Ice in turn makes use of turn defined in rfc 5766 60

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ICE in turn makes use of TURN, defined in RFC 5766 [60], which is a protocol for relaying media through a relay server. An IMS client that supports ICE will allocate relayed candidates at the TURN server and include the candidate information in the SDP offer/answer sent to the peer. The relayed candidates will be used as a last resort when the client and peer fail to establish a direct communication path. The communication between the client and the TURN server (this includes both the relayed media and the control information needed to setup the relayed candidates) can occur over UDP, TCP or TCP/TLS. By using TCP/TLS on port 443 (HTTPS) or TCP on port 80 (HTTP) the communication will appear as HTTP(S) to firewalls and will (typically) be allowed through. Using TCP instead of TLS/TCP reduces the overhead but will fail when the firewall performs DPI or if an HTTP proxy is present. An IMS client may be configured to use both TURN over TCP/80 and TURN over TLS/443, in such case, the client should prefer to use TURN over TCP/80 to avoid TLS overhead. ICE and TURN have later on been extended to also support TCP based media. ICE TCP is defined in RFC 6544 [59] and TURN TCP is defined in RFC 6062 [61]. One of the changes introduced in TURN TCP is that the multiple TCP connections are established between the client and TURN server: one for exchange of control information and one for each relayed TCP based media stream. All UDP based media streams are relayed over the same TCP connection that is used for the control information, just as in the original TURN protocol. The TURN server will use TCP/TLS on port 443 (HTTPS) or TCP over port 80 (HTTP) for all the connections. In order to reduce the TLS setup time when several TCP connections are established, the IMS client and TURN server may use the TLS session resumption feature. An IMS client that is configured to use an HTTP proxy uses the HTTP CONNECT method (see RFC 2817 [62]) to request the proxy to establish a TCP connection with the TURN server. Once the client has received a positive reply from the proxy that the TCP connection has been established, the client initates the TLS handshake with the TURN 3GPP 3GPP TS 33.203 V12.67.0 (2014-0609) 125 Release 12
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server and establishes the TLS tunnel. This procedure is repeated once for every TCP connection the client establishes with the TURN server. Note that the use of the HTTP CONNECT method is completely transparent to TURN server. Using ICE for firewall traversal is particularly suitable for IMS clients that already implement ICE for NAT traversal, since in this case only minimal changes are required to the client. Usage of ICE for IMS clients is specified in TS 23.228 [3] and TS 24.229 [8]. Note that there is no need to specify any keep-alive mechanism since this functionality is already included in ICE. The IMS client will send regular STUN keep-alives which ensures that the firewall pinholes are maintained.
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