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Internet Engineering Task Force (IETF)                        Y. NishidaRequest for Comments: 7829                            GE Global ResearchCategory: Standards Track                                   P. NatarajanISSN: 2070-1721                                            Cisco Systems                                                                 A. Caro                                                        BBN Technologies                                                                 P. Amer                                                  University of Delaware                                                              K. Nielsen                                                                Ericsson                                                              April 2016SCTP-PF: A Quick Failover Algorithm for theStream Control Transmission ProtocolAbstract   The Stream Control Transmission Protocol (SCTP) supports multihoming.   However, when the failover operation specified inRFC 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 inRFC 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 inRFC 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 2016Status 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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 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 2016Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .32.  Conventions and Terminology . . . . . . . . . . . . . . . . .53.  SCTP with Potentially Failed (SCTP-PF) Destination State  . .53.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .53.2.  Specification of the SCTP-PF Procedures . . . . . . . . .64.  Dormant State Operation . . . . . . . . . . . . . . . . . . .104.1.  SCTP Dormant State Procedure  . . . . . . . . . . . . . .115.  Primary Path Switchover . . . . . . . . . . . . . . . . . . .116.  Suggested SCTP Protocol Parameter Values  . . . . . . . . . .137.  Socket API Considerations . . . . . . . . . . . . . . . . . .137.1.  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  . .168.  Security Considerations . . . . . . . . . . . . . . . . . . .169.  MIB Considerations  . . . . . . . . . . . . . . . . . . . . .1710. References  . . . . . . . . . . . . . . . . . . . . . . . . .1710.1.  Normative References . . . . . . . . . . . . . . . . . .1710.2.  Informative References . . . . . . . . . . . . . . . . .18Appendix A.  Discussion of Alternative Approaches . . . . . . . .20A.1.  Reduce PMR  . . . . . . . . . . . . . . . . . . . . . . .20A.2.  Adjust RTO-Related Parameters . . . . . . . . . . . . . .21Appendix B.  Discussion of the Path-Bouncing Effect . . . . . . .21Appendix C.  SCTP-PF for SCTP Single-Homed Operation  . . . . . .22   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .22   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .231.  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.  TheNishida, et al.              Standards Track                    [Page 3]

RFC 7829                         SCTP-PF                      April 2016   performance deficiencies of the failover operation described inRFC4960, 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 inAppendix 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 20162.  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 State3.1.  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 20163.2.  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 inSection 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 ofSection 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 inSection 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 stateNishida, 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 ofSection 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 inSection 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 inSection 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 thatNishida, 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 inSection 4.1 during dormant state.   An SCTP-PF implementation MAY choose a different dormant state   operation than the one described inSection 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 20164.1.  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 ofSection 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) - 07.  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 Sections3 and5.  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 20167.1.  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 20167.2.  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 SCTPNishida, 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 inSection 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 fromNishida, 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.  References10.1.  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 201610.2.  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 2016Appendix A.  Discussion of Alternative Approaches   This section lists alternative approaches for the issues described in   this document.  Although these approaches do not require updatingRFC4960, 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 inSection 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 byNishida, 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 2016Authors' 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.comNishida, et al.              Standards Track                   [Page 23]