Subject: [multimob] Question for RFC 6224

From: Aaron Feng <[email-masked]>
Date: Mon, 29 Aug 2011 10:43:26 +0800

Hi!
    I have a quention for RFC 6224. In RFC 6224, under the scenario
where MLD Proxy instances are only deployed on MAGs, there may be a
problem. After receiving the MLD report message from MNs, the attached MAG
should be on behalf of the MNs sending MLD report message using the link
address of  the MAG itself as the L3 source address of data
packets. According to RFC 3810, MLDv2 protocol is used by an IPv6 router to
discover the presence of multicast listeners *on directly attached
links*. However, in
most cases, the LMA and MAG are not directly connected and they don't have
the same link address prefix, so maybe LMA will discard the MLD report
messages for not direct connection even the MLD report messages could be
transmitted through the LMA-MAG tunnel to LMA. In my opinion, at first, when
receiving packets from the corresponding MAG, the LMA should first
decapsulate the packets and then extract the MLD report messages to execute
check of direct connectionly, using source address prefix of the packets
which is the link address prefix of the MAG before adding or maintaining the
corresponding MFIB info. Is what I think resaonable? Give some opinions
please~

                                          Aaron Feng

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From: "Thomas C. Schmidt" <[email-masked]>
Date: Mon, 29 Aug 2011 10:59:32 +0200

Hi Aaron,

thanks for your comments.

There are two MLD reports involved here: one from MN to the MAG which 
are physically link-local, one from the MAG-Proxy to LMA which are 
virtually link-local by using a tunnel (normally generic as of RFC 2473).

You address the second one.

The answer should be as follows: The LMA operates in MLD router part on 
the tunnel interface and listens to appropriate groups (depending on the 
MLD version). MAG transmits MLD reports with link-locally scoped 
messages (link-local address, hop limit 1, router alert option) that are 
encapsulated in the tunnel and thus arrive at the LMA. The LMA tunnel 
interface endpoint will decapsulate the report and - according to RFC 
3810 - checks:

  "Upon reception of an MLD message that contains a Report, the router
    checks if the source address of the message is a valid link-local
    address, if the Hop Limit is set to 1, and if the Router Alert option
    is present in the Hop-By-Hop Options header of the IPv6 packet. "

This will all be true, so LMA will continue to process the report and 
build per interface states accordingly.

Could I explain that well enough?

Thomas

On 29.08.2011 04:43, Aaron Feng wrote:
> problem. After receiving the MLD report message from MNs, the attached
> MAG should be on behalf of the MNs sending MLD report message using the
> link address of  the MAG itself as the L3 source address of data
> packets. According to RFC 3810, MLDv2 protocol is used by an IPv6 router
> to discover the presence of multicast listeners _*on directly* *attached
> links*_. However, in most cases, the LMA and MAG are not directly
> connected and they don't have the same link address prefix, so maybe LMA
> will discard the MLD report messages for not direct connection even the
> MLD report messages could be transmitted through the LMA-MAG tunnel to
> LMA. In my opinion, at first, when receiving packets from the
> corresponding MAG, the LMA should first decapsulate the packets and then
> extract the MLD report messages to execute check of direct
> connectionly, using source address prefix of the packets which is the
> link address prefix of the MAG before adding or maintaining the

-- 

Prof. Dr. Thomas C. Schmidt
° Hamburg University of Applied Sciences                   Berliner Tor 7 °
° Dept. Informatik, Internet Technologies Group    20099 Hamburg, Germany °
° http://www.haw-hamburg.de/inet                   Fon: +49-40-42875-8452 °
° http://www.informatik.haw-hamburg.de/~schmidt    Fax: +49-40-42875-8409 °

---

From: Aaron Feng <[email-masked]>
Date: Mon, 29 Aug 2011 18:00:19 +0800

Hi, Thomas

 Thank you for your reply!

>
> There are two MLD reports involved here: one from MN to the MAG which are
> physically link-local, one from the MAG-Proxy to LMA which are virtually
> link-local by using a tunnel (normally generic as of RFC 2473).
>

Yes.

>
> The answer should be as follows: The LMA operates in MLD router part on the
> tunnel interface and listens to appropriate groups (depending on the MLD
> version). MAG transmits MLD reports with link-locally scoped messages
> (link-local address, hop limit 1, router alert option) that are encapsulated
> in the tunnel and thus arrive at the LMA. The LMA tunnel interface endpoint
> will decapsulate the report and - according to RFC 3810 - checks:
>
   I agree above.

   I think the divergence between us is how to comprehend the "valid
link-local address". In my opinion, a valid link-local address means more
than a legal link address, but also a *local* link-address, with the same
link-address prefix of the router which receives the MLD Report messages
(It's the LMA in such scenario, I think, the "local" is not for the MAG, but
for the LMA ). However, in most cases, MAGs and LMAs don't have the
same link-address prefix though they are virtually directly connected, but
after decapsulation at first, they aren't directly connected, which may
cause the failure of direct connection. That's what I worry about.

                                                             Aaron Feng

---

From: "Thomas C. Schmidt" <[email-masked]>
Date: Mon, 29 Aug 2011 13:17:13 +0200

Hi Aaron,

On 29.08.2011 12:00, Aaron Feng wrote:

>
>     The answer should be as follows: The LMA operates in MLD router part
>     on the tunnel interface and listens to appropriate groups (depending
>     on the MLD version). MAG transmits MLD reports with link-locally
>     scoped messages (link-local address, hop limit 1, router alert
>     option) that are encapsulated in the tunnel and thus arrive at the
>     LMA. The LMA tunnel interface endpoint will decapsulate the report
>     and - according to RFC 3810 - checks:
>
> link-local address". In my opinion, a valid link-local address means
> more than a legal link address, but also a _local_ link-address, with
> the same link-address prefix of the router which receives the MLD Report
> messages (It's the LMA in such scenario, I think, the "local" is not for
> the MAG, but for the LMA ). However, in most cases, MAGs and LMAs don't
> have the same link-address prefix though they are virtually directly
> connected, but after decapsulation at first, they aren't
> directly connected, which may cause the failure of direct connection.
> That's what I worry about.

An IPv6 link-local address always has the prefix FE80::/10. There is no 
additional "link-address prefix", as link-local addresses are not 
routable -> see RFC 4291.

Hope that resolves the issue.

Best regards,

Thomas
--

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From: Aaron Feng <[email-masked]>
Date: Mon, 29 Aug 2011 20:11:29 +0800

Hi, Thomas,

You are right, thank you!  :)

                   Aaron Feng