IPTV

Revision as of 18:46, 6 September 2006

IPTV is a name for a collection of technologies used to stream Television over the Internet. Essentially, it's the protocols RTC/RTCP or RTSP for streaming the video & audio, and SAP/SDP to announce the streams, all operating over UDP multicast.

Emulation

Currently, there are a few implementations of SAP/SDP and RTP/RTCP that can be used to emulate being on an IPTV system. The most advanced version of this would be VideoLAN, which will take any kind of file it can open and will stream it over RTP/RTCP or RTSP (In version 8.3+), with automatic announcement over SAP/SDP.

SAP/SDP

SAP

SAP is a method of sending "Session Announcements" over UDP Multicast. SAP uses Multicast address 224.2.127.254 port 9875 by default for sending announcements in the global scope.

A SAP announcement looks like this (Taken from the RFC):

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | V=1 |A|R|T|E|C|   auth len    |         msg id hash           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   :                originating source (32 or 128 bits)            :
   :                                                               :
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    optional authentication data               |
   :                              ....                             :
   *-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*
   |                      optional payload type                    |
   +                                         +-+- - - - - - - - - -+
   |                                         |0|                   |
   + - - - - - - - - - - - - - - - - - - - - +-+                   |
   |                                                               |
   :                            payload                            :
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   V: Version Number. The version number field MUST be set to 1 (SAPv2
      announcements which use only SAPv1 features are backwards
      compatible, those which use new features can be detected by other
      means, so the SAP version number doesn't need to change).

   A: Address type. If the A bit is 0, the originating source field
      contains a 32-bit IPv4 address.  If the A bit is 1, the
      originating source contains a 128-bit IPv6 address.

   R: Reserved. SAP announcers MUST set this to 0, SAP listeners MUST
      ignore the contents of this field.

   T: Message Type. If the T field is set to 0 this is a session
      announcement packet, if 1 this is a session deletion packet.

   E: Encryption Bit. If the encryption bit is set to 1, the payload of
      the SAP packet is encrypted.  If this bit is 0 the packet is not
      encrypted.  See section 7 for details of the encryption process.

   C: Compressed bit. If the compressed bit is set to 1, the payload is
      compressed using the zlib compression algorithm [3].  If the
      payload is to be compressed and encrypted, the compression MUST be
      performed first.

   Authentication Length. An 8 bit unsigned quantity giving the number
      of 32 bit words following the main SAP header that contain
      authentication data.  If it is zero, no authentication header is
      present.

   Authentication data containing a digital signature of the packet,
      with length as specified by the authentication length header
      field.  See section 8 for details of the authentication process.

   Message Identifier Hash. A 16 bit quantity that, used in combination
      with the originating source, provides a globally unique identifier
      indicating the precise version of this announcement.  The choice
      of value for this field is not specified here, except that it MUST
      be unique for each session announced by a particular SAP announcer
      and it MUST be changed if the session description is modified (and
      a session deletion message SHOULD be sent for the old version of
      the session).

      Earlier versions of SAP used a value of zero to mean that the hash
      should be ignored and the payload should always be parsed.  This
      had the unfortunate side-effect that SAP announcers had to study
      the payload data to determine how many unique sessions were being
      advertised, making the calculation of the announcement interval
      more complex that necessary.  In order to decouple the session
      announcement process from the contents of those announcements, SAP
      announcers SHOULD NOT set the message identifier hash to zero.

      SAP listeners MAY silently discard messages if the message
      identifier hash is set to zero.

   Originating Source. This gives the IP address of the original source
      of the message.  This is an IPv4 address if the A field is set to
      zero, else it is an IPv6 address.  The address is stored in
      network byte order.

      SAPv0 permitted the originating source to be zero if the message
      identifier hash was also zero.  This practise is no longer legal,
      and SAP announcers SHOULD NOT set the originating source to zero.
      SAP listeners MAY silently discard packets with the originating
      source set to zero.

The standard announcement type for SAP is SDP.

SDP

An SDP announcement looks like this:

v= (protocol version)
o= (owner/creator and session identifier).
s= (session name)
i=* (session information)
u=* (URI of description)
e=* (email address)
p=* (phone number)
c=* (connection information - not required if included in all media)
b=* (bandwidth information)

One or more time descriptions (see below)

z=* (time zone adjustments)
k=* (encryption key)
a=* (zero or more session attribute lines)

Zero or more media descriptions (see below)

Time description
t= (time the session is active)
r=* (zero or more repeat times)

Media description
m= (media name and transport address)
i=* (media title)
c=* (connection information - optional if included at session-level)
b=* (bandwidth information)
k=* (encryption key)
a=* (zero or more media attribute lines)

Example

v=0
o=mhandley 2890844526 2890842807 IN IP4 126.16.64.4
s=SDP Seminar
i=A Seminar on the session description protocol
u=http://www.cs.ucl.ac.uk/staff/M.Handley/sdp.03.ps
e=mjh@isi.edu (Mark Handley)
c=IN IP4 224.2.17.12/127
t=2873397496 2873404696
a=recvonly
m=audio 49170 RTP/AVP 0
m=video 51372 RTP/AVP 31
m=application 32416 udp wb
a=orient:portrait

For more information, please see the RFC

RTP/RTCP

RTP/RTCP

RTSP

RTSP

Note: The FreeBox patch that's in Trac uses RTSP to recieve IPTV streams from an ADSL box. However, this version uses a PLS "Playlist" provided by the box to discover channels, not SDP/SAP, thereby still needing a STB to tune. If we can make a version that uses SDP/SAP Discovery, this requirement will not be needed, and the "Tuner" can be made into a more generic "IPTV tuner".