazo Platform C4 Softswitch we decided to implement it on the foundations of the rtjson module. This module not only allows you to push the routing destination URI and the outbound proxy, but it also supports the normalization of the From and To headers, the insertion of additional headers as well as various settings related to the transaction management and its timers.

Together with the http_async_client Kamailio module, it offers a perfect solution to manage very complex and dynamic routing rules of SIP messages delegating the routing logic to an external, HTTP-based web service. The biggest advantage of http_async_client is that, as the name suggests, it is asynchronous: the HTTP request to the API server doesn't block the Kamailio worker process which is immediately released, ready to serve subsequent SIP message requests. As soon as the HTTP request completes and the http_async_client worker process managing it collects the response, it resumes the Kamailio transaction and calls the Kamailio route designated to manage the API responses, resuming the processing of the SIP message and its transaction.

In order to parse JSON response provided by the API we use the ubiquitous jansson module, extracting the rtjson payload from the JSON response of the API.


Kamailio side

The Wazo C4 Router is the component responsible for routing SIP messages in the Wazo class4 infrastructure. The relay of the SIP messages to be routed is managed by the RELAY_API route, defined as follows:

route[RELAY_API] {
    $http_req(all) = $null;
    $http_req(suspend) = 1;
    $http_req(timeout) = HTTP_API_TIMEOUT;
    $http_req(method) = "POST";
    $http_req(hdr) = "Content-Type: application/json";
    jansson_set("string", "event", "sip-routing", "$var(http_routing_query)");
    jansson_set("string", "source_ip", $avp(source_ip), "$var(http_routing_query)");
    jansson_set("int", "source_port", $avp(source_port), "$var(http_routing_query)");
    jansson_set("int", "auth", 0, "$var(http_routing_query)");
    jansson_set("string", "call_id", $ci, "$var(http_routing_query)");
    jansson_set("string", "from_name", $fn, "$var(http_routing_query)");
    jansson_set("string", "from_uri", $fu, "$var(http_routing_query)");
    jansson_set("string", "from_tag", $ft, "$var(http_routing_query)");
    jansson_set("string", "to_name", $tn, "$var(http_routing_query)");
    jansson_set("string", "to_uri", $tu, "$var(http_routing_query)");
    jansson_set("string", "to_tag", $tt, "$var(http_routing_query)");
    xlog("L_INFO","API ASYNC ROUTING REQUEST: $var(http_routing_query)\n");
    $http_req(body) = $var(http_routing_query);

The RELAY_API route creates an asynchronous HTTP request, processed by the http_async_query function provided by the http_async_module, building a JSON object containing all the information needed by the API to return the routing result according to the rtjson format.

We specify the current request shall be suspended ($http_req(suspend) = 1) and resumed in the RELAY_API_RESPONSE route when the response from API has been collected.

The response route is straightforward:

    if ($http_ok && $http_rs == 200) {
        xlog("L_INFO","API ROUTING RESPONSE: $http_rb\n");
        if (jansson_get("rtjson", $http_rb, "$var(rtjson)")) {
            if (!jansson_get("success", $var(rtjson), "$var(success)") || !$var(success)) {
                send_reply(404, "Not found");
            } else {
                # relay the message
    send_reply(500, "API Not Available");

If the status code of the response is 200 Ok and the response is a valid JSON containing the rtjson key, the corresponding value is pushed as routing configuration using the rtjson_init_routes and rtjson_push_routes functions from the rtjson module.

We also support rtjson-based routing in outgoing branches thanks to the following branch_route:

branch_route[MANAGE_BRANCH] {
    xdbg("new branch [$T_branch_idx] to $ru\n");

In case of branch failure, if the rtjson response contained more than one possible destination and we are using serial routing (as we do), we can try the next destination:

failure_route[MANAGE_FAILURE] {
    if (t_is_canceled()) {
    if (rtjson_next_route()) {

API side

The primary programming language at Wazo is Python, thus it was an obvious choice to develop the API for the Kamailio Router using it. We decided to use FastAPI, a modern, high-performance, Python 3.6+ web framework to build APIs. It is built on the top of asyncio and for I/O bound processes it promises similar performances to the ones you can achieve with golang, for instance.

Our web service uses a PostgreSQL to store the routing configuration and exposes not only the routing end-point but also the end-points needed to configure the Wazo C4 Router instances.

The Wazo Router Confd web service provides the /kamailio/rtjson end-point which is queried by the aforementioned RELAY_API Kamailio route.

Here you can find an example of the response returned by the API:

  "rtjson": {
    "success": true,
    "version": "1.0",
    "routing": "serial",
    "routes": [
        "dst_uri": "sip:carrier:5060",
        "path": "",
        "socket": "",
        "headers": {
          "from": {
            "display": "Caller",
            "uri": "sip:caller@domain.com"
          "to": {
            "display": "Callee",
            "uri": "sip:callee@domain.com"
          "extra": ""
        "branch_flags": 8,
        "fr_timer": 5000,
        "fr_inv_timer": 30000

We use serial routing, which means the routes are tried by Kamailio sequentially, one by one. Additional settings (eg. success) are ignored by the rtjson module and can be used to provide Kamailio with additional data fields related to the routing request.


After several stress test sessions in our C4 lab, we determined the overhead of the rtjson-based routing using the async_http_client module is negligible.

The main advantages of moving complex routing logic out of Kamailio to external APIs are:

  • Ease of unit testing the routing logic
  • More powerful and rich programming language than the Kamailio config file
  • Possibility to scale independently the SIP routing component (Kamailio) and the API web service (FastAPI/Python), in a true micro-service architecture.

The Wazo Platform C4 is in continuous development to offer: our goal is delivering a true cloud-native telecom solution with strong and solid foundations. Stay tuned!