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Adrian Georgescu, 02/18/2010 07:13 pm


= ICE Negotiation through OpenSIPS/Mediaproxy =

Background

[http://www.ietf.org/id/draft-ietf-mmusic-ice-19.txt ICE methodology] has been defined to solve the NAT traversal issue of RTP media as much as possible in the SIP end-points. Where the end-point topology does not allow, a TURN server has been standardized to provide end-points with a visible public presence that can be used to relay traffic to SIP 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'''.

MediaProxy module rewrites the SDP in order to present to the destination SIP User Agent a reachable IP:port number for sending RTP/RTCP. This has the dis-advantage that is always using a relay and is mutually exclusive with the use of ICE. The rewriting of the SDP and the missing candiate correspondent to the c= line of the SDP causes the target SIP end-point to reply with ICE missmatch and the ICE negotiation fails in its tracks.

Implementation

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

  • Creating a proper candidate based on the media relay IP:port allocation
  • Appending the candidate to the offer/answer of the SDP
  • Allows an administrator to set a policy about when to use of the relay candidate

OpenSIPS mediaproxy module has a new setting:

{{{
ice_priority_avp=$avp(s:ice_priority)
}}}

This module parameter defines which AVP should be used to set the ICE candidates priority. It can take 3 different values: "none", "low" or "high". None means no candidate will be added at all, and low or high affect on the priority field of the ICE candidate being inserted. Example:

{{{
route {
...
$avp(s:ice_priority) := "low";
...
}
}}}

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

{{{
if (ice_priority_avp != none and
SDP offer contains a=ice-pwd and a=ice-ufrag and a=candidate line(s) then
append to the SDP the following line:
a=candidate:R 1 UDP PRIORITY MP_IP MP_PORT typ relay
a=candidate:R 2 UDP PRIORITY MP_IP MP_RTCP_PORT typ relay
}}}

where

  • R (foundation) = we'll use 'R' to represent we are emulating 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_priority) low and POLICY = 126 if $avp(s:ice_priority) high.

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.