Subject: (IPng 7577) Multicast Listener Discovery timing question

From: Jean-Luc Richier <[email-masked]>
Date: Wed, 2 Jun 1999 11:39:28 +0200 (MET DST)

I have a question about draft-ietf-ipngwg-mld-01.txt about timing
considerations.

As I said in a previous mail, I suppose the rationale for using MLD
on sollicited node multicast groups is that even though multicast addresses
whose scope is 2 should not be routed, there can be level 2 bridges/switches
which are performing multicast selective forwarding, and thus engage in the
mld protocol.

draft-ietf-ipngwg-mld-01.txt states:

    When a node starts listening to a multicast address on an interface, it
    should immediately transmit an unsolicited Report for that address on
    that interface, in case it is the first listener on the link.  To cover
    the possibility of the initial Report being lost or damaged, it is
    recommended that it be repeated once or twice after short delays
    [Unsolicited Report Interval].  (A simple way to accomplish this is to
    send the initial Report and then act as if a Multicast-Address-Specific
    Query was received for that address, and set a timer appropriately).

    The Unsolicited Report Interval is the time between repetitions of a
    node's initial report of interest in a multicast address.  Default: 10
    seconds.

But RFC2461 and RFC2462 state:
      DupAddrDetectTransmits
                     Default: 1, but may be overridden by a link-type
                     specific value in the document that covers issues
                     related to the transmission of IP over a particular
                     link type (e.g., [IPv6-ETHER]).

                     Autoconfiguration also assumes the presence of the
                     variable RetransTimer as defined in [DISCOVERY].
                     For autoconfiguration purposes, RetransTimer
                     specifies the delay between consecutive Neighbor
                     Solicitation transmissions performed during
                     Duplicate Address Detection (if
                     DupAddrDetectTransmits is greater than 1), as well
                     as the time a node waits after sending the last
                     Neighbor Solicitation before ending the Duplicate
                     Address Detection process.

        RetransTimer   The time between retransmissions of Neighbor
                       Solicitation messages to a neighbor when
                       resolving the address or when probing the
                       reachability of a neighbor.

                       Default: RETRANS_TIMER milliseconds

            RETRANS_TIMER                 1,000 milliseconds

Therefore, even if one sets DupAddrDetectTransmits to 2 or 3, the Duplicate
    address detection algorithm will end before the retransmission of an
    ``initial Report being lost or damaged''.

Is it not a reliability problem ?

-- 
Jean-Luc RICHIER ([email-masked]  [email-masked])
Laboratoire Logiciels, Systemes et Reseaux (LSR-IMAG)
IMAG-CAMPUS, BP 72, F-38402 St Martin d'Heres Cedex
Tel : +33 4 76 82 72 32 Fax : +33 4 76 82 72 87
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