Stuart Cheshire Document: draft-cheshire-ipv4-acd-02.txt Apple Computer Expires 23rd February 2003 23rd August 2002 IPv4 Address Conflict Detection <draft-cheshire-ipv4-acd-02.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. 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) Expires 23rd February 2003 Cheshire [Page 1]
Internet Draft IPv4 Address Conflict Detection 23rd August 2002 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 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 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 Expires 23rd February 2003 Cheshire [Page 2]
Internet Draft IPv4 Address Conflict Detection 23rd August 2002 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. 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 '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. Expires 23rd February 2003 Cheshire [Page 3]
Internet Draft IPv4 Address Conflict Detection 23rd August 2002 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. 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. Expires 23rd February 2003 Cheshire [Page 4]
Internet Draft IPv4 Address Conflict Detection 23rd August 2002 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, using ARP probes. A host probes to see if an address is already in use by broadcasting an ARP request for the desired address. The client MUST fill in the 'sender hardware address' field of the ARP request with the hardware address of the interface through which it is sending the packet. The 'sender IP address' field MUST be set to all zeroes, to avoid polluting ARP caches in other hosts on the same link in the case where the address turns out to be already in use by another host. The 'target hardware address' field is ignored and SHOULD be set to all zeroes. The 'target IP address' field MUST be set to the address being probed. An ARP request constructed this way with an all-zero 'sender IP address' is referred to as an "ARP probe". When ready to begin probing, the host should then wait for a random time interval selected uniformly in the range zero to 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 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 Expires 23rd February 2003 Cheshire [Page 5]
Internet Draft IPv4 Address Conflict Detection 23rd August 2002 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 portable hand-held wireless 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. 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. Expires 23rd February 2003 Cheshire [Page 6]
Internet Draft IPv4 Address Conflict Detection 23rd August 2002 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 the host's own IP address, 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. (c) If a host has been configured such that it should not give up its address under any circumstances, perhaps because it is an important company server with a well-known IP address, 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 the 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 Expires 23rd February 2003 Cheshire [Page 7]
Internet Draft IPv4 Address Conflict Detection 23rd August 2002 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 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 Expires 23rd February 2003 Cheshire [Page 8]
Internet Draft IPv4 Address Conflict Detection 23rd August 2002 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 responses 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. 4. IANA Considerations This document has no IANA-related considerations. 5. 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: Bernard Aboba, Jim Busse, Pavani Diwanji, 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, Daniel Senie, Dieter Siegmund, Valery Smyslov and Ryan Troll. 6. Copyright Copyright (C) The Internet Society 23rd August 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 Expires 23rd February 2003 Cheshire [Page 9]
Internet Draft IPv4 Address Conflict Detection 23rd August 2002 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 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. 7. References [802.1d] ISO/IEC 10038, ANSI/IEEE Std 802.1D-1993 "MAC Bridges". [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. [RFC 2131] R. Droms, "Dynamic Host Configuration Protocol", RFC 2131, March 1997. [v4LL] S. Cheshire, B. Aboba, E. Guttman, "Dynamic Configuration of IPv4 Link-Local Addresses", draft-ietf-zeroconf-ipv4-linklocal-06.txt, Work in Progress, August 2002. 8. Author's Address Stuart Cheshire Apple Computer, Inc. 1 Infinite Loop Cupertino California 95014 USA Phone: +1 408 974 3207 EMail: rfc@stuartcheshire.org Expires 23rd February 2003 Cheshire [Page 10]
