Controlling TCP congestion

NETWORK WORLD NEWSLETTER: STEVE TAYLOR AND JIM METZLER ON WIDE
AREA NETWORKING
07/05/05
Today's focus: Controlling TCP congestion

Dear networking.world@gmail.com,

In this issue:

* TCP's Congestion Control mechanism
* Links related to Wide Area Networking
* Featured reader resource
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Today's focus: Controlling TCP congestion

By Steve Taylor and Jim Metzler

Last time, we discussed how TCP combines a flow control
parameter (Advertised Window) and a congestion control parameter
(Congestion Window) to determine how many bytes the sending
device can transmit (Effective Window). We also mentioned that
determining the optimum value for the Congestion Window is
somewhat tricky because network congestion changes over time.

TCP has multiple mechanisms to determine Congestion Window and
today, we'll describe one of those called Additive
Increase/Multiplicative Decrease (AI/MD). AI/MD is typically
used when the sending device is operating close to the available
capacity of the network.

The basic assumption made by AI/MD is that if the sender does
not receive an acknowledgement of a sent packet, the packet was
dropped by the network due to congestion. Hence, when faced with
a timeout, AI/MD assumes there is network congestion and cuts
Congestion Window in half. Subsequent timeouts result in
additional halving of Congestion Window, which leads to the
phrase "Multiplicative Decrease."

While AI/MD decreases Congestion Window rapidly, it increases
Congestion Window slowly. Every time that the source is
successful in sending a Congestion Window's worth of packets, it
adds the equivalent of one packet to Congestion Window.

Analysis of TCP throughput demonstrates that this slow increase
and rapid decrease in Congestion Window results in throughput
that resembles a saw tooth. This analysis also demonstrates that
network throughput can be significantly impacted by packet loss.

Future newsletters will examine some of the techniques that are
used to overcome the performance limitations of TCP.

RELATED EDITORIAL LINKS

The Transmission Control Protocol
http://mmnetworks.stanford.edu/waelnour/docs/TCP-doc.pdf
______________________________________________________________
To contact: Steve Taylor and Jim Metzler

Steve Taylor is president of Distributed Networking Associates
and publisher/editor-in-chief of Webtorials. For more detailed
information on most of the topics discussed in this newsletter,
connect to Webtorials <http://www.webtorials.com/>, the premier
site for Web-based educational presentations, white papers, and
market research. Taylor can be reached at
<mailto:taylor@webtorials.com>

Jim Metzler is the Vice President of Ashton, Metzler &
Associates, a consulting organization that focuses on leveraging
technology for business success. Jim assists vendors to refine
product strategies, service providers to deploy technologies and
services, and enterprises evolve their network infrastructure.
He can be reached at <mailto:jim@ashtonmetzler.com>
_______________________________________________________________
This newsletter is sponsored by Nokia
Empower Your Mobile Enterprise

Nokia believes that business mobility will fundamentally change
the way work gets done-and for the better. To allow the entire
organization to get the most from this paradigm shift in
productivity, Nokia Enterprise Solutions focuses on delivering
increased efficiency through enhanced mobility. Learn more by
downloading this white paper today!
http://www.fattail.com/redir/redirect.asp?CID=107733
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ARCHIVE LINKS

Archive of the WAN newsletter:
http://www.networkworld.com/newsletters/frame/index.html
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touted - find out if attendees agreed and if faster storage
solutions will soon be available. Click here:
<http://www.networkworld.com/news/2005/062005-data-recovery.html>
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