Stuart Cheshire Document: draft-ietf-zeroconf-ipv4-linklocal-02.txt Apple Computer Expires 1st September 2001 Bernard Aboba Microsoft 1st March 2001 Dynamic Configuration of IPv4 Link-Local Addresses 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 As the Internet Protocol continues to increase in popularity as a global communication system, it becomes increasingly valuable to be able to use familiar IP tools such as ftp for local communication as well. For example, two people with laptop computers with built-in wireless Ethernet may meet and wish to exchange files. It is desirable for these people to be able to use IP application software without the inconvenience of having to manually configure static IP addresses or set up a DHCP server [RFC 2131]. This document describes a method by which a host may automatically configure an interface with an IPv4 address in the 169.254/16 range that is valid for link-local communication on that interface. This is especially valuable in environments where no other configuration mechanism is available. Microsoft Windows 98 (and later) and Mac OS 8.5 (and later) already implement versions of this functionality. This document standardizes this protocol and simplifies it in one important way -- in previous implementations, link-local addresses were only available after a host had tried and failed to contact a DHCP server. This standard removes that restriction, making link-local addresses available all the time, independent of DHCP. Expires 1st September 2001 Cheshire & Aboba [Page 1] Internet Draft IPv4 Link-Local Addresses 1st March 2001 1. Introduction As the Internet Protocol continues to increase in popularity as a global communication system, it becomes increasingly valuable to be able to use familiar IP tools such as ftp for local communication as well. For example, two people with laptop computers with built-in wireless Ethernet may meet and wish to transfer files. It is desirable for these people to be able to use IP application software without the inconvenience of having to manually configure static IP addresses or set up a DHCP server [RFC 2131]. This document describes a method by which a host may automatically configure an interface with an IPv4 address in the 169.254/16 range that is valid for link-local communication on that interface. This is especially valuable in environments where no other configuration mechanism is available. Allocation of link-local IPv6 addresses is described in [RFC 2462]. Microsoft Windows 98 (and later) and Mac OS 8.5 (and later) already implement versions of this functionality. This document standardizes this protocol and simplifies it in one important way -- in previous implementations, link-local addresses were only available after a host had tried and failed to contact a DHCP server. This standard removes that restriction, making link-local addresses available all the time, independent of DHCP. Extremely simple devices may implement only IPv4 link-local addresses, without also being required to implement a DHCP client. Hosts and routers using addresses in the 169.254/16 range MUST follow the rules laid out in this document. This document will discuss claiming and defending addresses, maintaining link-local and routable addresses on the same interface, and multihoming issues. Note that addresses in the 169.254/16 range SHOULD NOT be configured manually or by a DHCP server, since doing so may cause a host to use an address in the 169.254/16 range without awareness of the special rules regarding duplicate detection and automatic configuration that pertain to addresses in this range. Administrators wishing to configure their own local addresses (using manual configuration, a DHCP server, or any other mechanism not described in this document) should use one of the existing private address ranges [RFC 1918], not the 169.254/16 range. Expires 1st September 2001 Cheshire & Aboba [Page 2] Internet Draft IPv4 Link-Local Addresses 1st March 2001 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 [RFC 2119]. This document uses the term "routable address" to refer to any unicast address outside the 169.254/16 range, including global addresses and private addresses such as Net 10/8 [RFC 1918]. Wherever this document uses the term "host" when describing use of link-local addresses, the text applies equally to routers using link-local addresses on any or all interfaces. Wherever this document uses the term "ARP packet" or "ARP packets", the text should be taken to apply uniformly to ARP request and reply packets. Anywhere that the text does not apply equally to both kinds of ARP packet, this document explicitly states either "ARP request" or "ARP reply". 1.2. Table of Contents 1. Introduction..................................................2 1.1 Conventions and Terminology Used in this Document.............3 1.2 Table of Contents.............................................3 1.3 Issues with Autoconfiguration.................................4 1.4 Supporting Multiple Addresses per Interface...................4 1.5 Supporting Multiple Interfaces................................4 2. IPv4 Link-Local Address Selection, Defense and Delivery.......5 2.1 Selecting Link-Local Addresses................................5 2.2 Claiming Link-Local Addresses.................................5 2.3 Address Collision Detection and Address Defense...............7 2.4 Source Address Selection......................................8 2.5 Link-Local Addresses Are Not Forwarded........................8 3. Considerations for Multiple Interfaces........................9 3.1 Example Illustrating Ambiguous Addressing....................11 3.2 Example Illustrating Acceptable Address Re-Use...............12 4. Considerations for Joining of Previously Separate Networks...12 5. Security Considerations......................................13 6. Acknowledgements.............................................14 7. Copyright....................................................14 8. References...................................................14 9. Authors' Addresses...........................................15 Appendix. Prior Implementations..................................16 Expires 1st September 2001 Cheshire & Aboba [Page 3] Internet Draft IPv4 Link-Local Addresses 1st March 2001 1.3. Issues with Autoconfiguration Implementations of IPv4 link-local address autoconfiguration MUST expect address collisions, and MUST be prepared to handle them grace- fully by automatically selecting a new address whenever a collision is detected, as described in Section 2. This requirement to detect and handle address collisions applies during the entire period that a host is using a 169.254/16 link-local address, not just during some 'startup phase' of initial interface configuration. For example, address collisions can occur well after a host has completed booting if two previously separate networks are joined, as described in Section 4. 1.4. Supporting Multiple Addresses per Interface IPv4 link-local addresses are independent from any other IPv4 addresses that a host may have. Each interface on a host may have a link-local address in addition to zero or more other addresses configured by other means (e.g. manually or via a DHCP server). There are several reasons why it is beneficial for a host to maintain link-local addresses in addition to any other addresses it may have. For example, a DHCP server may appear on a network where hosts are already communicating using link-local addresses, and it is beneficial for those already-established link-local TCP connections to continue working even after the hosts have configured additional routable addresses assigned by the DHCP server. Another example is that there may be networks where not all of the hosts have externally configured addresses. For example, a user with a wireless home network may have a laptop computer and an IP printer. The laptop computer may have both a self-configured link-local address and a DHCP-configured global address. The printer, in contrast, may have only a link-local address, because the user does not want the printer to be globally addressable. In this case, the laptop computer would access pages on the World Wide Web using its globally-routable address to communicate with servers world-wide, but print those web pages using its link-local address to communicate with its local printer. 1.5. Supporting Multiple Interfaces Hosts that support multi-homing have additional considerations if they wish to use IPv4 link-local addresses on more than one interface at a time. These are discussed in Section 3. Expires 1st September 2001 Cheshire & Aboba [Page 4] Internet Draft IPv4 Link-Local Addresses 1st March 2001 2. IPv4 Link-Local Address Selection, Defense and Delivery The following section explains the link-local address selection algorithm, how these addresses are defended and how IPv4 packets with link-local addresses are delivered. Windows 98 and Mac OS 8.5 hosts that already implement IPv4 address autoconfiguration are compatible with the rules presented in this section. However, should any interoperability problem be discovered, this document, not any prior implementation, defines the standard. 2.1 Selecting Link-Local Addresses When a host wishes to configure a link-local address, it selects an address using a random (or pseudo-random) number generator with a uniform distribution in the range from 169.254.1.0 to 169.254.254.255. The IPv4 network 169.254/16 is registered with the IANA for this purpose. The first 256 and last 256 addresses in the 169.254/16 network are reserved for future use and MUST NOT be selected by a host using this dynamic configuration mechanism. The pseudo-random number generation algorithm should be chosen so that different hosts do not generate the same sequence of numbers. Recommendations for pseudo-random number generators can be found in "Randomness Recommendations for Security" [RFC 1750]. If the host has access to persistent information that is different for each host, such as it's burned-in Ethernet hardware address, then the pseudo-random number generator SHOULD be seeded using a value derived from this information. This means that even without using any other persistent storage, a host will usually select the same link-local address each time it is booted, which can be convenient for debugging and other operational reasons. Seeding the pseudo-random number generator using the real-time clock is NOT suitable for this purpose, since a group of hosts that are all powered on at the same time might then all generate the same sequence. 2.2 Claiming Link-Local Addresses After it has selected an address, a host MUST test to see if the address is already in use before beginning to use it. On a network such as Ethernet that supports ARP, this test is done using ARP [RFC 826] probes. On link-layer network technologies that do not support ARP there may be equivalent mechanisms for determining whether a particular IP address is currently in use, but these kinds of networks are not discussed in this document. 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 its own interface hardware address as the sender's hardware address. The sender's IP address MUST be set to all zeroes, to avoid polluting ARP Expires 1st September 2001 Cheshire & Aboba [Page 5] Internet Draft IPv4 Link-Local Addresses 1st March 2001 caches in other hosts on the same link in the case where the address turns out to already be in use by another host. The target hardware address is ignored and SHOULD be set to all zeroes. The target IP address MUST be set to the address being probed. An ARP request constructed this way with an all-zero sender address is referred to as an "ARP probe". The appropriate number of ARP probes and the interval between them is implementation-dependent. For Ethernet at 10Mb/s, 100Mb/s or 1Gb/s (henceforth referred to simply as 'Ethernet'), four probes with a two-second wait after each probe is recommended (making a total of eight seconds). If during this period the host receives any ARP packet where the packet's 'sender IP address' is the address being probed for, then the host MUST treat this address as being in use by some other host, and MUST select a new pseudo-random address and repeat the process. In addition, if during this period the host receives any ARP packet where the packet's 'target IP address' is the address being probed for, and the packet's 'sender hardware address' is not the hardware address of the interface the host is configuring, then the host MUST similarly treat this as an address collision and select a new address as above. This can occur if two (or more) hosts by chance attempt to configure the same link-local address at the same time. A host should maintain a counter of the number of address collisions it has experienced in the process of trying to acquire an address, and if the number of collisions exceeds ten (on Ethernet) then the host MUST limit the rate at which it probes for new addresses to (on Ethernet) no more than one new address per second. This is to prevent catastrophic ARP storms in pathological failure cases, such as a rogue host that answers all ARP probes, causing legitimate hosts to go into an infinite loop attempting to select a usable address. After successfully configuring a link-local address, a host on Ethernet SHOULD broadcast two gratuitous ARPs, spaced two seconds apart. A gratuitous ARP 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 gratuitous ARPs 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. Hosts that are equipped with persistent storage MAY, for each interface, record the IP address they have selected, and on the next boot should use that address as their first candidate when probing. This increases the stability of addresses. For example, if a group of hosts are powered off at night, then when they are powered on the next morning they will all resume using the same addresses, instead of picking different addresses and potentially having to resolve conflicts that arise. Expires 1st September 2001 Cheshire & Aboba [Page 6] Internet Draft IPv4 Link-Local Addresses 1st March 2001 2.3 Address Collision Detection and Address Defense Address collision detection is not limited to the address selection phase, when a host is sending ARP probes. Address collision detection is an ongoing process that is in effect for as long as a host is using a link-local IPv4 address. At any time, if a host receives an ARP packet where the 'sender IP address' is the host's own IP address, but the 'sender hardware address' that does not match any of the host's own interface addresses, then this is a conflicting ARP packet, indicating an address collision. A host MUST respond to a conflicting ARP packet as described in either (a) or (b) below: (a) Upon receiving a conflicting ARP packet, a host MAY elect to immediately configure a new link-local IP address 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, by recording the time that the conflicting ARP packet was received, and then broadcasting one single gratuitous ARP request, giving its own IP and hardware addresses as the sender addresses of the ARP. Having done this, the host can then continue to use the address normally without any further special action. However, if this is not the first 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 configure a new link-local IP address as described above. This is necessary to ensure that two hosts do not get stuck in an endless loop both trying to defend the same address. A host MUST respond to conflicting ARP packets as described in either (a) or (b) above. A host MUST NOT ignore conflicting ARP packets. Forced address reconfiguration may be disruptive, causing TCP connections to be broken. However, it is expected that such disruptions will be rare, and if an inadvertent address duplication happens, then disruption of communication is inevitable, no matter how the addresses were assigned. It is not possible for two different hosts using the same IP address on the same network to operate reliably. Immediately configuring a new address as soon as the conflict is detected is the best way to restore useful communication as quickly as possible. The mechanism described above of broadcasting a single gratuitous ARP to defend the address mitigates the problem somewhat, by helping improve the chance that one of the two conflicting hosts may be able to retain its address. Expires 1st September 2001 Cheshire & Aboba [Page 7] Internet Draft IPv4 Link-Local Addresses 1st March 2001 All ARP packets (replies as well as requests) that contain a link- local sender address SHOULD be sent using link-level broadcast instead of link-level unicast. This aids timely detection of duplicate addresses. An example illustrating how this helps is given in Section 4. 2.4 Source Address Selection Since each interface on a host may have a link-local address in addition to zero or more other addresses configured by other means (e.g. manually or via a DHCP server), a host may have to make a choice about what source address to use when it sends a packet or initiates a TCP connection. If the destination address is in the 169.254/16 range, the host SHOULD use its link-local source address. If the destination address is a multicast address with link-local scope the host MAY use its link-local source address. If the destination address is a unicast address outside the 169.254/16 range, or a multicast address with scope larger than link- local, the host SHOULD NOT use its link-local source address, and should instead use its appropriate routable interface address. In the case where two hosts on the same link each have both a link- local address and another address configured via some other means, it is usually preferable to use the configured addresses when establishing new communications. Those addresses are more likely to remain stable than link-local addresses, which may change over time, as described in Section 2. 2.5 Link-Local Addresses Are Not Forwarded An IPv4 datagram whose source and/or destination addresses is in the 169.254/16 range MUST NOT be sent to any router for forwarding, and any network device receiving such a datagram MUST NOT forward it, regardless of the TTL in the IP header. This restriction also applies to multicast packets. IP datagrams with a link-local source address MUST NOT be forwarded off the local link even if they have a multicast destination address. Similar considerations apply at layers above IP. For example, DNS Resource Records containing link-local address SHOULD NOT be sent to hosts outside the link to which those link-local address apply. Automatically generated web pages SHOULD NOT contain links with embedded link-local addresses if those pages are viewable from hosts outside the local link where the addresses are valid. Since DNS treats Resource Record Sets [RFC 2181] as indivisible units Expires 1st September 2001 Cheshire & Aboba [Page 8] Internet Draft IPv4 Link-Local Addresses 1st March 2001 (e.g. for generating DNS reply packets, signatures, etc.) this means that RRSets SHOULD only contain A records where all the addresses have the same scope. Link-local and routable addresses SHOULD NOT be mixed in a single set. The non-forwarding rule means that hosts may assume that all 169.254/ 16 destination addresses are on-link and directly reachable. The 169.254/16 address range MUST NOT be subnetted. This specification utilizes ARP-based address collision detection, which functions by broadcasting on the local subnet. Since such broadcasts are not forwarded, were subnetting to be allowed then address conflicts could remain undetected. The non-forwarding rule is important because it is expected that many link-local-only devices will be extremely simple devices of the kind that currently use X10 [X10], USB [USB] or FireWire [IEEE 1394]. The designers of these devices assume that they will communicate only with other local devices, and implement a degree of security appropriate for that expected environment. (For example, a typical USB mouse does not have a password, nor does it encrypt its mouse- movement data, and in most environments this is acceptable.) Any network gateway device that blindly forwards the contents of link-local IP packets off the local network (or vice-versa) exposes these link-local-only devices to a much greater degree of risk than their designers may have planned for. This does not mean that link-local devices are forbidden from communication outside the local link. IP hosts that implement both link-local and conventional routable IP addresses may still use their routable addresses without restriction as they do today. Extremely simple IP devices that implement only link-local addresses may also communicate with hosts outside the local link, provided that such communication is mediated through a device capable of enforcing appropriate security controls. For example, a home heating system could be controlled via a Web server on the local network, where the Web server uses cryptographic methods to verify the authority of the remote user, and then uses link-local communication on the local network to communicate commands to the heating system's thermostat. Alternatively, a remote host could use a secure tunnelling protocol to establish access to a 'virtual interface' on the local link, via which it could then send and receive 'virtual' link-local packets just like any other host directly connected to that link. It should be understood that this mediated communication is not mandatory; it is an option afforded to designers of very simple devices who wish to implement only link-local addresses and thereby simplify their security assumptions. Any designer of a device desiring unmediated communication outside the local link need only implement today's conventional IP host software (e.g. a DHCP client) in order to enjoy the same degree of global addressability available to other conventional IP hosts on the same network. Expires 1st September 2001 Cheshire & Aboba [Page 9] Internet Draft IPv4 Link-Local Addresses 1st March 2001 3. Considerations for Multiple Interfaces A multi-homed host may elect to configure an IPv4 link-local address on only one of its active interfaces. In many situations this will be adequate. However, should a host wish to configure IPv4 link-local addresses on more than one of its active interfaces, there are some additional precautions it should take. Implementers who are not planning to support IPv4 link-local addresses simultaneously on multiple interfaces may skip this section. This section does not specify protocol requirements, but offers implementation advice which may aid deployment of link-local addresses on today's operating systems without requiring changes to application programming interfaces. A multi-homed host should ensure that all of its interfaces are configured with different link-local addresses. If the selection algorithm chooses an address that is already in use on one of the host's other interfaces, then the process should be repeated until a unique address is selected. A multi-homed host should also probe for, and defend, all of its link-local addresses on all of its active interfaces that are using link-local addresses. When bringing up a new interface, the host should first probe for all of its existing link-local addresses on that new interface. If any of the addresses are found to be in use already on the new link, then the interfaces in question must be reconfigured to select new unique link-local addresses. The host should then select a link-local address for the new interface, and probe on all of its active interfaces to verify that the address is unique. This uniqueness requirement exists to simplify host application software, which typically identifies connections using source and destination IP addresses, not interface names. Since link-local addresses are only unique per-link, hosts on different links could be using the same link-local address. By requiring uniqueness of source addresses on the multi-homed host, this ensures that TCP connections to hosts using the same link-local destination addresses on different links can be disambiguated by their different source addresses. Note that this also requires that the multi-homed host using link-local addresses on multiple interfaces MUST implement the "strong end system" model [RFC 1122] for packets going to link-local destination addresses, so that packets are only sent out from the interface that matches the source address in the packet. (The "weak end system" model may still be used for packets to other destinations.) When a multi-homed host receives an ARP packet on a particular interface with a sender IP address equal to one of the host's addresses, if the sender hardware address matches the hardware Expires 1st September 2001 Cheshire & Aboba [Page 10] Internet Draft IPv4 Link-Local Addresses 1st March 2001 address of *any* of the host's active interfaces it should be silently discarded and not considered a collision. This is because a user of a multi-homed host with two Ethernet interfaces may connect both interfaces to the same Ethernet hub, in which case the two interfaces will see each other's packets, and if it did not check and realize that the apparently conflicting ARP packets were coming from itself the host could erroneously conclude that all its addresses were in conflict. Another common example is a host with both wired and wireless Ethernet interfaces, in an environment where a wireless gateway is available, but (perhaps unknown to the user) is bridged onto the same wired Ethernet. 3.1 Example Illustrating Ambiguous Addressing Figure 1 shows a network topology where host A has an interface on link X with link-local address P, and another interface on link Y with link-local address Q. If we allowed there to be another host, B, on link X that also has address Q, then although there are no conflicts strictly within the scope of either link, when host A sends a UDP packet from source address P to destination address Q, it is ambiguous whether A intends to talk to itself, or to host B. By ensuring that all of a host's link-local addresses are distinct not only from each other, but also from all addresses currently active on all links that the host is connected to, we remove this ambiguity. | | | P ----- Q | |-----| A |-----| | ----- | | | | | X| |Y | | | | ----- | | | B |-----| | ----- Q | | | | Figure 1. Ambiguous Addressing Expires 1st September 2001 Cheshire & Aboba [Page 11] Internet Draft IPv4 Link-Local Addresses 1st March 2001 3.2 Example Illustrating Acceptable Address Re-Use Note that it is acceptable for different hosts on separate links to be using the same link-local address on their respective separate links. Figure 2 shows a network topology where host C on link X is using address R, while at the same time, host D on link Y is also using address R. This is entirely in keeping with the concept of link-local addresses. Link-local addresses are only unique amongst the member hosts of a single link. Hosts C and D are not on the same link, hence there is no requirement for them to have distinct addresses. Note that in this case, host A is still able to communicate with both hosts C and D, because a packet sent from source address P to destination address R travels on link X to host C, and a packet sent from source address Q to destination address R travels on link Y to host D. TCP connections are uniquely identified by the source and destination addresses and port numbers, not just by the destination address and port number alone. To support link-local addressing on multiple interfaces simultaneously, the network software API must allow applications to bind endpoints to a desired source address as well as specifying the desired destination address for a TCP connection. Networking implementations that only allow the destination address to be specified should limit themselves to configuring only one interface for link-local addressing. | | | P ----- Q | |-----| A |-----| | ----- | | | X| |Y | | ----- | | ----- | C |-----| |-----| D | ----- R | | R ----- | | Figure 2. Acceptable Address Re-Use 4. Considerations for Joining of Previously Separate Networks Hosts on disjoint network links may unknowingly configure the same link-local addresses. If these separate network links are later joined or bridged together, then there may be two hosts which are now on the same link, trying to use the same address. When either host attempts to communicate with any other host on the network, it will at some point broadcast an ARP packet which will enable the hosts in question to detect that there is a duplicate address. Expires 1st September 2001 Cheshire & Aboba [Page 12] Internet Draft IPv4 Link-Local Addresses 1st March 2001 When these address conflicts are detected, the subsequent forced reconfiguration may be disruptive, causing TCP connections to be broken. However, it is expected that such disruptions will be rare. It should be relatively uncommon for networks to be joined while hosts on those networks are active. Also, 65024 addresses are available for link-local use, so even when two small networks are joined, the chance of collision for any given host is fairly small. When joining two large networks there is a greater chance of collision, but large networks are not expected to rely heavily on link-local addresses for normal communication. Large networks are better managed by using existing mechanisms such as DHCP servers to allocate addresses. Sending ARP replies that have link-local sender addresses via broadcast instead of unicast ensures that these conflicts can be detected as soon as they become potential problems, but no sooner. For example, if two disjoint network links are joined, where hosts A and B have both configured the same address, X, they can remain in this state until A, B or some other host attempts to initiate communication. If some other host C now sends an ARP request for address X, and hosts A and B were to both reply with conventional unicast ARP replies, then host C might be confused, but A and B still wouldn't know there is a problem because neither would have seen the other's packet. Sending these replies via broadcast allows A and B see each other's conflicting ARP packets and respond accordingly. Note that sending periodic gratuitous ARPs in an attempt to detect these conflicts sooner is not necessary, wastes network bandwidth, and may actually be detrimental. For example, if the network links were joined only briefly, and were separated again before any new communication involving A or B were initiated, then the temporary conflict would have been benign and no forced reconfiguration would have been required. Triggering an unnecessary forced reconfiguration in this case would not serve any useful purpose. Hosts SHOULD NOT send periodic gratuitous ARPs. 5. Security Considerations The use of this functionality may open a network host to new attacks. In particular, a host that previously did not have an IP address, and no IP stack running, was not susceptible to IP-based attacks. By configuring a working address, the host may now be vulnerable to IP-based attacks. The ARP protocol [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. Expires 1st September 2001 Cheshire & Aboba [Page 13] Internet Draft IPv4 Link-Local Addresses 1st March 2001 6. Acknowledgements We'd like to thank (in alphabetical order) Donald Eastlake 3rd, Peter Ford, Erik Guttman, Myron Hattig, Hugh Holbrook, Richard Johnson, Satish Mundra, Thomas Narten, Daniel Senie, Valery Smyslov and Ryan Troll for their contributions. 7. Copyright Copyright (C) The Internet Society 8th March 2000. 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 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. References [IEEE 1394]IEEE Std 1394-1995, Standard for a High Performance Serial Bus [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. Expires 1st September 2001 Cheshire & Aboba [Page 14] Internet Draft IPv4 Link-Local Addresses 1st March 2001 [RFC 1122] R. Braden, "Requirements for Internet Hosts -- Communication Layers", RFC 1122, October 1989. [RFC 1750] D. Eastlake 3rd, S. Crocker and J. Schiller, "Randomness Recommendations for Security", RFC 1750, December 1994. [RFC 1918] Y. Rekhter et.al., "Address Allocation for Private Internets", RFC 1918, February 1996. [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. [RFC 2181] R. Elz and R. Bush, "Clarifications to the DNS Specification", RFC 2181, July 1997. [RFC 2462] S. Thomson and T. Narten, "IPv6 Stateless Address Autoconfiguration", RFC 2462, December 1998. [USB] Universal Serial Bus Implementers Forum [X10] X10 Ltd. 9. Authors' Addresses Stuart Cheshire Apple Computer, Inc. 1 Infinite Loop Cupertino California 95014 USA Phone: +1 408 974 3207 EMail: rfc@stuartcheshire.org Bernard Aboba Microsoft Corporation One Microsoft Way Redmond, WA 98052 USA Phone: +1 425 936 6605 Fax: +1 425 936 7329 EMail: bernarda@microsoft.com Expires 1st September 2001 Cheshire & Aboba [Page 15] Internet Draft IPv4 Link-Local Addresses 1st March 2001 Appendix. Prior Implementations This standard builds upon experience gained implementing link-local addresses in Windows 98 and Mac OS 8.5. These implementations had a number of drawbacks. These operating systems did not support more than one active IPv4 address at a time. For this reason, these implementations would only configure a link-local address after attempting to contact a DHCP server and failing. This meant link-local addresses were only available on systems configured to use DHCP, and on those only if DHCP failed. Because they only configure a link-local address after DHCP has failed, on isolated networks, boot time is delayed by the time it takes the DHCP client to time-out, which was 24 seconds for Windows and 15 seconds for Mac OS. Having failed to contact a DHCP server, these implementations retry once every five minutes. If the failure to contact a DHCP server was a transient problem such as a loose Ethernet cable, after correcting the error, it could take up to five minutes for the system to reconfigure with a server-assigned address. Because these operating systems did not support more than one active IPv4 address at a time, when they reconfigured with a server-assigned address, the self-configured link-local address would be lost. Any open TCP connections would be broken without warning, possibly causing user data loss, and the ability to communicate with simple devices that implement only link-local addressing would be lost. Expires 1st September 2001 Cheshire & Aboba [Page 16]