draft-ietf-6man-impatient-nud-00.txt   draft-ietf-6man-impatient-nud-01.txt 
6MAN WG E. Nordmark 6MAN WG E. Nordmark
Internet-Draft Cisco Systems, Inc. Internet-Draft Cisco Systems, Inc.
Expires: May 17, 2012 I. Gashinsky Updates: 4861 (if approved) I. Gashinsky
Yahoo! Expires: September 13, 2012 Yahoo!
November 14, 2011 March 12, 2012
Neighbor Unreachability Detection is too impatient Neighbor Unreachability Detection is too impatient
draft-ietf-6man-impatient-nud-00.txt draft-ietf-6man-impatient-nud-01.txt
Abstract Abstract
IPv6 Neighbor Discovery includes Neighbor Unreachability Detection. IPv6 Neighbor Discovery includes Neighbor Unreachability Detection.
That function is very useful when a host has an alternative, for That function is very useful when a host has an alternative, for
instance multiple default routers, since it allows the host to switch instance multiple default routers, since it allows the host to switch
to the alternative in short time. This time is 3 seconds after the to the alternative in short time. This time is 3 seconds after the
node starts probing. However, if there are no alternatives, this is node starts probing by default. However, if there are no
far too impatient. This document proposes an approach where an alternatives, this is far too impatient. This document specifies
implementation can choose the timeout behavior to be different based relaxed rules for Neighbor Discovery retransmissions that allows an
on whether or not there are alternatives. implementation to choose different timeout behavior based on whether
or not there are alternatives.
Status of this Memo Status of this Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
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Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
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time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 17, 2012. This Internet-Draft will expire on September 13, 2012.
Copyright Notice Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definition Of Terms . . . . . . . . . . . . . . . . . . . . . . 3 2. Definition Of Terms . . . . . . . . . . . . . . . . . . . . . . 4
3. Proposed Remedy . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Protocol Updates . . . . . . . . . . . . . . . . . . . . . . . 4
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6 4. Example Algorithm . . . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 6. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . . 6 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . . 6 8.1. Normative References . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6 8.2. Informative References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction 1. Introduction
IPv6 Neighbor Discovery [RFC4861] includes Neighbor Unreachability IPv6 Neighbor Discovery [RFC4861] includes Neighbor Unreachability
Detection, which detects when a neighbor is no longer reachable. The Detection (NUD), which detects when a neighbor is no longer
timeouts specified are very short (three transmissions spaced one reachable.The timeouts specified are very short (by default three
second apart). That can be appropriate when there are alternative transmissions spaced one second apart). That can be appropriate when
paths the packet can be sent. For example, if a host has multiple there are alternative paths over which the packets can be sent. For
default routers in its Default Router List, or if the host has a example, if a host has multiple default routers in its Default Router
Neigbor Cache Entry (NCE) created by a Redirect message. The effect List, or if the host has a Neighbor Cache Entry (NCE) created by a
of NUD reporting a failure in those cases is that the host will try Redirect message. The effect of NUD reporting a failure in those
the alternative; the next router in the Default Router List, or cases is that the host will try the alternative; the next router in
discard the NCE which will also send using a different router. the Default Router List, or discard the NCE which will also send
using a different router.
For that reason the timeouts where chosen to be short; this ensures For that reason the timeouts in [RFC4861] were chosen to be short;
that if a default router fails the host can use the next router in this ensures that if a default router fails the host can use the next
less than 45 seconds. router in less than 45 seconds (taking into account a default
ReachableTime of 30 seconds and the time spent in the DELAY state).
However, where there is no alternative there are several benefits in However, when there is no alternative there are several benefits in
making NUD try probing for a longer time. One of those benefits is making NUD try probing for a longer time. One of those benefits is
to be more robust against transient failures, such as spanning tree to be more robust against transient failures, such as spanning tree
recovergence and other layer 2 issues that can take many seconds to reconvergence and other layer 2 issues that can take many seconds to
resolve. Marking the NCE as unreachable in that case causes resolve. Marking the NCE as unreachable in that case causes
additional multicast on the network. Assuming there are IP packets additional multicast on the network. Assuming there are IP packets
to send, the lack of an NCE will result in multicast Neighbor to send, the lack of an NCE will result in multicast Neighbor
Solicitations every second instead of the unicast Neighbor Solicitations every second instead of the unicast Neighbor
Solicitations that NUD sends. Solicitations that NUD sends.
As a result IPv6 is operationally more brittle than IPv4. For IPv4 As a result IPv6 is operationally more brittle than IPv4. For IPv4
there is no mandatory time limit on the retransmission behavior for there is no mandatory time limit on the retransmission behavior for
ARP [RFC0826] which allows implementors to pick more robust schemes. ARP [RFC0826] which allows implementors to pick more robust schemes.
The following constant values in [RFC4861] seem to have been made The following constant values in [RFC4861] seem to have been made
part of IPv6 conformance testing: MAX_MULTICAST_SOLICIT, part of IPv6 conformance testing: MAX_MULTICAST_SOLICIT,
MAX_UNICAST_SOLICIT, RETRANS_TIMER. While such strict conformance MAX_UNICAST_SOLICIT, and RETRANS_TIMER. While such strict
testing seems consistent with the the specificiation, it means that conformance testing seems consistent with [RFC4861], it means that we
we need to update the standard if we want to allow IPv6 Neighbor need to update the standard if we want to allow IPv6 Neighbor
Discovery to be as operationally robust as ARP. Discovery to be as operationally robust as ARP.
This document updates RFC 4861 to relax the retransmission rules.
Additional motivations for making IPv6 Neighbor Discovery as robust Additional motivations for making IPv6 Neighbor Discovery as robust
as ARP are covered in [I-D.gashinsky-v6nd-enhance]. as ARP are covered in [I-D.gashinsky-v6nd-enhance].
2. Definition Of Terms 2. Definition Of Terms
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
3. Proposed Remedy 3. Protocol Updates
We can clarify that the giving up after three packets spaced one Giving up after three packets spaced one second apart is only
second apart is only REQUIRED when there is an alternative, such as REQUIRED when there is an alternative, such as an additional default
an additional default route or a redirect. route or a redirect.
If implementations transmit more than MAX_*CAST_SOLICIT packets they If implementations transmit more than MAX_*CAST_SOLICIT packets it
MAY use binary exponential backoff of the retransmit timer. This is SHOULD use (binary) exponential backoff of the retransmit timer.
so that if we end up with implementations that try for a very long This is to avoid any significant load due to a steady background
time we don't end up with a steady background level of level of retransmissions from implementations that try for a long
retransmissions. time.
However, even if there is no alternative, we still need to be able to However, even if there is no alternative, the protocol needs to be
handle the case when the link-layer address of the destination has able to handle the case when the link-layer address of the
changed. Thus at some point in time we need to switch to multicast destination has changed by switching to multicast Neighbor
Neighbor Solicitations. Solicitations at some point in time.
A possible way to describe a node behavior which captures all the In order to capture all the cases above this document introduces a
cases is to introduce a new, optional, UNREACHABLE state in the new UNREACHABLE state in the conceptual model described in [RFC4861].
conceptual model described in [RFC4861]. A NCE in the UNREACHABLE A NCE in the UNREACHABLE state retains the link-layer address, and
state retains the link-layer address, and IPv6 packets continue to be IPv6 packets continue to be sent to that link-layer address. But in
sent to that link-layer address. But the Neighbor Soliciations are the UNREACHABLE state the NUD Neighbor Solicitations are multicast,
multicast, using a timeout that follows a binary exponential backoff. using a timeout that follows a (binary) exponential backoff.
In the places where RFC4861 says to to discard/delete the NCE after N In the places where RFC4861 says to to discard/delete the NCE after N
probes (Section 7.3, 7.3.3 and Appendix C) we will instead transition probes (Section 7.3, 7.3.3 and Appendix C) we will instead transition
to the UNREACHABLE state. to the UNREACHABLE state.
If the Neighbor Cache Entry was created by a redirect, a node MAY If the Neighbor Cache Entry was created by a redirect, a node MAY
delete the NCE instead of changing its state to UNREACHABLE. In any 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 case, the node SHOULD NOT use an NCE created by a Redirect to send
packets if that NCE is in unreachable state. Packets should be sent packets if that NCE is in unreachable state. Packets should be sent
following the next-hop selection algorithm in section XXX which following the next-hop selection algorithm in section 5.2 in
disregards NCEs that are not reachable. [RFC4861] which disregards NCEs that are not reachable.
The default router selection in section 6.3.6 says to prefer default The default router selection in section 6.3.6 says to prefer default
routers that are "known to be reachable". For the purposes of that routers that are "known to be reachable". For the purposes of that
section, if the NCE for the router is in UNREACHABLE state, it is not section, if the NCE for the router is in UNREACHABLE state, it is not
known to be reachable. Thus the particular text in section 6.3.6 known to be reachable. Thus the particular text in section 6.3.6
which says "in any state other than INCOMPLETE" needs to be extended which says "in any state other than INCOMPLETE" needs to be extended
to say "in any state other than INCOMPLETE or UNREACHABLE". to say "in any state other than INCOMPLETE or UNREACHABLE".
Apart from the use of multicast NS instead of unicast NS, and the Apart from the use of multicast NS instead of unicast NS, and the
binary exponential backoff of the timer, the UNREACHABLE state works (binary) exponential backoff of the timer, the UNREACHABLE state
the same as the current PROBE state. works the same as the current PROBE state.
A node MAY garbage collect a Neighbor Cache Entry as any time as A node MAY garbage collect a Neighbor Cache Entry as any time as
specified in RFC 4861. This does not change with the introduction of specified in RFC 4861. This does not change with the introduction of
the UNREACHABLE state in the coneptual model. the UNREACHABLE state in the conceptual model.
The UNREACHABLE state is conceptual and not a required part of this The UNREACHABLE state is conceptual and not a required part of this
specification. A node merely needs to satisfy the externally specification. A node merely needs to satisfy the externally
observable behavior of this specificiation. observable behavior of this specification.
There is a non-obvious extension to the state machine description in There is a non-obvious extension to the state machine description in
Appendix C in RFC 4861 in the case for "NA, Solicited=1, Override=0. Appendix C in RFC 4861 in the case for "NA, Solicited=1, Override=0.
Different link-layer address than cached". There we need to add Different link-layer address than cached". There we need to add
"UNREACHABLE" to the current list of "STALE, PROBE, Or DELAY". That "UNREACHABLE" to the current list of "STALE, PROBE, Or DELAY". That
is, the NCE would be unchanged. Note that there is no corresponding is, the NCE would be unchanged. Note that there is no corresponding
change necessary to the text in section 7.2.5 since it is phrased change necessary to the text in section 7.2.5 since it is phrased
using "Otherwise" instead of explicitly listing the three states. using "Otherwise" instead of explicitly listing the three states.
The other state transitions described in Appendix C handle the The other state transitions described in Appendix C handle the
skipping to change at page 5, line 47 skipping to change at page 6, line 5
The binary exponential backoff SHOULD be clamped at some reasonable The binary exponential backoff SHOULD be clamped at some reasonable
maximum retransmit timeout, such as 60 seconds. And if there is no maximum retransmit timeout, such as 60 seconds. And if there is no
IPv6 packets sent using the UNREACHABLE NCE, then it makes sense to IPv6 packets sent using the UNREACHABLE NCE, then it makes sense to
stop the retransmits of the multicast NS until either the NCE is stop the retransmits of the multicast NS until either the NCE is
garbage collected, or there are IPv6 packets sent using the NCE. In garbage collected, or there are IPv6 packets sent using the NCE. In
essence the multicast NS and associated binary exponential backoff essence the multicast NS and associated binary exponential backoff
can be conditioned on the continued use of the NCE to send IPv6 can be conditioned on the continued use of the NCE to send IPv6
packets to the recorded link-layer address. packets to the recorded link-layer address.
A node MAY unicast the first few Neighbor Soliciation messages while A node MAY unicast the first few Neighbor Solicitation messages while
in UNREACHABLE state, but it MUST switch to multicast Neighbor in UNREACHABLE state, but it MUST switch to multicast Neighbor
Soliciations. Otherwise it would not detect a link-layer address Solicitations. Otherwise it would not detect a link-layer address
change for the target. change for the target.
4. Acknowledgements 4. Example Algorithm
This section is NOT normative, but specifies a simple implementation
which conforms with this document. The implementation is described
using operator configurable values that allows it to be configured in
a way to be compatible with the retransmission behavior in [RFC4861].
The operator can configure the values for MAX_*CAST_SOLICIT,
RETRANS_TIMER, and the new BACKOFF_MULTIPLE and MARK_UNREACHABLE.
This allows the implementation to be as simple as:
next_retrans = ($BACKOFF_MULTIPLE^$solicit_attempt_num)*$RetransTimer
+ jittered value.
After MARK_UNREACHABLE retransmissions the implementation would mark
the NCE UNREACHABLE and switch to multicast NUD probes.
The recommended behavior is to have 5 attempts, with timing spacing
of 0 (initial request), 1 second later, 3 seconds later, then 9, and
then 27, and switch to UNREACHABLE after 3 transmissions, which
represents:
MAX_UNICAST_SOLICIT=5
RETRANS_TIMER=1 (default)
BACKOFF_MULTIPLE=3
MARK_UNREACHABLE=3
After 3 retransmissions the implementation would mark the NCE
UNREACHABLE and switch to multicast NUD probes. Thus we enter
UNREACHABLE, and try any available alternative, after 4 seconds
compared to the current 2 seconds. That additional delay is small
compared to the default 30 seconds ReachableTime.
If BACKOFF_MULTIPLE=1, MARK_UNREACHABLE=3 and MAX_UNICAST_SOLICIT=3,
you would get the same behavior as in [RFC4861].
An Implementation following this algorithm would, if the request was
not answered at first due for example to a transitory condition,
retry immediately, and then back off for progressively longer
periods. This would allow for a reasonably fast resolution time when
the transitory condition clears.
Note that RetransTimer and ReachableTime are by default set from the
protocol constants RETRANS_TIMER and REACHABLE_TIME, but are
overridden by values advertised in Router Advertisements as specified
in [RFC4861]. That remains the case even with the protocol updates
specified in this document. The key values that the operator would
configure are BACKOFF_MULTIPLE, MAX_UNICAST_SOLICIT and
MAX_MULTICAST_SOLICIT.
It would be useful to have a maximum value for
($BACKOFF_MULTIPLE^$solicit_attempt_num)*$RetransTimer so that the
retransmissions are not too far apart. A value 60 seconds is
consistent with DHCP.
5. Acknowledgements
The comments from Thomas Narten and Philip Homburg have helped The comments from Thomas Narten and Philip Homburg have helped
improve this draft. improve this draft.
5. Security Considerations 6. Security Considerations
Relaxing the retransmission behavior for NUD has no impact on Relaxing the retransmission behavior for NUD has no impact on
security. In particular, it doesn't impact applying Secure Neighbor security. In particular, it doesn't impact applying Secure Neighbor
Discovery [RFC3971]. Discovery [RFC3971].
6. IANA Considerations 7. IANA Considerations
This are no IANA considerations for this document. This are no IANA considerations for this document.
7. References 8. References
7.1. Normative References 8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure [RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005. Neighbor Discovery (SEND)", RFC 3971, March 2005.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007. September 2007.
7.2. Informative References 8.2. Informative References
[I-D.gashinsky-v6nd-enhance] [I-D.gashinsky-v6nd-enhance]
Kumari, W., "Operational Neighbor Discovery Problems and Kumari, W., "Operational Neighbor Discovery Problems and
Enhancements.", draft-gashinsky-v6nd-enhance-00 (work in Enhancements.", draft-gashinsky-v6nd-enhance-00 (work in
progress), June 2011. progress), June 2011.
[RFC0826] Plummer, D., "Ethernet Address Resolution Protocol: Or [RFC0826] Plummer, D., "Ethernet Address Resolution Protocol: Or
converting network protocol addresses to 48.bit Ethernet converting network protocol addresses to 48.bit Ethernet
address for transmission on Ethernet hardware", STD 37, address for transmission on Ethernet hardware", STD 37,
RFC 826, November 1982. RFC 826, November 1982.
 End of changes. 29 change blocks. 
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