6MAN WG E. Nordmark
Internet-Draft Cisco Systems, Inc.
Updates: 4861 (if approved) I. Gashinsky
Intended status: Standards Track Yahoo!
Expires: April 4, 2013 Oct 2012
Neighbor Unreachability Detection is too impatient
draft-ietf-6man-impatient-nud-05.txt
Abstract
IPv6 Neighbor Discovery includes Neighbor Unreachability Detection.
That function is very useful when a host has an alternative, for
instance multiple default routers, since it allows the host to switch
to the alternative in short time. This time is 3 seconds after the
node starts probing by default. However, if there are no
alternatives, this is far too impatient. This document specifies
relaxed rules for Neighbor Discovery retransmissions that allows an
implementation to choose different timeout behavior based on whether
or not there are alternatives. This document updates RFC 4861.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 4, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definition Of Terms . . . . . . . . . . . . . . . . . . . . . . 4
3. Protocol Updates . . . . . . . . . . . . . . . . . . . . . . . 4
4. Example Algorithm . . . . . . . . . . . . . . . . . . . . . . . 6
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
IPv6 Neighbor Discovery [RFC4861] includes Neighbor Unreachability
Detection (NUD), which detects when a neighbor is no longer
reachable. The timeouts specified are very short (by default three
transmissions spaced one second apart). That can be appropriate when
there are alternative paths over 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. The effect of NUD reporting a failure in those
cases is that the host will try the alternative; the next router in
the Default Router List, or discard the NCE which will also send
using a different router.
For that reason the timeouts in [RFC4861] were chosen to be short;
this ensures that if a default router fails the host can use the next
router in less than 45 seconds (taking into account a default
ReachableTime of 30 seconds and the time spent in the DELAY state).
However, when there is no alternative there are several benefits in
making NUD try probing for a longer time. One of those benefits is
to be 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 every second instead of the unicast Neighbor
Solicitations that NUD sends.
As a result IPv6 Neighbor Discovery is operationally more brittle
than IPv4 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 we
need to update the standard if we want 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].
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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
Giving up after three packets spaced one second apart is only
REQUIRED when there is an alternative, such as an additional default
router or a redirect.
If implementations transmit more than MAX_*CAST_SOLICIT packets it
SHOULD use (binary) exponential backoff of the retransmit timer.
This is to avoid any significant load due to a steady background
level of retransmissions from implementations that try for a long
time.
Even if there is no alternative, the protocol needs to be able to
handle the case when the link-layer address of the destination 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].
A 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,
using a timeout that follows a (binary) exponential backoff.
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
to the UNREACHABLE state.
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
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
following the next-hop selection algorithm in section 5.2 in
[RFC4861] which disregards NCEs that are not reachable.
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
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
which says "in any state other than INCOMPLETE" needs to be extended
to say "in any state other than INCOMPLETE or UNREACHABLE".
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Apart from the use of multicast NS instead of unicast NS, and the
(binary) 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 RFC 4861. This does not change with the introduction of
the UNREACHABLE state in the conceptual model.
The UNREACHABLE state is conceptual and not a required part of this
specification. Just as for [RFC4861] a node merely needs to satisfy
the externally observable behavior of this specification.
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.
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 section 7.2.5 since it is phrased
using "Otherwise" instead of explicitly listing the three states.
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. RFC
4861 has this:
PROBE Retransmit timeout, Discard entry -
N or more
retransmissions.
That needs to be replaced by:
PROBE Retransmit timeout, Increase timeout UNREACHABLE
N or more Send multicast NS
retransmissions.
UNREACHABLE Retransmit timeout Increase timeout UNREACHABLE
Send multicast NS
The (binary) 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 makes sense 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 binary exponential
backoff can be applied on the condition of the continued use of the
NCE to send IPv6 packets to the recorded link-layer address.
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A node MAY unicast the first few Neighbor Solicitation messages even
while in UNREACHABLE state, but it MUST switch to multicast Neighbor
Solicitations sooner or later. Otherwise it would not detect 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
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, MAX_RETRANS_TIMER 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 transmissions the implementation would mark
the NCE UNREACHABLE and as result explore alternate next hops. After
MAX_UNICAST_SOLICIT the implementation would 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 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 recommended
behavior corresponds to:
MAX_UNICAST_SOLICIT=5
RETRANS_TIMER=1 (default)
MAX_RETRANS_TIMER=60
BACKOFF_MULTIPLE=3
MARK_UNREACHABLE=3
After 3 retransmissions the implementation would mark the NCE
UNREACHABLE. That results in trying an alternative, such as another
default router or ignoring a redirect as specified in [RFC4861].
With the above recommended values that would occur after 4 seconds
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after the first transmission compared to the 2 seconds using the
fixed scheme in [RFC4861]. That additional delay is small compared
to the default 30 seconds ReachableTime.
After 5 transmissions, i.e., 40 seconds after the initial
transmission, the recommended behavior is to switch to multicast NUD
probes. In the language of the state machine in [RFC4861] says that
a node MAY send unsolicited NS to handle that case, which is rather
infrequent in operational networks.
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_RETRANS_TIMER,
MAX_UNICAST_SOLICIT and MAX_MULTICAST_SOLICIT.
It is be useful to have a maximum value for
($BACKOFF_MULTIPLE^$solicit_attempt_num)*$RetransTimer so that the
retransmissions are not too far apart. The recommended value of 60
seconds for this MAX_RETRANS_TIMER is consistent with DHCPv6.
5. Acknowledgements
The comments from Thomas Narten, Philip Homburg, Joel Jaeggli, Hemant
Singh, and Tina Tsou have helped improve this draft.
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
Secure Neighbor Discovery [RFC3971].
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7. IANA Considerations
This are no IANA considerations for this document.
8. References
8.1. 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,
September 2007.
8.2. 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.
Authors' Addresses
Erik Nordmark
Cisco Systems, Inc.
510 McCarthy Blvd.
Milpitas, CA, 95035
USA
Phone: +1 408 527 6625
Email: nordmark@cisco.com
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Igor Gashinsky
Yahoo!
45 W 18th St
New York, NY
USA
Email: igor@yahoo-inc.com
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