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).

The HTS Showtime client can be found [http://trac.lonelycoder.com/hts/browser/trunk/showtime/tv/htsp.c here].

----

== Communication ==

This communication is currently implemented by using htsmsg:s. All strings are encoded as UTF-8.

There are two distinct ways for communication within HTSP.

Apart from this there is a number of messages that needs to be exchanged during login, see the login section below.

=== 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.

username         str  optional   Username, in combination with 'digest' this can be used to raise the privileges

                                 for the session in combination with invocation of a method. 

digest           bin  optional   Used to raise privileges.

}}}

The followings 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. 

}}}

=== 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). 

=== Login ===

Upon connect the server sends a welcome message looking like this:

{{{

method           str  required   Contains 'welcome'

htspversion      int  required   The server supports all versions of the protocol up to and including this number.

servername       str  required   Server software name.

serverversion    str  required   Server software version.

challenge        bin  required   32 bit randomized data used to generate authentication digests

}}}

The client is then expected to reply with:

{{{

method           str  required   Contains 'login'

htspversion      int  required   HTSP protocol version the client wish to use.

username         str  optional   Login username. If omitted the login is considered anonymous.

digest           bin  optional   SHA-1 hash of [password (not including terminating NUL)] + [challenge from welcome message]

}}}

The server will in turn reply with:

{{{

method           str  required   Contains 'loginAck'

noaccess         int  optional   The initial credentials did not grant access to any methods.

                                 If the client employs 'on-demand authentication' it will chose to ignore this.

error            str  optional   If set, an error occurred during login. The connection will be terminated by the server.

}}}

After this procedure normal communication may take place.

=== Authentication ===

In Tvheadend, each method has an associated access restriction. Currently there is only on restriction (Streaming). However, this may change in the future.

Privileges for these restrictions may be granted in two ways: Username + Password and/or Source IP address.

Therefore it is possible to gain permissions to the system without entering a username and password.

While this is really useful it also complicates the authentication schema a bit.

Upon connect the initial privileges will be raised based on the source address.

In principle it's possible to use two different authentication idioms with HTSP.

Depending on how your application works one or another may be more suitable.

While they do not really differ from a protocol point of view it's worth mentioning a bit about them here:

=== Initial login authentication ===

The client performs all of its authentication using the 'login' method.

It may choose to send:

 * Username and password: Privileges will be raised based on these credentials.

 * Username only: Privileges will be based on just the source address. The username will be used for various logging purposes.

 * Nothing: Privileges will be based on just the source address.

If no privileges are granted after the login message has been received by the server (i.e. both network and username + password based)

the server will reply with 'noaccess' set to 1. A client that only employs initial login should honor this flag and ask the

user for a username + password and retry by using the 'authenticate' method. I.e. it should not send the 'login' method again.

=== On-demand authentication ===

The client performs all of its authentication when it needs to.

When using this method, the client will check every RPC reply for the 'noaccess' field.

If it set to 1 it whould ask the user for username + password and retry the request but also

add 'username' and 'digest' to the original message. (See ''RPC request extra fields'' above)

Typically it would not send a username or digest during login.

----

= Client to Server (RPC) methods =

----

=== authenticate ===

This can be used to issue authentication without doing anything else.

If no privileges are gained it will return with 'noaccess' set to 1.

Request message fields:

{{{

None

}}}

Reply message fields:

{{{

None

}}}

----

=== 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 ===

Request information about the given event. An event typically corresponds to a program on a channel.

Request message fields:

{{{

eventId          int  required   Event ID.

}}}

Reply message fields:

{{{

start            int  required   Start time of event (Seconds since the epoch, in UTC)

stop             int  required   Ending time of event (Seconds since the epoch, in UTC)

title            str  required   Title of event.

description      str  required   Description of event. This can be quite huge and may also contain newlines.

nextEventId      int  optional   ID of next event on the same channel.

}}}

----

=== subscribe ===

Request subscription to the given channel. 

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.

}}}