Network Working Group                                         Y. Nishida
Internet-Draft                                        GE Global Research
Intended status: Standards Track                            P. Natarajan
Expires: September 10, 2015 January 18, 2016                                  Cisco Systems
                                                                 A. Caro
                                                        BBN Technologies
                                                                 P. Amer
                                                  University of Delaware
                                                              K. Nielsen
                                                                Ericsson
                                                           March 9,
                                                           July 17, 2015

               SCTP-PF: Quick Failover Algorithm in SCTP
                 draft-ietf-tsvwg-sctp-failover-10.txt
                 draft-ietf-tsvwg-sctp-failover-11.txt

Abstract

   One of the major advantages of SCTP is the support of 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. supports multi-homing.  However, if when the failover operation as
   specified in RFC4960 is followed, there can be a significant delay and
   performance degradation in the migration to the active destination addresses, thus severely
   reducing the effectiveness of the SCTP failover operation.

   This data transfer path failover.  To
   overcome this problem this document complements RFC4960 by specifies a quick failover
   algorithm (SCTP-PF) based on the introduction of a new path
   state, the Potentially Failed
   (PF) path state, and an associated new
   failover state in SCTP Path Management.

   The document also specifies a dormant state operation of SCTP.  This
   dormant state operation is required to apply during be followed by an SCTP-PF
   implementation, but it may equally well be applied by a network failure.  The algorithm
   defined is called standard
   RFC4960 SCTP Potentially Failed Algorithm, SCTP-PF for
   short.  In addition, implementation.

   Additionally, the document complements RFC4960 by introducing introduces an alternative switchover operation modes for the data transfer path
   management after the recovery of a failed primary path.  These modes
   can allow improvements in the performance of the operation switchback mode
   called Permanent Failover that will be beneficial in some
   network environments.  The implementation situations.
   This mode of the additional
   switchover operation modes is an optional part of SCTP-PF. applies to both a standard RFC4960 SCTP
   implementation as well as to a SCTP-PF implementation.

   The procedures defined in the document require only minimal
   modifications to the current RFC4960 specification.  The procedures are
   sender-side only and do not impact the SCTP receiver.

Status of 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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   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on September 10, 2015. January 18, 2016.

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  . . . . . . . . . . . .   4
   4.  SCTP with Potentially-Failed Destination State (SCTP-PF)  . .   5
     4.1.  SCTP-PF Concept   4
     3.1.  Overview  . . . . . . . . . . . . . . . . . . . . .   5
     4.2. . . .   4
     3.2.  Specification of the SCTP-PF Algorithm Procedures . . . . . . . . .   6
       4.2.1.   5
   4.  Dormant State Operation . . . . . . . . . . . . . . .  10
     4.3.  Permanent Failover  . . . . . . .   9
     4.1.  SCTP Dormant State Procedure  . . . . . . . . . . . .  12
       4.3.1.  Background . .  10
   5.  Permanent Failover  . . . . . . . . . . . . . . . . . . .  12
       4.3.2.  Permanent Failover Algorithm . .  11
   6.  Suggested SCTP Protocol Parameter Values  . . . . . . . . . .  12
   5.
   7.  Socket API Considerations . . . . . . . . . . . . . . . . . .  13
     5.1.  12
     7.1.  Support for the Potentially Failed Path State . . . . . .  14
     5.2.  13
     7.2.  Peer Address Thresholds (SCTP_PEER_ADDR_THLDS) Socket
           Option  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     5.3.  14
     7.3.  Exposing the Potentially Failed Path State
           (SCTP_EXPOSE_POTENTIALLY_FAILED_STATE) Socket Option  . .  16
   6.  15
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   7.  15
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   8.  16
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   11. Proposed Change of Status (to be Deleted before Publication)   17
   9.   16
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     9.1.
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     9.2.
     12.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Appendix A.  Discussions of Alternative Approaches  . . . . . . .  18
     A.1.  Reduce Path.Max.Retrans (PMR) . . . . . . . . . . . . . .  18
     A.2.  Adjust RTO related parameters . . . . . . . . . . . . . .  19
   Appendix B.  Discussions for Path Bouncing Effect . . . . . . . .  20  19
   Appendix C.  SCTP-PF for SCTP Single-homed Operation  . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   The Stream Control Transmission Protocol (SCTP) as specified in
   [RFC4960] supports multihoming multi homing at the transport layer -- an SCTP
   endpoint can bind to multiple IP addresses. layer.  SCTP's multihoming multi
   homing 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 timer-
   based
   timer-based retransmissions on a destination address before detecting
   a path failure.  The sender fails over to an alternate active
   destination address only after failure detection.  Until detecting the failover, path failure, the sender
   continues to transmit data on the failed
   path, path.  The prolonged time in
   which degrades the SCTP performance.  Concurrent Multipath
   Transfer (CMT) [IYENGAR06] is an proposed extension to [RFC4960] SCTP that
   allows the sender continues to transmit data on multiple paths simultaneously.
   Research [NATARAJAN09] shows that use a failed path severely degrades
   the current failure detection
   procedure worsens CMT performance during failover and can be
   significantly improved by employing a better failover algorithm.

   This of the protocol.  To address this problem, this
   document specifies an alternative failure detection and a quick failover
   procedure, the SCTP Potentially Failed algorithm, that improves algorithm (SCTP-PF) based on the
   performance
   introduction of SCTP multi-homed operation during a failover.

   For multi-homed new Potentially Failed path state in SCTP the operation after the recovery of a failed path equally well impacts the
   management.  The performance deficiencies of the protocol.  With [RFC4960] failover
   operation, and the
   procedures specified improvements obtainable from the introduction of a
   Potentially Failed state in [RFC4960], SCTP, were proposed and documented in
   [NATARAJAN09] for Concurrent Multipath Transfer SCTP will, after a [IYENGAR06].

   While SCTP-PF can accelerate failover from process and improve
   performance, the primary path, switch back risks that an SCTP endpoint enters in dormant state
   where all destination addresses are inactive can be increased.
   [RFC4960] leaves the protocol operation during dormant state to
   implementations and encourages to avoid entering the primary path for data transfer
   as soon state as this path becomes available again.  From a performance
   perspective, much as confirmed in research [CARO02], such a
   possible by careful tuning of the Path.Max.Retrans (PMR) and
   Association.Max.Retrans (AMR) parameters.  We specify a dormant state
   operation for SCTP-PF which makes SCTP-PF provide the same disruption
   tolerance as [RFC4960] despite that the dormant state may be entered
   more quickly.  The dormant state operation may equally well be
   applied by an [RFC4960] implementation and will here serve to provide
   added fault tolerance for situations where the tuning of the
   Path.Max.Retrans (PMR) and Association.Max.Retrans (AMR) parameters
   fail to provide adequate prevention of the entering of the dormant
   state.

   The operation after the recovery of a failed path equally well
   impacts the performance of the protocol.  With the procedures
   specified in [RFC4960] 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 again.  From a performance perspective
   such a forced switchback of the data transmission path is not optimal in general. can be
   suboptimal as the CWND towards the original primary destination
   address has to be rebuilt once data transfer resumes, [CARO02].  As
   an optional alternative to the switchback operation of [RFC4960],
   this document specifies an alternative Permanent Failover procedure
   which avoid such forced switchbacks of the data transfer path.  The
   Permanent Failover procedures operation was originally proposed by [CARO02].

   Additional discussion for alternative approaches that do not require
   modifications to [RFC4960], as well as discussion of path bouncing
   effects that might be caused by frequent switchover, are provided in
   the Appendices. [CARO02].

   While the Potentially Failed algorithm SCTP-PF primarily is motivated for
   improvement of by a desire to improve the SCTP
   multi-homed operation, the feature applies also to SCTP single-homed
   operation.  Here the algorithm serves to provide increased failure
   detection on idle associations, whereas the failover or switchback
   aspects of the algorithm will not be activated.  This is discussed in
   more detail in Appendix C.

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 SCTP as specified in [RFC4960]
   to be fixed by the approach described in this document.

   An SCTP endpoint can support multiple IP addresses.  Each SCTP
   endpoint exchanges the list of its usable addresses during the
   initial negotiation with its peer.  Then the endpoints select one
   address from the peer's list and use this as the primary destination
   address.  During normal transmission, an SCTP endpoint sends all user
   data to the primary destination address.  Also, it sends packets
   containing a HEARTBEAT chunk to all idle destination addresses at a
   certain interval to check the reachability of these destination
   addresses.  Idle destination addresses normally include all non-
   primary destination addresses.

   If a sender has multiple active destination addresses, it can
   retransmit data to an non-primary destination address, if 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 address to be active and clears the error counter for the
   destination address.  If it fails to receive acknowledgments, the
   error count for the destination address is increased.  If the error
   counter exceeds the tunable protocol parameter Path.Max.Retrans
   (PMR), the SCTP endpoint considers the destination address to be
   inactive.

   The failover process of SCTP is initiated when the primary path
   becomes inactive (the 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 starts using this as the destination address for sending data.
   If the primary path becomes active again, SCTP reverts to using the
   primary destination address for subsequent data transmissions and
   stop using the non-primary one.

   One issue with this failover process defined in [RFC4960] is that it
   usually takes a significant amount

   A brief description of time before SCTP switches to
   the new destination address.  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 non-primary path for initial data
   transmission.  This is because the recommended value for
   Path.Max.Retrans in the [RFC4960] is 5, which requires 6 consecutive
   timeouts before the failover takes place.  Before SCTP switches to
   the non-primary address, SCTP keeps trying to send packets to the
   primary address and only retransmitted packets are sent to the non-
   primary address and thus can be received by the receiver.  This slow
   failover process can cause significant performance degradation and is
   not acceptable in some situations.

   Another issue with RFC4960 failover and switchback operation is that
   once the primary path becomes active again, the traffic is
   unconditionally switched back to use this path.  This is not optimal
   in some situations.  This is further discussed in Section 4.3.

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

   To address the issues described in Section 3, this document extends
   SCTP path management scheme by adding the Potentially Failed state
   and associated protocol operation.  The algorithm is called SCTP
   Potentially Failed algorithm.  SCTP-PF the motivation for short.  The resulting SCTP
   path management operation is called SCTP Potentially Failed
   operation.

4.1.  SCTP-PF Concept

   The the introduction of the
   Potentially Failed state stems from including a discussion of alternative
   approaches to mitigate the
   following two observations about SCTP's failure detection procedure:

   o deficiencies of the [RFC4960] failover
   operation are given in the Appendices.  Discussion of path bouncing
   effects that might be caused by frequent switchover, are also
   provided there.

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

3.1.  Overview

   To minimize the performance impact during failover, the sender should
   avoid transmitting data to the a failed destination address as early as
   possible.  In the current [RFC4960] SCTP path management scheme, the sender
   stops transmitting data to a destination address only after the
   destination address is marked Failed (inactive).  Thus, inactive.  This process takes a smaller PMR value is better because
   significant amount of time as it requires the error counter of the sender can transition a
   destination address to exceed the Failed (inactive) state quicker.

   o  Smaller PMR values increase the chances Path.Max.Retrans (PMR) threshold.
   The issue cannot simply be mitigated by lowering of the PMR threshold
   because this may result in spurious failure detection where and unnecessary
   prevention of the sender incorrectly marks a destination address
      as Failed (inactive) during periods usage of temporary congestion.  As

      [RFC4960] recommends for a coupling preferred primary path as well as it,
   due to the coupled tuning of the PMR value Path.Max.Retrans (PMR) and the
      protocol parameter
   Association.Max.Retrans (AMR) value such
      spurious failure detection risks to carry over to spurious
      association failure detection and closure.  Larger PMR parameter values are
      preferable to avoid spurious failure detection.

   From the above observations it is clear that tuning the PMR value
   involves the following trade off -- 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 the
   performance impact during failure.

   SCTP-PF defined [RFC4960], may
   result in compromisation of the fault tolerance of SCTP.

   The solution provided in this document introduces is to extend the new Potentially
   Failed (PF) destination address state in SCTP's SCTP path
   management
   procedure.  The new scheme of [RFC4960] by the addition of the Potentially
   Failed (PF) destination address state
   applies to SCTP single-homed operation as well as to SCTP multi-homed
   operation.  The PF state was originally proposed to improve CMT
   performance [NATARAJAN09].  The PF state is an intermediate state in between the Active active and Failed states.  SCTP's failure detection
   procedure is modified to include
   inactive state of a destination address in [RFC4960] path management
   scheme, and let the failover of data transfer away from a destination
   address be driven by the entering of the PF state instead of by the
   entering of the inactive state.  Thereby SCTP may perform quick
   failover without compromising the overall fault tolerance of
   [RFC4960] SCTP.  At the same time, RTO-based HEARTBEAT probing is
   initiated towards a destination address once it enters PF state.
   Thereby SCTP may quickly ascertain whether network connectivity
   towards the destination address is broken or whether the failover was
   spurious.  In the case where the failover was spurious data transfer
   may quickly resume towards the original destination address.

   The new failure detection algorithm assumes that loss detected by a
   timeout implies either severe congestion or network connectivity
   failure en-route.  After a number of
   consecutive timeouts on a path, the sender is unsure, and marks the
   corresponding it assumes that by default a destination address is
   classified as in the PF state.  A PF
   destination address is not used for data transmission except when it
   is already at the only destination address available (e.g., for single-homed
   SCTP) or in other special cases (discussed below).  The new failure
   detection algorithm requires only sender-side changes.

4.2. occurrence of one first timeout.

3.2.  Specification of the SCTP-PF Algorithm Procedures

   The SCTP-PF operation is specified as follows:

   1.   The sender maintains a new tunable parameter SCTP Protocol Parameter
        called PotentiallyFailed.Max.Retrans (PFMR).  The RECOMMENDED value of
        PFMR is 0 when SCTP-PF is used.  The PFMR defines a
        the new intermediate PF threshold on the destination address
        error counter at exceed of which the destination address is
        classified as PF and related PF state actions are to be taken.  By standard
        RFC4960 semantics a destination address PF.  The RECOMMENDED value of PFMR is classified as
        Inactive once the error counter exceeds PMR. 0, but other
        values MAY be used.  Setting PFMR larger to or equal to PMR
        Path.Max.Retrans (PMR) does not result in definition of a PF
        threshold for the destination address.  I.e., PFMR set larger to
        or equal to PMR means that the destination
        address never will not be classified as PF. PF prior to reaching inactive
        state.

   2.   The error counter of an active destination address is
        incremented as specified in [RFC4960].  This means that the
        error counter of the destination address will be incremented
        each time the T3-rtx timer expires, or each time a HEARTBEAT
        chunk is sent when idle and not acknowledged within an RTO.
        When the value in the destination address error counter exceeds
        PFMR, the endpoint MUST mark the destination address as in the
        PF state.

   3.   The PFMR threshold defines the point the destination address no
        longer is considered a good candidate for data transmission and
        a SCTP-PF sender SHOULD NOT send data to destination addresses
        in PF state when alternative destination addresses in active
        state are available.  Specifically this means that:

        i  When there is outbound data to send and the destination
           address presently used for data transmission is in PF state,
           the sender SHOULD choose a destination address in active
           state, if one exists, and failover to deploy this destination
           address for data transmission.

        ii When retransmitting data that has timed out and the sender
           thus by [RFC4960], section 6.4.1, should attempt to pick a
           new destination address for data retransmission, the sender
           SHOULD choose an alternate destination transport address in
           active state if one exists.

        iii  When there is outbound data to send and the SCTP user
           explicitly requests to send data to a destination address in
           PF state, the sender SHOULD send the data to an alternate
           destination address in active state if one exists.

        When choosing among multiple destination address in active state
        the following considerations are given:

        A.  An SCTP sender should comply with [RFC4960], section 6.4.1,
            principles of choosing most divergent source-destination
            pairs compared with, for i.: the destination address in PF
            state that it performs a failover from, and for ii.: the
            destination address towards which the data timed out.  Rules
            for picking the most divergent source-destination pair are
            an implementation decision and are not specified within this
            document.

        B.  A SCTP-PF sender MAY choose to send data to a destination
            address in PF state, even if destination addresses in active
            state exist, have the SCTP-PF sender other means of
            information available that disqualifies the destination
            address in active state from being preferred.  However, the
            discussion of such mechanisms is outside of the scope of the
            SCTP_PF
            SCTP-PF operation specified in this document.

        In all cases, the sender MUST NOT change the state of chosen
        destination address, whether this state be active or PF, and it
        MUST NOT clear the error counter of the destination address as a
        result of choosing the destination address for data
        transmission.

   4.   When the destination addresses are all in PF state or some in PF
        state and some in inactive state, the sender MUST choose one
        destination address in PF state and transmit or retransmit data
        to this destination address using the following rules:

        A.  The sender SHOULD choose the destination in PF state with
            the lowest error count (fewest consecutive timeouts) for
            data transmission and transmit or retransmit data to this
            destination.

        B.  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
            [RFC4960], section 6.4.1, principles of choosing most
            divergent source-destination pairs when executing
            (potentially consecutive) retransmission.  Rules for picking
            the most divergent source-destination pair are an
            implementation decision and are not specified within this
            document.

        C.  A sender MAY choose to deploy other strategies than the
            above when choosing among multiple PF destinations have the
            SCTP-PF sender other means of information available that
            qualifies a particular destination address for being used.
            The SCTP-PF protocol operation specified in this document
            makes no assumption of the existence of such other means of
            information and specifies for the above as the default
            operation of an SCTP-PF sender.

        The sender MUST NOT change the state and the error counter of
        any destination address regardless of whether it has been chosen
        for transmission or not.

   5.   HEARTBEAT chunks   The HB.interval of the Path Heartbeat function of [RFC4960]
        MUST be send ignored for destination addresses in PF state.  Instead
        HEARTBEAT chunks are sent to destination addresses in PF state
        once per RTO.  HEARTBEAT chunks SHOULD be sent to destination
        addresses
        regardless in PF state, but the sending of HEARTBEATS MUST honor
        whether the Path Heartbeat function (Section 8.3 of [RFC4960])
        is enabled for the destination address or not.
        The HB.interval of  I.e., if the
        Path Heartbeat function of [RFC4960] MUST
        be ignored is disabled for the destination addresses address
        in question, HEARTBEATS MUST NOT be sent.  Note that when
        Heartbeat function is disabled, it may take longer to transition
        PF state, instead
        HEARTBEAT chunks destination to ACTIVE.

   6.   HEARTBEATs are sent to destination addresses in PF state
        once per RTO.  The HEARTBEAT sending begins upon that when a destination address reaches the PF
        state.  When a HEARTBEAT chunk is not acknowledged within the
        RTO, the sender increments the error counter and exponentially back
        backs off the RTO value.  If the error counter is less than PMR,
        the sender transmits another packet containing the HEARTBEAT
        chunk immediately after timeout expiration on the previous
        HEARTBEAT.  When data is being transmitted to a destination
        address in the PF state, the transmission of a HEARTBEAT chunk
        MAY be omitted in case receipt of a SACK of or a T3-rtx timer
        expiration on the outstanding data can provide equivalent information.  Likewise
        information, such as a case where the data chunk has transmitted
        to a single destination.  Likewise, the timeout of a HEARTBEAT
        chunk MAY be ignored if data is outstanding towards the
        destination address.

   6.

   7.   When the sender receives a HEARTBEAT ACK from a HEARTBEAT sent
        to a destination address in PF state, the sender MUST clear the
        error counter of the destination address and transition the
        destination address back to active state.  When the sender
        resumes data transmission on the destination address, it MUST do
        this following the prescriptions of Section 7.2 of [RFC4960].

   7.

   8.   Additional (PMR - PFMR) consecutive timeouts on a destination
        address in PF state confirm the path failure, upon which the
        destination address transitions to the inactive state.  As
        described in [RFC4960], the sender (i) SHOULD notify the ULP
        about this state transition, and (ii) transmit HEARTBEAT chunks
        to the inactive destination address at a lower HB.interval
        frequency as described in Section 8.3 of [RFC4960] (when this the
        Path Heartbeat function is enabled for the destination address).

   8.

   9.   Acknowledgments for chunks that have been transmitted to
        multiple destinations (i.e., a chunk which has been
        retransmitted to a different destination address than the
        destination address to which the chunk was first transmitted)
        MUST NOT clear the error count for an inactive destination
        address and MUST NOT transition a PF destination address in PF
        state 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 destination addresses SHOULD be
        considered for contribution to the congestion window growth
        towards the destination address 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 destination addresses)
        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 design of
        such an alternative approach is left to implementations.

   9.

   10.  Acknowledgments for chunks that has been transmitted to one
        destination address only MUST clear the error counter for the
        destination address and MUST transition a PF destination address in
        PF state back to Active state.  This situation can happen when
        new data is sent to a destination address in the PF state.  It
        can also happen in situations where the destination address is
        in the PF state due to the occurrence of a spurious T3-rtx timer
        and Acknowledgments 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.

   11.  When all destination addresses are in inactive state, and SCTP
        protocol operation thus is said to be in dormant state, the
        prescriptions given in Section 4 shall be followed.

   12.  The SCTP stack SHOULD should provide the ULP with the means to expose
        the PF state of its destinations as well as the means to notify
        the
        of state transitions from Active to PF, and vice-versa.  When
        doing this, such  However
        it is recommended that an SCTP stack MUST implementing SCTP-PF also
        allows for that the ULP is kept ignorant of the PF state of its
        destinations and the associated state transition.  For this
        reason is it recommended that an SCTP stack implementing SCTP-PF
        also should provide the ULP with the means to suppress exposure
        of PF state and the associated state
        transitions as well.

4.2.1. transitions.

4.  Dormant State Operation

   In a situation with complete disruption of the communication in
   between the SCTP Endpoints, the aggressive HEARTBEAT transmissions of
   SCTP-PF on destination addresses in PF state may make the association
   enter dormant state faster than a standard [RFC4960] SCTP
   implementation given the same setting of Path.Max.Retrans (PMR) and
   Association.Max.Retrans (AMR).  For example, an SCTP association with
   two destination addresses typically would reach dormant state in half
   the time of an [RFC4960] SCTP implementation in such situations.
   This is because a SCTP PF sender will send HEARTBEATS and data
   retransmissions in parallel with RTO intervals when there are
   multiple destinations addresses in PF state.  This argument pressumes presumes
   that RTO << HB.interval of [RFC4960].  One could use higher values of
   PMR, which makes the dormant state situations less likely to happen.
   The downside of increasing  With the PMR value is that destination address
   failure detections and notifications of such events to ULP is
   weakened.

   A design goal of SCTP-PF is that it should
   SCTP-PF shall provide the same level of disruption tolerance as an
   [RFC4960] SCTP implementation with the same Path.Max.Retrans (PMR)
   and Association.Max.Retrans (AMR)
   setting.  For this reason, setting, we prescribe for that an
   SCTP-PF implementation SHOULD perform the following
   operations operate as described below in
   Section 4.1 during dormant state, while this state.

   An SCTP-PF implementation MAY choose a different dormant state
   operation than the one described below in Section 4.1 provided that
   the solution chosen does not compromise the fault tolerance of the
   SCTP-PF operation.

   The below prescription for SCTP-PF dormant state handling SHOULD NOT
   be coupled to the value of the PFMR, but solely to the activation of
   SCTP-PF logic in an SCTP implementation.

   It is noted that the below dormant state operation is considered to
   provide added disruption tolerance also for an [RFC4960] SCTP
   implementation, and that it can be sensible for an [RFC4960] SCTP
   implementation
   decision in [RFC4960]. to follow this mode of operation.  For an [RFC4960]
   SCTP implementation the continuation of data transmission during
   dormant state makes the fault tolerance of SCTP be more robust
   towards situations where some, or all, alternative paths of an SCTP
   association approach, or reach, inactive state prior to that the
   primary path used for data transmission observes trouble.

4.1.  SCTP Dormant State Procedure

   a.  When the destination addresses are all in inactive state, state and data
       is available for transfer, the sender MUST choose one destination when
       and transmit data is transmitted. to this destination address.

   b.  The sender MUST NOT change the state of the chosen destination
       address (it remains in inactive state) and it MUST NOT clear the
       error counter of any the destination address regardless as a result of whether it has been chosen choosing
       the destination address for
       transmission or not.

   b. data transmission.

   c.  The sender SHOULD choose the destination in inactive state with
       the lowest error count (fewest consecutive timeouts) for data
       transmission.  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 where failure was observed.  Rules for picking
       the most divergent source-destination pair are an implementation
       decision and are not specified within this document.  To support
       differentiation of inactive destination addresses based on their
       error count SCTP will need to allow for increment of the
       destination address error counters up to some reasonable limit
       above PMR+1, thus changing the prescriptions of [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 that the
       strategy defined by here.  The strategy to prioritize the last
       active destination address,i.e., the destination address with the
       fewest error counts is optimal when some paths are permanently
       inactive, but suboptimal when a path instability is transient.

   An SCTP-PF implementation MAY keep the operation during dormant state
   an implementation decision, but it should be careful not to
   compromise the fault tolerance of the SCTP operation.

   The above prescriptions for SCTP-PF dormant state handling SHOULD NOT
   be coupled to the value of the PFMR, but solely to the activation of
   SCTP-PF logic in an SCTP implementation.  It is further noted that
   also a standard [RFC4960] SCTP implementation can use this mode of
   operation to improve the fault tolerance (which some implementations
   already do).

4.3.  Permanent Failover

   This section describes an OPTIONAL switchback feature called
   Permanent Failover which is beneficiary to deploy in certain
   situations.

4.3.1.  Background

   In [RFC4960], an SCTP sender migrates the traffic back to the
   original primary destination address once this address becomes active
   again.  As the CWND towards the original primary destination address
   has to be rebuilt once data transfer resumes, the switch back to use
   the original primary address is not always optimal.  Indeed [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, to prioritize the Permanent Failover operation was proposed in
   [CARO02].  When SCTP changes last
       active destination address, i.e., the destination address due to failover, with
       the fewest error counts is optimal when some paths are
       permanently inactive, but suboptimal when a path instability is
       transient.

5.  Permanent Failover

   The objective of the Permanent Failover operation allows is to allow the
   SCTP sender to continue data transmission on the a new working path even
   when the old primary destination address becomes active again.  This
   is achieved by having SCTP perform a switch over of the primary path
   to the alternative new working path rather than having SCTP switch back data transfer to if the
   (previous) primary path.

   The manner error counter 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
   exceeds a certain threshold.  This mode of operation to
   enforce traffic distribution over a number of network paths.  I.e.,
   load distribution of traffic from multiple SCTP associations may can be
   sought applied
   not only 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 SCTP-PF implementations, but also to support also, as alternative behavior, the Permanent
   Failover switch over modes of operation.

4.3.2.  Permanent Failover Algorithm [RFC4960]
   implementations.

   The Permanent Failover operation requires only sender side changes.
   The details are:

   1.  The sender maintains a new tunable parameter, called
       Primary.Switchover.Max.Retrans (PSMR).  The  For SCTP-PF
       implementations, the PSMR MUST be set greater or equal to the
       PFMR value.  For [RFC4960] implementations the PSMR MUST be set
       greater or equal to the PMR value.  Implementations MUST reject
       any other values of PSMR.

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

   3.  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 can be used as data transfer
       path as per normal path selection when the present data transfer
       path fails.

   4.  The present data transfer
       path fails.

   4.  For SCTP-PF, 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 SCTP-PF
       implementation of Permanent Failover MUST support the setting of
       PSMR = PFMR.  A SCTP-PF implementation of Permanent Failover MAY
       support setting of PSMR > PFMR.

   5.  For [RFC4960] SCTP, the recommended value of PSMR is PFMR PMR when
       Permanent Failover is used.  This means that no forced switchback
       to a previously failed primary path is performed.  An  A [RFC4960]
       SCTP implementation of Permanent Failover MUST support the
       setting of PSMR = PFMR. PMR An [RFC4960] SCTP implementation of
       Permanent Failover MAY support setting larger settings of PSMR > PFMR.

   5. PMR.

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

   To support optimal

   The manner of switch over operation that is most optimal in a wider range given
   scenario depends on the relative quality of a set primary path versus
   the quality of network scenarios, alternative paths available as well as it depends on
   the extent to which it proposed is desired for an SCTP-PF implementation to implement Permanent
   Failover 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, an optional feature.  The SCTP
   implementation of MAY support also, as alternative behavior, the
   Permanent Failover feature is optional for an SCTP-PF implementation. mode of operation and MAY enable it based on
   users' requests.

   For an SCTP implementation that implements Permanent Failover, this
   specification RECOMMENDS that the standard RFC4960 switchback
   operation is retained as the default operation.

5.

6.  Suggested SCTP Protocol Parameter Values

   This document does not alter the [RFC4960] value RECOMMENDATIONS for
   the SCTP Protocol Parameters defined in [RFC4960].

   The following protocol parameter is RECOMMENDED:

      PotentiallyFailed.Max.Retrans (PFMR) - 0

7.  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 SCTP-PF behavior. behavior as well as the Permanent Failover function.

   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 4. 3 and in Section 5.  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().

5.1.

7.1.  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.

5.2.

7.2.  Peer Address Thresholds (SCTP_PEER_ADDR_THLDS) Socket Option

   Applications can control the SCTP-PF behavior by getting or setting
   the number of consecutive timeouts before a peer address is
   considered potentially failed or unreachable unreachable.  The same socket option
   is used by applications to set and get the number of timeouts before
   the primary path is changed automatically. automatically by the Permanent Failover
   function.  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
      wild card 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,
      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.  Setting  For SCTP-PF, the setting
      of spt_pathcpthld < spt_pathpfthld should be rejected with errno
      set to EINVAL.  For [RFC4960] SCTP, the setting of spt_pathcpthld
      < spt_pathpfthld spt_pathmaxrxt should be rejected with errno set to EINVAL.  An  A
      SCTP-PF implementation MAY support only setting of spt_pathcpthld
      = spt_pathpfthld and spt_pathcpthld = 0xffff and a [RFC4960] SCTP
      implementation MAY support only setting of spt_pathcpthld =
      spt_pathmaxrxt and spt_pathcpthld = 0xffff.  In this case it these cases SCTP
      shall reject setting of other values with errno set to EINVAL.

5.3.

7.3.  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 5.1. 7.1.  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.

6.

8.  Security Considerations

   Security considerations for the use of SCTP and its APIs are
   discussed in [RFC4960] and [RFC6458].

   The logic described here is
   for sender-side only enabled introduced by configuration and this document does not have impact existing on-
   the-wire SCTP messages.  Also, this document does not introduce any
   impacts on protocol
   new on-the-wire SCTP messages that require new security
   considerations.

   SCTP-PF makes SCTP not only more robust during primary path failure/
   congestion but also more vulnerable to network connectivity/
   congestion attacks on the wire.  No new chunk type or new
   field parameter primary path.  SCTP-PF makes it easier for
   an attacker to trick SCTP to change data transfer path, since the
   duration of time that an attacker needs to compromise the network
   connectivity is much shorter than [RFC4960].  However, SCTP-PF does
   not required constitute a significant change in the duration of time and
   effort an attacker needs to keep SCTP away from the primary path.
   With the standard switchback operation [RFC4960] SCTP resumes data
   transfer on its primary path as soon as the next HEARTBEAT succeeds.

   On the other hand, usage of the Permanent Failover mechanism, does
   change the treat analysis.  This is because attackers can force a
   permanent change of the data transfer path by blocking the primary
   path until the switchover of the primary path is triggered by the
   Permanent Failover algorithm.  This especially will be the case when
   Permanent Failover is used together with SCTP-PF with the particular
   setting of PSMR = PFMR = 0, as Permanent Failover here happens
   already at the first RTO timeout experienced.  Users of the Permanent
   Failover mechanism should be aware of this fact.

   The event notification of path state transfer from active to
   potentially failed state and vice versa gives attackers an increased
   possibility to generate more local events.  However, it is assumed
   that event notifications are rate-limited in the implementation to
   address this document.

7. threat.

9.  IANA Considerations

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

8.

10.  Acknowledgements

   The authors wish to thank Michael Tuexen for his many invaluable
   comments and for his very substantial support with the making of this
   document.

11.  Proposed Change of Status (to be Deleted before Publication)

   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.

   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.

   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.

9.

12.  References

9.1.

12.1.  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.

12.2.  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 described 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, duration and
   in fact this is recommended in some research results [JUNGMAIER02]
   [GRINNEMO04] [FALLON08].  For example, if when Path.Max.Retrans=0,  However to significantly reduce the
   failover time it is required to go down (as with PFMR) to
   Path.Max.Retrans=0 and with this setting SCTP switches to another
   destination address already on a single timeout.
   This smaller value for Path.Max.Retrans can results timeout which may result in
   spurious
   failover, which might be failover.  Spurious failover is a problem.

   Unlike SCTP-PF, problem in [RFC4960] SCTP
   as the interval for heartbeat packets transmission of HEARTBEATS on the left primary path, unlike in
   SCTP-PF, is governed by 'HB.interval' even also during the failover
   process.  'HB.interval' is usually set in the order of seconds
   (recommended value is 30 seconds).  When seconds) and when the primary path becomes
   inactive, the next HEARTBEAT can may be transmitted only many seconds
   later.  Indeed as recommended, only 30 secs later.  Meanwhile, the
   primary path may since long have
   recovered. recovered, if it needed recovery at
   all (indeed the failover could be truely spurious).  In such
   situations, post failover, an endpoint is forced to wait on in the order
   of many seconds before the endpoint can resume transmission on the
   primary path and furthermore once it returns on the primary path the
   CWND needs to be rebuild anew - a process which the throughput
   already have had to suffer from on the alternate path.  However, using  Using a
   smaller value for 'HB.interval' might help this situation, but it will be the
   would result in a general waste of bandwidth as such more frequent
   HEARBEATING would take place also when there are no observed
   troubles.  The bandwidth overhead may be diminished by having the ULP
   use a smaller 'HB.interval' only on the path which at any given time
   is set to be the primary path, but this adds complication in most cases. the ULP.

   In addition, smaller Path.Max.Retrans values also affect the
   'Association.Max.Retrans' values. value.  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 addresses.  Else
   the SCTP sender considers its peer reachable even when all
   destinations are INACTIVE.  To INACTIVE and to avoid
   such inconsistent behavior an this dormant state operation,
   [RFC4960]  SCTP implementation SHOULD reduce Association.Max.Retrans
   accordingly whenever it reduces Path.Max.Retrans.  However, smaller
   Association.Max.Retrans value compromizes the fault tolerance of SCTP
   as it increases the 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 the
   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.

   The significant issue with this approach, however, is that even if
   the RTO.max is lowered to an optimal low value, then as long as the
   Path.Max.Retrans is kept at the [RFC4960] recommended value, the
   reduction of the RTO.max doesn't reduce the failover time
   sufficiently enough to prevent severe performance degradation during
   failover.

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 SCTP-PF results in a behavior already
   allowed by [RFC4960].

Appendix C.  SCTP-PF for SCTP Single-homed Operation

   For a single-homed SCTP association the only tangible effect of the
   activation of SCTP-PF operation is enhanced failure detection in
   terms of potential notification of the PF state of the sole
   destination address as well as, for idle associations, more rapid
   entering, and notification, of inactive state of the destination
   address and more rapid end-point failure detection.  It is believed
   that neither of these effects are harmful, provided adequate dormant
   state operation is implemented, and furthermore that they may be
   particularly useful for applications that deploys multiple SCTP
   associations for load balancing purposes.  The early notification of
   the PF state may be used for preventive measures as the entering of
   the PF state can be used as a warning of potential congestion.
   Depending on the PMR value, the aggressive HEARTBEAT transmission in
   PF state may speed up the end-point failure detection (exceed of AMR
   threshold on the sole path error counter) on idle associations in
   case where relatively large HB.interval value compared to RTO (e.g.
   30secs) is used.

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  164 80
   Sweden

   Email: karen.nielsen@tieto.com