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Versions: (draft-nishida-tsvwg-sctp-failover) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 RFC 7829

Network Working Group                                         Y. Nishida
Internet-Draft                                        GE Global Research
Intended status: Standards Track                            P. Natarajan
Expires: March 3, 2016                                     Cisco Systems
                                                                 A. Caro
                                                        BBN Technologies
                                                                 P. Amer
                                                  University of Delaware
                                                              K. Nielsen
                                                                Ericsson
                                                         August 31, 2015


               SCTP-PF: Quick Failover Algorithm in SCTP
                 draft-ietf-tsvwg-sctp-failover-12.txt

Abstract

   SCTP supports multi-homing.  However, when the failover operation
   specified in RFC4960 is followed, there can be significant delay and
   performance degradation in the data transfer path failover.  To
   overcome this problem this document specifies a quick failover
   algorithm (SCTP-PF) based on the introduction of a Potentially Failed
   (PF) state in SCTP Path Management.

   The document also specifies a dormant state operation of SCTP.  This
   dormant state operation is required to be followed by an SCTP-PF
   implementation, but it may equally well be applied by a standard
   RFC4960 SCTP implementation.

   Additionally, the document introduces an alternative switchback
   operation mode called Primary Path Switchover that will be beneficial
   in certain situations.  This mode of operation 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 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
   Task Force (IETF).  Note that other groups may also distribute




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   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 3, 2016.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   4
   3.  SCTP with Potentially Failed Destination State (SCTP-PF)  . .   4
     3.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Specification of the SCTP-PF Procedures . . . . . . . . .   5
   4.  Dormant State Operation . . . . . . . . . . . . . . . . . . .   9
     4.1.  SCTP Dormant State Procedure  . . . . . . . . . . . . . .  10
   5.  Primary Path Switchover . . . . . . . . . . . . . . . . . . .  11
   6.  Suggested SCTP Protocol Parameter Values  . . . . . . . . . .  12
   7.  Socket API Considerations . . . . . . . . . . . . . . . . . .  12
     7.1.  Support for the Potentially Failed Path State . . . . . .  13
     7.2.  Peer Address Thresholds (SCTP_PEER_ADDR_THLDS) Socket
           Option  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     7.3.  Exposing the Potentially Failed Path State
           (SCTP_EXPOSE_POTENTIALLY_FAILED_STATE) Socket Option  . .  15
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   11. Proposed Change of Status (to be Deleted before Publication)   17
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  17



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     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
   Appendix C.  SCTP-PF for SCTP Single-homed Operation  . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   The Stream Control Transmission Protocol (SCTP) specified in
   [RFC4960] supports multi-homing at the transport layer.  SCTP's
   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 retransmissions on a destination
   address before detecting a path failure.  Until detecting the path
   failure, the sender continues to transmit data on the failed path.
   The prolonged time in which [RFC4960] SCTP continues to use a failed
   path severely degrades the performance of the protocol.  To address
   this problem, this document specifies a quick failover algorithm
   (SCTP-PF) based on the introduction of a new Potentially Failed (PF)
   path state in SCTP path management.  The performance deficiencies of
   the [RFC4960] failover operation, and the improvements obtainable
   from the introduction of a Potentially Failed state in SCTP, were
   proposed and documented in [NATARAJAN09] for Concurrent Multipath
   Transfer SCTP [IYENGAR06].

   While SCTP-PF can accelerate failover process and improve
   performance, the 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 state as much as
   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



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   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 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 Primary Path Switchover
   procedure which avoid such forced switchbacks of the data transfer
   path.  The Primary Path Switchover operation was originally proposed
   in [CARO02].

   While SCTP-PF primarily is motivated by a desire to improve the
   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.

   A brief description of the motivation for the introduction of the
   Potentially Failed state including a discussion of alternative
   approaches to mitigate the 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 a failed destination address as early as
   possible.  In the [RFC4960] SCTP path management scheme, the sender
   stops transmitting data to a destination address only after the
   destination address is marked inactive.  This process takes a
   significant amount of time as it requires the error counter of the
   destination address to exceed the 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 and unnecessary
   prevention of the usage of a preferred primary path as well as it,
   due to the coupled tuning of the Path.Max.Retrans (PMR) and the
   Association.Max.Retrans (AMR) parameter values in [RFC4960], may
   result in compromisation of the fault tolerance of SCTP.



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   The solution provided in this document is to extend the SCTP path
   management scheme of [RFC4960] by the addition of the Potentially
   Failed (PF) state as an intermediate state in between the active and
   inactive state of a destination address in the [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 and it assumes that by default a destination address is
   classified as PF already at the occurrence of one first timeout.

3.2.  Specification of the SCTP-PF Procedures

   The SCTP-PF operation is specified as follows:

   1.   The sender maintains a new tunable SCTP Protocol Parameter
        called PotentiallyFailed.Max.Retrans (PFMR).  The PFMR defines
        the new intermediate PF threshold on the destination address
        error counter at exceed of which the destination address is
        classified as PF.  The RECOMMENDED value of PFMR is 0, but other
        values MAY be used.  Setting PFMR larger to or equal to
        Path.Max.Retrans (PMR) does not result in definition of a PF
        threshold for the destination address.  I.e., the destination
        address will not be classified as 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




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        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 addresses 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 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



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        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 destination addresses in PF state
            with same error count, the sender should let the choice
            among the multiple destination addresses in PF state 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 destinations in PF state
            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.   The HB.interval of the Path Heartbeat function of [RFC4960] MUST
        be 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 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.  I.e., if the
        Path Heartbeat function is disabled for the destination address
        in question, HEARTBEATS MUST NOT be sent.  Note that when
        Heartbeat function is disabled, it may take longer to transition
        a destination address in PF state back to active state.

   6.   HEARTBEATs are sent 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
        backs off the RTO value.  If the error counter is less than PMR,
        the sender transmits another packet containing the HEARTBEAT



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        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, 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.

   7.   When the sender receives a HEARTBEAT ACK from a HEARTBEAT sent
        to a destination address in PF state, the sender SHOULD clear
        the error counter of the destination address and transition the
        destination address back to active state.  When the sender
        resumes data transmission on a destination address after a
        transition of the destination address from PF to active state,
        it MUST do this following the prescriptions of Section 7.2 of
        [RFC4960].  In a situation where a HEARTBEAT ACK arrives while
        there is data outstanding towards the destination address to
        which the HEARTBEAT was sent, then an implementation MAY choose
        to not have the HEARTBEAT ACK reset the error counter, but have
        the error counter reset await the fate of the outstanding data
        transmission.  This situation can happen when data is sent to a
        destination address in PF state.

   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 the
        Path Heartbeat function is enabled for the destination address).

   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)
        SHOULD NOT clear the error count for an inactive destination
        address and SHOULD NOT transition a 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).  A SCTP sender MAY apply a different approach
        for the error count 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




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        such unequivocally information could be obtained.  The design of
        such an alternative approach is left to implementations.

   10.  Acknowledgments for data chunks that has been transmitted to one
        destination address only MUST clear the error counter for the
        destination address and MUST transition a 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-avis
        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.

   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 provide the ULP with the means to expose
        the PF state of its destinations as well as the means to notify
        of state transitions from active to PF, and vice-versa.  However
        it is recommended that an SCTP stack 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 it is 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.

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



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   retransmissions in parallel with RTO intervals when there are
   multiple destinations addresses in PF state.  This argument presumes
   that RTO << HB.interval of [RFC4960].  With the design goal that
   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, we prescribe for that an
   SCTP-PF implementation SHOULD operate as described below in
   Section 4.1 during dormant 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 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 and data
       is available for transfer, the sender MUST choose one destination
       and transmit data 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 the destination address as a result of choosing
       the destination address for 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



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       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.

5.  Primary Path Switchover

   The objective of the Primary Path Switchover operation is to allow
   the SCTP sender to continue data transmission on 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 new working path if the error counter of the primary path
   exceeds a certain threshold.  This mode of operation can be applied
   not only to SCTP-PF implementations, but also to [RFC4960]
   implementations.

   The Primary Path Switchover operation requires only sender side
   changes.  The details are:

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

   2.  When the path error counter on a set primary path exceeds PMR,
       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.  For SCTP-PF, the recommended value of PMR is PFMR when Primary
       Path Switchover operation mode is used.  This means that no
       forced switchback to a previously failed primary path is
       performed.  An SCTP-PF implementation of Primary Path Switchover



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       MUST support the setting of PMR = PFMR.  A SCTP-PF implementation
       of Primary Path Switchover MAY support setting of PMR > PFMR.

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

   6.  It MUST be possible to disable the Primary Path Switchover
       operation and obtain the standard switchback operation of
       [RFC4960].

   The manner of switch over operation that is most optimal in a given
   scenario depends on the relative quality of a set primary path versus
   the quality of alternative paths available as well as it depends on
   the extent to which it is desired for the mode of operation to
   enforce traffic distribution over a number of network paths.  I.e.,
   load distribution of traffic from multiple SCTP associations may be
   sought to be enforced by distribution of the set primary paths with
   [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 SCTP
   implementation MAY support also, as alternative behavior, the Primary
   Path Switchover mode of operation and MAY enable it based on users'
   requests.

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

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 as well as the Primary Path Switchover function.




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   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
   PMR parameters described in Section 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().

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:

   strict sctp_paddr_change {
     uint16_t spc_type;
     uint16_t spc_flags;
     uint32_t spc_length;
     strict 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:



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   strict sctp_paddrinfo {
     sctp_assoc_t spinfo_assoc_id;
     strict 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.

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.  The same socket option
   is used by applications to set and get the number of timeouts before
   the primary path is changed automatically by the Primary Path
   Switchover 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:

   strict sctp_paddrthlds {
     sctp_assoc_t spt_assoc_id;
     strict 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-romany 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.




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   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 0off 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 Erna set to RIVAL if a value different from
      0off is used in spt_pathcpthld.  For SCTP-PF, the setting of
      spt_pathcpthld < spt_pathpfthld should be rejected with Erna set
      to RIVAL.  For [RFC4960] SCTP, the setting of spt_pathcpthld <
      spt_pathmaxrxt should be rejected with Erna set to RIVAL.  A SCTP-
      PF implementation MAY support only setting of spt_pathcpthld =
      spt_pathpfthld and spt_pathcpthld = 0off and a [RFC4960] SCTP
      implementation MAY support only setting of spt_pathcpthld =
      spt_pathmaxrxt and spt_pathcpthld = 0off.  In these cases SCTP
      shall reject setting of other values with Erna set to RIVAL.

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 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:

   strict 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-romany 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.





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8.  Security Considerations

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

   The logic introduced by this document does not impact existing On-
   anthe-wire SCTP messages.  Also, this document does not introduce any
   new On-anthe-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 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 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 Primary Path Switchover 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
   Primary Path Switchover algorithm.  This especially will be the case
   when the Primary Path Switchover is used together with SCTP-PF with
   the particular setting of PMR = PFMR = 0, as Primary Path Switchover
   here happens already at the first RTO timeout experienced.  Users of
   the Primary Path Switchover 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 threat.

9.  IANA Considerations

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

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.




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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 G 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 Tel co
   signaling environments for several years with good results.

12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
              RFC 4960, DOI 10.17487/RFC4960, September 2007,
              <http://www.rfc-editor.org/info/rfc4960>.

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 Multi homing",
              MILCOM 2004 , 11 2004.

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




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

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

   [IYENGAR06]
              Iyengar, J., Amer, P., and R. Stewart, "Concurrent
              Multipath Transfer using SCTP Multi homing 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,
              DOI 10.17487/RFC6458, December 2011,
              <http://www.rfc-editor.org/info/rfc6458>.

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 and
   in fact this is recommended in some research results [JUNGMAIER02]
   [GRINNEMO04] [FALLON08].  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 which may result in
   spurious failover.  Spurious failover is a problem in [RFC4960] SCTP
   as the transmission of HEARTBEATS on the left primary path, unlike in



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   SCTP-PF, is governed by 'HB.interval' also during the failover
   process.  'HB.interval' is usually set in the order of seconds
   (recommended value is 30 seconds) and when the primary path becomes
   inactive, the next HEARTBEAT may be transmitted only many seconds
   later.  Indeed as recommended, only 30 secs later.  Meanwhile, the
   primary path may since long have recovered, if it needed recovery at
   all (indeed the failover could be truly spurious).  In such
   situations, post failover, an endpoint is forced to wait 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.  Using a
   smaller value for 'HB.interval' might help this situation, but it
   would result in a general waste of bandwidth as such more frequent
   HEARTBEATING 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 the ULP.

   In addition, smaller Path.Max.Retrans values also affect the
   'Association.Max.Retrans' value.  When the SCTP association's error
   count exceeds Association.Max.Retrans threshold, the SCTP sender
   considers the peer endpoint unreachable and terminates the
   association.  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 and to avoid 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 compromises 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



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   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



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   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











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

   Email: karen.nielsen@tieto.com












































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