draft-ietf-6man-impatient-nud-06.txt   draft-ietf-6man-impatient-nud-07.txt 
6MAN WG E. Nordmark 6MAN WG E. Nordmark
Internet-Draft Cisco Systems, Inc. Internet-Draft Arista Networks
Updates: 4861 (if approved) I. Gashinsky Updates: 4861 (if approved) I. Gashinsky
Intended status: Standards Track Yahoo! Intended status: Standards Track Yahoo!
Expires: October 26, 2013 April 24, 2013 Expires: April 24, 2014 October 21, 2013
Neighbor Unreachability Detection is too impatient Neighbor Unreachability Detection is too impatient
draft-ietf-6man-impatient-nud-06.txt draft-ietf-6man-impatient-nud-07.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 neighbor, That function is very useful when a host has an alternative neighbor,
for instance when there are multiple default routers, since it allows for instance when there are multiple default routers, since it allows
the host to switch to the alternative neighbor in short time. This the host to switch to the alternative neighbor in short time. This
time is 3 seconds after the node starts probing by default. However, time is 3 seconds after the node starts probing by default. However,
if there are no alternative neighbors, this is far too impatient. if there are no alternative neighbors, this is far too impatient.
This document specifies relaxed rules for Neighbor Discovery This document specifies relaxed rules for Neighbor Discovery
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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 October 26, 2013. This Internet-Draft will expire on April 24, 2014.
Copyright Notice Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the Copyright (c) 2013 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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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 . . . . . . . . . . . . . . . . . . . . . . . 8 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 8 6. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . . 8 8.1. Normative References . . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . . 8 8.2. Informative References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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 for NUD are very short (by default reachable. The timeouts specified for NUD are very short (by default
three transmissions spaced one second apart). These short can be three transmissions spaced one second apart). These short timeouts
appropriate when there are alternative neighbors to which the packets can be appropriate when there are alternative neighbors to which the
can be sent. For example, if a host has multiple default routers in packets can be sent. For example, if a host has multiple default
its Default Router List, or if the host has a Neighbor Cache Entry routers in its Default Router List or if the host has a Neighbor
(NCE) created by a Redirect message. In these cases, when NUD fails, Cache Entry (NCE) created by a Redirect message. In those cases,
the host will try the alternative neighbor; the next router in the when NUD fails, the host will try the alternative neighbor by redoing
Default Router List, or discard the NCE which will also send using a next-hop selection. That implies picking the next router in the
different router. Default Router List or discarding the redirect, respectively.
The timeouts specified in [RFC4861] were chosen to be short in order The timeouts specified in [RFC4861] were chosen to be short in order
to optimize for the scenarios where alternative neighbors are to optimize for the scenarios where alternative neighbors are
available. available.
However, when there is no alternative neighbor there are several However, when there is no alternative neighbor there are several
benefits in making NUD try probing for a longer time. One of those benefits in making NUD try probing for a longer time. One of those
benefits is to make NUD more robust against transient failures, such benefits is to make NUD more robust against transient failures, such
as spanning tree reconvergence and other layer 2 issues that can take as spanning tree reconvergence and other layer 2 issues that can take
many seconds to resolve. Marking the NCE as unreachable in that case many seconds to resolve. Marking the NCE as unreachable in that case
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in the face of degenerate conditions are covered in [RFC6583]. in the face of degenerate conditions are covered in [RFC6583].
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 needed Discarding the NCE after after three packets spaced one second apart
when an alternative neighbor is available, such as an additional is only needed when an alternative neighbor is available, such as an
default router or a redirect. additional default router or a redirect.
If implementations transmit more than MAX_UNICAST_SOLICIT/ If an implementation transmits more than MAX_UNICAST_SOLICIT/
MAX_MULTICAST_SOLICIT packets it SHOULD use exponential backoff of MAX_MULTICAST_SOLICIT packets then it SHOULD use exponential backoff
the retransmit timer. This is to avoid any significant load due to a of the retransmit timer. This is to avoid any significant load due
steady background level of retransmissions from implementations that to a steady background level of retransmissions from implementations
try for a long time. that retransmit a large number of NSes before discarding the NCE.
Even if there is no alternative neighbor, the protocol needs to be Even if there is no alternative neighbor, the protocol needs to be
able to handle the case when the link-layer address of the able to handle the case when the link-layer address of the neighbor/
destination has changed by switching to multicast Neighbor target has changed by switching to multicast Neighbor Solicitations
Solicitations at some point in time. at some point in time.
In order to capture all the cases above this document introduces a In order to capture all the cases above this document introduces a
new UNREACHABLE state in the conceptual model described in [RFC4861]. new UNREACHABLE state in the conceptual model described in [RFC4861].
A NCE in the UNREACHABLE state retains the link-layer address, and 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 IPv6 packets continue to be sent to that link-layer address. But in
the UNREACHABLE state the NUD Neighbor Solicitations are multicast the UNREACHABLE state the NUD Neighbor Solicitations are multicast
(to the solicited-node multicast address), using a timeout that (to the solicited-node multicast address), using a timeout that
follows an exponential backoff. follows an 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) this document instead
to the UNREACHABLE state. specifies a transition 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 [RFC4861], Section 5.2,
[RFC4861] which disregards NCEs that are not reachable. 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 [RFC4861], Section 6.3.6 says to
routers that are "known to be reachable". For the purposes of that prefer default routers that are "known to be reachable". For the
section, if the NCE for the router is in UNREACHABLE state, it is not purposes of that section, if the NCE for the router is in UNREACHABLE
known to be reachable. Thus the particular text in section 6.3.6 state, it is not known to be reachable. Thus the particular text in
which says "in any state other than INCOMPLETE" needs to be extended section 6.3.6 which says "in any state other than INCOMPLETE" needs
to say "in any state other than INCOMPLETE or UNREACHABLE". 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 Apart from the use of multicast NS instead of unicast NS, and the
exponential backoff of the timer, the UNREACHABLE state works the exponential backoff of the timer, the UNREACHABLE state works the
same as the current PROBE state. same as the current PROBE state.
A node MAY garbage collect a Neighbor Cache Entry at 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 freedom to garbage collect does not
the UNREACHABLE state in the conceptual model. 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 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 [RFC4861], Section 7.2.5, since it is
using "Otherwise" instead of explicitly listing the three states. phrased 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
introduction of the UNREACHABLE state without any change, since they introduction of the UNREACHABLE state without any change, since they
are described using "not INCOMPLETE". are described using "not INCOMPLETE".
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:
State Event Action New state State Event Action New state
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:
State Event Action New state State Event Action New state
PROBE Retransmit timeout, Increase timeout UNREACHABLE PROBE Retransmit timeout, Increase timeout UNREACHABLE
N or more Send multicast NS N retransmissions. Send multicast NS
retransmissions.
UNREACHABLE Retransmit timeout Increase timeout UNREACHABLE UNREACHABLE Retransmit timeout Increase timeout UNREACHABLE
Send multicast NS Send multicast NS
The exponential backoff SHOULD be clamped at some reasonable maximum The exponential backoff SHOULD be clamped at some reasonable maximum
retransmit timeout, such as 60 seconds (see MAX_RETRANS_TIMER below). retransmit timeout, such as 60 seconds (see MAX_RETRANS_TIMER below).
If there is no IPv6 packet sent using the UNREACHABLE NCE, then it If there is no IPv6 packet sent using the UNREACHABLE NCE, then it is
makes sense to stop the retransmits of the multicast NS until either RECOMMENDED to stop the retransmits of the multicast NS until either
the NCE is garbage collected or there are IPv6 packets sent using the the NCE is garbage collected or there are IPv6 packets sent using the
NCE. The multicast NS and associated exponential backoff can be NCE. The multicast NS and associated exponential backoff can be
applied on the condition of the continued use of the NCE to send IPv6 applied on the condition of the continued use of the NCE to send IPv6
packets to the recorded link-layer address. packets to the recorded link-layer address.
A node MAY unicast the first few Neighbor Solicitation messages even A node can unicast the first few Neighbor Solicitation messages even
while in UNREACHABLE state, but it MUST switch to multicast Neighbor while in UNREACHABLE state, but it MUST switch to multicast Neighbor
Solicitations sooner or later. Otherwise it would not detect a link- Solicitations within 60 seconds of the initial retransmission to be
layer address change for the target. The example below shows such able to handle a link-layer address change for the target. The
behavior. 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_UNICAST_SOLICIT, The operator can configure the values for MAX_UNICAST_SOLICIT,
MAX_MULTICAST_SOLICIT, RETRANS_TIMER, and the new BACKOFF_MULTIPLE, MAX_MULTICAST_SOLICIT, RETRANS_TIMER, and the new BACKOFF_MULTIPLE,
MAX_RETRANS_TIMER and MARK_UNREACHABLE. This allows the MAX_RETRANS_TIMER and MARK_UNREACHABLE. This allows the
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$RetransTimer * $JitterFactor where solicit_retrans_num is zero for $RetransTimer * $JitterFactor where solicit_retrans_num is zero for
the first transmission, and JitterFactor is a random value between the first transmission, and JitterFactor is a random value between
MIN_RANDOM_FACTOR and MAX_RANDOM_FACTOR [RFC4861] to avoid any MIN_RANDOM_FACTOR and MAX_RANDOM_FACTOR [RFC4861] to avoid any
synchronization of transmissions from different hosts. synchronization of transmissions from different hosts.
After MARK_UNREACHABLE transmissions the implementation would mark After MARK_UNREACHABLE transmissions the implementation would mark
the NCE UNREACHABLE and as result explore alternate next hops. After the NCE UNREACHABLE and as result explore alternate next hops. After
MAX_UNICAST_SOLICIT the implementation would switch to multicast NUD MAX_UNICAST_SOLICIT the implementation would switch to multicast NUD
probes. probes.
The recommended behavior is to have 5 attempts, with timing spacing The behavior of this example algorithm is to have 5 attempts, with
of 0 (initial request), 1 second later, 3 seconds after the first timing spacing of 0 (initial request), 1 second later, 3 seconds
retransmission, then 9, then 27, and switch to UNREACHABLE after the after the first retransmission, then 9, then 27, and switch to
first three transmissions. Thus relative to the time of the first UNREACHABLE after the first three transmissions. Thus relative to
transmissions the retransmissions would occur at 1 second, 4 seconds, the time of the first transmissions the retransmissions would occur
13 seconds, and finally 40 seconds. At 4 seconds from the first at 1 second, 4 seconds, 13 seconds, and finally 40 seconds. At 4
transmission the NCE would be marked UNREACHABLE. That recommended seconds from the first transmission the NCE would be marked
behavior corresponds to: UNREACHABLE. That 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. That results in trying an alternative neighbor, such as UNREACHABLE. That results in trying an alternative neighbor, such as
another default router or ignoring a redirect as specified in another default router or ignoring a redirect as specified in
[RFC4861]. With the above recommended values that would occur after [RFC4861]. With the above values that would occur after 4 seconds
4 seconds after the first transmission compared to the 2 seconds after the first transmission compared to the 2 seconds using the
using the fixed scheme in [RFC4861]. That additional delay is small fixed scheme in [RFC4861]. That additional delay is small compared
compared to the default 30,000 milliseconds ReachableTime. to the default 30,000 milliseconds ReachableTime.
After 5 transmissions, i.e., 40 seconds after the initial After 5 transmissions, i.e., 40 seconds after the initial
transmission, the recommended behavior is to switch to multicast NUD transmission, the example behavior is to switch to multicast NUD
probes. In the language of the state machine in [RFC4861] that probes. In the language of the state machine in [RFC4861] that
corresponds to the action "Discard entry". Thus any attempts to send corresponds to the action "Discard entry". Thus any attempts to send
future packets would result in sending multicast NS packets. An future packets would result in sending multicast NS packets. An
implementation MAY retain the backoff value as it switches to implementation MAY retain the backoff value as it switches to
multicast NUD probes. The potential downside of deferring switching multicast NUD probes. The potential downside of deferring switching
to multicast is that it would take longer for NUD to handle a change 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 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. changes their link-layer address while keeping the same IPv6 address.
However, [RFC4861] says that a node MAY send unsolicited NS to handle However, [RFC4861] says that a node MAY send unsolicited NS to handle
that case, which is rather infrequent in operational networks. 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, 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_RETRANS_TIMER, configure are BACKOFF_MULTIPLE, MAX_RETRANS_TIMER,
MAX_UNICAST_SOLICIT and MAX_MULTICAST_SOLICIT. MAX_UNICAST_SOLICIT and MAX_MULTICAST_SOLICIT.
It is 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. The recommended value of 60 retransmissions are not too far apart. The above value of 60 seconds
seconds for this MAX_RETRANS_TIMER is consistent with DHCPv6. 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, Tina Tsou, and Suresh Krishnan have helped improve this draft. Singh, Tina Tsou, Suresh Krishnan, and Murray Kucherawy 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
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.
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converting network protocol addresses to 48.bit Ethernet converting network protocol addresses to 48.bit Ethernet
address for transmission on Ethernet hardware", STD 37, address for transmission on Ethernet hardware", STD 37,
RFC 826, November 1982. RFC 826, November 1982.
[RFC6583] Gashinsky, I., Jaeggli, J., and W. Kumari, "Operational [RFC6583] Gashinsky, I., Jaeggli, J., and W. Kumari, "Operational
Neighbor Discovery Problems", RFC 6583, March 2012. Neighbor Discovery Problems", RFC 6583, March 2012.
Authors' Addresses Authors' Addresses
Erik Nordmark Erik Nordmark
Cisco Systems, Inc. Arista Networks
510 McCarthy Blvd. Santa Clara, CA
Milpitas, CA, 95035
USA USA
Phone: +1 408 527 6625 Email: nordmark@acm.org
Email: nordmark@cisco.com
Igor Gashinsky Igor Gashinsky
Yahoo! Yahoo!
45 W 18th St 45 W 18th St
New York, NY New York, NY
USA USA
Email: igor@yahoo-inc.com Email: igor@yahoo-inc.com
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