Transport layer flow control

Answered Question
Dec 28th, 2008

How does the receiver govern the amount of data sent by the sender? How and where is flow control adjusted?

I have this problem too.
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Correct Answer by Joseph W. Doherty about 7 years 11 months ago

The TCP flow conrol mechanism is fixed, however knowing how it works, we can influence it by how we drop (or ECN, if supported) packets, e.g. RED, police, etc. We can also "shape" traffic. Actual features vary per Cisco device.

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Giuseppe Larosa Mon, 12/29/2008 - 01:54

Hello Said,

if you refer to TCP flow control the receiver advertises in each ACK packet to the sender the current size of its available buffers for the session

see

http://www.cs.umd.edu/~shankar/417-F01/Slides/chapter3b/sld008.htm

So the window can change over time and it is called sliding window because it can increase or decrease over time

TCP uses bytes and not packets as a reference.

the field in the TCP header is window

see

http://www.freesoft.org/CIE/Course/Section4/8.htm

Hope to help

Giuseppe

Joseph W. Doherty Mon, 12/29/2008 - 05:04

Giuseppe's post describes how the receiver, at least in TCP, informs the sender of its available buffer space. This feature is mostly an all or nothing method of adjusting flow (although there are some network devices that "spoof" this information for flow contol).

TCP flow control also relies on sender adjusting itself via "slow start" and management of its congestion window. (NB: Giuseppe's first reference also provides information about these features of TCP.)

saidfrh Mon, 12/29/2008 - 06:53

So, the the TCP flow control mechanism is fixed and can not be adjusted? We have Cisco equipment. Is the above mechanism related to firewall, or router? So in both equipment the flow control can not be adjusted?

Thanks.

Correct Answer
Joseph W. Doherty Mon, 12/29/2008 - 07:46

The TCP flow conrol mechanism is fixed, however knowing how it works, we can influence it by how we drop (or ECN, if supported) packets, e.g. RED, police, etc. We can also "shape" traffic. Actual features vary per Cisco device.

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