### Description

Newer rtpengine versions support manipulating SDP "a=" lines directly. Although kamailio is quite versatile when it comes to editing SIP message body, this functionality is often rather frail, especially when forking and having to use msg_apply_changes several times. I believe it would be beneficial to be able to offload such functionality to rtpengine, especially if one wants to apply different manipulations per outgoing branch.

### Expected behavior There should be a way to issue rtpengine ng-control protocol commands compatible with the sdp-attr dictionary syntax.

#### Actual observed behavior Currently sdp-attr tokens are not properly evaluated. For example, **doing**:

rtpengine_manage("ICE=remove rtcp-mux-demux trust-address replace-origin replace-session-connection replace-SDP-version direction=internal direction=external sdp-attr-audio-substitute=$avp(fmtp_line) sdp-attr-audio-substitute=fmtp:101 0-15");

_[NOTE: $avp(fmtp_line) seems to expand in empty string here, which is a config error, but it doesn't affect the syntax demonstration in this example IMO]_ **results in**: ``` { "supports": [ "load limit" ], "sdp": "...", "ICE": "remove", "sdp-attr-audio-substitute": "", "sdp-attr-audio-substitute": "fmtp:101", "direction": [ "internal", "external" ], "flags": [ "trust-address", "0-15" ], "replace": [ "origin", "session-connection", "SDP-version" ], "rtcp-mux": [ "demux" ], "call-id": "...", "received-from": [ "IP4", "..." ], "from-tag": "...", "to-tag": "...", "command": "answer" } ```

### Possible Solutions Support the special syntax of sdp-attr as documented here: https://github.com/sipwise/rtpengine/blob/master/docs/ng_control_protocol.md

Here's one way to do it (I guess). In order to get this: ``` "sdp-attr" : { "audio" : { "add" : [ "ptime:20", "sendrecv" ], "substitute": [["fmtp:101 0-15" , "fmtp:126 0-16" ]] }, "video": { "remove" : [ "rtpmap:101 telephone-event/8000" ] }, "none" : { "substitute": [[ "sendrecv" , "sendonly" ], [ "ptime:20" , "ptime:40" ]] } } ``` Use a syntax similar to the following:

rtpengine_manage("... sdp-attr-audio-add=ptime:20 sdp-attr-audio-add=sendrecv sdp-attr-audio-substitute=fmtp:101 0-15 sdp-attr-audio-substitute=fmtp:101 0-16 sdp-attr-video-remove=rtpmap:101 telephone-event/8000 sdp-attr-none-substitute=sendrecv sdp-attr-none-substitute=sendonly sdp-attr-none-substitute=ptime:20 sdp-attr-none-substitute=ptime:40 ...");

It's not very pretty, but it could work. Caveats: * How to handle whitespace (e.g. there's a space in "fmtp:101 0-15" and in "rtpmap:101 telephone-event/8000" ) * substitute commands must always be in pairs, data type is a list of lists containing exactly two items as value in "substitute" key

Unfortunately my C skills are not up to this task, but if I can provide any other kind of help please let me know. Thanks!