Subject: Re: draft-ietf-ipngwg-uni-based-mcast-03.txt

From: Thomas Narten <[email-masked]>
Date: Thu, 27 Sep 2001 12:55:34 -0400

> If plen = 0, then isn't the "network prefix" portion 0 bits long?

That would be one interpretation. Another possibly would be that the
network prefix should have special meaning, other than looking at the
first plen bits. I guess I'm asking if it would make sense to do so,
for (say) debugging purposes.

> The SSM range should be FF3x::/12 (and this is mentioned in the SSM
> overview doc today).

draft-holbrook-ssm-arch-02.txt talks about reserving ff3x and ff2x for
SSM.

draft-ietf-ssm-overview-01.txt says:

>   Source-Specific Multicast (SSM) : This is the multicast service model
>   defined in [SSM-ARCH]. An IP datagram is transmitted by a source S to
>   an SSM destination address G, and receivers can receive this datagram
>   by subscribing to channel (S,G). SSM is derived from EXPRESS [EXPRESS]
>   and supports one-to-many multicast.The address range 232/8 has been
>   assigned by IANA [IANA-ALLOC] for SSM service in IPv4. For IPv6, the
>   range FF3x::/12 is defined for SSM services [SSM-IPV6].

But that range also covers the address that are defined in this
document, including non-SSM addresses! (And it doesn't cover the plen
field that defines whether the address is SSM or not!!) So things do
not seem to be in sync.

Thomas

---

From: "Dave Thaler" <[email-masked]>
Date: Thu, 27 Sep 2001 09:34:35 -0700

> -----Original Message-----
> From: Thomas Narten [mailto:[email-masked]]
> Sent: Thursday, September 27, 2001 7:44 AM
> To: [email-masked]; [email-masked]
> Subject: draft-ietf-ipngwg-uni-based-mcast-03.txt
> 
> I have  question about the references to SSM. In particular, the
> document says:
> 
> >    The unicast prefix-based IPv6 multicast address format supports
> >    Source-specific multicast addresses, as defined by [SSM ARCH].
To
> >    accomplish is, a node MUST:
> >
> >            o Set P = 1.
> >            o Set plen = 0.
> >            o Set network prefix = 0.
> >
> >    These settings indicate that the multicast address is being used
in
> >    source-specific multicast transmission.  The source address field
in
> >    the IPv6 header identifies the owner of the multicast address.
> 
> Is it required (or desirable) for the network prefix to be set to 0 as
> stated above? I.e., the test for the address being SSM would be
> something like:
> 
>            - SSM - All IPv6 SSM multicast addresses will have the
>               format FF3x::/96.

-Dave

---

From: "Brian Haberman" <[email-masked]>
Date: Thu, 27 Sep 2001 13:08:10 -0400

> for (say) debugging purposes.

The problem is that SSM requires all 128-bits to represent the source.
So, it seems simpler to indicate in some manner that the destination
multicast address is SSM and refer to the source address to determine
the "owner".

> draft-holbrook-ssm-arch-02.txt talks about reserving ff3x and ff2x for

I believe that this draft is in the process of being revised prior to
a last call.

> field that defines whether the address is SSM or not!!) So things do

So, since I am the common denominator between the two drafts, I must
claim responsibility.  The SSM overview draft should state that
FF3x::/32 is the SSM range for IPv6.

Brian

---

From: Thomas Narten <[email-masked]>
Date: Thu, 27 Sep 2001 13:19:58 -0400

> The problem is that SSM requires all 128-bits to represent the
  source.

To be clear, you mean all 128 bits of the source address in the IP
header. I'm not questioning that or suggesting otherwise.

> multicast address is SSM and refer to the source address to determine

Agree with this also.

But, does that mean it's best to require that the 64plus middle bits
of the SSM address be all zeros? Seems like it would be useful to
allow (or even suggest in the SHOULD sense) that the middle bits
identify the link. That way if you look at the multicast address
(independant of the source IP address) you can still glean some useful
information out of it.

Note: I'm asking this as a question. If other folks have opinions, I'd
like to hear them.

> FF3x::/32 is the SSM range for IPv6.

Either I still don't get it, or this isn't true either. Only if plen
== 0 is the address an SSM one. So you can't say that ff3x::/32 is the
SSM range, because the plen field is in the last octet  of that range.

Thomas

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From: Thomas Narten <[email-masked]>
Date: Thu, 27 Sep 2001 14:06:15 -0400

> SSM range, because the plen field is in the last octet  of that range.

I clearly didn't get it above; I agree that FF3x::/32 would be the SSM
range.

Thomas