Network Working Group                                         Y. Nishida
Internet-Draft                                        GE Global Research
Intended status: Experimental                               P. Natarajan
Expires: December 20, 2013 April 24, 2014                                    Cisco Systems
                                                                 A. Caro
                                                        BBN Technologies
                                                                 P. Amer
                                                  University of Delaware
                                                           June 18,
                                                        October 21, 2013

                    Quick Failover Algorithm in SCTP


   One of the major advantages in SCTP is supporting multi-homing
   communication.  If a multi-homed end-point has redundant network
   connections, SCTP sessions can have a good chance to survive from
   network failures by migrating inactive network to active one.
   However, if we follow the SCTP standard, there can be significant
   delay for the network migration.  During this migration period, SCTP
   cannot transmit much data to the destination.  This issue drastically
   impairs the usability of SCTP in some situations.  This memo
   describes the issue of SCTP failover mechanism and discuss its
   solutions which require minimal modification to the current standard.

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 December 20, 2013. April 24, 2014.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions and Terminology  . . . . . . . . . . . . . . . . .  4
   3.  Issue in SCTP Path Management Process  . . . . . . . . . . . .  5
   4.  Existing Solutions for Smooth Failover . . . . . . . . . . . .  6
     4.1.  Reduce Path.Max.Retrans  . . . . . . . . . . . . . . . . .  6
     4.2.  Adjust RTO related parameters  . . . . . . . . . . . . . .  7
   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  . . . . . . . . . . . . . . . . . . 11
   6.  Socket API Considerations  . . . . . . . . . . . . . . . . . . 12
     6.1.  Peer Address Thresholds (SCTP_PEER_ADDR_THLDS) socket
           option . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 15
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17

1.  Introduction

   The Stream Control Transmission Protocol (SCTP) [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 on a
   destination before detecting path failure.  The sender fails over to
   an alternate active destination only after failure detection.  Until
   failover, the sender transmits data on the failed path, degrading
   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 an alternative failure detection procedure for
   SCTP (and CMT) that improves SCTP (CMT) performance during failover.

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 SCTP Path Management Process

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

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

   When sender receives the acknowledgment for data or heartbeat packets
   from one of the destination addresses, it considers the destination
   is 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 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 new destination address from one of the active destinations
   and start using this address to send data.  If the primary path
   becomes active again, SCTP uses the primary destination for
   subsequent data transmissions and stop using non-primary one.

   An issue in 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.

4.  Existing Solutions for Smooth Failover

   The following approach are conceivable for the solutions of this

4.1.  Reduce Path.Max.Retrans

   If we choose smaller value for Path.Max.Retrans, we can shorten the
   duration of failover process.  In fact, this is recommended in some
   research results [JUNGMAIER02] [GRINNEMO04] [FALLON08].  For example,
   if we set Path.Max.Retrans to 0, SCTP switches to another destination
   on a single timeout.  However, smaller value for Path.Max.Retrans
   might cause spurious failover.  In addition, if we use smaller value
   for Path.Max.Retrans, we may also need to choose smaller value for
   'Association.Max.Retrans'.  The Association.Max.Retrans indicates the
   threshold for the total number of consecutive error count for the
   entire SCTP association.  If the total of the error count for all
   paths exceeds this value, the endpoint considers the peer endpoint
   unreachable and terminates the association.  According to the Section
   8.2 in [RFC4960], we should avoid having the value of
   Association.Max.Retrans larger than the summation of the
   Path.Max.Retrans of all the destination addresses.  Otherwise, even
   if all the destination addresses become inactive, the endpoint still
   considers the peer endpoint reachable.  The behavior in this
   situation is not defined in the RFC and depends on each
   implementation.  In order to avoid inconsistent behavior between
   implementations, we had better use smaller value for
   Association.Max.Retrans.  However, if we choose smaller value for
   Association.Max.Retrans, associations will prone to be terminated
   with minor congestion.

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

   The advantage of tuning Path.Max.Retrans is that it requires no
   modification to the current standard, although it needs to ignore
   several recommendations.  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.

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, although
   it needs to ignore several recommendations.  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

5.1.  SCTP-PF Description

   Our proposal 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 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.  After
   a single timeout on a path, a 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 >= 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 sender
       transmits an HB after the T3-timer expiry (more details in
       Section 5 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 this performance difference.

   6.  An 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).

5.2.  Effect of Path Bouncing

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

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

   Second, even if all communication paths between 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.

5.3.  Permanent Failover

   Post failover, an SCTP sender migrates back to the original primary
   destination once this destination becomes active.  The sender sets
   cwnd to the initial cwnd value and performs slow start.  [CARO02]
   shows that the switch over to the original primary may degrade SCTP
   performance compared to continuing data transmission on the same
   path, especially in scenarios where this path's characteristics are
   better.  In order to mitigate this performance degradation, permanent
   failover was proposed in [CARO02].  Permanent failover allows SCTP to
   remain the alternative path even if the primary path becomes active
   again.  We recommend that SCTP-PF should stick to the standard
   RFC4960 behavior, i.e., switch back to the original primary once this
   destination becomes active again.  Permanent failover may be
   considered in the future based on discussions and consensus within
   the community.

5.4.  Handling of Association Error Counter

   When multiple destinations are in the PF state, the sender may
   transmit heartbeats to multiple destinations at the same time.  This
   allows SCTP-PF sender to quickly track and respond to network status
   change compared to an SCTP sender.  However, when all PF destinations
   become unavailable, an SCTP-PF sender has outstanding HBs on all
   destinations compared to an SCTP sender and increases the count for
   the total number of consecutive retransmissions faster than the SCTP
   sender.  SCTP-PF's faster increase in the error count will result in
   association termination sooner than SCTP.  The key difference between
   SCTP and SCTP-PF with regard to this feature is whether checking path
   status sequentially or concurrently as the number of packets sent for
   probing is the same.

   For deployments where aggressive failure detection and association
   termination is not desired, we recommend suggest that AMR be set to the
   recommended maximum allowed value (sum of PMRs of all paths), to delay assoc
   termination during SCTP-PF.  Another option is to send retransmitted
   data or HB to only one PF destination at a time, but this approach
   may delay path status tracking.  To exclude HB timeouts from
   incrementing the error count can also be a solution, however, this
   requires an update to Section 8.1 and Section 8.3 of [RFC4960]. [RFC4960],
   otherwise special logics for error counter need to be implemented for

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 the quick
   failover behavior.

   Please note that this section is informational only.

   A socket API implementation based on [RFC6458] is extended by adding
   a new read/write socket option for the level IPPROTO_SCTP and the
   name SCTP_PEER_ADDR_THLDS as described below.  This socket option is
   used to read/write the value of PFMR parameter described in Section

   Support for the SCTP_PEER_ADDR_THLDS socket option needs also to be
   added to the function sctp_opt_info().

6.1.  Peer Address Thresholds (SCTP_PEER_ADDR_THLDS) socket option

   Applications can control the quick failover behavior by getting or
   setting the number of timeouts before a peer address is considered
   potentially failed or unreachable.

   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;

   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.
      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

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