SCTP-PF: Quick Failover Algorithm in SCTP
draft-ietf-tsvwg-sctp-failover-09

Network Working Group                                         Y. Nishida
Internet-Draft                                        GE Global Research
Intended status: Standards Track                            P. Natarajan
Expires: June 27, 2015                                     Cisco Systems
                                                                 A. Caro
                                                        BBN Technologies
                                                                 P. Amer
                                                  University of Delaware
                                                              K. Nielsen
                                                                Ericsson
                                                       December 24, 2014

               SCTP-PF: Quick Failover Algorithm in SCTP
                 draft-ietf-tsvwg-sctp-failover-09.txt

Abstract

   One of the major advantages of SCTP is the support of multi-homed
   communication.  A multi-homed SCTP end-point has the ability to
   withstand network failures by migrating the traffic from an inactive
   network to an active one.  However, if the failover operation as
   specified in [RFC4960] is followed, there can be a significant delay
   in the migration to the active destination addresses, thus severely
   reducing the effectiveness of the SCTP failover operation.

   This memo complements [RFC4960] by the introduction of the
   Potentially Failed path state and the associated new failover
   operation called SCTP-PF to apply during a network failure.  In
   addition, the memo complements [RFC4960] by introducing of
   alternative switchover operation modes for the data transfer path
   management after the recovery of a failed primary path.  These modes
   offers for more performance optimal operation in some network
   environments.  The implementation of the additional switchover
   operation modes is optional.

   The procedures defined in the document require only minimal
   modifications to the current 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
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   This Internet-Draft will expire on June 27, 2015.

Copyright Notice

   Copyright (c) 2014 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
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   publication of this document.  Please review these documents
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   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 . . . . . . . . . . . . . . . . .   3
   3.  Issues with the SCTP Path Management  . . . . . . . . . . . .   4
   4.  SCTP with Potentially-Failed Destination State (SCTP-PF)  . .   5
     4.1.  SCTP-PF Concept . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  SCTP-PF Algorithm in Detail . . . . . . . . . . . . . . .   6
     4.3.  Optional Feature: Permanent Failover  . . . . . . . . . .   9
   5.  Socket API Considerations . . . . . . . . . . . . . . . . . .  11
     5.1.  Support for the Potentially Failed Path State . . . . . .  11
     5.2.  Peer Address Thresholds (SCTP_PEER_ADDR_THLDS) Socket
           Option  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     5.3.  Exposing the Potentially Failed Path State
           (SCTP_EXPOSE_POTENTIALLY_FAILED_STATE) Socket Option  . .  13
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   8.  Proposed Change of Status (to be Deleted before Publication)   14
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Appendix A.  Discussions of Alternative Approaches  . . . . . . .  16
     A.1.  Reduce Path.Max.Retrans (PMR) . . . . . . . . . . . . . .  16

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     A.2.  Adjust RTO related parameters . . . . . . . . . . . . . .  16
   Appendix B.  Discussions for Path Bouncing Effect . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   The Stream Control Transmission Protocol (SCTP) as specified in
   [RFC4960] supports multihoming at the transport layer -- an SCTP
   endpoint can bind to multiple IP addresses.  SCTP's multihoming
   features include failure detection and failover procedures to provide
   network interface redundancy and improved end-to-end fault tolerance.

   In SCTP's current failure detection procedure, the sender must
   experience Path.Max.Retrans (PMR) number of consecutive failed timer-
   based retransmissions on a destination address before detecting a
   path failure.  The sender fails over to an alternate active
   destination address only after failure detection.  Until detecting
   the failover, the sender continues to transmit data on the failed
   path, which degrades the SCTP performance.  Concurrent Multipath
   Transfer (CMT) [IYENGAR06] is an extension to SCTP that allows the
   sender to transmit data on multiple paths simultaneously.  Research
   [NATARAJAN09] shows that the current failure detection procedure
   worsens CMT performance during failover and can be significantly
   improved by employing a better failover algorithm.

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

   Also the operation after the recovery of a failed path impacts the
   performance of the protocol.  With procedures specified in [RFC4960],
   SCTP will, after a failover from the primary path, switch back to the
   primary path for data transfer as soon as this path becomes available
   again.  From a performance perspective, as confirmed in research
   [CARO02], such a switchback of the data transmission path is not
   optimal in general.  As an optional alternative to the switchback
   operation of [RFC4960], this document specifies the Permanent
   Failover procedures proposed by [CARO02].

   Additional discussions for alternative approaches that do not require
   modifications to [RFC4960] and path bouncing effects that might be
   caused by frequent switchover are provided in the Appendices.

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].

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3.  Issues with the SCTP Path Management

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

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

   If a sender has multiple active destination addresses, it can
   retransmit data to an non-primary destination address, if the
   transmission to the primary times out.

   When a sender receives an acknowledgment for DATA or HEARTBEAT chunks
   sent to one of the destination addresses, it considers that
   destination address to be active and clears the error counter for the
   destination address.  If it fails to receive acknowledgments, the
   error count for the destination address is increased.  If the error
   counter exceeds the tunable protocol parameter Path.Max.Retrans
   (PMR), the SCTP endpoint considers the destination address to be
   inactive.

   The failover process of SCTP is initiated when the primary path
   becomes inactive (the error counter for the primary path exceeds
   Path.Max.Retrans).  If the primary path is marked inactive, SCTP
   chooses a new destination address from one of the active destinations
   and start using this address to send data to.  If the primary path
   becomes active again, SCTP uses the primary destination address for
   subsequent data transmissions and stop using the non-primary one.

   One issue with this failover process is that it usually takes a
   significant amount of time before SCTP switches to the new
   destination address.  Let's say the primary path on a multi-homed
   host becomes unavailable and the RTO value for the primary path at
   that time is around 1 second, it usually takes over 60 seconds before
   SCTP starts to use the non-primary path for initial data
   transmission.  This is because the recommended value for
   Path.Max.Retrans in the [RFC4960] is 5, which requires 6 consecutive
   timeouts before the failover takes place.  Before SCTP switches to
   the non-primary address, SCTP keeps trying to send packets to the
   primary address and only retransmitted packets are sent to the non-

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   primary address and thus can be received by the receiver.  This slow
   failover process can cause significant performance degradation and is
   not acceptable in some situations.

   Another issue is that once the primary path becomes active again, the
   traffic is switched back.  This is not optimal in some situations.
   This is further discussed in Section 4.3.

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

   To address the issues described in Section 3, this section extends
   SCTP path management scheme by adding the Potentially Failed state
   and the associated failover operation.  We use the term SCTP-PF to
   denote the resulting SCTP path management operation.

4.1.  SCTP-PF Concept

   SCTP-PF as defined stems from the following two observations about
   SCTP's failure detection procedure:

   o  To minimize the performance impact during failover, the sender
      should avoid transmitting data to the failed destination address
      as early as possible.  In the current SCTP path management scheme,
      the sender stops transmitting data to a destination destination
      only after the destination is marked Failed (inactive).  Thus, a
      smaller PMR value is better because the sender can transition a
      destination address to the Failed (inactive) state quicker.

   o  Smaller PMR values increase the chances of spurious failure
      detection where the sender incorrectly marks a destination address
      as Failed (inactive) during periods of temporary congestion.  As
      [RFC4960] recommends for a coupling of the PMR value and the
      protocol parameter Association.Max.Retrans (AMR) value such
      spurious failure detection risks to carry over to spurious
      association failure detection and closure.  Larger PMR values are
      preferable to avoid spurious failure detection.

   From the above observations it is clear that tuning the PMR value
   involves the following tradeoff -- a lower value improves performance
   but increases the chances of spurious failure detection, whereas a
   higher value degrades performance and reduces spurious failure
   detection in a wide range of path conditions.  Thus, tuning the
   association's PMR value is an incomplete solution to address the
   performance impact during failure.

   SCTP-PF defined in this document introduces a new "Potentially-
   Failed" (PF) destination state in SCTP's path management procedure.
   The PF state was originally proposed to improve CMT performance

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   [NATARAJAN09].  The PF state is an intermediate state between the
   Active and Failed states.  SCTP's failure detection procedure is
   modified to include the PF state.  The new failure detection
   algorithm assumes that loss detected by a timeout implies either
   severe congestion or failure en-route.  After a number of consecutive
   timeouts on a path, the sender is unsure, and marks the corresponding
   destination address as PF.  A PF destination address is not used for
   data transmission except in special cases (discussed below).  The new
   failure detection algorithm requires only sender-side changes.

4.2.  SCTP-PF Algorithm in Detail

   The SCTP-PF operation is specified as follows:

   1.   The sender maintains a new tunable parameter called Potentially-
        Failed.Max.Retrans (PFMR).  The RECOMMENDED value of PFMR = 0
        when SCTP-PF is used.  When PFMR is larger or equal to PMR,
        SCTP-PF is turned off.

   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 at times where a
        HEARTBEAT sent to an idle, active address is not acknowledged
        within an RTO.  When the value in the destination address error
        counter exceeds PFMR, the endpoint MUST mark the destination
        transport address as PF.

   3.   The sender SHOULD avoid data transmission to PF destination
        addresses.  When the destination addresses are all in PF state
        or some in PF state and some in inactive state, the sender MUST
        choose one destination address in PF state and transmit data to
        this destination.  The sender SHOULD choose the destination
        address in PF state with the lowest error count (fewest
        consecutive timeouts) for data transmission and transmit data to
        this destination.  When there are multiple PF destinations with
        same error count, the sender SHOULD let the choice among the
        multiple PF destination address 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.  This means that the sender SHOULD
        attempt to pick the most divergent source - destination pair
        from the last source - destination pair on which data were
        transmitted or retransmitted.  Rules for picking the most
        divergent source-destination pair are an implementation decision
        and are not specified within this document.  A sender may choose
        to deploy other strategies than the above when choosing among
        multiple PF destinations with equal error count.  In all cases,

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        the sender MUST NOT change the state of chosen destination
        address and it MUST NOT clear the destination's error counter as
        a result of choosing the destination address for data
        transmission.

   4.   HEARTBEAT chunks SHOULD be sent to PF destination(s) once per
        RTO, which requires to ignore HB.interval for PF destinations.
        If a HEARTBEAT chunk is not acknowledged, the sender SHOULD
        increment the error counter and exponentially back off the RTO
        value.  If error counter is less than PMR, the sender SHOULD
        transmit another packet containing HEARTBEAT chunk immediately
        after T3-timer expiration.  When data is transmitted to a PF
        destination, the transmission of HEARTBEAT chunk MAY be omitted
        as receipt of SACK chunks or a T3-rtx timer expiration can
        provide equivalent information.  It is RECOMMENDED that
        HEARTBEAT chunks are send to PF destinations regardless of
        whether the Path Heartbeat function (Section 8.3 of [RFC4960])
        is enabled for the destination address or not.

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

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

   7.   When all destinations are in inactive state (association dormant
        state) the sender MUST also choose one destination address to
        transmit data to.  The sender SHOULD choose the destination
        address in inactive state with the lowest error count (fewest
        consecutive timeouts) for data transmission and transmit data to
        this destination.  When there are multiple destination addresses
        with same error count in inactive state, the sender SHOULD
        attempt to pick the most divergent source - destination pair
        from the last source - destination pair on which data were
        transmitted or retransmitted following [RFC4960].  Rules for
        picking the most divergent source-destination pair are an
        implementation decision and are not specified within this
        document.  Therefore, a sender SHOULD allow for incrementing the

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        destination error counters up to some reasonable limit larger
        than 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 than the
        above.  For example, a sender could choose to prioritize the
        last active destination address during dormant state.  The
        strategy to prioritize the last active destination address is
        optimal when some paths are permanently inactive, but suboptimal
        when paths' instability is transient.  While the increment of
        the error counters above PMR+1 is a prerequisite for the error
        counter values to serve to guide the path selection in dormant
        state, then it is noted that by virtue of the introduction of
        the Potentially Failed state, one may deploy higher values of
        PMR without compromising the efficiency of the failover
        operation, and thus making the increase of path error counters
        above PMR+1 less critical as the dormant state will be less
        likely to happen.  The downside of increasing the PMR value
        relative to the AMR value, however, is that the per destination
        address failure detection and notification of such to ULP
        thereby is weakened.  In all cases the sender MUST NOT change
        the state of the chosen destination address and it MUST NOT
        clear the destination's error counter as a result of choosing
        the destination address for data transmission.

   8.   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 of an inactive destination
        address and SHOULD NOT transition a PF destination address back
        to Active state, since a sender cannot disambiguate whether the
        ACK was for the original transmission or the retransmission(s).
        The same ambiguity concerns the related congestion window
        growth.  The bytes of a newly acknowledged chunk which has been
        transmitted to multiple destination addresses SHOULD be
        considered for contribution to the congestion window growth
        towards the destination address where the chunk was last sent.
        The contribution of the ACKed bytes to the window growth is
        subject to the prescriptions described in Section 7.2 of
        [RFC4960] is fulfilled.  A SCTP sender MAY apply a different
        approach for both the error count handling and the congestion
        control growth handling based on unequivocally information on
        which destination (including multiple destination addresses) the
        chunk reached.  This document makes no reference to what such
        unequivocally information could consist of, neither how such

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

   9.   Acknowledgments for chunks that has been transmitted to one
        destination address only MUST clear the error counter of the
        destination address and MUST transition a PF destination address
        back to Active state.  This situation can happen when new data
        is sent to a destination address in PF state.  It can also
        happen in situations where the destination address is in PF
        state due to the occurrence of a spurious T3-rtx timer and
        Acknowledgments start to arrive for data sent prior to
        occurrence of the spurious T3-rtx and data has not yet been
        retransmitted towards other destinations.  This document does
        not specify special handling for detection of or reaction to
        spurious T3-rtx timeouts, e.g., for special operation vis-a-vis
        the congestion control handling or data retransmission operation
        towards a destination address which undergoes a transition from
        active to PF to active state due to a spurious T3-rtx timeout.
        But it is noted that this is an area which would benefit from
        additional attention, experimentation and specification for
        Single Homed SCTP as well as for Multi Homed SCTP protocol
        operation.

   10.  SCTP stack SHOULD provide the ULP with the means to expose the
        PF state of its destinations as well as the means to notify the
        state transitions from Active to PF, and vice-versa.  When doing
        this, such an SCTP stack MUST provide the ULP with the means to
        suppress exposure of PF state and associated state transitions
        as well.

4.3.  Optional Feature: Permanent Failover

   In [RFC4960], an SCTP sender migrates the traffic back to the
   original primary destination address once this address becomes active
   again.  As the CWND towards the original primary destination address
   has to be rebuilt once data transfer resumes, the switch back to use
   the original primary address is not always optimal.  Indeed [CARO02]
   shows that the switch back to the original primary may degrade SCTP
   performance compared to continuing data transmission on the same
   path, especially, but not only, in scenarios where this path's
   characteristics are better.  In order to mitigate this performance
   degradation, the Permanent Failover operation was proposed in
   [CARO02].  When SCTP changes the destination address due to failover,
   Permanent Failover operation allows SCTP sender to continue data
   transmission on the new working path even when the old primary
   destination address becomes active again.  This is achieved by having
   SCTP perform a switch over of the primary path to the alternative

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   working path rather than having SCTP switch back data transfer to the
   (previous) primary path.

   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, it is recommended
   for SCTP to support also, as alternative behavior, the Permanent
   Failover switch over modes of operation.

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

   1.  The sender maintains a new tunable parameter, called
       Primary.Switchover.Max.Retrans (PSMR).  The PSMR MUST be set
       greater or equal to the PFMR 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 MAY come in use as data transfer
       path as per normal path selection when the present data transfer
       path fails.

   4.  The recommended value of PSMR is PFMR when Permanent Failover is
       used.  This means that no forced switchback to a previously
       failed primary path is performed.  An implementation of Permanent
       Failover MUST support the setting of PSMR = PFMR.  An
       implementation of Permanent Failover MAY support setting of PSMR
       > PFMR.

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

   This specifications RECOMMENDS a default configuration that uses
   standard RFC4960 switchback, i.e., switch back to the old primary
   destination once the destination address becomes active again.
   However, to support optimal operation in a wider range of network

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   scenarios, an implementation MAY implement Permanent Failover
   operation as detailed above and MAY enable it based on network
   configurations or users' requests.

5.  Socket API Considerations

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

   Please note that this section is informational only.

   A socket API implementation based on [RFC6458] is, by means of the
   existing SCTP_PEER_ADDR_CHANGE event, extended to provide the event
   notification when a peer address enters or leaves the potentially
   failed state as well as the socket API implementation is extended to
   expose the potentially failed state of a peer address in the existing
   SCTP_GET_PEER_ADDR_INFO structure.

   Furthermore, two new read/write socket options for the level
   IPPROTO_SCTP and the name SCTP_PEER_ADDR_THLDS and
   SCTP_EXPOSE_POTENTIALLY_FAILED_STATE are defined as described below.
   The first socket option is used to control the values of the PFMR and
   PSMR parameters described in Section 4.  The second one controls the
   exposition of the potentially failed path state.

   Support for the SCTP_PEER_ADDR_THLDS and
   SCTP_EXPOSE_POTENTIALLY_FAILED_STATE socket options need also to be
   added to the function sctp_opt_info().

5.1.  Support for the Potentially Failed Path State

   As defined in [RFC6458], the SCTP_PEER_ADDR_CHANGE event is provided
   if the status of a peer address changes.  In addition to the state
   changes described in [RFC6458], this event is also provided, if a
   peer address enters or leaves the potentially failed state.  The
   notification as defined in [RFC6458] uses the following structure:

   struct sctp_paddr_change {
     uint16_t spc_type;
     uint16_t spc_flags;
     uint32_t spc_length;
     struct sockaddr_storage spc_aaddr;
     uint32_t spc_state;
     uint32_t spc_error;
     sctp_assoc_t spc_assoc_id;
   }

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   [RFC6458] defines the constants SCTP_ADDR_AVAILABLE,
   SCTP_ADDR_UNREACHABLE, SCTP_ADDR_REMOVED, SCTP_ADDR_ADDED, and
   SCTP_ADDR_MADE_PRIM to be provided in the spc_state field.  This
   document defines in addition to that the new constant
   SCTP_ADDR_POTENTIALLY_FAILED, which is reported if the affected
   address becomes potentially failed.

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

   struct sctp_paddrinfo {
     sctp_assoc_t spinfo_assoc_id;
     struct sockaddr_storage spinfo_address;
     int32_t spinfo_state;
     uint32_t spinfo_cwnd;
     uint32_t spinfo_srtt;
     uint32_t spinfo_rto;
     uint32_t spinfo_mtu;
   };

   [RFC6458] defines the constants SCTP_UNCONFIRMED, SCTP_ACTIVE, and
   SCTP_INACTIVE to be provided in the spinfo_state field.  This
   document defines in addition to that the new constant
   SCTP_POTENTIALLY_FAILED, which is reported if the peer address is
   potentially failed.

5.2.  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 and before the primary
   path is changed automatically.  This socket option uses the level
   IPPROTO_SCTP and the name SCTP_PEER_ADDR_THLDS.

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

   struct sctp_paddrthlds {
     sctp_assoc_t spt_assoc_id;
     struct sockaddr_storage spt_address;
     uint16_t spt_pathmaxrxt;
     uint16_t spt_pathpfthld;
     uint16_t spt_pathcpthld;
   };

   spt_assoc_id:  This parameter is ignored for one-to-one style
      sockets.  For one-to-many style sockets the application may fill

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      in an association identifier or SCTP_FUTURE_ASSOC.  It is an error
      to use SCTP_{CURRENT|ALL}_ASSOC in spt_assoc_id.

   spt_address:  This specifies which peer address is of interest.  If a
      wildcard address is provided, this socket option applies to all
      current and future peer addresses.

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

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

   spt_pathcpthld:  Each peer address of interest is not considered the
      primary remote address anymore, if its path error counter exceeds
      spt_pathcpthld.  Using a value of 0xffff disables the selection of
      a new primary peer address.  If an implementation does not support
      the automatically selection of a new primary address, it should
      indicate an error with errno set to EINVAL if a value different
      from 0xffff is used in spt_pathcpthld.  Setting of spt_pathcpthld
      < spt_pathpfthld should be rejected with errno set to EINVAL.  An
      implementation MAY support only setting of spt_pathcpthld =
      spt_pathpfthld and spt_pathcpthld = 0xffff.  In this case it shall
      reject setting of other values with errno set to EINVAL.

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

   Applications can control the exposure of the potentially failed path
   state in the SCTP_PEER_ADDR_CHANGE event and the
   SCTP_GET_PEER_ADDR_INFO as described in Section 5.1.  The default
   value is implementation specific.

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

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

   struct sctp_assoc_value {
     sctp_assoc_t assoc_id;
     uint32_t assoc_value;
   };

   assoc_id:  This parameter is ignored for one-to-one style sockets.
      For one-to-many style sockets the application may fill in an

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      association identifier or SCTP_FUTURE_ASSOC.  It is an error to
      use SCTP_{CURRENT|ALL}_ASSOC in assoc_id.

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

6.  Security Considerations

   Security considerations for the use of SCTP and its APIs are
   discussed in [RFC4960] and [RFC6458].  There are no new security
   considerations introduced in this document.

7.  IANA Considerations

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

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

   Initially this work looked to entail some changes of the Congestion
   Control (CC) operation of SCTP and for this reason the work was
   proposed as Experimental.  These intended changes of the CC operation
   have since been judged to be irrelevant and are no longer part of the
   specification.  As the specification entails no other potential
   harmful features, consensus exists in the WG to bring the work
   forward as PS.

   Initially concerns have been expressed about the possibility for the
   mechanism to introduce path bouncing with potential harmful network
   impacts.  These concerns are believed to be unfounded.  This issue is
   addressed in Appendix B.

   It is noted that the feature specified by this document is
   implemented by multiple SCTP SW implementations and furthermore that
   various variants of the solution have been deployed in Telco
   signaling environments for several years with good results.

9.  References

9.1.  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.

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9.2.  Informative References

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

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

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

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

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

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

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

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

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

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Appendix A.  Discussions of Alternative Approaches

   This section lists alternative approaches for the issues desribed in
   this document.  Although these approaches do not require to update
   RFC4960, we do not recommend them from the reasons described below.

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

   Smaller values for Path.Max.Retrans shorten the failover duration.
   In fact, this is recommended in some research results [JUNGMAIER02]
   [GRINNEMO04] [FALLON08].  For example, if when Path.Max.Retrans=0,
   SCTP switches to another destination address on a single timeout.
   This smaller value for Path.Max.Retrans can results in spurious
   failover, which might be a problem.

   Unlike SCTP-PF, the interval for heartbeat packets is governed by
   'HB.interval' even during failover process.  'HB.interval' is usually
   set in the order of seconds (recommended value is 30 seconds).  When
   the primary path becomes inactive, the next HB can be transmitted
   only seconds later.  Meanwhile, the primary path may have recovered.
   In such situations, post failover, an endpoint is forced to wait on
   the order of seconds before the endpoint can resume transmission on
   the primary path.  However, using smaller value for 'HB.interval'
   might help this situation, but it will be the waste of bandwidth in
   most cases.

   In addition, smaller Path.Max.Retrans values also affect
   'Association.Max.Retrans' values.  When the SCTP association's error
   count (sum of error counts on all ACTIVE paths) exceeds
   Association.Max.Retrans threshold, the SCTP sender considers the peer
   endpoint unreachable and terminates the association.  Therefore,
   Section 8.2 in [RFC4960] recommends that Association.Max.Retrans
   value should not be larger than the summation of the Path.Max.Retrans
   of each of the destination addresses, else the SCTP sender considers
   its peer reachable even when all destinations are INACTIVE.  To avoid
   such inconsistent behavior an SCTP implementation SHOULD reduce
   Association.Max.Retrans accordingly whenever it reduces
   Path.Max.Retrans.  However, smaller Association.Max.Retrans value
   increases chances of association termination during minor congestion
   events.

A.2.  Adjust RTO related parameters

   As several research results indicate, we can also shorten the
   duration of failover process by adjusting RTO related parameters
   [JUNGMAIER02] [FALLON08].  During failover process, RTO keeps being
   doubled.  However, if we can choose smaller value for RTO.max, we can
   stop the exponential growth of RTO at some point.  Also, choosing

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   smaller values for RTO.initial or RTO.min can contribute to keep RTO
   value small.

   Similar to reducing Path.Max.Retrans, the advantage of this approach
   is that it requires no modification to the current specification,
   although it needs to ignore several recommendations described in the
   Section 15 of [RFC4960].  However, this approach requires to have
   enough knowledge about the network characteristics between end
   points.  Otherwise, it can introduce adverse side-effects such as
   spurious timeouts.

Appendix B.  Discussions for Path Bouncing Effect

   The methods described in the document can accelerate the failover
   process.  Hence, they might introduce the path bouncing effect where
   the sender keeps changing the data transmission path frequently.
   This sounds harmful to the data transfer, however several research
   results indicate that there is no serious problem with SCTP in terms
   of path bouncing effect [CARO04] [CARO05].

   There are two main reasons for this.  First, SCTP is basically
   designed for multipath communication, which means SCTP maintains all
   path related parameters (CWND, ssthresh, RTT, error count, etc) per
   each destination address.  These parameters cannot be affected by
   path bouncing.  In addition, when SCTP migrates the data transfer to
   another path, it starts with the minimal or the initial CWND.  Hence,
   there is little chance for packet reordering or duplicating.

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

Authors' Addresses

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

   Email: nishida@wide.ad.jp

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

   Email: prenatar@cisco.com

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

   Email: acaro@bbn.com

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

   Email: amer@udel.edu

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

   Email: karen.nielsen@tieto.com

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