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rfc:rfc4436

Network Working Group B. Aboba Request for Comments: 4436 Microsoft Corporation Category: Standards Track J. Carlson

                                                      Sun Microsystems
                                                           S. Cheshire
                                                        Apple Computer
                                                            March 2006
            Detecting Network Attachment in IPv4 (DNAv4)

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 (2006).

Abstract

 The time required to detect movement between networks and to obtain
 (or to continue to use) an IPv4 configuration may be significant as a
 fraction of the total handover latency in moving between points of
 attachment.  This document synthesizes, from experience in the
 deployment of hosts supporting ARP, DHCP, and IPv4 Link-Local
 addresses, a set of steps known as Detecting Network Attachment for
 IPv4 (DNAv4), in order to decrease the handover latency in moving
 between points of attachment.

Aboba, et al. Standards Track [Page 1] RFC 4436 DNAv4 March 2006

Table of Contents

 1. Introduction ....................................................2
    1.1. Applicability ..............................................2
    1.2. Requirements ...............................................5
    1.3. Terminology ................................................5
 2. Overview ........................................................6
    2.1. Reachability Test ..........................................8
         2.1.1. Packet Format .......................................9
    2.2. IPv4 Address Acquisition ..................................10
    2.3. IPv4 Link-Local Addresses .................................11
    2.4. Manually Assigned Addresses ...............................12
 3. Security Considerations ........................................12
 4. References .....................................................13
    4.1. Normative References ......................................13
    4.2. Informative References ....................................13
 5. Acknowledgements ...............................................14

1. Introduction

 The time required to detect movement between networks and to obtain
 (or to continue to use) an operable IPv4 configuration may be
 significant as a fraction of the total handover latency in moving
 between points of attachment.
 This document synthesizes, from experience in the deployment of hosts
 supporting ARP [RFC826], DHCP [RFC2131], and IPv4 Link-Local
 addresses [RFC3927], a set of steps known as Detecting Network
 Attachment for IPv4 (DNAv4).  DNAv4 optimizes the (common) case of
 re-attachment to a network that one has been connected to previously
 by attempting to re-use a previous (but still valid) configuration,
 reducing the re-attachment time on LANs to a few milliseconds.  Since
 this procedure is dependent on the ARP protocol, it is not suitable
 for use on media that do not support ARP.

1.1. Applicability

 DHCP is an effective and widely adopted mechanism for a host to
 obtain an IP address for use on a particular network link, or to
 re-validate a previously obtained address via DHCP's INIT-REBOOT
 mechanism [RFC2131].
 When obtaining a new address, DHCP specifies that the client SHOULD
 use ARP to verify that the offered address is not already in use.
 The process of address conflict detection [ACD] can take as much as
 seven seconds.  In principle, this time interval could be shortened,

Aboba, et al. Standards Track [Page 2] RFC 4436 DNAv4 March 2006

 with the obvious trade-off: the less time a host spends waiting to
 see if another host is already using its intended address, the
 greater the risk of inadvertent address conflicts.
 Where the client successfully re-validates a previously obtained
 address using the INIT-REBOOT mechanism, the DHCP specification does
 not require the client to perform address conflict detection, so this
 seven-second delay does not apply.  However, the DHCP server may be
 slow to respond or may be down and not responding at all, so hosts
 could benefit from having an alternative way to quickly determine
 that a previously obtained address is valid for use on this
 particular link.
 When the client moves between networks, the address re-validation
 attempt may be unsuccessful; a DHCPNAK may be received in response to
 a DHCPREQUEST, causing the client to restart the configuration
 process by moving to the INIT state.  If an address previously
 obtained on the new network is still operable, DNAv4 enables the host
 to confirm the new configuration quickly, bypassing restart of the
 configuration process and conflict detection.
 The alternative mechanism specified by this document applies when a
 host has a previously allocated DHCP address, which was not returned
 to the DHCP server via a DHCPRELEASE message, and which still has
 time remaining on its lease.  In this case, the host may determine
 whether it has re-attached to the logical link where this address is
 valid for use, by sending a unicast ARP Request packet to a router
 previously known for that link (or, in the case of a link with more
 than one router, by sending one or more unicast ARP Request packets
 to one or more of those routers).
 The use of unicast ARP has a number of benefits.  One benefit is that
 unicast packets impose less burden on the network than broadcast
 packets, particularly on 802.11 networks where broadcast packets may
 be sent at rates as low as 1 Mb/sec.  Another benefit is that if the
 host is not on the link it hoped to find itself on, a broadcast ARP
 Request could pollute the ARP caches of peers on that link.  When
 using private addresses [RFC1918], another device could be
 legitimately using the same address, and a broadcast ARP Request
 could disrupt its communications, causing TCP connections to be
 broken, and similar problems.  Also, using a unicast ARP packet
 addressed to the MAC address of the router the host is expecting to
 find means that if the host is not on the expected link there will be
 no device with that MAC address, and the ARP packet will harmlessly
 disappear into the void without doing any damage.

Aboba, et al. Standards Track [Page 3] RFC 4436 DNAv4 March 2006

 These issues that define the applicability of DNAv4 lead us to a
 number of conclusions:
    o  DNAv4 is a performance optimization.  Its purpose is to speed
       up a process that may require as much as a few hundred
       milliseconds (DHCP INIT-REBOOT), as well as to reduce multi-
       second conflict detection delays when a host changes networks.
    o  As a performance optimization, it must not sacrifice
       correctness.  In other words, false positives are not
       acceptable.  DNAv4 must not conclude that a host has returned
       to a previously visited link where it has an operable IP
       address if this is not in fact the case.
    o  As a performance optimization, false negatives are acceptable.
       It is not an absolute requirement that this optimization
       correctly recognize a previously visited link in all possible
       cases.  For example, if a router ignores unicast ARP Requests,
       then DNAv4 will not be able to detect that it has returned to
       the same link in the future.  This is acceptable because the
       host still operates correctly as it did without DNAv4, just
       without the performance benefit.  Users and network operators
       who desire the performance improvement offered by DNAv4 can
       upgrade their routers to support DNAv4.
    o  As a performance optimization, where DNAv4 fails to provide a
       benefit, it should add little or no delay compared to today's
       DHCP processing.  In practice, this implies that DHCP
       processing needs to proceed in parallel.  Waiting for DNAv4 to
       fail before beginning DHCP processing can greatly increase
       total processing time, the opposite of the desired effect.
    o  Trials are inexpensive.  DNAv4 performs its checks using small
       unicast packets.  An IPv4 ARP packet on Ethernet is just 42
       octets, including the Ethernet header.  This means that the
       cost of an unsuccessful attempt is small, whereas the cost of a
       missed opportunity (having the right address available as a
       candidate and choosing not to try it for some reason) is large.
       As a result, the best strategy is often to try all available
       candidate configurations, rather than try to determine which
       candidates, if any, may be correct for this link, based on
       heuristics or hints.  For a heuristic to offer the prospect of
       being a potentially useful way to eliminate inappropriate
       configurations from the candidate list, that heuristic has to
       (a) be fast and inexpensive to compute, as compared to sending
       a 42-octet unicast packet, and (b) have high probability of not
       falsely eliminating a candidate configuration that could be
       found to be the correct one.

Aboba, et al. Standards Track [Page 4] RFC 4436 DNAv4 March 2006

    o  Time is limited.  If DNAv4 is to be effective in enabling low
       latency handoffs, it needs to complete in less than 10 ms.
       This implies that any heuristic used to eliminate candidate
       configurations needs to take at most a few milliseconds to
       compute.  This does not leave much room for heuristics based on
       observation of link-layer or Internet-layer traffic.

1.2. Requirements

 In this document, several words are used to signify the requirements
 of the specification.  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 to Indicate Requirement Levels" [RFC2119].

1.3. Terminology

 This document uses the following terms:
 ar$sha
    ARP packet field: Sender Hardware Address [RFC826].  The hardware
    (MAC) address of the originator of an ARP packet.
 ar$spa
    ARP packet field: Sender Protocol Address [RFC826].  For IP
    Address Resolution, this is the IPv4 address of the sender of the
    ARP packet.
 ar$tha
    ARP packet field: Target Hardware Address [RFC826].  The hardware
    (MAC) address of the target of an ARP packet.
 ar$tpa
    ARP packet field: Target Protocol Address [RFC826].  For IPv4
    Address Resolution, the IPv4 address for which one desires to know
    the hardware address.
 DHCP client
    A DHCP client or "client" is an Internet host using the Dynamic
    Host Configuration Protocol (DHCP) [RFC2131] to obtain
    configuration parameters, such as a network address.
 DHCP server
    A DHCP server or "server" is an Internet host that returns
    configuration parameters to DHCP clients.

Aboba, et al. Standards Track [Page 5] RFC 4436 DNAv4 March 2006

 Link
    A communication facility or medium over which network nodes can
    communicate.  Each link is associated with a minimum of two
    endpoints.  Each link endpoint has a unique link-layer identifier.
 Link Down
    An event provided by the link layer that signifies a state change
    associated with the interface's no longer being capable of
    communicating data frames; transient periods of high frame loss
    are not sufficient.  DNAv4 does not utilize "Link Down"
    indications.
 Link Layer
    Conceptual layer of control or processing logic that is
    responsible for maintaining control of the data link.  The data
    link layer functions provide an interface between the higher-layer
    logic and the data link.  The link layer is the layer immediately
    below IP.
 Link Up
    An event provided by the link layer that signifies a state change
    associated with the interface's becoming capable of communicating
    data frames.
 Point of Attachment
    The link endpoint on the link to which the host is currently
    connected.
 Routable address
    In this specification, the term "routable address" refers to any
    unicast IPv4 address other than an IPv4 Link-Local address.  This
    includes private addresses as specified in "Address Allocation for
    Private Internets" [RFC1918].
 Operable address
    In this specification, the term "operable address" refers either
    to a static IPv4 address, or an address assigned via DHCPv4 that
    has not been returned to the DHCP server via a DHCP RELEASE
    message, and whose lease has not yet expired.

2. Overview

 On connecting to a new point of attachment, the host responds to a
 "Link Up" indication from the link layer by carrying out the DNAv4
 procedure.
 For each network that it connects to, it is assumed that the host
 saves the following parameters to stable storage:

Aboba, et al. Standards Track [Page 6] RFC 4436 DNAv4 March 2006

 [1] The IPv4 and MAC address of one or more test nodes on the
     network.
 [2] The IPv4 configuration parameters, including the DHCP client
     identifier, assigned address, and lease expiration time.
 From the set of networks that have operable IPv4 addresses associated
 with them, the host selects a subset and attempts to confirm the
 configuration for each network, using the reachability test described
 in Section 2.1.
 For a particular network, the host SHOULD use the addresses of local
 routers (preferably default gateways) as its test nodes.  If more
 than one address is known, those addresses may be tested in parallel.
 In order to ensure configuration validity, the host SHOULD only
 configure routes for which the next hop address passes the
 reachability test.  Other routes SHOULD be re-learned.
 DNAv4 does not significantly increase the likelihood of an address
 conflict.  The reachability test is only carried out for a network
 when the host has previously completed conflict detection as
 recommended in Section 2.2 of the DHCP specification [RFC2131] and
 obtained an operable IPv4 configuration on that network.
 Restrictions on sending ARP Requests and Responses are described in
 Section 2.1.1.
 One case where DNAv4 does increase the likelihood of an address
 conflict is when:
    o  a DHCP server hands out an address lease,
    o  the host with that lease leaves the network,
    o  the DHCP server is power-cycled or crashes and is rebooted,
    o  the DHCP server, having failed to save leases to stable
       storage, assigns that same address to another host, and
    o  the first host returns and, having a still-valid lease with
       time remaining, proceeds to use its assigned address,
       conflicting with the new host that is now using that same
       address.
 While Section 4 of the DHCP specification [RFC2131] assumes that DHCP
 servers save their leases in persistent storage, almost no consumer-
 grade NAT gateway does so.  Short DHCP lease lifetimes can mitigate
 this risk, though this also limits the operable candidate
 configurations available for DNAv4 to try.

Aboba, et al. Standards Track [Page 7] RFC 4436 DNAv4 March 2006

2.1. Reachability Test

 The host skips the reachability test for a network if any of the
 following conditions are true:
 [a] The host does not have an operable routable IPv4 address on that
     network.  In this case, the reachability test cannot confirm that
     the host has an operable routable IPv4 address, so completing the
     reachability test would serve no purpose.
 [b] The host does not know the addresses of any test nodes on that
     network.  In this case, insufficient information is available to
     carry out the reachability test.
 [c] If DHCP authentication [RFC3118] is configured.  The reachability
     test utilizes ARP, which is insecure.  Hosts that have been
     configured to attempt DHCP authentication SHOULD NOT utilize the
     reachability test.  Security issues are discussed in Section 4.
 [d] The contents of the DHCP Client Identifier option that the client
     used to obtain the candidate configuration is different from the
     DHCP Client Identifier option the client intends to present on
     the interface in question.  In this case, it is anticipated that
     a DHCP server would NAK any request made by the client to acquire
     or extend the candidate configuration, since the two interfaces
     are presenting differing identities.
 If the reachability test is successful, the host SHOULD continue to
 use the operable routable IPv4 address associated with the confirmed
 network, without needing to re-acquire it.  Once a valid reachability
 test response is received, validation is complete, and additional
 responses should be discarded.
 If a DHCPv4 client is operational, it is RECOMMENDED that the host
 attempt to obtain IPv4 configuration via DHCPv4 in parallel with the
 reachability tests, with the host using the first answer returned.
 This ensures that the DNAv4 procedure will not result in additional
 delay in the case where reachability tests fail, or where sending a
 DHCPREQUEST from the INIT-REBOOT state, as described in Section 3.2
 and 4.3.2 of the DHCP specification [RFC2131], completes more quickly
 than the reachability tests.
 In situations where both DNAv4 and DHCP are used on the same link, it
 is possible that the reachability test will complete successfully,
 and then DHCP will complete later with a different result.  If this
 happens, the implementation SHOULD abandon the reachability test

Aboba, et al. Standards Track [Page 8] RFC 4436 DNAv4 March 2006

 results and use the DHCP result instead, unless the address confirmed
 via the reachability test has been manually assigned (see Section
 2.4).
 Where the reachability test does not return an answer, this is
 typically because the host is not attached to the network whose
 configuration is being tested.  In such circumstances, there is
 typically little value in aggressively retransmitting reachability
 tests that do not elicit a response.
 Where DNAv4 and DHCP are tried in parallel, one strategy is to
 forsake reachability test retransmissions and to allow only the DHCP
 client to retransmit.  In order to reduce competition between DNAv4
 and DHCP retransmissions, a DNAv4 implementation that retransmits may
 utilize the retransmission strategy described in Section 4.1 of the
 DHCP specification [RFC2131], scheduling DNAv4 retransmissions
 between DHCP retransmissions.
 If a response is received to any reachability test or DHCP message,
 pending retransmissions are canceled.  It is RECOMMENDED that a DNAv4
 implementation retransmit no more than twice.  To provide damping in
 the case of spurious "Link Up" indications, it is RECOMMENDED that
 the DNAv4 procedure be carried out no more than once a second.

2.1.1. Packet Format

 The reachability test is performed by sending a unicast ARP Request.
 The host MUST set the target protocol address (ar$tpa) to the IPv4
 address of the node being tested, and the sender protocol address
 field (ar$spa) to its own candidate IPv4 address.  The ARP Request
 MUST use the host MAC address as the source, and the test node MAC
 address as the destination.  The host includes its MAC address in the
 sender hardware address field (ar$sha) and sets the target hardware
 address field (ar$tha) to 0.
 If a valid ARP Reply is received, the MAC address in the sender
 hardware address field (ar$sha) in the ARP Reply is matched against
 the target hardware address field (ar$tpa) in the ARP Request, and
 the IPv4 address in the sender protocol address field (ar$spa) of the
 ARP Reply is matched against the target protocol address field
 (ar$tpa) in the ARP Request.  If a match is found, then the host
 continues to use that IPv4 address, subject to the lease re-
 acquisition and expiration behavior described in Section 4.4.5 of the
 DHCP specification [RFC2131].
 The risk of an address conflict is greatest when the host moves
 between private networks, since in this case the completion of
 conflict detection on the former network does not provide assurance

Aboba, et al. Standards Track [Page 9] RFC 4436 DNAv4 March 2006

 against an address conflict on the new network.  Until a host has
 confirmed the operability of its IPv4 configuration by receipt of a
 response to the reachability test, it SHOULD NOT respond to ARP
 Requests and SHOULD NOT broadcast ARP Requests containing its address
 within the sender protocol address field (ar$spa).
 Sending an ICMP Echo Request [RFC792] would not be an acceptable way
 of testing a candidate configuration, since sending any IP packet
 generally requires an ARP Request/Reply exchange and, as explained
 above, ARP packets may not be broadcast safely until after the
 candidate configuration has been confirmed.  Also, where a host moves
 from one private network to another, an ICMP Echo Request can result
 in an ICMP Echo Response even when the MAC address has changed, as
 long as the IPv4 address remains the same.  This can occur, for
 example, where a host moves from one home network using prefix
 192.168/16 to another one.  In addition, if the ping is sent with TTL
 > 1, then an ICMP Echo Response can be received from an off-link
 router.  As a result, if the MAC address of the test node is not
 checked, the host can mistakenly confirm attachment, potentially
 resulting in an address conflict.  As a result, sending an ICMP Echo
 Request SHOULD NOT be used as a substitute for the reachability test.

2.2. IPv4 Address Acquisition

 If the host has an operable routable IPv4 address on one or more
 networks, and if DHCPv4 is enabled on the interface, the host SHOULD
 attempt to acquire an IPv4 configuration using DHCPv4, in parallel
 with one or more reachability tests.  This is accomplished by
 entering the INIT-REBOOT state and sending a DHCPREQUEST to the
 broadcast address, as specified in Section 4.4.2 of the DHCP
 specification [RFC2131].
 If the host does not have an operable routable IPv4 address on any
 network, the host enters the INIT state and sends a DHCPDISCOVER
 packet to the broadcast address, as described in Section 4.4.1 of the
 DHCP specification [RFC2131].  If the host supports the Rapid Commit
 Option [RFC4039], it is possible that the exchange can be shortened
 from a 4-message exchange to a 2-message exchange.
 If the host does not receive a response to a DHCPREQUEST or
 DHCPDISCOVER, then it retransmits as specified in Section 4.1 of the
 DHCP specification [RFC2131].
 As discussed in Section 4.4.4 of the DHCP specification [RFC2131], a
 host in INIT or REBOOTING state that knows the address of a DHCP
 server may use that address in the DHCPDISCOVER or DHCPREQUEST rather
 than the IPv4 broadcast address.  In the INIT-REBOOT state, a
 DHCPREQUEST is sent to the broadcast address so that the host will

Aboba, et al. Standards Track [Page 10] RFC 4436 DNAv4 March 2006

 receive a response regardless of whether the previously configured
 IPv4 address is correct for the network to which it has connected.
 Sending a DHCPREQUEST to the unicast address in INIT-REBOOT state is
 not appropriate, since if the DHCP client has moved to another
 subnet, a DHCP server response cannot be routed back to the client
 since the DHCPREQUEST will bypass the DHCP relay and will contain an
 invalid source address.

2.3. IPv4 Link-Local Addresses

 DNAv4 applies only to previously configured addresses that had some
 lease lifetime associated with them, during which lifetime the
 address may be legitimately regarded as being reserved for exclusive
 use by the assigned host.  DHCP-assigned addresses fit this
 description, but IPv4 Link-Local address [RFC3927] do not, since IPv4
 Link-Local addresses are not handed out by an authoritative server
 and do not come with any guaranteed usable lifetime.
 A host's claim on an IPv4 Link-Local address is valid only as long as
 that host remains connected to the link, actively defending against
 probes for its chosen address.  As soon as a host shuts down, sleeps,
 or otherwise disconnects from a link, it immediately relinquishes any
 claim it may have had on any IPv4 Link-Local address on that link.  A
 host wishing to reclaim a previously used IPv4 Link-Local address
 MUST perform the full probing and announcement process required by
 "Dynamic Configuration of IPv4 Link-Local Addresses" [RFC3927] and
 MUST NOT attempt to use DNAv4 as a shortcut to bypass that process.
 Where the host does not have an operable routable IPv4 address on any
 network, the host MAY configure an IPv4 Link-Local address prior to
 entering the INIT state and sending a DHCPDISCOVER packet, as
 described in Section 2.3 of the DHCP specification [RFC2131].  Where
 a host can confirm that it remains connected to a network on which it
 possesses an operable routable IPv4 address, that address should be
 used, and the IPv4 Link-Local address is deprecated, as noted in
 Section 1.9 of the IPv4 Link-Local specification [RFC3927].
 Where a host has an operable routable IPv4 address on one or more
 networks but the reachability test cannot confirm the configuration
 and the DHCPv4 client does not receive a response after employing the
 retransmission algorithm, Section 3.2 of the DHCP specification
 [RFC2131] states that the client MAY choose to use the previously
 allocated network address and configuration parameters for the
 remainder of the unexpired lease.

Aboba, et al. Standards Track [Page 11] RFC 4436 DNAv4 March 2006

2.4. Manually Assigned Addresses

 An implementation may use DNAv4 to confirm the configuration of
 manually assigned addresses.  However, special consideration is
 required for this to produce reliable results, so it SHOULD NOT be
 enabled by default.
 For the purposes of DNAv4, manually assigned addresses may be treated
 as equivalent to DHCP-assigned addresses with an infinite lifetime.
 This does not significantly increase the probability of an address
 conflict as long as the manually assigned address is reserved by the
 DHCP server or is outside the scope of addresses assigned by a DHCP
 server.  However, where the manually assigned address is within an
 address scope utilized by a DHCP server, it is possible that the host
 will be unavailable when the DHCP server checks for a conflict prior
 to assigning the conflicting address.  In this case, a host utilizing
 DNAv4 could confirm an address that had been assigned to another
 host.
 Typically, an address is manually assigned on a network because a
 dynamically assigned address was not suitable for some reason.
 Therefore, where DNAv4 and DHCP are run in parallel and DNAv4
 confirms a manual configuration, it may be undesirable to allow this
 configuration to be overridden by DHCP, as described in Section 2.1.
 However, packet loss may cause the reachability test to fail while
 DHCP completes successfully, resulting in the host obtaining a
 dynamic address where a static address is desired.  In order to
 provide for reliable reconfirmation of manually assigned addresses,
 reachability tests for manual configurations require a more
 aggressive retransmission strategy than that detailed in Section 4.1
 of the DHCP specification [RFC2131].  For example, shorter
 retransmission intervals and more persistent retransmissions may be
 required.

3. Security Considerations

 Detecting Network Attachment for IPv4 (DNAv4) is based on ARP and
 DHCP and inherits the security vulnerabilities of these two
 protocols.
 ARP [RFC826] traffic is not secured, so an attacker gaining access to
 the network can spoof a response to the reachability test described
 in Section 2.1, leading the querier to conclude falsely that it is
 attached to a network that it is not connected to.
 Similarly, where DHCPv4 traffic is not secured, an attacker could
 masquerade as a DHCPv4 server, in order to convince the host that it
 was attached to a particular network.  This and other threats

Aboba, et al. Standards Track [Page 12] RFC 4436 DNAv4 March 2006

 relating to DHCPv4 are described in "Authentication for DHCP
 Messages" [RFC3118].
 The effect of these attacks will typically be limited to denial of
 service, unless the host utilizes its IP configuration for other
 purposes, such as security configuration or location determination.
 For example, a host that disables its personal firewall based on
 evidence that it had attached to a home network could be compromised
 by spoofing of the DNAv4 reachability test.  In general, adjustment
 of the security configuration based on DNAv4 is NOT RECOMMENDED.
 Hosts that depend on secure IP configuration SHOULD NOT use DNAv4 but
 SHOULD instead utilize DHCP authentication [RFC3118], possibly in
 combination with the Rapid Commit Option [RFC4039].

4. References

4.1. Normative References

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

4.2. Informative References

 [ACD]     Cheshire, S., "IPv4 Address Conflict Detection", Work in
           Progress, July 2005.
 [RFC792]  Postel, J., "Internet Control Message Protocol", STD 5, RFC
           792, September 1981.
 [RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
           and E. Lear, "Address Allocation for Private Internets",
           BCP 5, RFC 1918, February 1996.
 [RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP
           Messages", RFC 3118, June 2001.
 [RFC3927] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
           Configuration of IPv4 Link-Local Addresses", RFC 3927, May
           2005.

Aboba, et al. Standards Track [Page 13] RFC 4436 DNAv4 March 2006

 [RFC4039] Park, S., Kim, P., and B. Volz, "Rapid Commit Option for
           the Dynamic Host Configuration Protocol version 4
           (DHCPv4)", RFC 4039, March 2005.

5. Acknowledgements

 The authors would like to acknowledge Greg Daley of Monash
 University, Erik Guttman and Erik Nordmark of Sun Microsystems, Ralph
 Droms of Cisco Systems, Ted Lemon of Nominum, John Loughney of Nokia,
 Thomas Narten of IBM and David Hankins of ISC for contributions to
 this document.

Authors' Addresses

 Bernard Aboba
 Microsoft Corporation
 One Microsoft Way
 Redmond, WA 98052
 Phone: +1 425 818 4011
 Fax:   +1 425 936 7329
 EMail: bernarda@microsoft.com
 James Carlson
 Sun Microsystems, Inc
 1 Network Drive
 Burlington, MA  01803-2757
 USA
 Phone: +1 781 442 2084
 Fax:   +1 781 442 1677
 EMail: james.d.carlson@sun.com
 Stuart Cheshire
 Apple Computer, Inc.
 1 Infinite Loop
 Cupertino, California 95014, USA
 Phone: +1 408 974 3207
 EMail: rfc@stuartcheshire.org

Aboba, et al. Standards Track [Page 14] RFC 4436 DNAv4 March 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
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Aboba, et al. Standards Track [Page 15]

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