AP Buffer Sizing for TCP in 802.11 WLANs
Naeem Khademi
Networks and Distributed Systems Group; Department of Informatics; University of
Oslo, Norway
naeemk@ifi.uio.no
PhD Research Project Status Report
Abstract. This text’s aim is to report the status of the PhD research
project under the title of AP Buffer Sizing for TCP in 802.11 WLANs,
including the eventual deviations and changes in the project description
and its educational components, the currently achieved milestones and
the description of the research direction taking the state-of-the-art into
consideration. It also presents the currently achieved results in form of
academic publications concisely and updates the time-schedule of the
research plan and its sub-goals accordingly.
1
Introduction
This section is an introduction to the project description and presents the “problem statement” of the project. It also addresses the current project status.
1.1
Short Description
TCP is the dominant transport protocol in today’s Internet providing end-toend reliable data communication. TCP was initially designed with the goal of
optimizing bulk data transfer over the wired networks. While infrastructurebased 802.11 technology is becoming the prevalent medium of Internet access
on the access links, a number of issues arises regarding the TCP performance
in 802.11 WLANs, mainly due to the characteristics of the wireless channel
and MAC scheme (e.g. varying channel quality, limited bandwidth and DCF
mechanism). The cross-layer interaction of TCP and 802.11 PHY/MAC layers
can be deterministic to its performance in wireless networks. Access Point’s (AP)
downstream buffer is usually a bottleneck, and its size plays an important role
in determining the degree of fairness among TCP flows as well as their aggregate
throughput. Effects of buffer related issues in 802.11 WLANs have received little
attention in the literature. Exceptions include [6], which shows that appropriate
buffer sizing can restore TCP upload/download (direction-based) unfairness. The
major problem with such proposals is that the AP buffer size grows linearly with
the increase in number of TCP flows N inducing large amount of queuing delays
and therefore lower throughput.
Buffer sizing has been extensively studied in wired network routers. With
over-provisioning the bandwidth on the access links in recent years, delay has
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still remained as the major performance issue in the networks. Appenzeller et
al. [1] show that buffer sizes in core routers can be reduced significantly, without
any major degradation in network throughput. Recent results on buffer sizing
in Internet core routers suggest that in a network carrying TCP-Reno flows and
with the DropTail queue management scheme in intermediate routers, buffer
sizes can be reduced as the number of flows is increased yielding significant
reduction in RTT. Despite the extensive amount of research on buffer sizing in
wired networks, there has been very little done to identify and probably reduce
the buffer size needed in AP downstream queues. Among very few exceptions [4]
proposes an adaptive algorithm which uses elastic-size AP buffers to set a target
maximum queuing delay. This work also demonstrates that there does not exist
a fixed buffer size which can be used for sizing buffers in WLANs and instead
a dynamic buffer sizing scheme should be proposed. However the work in [4]
first: lacks the consideration of newer PHY/MAC schemes such as 802.11n and
relies on the EDCA function of 802.11e; second: it does not take different rate
adaptation mechanisms and therefore the changes in bit-rates into consideration.
1.2
Problem Statement
A dynamic buffer sizing scheme should be proposed, evaluated and deployed in
802.11 APs to limit the queuing delay while providing the maximum achievable
throughput taking into consideration different real-life factors such as deployed
rate adaptation mechanisms, channel quality, contention level and for different
PHY/MAC schemes including 802.11n.
1.3
Current Project Status
After studying the factors which play role in determining the buffer sizing requirements of APs in 802.11 networks and by focusing more explicitly on TCP
performance and dynamics under different link-layer rate adaptation mechanisms, we are now primarily investigating the TCP buffer sizing on wired access
links for different TCP variants (e.g. CUBIC, Westwood and HSTCP) through
a collaboration with Dr. David Ros from Networks, Security and Multimedia
Department, Telecom Bretagne, France [2]. This can be a good starting point
to update the buffer sizing models which are currently based on dynamics of
TCP NewReno flows and DropTail policy and provide a more general solution
than the one in [1] taking into consideration other TCP variants with Additive
Increase-Multiplicative Decrease (AIMD) behaviors different than the standard
NewReno. The prospectus derived model is later expected to be extended to
802.11 scenarios which have different buffer sizing requirements than the wired
networks and are more complex due to different issues e.g. flow-level fairness and
rate adaptation.
AP Buffer Sizing for TCP in 802.11 WLANs
2
3
Eventual Deviation and Changes
This section explains the deviations and changes in project descriptions and as
well its educational component.
2.1
Project Description
While the initial course of the research was designed to initially focus on the
AP buffer sizing in 802.11n WLANs, we realized that one first needs to understand which factors play a role in determining the appropriate AP buffer size
for TCP traffic in traditional 802.11 networks (e.g. 802.11b/g). The buffering
at the downstream queue of AP normally aims to compensate for the difference
between the typically higher link capacity (e.g. 100 Mbps Ethernet link) on the
wired side and lower capacity of the wireless channel (e.g. capacity associated
with maximum 54 Mbps bit-rate in 802.11b/g) and its service time is shaped
by the variety of inter-correlated factors on the wireless channel such as noise
and collision probabilities, modulation and coding schemes (PHY bit-rates), etc.
While the first two factors seem to be studied extensively in literature, little
attention has been paid to the TCP-related issues under different bit-rate selection mechanisms in 802.11 networks. To develop an AP buffer sizing scheme,
one needs to take all the above factors into consideration motivating us to study
the TCP performance over different bit-rate selection mechanisms.
Bit-rate selection or in other words Rate Adaptation (RA) is a mechanism
that carries out run-time prediction and selection of the most appropriate bitrate to provide the optimum system throughput under varying channel conditions. The choice of PHY bit-rate is critical to the systems performance in
802.11 WLANs. Several widely-known rate adaptation algorithms are proposed
and commonly deployed in 802.11 devices, however in most cases, mistaking
the collisions due to “contention” as indication of noise to some extent. The
cross-layer interaction of TCP, with different rate adaptation algorithms and
Distributed Coordination Function (DCF) in 802.11 WLANs is yet to be deeply
investigated. Therefore, we decided to start with studying and evaluating the
TCP performance coupling with these rate adaptation mechanism and tried to
identify which mechanism yields the best throughput under varieties of scenarios
e.g. upload versus download, different contention levels and RTTs. Our initial
approach was to consider 802.11b/g networks and it will be extended to a more
recent PHY/MAC scheme defined in 802.11n standard.
2.2
The Educational Component
We substituted the course INF9063-Programming heterogeneous multi-core architectures which was supposed to be taken on Fall 2011 with a course that is
more relevant to our research topic as INF9910CPS-Cyber Physical Systems. This
course explores state-of-the art principles, methods, and techniques for devising
cyber physical systems and it covers architectural and infrastructural principles
of CPS systems, wireless sensor networks and their architectures, protocols and
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techniques, Radio Frequency Identification (RFID) key components and systems
Internet of Things (IoT) for pervasive computing. INF9910CPS was successfully
passed on Fall 2010 and the curriculum was updated and approved by the department accordingly. Table 1 shows an updated version of the curriculum.
Based on this, by the end of Fall 2011 (3rd semester), all required courses in
curriculum are taken and successfully passed.
Table 1. Updated educational curriculum
Subject code
Subject title
MNSES9100
Science, Ethics and Society
INF9910CPS
Cyber Physical Systems
INF9050 Protocols and Routing in the Internet
INF9071
Performance in Distributed Systems
3
Semester credits
Fall 2010
5
Fall 2010
10
Spring 2011 10
Fall 2011
10
Publications
This sections overviews the currently published results of this PhD research as
well as the publications under review. Our first paper was accepted in 31 Jan 2011
(5.5 month after stating the project) and published in May 2011 in proceeding
of IFIP/TC6 Networking 2011, under the title of ”On the Uplink Performance
of TCP in Multi-rate 802.11 WLANs” [3]. The findings of this paper is shortly
explained in Section 3.1 . The full citation of the paper is as below:
– Naeem Khademi, Michael Welzl and Renato Lo Cigno, “On the Uplink Performance of TCP in Multi-rate 802.11 WLANs”, IFIP/TC6 NETWORKING
2011, Valencia, Spain
Our second publication under the title of ”Experimental Evaluation of TCP
Performance in Multi-rate 802.11 WLANs“ is aimed for the submission in the
thirteenth international symposium on a World of Wireless, Mobile and Multimedia networks WoWMoM 2012 [7] in 2 Dec 2011. The short description of our
findings in this paper is brought in Section 3.2.
3.1
On the Uplink Performance of TCP in Multi-rate 802.11
WLANs (Published in IFIP Networking’11)
This paper investigates the cross-layer interaction of TCP with a classic rate
adaptation mechanism (ARF) and DCF in 802.11 networks. Auto-Rate Fallback
(ARF) is a well-known mechanism in which the sender adapts its transmission
rate in response to link noise using up/down thresholds. ARF has been criticized
for not being able to distinguish MAC collisions from channel noise. It has been
previously shown in literature that, in the absence of noise and in the face of
AP Buffer Sizing for TCP in 802.11 WLANs
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collisions, ARF (and potentially other RA mechanisms) does not play a significant role for TCP’s downlink performance. However, the interactions of ARF,
DCF and uplink TCP have not yet been deeply investigated and this has become
the focal point of this paper. In this paper, we demonstrate our findings on the
impact of rate fallback caused by collisions in ARF on the uplink performance
of various TCP variants using simulations.
Our evaluation results show that ARF, as a sample of RA mechanisms that
wrongly react to frame collisions, can significantly impede the performance of
all major TCP variants (e.g. Reno, CUBIC, HSTCP and H-TCP) in uplink scenarios necessitating the need to investigate the uplink TCP performance under
commonly deployed RA mechanism in 802.11 devices in real-life and identify
the mechanism which differentiates the best, the frame errors due to collision
(contention) from noise.
3.2
Experimental Evaluation of TCP Performance in Multi-rate
802.11 WLANs (For Submission in WoWMOM’12)
While previous efforts have in literature, never included real-life measurements
of uplink traffic, in this paper we have taken an experimental approach by conducting real-life measurements in two different test-beds (NDlab and Emulab)
alongside with simulations, to study TCP performance coupled with different
RA mechanisms deployed in commonly used 802.11 devices. Our measurements
reveal that 1) most conventional RA mechanisms are unable to distinguish frame
collisions due to contention from channel noise/interference and will respond to
them negatively to some extent; 2) other than downlink TCP, uplink TCP can be
adversely affected by collision-triggered rate downshifts that some RA schemes
exhibit even in low-noise environments; 3) the relatively recent Minstrel RA
mechanism can counter this negative uplink behavior well, yielding almost equal
performance as in the downlink case.
We have presented our investigation of different RA mechanisms (AMRR,
SampleRate and Minstrel) through simulations and real-life measurements. Our
experiments show that uplink TCP performance can be affected by the choice of
RA mechanism in infrastructure-based WLANs, even in low-noise environments,
due to the inability to differentiate the source of frame loss. While AMRR and
SampleRate degrade the TCP performance in such scenarios except for very
high RTTs, Minstrel is able to keep the uplink performance at roughly the same
level as downlink in various different scenarios, e.g. with a varying number of
contending nodes, different RTT values and under different TCP congestion
control variants.
The good uplink performance of Minstrel over other RA mechanisms has
mainly two reasons: 1) instead of measuring the packet transmission time at a
rate which can have a misleading effect when the tx time of two rates are very
close, Minstrel estimates the potential throughput per rate, which also incorporates the delivery probability; 2) while bit-rate selection in SampleRate is too
sensitive towards single probing failures by probing on every 10th packet transmission, Minstrel distributes the sampling frames (a.k.a look-around frames) ran-
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domly around different rates and chooses the best throughput rate every 100 ms,
therefore having more sampling frames taking part in each rate selection process.
Random distribution of sampling frames in time and rate makes the bit-rate selection unbiased toward a single frame loss at the current rate or the rate being
probed, as the collision probability is homogeneous among different rates. Our
observations lead us to recommend Minstrel as the default RA mechanism for
deployment in 802.11 devices in low-noise environments. A future investigation,
focusing on noise in all its facets, could complement our work to draw a complete
picture of which RA mechanism to use.
4
Current Achievements
This section presents the current achievements of the PhD research until the time
of writing this report. Two publications are produced as the outcome of the research activities performed so far in which one is already published and another
will be submitted for review process shortly. The details are these publications
are brought in Section 3. We have been also able to setup an experimental platform for this research project (Section 4.1) and define and supervise a secondary
project as a master thesis (Section 4.2).
4.1
NDlab 802.11 Testbed
To conduct our real-life measurements, we designed and implemented an 802.11
testbed (NDlab) which is a cluster of concentrated nodes stacked in network
lab environments. The specifications of this test-bed is presented in Table 2.
For further details refer to the NDlab’s work-in-progress wikipage [5]. We are
currently in the process of extending the testbed to provide 802.11n support.
The current testbed setup potentially provides a platform for other interested
network researchers to conduct their measurement studies.
Table 2. NDlab test-bed setup
Test-bed
NDlab
PC
Dell OptiPlex GX620
CPU
Intel Pentium 4 CPU 3.00 GHz
Memory
1 GB
802.11 device D-Link DWL-G520 Compex WLM54AG AirForece One 54g
Chipset
AR5001X
AR5413
BCM4318
Driver
MADWIFI 0.9.4 or ath5k
b43
OS
FC14
Linux kernel
2.6.35.11
Node numbers
10
5
5
AP Buffer Sizing for TCP in 802.11 WLANs
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Fig. 1. NDlab 802.11 testbed
4.2
Master Thesis Projects
To provide a better platform to work on the PhD thesis topic, we announced
several master thesis projects that their outcome tools could be seen assisting in
process of investigating the materials related to the research topic. One of these
projects is already undertaken by a master student associated to ND/IFI and
supervised by the PhD candidate and Prof. Michael Welzl and expected to be
finished by the end of 2012. This project is concisely explained below.
A tool for determining the rate-adaptation mechanism of 802.11 cards
The rate-adaptation mechanism implemented in 802.11 products is left up to
the vendor, basically making it a proprietary feature. Rate-adaptive behavior
of wireless network cards is mostly unidentified due to the vendor-specific algorithms which makes a generic research in this field somewhat difficult and
product-specific. The goal of this thesis is to develop, test and demonstrate a
Linux-based benchmarking tool to determine the rate-adaptive behavior of different wireless cards and matching them to their associated algorithms. This
project is associated with Tor Martin Slåen.
5
Research Time Plan
This section explains the updated time plan of the PhD project. Table 3 demonstrates an updated version of the research time plan until the time of writing
this report, including the sub-goals and milestones assigned in the initial proposal, their status and comments about what actions actually have been taken
in relevant semesters.
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References
1. Guido Appenzeller, Isaac Keslassy, and Nick McKeown. Sizing router buffers. In
Proceedings of the 2004 conference on Applications, technologies, architectures, and
protocols for computer communications, SIGCOMM ’04, pages 281–292, New York,
NY, USA, 2004. ACM.
2. http://www.rennes.enst-bretagne.fr/~dros/.
3. Naeem Khademi, Michael Welzl, and Renato Lo Cigno. On the uplink performance
of TCP in multi-rate 802.11 WLANs. In Proceedings of the 10th international IFIP
TC 6 conference on Networking - Volume Part II, NETWORKING’11, pages 368–
378, 2011.
4. Tianji Li, D. Leith, and D. Malone. Buffer sizing for 802.11-based networks. Networking, IEEE/ACM Transactions on, 19(1):156 –169, feb. 2011.
5. http://nd-wifi-testbed.wiki.ifi.uio.no/.
6. S. Pilosof, Ramachandran Ramjee, D. Raz, Y. Shavitt, and Prasun Sinha. Understanding tcp fairness over wireless lan. In INFOCOM 2003. Twenty-Second Annual
Joint Conference of the IEEE Computer and Communications. IEEE Societies, volume 2, pages 863 – 872 vol.2, march-3 april 2003.
7. http://wowmom2012.it.uc3m.es/.
Table 3. Research Time Plan (by the end of 3rd semester)
Semester Milestone(s)
Literature Review
Fall’10
Preparation of PhD proposal
Status
Completed
Completed
Analytical modeling of AP Postponed
Spring’11 buffer sizing and its reduction
model
Comments
-TCP, 802.11 rate adaptation and
buffer sizing
-Approved by the department before starting the contract
-Simulation study of TCP over
multi-rate 802.11 WLANs
-Experimental evaluation of TCP
over 802.11 RA mechanisms
-Paper #1 published
-NDlab setup
Implementation of 802.11n in Master
-This is seen potentially suitable for
Fall’11 ns-2
project
a master project
Implementation of AP buffer Undergoing -More details on modifications in
sizing model in ns-2
with modifi- Section 1.3
cations
-Paper #2 submission
-Completing all courses in curriculum