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Document: draft-cheshire-ipv4-acd-03.txt                 Stuart Cheshire
Category: Standards Track                                 Apple Computer
Expires 9th June 2003                                  9th December 2002

                    IPv4 Address Conflict Detection
                    <draft-cheshire-ipv4-acd-03.txt>


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.  Internet-Drafts are
   working documents of the Internet Engineering Task Force (IETF),
   its areas, and its working groups.  Note that other groups may
   also distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents
   at any time.  It is inappropriate to use Internet-Drafts as
   reference material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html

   Distribution of this memo is unlimited.


Abstract

   When two hosts on the same link attempt to use the same IPv4 address
   at the same time (except in rare special cases where this has been
   arranged by prior coordination) problems ensue for one or both hosts.
   This document describes (i) a simple precaution that a host can take
   in advance to help prevent this misconfiguration from happening, and
   (ii) if this misconfiguration does occur, a simple mechanism by which
   a host can passively detect after-the-fact that it has happened, so
   that the host may respond to rectify the problem.

















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Table of Contents

   1.  Introduction....................................................2
   1.1 Conventions and Terminology Used in this Document...............4
   1.2 Relationship to RFC 826.........................................4
   1.3 Applicability...................................................6
   2.  Address Probing, Announcing, Conflict Detection and Defense.....6
   2.1 Probing an Address..............................................6
   2.2 Shorter Timeouts on Appropriate Network Technologies............7
   2.3 Announcing an Address...........................................8
   2.4 Ongoing Address Conflict Detection and Address Defense..........8
   2.5 Broadcast ARP Replies...........................................9
   3.  Security Considerations........................................10
   4.  Historical Note................................................11
   5.  IANA Considerations............................................11
   6.  Acknowledgements...............................................11
   7.  Copyright......................................................11
   8.  Normative References...........................................12
   9.  Informative References.........................................12
   10. Author's Address...............................................12


1. Introduction

   Historically, accidentally configuring two Internet hosts with the
   same IP address has often been an annoying and hard-to-diagnose
   problem.

   This is unfortunate, because the existing ARP protocol provides
   an easy way for a host to detect this kind of misconfiguration and
   report it to the user. The DHCP specification [RFC 2131] briefly
   mentions the role of ARP in detecting misconfiguration, as
   illustrated in the following three excerpts from RFC 2131:

   o  the client SHOULD probe the newly received address,
      e.g., with ARP.

   o  The client SHOULD perform a final check on the parameters
      (e.g., ARP for allocated network address)

   o  If the client detects that the address is already in use
      (e.g., through the use of ARP), the client MUST send
      a DHCPDECLINE message to the server

   Unfortunately, the DHCP specification does not give any guidance to
   implementers concerning the number of ARP packets to send, the
   interval between packets, the total time to wait before concluding
   that an address may safely be used, or indeed even which kinds of
   packets a host should be listening for, in order to make this
   determination. It leaves unspecified the action a host should take
   if, after concluding that an address may safely be used, it


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   subsequently discovers that it was wrong. It also fails to specify
   what precautions a DHCP client should take to guard against
   pathological failure cases, such as DHCP server that repeatedly
   OFFERs the same address, even though it has been DECLINEd multiple
   times.

   The authors of the DHCP specification may have thought the answers to
   these questions too obvious to mention, but unfortunately this leaves
   some of the burden of protocol design to each individual implementer.
   This document seeks to remedy this omission by clearly specifying the
   required actions for:

   1. Determining whether use of an address is likely to lead to an
      addressing conflict. This includes (a) the case where the address
      is already actively in use by another host on the same link, and
      (b) the case where two hosts are inadvertently about to begin
      using the same address, and both are simultaneously in the process
      of probing to determine whether the address may safely be used.

   2. Subsequent passive detection that another host on the network is
      inadvertently using the same address. Even if all hosts observe
      precautions to avoid using an address that is already in use,
      conflicts can still occur if two hosts are out of communication at
      the time of initial interface configuration. This could occur with
      wireless network interfaces if the hosts are temporarily out of
      range, or with Ethernet interfaces if the link between two
      Ethernet hubs is not functioning at the time of address
      configuration. A well-designed host will handle not only conflicts
      detected during interface configuration, but also conflicts
      detected later, for the entire duration of the time that the host
      is using the address.

   3. Rate-limiting of address acquisition attempts in the case of an
      excessive number of repeated conflicts.

   The utility of IPv4 Address Conflict Detection is not limited to DHCP
   clients. No matter how an address was configured, whether via manual
   entry by a human user, via information received from a DHCP server,
   or via any other source of configuration information, detecting
   conflicts is useful. Upon detecting a conflict, the configuring agent
   should be notified of the error. In the case where the configuring
   agent is a human user, that notification may take the form of an
   error message on a screen, an SNMP trap, or an error message sent via
   pager. In the case of a DHCP server, that notification takes the form
   of a DHCP DECLINE message sent to the server. In the case of
   configuration by some other kind of software, that notification takes
   the form of an error indication to the software in question, to
   inform it that the address it selected is in conflict with some other
   host on the network. The configuring software may choose to cease
   network operation, or it may automatically select a new address so
   that the host may re-establish IP connectivity as soon as possible.


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1.1. Conventions and Terminology Used in this Document

   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" [RFC 2119].

   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 [RFC
   826] 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 '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.

   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.


1.2 Relationship to RFC 826

   This document does not modify any of the protocol rules in RFC 826.
   It does not modify the packet format, or the meaning of any
   of the fields. The existing rules for "Packet Generation" and
   "Packet Reception" still apply exactly as specified in RFC 826.

   This document expands on RFC 826 by specifying some additional ARP
   packets that should be generated when an interface is configured, and
   an additional check that should be performed on each ARP packet that
   is received. This additional check does not impose a significant
   additional CPU burden on hosts, since every host implementing ARP
   is already required to process every received ARP packet according
   to the "Packet Reception" rules specified in RFC 826.









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   As specified in RFC 826, an ARP Request packet serves two functions,
   an assertion and a question:

   * Assertion:
     The fields "ar$sha" (Sender Hardware Address) and "ar$spa" (Sender
     Protocol Address) together serve as an assertion of a fact, that
     the stated Protocol Address is mapped to the stated Hardware
     Address.

   * Question:
     The fields "ar$tha" (Target Hardware Address, zero) and "ar$tpa"
     (Target Protocol Address) serve as a question, asking, for the
     stated Protocol Address, to which Hardware Address it is mapped.

   This document clarifies what it means to have one without the other.


1.2.1 ARP Probe

   This document standardizes the widely-used natural interpretation of
   an ARP Request where the Target Protocol Address is non-zero but the
   Sender Protocol Address is zero, namely that it is a question without
   an associated assertion (an "ARP Probe").


1.2.2 ARP Announcement

   This document standardizes the widely-used natural interpretation of
   an ARP Request where the Sender and Target Protocol Address fields
   contain the same address, namely that it is an assertion without an
   associated question (an "ARP Announcement").


1.2.3 Broadcast Replies

   In some applications of IPv4 Address Conflict Detection, it may be
   advantageous to deliver ARP replies using broadcast instead of
   unicast because this allows address conflicts to be detected sooner
   than might otherwise happen. This is not needed in most situations,
   but certain specialized uses of Address Conflict Detection such as
   "Dynamic Configuration of IPv4 Link-Local Addresses" [v4LL] can
   benefit from this optional capability.

   RFC 826 implies that replies to ARP requests are usually delivered
   using unicast, but it is also acceptable to deliver ARP replies using
   broadcast. The "Packet Reception" rules in RFC 826 specify that the
   content of the "ar$spa" field should be processed *before* examining
   the "ar$op" field, so any host that correctly implements the Packet
   Reception algorithm specified in RFC 826 will correctly handle ARP
   replies delivered via link-layer broadcast.



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1.3. Applicability

   The specifications in this document apply to any link-layer network
   technology that uses ARP [RFC 826] to map from IP addresses to
   link-layer hardware addresses.

   For the specifications in this document to be effective at detecting
   address conflicts, the other hosts on the link must correctly
   implement the ARP protocol as given in RFC 826. In particular, when
   a host receives an ARP Request where the Target Protocol Address of
   the ARP Request matches (one of) the host's IP address(es), then the
   host MUST reply with a properly formatted ARP reply.

   If a particular host fails to implement the ARP specification
   correctly, then that host is liable to be at greater risk of
   undetected address conflicts.


2. Address Probing, Announcing, Conflict Detection and Defense

   This section describes initial probing to safely determine whether
   an address is already in use, ongoing conflict checking, and optional
   use of broadcast ARP replies to provide faster conflict detection.


2.1 Probing an Address

   Before beginning to use an IP address (whether received from manual
   configuration, DHCP, or some other means), a host implementing this
   specification MUST test to see if the address is already in use,
   by broadcasting ARP Probe packets, as defined in Section 1.1.

   When ready to begin probing, the host should first wait for a random
   time interval selected uniformly in the range zero to two seconds,
   and should then send four probe packets, spaced two seconds apart.
   This initial random delay helps ensure that a large number of hosts
   powered on at the same time do not all send their initial probe
   packets simultaneously.

   If during this period, from the beginning of the probing process
   until two seconds after the last probe packet is sent, the host
   receives any ARP packet (request *or* reply) 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
   should indicate to the configuring agent (human operator, DHCP
   server, etc.) that the proposed address is not acceptable. 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
   any of the host's interfaces, then the host MUST similarly treat this
   as an address conflict and signal an error to the configuring agent


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   as above. This can occur if two (or more) hosts have, for whatever
   reason, been inadvertently configured with the same address, and both
   are simultaneously in the process of probing that address to see if
   it can safely be used.

   A host implementing this specification MUST maintain a counter of the
   number of conflicts it has experienced in the process of trying to
   configure an interface, and if the number of conflicts exceeds ten
   then the host MUST limit the rate at which it probes for new
   addresses to no more than one new address per minute. This is to
   prevent catastrophic ARP storms in pathological failure cases, such
   as a defective DHCP server that repeatedly assigns the same address
   to every host that asks for one.

   If, by two seconds after the transmission of the last ARP probe
   no conflicting ARP reply has been received, then the host has
   successfully determined that the desired address may be used safely.


2.2 Shorter Timeouts on Appropriate Network Technologies

   The time values specified above are intended for use on technologies
   such as Ethernet, where switches that implement Spanning Tree
   [802.1d] often silently discard all packets for several seconds. The
   time values specified above result in a delay of 8-10 seconds before
   a chosen IP address may be used. For a desktop machine using DHCP,
   this may not be a great problem, but for other types of device,
   particularly hand-held portable devices, a ten-second delay before
   networking services becomes available may not be acceptable. For this
   reason, shorter time values may be used on network technologies that
   allow the device to determine when the link has become active and can
   be reasonably trusted to deliver packets reliably. On these network
   technologies the recommended time values are: The host should first
   wait for a random time interval selected uniformly in the range 0-200
   milliseconds, and then send four probe packets, waiting 200
   milliseconds after each probe, making a total delay of 800-1000ms
   before a chosen IP address may be used.

   Should future versions of the IEEE Spanning Tree Protocol be enhanced
   to inform clients when the link is ready to begin forwarding packets,
   then the shorter time values may be used on these networks too.

   If the situation arises where some hosts on a network are using the
   optional shorter timeouts and others are not, this does not cause any
   problems. It simply means that some hosts will be able to configure
   their interfaces quicker than others. In the unlikely event that an
   address conflict is not detected during the short address probing
   phase, the conflict will still be detected by the Ongoing Address
   Conflict Detection described below in Section 2.4.




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2.3 Announcing an Address

   Having determined that a desired address may be used safely, a host
   implementing this specification MUST then announce that it is
   commencing to use this address by broadcasting two ARP announcements,
   spaced two seconds apart. This time interval is not modified by the
   shorter timeouts described above in Section 2.2. 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 IP 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. The host may begin legitimately using the IP
   address immediately after sending the first of the two ARP
   announcements, and the sending of the second ARP announcement may be
   completed asynchronously, concurrent with other networking operations
   the host may wish to perform.


2.4 Ongoing Address Conflict Detection and Address Defense

   Address conflict detection should not be limited to only the time of
   initial interface configuration, 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 address. At any time, if a host
   receives an ARP packet (request *or* reply) where the 'sender IP
   address' is (one of) the host's own IP address(es), but the 'sender
   hardware address' does not match any of the host's own interface
   addresses, then this is a conflicting ARP packet, indicating some
   other host also thinks it is validly using this address. To resolve
   the address conflict, a host must respond to a conflicting ARP packet
   as described in either (a), (b) or (c) below:

   (a) Upon receiving a conflicting ARP packet, a host MAY elect to
   immediately cease using the address, and signal an error to the
   configuring agent as described above, or

   (b) If a host currently has active TCP connections or other reasons
   to prefer to keep the same IP address, and it has not seen any other
   conflicting ARP packets recently (for Ethernet, within the last ten
   seconds) then it MAY elect to attempt to defend its address.
   To defend its address, the host first records the time that the
   conflicting ARP packet was received, and then broadcasts one single
   ARP announcement, giving its own IP and hardware addresses. Having
   done this, the host can then continue to use the address normally
   without any further special action. However, if this is not the first
   conflicting ARP packet the host has seen, and the time recorded for
   the previous conflicting ARP packet is recent (within ten seconds for
   Ethernet) then the host MUST immediately cease using this address and
   signal an error to the configuring agent 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.

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   (c) If a host has been configured such that it should not give up
   its address under any circumstances (perhaps because it is the kind
   of device that needs to have a well-known stable IP address, such
   as a link's default router, or a DNS server) then it MAY elect
   to defend its address indefinitely. If such a host receives a
   conflicting ARP packet, then it should take appropriate steps to
   log useful information such as source Ethernet address from the ARP
   packet, and inform an administrator of the problem. The number of
   such notifications should be appropriately controlled to prevent an
   excessive number of error reports being generated. If the host has
   not seen any other conflicting ARP packets recently (for Ethernet,
   within the last ten seconds) then it MUST record the time that the
   conflicting ARP packet was received, and then broadcast one single
   ARP announcement, giving its own IP and hardware addresses. Having
   done this, the host can then continue to use the address normally
   without any further special action. However, if this is not the first
   conflicting ARP packet the host has seen, and the time recorded for
   the previous conflicting ARP packet is recent (within ten seconds for
   Ethernet) then the host MUST NOT send another defensive ARP
   announcement. This is necessary to ensure that two misconfigured
   hosts do not get stuck in an endless loop flooding the network with
   broadcast traffic while they both try to defend the same address.

   A host wishing to provide reliable network operation must respond to
   conflicting ARP packets as described in (a), (b) or (c) above.
   Ignoring conflicting ARP packets results in seemingly random network
   failures which can be hard to diagnose and very frustrating for human
   users.

   Forced address reconfiguration may be disruptive, causing TCP
   connections to be broken. However, it is such disruptions should be
   exceedingly rare, and if inadvertent address duplication happens,
   then disruption of communication is inevitable. It is not possible
   for two different hosts using the same IP address on the same network
   to operate reliably. For most client machines that do not need a
   fixed IP address, immediately requesting the configuring agent (human
   user, DHCP client, etc.) to configure 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.


2.5 Broadcast ARP Replies

   In a carefully-run network with manually-assigned addresses, or
   a network with a reliable DHCP server and reliable DHCP clients,
   address conflicts should occur only in rare failure scenarios,
   so the passive monitoring described above in Section 2.4 is adequate.
   If two hosts are using the same IP address, then sooner or later one


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   or other host will broadcast an ARP request, which the other will
   see, allowing the conflict to be detected and consequently resolved.

   It is possible however, that a conflicting configuration may persist
   for a short time before it is detected. Suppose that two hosts A and
   B have been inadvertently assigned the same IP address X. Suppose
   further that at the time they were both probing to determine whether
   the address could safely be used, the communication link between them
   was non-functional for some reason, so neither detected the conflict
   at interface-configuration time. Suppose now that the communication
   link is restored, and a third host C broadcasts an ARP request for
   address X. Unaware of any conflict, both hosts A and B will send
   unicast ARP replies to host C. Host C will see both replies, and may
   be a little confused, but neither host A nor B will see the other's
   reply, and neither will immediately detect that there is a conflict
   to be resolved. Hosts A and B will continue to be unaware of the
   conflict until one or other broadcasts an ARP request of their own.

   If quicker conflict detection is desired, this MAY be achieved by
   having hosts send ARP replies using link-level broadcast, instead
   of sending only ARP requests via broadcast, and replies via unicast.
   This is not recommended for general use, but "Dynamic Configuration
   of IPv4 Link-Local Addresses" [v4LL] makes use of this optional
   capability, since in that case, detection of address conflicts using
   ARP is not merely a backup precaution to detect failures of some
   other configuration mechanism; detection of address conflicts using
   ARP is the sole configuration mechanism.

   Sending both requests and replies via broadcast potentially doubles
   the ARP traffic load on each host on the network. On many networks,
   ARP traffic is such an insignificant proportion of the total traffic
   that doubling it makes no practical difference. However, this may not
   be true of all networks, so broadcast ARP replies should not be used
   universally. Broadcast ARP replies should be used where the benefit
   of faster conflict detection outweighs the cost of slightly increased
   packet processing load on the participant network hosts.


3. Security Considerations

   The ARP protocol [RFC 826] 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.

   This specification makes this existing ARP vulnerability no worse,
   and in some ways makes it better: Instead of failing silently with no
   indication why, hosts implementing this specification are required to
   either attempt to reconfigure automatically, or if not that, at least
   inform the human user of what is happening.


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4. Historical Note

   A widely-known, but ineffective, duplicate address detection
   technique called "gratuitous ARP" is found in many deployed systems
   [Ste94].  This traditional gratuitous ARP implementation sends only
   an ARP Announcement per Section 2.3 of this document when an
   interface is first configured.  The result is that the victim (the
   existing address holder) logs an error, and the offender continues
   operation, often without even detecting the problem.  The
   administrator is expected to note the log message on the victim and
   repair the damage after the fact.

   Implementers of this specification should be aware that this flaw is,
   as of this writing, very widely deployed, and should take steps as
   described in Sections 2.1 and 2.4 to mitigate the problem.


5. IANA Considerations

   This document has no IANA-related considerations.


6. Acknowledgements

   This document arose as a result of discussions on link-local
   addressing, where it was not clear to many readers which elements of
   link-local address management were specific to that particular
   problem, and which elements were generic and applicable to all IPv4
   address configuration mechanisms. The following people made valuable
   comments in the course of that work and/or the subsequent editing
   of this document: Bernard Aboba, Randy Bush, Jim Busse, James
   Carlson, Pavani Diwanji, Ralph Droms, Donald Eastlake 3rd, Peter
   Ford, Spencer Giacalone, Josh Graessley, Erik Guttman, Myron Hattig,
   Hugh Holbrook, Richard Johnson, Kim Yong-Woon, Rod Lopez, Satish
   Mundra, Thomas Narten, Erik Nordmark, Howard Ridenour, Pekka Savola,
   Daniel Senie, Dieter Siegmund, Valery Smyslov and Ryan Troll.


7. Copyright

   Copyright (C) The Internet Society 2001-2002.
   All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other


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   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

8. Normative References

   [RFC 826]  D. Plummer, "An Ethernet Address Resolution Protocol -or-
              Converting Network Addresses to 48-bit Ethernet Address
              for Transmission on Ethernet Hardware", STD 37, RFC 826,
              November 1982.

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

9. Informative References

   [802.1d]   ISO/IEC 10038, ANSI/IEEE Std 802.1D-1993 "MAC Bridges".

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

   [Ste94]    W. Stevens, "TCP/IP Illustrated, Volume 1: The Protocols",
              Addison-Wesley, 1994.

   [v4LL]     S. Cheshire, B. Aboba, E. Guttman,
              "Dynamic Configuration of IPv4 Link-Local Addresses",
              draft-ietf-zeroconf-ipv4-linklocal-07.txt,
              Work in Progress, August 2002.

10. Author's Address

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

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

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