Network Working Group J. Babiarz
Internet-Draft X-G. Liu
Intended status: Informational Nortel
Expires: January 9, 2008 July 8, 2007
Simulations Results for 3sM
draft-babiarz-pcn-explicit-marking-01
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Abstract
This document describes the simulation setups and results for testing
the Three State PCN Marking approach. Simulations done to date,
demonstrate that the three state PCN marking approach has certain
ability to support admission control and flow termination of real-
time application flows at the congestion point of the PCN-enabled
network. The real-time traffic used in the simulation covers voice
and video traffic with large and small number of flows.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology used in this Document . . . . . . . . . . . . 4
1.2. Overview of Three State PCN Marking Approach . . . . . . . 5
2. General Description of the Simulation Setup . . . . . . . . . 5
2.1. Traffic Sources . . . . . . . . . . . . . . . . . . . . . 6
2.2. Multiple PCN Nodes . . . . . . . . . . . . . . . . . . . . 7
2.3. Traffic Control . . . . . . . . . . . . . . . . . . . . . 8
3. Performance of 3sM . . . . . . . . . . . . . . . . . . . . . . 8
3.1. Performance of Flow Termination . . . . . . . . . . . . . 8
3.1.1. Large Number of Flows in a Single Domain . . . . . . . 9
3.1.2. Small Number of Flows in a Single Domain . . . . . . . 11
3.1.3. Large Number of Flows in a Multi Domain . . . . . . . 12
3.1.4. Discussion of Parameter Settings . . . . . . . . . . . 13
3.2. Performance of Admission Control . . . . . . . . . . . . . 14
3.2.1. Simulation Results for Admission Control . . . . . . . 16
4. Simulation Results Prior to 68th IETF . . . . . . . . . . . . 18
4.1. Simulation Setup for Voice Traffic . . . . . . . . . . . . 19
4.2. Large Number of Voice Flows . . . . . . . . . . . . . . . 20
4.3. Small Number of Voice Flows . . . . . . . . . . . . . . . 21
4.4. Large Number of Voice Flows with Packet Loss . . . . . . . 23
4.5. Small Number of Voice Flows with Packet Loss . . . . . . . 24
4.6. Corner Voice Cases Studied . . . . . . . . . . . . . . . . 25
4.7. Simulation Setup for Video Traffic . . . . . . . . . . . . 25
4.8. Excess Load Marking Algorithm Used in Simulation . . . . . 27
5. Security Considerations . . . . . . . . . . . . . . . . . . . 28
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28
7. Informative References . . . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29
Intellectual Property and Copyright Statements . . . . . . . . . . 31
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1. Introduction
In a PCN-enabled network, each link is configured with an admissible
rate (AR). When the PCN traffic rate on a link exceeds its AR, the
corresponding PCN node re-marks all PCN packets on this link with an
"admission-stop" (AS) codepoint. The PCN egress nodes analyze the
packet markings, and if sufficiently many packets are AS-marked
within an ingress-egress aggregate, signal "admission-stop" for this
aggregate to the appropriate admission control entity to stop
admitting flows belonging to this aggregate so as to avoid the PCN
traffic rate to exceed AR. When the PCN egress nodes stop receiving
AS-marked packets, they signal "admission-continue" after some time
to allow admitting flows from the blocked aggregate.
Similarly, a supportable rate (SR) is configured for each link in a
PCN-enabled network. When the current PCN traffic rate on a link
exceeds its SR, the corresponding PCN node re-marks some of the PCN
packets on this link with an "excess-traffic" (ET) codepoint. The
PCN egress nodes pass the marking information to the appropriate flow
termination entity (e.g. at the respective PCN ingress nodes) to
terminate flows in order to reduce the PCN traffic rate of the SR-
overloaded link below its SR. The reader is referred to
[I-D.eardley-pcn-architecture] for details.
The purpose of this document is to evaluate the performance of the
proposed three state PCN marking (3sM) [I-D.babiarz-pcn-3sm]
approach. We provide an overview of the simulations setup and the
results of testing that were carried out. Simulation demonstrate
that the three state PCN marking (3sM) approach has certain ability
to support admission control and flow termination of real-time
application flows at the congestion point of the PCN-enabled network.
The simulation is based on modeling the real-time traffic of voice,
both constant bit rate and variable bit rate with silence
suppression, and variable rate MPEG-4 like video with large and small
number of flows. The preliminary key findings of the simulation are:
o Both the AR- and SR-meters are able to be adjusted to provide the
desired traffic control to a certain degree, i.e., limiting the
traffic in the network within some tolerance level for the test
cases.
o The setting of the two meters are usually not sensitive or can be
set proportional to the traffic load (BW and number of flows) in
the test cases.
o The effectiveness of the AR-meter and AS-marker is similar for a
single bottleneck node and a number of bottleneck nodes. (the SR-
meter and ET-marking has not been tested for this scenario, it is
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on the to do list.)
o The precise control of mixed VBR traffic for admission control is
difficult with small number of flows, as expected additional
mechanism may be needed.
o Flow termination using the proposed SR-metering and ET-marking:
* Is independent of the number of flows at the congestion point
or in an ingress-egress aggregate, works equality well on small
and large links.
* Works over a large range of network RTTs.
* Works well with packet loss.
* The desired setting for "s" marking slow-down parameter can be
determined based on a given formula.
* ET-marking identifies flows that are routed on overloaded
paths, therefore when multiple paths exist in a network the
edge nodes or explicitly informed which flows should be
terminated.
* ET-marking is proportional to the level of overload, the higher
the overload the more packets are marked.
* ET-marking has an exponential decay property.
1.1. Terminology used in this Document
Since the terminology for this work is evolving, we provide a brief
explanation of terms used in this document and the referenced
simulation results.
Preemption = flow termination
SR = Supportable Rate = Preemption Threshold
Preemption Level = traffic above this rate is marked as excess.
Same as Supportable Rate.
ET-marking = PM flag = explicit marking of packet to indicated
excess load. In the simulation, the router sets both ECN bits to
"11" in the IP header.
Preemption Time = round-trip-time (RTT) in the network +
processing time of termination of a flow. This is how long it
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takes before a marked flow to stops sending packets.
"s" slow-down marking factor = pcn_px = represents marking a
packet every "x" bytes of excess rate.
AR = Admissible Rate
AS-marking = Admission Stop-marking = marking to indicated that
additional new flows should be blocked.
1.2. Overview of Three State PCN Marking Approach
For AR metering, the proposed approach defines an AR-meter and AS-
marker based on a token bucket (TB) with threshold marking. The TB
has a bucket of size TB.size which is continuously filled with tokens
at rate TB.rate. The AR-meter and AS-marker consider only packets
that are not ET-marked. When a non-ET-marked PCN packet arrives, it
is re-marked to "AS" if the fill state of the bucket (TB.fill) in
tokens is smaller than its size (packet.size) in bytes; otherwise,
the fill state is reduced by packet.size tokens and if the fill state
is then smaller than the marking threshold (TB.threshold), the packet
is also re-marked to "AS" while if the fill state is then larger than
or equal to the marking threshold, the packet is not re-marked.
For SR metering, the proposed approach defines an SR-meter and ET-
marker based on a token bucket with tail marking and marking
frequency reduction (see Appendix A for explanation). The TB has a
bucket of size TB.size which is continuously filled with tokens at
rate TB.rate. When a non-ET-labelled PCN packet arrives, it is re-
marked with "ET" if the fill state of the bucket (TB.fill) in tokens
is smaller than its size (packet.size) in bytes and "s" additional
tokens are added to the bucket; otherwise, the fill state is reduced
by packet.size tokens. The slow-down parameter "s" reduces the
marking frequency of the mechanism. If an ET-marked packet arrives,
the TB's fill state is also incremented by "s".
2. General Description of the Simulation Setup
The simulation model used in our experiments consists of the traffic
sources, the PCN-enabled network nodes, and the traffic control loop
(admission control and flow termination entities) Figure 1.
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+-------------------------------------+
| Block/start admission or flow |
| termination signal with time lag |
V |
+-----------+ +---------------+ +---------+
| | | | | |
| Traffic | | PCN Node | | Traffic |
| Sources | ===>| with |===>| Control |
| | | AR/SR Meter & | | |
+-----------+ | Marker | +---------+
| |
+---------------+
Figure 1: PCN Marking Simulation Model
2.1. Traffic Sources
The traffic source model can generate voice or video flows (calls)
according to the Poisson arrival process with a given arrival rate.
A Poisson arrival can contain one or more flows. The arrival batch
size is a random variable with a given mean batch size. To model the
reroute events in the network, the traffic source model can also
generate flows at scheduled time points and/or within scheduled short
time intervals. The model also allows some flows to start together,
e.g., a voice flow plus a video flow.
Each flow has a random life-span (holding time) with a given mean
holding time.
During its life time, a flow periodically generates packets based on
a given codec and packetization scheme such as G.711 for voice over
IP. Depending on the type of application and codec used in
simulation, the packets from a flow can have fixed or variable sizes,
and the inter-arrival time between the packets can be fixed or
variable. To model the applications such as G.711 with silence
suppression, the packet generation of a flow can be described by an
ON-OFF process with given mean ON and OFF periods. With an ON-OFF
flow, the packet can only be generated in the ON-period of the flow.
An ON-OFF flow may start in either the ON or OFF state.
When a flow starts, it can delay the generation of its first packet
for some random time up to a given time limit, say 10 seconds. This
delay is used for modeling the media delay of the call setup process
for telephony application. To avoid unrealistic synchronization
effects in the network, in any case, the start of the first packet
from a flow is always randomized within a given small time interval
after the flow start time, which is independent of the media delay.
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The generation of packets for different flows are independent of each
other.
There can be mixed types of flows in the network. Each flow belongs
to a given traffic aggregate with a fixed route crossing the network.
Different types of flows can be in the same traffic aggregate.
When modeling flow admission control, after a flow starts, the
traffic source model will check if the traffic aggregate is blocked
for admission. If the aggregate is blocked, any new flows will
immediately be turned off (blocked) without generating any packet.
The blocking of a traffic aggregate for admission will not affect the
existing flows of the aggregate.
When modeling flow termination, the traffic source model may receive
signals for terminating particular flows. Upon receiving the flow
termination signal, the affected flow will immediately be turned off,
stopping generating of packets.
2.2. Multiple PCN Nodes
For multiple PCN nodes simulations, we use a "parking lot" model with
n nodes in tandem. A traffic aggregate uses a given segment of the
n-node tandem to cross the network, i.e., its traffic will enter the
network at node i and exit the network at node j (1<=i<=j<=n), where
all the nodes in the segment are consecutively numbered.
The node is configured with queue size and a given service rate or
link bandwidth. The queue can be configured to have finite or
unlimited buffer size. When a PCN packet arrives at a node, before
entering the queue, the packet is metered by the AR-meter and/or SR-
meter and re-marked if needed. The AR-meter and SR-meter are modeled
using the example pseudo codes provided in [I-D.babiarz-pcn-3sm].
After AR- and/or SR-metering, the packet enters the queue to be
forwarded or is discarded if the queue is full. After forwarding,
the packet will proceed to the next node or exit the network,
depending on the definition of its traffic aggregate. The packet
travel between nodes is instant.
Upon exiting the network, the packet will be checked by the traffic
control for its PCN marking and then destroyed. Based on the PCN
marking of the packet, the traffic control will decide if it needs to
signal the traffic model for a given type of traffic control: block
new flow, restart admission of a particular traffic aggregate and/or
terminate a particular flow. The signal to the traffic source model
will experience certain delay or round-trip-time (RTT). The RTT can
be modeled as a fixed time for all the flows or different RTT per
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each flow. (Term RTT signifies the delay between generating a packet
and receiving the corresponding traffic control feedback at the
traffic source. But, in our model, RTT does not include any queueing
delay experienced by the packet.)
2.3. Traffic Control
In the simulation for admission control, the "block admission signal"
will be sent whenever the traffic control sees an "AS" marking in the
packet. The receiver (ingress) of the signal controls admission of
all new flows within the aggregate and the same route. If the
traffic aggregate is currently not blocked, the receiving of the
block admission signal will trigger a "stop blocking timer" with a
preset timeout. At timeout, the traffic control will check if there
is one more block admission signal sent for the traffic aggregate
during the timeout period, and if so, it will restart the timer.
This process will repeat until there is no block admission signal
sent for the traffic aggregate in the past timeout period. At this
time, the traffic control will send the start admission signal for
the traffic aggregate to allow it to admit flows into the network
from now on.
In the simulation for flow termination, a flow termination signal
will be sent to the traffic source model for each ET-marked packet.
Since SR is by definition greater than AR, a flow termination signal
will also generate a block admission signal to the related traffic
aggregate if admission control is being modeled at the same time.
For more details of the simulation setup, see the case description
sections in this document.
3. Performance of 3sM
In this section we discuss the simulations results that where
performed in time for 69th IETF meeting. Graphical results of the
simulations can be viewed at
http://standards.nortel.com/pcn/3sM-Simulation-1.pdf [SIM1-07]. See
also Section 4 for simulations results that where done prior to 68th
IETF meeting.
3.1. Performance of Flow Termination
The following simulations were performed to measure how long it takes
for the defined mechanism in 3sM to reduce the aggregate traffic
after condition where significant overload of PCN traffic occurred on
a link (like after fast reroute of traffic due to link failure).
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The simulation setup emulates a condition where all PCN traffic is
rerouted instantaneously from the failed link on to a good link that
was at 50% or 100% of supportable rate (SR). The rerouted PCN
traffic from the failed link is equivalent to SR of the remaining
link, so that after reroute the load on the link is 150% and 200% of
SR.
Simulations were done with CBR and variable rate silence suppressed
voice traffic sources. Our traffic generation model produces many
individual flows that represented one of the codecs. Results are
recorded for the following codec mix:
o G.711CBR = G.711 with 20ms packetization time CBR, (200 bytes
packets sent every 20ms)
o 3VBR+CBR = 4 different code mix. 3 codecs running silence
suppression per ITU-T P.56, G.711 at 20ms (200 byte packets),
G.711 at 10ms (120 byte packets) and G.729 at 20ms (60 byte
packets). And one dual-rate codec that sends packets at constant
rate, 360 byte packets every 20ms. Each of the codec types
generates approximate 20% of SR traffic measured as a rate. Note,
that there is significantly more number of G.729VBR flows than
flows generated by the dual-rate codec. The traffic mix for 3VBR+
CBR produces a 15 to 1 bandwidth ratio; the highest flow rate is
15 times bigger than the lowest rate within the mix for this
simulation.
3.1.1. Large Number of Flows in a Single Domain
For these simulations, it was assumed that the RTT within a single
domain would be less than 50ms, therefore we simulated with 50ms as
the RTT. However, normally RTT between different ingress-egress
nodes will very, therefore typical results would produce shorter
delays than the corner case that we simulated using 50ms for delay
after marking for flow termination. Large number of flows, equates
approximately 500 to 4,250 flows depending on the codec mix used to
generate SR of 40 Mbps.
Parameter setting:
o Token bucket of the meter was configured to be 50,000 bytes in
size
o Supportable Rate = 40.0 Mbps
o FT-marking reduction factor "s" was set to 1064 bytes.
The table in Figure 2 summarizes the results of how long it took to
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terminate the excess traffic form 200% of SR to SR. Also we provide
the measured traffic rate and variation after flow termination was
completed. The rate of remaining traffic was measured over 12 second
period and results are recorded in table below as average, maximum,
minimum and the variances.
------------------------------------------------------------------
| | % Overload, time in sec. | Bandwidth in Mbps |
|----------+---------------------------+---------------------------|
| Traffic | 150% | 125% | 110% | 100% | AVG | Max | Min | Var |
|----------+------+------+------+------+------+------+------+------|
| G.711CBR | 0.15 | 0.20 | 0.25 | 0.50 | 40.0 | 40.0 | 40.0 | 0 |
|----------+------+------+------+------+------+------+------+------|
| 3VBR+CVR | 0.15 | 0.20 | 0.30 | ~2 | 38.9 | 41.7 | 36.7 | 5.00 |
------------------------------------------------------------------
Figure 2: Overload at 200% of SR with "s" set to 1064 bytes
The table in Figure 3 summarizes the results with FT-marking
reduction factor "s" set to 2064 bytes.
------------------------------------------------------------------
| | % Overload, time in sec. | Bandwidth in Mbps |
|----------+---------------------------+---------------------------|
| Traffic | 150% | 125% | 110% | 100% | AVG | Max | Min | Var |
|----------+------+------+------+------+------+------+------+------|
| G.711CBR | 0.15 | 0.30 | 0.45 | 1.20 | 40.0 | 40.0 | 40.0 | 0 |
|----------+------+------+------+------+------+------+------+------|
| 3VBR+CVR | 0.20 | 0.35 | 0.85 | ~3 | 38.9 | 41.8 | 36.3 | 5.52 |
------------------------------------------------------------------
Figure 3: Overload at 200% of SR with "s" set to 2064 bytes
The table in Figure 4 summarizes the results of how long it took to
terminate the excess traffic form 150% of SR to SR with "s" set to
2064 bytes.
------------------------------------------------------------------
| | % Overload, time in sec. | Bandwidth in Mbps |
|----------+---------------------------+---------------------------|
| Traffic | 150% | 125% | 110% | 100% | AVG | Max | Min | Var |
|----------+------+------+------+------+------+------+------+------|
| G.711CBR | - | 0.20 | 0.35 | 1.05 | 40.0 | 40.0 | 40.0 | 0 |
|----------+------+------+------+------+------+------+------+------|
| 3VBR+CVR | - | 0.20 | 0.40 | ~2 | 38.7 | 41.6 | 35.4 | 6.30 |
------------------------------------------------------------------
Figure 4: Overload at 150% of SR with "s" set to 2064 bytes
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3.1.2. Small Number of Flows in a Single Domain
For these simulations, it was assumed that the RTT within a single
domain would be less than 50ms, therefore we simulated with 50ms as
the RTT. However, normally RTT between different ingress-egress
nodes will very, therefore typical results would produce shorter
delays than the corner case that we simulated using 50ms for delay
after marking for flow termination. Small number of flows equates
approximately 10 to 30 flows depending on the codec mix used to
generate SR of 0.8 Mbps.
Parameter setting:
o Token bucket of the meter was configured to be 10,000 bytes in
size
o Supportable Rate = 0.8 Mbps
o FT-marking reduction factor "s" was set to 1064 bytes.
The table in Figure 5 summarizes the results of how long it took to
terminate the excess traffic form 200% of SR to SR. Also we provide
the measured traffic rate and variation after flow termination was
completed. The rate of remaining traffic was measured over 12 second
period and results are recorded in table below as average, maximum,
minimum and the variances.
------------------------------------------------------------------
| | % Overload, time in sec. | Bandwidth in Mbps |
|----------+---------------------------+---------------------------|
| Traffic | 150% | 125% | 110% | 100% | AVG | Max | Min | Var |
|----------+------+------+------+------+------+------+------+------|
| G.711CBR | 0.20 | 0.25 | 0.30 | 0.40 | 0.80 | 0.80 | 0.80 | 0 |
|----------+------+------+------+------+------+------+------+------|
| 3VBR+CVR | 0.25 | 0.35 | 0.40 | ~2 | 0.74 | 1.07 | 0.50 | 0.57 |
------------------------------------------------------------------
Figure 5: Overload at 200% of SR with "s" set to 1064 bytes
The table in Figure 6 summarizes the results with FT-marking
reduction factor "s" set to 2064 bytes.
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------------------------------------------------------------------
| | % Overload, time in sec. | Bandwidth in Mbps |
|----------+---------------------------+---------------------------|
| Traffic | 150% | 125% | 110% | 100% | AVG | Max | Min | Var |
|----------+------+------+------+------+------+------+------+------|
| G.711CBR | 0.30 | 0.40 | 0.60 | 0.65 | 0.80 | 0.80 | 0.80 | 0 |
|----------+------+------+------+------+------+------+------+------|
| 3VBR+CVR | 0.30 | 0.35 | 0.45 | ~4 | 0.74 | 1.05 | 0.51 | 0.54 |
------------------------------------------------------------------
Figure 6: Overload at 200% of SR with "s" set to 2064 bytes
3.1.3. Large Number of Flows in a Multi Domain
For these simulations, it was assumed that the RTT in a multi domain
network would be less than 200ms, therefore we simulated with 200ms
as the RTT. However, normally RTT between different ingress-egress
nodes will very, many flows would have shorter than 200ms RTT,
therefore typical results would produce shorter delays than the
corner case that we simulated using 200ms for delay after marking for
flow termination. Large number of flows equates approximately 500 to
4,250 flows depending on the codec mix used to generate SR of 40
Mbps. Performance results for RTT of 800ms can be found in [SIM-07].
Parameter setting:
Token bucket of the meter was configured to be 50,000 bytes in
size
Supportable Rate = 40.0 Mbps
FT-marking reduction factor "s" was set to 4064 bytes.
The table in Figure 7 summarizes the results of how long it took to
terminate the excess traffic form 200% of SR to SR. Also we provide
the measured traffic rate and variation after flow termination was
completed. The rate of remaining traffic was measured over 12 second
period and results are recorded in table below as average, maximum,
minimum and the variances.
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------------------------------------------------------------------
| | % Overload, time in sec. | Bandwidth in Mbps |
|----------+---------------------------+---------------------------|
| Traffic | 150% | 125% | 110% | 100% | AVG | Max | Min | Var |
|----------+------+------+------+------+------+------+------+------|
| G.711CBR | 0.45 | 0.65 | 0.90 | 1.6 | 40.0 | 40.0 | 40.0 | 0 |
|----------+------+------+------+------+------+------+------+------|
| 3VBR+CVR | 0.45 | 0.75 | 1.7 | ~4 | 38.9 | 42.3 | 36.1 | 6.24 |
------------------------------------------------------------------
Figure 7: Overload at 200% of SR with "s" set to 4064
3.1.4. Discussion of Parameter Settings
Token bucket sizes:
The size of the token bucket filters out short term rate
variations. Normally, larger token bucket is need for highly
variable traffic. The draw back of configuring token bucket too
big is that it will delay the start of FT-marking (flow
termination).
"s" FT-marking reduction factor:
The "s" parameter controls how often packets are marked when in
overload. SR-meter measures traffic that is in excess of SR and
FT-marker marks a packet ever "s" bytes of excess traffic. FT-
marking is proportional to the overload, the higher the overload
the higher the number of packets get FT-marked.
In our simulations we used the following equation to compute the
value for "s"; average rate of a flow * RTT * 2 = s; we used the
rate of G.711 at 20ms CBR codec for the average rate in the
calculations.
Making "s" too small leads to over flow termination due to the
delay in the response. A flow is terminated RTT after it is
marked.
As observed in simulations, this flow termination mechanism has
exponential decay property and to prevent over termination, the
period between ET-marking when PCN traffic rate is one flow above
SR needs to be greater than 2 * RTT. Making "s" too big leads to
longer termination time.
The "s" parameter has the biggest impact on how fast or slow
excess traffic is reduced.
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RTT - total delay for termination of flows (network + ingress and
egress processing delays.)
Since PCN is a responsive mechanism, node meters traffic and ET-
mark packets indicate the traffic is in excess of SR, the time
that it take for the indication that flow needs to be terminated
and the reduced load on to the overloaded link is what we call
RTT. RTT has direct impact on how fast the overload condition can
be eliminated.
3.2. Performance of Admission Control
The purpose of the simulation experiments with admission control is
to test the ability of the AR-meter and AS- marker of 3sM to support
admission control in a PCN-enabled network and to observe the
behavior of the AR-meter and AS-marker as a function of its settings
and the traffic and network environments.
For this purpose, we have performed the following preliminary
simulation tests:
o Erlang-B Test: test if the AR-meter and AS-marker can support
admission control similarly to the Erlang blocking system for CBR
traffic at a single node.
o Overload Protection Test: test the above with 2x base load and
everything else being the same.
o Multiple-congested-node Test: test the performance of the AR-meter
and AS-marker configured for a single node applies to a three-
identical congestion-node environment with CBR traffic of 2x base
load; traffic aggregate A1: route 1->2->3.
o Cross-traffic Test: similar to the above, with additional CBR
traffic aggregates from different routes carried in the ("parking-
lot") network, where the aggregate traffic at each node with
cross-traffic is increased proportionally to the combined load; 2
traffic aggregates are used, A1: route 1->2->3 with 2x base load,
A2: route 2->3->4 with base load.
o VBR Test: test the performance of the AR-meter and AS-marker
configured for Erlang-B Test applied to VBR traffic at a single
node, where the AR is set proportionally to the expected data rate
of the aggregate traffic sources.
o Traffic Mix Test: similar to the above, with combined VBR/CBR
traffic served by the single node.
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In all the above cases, the following settings are used unless
otherwise indicated.
Traffic settings:
o the base load defined as the number of the targeted flows to be
carried by the system, N, where N=10 for small load (S), N=50 for
medium load (M), and N=200 for large load (L);
o a Poisson arrival rate of Y=45*N flows (calls) per hour per base
load: i.e., Y=450 flows/hour for small load (S); Y=2250 flows/hour
for medium load (M); Y=9000 flows/hour for large load (L);
o a batch size of 1 flow per arrival;
o the mean holding time of 1 minute for each flow;
o the maximum media delay of 10 seconds;
o CBR traffic data rate of 80 kbps per flow with a fixed packet size
of 200 bytes (corresponding to G.711 with 20 milliseconds of frame
time for voice over IP);
o VBR traffic with exponentially distributed ON and OFF periods with
mean ON period of 1.004 seconds and mean OFF period of 1.590
seconds (corresponding to a voice codec with silence suppression);
o the traffic mix with 3 types of flows, each with N/3 flows: type 1
flow: G.711/20ms VBR (data rate 31 kbps); type 2 flow: G.711/10ms
VBR (data rate 37.2 kbps); type 3 flow: G.729/20ms VBR (data rate
9.3 kbps);
o all the packets entering the system to be PCN marked.
AR-meter settings:
o AR=TB.rate=the data rate of the base load times (N-1)/N;
o TB.size=20K bytes;
o TB.thershold=10K bytes
Admission control setting:
o stop blocking timer timeout = 1 second;
o RTT=50 milliseconds, fixed for all flows.
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Network node settings:
o link BW = 2 x the data rate of the traffic load seen at a link;
o unlimited buffer size , identical for all the nodes.
Simulation settings:
o the initial number of the flows activated equal to the Poisson
arrival rate in flows per hour x mean holding time in minutes /60;
o the warm-up period of 99 seconds;
o the observation period of 120 seconds;
o the observation interval of 50 milliseconds;
o simulation result measurement based on averaging 10 independent
samples, each with 120-seconds worth of statistics collected in
simulation.
3.2.1. Simulation Results for Admission Control
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Blocking BW Data Rate Worst
Probability Utilization (Mbps) Overshoot
S M L | S M L | S M L | S M L
------------------------------------------------------------------
Erlang-B 12% 0.3% 0% | 62% 72% 73% | 0.5 2.9 11.8 | 0% 4% 0%
Test
(Erlang-B
Theoretical 10% 1% 0% | 75% 75% 75% | 0.6 3 12)
------------------------------------------------------------------
2x Overload
Protection 37% 32% 40% | 80% 93% 97% | 0.6 3.7 15.6 |16% 20% 17%
Test
------------------------------------------------------------------
Multiple-
congestion- * * * | 80% 93% 97% | 0.6 3.7 15.5 | * * *
node Test (2x overload)
------------------------------------------------------------------
Cross-
Traffic * * * | 80% 95% 97% | 0.6 3.8 15.5 | * * *
Test (2x overload)
------------------------------------------------------------------
VBR Test 7% 4% 1% | 51% 71% 77% | 0.16 1.1 4.8 | 0% 0% 0%
(Erlang-B
Theoretical 10% 1% 0% | 75% 75% 75% | 0.24 1.2 4.7)
------------------------------------------------------------------
Traffic Mix 20% 11% 0.2% | 58% 71% 72% | 0.14 0.9 3.8 | 0% 0% 0%
Test
(Erlang-B
Theoretical 10% 1% 0% | 75% 75% 75% | 0.19 0.96 3.8)
------------------------------------------------------------------
*: not summarized at the time of preparing this draft; but they look
similar to the corresponding 2x Overload Protection Test results.
Observations
o For CBR traffic, the AR-meter and AS-marker can support blocking
performance similar to what is expected form Erlang blocking
system for small to large loads; as expected, the performance of
small load is not as good as for larger loads.
o Protection for 2x provisioned BW with CRR traffic: worst case:
<=20% overshoot for small to large loads.
o For the multiple congestion node and traffic crossing scenarios,
the AR-meter and AS-marker can provide similar protection to the
single node for small to large loads; this is expected since the
control is based on the average rate in all the cases.
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o Similar behaviors of the AR-meter and AS-marker are observed for
VBR and mixed traffic; as expected, the AR-meter and AS-marker
will need different settings for VBR/mixed traffic than those for
CBR traffic to improve performance.
The AR-meter and AS-marker settings used in the simulation are chosen
from a number of different settings. With different AR-meter and AS-
marker settings, the simulation results can be different in terms of
the number of flows carried by the system, the blocking probability,
BW utilization, overshoot, control reaction time, etc. This behavior
of the AR-meter and AS-marker suggests that the AR-meter and AS-
marker has certain ability to assist the admission control to limit
traffic load in the system to the desired level.
All the results shown in the above have considered the impact of the
media delay. Without the media delay, the performance of the
simulation is expected to improve according to our preliminary tests.
Conclusions
o AR meter parameters can be adjusted to provide the following
desired behaviors: (1) admit traffic to the expected data rate;
(2) reduce over-/under-shoot to some degree; (3) change reaction
time to some degree; (4) be applicable to a variety of traffic
characteristics and multiple congested-node network with cross-
traffic.
o Limitations observed: (a) difficult to avoid over-/under-shoot for
large media delay; (b) difficult to avoid over-/under-shoot for
VBR/mixed traffic with small load.
4. Simulation Results Prior to 68th IETF
This section captures the simulation work that was done prior to 68th
IETF meeting. Documented are explanation of our simulation setup and
results. Detailed explanations and graphed results from the
simulations can be viewed in [SIM-07]
(http://standards.nortel.com/pcn/Simulation_EPCN.pdf). In
Section 4.1 we provide a brief explanation of the simulations setup
that was used to test flow termination of constant and variable rate
(silence superseded) voice traffic, Section 4.2 to Section 4.6
discusses results of the voice-related simulation, and Section 4.7
briefly discusses the preliminary video-related simulation results.
All the simulations were performed using the token bucket algorithm
documented in Section 4.8.
Note: Since the terminology for this work is evolving, we provide a
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brief explanation of terms used in the simulation results.
Preemption = flow termination
Preemption Threshold = Supportable Rate
Preemption Level = traffic above this rate is marked as excess.
Same as Supportable Rate.
PM flag = explicit marking of packet to indicated excess load. In
the simulation, the router sets both ECN bits to "11" in the IP
header.
Preemption Time = RTT + processing time of termination of a flow.
This is how long it takes before a marked flow stops sending
packets.
pcn_px = represents marking a packet every "x" bytes of excess
rate.
pcn_tb = token bucket depth.
In our simulations, we graphically show architectural performance
comparison criteria for:
o Convergence time in response to a step overload.
o Convergence time in response to multiple steps of overload.
o Convergence time in response to packet loss.
4.1. Simulation Setup for Voice Traffic
Our simulations were done using OPNET see simulation results at
[SIM-07] (http://standards.nortel.com/pcn/Simulation_EPCN.pdf).
Pages 2 through 6 [SIM-07] provide details of the simulation setup:
o Pages 2 and 3 [SIM-07] describe simulation setup. The source
traffic generator (SRC) block produces flows and each flow has a
flow ID, with each flow sending packets at its codec configured
rate. Start time of packets between flows is asynchronous,
representing different sources. Codec mix and number of flows
enabled is programmable.
o Pages 4 and 5 [SIM-07] describe characteristics of the 4 voice
codecs used in the simulations and explanation of two methods used
to simulate fail in the network to cause flow termination
(preemption) to be invoked. During a failure, 100% of additional
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traffic is introduced on to the path (router that is performing
metering and marking of packets). The additional load was
introduced using two models. The first failure emulates a fast
reroute, were all traffic is switched instantaneously. The second
failure on the graph (occurring at 500 time intervals, or
approximately 25 seconds in the simulation) represents a condition
where reroutes takes some time. We configured the simulation so
that 80% of new traffic is added within 1 second and the remaining
20% within additional 5 seconds. Our simulations generated a
traffic mix ratio of up to 15 to 1 for voice. The highest sending
rate is 15 times the smallest.
4.2. Large Number of Voice Flows
First we provide simulation results when there are many flows at the
congestion point (internal router), 500 to 4,250 flows depending on
codec mix used. The violet trace on the graphs shows the number of
flows that are sending packets.
o The preemption marking threshold is set to 40Mbps, so when traffic
exceeds this rate packets are marked every "x" bytes of excess
rate.
o The forwarding rate is configures such that there is no packet
loss in these simulations. See Section 4.4 for results with
packet loss.
o We simulated with pcn_px = 2,064, 4,064 and 8,064 bytes sizes as
well with preemption time set to 50ms, 200ms and 800ms to see the
impact these parameters had on rate and behavior of flow
termination (preemption). See page 7 [SIM-07]
(http://standards.nortel.com/pcn/Simulation_EPCN.pdf) for more
details.
Pages 8 through 20 [SIM-07] show the simulation results. The left
side of graph shows aggregate bandwidth. The bottom of the graph
indicates time scale in 0.05 seconds resolution or 3 seconds between
vertical dashed lines. The right side of the graph shows number of
active flows (flows that are sending packets). The violet trace
shows number of active flows. The orange trace shows aggregated
transmitted rate that egresses the congested router. The blue trace
shows aggregated transmitted rate that is flowing into the router.
Note: The blue trace is only visible when there is packet loss. In
simulations where there is no packet loss the orange trace over-
writes the blue.
Observations for large (500 - 4,250) number of flows with no packet
loss:
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o The shorter the preemption time, the faster overload condition is
restored back to supportable rate.
o The smaller the pcn_px value (packet marked every "x" bytes of
excess traffic), the faster the overload condition is restored
back to supportable rate.
o Packets where marked and flows where terminated when ever excess
rate exceeded by pcn_px bytes the supportable rate.
o The marking and flow termination (preemption) produced exponential
decay behavior. When excess rate was high meaning many flows
needed to be terminated, the marking was frequent but as excess
load decreased so did the marking and flow termination frequency.
Produce a stable behavior for both constant rate and silence
suppressed voice traffic.
o Flow termination (preemption) of traffic generated by constant bit
rate codecs is faster than when silence suppression was used since
the model that we used to generate VBR voice had an exponential
distribution that generated mean on period of 1 second and mean
off period of 1.59 seconds (40 on / 60 off).
o With VBR voice, during reroute condition some active flows were in
silence mode (not sending any packets during off period that had
exponential distribution) as can be observed by rounded peak for
active flows during link failure. Therefore the total load was
not presented instantaneously.
o The defined token bucket measurement method, marked higher rate
flows more aggressively then lower rate flows. See page 15
[SIM-07] for details. This can also be observed that with mixed
codec the number of flows that can be supported after link fail is
higher then before.
4.3. Small Number of Voice Flows
Here (on slides 21 to 28) we provide simulation results when there
are small numbers of flows at the congestion point (internal router),
10 to 80 depending on codec mix used. The violet trace on the graphs
shows the number of flows that are sending packets.
o The preemption marking threshold is set to 800Kbps, so when
traffic exceeds this rate packets are marked every "x" bytes of
excess rate.
o The forwarding rate is configured such that there is no packet
loss in these simulations. See Section 4.5 for results with
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packet loss.
o We simulated with pcn_px = 2,064 and 8,064 bytes sizes as well
with preemption time set to 50ms, 200ms and 800ms to see the
impact these parameters had on rate and behavior of flow
termination (preemption). See page 21 [SIM-07] for more details.
Pages 22 through 28 [SIM-07] show the simulation results when there
are a low number of flows at the congested router.
Observations for small (10 - 80) number of flows with no packet loss:
o The shorter the preemption time, the faster overload condition is
restored back to supportable rate.
o The smaller pcn_px value (packet marked every "x" bytes of excess
traffic), the faster the overload condition is restored back to
supportable rate.
o Packets where marked and flows where terminated when ever excess
rate exceeded by pcn_px bytes the supportable rate.
o When excess rate was high meaning many flows needed to be
terminated, the marking was frequent but as excess load decreased
so did the marking and flow termination frequency. Produce a
stable behavior for both constant rate and silence supersede voice
traffic.
o Flow termination (preemption) of traffic generated by constant bit
rate codecs is faster than when silence suppression was used since
the model that we used to generate VBR voice had an exponential
distribution that generated "mean on period" of 1 second and "mean
off period" of 1.59 seconds (40 on / 60 off).
o With VBR voice, during reroute condition some active flows were in
silence mode (not sending any packets during off period that had
exponential distribution) as can be observed by rounded peak for
active flows during link failure. Therefore the total load was
not presented instantaneously.
o The defined token bucket measurement method, marked higher rate
flows more aggressively then lower rate flows. See [SIM-07] page
28 for details. This can also be observed that with mixed codec
the number of flows that can be supported after link fail is
higher then before.
The explicit marking behavior produced similar results when the
number of constant rate and variable rate (silence suppressed) voice
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flows was small or high. These simulation results would indicated
that for voice traffic this marking approach works independently of
number of flows at the congestion point.
4.4. Large Number of Voice Flows with Packet Loss
Now (see slides 29 to 38) we analyze the impact of packet loss has on
the explicate marking approach when there are many flows at the
congestion point (internal router), 500 to 4,250 depending on codec
mix used. The violet trace on the graphs shows the number of flows
that are sending packets.
o The preemption marking threshold is set to 40Mbps, so when traffic
exceeds this rate packets are marked every "x" bytes of excess
rate.
o The forwarding rate is configures to 48Mbps (introducing up to 40%
packet loss) or 40Mbps (introducing up to 50% packet loss). 50%
packet loss occurs when forwarding rate of service class =
supportable rate (or preemption level), current traffic level is
at supportable rate and 100% of additional traffic is added to
simulate traffic being switch or rerouted due to failure in the
network.
o We simulated with pcn_px = 8,064 bytes sizes as well with
preemption time set to 200ms and 800ms to see the impact these
parameters had on rate and behavior of flow termination
(preemption). See page 29 [SIM-07] for more details.
Pages 30 through 38 [SIM-07] show the simulation results.
Observations for large (500 - 4,250) number of flows with up to 40%
and 50% packet loss:
o As can be observed the flow termination behaved is similar to when
there was no packet loss, except that when there is packet loss
the time it takes to terminate sufficient number of flows to the
supportable rate (preemption threshold) takes longer. This is
because some of the marked packets are lost.
o We also observed that over preemption can occur see page 31
[SIM-07] for CBR (G.711 at 20ms) only traffic when pcn_px value of
8.064 bytes is used with preemption time of 800ms. Increasing
pcn_px or decreasing preemption time will remove the over
preemption condition for this traffic mix.
o This packet marking and flow termination approach works well even
under high packet loss conditions.
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4.5. Small Number of Voice Flows with Packet Loss
Now we analyze the impact of packet loss has on the explicate marking
approach when there are a small number of flows at the congestion
point (internal router), 10 to 80 depending on codec mix used. The
violet trace on the graphs shows the number of flows that are sending
packets.
o The preemption marking threshold is set to 800Kbps, so when
traffic exceeds this rate packets are marked every "x" bytes of
excess rate.
o The forwarding rate is configures 960Kbps (introducing up to 40%
packet loss) and 800Kbps (introducing up to 50% packet loss). 50%
packet loss occurs when forwarding rate of service class =
supportable rate (or preemption level), current traffic level is
at supportable rate and 100% of additional traffic is added to
simulate traffic being switch or rerouted due to failure in the
network.
o We simulated with pcn_px = 8,064 bytes sizes and preemption time
set to 800ms to see the impact these parameters had on rate and
behavior of flow termination (preemption). See page 39 [SIM-07]
for more details.
Pages 40 through 43 [SIM-07] show the simulation results when there
are a low number of flows with up to 40% and 50% packet loss at the
congested router.
Observations for small (10 - 80) number of flows with up to 40% and
50% packet loss:
o As can be observed the flow termination behaved is similar to when
there was no packet loss, except that when there is packet loss
the time it takes to terminate sufficient number of flows to the
supportable rate (preemption threshold) takes longer.
o We also observed that over preemption can occur see page 40
[SIM-07] for CBR (G.711 at 20ms) only traffic when pcn_px value of
8.064 bytes is used with preemption time of 800ms. Increasing
pcn_px or decreasing preemption time will remove the over
preemption condition for this traffic mix..
o This packet marking and flow termination approach works well even
under high packet loss conditions.
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4.6. Corner Voice Cases Studied
Now we want to look at some corner cases where this method starts to
breakdown. We looked at the configuration of parameters that caused
the following conditions:
o Over termination (preemption) of flows. This condition occurs
when pcn_px parameter is too small for the time that it takes to
terminate a flow (total preemption time). This condition is
noticeable when there is CBR only traffic flowing through the
router. Increasing pcn_px therefore slowing down flow termination
can eliminate any possibility of over terminating flows. This is
a parameter that can be configured by the network administrator.
See simulation results [SIM-07], pages 45-48 of examples of this
condition.
o Synchronization of packet marking. This conditional occurs for
CBR fixed packet size traffic at metering point and when pcn_px is
an even multiple of payload packet size, e.g., packet size = 200
bytes and pcn_px = 2,000 bytes. Page 49 [SIM-07] shows that
synchronization of marking condition. However, this undesirable
behavior does not break the mechanism, but it takes longer to
terminate flows.
o Preemption takes to long. This condition can be created if pcn_px
is configured to be x times larger than need. Page 50 [SIM-07]
shows the impact of setting pcn_px 2x bigger then needed.
4.7. Simulation Setup for Video Traffic
In this section, we briefly discuss the preliminary video-related
simulation results; for details, see pages 51-65 [SIM-07]
(http://standards.nortel.com/pcn/Simulation_EPCN.pdf).
The video simulation is based on the same token bucket algorithm as
the voice simulation discussed in the previous sections. The main
differences between our video simulation and voice simulation are the
traffic source model and the selection of the pcn_tb and pcn_px
values.
In the video simulation, the traffic source model is based on the
video model proposed by [Maglaris-88], which has the following
properties:
o a constant frame rate of F frames per sec (a fixed time interval
between frames),
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o a constant number of P pixels per frame,
o a random number of bits per frame calculated using the number of
compressed bits per pixel in the n-th frame modeled by a first-
order autoregressive Markov process.
In our simulation, the packetization of the bits is modeled as
follows,
o the MTU of the video packet is 1356 bytes, including 40 bytes of
the IP header;
o only the positive bits calculated from the above video model can
generate packets;
o the first 1316*8 bits of the total bits of a frame is packed into
the first MTU-sized packet; the second 1316*8 bits is packed into
the second MTU-sized packet; this is done until all the bits are
packed; the last packet likely smaller than MTU contains all the
remainder bits plus the 40-byte IP header;
o the packets generated from a frame are sent to the network one by
one at the end of the time interval of 1/F seconds with a per-
packet serialization time of (packet size / link speed);
o when a source starts, the first frame is generated at a random
time point in the 1/F-sec time interval.
In our current video simulation, only a single type of video source
is used for generating video flows, which has an expected average
data rate of 400Kbps. The following flow settings are considered,
similarly to those voice settings, where T is relative to the end of
the simulation warm-up period,
o Small sources with preemption threshold BW = 4Mpbs: start with 8
flows, add 10 flows at T = 3 sec; add another 10 flows at T = 24
sec;
o Medium sources with preemption threshold BW = 40Mpbs: start with
80 flows, add 100 flows at T = 3 sec; add another 100 flows at T =
24 sec;
o Large sources with preemption threshold BW = 200Mpbs: start with
400 flows, add 500 flows at T = 3 sec; add another 500 flows at T
= 24.
The simulation was run with these flow settings for three RTT (flow
termination) times of 50, 200, and 800ms and four token bucket-
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marking interval combinations,
o (pcn_tb = 400KB; pcn_px = 200KB);
o (pcn_tb = 200KB; pcn_px = 100KB);
o (pcn_tb = 300KB; pcn_px = 200KB);
o (pcn_tb = 250KB; pcn_px = 50KB).
In all the simulation runs, the forwarding rate of the router is set
as two times the preemption threshold BW, and the buffer has
unlimited space (i.e., there is no packet loss).
We have the following observations from the simulations,
o video flow preemption is achievable and behaves similarly to what
is observed in the voice simulations;
o the tested token bucket-marking interval combinations are
similarly effective across the flow settings and RTT cases with
combination (pcn_tb = 400 KB; pcn_px = 200 KB) seemingly the most
stable;
o It is difficult to measure the over-/under-preemption error, as
offered traffic is constantly changing. However, we believe that
(pcn_tb = 400 KB; pcn_px = 200 KB) provide more consistent results
then (pcn_tb = 250 KB; pcn_px = 50 KB) parameter settings.
4.8. Excess Load Marking Algorithm Used in Simulation
Below is the pseudo code of a token bucket algorithm that was used in
our simulations for metering and marking for flow termination
(preemption) of flows. This is an example of an metering and
preemption marking function that would reside in PCN capable routers.
Configuration parameters are per DSCP:
pcn_pt = traffic rate at preemption threshold in bits per second
pcn_tb = the size of token bucket in bytes for detection that
preemption threshold is exceeded
pcn_px = the measurement of excess rate, (sets ECN=11 every "x"
bytes of excess traffic)
Definition of terms used in the algorithm:
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delta_t is the time since the processing of the previous packet
for this token bucket
pktLen is the length of the packet being processed in unit of
bytes
Initialization of local variables:
tokenCountP = pcn_tb //initialize token bucket to max.
pcn_pt_B = pcn_pt / 8 //change preemption rate to bytes per second
Preempt_Level_Metering_Marking routine, with length of current packet
as input:
Preempt_Meter ( pktLen)
{
tokenCountP = tokenCountP + (delta_t * pcn_pt_B)
//this adds tokens to token bucket
tokenCountP = Min (tokenCountP, pcn_tb)
//keeps tb from growing pass full
tokenCountP = tokenCountP - pktLen //subtracts tx bytes from bucket
if (tokenCountP < = 0) //when tb becomes empty or negative
{
Set ECN = 11 //preemption mark packet, (Set ECN bits = 11)
tokenCountP = tokenCountP + pcn_px
//add "x" tokens to token bucket
}
return
} // End of Preempt_Meter().
Figure 9
5. Security Considerations
Not applicable for this draft.
6. Acknowledgements
The authors would like to thank the Dave McDysan for review of 00
version of this document and for his suggestions to make it more
complete.
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7. Informative References
[I-D.babiarz-pcn-3sm]
Babiarz, J., "Three State PCN Marking",
draft-babiarz-pcn-3sm-00 (work in progress), July 2007.
[I-D.eardley-pcn-architecture]
Eardley, P., "Pre-Congestion Notification Architecture",
draft-eardley-pcn-architecture-00 (work in progress),
June 2007.
[Maglaris-88]
Maglaris et al, "Performance Models of Statistical
Multiplexing in Packet Video Communications, IEEE
Transactions on Communications 36, pp. 834-844", July
1988.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001.
[SIM-07] Liu, X-G. and J. Babiarz, "Simulation Results for Explicit
PCN Marking and Flow Termination
(http://standards.nortel.com/pcn/Simulation_EPCN.pdf)",
February 2007.
[SIM1-07] Liu, X-G. and J. Babiarz, "Simulation Results for Three
State PCN Marking for Admission Control and Flow
Termination,
http://standards.nortel.com/pcn/3sM-Simulation-1.pdf",
July 2007.
Authors' Addresses
Jozef Z. Babiarz
Nortel
3500 Carling Avenue
Ottawa, Ont. K2H 8E9
Canada
Phone: +1-613-763-6098
Email: babiarz@nortel.com
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Xiao-Gao Liu
Nortel
3500 Carling Avenue
Ottawa, Ont. K2H 8E9
Canada
Phone: +1-613-763-7516
Email: xgliu@nortel.com
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Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
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Babiarz & Liu Expires January 9, 2008 [Page 31]
