MediaProxy ICE Support

Background

ICE methodology has been defined at IETF (RFC5245) to help end-points traverse NAT routers for RTP media by finding the shortest available path between the end-points. Where the network topology does not allow for direct media path between the end-points, a TURN server has been standardized to provide the end-points with a visible public candidate that can be used to relay traffic to the end-points behind an unfriendly NAT type (e.g. symmetric). TURN servers are not widely deployed today whereas solutions like SIP Proxies in tandem with RTP relays are.

RTP relays that work in tandem with SIP Proxies perform similar function to the TURN protocol except that the reservation is done by the SIP Proxy rather than by the user agent. The SIP User Agent does not need to support TURN protocol in order to use the relay, which makes it compatible with any SIP User Agent available today. Such solution used widely today is the tandem OpenSIPS/MediaProxy.

Until the addition of this functionality, MediaProxy module used to rewrite the SDP in order to present to the destination SIP User Agent a reachable IP:port number for sending RTP/RTCP. This had the dis-advantage that the session was always using a relay and it was mutually exclusive with the use of ICE. The rewriting of the SDP and the missing candiate correspondent to the c= line of the SDP caused the target SIP end-point to reply with ICE missmatch and the ICE negotiation always failed in its tracks.

Features

By using the current solution, the SIP Proxy administrator can decide whether ICE is allowed and which priority the MediaProxy candidate has on a per call basis. There is no need for deploying a TURN server anymore in order to support ICE negotiation between end-points.

The client needs to implement STUN and the latest ICE specification. Blink is a SIP client that is fully compliant with these requirements.

Implementation

To circumvent the failure of ICE negotiation, MediaProxy module in OpenSIPS is now:

  1. Creating a proper candidate based on the media relay IP:port allocation
  2. Appending the candidate to the offer/answer of the SDP
  3. Allowing an administrator to set a policy about how to use the relay candidate

OpenSIPS mediaproxy module has new settings:

ice_candidate="none" 
ice_candidate_avp="$avp(s:ice_priority)" 

"ice-candidate" defines a global setting whereas "ice_candidate_avp" allows the user to override that value by setting the specified AVP. It can take 3 different values: "none", "low-priority" or "high-priority". None means no candidate will be added at all, and low-priority or high-priority affect on the priority field of the ICE candidate being inserted. Example:

route {
    ...
    $avp(s:ice_candidate) := "low-priority";
    ...
}

For the initial INVITE and its reply, mediaproxy module now appends an ICE candidate to the SDP as follows:

1if ((ice_candidate != none and ice_candidate_avp != none) 
2        and SDP offer contains a=ice-pwd 
3        and a=ice-ufrag and a=candidate line(s) then: 
4        append to the SDP the following line:
5        a=candidate:R(random string) 1 UDP PRIORITY MP_IP MP_PORT typ relay
6        a=candidate:R(random string) 2 UDP PRIORITY MP_IP MP_RTCP_PORT typ relay

where

  • R(random string) = we'll use 'R' plus a random string as the foundation. The random string is the hex representation of the chosen relay IP address. Having the 'R' as the first letter is just a convention to represent a relayed candidate.
  • PRIORITY = 2!^24 * POLICY + 2!^8 * 65535 + 2!^0 * 256 - Candidate ID (1 for RTP, 2 for RTCP)
    • POLICY = 0 if $avp(s:ice_candidate) low-priority and POLICY = 130 if $avp(s:ice_candidate) high-priority. A value of 130 is being used as the high priority candidate type preference, as it will be even higher than host candidates priority.

NOTE: The AVP has precedence over the module parameter, that is, if module parameter is set to low-priority and the AVP is set to none, no candidate will be added.

If ICE negotiation succeeds and the MediaProxy relay candidate has not been selected by the end-points, MediaProxy deallocates its ports after the built-in timeout and does not terminate the dialog in case of missing BYEs. MediaProxy decides this, if both SDP offer and answer have ICE support and if STUN binding requests (which correspond to ICE probing) have been detected while no RTP has been subsequently sent through the relay.

Software versions

You need the following software versions to support ICE:

  • MediaProxy version >=2.4.2
  • OpenSIPS version >= 1.6.0

CDRTool version >=7.1.0 can be used to display the media traces.