RFC2101 - IPv4 Address Behaviour Today

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　　Network Working Group B. Carpenter
Request for Comments: 2101 J. Crowcroft
Category: Informational Y. Rekhter
IAB
February 1997

IPv4 Address Behaviour Today

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.

Abstract

The main purpose of this note is to clarify the current
interpretation of the 32-bit IP version 4 address space, whose
significance has changed substantially since it was originally
defined. A short section on IPv6 addresses mentions the main points
of similarity with, and difference from, IPv4.

Table of Contents

1. Introduction.................................................1
2. Terminology..................................................2
3. Ideal properties.............................................3
4. Overview of the current situation of IPv4 addresses..........4
4.1. Addresses are no longer globally unique locators.........4
4.2. Addresses are no longer all temporally unique............6
4.3. Multicast and Anycast....................................7
4.4. Summary..................................................8
5. IPv6 Considerations..........................................8
ANNEX: Current Practices for IPv4 Address Allocation & Routing..9
Security Considerations........................................10
Acknowledgements...............................................11
References.....................................................11
Authors' Addresses.............................................13

1. Introduction

The main purpose of this note is to clarify the current
interpretation of the 32-bit IP version 4 address space, whose
significance has changed substantially since it was originally
defined in 1981 [RFC791].

This clarification is intended to assist protocol designers, product
implementors, Internet service providers, and user sites. It aims to
avoid misunderstandings about IP addresses that can result from the
substantial changes that have taken place in the last few years, as a
result of the Internet's exponential growth.

A short section on IPv6 addresses mentions the main points of
similarity with, and difference from, IPv4.

2. Terminology

It is well understood that in computer networks, the concepts of
directories, names, network addresses, and routes are separate and
must be analysed separately [RFC1498]. However, it is also
necessary to sub-divide the concept of "network address" (abbreviated
to "address" from here on) into at least two notions, namely
"identifier" and "locator". This was perhaps less well understood
when RFC791 was written.

In this document, the term "host" refers to any system originating
and/or terminating IPv4 packets, and "router" refers to any system
forwarding IPv4 packets from one host or router to another.

For the purposes of this document, an "identifier" is a bit string
which is used throughout the lifetime of a communication session
between two hosts, to identify one of the hosts as far as the other
is concerned. Such an identifier is used to verify the source of
incoming packets as being truly the other end of the communication
concerned, e.g. in the TCP pseudo-header [RFC793] or in an IP
Security association [RFC1825]. Traditionally, the source IPv4
address in every packet is used for this.

Note that other definitions of "identifier" are sometimes used; this
document does not claim to discuss the general issue of the semantics
of end-point identifiers.

For the purposes of this document, a "locator" is a bit string which
is used to identify where a particular packet must be delivered, i.e.
it serves to locate the place in the Internet topology where the
destination host is attached. Traditionally, the destination IPv4
address in every packet is used for this. IP routing protocols
interpret IPv4 addresses as locators and construct routing tables
based on which routers (which have their own locators) claim to know
a route towards the locators of particular hosts.

Both identifiers and locators have requirements of uniqueness, but
these requirements are different. Identifiers must be unique with
respect to each set of inter-communicating hosts. Locators must be

unique with respect to each set of inter-communicating routers (which
we will call a routing "realm"). While locators must be unique within
a given routing realm, this uniqueness (but not routability) could
extend to more than one realm. Thus we can further distinguish
between a set of realms with unique locators versus a set of realms
with non-unique (overlapping) locators.

Both identifiers and locators have requirements of lifetime, but
these requirements are different. Identifiers must be valid for at
least the maximum lifetime of a communication between two hosts.
Locators must be valid only as long as the routing mechanisms so
require (which could be shorter or longer than the lifetime of a
communication).

It will be noted that it is a contingent fact of history that the
same address space and the same fields in the IP header (source and
destination addresses) are used by RFC791 and RFC793 for both
identifiers and locators, and that in the traditional Internet a
host's identifier is identical to its locator, as well as being
spatially unique (unambiguous) and temporally unique (constant).

These uniqueness conditions had a number of consequences for design
assumptions of routing (the infrastructure that IPv4 locators enable)
and transport protocols (that which depends on the IP connectivity).
Spatial uniqueness of an address meant that it served as both an
interface identifier and a host identifier, as well as the key to the
routing table. Temporal uniqueness of an address meant that there
was no need for TCP implementations to maintain state regarding
identity of the far end, other than the IP address. Thus IP addresses
could be used both for end-to-end IP security and for binding upper
layer sessions.

Generally speaking, the use of IPv4 addresses as locators has been
considered more important than their use as identifiers, and whenever
there has been a conflict between the two uses, the use as a locator
has prevailed. That is, it has been considered more useful to deliver
the packet, then worry about how to identify the end points, than to
provide identity in a packet that cannot be delivered. In other
words, there has been intensive work on routing protocols and little
concrete work on other aspects of address usage.

3. Ideal properties.

Whatever the constraints mentioned above, it is easy to see the ideal
properties of identifiers and locators. Identifiers should be
assigned at birth, never change, and never be re-used. Locators
should describe the host's position in the network's topology, and
should change whenever the topology changes.

Unfortunately neither of the these ideals are met by IPv4 addresses.
The remainder of this document is intended as a snapshot of the
current real situation.

4. Overview of the current situation of IPv4 addresses.

It is a fact that IPv4 addresses are no longer all globally unique
and no longer all have indefinite lifetimes.

4.1 Addresses are no longer globally unique locators

[RFC1918] shows how corporate networks, a.k.a. Intranets, may if
necessary legitimately re-use a subset of the IPv4 address space,
forming multiple routing realms. At the boundary between two (or
more) routing realms, we may find a spectrum of devices that
enables communication between the realms.

At one end of the spectrum is a pure Application Layer Gateway
(ALG). Such a device acts as a termination point for the
application layer data stream, and is visible to an end-user. For
example, when an end-user Ua in routing realm A wants to
communicate with an end-user Ub in routing realm B, Ua has first
to explicitly establish communication with the ALG that
interconnects A and B, and only via that can Ua establish
communication with Ub. We term such a gateway a "non-transparent"
ALG.

Another form of ALG makes communication through the ALG
transparent to an end user. Using the previous example, with a
"transparent" ALG, Ua would not be required to establish explicit
connectivity to the ALG first, before starting to communicate with
Ub. Such connectivity will be established transparently to Ua, so
that Ua would only see connectivity to Ub.

For completeness, note that it is not necessarily the case that
communicating via an ALG involves changes to the network header.
An ALG could be used only at the beginning of a session for the
purpose of authentication, after which the ALG goes away and
communication continues natively.

Both non-transparent and transparent ALGs are required (by
definition) to understand the syntax and semantics of the
application data stream. ALGs are very simple from the viewpoint
of network layer architecture, since they appear as Internet hosts
in each realm, i.e. they act as origination and termination points
for communication.

At the other end of the spectrum is a Network Address Translator
(NAT) [RFC1631]. In the context of this document we define a NAT
as a device that just modifies the network and the transport layer
headers, but does not understand the syntax/semantics of the
application layer data stream (using our terminology what is
described in RFC1631 is a device that has both the NAT and ALG
functionality).

In the standard case of a NAT placed between a corporate network
using private addresses [RFC1918] and the public Internet, that
NAT changes the source IPv4 address in packets going towards the
Internet, and changes the destination IPv4 address in packets
coming from the Internet. When a NAT is used to interconnect
routing realms with overlapping addresses, such as a direct
connection between two intranets, the NAT may modify both
addresses in the IP header. Since the NAT modifies address(es) in
the IP header, the NAT also has to modify the transport (e.g.,
TCP, UDP) pseudo-header checksum. Upon some introspection one
could observe that when interconnecting routing realms with
overlapping addresses, the set of operations on the network and
transport header performed by a NAT forms a (proper) subset of the
set of operations on the network and transport layer performed by
a transparent ALG.

By definition a NAT does not understand syntax and semantics of an
application data stream. Therefore, a NAT cannot support
applications that carry IP addresses at the application layer
(e.g., FTP with PORT or PASV command [RFC959]). On the other
hand, a NAT can support any application, as long as such an
application does not carry IP addresses at the application layer.
This is in contrast with an ALG that can support only the
applications coded into the ALG.

One can conclude that both NATs and ALGs have their own
limitations, which could constrain their usefulness. Combining NAT
and ALG functionality in a single device could be used to overcome
some, but not all, of these limitations. Such a device would use
the NAT functionality for the applications that do not carry IP
addresses, and would resort to the ALG functionality when dealing
with the applications that carry IP addresses. For example, such a
device would use the NAT functionality to deal with the FTP data
connection, but would use the ALG functionality to deal with the
FTP control connection. However, such a device will fail
completely handling an application that carries IP addresses, when
the device does not support the application via the ALG
functionality, but rather handles it via the NAT functionality.

Communicating through either ALGs or NATs involves changes to the
network header (and specifically source and destination
addresses), and to the transport header. Since IP Security
authentication headers assume that the addresses in the network
header are preserved end-to-end, it is not clear how one could
support IP Security-based authentication between a pair of hosts
communicating through either an ALG or a NAT. Since IP Security,
when used for confidentiality, encrypts the entire transport layer
end-to-end, it is not clear how an ALG or NAT could modify
encrypted packets as they require to. In other words, both ALGs
and NATs are likely to force a boundary between two distinct IP
Security domains, both for authentication and for confidentiality,
unless specific enhancements to IP Security are designed for this
purpose.

Interconnecting routing realms via either ALGs or NATs relies on
the DNS [RFC1035]. Specifically, for a given set of
(interconnected) routing realms, even if network layer addresses
are no longer unique across the set, fully qualified domain names
would need to be unique across the set. However, a site that is
running a NAT or ALG probably needs to run two DNS servers, one
inside and one outside the NAT or ALG, giving different answers to
identical queries. This is discussed further in [kre]. DNS
security [RFC2065] and some dynamic DNS updates [dns2] will
presumably not be valid across a NAT/ALG boundary, so we must
assume that the external DNS server acquires at least part of its
tables by some other mechanism.

To summarize, since RFC1918, we have not really changed the
spatial uniqueness of an address, so much as recognized that there
are multiple spaces. i.e. each space is still a routing realm
such as an intranet, possibly connected to other intranets, or the
Internet, by NATs or ALGs (see above discussion). The temporal
uniqueness of an address is unchanged by RFC1918.

4.2. Addresses are no longer all temporally unique

Note that as soon as address significance changes anywhere in the
address space, it has in some sense changed everywhere. This has
in fact already happened.

IPv4 address blocks were for many years assigned chronologically,
i.e. effectively at random with respect to network topology.
This led to constantly growing routing tables; this does not
scale. Today, hierarchical routing (CIDR [RFC1518], [RFC1519])
is used as a mechanism to improve scaling of routing within a
routing realm, and especially within the Internet (The Annex goes
into more details on CIDR).

Scaling capabilities of CIDR are based on the assumption that
address allocation reflects network topology as much as possible,
and boundaries for aggregation of addressing information are not
required to be fully contained within a single organization - they
may span multiple organizations (e.g., provider with its
subscribers). Thus if a subscriber changes its provider, then to
avoid injecting additional overhead in the Internet routing
system, the subscriber may need to renumber.

Changing providers is just one possible reason for renumbering.
The informational document [RFC1900] shows why renumbering is an
increasingly frequent event. Both DHCP [RFC1541] and PPP [RFC
1661] promote the use of dynamic address allocation.

To summarize, since the development and deployment of DHCP and
PPP, and since it is expected that renumbering is likely to become
a common event, IP address significance has indeed been changed.
Spatial uniqueness should be the same, so addresses are still
effective locators. Temporal uniqueness is no longer assured. It
may be quite short, possibly shorter than a TCP connection time.
In such cases an IP address is no longer a good identifier. This
has some impact on end-to-end security, and breaks TCP in its
current form.

4.3. Multicast and Anycast

Since we deployed multicast [RFC1112], we must separate the
debate over meaning of IP addresses into meaning of source and
destination addresses. A destination multicast address (i.e. a
locator for a topologically spread group of hosts) can traverse a
NAT, and is not necessarily restricted to an intranet (or to the
public Internet). Its lifetime can be short too.

The concept of an anycast address is of an address that
semantically locates any of a group of systems performing
equivalent functions. There is no way such an address can be
anything but a locator; it can never serve as an identifier as
defined in this document, since it does not uniquely identify
host. In this case, the effective temporal uniqueness, or useful
lifetime, of an IP address can be less than the time taken to
establish a TCP connection.

Here we have used TCP simply to illustrate the idea of an
association - many UDP based applications (or other systems
layered on IP) allocate state after receiving or sending a first
packet, based on the source and/or destination. All are affected
by absence of temporal uniqueness whereas only the routing
infrastructure is affected by spatial uniqueness changes.

4.4. Summary

Due to dynamic address allocation and increasingly frequent
network renumbering, temporal uniqueness of IPv4 addresses is no
longer globally guaranteed, which puts their use as identifiers
into severe question. Due to the proliferation of Intranets,
spatial uniqueness is also no longer guaranteed across routing
realms; interconnecting routing realms could be accomplished via
either ALGs or NATs. In principle such interconnection will have
less functionality than if those Intranets were directly
connected. In practice the difference in functionality may or may
not matter, depending on individual circumstances.

5. IPv6 Considerations

As far as temporal uniqueness (identifier-like behaviour) is
concerned, the IPv6 model [RFC1884] is very similar to the current
state of the IPv4 model, only more so. IPv6 will provide mechanisms
to autoconfigure IPv6 addresses on IPv6 hosts. Prefix changes,
requiring the global IPv6 addresses of all hosts under a given prefix
to change, are to be expected. Thus, IPv6 will amplify the existing
problem of finding stable identifiers to be used for end-to-end
security and for session bindings such as TCP state.

The IAB feels that this is unfortunate, and that the transition to
IPv6 would be an ideal occasion to provide upper layer end-to-end
protocols with temporally unique identifiers. The exact nature of
these identifiers requires further study.

As far as spatial uniqueness (locator-like behaviour) is concerned,
the IPv6 address space is so big that a shortage of addresses,
requiring an RFC1918-like approach and address translation, is
hardly conceivable. Although there is no shortage of IPv6 addresses,
there is also a well-defined mechanism for obtaining link-local and
site-local addresses in IPv6 [RFC1884, section 2.4.8]. These
properties of IPv6 do not prevent separate routing realms for IPv6,
if so desired (resulting in multiple security domains as well).
While at the present moment we cannot identify a case in which
multiple IPv6 routing realms would be required, it is also hard to
give a definitive answer to whether there will be only one, or more
than one IPv6 routing realms. If one hypothesises that there will be
more than one IPv6 routing realm, then such realms could be
interconnected together via ALGs and NATs. Considerations for such
ALGs and NATs appear to be identical to those for IPv4.

ANNEX: Current Practices for IPv4 Address Allocation & Routing

Initially IP address structure and IP routing were designed around
the notion of network number classes (Class A/B/C networks) [RFC
790]. In the earlier 90s growth of the Internet demanded significant
improvements in both the scalability of the Internet routing system,
as well as in the IP address space utilization. Classful structure
of IP address space and associated with it classful routing turned
out to be inadequate to meet the demands, so during 1992 - 1993
period the Internet adopted Classless Inter-Domain Routing (CIDR)
[RFC1380], [RFC1518], [RFC1519]. CIDR encompasses a new address
allocation architecture, new routing protocols, and a new structure
of IP addresses.

CIDR improves scalability of the Internet routing system by extending
the notion of hierarchical routing beyond the level of individual
subnets and networks, to allow routing information aggregation not
only at the level of individual subnets and networks, but at the
level of individual sites, as well as at the level of Internet
Service Providers. Thus an organization (site) could act as an
aggregator for all the destinations within the organization.
Likewise, a provider could act as an aggregator for all the
destinations within its subscribers (organizations directly connected
to the provider).

Extending the notion of hierarchical routing to the level of
individual sites and providers, and allowing sites and providers to
act as aggregators of routing information, required changes both to
the address allocation procedures, and to the routing protocols.
While in pre-CIDR days address allocation to sites was done without
taking into consideration the need to aggregate the addressing
information above the level of an individual network numbers, CIDR-
based allocation recommends that address allocation be done in such
a way as to enable sites and providers to act as aggregators of
addressing information - such allocation is called "aggregator
based". To benefit from the "aggregator based" address allocation,
CIDR introduces an inter-domain routing protocol (BGP-4) [RFC1771,
RFC1772] that provides capabilities for routing information
aggregation at the level of individual sites and providers.

CIDR improves address space utilization by eliminating the notion of
network classes, and replacing it with the notion of contiguous
variable size (power of 2) address blocks. This allows a better match
between the amount of address space requested and the amount of
address space allocated [RFC1466]. It also facilitates "aggregator
based" address allocation. Eliminating the notion of network classes
requires new capabilities in the routing protocols (both intra and
inter-domain), and IP forwarding. Specifically, the CIDR-capable

protocols are required to handle reachability (addressing)
information expressed in terms of variable length address prefixes,
and forwarding is required to implement the "longest match"
algorithm. CIDR implications on routing protocols are described in
[RFC1817].

The scaling capabilities of CIDR are based on the assumption that
address allocation reflects network topology as much as possible,
especially at the level of sites, and their interconnection with
providers, to enable sites and providers to act as aggregators. If a
site changes its provider, then to avoid injecting additional
overhead in the Internet routing system, the site may need to
renumber. While CIDR does not require every site that changes its
providers to renumber, it is important to stress that if none of the
sites that change their providers will renumber, the Internet routing
system might collapse due to the excessive amount of routing
information it would need to handle.

Maintaining "aggregator based" address allocation (to promote
scalable routing), and the need to support the ability of sites to
change their providers (to promote competition) demands practical
solutions for renumbering sites. The need to contain the overhead
in a rapidly growing Internet routing system is likely to make
renumbering more and more common [RFC1900].

The need to scale the Internet routing system, and the use of CIDR as
the primary mechanism for scaling, results in the evolution of
address allocation and management policies for the Internet. This
evolution results in adding the "address lending" policy as an
alternative to the "address ownership" policy [RFC2008].

IP addressing and routing have been in constant evolution since IP
was first specified [RFC791]. Some of the addressing and routing
principles have been deprecated, some of the principles have been
preserved, while new principles have been introduced. Current
Internet routing and addresses (based on CIDR) is an evolutionary
step that extends the use of hierarchy to maintain a routable global
Internet.

Security Considerations

The impact of the IP addressing model on security is discussed in
sections 4.1 and 5 of this document.

Acknowledgements

This document was developed in the IAB. Constructive comments were
received from Ran Atkinson, Jim Bound, Matt Crawford, Tony Li,
Michael A. Patton, Jeff Schiller. Earlier private communications from
Noel Chiappa helped to clarify the concepts of locators and
identifiers.

References

[RFC791] Postel, J., "Internet Protocol", STD 5, RFC791, September
1981.

[RFC790] Postel, J., "Assigned Numbers", September 1981.

[RFC959] Postel, J., and J. Reynolds, "File Transfer Protocol", STD
9, RFC959, October 1985.

[RFC1035] Mockapetris, P., "Domain Names - Implementation and
Specification", STD 13, RFC1035, November 1987.

[RFC1112] Deering, S., "Host Extensions for IP Multicasting", STD 5,
RFC1112, September 1989.

[RFC1380] Gross, P., and P. Almquist, "IESG Deliberations on Routing
and Addressing", RFC1380, November 1992.

[RFC1466] Gerich, E., "Guidelines for Management of IP Address
Space", RFC1466, May 1993.

[RFC1498] Saltzer, J., "On the Naming and Binding of Network
Destinations", RFC1498, August 1993 (originally published 1982).

[RFC1518] Rekhter, Y., and T. Li, "An Architecture for IP Address
Allocation with CIDR", RFC1518, September 1993.

[RFC1519] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless
Inter-Domain Routing (CIDR): an Address Assignment and Aggregation
Strategy", RFC1519, September 1993.

[RFC1541] Droms, R., "Dynamic Host Configuration Protocol", RFC
1541, October 1993.

[RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
RFC1661, July 1994.

[RFC1771] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC1771, March 1995.

[RFC1772] Rekhter, Y., and P. Gross, "Application of the Border
Gateway Protocol in the Internet", RFC1772, March 1995.

[RFC1817] Rekhter, Y., "CIDR and Classful Routing", RFC1817,
September 1995.

[RFC1825] Atkinson, R., "Security Architecture for the Internet
Protocol", RFC1825, September 1995.

[RFC1900] Carpenter, B., and Y. Rekhter, "Renumbering Needs Work",
RFC1900, February 1996.

[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
J., and E. Lear, "Address Allocation for Private Internets", RFC
1918, February 1996.

[RFC1933] Gilligan, R., and E. Nordmark, "Transition Mechanisms for
IPv6 Hosts and Routers", RFC1933, April 1996.

[RFC2008] Rekhter, Y., and T. Li, "Implications of Various Address
Allocation Policies for Internet Routing", RFC2008, October 1996.

[kre] Elz, R., et. al., "Selection and Operation of Secondary DNS
Servers", Work in Progress.

[RFC2065] Eastlake, E., and C. Kaufman, "Domain Name System Security
Extensions", RFC2065, January 1997.

[dns2] Vixie, P., et. al., "Dynamic Updates in the Domain Name System
(DNS UPDATE)", Work in Progress.

Authors' Addresses

Brian E. Carpenter
Computing and Networks Division
CERN
European Laboratory for Particle Physics
1211 Geneva 23, Switzerland

EMail: brian@dxcoms.cern.ch

Jon Crowcroft
Dept. of Computer Science
University College London
London WC1E 6BT, UK

EMail: j.crowcroft@cs.ucl.ac.uk

Yakov Rekhter
Cisco systems
170 West Tasman Drive
San Jose, CA, USA

Phone: +1 914 528 0090
Fax: +1 408 526-4952
EMail: yakov@cisco.com

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