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3rd/libomtnet/PROTOCOL.md

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+# Open Media Transport (OMT) Protocol 1.0
+
+## Introduction
+
+This guide should serve as an overview of the protocol used by Open Media Transport.
+
+Implementers should first look at using libomtnet or libomt as these are complete implementations covering the vast majority of platforms.
+
+## Basics
+
+Open Media Transport consists of three major components:
+
+1. TCP Protocol for sending/receiving video, audio and metadata.
+2. Special Metadata commands to control various aspects of the connection.
+3. DNS-SD for discovery (RFC 6763).
+
+## Data Types
+
+All data types are stored in Little-Endian byte order.
+This is in contrast to the often used Big-Endian network order.
+
+## TCP Protocol
+
+All data is encapsulated into a frame consisting of the following parts:
+
+### HEADER (16 bytes)
+
+BYTE Version // Must be 1
+
+BYTE FrameType // Metadata = 1, Video = 2, Audio = 4
+
+INT64 Timestamp // Timestamp where 1 second = 10,000,000
+
+UINT16 MetadataLength // Length of XML UTF-8 per-frame metadata including null character.
+
+INT32 DataLength //ExtendedHeader + Data length + MetadataLength, excluding this header
+
+### VIDEO EXTENDED HEADER (32 bytes) (Mandatory for video frame type)
+
+INT32 Codec //Video codec FourCC
+
+INT32 Width //Video width in pixels
+
+INT32 Height //Video height in pixels
+
+INT32 FrameRateN //Frame rate numerator/denominator in frames per second, for example 60/1 is 60 frames per second.
+
+INT32 FrameRateD 
+
+FLOAT32 AspectRatio //Display aspect ratio expressed as a ratio of width/height. For example 1.777777777777778 for 16/9
+
+INT32 Flags //Interlaced=1, Alpha=2, PreMultiplied=4, Preview=8, HighBitDepth=16
+
+INT32 ColorSpace //Color space flag. 601 for BT601, 709 for BT709, 0 for undefined (typically BT601 for SD, BT709 for HD)
+
+### AUDIO EXTENDED HEADER (24 bytes) (Mandatory for audio frame type)
+
+INT32 Codec //Audio codec FourCC, currently only 'FPA1' is supported which is 32bit floating point planar audio
+
+INT32 SampleRate //Audio sample rate
+
+INT32 SamplesPerChannel //Number of samples per channel stored in this frame
+
+INT32 Channels //Number of channels of audio
+
+UINT32 ActiveChannels //Bit field denoting the number of actual channels stored in this frames data out of the total Channels specified. This is so silent channels can be skipped, saving bandwidth.
+
+INT32 Reserved1 //Reserved for future use
+
+### DATA
+
+The frame data followed by the per-frame metadata
+
+### Latency considerations
+
+To optimize latency and prevent network stalls, implementers should ensure the receiver never blocks when accepting data.
+To achieve this one approach involves keeping a small frame queue with at least one frame reserved for the asynchronous network callback.
+That reserved frame can the be reused repeatedly if the separate decode thread is taking too long to process frames.
+
+## Metadata
+
+Metadata is stored as UTF-8 encoded, null terminated XML data.
+
+DataLength should always include the null character.
+
+Special metadata commands are used to control various aspects of the connection.
+These are fixed strings that must be specified exactly.
+This is an optimization so that the end point can employ simple string matching rather than full XML parsing.
+
+### Subscribe Commands
+
+A sender should not send any data until a subscribe command is received.
+
+\<OMTSubscribe Video="true" /\>
+
+Sent by a receiver to request the sender start sending video frames.
+
+\<OMTSubscribe Audio="true" /\>
+
+Sent by a receiver to request the sender start sending audio frames.
+
+\<OMTSubscribe Metadata="true" /\>
+
+### Preview Commands
+
+\<OMTSettings Preview="true" /\>
+\<OMTSettings Preview="false" /\>
+
+Enable/disable sending preview video data instead of the full resolution frame.
+
+### Tally Commands
+
+\<OMTTally Preview="true" Program="false" /\>
+\<OMTTally Preview="false" Program="true" /\>
+\<OMTTally Preview="true" Program="true" /\>
+\<OMTTally Preview="false" Program="false" /\>
+
+Sent by a receiver to indicate tally status.
+
+The sender should then combine this tally status with those set by other receivers and then broadcast this new combined tally to all receivers.
+
+This tally should also be sent to any new connections as well.
+
+### Suggested Quality
+
+\<OMTSettings Quality="Default" /\>
+
+Sent by receivers to indicate the preferred compression quality:
+
+Default,
+Low,
+Medium,
+High
+
+Senders may respond to this by gathering the preferred quality of all receivers, determining the highest quality requested and then adjusting the encoder to match.
+
+Default is Medium quality.
+
+### Sender Information
+
+\<OMTInfo ProductName="MyProduct" Manufacturer="MyCompany" Version="1.0" /\>
+
+Senders can optionally send information about the encoder to receivers when connected.
+
+## DNS-SD
+
+Open Media Transport uses the service type _omt._tcp
+The port should be the tcp port that sender is listening on
+The full service name should take the form HOSTNAME (Source Name)._omt._tcp.local
+
+## Discovery Server
+
+Discovery Server uses the same communication protocol and frame headers to send and receive XML data.
+
+The server does the following:
+
+1. Keep track of register/deregister XML requests from each client
+2. Determine the IP address to use for each registered source based on the client's connection ip.
+3. Repeat registered requests to all connected clients, including the client that submitted the request.
+4. Repeat all current registration requests to new clients.
+5. Remove all requests from a client that has disconnected, and repeat that removed request to all remaining clients.
+
+### Register XML
+
+\<OMTAddress>  
+\<Name>MYMACHINENAME (My Source Name)\</Name>  
+\<Port>1234\</Port>  
+\<Addresses>  
+\<Address>0.0.0.0\</Address>  
+\</Addresses>  
+\</OMTAddress>
+
+### DeRegister XML
+
+\<OMTAddress>  
+\<Name>MYMACHINENAME (My Source Name)\</Name>  
+\<Port>1234\</Port>  
+\<Removed>True\</Removed>  
+\</OMTAddress>
+
+### IP Addresses
+
+IP Addresses should be determined by the server to ensure only the address accessible to the server is used.
+Therefore when registering a source, the client provided Addresses portion should be ignored.