One document matched: draft-ietf-tsvwg-sctp-failover-03.xml
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<front>
<title abbrev="SCTP Quick Failover">Quick Failover Algorithm in
SCTP</title>
<author fullname="Yoshifumi Nishida" initials="Y.N" surname="Nishida">
<organization>GE Global Research</organization>
<address>
<postal>
<street>2623 Camino Ramon</street>
<city>San Ramon</city>
<region>CA</region>
<code>94583</code>
<country>USA</country>
</postal>
<email>nishida@wide.ad.jp</email>
</address>
</author>
<author fullname="Preethi Natarajan" initials="P.N" surname="Natarajan">
<organization>Cisco Systems</organization>
<address>
<postal>
<street>510 McCarthy Blvd</street>
<city>Milpitas</city>
<region>CA</region>
<code>95035</code>
<country>USA</country>
</postal>
<email>prenatar@cisco.com</email>
</address>
</author>
<author fullname="Armando Caro" initials="A.C" surname="Caro">
<organization>BBN Technologies</organization>
<address>
<postal>
<street>10 Moulton St.</street>
<city>Cambridge</city>
<region>MA</region>
<code>02138</code>
<country>USA</country>
</postal>
<email>acaro@bbn.com</email>
</address>
</author>
<author fullname="Paul D. Amer" initials="P.A" surname="Amer">
<organization>University of Delaware</organization>
<address>
<postal>
<street>Computer Science Department - 434 Smith Hall</street>
<city>Newark</city>
<region>DE</region>
<code>19716-2586</code>
<country>USA</country>
</postal>
<email>amer@udel.edu</email>
</address>
</author>
<author fullname="Karen E. E. Nielsen" initials="K.N" surname="Nielsen">
<organization>Ericsson</organization>
<address>
<postal>
<street>Kistavägen 25</street>
<city>Stockholm</city>
<region/>
<code>164 80</code>
<country>Sweden</country>
</postal>
<email>karen.nielsen@tieto.com</email>
</address>
</author>
<date/>
<abstract>
<t>One of the major advantages of SCTP is supporting multi-homed
communication. If a multi-homed end-point has a redundant network
connections, the SCTP associations have a good chance to survive network
failures by migrating traffic from inactive networks to active ones.
However, if the SCTP standard is followed, there can be a significant
delay during the migration. During this period, SCTP might not be able
to transmit much data to the peer. This issue drastically impairs the
usability of SCTP in some situations. This memo describes the issue of
the SCTP failover mechanism and specifies an alternative failover
procedure for SCTP that improves its performance during and after
failover. The procedures require only minimal modifications to the
current specification.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>The Stream Control Transmission Protocol (SCTP) as specified in <xref
target="RFC4960"/> supports multihoming at the transport layer -- an
SCTP association can bind to multiple IP addresses at each endpoint.
SCTP's multihoming features include failure detection and failover
procedures to provide network interface redundancy and improved
end-to-end fault tolerance.</t>
<t>In SCTP's current failure detection procedure, the sender must
experience Path.Max.Retrans (PMR) number of consecutive failed
retransmissions on a destination before detecting a path failure. The
sender fails over to an alternate active destination only after failure
detection. Until detecting the failover, the sender continues to
transmit data on the failed path, which degrades the SCTP performance.
Concurrent Multipath Transfer (CMT) <xref target="IYENGAR06"/> is an
extension to SCTP and allows the sender to transmit data on multiple
paths simultaneously. Research <xref target="NATARAJAN09"/> shows that
the current failure detection procedure worsens CMT performance during
failover and can be significantly improved by employing a better
failover algorithm.</t>
<t>This document specifies an alternative failure detection procedure
for SCTP (and CMT) that improves the SCTP (and CMT) performance during a
failover.</t>
<t>Also the operation after a failover impacts the performance of the
protocol. With <xref target="RFC4960"/> procedures, SCTP will, after a
failover from the primary path, switch back to use the primary path for
data transfer as soon as this path becomes available. From a performance
perspective, as confirmed in research <xref target="CARO02"/>, such a
switchback of the data transmission path is not optimal in general. As
an alternative option to the switchback operation of <xref
target="RFC4960"/>, this document specifies the support the Permanent
Failover switchover procedures proposed by <xref target="CARO02"/>.</t>
</section>
<section title="Conventions and Terminology">
<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="Issues with the SCTP Path Management">
<t>SCTP can utilize multiple IP addresses for a single SCTP association.
Each SCTP endpoint exchanges the list of its usable addresses during
initial negotiation with its peer. Then the endpoints select one address
from the peer's list and define this as the primary destination. During
normal transmission, SCTP sends all user data to the primary
destination. Also, it sends heartbeat packets to all idle destinations
at a certain interval to check the reachability of the path. Idle
destinations normally include all non-primary destinations.</t>
<t>If a sender has multiple active destination addresses, it can
retransmit data to secondary destination address, when the transmission
to the primary times out.</t>
<t>When a sender receives an acknowledgment for DATA or HEARTBEAT chunks
sent to one of the destination addresses, it considers that destination
to be active. If it fails to receive acknowledgments, the error count
for the address is increased. If the error counter exceeds the protocol
parameter 'Path.Max.Retrans', SCTP endpoint considers the address to be
inactive.</t>
<t>The failover process of SCTP is initiated when the primary path
becomes inactive (error counter for the primary path exceeds
Path.Max.Retrans). If the primary path is marked inactive, SCTP chooses
a new destination address from one of the active destinations and start
using this address to send data to. If the primary path becomes active
again, SCTP uses the primary destination for subsequent data
transmissions and stop using non-primary one.</t>
<t>One issue with this failover process is that it usually takes
significant amount of time before SCTP switches to the new destination.
Let's say the primary path on a multi-homed host becomes unavailable and
the RTO value for the primary path at that time is around 1 second, it
usually takes over 60 seconds before SCTP starts to use the secondary
path. This is because the recommended value for Path.Max.Retrans in the
standard is 5, which requires 6 consecutive timeouts before failover
takes place. Before SCTP switches to the secondary address, SCTP keeps
trying to send packets to the primary and only retransmitted packets are
sent to the secondary can be reached at the receiver. This slow failover
process can cause significant performance degradation and will not be
acceptable in some situations.</t>
<t>Another issue is that once the primary path is active again, the
traffic is switched back. This is not optimal in general.</t>
</section>
<section title="Existing Solutions for Smooth Failover">
<t>The following approaches are conceivable for the solutions of this
issue.</t>
<section title="Reduce Path.Max.Retrans (PMR)">
<t>Smaller values for Path.Max.Retrans shorten the failover duration.
In fact, this is recommended in some research results <xref
target="JUNGMAIER02"/> <xref target="GRINNEMO04"/> <xref
target="FALLON08"/>. For example, if when Path.Max.Retrans=0, SCTP
switches to another destination on a single timeout. However, smaller
value for Path.Max.Retrans also results in spurious failover. In
addition, smaller Path.Max.Retrans values also affect
'Association.Max.Retrans' values. When the SCTP association's error
count (sum of error counts on all ACTIVE paths) exceeds
Association.Max.Retrans threshold, the SCTP sender considers the peer
endpoint unreachable and terminates the association. Therefore,
Section 8.2 in <xref target="RFC4960"/> recommends that
Association.Max.Retrans value should not be larger than the summation
of the Path.Max.Retrans of each of the destination addresses, else the
SCTP sender considers its peer reachable even when all destinations
are INACTIVE. To avoid such inconsistent behavior an SCTP
implementation SHOULD reduce Association.Max.Retrans accordingly
whenever it reduces Path.Max.Retrans. However, smaller
Association.Max.Retrans value increases chances of association
termination during minor congestion events.</t>
<t>Another issue is that the interval of heartbeat packet:
'HB.interval' could be in the order of seconds (recommended value is
30 seconds). When the primary path becomes inactive, the next HB can
be transmitted only seconds later. Meanwhile, the primary path may
have recovered. In such situations, post failover, an endpoint is
forced to wait on the order of seconds before the endpoint can resume
transmission on the primary path.</t>
<t>The advantage of tuning Path.Max.Retrans is that it requires no
modification to the current standard. However, as we discuss above
tuning Path.Max.Retrans ignores several recommendations in <xref
target="RFC4960"/>. In addition, some research results indicate path
bouncing caused by spurious failover does not cause serious problems.
We discuss the effect of path bouncing in <xref
target="path_bouncing"/>.</t>
</section>
<section title="Adjust RTO related parameters">
<t>As several research results indicate, we can also shorten the
duration of failover process by adjusting RTO related parameters <xref
target="JUNGMAIER02"/> <xref target="FALLON08"/>. During failover
process, RTO keeps being doubled. However, if we can choose smaller
value for RTO.max, we can stop the exponential growth of RTO at some
point. Also, choosing smaller values for RTO.initial or RTO.min can
contribute to keep RTO value small.</t>
<t>Similar to reducing Path.Max.Retrans, the advantage of this
approach is that it requires no modification to the current
specification, although it needs to ignore several recommendations
described in the Section 15 of <xref target="RFC4960"/>. However, this
approach requires to have enough knowledge about the network
characteristics between end points. Otherwise, it can introduce
adverse side-effects such as spurious timeouts.</t>
</section>
</section>
<section anchor="SCTP_PF"
title="SCTP with Potentially-Failed Destination State (SCTP-PF)">
<section title="SCTP-PF Description">
<t>SCTP-PF stems from the following two observations about SCTP's
failure detection procedure: <list style="symbols">
<t>In order to minimize performance impact during failover, the
sender should avoid transmitting data to the failed destination as
early as possible. In the current SCTP path management scheme, the
sender stops transmitting data to a destination only after the
destination is marked Failed. Thus, a smaller PMR value is ideal
so that the sender transitions a destination to the Failed state
quicker.</t>
<t>Smaller PMR values increase the chances of spurious failure
detection where the sender incorrectly marks a destination as
Failed during periods of temporary congestion. Larger PMR values
are preferable to avoid spurious failure detection.</t>
</list></t>
<t>From the above observations it is clear that tweaking the PMR value
involves the following tradeoff -- a lower value improves performance
but increases the chances of spurious failure detection, whereas a
higher value degrades performance and reduces spurious failure
detection in a wide range of path conditions. Thus, tweaking the
association's PMR value is an incomplete solution to address
performance impact during failure.</t>
<t>This proposal introduces a new "Potentially-failed" (PF)
destination state in SCTP's path management procedure. The PF state
was originally proposed to improve CMT performance <xref
target="NATARAJAN09"/>. The PF state is an intermediate state between
Active and Failed states. SCTP's failure detection procedure is
modified to include the PF state. The new failure detection algorithm
assumes that loss detected by a timeout implies either severe
congestion or failure en-route. After a number of consecutive timeouts
on a path, the sender is unsure, and marks the corresponding
destination as PF. A PF destination is not used for data transmission
except in special cases (discussed below). The new failure detection
algorithm requires only sender-side changes. Details are: <list
style="numbers">
<t>The sender maintains a new tunable parameter called
Potentially-failed.Max.Retrans (PFMR). The recommended value of
PFMR = 0 when quick failover is used. When PFMR is larger or equal
to PMR, quick failover is turned off.</t>
<t>Each time the T3-rtx timer expires on an active destination,
the error counter of that destination address will be incremented.
When the value in the error counter exceeds PFMR, the endpoint
should mark the destination transport address as PF.</t>
<t>The sender SHOULD avoid data transmission to PF destinations.
When all destinations are in either PF or Inactive state, the
sender MAY either move the destination from PF to Active state
(and transmit data to the active destination) or the sender MAY
transmit data to a PF destination. In the former scenario, (i) the
sender MUST NOT notify the ULP about the state transition, and
(ii) MUST NOT clear the destination's error counter. It is
recommended that the sender picks the PF destination with least
error count (fewest consecutive timeouts) for data transmission.
In case of a tie (multiple PF destinations with same error count),
the sender MAY choose the last active destination.</t>
<t>Only heartbeats MUST be sent to PF destination(s) once per RTO.
This means the sender SHOULD ignore HB.interval for PF
destinations. If an heartbeat is unanswered, the sender increments
the error counter and exponentially backs off the RTO value. If
error counter is less than PMR, the sender SHOULD transmit another
heartbeat immediately after T3-timer expiration.</t>
<t>When the sender receives an heartbeat ACK from a PF
destination, the sender clears the destination's error counter and
transitions the PF destination back to Active state. The sender
should perform slow-start as specified in Section 7.2.1 of <xref
target="RFC4960"/> when it sends data on this destination.</t>
<t>Additional (PMR - PFMR) consecutive timeouts on a PF
destination confirm the path failure, upon which the destination
transitions to the Inactive state. As described in <xref
target="RFC4960"/>, the sender (i) SHOULD notify ULP about this
state transition, and (ii) transmit heartbeats to the Inactive
destination at a lower frequency as described in Section 8.3 of
<xref target="RFC4960"/>.</t>
<t>When all destinations are in the Inactive state, the sender
picks one of the Inactive destinations for data transmission. This
proposal recommends that the sender picks the Inactive destination
with least error count (fewest consecutive timeouts) for data
transmission. In case of a tie (multiple Inactive destinations
with same error count), the sender MAY choose the last active
destination.</t>
<!-- MT: Should this be specified? -->
<t>ACKs for retransmissions do not transition a PF destination
back to Active state, since a sender cannot disambiguate whether
the ack was for the original transmission or the
retransmission(s).</t>
<!--- MT: Is this new? Should it be done?-->
<t>SCTP shall provide the means to expose the PF state of its
destinations as well as SCTP SHOULD notify the ULP of the state
transitions from Active to PF and from PF to Active state. SCTP
can provide the means to suppress exposure of PF state and
association state transitions and in this case the ULP MAY make
SCTP suppress exposure of PF state to ULP. In this case the ULP
will rely solely on the <xref target="RFC4960"/> state machine
even if quick failover function is activated in SCTP.</t>
</list></t>
</section>
<section anchor="path_bouncing" title="Effect of Path Bouncing">
<t>The methods described above can accelerate the failover process.
Hence, they might introduce the path bouncing effect where the sender
keeps changing the data transmission path frequently. This sounds
harmful to the data transfer, however several research results
indicate that there is no serious problem with SCTP in terms of path
bouncing effect <xref target="CARO04"/> <xref target="CARO05"/>.</t>
<t>There are two main reasons for this. First, SCTP is basically
designed for multipath communication, which means SCTP maintains all
path related parameters (CWND, ssthresh, RTT, error count, etc) per
each destination address. These parameters cannot be affected by path
bouncing. In addition, when SCTP migrates the data transfer to another
path, it starts with the minimal or the initial CWND. Hence, there is
little chance for packet reordering or duplicating.</t>
<t>Second, even if all communication paths between the end-nodes share
the same bottleneck, the quick failover results in a behavior already
allowed by <xref target="RFC4960"/>.</t>
</section>
<section title="Permanent Failover">
<t>Post failover then, by <xref target="RFC4960"/> behavior, an SCTP
sender migrates the traffic back to the original primary destination
once this destination becomes active anew. As the CWND towards the
original primary destination has to be rebuilt once data transfer
resumes, the switch back to use the original primary path is not
always optimal. Indeed <xref target="CARO02"/> shows that the switch
over to the original primary may degrade SCTP performance compared to
continuing data transmission on the same path, especially, but not
only, in scenarios where this path's characteristics are better. In
order to mitigate this performance degradation, Permanent Failover
operation was proposed in <xref target="CARO02"/>. When SCTP changes
the destination due to failover, Permanent Failover marks it as new
primary. This means Permanent Failover allows SCTP sender to continue
data transmission to the path even after the old primary destination
becomes active again. This is achieved by having SCTP perform a
switchover of the primary path to an alternative working path rather
than having SCTP switch back data transfer to the (previous) primary
path.</t>
<t>The manner of switchover operation that is most optimal in a given
scenario depends on the relative quality of a set primary path versus
the quality of alternative paths available as well as it depends on
the extent to which it is desired for the mode of operation to enforce
traffic distribution over a number of network paths. I.e., load
distribution of traffic from multiple SCTP associations may be sought
to be enforced by distribution of the set primary paths with <xref
target="RFC4960"/> switchback operation. However as <xref
target="RFC4960"/> switchback behavior is suboptimal in certain
situations, especially in scenarios where a number of equally good
paths are available, it is recommended for SCTP to support also, as
alternative behavior, the Permanent Failover modes of operation where
forced switch back to a previously failed primary path is not always
performed. The Permanent Failover operation requires only sender side
changes. Details, as originally outlined in <xref target="CARO02"/>,
are:</t>
<t><list style="numbers">
<t>The sender maintains a new tunable parameter, called
Primary.Switchover.Max.Retrans (PSMR). When the path error counter
on a set primary path exceeds PSMR, the SCTP implementation
autonomously selects and sets a new primary path.</t>
<t>The primary path selected by the SCTP implementation shall be
the path which at the given time would be chosen for data
transfer. A previously failed primary path may come in use as data
transfer path as per normal path selection when the present data
transfer path fails.</t>
<t>The recommended value of PSMR is PFMR when Permanent failover
is used. This means that no forced switchback to a previously
failed primary path is performed.</t>
<t>It must be possible to disable the Permanent Failover and
obtain the standard switchback operation of <xref
target="RFC4960"/>.</t>
</list></t>
<t>We recommend that SCTP-PF should stick to the standard RFC4960
behavior as default, i.e., switch back to the old primary destination
once the destination becomes active again. However, implementors MAY
implement Permanent Failover and MAY enable it based on network
configurations or users' requests.</t>
</section>
</section>
<section title="Socket API Considerations">
<t>This section describes how the socket API defined in <xref
target="RFC6458"/> is extended to provide a way for the application to
control and observe the quick failover behavior.</t>
<t>Please note that this section is informational only.</t>
<t>A socket API implementation based on <xref target="RFC6458"/> is, by
means of the existing SCTP_PEER_ADDR_CHANGE event, extended to provide
the event notification when a peer address enters or leaves the
potentially failed state as well as the socket API implementation is
extended to expose the potentially failed state of a peer address in the
existing SCTP_GET_PEER_ADDR_INFO structure.</t>
<t>Furthermore, two new read/write socket options for the level
IPPROTO_SCTP and the name SCTP_PEER_ADDR_THLDS and
SCTP_EXPOSE_POTENTIALLY_FAILED_STATE are defined as described below.
The first socket option is used to control the values of the PFMR and
PSMR parameters described in <xref target="SCTP_PF"/>. The second one
controls the exposition of the potentially failed path state.</t>
<t>Support for the SCTP_PEER_ADDR_THLDS and
SCTP_EXPOSE_POTENTIALLY_FAILED_STATE socket options need also to be
added to the function sctp_opt_info().</t>
<section anchor="pf_support_api"
title="Support for the Potentially Failed Path State">
<t>As defined in <xref target="RFC6458"/>, the SCTP_PEER_ADDR_CHANGE
event is provided if the status of a peer address changes. In addition
to the state changes described in <xref target="RFC6458"/>, this event
is also provided, if a peer address enters or leaves the potentially
failed state. The notification as defined in <xref target="RFC6458"/>
uses the following structure:</t>
<figure>
<artwork><![CDATA[
struct sctp_paddr_change {
uint16_t spc_type;
uint16_t spc_flags;
uint32_t spc_length;
struct sockaddr_storage spc_aaddr;
uint32_t spc_state;
uint32_t spc_error;
sctp_assoc_t spc_assoc_id;
}
]]></artwork>
</figure>
<t><xref target="RFC6458"/> defines the constants SCTP_ADDR_AVAILABLE,
SCTP_ADDR_UNREACHABLE, SCTP_ADDR_REMOVED, SCTP_ADDR_ADDED, and
SCTP_ADDR_MADE_PRIM to be provided in the spc_state field. This
document defines in addition to that the new constant
SCTP_ADDR_POTENTIALLY_FAILED, which is reported if the affected
address becomes potentially failed.</t>
<t>The SCTP_GET_PEER_ADDR_INFO socket option defined in <xref
target="RFC6458"/> can be used to query the state of a peer address.
It uses the following structure:</t>
<figure>
<artwork><![CDATA[
struct sctp_paddrinfo {
sctp_assoc_t spinfo_assoc_id;
struct sockaddr_storage spinfo_address;
int32_t spinfo_state;
uint32_t spinfo_cwnd;
uint32_t spinfo_srtt;
uint32_t spinfo_rto;
uint32_t spinfo_mtu;
};
]]></artwork>
</figure>
<t><xref target="RFC6458"/> defines the constants SCTP_UNCONFIRMED,
SCTP_ACTIVE, and SCTP_INACTIVE to be provided in the spinfo_state
field. This document defines in addition to that the new constant
SCTP_POTENTIALLY_FAILED, which is reported if the peer address is
potentially failed.</t>
</section>
<section title="Peer Address Thresholds (SCTP_PEER_ADDR_THLDS) Socket Option">
<t>Applications can control the quick failover behavior by getting or
setting the number of consecutive timeouts before a peer address is
considered potentially failed or unreachable and before the primary
path is changed automatically. This socket option uses the level
IPPROTO_SCTP and the name SCTP_PEER_ADDR_THLDS.</t>
<t>The following structure is used to access and modify the
thresholds:</t>
<figure>
<artwork><![CDATA[
struct sctp_paddrthlds {
sctp_assoc_t spt_assoc_id;
struct sockaddr_storage spt_address;
uint16_t spt_pathmaxrxt;
uint16_t spt_pathpfthld;
uint16_t spt_pathcpthld;
};
]]></artwork>
</figure>
<t><list style="hanging">
<t hangText="spt_assoc_id:">This parameter is ignored for
one-to-one style sockets. For one-to-many style sockets the
application may fill in an association identifier or
SCTP_FUTURE_ASSOC. It is an error to use SCTP_{CURRENT|ALL}_ASSOC
in spt_assoc_id.</t>
<t hangText="spt_address:">This specifies which peer address is of
interest. If a wildcard address is provided, this socket option
applies to all current and future peer addresses.</t>
<t hangText="spt_pathmaxrxt:">Each peer address of interest is
considered unreachable, if its path error counter exceeds
spt_pathmaxrxt.</t>
<t hangText="spt_pathpfthld:">Each peer address of interest is
considered potentially failed, if its path error counter exceeds
spt_pathpfthld.</t>
<t hangText="spt_pathcpthld:">Each peer address of interest is not
considered the primary remote address anymore, if its path error
counter exceeds spt_pathcpthld. Using a value of 0xffff disables
the selection of a new primary peer address. If an implementation
does not support the automatically selection of a new primary
address, it should indicate an error with errno set to EINVAL if a
value different from 0xffff is used in spt_pathcpthld.</t>
</list></t>
</section>
<section title="Exposing the Potentially Failed Path State (SCTP_EXPOSE_POTENTIALLY_FAILED_STATE) Socket Option">
<t>Applications can control the exposure of the potentially failed
path state in the SCTP_PEER_ADDR_CHANGE event and the
SCTP_GET_PEER_ADDR_INFO as described in <xref
target="pf_support_api"/>. The default value is implementation
specific.</t>
<t>This socket option uses the level IPPROTO_SCTP and the name
SCTP_EXPOSE_POTENTIALLY_FAILED_STATE.</t>
<t>The following structure is used to control the exposition of the
potentially failed path state:</t>
<figure>
<artwork><![CDATA[
struct sctp_assoc_value {
sctp_assoc_t assoc_id;
uint32_t assoc_value;
};
]]></artwork>
</figure>
<t><list style="hanging">
<t hangText="assoc_id:">This parameter is ignored for one-to-one
style sockets. For one-to-many style sockets the application may
fill in an association identifier or SCTP_FUTURE_ASSOC. It is an
error to use SCTP_{CURRENT|ALL}_ASSOC in assoc_id.</t>
<t hangText="assoc_value:">The potentially failed path state is
exposed if and only if this parameter is non-zero.</t>
</list></t>
</section>
</section>
<section title="Security Considerations">
<t>There are no new security considerations introduced in this
document.</t>
</section>
<section title="IANA Considerations">
<t>This document does not create any new registries or modify the rules
for any existing registries managed by IANA.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.2119" ?>
<?rfc include="reference.RFC.4960" ?>
</references>
<references title="Informative References">
<?rfc include="reference.RFC.6458" ?>
<reference anchor="IYENGAR06" target="">
<front>
<title>Concurrent Multipath Transfer using SCTP Multihoming over
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</rfc>
| PAFTECH AB 2003-2026 | 2026-04-24 03:00:12 |