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Re: RFC 3484 Section 6 Rule 9

2008-06-02 06:55:14

Well, longest prefix match is kind of useful in some scenarios i think.

Imagine a site that is multihomed to two ISPs and has two PA address blocks.

Now, longest prefix match ensures that when a node of the multihomed 
site wants to contact any other customer of its own isps, it does 
perform the correct source address selection and that is likely to be 
critical for the communication to work, especially if the isps are doing 
ingress filtering (i am assuming that the intra site routing of the 
multihomed site will preffer the route through the ISP that owns the 
prefix contained in the destiantion address)

Even though this is one case and the problem is more general, i tend to 
think that this is an importnat case and things would break more if this 
rule didn't exist

Regards, marcelo

        Section 6 Rule 9 is DESTINATION address selection.  It
        provides absolutely no help when attempting to distingish
        a multi-homed destination that is not with your current
        ISP.  It also won't help once your current ISP has more
        than one prefix.  It doesn't help with PI clients connected
        to your current ISP.

        It biases what should be a random selection.

        There is no science that says a /30 match is better than a
        /28 or a /8 match.

        If one really wants to have directly connected clients of
        your ISP match then get a appropriate feed of prefixes and
        use it to build appropriate tables.  We have the technology
        to distribute sets of prefixes.

        Just don't attempt to have longest match do the just because
        it can't do it except for PA address and even then only
        when your ISP has a single prefix.  For any other senario
        it is biased garbage.
 
Mark Andrews escribió:
    This rule should not exist for IPv4 or IPv6.  Longest match
    does not make a good sorting critera for destination address
    selection.  In fact it has the opposite effect by concentrating
    traffic on particular address rather than spreading load.

    I received a request today asking us to break up DNS RRsets
    as a workaround to the rule.    Can we please get a errata
    entry for RFC 3484 stating that this rule needs to be ignored.

    Mark
  


-- 
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: Mark_Andrews(_at_)isc(_dot_)org
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