One document matched: draft-ietf-zeroconf-ipv4-linklocal-09.txt
Differences from draft-ietf-zeroconf-ipv4-linklocal-08.txt
ZEROCONF Working Group Stuart Cheshire
INTERNET-DRAFT Apple Computer
Category: Standards Track Bernard Aboba
<draft-ietf-zeroconf-ipv4-linklocal-09.txt> Microsoft Corporation
2 September 2003 Erik Guttman
Sun Microsystems
Dynamic Configuration of Link-Local IPv4 Addresses
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC 2026.
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.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
To participate in wide-area IP networking, a host needs to be
configured, either manually by the user or automatically from a
source on the network such as a DHCP server. Unfortunately, such
external configuration information may not always be available. It
is therefore beneficial for a host to be able to depend on a useful
subset of IP networking functions even when no configuration is
available. This document describes how a host may automatically
configure an interface with an IPv4 address within the 169.254/16
prefix that is valid for communication with other devices connected
to the same physical (or logical) link. Communication using Link-
Local IPv4 addresses is not suitable for communication with devices
not directly connected to the same physical (or logical) link.
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Table of Contents
1. Introduction.............................................. 3
1.1 Requirements ....................................... 3
1.2 Terminology ........................................ 3
1.3 Applicability....................................... 4
1.4 Application Layer Protocol Considerations........... 5
1.5 Autoconfiguration Issues ........................... 6
1.6 Alternate Use Prohibition .......................... 6
1.7 Multiple Addresses per Interface.................... 7
1.8 Multiple Interfaces................................. 7
1.9 Communication with Routable Addresses............... 7
2. Address Selection, Defense and Delivery................... 8
2.1 Link-Local Address Selection........................ 8
2.2 Claiming a Link-Local Address....................... 9
2.3 Shorter Timeouts ................................... 10
2.4 Announcing an Address............................... 11
2.5 Conflict Detection and Defense...................... 11
2.6 Address Usage and Forwarding Rules.................. 12
2.7 Link-Local Packets Are Not Forwarded................ 13
2.8 Link-Local Packets are Local........................ 14
2.9 Higher-Layer Protocol Considerations................ 15
2.10 Privacy Concerns.................................... 15
3. Considerations for Multiple Interfaces.................... 15
3.1 Scoped Addresses.................................... 16
3.2 Address Ambiguity................................... 17
3.3 Interaction with Hosts with Routable Addresses...... 17
3.4 Unintentional Autoimmunity.......................... 18
4. Healing of Network Partitions ............................ 18
5. Security Considerations................................... 19
6. Application Programming Considerations.................... 21
6.1 Address Changes, Failure and Recovery............... 21
6.2 Limited Forwarding of Locators...................... 21
6.3 Address Ambiguity................................... 21
7. Router Considerations..................................... 22
8. IANA Considerations....................................... 22
9. Constants ................................................ 22
10. References ............................................... 22
10.1 Normative References ............................... 22
10.2 Informative References ............................. 23
Acknowledgments .............................................. 24
Authors' Addresses ........................................... 24
Appendix A - Prior Implementations............................ 25
Intellectual Property Statement .............................. 28
Full Copyright Statement ..................................... 29
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1. Introduction
As the Internet Protocol continues to grow in popularity, it becomes
increasingly valuable to be able to use familiar IP tools such as FTP
not only for global communication, but for local communication as
well. For example, two people with laptop computers supporting IEEE
802.11 Wireless LANs [802.11] 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 [RFC2131].
This document describes a method by which a host may automatically
configure an interface with an IPv4 address in the 169.254/16 prefix
that is valid for Link-Local communication on that interface. This
is especially valuable in environments where no other configuration
mechanism is available. The IPv4 prefix 169.254/16 is registered
with the IANA for this purpose. Allocation of Link-Local IPv6
addresses is described in "IPv6 Stateless Address Autoconfiguration"
[RFC2462].
Link-Local communication using Link-Local IPv4 addresses is only
suitable for communication with other devices connected to the same
physical (or logical) link. Link-Local communication using Link-
Local IPv4 addresses is not suitable for communication with devices
not directly connected to the same physical (or logical) link.
Microsoft Windows 98 (and later) and Mac OS 8.5 (and later) already
support this capability. This document standardizes usage,
prescribing rules for how Link-Local IPv4 addresses MUST be treated
by hosts and routers. In particular, it describes how routers MUST
behave when receiving packets with IPv4 Link-Local addresses in the
source or destination address. With respect to hosts, it discusses
claiming and defending addresses, maintaining Link-Local and routable
IPv4 addresses on the same interface, and multihoming issues.
1.1. Requirements
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized. 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 [RFC2119].
1.2. Terminology
This document describes Link-Local addressing, for IPv4 communication
between two hosts on a single link. A set of hosts is considered to
be "on the same link", if:
- when any host A from that set sends a packet to any other
host B in that set, using unicast, multicast, or broadcast,
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the entire link-layer packet payload arrives unmodified,
and
- a broadcast sent over that link by any host from that set
of hosts can be received by every other host in that set
The link-layer *header* may be modified, such as in Token Ring Source
Routing [802.5], but not the link-layer *payload*. In particular, if
any device forwarding a packet modifies any part of the IP header or
IP payload then the packet is no longer considered to be on the same
link. This means that the packet may pass through devices such as
repeaters, bridges, hubs or switches and still be considered to be on
the same link for the purpose of this document, but not through a
device such as an IP router that decrements the TTL or otherwise
modifies the IP header.
This document uses the term "routable address" to refer to all
unicast IPv4 addresses outside the 169.254/16 prefix, including
global addresses and private addresses such as Net 10/8 [RFC1918],
all of which may be forwarded via routers.
Wherever this document uses the term "host" when describing use of
Link-Local IPv4 addresses, the text applies equally to routers using
Link-Local IPv4 addresses on any or all interfaces.
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 [RFC826]
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
速arget 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.
1.3. Applicability
This specification applies to all IEEE 802 Local Area Networks (LANs)
[802], including Ethernet [802.3], Token-Ring [802.5] and IEEE 802.11
wireless LANs [802.11], as well as to other link-layer technologies
that operate at data rates of at least 1 Mbps, have a round-trip
latency of at most one second, and support ARP [RFC826]. Wherever
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this document uses the term "IEEE 802", the text applies equally to
any of these network technologies.
Link-layer technologies that support ARP but operate at rates below 1
Mbps or latencies above one second may need to specify different
values for the following parameters described in Sections 2.2, 2.3
and 2.4:
(a) the number of, and interval between, ARP probes,
(b) the number of, and interval between, ARP announcements,
(c) the maximum rate at which address claiming may be attempted, and
(d) the time interval between conflicting ARPs below which a host
MUST reconfigure instead of attempting to defend its address.
Link-layer technologies that do not support ARP may be able to use
other techniques for determining whether a particular IP address is
currently in use. However, the application of claim-and-defend
mechanisms to such networks is outside the scope of this document.
This specification is intended for use with small ad-hoc networks - a
single link containing only a few hosts. Although 65024 Link-Local
IPv4 addresses are available in principle, attempting to use all
those addresses on a single link would result a high probability of
an address conflict, requiring a host to take an inordinate amount of
time to find an available address.
Network operators with more than 1300 hosts on a single link may want
to consider dividing that single link into two or more subnets. A
host connecting to a link that already has 1300 hosts, selecting a
Link-Local IPv4 address at random, has a 98% chance of selecting an
unused Link-Local IPv4 address on the first try. A host has a 99.96%
chance of selecting an unused Link-Local IPv4 address within two
tries. The probability that it will have to try more than ten times
is about 1 in 10^17.
1.4. Application Layer Protocol Considerations
Use of Link-Local IPv4 addresses in off-link communication is likely
to cause application failures. This can occur within any application
that includes embedded addresses, if a Link-Local IPv4 address is
embedded when communicating with a host that is not on the link.
Examples of applications that include embedded addresses are found in
[RFC3027]. This includes IPsec, Kerberos 4/5, X-
Windows/Xterm/Telnet, FTP, RSVP, SMTP, SIP, Real Audio, H.323, and
SNMP.
In order to prevent use of Link-Local IPv4 addresses in off-link
communication, the following cautionary measures are advised:
a. Routable addresses should be used within applications whenever
they are available.
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b. Names that are globally resolvable to routable addresses should be
used within applications whenever they are available. Names that are
resolvable only on the local link (such as through use of protocols
such as Link Local Multicast Name Resolution [LLMNR]) MUST NOT be
used in off-link communication. IPV4 addresses and names which can
only be resolved on the local link SHOULD NOT be forwarded, they
SHOULD only be sent when a Link-Local address is used as the source
address. This strong advice should hinder limited scope addresses
and names from leaving the context in which they apply.
c. Link-Local IPv4 addresses MUST NOT be configured in the DNS.
Link-Local IPv4 addresses and their dynamic configuration have
profound implications upon applications which use them. This is
discussed in Section 6. Many applications fundamentally assume that
addresses of communicating peers are routable, relatively unchanging
and unique. These assumptions no longer hold with Link-Local IPv4
addresses, or a mixture of Link-Local and routable IPv4 addresses.
Therefore while many applications will work properly with Link-Local
IPv4 addresses, or a mixture of Link-Local and routable IPv4
addresses, others may do so only after modification, or will exhibit
reduced or partial functionality.
In some cases it may be infeasible for the application to be modified
to operate under such conditions.
Link-Local IPv4 addresses should therefore only be used where stable,
routable addresses are not available (such as on ad hoc or isolated
networks) or in controlled situations where these limitations and
their impact on applications are understood and accepted.
1.5. Autoconfiguration Issues
Implementations of Link-Local IPv4 address autoconfiguration MUST
expect address collisions, and MUST be prepared to handle them
gracefully 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 IPv4 address, not
just during 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.
1.6. Alternate Use Prohibition
Note that addresses in the 169.254/16 prefix SHOULD NOT be configured
manually or by a DHCP server. Manual or DHCP configuration may cause
a host to use an address in the 169.254/16 prefix without following
the special rules regarding duplicate detection and automatic
configuration that pertain to addresses in this prefix. While
[RFC2131] indicates that a DHCP client SHOULD probe a newly received
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address with ARP, this is not mandatory. Similarly, while [RFC2131]
recommends that a DHCP server SHOULD probe an address using an ICMP
Echo Request before allocating it, this is also not mandatory, and
even if the server does this, Link-Local IPv4 addresses are not
routable, so a DHCP server not directly connected to a link cannot
detect whether a host on that link is already using the desired Link-
Local IPv4 address.
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 prefixes [RFC1918], not the 169.254/16 prefix.
1.7. Multiple Addresses per Interface
Having addresses of multiple different scopes assigned to an
interface, with no adequate way to determine in what circumstances
each address should be used, leads to complexity for applications and
confusion for users. A host with an address on a link can
communicate with all other devices on that link, whether those
devices use Link-Local addresses, or routable addresses.
For this reason, a host that obtains, or is configured with, a
routable address on an interface, SHOULD NOT attempt to configure a
Link-Local IPv4 address on the same interface.
Where a Link-Local IPv4 address has been configured on an interface,
and a routable address is later configured on the same interface, the
host MUST always use the routable address when initiating new
communications, and MUST cease advertising the availability of the
Link-Local IPv4 address through whatever mechanisms that address had
been made known to others.
A host SHOULD continue to use the Link-Local IPv4 address for
communications underway when the routable address was configured, and
MAY continue to accept new communications addressed to the Link-Local
IPv4 address.
1.8. Multiple Interfaces
Additional considerations apply to hosts that support more than one
active interface where one or more of these interfaces support Link-
Local IPv4 address configuration. These considerations are
discussed in Section 3.
1.9. Communication with Routable Addresses
There will be cases when devices with a configured Link-Local address
will need to communicate with a device with a routable address
configured on the same physical link, and vice versa. The rules in
Section 2.6 allow this communication.
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This allows, for example, a laptop computer with only a routable
address to communicate with web servers world-wide using its
globally-routable address while at the same time printing those web
pages on a local printer that has only a Link-Local IPv4 address.
2. Address Selection, Defense and Delivery
The following section explains the Link-Local IPv4 address selection
algorithm, how Link-Local IPv4 addresses are defended, and how IPv4
packets with Link-Local IPv4 addresses are delivered.
Windows and Mac OS hosts that already implement Link-Local IPv4
address auto-configuration 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. Link-Local Address Selection
When a host wishes to configure a Link-Local IPv4 address, it selects
an address using a pseudo-random number generator with a uniform
distribution in the range from 169.254.1.0 to 169.254.254.255. The
IPv4 prefix 169.254/16 is registered with the IANA for this purpose.
The first 256 and last 256 addresses in the 169.254/16 prefix are
reserved for future use and MUST NOT be selected by a host using this
dynamic configuration mechanism.
The pseudo-random number generation algorithm MUST be chosen so that
different hosts do not generate the same sequence of numbers. If the
host has access to persistent information that is different for each
host, such as its IEEE 802 MAC 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 IPv4 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 or any other information which is
(or may be) identical in every host is NOT suitable for this purpose,
because a group of hosts that are all powered on at the same time
might then all generate the same sequence, resulting in a never-
ending series of conflicts as the hosts move in lock-step though
exactly the same pseudo-random sequence, conflicting on every address
they probe.
Hosts that are equipped with persistent storage MAY, for each
interface, record the IPv4 address they have selected. On booting,
hosts with a previously recorded address 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
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addresses and potentially having to resolve conflicts that arise.
2.2. Claiming a Link-Local Address
After it has selected a Link-Local IPv4 address, a host MUST test to
see if the Link-Local IPv4 address is already in use before beginning
to use it. When a network interface transitions from an inactive to
an active state, the host does not have knowledge of what Link-Local
IPv4 addresses may currently be in use on that link, since the point
of attachment may have changed or the network interface may have been
inactive when a conflicting address was claimed.
Were the host to immediately begin using a Link-Local IPv4 address
which is already in use by another host, this would be disruptive to
that other host. Since it is possible that the host has changed its
point of attachment, a routable address may be obtainable on the new
network, and therefore it cannot be assumed that a Link-Local IPv4
address is to be preferred.
Before using the Link-Local IPv4 address (e.g. using it as the source
address in an IPv4 packet, or as the Sender IPv4 address in an ARP
packet) a host MUST perform the probing test described below to
achieve better confidence that using the Link-Local IPv4 address will
not cause disruption.
Examples of events that involve an interface becoming active include:
Reboot/startup
Wake from sleep (if network interface was inactive during sleep)
Bringing up previously inactive network interface
IEEE 802 hardware link-state change that indicates that a
cable was attached.
Association with a wireless base station.
A host MUST NOT perform this check periodically as a matter of
course. This would be a waste of network bandwidth, and is
unnecessary due to the ability of hosts to passively discover
conflicts, as described in Section 2.5.
2.2.1. Probe details
On a link-layer such as IEEE 802 that supports ARP, conflict
detection is done using ARP probes. On link-layer technologies that
do not support ARP other techniques may be available for determining
whether a particular IPv4 address is currently in use. However, the
application of claim-and-defend mechanisms to such networks is left
to a future 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 the
連ender hardware address' field of the ARP Request with the hardware
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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
連ender 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 PROBE_MIN to PROBE_MAX
seconds, and should then send three probe packets, spaced randomly,
PROBE_MIN to PROBE_MAX seconds apart.
If during this period, from the beginning of the probing process
until PROBE_MAX seconds after the last probe packet is sent, the host
receives any ARP packet (Request *or* Reply) where the packet's
連ender 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 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 collision and select a new address as above. This can
occur if two (or more) hosts attempt to configure the same Link-Local
IPv4 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 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 rogue host that answers all ARP
probes, causing legitimate hosts to go into an infinite loop
attempting to select a usable address.
If, by PROBE_MAX seconds after the transmission of the last ARP probe
no conflicting ARP Reply has been received, then the host has
successfully claimed the desired Link-Local IPv4 address.
2.3. Shorter timeouts
The time values specified above are intended for use on technologies
such as IEEE 802, 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 on an IEEE
802 LAN, 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
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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 800-1000 milliseconds before a chosen IPv4 address may be
used.
Should future versions of the IEEE 802 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.
2.4. Announcing an Address
The host MUST then announce its claimed address by broadcasting two
ARP announcements, spaced PROBE_MAX seconds apart. This time
interval is not modified by the shorter timeouts described above in
Section 2.3. 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 IPv4 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.
2.5. Conflict Detection and 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 (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 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 IPv4 address as described
above, or
(b) If a host currently has active TCP connections or other reasons
to prefer to keep the same IPv4 address, and it has not seen any
other conflicting ARP packets recently (for IEEE 802, 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 ARP announcement, giving its own IP and
hardware addresses as the sender addresses of the ARP. Having done
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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
IEEE 802) then the host MUST immediately cease using this address and
configure a new Link-Local IPv4 address 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.
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 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
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.
All ARP packets (*replies* as well as requests) that contain a Link-
Local 連ender IP address' MUST be sent using link-layer broadcast
instead of link-layer unicast. This aids timely detection of
duplicate addresses. An example illustrating how this helps is given
in Section 4.
2.6. Address Usage and Forwarding Rules
A host implementing this specification has additional rules to
conform to, whether or not it has an interface configured with a
Link-Local IPv4 address.
2.6.1. Source Address Usage
Since each interface on a host may have a Link-Local IPv4 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.
The host SHOULD use a routable address in preference to a Link-Local
IPv4 address except for communication to peers for which the host has
an existing TCP connection at the time in which the host obtained a
routable address configuration. For more details, see Section 1.7.
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If the host is multihomed, the decision as to which source address to
use is more difficult. This specification does not define how the
host operates in this case, although it describes the issues involved
and provides advice (see Section 3).
2.6.2. Forwarding Rules
If the destination address is in the 169.254/16 prefix (including the
169.254.255.255 broadcast address), then the sender MUST ARP for the
destination address and then send its packet directly to the
destination on the same physical link. This MUST be done whether the
interface is configured with a Link-local or a routable IPv4 address.
In many network stacks, achieving this functionality may be as simple
as adding a routing table entry indicating that 169.254/16 is
directly reachable on the local link.
The host MUST NOT send a packet with a Link-Local IPv4 destination
address to any router for forwarding.
If the destination address is a unicast address outside the
169.254/16 prefix, then the host SHOULD use an appropriate routable
source address, if it has one. If the host has no appropriate
routable source address, then it SHOULD ARP for the destination
address and then send its packet, with a link-local source IP address
and a routable destination IP address, directly to the destination on
the same physical link. In the case of a device with only a link-
local address, this requirement can be paraphrased as "ARP for
everything". In many network stacks, achieving this "ARP for
everything" behaviour may be as simple as having no primary IP router
configured, having the primary IP router address configured to
0.0.0.0, or having the primary IP router address set to be the same
as the host's own link-local IP address. In any event, the host MUST
NOT send a packet with a link-local source address to any router for
forwarding.
If the host is multihomed, determining the rules on how to forward to
a destination are more complex. This specification does not define
multihomed operation. Rather, the issues are explained and advice is
given on how to address known problems (see Section 3).
2.7. Link-Local Packets Are Not Forwarded
A sensible default for applications which are sending from a Link-
Local IPv4 address is to explicitly set the IPv4 TTL to 1. This is
not appropriate in all cases as some applications may require that
the IPv4 TTL be set to other values.
An IPv4 packet whose source and/or destination address is in the
169.254/16 prefix MUST NOT be sent to any router for forwarding, and
any network device receiving such a packet MUST NOT forward it,
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regardless of the TTL in the IPv4 header. Similarly, a router or
other host MUST NOT indiscriminately answer all ARP Requests for
addresses in the 169.254/16 prefix. A router may of course answer
ARP Requests for one or more Link-Local IPv4 address(es) that it has
legitimately claimed for its own use according to the claim-and-
defend protocol described in this document.
This restriction also applies to multicast packets. IPv4 packets with
a Link-Local source address MUST NOT be forwarded off the local link
even if they have a multicast destination address.
2.8. Link-Local Packets are Local
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 prefix 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
Link-Local-only devices will often be simple devices of the kind that
currently use X10 [X10], USB [USB] or FireWire [1394].
The designers of these devices currently assume that they will
communicate only with other local devices, and this allows them to
produce cost-effective devices by implementing a degree of security
appropriate for that expected environment. Any network gateway
device that blindly forwards the contents of Link-Local IPv4 packets
off the local link (or onto the local link) exposes simple 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 any
communication outside the local link. IP hosts that implement both
Link-Local and conventional routable IPv4 addresses may still use
their routable addresses without restriction as they do today.
Simple devices that implement only a Link-Local IPv4 address 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
thermostat that implements only a Link-Local IPv4 address could be
controlled from a remote Web browser, by having an intermediary on
the local network which accepts incoming HTTP connections, uses
appropriate cryptographic methods to verify the authority of the
remote user, and then uses Link-Local IPv4 packets to communicate
with the thermostat to get status and issue commands.
It should be understood that this mediated communication is not
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mandatory; it is an option afforded to designers of extremely simple
devices. 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 IPv4 hosts.
Such networked devices should of course implement a degree of
security appropriate to being connected to a global public network.
2.9. Higher-Layer Protocol Considerations
Similar considerations apply at layers above IP.
For example, designers of Web pages (including automatically
generated web pages) SHOULD NOT contain links with embedded Link-
Local IPv4 addresses if those pages are viewable from hosts outside
the local link where the addresses are valid.
As Link-Local IPv4 addresses may change at any time and have limited
scope, storing Link-Local IPv4 addresses in the DNS is not well
understood and is NOT RECOMMENDED.
2.10. Privacy Concerns
Another reason to restrict leakage of Link-Local IPv4 addresses
outside the local link is privacy concerns. If Link-Local IPv4
addresses are derived from a hash of the MAC address, some argue that
they could be indirectly associated with an individual, and thereby
used to track that individual's activities. Within the local link
the hardware addresses in the packets are all directly observable, so
as long as Link-Local IPv4 addresses don't leave the local link they
provide no more information to an intruder than could be gained by
direct observation of hardware addresses.
3. Considerations for Multiple Interfaces
These considerations apply whenever a host has multiple IP addresses
whether or not it has multiple physical interfaces. Other examples
of multiple interfaces include different logical endpoints (tunnels,
virtual private networks etc.) and multiple logical networks on the
same physical medium. This is often referred to as "multihoming".
Hosts which have more than one active interface and elect to
implement dynamic configuration of Link-Local IPv4 addresses on one
or more of those interfaces will face various problems. This section
lists these problems but does no more than indicate how one might
solve them. At the time of this writing, there is no silver bullet
which solves these problems in all cases, in a general way.
Implementors must think through these issues before implementing the
protocol specified in this document on a system which may have more
than one active interface as part of a TCP/IP stack capable of
multihoming.
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3.1. Scoped addresses
A host may be attached to more than one network at the same time. It
would be nice if there was a single address space used in every
network, but this is not the case. Addresses used in one network, be
it a network behind a NAT or a link on which Link-Local IPv4
addresses are used, cannot be used in another network and have the
same effect.
It would also be nice if addresses were not exposed to applications,
but they are. Most software using TCP/IP which await messages
receives from any interface at a particular port number, for a
particular transport protocol. Applications are generally only aware
(and care) that they have received a message. The application knows
the source address of the sender to whom the application will reply.
The first scoped address problem is source address selection. A
multihomed host has more than one address. Which address should be
used as the source address when sending to a particular destination?
This answer is usually answered by referring to a routing table,
which expresses which interface (with which address) to send, and how
to send (should one forward to a router, or send directly). The
choice is made complicated by scoped addresses because the address
range in which the destination lies may be ambiguous. The table may
not be able to yield a good answer. This problem is bound up with
next-hop selection, which is discussed in Section 3.2.
The second scoped address problem arises from scoped parameters
leaking outside their scope. This is discussed in Section 7.
It is possible to overcome these problems. One way is to expose scope
information to applications such that they are always aware of what
scope a peer is in. This way, the correct interface could be
selected, and a safe procedure could be followed with respect to
forwarding addresses and other scoped parameters. There are other
possible approaches. None of these methods have been standardized for
IPv4 nor are they specified in this document. A good API design
could mitigate the problems, either by exposing address scopes to
'scoped-address aware' applications or by cleverly encapsulating the
scoping information and logic so that applications do the right thing
without being aware of address scoping.
An implementer could undertake to solve these problems, but cannot
simply ignore them. With sufficient experience, it is hoped that
specifications will emerge explaining how to overcome scoped address
multihoming problems.
3.2. Address Ambiguity
This is a core problem with respect to Link-Local IPv4 addresses
configured on more than one interface. What should a host do when it
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needs to send to Link-Local destination L and L can be resolved using
ARP on more than one link?
One possibility is to support this only in the case where the
application specifically expresses which interface to send from.
There no standard or obvious solution to this problem. Existing
application software written for the Internet protocol suite is
largely incapable of dealing with address ambiguity. This does not
preclude an implementer from finding a solution, writing applications
which are able to use it, and providing a host which can support
dynamic configuration of Link-Local IPv4 addresses on more than one
interface. This solution will almost surely not be generally
applicable to existing software and transparent to higher layers,
however.
3.3. Interaction with Hosts with Routable Addresses
Attention is paid in this specification to transition from the use of
Link-Local IPv4 addresses to routable addresses (see Section 1.5).
The intention is to allow a host with a single interface to first
support Link-Local configuration then gracefully transition to the
use of a routable address. Since the host transitioning to the use of
a routable address will not advertise scoped address information, the
scoped address issues described in Section 3.1 will apply. A host
which conforms to this specification will know that a Link-Local IPv4
destination must be reached by forwarding to the destination, not to
a router, even if the host is sending from a routable address.
A host with a Link-Local IPv4 address may send to a destination which
does not have a Link-Local IPv4 address. If the host is not
multihomed, the procedure is simple and unambiguous: Using ARP and
forwarding directly to on-link destinations is the default route. If
the host is multihomed, however, the routing policy is more complex,
especially if one of the interfaces is configured with a routable
address and the default route is (sensibly) directed at a router
accessible through that interface. The following example illustrates
this problem and provides a common solution to it.
i1 +---------+ i2 i3 +-------+
ROUTER-------= HOST1 =---------= HOST2 |
link1 +-------=-+ link2 +-------+
In the figure above, HOST1 is connected to link1 and link2. Interface
i1 is configured with a routable address, while i2 is a Link-Local
IPv4 address. HOST1 has its default route set to ROUTER's address,
through i1. HOST1 will route to destinations in 169.254/16 to i2,
sending directly to the destination.
HOST2 has a configured (non-Link-Local) IPv4 address assigned to i3.
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Using a name resolution or service discovery protocol HOST1 can
discover HOST2's address. Since HOST2's address is not in 169.254/16,
HOST1's routing policy will send datagrams to HOST2 via i1, to the
ROUTER. Unless there is a route from ROUTER to HOST2, the datagrams
sent from HOST1 to HOST2 will not reach it.
One solution to this problem is for a host to attempt to reach any
host locally (using ARP) for which it receives an unreachable ICMP
error message (ICMP message codes 0, 1, 6 or 7, see [RFC792]). The
host tries all its attached links in a round robin fashion. This has
been implemented successfully for some IPv6 hosts, to circumvent
exactly this problem. In terms of this example, HOST1 upon failing to
reach HOST2 via the ROUTER, will attempt to forward to HOST2 via i2
and succeed.
It may also be possible to overcome this problem using techniques
described in section 3.2, or other means not discussed here. This
specification does not provide a standard solution, nor does it
preclude implementers from supporting multihomed configurations,
provided that they address the concerns in this section for the
applications which will be supported on the host.
3.4. Unintentional Autoimmunity
Care must be taken if a multihomed host can support more than one
interface on the same link, all of which support Link-Local IPv4
autoconfiguration. If these interfaces attempt to allocate the same
address, they will defend the host against itself - causing the
claiming algorithm to fail. The simplest solution to this problem is
to run the algorithm independently on each interface configured with
Link-Local IPv4 addresses.
In particular, ARP packets which appear to claim an address which is
assigned to a specific interface, indicate conflict only if they are
received on that interface and their hardware address is of some
other interface.
If a host has two interfaces on the same network, then claiming and
defending on those interfaces must ensure that they end up with
different addresses just as if they were on different hosts.
4. Healing of Network Partitions
Hosts on disjoint network links may configure the same Link-Local
IPv4 address. 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 an address conflict.
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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 IPv4 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 (defined as networks with a
substantial number of hosts per segment) there is a greater chance of
collision. In such networks, it is likely that the joining of
previously separated segments will result in one or more hosts
needing to change their Link-Local IPv4 address, with subsequent loss
of TCP connections. In cases where separation and re-joining is
frequent, as in remotely bridged networks, this could prove
disruptive. However, unless the number of hosts on the joined
segments is very large, the traffic resulting from the join and
subsequent address conflict resolution will be small.
Sending ARP replies that have Link-Local sender IPv4 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 Link-Local 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 IPv4 Link-Local Addresses 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.
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The ARP protocol [RFC826] 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.
NOTE: The existence of local links without physical security, such as
LANs with attached wireless base stations, means that expecting all
local links to be secure enough that normal precautions can be
dispensed with is an extremely dangerous practice, which will expose
users to considerable risks.
A host implementing Link-Local IPv4 configuration has the additional
vulnerability to selective reconfiguration and disruption. It is
possible for an on-link attacker to issue ARP packets which would
cause a host to break all its connections by switching to a new
address. The attacker could force the host implementing Link-Local
IPv4 configuration to select certain addresses, or prevent it from
ever completing address selection. This is a distinct threat from
that posed by spoofed ARPs, described in the preceding paragraph.
Implementations and users should also note that a node that gives up
an address and reconfigures, as required by section 2.5, allows the
possibility that another node can easily successfully hijack existing
TCP connections. Before abandoning an address due to a conflict,
hosts SHOULD actively attempt to reset any existing connections using
that address.
Implementers are advised that the Internet Protocol architecture
expects every networked device or host must implement security which
is adequate to protect the resources to which the device or host has
access, including the network itself, against known or credible
threats. Even though use of Link-Local IPv4 addresses may reduce the
number of threats to which a device is exposed, implementers of
devices supporting the Internet Protocol must not assume that a
customer's local network is free from security risks.
While there may be particular kinds of devices, or particular
environments, for which the security provided by the network is
adequate to protect the resources that are accessible by the device,
it would be misleading to make a general statement to the effect that
the requirement to provide security is reduced for devices using
Link-Local IPv4 addresses as a sole means of access.
In all cases, whether or not Link-Local IPv4 addresses are used, it
is necessary for implementers of devices supporting the Internet
Protocol to analyze the known and credible threats to which a
specific host or device might be subjected, and to the extent that it
is feasible, to provide security mechanisms which ameliorate or
reduce the risks associated with such threats.
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6. Application Programming Considerations
Use of Link-Local IPv4 autoconfigured addresses presents additional
challenges to writers of applications and may result in existing
application software failing.
6.1. Address Changes, Failure and Recovery
Link-Local IPv4 addresses used by an application may change over
time. Some application software encountering an address change will
completely fail. For example, client TCP connections will fail,
servers whose addresses change will have to be rediscovered, blocked
reads and writes will exit with an error condition, and so on.
Vendors producing application software which will be used on IP
implementations supporting Link-Local IPv4 address configuration
SHOULD detect and cope with address change events. Vendors producing
IPv4 implementations supporting Link-Local IPv4 address configuration
SHOULD expose address change events to applications.
6.2. Limited Forwarding of Locators
Link-Local IPv4 addresses MUST NOT be forwarded via an application
protocol (for example in a URL), to a destination which is not Link-
Local, on the same link. This is discussed further in Section 2.9 and
3.
Existing distributed application software which forwards address
information may fail. For example, FTP [RFC 959] passive mode
transmits the IPv4 address of the client. Assume a client starts up
and obtains its *passive* IPv4 configuration at a time when the host
has only a Link-Local address. Later, the host gets a global IP
address configuration (for one of its interfaces). The client uses
this global IPv4 address to contact an FTP server off of the local
link for which it had (or still has) a Link-Local IPv4 address
configured. If the FTP client transmits its passive IPv4
configuration to the FTP server, the FTP server will be unable to
reach the FTP client. The passive FTP operation will fail.
6.3. Address Ambiguity
Application software run on a multihomed host which supports Link-
Local IPv4 address configuration on more than one interface may fail.
This is because application software assumes that an IPv4 address is
unambiguous, that it can refer to only one host. Link-Local IPv4
addresses are unique only on a single link. A host attached to
multiple links can easily encounter a situation where the same
address is present on more than one interface, or first on one
interface, later on another; in any case associated with more than
one host. Most existing software is not prepared for this ambiguity.
In the future, application programming interfaces could be developed
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to prevent this problem. This issue is discussed in Section 3.
7. Router Considerations
A router which receives a packet with a Link-Local IPv4 destination
address on an interface which either has no Link-Local IPv4 address
configured or a different address than the destination of the packet
MUST NOT forward the packet. This will prevent forwarding of
packets back onto the network segment from which they originated and
to any other segment.
A router MUST NOT forward a packet with a Link-Local IPv4 source or
destination address, irrespective of the router's default route
configuration or routes obtained from dynamic routing protocols.
8. IANA Considerations
The following terms are used here with the meanings defined in BCP
26: "name space", "assigned value", "registration".
The following policies are used here with the meanings defined in BCP
26: "Private Use", "First Come First Served", "Expert Review",
"Specification Required", "IESG Approval", "IETF Consensus",
"Standards Action".
The IANA has allocated the prefix 169.254/16 for the use described in
this document. The first and last 256 addresses in this range
(169.254.0.x and 169.254.255.x) are allocated by Standards Action. No
other IANA services are required by this document.
9. Constants
The following timing constants are used in this protocol.
PROBE_MIN 1 second
PROBE_MAX 2 seconds
10. References
10.1. Normative References
[RFC792] Postel, J., "Internet Control Message Protocol", RFC 792,
September 1981.
[RFC826] 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.
[RFC1122] R. Braden, "Requirements for Internet Hosts -- Communication
Layers", RFC 1122, October 1989.
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[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
[RFC2434] Alvestrand, H. and T. Narten, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October
1998.
10.2. Informative References
[802] IEEE Standards for Local and Metropolitan Area Networks:
Overview and Architecture, ANSI/IEEE Std 802, 1990.
[802.1d] ISO/IEC 15802-3 Information technology - Telecommunications
and information exchange between systems - Local and
metropolitan area networks - Common specifications - Part 3:
Media access Control (MAC) Bridges, (also ANSI/IEEE Std
802.1D-1998), 1998.
[802.3] ISO/IEC 8802-3 Information technology - Telecommunications and
information exchange between systems - Local and metropolitan
area networks - Common specifications - Part 3: Carrier Sense
Multiple Access with Collision Detection (CSMA/CD) Access
Method and Physical Layer Specifications, (also ANSI/IEEE Std
802.3- 1996), 1996.
[802.5] ISO/IEC 8802-5 Information technology - Telecommunications and
information exchange between systems - Local and metropolitan
area networks - Common specifications - Part 5: Token ring
access method and physical layer specifications, (also
ANSI/IEEE Std 802.5-1998), 1998.
[802.11] Information technology - Telecommunications and information
exchange between systems - Local and metropolitan area
networks - Specific Requirements Part 11: Wireless LAN Medium
Access Control (MAC) and Physical Layer (PHY) Specifications,
IEEE Std. 802.11-1999, 1999.
[1394] Standard for a High Performance Serial Bus. Institute of
Electrical and Electronic Engineers, IEEE Standard 1394-1995,
1995.
[RFC1918] Y. Rekhter et.al., "Address Allocation for Private Internets",
RFC 1918, February 1996.
[RFC2131] R. Droms, "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[RFC2181] R. Elz and R. Bush, "Clarifications to the DNS Specification",
RFC 2181, July 1997.
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[RFC2462] S. Thomson and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[RFC3027] Holdrege, M. and P. Srisuresh, "Protocol Complications with
the IP Network Address Translator", RFC 3027, January 2001.
[LLMNR] Esibov, L., Aboba, B. and D. Thaler, "Linklocal Multicast Name
Resolution (LLMNR)", draft-ietf-dnsext-mdns-23.txt, Internet
draft (work in progress), August 2003.
[USB] Universal Serial Bus Implementers Forum <http://www.usb.org/>
[X10] X10 Ltd. <http://www.x10.com/>
Acknowledgments
We would like to thank (in alphabetical order) Jim Busse, Pavani
Diwanji, Donald Eastlake 3rd, Robert Elz, Peter Ford, Spencer
Giacalone, Josh Graessley, Myron Hattig, Hugh Holbrook, Christian
Huitema, Richard Johnson, Kim Yong-Woon, Rod Lopez, Keith Moore,
Satish Mundra, Thomas Narten, Erik Nordmark, Howard Ridenour, Daniel
Senie, Dieter Siegmund, Valery Smyslov and Ryan Troll for their
contributions.
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
Phone: +1 425 706 6605
EMail: bernarda@microsoft.com
Erik Guttman
Sun Microsystems
Eichhoelzelstr. 7
74915 Waibstadt Germany
Phone: +49 7263 911 701
Email: erik.guttman@sun.com
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Appendix A - Prior Implementations
A.1. Apple Mac OS 8.x and 9.x.
Mac OS chooses the IP address on a pseudo-random basis. The selected
address is saved in persistent storage for continued use after
reboot, when possible.
Mac OS sends nine DHCPDISCOVER packets, with an interval of two
seconds between packets. If no response is received from any of these
requests (18 seconds), it will autoconfigure.
Upon finding that a selected address is in use, Mac OS will select a
new random address and try again, at a rate limited to no more than
one attempt every two seconds.
Autoconfigured Mac OS systems check for the presence of a DHCP server
every five minutes. If a DHCP server is found but Mac OS is not
successful in obtaining a new lease, it keeps the existing
autoconfigured IP address. If Mac OS is successful at obtaining a
new lease, it drops all existing connections without warning. This
may cause users to lose sessions in progress. Once a new lease is
obtained, Mac OS will not allocate further connections using the
autoconfigured IP address.
Mac OS systems do not send packets addressed to a Link-Local address
to the default gateway if one is present; these addresses are always
resolved on the local segment.
Mac OS systems by default send all outgoing unicast packets with a
TTL of 255. All multicast and broadcast packets are also sent with a
TTL of 255 if they have a source address in the 169.254/16 prefix.
Mac OS implements media sense where the hardware (and driver
software) supports this. As soon as network connectivity is
detected, a DHCPDISCOVER will be sent on the interface. This means
that systems will immediately transition out of autoconfigured mode
as soon as connectivity is restored.
A.2. Apple Mac OS X Version 10.2
Mac OS X chooses the IP address on a pseudo-random basis. The
selected address is saved in memory so that it can be re-used during
subsequent autoconfiguration attempts during a single boot of the
system.
Autoconfiguration of a Link-Local address depends on the results of
the DHCP process. DHCP sends two packets, with timeouts of one and
two seconds. If no response is received (three seconds), it begins
autoconfiguration. DHCP continues sending packets in parallel for a
total time of 60 seconds.
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At the start of autoconfiguration, it generates 10 unique random IP
addresses, and probes each one in turn for 2 seconds. It stops
probing after finding an address that is not in use, or the list of
addresses is exhausted.
If DHCP is not successful, it waits five minutes before starting over
again. Once DHCP is successful, the autoconfigured Link-Local
address is given up. The Link-Local subnet, however, remains
configured.
Autoconfiguration is only attempted on a single interface at any
given moment in time.
Mac OS X ensures that the connected interface with the highest
priority is associated with the Link-Local subnet. Packets addressed
to a Link-Local address are never sent to the default gateway, if one
is present. Link-local addresses are always resolved on the local
segment.
Mac OS X implements media sense where the hardware and driver support
it. When the network media indicates that it has been connected, the
autoconfiguration process begins again, and attempts to re-use the
previously assigned Link-Local address. When the network media
indicates that it has been disconnected, the system waits four
seconds before de-configuring the Link-Local address and subnet. If
the connection is restored before that time, the autoconfiguration
process begins again. If the connection is not restored before that
time, the system chooses another interface to autoconfigure.
Mac OS X by default sends all outgoing unicast packets with a TTL of
255. All multicast and broadcast packets are also sent with a TTL of
255 if they have a source address in the 169.254/16 prefix.
A.3. Microsoft Windows 98/98SE
Windows 98/98SE systems choose their Link-Local IPv4 address on a
pseudo-random basis. This ensures that systems rebooting will obtain
the same autoconfigured address, unless a conflict is detected. The
address selection algorithm is based on computing a hash on the
interface's MAC address, so that a large collection of hosts should
obey the uniform probability distribution in choosing addresses
within the 169.254/16 address space.
When in INIT state, the Windows 98/98SE DHCP Client sends out a total
of 4 DHCPDISCOVERs, with an inter-packet interval of 6 seconds. When
no response is received after all 4 packets (24 seconds), it will
autoconfigure an address.
The autoconfigure retry count for Windows 98/98SE systems is 10.
After trying 10 autoconfigured IPv4 addresses, and finding all are
taken, the host will boot without an IPv4 address.
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Autoconfigured Windows 98/98SE systems check for the presence of a
DHCP server every five minutes. If a DHCP server is found but
Windows 98 is not successful in obtaining a new lease, it keeps the
existing autoconfigured Link-Local IPv4 address. If Windows 98/98SE
is successful at obtaining a new lease, it drops all existing
connections without warning. This may cause users to lose sessions in
progress. Once a new lease is obtained, Windows 98/98SE will not
allocate further connections using the autoconfigured Link-Local IPv4
address.
Windows 98/98SE systems with a Link-Local IPv4 address do not send
packets addressed to a Link-Local IPv4 address to the default gateway
if one is present; these addresses are always resolved on the local
segment.
Windows 98/98SE systems by default send all outgoing unicast packets
with a TTL of 128. There is no way a host can distinguish between
packets generated locally with a TTL of 128, and packets generated by
a remote attacker, deliberately constructed with a spoofed source
address in the 169.254/16 prefix, and a TTL higher than 128, such
that after passing through one or more misconfigured routers, the TTL
will have decremented to 128 by the time it reaches the target
network. TTL configuration is performed by setting the Windows
Registry Key
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services:\Tcpip\
Parameters\DefaultTTL of type REG_DWORD to the appropriate value.
However, this default TTL will apply to all packets so that it cannot
be used to set the default TTL of Link-Local IPv4 packets to one (1),
while allowing other packets to be sent with a TTL larger than one.
Windows 98/98SE systems do not implement media sense. This means that
network connectivity issues (such as a loose cable) may prevent a
system from contacting the DHCP server, thereby causing it to auto-
configure. When the connectivity problem is fixed (such as when the
cable is re-connected) the situation will not immediately correct
itself. Since the system will not sense the re-connection, it will
remain in autoconfigured mode until an attempt is made to reach the
DHCP server.
The DHCP server included with Windows 98SE Internet Connection
Sharing (ICS) (a NAT implementation) allocates out of the 192.168/16
private address space by default.
However, it is possible to change the allocation prefix via a
registry key, and no checks are made to prevent allocation out of the
Link-Local IPv4 prefix. When configured to do so, Windows 98SE ICS
will NAT packets from the Link-Local IPv4 prefix off the local link.
Windows 98SE ICS does not automatically route for the Link-Local IPv4
prefix, so that hosts obtaining addresses via DHCP cannot communicate
with autoconfigured-only devices.
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INTERNET-DRAFT Link-Local IPv4 2 September 2003
Other home gateways exist that allocate addresses out of the Link-
Local IPv4 prefix by default. Windows 98/98SE systems can use a
169.254/16 Link-Local IPv4 address as the source address when
communicating with non-Link-Local hosts. However, Windows 98/98SE
does not support router solicitation/advertisement. This means that
Windows 98/98SE systems will typically not be able to discover a
default gateway when in autoconfigured mode.
A.4. Windows XP, 2000 and ME
The autoconfiguration behavior of Windows XP, Windows 2000 and
Windows ME systems is identical to Windows 98/98SE except in the
following respects:
Media Sense
Router Discovery
Silent RIP
Windows XP, 2000 and ME implement media sense. As soon as network
connectivity is detected, a DHCPREQUEST or DHCPDISCOVER will be sent
on the interface. This means that systems will immediately
transition out of autoconfigured mode as soon as connectivity is
restored.
Windows XP, 2000 and ME also support router discovery, although it is
turned off by default. Windows XP and 2000 also support a RIP
listener. This means that it is possible to discover a default
gateway while in autoconfigured mode.
ICS on Windows XP/2000/ME behaves identically to Windows 98SE with
respect to address allocation and NATing of Link-Local prefixes.
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Cheshire, et al. Standards Track [Page 28]
INTERNET-DRAFT Link-Local IPv4 2 September 2003
be obtained from the IETF Secretariat.
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Open issues with this specification are tracked on the following web
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Expiration Date
This memo is filed as <draft-ietf-zeroconf-ipv4-linklocal-09.txt>, and
expires February 22, 2004.
Cheshire, et al. Standards Track [Page 29]
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