Network Working Group                                         Y. Nishida
Internet-Draft                                              WIDE Project
Intended status: Standards Track                            P. Natarajan
Expires: June 13, 2011                                     Cisco Systems
                                                       December 10, 2010

                    Quick Failover Algorithm in SCTP


   One of the major advantages in SCTP is supporting multi-homing
   communication.  If an 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 June 13, 2011.

Copyright Notice

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of

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   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions and Terminology  . . . . . . . . . . . . . . . . .  4
   3.  Issue in SCTP Path Management Process  . . . . . . . . . . . .  5
   4.  Solutions for Smooth Failover  . . . . . . . . . . . . . . . .  6
     4.1.  Reduce Path.Max.Retrans  . . . . . . . . . . . . . . . . .  6
     4.2.  Adjust RTO related parameters  . . . . . . . . . . . . . .  7
     4.3.  Introducing Potentially-failed Destination State in
           Failure Detection Algorithm  . . . . . . . . . . . . . . .  7
   5.  Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     5.1.  Effect of Path Bouncing  . . . . . . . . . . . . . . . . . 10
     5.2.  Permanent Failover . . . . . . . . . . . . . . . . . . . . 10
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15

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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 proceudre, 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.

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2.  Conventions and Terminology

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

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3.  Issue in SCTP Path Management Process

   SCTP can utilize multiple IP addresses for 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 destination of the
   primary path.  Basically, SCTP sends all data through this primary
   path for normal data transmissions.  Also, it sends heartbeat packets
   to other (non-primary) destinations at a certain interval to check
   the reachability of the path.

   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 primacy 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.

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4.  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.

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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.

4.3.  Introducing Potentially-failed Destination State in Failure
      Detection Algorithm

   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 is an intermediate
   state between Active and Failed states and was originally proposed to
   improve CMT performance [NATARAJAN09].  SCTP's failure detection

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   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:

   o  The sender maintains a new tunable parameter called Potentially-
      failed.Max.Retrans (PFMR).  An association's PFMR value MUST be
      lower than the association's PMR value.  The recommended value of
      PFMR = 0.

   o  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.

   o  The sender never transmits data to a PF destination.  However,
      when all destinations are in either PF or Inactive state, the
      sender SHOULD transition a destination marked PF to the active
      state and transmit data to this destination.  The destination's
      error counter MUST NOT be cleared during this state transition.
      It is recommended that the sender transitions the PF destination
      with least error count (fewest consecutive timeouts) to the active
      state.  In case of a tie (multiple PF destinations with same error
      count), the sender MAY choose the last active destination.

   o  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.
      An implementation MAY use protocol parameter 'PFHB.interval' for
      the interval of heartbeat transmissions.  If PFHB.interval is non-
      zero, a heartbeat packet is sent once per RTO of each destination
      address plus PFHB.interval with jittering of +/- 50% of the RTO
      value.  Use of PFHB.interval can reduce the frequency of failover,
      which might be useful where the characteristic of the paths are
      mostly equal.

   o  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

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      MUST NOT be notified to the ULP unless it is explicitly requested.
      This destination's cwnd is set to 1 MTU (TODO: or 2?  Needs more
      text discussing rationale; can revisit later?)

   o  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 notifiy 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].

   o  When all destinations are in the Inactive state, the sender
      transitions one of the destinations back to the Active state and
      continues data transmission to this destination.  This proposal
      recommends that the sender transitions the Inactive destination
      with least error count (fewest consecutive timeouts) to the active
      state.  In case of a tie (multiple Inactive destinations with same
      error count), the sender MAY choose the last active destination.

   o  Acks for retransmissions do not transition a PF destination back
      to the active state, since a sender cannot disambiguate whether
      the ack was for the original transmission or the

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5.  Discussion

5.1.  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.

5.2.  Permanent Failover

   When primary path becomes active again after failover, SCTP migrates
   back to the primary path.  After this, SCTP starts data transfer with
   minimal cwnd.  This is because SCTP must perform slow-start when it
   migrates to new path.  However, this might degrade the communication
   performance in case that the performance of the alternative path is
   relatively good.  In order to mitigate this effect of slow-start,
   permanent failover was proposed in [CARO02].  Permanent failover
   allows SCTP to remain the alternative path even if the primacy path
   becomes active again.  This approach can improve performance in some
   cases, however, it will require more detail analysis since it might
   impact on SCTP failover algorithm.  Since we prefer to keep the
   current behavior of the standard as possible, we recommend not to
   take this approach for now.

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6.  Security Considerations

   There are no new security considerations introduced in this document.

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7.  IANA Considerations

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

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8.  References

8.1.  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.

8.2.  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, "Peformance 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 scenrarios", World Multiconference on
              Systemics, Cybernetics and Informatics , 7 2002.

              Natarajan, P., Ekiz, N., Amer, P., and R. Stewart,

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              "Concurrent Multipath Transfer during Path Failure",
              Computer Communications , 5 2009.

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Authors' Addresses

   Yoshifumi Nishida
   WIDE Project
   Endo 5322
   Fujisawa, Kanagawa  252-8520


   Preethi Natarajan
   Cisco Systems
   425 E. Tasman Drive
   San Jose, CA  95134


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