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Network Working Group                                           B. Aboba
INTERNET-DRAFT                                                 D. Thaler
Category: Informational                            Microsoft Corporation
24 February 2007

               Principles of Internet Host Configuration

   By submitting this Internet-Draft, each author represents that any
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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   This document describes basic principles of Internet host
   configuration.  It covers issues relating to configuration of
   parameters that affect the Internet layer, as well as parameters
   affecting higher layer protocols.

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Table of Contents

1.  Introduction..............................................    3
      1.1 Terminology ........................................    3
2.  Principles ...............................................    5
      2.1 Minimize Configuration .............................    5
      2.2 Less is More .......................................    5
      2.3 Diversity is Not a Benefit .........................    6
      2.4 Lower Layer Independence ...........................    7
      2.5 Configuration is Not Access Control ................    8
3.  Additional Discussion ....................................    9
      3.1 General Purpose Mechanisms .........................    9
      3.2 Service Discovery Protocols ........................    9
      3.3 Fate Sharing .......................................   10
4.  Security Considerations ..................................   11
      4.1 Configuration Authentication .......................   12
5.  IANA Considerations ......................................   13
6.  References ...............................................   13
      6.1 Informative References .............................   13
Acknowledgments ..............................................   15
Authors' Addresses ...........................................   15
Full Copyright Statement .....................................   16
Intellectual Property ........................................   16

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1.  Introduction

   "Architectural Principles of the Internet" [RFC1958] documents
   architectural principles of the Internet.  This document describes
   principles of Internet host configuration.  It covers issues relating
   to configuration of parameters that affect the Internet layer, as
   well as parameters affecting higher layer protocols.

   In recent years, a number of architectural questions have arisen, for
   which we provide guidance to protocol developers:

      o What protocol layers and general approaches are most appropriate
        for configuration of various parameters.

      o The relationship between parameter configuration and service

      o The relationship between network access authentication and host

      o The role of link-layer protocols (including tunneling protocols)
        in Internet host configuration.

   The last point above is particularly important to address, since it
   can directly affect the properties of a link as seen by higher layers
   (for example, whether privacy extensions [RFC3041] are available to

1.1.  Terminology

   Internet layer configuration is defined as the configuration required
   to support the operation of the Internet layer.  This includes, for

IP address(es)
     IP address configuration includes both configuration of link-scope
     addresses as well as global addresses.  Configuration of IP
     addresses is an important step, since this enables a host to fill
     in the source address in the packets it sends, as well as to
     receive packets destined to that address.  As a result, the host
     can now receive unicast IP packets, rather requiring that IP
     packets be sent to the broadcast or multicast address.
     Configuration of an IP address also enables the use of IP
     fragmentation, since packets sent from the unknown address cannot
     be reliably reassembled (fragments from multiple hosts using the
     unknown address might be reassembled into a single IP packet).
     Configuration of an IP address also enables use of security
     facilities such as IPsec [RFC4301].

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Subnet prefix(es)
     Once a subnet prefix is configured, hosts with an IP address can
     now send and receive unicast IP packets from on-link hosts.

Default gateway(s)
     Once a default gateway is configured, hosts with an IP address can
     now send and receive unicast IP packets from off-link hosts.

Mobility agent(s)
     While Mobile IPv4 [RFC3344] and Mobile IPv6 [RFC3775] include their
     own mechanisms for locating home agents, it is also possible for
     mobile nodes to require dynamic home agent configuration.

Other parameters
     Internet layer parameter configuration also includes configuration
     of per-host (e.g. IP TTL, enabling/disabling of IP forwarding and
     source routing) and per-interface parameters (e.g. MTU).

Boot service configuration
     Boot service configuration is defined as the configuration
     necessary for a host to obtain and perhaps also to verify an
     appropriate boot image.  This is appropriate for diskless hosts
     looking to obtain a boot image via mechanisms such as TFTP
     [RFC1350], NFS [RFC3530] and iSCSI [RFC3720,RFC4173].  It also may
     be useful in situations where it is necessary to update the boot
     image of a host that supports a disk, such as in the Preboot
     eXecution Environment (PXE) [PXE][PXEOPT].  While strictly speaking
     boot services operate above the Internet layer, where boot service
     is used to obtain the Internet layer code, it may be considered
     part of Internet layer configuration.

Higher-layer configuration is defined as the configuration required to
support the operation of other components above the Internet layer.
This includes, for example:

Name Service Configuration
     Name service configuration includes the configuration required for
     the host to resolve names.  This includes configuration of the
     addresses of name resolution servers, including IEN 116, DNS, WINS,
     iSNS and NIS servers, and the setting of name resolution parameters
     such as the NetBIOS node type, the DNS domain and search list, etc.
     It may also include the transmission or setting of the host's own

     Once the host has completed name server configuration, it is able
     to resolve names.  This not only allows the host to communicate
     with other hosts whose IP address is not known, but to the extent
     that name services are utilized for service discovery, this also

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     enables the host to discover services available on the network or

Time Service Configuration
     Time service configuration includes configuration of servers for
     protocols such as SNTP and NTP.  Since accurate determination of
     the time may be important to operation of the applications running
     on the host (including security services), configuration of time
     servers may be a prerequisite for higher layer operation.  However,
     it is typically not a requirement for Internet layer configuration.

Other service configuration
     This can include discovery of additional servers and devices, such
     as printers, SIP proxies, etc.

2.  Principles

   This Section describes basic principles of Internet host

2.1.  Minimize Configuration

   Anything that can be configured can be misconfigured.  [RFC1958]
   Section 3.8 states: "Avoid options and parameters whenever possible.
   Any options and parameters should be configured or negotiated
   dynamically rather than manually."

   That is, to minimize the possibility of configuration errors,
   parameters should be automatically computed (or at least have
   reasonable defaults) whenever possible.  For example, TCP [RFC793]
   does not require configuration of the Maximum Segment Size, but is
   able to compute an appropriate value.

   Sometimes the means by which a parameter is automatically computed by
   a protocol involves message exchanges by which a protocol configures
   itself.  This is typical for capability negotiation, and often for
   discovery of other devices that implement the same protocol.

2.2.  Less is More

   The availability of standardized, simple mechanisms for general-
   purpose Internet host configuration is highly desirable.  [RFC1958]
   states, "Performance and cost must be considered as well as
   functionality" and "Keep it simple.  When in doubt during design,
   choose the simplest solution."

   To allow protocol support in more types of devices, it is important
   to minimize the footprint requirement.  For example, Internet hosts

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   span a wide range of devices, from embedded devices operating with
   minimal footprint to supercomputers.  Since the resources (memory,
   code size) available for host configuration may be very small, it is
   desirable for a host to able to configure itself in as simple a
   manner as possible.

   One interesting example is IP support in pre-boot execution
   environments.  Since by definition boot configuration is required in
   hosts that have not yet fully booted, it is often necessary for pre-
   boot code to be executed from ROM, with minimal available memory.  In
   PXE, prior to obtaining a boot image, the host is typically only be
   able to communicate using IP and UDP.  This is one reason why
   Internet layer configuration mechanisms typically depend only on IP
   and UDP.  After obtaining the boot image, the host will have the full
   facilities of TCP/IP available to it, including support for reliable
   transport protocols, IPsec, etc.

   In order to reduce complexity, it is desirable for Internet layer
   configuration mechanisms to avoid dependencies on higher layers.
   Since embedded hosts may wish to minimize the code included within a
   boot ROM, availability of higher layer facilities cannot be
   guaranteed during Internet layer configuration.  In fact, it cannot
   even be guaranteed that all Internet layer facilities will be
   available.  For example, IP fragmentation and reassembly may not work
   reliably until a host has obtained an IP address.

2.3.  Diversity is Not a Benefit

   The number of host configuration mechanisms should be minimized.
   Diversity in Internet host configuration mechanisms presents several

     As configuration diversity increases, it becomes likely that a host
     will not support the configuration mechanism(s) available on the
     network to which it has attached, creating interoperability

     In order to be able to interoperate, hosts need to implement all
     configuration mechanisms used on the media they support.  This
     increases the required footprint, a burden for embedded devices.

     To support diversity in host configuration mechanism(s), operators
     would need to support multiple configuration services to ensure
     that hosts connecting to their networks could configure themselves.
     This represents an additional expense for little discernible

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     As configuration diversity increases, hosts supporting multiple
     configuration mechanisms may spend increasing effort to determine
     which mechanism(s) are supported.  This adds to configuration

     Whenever multiple mechanisms are available, it is possible that
     multiple configuration(s) will be returned.  To handle this, hosts
     would need to merge potentially conflicting configurations.  This
     would require conflict resolution logic, such as ranking of
     potential configuration sources, increasing implementation

Additional traffic
     To limit configuration latency, hosts may simultaneously attempt to
     obtain configuration by multiple mechanisms, increasing on-the-wire

2.4.  Lower Layer Independence

   [RFC1958] states, "Modularity is good. If you can keep things
   separate, do so."

   It is becoming increasingly common for hosts to support multiple
   network access mechanisms, including dialup, wireless and wired local
   area networks, GPRS, CDMA 1X-RTT, etc.  As a result, it is desirable
   for hosts to be able to configure themselves on multiple networks
   without adding configuration code specific to a new link layer.

   As a result, it is highly desirable for Internet host configuration
   mechanisms to be independent of the underlying lower layer.  That is,
   the link layer protocol (whether it be a physical link, or a virtual
   tunnel link) should only be explicitly aware of link-layer parameters
   (although it may configure link-layer parameters - see Section 2.1).
   Introduction of lower layer dependencies increases the likelihood of
   interoperability problems and adds to the number of Internet layer
   configuration mechanisms that hosts need to implement.

   Lower layer dependencies can be best avoided by keeping Internet host
   configuration above the link layer, thereby enabling configuration to
   be handled for any link layer that supports IP.  In order to provide
   media independence, Internet host configuration mechanisms should be
   link-layer protocol independent.

   While there are examples of IP address assignment within the link

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   layer (such PPP IPv4CP [RFC1332]), the disadvantages of this approach
   have now become apparent.  The main disadvantages include the extra
   complexity of implementing different mechanisms on different link
   layers, and the difficulty in adding new parameters which would
   require defining a mechanism in each link layer protocol.

   For example, PPP IPv4CP and IPCP extensions for name service
   configuration [RFC1877] were developed at a time when DHCP [RFC2131]
   had not yet been widely implemented on access devices or in service
   provider networks.  However, in IPv6 where link layer independent
   mechanisms such as stateless address configuration [RFC2462] and
   DHCPv6 [RFC2131,RFC3736] are available, PPP IPv6CP [RFC2472] instead
   simply configures an Interface-Identifier which is similar to a MAC
   address.  In contrast, IKEv2 [RFC4306] repeats the same mistake as
   PPP IPv4CP by defining a Configuration Payload for Internet host
   configuration for both IPv4 and IPv6.

   As a result, extensions to link layer protocols for the purpose of
   Internet, Transport or Application layer configuration (including
   server configuration) should be avoided.  Such extensions can
   negatively affect the properties of a link as seen by higher layers.
   For example, if a link layer protocol (or tunneling protocol)
   configures individual IPv6 addresses and precludes using any other
   addresses, then this can break privacy extensions [RFC3041].  Hence
   applications that desire privacy extensions may not function well.
   Similar issues may arise for other types of addresses, such as
   Cryptographically Generated Addresses [RFC3972], as well.

   Avoidance of lower layer dependencies also applies even where the
   lower layer in question may be link independent.  For example, while
   Extensible Authentication Protocol (EAP) [RFC3748] may be run over
   any link satisfying the requirements of [RFC3748] Section 3.1, many
   link layers do not support EAP and therefore Internet layer
   configuration mechanisms with EAP dependencies would not usable on
   all links that support IP.

2.5.  Configuration is Not Access Control

   Network access authentication is a distinct problem from Internet
   host configuration.  Network access authentication is best handled
   independently of the configuration mechanisms in use for the Internet
   and higher layers.

   For example, attempting to control access by requiring authentication
   in order to obtain configuration parameters (such as an IP address)
   has little value if the user can manually configure the host.  Having
   an Internet (or higher) layer protocol authenticate clients is
   appropriate to prevent resource exhaustion of a scarce resource on

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   the server, but not for preventing rogue hosts from obtaining access
   to a link.  Note that client authentication is not required for
   Stateless DHCPv6 [RFC3736] since it does not result in allocation of
   any limited resources on the server.

3.  Additional Discussion

3.1.  General Purpose Mechanisms

   Protocols should either be self-configuring (especially where fate
   sharing is important), or use general-purpose configuration
   mechanisms.  Given the number of Internet host configuration
   mechanisms that have already been defined, and the problems resulting
   from the proliferation of these mechanisms, there is no apparent need
   for the development of additional general-purpose configuration

   When defining a new host parameter, protocol designers should first
   consider whether configuration is indeed necessary (see Section 2.1
   for further discussion).  If configuration is necessary, protocol
   designers should next consider:

      1. Where the authoritative source of information is.  For example,
         routers are authoritative for default gateway information, DNS
         servers are authoritative for DNS server information, etc.

      2. Which type of administrator would be the source of the
         information.  For example, router administrators and server
         administrators are often different sets of individuals.

      3. Whether the parameter is a per-interface parameter or a global
         parameter.  For example, most standard general purpose
         configuration protocols run on a per-interface basis and hence
         are more appropriate for per-interface parameters.

   Finally, protocol designers should choose to either make the protocol
   that needs the parameter be self-configuring, or use the most
   appropriate general purpose configuration mechanism (generally DHCP,
   but possibly a service discovery protocol as noted below in Section
   3.2).  The choice should be made taking into account all of the
   principles discussed in Section 2.

3.2.  Service Discovery Protocols

   Higher-layer configuration often includes configuring addresses of
   servers.  Hence the question arises as to how this differs from
   "service discovery" as provided by Service Discovery protocols such
   as SLPv2 [RFC2608].

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   In general-purpose configuration mechanisms such as DHCP, hosts on
   the same link are typically considered equivalent, and server
   instances are likewise considered equivalent.  In service discovery
   protocols, on the other hand, a host desires to find a server
   satisfying a particular set of criteria, where the set of criteria
   may vary by request.

   In addition, service discovery protocols such as SLPv2 can support
   discovery of servers on the Internet [RFC3832], not just those within
   the local network.  General-purpose configuration mechanisms such as
   DHCP, on the other hand, typically assume the server(s) in the local
   network contain the authoritative set of information.

   For the service discovery problem (i.e., where the criteria varies on
   a per-request basis, even from the same host), protocols should
   either be self-discovering (if fate sharing is critical), or use
   general purpose service discovery mechanisms.

   In order to avoid a dependency on multicast routing, it is necessary
   for a host to either restrict discovery to services on the local link
   or to discover the location of the Directory Agent (DA).  Therefore
   the use of service discovery protocols beyond the local link is
   typically dependent on a parameter configuration mechanism.  As a
   result, service discovery protocols are typically not appropriate for
   use in obtaining basic Internet layer configuration, although they
   can be used to obtain higher-layer configuration for parameters that
   don't meet the assumptions above made by general-purpose
   configuration mechanisms.

3.3.  Fate Sharing

   If a server (or set of servers) is needed to get a set of
   configuration parameters, "fate sharing" ([RFC1958] Section 2.3) is
   preserved if the servers are ones without which the parameters could
   not be used, even if they were obtained via other means.  For
   example, learning the default gateways from the gateways themselves
   via Router Advertisements provides perfect fate sharing.  That is,
   the parameters can be obtained if and only if they can actually be
   used.  Furthermore, the possibility of incorrect information being
   configured is minimized if there is only one machine which is
   authoritative for the information (i.e., there is no need to keep
   multiple authoritative servers in sync).

   While fate sharing is a desirable property of a configuration
   mechanism, existing configuration schemes typically only provide for
   fate sharing in limited circumstances.  When utilized to discover
   services on the local link, service discovery protocols typically
   provide for fate sharing, since hosts providing service information

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   typically also provide the services.  However, where service
   discovery is assisted by a DA, fate sharing is typically not
   supported.  The ability to discover services is dependent on whether
   the DA is operational, even though the DA is typically not involved
   in the delivery of the service.  Since the DA and service agents
   (SAs) can be out of synchronization, it is possible for the DA to
   provide service information that is no longer current.  For example,
   service descriptions provided to the DA by SAs might be included in
   response to service discovery queries even after the SAs were no
   longer operational.  Similarly, recently introduced services might
   not yet have been registered by the DA.

   Similar limitations exist for other server-based configuration
   mechanisms such as DHCP.  For example, typically DHCP servers do not
   check for the liveness of the configuration information they provide,
   or discover new configuration information automatically.   As a
   result, there is no guarantee that configuration information will be
   current.  "IPv6 Host configuration of DNS Server Information
   Approaches" [RFC4339] Section 3.3 discusses the use of well-known
   anycast addresses for discovery of DNS servers.  The use of anycast
   addresses enables fate sharing, even where the anycast address is
   provided by an unrelated server.  However, in order to be universally
   useful, this approach would require allocation of a well-known
   anycast address for each service.

4.  Security Considerations

   Today IP configuration is typically not secured.  For example, PPP
   IPv4CP [RFC1332] does not support secure negotiation, enabling an
   attacker with access to the link to subvert the negotiation.  DHCPv4
   [RFC2131] initially did not include support for security; this was
   added in [RFC3118].  DHCPv6 [RFC3736] does include security support.
   However, DHCP authentication is not yet widely implemented for either
   DHCPv4 or DHCPv6.

   A number of issues exist with various classes of parameters, as
   discussed in Section 2.6, [RFC3756] Section 4.2.7, [RFC3118] Section
   1.1, and [RFC3315] Section 23.  Given the potential vulnerabilities
   resulting from implementation of these options, it is currently
   common for hosts to restrict support for DHCP options to the minimum
   set required to provide basic TCP/IP configuration.

   Securing Internet layer configuration requires securing the protocol
   used to obtain it: Secure Neighbor Discovery (SEND) [RFC3971] for
   stateless address autoconfiguration, or DHCP authentication for
   stateful address configuration.

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4.1.  Configuration Authentication

   While most threats against configuration mechanisms result in denial-
   of-service, some parameters are more critical.  For example, since
   boot configuration determines the boot image to be run by the host, a
   successful attack on boot configuration could result in an attacker
   gaining complete control over a host.  As a result, it is
   particularly important that boot configuration be secured.

   Internet host configuration parameters fall into two categories:
   those that are necessary for basic IP unicast connectivity (Internet
   layer configuration), and those that aren't (Higher layer

   The techniques available for securing Internet layer configuration
   are inherently limited, since classic security protocols such as
   IPsec [RFC4301] or TLS [RFC4346] cannot be used since an IP address
   is not yet available.  Use of lower layer security mechanisms are
   limited by the "lower layer independence" principle.

   As a result, security mechanisms have typically been implemented
   within the configuration protocols themselves.  For example, IPv6
   supports SEcure Neighbor Discovery (SEND) [RFC3971], DHCPv4 supports
   DHCP authentication [RFC3118], and DHCPv6 supports an equivalent
   facility [RFC3315].

   Higher layer configuration parameters, however, typically do not have
   this problem.  When stateful DHCPv6 uses authentication for Internet
   layer configuration, higher-layer configuration parameters can be
   similarly secured.  However, even if a host does not support DHCPv6
   authentication, higher-layer configuration via Stateless DHCPv6
   [RFC2462] can still be secured with IPsec.  Possible exceptions to
   this may exist where security facilities may not yet be available
   until later in the boot process.  For example, it may be difficult to
   secure boot configuration even once the Internet layer has been
   configured, because security code may not become available until
   after boot configuration has been completed.  For example, Kerberos,
   IPsec or TLS may not yet be available.

   Finally, where public key cryptography is used to authenticate and
   integrity protect configuration, hosts need to be configured with
   trust anchors in order to validate received configuration messages.
   For a node which visits multiple administrative domains, acquiring
   the required trust anchors may be difficult.  This is left as an area
   for future work.

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5.  IANA Considerations

   This document has no actions for IANA.

6.  References

6.1.  Informative References

[PXE]     Henry, M. and M. Johnston, "Preboot Execution Environment
          (PXE) Specification", September 1999,

[PXEOPT]  Johnston, M., "DHCP Options for the Intel Preboot eXecution
          Environment (PXE)", draft-ietf-dhc-pxe-options-03.txt,
          Internet draft (work in progress), March 2006.

[RFC793]  Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
          September 1981.

[RFC1332] McGregor, G., "PPP Internet Control Protocol", RFC 1332,
          Merit, May 1992.

[RFC1350] Sollins, K., "The TFTP Protocol (Revision 2)", STD 33, RFC
          1350, July 1992.

[RFC1877] Cobb, S., "PPP Internet Protocol Control Protocol Extensions
          for Name Server Addresses", RFC 1877, December 1995.

[RFC1958] Carpenter, B., "Architectural Principles of the Internet", RFC
          1958, June 1996.

[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
          March 1997.

[RFC2462] Thomson, S. and T. Narten, "IPv6 Stateless Address
          Autoconfiguration", RFC 2462, December 1998.

[RFC2472] Haskin, D. and E. Allen, "IP Version 6 over PPP", RFC 2472,
          December 1998.

[RFC2608] Guttman, E., et al., "Service Location Protocol, Version 2",
          RFC 2608, June 1999.

[RFC3041] Narten, T. and R. Draves, "Privacy Extensions for Stateless
          Address Autoconfiguration in IPv6", RFC 3041, January 2001.

[RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
          RFC 3118, June 2001.

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[RFC3315] Droms, R., Ed., Bound, J., Volz,, B., Lemon, T., Perkins, C.
          and M. Carney, "Dynamic Host Configuration Protocol for IPv6
          (DHCPv6)", RFC 3315, July 2003.

[RFC3344] Perkins, C., "IP Mobility Support for IPv4", RFC 3344, August

[RFC3530] Shepler, S., Callaghan, B., Robinson, D., Thurlow, R., Beame,
          C., Eisler, M. and D. Noveck, "Network File System (NFS)
          version 4 Protocol", RFC 3530, April 2003.

[RFC3720] Satran, J., Meth, K., Sapuntzakis, C. Chadalapaka, M.  and E.
          Zeidner, "Internet Small Computer Systems Interface (iSCSI)",
          RFC 3720, April 2004.

[RFC3736] Droms, R., "Stateless Dynamic Host Configuration Protocol
          (DHCP) Service for IPv6", RFC 3736, April 2004.

[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J. and H.
          Levkowetz, "Extensible Authentication Protocol (EAP)", RFC
          3748, June 2004.

[RFC3756] Nikander, P., Kempf, J. and E. Nordmark, "IPv6 Neighbor
          Discovery (ND) Trust Models and Threats", RFC 3756, May 2004.

[RFC3775] Johnson, D., Perkins, C. and J. Arkko, "Mobility Support in
          IPv6", RFC 3775, June 2004.

[RFC3832] Zhao, W., Schulzrinne, H., Guttman, E., Bisdikian, C. and W.
          Jerome, "Remote Service Discovery in the Service Location
          Protocol (SLP) via DNS SRV", RFC 3832, July 2004.

[RFC3971] Arkko, J., Kempf, J., Sommerfeld, B., Zill, B. and P.
          Nikander, "SEcure Neighbor Discovery (SEND)", RFC 3971, March

[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)", RFC
          3972, March 2005.

[RFC4173] Sarkar, P., Missimer, D. and C. Sapuntzakis, "Bootstrapping
          Clients using the iSCSI Protocol", RFC 4173, September 2005.

[RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet
          Protocol", RFC 4301, December 2005.

[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC
          4306, December 2005.

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[RFC4339] Jeong, J., "IPv6 Host Configuration of DNS Server Information
          Approaches", RFC 4339, February 2006.

[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
          (TLS) Protocol Version 1.1", RFC 4346, April 2006.


   Bob Hinden and James Kempf provided valuable input on this document.

Authors' Addresses

   Bernard Aboba
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

   EMail: bernarda@microsoft.com
   Phone: +1 425 706 6605
   Fax:   +1 425 936 7329

   Dave Thaler
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

   EMail: dthaler@microsoft.com

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INTERNET-DRAFT      Principles of Host Configuration    24 February 2007

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