Internet DRAFT - draft-ietf-6man-impatient-nud
draft-ietf-6man-impatient-nud
6MAN WG E. Nordmark
Internet-Draft Arista Networks
Updates: 4861 (if approved) I. Gashinsky
Intended status: Standards Track Yahoo!
Expires: April 24, 2014 October 21, 2013
Neighbor Unreachability Detection is too impatient
draft-ietf-6man-impatient-nud-07.txt
Abstract
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 short time. This
time is 3 seconds after the node starts probing by default. However,
if there are no alternative neighbors, this 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 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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 24, 2014.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definition Of Terms . . . . . . . . . . . . . . . . . . . . . . 4
3. Protocol Updates . . . . . . . . . . . . . . . . . . . . . . . 4
4. Example Algorithm . . . . . . . . . . . . . . . . . . . . . . . 6
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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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 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
next-hop selection. That implies picking the next router in the
Default Router List or discarding the redirect, respectively.
The timeouts specified in [RFC4861] were chosen to be short in order
to optimize for the scenarios where alternative neighbors are
available.
However, when there is no alternative neighbor there are several
benefits in making NUD try probing for a longer time. One of those
benefits 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
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
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].
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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
Discarding the NCE after after three packets spaced one second apart
is only needed when an alternative neighbor is available, such as an
additional default router or a redirect.
If an implementation transmits more than MAX_UNICAST_SOLICIT/
MAX_MULTICAST_SOLICIT packets then it SHOULD use 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 NSes 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].
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
(to the solicited-node multicast address), using a timeout that
follows an 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) this document instead
specifies a 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 [RFC4861], Section 5.2,
which disregards NCEs that are not reachable.
The default router selection in [RFC4861], 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
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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
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 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 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 [RFC4861], 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:
State Event Action New state
PROBE Retransmit timeout, Discard entry -
N or more
retransmissions.
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
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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 the 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 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
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_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 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
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 behavior corresponds to:
MAX_UNICAST_SOLICIT=5
RETRANS_TIMER=1 (default)
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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 neighbor, such as
another default router or ignoring a redirect as specified in
[RFC4861]. With the above values that would occur after 4 seconds
after the first transmission compared to the 2 seconds using the
fixed scheme in [RFC4861]. That additional delay is small compared
to the default 30,000 milliseconds ReachableTime.
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 their 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
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].
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
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($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.
5. Acknowledgements
The comments from Thomas Narten, Philip Homburg, Joel Jaeggli, Hemant
Singh, Tina Tsou, Suresh Krishnan, and Murray Kucherawy 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].
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.
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Authors' Addresses
Erik Nordmark
Arista Networks
Santa Clara, CA
USA
Email: nordmark@acm.org
Igor Gashinsky
Yahoo!
45 W 18th St
New York, NY
USA
Email: igor@yahoo-inc.com
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