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Versions: 00 01 02 03                                                   
Network Working Group                                     Christian Vogt
Internet-Draft                                                  Ericsson
Expires:  January 14, 2010                                 July 13, 2009


           Qualifying the Harmfulness of Address Translation
           draft-vogt-address-translation-harmfulness-03.txt

Status of this Memo

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Abstract

   Address translation is widely considered harmful because it conflicts
   with design principles highly regarded within the Internet
   engineering community.  Still, address translation has become common
   practice despite technical problems because it constitutes an easy-



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   to-deploy solution to a set of common operational needs.  Since some
   of these needs will continue to exist in IP version 6, there is
   concern within the Internet engineering community about the potential
   proliferation of harmful technology from IP version 4 to IP version
   6.  This document investigates this concern.  It compares feasible
   address translator designs with respect to the harmful impact they
   may have, explains why the problems of address translation, as used
   today, are to a significant extent entailed by the shortage of global
   addresses in IP version 4, and shows how the problems can be
   mitigated in IP version 6.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Purposes of Address Translation  . . . . . . . . . . . . . . .  3
   3.  Functional Components of Address Translation . . . . . . . . .  5
   4.  Analysis of Problems and Possible Solutions  . . . . . . . . .  6
     4.1.  Impact on Host Reachability  . . . . . . . . . . . . . . .  6
     4.2.  Impact on Network Functioning  . . . . . . . . . . . . . .  8
   5.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   6.  Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . 11
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12



























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1.  Introduction

   One of the design principles most well-heeded by the Internet
   engineering community is that addresses have end-to-end validity and
   do not change in packets en route.  This principle is being
   challenged [HA2000][CA2000][CA1997][DA2002] by the widespread use of
   address translation on the Internet.  Address translators rewrite
   addresses in packets en route, typically at network borders, to
   satisfy network operators' desire for provider independence, network
   topology concealment, or conservation of global addresses.  The
   incentives for deploying address translation are strong, even though
   the technique, as used today for IP version 4, has profound drawbacks
   and hence is widely considered harmful.  Since the purposes of
   address translation are partly independent of the IP version, there
   is concern within the Internet engineering community about the
   potential proliferation of harmful technology from IP version 4 to IP
   version 6.

   This document investigates this concern by qualifying the harmfulness
   of feasible address translator designs in IP versions 4 and 6.  The
   document makes four contributions to this end:  First, it explains
   the purposes for which address translation is used, identifies the
   components of address translation that achieve these purposes, and
   distinguishes two main address translator designs based on the
   components.  Second, the document compares the problems that either
   address translator design may cause, and evaluates the cost of
   mitigating those problems.  Third, it infers that many of the
   problems of address translation as deployed today are not due to
   address rewriting as such.  They can rather be attributed to address
   overloading, a technique that helps conserving global addresses.
   Fourth, the document argues that, while address overloading is
   inevitable in IP version 4 due to the shortage of global addresses
   [HU2009], address translation in IP version 6 does not require
   address overloading and could hence, if designed rightly, be
   considerably less problematic than address translation in IP version
   4.


2.  Purposes of Address Translation

   Network operators frequently apply address translation to separate
   the "local" addresses they use inside their networks from the
   "global" addresses at which the networks are reachable from the
   Internet.  They do this for any of the following three purposes:

   o  Provider independence:  Network operators desire the flexibility
      to change providers at low cost, in order to avoid lock-in to any
      particular provider.



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   o  Topology concealment:  Network operators may want to hide a
      network's internal topology from the rest of the Internet for
      security reasons.

   o  Global address conservation:  Network operators see increasing
      pressure to conserve global IP version 4 addresses due to the
      imminent runout of unallocated global IP version 4 addresses.

   A use case of address translation to achieve provider independence is
   in networks that cannot afford, or are not eligible for, global
   provider-independent addresses.  Most residential networks and small
   enterprise and organizational networks belong to this group.  They
   must use addresses assigned by their providers and renumber in the
   event of a provider change.  Experience [CA2009] shows that the
   process of network renumbering often involves substantial manual
   labor, and is hence costly and time-consuming.  For example, routers
   and servers typically have statically configured addresses and
   therefore have to be renumbered manually.  Addresses may also have to
   be manually renumbered in applications, firewalls, and operations and
   management systems [CH2006].  Address translation eliminates the need
   to renumber without global provider-independent addresses.  It
   enables network operators to use local provider-independent addresses
   internally, while retaining external reachability at global addresses
   assigned by a provider.

   A security-related use case of address translation is for denial-of-
   service attack protection.  This mostly applies to large enterprise
   and organizational networks.  The standard, network-topological
   assignment of addresses provides remote hosts with a means to infer
   the topology of a network.  Attackers may use this information to
   identify attack targets.  For example, a denial-of-service attack
   against a server may more easily be executed via a host on the
   server's link, and such a host can typically be identified based on
   comparing its address to the address of the server in question.
   Firewalls can only partially mitigate this threat.  Although they
   defeat the systematic discovery of a network's internal topology
   through address scanning, addresses obtained from communications
   permitted by a firewall continue to reveal information about a
   network's topology.  Address translation can more thoroughly conceal
   the internal topology of a network, by mapping local and global
   addresses such that the topological structuring of local addresses
   cannot be derived from global addresses.

   The conservation of global addresses provides a third use case for
   address translation, which is of common interest among operators of
   IP version 4 networks.  The shortage of global IP version 4 addresses
   makes network expansion difficult, and hence can have a negative
   impact on revenue.  Address translation helps conserving global



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   addresses because it allows multiple hosts with separate local
   addresses to share one global address.


3.  Functional Components of Address Translation

   In order to accomplish the purposes identified above, address
   translation incorporates two functional components:

   o  Address rewriting:  The local and global addresses of a network
      are mapped, and swapped accordingly in packets leaving or entering
      the network.

   o  Address overloading:  Multiple local addresses are mapped onto a
      single global address.

   To enable demultiplexing of packets received at an overloaded global
   address back onto the right local address, address translators that
   use address overloading store address mappings as connection-specific
   "disambiguation state", and they use the connection initiator's port
   number in received packets as indexes into this state.  To ensure
   uniqueness of this port number across all connections handled by an
   address translator, the port number may have to be translated.  The
   port mapping is then stored as part of the corresponding
   disambiguation state.

   Address rewriting affords provider independence and topology
   concealment.  Provider independence is achieved through the
   decoupling of a network's local provider-independent addresses from
   the global addresses assigned by the network's provider.  The local
   addresses consequently do not need to change if the network changes
   providers, thus eliminating the need to renumber.  Topology
   concealment can be achieved either by overloading a single global
   address with a large set of local addresses, or by choosing, and
   keeping secret, a non-trivial permutation to be applied on topology-
   significant address bits during address rewriting.

   Simple address rewriting without address overloading requires at
   least one global address per host.  Local addresses then map one-to-
   one onto their corresponding global addresses.  To conserve global
   addresses, it is necessary for multiple hosts to share one global
   address.  This can be achieved by combining address rewriting with
   address overloading.

   Given that address overloading is required for only part of the use
   cases of address translation, two types of address translation can be
   distinguished:




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   o  One-to-one address translation, which consists of address
      rewriting without address overloading.  This achieves provider
      independence and topology concealment.

   o  Many-to-one address translation, which combines address rewriting
      with address overloading.  This achieves global address
      conservation in addition to provider independence and topology
      concealment.

   These two types of address translation will be evaluated separately
   throughout the rest of this document.


4.  Analysis of Problems and Possible Solutions

   Since address translation changes the way hosts are addressed and
   packets are forwarded, it has an impact on host reachability and
   network functioning.  The analysis below explains this impact for
   one-to-one and many-to-one address translation, identifies problems
   that may arise from it, and examines the feasibility and cost of
   mitigating the problems.

4.1.  Impact on Host Reachability

   Since hosts behind an address translator effectively have at least
   two addresses -- a local address and a global address --, peers must
   have a means to discover one of these addresses that they can reach.
   Which of the host's addresses are reachable by a given peer then
   depends on the location of the peer.  The peer must use the host's
   global address if all paths to the host lead through an address
   translator, and it should use the host's local address otherwise.
   Failure to choose the right address may lead to non-reachability of
   the host, or to sub-optimal routing, respectively.

   Address translation must consequently be accounted for in both of the
   two main address discovery methods -- DNS-based address discovery and
   host-based address referrals.  Authoritative DNS servers must refer
   peers to those addresses of a host that the peers can reach,
   preferably via an optimal path.  Hosts must be able to determine
   their global addresses for address referrals in packets they send to
   peers, and they must recognize their global addresses in packets
   received.  The following shows that, while both address discovery
   methods can be adapted to accommodate address translation, the cost
   and reliability of suitable solutions in either case depends
   significantly on the type of address translation.

   Appropriate configuration is sufficient to enable DNS-based address
   discovery in the presence of one-to-one address translation.  Since



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   without address overloading, a host's local and global addresses are
   both stable and unique, both can be associated with the host's name
   in the DNS through configuration of the authoritative DNS servers.

   For many-to-one address translation, DNS-based address discovery is
   more expensive to enable.  Since a host is reachable at a global
   address only after prior establishment of disambiguation state in an
   address translator, extra functionality is necessary in authoritative
   DNS servers to initiate this state establishment prior to referring a
   peer to the global addresses of a host.  This functionality can be
   either in hosts or in authoritative DNS servers.  Standard methods
   [UP2001][CH2008] exist for hosts to establish disambiguation state.
   Those define an interface for address translators through which
   applications can reserve particular global addresses and port
   numbers.  Disambiguation state establishment by authoritative DNS
   servers has been proposed in [PE2009].  The proposal calls for
   authoritative DNS servers to establish in a host's address translator
   a mapping between the host's local and global addresses when
   receiving a DNS query for the host's name, and for the address
   translator to bind this mapping to the peer's address and port number
   when receiving the first packet from the peer.

   Host-based address referrals require special host support to function
   in the presence of address translation.  Hosts must be enabled to
   discover their global addresses, and they must use and recognize
   their global addresses in referrals they send and receive.
   Furthermore, hosts behind a many-to-one address translator may have
   to establish demultiplexing state in the address translator prior to
   sending an address referral.  This is necessary when the address
   referral itself is sent via an overlay instead of to the peer
   directly, and hence cannot establish the necessary demultiplexing
   state in the address translator.  Applications that may send address
   referrals via overlays include those that use server-assisted peer-
   to-peer protocols or the Session Initiation Protocol.

   While the use and recognition of global addresses is specific to the
   application protocol, standard methods [RO2007][RO2009][RO2008]
   exists for hosts to discover their global addresses.  These methods
   were designed for many-to-one address translation, as it is the
   prevailing address translation type in the existing Internet, and
   they are hence appropriate to establish disambiguation state in
   address translators.  They discover a global address by inquiring of
   infrastructure what a packet's source address looks like after
   translation.

   Although the existing solutions to enable address discovery in the
   presence of many-to-one address translation would likewise apply to
   one-to-one address translation, solutions can be simpler in the case



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   of one-to-one address translation.  Since one-to-one address
   translation does not overload addresses, address discovery methods
   for one-to-one address translation do not have to establish
   disambiguation state in address translators.  For example, a
   simplified method for hosts to discover their global addresses is for
   access routers to announce address mapping rules, based on which
   hosts derive their global addresses given their local addresses.  In
   the case where addresses are translated by swapping their prefix, a
   mapping rule could be as simple as a pair of local and global address
   prefixes.  This discovery method would be comparable to the existing
   practice of auto-configuring addresses based on on-link address
   prefixes announced by access routers.

   Both DNS-based and referral-based address discovery are consequently
   more expensive and less reliable for many-to-one address translation
   than they can be for one-to-one address translation.  Apart from
   requiring extra complexity, many-to-one address translation assumes
   that connection initiation happens shortly after address discovery
   due to the establishment of disambiguation state during address
   discovery.  This assumption is not always the satisfied:
   Disambiguation state must expire when found unused to make room for
   new connections.  So if the first packet from a connection arrives at
   the address translator after the corresponding disambiguation state
   has expired, connection initiation fails.  For the same reason, many-
   to-one address translation does not allow peers to be configured with
   the global address of a host, since the necessary disambiguation
   state would likely be unavailable at the time the peer initiates a
   connection to this address.

4.2.  Impact on Network Functioning

   The addition of new components to a network, such as in the form of
   address translators, can impact the functioning of the network.  Two
   potential problems that may arise are the loss of generic forwarding
   support for all connection types, and the loss of network robustness.
   Forwarding support can be reduced to certain connection types if the
   new component relies on connection-specific properties in packets.
   This is the case for many-to-one address translators, which rely on
   modifiable port numbers.  Packets without port numbers are dropped,
   and so are packets with port numbers that are not modifiable due to
   encryption and authentication.  Network robustness can be reduced if
   the new component constitues a single point of failure.  This is the
   case for both types of address translators.  Address translators
   perform a function that connections depend on, and hence, if not
   provisioned redundantly, limit a network's ability to reroute traffic
   in the event of failures.  The following shows that, while both
   problems can be effectively mitigated, the cost and reliability of
   suitable solutions for either problem depend significantly on the



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   type of address translation.

   Loss of generic forwarding support for all connection types is a
   problem that is peculiar to many-to-one address translation, since
   address overloading requires packets to carry port numbers modifiable
   by an address translator for use as indexes into disambiguation
   state.  One-to-one address translation does not limit forwarding
   support because it does not rely on port numbers or other connection-
   specific properties in packets.

   A common method to retain support for all connection types in many-
   to-one address translation is to tunnel packets end-to-end in an
   extra layer of UDP.  This enables connections without accessible port
   numbers to pass through many-to-one address translators, because the
   new UDP header in packets adds the port numbers needed by the address
   translators.  Unfortunately, UDP tunneling cannot be expected to
   always be available.  It requires support at both ends of a
   connection, independent of whether the address of the peer is
   translated or not.  So if either the host which address is being
   translated or its peer does not support UDP tunneling, connections
   without modifiable port numbers cannot pass through many-to-one
   address translators.

   To avoid loss of network robustness due to the deployment of address
   translators, one must ensure that address translators do not
   constitute a single point of failure.  Depending on the type of
   address translation deployed, this may require one or both of the
   following:

   o  Redundancy of address translators:  Redundant provisioning of
      address translators on alternative paths is necessary to protect
      against failure of address translators.  It enables failover of
      connections from one path to another.

   o  Refreshes of disambiguation state:  Disambiguation state in many-
      to-one address translators must be refreshed periodically
      throughout the lifetime of the corresponding connection to prevent
      premature disposal of the disambiguation state.  Idle connections
      may otherwise be unable to resume.

   Redundancy is straightforward to provision for one-to-one address
   translators.  Since they operate without connection-specific state,
   redundantly provisioned one-to-one address translators can substitute
   for each other without prior synchronization, and hence can be
   deployed independently on alternative paths.  On the other hand,
   redundantly provisioned many-to-one address translators must be
   continuously synchronized because the disambiguation state they
   maintain is connection-specific.



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   Refreshes of disambiguation state are needed for many-to-one address
   translators.  Existing refresh methods [RO2008][RO2007] either ensure
   that connections periodically exchange "keep-alive" packets end to
   end, or they introduce infrastructure with which hosts behind address
   translators can exchange such packets.  Unfortunately, neither
   refresh method can be expected to always be available due to its
   prerequisites.  The methods either require support at both ends of a
   connection, or they require special infrastructure plus support in
   the host which address is being translated.  If none of these
   prerequisites is met, connections that go idle temporarily may be
   unable to resume.  Disambiguation state refreshes are not necessary
   for one-to-one address translation, since this functions without
   disambiguation state.


5.  Conclusion

   This document has shown that the harmfulness of address translation
   depends significantly on whether or not global addresses are
   overloaded with mappings to multiple local addresses.  Although
   address translation with and without address overloading can have an
   impact on host reachability and network functioning, resulting
   problems have been found to be especially intractable and expensive
   to solve if address overloading is used.

   Impacts on host reachability in one-to-one address translation, which
   functions without address overloading, can be compensated for by
   appropriate configuration of authoritative DNS servers and special
   support for address referrals in hosts behind an address translator.
   Many-to-one address translation, which does perform address
   overloading, in addition requires the hosts and authoritative DNS
   servers to interact with address translators.  This is necessary to
   establish disambiguation state that will subsequently permit an
   address translator to demultiplex packets received at an overloaded
   global address back onto the right local address.  Impacts on network
   functioning in one-to-one address translation can be compensated for
   by provisioning address translators redundantly.  Many-to-one address
   translation in addition requires synchronization of disambiguation
   state across redundantly provisioned address translators, periodic
   state refreshes, and UDP tunneling of connections without the
   modifiable port numbers that address translators need as indexes into
   their disambiguation state.

   Compensating for the impacts of address translation is hence
   significantly more expensive, both in deployment and in
   administration, when address translation is many-to-one compared to
   when it is one-to-one.  The higher complexity involved furthermore
   constitutes a source of potential failure on its own.  And extra



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   requirements for hosts and their peers reduce the likelihood that
   sufficient functionality will be available when needed.

   The advantageousness of one-to-one address translation over many-to-
   one address translation naturally leads to the question whether the
   former can satisfy the demand for address translation, as it has
   become apparent through the wide deployment of address translators in
   today's Internet.  Clearly, this depends on the availability of
   global addresses.  One-to-one address translation, which requires one
   global address per local address, is unsuitable for IP version 4,
   where the shortage of global addresses necessitates the use of
   address overloading.  For IP version 6, however, one-to-one address
   translation is suitable, as the sufficient number of global addresses
   here makes address overloading dispensable.  Address translation in
   IP version 6 could hence, if designed without address overloading, be
   considerably less harmful than address translation in IP version 4.


6.  Acknowledgment

   The author would like to thank Gonzalo Camarillo, Brian Carpenter,
   Tony Li, Joel Halpern, James Kempf, Alan Kavanagh, Suresh Krishnan,
   Zoltan Turanyi, and Wassim Haddad for their reviews of this document
   and their valuable comments.

   This document was generated using the xml2rfc tool.


7.  References

   [CA1997]  Carpenter, B., Crowcroft, J., and Y. Rekhter, "IPv4 Address
             Behaviour Today", RFC 2101, February 1997.

   [CA2000]  Carpenter, B., "Internet Transparency", RFC 2775,
             February 2000.

   [CA2009]  Carpenter, B., Atkinson, Ran., and H. Flinck, "Renumbering
             Still Needs Work", IETF Internet draft (work in progress),
             Februray 2009.

   [CH2006]  Chown, T., Thompson, M., Ford, A., and S. Venaas, "Things
             to Think About When Renumbering an IPv6 Network", IETF
             Internet draft (work in progress), September 2006.

   [CH2008]  Cheshire, S., Krochmal, M., and K. Sekar, "NAT Port Mapping
             Protocol (NAT-PMP)", IETF Internet draft (work in
             progress), April 2008.




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   [DA2002]  Daigle, L., "IAB Considerations for Unilateral Self-Address
             Fixing (UNSAF) Across Network Address Translation",
             RFC 3424, November 2002.

   [HA2000]  Hain, T., "Architectural Implications of NAT", RFC 2993,
             November 2000.

   [HU2009]  Huston, G., "IPv4 Address Report", online
             at http://www.potaroo.net/tools/ipv4, July 2009.

   [PE2009]  Perkins, C., "Translating IPv4 to IPv6 Based on Source IPv4
             Address", IETF Internet draft (work in progress),
             February 2009.

   [RO2007]  Rosenberg, J., "Interactive Connectivity Establishment
             (ICE): A Protocol for Network Address Translator (NAT)
             Traversal for Offer/Answer Protocols", IETF Internet
             draft (work in progress), October 2007.

   [RO2008]  Rosenberg, J., Mahy, R., and D. Wing, "Session Traversal
             Utilities for NAT (STUN)", RFC 5389, October 2008.

   [RO2009]  Rosenberg, J., Mahy, R., and P. Matthews, "Traversal Using
             Relays around NAT (TURN): Relay Extensions to Session
             Traversal Utilities for NAT (STUN)", IETF Internet
             draft (work in progress), July 2009.

   [UP2001]  "UPnP Forum Internet Gateway Device Protocol",
             November 2001.


Author's Address

   Christian Vogt
   Ericsson Research
   200 Holger Way
   San Jose, CA  95134
   United States

   Email:  christian.vogt@ericsson.com











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