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Network Working Group                                           B. Aboba
INTERNET-DRAFT                                                 D. Thaler
Category: Informational                                    Loa Andersson
Expires: September 5, 2009                               Stuart Cheshire
23 February 2009                             Internet Architecture Board

               Principles of Internet Host Configuration
                       draft-iab-ip-config-11.txt

Status of This Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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   This Internet-Draft will expire on August 27, 2009.

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   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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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.

Table of Contents

   1.  Introduction..............................................    3
         1.1 Terminology ........................................    3
         1.2 Internet Host Configuration ........................    4
   2.  Principles ...............................................    6
         2.1 Minimize Configuration .............................    7
         2.2 Less is More .......................................    7
         2.3 Minimize Diversity .................................    8
         2.4 Lower Layer Independence ...........................    9
         2.5 Configuration is Not Access Control ................   11
   3.  Additional Discussion ....................................   11
         3.1 Reliance on General Purpose Mechanisms .............   11
         3.2 Relationship between IP Configuration and
             Service Discovery ..................................   12
         3.3 Discovering Names vs. Addresses ....................   14
         3.4 Dual Stack Issues ..................................   15
         3.5 Relationship between Per-Interface and
             Per-Host Configuration .............................   16
   4.  Security Considerations ..................................   17
         4.1 Configuration Authentication .......................   17
   5.  IANA Considerations ......................................   19
   6.  References ...............................................   19
         6.1 Informative References .............................   19
   Acknowledgments ..............................................   23
   Appendix A - IAB Members .....................................   23
   Authors' Addresses ...........................................   24


















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1.  Introduction

   This document describes principles of Internet host [STD3]
   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 per-interface and per-host
        configuration.

      o The relationship between network access authentication and
        host configuration.

      o The desirability of supporting self-configuration of parameters
        or avoiding parameter configuration altogether.

      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 [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.




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   mobility agent
              Either a home agent or a foreign agent [RFC3344][RFC3775].

1.2.  Internet Host Configuration

1.2.1.  Internet Layer Configuration

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

   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 a vital step,
              since practically all of IP networking relies on the
              assumption that hosts have IP address(es) associated with
              (each of) their active network interface(s).  Used as the
              source address of an IP packet, these IP addresses
              indicate the sender of the packet; used as the destination
              address of a unicast IP packet, these IP addresses
              indicate the intended receiver.

              The only common example of IP-based protocols operating
              without an IP address involves address configuration, such
              as the use of DHCPv4 [RFC2131] to obtain an address.  In
              this case, by definition, DHCPv4 is operating before the
              host has an IPv4 address, so the DHCP protocol designers
              had the choice of either using IP without an IP address,
              or not using IP at all.  The benefits of making IPv4 self-
              reliant, configuring itself using its own IPv4 packets,
              instead of depending on some other protocol, outweighed
              the drawbacks of having to use IP in this constrained
              mode.  Use of IP for purposes other than address
              configuration can safely assume that the host will have
              one or more IP addresses, which may be self-configured
              link-local addresses [RFC3927][RFC4862], or other
              addresses configured via DHCP or other means.

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

   Default gateway(s)
              Once a default gateway is configured on an interface,



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              hosts with an IP address can send unicast IP packets to
              that gateway for forwarding to 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 utilize dynamic home
              agent configuration.

   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 disk-
              less 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 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.  While
              boot service parameters may be provided on a per-interface
              basis, loading and verification of a boot image affects
              behavior of the host as a whole.

   Other IP 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) to
              use in outgoing packets, enabling/disabling of IP
              forwarding and source routing, and Maximum Transmission
              Unit (MTU)).

1.2.2.  Higher 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 [IEN116], Domain
              Name System (DNS), Windows Internet Name Service (WINS),
              Internet Storage Name Service (iSNS) [RFC4171][RFC4174]



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              and Network Information Service (NIS) servers [RFC3898],
              and the setting of name resolution parameters such as the
              DNS domain and search list [RFC3397], the NetBIOS node
              type, 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.  While name service
              parameters can be provided on a per-interface basis, their
              configuration will typically affect behavior of the host
              as a whole.

   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.  While time service parameters can be
              provided on a per-interface basis, their configuration
              will typically affect behavior of the host as a whole.

   Other service configuration
              This can include discovery of additional servers and
              devices, such as printers, Session Initiation Protocol
              (SIP) proxies, etc.  This configuration will typically
              apply to the entire host.

2.  Principles

   This section describes basic principles of Internet host
   configuration.







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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 Path
   Maximum Transmission Unit (PMTU) can be discovered, as described in
   "Packetization Layer Path MTU Discovery" [RFC4821], "TCP Problems
   with Path MTU Discovery" [RFC2923], "Path MTU discovery" [RFC1191]
   and "Path MTU Discovery for IP version 6" [RFC1981].

   Having a protocol design with many configurable parameters increases
   the possibilities for misconfiguration of those parameters, resulting
   in failures or other sub-optimal operation.  Eliminating or reducing
   configurable parameters helps lessen this risk.  Where configurable
   parameters are necessary or desirable, protocols can reduce the risk
   of human error by making these parameters self-configuring, such as
   by using capability negotiation within the protocol, or by automated
   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.
   "Architectural Principles of the Internet" [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 many types of devices, it is important
   to minimize the footprint requirement.  For example, IP-based
   protocols are used on a wide range of devices,  from supercomputers
   to small low-cost devices running "embedded" operating systems.
   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.  Many hosts do not have enough space in this ROM
   for even a simple implementation of TCP, so in the Pre-boot Execution
   Environment (PXE) the task of obtaining a boot image is performed



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   using the User Datagram Protocol over IP (UDP/IP) [RFC768] instead.
   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 devices may be severely constrained on how much code
   they can fit within their ROM, designing a configuration mechanism in
   such a way that it requires the availability of higher layer
   facilities may make that configuration mechanism unusable in such
   devices.  In fact, it cannot even be guaranteed that all Internet
   layer facilities will be available.  For example, the minimal version
   of IP in a host's boot ROM may not implement IP fragmentation and
   reassembly.

2.3.  Minimize Diversity

   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          For maximum interoperability, a host would need to
                      implement all configuration mechanisms used on all
                      the link layers it supports.  This increases the
                      required footprint, a burden for embedded devices.
                      It also leads to lower quality, since testing
                      resources (both formal testing, and real-world
                      operational use) are spread more thinly -- the
                      more different configuration mechanisms a device
                      supports, the less testing each one is likely to
                      undergo.

   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.





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   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 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 attack surface without any potential
                      benefit.

2.4.  Lower Layer Independence

   "Architectural Principles of the Internet" [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 each 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,
   only the link layer protocol (whether it be a physical link, or a
   virtual tunnel link) should be explicitly aware of link-layer
   parameters (although it may configure them).  Introduction of lower
   layer dependencies increases the likelihood of interoperability
   problems and adds Internet layer configuration mechanisms that hosts
   need to implement.

   Lower layer dependencies can be best avoided by keeping Internet host



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   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 Internet layer configuration 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]), this approach
   has disadvantages.  These include the extra complexity of
   implementing different mechanisms on different link layers, and the
   difficulty in adding new higher-layer parameters which would require
   defining a mechanism in each link layer protocol.

   For example, "Internet Protocol Control Protocol (IPCP) Extensions
   for Name Service Configuration" [RFC1877] was developed prior to the
   definition of the DHCPINFORM message in "Dynamic Host Configuration
   Protocol" [RFC2131]; at that time Dynamic Host Configuration Protocol
   (DHCP) servers had not been widely implemented on access devices or
   deployed in service provider networks.  While the design of IPv4CP
   was appropriate in 1992, it should not be taken as an example that
   new link layer technologies should emulate.  Indeed, in order to
   "actively advance PPP's most useful extensions to full standard,
   while defending against further enhancements of questionable value",
   "IANA Considerations for the Point-to-Point Protocol (PPP)" [RFC3818]
   changed the allocation of PPP protocol numbers (including IPv4CP
   extensions) so as to no longer be "first come first served."

   In IPv6 where link-layer-independent mechanisms such as stateless
   autoconfiguration [RFC4862] and stateless DHCPv6 [RFC3736] are
   available, PPP IPv6CP [RFC5072] configures an Interface-Identifier
   which is similar to a MAC address.  This enables PPP IPv6CP to avoid
   duplicating DHCPv6 functionality.

   However, 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.  While the
   IKEv2 approach reduces the number of exchanges, "Dynamic Host
   Configuration Protocol (DHCPv4) Configuration of IPsec Tunnel Mode"
   [RFC3456] points out that leveraging DHCP has advantages in terms of
   address management integration, address pool management,
   reconfiguration and fail-over.

   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



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   individual IPv6 addresses and precludes using any other addresses,
   then applications that want to use privacy extensions [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) may be run over any link satisfying its
   requirements (see [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 links that support IP
   but not EAP.

2.5.  Configuration is Not Access Control

   Network access authentication and authorization 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.

   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.
   Network administrators who wish to control access to a link can
   achieve this better using technologies like Port Based Network Access
   Control [IEEE-802.1X].  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.  Reliance on 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.

   Taking into account the general-purpose configuration mechanisms
   currently available, we see little need for 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).



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   If configuration is necessary, in addition to considering fate
   sharing (see Section 3.2.1), 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 need is to configure a set of interchangeable
         servers or to select a server satisfying a particular set
         of criteria.  See Section 3.2.

      3. Whether IP address(es) should configured or name(s).
         See Section 3.3.

      4. If IP address(es) are configured, whether IPv4 and
         IPv6 addresses should be configured simultaneously or
         separately.  See Section 3.4.

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

      6. How per-interface configuration affects host-wide behavior.
         For example, whether the host should select a subset
         of the per-interface configurations, or whether the
         configurations are to merged, and if so, how this is
         done.  See Section 3.5.

3.2.  Relationship between IP Configuration and 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 DNS-Based
   Service Discovery (DNS-SD) [DNS-SD].

   In Internet host configuration mechanisms such as DHCP, if multiple
   server instances are provided, they are considered interchangeable.
   For example, in a list of time servers, the servers are considered
   interchangeable because they all provide the exact same service --
   telling you the current time.  In a list of local caching DNS
   servers, the servers are considered interchangeable because they all
   should give you the same answer to any DNS query.  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.  When printing a document, it is not the case that any



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   printer will do.  The speed, capabilities, and physical location of
   the printer matter to the user.

   Information learned via DHCP is typically learned once, at boot time,
   and after that may be updated only infrequently (e.g. on DHCP lease
   renewal), if at all.  This makes it appropriate for information that
   is relatively static and unchanging over these time intervals.  Boot-
   time discovery of server addresses is appropriate for service types
   where there are a small number of interchangeable servers that are of
   interest to a large number of clients.  For example, listing time
   servers in a DHCP packet is appropriate because an organization may
   typically have only two or three time servers, and most hosts will be
   able to make use of that service.  Listing all the printers or file
   servers at an organization is a lot less useful, because the list may
   contain hundreds or thousands of entries, and on a given day a given
   user may not use any of the printers in that list.

   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-Based Service Discovery
   [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 those parameters are ones that cannot be usefully used
   without those servers being available.  In this case, successfully
   obtaining those parameters via other means has little benefit if they
   cannot be used because the required servers are not available.  The



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   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 a number of situations fate sharing may not be
   possible.  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, this is no longer the case when service discovery
   is assisted by a Directory Agent (DA).  First of all, the DA's list
   of operational servers may not be current, so that it is possible for
   the DA to provide clients with service information that is out of
   date.  For example, a DA's response to a client's service discovery
   query may contain stale information about servers that are no longer
   operational.  Similarly, recently introduced servers might not yet
   have registered themselves with the DA.  Furthermore, the use of a DA
   for service discovery also introduces a dependency on whether the DA
   is operational, even though the DA is typically not involved in the
   delivery of the service.

   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 one or more well-known
   anycast addresses for each service.  Configuration of more than one
   anycast address is desirable to allow the client to fail over faster
   than would be possible from routing protocol convergence.

3.3.  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.



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   It is typically more efficient to obtain the list of addresses
   directly, since this avoids the extra name resolution steps and
   accompanying latency.  On the other hand, 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.

   In providing 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.

3.4.  Dual Stack Issues

   One use for learning a list of interchangeable server addresses is
   for fault tolerance, in case one or more of the servers are
   unresponsive.  Hosts will typically try the addresses in turn, only
   attempting to use the second and subsequent addresses in the list if
   the first one fails to respond quickly enough.  In such cases, having
   the list sorted in order of expected likelihood of success will help
   clients get results faster.  For hosts that support both IPv4 and
   IPv6, it is desirable to obtain both IPv4 and IPv6 server addresses
   within a single list.  Obtaining IPv4 and IPv6 addresses in separate
   lists, without indicating which server(s) they correspond to,
   requires the host to use a heuristic to merge the lists.

   For example, assume there are two servers, A and B, each with one
   IPv4 address and one IPv6 address.  If the first address the host
   should try is (say) the IPv6 address of server A, then the second
   address the host should try, if the first one fails, would generally
   be the IPv4 address of server B.  This is because the failure of the
   first address could either be due to server A being down, or due to
   some problem with the host's IPv6 address, or due to a problem with
   connectivity to server A.  Trying the IPv4 address next is preferred
   since the reachability of the IPv4 address is independent of all
   potential failure causes.

   If the list of IPv4 server addresses were obtained separate from the
   list of IPv6 server addresses, a host trying to merge the lists would
   not know which IPv4 addresses belonged to the same server as the IPv6



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   address it just tried.  This can be solved either by explicitly
   distinguishing which addresses belong to which server or, more
   simply, by configuring the host with a combined list of both IPv4 and
   IPv6 addresses.  Note that the same issue can arise with any
   mechanism (e.g. DHCP, DNS, etc.) for obtaining server IP addresses.

   Configuring a combined list of both IPv4 and IPv6 addresses gives the
   configuration mechanism control over the ordering of addresses, as
   compared with configuring a name and allowing the host resolver to
   determine the address list ordering.  See "DHCP Dual-Stack Issues"
   [RFC4477] for more discussion of dual-stack issues in the context of
   DHCP.

3.5.  Relationship between Per-Interface and Per-Host Configuration

   Parameters that are configured or acquired on a per-interface basis
   can affect behavior of the host as a whole.  Where only a single
   configuration can be applied to a host, the host may need to
   prioritize the per-interface configuration information in some way
   (e.g. most trusted to least trusted).  If the host needs to merge
   per-interface configuration to produce a host-wide configuration, it
   may need to take the union of the per-host configuration parameters
   and order them in some way (e.g. highest speed interface to lowest
   speed interface).  Which procedure is to be applied and how this is
   accomplished may vary depending on the parameter being configured.
   Examples include:

   Boot service configuration
              While boot service configuration can be provided on
              multiple interfaces, a given host may be limited in the
              number of boot loads that it can handle simultaneously.
              For example, a host not supporting virtualization may only
              be capable of handling a single boot load at a time, or a
              host capable of supporting N virtual machines may only be
              capable of handling up to N simultaneous boot loads.  As a
              result, a host may need to select which boot load(s) it
              will act on, out of those configured on a per-interface
              basis.  This requires that the host prioritize them (e.g.
              most trusted to least trusted).

   Name service configuration
              While name service configuration is provided on a per-
              interface basis, name resolution configuration typically
              will affect behavior of the host as a whole.  For example,
              given the configuration of DNS server addresses and
              searchlist parameters on each interface, the host
              determines what sequence of name service queries is to be
              sent on which interfaces.



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   Since the algorithms used to determine per-host behavior based on
   per-interface configuration can affect interoperability, it is
   important for these algorithms to be understood by implementers.  We
   therefore recommend that documents defining per-interface mechanisms
   for acquiring per-host configuration (e.g. DHCP or IPv6 Router
   Advertisement options) include guidance on how to deal with multiple
   interfaces.  This may include discussions of the following items:

      1. Merging.  How are per-interface configurations combined to
         produce a per-host configuration? Is a single configuration
         selected, or is the union of the configurations taken?

      2. Prioritization.  Are the per-interface configurations
         prioritized as part of the merge process?  If so, what are
         some of the considerations to be taken into account in
         prioritization?

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 such as those
   described in "New trojan in mass DNS hijack" [DNSTrojan].  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,
   hosts often 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.  While it is technically possible to perform a very
   limited subset of IP networking operations without an IP address, the
   capabilities are severely restricted.  A host without an IP address



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   cannot receive conventional unicast IP packets, only IP packets sent
   to the broadcast or a multicast address.  Configuration of an IP
   address enables the use of IP fragmentation; 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.  Without an IP address, it is not possible to take
   advantage of security facilities such as IPsec, specified in
   "Security Architecture for the Internet Protocol" [RFC4301], or
   Transport Layer Security (TLS) [RFC5246].  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.

   Where configuration packets are only expected to originate on
   particular links or from particular hosts, filtering can help control
   configuration spoofing.  For example, a Network Access Server (NAS)
   might only permit incoming configuration traffic (such as IPv6 Router
   Advertisement packets (ICMP Type 134), or DHCP packets sent to the
   client port (68 for DHCPv4, 546 for DHCPv6)) originating over a link
   towards authorized configuration sources.  To prevent spoofing,
   communication between the DHCP relay and servers can be authenticated
   and integrity protected using a mechanism such as IPsec.

   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



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   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.

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

[DNSTrojan]
          Goodin, D., "New trojan in mass DNS hijack", The Register,
          December 5, 2008, http://www.theregister.co.uk/2008/12/05/
          new_dnschanger_hijacks/

[IEN116]  J. Postel, "Internet Name Server", IEN 116, August 1979,
          http://www.ietf.org/rfc/ien/ien116.txt

[IEEE-802.1X]
          Institute of Electrical and Electronics Engineers, "Local and
          Metropolitan Area Networks: Port-Based Network Access
          Control", IEEE Standard 802.1X-2004, December 2004.

[DNS-SD]  Cheshire, S., and M. Krochmal, "DNS-Based Service Discovery",
          Internet-Draft (work in progress), draft-cheshire-dnsext-dns-
          sd-05.txt, September 2008.

[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



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[RFC768]  Postel, J., "User Datagram Protocol", RFC 768, August, 1980.

[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.

[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
          November 1990.

[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.

[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery for
          IP version 6", RFC 1981, August 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.

[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery", RFC 2923,
          September 2000.

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

[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.




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[RFC3397] Aboba, B. and S. Cheshire, "Dynamic Host Configuration
          Protocol (DHCP) Domain Search Option", RFC 3397, November
          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.

[RFC3818] Schryver, V., "IANA Considerations for the Point-to-Point
          Protocol (PPP)", RFC 3818, BCP 88, 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.

[RFC3898] Kalusivalingam, V., "Networking Information Service (NIS)
          Configuration Options for Dynamic Host Configuration Protocol
          for IPv6 (DHCPv6)", RFC 3898, October 2004.

[RFC3927] Cheshire, S., Aboba, B. and E. Guttman, "Dynamic Configuration
          of IPv4 Link-Local Addresses", RFC 3927, May 2005.

[RFC3971] Arkko, J., Kempf, J., Sommerfeld, B., Zill, B. and P.
          Nikander, "SEcure Neighbor Discovery (SEND)", RFC 3971, March
          2005.





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

[RFC4171] Tseng, J., Gibbons, K., Travostino, F., Du Laney, C. and J.
          Souza, "Internet Storage Name Service (iSNS), RFC 4171,
          September 2005.

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

[RFC4174] Monia, C., Tseng, J. and K. Gibbons, "The IPv4 Dynamic Host
          Configuration Protocol (DHCP) Option for the Internet Storage
          Name Service", RFC 4174, 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.

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

[RFC4477] Chown, T., Venaas, S. and C. Strauf, "Dynamic Host
          Configuration Protocol (DHCP): IPv4 and IPv6 Dual-Stack
          Issues", RFC 4477, May 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.

[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
          Discovery", RFC 4821, March 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.





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[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
          (TLS) Protocol Version 1.2", RFC 5246, August 2008.

[STD3]    Braden, R., "Requirements for Internet Hosts -- Communication
          Layers", STD 3, RFC 1122, and "Requirements for Internet Hosts
          -- Application and Support", STD 3, RFC 1123, October 1989.

Acknowledgments

   Elwyn Davies, Bob Hinden, Pasi Eronen, Jari Arkko, Pekka Savola,
   James Kempf, Ted Hardie and Alfred Hoenes provided valuable input on
   this document.

Appendix A - IAB Members at the time of this writing

   Loa Andersson
   Gonzalo Camarillo
   Stuart Cheshire
   Russ Housley
   Olaf Kolkman
   Gregory Lebovitz
   Barry Leiba
   Kurtis Lindqvist
   Andrew Malis
   Danny McPherson
   David Oran
   Dave Thaler
   Lixia Zhang























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Authors' Addresses

   Bernard Aboba
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

   EMail: bernarda@microsoft.com

   Dave Thaler
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

   EMail: dthaler@microsoft.com

   Loa Andersson
   Acreo AB

   EMail: loa@pi.nu

   Stuart Cheshire
   Apple Computer, Inc.
   1 Infinite Loop
   Cupertino, CA 95014

   EMail: cheshire@apple.com
























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