Tvheadend

= Home Tv Streaming Protocol (HTSP) =

== General ==

HTSP is a TCP based protocol primarily intended for streaming of live TV and related meta data such as channels, group of channels (called tags in HTSP) and electronic program guide (EPG) information.

The transmission and reception of a channel over HTSP is referred to a subscription. A single HTSP session can handle as many concurrent subscriptions as the bandwidth and CPU permits.

The HTSP server in tvheadend has a payload-aware scheduler for prioritizing more important packets (such as I-frames) before less important ones (such as B-frames). This makes HTSP is suitable for long-distance transmissions and/or paths with non-perfect delivery.

(It has been tested with a server in Stockholm and the client in Berlin).

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== Communication ==

There are two distinct ways for communication within HTSP.

=== RPC communication ===

There is a normal RPC way of doing things. I.e. the client sends a request and the server responds with a reply. All the RPC methods are listed below as the 'Client to Server' methods. Apart from all message fields listed within each message type the client can add an additional field:

RPC request extra fields:

{{{

seq              int  optional   Sequence number. This field will be echoed back by the server in the reply.

}}}

This field should be used by the client to match the reply with the request.

All replies are guaranteed to arrive in the same order as the requests.

Even so, probably the best way to implement the request-reply client is by taking advantage of the 'seq' field.

RPC reply extra fields:

{{{

seq              int  optional   Sequence number. Same as in the request.

error            str  optional   If present an error has occurred and the text describes the error.

noaccess         int  optional   If present and set to '1' the user is prohibited from invoking the method due to 

                                 access restrictions. 

                                 The client should first try to authenticate and then try to re-invoke the method.

challenge        bin  optional   If 'noaccess' is set the server also provides a challenge to be used in the, if

                                 the user decides so, upcoming 'authentication' request.

}}}

=== Streaming communication ===

For streaming of live TV and various related messages the server will continuously push data to the client.

These messages are referred to as asynchronous messages and always have the 'method' field set and never have the 'seq' field set.

Also, the client can enable an additional asyncMetadata mode and by doing so it will be notified by the server when meta data changes. (EPG updates, creation of channels and tags, etc). 

=== Authentication ===

If the client receives a reply with the 'noaccess' field set it means that it is not allowed to invoke the method.

The client can try to 'up' it's access by invoking the 'authenticate' method and then try to re-invoke the original method.

A typical client would do something like this:

{{{

 sendRequest() {

 while(1) {

  reply = send(request)

  if(reply.noaccess == 0) {

   return reply}

  else {

    do {

      ask-user-for-credentials-and-or-get-them-from-keychain

      request.username = name of user

      request.digest = sha1(password + reply.challenge)

      reply = send(request);

    } while(reply.noaccess == 1)

  }

}}}

The challenge will always 

----

== Flow of a subscription ==

When the client wish to receive a channel it starts by sending a 'subscribe' request. The server responds

----

= Client to Server (RPC) methods =

----

=== getChallenge ===

Request a 32 byte challenge used to generate a authentication digest.

Request message fields:

{{{

None

}}}

Reply message fields:

{{{

challenge        bin  required   32 bytes of random challenge to be used in next authentication request from client.

}}}

----

=== authenticate ===

Request message fields:

{{{

username         str  required   Username.

digest           bin  required   SHA-1 hash of [password (not including terminating NUL)] + [challenge]

}}}

Reply message fields:

{{{

noaccess         int  optional   Set to '1' if access was denied.

challenge        bin  optional   32 bytes of random challenge to be used in next authentication request from client.

                                 Always present if 'noaccess' is set. This is supplied so the client does not have

                                 to issue an extra 'getChallenge' request.

}}}

----

=== enableAsyncMetadata ===

When this is enabled the client will get continuous updates from the server about channels and tags.

This also includes creation and deletion of channels and tags. 

An interactive application that presents the user with information about channels and tags would probably want to switch to this mode.

Request message fields:

{{{

None

}}}

Reply message fields:

{{{

None

}}}

----

=== getEvent ===

----

=== subscribe ===

Request subscription to the given channel. A request for a subscription is always successful.

When the server is ready to transmit data it will first send a 'subscriptionStart' message.

Request message fields:

{{{

channelId        int  required   ID for channel. 

subscriptionId   int  required   Subscription ID. Selected by client. This value is not interpreted by the server in any form. 

                                 The value is used from now on in all messages related to the subscription.

}}}

Reply message fields:

{{{

None.

}}}

----

=== unsubscribe ===

Stop a subscription.

Attributes

{{{

subscriptionId   int  required   Subscription ID.

}}}

Reply message fields:

{{{

None.

}}}

----

= Server to Client methods =

----

=== channelAdd ===

A new channel has been created on the server.

Message fields:

{{{

channelId        int   required   ID of channel.

channelName      str   required   Name of channel.

channelIcon      str   required   URL to an icon representative for the channel.

eventId          int   optional   ID of the current (or next to be) event on this channel.

tags             int[] optional   Tags this channel is mapped to.

}}}

----

=== channelUpdate ===

A channel has been updated on the server. All fields will be sent even if they are not changed. Most clients can process this and the 'channelAdd' message

with the very same code.

Message fields:

{{{

channelId        int   required   ID of channel.

channelName      str   required   Name of channel.

channelIcon      str   optioanl   URL to an icon representative for the channel.

eventId          int   optional   ID of the current (or next to be) event on this channel.

tags             int[] required   Tags this channel is mapped to.

}}}

----

=== channelDelete ===

A channel has been deleted on the server.

This message is only sent if session is in asynchronous mode.

Message fields:

{{{

channelId        int   required   ID of channel.

}}}

----

=== tagAdd ===

A new tag has been created on the server.

Message fields:

{{{

tagId            int   required   ID of tag.

tagName          str   required   Name of tag.

tagIcon          str   optional   URL to an icon representative for the channel.

channels         int[] required   Channels this tag is mapped to.

}}}

----

=== tagUpdate ===

A tag has been updated on the server.

Message fields:

{{{

tagId            int   required   ID of tag.

tagName          str   required   Name of tag.

tagIcon          str   optional   URL to an icon representative for the channel.

channels         int[] required   Channels this tag is mapped to.

}}}

----

=== tagDelete ===

A tag has been deleted from the server.

Message fields:

{{{

tagId            str   required   ID of tag.

}}}

----

=== subscriptionStart ===

Message fields:

{{{

subscriptionId   int   required   Subscription ID.

streams          msg[] required   Array of messages with stream information

'streams' message:

index            int   required   Index for this stream

type             str   required   Type of stream

language         str   optional   Language for stream

Stream types:

    AC3                           AC3 audio

    MPEG2AUDIO                    MPEG2 audio (MP2)

    MPEG2VIDEO                    MPEG2 video

    H264                          H264 video

}}}

----

=== subscriptionStop ===

Message fields:

{{{

subscriptionId   int   required   Subscription ID.

reason           str   optional   Reason for subscription stop.

}}}

----

=== subscriptionStatus ===

Message fields:

{{{

subscriptionId   int   required   Subscription ID.

status           str   optional   English clear text of status. Absence of this field means that the status is OK. 

}}}

----

=== queueStatus ===

The queueStatus message is sent every second during normal data delivery.

The transmit scheduler have different drop thresholds for different frame types.

If congestion occurs it will favor dropping B-frames before P-frames before I-frames.

All audio is recognized as I-frames. 

Message fields:

{{{

subscriptionId   int   required   Subscription ID.

packets          int   required   Number of data packets in queue.

bytes            int   required   Number of bytes in queue.

delay            int   required   Estimated delay of queue (in µs)

Bdrops           int   required   Number of B-frames dropped

Pdrops           int   required   Number of P-frames dropped

Idrops           int   required   Number of I-frames dropped

}}}

----

=== muxpkt ===

Streaming data.

Message fields:

{{{

subscriptionId   int   required   Subscription ID.

frametype        int   required   Type of frame as ASCII value: 'I', 'P', 'B'

stream           int   required   Stream index. Corresponds to the streams reported in the subscriptionStart message.

dts              int   required   Decode Time Stamp in µs.

pts              int   required   Presentation Time Stamp in µs.

duration         int   required   Duration of frame in µs.

payload          bin   required   Actual frame data.

}}}