Subject: [MMUSIC] SDP Grammar for multicast-address in sdp-new-06

From: Bryan Thale <[email-masked]>
Date: Tue, 19 Mar 2002 17:33:32 -0600

Is the SDP grammar for multicast-address and IP4-multicast on page 36 of
sdp-new-06 correct?  The way the textual description of the Connection
Data field on pages 13 and 14 reads,  <ttl> and optional <number of
addresses> fields should follow IP6 multicast addresses the same as for
IP4 multicast addresses.

Currently in sdp-new-06:

multicast-address =   IP4-multicast / IP6-multicast

IP4-multicast =       m1 3*( "." decimal-uchar )
                      "/" ttl [ "/" integer ]
                      ; IPv4 multicast addresses may be in the
                      ; range 224.0.0.0 to 239.255.255.255

Shouldn't it read like::

multicast-address =   (IP4-multicast / IP6-multicast)
                      "/" ttl [ "/" integer ]

IP4-multicast =       m1 3*( "." decimal-uchar )
                      ; IPv4 multicast addresses may be in the
                      ; range 224.0.0.0 to 239.255.255.255

Just checking,
Bryan.

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Motorola Labs, Networking and Infrastructure Research
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From: Gonzalo Camarillo <[email-masked]>
Date: Mon, 25 Mar 2002 23:34:18 +0200

Hi,

IPv6 multicast addresses are not scoped using the TTL (besides, the
field is not called TTL anymore in IPv6).

Regards,

Gonzalo

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Gonzalo Camarillo                    Phone :   +1 212 939 71 71
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From: Bryan Thale <[email-masked]>
Date: Mon, 25 Mar 2002 16:04:33 -0600

> field is not called TTL anymore in IPv6).

Ah.   Ok, but isn't the <number of addresses> field still necessary?  So the
grammar should read:

                      [ "/" integer ]

                      "/" ttl

IP6-multicast =       hexpart
                      ; IPv6 address starting with FF

Note to draft authors:  The description of the Connection Data field in
sdp-new-06, page 14, shows an example of a c= field with an IPv6 address and
the TTL field.

Bryan.

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From: Gonzalo Camarillo <[email-masked]>
Date: Tue, 26 Mar 2002 00:16:45 +0200

Hi,

The idea was not to use the host count but rather IPv6 scopes.

Regards,

Gonzalo

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From: Bryan Thale <[email-masked]>
Date: Mon, 25 Mar 2002 17:46:55 -0600

> The idea was not to use the host count but rather IPv6 scopes.

Sorry to be so dense, but how do you use IPv6 scopes to specify a block of
multicast group addresses?

The text seems to say that

c=IN IP6 FF15::101/3

is semantically equivalent to:

c=IN IP6 FF15::101
c=IN IP6 FF15::102
c=IN IP6 FF15::103

Is this no longer the case?

Bryan.

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From: Colin Perkins <[email-masked]>
Date: Mon, 25 Mar 2002 20:01:34 -0500

There are actually two issues:

 - Should SDP allow specification of the hop-count field for IPv6 multicast?
 - How do we specify ranges of IPv6 multicast addresses for layered coding?

I think the answer to the first question is "no", and we should prefer
scoped addresses. This leads to the second issue, since elimination of 
the hop-count field would imply that:

	c=IN IP6 FF15::101/3

means a layered session of three addresses (as noted below), but

	c=IN IP4 224.2.2.2/3

would be a TTL scoped single address session. Having fields appear the same
but have different meanings depending on the address type may be confusing.
This is also an issue with the SDP/IPv6 draft, which seems to make layered
coding illegal as specified. 

Is the potential confusion for readers acceptable? (the BNF is unambiguous)
or should we mandate that a hop count is included for IPv6?

Colin

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From: Bryan Thale <[email-masked]>
Date: Tue, 26 Mar 2002 01:03:35 -0600

I don't think you want to mandate a do-nothing field for IP6 addresses.  There
seems to be an "out" available in the text.  From page 13:

> The second sub-field is the address type.  This allows SDP to be used
> for sessions that are not IP based.  Currently only IP4 and IP6 are
> defined.
>
> The third sub-field is the connection address.  Optional extra sub-
> fields may be added after the connection address depending on the value
> of the <address type> field.
>

That seems to read as if it is perfectly acceptable to have the <ttl> field
defined for IP4 and not for IP6 address types.

Maybe the <number of addresses> field could be made mandatory for IP4 multicast
addresses in all cases, even the normal case of 1.  Then IP4 multicast
addresses would always have 3 fields while IP6 multicast addresses have at most
2.  Another approach might be to use a different field separator for the <ttl>
field. Maybe

c=IN IP4 224.2.2.2!127/3

On the other hand, how important is it that human readers of an SDP PDU don't
confuse

c=IN IP4 224.2.2.2/3

with

c=IN IP4 224.2.2.2/x/3

?  The reader just has to remember that IP4 multicast addresses have a
mandatory <ttl> field and an optional <number of addresses> field so the
address is always unambiguous.

Bryan.

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From: Gonzalo Camarillo <[email-masked]>
Date: Tue, 26 Mar 2002 18:46:33 +0200

Hi,

I agree with Bryan. As long as the spec is clear, both solutions are
alright. I do not know, though, if always adding "/1" to IPv4 multicast
addresses would cause interoperability problems (it shouldn't).

Regards,

Gonzalo

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From: Colin Perkins <[email-masked]>
Date: Tue, 26 Mar 2002 14:58:52 -0500

Hi,

Okay, unless there are objections, I will update the BNF to allow 
	c=IN IP6 FF15::101/3 
to be a layered multicast session. I will also send a last call comment on
draft-ietf-mmusic-sdp-ipv6-03.txt suggesting the equivalent change is made
there.

Colin