IPv4 Address Behaviour Today
RFC 2101

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 [RFC 791].

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   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 [RFC 1498].  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 RFC 791 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 [RFC 793] or in an IP
   Security association [RFC 1825].  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

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   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 RFC 791 and RFC 793 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.

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   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

      [RFC 1918] 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.

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      At the other end of the spectrum is a Network Address Translator
      (NAT) [RFC 1631]. 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 [RFC 1918] 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 [RFC 959]). 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.

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      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 [RFC 1035].  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 [RFC 2065] 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 RFC 1918, 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 RFC 1918.

   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 [RFC 1518], [RFC 1519])
      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).

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      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 [RFC 1900] shows why renumbering is an
      increasingly frequent event.  Both DHCP [RFC 1541] 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 [RFC 1112], 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.

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   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 [RFC 1884] 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 RFC 1918-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 [RFC 1884, 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.

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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)
   [RFC 1380], [RFC 1518], [RFC 1519].  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) [RFC 1771,
   RFC 1772] 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 [RFC 1466]. 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

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   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
   [RFC 1817].

   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 [RFC 1900].

   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 [RFC 2008].

   IP addressing and routing have been in constant evolution since IP
   was first specified [RFC 791]. 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.

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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

   [RFC 791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
   1981.

   [RFC 790] Postel, J., "Assigned Numbers", September 1981.

   [RFC 959] Postel, J., and J. Reynolds, "File Transfer Protocol", STD
   9, RFC 959, October 1985.

   [RFC 1035] Mockapetris, P., "Domain Names - Implementation and
   Specification", STD 13, RFC 1035, November 1987.

   [RFC 1112] Deering, S., "Host Extensions for IP Multicasting", STD 5,
   RFC 1112, September 1989.

   [RFC 1380] Gross, P., and P. Almquist, "IESG Deliberations on Routing
   and Addressing", RFC 1380, November 1992.

   [RFC 1466] Gerich, E., "Guidelines for Management of IP Address
   Space", RFC 1466, May 1993.

   [RFC 1498] Saltzer, J., "On the Naming and Binding of Network
   Destinations", RFC 1498, August 1993 (originally published 1982).

   [RFC 1518] Rekhter, Y., and T. Li, "An Architecture for IP Address
   Allocation with CIDR", RFC 1518, September 1993.

   [RFC 1519] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless
   Inter-Domain Routing (CIDR): an Address Assignment and Aggregation
   Strategy", RFC 1519, September 1993.

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

   [RFC 1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
   RFC 1661, July 1994.

   [RFC 1771] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4
   (BGP-4)", RFC 1771, March 1995.

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   [RFC 1772] Rekhter, Y., and P. Gross, "Application of the Border
   Gateway Protocol in the Internet", RFC 1772, March 1995.

   [RFC 1817] Rekhter, Y., "CIDR and Classful Routing", RFC 1817,
   September 1995.

   [RFC 1825] Atkinson, R., "Security Architecture for the Internet
   Protocol", RFC 1825, September 1995.

   [RFC 1900] Carpenter, B., and Y. Rekhter, "Renumbering Needs Work",
   RFC 1900, February 1996.

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

   [RFC 1933] Gilligan, R., and E. Nordmark, "Transition Mechanisms for
   IPv6 Hosts and Routers", RFC 1933, April 1996.

   [RFC 2008] Rekhter, Y., and T. Li, "Implications of  Various Address
   Allocation Policies for Internet Routing", RFC 2008, October 1996.

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

   [RFC 2065] Eastlake, E., and C. Kaufman, "Domain Name System Security
   Extensions", RFC 2065, January 1997.

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

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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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