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

["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 //



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

---