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Network Working Group                                           B. Aboba
INTERNET-DRAFT                                                 D. Thaler
Category: Informational                            Microsoft Corporation
Expires: November 25, 2008                                 Loa Andersson
25 May 2008                                                     Acreo AB
                                             Internet Architecture Board

               Principles of Internet Host Configuration
                       draft-iab-ip-config-04.txt

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
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   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on November 25, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2008).

Abstract

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









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   Table of Contents

1.  Introduction..............................................    3
      1.1 Terminology ........................................    3
2.  Principles ...............................................    6
      2.1 Minimize Configuration .............................    6
      2.2 Less is More .......................................    6
      2.3 Diversity is Not a Benefit .........................    7
      2.4 Lower Layer Independence ...........................    8
      2.5 Configuration is Not Access Control ................    9
3.  Additional Discussion ....................................   10
      3.1 General Purpose Mechanisms .........................   10
      3.2 Service Discovery ..................................   10
4.  Security Considerations ..................................   13
      4.1 Configuration Authentication .......................   13
5.  IANA Considerations ......................................   14
6.  References ...............................................   14
      6.1 Informative References .............................   14
Acknowledgments ..............................................   17
Appendix A - IAB Members .....................................   17
Authors' Addresses ...........................................   18
Full Copyright Statement .....................................   18
Intellectual Property ........................................   19




























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1.  Introduction

   This document describes principles of Internet host configuration.
   It covers issues relating to configuration of Internet layer
   parameters, 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
        discovery.

      o The relationship between network access authentication and host
        configuration.

      o The role of link-layer protocols and tunneling protocols
        in Internet host configuration.

   The role of the link-layer and tunneling protocols is particularly
   important, since it can affect the properties of a link as seen by
   higher layers (for example, whether privacy extensions specified in
   "Privacy Extensions for Stateless Address Autoconfiguration in IPv6"
   [RFC4941] are available to applications).

1.1.  Terminology


link      A communication facility or medium over which nodes can
          communicate at the link-layer, i.e., the layer immediately
          below IP.  Examples are Ethernets (simple or bridged), PPP
          links, X.25, Frame Relay, or ATM networks as well as Internet
          (or higher) layer "tunnels", such as tunnels over IPv4 or IPv6
          itself.

on link   An address that is assigned to an interface on a specified
          link.

off link  The opposite of "on link"; an address that is not assigned to
          any interfaces on the specified link.

   Internet layer configuration is defined as the configuration required
   to support the operation of the Internet layer.  This includes IP
   address(es), subnet prefix(es), default gateway(s), mobility
   agent(s), boot service configuration and other parameters.




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IP address(es)
          Internet Protocol (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 receive
          unicast IP packets, rather than 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 (since 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 Internet
          layer security facilities such as IPsec specified in "Security
          Architecture for the Internet Protocol" [RFC4301].

Subnet prefix(es)
          Once a subnet prefix is configured, hosts with an IP address
          can exchange unicast IP packets with on-link hosts.

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

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 utilize dynamic home agent
          configuration.

Other parameters
          Internet layer parameter configuration also includes
          configuration of per-host parameters (e.g. hostname) and per-
          interface parameters (e.g.  IP Time-To-Live (TTL),
          enabling/disabling of IP forwarding and source routing, and
          Maximum Transmission Unit (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
          the Trivial File Transfer Protocol (TFTP) [RFC1350], Network
          File System (NFS) [RFC3530] and Internet Small Computer
          Systems Interface (iSCSI) [RFC3720][RFC4173].  It also may be
          useful in situations where it is necessary to update the boot



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          image of a host that supports a disk, such as in the Preboot
          eXecution Environment (PXE) [PXE][RFC4578].  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
          The configuration required for the host to resolve names.
          This includes configuration of the addresses of name
          resolution servers, including IEN 116, Domain Name Service
          (DNS), Windows Internet Name Service (WINS), Internet Storage
          Name Service (iSNS) and Network Information Service (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 name.  Note that Link local name resolution services (such
          as NetBIOS [RFC1001], Link Local Multicast Name Resolution
          (LLMNR) [RFC4795] and multicast DNS (mDNS) [mDNS]) typically
          do not require configuration.

          Once the host has completed name service configuration, it is
          capable of resolving names using name resolution protocols
          that require configuration.  This not only allows the host to
          communicate with off-link hosts whose IP address is not known,
          but to the extent that name services requiring configuration
          are utilized for service discovery, this also enables the host
          to discover services available on the network or elsewhere.

Time Service Configuration
          Time service configuration includes configuration of servers
          for protocols such as the Simple Network Time Protocol (SNTP)
          and the Network Time Protocol (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, Session Initiation Protocol (SIP) proxies,
          etc.





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2.  Principles

   This section describes basic principles of Internet host
   configuration.

2.1.  Minimize Configuration

   Anything that can be configured can be misconfigured.  "Architectural
   Principles of the Internet" [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, the
   Transmission Control Protocol (TCP) [RFC793] does not require
   configuration of the Maximum Segment Size, but is able to compute an
   appropriate value.

   Some protocols support self-configuration mechanisms, such as
   capability negotiation or discovery of other hosts 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.  RFC 1958
   [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
   span a wide range of devices, from embedded devices operating with
   minimal footprint to supercomputers.  Since the resources (e.g.
   memory and code size) available for host configuration may be very
   small, it is desirable for a host to be 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 Read Only Memory (ROM), with minimal
   available memory.  In the Pre-boot Execution Environment (PXE), prior
   to obtaining a boot image, the host is typically only able to
   communicate using IP and the User Datagram Protocol (UDP).  This is
   one reason why Internet layer configuration mechanisms typically
   depend only on IP and UDP.  After obtaining the boot image, the host



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   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 Read-Only Memory (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
   problems:

Interoperability    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
                    problems.

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

Redundancy          To support diversity in host configuration
                    mechanisms, 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 benefit.

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

Conflicts           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



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                    implementation complexity.

Additional traffic  To limit configuration latency, hosts may
                    simultaneously attempt to obtain configuration by
                    multiple mechanisms.  This can result in increasing
                    on-the-wire traffic, both from use of multiple
                    mechanisms as well as from retransmissions within
                    configuration mechanisms not implemented on the
                    network.

Security            Support for multiple configuration mechanisms
                    increases the potential for security vulnerabilities
                    in one of the mechanisms.

2.4.  Lower Layer Independence

   RFC 1958 [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, wireless metropolitan and wide area networks, etc.
   The proliferation of network access mechanisms makes it 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
   layer (such as in the Point-to-Point Protocol (PPP) IPv4CP [RFC1332]
   and  "Mobile radio interface Layer 3 specification; Core network
   protocols; Stage 3 (Release 5)" [3GPP-24.008]), the disadvantages of
   this approach have now become apparent.  This includes the extra
   complexity of implementing different mechanisms on different link
   layers, and the difficulty in adding new parameters which would



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   require defining a mechanism in each link layer protocol.

   For example, PPP IPv4CP and "Internet Protocol Control Protocol
   (IPCP) Extensions for Name Service Configuration" [RFC1877] were
   developed at a time when the Dynamic Host Configuration Protocol
   (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 "IPv6 Stateless Address
   Autoconfiguration" [RFC4862] and DHCPv6 [RFC3736] are available, PPP
   IPv6CP [RFC5072] instead simply configures an Interface-Identifier
   which is similar to a MAC address.  This enables PPP IPv6CP to avoid
   duplicating DHCPv6 functionality.

   In contrast, Internet Key Exchange Version 2 (IKEv2) [RFC4306]
   utilizes the same approach as PPP IPv4CP by defining a Configuration
   Payload for Internet host configuration for both IPv4 and IPv6.  As
   pointed out in "Dynamic Host Configuration Protocol (DHCPv4)
   Configuration of IPsec Tunnel Mode" [RFC3456], leveraging DHCP has
   advantages in terms of address management integration, address pool
   management, reconfiguration and fail-over.  On the other hand, the
   IKEv2 approach reduces the number of exchanges.

   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 applications that desire "Privacy Extensions for Stateless
   Address Autoconfiguration in IPv6" [RFC4941] may not function well.
   Similar issues may arise for other types of addresses, such as
   Cryptographically Generated Addresses [RFC3972].

   Avoiding lower layer dependencies is desirable even where the lower
   layer is link independent.  For example, while the 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 be usable on all links
   that support IP.

2.5.  Configuration is Not Access Control

   Network access authentication and authorizaton is a distinct problem
   from Internet host configuration.  Therefore network access
   authentication and authorization is best handled independently of the
   Internet and higher layer configuration mechanisms.




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   Having an Internet (or higher) layer protocol authenticate clients is
   appropriate to prevent resource exhaustion of a scarce resource on
   the server (such as IP addresses or prefixes), but not for preventing
   hosts from obtaining access to a link.  If the user can manually
   configure the host, requiring authentication in order to obtain
   configuration parameters (such as an IP address) has little value.
   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 (such as DHCP or a service discovery protocol, as noted in
   Section 3.2).  The choice should be made taking into account the
   architectural principles discussed in Section 2.

   Given the number of Internet host configuration mechanisms that have
   already been defined, there is no justification for hard coding of
   service IP addresses or domain names.  Taking into account the
   problems resulting from the proliferation of these mechanisms, there
   is no apparent need for the development of additional general-purpose
   configuration mechanisms.

   When defining a new host parameter, protocol designers should first
   consider whether configuration is indeed necessary (see Section 2.1).
   If configuration is necessary, in addition to considering fate
   sharing (see Section 3.3), protocol designers should consider:

      1. The organizational implications for administrators.  For
         example, routers and servers are often administered by
         different sets of individuals, so that configuring a router
         with server parameters may require cross-group collaboration.

      2. Whether the parameter is a per-interface or a global
         parameter.  For example, configuration protocols
         such as DHCP run on a per-interface basis and hence
         are more appropriate for per-interface parameters.

3.2.  Service Discovery

   Higher-layer configuration often includes configuring server
   addresses.  The question arises as to how this differs from "service
   discovery" as provided by Service Discovery protocols such as the
   Service Location Protocol Version 2 (SLPv2) [RFC2608] or Domain Name



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   Service Service Discovery (DNS-SD) [DNS-SD].

   In Internet host configuration mechanisms such as DHCP, server
   instances are considered equivalent.  In service discovery protocols,
   on the other hand, a host desires to find a server satisfying a
   particular set of criteria, which may vary by request.

   Service discovery protocols can support discovery of servers on the
   Internet, not just those within the local administrative domain.  For
   example, see "Remote Service Discovery in the Service Location
   Protocol (SLP) via DNS SRV" [RFC3832] and [DNS-SD].  Internet host
   configuration mechanisms such as DHCP, on the other hand, typically
   assume the server(s) in the local administrative domain 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 a Directory Agent (DA).  Since the DA
   may not be available on the local link, service discovery beyond the
   local link is typically dependent on a mechanism for configuring the
   DA address or name.  As a result, service discovery protocols can
   typically not be relied upon for obtaining basic Internet layer
   configuration, although they can be used to obtain higher-layer
   configuration parameters.

3.2.1.  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.  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).  For example, learning default gateways via Router
   Advertisements provides perfect fate sharing.  That is, gateway
   addresses can be obtained if and only if they can actually be used.
   Similarly, obtaining DNS server configuration from a DNS server would
   provide fate sharing since the configuration would only be obtainable
   if the DNS server were available.

   While fate sharing is a desirable property of a configuration
   mechanism, in many situations fate sharing is imperfect or



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   unavailable.  When utilized to discover services on the local link,
   service discovery protocols typically provide for fate sharing, since
   hosts providing service information typically also provide the
   services.  However, where service discovery is assisted by a DA, 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's list of operational servers
   may not be current, it is possible for the DA to provide clients with
   service information that is out of date.  For example, service
   descriptions provided to the DA by servers might be included in
   response to service discovery queries sent by clients even after the
   servers were no longer operational.  Similarly, recently introduced
   servers might not yet have registered themselves with the DA.  Thus,
   fate sharing can be imperfect.

   Similar limitations exist for other server-based configuration
   mechanisms such as DHCP.  Typically DHCP servers do not check for the
   liveness of the configuration information they provide, or do not
   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.

3.2.2.  Discovering Names vs. Addresses

   In discovering servers other than name resolution servers, it is
   possible to either discover the IP addresses of the server(s), or to
   discover names, each of which may resolve to a list of addresses.

   It is typically more efficient to obtain the list of addresses
   directly, since this avoids the extra name resolution steps and
   accompanying latency.  However, where servers are mobile, the name to
   address binding may change, requiring a fresh set of addresses to be
   obtained.  Where the configuration mechanism does not support fate
   sharing (e.g. DHCP), providing a name rather than an address can
   simplify operations, assuming that the server's new address is
   manually or automatically updated in the DNS; in this case there is
   no need to re-do parameter configuration, since the name is still
   valid.  Where fate sharing is supported (e.g. service discovery
   protocols), a fresh address can be obtained by re-initiating
   parameter configuration.




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   In obtaining the IP addresses for a set of servers, it is desirable
   to distinguish which IP addresses belong to which servers.   If a
   server IP address is unreachable, this enables the host to try the IP
   address of another server, rather than another IP address of the same
   server, in case the server is down.   This can be enabled by
   distinguishing which addresses belong to the same server.

4.  Security Considerations

   Secure IP configuration presents a number of challenges.  In addition
   to denial-of-service and man-in-the-middle attacks, attacks on
   configuration mechanisms may target particular parameters.  For
   example, attackers may target DNS server configuration in order to
   support subsequent phishing or pharming attacks.  A number of issues
   exist with various classes of parameters, as discussed in Section
   2.6, "IPv6 Neighbor Discovery (ND) Trust Models and Threats"
   [RFC3756] Section 4.2.7, "Authentication for DHCP Messages" [RFC3118]
   Section 1.1, and "Dynamic Host Configuration Protocol for IPv6
   (DHCPv6)" [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.

   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.
   Approaches to boot configuration security are described in
   "Bootstrapping Clients using the Internet Small Computer System
   Interface (iSCSI) Protocol" [RFC4173] and "Preboot Execution
   Environment (PXE) Specification" [PXE].

4.1.  Configuration Authentication

   The techniques available for securing Internet layer configuration
   are limited, since transmission layer security protocols such as
   IPsec [RFC4301] or Transport Layer Security (TLS) [RFC4346] cannot be
   used until an IP address has been configured.  As a result,
   configuration security is typically implemented within the
   configuration protocols themselves.

   PPP [RFC1661] does not support secure negotiation within IPv4CP
   [RFC1332] or IPv6CP [RFC5072], enabling an attacker with access to
   the link to subvert the negotiation.  In contrast, IKEv2 [RFC4306]
   provides encryption, integrity and replay protection for
   configuration exchanges.

   In situations where link layer security is provided, and the Network



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   Access Server (NAS) acts as a DHCP relay or server, protection can be
   provided against rogue DHCP servers, provided that the NAS filters
   incoming DHCP packets from unauthorized sources.  However, explicit
   dependencies on lower layer security mechanisms are limited by the
   "lower layer independence" principle (see Section 2.4).

   Internet layer secure configuration mechanisms include SEcure
   Neighbor Discovery (SEND) [RFC3971] for IPv6 stateless address
   autoconfiguration [RFC4862], or DHCP authentication for stateful
   address configuration.  DHCPv4 [RFC2131] initially did not include
   support for security; this was added in "Authentication for DHCP
   Messages" [RFC3118].  DHCPv6 [RFC3315] included security support.
   However, DHCP authentication is not widely implemented for either
   DHCPv4 or DHCPv6.

   Higher layer configuration can make use of a wider range of security
   techniques.  When DHCP authentication is supported, higher-layer
   configuration parameters provided by DHCP can be secured.  However,
   even if a host does not support DHCPv6 authentication, higher-layer
   configuration via Stateless DHCPv6 [RFC3736] can still be secured
   with IPsec.

   Possible exceptions can exist where security facilities are not
   available until later in the boot process.  It may be difficult to
   secure boot configuration even once the Internet layer has been
   configured, if security functionality is not available until after
   boot configuration has been completed.  For example, it is possible
   that Kerberos, IPsec or TLS will not be available until later in the
   boot process; see "Bootstrapping Clients using the Internet Small
   Computer System Interface (iSCSI) Protocol" [RFC4173] for discussion.

   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
   that visits multiple administrative domains, acquiring the required
   trust anchors may be difficult.  This is left as an area for future
   work.

5.  IANA Considerations

   This document has no actions for IANA.

6.  References

6.1.  Informative References

[3GPP-24.008]
          3GPP TS 24.008 V5.8.0, "Mobile radio interface Layer 3



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          specification; Core network protocols; Stage 3 (Release 5)",
          June 2003.

[DNS-SD]  Cheshire, S., and M. Krochmal, "DNS-Based Service Discovery",
          Internet-Draft (work in progress), draft-cheshire-dnsext-dns-
          sd-04.txt, August 2006.

[mDNS]    Cheshire, S. and M. Krochmal, "Multicast DNS", June 2005.
          http://files.multicastdns.org/draft-cheshire-dnsext-
          multicastdns.txt

[PXE]     Henry, M. and M. Johnston, "Preboot Execution Environment
          (PXE) Specification", September 1999,
          http://www.pix.net/software/pxeboot/archive/pxespec.pdf

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

[RFC1001] NetBIOS Working Group in the Defense Advanced Research
          Projects Agency, Internet Activities Board, and End-to-End
          Services Task Force, "Protocol standard for a NetBIOS service
          on a TCP/UDP transport: Concepts and methods", STD 19, RFC
          1001, March 1987.

[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.

[RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC
          1661, July 1994.

[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.

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

[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
          2002.

[RFC3456] Patel, B., Aboba, B., Kelly, S. and V. Gupta, "Dynamic Host
          Configuration Protocol (DHCPv4) Configuration of IPsec Tunnel
          Mode", RFC 3456, January 2003.

[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
          2005.

[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.



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

[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.

[RFC4578] Johnston, M. and S. Venaas, "Dynamic Host Configuration
          Protocol (DHCP) Options for the Intel Preboot eXecution
          Environment (PXE)", RFC 4578, November 2006.

[RFC4795] Aboba, B., Thaler, D. and L. Esibov, "Link-Local Multicast
          Name Resolution (LLMNR)", RFC 4795, January 2007.

[RFC4862] Thomson, S., Narten, T. and T. Jinmei, "IPv6 Stateless Address
          Autoconfiguration", RFC 4862, September 2007.

[RFC4941] Narten, T., Draves, R. and S. Krishnan, "Privacy Extensions
          for Stateless Address Autoconfiguration in IPv6", RFC 4941,
          September 2007.

[RFC5072] Varada, S., Haskins D. and E. Allen, "IP Version 6 over PPP",
          RFC 5072, September 2007.

Acknowledgments

   Bob Hinden, Pasi Eronen, Jari Arkko, Pekka Savola and James Kempf
   provided valuable input on this document.

Appendix A - IAB Members at the time of this writing

   Loa Andersson
   Leslie Daigle
   Elwyn Davies
   Kevin Fall
   Olaf Kolkman
   Barry Leiba
   Kurtis Lindqvist
   Danny McPherson
   David Oran
   Eric Rescorla
   Dave Thaler
   Lixia Zhang






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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

   Loa Andersson
   Acreo AB

   EMail: loa@pi.se

Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.













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Intellectual Property

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Acknowledgment

   Funding for the RFC Editor function is currently provided by the
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