More clever RTP transport remote address switch

[bennylp]

Currently the UDP and ICE media transport has the capability to switch destination RTP/RTCP address to the source address of the RTP/RTCP packets, if they are different than the original address set in SDP received from remote.

It may be better to put this functionality in the stream instead, for the following reason:

1. the stream has more information about the packet, such as RTP pt, SSRC, and info from RTCP statistic
2. some application that uses media transport wants to inspect and possibly filter the source address
3. some firewall such as Microsoft Forefront Threat Management Gateway wants us to send the RTP to the address in SDP, regardless of the source address of the RTP packet

[bennylp]

There is a temptation to support multiple callback users, so that multiple applications (e.g. stream and PJSUA-LIB) can peek the incoming RTP/RTCP packets. Some sample usage of this includes:

• supporting STUN keep-alive (done by pjsua-lib)
• packet filtering
• PoC (by capturing RTCP traffic)

Sure this can be done by creating transport adapter, but utilizing multiple callback users will make it lot easier for application (e.g. application doesn't need to handle transport creation in PJSUA-LIB).

Pending for further review. Moving to 1.4 milestone.

[bennylp]

Enhancement of this behavior in #1366

[ming]

In 5752:

Fixed #865: More clever RTP transport remote address switch

The summary of changes:

• To solve no 2:

Add callback rtp_cb2(pjmedia_tp_cb_param *param) which allows application to get more info from the media transport, such as the packet's source address.

• To solve no 3:

Add compile time option PJMEDIA_TRANSPORT_SWITCH_REMOTE_ADDR (by default enabled). Currently, there are already runtime options PJMEDIA_UDP_NO_SRC_ADDR_CHECKING and PJMEDIA_ICE_NO_SRC_ADDR_CHECKING, but there are a few drawbacks:

• the options are not exported to the higher level, such as stream, or pjsua.
• the options are separate for each transport, UDP and ICE, there's no single option to do this.

• To solve no 1:

Using the new rtp_cb2() callback, move the functionality to check the packet's source address to the stream/video stream.
By checking the RTP pt and SSRC, there are a few advantages:

• When receiving packets from multiple sources, stream can choose the packet with the correct SSRC as advertised from the SDP, and discard the others (see also ticket #1366).
• If remote address switch is enabled, a faster switch can be achieved as soon as packet with correct ssrc is received, instead of waiting for several consecutive packets (according to setting PJMEDIA_RTP_NAT_PROBATION_CNT).