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RFC 4436

Detecting Network Attachment in IPv4 (DNAv4)

Pages: 15
Proposed Standard

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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).


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.
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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,
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   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.
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   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.
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      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.
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      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"

   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

   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:
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   [1] The IPv4 and MAC address of one or more test nodes on the

   [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

   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.
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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
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   results and use the DHCP result instead, unless the address confirmed
   via the reachability test has been manually assigned (see Section

   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
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   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
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   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.
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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
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   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.
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   [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: James Carlson Sun Microsystems, Inc 1 Network Drive Burlington, MA 01803-2757 USA Phone: +1 781 442 2084 Fax: +1 781 442 1677 EMail: Stuart Cheshire Apple Computer, Inc. 1 Infinite Loop Cupertino, California 95014, USA Phone: +1 408 974 3207 EMail:
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