Network Working Group                                         Y. Nishida
Internet-Draft                                        GE Global Research
Intended status: Experimental                               P. Natarajan
Expires: September 3, December 30, 2014                                 Cisco Systems
                                                                 A. Caro
                                                        BBN Technologies
                                                                 P. Amer
                                                  University of Delaware
                                                              K. Nielsen
                                                                Ericsson
                                                           March 2,
                                                           June 28, 2014

                    Quick Failover Algorithm in SCTP
                 draft-ietf-tsvwg-sctp-failover-03.txt
                 draft-ietf-tsvwg-sctp-failover-04.txt

Abstract

   One of the major advantages of SCTP is supporting that it supports multi-homed
   communication.  If a  A multi-homed SCTP end-point has a redundant network
   connections, the SCTP associations have a good chance ability to survive
   withstand network failures by migrating the traffic from an inactive networks
   network to an active ones. one.  However, if the SCTP standard [RFC4960] specified
   failover operation is followed, followed there can be a significant delay during in
   the migration.  During this period, SCTP
   might not be able to transmit much data migration to the peer.  This issue
   drastically impairs active destination addresses, thus severely
   reducing the usability effectiveness of SCTP in some situations.  This multi-homed operation.

   The memo describes complements RFC4960 by the issue introduction of the SCTP failover mechanism Potentially
   Failed state and associated new Quick Failover operation to apply
   during network failure and specifies
   an alternative failover procedure for SCTP that improves its senders to support this
   more performance optimal failover procedure as an add-on to the
   [RFC4960] failover operation.  The memo in addition updates [RFC4960]
   by clarifying a number of issues related to the path management
   operation during and periods of partial or complete network failures.
   Finally the memo complements [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.

   The procedures defined require only minimal modifications to the
   current specification.  The procedures are sender-side only and do
   not impact the SCTP receiver.

Status of this This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on September 3, December 30, 2014.

Copyright Notice

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   4
   3.  Issues with the SCTP Path Management  . . . . . . . . . . . . .  5   4
   4.  Existing Solutions for Smooth Failover  SCTP with Potentially-Failed Destination State (SCTP-PF)  . .   5
     4.1.  SCTP-PF Description . . . . . . . . . .  6
     4.1.  Reduce Path.Max.Retrans (PMR) . . . . . . . . .   5
     4.2.  Permanent Failover  . . . . .  6
     4.2.  Adjust RTO related parameters . . . . . . . . . . . . . .  6   9
   5.  SCTP with Potentially-Failed Destination State (SCTP-PF)  Socket API Considerations . . . .  8
     5.1.  SCTP-PF Description . . . . . . . . . . . . . .  10
     5.1.  Support for the Potentially Failed Path State . . . . . .  8  11
     5.2.  Effect of Path Bouncing  Peer Address Thresholds (SCTP_PEER_ADDR_THLDS) Socket
           Option  . . . . . . . . . . . . . . . . . 10
     5.3.  Permanent Failover . . . . . . . .  12
     5.3.  Exposing the Potentially Failed Path State
           (SCTP_EXPOSE_POTENTIALLY_FAILED_STATE) Socket Option  . .  13
   6.  Security Considerations . . . . . . . . . . 10
   6.  Socket API Considerations  . . . . . . . . . . . . . . . . . . 12
     6.1.  Support for the Potentially Failed Path State  . . . . . . 12
     6.2.  Peer Address Thresholds (SCTP_PEER_ADDR_THLDS) Socket
           Option . . . . . . . .  13
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . 13
     6.3.  Exposing the Potentially Failed Path State
           (SCTP_EXPOSE_POTENTIALLY_FAILED_STATE) Socket Option . . .  14
   7.  Security Considerations
   8.  References  . . . . . . . . . . . . . . . . . . . 15
   8.  IANA Considerations . . . . . .  14
     8.1.  Normative References  . . . . . . . . . . . . . . . 16
   9.  References . . .  14
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Appendix A.  Discussions of Alternative Approaches  . . . . . . 17
     9.1.  Normative References .  15
     A.1.  Reduce Path.Max.Retrans (PMR) . . . . . . . . . . . . . .  15
     A.2.  Adjust RTO related parameters . . . . 17
     9.2.  Informative References . . . . . . . . . .  16
   Appendix B.  Discussions for Path Bouncing Effect . . . . . . . . 17  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 19  17

1.  Introduction

   The Stream Control Transmission Protocol (SCTP) as specified in
   [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.

   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) [IYENGAR06] is
   an extension to SCTP and allows the sender to transmit data on
   multiple paths simultaneously.  Research [NATARAJAN09] shows that the
   current failure detection procedure worsens CMT performance during
   failover and can be significantly improved by employing a better
   failover algorithm.

   This document specifies an alternative failure detection procedure
   for SCTP (and CMT) that improves the SCTP (and CMT) performance during a failover.

   Also the operation after a failover impacts the performance of the
   protocol.  With [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 [CARO02], such a switchback of
   the data transmission path is not optimal in general.  As an optional
   alternative option to the switchback operation of [RFC4960], this document
   specifies the for SCTP to support the Permanent Failover switchover
   procedures proposed by [CARO02].

2.  Conventions and Terminology

   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 [RFC2119].

3.  Issues with the SCTP Path Management

   This section describes issues in the current SCTP to be fixed by the
   approach described in this document.

   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.

   If a sender has multiple active destination addresses, it can
   retransmit data to secondary destination address, when the
   transmission to the primary times out.

   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.

   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.

   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.

   Another issue is that once the primary path is active again, the
   traffic is switched back.  This is not optimal in general. some situations.
   This is further discussed in Section 4.2.

4.  Existing Solutions for Smooth Failover

   The following approaches are conceivable for  SCTP with Potentially-Failed Destination State (SCTP-PF)

   In order to address the solutions of this
   issue. issues described in Section 3, We propose to
   update SCTP path management scheme as follows.

4.1.  Reduce Path.Max.Retrans (PMR)

   Smaller values for Path.Max.Retrans shorten  SCTP-PF Description

   SCTP-PF stems from the failover duration. following two observations about SCTP's
   failure detection procedure:

   o  In fact, this is recommended in some research results [JUNGMAIER02]
   [GRINNEMO04] [FALLON08].  For example, if when Path.Max.Retrans=0, 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 switches path management scheme,
      the sender stops transmitting data to another a destination on only after the
      destination is marked Failed (inactive).  Thus, a single timeout.  However, smaller PMR
      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, is ideal so that the SCTP sender considers the peer
   endpoint unreachable and terminates transitions a destination to the association.  Therefore,
   Section 8.2 in [RFC4960] recommends that Association.Max.Retrans
   value should not be larger than
      Failed (inactive) state quicker.

   o  Smaller PMR values increase the summation chances of spurious failure
      detection where the Path.Max.Retrans sender incorrectly marks a destination as
      Failed (inactive) during periods of each temporary congestion.  As
      [RFC4960] recommends for a coupling of the destination addresses, else PMR value and the SCTP sender considers
   its peer reachable even when all destinations AMR
      value such spurious failure detection risks to carry over to
      spurious association failure detection and closure.  Larger PMR
      values are INACTIVE.  To preferable to avoid
   such inconsistent behavior an SCTP implementation SHOULD reduce
   Association.Max.Retrans accordingly whenever spurious failure detection.

   From the above observations it reduces
   Path.Max.Retrans.  However, smaller Association.Max.Retrans value
   increases chances of association termination during minor congestion
   events.

   Another issue is clear that tweaking the interval of heartbeat packet: 'HB.interval'
   could be in PMR value
   involves the order of seconds (recommended following tradeoff -- a lower value is 30 seconds).
   When improves performance
   but increases the primary chances of spurious failure detection, whereas a
   higher value degrades performance and reduces spurious failure
   detection in a wide range of path becomes inactive, the next HB can be
   transmitted only seconds later.  Meanwhile, conditions.  Thus, tweaking the primary path may have
   recovered.  In such situations, post failover, an endpoint
   association's PMR value is forced an incomplete solution to wait on the order of seconds before the endpoint can resume
   transmission on address
   performance impact during failure.

   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 [NATARAJAN09].  The PF
   state is an intermediate state between Active and Failed states.

   SCTP's failure detection procedure is modified to include the primary path. PF
   state.  The advantage new failure detection algorithm assumes that loss
   detected by a timeout implies either severe congestion or failure en-
   route.  After a number of tuning Path.Max.Retrans consecutive timeouts on a path, the sender
   is that it requires no
   modification to unsure, and marks the current standard.  However, corresponding destination as we discuss above
   tuning Path.Max.Retrans ignores several recommendations in [RFC4960].
   In addition, some research results indicate path bouncing caused by
   spurious failover does PF.  A PF
   destination is not cause serious problems.  We discuss the
   effect of path bouncing used for data transmission except in Section 5.2.

4.2.  Adjust RTO related parameters

   As several research results indicate, we can also shorten the
   duration of failover process by adjusting RTO related parameters
   [JUNGMAIER02] [FALLON08].  During failover process, RTO keeps being
   doubled.  However, if we can choose smaller special cases
   (discussed below).  The new failure detection algorithm requires only
   sender-side changes.  The details are:

   1.   The sender maintains a new tunable parameter called Potentially-
        Failed.Max.Retrans (PFMR).  The recommended value for RTO.max, we can
   stop the exponential growth of RTO at some point.  Also, choosing
   smaller values for RTO.initial PFMR = 0
        when quick failover is used.  When PFMR is larger or RTO.min can contribute to keep RTO
   value small.

   Similar equal to reducing Path.Max.Retrans, the advantage of this approach
        PMR, quick failover is that it requires no modification to the current specification,
   although it needs to ignore several recommendations described turned off.

   2.   The error counter of an active destination address is
        incremented as specified in [RFC4960].  This means that the
   Section 15
        error counter of [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.

5.  SCTP with Potentially-Failed Destination State (SCTP-PF)

5.1.  SCTP-PF Description

   SCTP-PF stems from destination address will be incremented
        each time the following two observations about SCTP's
   failure detection procedure:

   o  In order T3-rtx timer expires, or at times where a
        HEARTBEAT sent to minimize performance impact during failover, an idle, active address is not acknowledged
        within an RTO.  When the
      sender should avoid transmitting data to value in the failed destination as
      early as possible.  In address error
        counter exceeds PFMR, the current SCTP path management scheme, endpoint MUST mark the destination
        transport address as PF.

   3.   The sender stops transmitting SHOULD avoid data transmission to a destination only after PF destinations.
        When the
      destination is marked Failed.  Thus, a smaller PMR value is ideal
      so that destinations are all in PF state or some in PF state
        and some in inactive state, the sender transitions a MUST choose one
        destination in PF state and transmit data to this destination.
        The sender SHOULD choose the Failed destination in PF state
      quicker.

   o  Smaller PMR values increase the chances with least
        error count (fewest consecutive timeouts) for data transmission
        and transmit data to this destination.  In case of spurious failure
      detection where multiple PF
        destinations with same error count, the sender incorrectly marks a SHOULD let the
        choice among the multiple PF destination as
      Failed during periods with equal error count
        be based on the [RFC4960], section 6.4.1, principles of temporary congestion.  Larger PMR values 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 preferable an implementation
        decision and are not specified within this document.  A sender
        MAY choose to avoid spurious failure detection.

   From deploy other strategies than the above observations it is clear that tweaking the PMR value
   involves when
        choosing among multiple PF destinations with equal error count.
        In all cases the following tradeoff -- a lower value improves performance
   but increases sender MUST NOT change the chances state of spurious failure detection, whereas a
   higher value degrades performance chosen
        destination and reduces spurious failure
   detection in it MUST NOT clear the destination's error
        counter as a wide range result of path conditions.  Thus, tweaking choosing the
   association's PMR value is an incomplete solution to address
   performance impact during failure.

   This proposal introduces a new "Potentially-failed" (PF) destination
   state in SCTP's path management procedure.  The PF state was
   originally proposed for data
        transmission.

   4.   Heartbeats SHOULD be sent to improve CMT performance [NATARAJAN09].  The PF
   state is an intermediate state between Active and Failed states.
   SCTP's failure detection procedure is modified to include destination(s) once per RTO.
        This means the sender MUST ignore HB.interval for 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,
        destinations.  If an heartbeat is unanswered, the sender
   is unsure, SHOULD
        increment the error counter and marks exponentially back off the corresponding destination as PF.  A PF
   destination RTO
        value.  If error counter is not used for data transmission except in special cases
   (discussed below).  The new failure detection algorithm requires only
   sender-side changes.  Details are:

   1.  The less than PMR, the sender maintains a new tunable parameter called Potentially-
       failed.Max.Retrans (PFMR).  The recommended value of PFMR = 0
       when quick failover is used. SHOULD
        transmit another heartbeat immediately after T3-timer
        expiration.  When PFMR data is larger or equal transmitted to
       PMR, quick failover is turned off.

   2.  Each time a PF destination the
        transmittal of heartbeats may be omitted as SACK or T3-rtx timer expires on an active destination,
        expiration can provide equivalent information.  This
        specification recommends that heartbeats be send to PF
        destinations independently from whether the
       error counter Path Heartbeat
        function (Section 8.3 of that [RFC4960]) is enabled for the
        destination address will be incremented. or not.

   5.   When the value in sender receives an heartbeat ACK from a PF destination,
        the sender MUST clear the destination's error counter exceeds PFMR, the endpoint
       should mark and
        transition the PF destination transport address as PF.

   3.  The sender SHOULD avoid data transmission back to PF destinations. Active state.  When all destinations are in either PF or Inactive state, the
        sender MAY either move resumes data transmission on the destination from it MUST do
        this following the prescriptions of Section 7.2 of [RFC4960].

   6.   Additional (PMR - PFMR) consecutive timeouts on a PF to Active state
       (and transmit data to destination
        confirm the active destination) or path failure, upon which the sender MAY
       transmit data destination transitions
        to a PF destination.  In the former scenario, (i) Inactive state.  As described in [RFC4960], the sender MUST NOT
        (i) SHOULD notify the ULP about the this state transition, and (ii) MUST NOT clear
        transmit heartbeats to the destination's error counter.  It Inactive destination at a lower
        frequency as described in Section 8.3 of [RFC4960] (when this
        function is
       recommended that enabled for the destination address).

   7.   When all destinations are in inactive state (association dormant
        state) the sender MUST also choose one destination to transmit
        data to.  The sender picks SHOULD choose the PF destination in inactive
        state with least error count (fewest consecutive timeouts) for
        data transmission. transmission and transmit data to this destination.  In
        case of a tie (multiple PF multiple destinations with same error
       count), count in inactive
        state, the sender MAY choose SHOULD attempt to pick the most divergent
        source - destination pair from the last active destination.

   4.  Only heartbeats MUST be sent to PF destination(s) once per RTO.
       This means source - destination
        pair on which data were transmitted or retransmitted following
        [RFC4960].  Rules for picking the most divergent source-
        destination pair are an implementation decision and are not
        specified within this document.  In order to support this
        prescription a sender SHOULD ignore HB.interval allow for PF
       destinations.  If an heartbeat is unanswered, the sender
       increments increment of the
        destination error counter and exponentially backs off counters up to some reasonable limit above
        PMR+1, thus changing the RTO
       value.  If error counter prescriptions of [RFC4960], section
        8.3, in this respect.  The exact limit to apply is less not specified
        in this document but it is considered reasonable to require for
        such to be an order of magnitude higher than PMR, the PMR value.  A
        sender SHOULD
       transmit another heartbeat immediately after T3-timer expiration.

   5.  When MAY choose to deploy other strategies than the sender receives an heartbeat ACK from above.
        For example, a PF destination,
       the sender clears the destination's error counter and transitions could choose to prioritize the PF last active
        destination back to Active during dormant state.  The sender should
       perform slow-start as specified in Section 7.2.1 of [RFC4960]
       when it sends data on this destination.

   6.  Additional (PMR - PFMR) consecutive timeouts on a PF strategy to prioritize
        the last active destination
       confirm is optimal when some paths are
        permanently inactive, but suboptimal when paths' instability is
        transient.  While the path failure, upon which increment of the destination transitions error counters above
        PMR+1 is a prerequisite for the error counter values to serve to
        guide the Inactive state.  As described path selection in [RFC4960], dormant state, then it is noted that
        by virtue of the sender (i)
       SHOULD notify ULP about this state transition, introduction of the Potentially Failed state,
        one may deploy higher values of PMR without compromising the
        efficiency of the failover operation, and (ii) transmit
       heartbeats 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 Inactive PMR value relative to the AMR value, however, is
        that the per destination at a lower frequency as
       described in Section 8.3 address failure detection and
        notification of [RFC4960].

   7.  When such to ULP thereby is weakened.  In all destinations are in the Inactive state, cases
        the sender picks
       one MUST NOT change the state of the Inactive destinations for data transmission.  This
       proposal recommends that chosen destination
        and it MUST NOT clear the sender picks destination's error counter as a
        result of choosing 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.

   8.   ACKs for retransmissions do not chunks which 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).

   9.  SCTP shall provide the means to expose  The same ambiguity concerns the PF state related
        congestion window growth.  In this respect then it is specified
        that bytes of its a newly acknowledged chunk which has been
        transmitted to multiple destinations as well as SCTP SHOULD notify SHOULD, when the ULP of conditions
        for such contribution is fulfilled following the state
       transitions from Active to PF and from PF prescriptions
        of Section 7.2 of [RFC4960], contribute to Active state.  SCTP
       can provide the means congestion window
        growth towards the destination to suppress exposure of PF state and which the chunk was last
        transmitted.  A SCTP sender MAY apply a different approach for
        both the error count handling as well as the congestion control
        growth handling does it have unequivocally information as to
        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.

   9.   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.

   10.  SCTP SHOULD provide the means to expose the PF state of its
        destinations as well as the means to notify the ULP of the state
        transitions from Active to PF and from PF to Active state . When
        doing such SCTP MUST 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  If
        exposure of PF state is suppressed, the ULP will rely solely on
        the [RFC4960] state machine even if quick
       failover Quick Failover function is
        activated in SCTP.

5.2.  Effect of Path Bouncing

   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 [CARO04] [CARO05].

   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.

   Second, even if all communication paths between the end-nodes share
   the same bottleneck, the quick failover results in a behavior already
   allowed by [RFC4960].

5.3.

4.2.  Permanent Failover

   Post failover then, by [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 [CARO02] shows that the switch over 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
   [CARO02].  When SCTP changes the destination due to failover,
   Permanent Failover marks it as new primary.  This means Permanent
   Failover operation allows SCTP sender to continue data
   transmission to on the new working path even after if the old primary
   destination becomes active again.  This is achieved by having SCTP
   perform a switchover switch over of the primary path to
   an the alternative working
   path rather than having SCTP switch back data transfer to the
   (previous) primary path.

   The manner of switchover 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
   [RFC4960] switchback operation.  However as [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 where forced switch back to a previously
   failed primary path is not always performed. operation.

   The Permanent Failover operation requires only sender side changes.  Details, as originally
   outlined in [CARO02],
   The details are:

   1.  The sender maintains a new tunable parameter, called
       Primary.Switchover.Max.Retrans (PSMR).  The PSMR SHOULD be set
       greater or equal to the PFMR value.  Any setting of PSMR < PFMR
       MUST be rejected by the implementation.

   2.  When the path error counter on a set primary path exceeds PSMR,
       the SCTP implementation autonomously selects and sets a new
       primary path.

   2.

   3.  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.

   3.

   4.  The recommended value of PSMR is PFMR when Permanent failover Failover is
       used.  This means that no forced switchback to a previously
       failed primary path is performed.

   4.  It must be possible to disable the  An implementation of Permanent
       Failover and obtain
       the standard switchback operation MUST support set of [RFC4960].

   We PSMR = PFMR.  An implementation of
       Permanent Failover MAY support setting of PSMR > PFMR.

   5.  It MUST be possible to disable the Permanent Failover and obtain
       the standard switchback operation of [RFC4960].

   We recommend that SCTP-PF should stick sticks to the standard RFC4960 switchback
   behavior as default, i.e., switch back to the old primary destination
   once the destination becomes active again.  However, implementors  However in order 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.

6.

5.  Socket API Considerations

   This section describes how the socket API defined in [RFC6458] is
   extended to provide a way for the application to control and observe
   the quick failover behavior.

   Please note that this section is informational only.

   A socket API implementation based on [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.

   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 Section 5. 4.  The second one controls the
   exposition of the potentially failed path state.

   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().

6.1.

5.1.  Support for the Potentially Failed Path State

   As defined in [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 [RFC6458], this event is also provided, if a
   peer address enters or leaves the potentially failed state.  The
   notification as defined in [RFC6458] uses the following structure:

   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;
   }

   [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.

   The SCTP_GET_PEER_ADDR_INFO socket option defined in [RFC6458] can be
   used to query the state of a peer address.  It uses the following
   structure:

   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;
   };

   [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.

6.2.

5.2.  Peer Address Thresholds (SCTP_PEER_ADDR_THLDS) Socket Option

   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.

   The following structure is used to access and modify the thresholds:

   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;
   };

   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.

   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.

   spt_pathmaxrxt:  Each peer address of interest is considered
      unreachable, if its path error counter exceeds spt_pathmaxrxt.

   spt_pathpfthld:  Each peer address of interest is considered
      potentially failed, if its path error counter exceeds
      spt_pathpfthld.

   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.

6.3.  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.

5.3.  Exposing the Potentially Failed Path State
      (SCTP_EXPOSE_POTENTIALLY_FAILED_STATE) Socket Option

   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 Section 6.1. 5.1.  The default
   value is implementation specific.

   This socket option uses the level IPPROTO_SCTP and the name
   SCTP_EXPOSE_POTENTIALLY_FAILED_STATE.

   The following structure is used to control the exposition of the
   potentially failed path state:

   struct sctp_assoc_value {
     sctp_assoc_t assoc_id;
     uint32_t assoc_value;
   };

   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.

   assoc_value:  The potentially failed path state is exposed if and
      only if this parameter is non-zero.

7.

6.  Security Considerations

   There are no new security considerations introduced in this document.

8.

7.  IANA Considerations

   This document does not create any new registries or modify the rules
   for any existing registries managed by IANA.

9.

8.  References

9.1.

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol", RFC
              4960, September 2007.

9.2.

8.2.  Informative References

   [CARO02]   Caro Jr., A., Iyengar, J., Amer, P., Heinz, G., and R.
              Stewart, "A Two-level Threshold Recovery Mechanism for
              SCTP", Tech report, CIS Dept, University of Delaware , 7
              2002.

   [CARO04]   Caro Jr., A., Amer, P., and R. Stewart, "End-to-End
              Failover Thresholds for Transport Layer Multihoming",
              MILCOM 2004 , 11 2004.

   [CARO05]   Caro Jr., A., "End-to-End Fault Tolerance using Transport
              Layer Multihoming", Ph.D Thesis, University of Delaware ,
              1 2005.

   [FALLON08]
              Fallon, S., Jacob, P., Qiao, Y., Murphy, L., Fallon, E.,
              and A. Hanley, "SCTP Switchover Performance Issues in WLAN
              Environments", IEEE CCNC 2008, 1 2008.

   [GRINNEMO04]
              Grinnemo, K-J. and A. Brunstrom, "Performance of SCTP-
              controlled failovers in M3UA-based SIGTRAN networks",
              Advanced Simulation Technologies Conference , 4 2004.

   [IYENGAR06]
              Iyengar, J., Amer, P., and R. Stewart, "Concurrent
              Multipath Transfer using SCTP Multihoming over Independent
              End-to-end Paths.", IEEE/ACM Trans on Networking 14(5), 10
              2006.

   [JUNGMAIER02]
              Jungmaier, A., Rathgeb, E., and M. Tuexen, "On the use of
              SCTP in failover scenarios", World Multiconference on
              Systemics, Cybernetics and Informatics , 7 2002.

   [NATARAJAN09]
              Natarajan, P., Ekiz, N., Amer, P., and R. Stewart,
              "Concurrent Multipath Transfer during Path Failure",
              Computer Communications , 5 2009.

   [RFC6458]  Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
              Yasevich, "Sockets API Extensions for the Stream Control
              Transmission Protocol (SCTP)", RFC 6458, December 2011.

Appendix A.  Discussions of Alternative Approaches

   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.

A.1.  Reduce Path.Max.Retrans (PMR)

   Smaller values for Path.Max.Retrans shorten the failover duration.
   In fact, this is recommended in some research results [JUNGMAIER02]
   [GRINNEMO04] [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.

   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.

   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 [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.

A.2.  Adjust RTO related parameters

   As several research results indicate, we can also shorten the
   duration of failover process by adjusting RTO related parameters
   [JUNGMAIER02] [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.

   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 [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.

Appendix B.  Discussions for Path Bouncing Effect

   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 [CARO04] [CARO05].

   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.

   Second, even if all communication paths between the end-nodes share
   the same bottleneck, the quick failover results in a behavior already
   allowed by [RFC4960].

Authors' Addresses

   Yoshifumi Nishida
   GE Global Research
   2623 Camino Ramon
   San Ramon, CA  94583
   USA

   Email: nishida@wide.ad.jp

   Preethi Natarajan
   Cisco Systems
   510 McCarthy Blvd
   Milpitas, CA  95035
   USA

   Email: prenatar@cisco.com

   Armando Caro
   BBN Technologies
   10 Moulton St.
   Cambridge, MA  02138
   USA

   Email: acaro@bbn.com

   Paul D. Amer
   University of Delaware
   Computer Science Department - 434 Smith Hall
   Newark, DE  19716-2586
   USA

   Email: amer@udel.edu

   Karen E. E. Nielsen
   Ericsson
   Kistavaegen 25
   Stockholm,
   Stockholm  164 80
   Sweden

   Email: karen.nielsen@tieto.com