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rfc:rfc3927

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.

Cheshire, et al. Standards Track [Page 1] RFC 3927 IPv4 Link-Local May 2005

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

Cheshire, et al. Standards Track [Page 2] RFC 3927 IPv4 Link-Local May 2005

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:

Cheshire, et al. Standards Track [Page 3] RFC 3927 IPv4 Link-Local May 2005

  1. 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
  1. 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.

Cheshire, et al. Standards Track [Page 4] RFC 3927 IPv4 Link-Local May 2005

 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%

Cheshire, et al. Standards Track [Page 5] RFC 3927 IPv4 Link-Local May 2005

 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.

Cheshire, et al. Standards Track [Page 6] RFC 3927 IPv4 Link-Local May 2005

    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.

Cheshire, et al. Standards Track [Page 7] RFC 3927 IPv4 Link-Local May 2005

 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.

Cheshire, et al. Standards Track [Page 8] RFC 3927 IPv4 Link-Local May 2005

    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.

Cheshire, et al. Standards Track [Page 9] RFC 3927 IPv4 Link-Local May 2005

 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.

Cheshire, et al. Standards Track [Page 10] RFC 3927 IPv4 Link-Local May 2005

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.

Cheshire, et al. Standards Track [Page 11] RFC 3927 IPv4 Link-Local May 2005

 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.

Cheshire, et al. Standards Track [Page 12] RFC 3927 IPv4 Link-Local May 2005

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

Cheshire, et al. Standards Track [Page 13] RFC 3927 IPv4 Link-Local May 2005

 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

Cheshire, et al. Standards Track [Page 14] RFC 3927 IPv4 Link-Local May 2005

 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

Cheshire, et al. Standards Track [Page 15] RFC 3927 IPv4 Link-Local May 2005

 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.

Cheshire, et al. Standards Track [Page 16] RFC 3927 IPv4 Link-Local May 2005

 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].

Cheshire, et al. Standards Track [Page 17] RFC 3927 IPv4 Link-Local May 2005

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.

Cheshire, et al. Standards Track [Page 18] RFC 3927 IPv4 Link-Local May 2005

 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

Cheshire, et al. Standards Track [Page 19] RFC 3927 IPv4 Link-Local May 2005

 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.

Cheshire, et al. Standards Track [Page 20] RFC 3927 IPv4 Link-Local May 2005

 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.

Cheshire, et al. Standards Track [Page 21] RFC 3927 IPv4 Link-Local May 2005

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

Cheshire, et al. Standards Track [Page 22] RFC 3927 IPv4 Link-Local May 2005

 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

Cheshire, et al. Standards Track [Page 23] RFC 3927 IPv4 Link-Local May 2005

 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

Cheshire, et al. Standards Track [Page 24] RFC 3927 IPv4 Link-Local May 2005

 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.

Cheshire, et al. Standards Track [Page 25] RFC 3927 IPv4 Link-Local May 2005

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.

Cheshire, et al. Standards Track [Page 26] RFC 3927 IPv4 Link-Local May 2005

 [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.

Cheshire, et al. Standards Track [Page 27] RFC 3927 IPv4 Link-Local May 2005

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.

Cheshire, et al. Standards Track [Page 28] RFC 3927 IPv4 Link-Local May 2005

 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

Cheshire, et al. Standards Track [Page 29] RFC 3927 IPv4 Link-Local May 2005

 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.

Cheshire, et al. Standards Track [Page 30] RFC 3927 IPv4 Link-Local May 2005

 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.

Cheshire, et al. Standards Track [Page 31] RFC 3927 IPv4 Link-Local May 2005

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

Cheshire, et al. Standards Track [Page 32] RFC 3927 IPv4 Link-Local May 2005

Full Copyright Statement

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Cheshire, et al. Standards Track [Page 33]

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