One document matched: draft-ietf-tsvwg-sctp-failover-08.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 that it supports multi-homed
communication. A multi-homed SCTP end-point has the ability to withstand
network failures by migrating the traffic from an inactive network to an
active one. However, if the <xref target="RFC4960"/> specified failover
operation is followed there can be a significant delay in the migration
to the active destination addresses, thus severely reducing the
effectiveness of SCTP multi-homed operation.</t>
<t>The memo complements RFC4960 by the introduction of the Potentially
Failed state and associated new Quick Failover operation to apply during
network failure and specifies for SCTP senders to support this more
performance optimal failover procedure as an add-on to the <xref
target="RFC4960"/> failover operation. The memo in addition complements
<xref target="RFC4960"/> by introduction of alternative switchover
operation modes for the data transfer path management after a failover.
These operation modes offer for more performance optimal operation in
some network environments. From the perspective of this memo the
implementation of the additional switchover operation modes is
considered optional.</t>
<t>The procedures defined require only minimal modifications to the
current specification. The procedures are sender-side only and do not
impact the SCTP receiver.</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 that improves the SCTP 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 optional alternative to the switchback operation of <xref
target="RFC4960"/>, this document specifies for SCTP to support the
Permanent Failover switchover procedures proposed by <xref
target="CARO02"/>. Additional discussions for alternative approach that
does not require modifications to <xref target="RFC4960"/> and path
bouncing effects that might be caused by frequent switchover are
provided in Appendix.</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 anchor="SCTP_issues" title="Issues with the SCTP Path Management">
<t>This section describes issues in the current SCTP to be fixed by the
approach described in this document.</t>
<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 and can thus 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 some situations. This
is further discussed in <xref target="permanent_failover"/>.</t>
</section>
<section anchor="SCTP_PF"
title="SCTP with Potentially-Failed Destination State (SCTP-PF)">
<t>To address the issues described in <xref target="SCTP_issues"/>, this
section updates SCTP path management scheme with the Potentially Failed
state and associated Quick Failover operation. We use the term SCTP-PF
to denote the resulting SCTP path management operation.</t>
<section title="SCTP-PF Concept">
<t>SCTP-PF as defined stems from the following two observations about
SCTP's failure detection procedure: <list style="symbols">
<t>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 (inactive). Thus, a smaller PMR value
is ideal so that the sender transitions a destination to the
Failed (inactive) state quicker.</t>
<t>Smaller PMR values increase the chances of spurious failure
detection where the sender incorrectly marks a destination as
Failed (inactive) during periods of temporary congestion. As <xref
target="RFC4960"/> recommends for a coupling of the PMR value and
the AMR value such spurious failure detection risks to carry over
to spurious association failure detection and closure. Larger PMR
values are preferable to avoid spurious failure detection.</t>
</list></t>
<t>From the above observations it is clear that tuning 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, tuning the
association's PMR value is an incomplete solution to address
performance impact during failure.</t>
<t>This new method 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.</t>
</section>
<section title="SCTP-PF Algorithm Detail">
<t>SCTP PF operation is specified as follows: <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>The error counter of an active destination address is
incremented as specified in <xref target="RFC4960"/>. This means
that the error counter of the destination address will be
incremented each time the T3-rtx timer expires, or at times where
a HEARTBEAT sent to an idle, active address is not acknowledged
within an RTO. When the value in the destination address error
counter exceeds PFMR, the endpoint MUST mark the destination
transport address as PF.</t>
<t>The sender SHOULD avoid data transmission to PF destinations.
When the destinations are all in PF state or some in PF state and
some in inactive state, the sender MUST choose one destination in
PF state and transmit data to this destination. The sender SHOULD
choose the destination in PF state with the lowest error count
(fewest consecutive timeouts) for data transmission and transmit
data to this destination. When there are multiple PF destinations
with same error count, the sender SHOULD let the choice among the
multiple PF destination with equal error count be based on the
<xref target="RFC4960"/>, section 6.4.1, principles of choosing
most divergent source-destination pairs when executing
(potentially consecutive) retransmission. This means that the
sender SHOULD attempt to pick the most divergent source -
destination pair from the last source - destination pair on which
data were transmitted or retransmitted. Rules for picking the most
divergent source-destination pair are an implementation decision
and are not specified within this document. A sender may choose to
deploy other strategies than the above when choosing among
multiple PF destinations with equal error count. In all cases the
sender MUST NOT change the state of chosen destination and it MUST
NOT clear the destination's error counter as a result of choosing
the destination for data transmission.</t>
<t>Heartbeats SHOULD be sent to PF destination(s) once per RTO.
This means the sender MUST ignore HB.interval for PF destinations.
If an heartbeat is unanswered, the sender SHOULD increment the
error counter and exponentially back off the RTO value. If error
counter is less than PMR, the sender SHOULD transmit another
heartbeat immediately after T3-timer expiration. When data is
transmitted to a PF destination, the transmission of heartbeats
may be omitted as SACK or T3-rtx timer expiration can provide
equivalent information. It is RECOMMENDED that heartbeats be send
to PF destinations regardless of whether the Path Heartbeat
function (Section 8.3 of <xref target="RFC4960"/>) is enabled for
the destination address or not.</t>
<t>When the sender receives an heartbeat ACK from a PF
destination, the sender MUST clear the destination's error counter
and transition the PF destination back to Active state. When the
sender resumes data transmission on the destination it MUST do
this following the prescriptions of Section 7.2 of <xref
target="RFC4960"/>.</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"/> (when this function is enabled for the
destination address).</t>
<t>When all destinations are in inactive state (association
dormant state) the sender MUST also choose one destination to
transmit data to. The sender SHOULD choose the destination in
inactive state with the lowest error count (fewest consecutive
timeouts) for data transmission and transmit data to this
destination. When there are multiple destinations with same error
count in inactive state, the sender SHOULD attempt to pick the
most divergent source - destination pair from the last source -
destination pair on which data were transmitted or retransmitted
following <xref target="RFC4960"/>. Rules for picking the most
divergent source-destination pair are an implementation decision
and are not specified within this document. Therefore, a sender
SHOULD allow for incrementing the destination error counters up to
some reasonable limit larger than PMR+1, thus changing the
prescriptions of <xref target="RFC4960"/>, section 8.3, in this
respect. The exact limit to apply is not specified in this
document but it is considered reasonable to require for such to be
an order of magnitude higher than the PMR value. A sender MAY
choose to deploy other strategies than the above. For example, a
sender could choose to prioritize the last active destination
during dormant state. The strategy to prioritize the last active
destination is optimal when some paths are permanently inactive,
but suboptimal when paths’ instability is transient. While
the increment of the error counters above PMR+1 is a prerequisite
for the error counter values to serve to guide the path selection
in dormant state, then it is noted that by virtue of the
introduction of the Potentially Failed state, one may deploy
higher values of PMR without compromising the efficiency of the
failover operation, and thus making the increase of path error
counters above PMR+1 less critical as the dormant state will be
less likely to happen. The downside of increasing the PMR value
relative to the AMR value, however, is that the per destination
address failure detection and notification of such to ULP thereby
is weakened. In all cases the sender MUST NOT change the state of
the chosen destination and it MUST NOT clear the destination's
error counter as a result of choosing the destination for data
transmission.</t>
<!-- MT: Should this be specified? -->
<t>ACKs for chunks that have been transmitted to multiple
destinations (i.e., a chunk which has been retransmitted to a
different destination than the destination to which the chunk was
first transmitted) SHOULD NOT clear the error count of an inactive
destination and SHOULD 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). The
same ambiguity concerns the related congestion window growth. The
bytes of a newly acknowledged chunk which has been transmitted to
multiple destinations SHOULD be considered for contribution to the
congestion window growth towards the destination where the chunk
was last sent. The contribution of the acked bytes to the window
growth is subject to the prescriptions described in Section 7.2 of
[RFC4960] is fulfilled. A SCTP sender MAY apply a different
approach for both the error count handling and the congestion
control growth handling based on unequivocally information on
which destination (including multiple destinations) the chunk
reached. This document makes no reference to what such
unequivocally information could consist of, neither how such
unequivocally information could be obtained. The implementation of
such an alternative approach is left to implementations.</t>
<!--- MT: Is this new? Should it be done?-->
<t>ACKs for chunks which has been transmitted to one destination
address only MUST clear the error counter of the destination
address and MUST transition a PF destination back to Active state.
This situation can happen when new data is sent to a destination
address in PF state. It can also happen in situations where the
destination address is in PF state due to the occurrence of a
spurious T3-rtx timer and ACKs start to arrive for data sent prior
to occurrence of the spurious T3-rtx and data has not yet been
retransmitted towards other destinations. This document does not
specify special handling for detection of or reaction to spurious
T3-rtx timeouts, e.g., for special operation vis-a-vis the
congestion control handling or data retransmission operation
towards a destination address which undergoes a transition from
active to PF to active state due to a spurious T3-rtx timeout. But
it is noted that this is an area which would benefit from
additional attention, experimentation and specification for Single
Homed SCTP as well as for Multi Homed SCTP protocol operation.</t>
<t>SCTP stack SHOULD provide the ULP with the means to expose the
PF state of its destinations as well as the means to notify the
state transitions from Active to PF, and vice-versa. When doing
this, such SCTP stack MUST provide the ULP with the means to
suppress exposure of PF state and association state transitions as
well.</t>
</list></t>
</section>
<section anchor="permanent_failover"
title="Optional Feature: Permanent Failover">
<t>In <xref target="RFC4960"/>, an SCTP sender migrates the traffic
back to the original primary destination once this destination becomes
active again. 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 back 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 operation allows SCTP sender to continue
data transmission on the new working path even if the old primary
destination becomes active again. This is achieved by having SCTP
perform a switch over of the primary path to the alternative working
path rather than having SCTP switch back data transfer to the
(previous) primary path.</t>
<t>The manner of switch over 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 switch over modes of
operation.</t>
<t>The Permanent Failover operation requires only sender side changes.
The details are:</t>
<t><list style="numbers">
<t>The sender maintains a new tunable parameter, called
Primary.Switchover.Max.Retrans (PSMR). The PSMR MUST be set
greater or equal to the PFMR value. Implementations MUST reject
any other values of PSMR.</t>
<t>When the path error counter on a set primary path exceeds PSMR,
the SCTP implementation MUST autonomously select and set a new
primary path.</t>
<t>The primary path selected by the SCTP implementation MUST 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. An implementation of Permanent
Failover MUST support the setting of PSMR = PFMR. An
implementation of Permanent Failover MAY support setting of PSMR
> PFMR.</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>This specifications RECOMMENDS a default configuration that uses
standard RFC4960 switchback, i.e., switch back to the old primary
destination once the destination becomes active again. However, to
support optimal operation in a wider range of network scenarios, an
implementation MAY implement Permanent Failover operation as detailed
above 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. Setting of
spt_pathcpthld < spt_pathpfthld should be rejected with errno
set to EINVAL. An implementation MAY support only setting of
spt_pathcpthld = spt_pathpfthld and spt_pathcpthld = 0xffff. In
this case it shall reject setting of other values with errno set
to EINVAL.</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>Security considerations for the use of SCTP and its APIs are
discussed in <xref target="RFC4960"/> and <xref target="RFC6458"/>.
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>
<section title="Proposed Change of Status (to be Deleted before Publication)">
<t> Initially this work looked to entail some changes of the Congestion
Control (CC) operation of SCTP and for this reason the work was proposed
as Experimental. These intended changes of the CC operation have since
been judged to be irrelevant and are no longer part of the
specification. As the specification entails no other potential harmful
features, consensus exists in the wg to bring the work forward as PS.
</t>
<t>Initially concerns have been expressed about the possibility for the
mechanism to introduce path bouncing with potential harmful network
impacts. These concerns are believed to be unfounded. This issue is
addressed in Appendix B. </t>
<t>It is noted that the feature specified by this document is
implemented by multiple SCTP SW implementations and furthermore that
various variants of the solution have been deployed in Telco signaling
environments for several years with good results. </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
Independent End-to-end Paths.</title>
<author fullname="" initials="J." surname="Iyengar">
<organization/>
</author>
<author fullname="" initials="P." surname="Amer">
<organization/>
</author>
<author fullname="" initials="R." surname="Stewart">
<organization/>
</author>
<date month="10" year="2006"/>
</front>
<seriesInfo name="IEEE/ACM Trans on Networking" value="14(5)"/>
</reference>
<reference anchor="NATARAJAN09" target="">
<front>
<title>Concurrent Multipath Transfer during Path Failure</title>
<author fullname="" initials="P." surname="Natarajan">
<organization/>
</author>
<author fullname="" initials="N." surname="Ekiz">
<organization/>
</author>
<author fullname="" initials="P." surname="Amer">
<organization/>
</author>
<author fullname="" initials="R." surname="Stewart">
<organization/>
</author>
<date month="5" year="2009"/>
</front>
<seriesInfo name="Computer Communications" value=""/>
</reference>
<reference anchor="JUNGMAIER02" target="">
<front>
<title>On the use of SCTP in failover scenarios</title>
<author fullname="" initials="A." surname="Jungmaier">
<organization/>
</author>
<author fullname="" initials="E." surname="Rathgeb">
<organization/>
</author>
<author fullname="" initials="M." surname="Tuexen">
<organization/>
</author>
<date month="7" year="2002"/>
</front>
<seriesInfo name="World Multiconference on Systemics, Cybernetics and Informatics"
value=""/>
</reference>
<reference anchor="GRINNEMO04" target="">
<front>
<title>Performance of SCTP-controlled failovers in M3UA-based
SIGTRAN networks</title>
<author fullname="" initials="K-J" surname="Grinnemo">
<organization/>
</author>
<author fullname="" initials="A." surname="Brunstrom">
<organization/>
</author>
<date month="4" year="2004"/>
</front>
<seriesInfo name="Advanced Simulation Technologies Conference"
value=""/>
</reference>
<reference anchor="FALLON08" target="">
<front>
<title>SCTP Switchover Performance Issues in WLAN
Environments</title>
<author fullname="" initials="S." surname="Fallon">
<organization/>
</author>
<author fullname="" initials="P." surname="Jacob">
<organization/>
</author>
<author fullname="" initials="Y." surname="Qiao">
<organization/>
</author>
<author fullname="" initials="L." surname="Murphy">
<organization/>
</author>
<author fullname="" initials="E." surname="Fallon">
<organization/>
</author>
<author fullname="" initials="A." surname="Hanley">
<organization/>
</author>
<date month="1" year="2008"/>
</front>
<seriesInfo name="IEEE CCNC" value="2008"/>
</reference>
<reference anchor="CARO04" target="">
<front>
<title>End-to-End Failover Thresholds for Transport Layer
Multihoming</title>
<author fullname="" initials="A." surname="Caro Jr.">
<organization/>
</author>
<author fullname="" initials="P." surname="Amer">
<organization/>
</author>
<author fullname="" initials="R." surname="Stewart">
<organization/>
</author>
<date month="11" year="2004"/>
</front>
<seriesInfo name="MILCOM 2004" value=""/>
</reference>
<reference anchor="CARO05" target="">
<front>
<title>End-to-End Fault Tolerance using Transport Layer
Multihoming</title>
<author fullname="" initials="A." surname="Caro Jr.">
<organization/>
</author>
<date month="1" year="2005"/>
</front>
<seriesInfo name="Ph.D Thesis, University of Delaware" value=""/>
</reference>
<reference anchor="CARO02" target="">
<front>
<title>A Two-level Threshold Recovery Mechanism for SCTP</title>
<author fullname="" initials="A." surname="Caro Jr.">
<organization/>
</author>
<author fullname="" initials="J." surname="Iyengar">
<organization/>
</author>
<author fullname="" initials="P." surname="Amer">
<organization/>
</author>
<author fullname="" initials="G." surname="Heinz">
<organization/>
</author>
<author fullname="" initials="R." surname="Stewart">
<organization/>
</author>
<date month="7" year="2002"/>
</front>
<seriesInfo name="Tech report, CIS Dept, University of Delaware"
value=""/>
</reference>
</references>
<section anchor="alternative_approach"
title="Discussions of Alternative Approaches">
<t>This section lists alternative approaches for the issues desribed in
this document. Although these approaches do not require to update
RFC4960, we do not recommend them from the reasons described below.</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. This smaller
value for Path.Max.Retrans can results in spurious failover, which
might be a problem.</t>
<t>Unlike SCTP-PF, the interval for heartbeat packets is governed by
'HB.interval' even during failover process. 'HB.interval' is usually
set 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. However, using smaller value for 'HB.interval' might help this
situation, but it will be the waste of bandwidth in most cases.</t>
<t>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>
</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="path_bouncing"
title="Discussions for Path Bouncing Effect">
<t>The methods described in the document 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>
</back>
</rfc>
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