Subject: T flag for an IPv6 multicast address with permanent group ID

From: JINMEI Tatuya / 神明達哉<[email-masked]>
Date: Wed, 21 Dec 2005 00:30:41 +0900

I'm confused about the T flag for an IPv6 multicast address with a
permanent group ID and would like to solicit clarification...

According to Section 4.2 of RFC3307:

   Permanent group IDs are allocated on an Expert Review basis, in the
   range 0x40000000 to 0x7FFFFFFF.  These permanent group IDs are meant
   to be used in IPv6 multicast addresses, defined in [UNIMCAST].
(UNIMCAST=RFC3306)

I interpret this as the corresponding IPv6 multicast address being
based on a unicast network prefix as defined in RFC3306.  Thus the P
flag of the 4-bit flags field must be 1, and then the T flag must also
be 1 as specified in RFC3306:

         o  If P = 1, T MUST be set to 1, otherwise the setting of the T
            bit is defined in Section 2.7 of [ADDRARCH].

Meanwhile, Section 4.3 of RFC3307 reads:

   It
   should be noted that the high-order bit of the Group ID will be the
   same value as the T flag.

Doesn't this contradict the relationship between the T flag and
permanent group IDs?  In fact, the high-order bit of a permanent group
ID is 0 while the T flag must be 1 as I explained above.

Or am I missing something?

					JINMEI, Tatuya
					Communication Platform Lab.
					Corporate R&D Center, Toshiba Corp.
					[email-masked]

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From: Brian Haberman <[email-masked]>
Date: Tue, 20 Dec 2005 11:34:09 -0500

> ID is 0 while the T flag must be 1 as I explained above.

Section 4.3 only deals with multicast addresses allocated dynamically.
That is, they are administered locally either by an allocation server
(as described in Section 4.3.1) or by an end-host (Section 4.3.2).

Are you considering the case where addresses with permanent group
IDs are allocated by a server?  We didn't consider that scenario.

Multicast Addresses created using permanent group IDs (Section 4.2)
does not mention the T flag mainly because addresses generated
by RFC 3306 are not permanent.  Rather, they only have a lifetime
equivalent to the lifetime of the embedded prefix.

Does that clarify things?

Regards,
Brian

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From: JINMEI Tatuya / 神明達哉<[email-masked]>
Date: Wed, 21 Dec 2005 14:23:52 +0900

Hello, thanks for the prompt response.

>>>>> On Tue, 20 Dec 2005 11:34:09 -0500, 
>>>>> Brian Haberman <[email-masked]> said:

> (as described in Section 4.3.1) or by an end-host (Section 4.3.2).

Hmm, so the following phrase of Section 4.3

  the high-order bit of the Group ID will be the same value as the T flag

just means "the high-order bit of the Group ID of a dynamic IPv6
multicast address will be the same value as the T flag of the dynamic
IPv6 multicast address".

I originally thought this is an 'if-and-only-if' relationship, that
is,

  - if the high-order bit of the Group ID is 1, then the T flag is
    also 1, and
  - if the high-order bit of the Group ID is 0, then the T flag is
    also 0.

But I understand that's not the intention of the RFC.  (Then I
personally wonder whether this really "should be noted", though...)

Anyway, I think it's not clear.  Thanks,

					JINMEI, Tatuya