6MAN WG E. Nordmark
Internet-Draft Cisco Systems, Inc.
Expires: January 8, 2012 I. Gashinsky
Yahoo!
July 7, 2011
Neighbor Unreachability Detection is too impatient
draft-nordmark-6man-impatient-nud-01.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. However, if there are no alternatives, this is
far too impatient. This document proposes an approach where an
implementation can choose the timeout behavior to be different based
on whether or not there are alternatives.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 8, 2012.
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Proposed Remedy . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6.1. Normative References . . . . . . . . . . . . . . . . . . . 6
6.2. Informative References . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6
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1. Introduction
IPv6 Neighbor Discovery [RFC4861] includes Neighbor Unreachability
Detection, which detects when a neighbor is no longer reachable. The
timeouts specified are very short (three transmissions spaced one
second apart). That can be appropriate when there are alternative
paths the packet can be sent. For example, if a host has multiple
default routers in its Default Router List, or if the host has a
Neigbor 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 where chosen to be short; this ensures
that if a default router fails the host can use the next router in
less than 45 seconds.
However, where 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
recovergence 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 is operationally more brittle than IPv4. 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, RETRANS_TIMER. While such strict conformance
testing seems consistent with the the specificiation, it means that
we need to update the standard if we want to allow IPv6 Neighbor
Discovery to be as operationally robust as ARP.
Additional motivations for making IPv6 Neighbor Discovery as robust
as ARP are covered in [I-D.gashinsky-v6nd-enhance].
2. Proposed Remedy
We can clarify that the giving up after three packets spaced one
second apart is only REQUIRED when there is an alternative, such as
an additional default route or a redirect.
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If implementations transmit more than MAX_*CAST_SOLICIT packets they
MAY use binary exponential backoff of the retransmit timer. This is
so that if we end up with implementations that try for a very long
time we don't end up with a steady background level of
retransmissions.
However, even if there is no alternative, we still need to be able to
handle the case when the link-layer address of the destination has
changed. Thus at some point in time we need to switch to multicast
Neighbor Solicitations.
A possible way to describe a node behavior which captures all the
cases is to introduce a new, optional, 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 the Neighbor Soliciations 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 XXX 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".
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 as any time as
specified in RFC 4861. This does not change with the introduction of
the UNREACHABLE state in the coneptual model.
The UNREACHABLE state is conceptual and not a required part of this
specification. A node merely needs to satisfy the externally
observable behavior of this specificiation.
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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, Double timeout UNREACHABLE
N or more Send multicast NS
retransmissions.
UNREACHABLE Retransmit timeout Double timeout UNREACHABLE
Send multicast NS
The binary exponential backoff SHOULD be clamped at some reasonable
maximum retransmit timeout, such as 60 seconds. And if there is no
IPv6 packets 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. In
essence the multicast NS and associated binary exponential backoff
can be conditioned on the continued use of the NCE to send IPv6
packets to the recorded link-layer address.
A node MAY unicast the first few Neighbor Soliciation messages while
in UNREACHABLE state, but it MUST switch to multicast Neighbor
Soliciations. Otherwise it would not detect a link-layer address
change for the target.
3. Acknowledgements
The comments from Thomas Narten and Philip Homburg have helped
improve this draft.
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4. Security Considerations
Relaxing the retransmission behavior for NUD has no impact on
security. In particular, it doesn't impact applying Secure Neighbor
Discovery [RFC3971].
5. IANA Considerations
This are no IANA considerations for this document.
6. References
6.1. Normative References
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC 4443, March 2006.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
6.2. Informative References
[I-D.gashinsky-v6nd-enhance]
Kumari, W., "Operational Neighbor Discovery Problems and
Enhancements.", draft-gashinsky-v6nd-enhance-00 (work in
progress), June 2011.
[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.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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Authors' Addresses
Erik Nordmark
Cisco Systems, Inc.
510 McCarthy Blvd.
Milpitas, CA, 95035
USA
Phone: +1 408 527 6625
Email: nordmark@cisco.com
Igor Gashinsky
Yahoo!
45 W 18th St
New York, NY
USA
Email: igor@yahoo-inc.com
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