Network Working Group E. Lear
Request for Comments: 1627 Silicon Graphics, Inc.
Category: Informational E. Fair
Apple Computer, Inc.
D. Crocker
Silicon Graphics, Inc.
T. Kessler
Sun Microsystems, Inc.
July 1994
Network 10 Considered Harmful
(Some Practices Shouldn't be Codified)
Status of this Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
SUMMARY
Re-use of Internet addresses for private IP networks is the topic of
the recent RFC 1597 [1]. It reserves a set of IP network numbers,
for (re-)use by any number of organizations, so long as those
networks are not routed outside any single, private IP network. RFC
1597 departs from the basic architectural rule that IP addresses must
be globally unique, and it does so without having had the benefit of
the usual, public review and approval by the IETF or IAB. This
document restates the arguments for maintaining a unique address
space. Concerns for Internet architecture and operations, as well as
IETF procedure, are explored.
INTRODUCTION
Growth in use of Internet technology and in attachments to the
Internet have taken us to the point that we now are in danger of
running out of unassigned IP network numbers. Initially, numbers
were formally assigned only when a network was about to be attached
to the Internet. This caused difficulties when initial use of IP
substantially preceded the decision and permission to attach to the
Internet. In particular, re-numbering was painful. The lesson that
we learned was that every IP address ought to be globally unique,
independent of its attachment to the Internet. This makes it
possible for any two network entities to communicate, no matter where
either might be located. This model is the result of a decades-long
evolution, through which the community realized how painful it can be
to convert a network of computers to use an assigned number after
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using random or default addresses found on computers just out of the
box. RFC 1597 abrogates this model without benefit of general IETF
community discussion and consensus, leaving policy and operational
questions unasked and unanswered.
KEEP OUR EYES ON THE PRIZE: AN ARCHITECTURAL GOAL AND VIOLATION
A common -- if not universal -- ideal for the future of IP is for
every system to be globally accessible, given the proper security
mechanisms. Whether such systems comprise toasters, light switches,
utility power poles, field medical equipment, or the classic examples
of "computers", our current model of assignment is to ensure that
they can interoperate.
In order for such a model to work there must exist a globally unique
addressing system. A common complaint throughout the community is
that the existing security in host software does not allow for every
(or even many) hosts in a corporate environment to have direct IP
access. When this problem is addressed through proper privacy and
authentication standards, non-unique IP addresses will become a
bottleneck to easy deployment if the recommendations in RFC 1597 are
followed.
The IP version 4 (IPv4) address space will be exhausted. The
question is simply: when?
If we assert that all IP addresses must be unique globally, connected
or not, then we will run out of IP address space soon.
If we assert that only IP addresses used on the world-wide Internet
need to be globally unique, then we will run out of IP address space
later.
It is absolutely key to keep the Internet community's attention
focused on the efforts toward IP next generation (IPng), so that we
may transcend the limitations of IPv4. RFC 1597 produces apparent
relief from IPv4 address space exhaustion by masking those networks
that are not connecting to the Internet, today. However, this
apparent relief will likely produce two results: complacency on the
large part of the community that does not take the long term view,
and a very sudden IP address space exhaustion at some later date.
Prior to IPng deployment, it is important to preserve all the
semantics that make both the Internet and Internet technology so very
valuable for interoperability. Apple Computer, IBM, and Motorola
could not collaborate as easily as they have to produce the PowerPC
without uniquely assigned IP addresses. The same can be said of the
Silicon Graphics merger with MIPS. There are many, many more examples
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that can be cited.
It should be noted that a scheme similar to RFC 1597 can be
implemented at the time that we actually run out of assignable IPv4
address space; it simply requires that those organizations which have
been assigned addresses but are not yet connected to the Internet
return their addresses to IANA. It is important that the IAB (and
IANA as its agent) reassert their ownership of the IP address space
now, to preclude challenges to this type of reassignment.
OPERATIONAL ISSUES
RFC 1597 Implementations
Methods are needed to ensure that the remaining addresses are
allocated and used frugally. Due to the current problems, Internet
service providers have made it increasingly difficult for
organizations to acquire public IP network numbers. Private networks
have always had the option of using addresses not assigned to them by
appropriate authorities. We do not know how many such networks
exist, because by their nature they do not interact with the global
Internet. By using a random address, a company must take some care
to ensure it is able to route to the properly registered owner of
that network.
RFC 1597 proposes to solve the routing problem by assigning numbers
that will never be used outside of private environments. Using such
standard numbers introduces a potential for clashes in another way.
If two private networks follow RFC 1597 and then later wish to
communicate with each other, one will have to renumber. The same
problem occurs if a private network wishes to become public. The
likely cost of renumbering is linear to the number of hosts on a
network. Thus, a large company with 10,000 hosts on a network could
incur considerable expense if it either merged with another company
or joined the Internet in such a way as to allow all hosts to
directly access the outside network.
The probability of address clashes occurring over time approach 100%
with RFC 1597. Picking a random network number reduces the chances
of having to renumber hosts, but introduces the routing problems
described above. Best of all, retrieving assigned numbers from the
appropriate authority in the first place eliminates both existing and
potential address conflicts at the cost of using a part of the
address space.
Apple Computer once believed that none of its internal systems would
ever speak IP directly to the outside world, and as such, network
operations picked IP class A network 90 out of thin air to use.
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Apple is only now recovering from this error, having renumbered some
5,000 hosts to provide them with "desktop" Internet access. Unless
the Internet community reaffirms its commitment to a globally unique
address space, we condemn many thousands of organizations to similar
pain when they too attempt to answer the call of the global Internet.
Another timely example of problems caused by RFC 1597 is Sun's use of
Internet multicasting. Sun selectively relays specific multicast
conferences. This has the effect of making many hosts at Sun visible
to the Internet, even though they are not addressable via IP unicast
routing. If they had non-global addresses this would not work at
all. It is not possible to predict which machines need global
addresses in advance. Silicon Graphics has a similar configuration,
as is likely for others, as well.
Some might argue that assigning numbers to use for private networks
will prevent accidental leaks from occurring through some sort of
convention a'la Martian packets. While the proposal attempts to
create a standard for "private" address use, there is absolutely no
way to ensure that other addresses are not also used.
Hence, the "standard" becomes nothing but a misleading heuristic. In
fact, it is essential that routers to the global Internet advertise
networks based only on explicit permission, rather than refusing to
advertise others based on implicit prohibition, as supported by the
policy formally created in RFC 1597.
Security Issues
Administrators will have a hard time spotting unauthorized networks,
when their network has been breached (either intentionally or
unintentionally) because the other networks might have the same
numbers as those normally in the routing tables. More over, an
inadvertent connection could possibly have a double whammy effect of
partitioning two operational networks.
It is worth emphasizing that IP providers should filter out all but
authorized networks. Such a practice would not only prevent
accidents but also enhance the security of the Internet by reducing
the potential number of points of attack.
Internet multicasting adds a new dimension to security. In some
cases it may possible to allow multicasting through firewalls that
completely restrict unicast routing. Otherwise unconnected networks
might well need unique addresses, as illustrated in the example
above.
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Problems with Examples
RFC 1597 gives several examples of IP networks that need not have
globally unique address spaces. Each of those cases is plausible,
but that does not make it legitimate to ENCOURAGE non-uniqueness of
the addresses. In fact, it is equally plausible that globally unique
IP addresses will be required, for every one of the scenarios
described in RFC 1597:
- Airport displays are public information and multicasting beyond the
airport might be useful.
- An organization's machines which, today, do not need global
connectivity might need it tomorrow. Further, merging
organizations creates havoc when the addresses collide.
- Current use of firewalls is an artifact of limitations in the
technology. Let's fix the problem, not the symptom.
- Inter-organization private links do not generate benefit from being
any more correct in guessing which machines want to interact than
is true for general Internet access.
This is another point that warrants repetition: the belief that
administrators can predict which machines will need Internet access
is quite simply wrong. We need to reduce or eliminate the penalties
associated with that error, in order to encourage as much Internet
connectivity as operational policies and technical security permit.
RFC 1597 works very much against this goal.
Problems With "Advantages" And More Disadvantages
RFC 1597 claims that Classless Inter-Domain Routing (CIDR) will
require enterprises to renumber their networks. In the general case,
this will only involve those networks that are routed outside of
enterprises. Since RFC 1597 addresses private enterprise networks,
this argument does not apply.
The authors mention that DCHP-based tools [2] might help network
number transition. However, it is observed that by and large such
tools are currently only "potential" in nature.
Additionally, with the onslaught of ISDN, slip, and PPP in host
implementations, the potential for a workstation to become a router
inadvertently has never been greater. Use of a common set of
addresses for private networks virtually assures administrators of
having their networks partitioned, if they do not take care to
carefully control modem connections.
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Finally, RFC 1597 implies that it may be simple to change a host's IP
address. For a variety of reasons this may not be the case, and it
is not the norm today. For example, a host may be well known within
a network. It may have long standing services such as NFS, which
would cause problems for clients were its address changed. A host
may have software licenses locked by IP address. Thus, migrating a
host from private to global addressing may prove difficult. At the
very least, one should be careful about addressing well known hosts.
POLICY ISSUES
IANA Has Overstepped Their Mandate
For many years, IANA has followed an assignment policy based on the
expectation of Internet connectivity for ALL assignees. As such it
serves to encourage interconnectivity. IANA assignment of the
network numbers listed in RFC 1597 serves to formally authorize
behavior contrary to this accepted practice. Further, this change
was effected without benefit of community review and approval.
RFC 1597 specifies a new operational requirement explicitly: network
service providers must filter the IANA assigned network numbers
listed in RFC 1597 from their routing tables. This address space
allocation is permanently removed from being used on the Internet.
As we read RFC 1601 [3], this action is not within the purview of
IANA, which should only be assigning numbers within the current
standards and axioms that underlie the Internet. IP network numbers
are assigned uniquely under the assumption that they will be used on
the Internet at some future date. Such assignments violate that
axiom, and constitute an architectural change to the Internet. RFC
1602 [4] and RFC 1310 [5] also contain identical wording to this
effect in the section that describes IANA.
While RFC 1597 contains a view worthy of public debate, it is not
ready for formal authorization. Hence, we strongly encourage IANA to
withdraw its IP address assignments documented by RFC 1597 forthwith.
The IAB should review the address assignment policies and procedures
that compose IANA's mandate, and reaffirm the commitment to a
globally unique IP address space.
COMMENTS AND CONCLUSIONS
The Internet technology and service is predicated on a global address
space. Members of the Internet community have already experienced
and understood the problems and pains associated with uncoordinated
private network number assignments. In effect the proposal attempts
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to codify uncoordinated behavior and alter the accepted Internet
addressing model. Hence, it needs to be considered much more
thoroughly.
RFC 1597 gives the illusion of remedying a problem, by creating
formal structure to a long-standing informal practice. In fact, the
structure distracts us from the need to solve these very real
problems and does not even provide substantive aid in the near-term.
In the past we have all dreaded the idea of having any part of the
address space re-used. Numerous luminaries have both written and
spoke at length, explaining why it is we want direct connections from
one host to another. Before straying from the current architectural
path, we as a community should revisit the reasoning behind the
preaching of unique addressing. While RFC 1597 attempts to change
this model, its costs and limitations for enterprises can be
enormous, both in the short and long term.
REFERENCES
[1] Rekhter, Y., Moskowitz, B., Karrenberg, D., and G. de Groot,
"Address Allocation for Private Internets", T.J. Watson Research
Center, IBM Corp., Chrysler Corp., RIPE NCC, RFC 1597, March
1994.
[2] Droms, R., "Dynamic Host Configuration Protocol", RFC 1541,
Bucknell University, October 1993.
[3] Huitema, C., "Charter of the Internet Architecture Board (IAB)",
RFC 1601, IAB, March 1994.
[4] Internet Architecture Board, Internet Engineering Steering
Group, "The Internet Standards Process -- Revision 2", IAB,
IESG, RFC 1602, March 1994.
[5] Internet Activities Board, "The Internet Standards Process", RFC
1310, IAB, March 1992.
[6] Internet Activities Board, "Summary of Internet Architecture
Discussion", Notes available from ISI, [ftp.isi.edu:
pub/IAB/IABmins.jan91Arch.txt], IAB, January 1991.
SECURITY CONSIDERATIONS
See the section, "Security Issues".
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AUTHORS' ADDRESSES
Eliot Lear
Silicon Graphics, Inc.
2011 N. Shoreline Blvd.
Mountain View, CA
94043-1389
Phone: +1 415 390 2414
EMail: lear@sgi.com
Erik Fair
Apple Computer, Inc.
1 Infinite Loop
Cupertino, CA 95014
Phone: +1 408 974 1779
EMail: fair@apple.com
Dave Crocker
Silicon Graphics, Inc.
2011 N. Shoreline Blvd.
Mountain View, CA
94043-1389
Phone: +1 415 390 1804
EMail: dcrocker@sgi.com
Thomas Kessler
Sun Microsystems Inc.
Mail Stop MTV05-44
2550 Garcia Ave.
Mountain View, CA 94043
Phone: +1 415 336 3145
EMail: kessler@eng.sun.com
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