Subject: Re: [Softwires] 6rd multicast

From: "Lee, Yiu" <[email-masked]>
Date: Fri, 7 Jan 2011 23:55:50 +0000

Hi Tom,

See comments inline:

On 1/7/11 5:17 PM, "Tom Taylor" <[email-masked]> wrote:

>Thanks for your comments. Responses below. BTW, we are going to follow a
>suggestion to write this draft as a more general mechanism, to cover any
>case where the host and the target network speak IPvx and the access
>network between speaks IPvy. We'll try to frame it so the host-end
>function (still trying to think of a name) can reside either in the CPE
>or in the provider IP edge.

[YL] Looking forward to it.

>[PTT] You don't need a mapping for every v6 multicast group, only the
>ones that someone in your network wants to join. So it's an engineering
>issue -- how many addresses do you set aside in the address pool to
>handle your demand?

[YL] This will require provision in the TF which is an overhead.

>
>[PTT] No, the new protocol is used only to request a mapping from <S,G>
>in the target network to <S',G'> in the access network. The 6rd CE does
>indeed translate the MLDv2 to IGMP directly (you must have missed a step
>in your reading), but it needs to know the access network view of the
>address pair (i.e., <S',G'>) before doing so.

[YL] Ok. I read the step now:

"The 6rd CE interworks between the MLDv2 request it received and an IGMPv3
request."

>
>[PTT] Let's start with the <S,G> in the target network (IPv6 network, in
>our current draft). S is wild-carded if the multicast session is
>any-source (ASM). If the multicast session is specific-source (SSM),
>then S in the target network is a unicast IPv6 address. The translator
>maps that to an IPv4 unicast address S' from its pool, and the 6rd CE
>uses S' in its IGMP request.

[YL] Since the draft specified mentioned only IGMPv3, I thought it worked
only on SSM. I think the text needed to update.

>
>[PTT] That's a point we hadn't thought of -- one of the details that
>have to be sorted out, along with the question of how the PIM-Join finds
>the 6rd-BR in the first place. For SSM, the answer is fairly simple: the
>translator sets S' to the unicast address of the 6rd-BR, as you
>suggested in an earlier comment. This doesn't work for ASM.

[YL] One solution is to ask TF to give all the mappings to the BR (or the
TF is collocated with the BR). For the get the PIMv4-Join question, you
can make the BR the RP of the special v4 groups which are provisioned for
v6 translation.

Regards,
Yiu