draft-ietf-6man-impatient-nud-02.txt   draft-ietf-6man-impatient-nud-03.txt 
6MAN WG E. Nordmark 6MAN WG E. Nordmark
Internet-Draft Cisco Systems, Inc. Internet-Draft Cisco Systems, Inc.
Updates: 4861 (if approved) I. Gashinsky Updates: 4861 (if approved) I. Gashinsky
Expires: January 2, 2013 Yahoo! Intended status: Standards Track Yahoo!
Jul 2012 Expires: April 4, 2013 Oct 2012
Neighbor Unreachability Detection is too impatient Neighbor Unreachability Detection is too impatient
draft-ietf-6man-impatient-nud-02.txt draft-ietf-6man-impatient-nud-03.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 by default. However, if there are no node starts probing by default. However, if there are no
alternatives, this is far too impatient. This document specifies alternatives, this is far too impatient. This document specifies
relaxed rules for Neighbor Discovery retransmissions that allows an relaxed rules for Neighbor Discovery retransmissions that allows an
implementation to choose different timeout behavior based on whether implementation to choose different timeout behavior based on whether
or not there are alternatives. or not there are alternatives. This document updates RFC 4861.
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
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
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
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
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 January 2, 2013. This Internet-Draft will expire on April 4, 2013.
Copyright Notice Copyright Notice
Copyright (c) 2012 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.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . . 7 8.1. Normative References . . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . . 8 8.2. Informative References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction 1. Introduction
IPv6 Neighbor Discovery [RFC4861] includes Neighbor Unreachability IPv6 Neighbor Discovery [RFC4861] includes Neighbor Unreachability
Detection (NUD), which detects when a neighbor is no longer Detection (NUD), which detects when a neighbor is no longer
reachable.The timeouts specified are very short (by default three reachable. The timeouts specified are very short (by default three
transmissions spaced one second apart). That can be appropriate when transmissions spaced one second apart). That can be appropriate when
there are alternative paths over which the packets can be sent. For there are alternative paths over which the packets can be sent. For
example, if a host has multiple default routers in its Default Router 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 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 Redirect message. The effect of NUD reporting a failure in those
cases is that the host will try the alternative; the next router in 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 the Default Router List, or discard the NCE which will also send
using a different router. using a different router.
For that reason the timeouts in [RFC4861] were chosen to be short; For that reason the timeouts in [RFC4861] were chosen to be short;
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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. Protocol Updates 3. Protocol Updates
Giving up after three packets spaced one second apart is only Giving up after three packets spaced one second apart is only
REQUIRED when there is an alternative, such as an additional default REQUIRED when there is an alternative, such as an additional default
route or a redirect. router or a redirect.
If implementations transmit more than MAX_*CAST_SOLICIT packets it If implementations transmit more than MAX_*CAST_SOLICIT packets it
SHOULD use (binary) exponential backoff of the retransmit timer. SHOULD use (binary) exponential backoff of the retransmit timer.
This is to avoid any significant load due to a steady background This is to avoid any significant load due to a steady background
level of retransmissions from implementations that try for a long level of retransmissions from implementations that try for a long
time. time.
Even if there is no alternative, the protocol needs to be able to 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 handle the case when the link-layer address of the destination has
changed by switching to multicast Neighbor Solicitations at some changed by switching to multicast Neighbor Solicitations at some
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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 (binary) exponential backoff of the timer, the UNREACHABLE state
works 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 at 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 conceptual 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. Just as for [RFC4861] a node merely needs to satisfy
observable behavior of this specification. the externally 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
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That needs to be replaced by: That needs to be replaced by:
PROBE Retransmit timeout, Double timeout UNREACHABLE PROBE Retransmit timeout, Double timeout UNREACHABLE
N or more Send multicast NS N or more Send multicast NS
retransmissions. retransmissions.
UNREACHABLE Retransmit timeout Double timeout UNREACHABLE UNREACHABLE Retransmit timeout Double timeout UNREACHABLE
Send multicast NS Send multicast NS
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. If there is no IPv6 maximum retransmit timeout, such as 60 seconds (MAX_RETRANS_TIMER
packets sent using the UNREACHABLE NCE, then it makes sense to stop below). If there is no IPv6 packet sent using the UNREACHABLE NCE,
the retransmits of the multicast NS until either the NCE is garbage then it makes sense to stop the retransmits of the multicast NS until
collected or there are IPv6 packets sent using the NCE. The either the NCE is garbage collected or there are IPv6 packets sent
multicast NS and associated binary exponential backoff can be applied using the NCE. The multicast NS and associated binary exponential
on the condition of the continued use of the NCE to send IPv6 packets backoff can be applied on the condition of the continued use of the
to the recorded link-layer address. NCE to send IPv6 packets to the recorded link-layer address.
A node MAY unicast the first few Neighbor Solicitation 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
Solicitations. 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. Example Algorithm 4. Example Algorithm
This section is NOT normative, but specifies a simple implementation This section is NOT normative, but specifies a simple implementation
which conforms with this document. The implementation is described which conforms with this document. The implementation is described
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RETRANS_TIMER, and the new BACKOFF_MULTIPLE and MARK_UNREACHABLE. RETRANS_TIMER, and the new BACKOFF_MULTIPLE and MARK_UNREACHABLE.
This allows the implementation to be as simple as: This allows the implementation to be as simple as:
next_retrans = ($BACKOFF_MULTIPLE^$solicit_attempt_num)*$RetransTimer next_retrans = ($BACKOFF_MULTIPLE^$solicit_attempt_num)*$RetransTimer
+ jittered value. + jittered value.
After MARK_UNREACHABLE retransmissions the implementation would mark After MARK_UNREACHABLE retransmissions the implementation would mark
the NCE UNREACHABLE and switch to multicast NUD probes. the NCE UNREACHABLE and switch to multicast NUD probes.
The recommended behavior is to have 5 attempts, with timing spacing The recommended behavior is to have 5 attempts, with timing spacing
of 0 (initial request), 1 second later, 3 seconds later, then 9, and of 0 (initial request), 1 second later, 3 seconds later, then 9, then
then 27, and switch to UNREACHABLE after 3 transmissions, which 27, then 60, then 60 and switch to UNREACHABLE after the three first
represents: transmissions. Thus relative to the time of the first transmissions
the retransmissions would occur at 1 second, 4 seconds, 13 seconds,
40 seconds, then 100 seconds, and finally 160 seconds. At 4 seconds
from the first transmission the NCE would be marked UNREACHABLE.
That recommended behavior corresponds to:
MAX_UNICAST_SOLICIT=5 MAX_UNICAST_SOLICIT=5
RETRANS_TIMER=1 (default) RETRANS_TIMER=1 (default)
MAX_RETRANS_TIMER=60
BACKOFF_MULTIPLE=3 BACKOFF_MULTIPLE=3
MARK_UNREACHABLE=3 MARK_UNREACHABLE=3
After 3 retransmissions the implementation would mark the NCE After 3 retransmissions the implementation would mark the NCE
UNREACHABLE and switch to multicast NUD probes. Thus we enter UNREACHABLE and switch to multicast NUD probes. Thus we enter
UNREACHABLE, and try any available alternative, after 4 seconds UNREACHABLE, and try any available alternative, after 4 seconds
compared to the current 2 seconds. That additional delay is small compared to the current 2 seconds. That additional delay is small
compared to the default 30 seconds ReachableTime. compared to the default 30 seconds ReachableTime.
If BACKOFF_MULTIPLE=1, MARK_UNREACHABLE=3 and MAX_UNICAST_SOLICIT=3, If BACKOFF_MULTIPLE=1, MARK_UNREACHABLE=3 and MAX_UNICAST_SOLICIT=3,
you would get the same behavior as in [RFC4861]. you would get the same behavior as in [RFC4861].
An Implementation following this algorithm would, if the request was An implementation following this algorithm would, if the request was
not answered at first due for example to a transitory condition, not answered at first due for example to a transitory condition,
retry immediately, and then back off for progressively longer retry immediately, and then back off for progressively longer
periods. This would allow for a reasonably fast resolution time when periods. This would allow for a reasonably fast resolution time when
the transitory condition clears. the transitory condition clears.
Note that RetransTimer and ReachableTime are by default set from the Note that RetransTimer and ReachableTime are by default set from the
protocol constants RETRANS_TIMER and REACHABLE_TIME, but are protocol constants RETRANS_TIMER and REACHABLE_TIME, but are
overridden by values advertised in Router Advertisements as specified overridden by values advertised in Router Advertisements as specified
in [RFC4861]. That remains the case even with the protocol updates in [RFC4861]. That remains the case even with the protocol updates
specified in this document. The key values that the operator would specified in this document. The key values that the operator would
configure are BACKOFF_MULTIPLE, MAX_UNICAST_SOLICIT and configure are BACKOFF_MULTIPLE, MAX_UNICAST_SOLICIT and
MAX_MULTICAST_SOLICIT. MAX_MULTICAST_SOLICIT.
It would be useful to have a maximum value for It would be useful to have a maximum value for
($BACKOFF_MULTIPLE^$solicit_attempt_num)*$RetransTimer so that the ($BACKOFF_MULTIPLE^$solicit_attempt_num)*$RetransTimer so that the
retransmissions are not too far apart. A value 60 seconds is retransmissions are not too far apart. A value 60 seconds is
consistent with DHCP. consistent with DHCP.
5. Acknowledgements 5. Acknowledgements
The comments from Thomas Narten, Philip Homburg, and Joel Jaeggli The comments from Thomas Narten, Philip Homburg, Joel Jaeggli, and
have helped improve this draft. Hemant Singh have helped improve this draft.
6. Security Considerations 6. Security Considerations
Relaxing the retransmission behavior for NUD is belived to have no Relaxing the retransmission behavior for NUD is believed to have no
impact on security. In particular, it doesn't impact the application impact on security. In particular, it doesn't impact the application
Secure Neighbor Discovery [RFC3971]. Secure Neighbor Discovery [RFC3971].
7. IANA Considerations 7. IANA Considerations
This are no IANA considerations for this document. This are no IANA considerations for this document.
8. References 8. References
8.1. Normative References 8.1. Normative References
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