All things related to multicast

[Alan Crosswell <>]

>Alan, I beg your pardon!  I thought you ment the session descriptor. We 
>don't currently support modification of the i field. But yes, we could allow 
>modification of this, but please help me understand why.

Why would I want to modify the i field?  To put information in it!
Just take a look at a typical SDR display such as using IPTV Viewer.
Every session has a name (s=) and a description (i=), except all the
Vbrick sessions which have the generic Vbrick message.  Here are some
examples:

nn2k-gw>sho ip sdr "Archaeology Channel (MPEG-1)"
SAP Cache - 154 entries
Session Name: Archaeology Channel (MPEG-1)
  Description: University of Oregon and the Archaeology project are testing n
               ew content on this channel.
...

nn2k-gw>sho ip sdr "ENSSAT"
SAP Cache - 154 entries
Session Name: ENSSAT
  Description: Essais multicast entre ENSSAT Lannion et le CNAM de
  Nantes
...

nn2k-gw>sho ip sdr "CSPAN via Nwstrn U."
SAP Cache - 154 entries
Session Name: CSPAN via Nwstrn U.
  Description: VBrick streaming video
...

As you can see, the Vbrick does not provide useful additional
information that we customers would like to put there.

To quote RFC-2327:

     i=<session description>

    The "i=" field is information about the session.  There may be at
       most one session-level "i=" field per session description, and at
       most one "i=" field per media. Although it may be omitted, this is
       discouraged for session announcements, and user interfaces for
       composing sessions should require text to be entered.  If it is
       present it must contain ISO 10646 characters (but see also the
       `charset' attribute below).

       A single "i=" field can also be used for each media definition.  In
       media definitions, "i=" fields are primarily intended for labeling
       media streams. As such, they are most likely to be useful when a
       single session has more than one distinct media stream of the same
       media type.  An example would be two different whiteboards, one for
       slides and one for feedback and questions.

While you are at it, there are other optional SDP tags.  The one that
comes immediately to mind is the URL (u=):

     u=<URI>

  o A URI is a Universal Resource Identifier as used by WWW clients

     o The URI should be a pointer to additional information about the
       conference

     o This field is optional, but if it is present it should be
       specified before the first media field

     o No more than one URI field is allowed per session description


Thanks.
/a