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RFC 3927 - Dynamic Configuration of IPv4 Link-Local Addresses


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Network Working Group                                        S. Cheshire
Request for Comments: 3927                                Apple Computer
Category: Standards Track                                       B. Aboba
                                                   Microsoft Corporation
                                                              E. Guttman
                                                        Sun Microsystems
                                                                May 2005

           Dynamic Configuration of IPv4 Link-Local Addresses

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   To participate in wide-area IP networking, a host needs to be
   configured with IP addresses for its interfaces, either manually by
   the user or automatically from a source on the network such as a
   Dynamic Host Configuration Protocol (DHCP) server.  Unfortunately,
   such address configuration information may not always be available.
   It is therefore beneficial for a host to be able to depend on a
   useful subset of IP networking functions even when no address
   configuration is available.  This document describes how a host may
   automatically configure an interface with an IPv4 address within the
   169.254/16 prefix that is valid for communication with other devices
   connected to the same physical (or logical) link.

   IPv4 Link-Local addresses are not suitable for communication with
   devices not directly connected to the same physical (or logical)
   link, and are only used where stable, routable addresses are not
   available (such as on ad hoc or isolated networks).  This document
   does not recommend that IPv4 Link-Local addresses and routable
   addresses be configured simultaneously on the same interface.

Table of Contents

   1.  Introduction. . . . . . . . . . . . . . . . . . . . . . . . .  3
       1.1.  Requirements. . . . . . . . . . . . . . . . . . . . . .  3
       1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . .  3
       1.3.  Applicability . . . . . . . . . . . . . . . . . . . . .  5
       1.4.  Application Layer Protocol Considerations . . . . . . .  6
       1.5.  Autoconfiguration Issues. . . . . . . . . . . . . . . .  7
       1.6.  Alternate Use Prohibition . . . . . . . . . . . . . . .  7
       1.7.  Multiple Interfaces . . . . . . . . . . . . . . . . . .  8
       1.8.  Communication with Routable Addresses . . . . . . . . .  8
       1.9.  When to configure an IPv4 Link-Local Address. . . . . .  8
   2.  Address Selection, Defense and Delivery . . . . . . . . . . .  9
       2.1.  Link-Local Address Selection. . . . . . . . . . . . . . 10
       2.2.  Claiming a Link-Local Address . . . . . . . . . . . . . 11
       2.3.  Shorter Timeouts. . . . . . . . . . . . . . . . . . . . 13
       2.4.  Announcing an Address . . . . . . . . . . . . . . . . . 13
       2.5.  Conflict Detection and Defense. . . . . . . . . . . . . 13
       2.6.  Address Usage and Forwarding Rules. . . . . . . . . . . 14
       2.7.  Link-Local Packets Are Not Forwarded. . . . . . . . . . 16
       2.8.  Link-Local Packets are Local. . . . . . . . . . . . . . 16
       2.9.  Higher-Layer Protocol Considerations. . . . . . . . . . 17
       2.10. Privacy Concerns. . . . . . . . . . . . . . . . . . . . 17
       2.11. Interaction between DHCPv4 and IPv4 Link-Local
             State Machines. . . . . . . . . . . . . . . . . . . . . 17
   3.  Considerations for Multiple Interfaces. . . . . . . . . . . . 18
       3.1.  Scoped Addresses. . . . . . . . . . . . . . . . . . . . 18
       3.2.  Address Ambiguity . . . . . . . . . . . . . . . . . . . 19
       3.3.  Interaction with Hosts with Routable Addresses. . . . . 20
       3.4.  Unintentional Autoimmune Response . . . . . . . . . . . 21
   4.  Healing of Network Partitions . . . . . . . . . . . . . . . . 22
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . 23
   6.  Application Programming Considerations. . . . . . . . . . . . 24
       6.1.  Address Changes, Failure and Recovery . . . . . . . . . 24
       6.2.  Limited Forwarding of Locators. . . . . . . . . . . . . 24
       6.3.  Address Ambiguity . . . . . . . . . . . . . . . . . . . 25
   7.  Router Considerations . . . . . . . . . . . . . . . . . . . . 25
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
   9.  Constants . . . . . . . . . . . . . . . . . . . . . . . . . . 26
   10. References. . . . . . . . . . . . . . . . . . . . . . . . . . 26
       10.1. Normative References. . . . . . . . . . . . . . . . . . 26
       10.2. Informative References. . . . . . . . . . . . . . . . . 26
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 27
   Appendix A - Prior Implementations. . . . . . . . . . . . . . . . 28

1.  Introduction

   As the Internet Protocol continues to grow in popularity, it becomes
   increasingly valuable to be able to use familiar IP tools such as FTP
   not only for global communication, but for local communication as
   well.  For example, two people with laptop computers supporting IEEE
   802.11 Wireless LANs [802.11] may meet and wish to exchange files.
   It is desirable for these people to be able to use IP application
   software without the inconvenience of having to manually configure
   static IP addresses or set up a DHCP server [RFC2131].

   This document describes a method by which a host may automatically
   configure an interface with an IPv4 address in the 169.254/16 prefix
   that is valid for Link-Local communication on that interface.  This
   is especially valuable in environments where no other configuration
   mechanism is available.  The IPv4 prefix 169.254/16 is registered
   with the IANA for this purpose.  Allocation of IPv6 Link-Local
   addresses is described in "IPv6 Stateless Address Autoconfiguration"
   [RFC2462].

   Link-Local communication using IPv4 Link-Local addresses is only
   suitable for communication with other devices connected to the same
   physical (or logical) link.  Link-Local communication using IPv4
   Link-Local addresses is not suitable for communication with devices
   not directly connected to the same physical (or logical) link.

   Microsoft Windows 98 (and later) and Mac OS 8.5 (and later) already
   support this capability.  This document standardizes usage,
   prescribing rules for how IPv4 Link-Local addresses are to be treated
   by hosts and routers.  In particular, it describes how routers are to
   behave when receiving packets with IPv4 Link-Local addresses in the
   source or destination address.  With respect to hosts, it discusses
   claiming and defending addresses, maintaining Link-Local and routable
   IPv4 addresses on the same interface, and multi-homing issues.

1.1.  Requirements

   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 "Key words for use in
   RFCs" [RFC2119].

1.2.  Terminology

   This document describes Link-Local addressing, for IPv4 communication
   between two hosts on a single link.  A set of hosts is considered to
   be "on the same link", if:

   -  when any host A from that set sends a packet to any other host B
      in that set, using unicast, multicast, or broadcast, the entire
      link-layer packet payload arrives unmodified, and

   -  a broadcast sent over that link by any host from that set of hosts
      can be received by every other host in that set

   The link-layer *header* may be modified, such as in Token Ring Source
   Routing [802.5], but not the link-layer *payload*.  In particular, if
   any device forwarding a packet modifies any part of the IP header or
   IP payload then the packet is no longer considered to be on the same
   link.  This means that the packet may pass through devices such as
   repeaters, bridges, hubs or switches and still be considered to be on
   the same link for the purpose of this document, but not through a
   device such as an IP router that decrements the TTL or otherwise
   modifies the IP header.

   This document uses the term "routable address" to refer to all valid
   unicast IPv4 addresses outside the 169.254/16 prefix that may be
   forwarded via routers.  This includes all global IP addresses and
   private addresses such as Net 10/8 [RFC1918], but not loopback
   addresses such as 127.0.0.1.

   Wherever this document uses the term "host" when describing use of
   IPv4 Link-Local addresses, the text applies equally to routers when
   they are the source of or intended destination of packets containing
   IPv4 Link-Local source or destination addresses.

   Wherever this document uses the term "sender IP address" or "target
   IP address" in the context of an ARP packet, it is referring to the
   fields of the ARP packet identified in the ARP specification [RFC826]
   as "ar$spa" (Sender Protocol Address) and "ar$tpa" (Target Protocol
   Address) respectively.  For the usage of ARP described in this
   document, each of these fields always contains an IP address.

   In this document, the term "ARP Probe" is used to refer to an ARP
   Request packet, broadcast on the local link, with an all-zero 'sender
   IP address'.  The 'sender hardware address' MUST contain the hardware
   address of the interface sending the packet.  The 'target hardware
   address' field is ignored and SHOULD be set to all zeroes.  The
   'target IP address' field MUST be set to the address being probed.

   In this document, the term "ARP Announcement" is used to refer to an
   ARP Request packet, broadcast on the local link, identical to the ARP
   Probe described above, except that both the sender and target IP
   address fields contain the IP address being announced.

   Constants are introduced in all capital letters.  Their values are
   given in Section 9.

1.3.  Applicability

   This specification applies to all IEEE 802 Local Area Networks (LANs)
   [802], including Ethernet [802.3], Token-Ring [802.5] and IEEE 802.11
   wireless LANs [802.11], as well as to other link-layer technologies
   that operate at data rates of at least 1 Mbps, have a round-trip
   latency of at most one second, and support ARP [RFC826].  Wherever
   this document uses the term "IEEE 802", the text applies equally to
   any of these network technologies.

   Link-layer technologies that support ARP but operate at rates below 1
   Mbps or latencies above one second may need to specify different
   values for the following parameters:

   (a) the number of, and interval between, ARP probes, see PROBE_NUM,
       PROBE_MIN, PROBE_MAX defined in Section 2.2.1

   (b) the number of, and interval between, ARP announcements, see
       ANNOUNCE_NUM and ANNOUNCE_INTERVAL defined in Section 2.4

   (c) the maximum rate at which address claiming may be attempted, see
       RATE_LIMIT_INTERVAL and MAX_CONFLICTS defined in Section 2.2.1

   (d) the time interval between conflicting ARPs below which a host
       MUST reconfigure instead of attempting to defend its address, see
       DEFEND_INTERVAL defined in Section 2.5

   Link-layer technologies that do not support ARP may be able to use
   other techniques for determining whether a particular IP address is
   currently in use.  However, the application of claim-and-defend
   mechanisms to such networks is outside the scope of this document.

   This specification is intended for use with small ad hoc networks --
   a single link containing only a few hosts.  Although 65024 IPv4
   Link-Local addresses are available in principle, attempting to use
   all those addresses on a single link would result in a high
   probability of address conflicts, requiring a host to take an
   inordinate amount of time to find an available address.

   Network operators with more than 1300 hosts on a single link may want
   to consider dividing that single link into two or more subnets.  A
   host connecting to a link that already has 1300 hosts, selecting an
   IPv4 Link-Local address at random, has a 98% chance of selecting an
   unused IPv4 Link-Local address on the first try.  A host has a 99.96%

   chance of selecting an unused IPv4 Link-Local address within two
   tries.  The probability that it will have to try more than ten times
   is about 1 in 10^17.

1.4.  Application Layer Protocol Considerations

   IPv4 Link-Local addresses and their dynamic configuration have
   profound implications upon applications which use them.  This is
   discussed in Section 6.  Many applications fundamentally assume that
   addresses of communicating peers are routable, relatively unchanging
   and unique.  These assumptions no longer hold with IPv4 Link-Local
   addresses, or a mixture of Link-Local and routable IPv4 addresses.

   Therefore while many applications will work properly with IPv4 Link-
   Local addresses, or a mixture of Link-Local and routable IPv4
   addresses, others may do so only after modification, or will exhibit
   reduced or partial functionality.

   In some cases it may be infeasible for the application to be modified
   to operate under such conditions.

   IPv4 Link-Local addresses should therefore only be used where stable,
   routable addresses are not available (such as on ad hoc or isolated
   networks) or in controlled situations where these limitations and
   their impact on applications are understood and accepted.  This
   document does not recommend that IPv4 Link-Local addresses and
   routable addresses be configured simultaneously on the same
   interface.

   Use of IPv4 Link-Local addresses in off-link communication is likely
   to cause application failures.  This can occur within any application
   that includes embedded addresses, if an IPv4 Link-Local address is
   embedded when communicating with a host that is not on the link.
   Examples of applications that embed addresses include IPsec, Kerberos
   4/5, FTP, RSVP, SMTP, SIP, X-Windows/Xterm/Telnet, Real Audio, H.323,
   and SNMP [RFC3027].

   To preclude use of IPv4 Link-Local addresses in off-link
   communication, the following cautionary measures are advised:

   a. IPv4 Link-Local addresses MUST NOT be configured in the DNS.
      Mapping from IPv4 addresses to host names is conventionally done
      by issuing DNS queries for names of the form,
      "x.x.x.x.in-addr.arpa."  When used for link-local addresses, which
      have significance only on the local link, it is inappropriate to
      send such DNS queries beyond the local link.  DNS clients MUST NOT
      send DNS queries for any name that falls within the
      "254.169.in-addr.arpa." domain.

      DNS recursive name servers receiving queries from non-compliant
      clients for names within the "254.169.in-addr.arpa." domain MUST
      by default return RCODE 3, authoritatively asserting that no such
      name exists in the Domain Name System.

   b. Names that are globally resolvable to routable addresses should be
      used within applications whenever they are available.  Names that
      are resolvable only on the local link (such as through use of
      protocols such as Link Local Multicast Name Resolution [LLMNR])
      MUST NOT be used in off-link communication.  IPv4 addresses and
      names that can only be resolved on the local link SHOULD NOT be
      forwarded beyond the local link.  IPv4 Link-Local addresses SHOULD
      only be sent when a Link-Local address is used as the source
      and/or destination address.  This strong advice should hinder
      limited scope addresses and names from leaving the context in
      which they apply.

   c. If names resolvable to globally routable addresses are not
      available, but the globally routable addresses are, they should be
      used instead of IPv4 Link-Local addresses.

1.5.  Autoconfiguration Issues

   Implementations of IPv4 Link-Local address autoconfiguration MUST
   expect address conflicts, and MUST be prepared to handle them
   gracefully by automatically selecting a new address whenever a
   conflict is detected, as described in Section 2.  This requirement to
   detect and handle address conflicts applies during the entire period
   that a host is using a 169.254/16 IPv4 Link-Local address, not just
   during initial interface configuration.  For example, address
   conflicts can occur well after a host has completed booting if two
   previously separate networks are joined, as described in Section 4.

1.6.  Alternate Use Prohibition

   Note that addresses in the 169.254/16 prefix SHOULD NOT be configured
   manually or by a DHCP server.  Manual or DHCP configuration may cause
   a host to use an address in the 169.254/16 prefix without following
   the special rules regarding duplicate detection and automatic
   configuration that pertain to addresses in this prefix.  While the
   DHCP specification [RFC2131] indicates that a DHCP client SHOULD
   probe a newly received address with ARP, this is not mandatory.
   Similarly, while the DHCP specification recommends that a DHCP server
   SHOULD probe an address using an ICMP Echo Request before allocating
   it, this is also not mandatory, and even if the server does this,
   IPv4 Link-Local addresses are not routable, so a DHCP server not
   directly connected to a link cannot detect whether a host on that
   link is already using the desired IPv4 Link-Local address.

   Administrators wishing to configure their own local addresses (using
   manual configuration, a DHCP server, or any other mechanism not
   described in this document) should use one of the existing private
   address prefixes [RFC1918], not the 169.254/16 prefix.

1.7.  Multiple Interfaces

   Additional considerations apply to hosts that support more than one
   active interface where one or more of these interfaces support IPv4
   Link-Local address configuration.  These considerations are discussed
   in Section 3.

1.8.  Communication with Routable Addresses

   There will be cases when devices with a configured Link-Local address
   will need to communicate with a device with a routable address
   configured on the same physical link, and vice versa.  The rules in
   Section 2.6 allow this communication.

   This allows, for example, a laptop computer with only a routable
   address to communicate with web servers world-wide using its
   globally-routable address while at the same time printing those web
   pages on a local printer that has only an IPv4 Link-Local address.

1.9.  When to configure an IPv4 Link-Local address

   Having addresses of multiple different scopes assigned to an
   interface, with no adequate way to determine in what circumstances
   each address should be used, leads to complexity for applications and
   confusion for users.  A host with an address on a link can
   communicate with all other devices on that link, whether those
   devices use Link-Local addresses, or routable addresses.  For these
   reasons, a host SHOULD NOT have both an operable routable address and
   an IPv4 Link-Local address configured on the same interface.  The
   term "operable address" is used to mean an address which works
   effectively for communication in the current network context (see
   below).  When an operable routable address is available on an
   interface, the host SHOULD NOT also assign an IPv4 Link-Local address
   on that interface.  However, during the transition (in either
   direction) between using routable and IPv4 Link-Local addresses both
   MAY be in use at once subject to these rules:

      1. The assignment of an IPv4 Link-Local address on an interface is
         based solely on the state of the interface, and is independent
         of any other protocols such as DHCP.  A host MUST NOT alter its
         behavior and use of other protocols such as DHCP because the
         host has assigned an IPv4 Link-Local address to an interface.

      2. If a host finds that an interface that was previously
         configured with an IPv4 Link-Local address now has an operable
         routable address available, the host MUST use the routable
         address when initiating new communications, and MUST cease
         advertising the availability of the IPv4 Link-Local address
         through whatever mechanisms that address had been made known to
         others.  The host SHOULD continue to use the IPv4 Link-Local
         address for communications already underway, and MAY continue
         to accept new communications addressed to the IPv4 Link-Local
         address.  Ways in which an operable routable address might
         become available on an interface include:

               * Manual configuration
               * Address assignment through DHCP
               * Roaming of the host to a network on which a previously
                 assigned address becomes operable

      3. If a host finds that an interface no longer has an operable
         routable address available, the host MAY identify a usable IPv4
         Link-Local address (as described in section 2) and assign that
         address to the interface.  Ways in which an operable routable
         address might cease to be available on an interface include:

               * Removal of the address from the interface through
                 manual configuration
               * Expiration of the lease on the address assigned through
                 DHCP
               * Roaming of the host to a new network on which the
                 address is no longer operable.

   The determination by the system of whether an address is "operable"
   is not clear cut and many changes in the system context (e.g.,
   router changes) may affect the operability of an address.  In
   particular roaming of a host from one network to another is likely --
   but not certain -- to change the operability of a configured address
   but detecting such a move is not always trivial.

   "Detection of Network Attachment (DNA) in IPv4" [DNAv4] provides
   further discussion of address assignment and operability
   determination.

2.  Address Selection, Defense and Delivery

   The following section explains the IPv4 Link-Local address selection
   algorithm, how IPv4 Link-Local addresses are defended, and how IPv4
   packets with IPv4 Link-Local addresses are delivered.

   Windows and Mac OS hosts that already implement Link-Local IPv4
   address auto-configuration are compatible with the rules presented in
   this section.  However, should any interoperability problem be
   discovered, this document, not any prior implementation, defines the
   standard.

2.1.  Link-Local Address Selection

   When a host wishes to configure an IPv4 Link-Local address, it
   selects an address using a pseudo-random number generator with a
   uniform distribution in the range from 169.254.1.0 to 169.254.254.255
   inclusive.

   The IPv4 prefix 169.254/16 is registered with the IANA for this
   purpose.  The first 256 and last 256 addresses in the 169.254/16
   prefix are reserved for future use and MUST NOT be selected by a host
   using this dynamic configuration mechanism.

   The pseudo-random number generation algorithm MUST be chosen so that
   different hosts do not generate the same sequence of numbers.  If the
   host has access to persistent information that is different for each
   host, such as its IEEE 802 MAC address, then the pseudo-random number
   generator SHOULD be seeded using a value derived from this
   information.  This means that even without using any other persistent
   storage, a host will usually select the same IPv4 Link-Local address
   each time it is booted, which can be convenient for debugging and
   other operational reasons.  Seeding the pseudo-random number
   generator using the real-time clock or any other information which is
   (or may be) identical in every host is NOT suitable for this purpose,
   because a group of hosts that are all powered on at the same time
   might then all generate the same sequence, resulting in a never-
   ending series of conflicts as the hosts move in lock-step through
   exactly the same pseudo-random sequence, conflicting on every address
   they probe.

   Hosts that are equipped with persistent storage MAY, for each
   interface, record the IPv4 address they have selected.  On booting,
   hosts with a previously recorded address SHOULD use that address as
   their first candidate when probing.  This increases the stability of
   addresses.  For example, if a group of hosts are powered off at
   night, then when they are powered on the next morning they will all
   resume using the same addresses, instead of picking different
   addresses and potentially having to resolve conflicts that arise.

2.2.  Claiming a Link-Local Address

   After it has selected an IPv4 Link-Local address, a host MUST test to
   see if the IPv4 Link-Local address is already in use before beginning
   to use it.  When a network interface transitions from an inactive to
   an active state, the host does not have knowledge of what IPv4 Link-
   Local addresses may currently be in use on that link, since the point
   of attachment may have changed or the network interface may have been
   inactive when a conflicting address was claimed.

   Were the host to immediately begin using an IPv4 Link-Local address
   which is already in use by another host, this would be disruptive to
   that other host.  Since it is possible that the host has changed its
   point of attachment, a routable address may be obtainable on the new
   network, and therefore it cannot be assumed that an IPv4 Link-Local
   address is to be preferred.

   Before using the IPv4 Link-Local address (e.g., using it as the
   source address in an IPv4 packet, or as the Sender IPv4 address in an
   ARP packet) a host MUST perform the probing test described below to
   achieve better confidence that using the IPv4 Link-Local address will
   not cause disruption.

   Examples of events that involve an interface becoming active include:

      Reboot/startup
      Wake from sleep (if network interface was inactive during sleep)
      Bringing up previously inactive network interface
      IEEE 802 hardware link-state change (appropriate for the
           media type and security mechanisms which apply) indicates
           that an interface has become active.
      Association with a wireless base station or ad hoc network.

   A host MUST NOT perform this check periodically as a matter of
   course.  This would be a waste of network bandwidth, and is
   unnecessary due to the ability of hosts to passively discover
   conflicts, as described in Section 2.5.

2.2.1.  Probe details

   On a link-layer such as IEEE 802 that supports ARP, conflict
   detection is done using ARP probes.  On link-layer technologies that
   do not support ARP other techniques may be available for determining
   whether a particular IPv4 address is currently in use.  However, the
   application of claim-and-defend mechanisms to such networks is
   outside the scope of this document.

   A host probes to see if an address is already in use by broadcasting
   an ARP Request for the desired address.  The client MUST fill in the
   'sender hardware address' field of the ARP Request with the hardware
   address of the interface through which it is sending the packet.  The
   'sender IP address' field MUST be set to all zeroes, to avoid
   polluting ARP caches in other hosts on the same link in the case
   where the address turns out to be already in use by another host.
   The 'target hardware address' field is ignored and SHOULD be set to
   all zeroes.  The 'target IP address' field MUST be set to the address
   being probed.  An ARP Request constructed this way with an all-zero
   'sender IP address' is referred to as an "ARP Probe".

   When ready to begin probing, the host should then wait for a random
   time interval selected uniformly in the range zero to PROBE_WAIT
   seconds, and should then send PROBE_NUM probe packets, each of these
   probe packets spaced randomly, PROBE_MIN to PROBE_MAX seconds apart.
   If during this period, from the beginning of the probing process
   until ANNOUNCE_WAIT seconds after the last probe packet is sent, the
   host receives any ARP packet (Request *or* Reply) on the interface
   where the probe is being performed where the packet's 'sender IP
   address' is the address being probed for, then the host MUST treat
   this address as being in use by some other host, and MUST select a
   new pseudo-random address and repeat the process.  In addition, if
   during this period the host receives any ARP Probe where the packet's
   'target IP address' is the address being probed for, and the packet's
   'sender hardware address' is not the hardware address of the
   interface the host is attempting to configure, then the host MUST
   similarly treat this as an address conflict and select a new address
   as above.  This can occur if two (or more) hosts attempt to configure
   the same IPv4 Link-Local address at the same time.

   A host should maintain a counter of the number of address conflicts
   it has experienced in the process of trying to acquire an address,
   and if the number of conflicts exceeds MAX_CONFLICTS then the host
   MUST limit the rate at which it probes for new addresses to no more
   than one new address per RATE_LIMIT_INTERVAL.  This is to prevent
   catastrophic ARP storms in pathological failure cases, such as a
   rogue host that answers all ARP probes, causing legitimate hosts to
   go into an infinite loop attempting to select a usable address.

   If, by ANNOUNCE_WAIT seconds after the transmission of the last ARP
   Probe no conflicting ARP Reply or ARP Probe has been received, then
   the host has successfully claimed the desired IPv4 Link-Local
   address.

2.3.  Shorter Timeouts

   Network technologies may emerge for which shorter delays are
   appropriate than those required by this document.  A subsequent IETF
   publication may be produced providing guidelines for different values
   for PROBE_WAIT, PROBE_NUM, PROBE_MIN and PROBE_MAX on those
   technologies.

2.4.  Announcing an Address

   Having probed to determine a unique address to use, the host MUST
   then announce its claimed address by broadcasting ANNOUNCE_NUM ARP
   announcements, spaced ANNOUNCE_INTERVAL seconds apart.  An ARP
   announcement is identical to the ARP Probe described above, except
   that now the sender and target IP addresses are both set to the
   host's newly selected IPv4 address.  The purpose of these ARP
   announcements is to make sure that other hosts on the link do not
   have stale ARP cache entries left over from some other host that may
   previously have been using the same address.

2.5.  Conflict Detection and Defense

   Address conflict detection is not limited to the address selection
   phase, when a host is sending ARP probes.  Address conflict detection
   is an ongoing process that is in effect for as long as a host is
   using an IPv4 Link-Local address.  At any time, if a host receives an
   ARP packet (request *or* reply) on an interface where the 'sender IP
   address' is the IP address the host has configured for that
   interface, but the 'sender hardware address' does not match the
   hardware address of that interface, then this is a conflicting ARP
   packet, indicating an address conflict.

   A host MUST respond to a conflicting ARP packet as described in
   either (a) or (b) below:

   (a) Upon receiving a conflicting ARP packet, a host MAY elect to
   immediately configure a new IPv4 Link-Local address as described
   above, or

   (b) If a host currently has active TCP connections or other reasons
   to prefer to keep the same IPv4 address, and it has not seen any
   other conflicting ARP packets within the last DEFEND_INTERVAL
   seconds, then it MAY elect to attempt to defend its address by
   recording the time that the conflicting ARP packet was received, and
   then broadcasting one single ARP announcement, giving its own IP and
   hardware addresses as the sender addresses of the ARP.  Having done
   this, the host can then continue to use the address normally without
   any further special action.  However, if this is not the first

   conflicting ARP packet the host has seen, and the time recorded for
   the previous conflicting ARP packet is recent, within DEFEND_INTERVAL
   seconds, then the host MUST immediately cease using this address and
   configure a new IPv4 Link-Local address as described above.  This is
   necessary to ensure that two hosts do not get stuck in an endless
   loop with both hosts trying to defend the same address.

   A host MUST respond to conflicting ARP packets as described in either
   (a) or (b) above.  A host MUST NOT ignore conflicting ARP packets.

   Forced address reconfiguration may be disruptive, causing TCP
   connections to be broken.  However, it is expected that such
   disruptions will be rare, and if inadvertent address duplication
   happens, then disruption of communication is inevitable, no matter
   how the addresses were assigned.  It is not possible for two
   different hosts using the same IP address on the same network to
   operate reliably.

   Before abandoning an address due to a conflict, hosts SHOULD actively
   attempt to reset any existing connections using that address.  This
   mitigates some security threats posed by address reconfiguration, as
   discussed in Section 5.

   Immediately configuring a new address as soon as the conflict is
   detected is the best way to restore useful communication as quickly
   as possible.  The mechanism described above of broadcasting a single
   ARP announcement to defend the address mitigates the problem
   somewhat, by helping to improve the chance that one of the two
   conflicting hosts may be able to retain its address.

   All ARP packets (*replies* as well as requests) that contain a Link-
   Local 'sender IP address' MUST be sent using link-layer broadcast
   instead of link-layer unicast.  This aids timely detection of
   duplicate addresses.  An example illustrating how this helps is given
   in Section 4.

2.6.  Address Usage and Forwarding Rules

   A host implementing this specification has additional rules to
   conform to, whether or not it has an interface configured with an
   IPv4 Link-Local address.

2.6.1.  Source Address Usage

   Since each interface on a host may have an IPv4 Link-Local address in
   addition to zero or more other addresses configured by other means
   (e.g., manually or via a DHCP server), a host may have to make a

   choice about what source address to use when it sends a packet or
   initiates a TCP connection.

   Where both an IPv4 Link-Local and a routable address are available on
   the same interface, the routable address should be preferred as the
   source address for new communications, but packets sent from or to
   the IPv4 Link-Local address are still delivered as expected.  The
   IPv4 Link-Local address may continue to be used as a source address
   in communications where switching to a preferred address would cause
   communications failure because of the requirements of an upper-layer
   protocol (e.g., an existing TCP connection).  For more details, see
   Section 1.7.

   A multi-homed host needs to select an outgoing interface whether or
   not the destination is an IPv4 Link-Local address.  Details of that
   process are beyond the scope of this specification.  After selecting
   an interface, the multi-homed host should send packets involving IPv4
   Link-Local addresses as specified in this document, as if the
   selected interface were the host's only interface.  See Section 3 for
   further discussion of multi-homed hosts.

2.6.2.  Forwarding Rules

   Whichever interface is used, if the destination address is in the
   169.254/16 prefix (excluding the address 169.254.255.255, which is
   the broadcast address for the Link-Local prefix), then the sender
   MUST ARP for the destination address and then send its packet
   directly to the destination on the same physical link.  This MUST be
   done whether the interface is configured with a Link-Local or a
   routable IPv4 address.

   In many network stacks, achieving this functionality may be as simple
   as adding a routing table entry indicating that 169.254/16 is
   directly reachable on the local link.  This approach will not work
   for routers or multi-homed hosts.  Refer to section 3 for more
   discussion of multi-homed hosts.

   The host MUST NOT send a packet with an IPv4 Link-Local destination
   address to any router for forwarding.

   If the destination address is a unicast address outside the
   169.254/16 prefix, then the host SHOULD use an appropriate routable
   IPv4 source address, if it can.  If for any reason the host chooses
   to send the packet with an IPv4 Link-Local source address (e.g., no
   routable address is available on the selected interface), then it
   MUST ARP for the destination address and then send its packet, with

   an IPv4 Link-Local source address and a routable destination IPv4
   address, directly to its destination on the same physical link.  The
   host MUST NOT send the packet to any router for forwarding.

   In the case of a device with a single interface and only an Link-
   Local IPv4 address, this requirement can be paraphrased as "ARP for
   everything".

   In many network stacks, achieving this "ARP for everything" behavior
   may be as simple as having no primary IP router configured, having
   the primary IP router address configured to 0.0.0.0, or having the
   primary IP router address set to be the same as the host's own Link-
   Local IPv4 address.  For suggested behavior in multi-homed hosts, see
   Section 3.

2.7.  Link-Local Packets Are Not Forwarded

   A sensible default for applications which are sending from an IPv4
   Link-Local address is to explicitly set the IPv4 TTL to 1.  This is
   not appropriate in all cases as some applications may require that
   the IPv4 TTL be set to other values.

   An IPv4 packet whose source and/or destination address is in the
   169.254/16 prefix MUST NOT be sent to any router for forwarding, and
   any network device receiving such a packet MUST NOT forward it,
   regardless of the TTL in the IPv4 header.  Similarly, a router or
   other host MUST NOT indiscriminately answer all ARP Requests for
   addresses in the 169.254/16 prefix.  A router may of course answer
   ARP Requests for one or more IPv4 Link-Local address(es) that it has
   legitimately claimed for its own use according to the claim-and-
   defend protocol described in this document.

   This restriction also applies to multicast packets.  IPv4 packets
   with a Link-Local source address MUST NOT be forwarded outside the
   local link even if they have a multicast destination address.

2.8.  Link-Local Packets are Local

   The non-forwarding rule means that hosts may assume that all
   169.254/16 destination addresses are "on-link" and directly
   reachable.  The 169.254/16 address prefix MUST NOT be subnetted.
   This specification utilizes ARP-based address conflict detection,
   which functions by broadcasting on the local subnet.  Since such
   broadcasts are not forwarded, were subnetting to be allowed then
   address conflicts could remain undetected.

   This does not mean that Link-Local devices are forbidden from any
   communication outside the local link.  IP hosts that implement both
   Link-Local and conventional routable IPv4 addresses may still use
   their routable addresses without restriction as they do today.

2.9.  Higher-Layer Protocol Considerations

   Similar considerations apply at layers above IP.

   For example, designers of Web pages (including automatically
   generated web pages) SHOULD NOT contain links with embedded IPv4
   Link-Local addresses if those pages are viewable from hosts outside
   the local link where the addresses are valid.

   As IPv4 Link-Local addresses may change at any time and have limited
   scope, IPv4 Link-Local addresses MUST NOT be stored in the DNS.

2.10.  Privacy Concerns

   Another reason to restrict leakage of IPv4 Link-Local addresses
   outside the local link is privacy concerns.  If IPv4 Link-Local
   addresses are derived from a hash of the MAC address, some argue that
   they could be indirectly associated with an individual, and thereby
   used to track that individual's activities.  Within the local link
   the hardware addresses in the packets are all directly observable, so
   as long as IPv4 Link-Local addresses don't leave the local link they
   provide no more information to an intruder than could be gained by
   direct observation of hardware addresses.

2.11.  Interaction between DHCPv4 client and IPv4 Link-Local State
       Machines

   As documented in Appendix A, early implementations of IPv4 Link-Local
   have modified the DHCP state machine.  Field experience shows that
   these modifications reduce the reliability of the DHCP service.

   A device that implements both IPv4 Link-Local and a DHCPv4 client
   should not alter the behavior of the DHCPv4 client to accommodate
   IPv4 Link-Local configuration.  In particular configuration of an
   IPv4 Link-Local address, whether or not a DHCP server is currently
   responding, is not sufficient reason to unconfigure a valid DHCP
   lease, to stop the DHCP client from attempting to acquire a new IP
   address, to change DHCP timeouts or to change the behavior of the
   DHCP state machine in any other way.

   Further discussion of this issue is provided in "Detection of Network
   Attachment (DNA) in IPv4" [DNAv4].

3.  Considerations for Multiple Interfaces

   The considerations outlined here also apply whenever a host has
   multiple IP addresses, whether or not it has multiple physical
   interfaces.  Other examples of multiple interfaces include different
   logical endpoints (tunnels, virtual private networks etc.) and
   multiple logical networks on the same physical medium.  This is often
   referred to as "multi-homing".

   Hosts which have more than one active interface and elect to
   implement dynamic configuration of IPv4 Link-Local addresses on one
   or more of those interfaces will face various problems.  This section
   lists these problems but does no more than indicate how one might
   solve them.  At the time of this writing, there is no silver bullet
   which solves these problems in all cases, in a general way.
   Implementors must think through these issues before implementing the
   protocol specified in this document on a system which may have more
   than one active interface as part of a TCP/IP stack capable of
   multi-homing.

3.1.  Scoped Addresses

   A host may be attached to more than one network at the same time.  It
   would be nice if there was a single address space used in every
   network, but this is not the case.  Addresses used in one network, be
   it a network behind a NAT or a link on which IPv4 Link-Local
   addresses are used, cannot be used in another network and have the
   same effect.

   It would also be nice if addresses were not exposed to applications,
   but they are.  Most software using TCP/IP which await messages
   receives from any interface at a particular port number, for a
   particular transport protocol.  Applications are generally only aware
   (and care) that they have received a message.  The application knows
   the address of the sender to which the application will reply.

   The first scoped address problem is source address selection.  A
   multi-homed host has more than one address.  Which address should be
   used as the source address when sending to a particular destination?
   This question is usually answered by referring to a routing table,
   which expresses on which interface (with which address) to send, and
   how to send (should one forward to a router, or send directly).  The
   choice is made complicated by scoped addresses because the address
   range in which the destination lies may be ambiguous.  The table may
   not be able to yield a good answer.  This problem is bound up with
   next-hop selection, which is discussed in Section 3.2.

   The second scoped address problem arises from scoped parameters
   leaking outside their scope.  This is discussed in Section 7.

   It is possible to overcome these problems.  One way is to expose
   scope information to applications such that they are always aware of
   what scope a peer is in.  This way, the correct interface could be
   selected, and a safe procedure could be followed with respect to
   forwarding addresses and other scoped parameters.  There are other
   possible approaches.  None of these methods have been standardized
   for IPv4 nor are they specified in this document.  A good API design
   could mitigate the problems, either by exposing address scopes to
   'scoped-address aware' applications or by cleverly encapsulating the
   scoping information and logic so that applications do the right thing
   without being aware of address scoping.

   An implementer could undertake to solve these problems, but cannot
   simply ignore them.  With sufficient experience, it is hoped that
   specifications will emerge explaining how to overcome scoped address
   multi-homing problems.

3.2.  Address Ambiguity

   This is a core problem with respect to IPv4 Link-Local destination
   addresses being reachable on more than one interface.  What should a
   host do when it needs to send to Link-Local destination L and L can
   be resolved using ARP on more than one link?

   Even if a Link-Local address can be resolved on only one link at a
   given moment, there is no guarantee that it will remain unambiguous
   in the future.  Additional hosts on other interfaces may claim the
   address L as well.

   One possibility is to support this only in the case where the
   application specifically expresses which interface to send from.

   There is no standard or obvious solution to this problem.  Existing
   application software written for the IPv4 protocol suite is largely
   incapable of dealing with address ambiguity.  This does not preclude
   an implementer from finding a solution, writing applications which
   are able to use it, and providing a host which can support dynamic
   configuration of IPv4 Link-Local addresses on more than one
   interface.  This solution will almost surely not be generally
   applicable to existing software and transparent to higher layers,
   however.

   Given that the IP stack must have the outbound interface associated
   with a packet that needs to be sent to a Link-Local destination
   address, interface selection must occur.  The outbound interface

   cannot be derived from the packet's header parameters such as source
   or destination address (e.g., by using the forwarding table lookup).
   Therefore, outbound interface association must be done explicitly
   through other means.  The specification does not stipulate those
   means.

3.3.  Interaction with Hosts with Routable Addresses

   Attention is paid in this specification to transition from the use of
   IPv4 Link-Local addresses to routable addresses (see Section 1.5).
   The intention is to allow a host with a single interface to first
   support Link-Local configuration then gracefully transition to the
   use of a routable address.  Since the host transitioning to the use
   of a routable address may temporarily have more than one address
   active, the scoped address issues described in Section 3.1 will
   apply.  When a host acquires a routable address, it does not need to
   retain its Link-Local address for the purpose of communicating with
   other devices on the link that are themselves using only Link-Local
   addresses: any host conforming to this specification knows that
   regardless of source address an IPv4 Link-Local destination must be
   reached by forwarding directly to the destination, not via a router;
   it is not necessary for that host to have a Link-Local source address
   in order to send to a Link-Local destination address.

   A host with an IPv4 Link-Local address may send to a destination
   which does not have an IPv4 Link-Local address.  If the host is not
   multi-homed, the procedure is simple and unambiguous: Using ARP and
   forwarding directly to on-link destinations is the default route.  If
   the host is multi-homed, however, the routing policy is more complex,
   especially if one of the interfaces is configured with a routable
   address and the default route is (sensibly) directed at a router
   accessible through that interface.  The following example illustrates
   this problem and provides a common solution to it.

                         i1 +---------+ i2   i3 +-------+
               ROUTER-------=  HOST1  =---------= HOST2 |
                      link1 +---------+  link2  +-------+

   In the figure above, HOST1 is connected to link1 and link2.
   Interface i1 is configured with a routable address, while i2 is an
   IPv4 Link-Local address.  HOST1 has its default route set to ROUTER's
   address, through i1.  HOST1 will route to destinations in 169.254/16
   to i2, sending directly to the destination.

   HOST2 has a configured (non-Link-Local) IPv4 address assigned to i3.

   Using a name resolution or service discovery protocol HOST1 can
   discover HOST2's address.  Since HOST2's address is not in
   169.254/16, HOST1's routing policy will send datagrams to HOST2 via
   i1, to the ROUTER.  Unless there is a route from ROUTER to HOST2, the
   datagrams sent from HOST1 to HOST2 will not reach it.

   One solution to this problem is for a host to attempt to reach any
   host locally (using ARP) for which it receives an unreachable ICMP
   error message (ICMP message codes 0, 1, 6 or 7 [RFC792]).  The host
   tries all its attached links in a round robin fashion.  This has been
   implemented successfully for some IPv6 hosts, to circumvent exactly
   this problem.  In terms of this example, HOST1 upon failing to reach
   HOST2 via the ROUTER, will attempt to forward to HOST2 via i2 and
   succeed.

   It may also be possible to overcome this problem using techniques
   described in section 3.2, or other means not discussed here.  This
   specification does not provide a standard solution, nor does it
   preclude implementers from supporting multi-homed configurations,
   provided that they address the concerns in this section for the
   applications which will be supported on the host.

3.4.  Unintentional Autoimmune Response

   Care must be taken if a multi-homed host can support more than one
   interface on the same link, all of which support IPv4 Link-Local
   autoconfiguration.  If these interfaces attempt to allocate the same
   address, they will defend the host against itself -- causing the
   claiming algorithm to fail.  The simplest solution to this problem is
   to run the algorithm independently on each interface configured with
   IPv4 Link-Local addresses.

   In particular, ARP packets which appear to claim an address which is
   assigned to a specific interface, indicate conflict only if they are
   received on that interface and their hardware address is of some
   other interface.

   If a host has two interfaces on the same link, then claiming and
   defending on those interfaces must ensure that they end up with
   different addresses just as if they were on different hosts.  Note
   that some of the ways a host may find itself with two interfaces on
   the same link may be unexpected and non-obvious, such as when a host
   has Ethernet and 802.11 wireless, but those two links are (possibly
   even without the knowledge of the host's user) bridged together.

4.  Healing of Network Partitions

   Hosts on disjoint network links may configure the same IPv4 Link-
   Local address.  If these separate network links are later joined or
   bridged together, then there may be two hosts which are now on the
   same link, trying to use the same address.  When either host attempts
   to communicate with any other host on the network, it will at some
   point broadcast an ARP packet which will enable the hosts in question
   to detect that there is an address conflict.

   When these address conflicts are detected, the subsequent forced
   reconfiguration may be disruptive, causing TCP connections to be
   broken.  However, it is expected that such disruptions will be rare.
   It should be relatively uncommon for networks to be joined while
   hosts on those networks are active.  Also, 65024 addresses are
   available for IPv4 Link-Local use, so even when two small networks
   are joined, the chance of conflict for any given host is fairly
   small.

   When joining two large networks (defined as networks with a
   substantial number of hosts per segment) there is a greater chance of
   conflict.  In such networks, it is likely that the joining of
   previously separated segments will result in one or more hosts
   needing to change their IPv4 Link-Local address, with subsequent loss
   of TCP connections.  In cases where separation and re-joining is
   frequent, as in remotely bridged networks, this could prove
   disruptive.  However, unless the number of hosts on the joined
   segments is very large, the traffic resulting from the join and
   subsequent address conflict resolution will be small.

   Sending ARP replies that have IPv4 Link-Local sender addresses via
   broadcast instead of unicast ensures that these conflicts can be
   detected as soon as they become potential problems, but no sooner.
   For example, if two disjoint network links are joined, where hosts A
   and B have both configured the same Link-Local address, X, they can
   remain in this state until A, B or some other host attempts to
   initiate communication.  If some other host C now sends an ARP
   request for address X, and hosts A and B were to both reply with
   conventional unicast ARP replies, then host C might be confused, but
   A and B still wouldn't know there is a problem because neither would
   have seen the other's packet.  Sending these replies via broadcast
   allows A and B to see each other's conflicting ARP packets and
   respond accordingly.

   Note that sending periodic gratuitous ARPs in an attempt to detect
   these conflicts sooner is not necessary, wastes network bandwidth,
   and may actually be detrimental.  For example, if the network links
   were joined only briefly, and were separated again before any new

   communication involving A or B were initiated, then the temporary
   conflict would have been benign and no forced reconfiguration would
   have been required.  Triggering an unnecessary forced reconfiguration
   in this case would not serve any useful purpose.  Hosts SHOULD NOT
   send periodic gratuitous ARPs.

5.  Security Considerations

   The use of IPv4 Link-Local Addresses may open a network host to new
   attacks.  In particular, a host that previously did not have an IP
   address, and no IP stack running, was not susceptible to IP-based
   attacks.  By configuring a working address, the host may now be
   vulnerable to IP-based attacks.

   The ARP protocol [RFC826] is insecure.  A malicious host may send
   fraudulent ARP packets on the network, interfering with the correct
   operation of other hosts.  For example, it is easy for a host to
   answer all ARP requests with replies giving its own hardware address,
   thereby claiming ownership of every address on the network.

   NOTE: There are certain kinds of local links, such as wireless LANs,
   that provide no physical security.  Because of the existence of these
   links it would be very unwise for an implementer to assume that when
   a device is communicating only on the local link it can dispense with
   normal security precautions.  Failure to implement appropriate
   security measures could expose users to considerable risks.

   A host implementing IPv4 Link-Local configuration has an additional
   vulnerability to selective reconfiguration and disruption.  It is
   possible for an on-link attacker to issue ARP packets which would
   cause a host to break all its connections by switching to a new
   address.  The attacker could force the host implementing IPv4 Link-
   Local configuration to select certain addresses, or prevent it from
   ever completing address selection.  This is a distinct threat from
   that posed by spoofed ARPs, described in the preceding paragraph.

   Implementations and users should also note that a node that gives up
   an address and reconfigures, as required by section 2.5, allows the
   possibility that another node can easily and successfully hijack
   existing TCP connections.

   Implementers are advised that the Internet Protocol architecture
   expects every networked device or host must implement security which
   is adequate to protect the resources to which the device or host has
   access, including the network itself, against known or credible
   threats.  Even though use of IPv4 Link-Local addresses may reduce the

   number of threats to which a device is exposed, implementers of
   devices supporting the Internet Protocol must not assume that a
   customer's local network is free from security risks.

   While there may be particular kinds of devices, or particular
   environments, for which the security provided by the network is
   adequate to protect the resources that are accessible by the device,
   it would be misleading to make a general statement to the effect that
   the requirement to provide security is reduced for devices using IPv4
   Link-Local addresses as a sole means of access.

   In all cases, whether or not IPv4 Link-Local addresses are used, it
   is necessary for implementers of devices supporting the Internet
   Protocol to analyze the known and credible threats to which a
   specific host or device might be subjected, and to the extent that it
   is feasible, to provide security mechanisms which ameliorate or
   reduce the risks associated with such threats.

6.  Application Programming Considerations

   Use of IPv4 Link-Local autoconfigured addresses presents additional
   challenges to writers of applications and may result in existing
   application software failing.

6.1.  Address Changes, Failure and Recovery

   IPv4 Link-Local addresses used by an application may change over
   time.  Some application software encountering an address change will
   fail.  For example, existing client TCP connections will be aborted,
   servers whose addresses change will have to be rediscovered, blocked
   reads and writes will exit with an error condition, and so on.

   Vendors producing application software which will be used on IP
   implementations supporting IPv4 Link-Local address configuration
   SHOULD detect and cope with address change events.  Vendors producing
   IPv4 implementations supporting IPv4 Link-Local address configuration
   SHOULD expose address change events to applications.

6.2.  Limited Forwarding of Locators

   IPv4 Link-Local addresses MUST NOT be forwarded via an application
   protocol (for example in a URL), to a destination that is not on the
   same link.  This is discussed further in Sections 2.9 and 3.

   Existing distributed application software that forwards address
   information may fail.  For example, FTP [RFC959] (when not using
   passive mode) transmits the IP address of the client.  Suppose a
   client starts up and obtains its IPv4 configuration at a time when it

   has only a Link-Local address.  Later, the host gets a global IP
   address, and the client contacts an FTP server outside the local
   link.  If the FTP client transmits its old Link-Local address instead
   of its new global IP address in the FTP "port" command, then the FTP
   server will be unable to open a data connection back to the client,
   and the FTP operation will fail.

6.3.  Address Ambiguity

   Application software run on a multi-homed host that supports IPv4
   Link-Local address configuration on more than one interface may fail.

   This is because application software assumes that an IPv4 address is
   unambiguous, that it can refer to only one host.  IPv4 Link-Local
   addresses are unique only on a single link.  A host attached to
   multiple links can easily encounter a situation where the same
   address is present on more than one interface, or first on one
   interface, later on another; in any case associated with more than
   one host.  Most existing software is not prepared for this ambiguity.
   In the future, application programming interfaces could be developed
   to prevent this problem.  This issue is discussed in Section 3.

7.  Router Considerations

   A router MUST NOT forward a packet with an IPv4 Link-Local source or
   destination address, irrespective of the router's default route
   configuration or routes obtained from dynamic routing protocols.

   A router which receives a packet with an IPv4 Link-Local source or
   destination address MUST NOT forward the packet.  This prevents
   forwarding of packets back onto the network segment from which they
   originated, or to any other segment.

8.  IANA Considerations

   The IANA has allocated the prefix 169.254/16 for the use described in
   this document.  The first and last 256 addresses in this range
   (169.254.0.x and 169.254.255.x) are allocated by Standards Action, as
   defined in "Guidelines for Writing an IANA" (BCP 26) [RFC2434].  No
   other IANA services are required by this document.

9.  Constants

   The following timing constants are used in this protocol; they are
   not intended to be user configurable.

   PROBE_WAIT           1 second   (initial random delay)
   PROBE_NUM            3          (number of probe packets)
   PROBE_MIN            1 second   (minimum delay till repeated probe)
   PROBE_MAX            2 seconds  (maximum delay till repeated probe)
   ANNOUNCE_WAIT        2 seconds  (delay before announcing)
   ANNOUNCE_NUM         2          (number of announcement packets)
   ANNOUNCE_INTERVAL    2 seconds  (time between announcement packets)
   MAX_CONFLICTS       10          (max conflicts before rate limiting)
   RATE_LIMIT_INTERVAL 60 seconds  (delay between successive attempts)
   DEFEND_INTERVAL     10 seconds  (minimum interval between defensive
                                    ARPs).

10.  References

10.1.  Normative References

   [RFC792]  Postel, J., "Internet Control Message Protocol", STD 5, RFC
             792, September 1981.

   [RFC826]  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.

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

   [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
             IANA Considerations Section in RFCs", BCP 26, RFC 2434,
             October 1998.

10.2.  Informative References

   [802]     IEEE Standards for Local and Metropolitan Area Networks:
             Overview and Architecture, ANSI/IEEE Std 802, 1990.

   [802.3]   ISO/IEC 8802-3 Information technology - Telecommunications
             and information exchange between systems - Local and
             metropolitan area networks - Common specifications - Part
             3:  Carrier Sense Multiple Access with Collision Detection
             (CSMA/CD) Access Method and Physical Layer Specifications,
             (also ANSI/IEEE Std 802.3- 1996), 1996.

   [802.5]   ISO/IEC 8802-5 Information technology - Telecommunications
             and information exchange between systems - Local and
             metropolitan area networks - Common specifications - Part
             5: Token ring access method and physical layer
             specifications, (also ANSI/IEEE Std 802.5-1998), 1998.

   [802.11]  Information technology - Telecommunications and information
             exchange between systems - Local and metropolitan area
             networks - Specific Requirements Part 11:  Wireless LAN
             Medium Access Control (MAC) and Physical Layer (PHY)
             Specifications, IEEE Std. 802.11-1999, 1999.

   [RFC959]  Postel, J. and J. Reynolds, "File Transfer Protocol", STD
             9, RFC 959, October 1985.

   [RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
             and E. Lear, "Address Allocation for Private Internets",
             BCP 5, RFC 1918, February 1996.

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

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

   [RFC3027] Holdrege, M. and P. Srisuresh, "Protocol Complications with
             the IP Network Address Translator", RFC 3027, January 2001.

   [DNAv4]   Aboba, B., "Detection of Network Attachment (DNA) in IPv4",
             Work in Progress, July 2004.

   [LLMNR]   Esibov, L., Aboba, B. and D. Thaler, "Linklocal Multicast
             Name Resolution (LLMNR)", Work in Progress, June 2004.

Acknowledgments

   We would like to thank (in alphabetical order) Jim Busse, Pavani
   Diwanji, Donald Eastlake 3rd, Robert Elz, Peter Ford, Spencer
   Giacalone, Josh Graessley, Brad Hards, Myron Hattig, Hugh Holbrook,
   Christian Huitema, Richard Johnson, Kim Yong-Woon, Mika Liljeberg,
   Rod Lopez, Keith Moore, Satish Mundra, Thomas Narten, Erik Nordmark,
   Philip Nye, Howard Ridenour, Daniel Senie, Dieter Siegmund, Valery
   Smyslov, and Ryan Troll for their contributions.

Appendix A - Prior Implementations

A.1.  Apple Mac OS 8.x and 9.x.

   Mac OS chooses the IP address on a pseudo-random basis.  The selected
   address is saved in persistent storage for continued use after
   reboot, when possible.

   Mac OS sends nine DHCPDISCOVER packets, with an interval of two
   seconds between packets.  If no response is received from any of
   these requests (18 seconds), it will autoconfigure.

   Upon finding that a selected address is in use, Mac OS will select a
   new random address and try again, at a rate limited to no more than
   one attempt every two seconds.

   Autoconfigured Mac OS systems check for the presence of a DHCP server
   every five minutes.  If a DHCP server is found but Mac OS is not
   successful in obtaining a new lease, it keeps the existing
   autoconfigured IP address.  If Mac OS is successful at obtaining a
   new lease, it drops all existing connections without warning.  This
   may cause users to lose sessions in progress.  Once a new lease is
   obtained, Mac OS will not allocate further connections using the
   autoconfigured IP address.

   Mac OS systems do not send packets addressed to a Link-Local address
   to the default gateway if one is present; these addresses are always
   resolved on the local segment.

   Mac OS systems by default send all outgoing unicast packets with a
   TTL of 255.  All multicast and broadcast packets are also sent with a
   TTL of 255 if they have a source address in the 169.254/16 prefix.

   Mac OS implements media sense where the hardware (and driver
   software) supports this.  As soon as network connectivity is
   detected, a DHCPDISCOVER will be sent on the interface.  This means
   that systems will immediately transition out of autoconfigured mode
   as soon as connectivity is restored.

A.2.  Apple Mac OS X Version 10.2

   Mac OS X chooses the IP address on a pseudo-random basis.  The
   selected address is saved in memory so that it can be re-used during
   subsequent autoconfiguration attempts during a single boot of the
   system.

   Autoconfiguration of a Link-Local address depends on the results of
   the DHCP process.  DHCP sends two packets, with timeouts of one and
   two seconds.  If no response is received (three seconds), it begins
   autoconfiguration.  DHCP continues sending packets in parallel for a
   total time of 60 seconds.

   At the start of autoconfiguration, it generates 10 unique random IP
   addresses, and probes each one in turn for 2 seconds.  It stops
   probing after finding an address that is not in use, or the list of
   addresses is exhausted.

   If DHCP is not successful, it waits five minutes before starting over
   again.  Once DHCP is successful, the autoconfigured Link-Local
   address is given up.  The Link-Local subnet, however, remains
   configured.

   Autoconfiguration is only attempted on a single interface at any
   given moment in time.

   Mac OS X ensures that the connected interface with the highest
   priority is associated with the Link-Local subnet.  Packets addressed
   to a Link-Local address are never sent to the default gateway, if one
   is present.  Link-local addresses are always resolved on the local
   segment.

   Mac OS X implements media sense where the hardware and driver support
   it.  When the network media indicates that it has been connected, the
   autoconfiguration process begins again, and attempts to re-use the
   previously assigned Link-Local address.  When the network media
   indicates that it has been disconnected, the system waits four
   seconds before de-configuring the Link-Local address and subnet.  If
   the connection is restored before that time, the autoconfiguration
   process begins again.  If the connection is not restored before that
   time, the system chooses another interface to autoconfigure.

   Mac OS X by default sends all outgoing unicast packets with a TTL of
   255.  All multicast and broadcast packets are also sent with a TTL of
   255 if they have a source address in the 169.254/16 prefix.

A.3.  Microsoft Windows 98/98SE

   Windows 98/98SE systems choose their IPv4 Link-Local address on a
   pseudo-random basis.  The address selection algorithm is based on
   computing a hash on the interface's MAC address, so that a large
   collection of hosts should obey the uniform probability distribution
   in choosing addresses within the 169.254/16 address space.  Deriving

   the initial IPv4 Link-Local address from the interface's MAC address
   also ensures that systems rebooting will obtain the same
   autoconfigured address, unless a conflict is detected.

   When in INIT state, the Windows 98/98SE DHCP Client sends out a total
   of 4 DHCPDISCOVERs, with an inter-packet interval of 6 seconds.  When
   no response is received after all 4 packets (24 seconds), it will
   autoconfigure an address.

   The autoconfigure retry count for Windows 98/98SE systems is 10.
   After trying 10 autoconfigured IPv4 addresses, and finding all are
   taken, the host will boot without an IPv4 address.

   Autoconfigured Windows 98/98SE systems check for the presence of a
   DHCP server every five minutes.  If a DHCP server is found but
   Windows 98 is not successful in obtaining a new lease, it keeps the
   existing autoconfigured IPv4 Link-Local address.  If Windows 98/98SE
   is successful at obtaining a new lease, it drops all existing
   connections without warning.  This may cause users to lose sessions
   in progress.  Once a new lease is obtained, Windows 98/98SE will not
   allocate further connections using the autoconfigured IPv4 Link-Local
   address.

   Windows 98/98SE systems with an IPv4 Link-Local address do not send
   packets addressed to an IPv4 Link-Local address to the default
   gateway if one is present; these addresses are always resolved on the
   local segment.

   Windows 98/98SE systems by default send all outgoing unicast packets
   with a TTL of 128.  TTL configuration is performed by setting the
   Windows Registry Key
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services:\Tcpip\
   Parameters\DefaultTTL of type REG_DWORD to the appropriate value.
   However, this default TTL will apply to all packets.  While this
   facility could be used to set the default TTL to 255, it cannot be
   used to set the default TTL of IPv4 Link-Local packets to one (1),
   while allowing other packets to be sent with a TTL larger than one.

   Windows 98/98SE systems do not implement media sense.  This means
   that network connectivity issues (such as a loose cable) may prevent
   a system from contacting the DHCP server, thereby causing it to
   auto-configure.  When the connectivity problem is fixed (such as when
   the cable is re-connected) the situation will not immediately correct
   itself.  Since the system will not sense the re-connection, it will
   remain in autoconfigured mode until an attempt is made to reach the
   DHCP server.

   The DHCP server included with Windows 98SE Internet Connection
   Sharing (ICS) (a NAT implementation) allocates out of the 192.168/16
   private address space by default.

   However, it is possible to change the allocation prefix via a
   registry key, and no checks are made to prevent allocation out of the
   IPv4 Link-Local prefix.  When configured to do so, Windows 98SE ICS
   will rewrite packets from the IPv4 Link-Local prefix and forward them
   beyond the local link.  Windows 98SE ICS does not automatically route
   for the IPv4 Link-Local prefix, so that hosts obtaining addresses via
   DHCP cannot communicate with autoconfigured-only devices.

   Other home gateways exist that allocate addresses out of the IPv4
   Link-Local prefix by default.  Windows 98/98SE systems can use a
   169.254/16 IPv4 Link-Local address as the source address when
   communicating with non-Link-Local hosts.  Windows 98/98SE does not
   support router solicitation/advertisement.  Windows 98/98SE systems
   will not automatically discover a default gateway when in
   autoconfigured mode.

A.4.  Windows XP, 2000, and ME

   The autoconfiguration behavior of Windows XP, Windows 2000, and
   Windows ME systems is identical to Windows 98/98SE except in the
   following respects:

   Media Sense
   Router Discovery
   Silent RIP

   Windows XP, 2000, and ME implement media sense.  As soon as network
   connectivity is detected, a DHCPREQUEST or DHCPDISCOVER will be sent
   on the interface.  This means that systems will immediately
   transition out of autoconfigured mode as soon as connectivity is
   restored.

   Windows XP, 2000, and ME also support router discovery, although it
   is turned off by default.  Windows XP and 2000 also support a RIP
   listener.  This means that they may inadvertently discover a default
   gateway while in autoconfigured mode.

   ICS on Windows XP/2000/ME behaves identically to Windows 98SE with
   respect to address allocation and NATing of Link-Local prefixes.

Authors' Addresses

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

   Phone: +1 408 974 3207
   EMail: rfc@stuartcheshire.org

   Bernard Aboba
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

   Phone: +1 425 818 4011
   EMail: bernarda@microsoft.com

   Erik Guttman
   Sun Microsystems
   Eichhoelzelstr. 7
   74915 Waibstadt Germany

   Phone: +49 7263 911 701
   EMail: erik@spybeam.org

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