RE: Trying to find words to describe how PIDs are used.....

[P.Pillai@Bradford.ac.uk]

Hi Art,

Thanx a lot for your comments.

See inline.

Regards
Prashant

Quoting "Allison, Art" <AAllison@nab.org>:

> Well, indeed it is important to define terms.
> At the top level; per 13818-1, there are Program Streams and Transport
> Streams.
> First, put aside Program Streams. (From 13818-1:"The Program Stream is
> designed for use in relatively error-free environments and is suitable
> for applications which may involve software processing of system
> information such as interactive multi-media applications. Program Stream
> packets may be of variable and relatively great length.")
> Continuing from 13818-1:
> "The Transport Stream combines one or more programs with one or more
> independent time bases into a single stream. PES packets made up of
> elementary streams that form a program share a common timebase. The
> Transport Stream is designed for use in environments where errors are
> likely, such as storage or transmission in lossy or noisy media.
> Transport Stream packets are 188 bytes in length."
>
> A Packetized Elementary stream can be constructed from a continuous
> sequence of PES packets of one elementary stream with one stream ID.
>
> So 'MPEG-2 Stream' is indeed ambiguous in meaning.
> Notwithstanding the mental discord, I attempted modifications in
> <yourspeak> below demarked by //
>
PP: I absolutely agree with you. It is important to be careful in usingdefining
the stream.

>
> _____________
> Art Allison
> Director, Advanced Engineering
> Science & Technology
> National Association of Broadcasters
> 1771 N Street, NW
> Washington, D.C. 20036
> Phone: 202.429.5418
> Fax: 202.777.4981
> aallison@nab.org
>
> The National Association of Broadcasters is a trade association that
> advocates on behalf of more than 8,300 free, local radio and television
> stations and also broadcast networks before Congress, the Federal
> Communications Commission and the Courts.
>
> -----Original Message-----
> From: owner-ipdvb@erg.abdn.ac.uk [mailto:owner-ipdvb@erg.abdn.ac.uk] On
> Behalf Of Gorry Fairhurst
> Sent: Wednesday, August 30, 2006 12:57 PM
> To: ipdvb@erg.abdn.ac.uk; P.Pillai@Bradford.ac.uk; mnoist@cosy.sbg.ac.at
> Cc: S.Iyengar
> Subject: Trying to find words to describe how PIDs are used.....
>
>
> Repost: My last posting to the list seemed to have gone astray, so I
> shall try an updated repost.
>
> I was trying to use some of the email from others to propose some text
> that answered the second set of questions.
>
>  > 2) Can  we determine precisely what we man by a "Stream".
> //AA>I think a modifier should always be used as this word is overloaded
> and usage here does not directly map to MPEG-2 systems.//

PP:Yes, It is important to define the stream properly to be able to clearly show
what is the aim of the security extensions.

>  > Does a Stream always only have ONE originating source?
>  //AA>I think that is so... if not then the meaning is lost on me.//

PP: This is true. But here again whatis the Stream. If this is refering to the
elementray stream then yes it has a single originating source, but it is
important to note that the PID associated with it may not be unique.

>  > That is, does the PID imply a specific intended source?
> //AA>No, because as you cover below, these PIDs are ephemeral and
> changeable by any multiplexer.
> If the PID is anounced in a PMT, it can change with each PMT section,
> although typically it does not.//

PP: Agreed. PID Mapping can take place in any multiplexer in between.

> Here is some draft text, that could perhaps be placed at the end of
> section 3.2. I'd be very pleased to receive comments/corrections so we
> converge on some good description of this, since I think this relates
> directly to the need for authentication.
>
> THOUGHTS??? Comments and corrections please...
>
> Best wishes,
>
> Gorry
>
>
> ----
>
> In a MPEG-2 Transmission network, the originating source of MPEG-2 TS
> Packets is either a L2 interface device (media encoder, encapsulation
> gateway, etc) or a L2 network device (TS multiplexor, etc). These
> devices may, but do not necessarily, have an associated IP address. In
> the case of an encapsulation gateway (e.g. ULE sender), the device may
> operate at L2 or L3, and is not normally the originator of an IP traffic
> flow, and usually the IP source address of the packets that it forwards
> do not correspond to an IP address associated with the device. When
> authentication of the IP source is required this must be provided by
> IPsec, TLS, etc. operating at a higher layer.
>
> The TS Packets are carried to the Receiver over a physical layer that
> usually includes Forward Error Correction //(which can time shift data
> by interleaving data from multiple unrelated TS packets)// and
> synchronisation processing that makes injection of single TS Packets
> very difficult. Replacement of a sequence of packets is difficult, but
> possible.
>
> Each Receiver needs to identify a TS Logical Channel (or MPEG-2
> //Elementary// Stream) to reassemble the fragments of PDUs sent by a L2
> source [RFC4259]. In an
> MPEG-2 TS, this association is made via the Packet Identifier, PID
> [ISO-MPEG]. At the sender, each source associates a locally unique set
> of PID values with each stream it originates. However, there is no
> required relationship between the PID value used at the sender and that
> received at the Receiver.  Network devices may re-number the PID values
> associated with one or more TS Logical Channels (Streams) to prevent
> clashes at a multiplexor between input Streams with the same PID carried
> on different input multiplexes //(updating the PMT[ISO-MPEG] if the data
> is reference in such)//.  A device may also modify and/or insert new SI
> data into the control plane (also sent as TS Packets identified by PID
> value).
>
> The Stream of TS Packets carried in a multiplex are usually received by
> many Receivers. One method is to secure the entire Stream at the MPEG-2
> TS level. This approach is well-suited to TV-transmission, data-push,
> etc, where the PID carries one or a set of flows with similar security
> requirements. Where the Stream carries a set of IP traffic flows to
> different destinations with a range of properties (multicast, unicast,
> etc) this it is often not appropriate to provide IP confidentiality
> services for the entire Stream. A finer-grain control is required that
> at least allows control to the level of a single MAC/NPA address.
> However, there is only one valid source of data for each MPEG-2
> //Elementary// Stream (i.e. PID) //for some duration//. //The duration
> may be only until the next PMT arrives in some systems. It may be until
> the next annuncement/discovery data is received in other systems.//
> Although an attacker that is able to modify the content of the received
> multiplex (e.g. replay data) could inject data locally with an arbitrary
> PID value.
>
> ---
>


-- 
Prashant Pillai
Research Assistant
School of Engineering, Design and Technology
University of Bradford
Bradford, BD7 1DP
West Yorkshire
United Kingdom
Phone: 0044-1274-233720
email: p.pillai@bradford.ac.uk
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