8. Redirect Function
This section describes the functions related to the sending and
processing of Redirect messages.
Redirect messages are sent by routers to redirect a host to a better
first-hop router for a specific destination or to inform hosts that a
destination is in fact a neighbor (i.e., on-link). The latter is
accomplished by having the ICMP Target Address be equal to the ICMP
A router MUST be able to determine the link-local address for each of
its neighboring routers in order to ensure that the target address in
a Redirect message identifies the neighbor router by its link-local
address. For static routing, this requirement implies that the next-
hop router's address should be specified using the link-local address
of the router. For dynamic routing, this requirement implies that
all IPv6 routing protocols must somehow exchange the link-local
addresses of neighboring routers.
8.1. Validation of Redirect Messages
A host MUST silently discard any received Redirect message that does
not satisfy all of the following validity checks:
- IP Source Address is a link-local address. Routers must use
their link-local address as the source for Router Advertisement
and Redirect messages so that hosts can uniquely identify
- The IP Hop Limit field has a value of 255, i.e., the packet
could not possibly have been forwarded by a router.
- ICMP Checksum is valid.
- ICMP Code is 0.
- ICMP length (derived from the IP length) is 40 or more octets.
- The IP source address of the Redirect is the same as the current
first-hop router for the specified ICMP Destination Address.
- The ICMP Destination Address field in the redirect message does
not contain a multicast address.
- The ICMP Target Address is either a link-local address (when
redirected to a router) or the same as the ICMP Destination
Address (when redirected to the on-link destination).
- All included options have a length that is greater than zero.
The contents of the Reserved field, and of any unrecognized options,
MUST be ignored. Future, backward-compatible changes to the protocol
may specify the contents of the Reserved field or add new options;
backward-incompatible changes may use different Code values.
The contents of any defined options that are not specified to be used
with Redirect messages MUST be ignored and the packet processed as
normal. The only defined options that may appear are the Target
Link-Layer Address option and the Redirected Header option.
A host MUST NOT consider a redirect invalid just because the Target
Address of the redirect is not covered under one of the link's
prefixes. Part of the semantics of the Redirect message is that the
Target Address is on-link.
A redirect that passes the validity checks is called a "valid
8.2. Router Specification
A router SHOULD send a redirect message, subject to rate limiting,
whenever it forwards a packet that is not explicitly addressed to
itself (i.e., a packet that is not source routed through the router)
- the Source Address field of the packet identifies a neighbor,
- the router determines (by means outside the scope of this
specification) that a better first-hop node resides on the same
link as the sending node for the Destination Address of the
packet being forwarded, and
- the Destination Address of the packet is not a multicast
The transmitted redirect packet contains, consistent with the message
format given in Section 4.5:
- In the Target Address field: the address to which subsequent
packets for the destination should be sent. If the target is a
router, that router's link-local address MUST be used. If the
target is a host, the target address field MUST be set to the
same value as the Destination Address field.
- In the Destination Address field: the destination address of the
invoking IP packet.
- In the options:
o Target Link-Layer Address option: link-layer address of the
target, if known.
o Redirected Header: as much of the forwarded packet as can
fit without the redirect packet exceeding the minimum MTU
required to support IPv6 as specified in [IPv6].
A router MUST limit the rate at which Redirect messages are sent, in
order to limit the bandwidth and processing costs incurred by the
Redirect messages when the source does not correctly respond to the
Redirects, or the source chooses to ignore unauthenticated Redirect
messages. More details on the rate-limiting of ICMP error messages
can be found in [ICMPv6].
A router MUST NOT update its routing tables upon receipt of a
8.3. Host Specification
A host receiving a valid redirect SHOULD update its Destination Cache
accordingly so that subsequent traffic goes to the specified target.
If no Destination Cache entry exists for the destination, an
implementation SHOULD create such an entry.
If the redirect contains a Target Link-Layer Address option, the host
either creates or updates the Neighbor Cache entry for the target.
In both cases, the cached link-layer address is copied from the
Target Link-Layer Address option. If a Neighbor Cache entry is
created for the target, its reachability state MUST be set to STALE
as specified in Section 7.3.3. If a cache entry already existed and
it is updated with a different link-layer address, its reachability
state MUST also be set to STALE. If the link-layer address is the
same as that already in the cache, the cache entry's state remains
If the Target and Destination Addresses are the same, the host MUST
treat the Target as on-link. If the Target Address is not the same
as the Destination Address, the host MUST set IsRouter to TRUE for
the target. If the Target and Destination Addresses are the same,
however, one cannot reliably determine whether the Target Address is
a router. Consequently, newly created Neighbor Cache entries should
set the IsRouter flag to FALSE, while existing cache entries should
leave the flag unchanged. If the Target is a router, subsequent
Neighbor Advertisement or Router Advertisement messages will update
Redirect messages apply to all flows that are being sent to a given
destination. That is, upon receipt of a Redirect for a Destination
Address, all Destination Cache entries to that address should be
updated to use the specified next-hop, regardless of the contents of
the Flow Label field that appears in the Redirected Header option.
A host MUST NOT send Redirect messages.
9. Extensibility - Option Processing
Options provide a mechanism for encoding variable length fields,
fields that may appear multiple times in the same packet, or
information that may not appear in all packets. Options can also be
used to add additional functionality to future versions of ND.
In order to ensure that future extensions properly coexist with
current implementations, all nodes MUST silently ignore any options
they do not recognize in received ND packets and continue processing
the packet. All options specified in this document MUST be
recognized. A node MUST NOT ignore valid options just because the ND
message contains unrecognized ones.
The current set of options is defined in such a way that receivers
can process multiple options in the same packet independently of each
other. In order to maintain these properties, future options SHOULD
follow the simple rule:
The option MUST NOT depend on the presence or absence of any other
options. The semantics of an option should depend only on the
information in the fixed part of the ND packet and on the
information contained in the option itself.
Adhering to the above rule has the following benefits:
1) Receivers can process options independently of one another. For
example, an implementation can choose to process the Prefix
Information option contained in a Router Advertisement message
in a user-space process while the link-layer address option in
the same message is processed by routines in the kernel.
2) Should the number of options cause a packet to exceed a link's
MTU, multiple packets can carry subsets of the options without
any change in semantics.
3) Senders MAY send a subset of options in different packets. For
instance, if a prefix's Valid and Preferred Lifetime are high
enough, it might not be necessary to include the Prefix
Information option in every Router Advertisement. In addition,
different routers might send different sets of options. Thus, a
receiver MUST NOT associate any action with the absence of an
option in a particular packet. This protocol specifies that
receivers should only act on the expiration of timers and on the
information that is received in the packets.
Options in Neighbor Discovery packets can appear in any order;
receivers MUST be prepared to process them independently of their
order. There can also be multiple instances of the same option in a
message (e.g., Prefix Information options).
If the number of included options in a Router Advertisement causes
the advertisement's size to exceed the link MTU, the router can send
multiple separate advertisements, each containing a subset of the
The amount of data to include in the Redirected Header option MUST be
limited so that the entire redirect packet does not exceed the
minimum MTU required to support IPv6 as specified in [IPv6].
All options are a multiple of 8 octets of length, ensuring
appropriate alignment without any "pad" options. The fields in the
options (as well as the fields in ND packets) are defined to align on
their natural boundaries (e.g., a 16-bit field is aligned on a 16-bit
boundary) with the exception of the 128-bit IP addresses/prefixes,
which are aligned on a 64-bit boundary. The link-layer address field
contains an uninterpreted octet string; it is aligned on an 8-bit
The size of an ND packet including the IP header is limited to the
link MTU. When adding options to an ND packet, a node MUST NOT
exceed the link MTU.
Future versions of this protocol may define new option types.
Receivers MUST silently ignore any options they do not recognize and
continue processing the message.
10. Protocol Constants
MAX_INITIAL_RTR_ADVERT_INTERVAL 16 seconds
MAX_INITIAL_RTR_ADVERTISEMENTS 3 transmissions
MAX_FINAL_RTR_ADVERTISEMENTS 3 transmissions
MIN_DELAY_BETWEEN_RAS 3 seconds
MAX_RA_DELAY_TIME .5 seconds
MAX_RTR_SOLICITATION_DELAY 1 second
RTR_SOLICITATION_INTERVAL 4 seconds
MAX_RTR_SOLICITATIONS 3 transmissions
MAX_MULTICAST_SOLICIT 3 transmissions
MAX_UNICAST_SOLICIT 3 transmissions
MAX_ANYCAST_DELAY_TIME 1 second
MAX_NEIGHBOR_ADVERTISEMENT 3 transmissions
REACHABLE_TIME 30,000 milliseconds
RETRANS_TIMER 1,000 milliseconds
DELAY_FIRST_PROBE_TIME 5 seconds
Additional protocol constants are defined with the message formats in
All protocol constants are subject to change in future revisions of
The constants in this specification may be overridden by specific
documents that describe how IPv6 operates over different link layers.
This rule allows Neighbor Discovery to operate over links with widely
varying performance characteristics.
11. Security Considerations
Neighbor Discovery is subject to attacks that cause IP packets to
flow to unexpected places. Such attacks can be used to cause denial
of service but also allow nodes to intercept and optionally modify
packets destined for other nodes. This section deals with the main
threats related to Neighbor Discovery messages and possible security
mechanisms that can mitigate these threats.
11.1. Threat Analysis
This section discusses the main threats associated with Neighbor
Discovery. A more detailed analysis can be found in [PSREQ]. The
main vulnerabilities of the protocol fall under three categories:
- Denial-of-Service (DoS) attacks.
- Address spoofing attacks.
- Router spoofing attacks.
An example of denial of service attacks is that a node on the link
that can send packets with an arbitrary IP source address can both
advertise itself as a default router and also send "forged" Router
Advertisement messages that immediately time out all other default
routers as well as all on-link prefixes. An intruder can achieve
this by sending out multiple Router Advertisements, one for each
legitimate router, with the source address set to the address of
another router, the Router Lifetime field set to zero, and the
Preferred and Valid lifetimes set to zero for all the prefixes. Such
an attack would cause all packets, for both on-link and off-link
destinations, to go to the rogue router. That router can then
selectively examine, modify, or drop all packets sent on the link.
The Neighbor Unreachability Detection (NUD) will not detect such a
black hole as long as the rogue router politely answers the NUD
probes with a Neighbor Advertisement with the R-bit set.
It is also possible for any host to launch a DoS attack on another
host by preventing it from configuring an address using [ADDRCONF].
The protocol does not allow hosts to verify whether the sender of a
Neighbor Advertisement is the true owner of the IP address included
in the message.
Redirect attacks can also be achieved by any host in order to flood a
victim or steal its traffic. A host can send a Neighbor
Advertisement (in response to a solicitation) that contains its IP
address and a victim's link-layer address in order to flood the
victim with unwanted traffic. Alternatively, the host can send a
Neighbor Advertisement that includes a victim's IP address and its
own link-layer address to overwrite an existing entry in the sender's
destination cache, thereby forcing the sender to forward all of the
victim's traffic to itself.
The trust model for redirects is the same as in IPv4. A redirect is
accepted only if received from the same router that is currently
being used for that destination. If a host has been redirected to
another node (i.e., the destination is on-link), there is no way to
prevent the target from issuing another redirect to some other
destination. However, this exposure is no worse than it was before
being redirected; the target host, once subverted, could always act
as a hidden router to forward traffic elsewhere.
The protocol contains no mechanism to determine which neighbors are
authorized to send a particular type of message (e.g., Router
Advertisements); any neighbor, presumably even in the presence of
authentication, can send Router Advertisement messages thereby being
able to cause denial of service. Furthermore, any neighbor can send
proxy Neighbor Advertisements as well as unsolicited Neighbor
Advertisements as a potential denial-of-service attack.
Many link layers are also subject to different denial-of-service
attacks such as continuously occupying the link in CSMA/CD (Carrier
Sense Multiple Access with Collision Detection) networks (e.g., by
sending packets closely back-to-back or asserting the collision
signal on the link), or originating packets with somebody else's
source MAC address to confuse, e.g., Ethernet switches. On the other
hand, many of the threats discussed in this section are less
effective, or non-existent, on point-to-point links, or cellular
links where a host shares a link with only one neighbor, i.e., the
11.2. Securing Neighbor Discovery Messages
The protocol reduces the exposure to the above threats in the absence
of authentication by ignoring ND packets received from off-link
senders. The Hop Limit field of all received packets is verified to
contain 255, the maximum legal value. Because routers decrement the
Hop Limit on all packets they forward, received packets containing a
Hop Limit of 255 must have originated from a neighbor.
Cryptographic security mechanisms for Neighbor Discovery are outside
the scope of this document and are defined in [SEND]. Alternatively,
IPsec can be used for IP layer authentication [IPv6-SA]. The use of
the Internet Key Exchange (IKE) is not suited for creating dynamic
security associations that can be used to secure address resolution
or neighbor solicitation messages as documented in [ICMPIKE].
In some cases, it may be acceptable to use statically configured
security associations with either [IPv6-AUTH] or [IPv6-ESP] to secure
Neighbor Discovery messages. However, it is important to note that
statically configured security associations are not scalable
(especially when considering multicast links) and are therefore
limited to small networks with known hosts. In any case, if either
[IPv6-AUTH] or [IPv6-ESP] is used, ND packets MUST be verified for
the purpose of authentication. Packets that fail authentication
checks MUST be silently discarded.
12. Renumbering Considerations
The Neighbor Discovery protocol together with IPv6 Address
Autoconfiguration [ADDRCONF] provides mechanisms to aid in
renumbering -- new prefixes and addresses can be introduced and old
ones can be deprecated and removed.
The robustness of these mechanisms is based on all the nodes on the
link receiving the Router Advertisement messages in a timely manner.
However, a host might be turned off or be unreachable for an extended
period of time (i.e., a machine is powered down for months after a
project terminates). It is possible to preserve robust renumbering
in such cases, but it does place some constraints on how long
prefixes must be advertised.
Consider the following example in which a prefix is initially
advertised with a lifetime of 2 months, but on August 1st it is
determined that the prefix needs to be deprecated and removed due to
renumbering by September 1st. This can be done by reducing the
advertised lifetime to 1 week starting on August 1st, and as the
cutoff gets closer, the lifetimes can be made shorter until by
September 1st the prefix is advertised with a lifetime of 0. The
point is that, if one or more nodes were unplugged from the link
prior to September 1st, they might still think that the prefix is
valid since the last lifetime they received was 2 months. Thus, if a
node was unplugged on July 31st, it thinks the prefix is valid until
September 30th. If that node is plugged back in prior to September
30th, it may continue to use the old prefix. The only way to force a
node to stop using a prefix that was previously advertised with a
long lifetime is to have that node receive an advertisement for that
prefix that changes the lifetime downward. The solution in this
example is simple: continue advertising the prefix with a lifetime of
0 from September 1st until October 1st.
In general, in order to be robust against nodes that might be
unplugged from the link, it is important to track the furthest into
the future that a particular prefix can be viewed as valid by any
node on the link. The prefix must then be advertised with a 0
lifetime until that point in the future. This "furthest into the
future" time is simply the maximum, over all Router Advertisements,
of the time the advertisement was sent, plus the prefix's lifetime
contained in the advertisement.
The above has an important implication on using infinite lifetimes.
If a prefix is advertised with an infinite lifetime, and that prefix
later needs to be renumbered, it is undesirable to continue
advertising that prefix with a zero lifetime forever. Thus, either
infinite lifetimes should be avoided or there must be a limit on how
long of a time a node can be unplugged from the link before it is
plugged back in again. However, it is unclear how the network
administrator can enforce a limit on how long time hosts such as
laptops can be unplugged from the link.
Network administrators should give serious consideration to using
relatively short lifetimes (i.e., no more than a few weeks). While
it might appear that using long lifetimes would help ensure
robustness, in reality, a host will be unable to communicate in the
absence of properly functioning routers. Such routers will be
sending Router Advertisements that contain appropriate (and current)
prefixes. A host connected to a network that has no functioning
routers is likely to have more serious problems than just a lack of a
valid prefix and address.
The above discussion does not distinguish between the preferred and
valid lifetimes. For all practical purposes, it is probably
sufficient to track the valid lifetime since the preferred lifetime
will not exceed the valid lifetime.
13. IANA Considerations
This document does not require any new ICMPv6 types or codes to be
allocated. However, existing ICMPv6 types have been updated to point
to this document instead of RFC 2461. The procedure for the
assignment of ICMPv6 types/codes is described in Section 6 of
This document continues to use the following ICMPv6 message types
introduced in RFC 2461 and already assigned by IANA:
Message name ICMPv6 Type
Router Solicitation 133
Router Advertisement 134
Neighbor Solicitation 135
Neighbor Advertisement 136
This document continues to use the following Neighbor Discovery
option types introduced in RFC 2461 and already assigned by IANA:
Option Name Type
Source Link-Layer Address 1
Target Link-Layer Address 2
Prefix Information 3
Redirected Header 4
Neighbor Discovery option types are allocated using the following
1. The IANA should allocate and permanently register new option types
from IETF RFC publication. This is for all RFC types including
standards track, informational, and experimental status that
originate from the IETF and have been approved by the IESG for
2. IETF working groups with working group consensus and area director
approval can request reclaimable Neighbor Discovery option type
assignments from the IANA. The IANA will tag the values as
"reclaimable in future".
The "reclaimable in the future" tag will be removed when an RFC is
published documenting the protocol as defined in 1). This will make
the assignment permanent and update the reference on the IANA Web
At the point where the option type values are 85% assigned, the IETF
will review the assignments tagged "reclaimable in the future" and
inform the IANA which ones should be reclaimed and reassigned.
3. Requests for new option type value assignments from outside the
IETF are only made through the publication of an IETF document, per
1) above. Note also that documents published as "RFC Editor
contributions" [RFC3667] are not considered to be IETF documents.
14.1. Normative References
[ADDR-ARCH] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[ICMPv6] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", RFC 4443,
[IPv6] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
14.2. Informative References
[ADDRCONF] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
[ADDR-SEL] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[ARP] Plummer, D., "Ethernet Address Resolution Protocol: Or
Converting Network Protocol Addresses to 48.bit Ethernet
Address for Transmission on Ethernet Hardware", STD 37,
RFC 826, November 1982.
[ASSIGNED] Reynolds, J., Ed., "Assigned Numbers: RFC 1700 is
Replaced by an On-line Database", RFC 3232, January
[DHCPv6] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC 3315, July 2003.
[HR-CL] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[ICMPIKE] Arkko, J., "Effects of ICMPv6 on IKE", Work in Progress,
[ICMPv4] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
[IPv6-3GPP] Wasserman, M., Ed., "Recommendations for IPv6 in Third
Generation Partnership Project (3GPP) Standards", RFC
3314, September 2002.
[IPv6-CELL] Arkko, J., Kuijpers, G., Soliman, H., Loughney, J., and
J. Wiljakka, "Internet Protocol Version 6 (IPv6) for
Some Second and Third Generation Cellular Hosts", RFC
3316, April 2003.
[IPv6-ETHER] Crawford, M., "Transmission of IPv6 Packets over
Ethernet Networks", RFC 2464, December 1998.
[IPv6-SA] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[IPv6-AUTH] Kent, S., "IP Authentication Header", RFC 4302, December
[IPv6-ESP] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
4303, December 2005.
[IPv6-NBMA] Armitage, G., Schulter, P., Jork, M., and G. Harter,
"IPv6 over Non-Broadcast Multiple Access (NBMA)
networks", RFC 2491, January 1999.
[LD-SHRE] Hinden, R. and D. Thaler, "IPv6 Host-to-Router Load
Sharing", RFC 4311, November 2005.
[MIPv6] Johnson, D., Perkins, C., and J. Arkko, "Mobility
Support in IPv6", RFC 3775, June 2004.
[MLD] Deering, S., Fenner, W., and B. Haberman, "Multicast
Listener Discovery (MLD) for IPv6", RFC 2710, October
[MLDv2] Vida, R., Ed., and L. Costa, Ed., "Multicast Listener
Discovery Version 2 (MLDv2) for IPv6", RFC 3810, June
[PSREQ] Nikander, P., Ed., Kempf, J., and E. Nordmark, "IPv6
Neighbor Discovery (ND) Trust Models and Threats", RFC
3756, May 2004.
[RAND] Eastlake, D., 3rd, Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC
4086, June 2005.
[RDISC] Deering, S., Ed., "ICMP Router Discovery Messages", RFC
1256, September 1991.
[RFC3667] Bradner, S., "IETF Rights in Contributions", RFC 3667,
[RTSEL] Draves, R. and D. Thaler, "Default Router Preferences
and More-Specific Routes", RFC 4191, November 2005.
[SH-MEDIA] Braden, B., Postel, J., and Y. Rekhter, "Internet
Architecture Extensions for Shared Media", RFC 1620, May
[SEND] Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
"SEcure Neighbor Discovery (SEND)", RFC 3971, March
[SYNC] S. Floyd, V. Jacobson, "The Synchronization of Periodic
Routing Messages", IEEE/ACM Transactions on Networking,
April 1994. ftp://ftp.ee.lbl.gov/papers/sync_94.ps.Z
Appendix A: Multihomed Hosts
There are a number of complicating issues that arise when Neighbor
Discovery is used by hosts that have multiple interfaces. This
section does not attempt to define the proper operation of multihomed
hosts with regard to Neighbor Discovery. Rather, it identifies
issues that require further study. Implementors are encouraged to
experiment with various approaches to making Neighbor Discovery work
on multihomed hosts and to report their experiences. Further work
related to this problem can be found in [RTSEL].
If a multihomed host receives Router Advertisements on all of its
interfaces, it will (probably) have learned on-link prefixes for the
addresses residing on each link. When a packet must be sent through
a router, however, selecting the "wrong" router can result in a
suboptimal or non-functioning path. There are number of issues to
1) In order for a router to send a redirect, it must determine that
the packet it is forwarding originates from a neighbor. The
standard test for this case is to compare the source address of
the packet to the list of on-link prefixes associated with the
interface on which the packet was received. If the originating
host is multihomed, however, the source address it uses may
belong to an interface other than the interface from which it
was sent. In such cases, a router will not send redirects, and
suboptimal routing is likely. In order to be redirected, the
sending host must always send packets out the interface
corresponding to the outgoing packet's source address. Note
that this issue never arises with non-multihomed hosts; they
only have one interface. Additional discussion on this topic
can be found in RFC 1122 under Section 220.127.116.11.
2) If the selected first-hop router does not have a route at all
for the destination, it will be unable to deliver the packet.
However, the destination may be reachable through a router on
one of the other interfaces. Neighbor Discovery does not
address this scenario; it does not arise in the non-multihomed
3) Even if the first-hop router does have a route for a
destination, there may be a better route via another interface.
No mechanism exists for the multihomed host to detect this
If a multihomed host fails to receive Router Advertisements on one or
more of its interfaces, it will not know (in the absence of
configured information) which destinations are on-link on the
affected interface(s). This leads to the following problem: If
Router Advertisements are received on some, but not all, interfaces,
a multihomed host could choose to only send packets out on the
interfaces on which it has received Router Advertisements. A key
assumption made here, however, is that routers on those other
interfaces will be able to route packets to the ultimate destination,
even when those destinations reside on the subnet to which the sender
connects, but has no on-link prefix information. Should the
assumption be FALSE, communication would fail. Even if the
assumption holds, packets will traverse a suboptimal path.
Appendix B: Future Extensions
Possible extensions for future study are:
o Using dynamic timers to be able to adapt to links with widely
varying delay. Measuring round-trip times, however, requires
acknowledgments and sequence numbers in order to match received
Neighbor Advertisements with the actual Neighbor Solicitation that
triggered the advertisement. Implementors wishing to experiment
with such a facility could do so in a backwards-compatible way by
defining a new option carrying the necessary information. Nodes
not understanding the option would simply ignore it.
o Adding capabilities to facilitate the operation over links that
currently require hosts to register with an address resolution
server. This could, for instance, enable routers to ask hosts to
send them periodic unsolicited advertisements. Once again, this
can be added using a new option sent in the Router Advertisements.
o Adding additional procedures for links where asymmetric and non-
transitive reachability is part of normal operations. Such
procedures might allow hosts and routers to find usable paths on,
e.g., radio links.
Appendix C: State Machine for the Reachability State
This appendix contains a summary of the rules specified in Sections
7.2 and 7.3. This document does not mandate that implementations
adhere to this model as long as their external behavior is consistent
with that described in this document.
When performing address resolution and Neighbor Unreachability
Detection the following state transitions apply using the conceptual
State Event Action New state
- Packet to send. Create entry. INCOMPLETE
Send multicast NS.
Start retransmit timer
INCOMPLETE Retransmit timeout, Retransmit NS INCOMPLETE
less than N Start retransmit
INCOMPLETE Retransmit timeout, Discard entry -
N or more Send ICMP error
INCOMPLETE NA, Solicited=0, Record link-layer STALE
Override=any address. Send queued
INCOMPLETE NA, Solicited=1, Record link-layer REACHABLE
Override=any address. Send queued
INCOMPLETE NA, Solicited=any, Update content of unchanged
Override=any, No IsRouter flag
- NS, RS, Redirect - -
No link-layer address
!INCOMPLETE NA, Solicited=1, - REACHABLE
address as cached.
!INCOMPLETE NA, Solicited=any, Update content of unchanged
Override=any, No IsRouter flag.
REACHABLE NA, Solicited=1, - STALE
address than cached.
STALE, PROBE NA, Solicited=1, - unchanged
Or DELAY Override=0
address than cached.
!INCOMPLETE NA, Solicited=1, Record link-layer REACHABLE
Override=1 address (if
!INCOMPLETE NA, Solicited=0, - unchanged
!INCOMPLETE NA, Solicited=0, - unchanged
address as cached.
!INCOMPLETE NA, Solicited=0, Record link-layer STALE
address than cached.
!INCOMPLETE upper-layer reachability - REACHABLE
REACHABLE timeout, more than - STALE
N seconds since
STALE Sending packet Start delay timer DELAY
DELAY Delay timeout Send unicast NS probe PROBE
Start retransmit timer
PROBE Retransmit timeout, Retransmit NS PROBE
less than N
PROBE Retransmit timeout, Discard entry -
N or more
The state transitions for receiving unsolicited information other
than Neighbor Advertisement messages apply to either the source of
the packet (for Neighbor Solicitation, Router Solicitation, and
Router Advertisement messages) or the target address (for Redirect
messages) as follows:
State Event Action New state
- NS, RS, RA, Redirect Create entry. STALE
INCOMPLETE NS, RS, RA, Redirect Record link-layer STALE
address. Send queued
!INCOMPLETE NS, RS, RA, Redirect Update link-layer STALE
Different link-layer address
address than cached.
INCOMPLETE NS, RS No link-layer - unchanged
!INCOMPLETE NS, RS, RA, Redirect - unchanged
address as cached.
Appendix D: Summary of IsRouter Rules
This appendix presents a summary of the rules for maintaining the
IsRouter flag as specified in this document.
The background for these rules is that the ND messages contain,
either implicitly or explicitly, information that indicates whether
or not the sender (or Target Address) is a host or a router. The
following assumptions are used:
- The sender of a Router Advertisement is implicitly assumed to be a
- Neighbor Solicitation messages do not contain either an implicit
or explicit indication about the sender. Both hosts and routers
send such messages.
- Neighbor Advertisement messages contain an explicit "IsRouter
flag", the R-bit.
- The target of the redirect, when the target differs from the
destination address in the packet being redirected, is implicitly
assumed to be a router. This is a natural assumption since that
node is expected to be able to forward the packets towards the
- The target of the redirect, when the target is the same as the
destination, does not carry any host vs. router information. All
that is known is that the destination (i.e., target) is on-link
but it could be either a host or a router.
The rules for setting the IsRouter flag are based on the information
content above. If an ND message contains explicit or implicit
information, the receipt of the message will cause the IsRouter flag
to be updated. But when there is no host vs. router information in
the ND message, the receipt of the message MUST NOT cause a change to
the IsRouter state. When the receipt of such a message causes a
Neighbor Cache entry to be created, this document specifies that the
IsRouter flag be set to FALSE. There is greater potential for
mischief when a node incorrectly thinks a host is a router, than the
other way around. In these cases, a subsequent Neighbor
Advertisement or Router Advertisement message will set the correct
Appendix E: Implementation Issues
E.1. Reachability Confirmations
Neighbor Unreachability Detection requires explicit confirmation that
a forward-path is functioning properly. To avoid the need for
Neighbor Solicitation probe messages, upper-layer protocols should
provide such an indication when the cost of doing so is small.
Reliable connection-oriented protocols such as TCP are generally
aware when the forward-path is working. When TCP sends (or receives)
data, for instance, it updates its window sequence numbers, sets and
cancels retransmit timers, etc. Specific scenarios that usually
indicate a properly functioning forward-path include:
- Receipt of an acknowledgment that covers a sequence number (e.g.,
data) not previously acknowledged indicates that the forward path
was working at the time the data was sent.
- Completion of the initial three-way handshake is a special case of
the previous rule; although no data is sent during the handshake,
the SYN flags are counted as data from the sequence number
perspective. This applies to both the SYN+ACK for the active open
and the ACK of that packet on the passively opening peer.
- Receipt of new data (i.e., data not previously received) indicates
that the forward-path was working at the time an acknowledgment
was sent that advanced the peer's send window that allowed the new
data to be sent.
To minimize the cost of communicating reachability information
between the TCP and IP layers, an implementation may wish to rate-
limit the reachability confirmations its sends IP. One possibility
is to process reachability only every few packets. For example, one
might update reachability information once per round-trip time, if an
implementation only has one round-trip timer per connection. For
those implementations that cache Destination Cache entries within
control blocks, it may be possible to update the Neighbor Cache entry
directly (i.e., without an expensive lookup) once the TCP packet has
been demultiplexed to its corresponding control block. For other
implementations, it may be possible to piggyback the reachability
confirmation on the next packet submitted to IP assuming that the
implementation guards against the piggybacked confirmation becoming
stale when no packets are sent to IP for an extended period of time.
TCP must also guard against thinking "stale" information indicates
current reachability. For example, new data received 30 minutes
after a window has opened up does not constitute a confirmation that
the path is currently working; it merely indicates that 30 minutes
ago the window update reached the peer, i.e., the path was working at
that point in time. An implementation must also take into account
TCP zero-window probes that are sent even if the path is broken and
the window update did not reach the peer.
For UDP-based applications (Remote Procedure Call (RPC), DNS), it is
relatively simple to make the client send reachability confirmations
when the response packet is received. It is more difficult and in
some cases impossible for the server to generate such confirmations
since there is no flow control, i.e., the server cannot determine
whether a received request indicates that a previous response reached
Note that an implementation cannot use negative upper-layer advice as
a replacement for the Neighbor Unreachability Detection algorithm.
Negative advice (e.g., from TCP when there are excessive
retransmissions) could serve as a hint that the forward path from the
sender of the data might not be working. But it would fail to detect
when the path from the receiver of the data is not functioning,
causing none of the acknowledgment packets to reach the sender.
Appendix F: Changes from RFC 2461
o Removed references to IPsec AH and ESP for securing messages or as
part of validating the received message.
o Added Section 3.3.
o Updated Section 11 to include more detailed discussion on threats,
IPsec limitations, and use of SEND.
o Removed the on-link assumption in Section 5.2 based on RFC 4942,
"IPv6 Neighbor Discovery On-Link Assumption Considered Harmful".
o Clarified the definition of the Router Lifetime field in Section
o Updated the text in Sections 4.6.2 and 6.2.1 to indicate that the
preferred lifetime must not be larger than valid lifetime.
o Removed the reference to stateful configuration and added reference
for DHCPv6 instead.
o Added the IsRouter flag definition to Section 6.2.1 to allow for
mixed host/router behavior.
o Allowed mobile nodes to be exempt from adding random delays before
sending an RS during a handover.
o Updated the definition of the prefix length in the prefix option.
o Updated the applicability to NBMA links in the introduction and
added references to 3GPP RFCs.
o Clarified that support for load balancing is limited to routers.
o Clarified router behavior when receiving a Router Solicitation
without Source Link-Layer Address Option (SLLAO).
o Clarified that inconsistency checks for CurHopLimit are done for
non-zero values only.
o Rearranged Section 7.2.5 for clarity, and described the processing
when receiving the NA in INCOMPLETE state.
o Added clarifications in Section 7.2 on how a node should react upon
receiving a message without SLLAO.
o Added new IANA section.
o Miscellaneous editorials.
The authors of RFC 2461 would like to acknowledge the contributions
of the IPV6 working group and, in particular, (in alphabetical order)
Ran Atkinson, Jim Bound, Scott Bradner, Alex Conta, Stephen Deering,
Richard Draves, Francis Dupont, Robert Elz, Robert Gilligan, Robert
Hinden, Tatuya Jinmei, Allison Mankin, Dan McDonald, Charles Perkins,
Matt Thomas, and Susan Thomson.
The editor of this document (Hesham Soliman) would like to thank the
IPV6 working group for the numerous contributions to this revision --
in particular (in alphabetical order), Greg Daley, Elwyn Davies,
Ralph Droms, Brian Haberman, Bob Hinden, Tatuya Jinmei, Pekka Savola,
Fred Templin, and Christian Vogt.
P.O. Box 12195
Research Triangle Park, NC 27709-2195
Phone: +1 919 254 7798
Sun Microsystems, Inc.
17 Network Circle
Menlo Park, CA 94025
Phone: +1 650 786 2921
Fax: +1 650 786 5896
William Allen Simpson
Computer Systems Consulting Services
Madison Heights, Michigan 48071
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