Network Working Group Y. Nishida
Internet-Draft WIDE Project
Intended status: Standards Track December 13, 2009
Expires: June 16, 2010
Quick Failover Algorithm in SCTP
draft-nishida-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
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Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
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document authors. All rights reserved.
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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. 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
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 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 connections, 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 describes 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] which allows SCTP
utilize multiple path simultaneously for data transmission can be an
another approach to solve this issue. CMT with sophisticated multi-
homing communication control may bring the ideal solution, however,
it might require to add a lot of additional functions to the current
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 non-primary 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 30 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 5
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 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 which might cause bouncing paths. 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 turning 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. 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 "Potential
Failure" state in failure detection algorithm in SCTP. Potential
Failure state is the intermediate state between Active and Inactive.
It indicates that the path is possibly inactive, but not confirmed
yet. By using this state, SCTP can respond network failure quickly,
while it can preserve a conservative policy of marking path as
inactive. The idea of using Potential Failure state is 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 marks the path as Potential Failure. (we might need to have new
threshold value for error counter to be conservative to migrate from
Active to Potential Failure. But, we choose this way for now)
If the primary path is marked Potential Failure, SCTP chooses new
destination address from one of the active destinations and start
using this address to send data. SCTP endpoints should not send any
data packet to paths in Potential Failure state, however, it can send
heartbeat packets at a certain interval. To allow quick recover from
Potential Failure 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. 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 paths
become Potential Failure state, SCTP endpoint should not send any
data to its peer, while it can send heartbeat packets. Except the
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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 Potential Failure state. When the primary
path is in Active or Inactive, the behavior is completely the same as
that of the current standard. Hence, the influences of the algorithm
will be limited.
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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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Author's Address
Yoshifumi Nishida
WIDE Project
Endo 5322
Fujisawa, Kanagawa 252-8520
Japan
Email: nishida@wide.ad.jp
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