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Zero Configuration Networking                                A. Williams
Internet-Draft                                                  Motorola
Expires: March 20, 2003                               September 19, 2002


          Requirements for Automatic Configuration of IP Hosts
                    draft-ietf-zeroconf-reqts-12.txt

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
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   The list of current Internet-Drafts can be accessed at http://
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   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on March 20, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   Many common TCP/IP protocols such as DHCP [RFC2131], DNS
   [RFC1034][RFC1035], MADCAP [RFC2730], and LDAP [RFC2251] must be
   configured and maintained by an administrative staff.  This is
   unacceptable for emerging networks such as home networks, automobile
   networks, airplane networks, or ad hoc networks at conferences,
   emergency relief stations, and many others.  Such networks may be
   nothing more than two isolated laptop PCs connected via a wireless
   LAN.  For all these networks, an administrative staff will not exist
   and the users of these networks neither have the time nor inclination
   to learn network administration skills.  Instead, these networks need
   protocols that require zero user configuration and administration.
   This document is part of an effort to define such zero configuration



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   (zeroconf) protocols.

   Before embarking on defining zeroconf protocols, protocol
   requirements are needed.  This document states the zeroconf protocol
   requirements for four protocol areas; they are: IP interface
   configuration, translation between host name and IP address, IP
   multicast address allocation, and service discovery.  This document
   does not define specific protocols, just requirements.   The
   requirements for these four areas result from examining everyday use
   or scenarios of these protocols.

Table of Contents

   1.      Introduction . . . . . . . . . . . . . . . . . . . . . . .  4
   1.1     Key Words  . . . . . . . . . . . . . . . . . . . . . . . .  4
   1.2     Reading This Document  . . . . . . . . . . . . . . . . . .  4
   1.3     Zeroconf Coexistence . . . . . . . . . . . . . . . . . . .  5
   1.4     Scalability  . . . . . . . . . . . . . . . . . . . . . . .  5
   1.5     Routable Protocol Requirement  . . . . . . . . . . . . . .  5
   1.6     Maintaining Consistent Network State . . . . . . . . . . .  6
   2.      Scenarios  . . . . . . . . . . . . . . . . . . . . . . . .  6
   2.1     Addition and Removal of Devices  . . . . . . . . . . . . .  6
   2.2     Network Coalescing and Partitioning  . . . . . . . . . . .  7
   3.      Requirements . . . . . . . . . . . . . . . . . . . . . . .  8
   3.1     IP Interface Configuration . . . . . . . . . . . . . . . .  8
   3.1.1   IPv6 Considerations  . . . . . . . . . . . . . . . . . . . 10
   3.2     Translation between Host name and IP Address . . . . . . . 10
   3.2.1   IPv6 Considerations  . . . . . . . . . . . . . . . . . . . 11
   3.2.2   Relationship to the DNS  . . . . . . . . . . . . . . . . . 11
   3.2.2.1 Close coupling . . . . . . . . . . . . . . . . . . . . . . 12
   3.2.2.2 Completely orthogonal  . . . . . . . . . . . . . . . . . . 12
   3.2.2.3 API compatible . . . . . . . . . . . . . . . . . . . . . . 12
   3.3     IP Multicast Address Allocation Scenarios  . . . . . . . . 13
   3.3.1   Scope Enumeration  . . . . . . . . . . . . . . . . . . . . 14
   3.3.2   Address Allocation . . . . . . . . . . . . . . . . . . . . 14
   3.3.3   Multiple Sources . . . . . . . . . . . . . . . . . . . . . 15
   3.3.4   IPv6 Considerations  . . . . . . . . . . . . . . . . . . . 15
   3.4     Service Discovery Scenarios  . . . . . . . . . . . . . . . 15
   3.4.1   Printer Service  . . . . . . . . . . . . . . . . . . . . . 16
   3.4.2   IPv6 Considerations  . . . . . . . . . . . . . . . . . . . 16
   4.      Security Considerations  . . . . . . . . . . . . . . . . . 16
   4.1     The Basic in/out Security Policy . . . . . . . . . . . . . 17
   4.2     Security Scenarios . . . . . . . . . . . . . . . . . . . . 18
   4.2.1   Use on an isolated, secure network link  . . . . . . . . . 18
   4.2.2   Use on a shared (insecure) link  . . . . . . . . . . . . . 18
   4.2.3   Use in conjunction with configured protocols . . . . . . . 18
   5.      IANA Considerations  . . . . . . . . . . . . . . . . . . . 19
   6.      Acknowledgments  . . . . . . . . . . . . . . . . . . . . . 19



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           Normative References . . . . . . . . . . . . . . . . . . . 19
           Informative References . . . . . . . . . . . . . . . . . . 19
           Author's Address . . . . . . . . . . . . . . . . . . . . . 21
           Full Copyright Statement . . . . . . . . . . . . . . . . . 22















































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1. Introduction

   A zeroconf protocol is able to operate correctly in the absence of
   configured information from either a user or infrastructure services
   such as conventional DHCP [RFC2131] or DNS [RFC1034][RFC1035]
   servers.  Zeroconf protocols may use configured information, when it
   is available, but do not rely on it being present.  For example, the
   use of MAC addresses (i.e.  layer two addresses) as parameters in
   zeroconf protocols is generally acceptable because they are globally
   unique and readily available on most devices of interest.

   The benefits of zeroconf protocols over existing configured protocols
   are an increase in the ease-of-use for end-users and a simplification
   of the infrastructure necessary to operate protocols.

   This document discusses requirements for zeroconf protocols in four
   areas:

   o  IP interface configuration

   o  Translation between host name and IP address

   o  IP multicast address allocation

   o  Service discovery

   Security considerations are also discussed.

1.1 Key Words

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

1.2 Reading This Document

   Introduction, Scenarios, Requirements, and Security Considerations
   are the major sections of this document.

   The Scenarios & Requirements sections walk through protocol scenarios
   and then lists the requirements of the protocol needed to accomplish
   the scenario.

   The Security Consideration section lists security issues with
   zeroconf protocols.

   Requirements dispersed throughout this document begin with the text
   "Requirements:" or "Requirement:" on a single line, which is followed



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   by a bulleted list of requirements.

1.3 Zeroconf Coexistence

   It is not necessary to simultaneously use zeroconf protocols in all
   four areas (i.e.  IP interface configuration, translation between
   host name and IP address, IP multicast address allocation, service
   discovery).  For example, it may make sense on some networks to
   provide a DHCP server for configured IP interface configuration, but
   perform translation between host name and IP address using a zeroconf
   protocol.

   Given that zeroconf protocols may be deployed on existing configured
   networks, care must be taken in their design to ensure minimum
   disruption to existing networks and applications.  Particular
   consideration should be given to the security implications of
   deploying zeroconf protocols in conjunction with standard configured
   network protocols.

   Requirements:

   o  Zeroconf protocols MUST minimise their impact on existing
      networks.

   o  Zeroconf protocols SHOULD minimise their impact on existing
      applications.

   o  Zeroconf protocols MUST NOT be any less secure than related
      current IETF-Standard protocols.


1.4 Scalability

   The primary reasons to deploy Zeroconf protocols are simplicity and
   ease-of-use.  Scalability is important but it is a secondary goal.
   Thus, scalability should not detract from the primary goals of
   simplicity and ease-of-use.

1.5 Routable Protocol Requirement

   Zeroconf protocols are not inherently limited to a single IP subnet.
   If a protocol is intended to span multiple IP subnets it MUST NOT use
   broadcasts or link-local addressing.

   Requirement:

   o  Protocols intended to span multiple IP subnets MUST NOT use
      broadcasts or link-local addressing.



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1.6 Maintaining Consistent Network State

   Many networks undergo change during their lifetime.  For example,
   hosts may be added and removed, network segments may be re-arranged,
   and devices may change names or run different services.  In a
   configured network an administrator ensures that protocol parameters
   are updated to reflect these changes and is responsible for ensuring
   network consistency.

   In contrast, zeroconf protocols must adapt to changing network
   conditions.  Zeroconf protocols must be able to resolve conflicts and
   return the network to a consistent state after changes in network
   topology or other events.

   Requirement:

   o  Zeroconf protocols MUST restore the network to a consistent state
      in a timely fashion when changes in network topology or other
      events occur.

   This is a general requirement applicable to all zeroconf protocols.
   It should be kept in mind when considering the scenarios in Section 2
   and will be applied to derive requirements for each zeroconf protocol
   area considered in Section 3.

2. Scenarios

   The scenarios described in the next few sections highlight the
   general characteristics of the zeroconf protocol environment.  Each
   zeroconf protocol needs to deal with the following aspects of the
   zeroconf environment.

2.1 Addition and Removal of Devices

   Zeroconf protocols are expected to be useful in networks where hosts
   come and go.  Hosts using zeroconf protocols must not assume that
   network resources assigned to them (e.g.  address assignments, names,
   etc) will be appropriate for networks they subsequently join.  In
   addition, network resources allocated to a host should be reclaimed
   once it leaves the network.

   Requirements:

   o  Zeroconf protocols MUST support mechanisms to probe whether a
      network resource is currently in use.

   o  Hosts using zeroconf protocols MUST validate allocated network
      resources when moving to a new network or powering up.



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   o  Zeroconf protocols MUST support timely reclamation of any network
      resources they allocate.

   Implication:

   o  The information needed to allocate network resources must arrive
      in the network along with the host.


2.2 Network Coalescing and Partitioning

   Inevitably, two or more operational networks using zeroconf protocols
   will be connected together, creating a single merged network.  Prior
   to the merge, each zeroconf network has independently allocated
   resources (e.g.  addresses, names, etc).  After merging, two hosts in
   the merged network may end up using the same network resource, thus
   potentially creating conflicts.

   In general a network merge "event" cannot be detected.  For example,
   the installation of a layer-2 bridge between two zeroconf networks
   does not result in network interfaces going up and down on the hosts,
   which would be an indication that they should re-validate or
   reconfigure the network resources they are using.

   Implication:

   o  It is not sufficient to rely on hosts detecting conflicts during
      power on or movement from network to network.  Rather, detection
      and resolution of network conflicts is an ongoing part of zeroconf
      protocol operation.

   Requirement:

   o  Zeroconf protocols MUST resolve network resource conflicts in a
      timely manner and on an ongoing basis.

   Zeroconf protocols that detect and resolve network resource conflicts
   on an ongoing basis will benefit from increased robustness in the
   face of poor implementation, and varying network conditions.

   A zeroconf network may also be split into two or more smaller
   independent networks.  The requirement from Section 2.1 that network
   resources be reclaimed in a timely fashion also applies in this case.
   Since network merging increases the potential for network conflicts,
   it may be prudent to ensure that network resources associated with
   hosts are not immediately re-claimed for re-use.  Any network which
   periodically joins and partitions with another zeroconf network will
   benefit from this behaviour.  An example is an IP network in a car



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   joining with the home network whilst the car is parked in the garage
   and partitioning when it is driven away.

   Requirement:

   o  Zeroconf protocols SHOULD NOT immediately reuse network resources
      as soon as they become available.

   o  Network resources SHOULD be allocated in a way that minimises the
      probability that two hosts will be allocated the same resource.

   o  Network resources SHOULD be allocated in a way that increases the
      chances of a particular host being allocated the same resource
      should it leave and rejoin the network.


3. Requirements

   This section contains a subsection for each of the four protocol
   areas.  Within each subsection, terms and assumptions are followed by
   specific zeroconf protocol requirements in that area.  Each
   subsection ends with IPv6 considerations.

3.1 IP Interface Configuration

   In this document, IP interface configuration always includes the
   configuration of an IP address and netmask; it may include some
   routing information (such as default route).  IP interface
   configuration is needed before almost any IP communication can take
   place.

   Terms:

   IP subnet: A single logical IP network that may span multiple link
      layer networks.  All IP hosts on the IP subnet communicate without
      any layer 3 forwarding device (e.g.  router).

   internetwork: Multiple IP subnets connected by routers.

   network: a context sensitive term that may apply to one or more of
      the terms: a link layer network, an IP subnet, or an internetwork.

   bridge: a networking device that connects two link layer networks by
      using only link-layer protocols (e.g.  Ethernet).

   IP interface configuration must take place before an IP packet can be
   sent from one host to another.  This section requires that sufficient
   information be provided by a zeroconf interface configuration



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   protocol to allow IP packets to be sent to a unicast destination IP
   address on the same subnet as the sender, and on a different subnet
   to the sender.

   Requirements: A zeroconf IP interface configuration protocol

   o  MUST configure an appropriate netmask.

   o  MUST allocate unique IP addresses within an IP subnet.

   o  MUST allow configuration of zero or more gateways (for the
      internetwork scenario).

   o  MUST have unique IP subnets within an internetwork (This is only
      for the case when there is one or more router attached in the
      network where autoconfigured hosts.  How routers obtain their
      configuration is beyond of the scope of this document.)

   The following requirements are derived from applying Section 2.1 and
   Section 2.2 to IP interface configuration.

   Requirements: A zeroconf IP interface configuration protocol

   o  MUST be capable of discovering whether an IP address is currently
      in use.

   o  Hosts using a zeroconf interface configuration protocol MUST
      validate allocated IP addresses when moving to a new network or
      powering up.

   o  MUST support timely reclamation of allocated IP addresses.

   o  MUST resolve IP address conflicts in a timely manner and on an
      ongoing basis.

   o  SHOULD NOT immediately reuse IP addresses as soon as they become
      available.

   o  IP addresses SHOULD be allocated in a way that minimises the
      probability that two hosts will be allocated the same address.

   o  IP addresses SHOULD be allocated in a way that increases the
      chances of a particular host being allocated the same address
      should it leave and rejoin the network.







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3.1.1 IPv6 Considerations

   IPv6 provides a mechanism that allows a host to generate a link-local
   IP address Autoconfiguration[RFC2462].  Thus a zeroconf IP interface
   configuration solution for generating link-local addresses already
   exists for hosts using IPv6.

3.2 Translation between Host name and IP Address

   A zeroconf name resolution protocol allows users to refer to their
   devices by name rather than IP address.  Host names are more user
   friendly than IP addresses and host names have a greater chance of
   remaining unchanged over time.  A zeroconf device connected to
   different networks is quite likely to use different IP addresses,
   however a host name may stay the same.  For applications like web
   browsers which store bookmarks and histories, use of names rather
   than IP addresses is beneficial.

   In a zeroconf network, the information required to construct a host
   name must enter the network with the device.  It is expected that
   users will configure names into devices, or that devices will come
   with a pre-configured name.  Devices may also construct a name from a
   MAC address or serial number.  How this is to be achieved is outside
   the scope of this document.

   Terms:

   host name: A textual name that allows a user to refer to a host by
      name rather than IP address.

   Requirements:

   o  A zeroconf name resolution protocol MUST allow host names to be
      mapped into IP addresses.

   o  A zeroconf name resolution protocol SHOULD allow IP addresses to
      be mapped back to names.

   o  Because hosts can connect and disconnect from a network at any
      time, the failure to resolve a name at some time MUST NOT be taken
      as an indication that the name will remain invalid for any length
      of time.

   o  A zeroconf name resolution protocol SHOULD support resolution of
      names on multiple IP subnets connected by a router.

   The following requirements are derived from applying Section 2.1 and
   Section 2.2 to zeroconf name resolution.



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   Requirements:

   o  A zeroconf name resolution protocol MUST support mechanisms to
      probe whether a host name is currently in use.

   o  A host moving to a new network or powering up MUST ensure that all
      names it will respond to do not conflict with names already in
      use.

   o  Zeroconf name resolution protocols MUST allow timely re-use of
      hostnames.

   o  Zeroconf name resolution protocols MUST resolve host name
      conflicts in a timely manner and on an ongoing basis.

   o  Conflict detection procedures (such as probing for the existence
      of a desired host name) MUST NOT prevent valid hostnames from
      being resolved.

   o  Zeroconf name resolution protocols SHOULD NOT immediately reuse
      host names as soon as they become available.

   o  Host names SHOULD be chosen in a way that minimises the
      probability that two hosts will use the same name.  Note that this
      is out of scope of the name resolution protocol itself.


3.2.1 IPv6 Considerations

   Protocols to perform translation between host name and IP address
   have no known zeroconf-related differences for IPv4 and IPv6.

3.2.2 Relationship to the DNS

   Zeroconf name resolution protocols cannot be directly equated with
   the DNS even though they may have a number of similarities.  For
   example, the DNS protocols as deployed today rely on a server
   infrastructure that may not be present in a zeroconf environment.
   Host names used in zeroconf networks are inherently locally scoped
   whereas DNS names are global and unique by design.

   At the time of writing, consensus on how zeroconf name resolution
   protocols should interact with the DNS has not been reached.  The
   next sections will attempt to capture the flavour of the different
   approaches that have proposed.






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3.2.2.1 Close coupling

   In this approach an application may look up a DNS name (e.g.
   "www.someco.com") using an existing API and receive and answer from a
   zeroconf name resolution protocol.  The zeroconf name resolution
   protocol makes use of existing on-the-wire DNS formats, resource
   record definitions, and namespace.  Some names may have a DNS suffix
   that identifies them as being local in scope.

   Issues yet to be resolved with this approach relate to security and
   consistency.  If the zeroconf name resolution involves multicasting
   the request on a local network then the risk of spoofed responses to
   global DNS names like "www.someco.com" is increased.  If the
   namespace is the same, then doing a zeroconf name lookup should
   return results consistent with DNS lookup for the same name.  What is
   meant by this consistency is not agreed.  Should the zeroconf lookup
   only be used when the DNS lookup has failed?  Should that lookup
   reflect what would have been returned by the DNS?  How should the
   probing for uniqueness of a zeroconf name relate to updating of a DNS
   record?

3.2.2.2 Completely orthogonal

   Another approach is to ensure that the zeroconf namespace and the DNS
   namespace are completely orthogonal.  There is therefore no
   possibility of any application using the DNS via existing APIs
   behaving differently after a zeroconf name resolution protocol is
   deployed.  Applications would need to explicitly use a zeroconf name
   lookup API in order to resolve names using a zeroconf lookup
   protocol.  Conversely, existing applications will derive no benefit
   from zeroconf protocols unless they are re-written.  Deployment of a
   zeroconf name resolution protocol would necessitate application
   upgrade.

3.2.2.3 API compatible

   In this approach the zeroconf namespace is distinct from the DNS
   namespace however zeroconf names may be resolved using an existing
   API.  A number of operating systems use generalised name service
   interfaces that transparently allow a variety of name lookup
   protocols to be used when resolving hostnames for applications.  A
   zeroconf name resolution protocol could be incorporated into such a
   system in a straightforward manner as just one more namespace and
   lookup protocol.

   Again, there is increased risk of spoofed responses if the multicast
   zeroconf name resolution protocol is used to resolve
   "www.someco.com".  One possible way of minimising the security risk



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   is to ensure that locally scoped names are distinguishable from DNS
   names, perhaps via a known reserved DNS suffix or by virtue of not
   containing a dot.  A multicast zeroconf resolution protocol could
   then avoid making requests for names which look like global DNS
   names.  Alternatively, we could require that zeroconf name lookups
   only be performed when the equivalent DNS lookup has failed.

3.3 IP Multicast Address Allocation Scenarios

   IP Multicast is used to conserve bandwidth for multi-receiver bulk-
   delivery applications, such as audio, video, or news.

   IP Multicast is also used to perform a logical addressing function.
   For example, when a host needs to communicate with local routers, it
   can send packets to the all-routers multicast address without having
   to know in advance the IP address(es) of the router(s).

   IPv4 multicast addresses range from 224.0.0.0 to 239.255.255.255.

   [RFC2365] defined multicast scopes are:

        node-local           (unspecified for IPv4)
        link-local           (224.0.0.0/24)
        local                (239.255.0.0/16)
        site-local           (unspecified for IPv4)
        organizational-local (239.192.0.0/14)
        global               (224.0.1.0-238.255.255.255)

   A relative assignment is an integer offset from the highest address
   in the scope and represents a 32-bit address.  For example, within
   the local scope, 239.255.255.0/24 is reserved for relative
   allocations.

   Source-Specific Multicast addresses are 232.0.0.0 to 232.255.255.255
   [I-D.ietf-ssm-arch].

   Unicast-prefix-based IPv6 multicast addresses are defined, as well as
   source-specific multicast addresses for IPv6[RFC3306].

   Assumptions:

   o  The node-local and SSM addresses require no protocol or
      interaction between multiple hosts, thus are not mentioned further
      in this document.

   o  Global and organizational scoped addresses are meant for networks
      of a greater scale than zeroconf protocols, thus are not mentioned
      further in this document.



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   o  Only local, link-local and site-local scopes are considered
      further in this document.

   o  If it is desirable to restrict multicast packets from entering and
      leaving a multicast scope boundary, it is assumed that the router
      at the boundary is a "boundary router" as described in [RFC2365].

   Scenarios are scope enumeration, address allocation, and multiple
   sources.

3.3.1 Scope Enumeration

   Applications that leave the choice of scope up to the user require
   the ability to enumerate what scopes the host is operating within.
   In addition, services that are assigned relative addresses require
   the ability to enumerate what scopes the host is within; only then
   will a host be able to apply the relative address to a scope.

   Requirements: Application support software used to autoconfigure
   multicast addresses

   o  MUST list which of the scopes (local, link-local, or site-local)
      are available for hosts.

   o  MUST list per-scope address ranges that may be allocated.


3.3.2 Address Allocation

   IP multicast address allocation (local, link-local and site-local
   scopes only) requires an application to be able to request the use of
   a suitable multicast address.  Coordination among applications must
   occur to avoid conflicting allocations of the same address.  This
   coordination must span the entire scope respective to the address.
   When an allocated address is no longer required, that address MUST
   become available for use again.

   Requirements: A zeroconf multicast address allocation protocol

   o  MUST select a multicast address.

   o  MUST prevent conflicting allocations of the same address.

   o  MUST allow the multicast address to become available after the
      address is no longer in use.






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3.3.3 Multiple Sources

   An intercom system inside a home is an example of a multiple source
   IP multicast application.  In this type of application, several
   sources may be sending packets destined to the same IP multicast
   address.

   This multiple source example illustrates the problem that a
   particular address may continue to be valid, even after the host that
   initially allocated the address is no longer present; the zeroconf
   multicast address allocation must correctly support this type of
   operation.  In other words, if a host allocates a multicast address,
   then leaves the multicast group, some other host must defend the
   address.

   Requirements:

   o  A host other than the allocating host MUST be able to defend or
      otherwise maintain the allocation of a multicast address.


3.3.4 IPv6 Considerations

   To date, no range has been reserved for dynamic allocation of Link-
   scoped addresses in IPv4.  Hence, unless such a range is reserved,
   dynamic allocation of link-scoped addresses applies only to IPv6.

3.4 Service Discovery Scenarios

   Service discovery protocols allow users or software to discover and
   select among available services.  This removes the requirement that
   the user or client software know a server's location in advance in
   order for the client to communicate with the server.

   Terms:

   service: a particular logical function that may be invoked via some
      network protocol, such as printing, storing a file on a remote
      disk, or even perhaps requesting delivery of a pizza.

   service characteristics: Characteristics provide a finer granularity
      of description to differentiate services beyond just the service
      type.  For example if the service type is printer, the
      characteristics may be color, pages printed per second, location,
      etc.

   service discovery protocol: A service discovery protocol enables
      clients to discover servers (or peers to find other peers) of a



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      particular service.  A service discovery protocol is an
      application layer protocol that relies on network and transport
      protocol layers.

   service protocol: A service protocol is used between the client and
      the server after service discovery is complete.

   The scenarios are the discovery of a simple printer service.

3.4.1 Printer Service

   Network-enabled printers allow various network clients to submit
   print jobs.  A service discovery protocol MUST allow a printer
   service to be discovered by devices needing to print.  This requires
   a service type as well as a service identifier to distinguish
   instances of a single service type.  Service discovery MUST be
   independent from any particular printing protocol such as lpd, raw-
   tcp, ipp.

   Printers vary in their characteristics such as location, status, dots
   per inch, etc.  Discovering a service based on these characteristics
   SHOULD be part of the service discovery protocol.

   Service discovery MUST complete in a timely (10s of seconds) manner.

   Requirements:

   o  MUST allow a service to be discovered.

   o  MUST discover via service identifier and/or service type.

   o  MUST discover services without use of a service-specific protocol.

   o  SHOULD discover via service characteristics.

   o  MUST complete in a timely (10s of seconds) manner.


3.4.2 IPv6 Considerations

   Service discovery protocols have no zeroconf related differences for
   IPv4 and IPv6.

4. Security Considerations

   Zeroconf protocols are intended to operate in a local scope, in
   networks containing one or more IP subnets, and potentially in
   parallel with standard configured network protocols.



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   Application protocols running on networks employing zeroconf
   protocols will be subject to the same sets of security issues
   identified for standard configured networks.  Examples are: denial of
   service due to the unauthenticated nature of IPv4 ARP and lack of
   confidentiality unless IPSec-ESP, TLS, or similar is used.  However,
   networks employing zeroconf protocols do have different security
   characteristics and the subsequent sections attempt to draw out some
   of the implications.

   Security schemes usually rely on some sort of configuration.
   Security mechanisms for zeroconf network protocols should be designed
   in keeping with the spirit of zeroconf, thus making it easy for the
   user to exchange keys, set policy, etc.  It is preferable that a
   single security mechanism be employed that will allow simple
   configuration of all the various security parameters that may be
   required.

   Generally speaking, security mechanisms in IETF protocols are
   mandatory to implement.  A particular implementation might permit a
   network administrator to turn off a particular security mechanism
   operationally.  However, implementations should be "secure out of the
   box" and have a safe default configuration.

   Zeroconf protocols MUST NOT be any less secure than related current
   IETF-Standard protocols.  This consideration overrides the goal of
   allowing systems to obtain configuration automatically.  Security
   threats to be considered include both active attacks (e.g.  denial of
   service) and passive attacks (e.g.  eavesdropping).  Protocols that
   require confidentiality and/or integrity should include integrated
   confidentiality and/or integrity mechanisms or should specify the use
   of existing standards-track security mechanisms (e.g.  TLS [RFC2246],
   ESP [RFC1827], AH [RFC2402]) appropriate to the threat.

4.1 The Basic in/out Security Policy

   The claim/collide approach to resource allocation is attractive in
   the zeroconf environment.  To operate securely, hosts allocating
   resources need to ensure that messages indicating that a network
   resource is in use are authenticated.  Hosts soliciting for a name or
   service must also be able to authenticate the responses they receive.
   A message is considered "authenticated" if it can be proved to have
   been sent by a member of a group of devices running zeroconf
   protocols.  Note that in general, devices running zeroconf protocols
   must trust the other devices in the group because any device may
   claim to be using an address or name, or advertising a service.
   Zeroconf security mechanisms must at a minimum be able to distinguish
   between messages originating from a device "inside" the group or a
   device "outside" the group.



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   Requirement:

   o  Security schemes for zeroconf protocols MUST be able to implement
      a basic "inside/outside" security policy.

   Access control mechanisms within a zeroconf network are beyond the
   scope of this document.

4.2 Security Scenarios

   In the next few sections, several scenarios are examined to highlight
   the security implications of employing zeroconf protocols in various
   environments.

4.2.1 Use on an isolated, secure network link

   In this scenario all the devices in the network are connected to a
   secure link (e.g.  a control network in a car, or a private network
   between two devices used for configuration).  The link might be a
   separate physical wire or shared media with layer-2 security enabled
   (e.g.  wireless or power-line networks).  Any host that has access to
   the link is trusted and any packet received from the secure link is
   considered to be authenticated.  In this case, the inside/outside
   policy is implemented by controlling access to the link itself.

4.2.2 Use on a shared (insecure) link

   In this scenario, a group of devices use zeroconf protocols on
   link(s) they share with other devices who are NOT part of the group.
   Various pieces of network configuration MUST be consistent across all
   hosts sharing the link (e.g.  addresses, netmask, routing
   information) in order for communication to occur at all.
   Consequently, hosts inside the group are potentially at risk from
   hosts outside their group when they try to configure such information
   (e.g.  via ARP insecurity).  Host names and service identifiers MUST
   be unique within a group, but with authentication may not need to be
   unique across the link.  Assuming that requests and responses can be
   associated with a group and authenticated, solicitations and
   responses for host names and services that are not located inside the
   group can be ignored.

4.2.3 Use in conjunction with configured protocols

   Zeroconf protocols are likely to be used in conjunction with
   configured network protocols.  In general, zeroconf protocols must
   not allocate resources from the same address or name spaces used by
   configured network protocols.  Locally allocated zeroconf network
   resources must not mask global assigned resources.



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5. IANA Considerations

   No known IANA considerations arise from this document.

6. Acknowledgments

   Thanks to Peter Ford and Stuart Cheshire for hosting the NITS
   (Networking In The Small) BOF that was the catalyst to forming the
   Zeroconf Working Group.

   Thanks to Erik Guttman and Stuart Cheshire for forming and chairing
   the Zeroconf Working Group, which is responsible for this document.

   Thanks to Erik Guttman for providing key input to the service
   discovery and the security sections.

   Thanks to Dave Thaler for providing key input to the IP multicast
   address allocation sections.

   Thanks to Stuart Cheshire for providing key input to the introduction
   and IP interface configuration sections.

   Thanks to Myron Hattig for acting as editor for previous versions of
   this document.

   Additional recognition goes the following people for their
   influential contributions to this document and its predecessors:
   Brent Miller, Thomas Narten, Marcia Peters, Bill Woodcock, Bob Quinn,
   John Tavs, Matt Squire, Daniel Senie, Cuneyt Akinlar, Karl Auerbach,
   Kanchei Loa, Dongyan Wang, James Kempf, Yaron Goland, and Bernard
   Aboba, Ran Atkinson.

Normative References

Informative References

   [I-D.ietf-ssm-arch]  Cain, B. and H. Holbrook, "Source-Specific
                        Multicast for IP", draft-ietf-ssm-arch-00 (work
                        in progress), February 2002.

   [RFC0826]            Plummer, D., "Ethernet Address Resolution
                        Protocol: Or converting network protocol
                        addresses to 48.bit Ethernet address for
                        transmission on Ethernet hardware", STD 37, RFC
                        826, November 1982.

   [RFC1034]            Mockapetris, P., "Domain names - concepts and
                        facilities", STD 13, RFC 1034, November 1987.



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   [RFC1035]            Mockapetris, P., "Domain names - implementation
                        and specification", STD 13, RFC 1035, November
                        1987.

   [RFC1123]            Braden, R., "Requirements for Internet Hosts -
                        Application and Support", STD 3, RFC 1123,
                        October 1989.

   [RFC1827]            Atkinson, R., "IP Encapsulating Security Payload
                        (ESP)", RFC 1827, August 1995.

   [RFC2026]            Bradner, S., "The Internet Standards Process --
                        Revision 3", BCP 9, RFC 2026, October 1996.

   [RFC2119]            Bradner, S., "Key words for use in RFCs to
                        Indicate Requirement Levels", BCP 14, RFC 2119,
                        March 1997.

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

   [RFC2246]            Dierks, T., Allen, C., Treese, W., Karlton, P.,
                        Freier, A. and P. Kocher, "The TLS Protocol
                        Version 1.0", RFC 2246, January 1999.

   [RFC2251]            Wahl, M., Howes, T. and S. Kille, "Lightweight
                        Directory Access Protocol (v3)", RFC 2251,
                        December 1997.

   [RFC2365]            Meyer, D., "Administratively Scoped IP
                        Multicast", BCP 23, RFC 2365, July 1998.

   [RFC2401]            Kent, S. and R. Atkinson, "Security Architecture
                        for the Internet Protocol", RFC 2401, November
                        1998.

   [RFC2402]            Kent, S. and R. Atkinson, "IP Authentication
                        Header", RFC 2402, November 1998.

   [RFC2461]            Narten, T., Nordmark, E. and W. Simpson,
                        "Neighbor Discovery for IP Version 6 (IPv6)",
                        RFC 2461, December 1998.

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

   [RFC2535]            Eastlake, D., "Domain Name System Security



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                        Extensions", RFC 2535, March 1999.

   [RFC2541]            Eastlake, D., "DNS Security Operational
                        Considerations", RFC 2541, March 1999.

   [RFC2608]            Guttman, E., Perkins, C., Veizades, J. and M.
                        Day, "Service Location Protocol, Version 2", RFC
                        2608, June 1999.

   [RFC2730]            Hanna, S., Patel, B. and M. Shah, "Multicast
                        Address Dynamic Client Allocation Protocol
                        (MADCAP)", RFC 2730, December 1999.

   [RFC2931]            Eastlake, D., "DNS Request and Transaction
                        Signatures ( SIG(0)s)", RFC 2931, September
                        2000.

   [RFC3306]            Haberman, B. and D. Thaler, "Unicast-Prefix-
                        based IPv6 Multicast Addresses", RFC 3306,
                        August 2002.


Author's Address

   Aidan Williams
   Motorola Australian Research Centre
   Locked Bag 5028
   Botany, NSW  1455
   Australia

   Phone: +61 2 9666 0500
   EMail: Aidan.Williams@motorola.com
   URI:   http://www.motorola.com.au/marc/


















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Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
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   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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