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
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Copyright Notice
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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. 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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