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rfc:rfc7829

Internet Engineering Task Force (IETF) Y. Nishida Request for Comments: 7829 GE Global Research Category: Standards Track P. Natarajan ISSN: 2070-1721 Cisco Systems

                                                               A. Caro
                                                      BBN Technologies
                                                               P. Amer
                                                University of Delaware
                                                            K. Nielsen
                                                              Ericsson
                                                            April 2016
            SCTP-PF: A Quick Failover Algorithm for the
                Stream Control Transmission Protocol

Abstract

 The Stream Control Transmission Protocol (SCTP) supports multihoming.
 However, when the failover operation specified in RFC 4960 is
 followed, there can be significant delay and performance degradation
 in the data transfer path failover.  This document specifies a quick
 failover algorithm and introduces the SCTP Potentially Failed
 (SCTP-PF) destination state in SCTP Path Management.
 This document also specifies a dormant state operation of SCTP that
 is required to be followed by an SCTP-PF implementation, but it may
 equally well be applied by a standard SCTP implementation, as
 described in RFC 4960.
 Additionally, this 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 SCTP implementation and an SCTP-PF implementation.
 The procedures defined in the document require only minimal
 modifications to the specification in RFC 4960.  The procedures are
 sender-side only and do not impact the SCTP receiver.

Nishida, et al. Standards Track [Page 1] RFC 7829 SCTP-PF April 2016

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7829.

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.

Nishida, et al. Standards Track [Page 2] RFC 7829 SCTP-PF April 2016

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
 2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   5
 3.  SCTP with Potentially Failed (SCTP-PF) Destination State  . .   5
   3.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.2.  Specification of the SCTP-PF Procedures . . . . . . . . .   6
 4.  Dormant State Operation . . . . . . . . . . . . . . . . . . .  10
   4.1.  SCTP Dormant State Procedure  . . . . . . . . . . . . . .  11
 5.  Primary Path Switchover . . . . . . . . . . . . . . . . . . .  11
 6.  Suggested SCTP Protocol Parameter Values  . . . . . . . . . .  13
 7.  Socket API Considerations . . . . . . . . . . . . . . . . . .  13
   7.1.  Support for the Potentially Failed Path State . . . . . .  14
   7.2.  Peer Address Thresholds (SCTP_PEER_ADDR_THLDS) Socket
         Option  . . . . . . . . . . . . . . . . . . . . . . . . .  15
   7.3.  Exposing the Potentially Failed Path State
         (SCTP_EXPOSE_POTENTIALLY_FAILED_STATE) Socket Option  . .  16
 8.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
 9.  MIB Considerations  . . . . . . . . . . . . . . . . . . . . .  17
 10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
   10.1.  Normative References . . . . . . . . . . . . . . . . . .  17
   10.2.  Informative References . . . . . . . . . . . . . . . . .  18
 Appendix A.  Discussion of Alternative Approaches . . . . . . . .  20
   A.1.  Reduce PMR  . . . . . . . . . . . . . . . . . . . . . . .  20
   A.2.  Adjust RTO-Related Parameters . . . . . . . . . . . . . .  21
 Appendix B.  Discussion of the Path-Bouncing Effect . . . . . . .  21
 Appendix C.  SCTP-PF for SCTP Single-Homed Operation  . . . . . .  22
 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  22
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1. Introduction

 The Stream Control Transmission Protocol (SCTP) specified in
 [RFC4960] supports multihoming at the transport layer.  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.  Until detecting the path
 failure, the sender continues to transmit data on the failed path.
 The prolonged time in which SCTP as described in [RFC4960] continues
 to use a failed path severely degrades the performance of the
 protocol.  To address this problem, this document specifies a quick
 failover algorithm called "SCTP-PF" based on the introduction of a
 new Potentially Failed (PF) path state in SCTP path management.  The

Nishida, et al. Standards Track [Page 3] RFC 7829 SCTP-PF April 2016

 performance deficiencies of the failover operation described in RFC
 4960, and the improvements obtainable from the introduction of a PF
 state in SCTP, were proposed and documented in [NATARAJAN09] for
 Concurrent Multipath Transfer SCTP [IYENGAR06].
 While SCTP-PF can accelerate the failover process and improve
 performance, the risk that an SCTP endpoint might enter the dormant
 state where all destination addresses are inactive can be increased.
 [RFC4960] leaves the protocol operation during dormant state to
 implementations and encourages avoiding entering the state as much as
 possible by careful tuning of the 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 the fact that the dormant state may be entered more
 quickly.  The dormant state operation may equally well be applied by
 an implementation of [RFC4960] and will serve here to provide added
 fault tolerance for situations where the tuning of the PMR and 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
 [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 Congestion Window (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 that avoids such forced switchbacks of the data transfer
 path.  The Primary Path Switchover operation was originally proposed
 in [CARO02].
 While SCTP-PF is primarily motivated by a desire to improve the
 multihomed operation, the feature also applies 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 PF
 state, including a discussion of alternative approaches to mitigate
 the deficiencies of the failover operation in [RFC4960], are given in
 the appendices.  Discussion of path-bouncing effects that might be
 caused by frequent switchovers are also provided there.

Nishida, et al. Standards Track [Page 4] RFC 7829 SCTP-PF April 2016

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 (SCTP-PF) Destination State

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 SCTP path management scheme described in [RFC4960],
 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 PMR threshold.  The issue cannot
 simply be mitigated by lowering 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 PMR and the AMR parameter values in [RFC4960], lowering
 the PMR threshold may result in lowering the AMR threshold, which
 would result in a 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 PF state as an
 intermediate state in between the active and inactive state of a
 destination address in the path management scheme of [RFC4960], 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 SCTP as described
 in [RFC4960].  At the same time, HEARTBEAT probing based on
 Retransmission Timeout (RTO) 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 be
 classified as PF at the occurrence of the first timeout.

Nishida, et al. Standards Track [Page 5] RFC 7829 SCTP-PF April 2016

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 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 Timer T3 retransmission
      (T3-rtx) timer expires, or each time a HEARTBEAT chunk is sent
      when idle and not acknowledged within an RTO.  When the value in
      the destination address error counter exceeds PFMR, the endpoint
      MUST mark the destination address as in the PF state.
 3.   An 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 Section 6.4.1 of [RFC4960], when the
             sender retransmits data that has timed out, they 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.

Nishida, et al. Standards Track [Page 6] RFC 7829 SCTP-PF April 2016

      When choosing among multiple destination addresses in active
      state, an SCTP sender will follow the guiding principles of
      Section 6.4.1 of [RFC4960] by choosing the most divergent
      source-destination pairs compared with, for (the aforementioned
      points i and ii):
      i.    the destination address in PF state that it performs a
            failover from, and
      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 the 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 are
      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:
      i.    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.
      ii.   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 principles of choosing
            the most divergent source-destination pairs when executing
            (potentially consecutive) retransmission outlined in
            Section 6.4.1 of [RFC4960].  Rules for picking the most
            divergent source-destination pairs 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

Nishida, et al. Standards Track [Page 7] RFC 7829 SCTP-PF April 2016

      once per RTO.  HEARTBEAT chunks SHOULD be sent to destination
      addresses in PF state, but the sending of HEARTBEATs MUST honor
      whether or not the Path Heartbeat function (Section 8.3 of
      [RFC4960]) is enabled for the destination address.  That is, if
      the Path Heartbeat function is disabled for the destination
      address in question, HEARTBEATs MUST NOT be sent.  Note that
      when the Path 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 the case where the receipt of a Selective
      Acknowledgment (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
      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 SHOULD (i) notify the Upper
      Layer Protocol (ULP) about this state transition, and (ii)

Nishida, et al. Standards Track [Page 8] RFC 7829 SCTP-PF April 2016

      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 that 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).  An 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 have 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 that 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 that would
      benefit from additional attention, experimentation, and
      specification for single-homed SCTP as well as for multihomed
      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

Nishida, et al. Standards Track [Page 9] RFC 7829 SCTP-PF April 2016

      an SCTP stack implementing SCTP-PF also allows for the ULP to be
      kept ignorant of the PF state of its destinations and the
      associated state transitions, thus allowing for retention 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 provide 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 SCTP implementation of
 [RFC4960] given the same setting of PMR and AMR.  For example, an
 SCTP association with two destination addresses would typically reach
 dormant state in half the time of an SCTP implementation of [RFC4960]
 in such situations.  This is because an 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 a standard SCTP implementation with the same PMR and AMR
 setting, we prescribe that an SCTP-PF implementation SHOULD operate
 as described in Section 4.1 during dormant state.
 An SCTP-PF implementation MAY choose a different dormant state
 operation than the one described in Section 4.1 provided that the
 solution chosen does not decrease the fault tolerance of the SCTP-PF
 operation.
 The prescription below 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 can also provide
 enhanced disruption tolerance to a standard SCTP implementation that
 doesn't support SCTP-PF.  Thus, it can be sensible for a standard
 SCTP implementation to follow this mode of operation.  For a standard
 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.

Nishida, et al. Standards Track [Page 10] RFC 7829 SCTP-PF April 2016

4.1. SCTP Dormant State Procedure

 1.  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.
 2.  The sender MUST NOT change the state of the chosen destination
     address (it remains in inactive state) and MUST NOT clear the
     error counter of the destination address as a result of choosing
     the destination address for data transmission.
 3.  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 the 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
     incrementing of the destination address error counters up to some
     reasonable limit above PMR+1, thus changing the prescriptions of
     Section 8.3 of [RFC4960] in this respect.  The exact limit to
     apply is not specified in this document, but it is considered
     reasonable enough to require that the limit be an order of
     magnitude higher than the PMR value.  A sender MAY choose to
     deploy other strategies than 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
     path instability is transient.

5. Primary Path Switchover

 The objective of the Primary Path Switchover operation is to allow
 the SCTP sender to continue data transmission on a new working path
 even when the old primary destination address becomes active again.
 This is achieved by having SCTP perform a 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 implementations of
 [RFC4960].

Nishida, et al. Standards Track [Page 11] RFC 7829 SCTP-PF April 2016

 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 than or equal to
     the PFMR value.  For implementations of [RFC4960], the PSMR MUST
     be set greater than 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 that, at the given time, would be chosen for data transfer.
     A previously failed primary path can be used as a 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.  An SCTP-PF
     implementation of Primary Path Switchover MAY support setting of
     PSMR > PFMR.
 5.  For standard 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
     standard SCTP implementation of Primary Path Switchover MUST
     support the setting of PSMR = PMR.  A standard SCTP
     implementation of Primary Path Switchover MAY support larger
     settings of PSMR > PMR.
 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.  That is, load
 distribution of traffic from multiple SCTP associations may be
 enforced by distribution of the set primary paths with the switchback
 operation of [RFC4960].  However, as switchback behavior of [RFC4960]

Nishida, et al. Standards Track [Page 12] RFC 7829 SCTP-PF April 2016

 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
 switchback operation of [RFC4960] be retained as the default
 operation.

6. Suggested SCTP Protocol Parameter Values

 This document does not alter the 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 PF state as
 well as the socket API implementation is extended to expose the PF
 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 Sections 3 and 5.  The second one
 controls the exposition of the PF path state.
 Support for the SCTP_PEER_ADDR_THLDS and
 SCTP_EXPOSE_POTENTIALLY_FAILED_STATE socket options also needs to be
 added to the function sctp_opt_info().

Nishida, et al. Standards Track [Page 13] RFC 7829 SCTP-PF April 2016

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 PF 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 the new additional constant
 SCTP_ADDR_POTENTIALLY_FAILED, which is reported if the affected
 address becomes PF.
 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 the new additional constant SCTP_POTENTIALLY_FAILED,
 which is reported if the peer address is PF.

Nishida, et al. Standards Track [Page 14] RFC 7829 SCTP-PF April 2016

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 PF 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
    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 PF, 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 automatic 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.  For SCTP-PF, the setting
    of spt_pathcpthld < spt_pathpfthld should be rejected with errno
    set to EINVAL.  For standard SCTP, the setting of spt_pathcpthld <
    spt_pathmaxrxt should be rejected with errno set to EINVAL.  An
    SCTP-PF implementation may support only setting of spt_pathcpthld
    = spt_pathpfthld and spt_pathcpthld = 0xffff and a standard SCTP

Nishida, et al. Standards Track [Page 15] RFC 7829 SCTP-PF April 2016

    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 PF 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 PF
 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 PF 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/
 congestion attacks on the primary path.  SCTP-PF makes it easier for
 an attacker to trick SCTP into changing the data transfer path, since
 the duration of time that an attacker needs to negatively influence
 the network connectivity is much shorter than used in [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

Nishida, et al. Standards Track [Page 16] RFC 7829 SCTP-PF April 2016

 the primary path.  With the standard switchback operation in
 [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 will
 especially 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 PF 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. References

10.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC4960]  Stewart, R., Ed., "Stream Control Transmission Protocol",
            RFC 4960, DOI 10.17487/RFC4960, September 2007,
            <http://www.rfc-editor.org/info/rfc4960>.

Nishida, et al. Standards Track [Page 17] RFC 7829 SCTP-PF April 2016

10.2. Informative References

 [CARO02]   Caro, 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,
            July 2002.
 [CARO04]   Caro, A., Amer, P., and R. Stewart, "End-to-End Failover
            Thresholds for Transport Layer Multihoming", MILCOM 2004,
            DOI 10.1109/MILCOM.2004.1493253, November 2004.
 [CARO05]   Caro, A., "End-to-End Fault Tolerance using Transport
            Layer Multihoming", Ph.D. Thesis, University of Delaware,
            DOI 10.1007/BF03219970, January 2005.
 [FALLON08]
            Fallon, S., Jacob, P., Qiao, Y., Murphy, L., Fallon, E.,
            and A. Hanley, "SCTP Switchover Performance Issues in WLAN
            Environments", IEEE CCNC, DOI 10.1109/ccnc08.2007.131,
            January 2008.
 [GRINNEMO04]
            Grinnemo, K-J. and A. Brunstrom, "Performance of SCTP-
            controlled failovers in M3UA-based SIGTRAN networks",
            Advanced Simulation Technologies Conference, April 2004.
 [IYENGAR06]
            Iyengar, J., Amer, P., and R. Stewart, "Concurrent
            Multipath Transfer using SCTP Multihoming over Independent
            End-to-end Paths", IEEE/ACM Transactions on Networking,
            DOI 10.1109/TNET.2006.882843, October 2006.
 [JUNGMAIER02]
            Jungmaier, A., Rathgeb, E., and M. Tuexen, "On the use of
            SCTP in failover scenarios", World Multiconference on
            Systemics, Cybernetics and Informatics, July 2002.
 [NATARAJAN09]
            Natarajan, P., Ekiz, N., Amer, P., and R. Stewart,
            "Concurrent Multipath Transfer during Path Failure",
            Computer Communications, DOI 10.1016/j.comcom.2009.05.001,
            May 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>.

Nishida, et al. Standards Track [Page 18] RFC 7829 SCTP-PF April 2016

 [RFC6458]  Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
            Yasevich, "Sockets API Extensions for the Stream Control
            Transmission Protocol (SCTP)", RFC 6458,
            DOI 10.17487/RFC6458, December 2011,
            <http://www.rfc-editor.org/info/rfc6458>.

Nishida, et al. Standards Track [Page 19] RFC 7829 SCTP-PF April 2016

Appendix A. Discussion of Alternative Approaches

 This section lists alternative approaches for the issues described in
 this document.  Although these approaches do not require updating RFC
 4960, we do not recommend them for the reasons described below.

A.1. Reduce PMR

 Smaller values for Path.Max.Retrans shorten the failover duration and
 in fact, this is recommended in some research results [JUNGMAIER02],
 [GRINNEMO04], and [FALLON08].  However, to significantly reduce the
 failover time, it is required to go down (as with PFMR) to
 Path.Max.Retrans=0 and, with this setting, SCTP switches to another
 destination address already on a single timeout that may result in
 spurious failover.  Spurious failover is a problem in standard 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: as recommended,
 only 30 seconds later.  Meanwhile, the primary path may have long
 since 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 rebuilt
 anew -- a process that the throughput already 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 that, at
 any given time, is set to be the primary path; however, 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 the
 Association.Max.Retrans value should not be larger than the summation
 of the Path.Max.Retrans of each of the destination addresses.

Nishida, et al. Standards Track [Page 20] RFC 7829 SCTP-PF April 2016

 Otherwise, the SCTP sender considers its peer reachable even when all
 destinations are INACTIVE.  To avoid this dormant state operation,
 standard 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 the failover process by adjusting the RTO-related
 parameters [JUNGMAIER02] and [FALLON08].  During the failover
 process, RTO keeps being doubled.  However, if we can choose a
 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 keeping the RTO value small.
 Similar to reducing Path.Max.Retrans, the advantage of this approach
 is that it requires no modification to the current specification,
 although it needs to ignore several recommendations described in
 Section 15 of [RFC4960].  However, this approach requires having
 enough knowledge about the network characteristics between endpoints.
 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, as long as the
 Path.Max.Retrans is kept at the recommended value from [RFC4960], the
 reduction of the RTO.max doesn't reduce the failover time
 sufficiently enough to prevent severe performance degradation during
 failover.

Appendix B. Discussion of the Path-Bouncing Effect

 The methods described in the document can accelerate the failover
 process.  Hence, they might introduce a path-bouncing effect in which
 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 the path-bouncing effect (see [CARO04] and [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

Nishida, et al. Standards Track [Page 21] RFC 7829 SCTP-PF April 2016

 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 endpoint failure detection.  It is believed
 that neither of these effects are harmful, provided adequate dormant
 state operation is implemented.  Furthermore, it is believed that
 they may be particularly useful for applications that deploy multiple
 SCTP associations for load-balancing purposes.  The early
 notification of the PF state may be used for preventive measures as
 the entering of the PF state can be used as a warning of potential
 congestion.  Depending on the PMR value, the aggressive HEARTBEAT
 transmission in PF state may speed up the endpoint failure detection
 (exceed of AMR threshold on the sole path error counter) on idle
 associations in the case with a relatively large HB.Interval value
 compared to RTO (e.g., 30 seconds) is used.

Acknowledgments

 The authors would like to acknowledge members of the IETF Transport
 Area Working Group (tsvwg) for continuing discussions on this
 document and insightful feedback, and we appreciate continuous
 encouragement and suggestions from the Chairs of the tsvwg.  We
 especially wish to thank Michael Tuexen for his many invaluable
 comments and for his substantial supports with the making of the
 document.

Nishida, et al. Standards Track [Page 22] RFC 7829 SCTP-PF April 2016

Authors' Addresses

 Yoshifumi Nishida
 GE Global Research
 2623 Camino Ramon
 San Ramon, CA  94583
 United States
 Email: nishida@wide.ad.jp
 Preethi Natarajan
 Cisco Systems
 510 McCarthy Blvd.
 Milpitas, CA  95035
 United States
 Email: prenatar@cisco.com
 Armando Caro
 BBN Technologies
 10 Moulton St.
 Cambridge, MA  02138
 United States
 Email: acaro@bbn.com
 Paul D. Amer
 University of Delaware
 Computer Science Department - 434 Smith Hall
 Newark, DE  19716-2586
 United States
 Email: amer@udel.edu
 Karen E. E. Nielsen
 Ericsson
 Kistavaegen 25
 Stockholm  164 80
 Sweden
 Email: karen.nielsen@tieto.com

Nishida, et al. Standards Track [Page 23]

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