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Versions: 00 01 02 03 04 05 draft-ietf-tsvwg-sctp-failover

Network Working Group                                         Y. Nishida
Internet-Draft                                              WIDE Project
Intended status: Standards Track                               Natarajan
Expires: August 28, 2010                                   Cisco Systems
                                                       February 24, 2010


                    Quick Failover Algorithm in SCTP
                  draft-nishida-tsvwg-sctp-failover-00

Abstract

   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

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on August 28, 2010.

Copyright Notice




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   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the BSD License.


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.  Introduce Potential Failure Status in Failure
           Detection Algorithm  . . . . . . . . . . . . . . . . . . .  7
   5.  Discussion . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     5.1.  Effect of Path Bouncing  . . . . . . . . . . . . . . . . .  9
     5.2.  Permanent Failover . . . . . . . . . . . . . . . . . . . .  9
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   8.  Normative References . . . . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13




















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1.  Introduction

   Multihoming support is one of the major advantage of SCTP which is
   not supported in other transport protocols such as TCP or UDP.  If an
   multi-homed end-point has redundant network interfaces, SCTP sessions
   can survive from the network failures by migrating inactive path to
   active one.  This feature can be expected to be a driving force for
   deploying SCTP, however, because of minor issues in the SCTP
   specification, most of SCTP sessions will have significant delay to
   failover and will cause significant performance degradation during
   the failover process.  We believe this issue is impairing the
   usability of SCTP and it is important to address it to make SCTP more
   efficient and attractive.

   In this memo, we describe the issue of SCTP failover process and
   discuss the solutions.  Our main focus is to propose a solution that
   does not require major modification to the current standard.  Using
   Concurrent Multipath Transfer (CMT) [IYENGAR06] allows SCTP to
   utilize multiple paths simultaneously for data transmission.  While
   CMT can reduce the impact of path failures, CMT is not yet a
   standard.  In addition, some may not want concurrent data transfer
   feature, but want to use smooth failover feature in SCTP.  From this
   reason, we believe the proposals in this document can be useful and
   meaningful.



























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2.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   Since this document describes a potential risk in NewReno, it uses
   the same terminology and definitions in RFC4690.  [RFC4690].











































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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
   issue.

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
   increased.

   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.  Introduce Potential Failure Status in Failure Detection Algorithm

   As seen above, one difficulty of tuning Path.Max.Retrans is that it
   is required to meet the following two inconsistent requirements.

   o  In order to respond network failure quickly, we need to mark a
      path as inactive as soon as we detect failure.

   o  In order to make an association persistent and robust against
      network failure, we need to be conservative to mark a path as
      inactive.

   To satisfy these requirements, we propose to introduce "Potentially-
   failed" (PF) destination state in failure detection algorithm in
   SCTP.  PF state is the intermediate state between Active and
   Inactive.  It indicates that the path is possibly inactive, but not
   confirmed yet.  By using the PF state, SCTP can respond to network
   failures quickly, while preserving a conservative policy of marking
   path as inactive.  The idea of using PF state was originally proposed
   in [NATARAJAN08] for CMT.

   In this algorithm, 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, SCTP endpoint increment the error count for the path
   and transitions the destination to the PF state. (we might need to
   have new threshold value for error counter to be conservative to
   migrate from Active to PF.  But, we choose this way for now)

   If the primary path is marked PF, SCTP chooses new destination
   address from one of the active destinations and starts using this



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   address to send data.  SCTP endpoints should not send any data packet
   to destinations in the PF state, however, it can send heartbeat
   packets at a certain interval.  To allow quick recover from the PF
   state, we also propose to introduce a new protocol parameter
   'PFHB.Interval'.  PFHB.interval is used to determine the interval of
   heartbeat packets.  It is recommended that a heartbeat packet is sent
   once per RTO of each destination address plus PFHB.interval with
   jittering of +/- 50% of the RTO value.  (Preethi: wondering why we
   need jittering?)  It is also recommended to use relatively smaller
   value than HB.interval for PFHB.interval.

   If the heartbeat is answered, SCTP marks the path Active again.  If
   unanswered, SCTP increments the error count and use an exponential
   backoff algorithm to increase the RTO.  If the error count exceeds
   Path.Max.Retrans, the path is marked as Inactive.  If all
   destinations are marked PF, SCTP endpoint can choose one destination
   to send data to its peer.  How SCTP chooses a path is implementation
   specific.  One possibility is to select the destination with the
   least error count.  Once a PF destination is chosen for data
   transmission, the chosen destination must be transitioned from PF to
   the Active state.  Except the use of PFHB.interval, other rules of
   sending heartbeats are completely the same as those of the standard.

   The advantage of this approach is that we can keep the same values
   for Path.Max.Retrans, Association.Max.Retrans and HB.interval used in
   the current implementations, while it can respond network failure
   quickly.  In addition, new transmission algorithm becomes effective
   only when the path is in the PF state.  When the primary path is in
   Active or Inactive, the behavior is completely the same as that of
   the current standard.  When the failure detection threshold is most
   aggressive (PMR=0), both SCTP and SCTP-PF detect path failure after
   the first timeout.  Specifically, SCTP-PF's failure detection does
   not involve the PF state transition and is equivalent to SCTP's
   failure detection procedure.  In other words, when PMR=0, both SCTP
   and SCTP-PF perform similarly during path failure.  As PMR increases,
   SCTP's failure detection takes longer and the performance difference
   between SCTP and SCTP-PF widens (SCTP-PF performs better)."














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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.  Normative 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.

   [FALLON08]
              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.

   [GRINNEMO04]
              Grinnemo, K-J. and A. Brunstrom, "Peformance of SCTP-
              controlled failovers in M3UA-based SIGTRAN networks",
              Advanced Simulation Technologies Conference , 4 2004.

   [IYENGAR06]
              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.

   [JUNGMAIER02]
              Jungmaier, A., Rathgeb, E., and M. Tuexen, "On the use of
              SCTP in failover scenrarios", World Multiconference on
              Systemics, Cybernetics and Informatics , 7 2002.

   [NATARAJAN08]
              Natarajan, P., Ekiz, N., Iyengar, J., Amer, P., and R.
              Stewart, "Concurrent Multipath Transfer using SCTP
              Multihoming: Introducing Potentially-failed Destination
              State", IFIP Networking , 5 2008.

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

   [RFC4690]  Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and
              Recommendations for Internationalized Domain Names
              (IDNs)", RFC 4690, September 2006.



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Authors' Addresses

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

   Email: nishida@wide.ad.jp


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

   Email: prenatar@cisco.com

































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