draft-ietf-6man-impatient-nud-04.txt   draft-ietf-6man-impatient-nud-05.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
Intended status: Standards Track Yahoo! Intended status: Standards Track Yahoo!
Expires: April 4, 2013 Oct 2012 Expires: April 4, 2013 Oct 2012
Neighbor Unreachability Detection is too impatient Neighbor Unreachability Detection is too impatient
draft-ietf-6man-impatient-nud-04.txt draft-ietf-6man-impatient-nud-05.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
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definition Of Terms . . . . . . . . . . . . . . . . . . . . . . 4 2. Definition Of Terms . . . . . . . . . . . . . . . . . . . . . . 4
3. Protocol Updates . . . . . . . . . . . . . . . . . . . . . . . 4 3. Protocol Updates . . . . . . . . . . . . . . . . . . . . . . . 4
4. Example Algorithm . . . . . . . . . . . . . . . . . . . . . . . 6 4. Example Algorithm . . . . . . . . . . . . . . . . . . . . . . . 6
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 7 6. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . . 7 8.1. Normative References . . . . . . . . . . . . . . . . . . . 8
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
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the UNREACHABLE state the NUD Neighbor Solicitations are multicast, the UNREACHABLE state the NUD Neighbor Solicitations are multicast,
using a timeout that follows a (binary) exponential backoff. using a timeout that follows a (binary) exponential backoff.
In the places where RFC4861 says to to discard/delete the NCE after N 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 probes (Section 7.3, 7.3.3 and Appendix C) we will instead transition
to the UNREACHABLE state. to the UNREACHABLE state.
If the Neighbor Cache Entry was created by a redirect, a node MAY 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 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 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 packets if that NCE is in UNREACHABLE state. Packets should be sent
following the next-hop selection algorithm in section 5.2 in following the next-hop selection algorithm in section 5.2 in
[RFC4861] which disregards NCEs that are not reachable. [RFC4861] which disregards NCEs that are not reachable.
The default router selection in section 6.3.6 says to prefer default 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 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".
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There is also the more obvious change already described above. RFC There is also the more obvious change already described above. RFC
4861 has this: 4861 has this:
PROBE Retransmit timeout, Discard entry - PROBE Retransmit timeout, Discard entry -
N or more N or more
retransmissions. retransmissions.
That needs to be replaced by: That needs to be replaced by:
PROBE Retransmit timeout, Double timeout UNREACHABLE PROBE Retransmit timeout, Increase 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 Increase 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 (MAX_RETRANS_TIMER maximum retransmit timeout, such as 60 seconds (see MAX_RETRANS_TIMER
below). If there is no IPv6 packet sent using the UNREACHABLE NCE, 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 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 either the NCE is garbage collected or there are IPv6 packets sent
using the NCE. The multicast NS and associated binary exponential using the NCE. The multicast NS and associated binary exponential
backoff can be applied on the condition of the continued use of the backoff can be applied on the condition of the continued use of the
NCE to send IPv6 packets 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 even
in UNREACHABLE state, but it MUST switch to multicast Neighbor while in UNREACHABLE state, but it MUST switch to multicast Neighbor
Solicitations. Otherwise it would not detect a link-layer address Solicitations sooner or later. Otherwise it would not detect a link-
change for the target. layer address change for the target. The example below shows such
behavior.
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
using operator configurable values that allows it to be configured in using operator configurable values that allows it to be configured in
a way to be compatible with the retransmission behavior in [RFC4861]. a way to be compatible with the retransmission behavior in [RFC4861].
The operator can configure the values for MAX_*CAST_SOLICIT, The operator can configure the values for MAX_*CAST_SOLICIT,
RETRANS_TIMER, and the new BACKOFF_MULTIPLE and MARK_UNREACHABLE. RETRANS_TIMER, and the new BACKOFF_MULTIPLE, MAX_RETRANS_TIMER and
This allows the implementation to be as simple as: MARK_UNREACHABLE. 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 transmissions the implementation would mark
the NCE UNREACHABLE and switch to multicast NUD probes. 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 The recommended behavior is to have 5 attempts, with timing spacing
of 0 (initial request), 1 second later, 3 seconds later, then 9, then of 0 (initial request), 1 second later, 3 seconds after the first
27, then 60, then 60 and switch to UNREACHABLE after the three first retransmission, then 9, then 27, and switch to UNREACHABLE after the
transmissions. Thus relative to the time of the first transmissions first three transmissions. Thus relative to the time of the first
the retransmissions would occur at 1 second, 4 seconds, 13 seconds, transmissions the retransmissions would occur at 1 second, 4 seconds,
40 seconds, then 100 seconds, and finally 160 seconds. At 4 seconds 13 seconds, and finally 40 seconds. At 4 seconds from the first
from the first transmission the NCE would be marked UNREACHABLE. transmission the NCE would be marked UNREACHABLE. That recommended
That recommended behavior corresponds to: behavior corresponds to:
MAX_UNICAST_SOLICIT=5 MAX_UNICAST_SOLICIT=5
RETRANS_TIMER=1 (default) RETRANS_TIMER=1 (default)
MAX_RETRANS_TIMER=60 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. That results in trying an alternative, such as another
UNREACHABLE, and try any available alternative, after 4 seconds default router or ignoring a redirect as specified in [RFC4861].
compared to the current 2 seconds. That additional delay is small With the above recommended values that would occur after 4 seconds
compared to the default 30 seconds ReachableTime. 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, 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_RETRANS_TIMER,
MAX_MULTICAST_SOLICIT. MAX_UNICAST_SOLICIT and MAX_MULTICAST_SOLICIT.
It would be useful to have a maximum value for It is 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. The recommended value of 60
consistent with DHCP. seconds for this MAX_RETRANS_TIMER is consistent with DHCPv6.
5. Acknowledgements 5. Acknowledgements
The comments from Thomas Narten, Philip Homburg, Joel Jaeggli, Hemant The comments from Thomas Narten, Philip Homburg, Joel Jaeggli, Hemant
Singh, and Tina Tsou have helped improve this draft. Singh, and Tina Tsou have helped improve this draft.
6. Security Considerations 6. Security Considerations
Relaxing the retransmission behavior for NUD is believed 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
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