One document matched: draft-ietf-6man-impatient-nud-00.xml

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<rfc ipr="trust200902" docName="draft-ietf-6man-impatient-nud-00.txt">
	
<front>
<title abbrev="NUD is too impatient">
Neighbor Unreachability Detection is too impatient
</title>

<author initials="E" surname="Nordmark" fullname="Erik Nordmark">
  <organization>Cisco Systems, Inc.</organization>
  <address>
    <postal>
    <street>510 McCarthy Blvd.</street>
    <city>Milpitas, CA, 95035</city>
    <country>USA</country>
    </postal>
    <phone>+1 408 527 6625</phone>
    <email>nordmark@cisco.com</email>
  </address>
</author>

<author initials="I" surname="Gashinsky" fullname="Igor Gashinsky">
  <organization>Yahoo!</organization>
  <address>
    <postal>
      <street>45 W 18th St</street>
      <city>New York, NY</city>
      <country>USA</country>
    </postal>
    <email>igor@yahoo-inc.com</email>
  </address>
</author>


<date month="November" year="2011"/>
<workgroup>6MAN WG</workgroup>
<keyword>6MAN</keyword>

<abstract>
<t>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.</t>
</abstract>
</front>

<middle>

<section title="Introduction">
<t>
IPv6 Neighbor Discovery <xref target="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.</t>

<t>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.</t>

<t>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.</t>

<t>As a result IPv6 is operationally more brittle than IPv4. For IPv4 there
is no mandatory time limit on the retransmission behavior for ARP 
<xref target="RFC0826"/> which allows implementors to pick more robust schemes.</t>

<t>The following constant values in <xref target="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.</t>

<t>Additional motivations for making IPv6 Neighbor Discovery as robust as ARP
are covered in <xref target="I-D.gashinsky-v6nd-enhance"/>.</t>

</section>

<section title="Definition Of Terms">

<t>
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 <xref target="RFC2119"/>.
</t>
</section>

<section title="Proposed Remedy">

<t>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.</t>

<t>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.</t>

<t>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.</t>

<t>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 <xref target="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.</t>

<t>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.</t>

<t>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.</t>

<t>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".</t>

<t>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.</t>

<t>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.</t>

<t>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.</t>

<t>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.</t>

<t>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".
</t>

<t>There is also the more obvious change already described above. RFC 4861 
has this:</t>
<figure>
 <artwork><![CDATA[
PROBE           Retransmit timeout,     Discard entry         -
                N or more
                retransmissions.
]]></artwork>
</figure>

<t>That needs to be replaced by:</t>
<figure>
 <artwork><![CDATA[
PROBE           Retransmit timeout,     Double timeout    UNREACHABLE
                N or more               Send multicast NS
                retransmissions.

UNREACHABLE     Retransmit timeout      Double timeout    UNREACHABLE
                                        Send multicast NS
]]></artwork>
</figure>

<t>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.</t>

<t>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.</t>

</section>

<section title="Acknowledgements">

<t>The comments from Thomas Narten and Philip Homburg 
have helped improve this draft.</t>

</section>

<section title="Security Considerations">

<t>Relaxing the retransmission behavior for NUD has no impact on security.
In particular, it doesn't impact applying Secure Neighbor Discovery
<xref target="RFC3971"/>.</t>

</section>

<section title="IANA Considerations">

<t>This are no IANA considerations for this document.</t>

</section>

</middle>

<back>
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&rfc2119;
&rfc3971;
&rfc4861;
</references>

<references title="Informative References">
&rfc0826;
&v6nd-enhance;
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</back>
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