draft-ietf-ipv6-rfc2462bis-01.txt   draft-ietf-ipv6-rfc2462bis-02.txt 
IETF IPv6 Working Group S. Thomson IETF IPv6 Working Group S. Thomson
Internet-Draft Cisco Internet-Draft Cisco
Expires: December 13, 2004 T. Narten Expires: December 16, 2004 T. Narten
IBM IBM
T. Jinmei T. Jinmei
Toshiba Toshiba
H. Soliman June 17, 2004
Flarion Technologies
June 14, 2004
IPv6 Stateless Address Autoconfiguration IPv6 Stateless Address Autoconfiguration
draft-ietf-ipv6-rfc2462bis-01.txt draft-ietf-ipv6-rfc2462bis-02.txt
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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skipping to change at page 1, line 36 skipping to change at page 1, line 34
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
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This Internet-Draft will expire on December 13, 2004. This Internet-Draft will expire on December 16, 2004.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved. Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract Abstract
This document specifies the steps a host takes in deciding how to This document specifies the steps a host takes in deciding how to
autoconfigure its interfaces in IP version 6. The autoconfiguration autoconfigure its interfaces in IP version 6. The autoconfiguration
process includes creating a link-local address and verifying its process includes creating a link-local address and verifying its
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5.5.4 Address Lifetime Expiry . . . . . . . . . . . . . . . . . . 20 5.5.4 Address Lifetime Expiry . . . . . . . . . . . . . . . . . . 20
5.6 Configuration Consistency . . . . . . . . . . . . . . . . . 21 5.6 Configuration Consistency . . . . . . . . . . . . . . . . . 21
5.7 Retaining Configured Addresses for Stability . . . . . . . . 21 5.7 Retaining Configured Addresses for Stability . . . . . . . . 21
6. SECURITY CONSIDERATIONS . . . . . . . . . . . . . . . . . . 21 6. SECURITY CONSIDERATIONS . . . . . . . . . . . . . . . . . . 21
7. IANA CONSIDERATIONS . . . . . . . . . . . . . . . . . . . . 22 7. IANA CONSIDERATIONS . . . . . . . . . . . . . . . . . . . . 22
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 22 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 22
Normative References . . . . . . . . . . . . . . . . . . . . 22 Normative References . . . . . . . . . . . . . . . . . . . . 22
Informative References . . . . . . . . . . . . . . . . . . . 23 Informative References . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 23 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 23
A. LOOPBACK SUPPRESSION & DUPLICATE ADDRESS DETECTION . . . . . 24 A. LOOPBACK SUPPRESSION & DUPLICATE ADDRESS DETECTION . . . . . 24
B. CHANGES SINCE RFC 1971 . . . . . . . . . . . . . . . . . . . 26 B. CHANGES SINCE RFC 1971 . . . . . . . . . . . . . . . . . . . 25
C. CHANGE HISTORY . . . . . . . . . . . . . . . . . . . . . . . 26 C. CHANGE HISTORY . . . . . . . . . . . . . . . . . . . . . . . 26
Intellectual Property and Copyright Statements . . . . . . . 29 Intellectual Property and Copyright Statements . . . . . . . 29
1. Introduction 1. Introduction
This document specifies the steps a host takes in deciding how to This document specifies the steps a host takes in deciding how to
autoconfigure its interfaces in IP version 6. The autoconfiguration autoconfigure its interfaces in IP version 6. The autoconfiguration
process includes creating a link-local address and verifying its process includes creating a link-local address and verifying its
uniqueness on a link, determining what information can be uniqueness on a link, determining what information can be
autoconfigured (addresses, other information, or both), and in the autoconfigured (addresses, other information, or both), and in the
case of addresses, whether they can be obtained through the stateless case of addresses, whether they can be obtained through the stateless
mechanism, the stateful mechanism, or both. This document defines the mechanism, the stateful mechanism, or both. This document defines the
process for generating a link-local address, the process for process for generating a link-local address, the process for
generating global addresses via stateless address autoconfiguration, generating global addresses via stateless address autoconfiguration,
and the Duplicate Address Detection procedure. The details of and the Duplicate Address Detection procedure. The details of
autoconfiguration using the stateful protocol is specified in RFC autoconfiguration using the stateful protocol is specified in RFC
3315 [7] and RFC 3736 [8]. 3315 [6] and RFC 3736 [7].
IPv6 defines both a stateful and stateless address autoconfiguration IPv6 defines both a stateful and stateless address autoconfiguration
mechanism. Stateless autoconfiguration requires no manual mechanism. Stateless autoconfiguration requires no manual
configuration of hosts, minimal (if any) configuration of routers, configuration of hosts, minimal (if any) configuration of routers,
and no additional servers. The stateless mechanism allows a host to and no additional servers. The stateless mechanism allows a host to
generate its own addresses using a combination of locally available generate its own addresses using a combination of locally available
information and information advertised by routers. Routers advertise information and information advertised by routers. Routers advertise
prefixes that identify the subnet(s) associated with a link, while prefixes that identify the subnet(s) associated with a link, while
hosts generate an "interface identifier" that uniquely identifies an hosts generate an "interface identifier" that uniquely identifies an
interface on a subnet. An address is formed by combining the two. In interface on a subnet. An address is formed by combining the two. In
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DHCPv6 server. Servers maintain a database that keeps track of which DHCPv6 server. Servers maintain a database that keeps track of which
addresses have been assigned to which hosts. The stateful addresses have been assigned to which hosts. The stateful
autoconfiguration protocol allows hosts to obtain addresses, other autoconfiguration protocol allows hosts to obtain addresses, other
configuration information or both from a server. Stateless and configuration information or both from a server. Stateless and
stateful autoconfiguration complement each other. For example, a host stateful autoconfiguration complement each other. For example, a host
can use stateless autoconfiguration to configure its own addresses, can use stateless autoconfiguration to configure its own addresses,
but use stateful autoconfiguration to obtain other information. but use stateful autoconfiguration to obtain other information.
To obtain other configuration information without configuring To obtain other configuration information without configuring
addresses in the stateful autoconfiguration model, a subset of DHCPv6 addresses in the stateful autoconfiguration model, a subset of DHCPv6
will be used [8]. While the model is called "stateful" here in order will be used [7]. While the model is called "stateful" here in order
to highlight the contrast to the stateless protocol defined in this to highlight the contrast to the stateless protocol defined in this
document, the intended protocol is also defined to work in a document, the intended protocol is also defined to work in a
stateless fashion. This is based on a result, through operational stateless fashion. This is based on a result, through operational
experiments, that all known "other" configuration information can be experiments, that all known "other" configuration information can be
managed by a stateless server, that is, a server that does not managed by a stateless server, that is, a server that does not
maintain state of each client that the server provides with the maintain state of each client that the server provides with the
configuration information. configuration information.
The stateless approach is used when a site is not particularly The stateless approach is used when a site is not particularly
concerned with the exact addresses hosts use, so long as they are concerned with the exact addresses hosts use, so long as they are
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autoconfiguration may still be available even if no routers are autoconfiguration may still be available even if no routers are
present. present.
Routers send Router Advertisements periodically, but the delay Routers send Router Advertisements periodically, but the delay
between successive advertisements will generally be longer than a between successive advertisements will generally be longer than a
host performing autoconfiguration will want to wait [5]. To obtain an host performing autoconfiguration will want to wait [5]. To obtain an
advertisement quickly, a host sends one or more Router Solicitations advertisement quickly, a host sends one or more Router Solicitations
to the all-routers multicast group. Router Advertisements contain two to the all-routers multicast group. Router Advertisements contain two
flags indicating what type of stateful autoconfiguration (if any) is flags indicating what type of stateful autoconfiguration (if any) is
available. A "managed address configuration (M)" flag indicates available. A "managed address configuration (M)" flag indicates
whether hosts can use stateful autoconfiguration [7] to obtain whether hosts can use stateful autoconfiguration [6] to obtain
addresses. An "other stateful configuration (O)" flag indicates addresses. An "other stateful configuration (O)" flag indicates
whether hosts can use stateful autoconfiguration [8] to obtain whether hosts can use stateful autoconfiguration [7] to obtain
additional information (excluding addresses). additional information (excluding addresses).
The details of how a host may use the M flags, including any use of The details of how a host may use the M flags, including any use of
the "on" and "off" transitions for this flag, to control the use of the "on" and "off" transitions for this flag, to control the use of
the stateful protocol for address assignment will be described in a the stateful protocol for address assignment will be described in a
separate document. Similarly, the details of how a host may use the O separate document. Similarly, the details of how a host may use the O
flags, including any use of the "on" and "off" transitions for this flags, including any use of the "on" and "off" transitions for this
flag, to control the use of the stateful protocol for getting other flag, to control the use of the stateful protocol for getting other
configuration information will be described in a separate document. configuration information will be described in a separate document.
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Duplicate Address Detection for the link-local address and skip Duplicate Address Detection for the link-local address and skip
the test for the global address using the same interface the test for the global address using the same interface
identifier as that of the link-local address. Whereas this identifier as that of the link-local address. Whereas this
document does not invalidate such implementations, this kind of document does not invalidate such implementations, this kind of
"optimization" is NOT RECOMMENDED, and new implementations MUST "optimization" is NOT RECOMMENDED, and new implementations MUST
NOT do that optimization. This optimization came from the NOT do that optimization. This optimization came from the
assumption that all of an interface's addresses are generated from assumption that all of an interface's addresses are generated from
the same identifier. However, the assumption does actually not the same identifier. However, the assumption does actually not
stand; new types of addresses have been introduced where the stand; new types of addresses have been introduced where the
interface identifiers are not necessarily the same for all unicast interface identifiers are not necessarily the same for all unicast
addresses on a single interface [10] [11]. Requiring to perform addresses on a single interface [9] [10]. Requiring to perform
Duplicate Address Detection for all unicast addresses will make Duplicate Address Detection for all unicast addresses will make
the algorithm robust for the current and future such special the algorithm robust for the current and future such special
interface identifiers. interface identifiers.
The procedure for detecting duplicate addresses uses Neighbor The procedure for detecting duplicate addresses uses Neighbor
Solicitation and Advertisement messages as described below. If a Solicitation and Advertisement messages as described below. If a
duplicate address is discovered during the procedure, the address duplicate address is discovered during the procedure, the address
cannot be assigned to the interface. If the address is derived from cannot be assigned to the interface. If the address is derived from
an interface identifier, a new identifier will need to be assigned to an interface identifier, a new identifier will need to be assigned to
the interface, or all IP addresses for the interface will need to be the interface, or all IP addresses for the interface will need to be
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message to be sent, the node SHOULD delay joining the solicited-node message to be sent, the node SHOULD delay joining the solicited-node
multicast address by a random delay between 0 and multicast address by a random delay between 0 and
MAX_RTR_SOLICITATION_DELAY if the address being checked is configured MAX_RTR_SOLICITATION_DELAY if the address being checked is configured
by a router advertisement message sent to a multicast address. The by a router advertisement message sent to a multicast address. The
delay will avoid similar congestion when multiple nodes are going to delay will avoid similar congestion when multiple nodes are going to
configure addresses by receiving a same single multicast router configure addresses by receiving a same single multicast router
advertisement. advertisement.
Note that the delay for joining the multicast address implicitly Note that the delay for joining the multicast address implicitly
means delaying transmission of the corresponding MLD report message means delaying transmission of the corresponding MLD report message
[12]. Since RFC 2710 [12] does not request a random delay to avoid [11]. Since RFC 2710 [11] does not request a random delay to avoid
race conditions, just delaying Neighbor Solicitation would cause race conditions, just delaying Neighbor Solicitation would cause
congestion by the MLD report messages. The congestion would then congestion by the MLD report messages. The congestion would then
prevent MLD-snooping switches from working correctly, and, as a prevent MLD-snooping switches from working correctly, and, as a
result, prevent Duplicate Address Detection from working. The result, prevent Duplicate Address Detection from working. The
requirement to include the delay for the MLD report in this case requirement to include the delay for the MLD report in this case
avoids this scenario. avoids this scenario.
In order to improve the robustness of the Duplicate Address Detection In order to improve the robustness of the Duplicate Address Detection
algorithm, an interface MUST receive and process datagrams sent to algorithm, an interface MUST receive and process datagrams sent to
the all-nodes multicast address or solicited-node multicast address the all-nodes multicast address or solicited-node multicast address
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5.5.1 Soliciting Router Advertisements 5.5.1 Soliciting Router Advertisements
Router Advertisements are sent periodically to the all-nodes Router Advertisements are sent periodically to the all-nodes
multicast address. To obtain an advertisement quickly, a host sends multicast address. To obtain an advertisement quickly, a host sends
out Router Solicitations as described in RFC 2461 [5]. out Router Solicitations as described in RFC 2461 [5].
5.5.2 Absence of Router Advertisements 5.5.2 Absence of Router Advertisements
Even if a link has no routers, stateful autoconfiguration to obtain Even if a link has no routers, stateful autoconfiguration to obtain
addresses and other configuration information may still be available, addresses and other configuration information may still be available,
and hosts may want to use the mechanism. and hosts may want to use the mechanism. From the perspective of
autoconfiguration, a link has no routers if no Router Advertisements
are received after having sent a small number of Router Solicitations
as described in RFC 2461 [5].
Note that it is possible that there is no router on the link in this
sense but there is a node that has the ability to forward packets. In
this case, the forwarding node's address must be manually configured
in hosts to be able to send packets off-link, since the only
mechanism to configure the default router's address automatically is
the one using router advertisements.
5.5.3 Router Advertisement Processing
For each Prefix-Information option in the Router Advertisement:
a) If the Autonomous flag is not set, silently ignore the Prefix
Information option.
b) If the prefix is the link-local prefix, silently ignore the
Prefix Information option.
c) If the preferred lifetime is greater than the valid lifetime,
silently ignore the Prefix Information option. A node MAY wish to
log a system management error in this case.
d) If the prefix advertised is not equal to the prefix of an address
configured by stateless autoconfiguration already in the list of
addresses associated with the interface (where "equal" means the
two prefix lengths are the same and the first prefix-length bits
of the prefixes are identical), and the Valid Lifetime is not 0,
form an address (and add it to the list) by combining the
advertised prefix with the link's interface identifier as follows:
| 128 - N bits | N bits |
+---------------------------------------+------------------------+
| link prefix | interface identifier |
+----------------------------------------------------------------+
If the sum of the prefix length and interface identifier length
does not equal 128 bits, the Prefix Information option MUST be
ignored. An implementation MAY wish to log a system management
error in this case. The length of the interface identifier is
defined in a separate link-type specific document, which should
also be consistent with the address architecture [4] (see Section
2).
It is the responsibility of the system administrator to insure
that the lengths of prefixes contained in Router Advertisements
are consistent with the length of interface identifiers for that
link type. It should be noted, however, that this does not mean
the advertised prefix length is meaningless. In fact, the
advertised length has non trivial meaning for on-link
determination in RFC 2461 [5] where the sum of the prefix length
and the interface identifier length may not be equal to 128. Thus,
it should be safe to validate the advertised prefix length here,
in order to detect and avoid a configuration error specifying an
invalid prefix length in the context of address autoconfiguration.
Note that a future revision of the address architecture [4] and a
future link-type specific document, which will still be consistent
with each other, could potentially allow for an interface
identifier of length other than the value defined in the current
documents. Thus, an implementation should not assume a particular
constant. Rather, it should expect any lengths of interface
identifiers.
If an address is formed successfully, the host adds it to the list
of addresses assigned to the interface, initializing its preferred
and valid lifetime values from the Prefix Information option.
e) If the advertised prefix is equal to the prefix of an address
configured by stateless autoconfiguration in the list, the
preferred lifetime of the address is reset to the Preferred
Lifetime in the received advertisement. The specific action to
perform for the valid lifetime of the address depends on the Valid
Lifetime in the received advertisement and the remaining time to
the valid lifetime expiration of the previously autoconfigured
address. We call the remaining time "RemainingLifetime" in the
following discussion:
1. If the received Valid Lifetime is greater than 2 hours or
greater than RemainingLifetime, set the valid lifetime of the
corresponding address to the advertised Valid Lifetime.
2. If RemainingLifetime is less than or equal to 2 hours, ignore
the Prefix Information option with regards to the valid
lifetime, unless the Router Advertisement from which this
option was obtained has been authenticated (e.g., via IP
security [1]). If the Router Advertisement was authenticated,
the valid lifetime of the corresponding address should be set
to the Valid Lifetime in the received option.
3. Otherwise, reset the valid lifetime of the corresponding
address to two hours.
The above rules address a specific denial of service attack in
which a bogus advertisement could contain prefixes with very small
Valid Lifetimes. Without the above rules, a single unauthenticated
advertisement containing bogus Prefix Information options with
short Valid Lifetimes could cause all of a node's addresses to
expire prematurely. The above rules ensure that legitimate
advertisements (which are sent periodically) will "cancel" the
short Valid Lifetimes before they actually take effect.
Note that the preferred lifetime of the corresponding address is
always reset to the Preferred Lifetime in the received Prefix
Information option, regardless of whether the valid lifetime is
also reset or ignored. The difference comes from the fact that the
possible attack for the preferred lifetime is relatively minor.
Additionally, it is even undesirable to ignore the preferred
lifetime when a valid administrator wants to deprecate a
particular address by sending a short preferred lifetime (and the
valid lifetime is ignored by accident).
5.5.4 Address Lifetime Expiry
A preferred address becomes deprecated when its preferred lifetime
expires. A deprecated address SHOULD continue to be used as a source
address in existing communications, but SHOULD NOT be used to
initiate new communications if an alternate (non-deprecated) address
of sufficient scope can easily be used instead.
Note that the feasibility of initiating new communication using a
non-deprecated address may be an application-specific decision, as
only the application may have knowledge about whether the (now)
deprecated address was (or still is) in use by the application. For
example, if an application explicitly specifies the protocol stack to
use a deprecated address as a source address, the protocol stack must
accept that; the application might request it because that IP address
is used for in higher-level communication and there might be a
requirement that the multiple connections in such a grouping use the
same pair of IP addresses.
IP and higher layers (e.g., TCP, UDP) MUST continue to accept and
process datagrams destined to a deprecated address as normal since a
deprecated address is still a valid address for the interface. In the
case of TCP, this means TCP SYN segments sent to a deprecated address
are responded to using the deprecated address as a source address in
the corresponding SYN-ACK (if the connection would otherwise be
allowed).
An implementation MAY prevent any new communication from using a
deprecated address, but system management MUST have the ability to
disable such a facility, and the facility MUST be disabled by
default.
Other subtle cases should also be noted about source address
selection. For example, the above description does not clarify which
address should be used between a deprecated, smaller-scope address
and a non-deprecated, enough scope address. The details of the
address selection including this case are described in RFC 3484 [8]
and beyond the scope of this document.
An address (and its association with an interface) becomes invalid
when its valid lifetime expires. An invalid address MUST NOT be used
as a source address in outgoing communications and MUST NOT be
recognized as a destination on a receiving interface.
5.6 Configuration Consistency
It is possible for hosts to obtain address information using both
stateless and stateful protocols since both may be enabled at the
same time. It is also possible that the values of other
configuration parameters such as MTU size and hop limit will be
learned from both Router Advertisements and the stateful
autoconfiguration protocol. If the same configuration information is
provided by multiple sources, the value of this information should be
consistent. However, it is not considered a fatal error if
information received from multiple sources is inconsistent. Hosts
accept the union of all information received via the stateless and
stateful protocols. If inconsistent information is learned different
sources, the most recently obtained values always have precedence
over information learned earlier.
5.7 Retaining Configured Addresses for Stability
An implementation that has stable storage may want to retain
addresses in the storage when the addresses were acquired using
stateless address autoconfiguration. Assuming the lifetimes used are
reasonable, this technique implies that a temporary outage (less than
the valid lifetime) of a router will never result in the node losing
its global address even if the node were to reboot. When this
technique is used, it should also be noted that the expiration times
of the preferred and valid lifetimes must be retained, in order to
prevent the use of an address after it has become deprecated or
invalid.
Further details on this kind of extension are beyond the scope of
this document.
6. SECURITY CONSIDERATIONS
Stateless address autoconfiguration allows a host to connect to a
network, configure an address and start communicating with other
nodes without ever registering or authenticating itself with the
local site. Although this allows unauthorized users to connect to
and use a network, the threat is inherently present in the Internet
architecture. Any node with a physical attachment to a network can
generate an address (using a variety of ad hoc techniques) that
provides connectivity.
The use of stateless address autoconfiguration and Duplicate Address
Detection opens up the possibility of several denial of service
attacks. For example, any node can respond to Neighbor Solicitations
for a tentative address, causing the other node to reject the address
as a duplicate. A separate document [12] discusses details about
these attacks. These attacks can be addressed by requiring that
Neighbor Discovery packets be authenticated [1]. However, it should
be noted that [12] points out the use of IP security is not always
feasible depending on network environments.
7. IANA CONSIDERATIONS
This document has no actions for IANA.
8. Acknowledgements
The authors would like to thank the members of both the IPNG (which
is now IPV6) and ADDRCONF working groups for their input. In
particular, thanks to Jim Bound, Steve Deering, Richard Draves, and
Erik Nordmark. Thanks also goes to John Gilmore for alerting the WG
of the "0 Lifetime Prefix Advertisement" denial of service attack
vulnerability; this document incorporates changes that address this
vulnerability.
A number of people have contributed to identifying issues on a
previous version of this document and to proposing resolutions to the
issues, on which this version is based. In addition to those listed
above, the contributors include Jari Arkko, Brian E Carpenter,
Gregory Daley, Ralph Droms, Christian Huitema, Soohong Daniel Park,
Markku Savela, and Pekka Savola.
Normative References
[1] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
November 1998.
[2] Crawford, M., "A Method for the Transmission of IPv6 Packets
over Ethernet Networks", RFC 2464, December 1998.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
[4] Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6)
Addressing Architecture", RFC 3513, April 2003.
[5] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for
IP Version 6 (IPv6)", RFC 2461, December 1998.
Informative References
[6] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and M.
Carney, "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003.
[7] Droms, R., "Stateless Dynamic Host Configuration Protocol
(DHCP) Service for IPv6", RFC 3736, April 2004.
[8] Draves, R., "Default Address Selection for Internet Protocol
version 6 (IPv6)", RFC 3484, February 2003.
[9] Narten, T. and R. Draves, "Privacy Extensions for Stateless
Address Autoconfiguration in IPv6", RFC 3041, January 2001.
[10] Aura, T., "Cryptographically Generated Addresses (CGA)",
draft-ietf-send-cga-06.txt (work in progress), April 2004.
[11] Deering, S., Fenner, W. and B. Haberman, "Multicast Listener
Discovery (MLD) for IPv6", RFC 2710, October 1999.
[12] Nikander, P., Kempf, J. and E. Nordmark, "IPv6 Neighbor
Discovery (ND) Trust Models and Threats", RFC 3756, May 2004.
[13] Deering, S., "Host Extensions for IP Multicasting", RFC 1112,
August 1989.
[14] IEEE, "Wireless LAN Medium Access Control (MAC) and Physical
Layer (PHY) Specifications", ANSI/IEEE STd 802.11, August 1999.
Authors' Addresses
Susan Thomson
Cisco Systems
EMail: sethomso@cisco.com
Thomas Narten
IBM Corporation
P.O. Box 12195
Research Triangle Park, NC 27709-2195
USA
Phone: +1 919-254-7798
EMail: narten@us.ibm.com
Tatuya Jinmei
Corporate Research & Development Center, Toshiba Corporation
1 Komukai Toshiba-cho, Saiwai-ku
Kawasaki-shi, Kanagawa 212-8582
Japan
Phone: +81 44-549-2230
EMail: jinmei@isl.rdc.toshiba.co.jp
Appendix A. LOOPBACK SUPPRESSION & DUPLICATE ADDRESS DETECTION
Determining whether a received multicast solicitation was looped back
to the sender or actually came from another node is implementation-
dependent. A problematic case occurs when two interfaces attached to
the same link happen to have the same identifier and link-layer
address, and they both send out packets with identical contents at
roughly the same time (e.g., Neighbor Solicitations for a tentative
address as part of Duplicate Address Detection messages). Although a
receiver will receive both packets, it cannot determine which packet
was looped back and which packet came from the other node by simply
comparing packet contents (i.e., the contents are identical). In this
particular case, it is not necessary to know precisely which packet
was looped back and which was sent by another node; if one receives
more solicitations than were sent, the tentative address is a
duplicate. However, the situation may not always be this
straightforward.
The IPv4 multicast specification [13] recommends that the service
interface provide a way for an upper-layer protocol to inhibit local
delivery of packets sent to a multicast group that the sending host
is a member of. Some applications know that there will be no other
group members on the same host, and suppressing loopback prevents
them from having to receive (and discard) the packets they themselves
send out. A straightforward way to implement this facility is to
disable loopback at the hardware level (if supported by the
hardware), with packets looped back (if requested) by software. On
interfaces in which the hardware itself suppresses loopbacks, a node
running Duplicate Address Detection simply counts the number of
Neighbor Solicitations received for a tentative address and compares
them with the number expected. If there is a mismatch, the tentative
address is a duplicate.
In those cases where the hardware cannot suppress loopbacks, however,
one possible software heuristic to filter out unwanted loopbacks is
to discard any received packet whose link-layer source address is the
same as the receiving interface's. There is even a link-layer
specification that requires to discard any such packets [14].
Unfortunately, use of that criteria also results in the discarding of
all packets sent by another node using the same link-layer address.
Duplicate Address Detection will fail on interfaces that filter
received packets in this manner:
o If a node performing Duplicate Address Detection discards received
packets having the same source link-layer address as the receiving
interface, it will also discard packets from other nodes also
using the same link-layer address, including Neighbor
Advertisement and Neighbor Solicitation messages required to make
Duplicate Address Detection work correctly. This particular
problem can be avoided by temporarily disabling the software
suppression of loopbacks while a node performs Duplicate Address
Detection, if it is possible to disable the suppression.
o If a node that is already using a particular IP address discards
received packets having the same link-layer source address as the
interface, it will also discard Duplicate Address
Detection-related Neighbor Solicitation messages sent by another
node also using the same link-layer address. Consequently,
Duplicate Address Detection will fail, and the other node will
configure a non-unique address. Since it is generally impossible
to know when another node is performing Duplicate Address
Detection, this scenario can be avoided only if software
suppression of loopback is permanently disabled.
Thus, to perform Duplicate Address Detection correctly in the case
where two interfaces are using the same link-layer address, an
implementation must have a good understanding of the interface's
multicast loopback semantics, and the interface cannot discard
received packets simply because the source link-layer address is the
same as the interfaces. It should also be noted that a link-layer
specification can conflict with the condition necessary to make
Duplicate Address Detection work.
Appendix B. CHANGES SINCE RFC 1971
o Changed document to use term "interface identifier" rather than
"interface token" for consistency with other IPv6 documents.
o Clarified definition of deprecated address to make clear it is OK
to continue sending to or from deprecated addresses.
o Added rules to Section 5.5.3 Router Advertisement processing to
address potential denial-of-service attack when prefixes are
advertised with very short Lifetimes.
o Clarified wording in Section 5.5.4 to make clear that all upper
layer protocols must process (i.e., send and receive) packets sent
to deprecated addresses.
Appendix C. CHANGE HISTORY
Changes since RFC 2462 are:
o Fixed a typo in Section 2.
o Updated references and categorized them into normative and
informative ones.
o Removed redundant code in denial of service protection in Section
5.5.3.
o Clarified that a unicasted NS or NA should be discarded while
performing Duplicate Address Detection.
o Replaced the word "StoredLifetime" with "RemainingLifetime" with a
precise definition to avoid confusion.
o Removed references to site-local and revise wording around the
keyword.
o Added a note about source address selection with regards to
deprecated vs insufficient-scope addresses, etc. Added a reference
to RFC 3484 for further details.
o Clarified what "new communication" means in Section 5.5.4.
o Added a new subsection (5.7) to mention the possibility to use
stable storage to retain configured addresses for stability.
o Revised the Security Considerations section with a reference to
RFC 3756 and a note that the use of IP security is not always
feasible.
o Added a note with a reference in Appendix A about the case where a
link-layer filtering conflicts with a condition to make DAD work
correctly.
o Specified that a node performing Duplicate Address Detection delay
joining the solicited-node multicast group, not just delay sending
Neighbor Solicitations, explaining the detailed reason.
o Clarified the reason why the interface should be disabled after an
address duplicate is detected. Also clarified that the interface
may continue to be used if the interface identifier is not based
on the hardware address.
o Clarified that the preferred lifetime for an existing configured
address is always reset to the advertised value by Router
Advertisement.
o Updated the description of interface identifier considering the
latest format.
Changes since draft-ietf-ipv6-rfc2462bis-00.txt are:
o Clarified how the length of interface identifiers should be
determined, described the relationship with the prefix length
advertised in Router Advertisements, and avoided using a
particular length hard-coded in this document.
o Added a note when an implementation uses stable storage for
autoconfigured addresses.
o Resolved conflict with the Multicast Listener Discovery
specification about random delay for the first packet from the
host.
o Clarified the semantics of the M and O flags based on the latest
standard and operational status. In particular, clarified that
these flags show the availability of the stateful protocol instead
of a trigger to invoke the stateful protocol. ManagedFlag and
OtherConfigFlag, which were implementation-internal variables,
were removed accordingly.
o Recommended to perform Duplicate Address Detection for all unicast
addresses more strongly, considering a variety of different
interface identifiers, while keeping care of existing
implementations.
o Added a requirement for a random delay befor sending Neighbor
Solicitations for Duplicate Address Detection if the address being
checked is configured by a multicasted Router Advertisements.
o Clarified that the prefix comparison in Section 5.5.3 is based on
exact match. Also clarified the comparison described in this
document concentrates on addresses configured by the stateless
mechanism.
o Revisited the author list.
o Added IANA Considerations Section.
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 End of changes. 

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