WebSocket Module

Peter Dunkley

Crocodile RCS Ltd

Table of Contents

1. Admin Guide
1. Overview
2. How it works
2.1. Initiating a connection
2.2. SIP message routing
2.3. MSRP message routing
3. Dependencies
3.1. Kamailio Modules
3.2. External Libraries or Applications
4. Parameters
4.1. keepalive_mechanism (integer)
4.2. keepalive_timeout (integer)
4.3. keepalive_processes (integer)
4.4. keepalive_interval (integer)
4.5. ping_application_data (string)
4.6. sub_protocols (integer)
4.7. cors_mode (integer)
4.8. verbose_list (int)
5. Functions
5.1. ws_handle_handshake()
5.2. ws_close([status, reason[, connection_id]])
6. RPC Commands
6.1. ws.dump
6.2. ws.close
6.3. ws.ping
6.4. ws.pong
6.5. ws.disable
6.6. ws.enable
7. Event routes
7.1. websocket:closed
8. Exported Pseudo Variables
8.1. $ws_conid

List of Examples

1.1. event_route[xhttp:request]
1.2. WebSocket SIP Routing
1.3. Set keepalive_mechanism parameter
1.4. Set keepalive_timeout parameter
1.5. Set keepalive_processes parameter
1.6. Set keepalive_interval parameter
1.7. Set ping_application_data parameter
1.8. Set sub_protocols parameter
1.9. Set cors_mode parameter
1.10. Set verbose_list parameter
1.11. ws_handle_handshake usage
1.12. ws_close usage
1.13. event_route[websocket:closed] usage
1.14. $ws_conid usage

Chapter 1. Admin Guide

1. Overview

This module implements a WebSocket (RFC 6455) server and provides connection establishment (handshaking), management (including connection keep-alive), and framing for the SIP and MSRP WebSocket sub-protocols (RFC 7118 and RFC 7977).

The module supports WebSockets (ws) and secure WebSockets (wss) transports

2. How it works

2.1. Initiating a connection

A WebSocket connection is initiated with an HTTP GET. The xhttp module is used to handle this GET and call the Section 5.1, “ ws_handle_handshake() exported function.

event_route[xhttp:request] should perform some validation of the HTTP headers before calling Section 5.1, “ ws_handle_handshake(). The event_route can also be used to make sure the HTTP GET has the correct URI, perform HTTP authentication on the WebSocket connection, and check the Origin header (RFC 6454) to ensure a browser-based SIP UA or MSRP client has been downloaded from the correct location.

Example 1.1. event_route[xhttp:request]

...
loadmodule "sl.so"
loadmodule "xhttp.so"
loadmodule "msrp.so"  # Only required if using MSRP over WebSockets
loadmodule "websocket.so"
...
event_route[xhttp:request] {
        set_reply_close();
        set_reply_no_connect();

        if ($Rp != 80
#!ifdef WITH_TLS
            && $Rp != 443
#!endif
        ) {

                xlog("L_WARN", "HTTP request received on $Rp\n");
                xhttp_reply("403", "Forbidden", "", "");
                exit;
        }

        xlog("L_DBG", "HTTP Request Received\n");

        if ($hdr(Upgrade)=~"websocket"
                        && $hdr(Connection)=~"Upgrade"
                        && $rm=~"GET") {

                # Validate Host - make sure the client is using the correct
                # alias for WebSockets
                if ($hdr(Host) == $null || !is_myself("sip:" + $hdr(Host))) {
                        xlog("L_WARN", "Bad host $hdr(Host)\n");
                        xhttp_reply("403", "Forbidden", "", "");
                        exit;
                }

                # Optional... validate Origin - make sure the client is from an
                # authorised website.  For example,
                #
                # if ($hdr(Origin) != "http://communicator.MY_DOMAIN"
                #     && $hdr(Origin) != "https://communicator.MY_DOMAIN") {
                #       xlog("L_WARN", "Unauthorised client $hdr(Origin)\n");
                #       xhttp_reply("403", "Forbidden", "", "");
                #       exit;
                # }

                # Optional... perform HTTP authentication

                # ws_handle_handshake() exits (no further configuration file
                # processing of the request) when complete.
                if (ws_handle_handshake())
		{
			# Optional... cache some information about the
			# successful connection
			exit;
		}
        }

        xhttp_reply("404", "Not found", "", "");
}
...

2.2. SIP message routing

SIP over WebSockets uses invalid URIs in routing headers (Contact:, Record-Route:, and Via:) because a JavaScript stack running in a browser has no way to determine the local address from which the WebSocket connection is made. This means that the routing headers cannot be used for request or response routing in the normal manner.

RFC 7118 - The WebSocket Protocol as a Transport for the Session Initiation Protocol - states that SIP WebSocket Clients and the SIP registrar should implement SIP Outbound (RFC 5626) and Path header support (RFC 3327) to enable requests and responses to be correctly routed. Kamailio has a module called "Outbound" for this functionality.

The nathelper module functions (nat_uac_test(), fix_nated_register(), add_contact_alias(), and handle_ruri_alias()) and the Kamailio core force_rport() can be used to ensure correct routing of SIP WebSocket requests without using Outbound and Path.

Example 1.2. WebSocket SIP Routing

...
loadmodule "sl.so"
loadmodule "tm.so"
...
loadmodule "websocket.so"
...
request_route {

        # per request initial checks
        route(REQINIT);

        if (nat_uac_test(64)) {
                # Do NAT traversal stuff for requests from a WebSocket
                # connection - even if it is not behind a NAT!
                # This won't be needed in the future if Kamailio and the
                # WebSocket client support Outbound and Path.
                force_rport();
                if (is_method("REGISTER"))
                        fix_nated_register();
                else {
                        if (!add_contact_alias()) {
                                xlog("L_ERR", "Error aliasing contact <$ct>\n");
                                sl_send_reply("400", "Bad Request");
                                exit;
                        }
                }
        }

        if (!is_method("REGISTER"))
                t_on_reply("WS_REPLY");
...
# Handle requests within SIP dialogs
route[WITHINDLG] {
        if (has_totag()) {
                # sequential request withing a dialog should
                # take the path determined by record-routing
                if (loose_route()) {
                        if ($du == "") {
                                if (!handle_ruri_alias()) {
                                        xlog("L_ERR", "Bad alias <$ru>\n");
                                        sl_send_reply("400", "Bad Request");
                                        exit;
                                }
                        }
                        route(RELAY);
                } else {
                        if ( is_method("ACK") ) {
...
onreply_route[WS_REPLY] {
        if (nat_uac_test(64)) {
                # Do NAT traversal stuff for replies to a WebSocket connection
		# - even if it is not behind a NAT!
                # This won't be needed in the future if Kamailio and the
		# WebSocket client support Outbound and Path.
                add_contact_alias();
        }
}
...

2.3. MSRP message routing

MSRP over WebSocket clients create invalid local URIs for use in Path headers (From-Path: and To-Path:) because a JavaScript stack running in a browser has no way to determine the local address from which the WebSocket connection is made. This is OK because MSRP over WebSocket clients MUST use an MSRP relay and it is the MSRP relay's responsibility to select the correct connection to the client based on the MSRP URIs that it has created (and maintains a mapping for).

3. Dependencies

3.1. Kamailio Modules

The following module must be loaded before this module:

  • sl.

  • tm.

The following modules are required to make proper use of this module:

  • nathelper or outbound.

  • xhttp.

The following module is required to use the secure WebSocket (wss) scheme:

  • tls.

The following module is required to support MSRP over WebSockets:

  • msrp.

3.2. External Libraries or Applications

The following libraries must be installed before running Kamailio with this module loaded:

  • OpenSSL.

  • GNU libunistring.

4. Parameters

4.1. keepalive_mechanism (integer)

The keep-alive mechanism to use for WebSocket connections.

Note

If nathelper is only being used for WebSocket connections then nathelper NAT pinging is not required. If nathelper is used for WebSocket connections and TCP/TLS aliasing/NAT-traversal then WebSocket keep-alives are not required.

  • 0 - no WebSocket keep-alives

  • 1 - Ping WebSocket keep-alives

  • 2 - Pong WebSocket keep-alives

Default value is 1.

Example 1.3. Set keepalive_mechanism parameter

...
modparam("websocket", "keepalive_mechanism", 0)
...

4.2. keepalive_timeout (integer)

The time (in seconds) after which to send a keep-alive on idle WebSocket connections.

Default value is 180.

Example 1.4. Set keepalive_timeout parameter

...
modparam("websocket", "keepalive_timeout", 30)
...

4.3. keepalive_processes (integer)

The number of processes to start to perform WebSocket connection keep-alives.

Default value is 1.

Example 1.5. Set keepalive_processes parameter

...
modparam("websocket", "keepalive_processes", 2)
...

4.4. keepalive_interval (integer)

The number of seconds between each keep-alive process run

Default value is 1.

Example 1.6. Set keepalive_interval parameter

...
modparam("websocket", "keepalive_interval", 2)
...

4.5. ping_application_data (string)

The application data to use in keep-alive Ping and Pong frames.

Default value is Kamailio Server: header content

Example 1.7. Set ping_application_data parameter

...
modparam("websocket", "ping_application_data", "WebSockets rock")
...

4.6. sub_protocols (integer)

A bitmap that allows you to control the sub-protocols supported by the WebSocket server.

  • 1 - sip (RFC 7118)

  • 2 - msrp (RFC 7977) - msrp.so must be loaded before websocket.so

Default value is 1 when msrp.so is not loaded 3 when msrp.so is loaded.

Example 1.8. Set sub_protocols parameter

...
modparam("websocket", "sub_protocols", 2)
...

4.7. cors_mode (integer)

This parameter lets you set the "Cross-origin resource sharing" behaviour of the WebSocket server.

  • 0 - Do not add an "Access-Control-Allow-Origin:" header to the response accepting the WebSocket handshake.

  • 1 - Add a "Access-Control-Allow-Origin: *" header to the response accepting the WebSocket handshake.

  • 2 - Add a "Access-Control-Allow-Origin:" header containing the same body as the "Origin:" header from the request to the response accepting the WebSocket handshake. If there is no "Origin:" header in the request no header will be added to the response.

Default value is 0.

Example 1.9. Set cors_mode parameter

...
modparam("websocket", "cors_mode", 2)
...

4.8. verbose_list (int)

Allows to enable/disable the printing of debug messages when getting the list of websocket connections. If enabled, it prints debug messages every second for ping operations.

Default value is 0 (disabled).

Example 1.10. Set verbose_list parameter

...
modparam("websocket", "verbose_list", 1)
...

5. Functions

5.1.  ws_handle_handshake()

This function checks an HTTP GET request for the required headers and values, and (if successful) upgrades the connection from HTTP to WebSocket.

This function can be used from ANY_ROUTE (but will only work in event_route[xhttp:request]).

Note

This function returns 0, stopping all further processing of the request, when there is a problem.

Example 1.11. ws_handle_handshake usage

...
ws_handle_handshake();
...

5.2.  ws_close([status, reason[, connection_id]])

This function closes a WebSocket connection.

The function returns -1 if there is an error and 1 if it succeeds.

The meaning of the parameters is as follows:

  • status - an integer indicating the reason for closure.

  • reason - a string describing the reason for closure.

  • connection_id - the connection to close. If not specified the connection the current message arrived on will be closed.

Note

status and reason values SHOULD correspond to the definitions in section 7.4 of RFC 6455. If these parameters are not used the defaults of "1000" and "Normal closure" will be used.

This function can be used from ANY_ROUTE.

Example 1.12. ws_close usage

...
ws_close(4000, "Because I say so");
...

6. RPC Commands

6.1. ws.dump

Provides the details of the first 50 WebSocket connections.

Name: ws.dump

Parameters:

  • order (optional) - id_hash, used_desc, or used_asc.

Note

If no parameter is provided id_hash order is used.

RPC Command Usage:

...
kamcmd ws.dump used_asc
...

6.2. ws.close

Starts the close handshake for the specified WebSocket connection.

Name: ws.close

Parameters:

  • id - WebSocket connection ID.

RPC Command Usage:

...
kamcmd ws.close: 1
...

6.3. ws.ping

Sends a Ping frame on the specified WebSocket connection.

Name: ws.ping

Parameters:

  • id - WebSocket connection ID.

RPC Command Usage:

...
kamcmd ws.ping 1
...

6.4. ws.pong

Sends a Pong frame on the specified WebSocket connection.

Name: ws.pong

Parameters:

  • id - WebSocket connection ID.

RPC Command Usage:

...
kamcmd ws.pong 1
...

6.5. ws.disable

Disables WebSockets preventing new connections from being established.

Name: ws.disable

Parameters: none

RPC Command Usage:

...
kamcmd ws.disable
...

6.6. ws.enable

Enables WebSockets allowing new connections to be established.

Note

WebSockets are enabled at start-up.

Name: ws.enable

Parameters: none

RPC Command Format:

...
kamcmd ws.enable
...

7. Event routes

7.1.  websocket:closed

When defined, the module calls event_route[websocket:closed] when a connection closes. The connection may be identified using the the $si and $sp pseudo-variables.

Example 1.13. event_route[websocket:closed] usage

...
event_route[websocket:closed] {
	xlog("L_INFO", "WebSocket connection from $si:$sp has closed\n");
}
...

8. Exported Pseudo Variables

8.1. $ws_conid

Connection id of closed websocket connection. Can only be used in websocket:closed event route.

Example 1.14. $ws_conid usage

...
event_route[websocket:closed] {
        xlog("L_INFO", "WebSocket connection with id $ws_conid has closed\n");
}
...