One document matched: draft-elloumi-diffserv-threevstwo-00.txt
July, 1999
Expires January, 2000
Usefullness of three drop precedences in Assured Forwarding service
Status of this Memo
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Abstract
This informational memo points out advantages of using three drop
precedences (compared to two) in the Assured Forwarding (AF) Per-
Hop-Behavior (PHB) when the traffic is composed of responsive TCP and
non responsive UDP traffic. Our simulation results show that while
two drop precedences can, in some cases, guarantee to an aggregate
consisting of TCP traffic the full utilization of the reserved
bandwidth, they cannot allow to access the unreserved bandwidth in
the presence of high bit rate unresponsive UDP traffic. When three
drop precedences are appropriately used TCP aggregates can fully use
the minimum reserved bandwidth and share the unreserved one.
1. Introduction
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Internet Draft draft-elloumi-diffserv-threevstwo-00.txt June 1999
The Assured Forwarding PHB [RFC2597] group is intended to provide
high probability packets forwarding to traffic not exceeding the
subscribed profile (in-profile). Users are authorised to exceed the
subscribed profile, however the excess traffic (out-of-profile) is
not delivered with a high probability as the in-profile one. The data
traffic is subject to marking in order to provide a low drop
probability for in-profile traffic and a higher drop probability for
out-of-profile one. The AF PHB specifies three drop precedences
within each of four defined traffic classes. While the use of 3
levels of drop precedences was mainly proposed to prevent non
responsive traffic from getting more than its fair share of the non
reserved bandwidth, several contributions presented simulations where
the use of only two drop precedences is enough to achieve the
protection of TCP traffic [Goyal1, Goyal2]. In [Ibanez, Yeom]
simulation results show that 2 drop precedences are not enough to
fully utilize the reserved bandwidth. In this memo, we present some
simulation configurations in which the use of three levels of drop
precedences can be used to allow TCP traffic to achieve its reserved
bandwidth.
2. Simulation configuration
Our simulation configuration is depicted in Figure 2.1. The network
is composed of 4 pairs of interconnected LANs. Each LAN is attached
to a customer router (C1-C4, C1'-C4'). Customer routers are connected
via a 139 Mbps link to the backbone composed of a single bottleneck
link between the edge routers ER1 and ER2. In LAN1, LAN2 and LAN3
(connected respectively to C1, C2 and C3) a set of 10 TCP-SACK
sources are performing infinite file transfer to destinations in
LAN1', LAN2' and LAN3' (connected respectively to C1', C2' and C3').
The MSS for TCP is set to 536 Bytes. In LAN4 (connected to C4) a UDP
source is sending fixed sized packets of 576 Bytes to a UDP
destination in LAN4' (connected to C4'). The inter packets sending
time is exponentially distributed. The mean UDP rate varies for the
different simulations from 0 Mbps up to 60 Mbps. The customer routers
perform traffic conditioning: packets are marked with the appropriate
drop precedence according to a certain traffic profile. In order to
compare the performance of two color and three color markers we use
the Token Bucket Marker (TBM) allowing two drop precedences and the
two rate Three Color Marker (trTCM) allowing three drop precedences
[Heinanen].
With the TBM, tokens are generated at a constant rate, i.e. the
Committed Information Rate (CIR), up to a maximum value, i.e the
Committed Burst Size (CBS). When a packet arrives, the bucket
occupancy is decremented by the packet size and the packet is marked
with the drop precedence 0 if enough tokens are available. The packet
is marked with the drop precedence 1 otherwise. In the latter case
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Internet Draft draft-elloumi-diffserv-threevstwo-00.txt June 1999
the bucket occupancy is not decremented.
With the trTCM, two buckets, with sizes CBS and Peak Burst Size
(PBS), and two generation rates, CIR and Peak Information Rate (PIR),
are used. Tokens in each bucket are generated at a constant rate: CIR
and PIR respectively. Two modes of operation can be considered: the
color blind and the color aware mode. In our simulation we used only
the color blind mode in which each arriving packet is forwarded with
the drop precedence 0 if the packet size is less than the first and
the second bucket occupancy (both buckets are decremented by the
packet size), with drop precedence 1 if the packet size is more than
the the first bucket occupancy but less than the the second (the
second bucket is decremented by the packet size) and finally with
drop precedence 2 if the packet size is more than the first and the
second bucket occupancy.
2.5 ms, 139 Mbps 2.5 ms, 139 Mbps
<------------> <------------>
\+---+ +---+/
-| C1|--------- ---------|C1'|-
/+---+ | +----------+ +----------+ | +---+\
\+---+ |---| | | |---| +---+/
-| C2|------ | | | | ------|C2'|-
/+---+ |------| | | |------| +---+\
\+---+ |------| ER1 |-----| ER2 |------| +---+/
-| C3|------ | | | | ------|C3'|-
/+---+ |---| | | |---| +---+\
\+---+ | +----------+ +----------+ | +---+/
-| C4|--------- <-----> ---------|C4'|-
/+---+ 10 ms, 34 Mbps +---+\
Figure 2.1: Simulation model
The simulation parameters of the TBM and trTCM are, respectively,
given in Table 2.1 and Table 2.2. We configured the simulation
parameters for the trTCM such that all the excess traffic for TCP
sources is marked with drop precedence 1 and all the excess traffic
from the UDP source is marked with drop precedence 2. This is done by
setting the PIR for customers running TCP sources to a very high
value (equal to the link rate) and to the CIR for the customer
running the UDP source.
Table 2.1: Simulation parameters for TBM
Scenario 1 Scenario 2 Scenario 3
CBS (C1, C2, C3, C4) 5 KB 10 KB 50 KB
CIR (C1, C2, C3, C4) 2 Mbps 2 Mbps 2 Mbps
Table 2.2: Simulation parameters for trTCM
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Scenario 1 Scenario 2 Scenario 3
CBS (C1, C2, C3, C4) 5 KB 10 KB 50 KB
PBS (C1, C2, C3, C4) 5 KB 10 KB 50 KB
CIR (C1, C2, C3, C4) 2 Mbps 2 Mbps 2 Mbps
PIR (C1, C2, C3) 139 Mbps 139 Mbps 139 Mbps
PIR (C4) 2 Mbps 2 Mbps 2 Mbps
Routers ER1 and ER2 implement the RIO algorithm [Clark] that we have
extended to support three drop precedences instead of two. The
parameter maxp was set to 0.02, 0.05 and 0.1 for drop precedence 0, 1
and 2 respectively. The thresholds (minth, maxth) in KB for drop
precedences 0, 1 and 2 are respectively (45, 90), (90, 180) and (180,
360). For each drop precedence, i, an average queue length is
calculated using the current buffer occupancy of all packets with
drop precedences ranging from 0 to i. As in [Floyd], the weight to
calculate the average queue length is set to 0.002.
3. Simulation Results
In this section we present simulation results obtained with 2 and 3
drop precedences. Tables 3.1, 3.2 and 3.3 give the simulation results
for Scenario 1, Scenario 2 and Scenario 3, respectively, described in
Section 2.
From Tables 3.1, 3.2 and 3.3 we can see that when only two drop
precedences are used the TCP aggregates are, in most cases, only able
to consume their reserved bandwidth (i.e. the CIR) and a portion of
the unreserved bandwidth if the UDP sending rate is less than its CIR
plus the unreserved bandwidth. For low values of CBS (Table 3.1) TCP
aggregates are not able to fully utilize the CIR when the UDP sending
rate exceeds its CIR plus the unreserved bandwidth.
When 3 drop precedences are used the UDP source is only able to fully
utilize its CIR. The unreserved bandwidth is fully utilized by TCP
aggregates. In addition the obtained results do not vary when the CBS
varies (Tables 3.1, 3.2 and 3.3).
Table 3.1: Simulation results for Scenario 1.
------------------------------------------------------------------
| with two drop precedences | with three drop precedences
UDP rate| Sum(TCP Throu.)| UDP Throu.| Sum(TCP Throu.)| UDP Throu.
------------------------------------------------------------------
0 33.98 - 33.98 -
5 29.05 4.92 31.96 2.03
10 24.20 9.90 31.93 2.05
15 19.34 14.65 31.97 2.02
20 14.66 19.32 31.94 2.05
25 10.62 23.31 31.94 2.04
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30 7.80 26.11 31.93 2.06
35 6.34 27.56 31.94 2.04
40 5.60 28.30 31.93 2.06
45 5.29 28.61 31.93 2.05
50 5.22 28.70 31.93 2.04
55 5.14 28.77 31.92 2.06
60 5.01 28.91 31.93 2.05
Table 3.2: Simulation results for Scenario 2.
------------------------------------------------------------------
| with two drop precedences | with three drop precedences
UDP rate| Sum(TCP Throu.)| UDP Throu.| Sum(TCP Throu.)| UDP Throu.
------------------------------------------------------------------
0 33.98 - 33.98 -
5 29.06 4.91 31.95 2.03
10 24.18 9.79 31.95 2.03
15 19.29 14.68 31.95 2.03
20 14.71 19.32 31.93 2.05
25 10.60 23.38 31.95 2.03
30 7.96 25.94 31.90 2.07
35 6.81 27.08 31.94 2.04
40 6.36 27.53 31.94 2.04
45 6.15 27.74 31.95 2.02
50 5.97 27.92 31.91 2.06
55 5.87 28.01 31.90 2.08
60 5.78 28.50 31.93 2.05
Table 3.3: Simulation results for Scenario 3.
------------------------------------------------------------------
| with two drop precedences | with three drop precedences
UDP rate| Sum(TCP Throu.)| UDP Throu.| Sum(TCP Throu.)| UDP Throu.
------------------------------------------------------------------
0 33.98 - 33.98 -
5 29.03 4.97 31.97 2.04
10 24.15 9.85 31.94 2.05
15 19.33 14.67 31.97 2.04
20 14.69 19.31 31.95 2.06
25 10.52 23.43 31.97 2.04
30 7.89 26.05 31.94 2.07
35 6.90 27.02 31.95 2.06
40 6.52 27.40 31.93 2.05
45 6.29 27.61 31.93 2.07
50 6.25 27.66 31.91 2.09
55 6.19 27.71 31.92 2.09
60 6.08 27.80 31.91 2.09
4. Simulations with large RTT and large CIR
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In this paragraph we study the effect of having a large RTT (large
propagation delay) and a large value for the CIR. The simulation
model studied is similar to the one presented in paragraph 2. The
only diffrences are: the propagation delay of the link between
customer 2 and ER1 is set to 100 ms instead of 2.5 ms and customer 3
has a CIR of 20 Mbps instead of 2 Mbps. The UDP source is sending
fixed sized packets of 576 Bytes with an inter packet sending time
exponentially distributed. The mean rate is set to 25 Mbps for all
simulations. We performed 5 sets of simulations with the following
CBS values: 5, 10, 50, 100, 500 KB. The simulation parameters are
summarized in Table 4.1. The remaining parameters are set as in
section 2. We also performed simulations with the Rate Adaptive
Shaper (RAS) [Bonaventure] which is combined with the TBM and the
trTCM. The RAS is used in front of the marker and aims at increasing
the number of low drop precedence packets by reducing the burstiness
of the traffic.
Tables 4.2, 4.3, 4.4 and 4.5 give the simulation results.
From Table 4.2 we can see that with only two drop precedences
customer 3 is never (for the used values of CBS) able to use its
reserved bandwidth (i.e. CIR). Customer 2 succeeds to use its CIR for
high values of the CBS (50, 100, 500 KB).
When 2 drop precedences are used in combination with the RAS (see
Table 4.3) mostly all customers, except customer 3 when the CBS is
too small, can fully utilize their CIR. The use of the shaper allows
the customers to obtain an aggregate throughput close to the CIR but
the unreserved bandwidth is mostly utilized by the UDP source.
When 3 drop precedences are used (see Table 4.4) it appears clearly
that all customers obtain an aggregate throughput which is higher
than the CIR. The unreserved bandwidth is mostly used by the TCP
aggregates. In addition the obtained aggregate throughput do not vary
a lot when the CBS vary.
Finally the use of three drop precedences combined with the RAS gives
results very close to ones obtained without the RAS for this
configuration.
From this set of simulations we have shown the following properties:
- The use of 2 drop precedences do not allow, in the presence of UDP
traffic, the TCP aggregates to utilize the unreserved bandwidth which
is mostly consumed by the UDP source. TCP aggregates can fully
utilize their CIR when the appropriate CBS values are used. The use
of the RAS can help TCP aggregates to utilize their CIR much better
but not the unreserved bandwidth.
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Internet Draft draft-elloumi-diffserv-threevstwo-00.txt June 1999
- The use of 3 drop precedences allows the TCP aggregates to share
the unreserved bandwidth. In addition the obtained throughput does
not vary a lot with CBS variations.
Table 4.1: Simulation parameters
customer 1: CIR of 2 Mbps
customer 2: CIR of 2 Mbps, large RTT
customer 3: CIR of 20 Mbps
customer 4: CIR of 2 Mbps, UDP of 25 Mbps
CBS : 5, 10, 50, 100, 500 KB
Table 4.2: Simulation results with 2 drop precedences
CBS customer 1 customer 2 customer 3 customer 4
5 2.39 1.65 13.59 16.35
10 2.36 1.86 14.05 15.71
50 2.21 2.00 18.42 11.35
100 2.21 2.02 18.53 11.22
500 2.26 2.12 19.41 10.20
Table 4.3: Simulation results with 2 drop precedences - shaper
CBS customer 1 customer 2 customer 3 customer 4
5 2.09 1.99 19.07 10.84
10 2.08 1.99 19.32 10.59
50 2.05 1.99 19.83 10.11
100 2.10 1.99 19.80 10.08
500 2.05 1.99 19.83 10.11
Table 4.4: Simulation results with 3 drop precedences
CBS customer 1 customer 2 customer 3 customer 4
5 6.80 2.98 21.96 2.23
10 6.65 2.96 22.22 2.15
50 6.54 3.03 22.14 2.28
100 6.36 3.05 22.23 2.34
500 6.37 2.97 22.46 2.19
Table 4.5: Simulation results with 3 drop precedences - shaper
CBS customer 1 customer 2 customer 3 customer 4
5 6.64 3.36 21.72 2.26
10 6.01 3.56 22.19 2.23
50 6.03 3.54 22.15 2.26
100 6.23 3.40 22.12 2.23
500 5.89 3.64 22.18 2.27
5. Conclusion
Simulation results presented in this memo can be summarized as
follows:
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- When only two drop precedences are used TCP aggregates can, in most
cases, fully use their reserved bandwidth if the markers' parameters
are appropriately set (CBS value). However the unreserved bandwidth
is mostly used by UDP sources.
- The use of three drop precedences allows the TCP aggregates to
fully use their reserved bandwidth and to share the unreserved one.
In addition the obtained throughputs do not vary a lot with the
variations of the CBS parameter. However the unreserved bandwidth is
not fairly distributed among the TCP aggregates.
6. References
[Bonaventure]
O.Bonaventure, S. De Cnodder, A rate adaptive shaper for dif-
ferentiated services. Internet draft, IETF, draft-bonaventure-
diffserv-rashaper-00.txt. Work in progress.
[Clark] D. D. Clark, W. Fang, Explicit Allocation of Best-Effort Packet
Delivery Service. IEEE/ACM Trans. on Networking, Vol. 6, No. 4,
August 1998.
[Floyd] S. Floyd, V. Jacobson, Random Early Detection Gateways for
Congestion Avoidance. IEEE/ACM Transactions on Networking,
August 1993.
[Ibanez]J. Ibanez, K. Nichols, Preliminary Simulation Evaluation of an
Assured Service. Internet draft, IETF, draft-ibanez-diffserv-
assured-eval-00.txt. Work in progress.
[Goyal1]M. Goyal, P. Misra, R. Jain, Effect of Number of Drop Pre-
cedences in Assured Forwarding. Internet draft, IETF, draft-
goyal-dpsty-diffserv-00.txt. Work in progress.
[Goyal2]M. Goyal, P. Misra, R. Jain, Effect of number of drop pre-
cedences in Assured Forwarding. Available from
http://www.cis.ohio-state.edu/~
jain/papers/dpstdy_globecom99.htm
[Heinanen]
J. Heinanen, and R. Guerin, A Two Rate Three Color Marker.
Elloumi, De Cnodder, Pauwels Drop precedences in AF [Page 8]
Internet Draft draft-elloumi-diffserv-threevstwo-00.txt June 1999
Internet draft, IETF, draft-heinanen-diffserv-trtcm-01.txt.
Work in progress.
[RFC2597]J. Heinanen, F. Baker, W. Weiss, J. Wrockawski, Assured For-
warding PHB Group. RFC 2597, June 1999.
[Yeom] I. Yeom, A. L. N. Reddy, Realizing throughput guarantees in a
differentiated services network. IEEE ICMCS'99, June 1999.
Acknowledgments
The authors would like to thank Jordi Nelissen for several dis-
cussions.
Authors' Addresses
Omar Elloumi
Alcatel Corporate Research Center
Fr. Wellesplein 1, B-2018 Antwerpen, Belgium.
Phone : 32-3-240-7833
Fax : 32-3-240-9932
E-mail: Omar.Elloumi@alcatel.be
Stefaan De Cnodder
Alcatel Corporate Research Center
Fr. Wellesplein 1, B-2018 Antwerpen, Belgium.
Phone : 32-3-240-8515
Fax : 32-3-240-9932
E-mail: stefaan.de_cnodder@alcatel.be
Kenny Pauwels
Alcatel Corporate Research Center
Fr. Wellesplein 1, B-2018 Antwerpen, Belgium.
Phone : 32-3-241-5941
Fax : 32-3-240-9932
E-mail: kenny.pauwels@alcatel.be
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