All things related to multicast

["Richard Mavrogeanes" <>]

For your information, the VB3000 (MPEG-1) sends SAPs at fixed intervals.  You 
can configure the interval as desired.  The VB6000 (MPEG-2) sends SAPs at 
either fixed intervals, or per the RFC referenced.


-----Original Message-----
From: Lucy E. Lynch 
[mailto:]
Sent: Monday, May 13, 2002 2:45 PM
To: James O. Whitlock
Cc: 
;
 
;

Subject: Re: USA WNY-HPNVI 24/7 Live Surgery at 1330 EST (fwd)



On Mon, 13 May 2002, James O. Whitlock wrote:

> Agreed.  Do you have a good means to zoom in on the appropriate RFC?
> I can launch some enquiries but it's really not my specialty.  It
> may also be worth checking what others in the commercial market are
> doing.  I'll have some VBrick Systems encoders permanently stitched
> into our public facilities over the next few months and will be sure
> to exercise that dimension.  Ditto on MS WMP implementations ATP.  If
> they all fail to limit message length, that becomes a something of de-
> facto standard that we have to live with.  At least, in my experience
> to date.  In any case, whereto next?  The solution will certainly not
> be chasing everyone down who happens to use a long announcement message.
>
> -- Jim
>
>

Jim -

I believe the relevant IETF document is RFC2974 - Session Announcement
Protocol (experimental)

We can take a look at the XMIM cade (based on SDR) here at UO - but
if folks on the wg-multicast list have wisdom to offer about current
versions of SDR (mash etc.) or hpw this is handled for IP/TV or
vbrick please say so!

- lel

key points:

"Announcements are made by periodic multicast to the group.  The base
interval between announcements is derived from the number of
announcements being made in that group, the size of the announcement
and the configured bandwidth limit.  The actual transmission time is
derived from this base interval as follows:

   1. The announcer initializes the variable tp to be the last time a
      particular announcement was transmitted (or the current time if
      this is the first time this announcement is to be made).

   2. Given a configured bandwidth limit in bits/second and an
      announcement of ad_size bytes, the base announcement interval
      in seconds is

             interval =max(300; (8*no_of_ads*ad_size)/limit)

   3. An offset is calculated based on the base announcement interval

             offset= rand(interval* 2/3)-(interval/3)

   4. The next transmission time for an announcement derived as

             tn =tp+ interval+ offset"

and

"It is desirable for the payload to be sufficiently small that SAP
packets do not get fragmented by the underlying network.
Fragmentation has a loss multiplier effect, which is known to
significantly affect the reliability of announcements.  It is

RECOMMENDED that SAP packets are smaller than 1kByte in length,
although if it is known that announcements will use a network with a
smaller MTU than this, then that SHOULD be used as the maximum
recommended packet size."

Authors:

Mark Handley
AT&T Center for Internet Research at ICSI,
International Computer Science Institute,
1947 Center Street, Suite 600,
Berkeley, CA 94704, USA

EMail: 


Colin Perkins
USC Information Sciences Institute
4350 N. Fairfax Drive, Suite 620
Arlington, VA 22203, USA

EMail: 


Edmund Whelan
Department of Computer Science,
University College London,
Gower Street,
London, WC1E 6BT, UK

EMail: