|Title||Neighbor Unreachability Detection Is Too Impatient
|Author||E. Nordmark, I.
Internet Engineering Task Force (IETF) E. Nordmark
Request for Comments: 7048 Arista Networks
Updates: 4861 I. Gashinsky
Category: Standards Track Yahoo!
ISSN: 2070-1721 January 2014
Neighbor Unreachability Detection Is Too Impatient
IPv6 Neighbor Discovery includes Neighbor Unreachability Detection.
That function is very useful when a host has an alternative neighbor
-- for instance, when there are multiple default routers -- since it
allows the host to switch to the alternative neighbor in a short
time. By default, this time is 3 seconds after the node starts
probing. However, if there are no alternative neighbors, this
timeout behavior is far too impatient. This document specifies
relaxed rules for Neighbor Discovery retransmissions that allow an
implementation to choose different timeout behavior based on whether
or not there are alternative neighbors. This document updates RFC
Status of This Memo
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(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
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Table of Contents
1. Introduction ....................................................2
2. Definition of Terms .............................................3
3. Protocol Updates ................................................3
4. Example Algorithm ...............................................6
5. Acknowledgements ................................................7
6. Security Considerations .........................................8
7. References ......................................................8
7.1. Normative References .......................................8
7.2. Informative References .....................................8
IPv6 Neighbor Discovery [RFC4861] includes Neighbor Unreachability
Detection (NUD), which detects when a neighbor is no longer
reachable. The timeouts specified for NUD are very short (by
default, three transmissions spaced one second apart). These short
timeouts can be appropriate when there are alternative neighbors to
which the packets can be sent -- for example, if a host has multiple
default routers in its Default Router List or if the host has a
Neighbor Cache Entry (NCE) created by a Redirect message. In those
cases, when NUD fails, the host will try the alternative neighbor by
redoing the next-hop selection. That implies picking the next router
in the Default Router List or discarding the NCE created by a
Redirect message, respectively.
The timeouts specified in [RFC4861] were chosen to be short in order
to optimize scenarios where alternative neighbors are available.
However, when there is no alternative neighbor, there are several
benefits to making NUD probe for a longer time. One benefit is to
make NUD more robust against transient failures, such as spanning
tree reconvergence and other layer 2 issues that can take many
seconds to resolve. Marking the NCE as unreachable, in that case,
causes additional multicast on the network. Assuming there are IP
packets to send, the lack of an NCE will result in multicast Neighbor
Solicitations being sent (to the solicited-node multicast address)
every second instead of the unicast Neighbor Solicitations that NUD
As a result, IPv6 Neighbor Discovery is operationally more brittle
than the IPv4 Address Resolution Protocol (ARP). For IPv4, there is
no mandatory time limit on the retransmission behavior for ARP
[RFC0826], which allows implementors to pick more robust schemes.
The following constant values in [RFC4861] seem to have been made
part of IPv6 conformance testing: MAX_MULTICAST_SOLICIT,
MAX_UNICAST_SOLICIT, and RETRANS_TIMER. While such strict
conformance testing seems consistent with [RFC4861], it means that
the standard needs to be updated to allow IPv6 Neighbor Discovery to
be as robust as ARP.
This document updates RFC 4861 to relax the retransmission rules.
Additional motivations for making IPv6 Neighbor Discovery more robust
in the face of degenerate conditions are covered in [RFC6583].
2. Definition of Terms
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].
3. Protocol Updates
Discarding the NCE after three packets spaced one second apart is
only needed when an alternative neighbor is available, such as an
additional default router or discarding an NCE created by a Redirect.
If an implementation transmits more than MAX_UNICAST_SOLICIT/
MAX_MULTICAST_SOLICIT packets, then it SHOULD use the exponential
backoff of the retransmit timer. This is to avoid any significant
load due to a steady background level of retransmissions from
implementations that retransmit a large number of Neighbor
Solicitations (NS) before discarding the NCE.
Even if there is no alternative neighbor, the protocol needs to be
able to handle the case when the link-layer address of the neighbor/
target has changed by switching to multicast Neighbor Solicitations
at some point in time.
In order to capture all the cases above, this document introduces a
new UNREACHABLE state in the conceptual model described in [RFC4861].
An NCE in the UNREACHABLE state retains the link-layer address, and
IPv6 packets continue to be sent to that link-layer address. But in
the UNREACHABLE state, the NUD Neighbor Solicitations are multicast
(to the solicited-node multicast address), using a timeout that
follows an exponential backoff.
In the places where [RFC4861] says to discard/delete the NCE after N
probes (Sections 7.3 and 7.3.3, and Appendix C), this document
instead specifies a transition to the UNREACHABLE state.
If the Neighbor Cache Entry was created by a Redirect message, a node
MAY delete the NCE instead of changing its state to UNREACHABLE. In
any case, the node SHOULD NOT use an NCE created by a Redirect to
send packets if that NCE is in the UNREACHABLE state. Packets should
be sent following the next-hop selection algorithm in [RFC4861],
Section 5.2, which disregards NCEs that are not reachable.
Section 6.3.6 of [RFC4861] indicates that default routers that are
"known to be reachable" are preferred. For the purposes of that
section, if the NCE for the router is in the UNREACHABLE state, it is
not known to be reachable. Thus, the particular text in
Section 6.3.6 that says "in any state other than INCOMPLETE" needs to
be extended to say "in any state other than INCOMPLETE or
Apart from the use of multicast NS instead of unicast NS, and the
exponential backoff of the timer, the UNREACHABLE state works the
same as the current PROBE state.
A node MAY garbage collect a Neighbor Cache Entry at any time as
specified in [RFC4861]. This freedom to garbage collect does not
change with the introduction of the UNREACHABLE state in the
conceptual model. An implementation MAY prefer garbage collecting
UNREACHABLE NCEs over other NCEs.
There is a non-obvious extension to the state-machine description in
Appendix C of [RFC4861] in the case for "NA, Solicited=1, Override=0.
Different link-layer address than cached". There we need to add
"UNREACHABLE" to the current list of "STALE, PROBE, Or DELAY". That
is, the NCE would be unchanged. Note that there is no corresponding
change necessary to the text in [RFC4861], Section 7.2.5, since it is
phrased using "Otherwise" instead of explicitly listing the three
The other state transitions described in Appendix C handle the
introduction of the UNREACHABLE state without any change, since they
are described using "not INCOMPLETE".
There is also the more obvious change already described above.
[RFC4861] has this:
State Event Action New state
PROBE Retransmit timeout, Discard entry -
N or more
That needs to be replaced by:
State Event Action New state
PROBE Retransmit timeout, Increase timeout UNREACHABLE
N retransmissions. Send multicast NS
UNREACHABLE Retransmit timeout Increase timeout UNREACHABLE
Send multicast NS
The exponential backoff SHOULD be clamped at some reasonable maximum
retransmit timeout, such as 60 seconds (see MAX_RETRANS_TIMER below).
If there is no IPv6 packet sent using the UNREACHABLE NCE, then it is
RECOMMENDED to stop the retransmits of the multicast NS until either
the NCE is garbage collected or there are IPv6 packets sent using the
NCE. The multicast NS and associated exponential backoff can be
applied on the condition of continued use of the NCE to send IPv6
packets to the recorded link-layer address.
A node can unicast the first few Neighbor Solicitation messages even
while in the UNREACHABLE state, but it MUST switch to multicast
Neighbor Solicitations within 60 seconds of the initial
retransmission to be able to handle a link-layer address change for
the target. The example below shows such behavior.
4. Example Algorithm
This section is NOT normative but specifies a simple implementation
that conforms with this document. The implementation is described
using operator-configurable values that allow it to be configured to
be compatible with the retransmission behavior in [RFC4861]. The
operator can configure the values for MAX_UNICAST_SOLICIT,
MAX_MULTICAST_SOLICIT, RETRANS_TIMER, and the new BACKOFF_MULTIPLE,
MAX_RETRANS_TIMER, and MARK_UNREACHABLE. This allows the
implementation to be as simple as:
next_retrans = ($BACKOFF_MULTIPLE ^ $solicit_retrans_num) *
$RetransTimer * $JitterFactor where solicit_retrans_num is zero for
the first transmission, and JitterFactor is a random value between
MIN_RANDOM_FACTOR and MAX_RANDOM_FACTOR [RFC4861] to avoid any
synchronization of transmissions from different hosts.
After MARK_UNREACHABLE transmissions, the implementation would mark
the NCE UNREACHABLE and as a result explore alternate next hops.
After MAX_UNICAST_SOLICIT, the implementation would switch to
multicast NUD probes.
The behavior of this example algorithm is to have 5 attempts, with
time spacing of 0 (initial request), 1 second later, 3 seconds after
the first retransmission, then 9, then 27, and switch to UNREACHABLE
after the first three transmissions. Thus, relative to the time of
the first transmissions, the retransmissions would occur at 1 second,
4 seconds, 13 seconds, and finally 40 seconds. At 4 seconds from the
first transmission, the NCE would be marked UNREACHABLE. That
behavior corresponds to:
After 3 retransmissions, the implementation would mark the NCE
UNREACHABLE. That results in trying an alternative neighbor, such as
another default router, or ignoring an NCE created by a Redirect as
specified in [RFC4861]. With the above values, that would occur
after 4 seconds following the first transmission compared to the
2 seconds using the fixed scheme in [RFC4861]. That additional
delay is small compared to the default ReachableTime of
After 5 transmissions, i.e., 40 seconds after the initial
transmission, the example behavior is to switch to multicast NUD
probes. In the language of the state machine in [RFC4861], that
corresponds to the action "Discard entry". Thus, any attempts to
send future packets would result in sending multicast NS packets. An
implementation MAY retain the backoff value as it switches to
multicast NUD probes. The potential downside of deferring switching
to multicast is that it would take longer for NUD to handle a change
in a link-layer address, i.e., the case when a host or a router
changes its link-layer address while keeping the same IPv6 address.
However, [RFC4861] says that a node MAY send unsolicited NS to handle
that case, which is rather infrequent in operational networks. In
any case, the implementation needs to follow the "SHOULD" in
Section 3 to switch to multicast solutions within 60 seconds after
the initial transmission.
If BACKOFF_MULTIPLE=1, MARK_UNREACHABLE=3, and MAX_UNICAST_SOLICIT=3,
you would get the same behavior as in [RFC4861].
If the request was not answered at first -- due, for example, to a
transitory condition -- an implementation following this algorithm
would retry immediately and then back off for progressively longer
periods. This would allow for a reasonably fast resolution time when
the transitory condition clears.
Note that RetransTimer and ReachableTime are by default set from the
protocol constants RETRANS_TIMER and REACHABLE_TIME but are
overridden by values advertised in Router Advertisements as specified
in [RFC4861]. That remains the case even with the protocol updates
specified in this document. The key values that the operator would
configure are BACKOFF_MULTIPLE, MAX_RETRANS_TIMER,
MAX_UNICAST_SOLICIT, and MAX_MULTICAST_SOLICIT.
It is useful to have a maximum value for
($BACKOFF_MULTIPLE^$solicit_attempt_num)*$RetransTimer so that the
retransmissions are not too far apart. The above value of 60 seconds
for this MAX_RETRANS_TIMER is consistent with DHCPv6.
The comments from Thomas Narten, Philip Homburg, Joel Jaeggli, Hemant
Singh, Tina Tsou, Suresh Krishnan, and Murray Kucherawy have helped
improve this document.
6. Security Considerations
Relaxing the retransmission behavior for NUD is believed to have no
impact on security. In particular, it doesn't impact the application
of Secure Neighbor Discovery [RFC3971].
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
7.2. Informative References
[RFC0826] 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.
[RFC6583] Gashinsky, I., Jaeggli, J., and W. Kumari, "Operational
Neighbor Discovery Problems", RFC 6583, March 2012.
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