Subject: Re: [manet] TCP Retransmission and Goodput : An experimental study

From: Vikas Kawadia <[email-masked]>
Date: Fri, 03 Dec 2004 11:02:51 -0500

We studied TCP performance over a real 802.11b ad hoc network testbed
extensively. We studied topologies of up to 6 hops. We used Cisco cards,
whose antennae were covered with copper to scale down their range.

Several repetitions of the experiments were conducted, and the results were
subjected to statistical analysis. We measured throughput, delay and delay
jitter and collected several other dumps.

Among other things we found that switching off RTS-CTS handshake always
improved throughput. There are several factors that come into play when 
RTS-CTS
and carrier sensing are both on, and it is difficult to argue 
qualitatively what the net
impact on throughput would be. Please see page 57 of the document 
referred below for a discussion.

To reduce end to end delay and delay jitter other mechanisms are
needed. We study 'congestion window clamping' and find that it helps
tremendously. We suggest clamping the congestion window of each flow to 3/2
times the number of hops for that flow.

We also study the effect of the SACK option, MTU, queue lengths, MAC retry
limits, MAC data rates and other parameters.

A paper is under preparation. Meanwhile please see chapter 3 of my 
thesis which
can be found at:
http://black.csl.uiuc.edu/~prkumar/ps_files/04_07_kawadia_thesis.pdf

Thanks
-vikas

Vikas Kawadia
BBN Technologies

------------------------------------------------------------------------

    * /To/: "Farrag, Osama" <Osama.Farrag at jhuapl.edu
      <mailto:[email-masked]>>
    * /Subject/: Re: [manet] TCP Retransmission and Goodput
    * /From/: Ram Ramanathan <ramanath at bbn.com
      <mailto:[email-masked]>>
    * /Date/: Tue, 23 Nov 2004 18:23:07 -0500
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------------------------------------------------------------------------
Is this really true in practice -- that 802.11 networks have better 
goodput when RTS/CTS is turned on?

Researchers doing experimental work with testbeds more often than not 
report better throughput with RTS/CTS turned off (at least those I've 
spoken to), hidden terminals notwithstanding.

While RTS/CTS solves the hidden terminal problem, it does introduce 
exposed terminals, and the overhead of the handshake. And in some cases, 
the negatives may be more than the positives. For instance, at high data 
rates (e.g. 54 Mbps) the time for RTS/CTS as a fraction of total packet 
transfer time gets to be significant, especially if RTS/CTS are sent at 
lower data rate as is typical.

I'm not disputing your statement, but do you (or anyone on this list) 
know of any study that corrobarates this statement? Are there 
comprehensive simulation or testbed based studies on this and does one 
differ from the other in conclusions? If so, I'd appreciate a pointer.

It would be good for the community to really understand the benefits of 
RTS/CTS for ad hoc networks.

cheers,

-Ram

On Nov 23, 2004, at 5:23 PM, Farrag, Osama wrote:

Ehsan,

    There could be many factors, in general 802.11 networks perform
    better in terms goodput when RTS/CTS handshake is used.

    a) RTS/CTS mode allows nodes to detect collision faster than in
    Data/ACK mode because Data messages are much longer than RTS
    messages. So the channel revert to idle state faster and nodes will
    start contending again in shorter time.

    b) Probability of collisions from Hidden node problem is reduced
    when RTS/CTS handshake is used. Less collision results in better
    throughput.

Hope that helps.

Best Regards

Osama Farrag

Johns Hopkins University/ APL

    -----Original Message-----
    From:manet-bounces at ietf.org [mailto:manet-bounces at ietf.org]On
    Behalf OfEhsan Hamadani
    Sent: Tuesday, November 23, 2004 4:34 PM
    To:manet at ietf.org
    Subject:[manet] TCP Retransmission and Goodput

Hi all,

    I am simulating a 4 hop static network with one TCP flow between the
    sender and receiver. There are 2 scenarios: One scenario (let’s call
    it scenario 1) is based on RTS/CTS message exchange for packets
    larger than 1024 Bytes and the other one (scenario 2) does not use
    any RTS/CTS to send data over network.

    When I monitor thebehaviourof TCP in the network, I come across the
    following issue:

    While scenario 1 has less average congestion window size and flight
    size (number of sent and unacknowledged data in network) with higher
    number of TCP retransmission in comparison to scenario 2, the
    goodput of scenario 1 is greater than scenario 2!!!

    For measuring the goodput, I am monitoring the sent sequence number
    by sender. So, for instance, at time 50sec, the sender in scenario 1
    is sending packet with sequence number 10000 while the sender in
    scenario 2 is sending packet with sequence number 6000.

Does anyone know the reason behind this?

Thanks for your help

Ehsan

-----------------------------------------

Ehsan Hamadani, PhD student

SchoolofEngineeringand Math Science

City University

London,EC1V 0HB

United Kingdom

Office:+44 (0)207 040 3886

 
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