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Re: comments on draft-ietf-l3vpn-ppvpn-mcast-reqts-03



Pekka, Yakov, folks,


Yakov Rekhter:

> > 5.1.11.  RP Engineering
> > 
> >     When PIM-SM (or bidir-PIM) is used in ASM mode on the VPN customer
> >     side, the location of the RP has to be chosen.  In some cases this
> >     engineering problem is not trivial: for instance, if sources and
> >     receivers are located in VPN sites that are different than that of
> >     the RP, then traffic may flow twice through the SP network and the
> >     CE-PE link of the RP (from source to RP, and then from RP to
> >     receivers) ; this is obviously not ideal.  A multicast VPN solution
> >     SHOULD propose a way to help on solving this RP engineering issue.
> > 
> > ==> I'm not sure if this specific problem exists in practice.  If the 
> > RP sends Register-Stop, the multicast traffic is forwarded natively, 
> > and the data will short-cut inside the site without going to the RP 
> > and then back.  I.e., there should not be two times traffic once the 
> > forwarding is native.
> 
> Well...
> 1) even such transient peaks may not be something that you'll want
> 2) what if you lose connectivity to the remote site hosting the RP, are
> you happy to lose local multicast on every other site?
> 3) what if you want to stick with RPT ? 
>    [okay, maybe not so relevant, but what if the VPN customer wants to
>     do this ?]

Wrt your second point I think it would be highly desirable for a
solution to maintain multicast service among all the sites within
an MVPN that maintain connectivity among themselves, even when the
site(s) hosting the RP lose their connectivity.

Likewise, it would be highly desirable for a site that loses
connectivity to the service provider to maintain multicast service
within that site.

I would suggest to include both of the above points in the requirements
document.

I do agree that the unavailability of the RP may be a stronger reason that transient bandwidth peaks happening when forwarding switches from PIM-register-encapsulated to native.

However, the initial reason for this section 5.1.11 is that:
a) it seems that some VPN clients would be happy to _not_ have to bother with running RP(s) 
b) it seems that some providers may happily host the RP function
So I don't want focus to get lost...

Below is my suggestion for a reshaped section 5.1.11.
It aims at distinguishing between the three different problems that we may have: RP outsourcing, RP availability, RP location.

Feel free to comment.

Cheers,

-Thomas

----------------------------

5.1.11. RP Engineering
When PIM-SM (or bidir-PIM) is used in ASM mode on the VPN customer side, the RP function (or RP-address in the case of bidir-PIM) has to be associated to a node running PIM, and configured on this node.





5.1.11.1. RP Outsourcing

In the case of PIM-SM in ASM mode, engineering of the RP function requires the deployment of specific protocols and associated configurations. Some service provider may offer to manage customer's multicast protocol operation on behalf of them. This implies that it is necessary to consider cases where the customer's RPs are outsourced (e.g., on PEs). Consequently, a VPN solution MAY support the hosting of the RP function in a VR or VRF.





5.1.11.2. RP Availability

Availability of the RP function (or address) is required for proper operation of PIM-SM (ASM mode) and bidir-PIM. Loss of connectivity to the RP from a receiver or source will impact the multicast service. For this reason different mecanisms exist, such as BSR(Bhaskar, N., “Bootstrap Router (BSR) Mechanism for PIM,” October 2005.) [I-D.ietf-pim-sm-bsr] or anycast-RP (MSDP based(Kim, D., Kilmer, H., and D. Farinacci, “Anycast RP mechanism using PIM and MSDP,” May 2001.) [I-D.ietf-mboned-anycast-rp] or PIM based(Farinacci, D. and Y. Cai, “Anycast-RP using PIM,” August 2005.) [I-D.ietf-pim-anycast-rp]).

These protocols and procedures SHOULD work transparently through a multicast VPN, and MAY, if relevant, be implemented in a VRF/VR.

Moreover, a multicast VPN solution MAY improve the robustness of the ASM multicast service regarding loss of connnectivity to the RP, by providing specific features that help :
a) maintaining ASM multicast service among all the sites within an MVPN that maintain connectivity among themselves, even when the site(s) hosting the RP lose their connectivity to the MVPN
b) maintaining ASM multicast service within any site that loses connectivity to the service provider





5.1.11.3. RP Location
In the case of PIM-SM, when a source starts to emit traffic toward a group (in ASM mode), if sources and receivers are located in VPN sites that are different than that of the RP, then traffic may transiently flow twice through the SP network and the CE-PE link of the RP (from source to RP, and then from RP to receivers). This traffic peak, even short, may no be convenient depending on the traffic and link bandwidth.

Thus, a VPN solution MAY provide features that solve or help mitigate this potential issue.