Network Working Group Y. Nishida
Internet-Draft WIDE Project
Intended status: Standards Track P. Natarajan
Expires: January 7, 2012 Cisco Systems
July 6, 2011
Quick Failover Algorithm in SCTP
draft-nishida-tsvwg-sctp-failover-03
Abstract
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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 7, 2012.
Copyright Notice
Copyright (c) 2011 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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
Nishida & Natarajan Expires January 7, 2012 [Page 1]
Internet-Draft SCTP Quick Failover July 2011
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified 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. 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
Nishida & Natarajan Expires January 7, 2012 [Page 2]
Internet-Draft SCTP Quick Failover July 2011
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.
Nishida & Natarajan Expires January 7, 2012 [Page 3]
Internet-Draft SCTP Quick Failover July 2011
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].
Nishida & Natarajan Expires January 7, 2012 [Page 4]
Internet-Draft SCTP Quick Failover July 2011
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
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.
Nishida & Natarajan Expires January 7, 2012 [Page 5]
Internet-Draft SCTP Quick Failover July 2011
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.
Nishida & Natarajan Expires January 7, 2012 [Page 6]
Internet-Draft SCTP Quick Failover July 2011
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
quicker.
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
Nishida & Natarajan Expires January 7, 2012 [Page 7]
Internet-Draft SCTP Quick Failover July 2011
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:
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
Nishida & Natarajan Expires January 7, 2012 [Page 8]
Internet-Draft SCTP Quick Failover July 2011
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 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].
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
retransmission(s).
Nishida & Natarajan Expires January 7, 2012 [Page 9]
Internet-Draft SCTP Quick Failover July 2011
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.
Nishida & Natarajan Expires January 7, 2012 [Page 10]
Internet-Draft SCTP Quick Failover July 2011
6. Security Considerations
There are no new security considerations introduced in this document.
Nishida & Natarajan Expires January 7, 2012 [Page 11]
Internet-Draft SCTP Quick Failover July 2011
7. IANA Considerations
This document does not create any new registries or modify the rules
for any existing registries managed by IANA.
Nishida & Natarajan Expires January 7, 2012 [Page 12]
Internet-Draft SCTP Quick Failover July 2011
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.
[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, "Performance 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 scenarios", World Multiconference on
Systemics, Cybernetics and Informatics , 7 2002.
[NATARAJAN09]
Natarajan, P., Ekiz, N., Amer, P., and R. Stewart,
Nishida & Natarajan Expires January 7, 2012 [Page 13]
Internet-Draft SCTP Quick Failover July 2011
"Concurrent Multipath Transfer during Path Failure",
Computer Communications , 5 2009.
Nishida & Natarajan Expires January 7, 2012 [Page 14]
Internet-Draft SCTP Quick Failover July 2011
Authors' Addresses
Yoshifumi Nishida
WIDE Project
Endo 5322
Fujisawa, Kanagawa 252-8520
Japan
Email: nishida@wide.ad.jp
Preethi Natarajan
Cisco Systems
510 McCarthy Blvd
Milpitas, CA 95035
USA
Email: prenatar@cisco.com
Nishida & Natarajan Expires January 7, 2012 [Page 15]