Internet Engineering Task Force D. Jovev
InternetDraft M. Proshin
Intended status: Standards Track Ericsson
Expires: November 15, 2018 May 14, 2018
Determining SCTP's Retransmission Timer
draftjovevtsvwgsctprto02
Abstract
This document defines a modification in the RFC 4960 [RFC4960]
defined Stream Control Transmission Protocol's (SCTP's)
Retransmission Timer (RTO) calculation method.
The modification is aimed to reduce the frequency of spurious T3
timeouts, which are caused by underestimated RTO values, derived by
the [RFC4960] defend RTO calculation method. The proposed
modification aligns the RTO calculation method with the
characteristics of the statistical estimator algorithms, which are
used for SRTT and RTTVAR calculation, the SCTP protocol data transfer
rules and the characteristics of the data packets' arrival pattern in
the telecom signalling networks.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions and Terminology . . . . . . . . . . . . . . . 3
2. Problem description . . . . . . . . . . . . . . . . . . . . . 3
3. The modified algorithm for RTO Calculation . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Normative References . . . . . . . . . . . . . . . . . . 8
6.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Technical background for the modifications in the
RTO calculation algorithm . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction
Like TCP, the SCTP's reliable transfer of data is ensured by limiting
the time in which the acknowledgement for the reception of the
transmitted data is received, after which expiration all
unacknowledged data is retransmitted. The duration of this timer is
referred to as Retransmission Timeout (RTO) and the actual timer is
called T3rtx or just T3.
The expiration of the T3 timer not only invokes retransmission of the
unacknowledged data it also drastically reduces the congestion window
(cwnd) to 1 MTU, which are both undesirable actions: data
retransmission increases the amount of sent data in the network, and
1 MTU cwnd drastically reduces the SCTP association transmission
capacity. Because of that, determining an RTO value which reflects
the highest RTT, or the highest feedback time, as more appropriately
called in [ALLMAN99], is critical for reducing the probability of
spurious T3 timeouts, which is critically important for stable SCTP
operation.
Namely, while in the conventional file transfer applications the
transport layer transmission capacity reduction, due to T3 timeouts,
only prolongs the time for completion of the file transfer, in the
telecom signalling networks it often results in false congestion
i.e., congestion caused by SCTP transmission capacity reduction not
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by traffic increase, which can lead to unrepairable loss of data that
adversely affects the services provided by the telecom networks.
This document defines a modification in the [RFC4960] defined SCTP's
Retransmission Timer (RTO) calculation method. The modification is
aimed to reduce the frequency of spurious T3 timeouts, which are
caused by underestimated RTO values, by adjusting the RTO calculation
method to the characteristics of the statistical estimator
algorithms, which are used for SRTT and RTTVAR calculation, and to
the SCTP protocol data transfer rules and the characteristics of the
data packets' arrival pattern in the telecom signalling networks.
The modified RTO calculation affects only the sender side and it does
not require introduction of new protocol variables or parameters nor
change of the [RFC4960] recommended values for the existing RTO
related protocol parameters.
The motivations for the modification in the [RFC4960] algorithm for
RTO calculation are outlined in Section 2. The actual modification
in the [RFC4960] algorithm for RTO calculation is specified in
Section 3 whereas the technical background for the modification is
elaborated in the Appendix A.
1.1. 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 RFC 2119 [RFC2119].
2. Problem description
The [RFC4960] defined process for RTO determination consists of two
steps.
In the first step, using RTT measurements as input data, a calculated
RTO value is derived from the mean/smooth RTT (SRTT) and RTT
variation (RTTVAR) values, which are determined using a statistical
estimator algorithm, originally published in [JAC88], and then, in
the second step, the used RTO is determined as:
RTO < min(RTO.Max, max(calculated RTO, RTO.Min)),
where RTO.Min and RTO.Max are configurable protocol parameters with
[RFC4960] recommended values of 1 sec and 60 seconds.
By applying the [RFC4960] RTO calculation rules, the RTO value that
will be used for the T3 timer will be:
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* The value of the RTO.Min  if the calculated RTO is below
RTO.Min.
* The calculated RTO  if the calculated RTO is above RTO.Min but
below RTO.Max.
* The value of the RTO.Max  if the calculated RTO is above
RTO.Max.
Diagram in Figure 1 illustrates the outcome of the above RTO
determination rules.
Used RTO
^

RTO.MAX +. . . . . .+
 / .
 / .
 / .
 / .
 / .
RTO.Min ++ .
 . .
 . .
 . .
 . .
+++>
Calculated RTO
RTO.Min RTO.Max
Figure 1: Relation between the calculated and used RTO values
The SCTP protocol has been operating in the telecom networks for more
than fifteen years and spurious T3 timeouts have been one of the most
frequently reported problems.
The results of the analysis of the spurious T3 timeouts problems,
reported from the operating networks, indicated that the spurious T3
timeouts frequency increases when the SRTT value is closer to the
RTO.Min value to the point where the association becomes unstable if
the SRTT is longer than the RTO.Min value. The analysis of these
problems also showed that the reported spurious T3 timeouts problems
were resolved only by increasing the RTO.Min value well above the
SRTT value.
The fact that the spurious T3 timeouts were successfully prevented
only by setting the RTO.Min value considerably above the SRTT value,
leads to conclusion that the RTO values, which are derived by the
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[RFC4960] defend rules, are inadequate for the RTT variation pattern
in the telecom signalling networks.
In other words, the fact that the SCTP association operation is
stable only when the RTO.Min value is well above the SRTT value,
makes the RTO calculation, which is specified by the [RFC4960]
section 6.3.1. rules C1 C2 and C3, seemingly redundant.
To help visualise the problem, let assume, hypothetically, that the
packets transmission pattern consists of high packet rate sequences
longer than 500 msec with, for example, 200 packets/sec, which
separated by 50 to 80 ms "idle" gaps. For such packet rate pattern,
the statistical estimator algorithm for RTTVAR will produce a very
low RTTVAR values, very likely well below 5 msec, because, during the
long high packet rate sequences, the SACK delay will vary around 5
msec due to packet rate of 200 packets/sec.
Consequently, with the [RFC4960] RTO calculation rule:
RTO < max(SRTT + 4 * RTTVAR, RTO.Min),
the RTO margin to absorb unexpected SACK delays, in this hypothetical
case 50 to 80 msec due to the packet transmission gaps, is determined
by the difference between the calculated RTO value and the measured
(calculated) SRTT.
Since in case of low RTTVAR values the RTO is determined by the
RTO.Min parameter, the RTO margin will be equal to the difference
between the RTO.Min and SRTT (RTO margin = RTO.Min  SRTT). Thus, as
illustrated in Figure 2, the [RFC4960] RTO calculation rules produce
robust RTO values only when the SRTT is well below RTO.Min parameter
value, which is the root cause of the problem.
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RTO margin
^

RTO.Min +
 \
 \
 \
 \
 \
 \
 \
++>
0 SRTT
RTO.Min
Figure 2: Relation between the RTO margin and SRTT
To rectify this anomaly, this document introduces modification in the
[RFC4960] algorithm for RTO calculation. The actual modification is
specified in Section 3 and it includes only change in the use of the
RTO.Min protocol parameter; the technical background for the
modification is elaborated in the Appendix A.
3. The modified algorithm for RTO Calculation
The modified rules governing the computation of SRTT, RTTVAR and RTO
are as follows:
C1) Until an RTT measurement has been made for a packet sent to
the given destination transport address, set RTO to the
protocol parameter 'RTO.Initial'.
C2) When the first RTT measurement R is made, set
SRTT < R,
RTTVAR < R/2, and
RTO < SRTT + max(4 * RTTVAR, RTO.Min).
C3) When a new RTT measurement R' is made, set
RTTVAR < (1  RTO.Beta) * RTTVAR + RTO.Beta * SRTT  R'
and
SRTT < (1  RTO.Alpha) * SRTT + RTO.Alpha * R'
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Note: The value of SRTT used in the update to RTTVAR is its
value before updating SRTT itself using the second
assignment.
After the SRTT and RTTVAR computation, update RTO:
RTO < SRTT + max(4 * RTTVAR, RTO.Min).
C4) When data is in flight and when allowed by rule C5 below, a
new RTT measurement MUST be made each round trip.
Furthermore, new RTT measurements SHOULD be made no more than
once per round trip for a given destination transport
address. There are two reasons for this recommendation:
First, it appears that measuring more frequently often does
not in practice yield any significant benefit [ALLMAN99];
second, if measurements are made more often, then the values
of RTO.Alpha and RTO.Beta in rule C3 above should be adjusted
so that SRTT and RTTVAR still adjust to changes at roughly
the same rate (in terms of how many round trips it takes them
to reflect new values) as they would if making only one
measurement per roundtrip and using RTO.Alpha and RTO.Beta
as given in rule C3. However, the exact nature of these
adjustments remains a research issue.
C5) Karn's algorithm: RTT measurements MUST NOT be made using
packets that were retransmitted (and thus for which it is
ambiguous whether the reply was for the first instance of the
chunk or for a later instance).
IMPLEMENTATION NOTE: RTT measurements should only be made
using a chunk with TSN r if no chunk with TSN less than or
equal to r is retransmitted since r is first sent.
C6) A maximum value may be placed on RTO provided it is at least
RTO.max seconds.
There is no requirement for the clock granularity G used for
computing RTT measurements and the different state variables, other
than:
G1) Whenever RTTVAR is computed, if RTTVAR = 0, then adjust RTTVAR <
G.
Experience [ALLMAN99] has shown that finer clock granularities (<=
100 msec) perform somewhat better than more coarse granularities.
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4. IANA Considerations
This document does not create any new registries or modify the rules
for any existing registries managed by IANA.
5. Security Considerations
This document does not add any security considerations to those given
in [RFC4960].
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
September 2007, .
6.2. Informative References
[ALLMAN99]
Mark Allman and Vern Paxson, "On Estimating EndtoEnd
Network Path Properties", 1999,
.
[JAC88] Van Jacobson and Michael J. Karels , "Congestion Avoidance
and Control", November 1988,
.
Appendix A. Technical background for the modifications in the RTO
calculation algorithm
As indicated in Section 2, with the [RFC4960] RTO calculation rules,
the frequency of spurious T3 timeouts increases when the SRTT value
is close to the RTO.Min value to the point where, under heavy load,
the association becomes unstable if the SRTT is longer than the
RTO.Min value.
The reasons for such outcome can be contributed to the following
factors:
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a) The characteristic of the statistical estimator algorithms for
SRTT and RTTVAR calculation;
b) The anomalies in the distribution of the RTT measurement
values caused by the [RFC4960] SACK generation rules,
specifically, the delay of SACK sending; and
c) Inappropriate solution for protection against underestimated
RTO values.
The characteristics of the statistical estimator algorithms for SRTT
and RTTVAR, which are the foundation for RTO calculation, are well
known and widely investigated in terms of improving the outcome
(reduction of spurious T3 timeouts) by adjustment of the statistical
estimator algorithms' configurable parameters. For example, the
investigation results published in [ALLMAN99] indicate that lower
gain factors RTO.Alpha and RTO.Beta, in the SRTT and RTTVAR
calculations formulas, reduces the probability of computing a low RTO
value that will result in T3 timeout. The same source also states
that lower spurious T3 timeouts probability is also achieved by
increasing the RTTVAR component i.e., the value of the factor K in
the RTO calculation formula:
RTO < SRTT + K * RTTVAR.
This behaviour can be related to the wellknown characteristic of the
statistical estimator algorithms for SRTT and RTTVAR estimation,
which can be described as follows: If the RTT measurements values
converge to a single RTT value, the calculated RTTVAR converge to
zero (0) and the calculated RTO converge to SRTT. As a result, a
relatively short sequence of moderately low RTT values, which are
within the RTT values range, simultaneously lowers the SRTT and
RTTVAR values to the point where the calculated RTO value is below
the highest value in the RTT variation range, which may result in
spurious T3 timeout if the next RTT is at the top of the RTT
variation range.
This 'problem' is further exacerbated by the SCTP protocol rules for
sending SACK which allow SACK delay of up to 500 msec. Namely, the
SACK delay rules, combined with burst nature of the data packets'
arrival pattern in the telecom signalling networks, drastically
increase the jitteriness of the RTT measurements. That, in turn,
adversely affect the results obtained by statistical estimator
algorithms for SRTT and RTTVAR calculations in terms of
underestimated RTO values that are prone to spurious T3 timeouts.
Obviously, and as proven in the operating networks, an RTO determined
by application of rule C6, with an RTO.Min value in seconds,
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practically eliminates underestimated RTO values and with that the
spurious T3 timeouts. That is because the 1 second RTO will be well
above the delay inserted by the terrestrial transport networks, which
operate with latency below 100 msec, and because the SACK delay is
also well below 1 second.
However, an RTO value in seconds, coupled with the RTO backoff rule
RTO < RTO * 2, results in too long detection of remote endpoint
failure or complete failure of the physical layer. For example, with
the [RFC4960] recommended RTO.Min of 1 second, RTO.Max of 60 seconds
and Association.Max.Retrans of 4 attempts, the association closure
time will be 31 seconds, which is an unacceptably long time that,
under high load, can potentially destabilise the operation of the
network.
Namely, in the telecom networks where the client nodes are connected
to redundant server nodes and where multiple load sharing SCTP
associations are used between the nodes, a timely detection of the
SCTP remote peer endpoint failure, or complete failure of the
physical layer, is critical to enables failover to the redundant
resources.
Thus, instead of using an arbitrary long RTO defend by RTO.Min
parameter, which practically makes the calculated RTO value by rules
C1, C2 and C3 redundant, the RTO value should reflect, as close as
possible, the real conditions in the network in terms of the time to
transport the packets between two endpoints, the time delays induced
by the SCTP protocol rules and to also include adequate additional
time as protection against underestimated RTO values. To achieve
that, the subsequent paragraphs first analyse the characteristics of
the RTT components and then specify a modified RTO calculation
algorithm which is derived from the characteristics of the
statistical estimator algorithms for SRTT and RTTVAR and the
characteristics of the RTT components.
Specifically, an RTT measurement starts at transmission of data, or
at transmission of HEARTBEAT, and it is completed at reception of the
corresponding SACK or HEARTBEAT ACK from the remote peer endpoint.
The RTT measurements results, which are based on data transfer and
SACK reception, will be influenced by the following main components:
a) Transport network's physical layer propagation times in
forward and backward directions.
b) IP network layer IP packets' sending, receiving and processing
times in forward and backward directions.
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c) The time to send, receive and process SCTP packet at the
transmitting and receiving SCTP endpoints.
d) SACK sending delay when SACK is not sent for every received
packet.
A similar RTT structuring can be constructed for the RTT measurements
based on HEARTBEAT and HEARTBEAT ACK however, since HEARTBEAT ACK is
sent for every HEARTBEAT with no delay, the HEARTBEAT based RTT
estimation is less 'challenging' and it will not be examined in
detail in this document.
The component 'a)', the transport network's physical layer
propagation time is a stable component determined primarily by the
length of the connection between two endpoints and to a very small
degree by the nature of the physical medium (coper, coax cable, radio
link, etc.). This component determines the theoretical/absolute
minimum RTT time and it changes only when the physical properties of
the connection, primarily the length, are changed.
The components 'b)' and 'c)', the IP network layer and SCTP endpoints
packets sending, receiving and processing times are proportional to
the traffic level (A) by factor 1/(1A), which is the mean value of
the waiting queues length. However, the actual time durations are
derived as a product of the waiting queue length (the number of
packets waiting to be processed) and the time to process a packet
(the time to transmit/receive packet or the time to process a packet
by the protocol stack's layers). Since the waiting queues' lengths
are variable the aggregated time to send, receive and process SCTP
packet will be variable too. Because the networks' load variation's
gradient is generally small and because the telecom networks'
signalling traffic is normally carried over high speed IP backbone
networks with engineered capacity i.e., with no congestion, the
variation of this timing components values will be significantly
smaller than the variation range due to SACK delay.
The time component due to bullet 'd)' is the delay time inserted by
the SCTP protocol rules and it is applicable only when the SACK is
not returned on every packet.
Namely, when SACK is returned on every received packet, the RTT
measurement value R is determined only by the combined time from
components 'a)', 'b)' and 'c)', which in this context will be called
NRTT (Network RTT). However, when the SACK is not returned on every
packet i.e., when the SACK is returned on every 'Nth' received
packet, and N > 1, the RTT measurement value R is determined by NRTT
and the allowed SACK delay time.
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Specifically, if the packets' arrival rate/frequency F is low,
relative to the value of the protocol parameter SACK delay timer
(SACK.Delay.timer), i.e., if the relation
(N  1) * 1/F >= SACK.Delay.timer
is true, the RTT measurement value will be determined by the NRTT and
the SACK.Delay. In that case, the RTT measurement value R can be
expressed as follows:
R = NRTT + SACK.Delay.timer.
Alternatively, if the packets' arrival rate F is high, relative to
the SACK.Delay, i.e., if the inequation
(N  1) * 1/F < SACK.Delay.timer
is true, the RTT measurement value will be determined by the NRTT and
the time to receive the number of packets required to trigger sending
of SACK. In that case, the RTT measurement value can be expressed as
follows:
R = NRTT + (N  1) * 1/F.
Since by the [RFC4960] specifications the number of received packets
that is required to trigger sending of SACK is limited to 2 (N = 2),
the expression for the RTT measurement value can be simplified as
follows:
R = NRTT + 1/F.
Thus, in general, the RTT measurement value can be expressed as
follows:
R = NRTT + min(SACK.Delay.timer, 1/F).
In other words, for any packet arrival rate F, the shortest RTT
measurement value is greater than the NRTT and the longest RTT
measurement value does not exceed NRTT plus SACK.Delay i.e., the
following relation is true:
NRTT + 1/maxF < R <= NRTT + SACK.Delay.timer,
where maxF is the highest packets arrival rate. Consequently, the
range of the RTT measurements R is given by the following relation:
NRTT + 1/maxF <= R <= NRTT + SACK.Delay.timer,
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Or in other words, the values of the RTT measurements R will be
between a minimum value (minR) that is determined as:
minR = NRTT + 1/maxF,
and a maximum value (maxR) that is determined as:
maxR = NRTT + SACK.Delay.timer.
The above presented RTT related relations are illustrated in
Figure 3.
R values
range
/\
NRTT minR maxR
###>
0 \/ R
1/maxF
\/
SACK.Delay.timer
Figure 3: The expected values range of the RTT measurements R
The above analysis also shows that the SACK delay, in practical
terms, significantly increases the RTT (R'), which leads to
conclusion that the calculated SRTT (mean RTT) by formula:
SRTT < (1  RTO.Alpha) * SRTT + RTO.Alpha * R';
converges to a value greater than NRTT + 1/maxF i.e., to a value
greater than the lowest RTT, regardless of the variation pattern of
the measured RTTs.
At that same time, the above analysis shows that the SACK delay
significantly increases the RTT measurement (R') variation range but
it does not alter the RTTVAR convergence to 0, or rather low values
when calculated by formula:
RTTVAR < (1  RTO.Beta) * RTTVAR + RTO.Beta * SRTT  R'.
Or in other words, the RTTVAR calculation can still yield low values
even though the SACK delay increases the RTT measurement (R')
variation range (refer to Figure 3).
That, combined with the fact that RTTVAR contribution to the RTO
value is 4 times of SRTT (RTO < SRTT + 4 * RTTVAR), leads to
conclusion that the RTO underestimations are primarily due to low
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RTTVAR values. Thus, instead of setting low threshold for the
calculated RTO, which is the role of rule C6, the compensation for
underestimated RTOs should be achieved by setting low threshold for
RTTVAR as follows:
After calculating RTTVAR by formula:
RTTVAR < (1  RTO.Beta) * RTTVAR + RTO.Beta * SRTT  R',
if RTTVAR is less than RTTVAR.Min set RTTVAR to RTTVAR.Min.
Or by altering the RTO calculation formula as follows:
RTO < SRTT + max(4 * RTTVAR, RTTVAR.Min).
However, to avoid introduction of new protocol parameter, and because
the existing RTO.Min protocol parameter is no longer used, RTO.Min
can take the role of the RTTVAR.Min. In that case, the RTO
calculation formula will be expressed as follows:
RTO < SRTT + max(4 * RTTVAR, RTO.Min).
The above formula ensures that, in case of low RTTVAR values, the RTO
margin to absorb unexpected SACK delays is determined by the RTO.Min
(the RTTVAR.Min alias) only, thus, it is constant and independent of
the SRTT (refer to the illustration in Figure 4).
RTO margin
^

RTO.Min +







+>
0 SRTT
Figure 4: Relation between the RTO margin and SRTT with the new RTO
calculation rules
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Since the RTT variation range introduced by SACK delay is predictable
i.e., the RTT variation range introduced by SACK delay is, in
practical terms, determined by the SACK delay time (refer to
Figure 2), the value of the RTTVAR low threshold should be determined
based on the SACK delay time used at the remote peer.
The [RFC4960] recommended value for RTO.Min does not require change
when the RTO.Min is used as RTTVAR low threshold in the above
modified formula for RTO calculation. Namely, the recommended 1 sec
correspond to 2 times the allowed SACK delay time, which is 500 msec.
Authors' Addresses
Dimitar Jovev
Ericsson
818 Bourke St.
Melbourne, Victoria 3008
Australia
Email: dimitar.jovev@gmail.com
Maksim Proshin
Ericsson
Kistavaegen 25
Stockholm 164 80
Sweden
Email: mproshin@tieto.mera.ru
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