Internet DRAFT - draft-ietf-cidrd-myth
draft-ietf-cidrd-myth
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CIDRD Working Group D. Crocker
Internet Draft Brandenburg Consulting
<draft-ietf-cidrd-myth-00.txt> August 1995
The Myth of Topological Hierarchy:
Comments on <draft-ietf-cidrd-ownership-01.txt>
STATUS OF THIS MEMO
This document is an Internet Draft. Internet Drafts
are working documents of the Internet Engineering Task
Force (IETF), its Areas, and its Working Groups. Note
that other groups may also distribute working documents as
Internet Drafts.
Internet Drafts are draft documents valid for a maximum
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or to cite them other than as a ``working draft'' or ``work
in progress.''
Please check the 1id-abstracts.txt listing contained in
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Draft.
OVERVIEW
This note offers comments on the technical and
operational aspects of the proposal for large-scale use of
"address leasing" recommended in "On the Implications of
Address Ownership for Internet Routing", I-D <draft-ietf-
cidrd-ownership-01.txt>, by Rekhter & Li. The draft has
been produced within the cidrd working group and is intended
for publication as a Best Current Practices official IETF
document.
The "Ownership" document begins by discussing issues in
the use of addresses for routing. In particular it notes
that router memory and computation resources are neither
free nor infinite. It further notes that "flat" address
spaces are intolerable for an exponentially growing service
like the Internet. In short, the document re-asserts the
requirement for more efficient use of router storage and
computation.
The document goes on to describe the nature and
benefits of hierarchical addressing. It then, incorrectly,
asserts that the Internet topology reflects a hierarchy and
that addresses must be kept aligned with the hierarchy.
This requirement is used to assert the need for enforcing
addressing changes when (some) topological changes take
place. The document makes no effort to deal with the very
real difficulties this model creates for multi-homed
organizations, including local service providers. Working
group discussions have left the issue with citations to the
original CIDR document, but it offers no real guidance
either, since it largely presumes the NSFNet as the top of
the Internet's topology.
The document asserts the model of customer "leasing" of
addressing rather than "owning" them but does not discuss
the problems this creates with large-scale requirements for
renumbering, generally viewed as difficult, but potentially
quite insidious for local providers who change transit
providers.
Overall <draft-ietf-cidrd-ownership-01.txt> lacks
answers for problems of large-scale renumbering. It also
lack sufficient operational detail to serve as an adequate
description of current practices.
DETAILED COMMENTS
Comments follow the document's section headings:
1 Abstract
Paragraph 2 offers the document as a treatise or review
of issues, citing a "focus" on one approach. The primary
principle that the paper observes is that flat address
spaces don't scale well. In fact the paper is simply a
direct proposal for a particular scheme and should be
characterized as such. It makes no effort to consider
alternatives other than flat addressing nor even to consider
the full impact of its own proposal.
The title of the paper is not helpful and may well be
misleading. It introduces the concept of address "leasing",
rather than only discussing some drawbacks to permanent
"ownership". Hence I suggest that the title of the paper be
changed to something like "Proposal for General Use of
Address Prefix Leasing"
2 Address allocation
Paragraph 3 appears to claim that address ownership has
not actually been the policy for more than 15 years and
therefore implies that the idea of address leasing is not
new and frankly experimental. This is at best incorrect and
at worst misleading. For more than 15 years user sites have
been given permanent network numbers and have been free to
attach via any available provider using those numbers. It
is only quite recently that providers have begun to require
use of CIDR space and/or change addresses when changing
providers.
The paper should not attempt to suggest to the
reader that leasing is anything other than a fundamental
change in Internet address administration policy. As such,
the paper should acknowledge and emphasize the frankly
experimental nature of leasing done on a global scale.
(Yes, I did say experimental. Contrary to the
comments on the cidrd mailing list there has been no large
scale use of a leasing policy and no real analysis of its
effect. When claims are made to the contrary, they seem to
hinge on a) the lack of changes needed to the large transit
providers and their routers and b) a long history of
organizations occasionally changing their IP network
numbers. A side effect of this is that some other schemes
which have been handily dismissed as experimental probably
warrant more equal consideration. One more presumption that
warrants attention is that the Internet does not have time
to consider alternatives. (The facile counter to this
concern is to ask whether the Internet has time to see CIDR
and address leasing fail, eliminate local providers, or
otherwise kill the Internet?)
3 Addressing and routing (no comments)
4 Address ownership
Paragraph 4 refers to space complexity for "the
different schemes". Which schemes? The paper only
discusses totally flat and totally hierarchical schemes. Is
this total of 2 schemes what is being cited?
In the next paragraph the paper states the importance
of having an address reflect the topology of the network.
Since the Internet is not a simple tree, but instead is a
messy mesh one must ask by what procrustean process the
Internet is to be represented by (relatively) stable
hierarchical addresses? It is certainly true that a graph
can be represented as a tree by taking a particular view of
it but that representation only holds from that view. Such
a view is applicable to snapshots needed for when making
routing decisions but not for globally-absolute addressing.
The paper also says that changing an address is
required as the network topology changes. This simply isn't
true or, at least, is not what is being done or proposed.
Internet topology changes all the time and sites are not
required to change their addresses. At best this paragraph
is seriously imprecise. At worst it is seriously wrong.
The end effect is to aid in the myth of Internet topological
hierarchy. We need to reverse this misleading view.
The seventh paragraph discusses routing table
exceptions. (The second sentence is awkward and should be
re-written.) Exceptions are those entries which do not
conform to the hierarchical model. The paragraph
acknowledges that the Internet isn't strictly hierarchical
but it then entirely misses the implication. For example it
fails to note the effect of large-scale occurrence of multi-
homing by local providers and users.
The last paragraph in the section makes quick mention
of a possible impact on organizations which "sub-lease"
numbers. Presumably this means that a local provider which
changes transit providers will be forced to force its
customers to renumber? While large transit providers well
might not view this impact as substantial I suggest that it
is, in fact, quite serious and deserves rather more
discussion. For example CIDR and address leasing is likely
to create a basic barrier to entry for local providers.
("Barrier to entry" is offered as a euphemism for "put all
of them out of business.") They might not approve of such
an effect.
5 Recommendations
The first paragraph contains the disclaimer "Existing
address assignments and allocations are outside the scope of
this document." However the handling of existing addresses
has very much been a part of working group discussions and
appears to be very much a part of the intent behind pursuit
of the address leasing model. In particular, working group
discussions have cited CIDR as no longer being inadequate
when applied to new addresses only and that recovering
existing addresses is required. Please note that I said
"required". That, at least, is the message that has been
communicated in recent working group discussions. If that
message is wrong then we need to hear rather clearer
explanations for the need to promulgate a leasing policy.
The proposal needs to attend to this issue directly and
should not pretend that it is anything other than an
immediate and pressing concern.
The next-to-last paragraph acknowledges that users
might incur some cost when renumbering, but the paragraph
entirely elides further discussion of the issue, instead
simply saying that organizations need to consider tradeoffs.
I suggest that a paper which is proposing a major change for
the Internet is obligated to offer rather more detail about
its impact. It needs to list and discuss those tradeoffs.
At the least that will demonstrate detailed understanding of
the proposed policy's impact.
The last paragraph cites DHCP. It appears to suggest
that renumbering tools are a) adequate, and b) adequately
deployed. The paper needs to discuss both of these
assumptions in detail, particularly since there is a
substantial constituency which believes that both
assumptions are entirely incorrect.
6 Conclusions
The first paragraph cites concerns for routing system
scaling. However there is no concern expressed for Internet
local providers or users. The paper needs substantial
enhancement to consider the breadth of the proposal's
effect. In fact I strongly urge than any proposal which
attempts major change to the Internet, as does this one, be
required to offer an operational "environmental impact
statement." Equally strongly I suggest that this include a
transition plan, as we require for other, equally massive
changes.
RECOMMENDATIONS
The Ownership document needs substantial re-working.
In particular it should treat multi-homing and renumbering
seriously and thoroughly.
It is time to consider alternatives to CIDR. CIDR was
chosen with the expectation that it would be a sufficient
near-term answer for routing table compression. After some
considerable initial success it is proving inadequate. The
latest round of effort is attempting to bolster an
inadequate technology in a fashion which is a) inappropriate
for frequent connectivity styles, and b) creates the very
real possibility of forcing an entire class of Internet
provider out of business.
