Android’s RTP implementation

Although still being not really mature, Android is supporting RTSP streaming for a long time. In theory, it’s very trivial to play an RTSP link with MediaPlayer controller.

MediaPlayer player = new MediaPlayer("rtsp://...");
player.prepare();
player.start();

But in practice, MediaPlayer implementation is not fair enough to give you responses and you basically dont know what’s going on since your media is not playing. I will be generally talking about network layer, so you will have a basic idea how to configure your media servers.

RTSP and RTP

Generally we call it RTSP. But RTSP streaming has two phases: RTSP and mostly RTP to transform actual media data. RTSP is a stateful protocol. While making the first connection, it agrees on a bunch of details and exchanges data about the media being served between client and server. These are done with a family of directives.  These directives are sent on TCP 554. The RTSP flow includes OPTIONS, DESCRIBE, SETUP, PLAY/PAUSE/etc. On the request made for SETUP directive, client specifies what transform protocol it’ll support (in this case, it’s RTP) and on which protocol and which port. Android clients choose UDP and a range starting form 15000 to 65k. This range may change from phone to phone, manufacturer to manufacturer. Summary: There is absolutely no standard at  all. If you look at native MediaPlayer implementation in Android codebase, you will see no specific range as well. So, it’s very likely for you to have trouble. Another bad point is, RTP is usually supported on a port range between 9k-15k on TCP (e.g. Blackberry devices). And if you read tips and tricks about configuring a server, you won’t be able to catch the Android fact.

Note: This post was a draft for about a year, I reviewed it and posted. There’s nothing over-dated according to my practical knowledge. If you are against me, contact me for fixes.

Edit: Some phones fallback to TCP based streaming when UDP is not available.

3 thoughts on “Android’s RTP implementation”

  1. Daniel J. says:

    Insideful again! But isnt UDP too lossy to make it the default streaming protocol? I think the SDK should include libraries to support both and it should be the application’s itself to decide on depending on performance contraints.

    Reply
  2. Burcu Dogan says:

    There is one thing I’m aware of: You wouldnt want to mess up with windowing for a live stream on a mobile connection that is not promised to be up continuously. If you are on a mobile network, you usually face the dropped line. “Slow start” will ruin it better than UDP loss.

    Reply
  3. Juden Crayford says:

    A single question here. Is that means “there are no RTP port range for Android”? Thanks to the level of fragmentation of Android. We spent hours to understand what the problem is.

    Reply

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