Subject: [magma] IGMP Proxy Querier-based upstream forwarding

From: James Lingard <[email-masked]>
Date: Mon, 19 Apr 2004 18:24:25 +0100

Hi,

The IGMP Proxy draft describes how multicast packets are only forwarded *to*
a downstream interface if the proxy device is the IGMP Querier on that
downstream interface.  However, the draft is not clear that packets
*received* on a downstream interface must not be forwarded unless the proxy
device is the IGMP Querier on that interface.  I believe this also is
necessary in order to avoid forwarding loops.

For example, consider the network shown in the following diagram (taken from
the draft), where both A and B are proxy devices, and A is the Querier on
LAN 2.

           LAN 1  --------------------------------------
                  Upstream |              | Upstream
                           A              B
                Downstream |              | Downstream
           LAN 2  --------------------------------------

Any traffic sent to group G by a source host on LAN 2 will be forwarded onto
LAN 1 by router A.  If router B also forwards this packet onto LAN 1, then
if LAN 2 contains a receiver for group G, this second copy of the packet
will be forwarded back onto LAN 2 by router A, and back onto LAN 1 by router
B... and we have a loop.

Assuming my reasoning is correct, I have listed below some suggested
clarifications to the draft.  Please feel free to use these as is or to
modify them however you wish.

Thanks,
James.

==========================================================================
*   Section 3, page 4/5.

Original text
>>   When the proxy device receives a packet destined for a multicast
>>   group (channel in Source-Specific Multicast (SSM)), it uses a list
>>   consisting of the upstream interface and any downstream interface
>>   which has a subscription pertaining to this packet and on which it is
>>   the IGMP/MLD Querier.  This list may be built dynamically or cached.
>>   It removes the interface on which this packet arrived from the list
>>   and forwards the packet to the remaining interfaces (this may include
>>   the upstream interface).

Suggested replacement
>>   When the proxy device receives a packet destined for a multicast
>>   group (channel in Source-Specific Multicast (SSM)), the packet is
>>   accepted for forwarding if it arrives on the upstream interface or on
>>   a downstream interface for which the proxy device is the IGMP/MLD
>>   Querier.  Multicast packets arriving on other downstream interfaces
>>   are not forwarded.
>>   
>>   To determine where to forward an accepted multicast packet, the proxy
>>   device uses a list consisting of the upstream interface and any
>>   downstream interface which has a subscription pertaining to this
>>   packet and on which it is the IGMP/MLD Querier.

==========================================================================
*   Section 3, page 5.

Original text
>>   On a link with only one IGMP/MLD-based forwarding
>>   device, this rule MAY be disabled (i.e. the device MAY be configured
>>   to forward packets to an interface on which it is not the querier).

Suggested replacement
>>   On a link with only one IGMP/MLD-based forwarding
>>   device, this rule MAY be disabled (i.e. the device MAY be configured
>>   to forward packets to and from an interface on which it is not the 
>>   querier).

==========================================================================
*   Section 3, page 6.

Original text
>>   So proxy
>>   device A will forward packets to LAN 2 only if A is the querier. 

Suggested replacement
>>   So proxy
>>   device A will forward packets to and from LAN 2 only if A is the
>>   querier. 

==========================================================================
*   Section 4.2, page 8.

Original text
>>   A proxy device forwards packets received
>>   on any downstream interface to the upstream interface, and to each
>>   downstream interface other than the incoming interface based upon the
>>   downstream interfaces' subscriptions and whether or not this proxy
>>   device is the IGMP/MLD Querier on each interface.

Suggested replacement
>>   A proxy device forwards packets received
>>   on a downstream interface on which it is the IGMP/MLD Querier to the
>>   upstream interface, and to each downstream interface other than the
>>   incoming interface based upon the downstream interfaces'
>>   subscriptions and whether or not this proxy device is the IGMP/MLD
>>   Querier on each interface.  A proxy device does not forward packets
>>   received on a downstream interface on which it is not the IGMP/MLD
>>   Querier.

--
James Lingard
Data Connection Ltd (DCL)
http://www.dataconnection.com/

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