grow: Re: I-D ACTION:draft-ietf-grow-rfc1519bis-02.txt

[Pekka Savola <[e-mail]>]

On Mon, 13 Jun 2005 [e-mail] wrote:
> A New Internet-Draft is available from the on-line Internet-Drafts directories.
> This draft is a work item of the Global Routing Operations Working Group of the IETF.
>
> 	Title		: Classless Inter-Domain Routing (CIDR): The Internet
> 			  Address Assignment and Aggregation Plan
> 	Author(s)	: T. Li, V. Fuller
> 	Filename	: draft-ietf-grow-rfc1519bis-02.txt
> 	Pages		: 28
> 	Date		: 2005-6-13

Sorry I couldn't find the time to look at this during WGLC, so maybe 
you should consider these as IETF LC comments.

In general I think the doc is good, but there are a couple of places 
which weren't updated since 1519 which needed a slight polish, and a 
couple of pieces of text (like on multihoming) which left me wondering 
whether they even belong to this document (at least in that extent). 
The problem could be that someone thinks the text here is normative on 
how you should or should not multihome, how IANA should allocate 
addresses, etc., and we don't want to create that perception.

Below..

substantial
-----------

  The IANA makes allocations from this pool to Regional
    Internet Registries, as required.  These allocations are made in
    contiguous bit-aligned blocks of 2^24 addresses (a.k.a. /8 prefixes).

==> I do not think the latter sentence is appropriate in a standards track
document.  We should not be micro-managing the IANA allocation procedures in
this document; if the IANA wanted to change to allocate e.g., /9 or /7, 
someone could come along and point to this doc, stating that the IETF
doesn't support such allocations.  The text like this should be reworded so
that the /8 prefixes can not be read as normative text but rather just the
status as of writing this document.

    o  An organization which is multi-homed.  Because a multi-homed
       organization must be advertised into the system by each of its
       service providers, it is often not feasible to aggregate its
       routing information into the address space of any one of those
       providers.  Note that the organization still may receive its
       address assignment out of a service provider's address space
       (which has other advantages),

                                    but a route to the organization's
       prefix must still be explicitly advertised by all of its service
       providers.  For this reason, the global routing cost for a multi-
       homed organization is generally the same as it was prior to the
       adoption of CIDR.

==> this document describes the multihoming approaches at quite bit of
length, and I'm not sure if such are appropriate for a standards track
document.  Certainly, the practices do change, and the text above "must ..
be advertised by all.." is not correct.  As was discussed in section 5.2, if
the site is using one ISP as the primary, and is using a more specific
prefix, there exists a valid case of multihoming where advertising the
prefix equally through both ISPs is not required.

It would be useful to consider to which degree the language of what must be
done in multihoming scenarios is needed in this doc, but if it is needed,
the tone should possibly be watered down a bit to also address the
cornercases like above.

               Also, since the routing cost
    associated with assigning a multi-homed site out of a service
    provider's address space is no greater than the old method of
    sequential number assignment by a central authority, it makes sense
    to assign all end-site address space out of blocks allocated to
    service providers.

==> the routing cost might be the same either way (more specific vs an
independent address block), but there are other significant considerations
(which is one reason, above, why this kind of text might be controversial in
a standards track document on CIDR; a BCP on multihoming mechanisms might be
a better place).  Specifically, when ISPs employ BGP or IP prefix filtering,
to block outside advertisements (or packets) from their own address space,
multihomed more specifics from your own block are exceptions you must
handle.

More specifics are also less elegant because they create a perception of
routing swamp -- it is more difficult to pursue "bad advertisements" when
there is legitimate or recommended use of advertising more specifics.

....

4.1  Rules for route advertisement

    1.  Routing to all destinations must be done on a longest-match basis
        only. [...]

==> this is an overly simplistic statement.  Shouldn't you rather say that
longest-match basis must always be the _first_ route selection criteria? (by
the way some multicast RPF techniques allow overriding this AFAIR) --
otherwise the text is confusing about the other route selection criteria
(such as traffic class for class-based routing, protocol distance, etc.)

Note that the degenerate route to
    prefix 0.0.0.0/0 is used as a default route and MUST be accepted by
    all implementations.  Further, to protect against accidental
    advertisements of this route via the inter-domain protocol, this
    route should only be advertised when a router is explicitly
    configured to do so - never as a non-configured, "default" option.

==> I do not think the "Further, ..." statement is appropriate here -- and I
don't think the vendors actually implement this stuff.  I suggest just
removing the last sentence completely.

Multi-homed networks are always explicitly advertised
    by every service provider through which they are routed even if they
    are a specific subset of one service provider's aggregate (if they
    are not, they clearly must be explicitly advertised).  It may seem as
    if the "primary" service provider could advertise the multi-homed
    site implicitly as part of its aggregate, but the assumption that
    longest-match routing is always done causes this not to work.

==> see above; not sure if this text is appropriate or useful in this kind
of doc (in any case, the same thing seems to be said in different ways in
about 3 different places in the doc)

    These six sites should be represented as six prefixes of varying size
    within the provider IGP.  If, for some reason, the provider were to
    use an obsolete IGP that doesn't support classless routing or
    variable-length subnets, then then explicit routes all /24s will have
    to be carried.

==> what's your definition of IGP?  typically you don't carry customer
routes or even your own aggregates in your IGP, so the text could probably
use refreshing.

    See [RFC2317] for a much more detailed discussion of DNS delegation
    with classless addressing.

==> "much more detailed discussion" indeed -- this doc doesn't really
address the beef of the classless DNS delegation, i.e., assignments on
boundaries other than 8 bits.  I'd cut down the amount of DNS text that
currently exists or put in an example of about /26, /27, or /30 reverse dns
classless delegation.



editorial
---------

    o  RFC 2036: Observations on the use of Components of the Class A
       Address Space within the Internet

[...]

    o  RFC 1518: An Architecture for IP Address Allocation with CIDR

==> 1518 should probably be put in the right numerical place?

   Rule #1 guarantees that the routing algorithm used is consistent
    across implementations and consistent with other routing protocols,
    such as OSPF.

==> "_other_ routing protocols" ? so, I guess the document is implicitly
written about BGP?  Rewording needed..

When that traffic gets to the "child", however,
    the mid-level *must not* follow the route 192.168.0.0/16 back up to
    the "parent" ...

==> the first use of the term "mid-level".  what do you mean?  clarify.
(btw, the paragraph was difficult to understand though it described
something that's blindingly obvious right now.  Maybe it could use a
rephrasing.)

  This can be a useful
    tool for reducing the amount of routing state that an AS must carry
    and propagate to its customers and neighbors, proxy aggregation can
    also create inconsistencies in global routing state.

==> insert "however" or something before "proxy aggregation" ?

  Assuming
    a best common practice for network administrators to exchange lists
    of prefixes to accept from one and other,

==> s/to/is to/ ?

5.  Example of new address assignments and routing

==> remove "new" ?

    2.  Acceleration of the exponential trend in late 1993 and early 1994
        as CIDR supernet" blocks were first assigned by the NIC and

==> missing "

   this base "root" collection.  There is reason to believe that many of
    these additional entries are exist to solve problems of regional or
    even local scope and should not need to be globally propagated.

==> remove "are" or "exist"
==> btw, most of these actually don't solve any problem at all, but are just
useless junk :)






-- 
Pekka Savola                 "You each name yourselves king, yet the
Netcore Oy                    kingdom bleeds."
Systems. Networks. Security. -- George R.R. Martin: A Clash of Kings
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