Internet Engineering Task Force M. Menth
Internet-Draft F. Lehrieder
Expires: January 5, 2009 University of Wuerzburg
July 4, 2008
Performance Evaluation of PCN-Based Algorithms
draft-menth-pcn-performance-03
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Abstract
This document presents a summary of performance studies for PCN-based
admission control and flow termination. The numerical results were
obtained by simulation or mathematical analysis.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Comparison of Marking Algorithms for PCN-Based Admission
Control . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Definition of Simulated Entities and Simulation Setup . . 7
3.1.1. Metering and Marking Mechanisms . . . . . . . . . . . 7
3.1.2. Congestion Level Estimator . . . . . . . . . . . . . . 7
3.1.3. Simulation Setup . . . . . . . . . . . . . . . . . . . 8
3.2. Impact of the Marking Threshold T and the Queue Size S . . 9
3.3. Two Marking Strategies with Different Admission
Control Policies . . . . . . . . . . . . . . . . . . . . . 9
3.3.1. Marking with Clear Decisions (MCD) . . . . . . . . . . 9
3.3.2. Marking with Early Warning (MEW) . . . . . . . . . . . 9
3.4. Impact of Ramp Marking . . . . . . . . . . . . . . . . . . 9
3.5. Impact of the Memory M of the Congestion Level
Estimator . . . . . . . . . . . . . . . . . . . . . . . . 10
3.6. Impact of Traffic Characteristics . . . . . . . . . . . . 10
3.7. Response Time of the Marking to Sudden Overload . . . . . 11
3.8. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . 11
4. Performance Evaluation of Admission Control Methods . . . . . 13
4.1. PBAC . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.2. OBAC . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.3. CLEBAC . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.4. Other Observations . . . . . . . . . . . . . . . . . . . . 14
5. Performance Evaluation of Measured Rate Termination (MRT) . . 15
5.1. Two Options for MRT . . . . . . . . . . . . . . . . . . . 15
5.1.1. Direct MRT . . . . . . . . . . . . . . . . . . . . . . 15
5.1.2. Indirect MRT . . . . . . . . . . . . . . . . . . . . . 15
5.2. Impact of Packet Loss . . . . . . . . . . . . . . . . . . 16
5.2.1. Direct MRT under Lose & Mark . . . . . . . . . . . . . 16
5.2.2. Indirect MRT under Lose & Mark . . . . . . . . . . . . 16
5.2.3. Direct MRT under Mark & Lose . . . . . . . . . . . . . 16
5.2.4. Indirect MRT under Mark & Lose . . . . . . . . . . . . 17
5.2.5. Summary . . . . . . . . . . . . . . . . . . . . . . . 17
5.3. Unintended Traffic Termination with Indirect MRT
through Badly Aligned Measurement Intervals . . . . . . . 17
5.3.1. Experiments with Almost CBR Traffic . . . . . . . . . 18
5.3.2. Experiments with VBR Traffic . . . . . . . . . . . . . 18
5.3.3. Experiments with On/Off Traffic . . . . . . . . . . . 18
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5.3.4. Experiments with Rerouted Traffic . . . . . . . . . . 19
5.3.5. Summary . . . . . . . . . . . . . . . . . . . . . . . 19
6. Performance Evaluation of Marked Flow Termination . . . . . . 20
6.1. CMFT . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.2. Flow-based EMFT (F-EMFT) . . . . . . . . . . . . . . . . . 21
6.3. Aggregate-based EMFT (A-EMFT) . . . . . . . . . . . . . . 21
6.4. General Performance of MFT Methods . . . . . . . . . . . . 22
6.5. Comparison of CMFT, F-EMFT, and A-EMFT . . . . . . . . . . 22
6.5.1. Key Benefits of MFT . . . . . . . . . . . . . . . . . 22
6.5.2. Unknown Traffic Characteristics . . . . . . . . . . . 23
6.5.3. Implementation and Configuration Complexity . . . . . 23
6.5.4. Fairness Issues . . . . . . . . . . . . . . . . . . . 23
6.5.5. Termination Priorities and Policies . . . . . . . . . 24
6.5.6. Marking Support from Simple ECN Nodes . . . . . . . . 24
6.5.7. Compatibility with Existing Hardware . . . . . . . . . 24
7. Performance Evaluation of Marked Flow Termination (MFT)
with Multiple Bottleneck Links . . . . . . . . . . . . . . . . 25
7.1. Several Serial Links Carrying Only a Common Aggregate . . 25
7.2. Two Serial Links Carrying a Common Aggregate with
Cross Traffic on the Second Link . . . . . . . . . . . . . 26
7.3. Two Serial Links Carrying a Common Aggregate with
Cross Traffic on the First Link . . . . . . . . . . . . . 26
7.4. Two Serial Links Carrying a Common Aggregate with
Cross Traffic on Both Links . . . . . . . . . . . . . . . 27
7.5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 28
8. Performance Evaluation of a Marking Converter for Excess
Marking . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
8.1. Simulation Setup . . . . . . . . . . . . . . . . . . . . . 29
8.2. Results . . . . . . . . . . . . . . . . . . . . . . . . . 29
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
10. Security Considerations . . . . . . . . . . . . . . . . . . . 32
11. Changes from Previous Revisions . . . . . . . . . . . . . . . 33
11.1. Changes from Version -00 to Version -01 . . . . . . . . . 33
11.2. Changes from Version -01 to Version -02 . . . . . . . . . 33
11.3. Changes from Version -02 to Version -03 . . . . . . . . . 33
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34
12.1. Normative References . . . . . . . . . . . . . . . . . . . 34
12.2. Informative References . . . . . . . . . . . . . . . . . . 34
12.3. Other References . . . . . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 36
Intellectual Property and Copyright Statements . . . . . . . . . . 37
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1. Introduction
Pre-congestion notification (PCN) is based on the idea of marking
packets when a certain load threshold on a link is exceeded by PCN
traffic. Then, the marking of a packet at the PCN egress node
provides information whether the rate threshold of at least one link
of the path over which the packet was carried was exceeded by PCN
traffic. This information can be used for admission control and flow
termination. Several approaches such as Single-Marking (SM)
[I-D.charny-pcn-single-marking], CL
[I-D.briscoe-tsvwg-cl-architecture], 3SM [I-D.babiarz-pcn-3sm], EMFT
[I-D.menth-pcn-emft] have been proposed for that purpose. An
overview of the basic concept is given in
[I-D.ietf-pcn-architecture].
The University of Wuerzburg is conducting performance studies to
understand basic mechanisms and to compare different approaches.
This document is intended to collect and present summaries of
performance results documented in more detail in technical papers
that are available online. Currently, it covers the following
studies.
o A summary of the results of [TR437] is presented in Section 3.
[TR437] studies the impact of virtual queue (token bucket)
parameters on marking results for threshold and ramp marking and
gives a comparison.
o A summary of the results in [Menth08-AC] regarding admission
control methods is presented in Section 4.
o A summary of the results in [Menth07] regarding "Performance
Evaluation of Measured Rate Termination (MRT)" is presented in
Section 5.
o A summary of the results in [Menth08-MFT] regarding "Performance
Evaluation of Marked Flow Termination (MFT) on a Single Link" is
presented in Section 6.
o A summary of the results in [Menth07] regarding "Performance
Evaluation of Marked Flow Termination (MFT) with Multiple
Bottleneck Links" is presented in Section 7.
o A summary of our simulation results regarding the marking
converter proposed in [I-D.menth-pcn-marking-converter] is
presented in Section 8.
The next section clarifies some terminology issues.
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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].
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2. Terminology
The terminology used in this document conforms to the topology of
[I-D.ietf-pcn-architecture].
We use the following exceptions for better readability and provide
the synonyms defined in [I-D.ietf-pcn-architecture].
o Admissible rate: PCN-lower-rate
o Supportable rate: PCN-upper-rate
o Admission-stop marking: first encoding or PCN-lower-rate-marking
o Excess-traffic marking: second encoding or PCN-upper-rate-marking
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3. Comparison of Marking Algorithms for PCN-Based Admission Control
The following presents a short summary of [TR437] without the graphs
and exact numerical results that are provided in the technical
report. The interested reader is referred to that document.
3.1. Definition of Simulated Entities and Simulation Setup
In this study, we investigate the behaviour of different metering and
marking algorithms under different configuration and use a congestion
level estimator to observe the packet markings.
3.1.1. Metering and Marking Mechanisms
PCN requires metering and marking algorithms in the interior nodes.
[TR437] defines
o threshold marking and
o ramp marking
based on a virtual queue (VQ), but there are equivalent descriptions
based on token buckets.
The parameters are
o the size S of the VQ,
o the rate R of the VQ,
o the marking threshold T for threshold marking, which is also the
upper threshold for ramp marking,
o the marking threshold T_ramp, which is the lower threshold for
ramp marking
3.1.2. Congestion Level Estimator
Furthermore, a congestion level estimator is defined that calculates
a congestion level estimate (CLE) at the PCN egress node based on an
exponentially weighted moving average (EWMA). Marked packets count 1
and unmarked packets count 0. The CLE is computed as
CLE = w o CLE + (1 - w) o X
where X is the observed packet marking and w<1 is the weight
parameter. If w is large, CLE has a long memory M, if it is low, CLE
has a short memory M. The time between CLE updates also influences
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the memory M. A formal definition of the memory M is given in 3.4.2
of [TR437]. The CLE is used to observe the packet markings of the
simulations.
3.1.3. Simulation Setup
We simulate a single link scenario. Packets from n independent,
homogeneous traffic sources are multiplexed onto a single link with
infinite bandwidth and pass a meter and marker. The markings are
evaluated by a subsequent congestion level estimator.
If not mentioned differently, we simulate around n = 100 homogeneous
flows for sufficiently long time to obtain reliable results.
However, we omit confidence intervals in all our graphs for the sake
of clarity. We choose a Gamma distribution to generate the inter-
arrival times A between consecutive packets within a flow with a mean
of E[A] = 20 ms and a coefficient of variation of cvar[A] = 0.1. The
packet sizes B are independent and distributed according to a
deterministic phase of 30 bytes plus a negative binomial
distribution. Their overall mean is E[B] = 60 bytes and their
coefficient of variation is cvar[B] = 0.5. The values for E[A] and
E[B] are motivated by typical voice connections that periodically
send every 20 ms a packet with 20 bytes payload using a 40 bytes IP/
UDP/RTP header. However, our flow model is not periodic and has
variable packet sizes. We use it for two reasons. The simulation of
multiplexed, strictly periodic traffic requires special care due to
the non-ergodicity of the system and is very time consuming.
Therefore, we relax cvar[A] = 0.0 to cvar[A] = 0.1. Furthermore, we
use cvar[B] = 0.5 instead of cvar[B] = 0.0 because realtime traffic
consists of packets from different applications with and without
compression which leads to different packet sizes.
However, our findings are general and do not depend on special
parameter settings. The rate of the virtual queue is R = 2.4 Mbit/s
such that at most 100 flows can pass unmarked. The congestion level
estimator implements an exponentially weighted moving average (EWMA)
and counts packets with admission-stop marks as 1 and those without
as 0. As mentioned previously, its memory M depends on the packet
rate and the weight parameter w such that w needs to be adapted to
the desired M and the packet frequency in the experiment for which we
take the maximum packet rate that can pass unmarked. Thus, we set
the weight parameter to w = 0.998 which corresponds to a memory of
0.1 s when 100 default flows are active. If the packet rate changes
due to more bursty traffic, we adapt the weight parameter w to
achieve the same memory.
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3.2. Impact of the Marking Threshold T and the Queue Size S
We measure the percentage of marked packets depending on the PCN rate
(number of flows n) and the queue parameters size S and marking
threshold T. The ideal marker marks
1. no packets if the PCN rate is below the VQ rate R and
2. all packets if it is above.
We found out that
1. is increasingly achieved with increasing threshold T and
2. is increasingly achieved with increasing remaining queue size
S-T.
3.3. Two Marking Strategies with Different Admission Control Policies
We construct threshold markers with two different CLE characteristics
(=function describing the percentage of marked packets depending on
PCN rate).
3.3.1. Marking with Clear Decisions (MCD)
Marking with clear decisions (MCD) means that the above objectives
(1) and (2) are well achieved. This can be obtained for threshold
marking with a large marking threshold T and a large remaining queue
size S - T. Then, hardly any fluctuations in marking are observed.
3.3.2. Marking with Early Warning (MEW)
Marking with early warning (MEW) means that (3) the percentage of
marked packets already increases when the PCN rate approaches the VQ
rate and (4) is 100% when the PCN rate is above the VQ rate. This
can be obtained for threshold marking with a small marking threshold
T and a large remaining queue size S - T.
3.4. Impact of Ramp Marking
Ramp marking already marks packets probabilistically if the virtual
queue length is below the marking threshold T. Therefore, it marks
more packets than threshold marking with the same marking threshold T
and queue size S. In our study we always set the lower marking
threshold to T_ramp = 0. We found out that ramp marking with this
configuration cannot achieve MCD because it marks a small percentage
of packets when the PCN rate is below the VQ rate, but it can well
achieve MEW. MEW can be achieved both with threshold and ramp
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marking, but threshold marking requires a smaller threshold parameter
T to get the same marking results as with ramp marking.
3.5. Impact of the Memory M of the Congestion Level Estimator
The memory M of the congestion level estimator does not have an
impact on the percentage of marked packets that were observed over
the simulation time, but it impacts the degree to which the CLE
fluctuates. If the memory is long, the fluctuation of CLE is small.
If the memory M is short, the fluctuation of CLE is large. When we
configure the queue for MCD, i.e., the threshold T and the remaining
queue size S-T were chosen sufficiently large, the CLE is almost 0
for PCN rates smaller than the VQ rate and it is 1 for PCN rates
larger than the VQ rate. This holds even for a very small memory M
of the congestion level estimator.
3.6. Impact of Traffic Characteristics
Traffic characteristics have a significant impact on the marking
result.
o Decreased variance of packet sizes: no impact on the CLE
characteristics in case of MCD, slightly lower curves in case of
MEW
o Increased variance of packet sizes: little impact on the CLE
characteristics in case of MCD, significantly higher curves in
case of MEW and larger fluctuation of CLE
o Increased aggregation level: no impact on the CLE characteristics
in case of MCD, slightly higher curves in case of MEW and less
fluctuation of CLE
o Increased variance of inter-arrival times: little impact on the
CLE characteristics in case of MCD, slightly higher curves in case
of MEW and larger fluctuation of CLE
o Increased burstiness (fewer but larger packets): little impact on
the CLE characteristics in case of MCD, significantly higher
curves in case of MEW and large fluctuations of CLE
o On/off traffic instead of continuous flows: large impact on the
CLE characteristics in case of MCD and MEW, in particular very
large fluctuations of the CLE
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3.7. Response Time of the Marking to Sudden Overload
Large marking thresholds T and remaining queue sizes S-T lead to
stable marking results for MCD, but large parameters slow down the
reaction time of the marker when the PCN rate exceeds the VQ rate.
3.8. Conclusion
One option for pre-congestion notification (PCN) based admission
control requires that all packets are marked if the current link rate
exceeds a pre-configured admissible rate. This can be achieved by
virtual queue based marking algorithms such as simple threshold
marking or more complex ramp marking.
The objective of [TR437] was to study how marking algorithms can
support admission control in order to limit the utilization of the
links of a network. We did not consider the use of marking
algorithms to support admission control in order to limit the packet
delay because we assume that PCN will be used in high-speed networks
where packet delay caused by queuing is negligible as long as link
utilizations are moderate.
We investigated the influence of the parameters of the marking
algorithms on their marking results which are translated into a
congestion level estimate (CLE) using EWMA-based averaging. We
showed that two different marking strategies can be pursued: marking
such that the CLE leads to clear decisions (MCD) and marking such
that the CLE yields early warning (MEW) when the rate of PCN traffic
on a link approaches its admissible rate. We provided
recommendations for the configuration of the marking threshold T and
the size S of the virtual queue in both cases. Ramp marking
increases the level of early warning compared to threshold marking,
but this can be approximated by smaller marking thresholds for simple
threshold marking such that there is no obvious need for ramp
marking.
The CLE values for MEW fluctuate, therefore, it is difficult to infer
the exact, current traffic rate from the CLE values which is required
to take advantage of early warning. A sensitivity study revealed
that the average CLE values for MEW depend heavily on the traffic
characteristics. This makes the use of early warning difficult:
either the marking parameters need to be adapted to produce similar
warnings for different traffic types or the mechanism taking early
warning into account requires knowledge about the traffic
characteristics to correctly interpret the CLE level. In contrast,
CLE values for MCD show hardly any variation and are robust against
different traffic types.
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For the sake of simplicity, we advocate for the use of MCD for PCN
based admission control instead of MEW because the interpretation of
early warning is difficult due to its high variation and dependency
on traffic characteristics. Furthermore, we think that ramp marking
is not needed for PCN since similar markings can be obtained by
appropriately configured threshold marking and we do not see any
benefit that justifies the implementation complexity of ramp marking.
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4. Performance Evaluation of Admission Control Methods
Two marking methods have been proposed to support admission control:
excess marking and exhaustive marking. Excess marking marks only
those packets that exceed the admissible threshold while exhaustive
marking marks all packets when the admissible threshold is exceeded.
Three methods have been discussed to evaluate the markings and
translate them into admission control decisions: probe-based
admission control (PBAC, 3sm), observation-based admission control
(OBAC, 3sm), and CLE-based admission control (CLEBAC, 3sm, SM, CL).
PBAC sends probe packets for a prospective flow through the network
and admits the flow if all probe packets are received unmarked at the
PCN egress node. OBAC and CLEBAC use the concept of ingress-egress
aggregates (IEAs). OBAC stops admission of further flows for a
specific aggregate when the PCN egress node has received a marked
packet for the IEA while CLEBAC stops only when the fraction of
marked packets in the IEA was high enough.
We have investigated the combination of all marking and AC algorithms
and provide high-level results. The details can be found in
[Menth08-AC].
4.1. PBAC
PBAC has only one configuration parameter which is the number of
probe packets that are sent upon admission request of a flow. In
combination with exhaustive marking, a single probe packet is enough
for a reliable admission decision. In combination with excess
marking, many probe packets (about 100) are required. The exact
number depends on the overload and the tolerable error probability.
4.2. OBAC
OBAC also has only one configuration parameter. It is the minimum
block duration, i.e. the minimum time the IEA stays blocking after
its PCN egress node observed the last marked packet. With excess
marking OBAC starts blocking when the current rate is slightly above
the admissible rate. This effect becomes clearer for more bursty
traffic. We recommend a minimum block duration of 200 ms because
smaller values lead to fast oscillations of the IEA between blocking
and accepting state. With exhaustive marking, OBAC already blocks
when the PCN traffic rate is close to the admissible rate and the
blocking behaviour is rather independent of the minimum block
duration.
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4.3. CLEBAC
CLEBAC measures the fraction of marked bytes over a measurement
interval and this fraction is called the congestion level estimate
(CLE). CLEBAC has three configuration parameters. The duration of
the measurement interval over which the CLE is computed, the
admission-continue, and the admission-stop CLE threshold. For excess
marking, the duration of the measurement interval should be at least
200 ms to avoid oscillations of the block/admit state of the IEA.
The IEA blocks when the PCN rate is above the admissible rate and
late blocking increases with increasing admission-stop threshold.
Also more bursty traffic increases late blocking. An admission-
continue threshold of 0 shows good blocking results. With exhaustive
marking, the blocking behaviour of CLEBAC is rather independent of
the admission-continue and admission-stop thresholds and small
measurement intervals of 50 ms can be used without leading to strong
oscillations of the admit/block state of an IEA.
4.4. Other Observations
With normal excess marking, the marking probability of a packet
increases with its size in case of overload. The average packet
marking probability decreases with increasing variance of the packet
size. Large packets are more likely to be marked than small packets
and when marked packets tend to be large, excess marking marks fewer
packets. Therefore, it is important that CLE is based on bytes and
not on packet numbers.
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5. Performance Evaluation of Measured Rate Termination (MRT)
Measured rate termination (MRT) assumes that PCN interior nodes mark
packets with "excess-traffic" (ET) when they exceed the supportable
rate (SR) of a link with some tolerance. This marking is explained
in [I-D.babiarz-pcn-3sm] and used in the CL architecture
[I-D.briscoe-tsvwg-cl-architecture]. The PCN egress node measures
the rate of marked and unmarked packets and communicates them to the
PCN ingress node. Based on this information, the PCN ingress node
calculates the rate that needs to be terminated and chooses an
appropriate set of flows for termination. This is not a trivial task
since the rate of the flows are possibly unknown, but we do not
further study this issue. There are two options for MRT: direct and
indirect MRT.
5.1. Two Options for MRT
5.1.1. Direct MRT
The PCN egress node measures the rate of ET-marked packets per
ingress-egress aggregate. If this excess traffic rate (ETR) is
larger than zero, the PCN egress node communicates it to the
corresponding PCN ingress node using a "traffic-reduction" message.
Then, the PCN ingress node terminates sufficiently many flows to
achieve a reduction of that rate. A minimum interval, a so-called
inter-termination time, is required between consecutive rate-
reduction messages because it takes some time until the effect of the
previous termination action becomes visible in the measured rate of
ET-marked traffic at the PCN egress node. Direct MRT is used as a
non-standard option to terminate traffic in the E3Tunnel deployment
model of 3sm [I-D.babiarz-pcn-3sm].
5.1.2. Indirect MRT
The marking is again the same as in the CL architecture. In contrast
to direct MRT, with indirect MRT the PCN egress node measures the
rate of traffic that is not marked with ET (nETR) per ingress-egress-
aggregate. This is the so-called sustainable rate and the PCN egress
node immediately sends it to the PCN ingress node. The PCN ingress
node measures the rate of traffic to be transmitted per ingress-
egress aggregate, the so-called ingress rate (IR). When the PCN
ingress node receives the sustainable rate from the PCN egress node,
it calculates the difference between the timely corresponding ingress
rate and sustainable rate. This difference is positive if traffic
had been ET-marked because then the ingress rate is larger than the
sustainable rate. If the difference is positive, it is used as an
estimate for the traffic that should be terminated. Indirect MRT is
used as the preferred option in [I-D.briscoe-tsvwg-cl-architecture].
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5.2. Impact of Packet Loss
Packet loss has an impact on the rate of marked and unmarked packets
received by the PCN egress node. In addition, the fraction of marked
and unmarked packets depends on whether packets are first lost or
marked. We illustrate this issue on a link with 8 Mbit/s and its
supportable rate being set to 4 Mbit/s. Due to a reroute or some
other reason, the link is suddenly confronted with a traffic rate of
16 Mbit/s. We consider the behaviour of direct and indirect MRT
under the conditions that traffic is first lost and then marked (L&M)
or first marked and then lost (M&R).
5.2.1. Direct MRT under Lose & Mark
Under L&M, 8 Mbit/s are lost. 4 Mibt/s out of the remaining 8 Mbit/s
are AS-marked and the remaining 4 Mbit/s are ET-marked. As a
consequence, 4 Mbit/s are terminated such that 12 Mbit/s will remain
for the next round. Then, 4 Mbit/s are lost. 4 Mibt/s out of the
remaining 8 Mbit/s are AS-marked and the remaining 4 Mbit/s are ET-
marked. As a consequence, 4 Mbit/s are terminated such that 8 Mbit/s
will remain for the next round. Then, no traffic is lost anymore,
but out of the 8 Mbit/s 4 Mbit/s are AS-marked and the remaining 4
Mbit/s are ET-marked. When these 4 Mbit/s have been terminated,
there is no overload anymore.
5.2.2. Indirect MRT under Lose & Mark
Under L&M, 8 Mbit/s are lost. 4 Mibt/s out of the remaining 8 Mbit/s
are AS-marked and the remaining 4 Mbit/s are ET-marked. As a
consequence, there is a sustainable rate of 4 Mbit/s such that 12
Mbit/s are terminated. Thus, the overload is already removed after a
single termination step.
5.2.3. Direct MRT under Mark & Lose
Under M&L, 4 Mbit/s out of the initial 16 Mbit/s are AS-marked and
the remaining 12 Mbit/s are ET-marked. Then, 8/16 of the traffic is
lost such that 6 Mbit/s arrive with ET-marks. After 6 Mbit/s are
terminated, the system is still confronted with a load of 10Mbit/s
out of which 4Mbit/s are AS-marked and the remaining 6 Mbit/s are ET-
marked. Then, 2/10 of the traffic is lost such that 5.2 Mbit/s
arrive with ET-marks. After 5.2 Mbit/s are terminated, the system is
confronted with a load of 4.8 Mbit/s out of which 4 Mibt/s are AS-
marked and the remaining 0.8 Mbit/s are ET-marked. As no loss occurs
anymore, these 0.8 Mbit/s are terminated such that there is no
overload anymore.
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5.2.4. Indirect MRT under Mark & Lose
Under M&L, 4 Mbit/s out of the initial 16 Mbit/s are AS-marked and
the remaining 12 Mbit/s are ET-marked. Then, 8/16 of the traffic is
lost such that 6 Mbit/s arrive with ET-marks. As a consequence,
there is a sustainable rate of 2 Mbit/s such that 14 Mbit/s are
terminated. Thus, the overload is already removed after a single
termination step, but with a substantial amount of overtermination.
5.2.5. Summary
Table 2 summarizes the termination behavior of direct and indirectMRT
under L&M and M&L conditions. With indirectMRT, sudden SR-overload
is removed after a single termination step while for direct MRT, the
removal of SR-overload requires several termination steps. When
packet loss occurs before packets are marked by the meter and marker,
indirect MRT works well. However, when packets are metered and
marked before they are lost, then indirectMRT can lead to substantial
overtermination.
Table 2: Impact of the order of packet marking and loss on the
termination behavior with direct and indirect MRT.
+-----------------------------------------------------------------+
| Time | Lose&Mark | Lose&Mark | Mark&Lose | Mark&Lose |
| | Direct MRT| Ind. MRT | Direct MRT| Ind. MRT |
+-----------------------------------------------------------------+
|Start of overload|16.0 Mbit/s|16.0 Mbit/s|16.0 Mbit/s|16.0 Mbit/s|
|After 1st term. |12.0 Mbit/s| 4.0 Mbit/s|10.0 Mbit/s| 2.0 Mbit/s|
|After 2nd term. | 8.0 Mbit/s| 4.0 Mbit/s| 5.2 Mbit/s| 2.0 Mbit/s|
|After 3rd term. | 4.0 Mbit/s| 4.0 Mbit/s| 4.0 Mbit/s| 2.0 Mbit/s|
+-----------------------------------------------------------------+
5.3. Unintended Traffic Termination with Indirect MRT through Badly
Aligned Measurement Intervals
Indirect MRT measures the rate of non-ET marked traffic (nETR) at the
egress node and compares it with the rate of the PCN traffic at the
ingress node (IR). This implies that the same packets are measured
in the corresponding measurement intervals which is hard to achieve.
When the delay between PCN ingress and egress node is some fixed
transmission delay X, the measurement interval at the PCN egress node
needs to start X time later than the corresponding one at the PCN
ingress node and it must have the same length to observe the same set
of packets. However, X is not exactly fixed due to queuing and other
effects. Therefore, exact alignment cannot be achieved. This
possibly leads to wrong rate differences when results from badly
aligned measurement intervals are compared. In this section, we
investigate its impact as it can lead to unintended traffic
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termination.
We use a measurement interval length of 100 ms and the misalignment
is 50 ms such that the first packet in the measurement interval a the
PCN egress is contained in the measurement interval at the PCN
ingress.
5.3.1. Experiments with Almost CBR Traffic
First, we use exactly periodic constant bit rate (CBR) traffic and
several variants of almost periodic or almost CBR traffic. Those are
CBR traffic with at most 1 ms uniformly distributed packet arrival
delays and constant packet sizes, exactly periodic traffic with
little variation in packet sizes (coefficient of variation cvar[B] =
0.1), and almost periodic traffic with little variation in packet
inter-arrival times (coefficient of variation cvar[A] = 0.1) and
constant packet sizes. We consider n = 200 flows and the supportable
rate is set to infinity such that no packet is marked. Experiments
show that in case of exact periodicity and equal packet sizes, no
flows are terminated. This is different, when little variation is
introduced where flows are continuously terminated without any packet
being lost or ET-marked.
To repair this, we enhance the indirect MRT method by the side
condition, that flows are only terminated if a tolerance of Tf is
exceeded. This tolerance is usually given as a rate, but we think of
it as a number of flows as we deal only with equal rate flows in this
section. Experiments show that a tolerance of T_f = 1 flow removes
the unintended flow termination effect.
5.3.2. Experiments with VBR Traffic
We now conduct the same experiment with more variable traffic and
study the impact of different tolerance values T_f on unintended
traffic termination. We performed experiments for exactly periodic
flows having packet sizes with the same mean E[B] = 200 bytes but a
coefficient of variation of cvar[B] = 0.5 and experiments for flows
with constant packet sizes but inter-arrival times with the same mean
E[A] = 20 ms but a coefficient of variation of cvar[A] = 0.5. In
both cases, a tolerance value of T_f = 1 flow cannot avoid unintended
flow termination and even a tolerance of T_f = 5 flows cannot fully
remove it.
5.3.3. Experiments with On/Off Traffic
We now look at on/off traffic, i.e. traffic sources have
exponentially distributed on and off phases during which they send
CBR traffic or are silent. We performed experiments for mean phase
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durations of 0.5 s and 5 s, respectively. Again, we see substantial
unintended flow termination that is mitigated by an increasing
tolerance value T_f.
5.3.4. Experiments with Rerouted Traffic
In the presence of network failures, large amounts of traffic are
rerouted, i.e. shifted to other links. The PCN ingress and egress
nodes perform rate measurement using intervals of length of 100 ms
starting at a global time of 0 s. We simulate what happens in such a
scenario at the PCN ingress node. At time 50 ms a reroute of 8Mbit/s
occurs at the PCN ingress node and after another 50 ms the traffic
arrives at the PCN egress node. At that time, the PCN egress node
measures a sustainable rate of 0 Mbit/s for its first measurement
interval and sends this value to the PCN ingress node. At time 150
ms this value arrives there and the PCN ingress node compares the
ingress rate of 4 Mbit/s measured in the first measurement interval
with the sustainable rate of 0 Mbit/s. As a consequence, it
terminates 4 Mbit/s.
5.3.5. Summary
The disadvantage of indirect MRT is that the measurement intervals at
the PCN ingress and egress nodes require some timely alignment.
Otherwise, unintended small positive differences can occur without
any traffic being ET-marked or lost which results in unintended
traffic termination. Therefore, the traffic rate to be terminated
needs to exceed a tolerance threshold $T_f$ before traffic is really
terminated. However, it turns out that the value of that threshold
should increase with increasing traffic variability. Moreover, in
case of sudden extra traffic as it can occur with reroutes, a
substantial fraction of the traffic can be terminated without any
packets being ET-marked.
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6. Performance Evaluation of Marked Flow Termination
Marked flow termination (MFT) is used in 3sm [I-D.babiarz-pcn-3sm].
In contrast to measured rate termination (MRT) it requires that the
metering and marking algorithms in interior nodes mark only some of
the traffic that exceeds the supportable rate. This is done by
adding a slowdown factor of "S" tokens to the bucket whenever a
packet is marked with ET. This is called marking frequency reduction
(MFR).
6.1. CMFT
To work properly, CMFT should use packet size independent marking and
proportional marking frequency reduction. That means, the marker
should add
I_alpha=(2*E[DT]/E[A]-1)/alpha*B
bytes to the token bucket of the marker to slow down the marking
frequency.
o E[DT] is the average flow termination delay
o E[A] is the average interarrival time of packets within flows
o E[R] is the average flow rate
o B is the size of a packet
As a result, one packet is marked for
sigma_b= 2 *E[DT ] * E[R] / alpha
bytes that were above the supportable rate during an overload period
where the PCN rate exceeds the supportable rate. The PCN edge nodes
terminate all flows with at least one marked packet. Performance
studies showed the following results:
o alpha is the termination aggressiveness and controls the
termination speed. alpha>1 leads to overtermination while alpha<1
slows down the termination process. It is a configuration
parameter allowing to control the termination speed.
o The termination process is independent of packet sizes.
o The termination process depends on the average interarrival E[A]
time of packets within flows.
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o Flows with shorter interarrival times have a larger probability to
be marked and terminated.
o The termination process also depends on the variance of
interarrival times of different flows.
6.2. Flow-based EMFT (F-EMFT)
With F-EMFT, a PCN endpoint has a credit counter for each flow which
is reduced by the size of arriving marked packets. If a marked
packet arrives and the credit counter is 0 or negative, the flow is
terminated. Performance studies showed the following results:
o The credit counter size should be initialized randomly according
to an exponential distribution. The average value should be set
to sigma_b.
o The termination behaviour is independent of the packet
interarrival time and the packet sizes of a flow.
o Stochastic flow termination priorities can be implemented by using
different alpha for the initialization of the credit counters.
6.3. Aggregate-based EMFT (A-EMFT)
With A-EMFT, a PCN egress node has a credit counter for each IEA
which is reduced by the size of arriving marked packets. If a marked
packet arrives and the credit counter is 0 or negative, the flow is
terminated. When a flow is terminated, an increment of sigma_b is
added to the credit counter. Performance studies showed the
following results:
o The termination behaviour is rather independent of the number of
flows within the IEA for 1, 10, 50, 200 tested flows.
o The termination behaviour is insensitive against packet
interarrival times and packet sizes of flows.
o Flows with shorter packet interarrival times have larger marking
and termination probabilities.
o Termination policies (e.g. "terminate all small/large flows first"
and others) can be enforced stochastically by keeping a pool of
flows that have recently been marked. When a flow needs to be
terminated, a flow of this pool can be chosen. This works well
when the number of flows in the IEA is large.
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6.4. General Performance of MFT Methods
We consider the common performance of the 3 termination methods:
CMFT, F-EMFT, A-EMFT.
o The termination process of all methods depends on the average
termination delay E[DT] of flows. If flows have different
termination delays, their termination probability is still the
same.
o The time to remove the overload increases with the overload
intensity. However, 100% more overload requires about E[DT] more
time. This holds for all considered methods.
o Statistical effects influence the termination behaviour of all
three methods. However, it is well predictable. The behaviour of
F-EMFT is more variable than the one of CMFT and A-EMFT.
o When a link is used by several IEA, about the same fraction of
traffic should be terminated from each IEA. We call this
termination fairness among aggregates. A-EMFT is visibly fairer
than CMFT and F-EMFT.
o Impact of Traffic Characteristics: We studied the impact of
strongly varying packet sizes and inter-arrival times, but they
had a rather negligible impact on the termination behavior. The
same holds for on/off traffic with exponentially distributed on/
off phase durations and for different average values of these
durations.
6.5. Comparison of CMFT, F-EMFT, and A-EMFT
We highlight the key benefits of MFT and discuss the pros and cons of
CMFT, F-EMFT, and A-EMFT under challenging conditions.
6.5.1. Key Benefits of MFT
MFT methods consecutively terminate only ET-marked flows. This has
several key advantages compared to measured rate termination (MRT) as
suggested in the CL and SM proposal. (1) CMFT and F-EMFT do not
require IEAs. This is necessary for end-to-end PCN. If IEAs are
available, A-EMFT can take advantage of them. (2) MFT works well with
multipath routing, i.e. when flows of a single IEA are carried over
different paths. As MFT terminates only ET-marked flows, it
decreases in any case the load on SR-pre-congested paths which is
different for MRT. (3) Unlike MRT, edge nodes of MFT do not need
measurements that are error-prone due to stochastic variations in
case of low aggregation. (4) MFT decreases the SR-overload only
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gradually such that wrong rate estimates of terminated flows are
compensated by more or less frequent termination of additional flows.
When MRT underestimates flow rates, overtermination occurs as there
is no possibility to correct the termination result.
6.5.2. Unknown Traffic Characteristics
CMFT requires estimates for the average packet inter-arrival time
within flows E[A], the average flow termination delay E[DT], for the
configuration of the marking algorithm of PCN nodes while F/A-EMFT
need only an estimate for E[DT] when we assume that the rates of the
flows are known by the PCN egress nodes or endpoints. Therefore, the
termination behavior is harder to control for CMFT than for F/A-EMFT.
6.5.3. Implementation and Configuration Complexity
CMFT and F-EMFT are simple to implement in the sense that they do not
need IEAs. This is an advantage since IEAs need extra data
structures and it is sometimes difficult to associate flows with
correct IEAs because it is not a trivial to derive the PCN ingress
and egress node for a flow. The termination function of CMFT is very
simple while F/A-EMFT needs initialization and maintenance of credit
counters per flow. With CMFT, the stretch factor ba of the marking
algorithm requires an estimate of the mean packet inter-arrival time
E[A] within flows and the mean flow termination delay E[DT]. The
parameters may be different in different nodes. In contrast, F/A-
EMFT require only an estimate for E[DT] in their egress nodes or
endpoints for the initialization of credit counters and the
calculation of the increments.
In case of end-to-end PCN, the E[DT] must be globally the same value
for the sake of fair termination probabilities. This is especially
difficult for F-EMFT if many distributed endpoints are under the
control of a user instead of an operator. It may be more feasible
for A-EMFT as PCN egress nodes are under the control of operators.
The setting of the aggressiveness a raises similar security issues.
6.5.4. Fairness Issues
Flows with a higher packet rate than others have a higher termination
probability with CMFT and A-EMFT even if they have the same rate. In
contrast, F-EMFT leads to fair termination. A-EMFT balances the
percentage of terminated traffic for different sets of flows when
they are grouped by IEAs. CMFT and F-EMFT do not operate on IEAs and
cannot enforce equal termination among traffic aggregates in a simple
way.
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6.5.5. Termination Priorities and Policies
Both F-EMFT and A-EMFT can implement stochastic termination
priorities by modifying the aggressiveness alpha for a set of flows.
A-EMFT is most flexible with stochastic enforcement of general
termination termination policies that even compensate increased flow
termination probabilities due to short packet inter-arrival times.
CMFT does not offer obvious termination policies. Additional
mechanisms can enable termination priorities. E.g. ET-marked flows
with low termination priority may be terminated only with a certain
probability smaller than 1. However, this adds additional complexity
to egress nodes or endpoints and raises also configuration and
security questions.
6.5.6. Marking Support from Simple ECN Nodes
We cannot expect that all nodes in the Internet will upgrade to PCN.
Therefore, it is desirable to get appropriate feedback from ECN nodes
that are not PCN-capable. When the PCN codepoints NP, AS, and ET are
chosen as ECT(0), ECT(1), and CE, non-PCN-capable ECN nodes indicate
set the markings of some packets to CE, i.e. ET, and thereby
terminate excess flows. The effect of this feature is more
aggressive with CMFT than for F/A-EMFT. However, it is not yet clear
whether this mechanism is helpful or counterproductive.
6.5.7. Compatibility with Existing Hardware
Current hardware offers simple excess marking, but not marking
frequency reduction (MFR), proportional MFR (PMFR) or packet size
independent marking (PSIM) as required by PCN nodes to support CMFT.
Thus CMFT needs new metering and marking features in routers. A/F-
EMFT requires excess marking with PSIM, but PSIM is only required to
achieve equal termination probabilities for flows independent of
their packet size. Therefore, the roll-out of A/F-EMFT could start
without waiting for new router features to be deployed and PSIM may
be added as an improvement by future updates.
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7. Performance Evaluation of Marked Flow Termination (MFT) with
Multiple Bottleneck Links
In the presence of several bottleneck links, flows can be ET-marked
by different interior nodes. As a consequence, an overloaded link
loses flows whose packets were ET-marked by itself, but it also loses
flows whose packets were ET-marked by another overloaded link. This
impacts the termination speed and possibly leads to unintended
overtermination. To avoid this effect, we propose to remove S bytes
from the virtual queue for each ET-marked packet sent over the
corresponding link (general MFR) and not only for each packet that
has been ET-marked by that link itself (local MFR). We illustrate
the impact of this change to the metering algorithm.
7.1. Several Serial Links Carrying Only a Common Aggregate
We start with a single aggregate crossing several links as shown in
Figure 1. All links have a supportable rate of 4Mbit/s such that 125
flows can be supported. However, the observed aggregate initially
carries 8 Mbit/s. Simulation results show that with local MFR, the
aggregate rate is reduced to 3.0 Mbit/s, 2.15 Mbit/s, and to 1.6
Mbit/s in case of 2, 3, and 4 consecutive links. In contrast, with
global MFR, the aggregate rate decreases only to 3.9 Mbit/s in all
studied scenarios. Thus, we observe a significant degree of
overtermination for local MFR while with global MFR the reduction of
the traffic rate meets the expected value quite well and
independently of the number of serial bottlenecks.
+-----------+ +-----------+ +-----------+
Aggregate a0 ---| |---| |---| |---
+-----------+ +-----------+ +-----------+
Link l0 Link l1 Link l2
Several serial links carry only a common aggregate.
Figure 1
To reconstruct this experiment, it is important to avoid the
synchronization of the consecutive meters which is achieved when they
are configured with (a) the same supportable rate (SR) and (b) the
same supportable burst size (SBS) and (c) the same slowdown factor S,
and (d) if there is no cross traffic at all. In our first experiment
depicted in Figure Figure 1, we have chosen a slightly different SR
for each link. In the other experiments that are depicted in
Figure 2- Figure 4, the links carry cross traffic such that
precondition (d) for the synchronization effect is violated anyway.
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7.2. Two Serial Links Carrying a Common Aggregate with Cross Traffic on
the Second Link
We consider two serial links carrying an aggregate a0 and the second
link carrying cross traffic from an aggregate a1 (cf. Figure 2).
+-----------+ +-----------+
---| |---| |--- Aggregate a0
+-----------+ | |--- Aggregate a1
/+-----------+
Link l0 / Link l1
Two serial links carrying a common aggregate with cross traffic on
the second link.
Figure 2
The first link l0 has SR(l0) = 4 Mbit/s and the second link l1 has
SR(l1) = 8Mbit/s. Initially, there is an SR-overload of 100% since
both aggregates carry 8 Mbit/s. Simulation results show that the
rate of aggregate a0 decreases from 8 Mbit/s to 2.9 Mibt/s with local
MFR and to 3.0 Mbit/s with global MFR. The rate of aggregate a1
(cross traffic) is reduced from 8 Mibt/s to 4.4 Mbit/s with local MFR
and to 4.8 Mbit/s with global MFR. As a consequence, the remaining
rate on the second link l1 is 7.3Mbit/s with localMFR and 7.8Mbit/s
with globalMFR. Thus, global MFT leads to visibly less terminated
traffic in this experiment.
7.3. Two Serial Links Carrying a Common Aggregate with Cross Traffic on
the First Link
We consider two serial links carrying an aggregate a0 and the first
link carrying cross traffic from an aggregate a1 (cf. Figure 3).
+-----------+ +-----------+
Aggregate a0 ---| |---| |---
| | +-----------+
/+-----------+\
Aggregate a1 / Link l0 \ Link l1
Two serial links carrying a common aggregate with cross traffic on
the first link.
Figure 3
We consider two serial links carrying an aggregate a0 and the first
link carrying cross traffic from an aggregate a1 (cf. Figure 3). The
first link l0 has SR(l0) = 8 Mbit/s and the second link l1 has SR(l1)
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= 4 Mbit/s. Initially, there is an SR-overload of 100% since both
aggregates carry 8 Mbit/s. Initially, there is an SR-overload of
100% since both aggregates carry 8 Mbit/s. Simulation results show
that the rate of aggregate a0 decreases from 8 Mbit/s to 2.8 Mibt/s
with local MFR and to 3.8 Mbit/s with global MFR. The rate of
aggregate a1 (cross traffic) is reduced from 8 Mibt/s to 4.5 Mbit/s
with local MFR and to 4.0 Mbit/s with global MFR. As a consequence,
the remaining rate on the second link l0 is 7.3 Mbit/s with local MFR
and 7.8 Mbit/s with global MFR. Thus, global MFT leads to visibly
less terminated traffic in this experiment and to a fairer treatment
of flows crossing a different numbers of bottlenecks.
7.4. Two Serial Links Carrying a Common Aggregate with Cross Traffic on
Both Links
We consider two serial links carrying a common aggregate with cross
traffic on both links (cf. Figure 4).
+-----------+ +-----------+
Aggregate a0 ---| |-----| |---
Aggregate a1 ---| | | |--- Aggregate a2
+-----------+\ /+-----------+
Link l0 \ / Link l1
Two serial links carrying a common aggregate with cross traffic on
both links.
Figure 4
We consider two serial links carrying an aggregate a0. The first
link carries cross traffic from aggregate a1 and the second link
carries cross traffic from aggregate a2 (cf. Figure 4). Both links
have a supportable rate of 8 Mbit/s. Initially, there is an SR-
overload of 100% since all three aggregates carry 8 Mbit/s.
Simulation results show that the rate of aggregate a0 decreases from
8 Mbit/s to 2.6 Mibt/s with local MFR and to 2.7 Mbit/s with global
MFR. The rate of aggregate a1 (cross traffic on link l0) is reduced
from 8 Mibt/s to 4.5 Mbit/s with local MFR and to 4.3 Mbit/s with
global MFR and the one of aggregate a2 (cross traffic on link l1) is
reduced from 8 Mibt/s to 4.3 Mbit/s with local MFR and to 5.1 Mbit/s
with global MFR. As a consequence, the remaining rate on the first
link l0 is 7.2 Mbit/s with local MFR and 7.8 Mbit/s with global MFR
and the one on the second link l1 is 7.2 Mbit/s with local MFR and
7.1 Mbit/s with global MFR. Thus, global MFT leads to visibly less
terminated cross traffic on the second link in this experiment.
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7.5. Summary
Global MFR leads in all studied experiments to less terminated
traffic than local MFR. Especially in case of several serial links
carrying only a single aggregate the degree of overtermination is
tremendous with local MFR while global MFR does not lead to
overtermination in that scenario. In other experiments with cross
traffic the difference between local and global MFR is still visible
but less dramatic. Nevertheless, global MFR is clearly better than
localMFR. However, multiple simultaneous bottleneck links are rather
unlikely and, in particular, the situation with several serial
bottleneck links where tremendous overtermination with local MFR
occurs is rather pathologic. Therefore, local MFR may still be used
instead of global MFR if it reduces implementation complexity.
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8. Performance Evaluation of a Marking Converter for Excess Marking
Both F-EMFT and A-EMFT rely on the assumption that all packets
exceeding a supportable rate SR(l) on a link l of the PCN-domain are
ET-marked (cf. Section 6). This means that excess marking is
performed with the supportable rate SR as reference rate.
In contrast, the Single-Marking (SM) draft
([I-D.charny-pcn-single-marking]) proposes not to use ET-marks for
flow termination but only AS-marks. This save one codepoint in the
IP-header. It proposes to AS-mark all packets exceeding the
admissible rate AR, i.e. to perform excess marking with regard to AR
and the AR-overlaod corresponds directly to the rate of AS-marked
packets. In order to estimate SR-overload, SR(l)=u*AR is set to a
fixed multiple u of AR where u is a domain-wide constant.
[I-D.menth-pcn-marking-converter] proposes an algorithm which
converts marked AR-overload into marked SR-overload. It makes flow
termination mechanisms requiring SR-overload (e.g. F-EMFT and
A-EMFT) applicable in networks that mark AR-overload only. Here, we
evaluate the performance by simulation experiments.
8.1. Simulation Setup
We simulate 200 periodic real-time traffic source with a packet
inter-arrival time of 20 ms and a packet size of 200 byte, i.e.,
every flow has a bandwidth of 80 kbit/s and all flows together result
in a traffic rate of 16 Mbit/s. The traffic source are equally
distributed among n_Agg = (1,10,50,100,200) ingress-egress-
aggregates. All aggregates use the bottleneck link with an
admissible rate of AR=5 Mbit/s and u=1.6. Hence, the supportable
rate is SR=8 Mbit/s so that only half of the traffic can be supported
by the bottleneck link.
The bottleneck link marks AR-overload as proposed in the SM draft.
At the PCN egress node, the marking converter converts AR-overload
into SR-overload in a first step. In a second step, it performs
F-EMFT (for n_Agg=200) and A-EMFT (for n_Agg<200) based on the SR-
overload with an aggressiveness alpha=1.
8.2. Results
We investigate the time until overload is reduced and the amount of
overtermination. Overtermination occurs when the traffic rate on the
link is significantly below the supportable rate after termination.
We analyze the impact of bucket size S of the marking converter and
the number of aggregates sharing the bottleneck link. In summary,
first simulations give a proof of concept. However, we also observed
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some limitations.
For n_Agg<=10, we get fast termination and no overtermination even in
case of small bucket sizes S. The termination behavior is very
similar to the one described in Section 6.
However, we observe a considerable amount of overtermination (up to
20 % of SR) if many aggregates share a common bottleneck link. The
main reason of this phenomenon is that marked and unmarked packets
are distributed randomly over all aggregates. Consequently, one
aggregate a1 can suffer from a rate marked packets significantly
higher than the average while another one receives only a few marked
packets. Hence, flows can be terminated on aggregate one because the
PCN egress node detected to many marked packets.
We tried to minimize that effect be choosing larger bucket sizes S
because a large bucket can compensate fluctuations of the rate of
marked packets up to a certain degree. But S cannot be increased to
arbitrarly large values as this delays the reaction of the marking
converter in case of sudden overload. Our simulations showed that
even bucket sizes which delayed the reaction several seconds could
not prevent considerable overtermination. As a consequence, using
the marking converter is difficult in a scenario where many
aggregates share a single bottleneck link.
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9. IANA Considerations
TBD
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10. Security Considerations
TBD
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11. Changes from Previous Revisions
11.1. Changes from Version -00 to Version -01
o Added section on "Performance Evaluation of Admission Control with
Single-Marking"
o Added section on "Performance Evaluation of Measured Rate
Termination (MRT)"
o Added section on "Performance Evaluation of Marked Flow
Termination on a Single Link"
o Added section on "Performance Evaluation of Marked Flow
Termination with Multiple Bottleneck Links"
11.2. Changes from Version -01 to Version -02
o Completely updated Section 4
o Completely updated Section 6
11.3. Changes from Version -02 to Version -03
o Added section "Performance Evaluation of a Marking Converter for
Excess Marking"
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12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
12.2. Informative References
[I-D.babiarz-pcn-3sm]
Babiarz, J., Liu, X., Chan, K., and M. Menth, "Three State
PCN Marking", draft-babiarz-pcn-3sm-01 (work in progress),
November 2007.
[I-D.briscoe-tsvwg-cl-architecture]
Briscoe, B., "An edge-to-edge Deployment Model for Pre-
Congestion Notification: Admission Control over a
DiffServ Region", draft-briscoe-tsvwg-cl-architecture-04
(work in progress), October 2006.
[I-D.charny-pcn-single-marking]
Charny, A., Zhang, X., Faucheur, F., and V. Liatsos, "Pre-
Congestion Notification Using Single Marking for Admission
and Termination", draft-charny-pcn-single-marking-03
(work in progress), November 2007.
[I-D.ietf-pcn-architecture]
Eardley, P., "Pre-Congestion Notification Architecture",
draft-ietf-pcn-architecture-03 (work in progress),
February 2008.
[I-D.menth-pcn-emft]
Menth, M., Lehrieder, F., Eardley, P., Charny, A., and J.
Babiarz, "Edge-Assisted Marked Flow Termination",
draft-menth-pcn-emft-00 (work in progress), February 2008.
[I-D.menth-pcn-marking-converter]
Menth, M. and F. Lehrieder, "Marking Converter for Excess-
Marked Traffic", July 2008.
12.3. Other References
[Menth07] Menth, M. and F. Lehrieder, "Performance Evaluation of
PCN-Based Admission Control and Flow Termination (work in
progress)", November 2007, <http://
www3.informatik.uni-wuerzburg.de/staff/menth/Publications/
Menth07-PCN-Eval.pdf>.
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[Menth08-AC]
Menth, M. and F. Lehrieder, "Performance Evaluation of
PCN-Based Admission Control", February 2008, <http://
www3.informatik.uni-wuerzburg.de/staff/menth/Publications/
Menth08-PCN-AC.pdf>.
[Menth08-MFT]
Menth, M. and F. Lehrieder, "Termination Methods for End-
to-End PCN-Based Flow Control", February 2008, <http://
www3.informatik.uni-wuerzburg.de/staff/menth/Publications/
Menth08-PCN-MFT.pdf>.
[TR437] Menth, M. and F. Lehrieder, "Comparison of Marking
Algorithms for PCN-Based Admission Control, Technical
Report No. 437", October 2007, <http://
www-info3.informatik.uni-wuerzburg.de/TR/tr437.pdf>.
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Authors' Addresses
Michael Menth
University of Wuerzburg
Am Hubland
Wuerzburg D-97074
Germany
Phone: +49-931-888-6644
Email: menth@informatik.uni-wuerzburg.de
Frank Lehrieder
University of Wuerzburg
Am Hubland
Wuerzburg D-97074
Germany
Phone: +49-931-888-6634
Email: lehrieder@informatik.uni-wuerzburg.de
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