Network Working Group                                         Y. Nishida
Internet-Draft                                        GE Global Research
Intended status: Experimental                               P. Natarajan
Expires: April 24, September 3, 2014                                 Cisco Systems
                                                                 A. Caro
                                                        BBN Technologies
                                                                 P. Amer
                                                  University of Delaware
                                                        October 21, 2013
                                                              K. Nielsen
                                                           March 2, 2014

                    Quick Failover Algorithm in SCTP


   One of the major advantages in of SCTP is supporting multi-homing multi-homed
   communication.  If a multi-homed end-point has a redundant network
   connections, the SCTP sessions can associations have a good chance to survive from
   network failures by migrating traffic from inactive network networks to
   active one. ones.  However, if we follow the SCTP standard, standard is followed, there can be
   a significant delay for during the network migration.  During this migration period, SCTP
   might not be able to transmit much data to the destination. peer.  This issue
   drastically impairs the usability of SCTP in some situations.  This
   memo describes the issue of the SCTP failover mechanism and discuss specifies
   an alternative failover procedure for SCTP that improves its
   solutions which
   performance during and after failover.  The procedures require only
   minimal modification modifications to the current standard. specification.

Status of this Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on April 24, September 3, 2014.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions and Terminology  . . . . . . . . . . . . . . . . .  4
   3.  Issue in  Issues with the SCTP Path Management Process . . . . . . . . . . . . .  5
   4.  Existing Solutions for Smooth Failover . . . . . . . . . . . .  6
     4.1.  Reduce Path.Max.Retrans  . . . (PMR)  . . . . . . . . . . . . . .  6
     4.2.  Adjust RTO related parameters  . . . . . . . . . . . . . .  7  6
   5.  Proposed Solution:  SCTP with Potentially-Failed Destination State (SCTP-PF) . . . . . . . . . . . . . . . . .  8
     5.1.  SCTP-PF Description  . . . . . . . . . . . . . . . . . . .  8
     5.2.  Effect of Path Bouncing  . . . . . . . . . . . . . . . . . 10
     5.3.  Permanent Failover . . . . . . . . . . . . . . . . . . . . 10
     5.4.  Handling of Association Error Counter  . . . .
   6.  Socket API Considerations  . . . . . . 11
   6.  Socket API Considerations . . . . . . . . . . . . 12
     6.1.  Support for the Potentially Failed Path State  . . . . . . 12
     6.2.  Peer Address Thresholds (SCTP_PEER_ADDR_THLDS) socket
           option Socket
           Option . . . . . . . . . . . . . . . . . . . . . . . . . . 12 13
     6.3.  Exposing the Potentially Failed Path State
           (SCTP_EXPOSE_POTENTIALLY_FAILED_STATE) Socket Option . . . 14
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13 15
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14 16
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 15 17
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 15 17
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 15 17
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17 19

1.  Introduction

   The Stream Control Transmission Protocol (SCTP) as specified in
   [RFC4960] natively supports multihoming at the transport layer -- an SCTP
   association can bind to multiple IP addresses at each endpoint.
   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 timeouts failed
   retransmissions on a destination before detecting a path failure.
   The sender fails over to an alternate active destination only after
   failure detection.  Until detecting the failover, the sender transmits
   continues to transmit data on the failed path, degrading which degrades the
   SCTP performance.  Concurrent Multipath Transfer (CMT) [IYENGAR06] is
   an extension to SCTP and allows the sender to transmit data on
   multiple paths simultaneously.  Research [NATARAJAN09] shows that the
   current failure detection procedure worsens CMT performance during
   failover and can be significantly improved by employing a better
   failover algorithm.

   This document proposes specifies an alternative failure detection procedure
   for SCTP (and CMT) that improves the SCTP (CMT) (and CMT) performance
   during a failover.

   Also the operation after a failover impacts the performance of the
   protocol.  With [RFC4960] procedures, 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.  From a performance
   perspective, as confirmed in research [CARO02], such a switchback of
   the data transmission path is not optimal in general.  As an
   alternative option to the switchback operation of [RFC4960], this
   document specifies the support the Permanent Failover switchover
   procedures proposed by [CARO02].

2.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Issue in  Issues with the SCTP Path Management Process

   SCTP can utilize multiple IP addresses for a single SCTP association.
   Each SCTP endpoint exchanges the list of available its usable addresses on the
   node during
   initial negotiation.  After this, negotiation with its peer.  Then the endpoints select one
   address from the peer's list and define this as the primary
   destination.  During normal transmission, SCTP sends all user data to
   the primary destination.  Also, it sends heartbeat packets to other (non-primary) all
   idle destinations at a certain interval to check the reachability of
   the path.  Idle destinations normally include all non-primary

   If a sender has multiple active destination addresses, it can
   retransmit data to secondary destination address address, when the
   transmission to the primary times out.

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

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


   One issue in with this failover process is that it usually takes
   significant amount of time before SCTP switches to the new
   destination.  Let's say the primary path on a multi-homed host
   becomes unavailable and the RTO value for the primary path at that
   time is around 1 second, it usually takes over 60 seconds before SCTP
   starts to use the secondary path.  This is because the recommended
   value for Path.Max.Retrans in the standard is 5, which requires 6
   consecutive timeouts before failover takes place.  Before SCTP
   switches to the secondary address, SCTP keeps trying to send packets
   to the primary and only retransmitted packets are sent to the
   secondary can be reached at the receiver.  This slow failover process
   can cause significant performance degradation and will not be
   acceptable in some situations.


   Another issue is that once the primary path is active again, the
   traffic is switched back.  This is not optimal in general.

4.  Existing Solutions for Smooth Failover

   The following approach approaches are conceivable for the solutions of this

4.1.  Reduce Path.Max.Retrans

   If we choose smaller value (PMR)

   Smaller values for Path.Max.Retrans, we can Path.Max.Retrans shorten the
   duration of failover process. duration.
   In fact, this is recommended in some research results [JUNGMAIER02]
   [GRINNEMO04] [FALLON08].  For example, if we set Path.Max.Retrans to 0, when Path.Max.Retrans=0,
   SCTP switches to another destination on a single timeout.  However,
   smaller value for Path.Max.Retrans
   might cause also results in spurious failover.
   In addition, if we use smaller value
   for Path.Max.Retrans, we may Path.Max.Retrans values also need to choose smaller value for
   'Association.Max.Retrans'.  The Association.Max.Retrans indicates the
   threshold for affect
   'Association.Max.Retrans' values.  When the total number of consecutive SCTP association's error
   count for the
   entire SCTP association.  If the total (sum of the error count for counts on all
   paths ACTIVE paths) exceeds this value,
   Association.Max.Retrans threshold, the endpoint SCTP sender considers the peer
   endpoint unreachable and terminates the association.  According to the  Therefore,
   Section 8.2 in [RFC4960], we should avoid having the value of [RFC4960] recommends that Association.Max.Retrans
   value should not be larger than the summation of the Path.Max.Retrans
   of all the destination addresses.  Otherwise, even
   if all each of the destination addresses become inactive, addresses, else the endpoint still SCTP sender considers the
   its peer endpoint reachable.  The behavior in this
   situation is not defined in the RFC and depends on each
   implementation.  In order to reachable even when all destinations are INACTIVE.  To avoid
   such inconsistent behavior between
   implementations, we had better use smaller value for
   Association.Max.Retrans. an SCTP implementation SHOULD reduce
   Association.Max.Retrans accordingly whenever it reduces
   Path.Max.Retrans.  However, if we choose smaller Association.Max.Retrans value for
   Association.Max.Retrans, associations will prone to be terminated
   increases chances of association termination during minor congestion. congestion

   Another issue is that the interval of heartbeat packet: 'HB.interval'
   may not
   could be small. in the order of seconds (recommended value is 30 seconds) This means once
   failover takes place, an endpoint might need a certain amount of time
   to use seconds).
   When the primary path again.  This becomes inactive, the next HB can cause undesirable effects in
   case of spurious failover.  If we choose smaller value for
   HB.interval, be
   transmitted only seconds later.  Meanwhile, the traffic used for primary path probing in a session will be

   The advantage of tuning Path.Max.Retrans may have
   recovered.  In such situations, post failover, an endpoint is that it requires no
   modification forced
   to wait on the current standard, although it needs order of seconds before the endpoint can resume
   transmission on the primary path.

   The advantage of tuning Path.Max.Retrans is that it requires no
   modification to ignore the current standard.  However, as we discuss above
   tuning Path.Max.Retrans ignores several recommendations. recommendations in [RFC4960].
   In addition, some research results indicate path bouncing caused by
   spurious failover does not cause serious problems.  We discuss the
   effect of path bouncing in the section 5. Section 5.2.

4.2.  Adjust RTO related parameters

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

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

5.  Proposed Solution:  SCTP with Potentially-Failed Destination State (SCTP-PF)

5.1.  SCTP-PF Description

   Our proposal

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

   o  In order to minimize performance impact during failover, the
      sender should avoid transmitting data to the failed destination as
      early as possible.  In the current SCTP path management scheme,
      the sender stops transmitting data to a destination only after the
      destination is marked Failed.  Thus, a smaller PMR value is ideal
      so that the sender transitions a destination to the Failed state

   o  Smaller PMR values increase the chances of spurious failure
      detection where the sender incorrectly marks a destination as
      Failed during periods of temporary congestion.  Larger PMR values
      are preferable to avoid spurious failure detection.

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

   We propose

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

   1.  The sender maintains a new tunable parameter called Potentially-
       failed.Max.Retrans (PFMR).  The recommended value of PFMR = 0
       when quick failover is used.  When an association's PFMR >= is larger or equal to
       PMR, quick failover is turned off.

   2.  Each time the T3-rtx timer expires on an active or idle destination, the
       error counter of that destination address will be incremented.

       When the value in the error counter exceeds PFMR, the endpoint
       should mark the destination transport address as PF.  SCTP MUST NOT send any notification to the upper layer
       about the Active to PF state transition.

   3.  The sender SHOULD avoid data transmission to PF destinations.
       When all destinations are in either PF or Inactive state, the
       sender MAY either move the destination from PF to Active state
       (and transmit data to the active destination) or the sender MAY
       transmit data to a PF destination.  In the former scenario, (i)
       the sender MUST NOT notify the ULP about the state transition,
       and (ii) MUST NOT clear the destination's error counter.  It is
       recommended that the sender picks the PF destination with least
       error count (fewest consecutive timeouts) for data transmission.
       In case of a tie (multiple PF destinations with same error
       count), the sender MAY choose the last active destination.

   4.  Only heartbeats MUST be sent to PF destination(s) once per RTO.
       This means the sender SHOULD ignore HB.interval for PF
       destinations.  If an heartbeat is unanswered, the sender
       increments the error counter and exponentially backs off the RTO
       value.  If error counter is less than PMR, the sender SHOULD
       transmit another heartbeat immediately after T3-timer expiration.

   5.  When the sender receives an heartbeat ACK from a PF destination,
       the sender clears the destination's error counter and transitions
       the PF destination back to Active state.  This state transition
       MUST NOT be notified to the ULP.  This destination's cwnd is set
       to 1 MTU.  Note that in scenarios where the destination was
       temporarily congested during the T3-timer expiration, an SCTP
       sender transmits 1 MTU worth of data while an SCTP-PF  The sender
       transmits an HB after the T3-timer expiry (more details should
       perform slow-start as specified in Section 5 7.2.1 of [NATARAJAN09]).  The SCTP sender has 1 RTT head-
       start in cwnd evolution compared to SCTP-PF sender.  An SCTP-PF
       sender may set cwnd to 2 MTUs after receiving HB-ACK in order to
       offset [RFC4960]
       when it sends data on this performance difference. destination.

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

   7.  When all destinations are in the Inactive state, the sender picks
       one of the Inactive destinations for data transmission.  This
       proposal recommends that the sender picks the Inactive
       destination with least error count (fewest consecutive timeouts)
       for data transmission.  In case of a tie (multiple Inactive
       destinations with same error count), the sender MAY choose the
       last active destination.

   8.  ACKs for retransmissions do not transition a PF destination back
       to Active state, since a sender cannot disambiguate whether the
       ack was for the original transmission or the retransmission(s).

   9.  SCTP shall provide the means to expose the PF state of its
       destinations as well as SCTP SHOULD notify the ULP of the state
       transitions from Active to PF and from PF to Active state.  SCTP
       can provide the means to suppress exposure of PF state and
       association state transitions and in this case the ULP MAY make
       SCTP suppress exposure of PF state to ULP.  In this case the ULP
       will rely solely on the [RFC4960] state machine even if quick
       failover function is activated in SCTP.

5.2.  Effect of Path Bouncing

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

   There are two main reasons for this.  First, SCTP is basically
   designed for multipath communication, which means SCTP maintains all
   path related parameters (cwnd, (CWND, ssthresh, RTT, error count, etc) per
   each destination address.  These parameters cannot be affected by
   path bouncing.  In addition, when SCTP migrates the data transfer to
   another path, it starts with the minimal cwnd because of slow-start. 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 proposed method does not make situations worse.
   In case of congestion, the current standard tries to transmit data
   packets to the primary during failover, while the proposed method
   tries to explore other destinations.  In any case, the same amount of
   data packets sent to the same bottleneck. quick failover results in a behavior already
   allowed by [RFC4960].

5.3.  Permanent Failover

   Post failover, failover then, by [RFC4960] behavior, an SCTP sender migrates
   the traffic back to the original primary destination once this
   destination becomes active.  The sender sets
   cwnd active anew.  As the CWND towards the original
   primary destination has to be rebuilt once data transfer resumes, the initial cwnd value and performs slow start.
   switch back to use the original primary path is not always optimal.
   Indeed [CARO02] shows that the switch over to the original primary
   may degrade SCTP performance compared to continuing data transmission
   on the same path, especially especially, but not only, in scenarios where this
   path's characteristics are better.  In order to mitigate this
   performance degradation, permanent
   failover Permanent Failover operation was proposed in
   [CARO02].  When SCTP changes the destination due to failover,
   Permanent failover Failover marks it as new primary.  This means Permanent
   Failover allows SCTP sender to continue data transmission to
   remain the alternative path
   even if after the old primary path destination becomes active again.  We recommend that SCTP-PF should stick to  This is
   achieved by having SCTP perform a switchover of the standard
   RFC4960 behavior, i.e., primary path to
   an alternative working path rather than having SCTP switch back data
   transfer to the original (previous) primary once this
   destination becomes active again.  Permanent failover may be
   considered path.

   The manner of switchover operation that is most optimal in the future based a given
   scenario depends on discussions and consensus within the community.

5.4.  Handling of Association Error Counter

   When multiple destinations are in relative quality of a set primary path versus
   the PF state, quality of alternative paths available as well as it depends on
   the sender may
   transmit heartbeats extent to multiple destinations at which it is desired for the same time.  This
   allows SCTP-PF sender to quickly track and respond mode of operation to
   enforce traffic distribution over a number of network status
   change compared to an paths.  I.e.,
   load distribution of traffic from multiple SCTP sender.  However, when all PF destinations
   become unavailable, an SCTP-PF sender has outstanding HBs on all
   destinations compared associations may be
   sought to an SCTP sender and increases the count for
   the total number be enforced by distribution of consecutive retransmissions faster than the SCTP
   sender.  SCTP-PF's faster increase set primary paths with
   [RFC4960] switchback operation.  However as [RFC4960] switchback
   behavior is suboptimal in the error count will result certain situations, especially in
   association termination sooner than SCTP.  The key difference between scenarios
   where a number of equally good paths are available, it is recommended
   for SCTP and SCTP-PF with regard to this feature is whether checking path
   status sequentially or concurrently support also, as alternative behavior, the number Permanent
   Failover modes of packets sent for
   probing is the same.

   For deployments operation where aggressive failure detection and association
   termination forced switch back to a previously
   failed primary path is not desired, we suggest that AMR be always performed.  The Permanent Failover
   operation requires only sender side changes.  Details, as originally
   outlined in [CARO02], are:

   1.  The sender maintains a new tunable parameter, called
       Primary.Switchover.Max.Retrans (PSMR).  When the path error
       counter on a set to primary path exceeds PSMR, the SCTP
       implementation autonomously selects and sets a new primary path.

   2.  The primary path selected by the SCTP implementation shall be the
       path which at the given time would be chosen for data transfer.
       A previously failed primary path may come in use as data transfer
       path as per normal path selection when the present data transfer
       path fails.

   3.  The recommended maximum value (sum of PMRs of all paths), to delay assoc
   termination during SCTP-PF.  Another option PSMR is PFMR when Permanent failover is
       used.  This means that no forced switchback to send retransmitted
   data or HB to only one PF destination at a time, but this approach
   may delay previously
       failed primary path status tracking.  To exclude HB timeouts from
   incrementing the error count can also is performed.

   4.  It must be a solution, however, this
   requires an update possible to Section 8.1 disable the Permanent Failover and Section 8.3 obtain
       the standard switchback operation of [RFC4960],
   otherwise special logics for error counter need [RFC4960].

   We recommend that SCTP-PF should stick to be implemented for
   SCTP-PF. the standard RFC4960
   behavior as default, i.e., switch back to the old primary destination
   once the destination becomes active again.  However, implementors MAY
   implement Permanent Failover and MAY enable it based on network
   configurations or users' requests.

6.  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 quick failover behavior.

   Please note that this section is informational only.

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

   Furthermore, two new read/write socket option options for the level
   IPPROTO_SCTP and the
   name SCTP_PEER_ADDR_THLDS as described below. name SCTP_PEER_ADDR_THLDS and
   SCTP_EXPOSE_POTENTIALLY_FAILED_STATE are defined as described below.
   The first socket option is used to control the values of the PFMR and
   PSMR parameters described in Section 5.  The second one controls the
   exposition of the potentially failed path state.

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

6.1.  Support for the Potentially Failed Path State

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

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

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

   The SCTP_GET_PEER_ADDR_INFO socket option is defined in [RFC6458] can be
   used to read/write query the value state of PFMR parameter described in Section

   Support for a peer address.  It uses the SCTP_PEER_ADDR_THLDS socket option needs also following

   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
   added provided in the spinfo_state field.  This
   document defines in addition to that the function sctp_opt_info().

6.1. new constant
   SCTP_POTENTIALLY_FAILED, which is reported if the peer address is
   potentially failed.

6.2.  Peer Address Thresholds (SCTP_PEER_ADDR_THLDS) socket option Socket Option

   Applications can control the quick failover behavior by getting or
   setting the number of consecutive timeouts before a peer address is
   considered potentially failed or unreachable. unreachable and before the primary
   path is changed automatically.  This socket option uses the level

   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 for this query. SCTP_FUTURE_ASSOC.  It is an error
      to use SCTP_{CURRENT|ALL}_ASSOC in spt_assoc_id.

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

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

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

   spt_pathcpthld:  Each peer address of interest is not considered the
      primary remote address anymore, if its path error counter exceeds
      spt_pathcpthld.  Using a value of 0xffff disables the selection of
      a new primary peer address.  If an implementation does not support
      the automatically selection of a new primary address, it should
      indicate an error with errno set to EINVAL if a value different
      from 0xffff is used in spt_pathcpthld.

6.3.  Exposing the Potentially Failed Path State

   Applications can control the exposure of the potentially failed path
   state in the SCTP_PEER_ADDR_CHANGE event and the
   SCTP_GET_PEER_ADDR_INFO as described in Section 6.1.  The default
   value is implementation specific.

   This socket option uses the level IPPROTO_SCTP and the name

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

   struct sctp_assoc_value {
     sctp_assoc_t assoc_id;
     uint32_t assoc_value;

   assoc_id:  This parameter is ignored for one-to-one style sockets.
      For one-to-many style sockets the application may fill in an
      association identifier or SCTP_FUTURE_ASSOC.  It is an error to
      use SCTP_{CURRENT|ALL}_ASSOC in assoc_id.

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

7.  Security Considerations

   There are no new security considerations introduced in this document.

8.  IANA Considerations

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

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol",
              RFC 4960, September 2007.

9.2.  Informative References

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

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

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

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

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

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

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

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

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

Authors' Addresses

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


   Preethi Natarajan
   Cisco Systems
   510 McCarthy Blvd
   Milpitas, CA  95035


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


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


   Karen E. E. Nielsen
   Kistavaegen 25
   Stockholm,   164 80