[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

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: August 20, 2016                                   Cisco Systems
                                                                 A. Caro
                                                        BBN Technologies
                                                                 P. Amer
                                                  University of Delaware
                                                              K. Nielsen
                                                                Ericsson
                                                       February 17, 2016


               SCTP-PF: Quick Failover Algorithm in SCTP
                 draft-ietf-tsvwg-sctp-failover-16.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




Nishida, et al.          Expires August 20, 2016                [Page 1]


Internet-Draft                   SCTP-PF                   February 2016


   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 August 20, 2016.

Copyright Notice

   Copyright (c) 2016 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 . . . . . . . . . . . . . . . . . . .  15
   9.  MIB Considerations  . . . . . . . . . . . . . . . . . . . . .  16
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   12. Proposed Change of Status (to be Deleted before Publication)   17
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17



Nishida, et al.          Expires August 20, 2016                [Page 2]


Internet-Draft                   SCTP-PF                   February 2016


     13.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     13.2.  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 the 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 also impacts the
   performance of the protocol.  With the procedures specified in



Nishida, et al.          Expires August 20, 2016                [Page 3]


Internet-Draft                   SCTP-PF                   February 2016


   [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 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 switchovers, 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.  Also due to the
   coupled tuning of the Path.Max.Retrans (PMR) and the
   Association.Max.Retrans (AMR) parameter values in [RFC4960], lowering




Nishida, et al.          Expires August 20, 2016                [Page 4]


Internet-Draft                   SCTP-PF                   February 2016


   of the PMR threshold may result in lowering of the AMR threshold,
   which would result in decrease of the fault tolerance of SCTP.

   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 negatively impacting 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.  It recommends that by default a destination address is
   classified as PF at the occurrence of the 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.  When this threshold is exceeded the destination
        address is classified as PF.  The RECOMMENDED value of PFMR is
        0.  If PFMR is set to be greater than or equal to
        Path.Max.Retrans (PMR), the resulting PF threshold will be so
        high that the destination address will reach the inactive state
        before it can be classified as PF.

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





Nishida, et al.          Expires August 20, 2016                [Page 5]


Internet-Draft                   SCTP-PF                   February 2016


   3.   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 use this destination address for
           data transmission.

        ii As specified in [RFC4960] section 6.4.1, when the sender
           retransmits data that has timed out, it should attempt to
           pick a new destination address for data retransmission.  In
           this case, 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 an SCTP sender will follow the guiding principles of
        section 6.4.1 of [RFC4960] 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.

        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 SHOULD 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.




Nishida, et al.          Expires August 20, 2016                [Page 6]


Internet-Draft                   SCTP-PF                   February 2016


        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.

        The sender MUST NOT change the state and the error counter of
        any destination addresses as the result of the selection.

   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
        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 where the receipt of a SACK of the data
        or a T3-rtx timer expiration on the data can provide equivalent
        information, such as the case where the data chunk has been
        transmitted to a single destination address only.  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.  However, there may be
        a situation where HEARTBEAT chunks can go through while DATA
        chunks cannot.  Hence, 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



Nishida, et al.          Expires August 20, 2016                [Page 7]


Internet-Draft                   SCTP-PF                   February 2016


        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.  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].

   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 move 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 clear the error counter
        and move a destination address back to active state by
        information other than acknowledgments, when it can uniquely
        determine which destination, among multiple destination
        addresses, the chunk reached.  This document makes no reference
        to what such information could consist of, nor how such
        information could be obtained.

   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-a-vis
        the congestion control handling or data retransmission operation
        towards a destination address which undergoes a transition from



Nishida, et al.          Expires August 20, 2016                [Page 8]


Internet-Draft                   SCTP-PF                   February 2016


        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 expose the PF state of its destination
        addresses to the ULP as well as provide the means to notify the
        ULP of state transitions of its destination addresses 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 transitions, thus allowing for retain of the
        simpler state transition model of RFC4960 in the ULP.  For this
        reason it is recommended that an SCTP stack implementing SCTP-PF
        also provides the ULP with the means to suppress exposure of the
        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
   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 decrease the fault tolerance of the
   SCTP-PF operation.





Nishida, et al.          Expires August 20, 2016                [Page 9]


Internet-Draft                   SCTP-PF                   February 2016


   The below prescription for SCTP-PF dormant state handling MUST 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 before 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
       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 the limit to be an order of magnitude higher than the
       PMR value.  A sender MAY choose to deploy other strategies that
       the strategy defined 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.





Nishida, et al.          Expires August 20, 2016               [Page 10]


Internet-Draft                   SCTP-PF                   February 2016


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 switchover 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 (PSMR).  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.  For SCTP-PF, the recommended value of PSMR 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
       MUST support the setting of PSMR = PFMR.  A SCTP-PF
       implementation of Primary Path Switchover MAY support setting of
       PSMR > PFMR.

   5.  For [RFC4960] SCTP, the recommended value of PSMR 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 PSMR = PMR.  An [RFC4960] SCTP
       implementation of Primary Path Switchover MAY support larger
       settings of PSMR > PMR.





Nishida, et al.          Expires August 20, 2016               [Page 11]


Internet-Draft                   SCTP-PF                   February 2016


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

   The manner of switchover operation that is most optimal in a given
   scenario depends on the relative quality of a set primary path versus
   the quality of alternative paths available as well as 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 applications'
   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 recommendation 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.

   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



Nishida, et al.          Expires August 20, 2016               [Page 12]


Internet-Draft                   SCTP-PF                   February 2016


   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 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:

   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;
   };




Nishida, et al.          Expires August 20, 2016               [Page 13]


Internet-Draft                   SCTP-PF                   February 2016


   [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:

   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, 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



Nishida, et al.          Expires August 20, 2016               [Page 14]


Internet-Draft                   SCTP-PF                   February 2016


      from 0xffff is used in spt_pathcpthld.  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_pathmaxrxt should be rejected with errno set to EINVAL.  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 these cases SCTP
      shall reject setting of other values with errno set to EINVAL.

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:

   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.

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 SCTP
   messages on the wire.  Also, this document does not introduce any new
   SCTP messages on the wire 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/



Nishida, et al.          Expires August 20, 2016               [Page 15]


Internet-Draft                   SCTP-PF                   February 2016


   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 negatively influence 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 threat analysis.  This is because on-path 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 PSMR = 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.  MIB Considerations

   SCTP-PF introduces new SCTP algorithms for failover and switchback
   with associated new state parameters.  It is recommended that the
   SCTP-MIB defined in [RFC3873] is updated to support the management of
   the SCTP-PF implementation.  This can be done by extending the
   sctpAssocRemAddrActive field of the SCTPAssocRemAddrTable to include
   information of the PF state of the destination address and by adding
   new fields to the SCTPAssocRemAddrTable supporting
   PotentiallyFailed.Max.Retrans (PFMR) and
   Primary.Switchover.Max.Retrans (PSMR) parameters.

10.  IANA Considerations

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

11.  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.



Nishida, et al.          Expires August 20, 2016               [Page 16]


Internet-Draft                   SCTP-PF                   February 2016


12.  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 telephony
   signaling environments for several years with good results.

13.  References

13.1.  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.

13.2.  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.







Nishida, et al.          Expires August 20, 2016               [Page 17]


Internet-Draft                   SCTP-PF                   February 2016


   [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.

   [RFC3873]  Pastor, J. and M. Belinchon, "Stream Control Transmission
              Protocol (SCTP) Management Information Base (MIB)", RFC
              3873, DOI 10.17487/RFC3873, September 2004,
              <http://www.rfc-editor.org/info/rfc3873>.

   [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 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



Nishida, et al.          Expires August 20, 2016               [Page 18]


Internet-Draft                   SCTP-PF                   February 2016


   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
   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 decreases 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.





Nishida, et al.          Expires August 20, 2016               [Page 19]


Internet-Draft                   SCTP-PF                   February 2016


   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



Nishida, et al.          Expires August 20, 2016               [Page 20]


Internet-Draft                   SCTP-PF                   February 2016


   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







Nishida, et al.          Expires August 20, 2016               [Page 21]


Internet-Draft                   SCTP-PF                   February 2016


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

   Email: karen.nielsen@tieto.com












































Nishida, et al.          Expires August 20, 2016               [Page 22]


Html markup produced by rfcmarkup 1.128, available from https://tools.ietf.org/tools/rfcmarkup/