Please Whitelist This Site?

I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :)

If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads.

If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK.

Thanks for your understanding!

Sincerely, Charles Kozierok
Author and Publisher, The TCP/IP Guide


NOTE: Using software to mass-download the site degrades the server and is prohibited.
If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The Book is Here... and Now On Sale!

Enjoy The TCP/IP Guide? Get the complete PDF!
The TCP/IP Guide

Custom Search







Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Transport Layer Protocols
           9  Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)
                9  TCP/IP Transmission Control Protocol (TCP)
                     9  TCP Reliability and Flow Control Features and Protocol Modifications

Previous Topic/Section
TCP "Silly Window Syndrome" and Changes To the Sliding Window System For Avoiding Small-Window Problems
Previous Page
Pages in Current Topic/Section
12
3
Next Page
Summary Comparison of TCP/IP Transport Layer Protocols (UDP and TCP)
Next Topic/Section

TCP Congestion Handling and Congestion Avoidance Algorithms
(Page 3 of 3)

Fast Retransmit

We’ve already seen in our look at TCP segment retransmission that when segments are received by a device out of order (meaning, non-contiguously), the recipient will only acknowledge the ones received contiguously. The Acknowledgment Number will specify the sequence number of the byte it expects to receive next. So, in the example given in that topic, Segment #1 and #2 were acknowledged while #4 was not, because #3 was not received.

It is possible for a TCP device to in fact respond with an acknowledgment when it receives an out-of-order segment, simply “reiterating” that it is stuck waiting for a particular byte number. So, when the client in that example receives Segment #4 and not Segment #3, it could send back an Acknowledgment saying “I am expecting the first byte of Segment #3 next”.

Now, suppose this happens over and over. The server, not realizing that Segment #3 was lost, sends Segment #5, #6 and so on. Each time one is received, the client sends back an acknowledgment specifying the first byte number of Segment #3. Eventually, the server can reasonably conclude that Segment #3 is lost, even if its retransmission timer has not expired.

The Fast Retransmit feature dictates that if three or more of these acknowledgments are received, all saying “I want the segment starting with byte #N”, then it's probable that the segment starting with byte #N has been lost, usually because it was dropped due to congestion. In this case, the device will immediately retransmit the missing segment without going through the normal retransmission queue process. This improves performance by eliminating delays that would suspend effective data flow on the link.

Fast Recovery

When Fast Retransmit is used to re-send a lost segment, the device using it performs Congestion Avoidance, but does not use Slow Start to increase the transmission rate back up again. The rationale for this is that since multiple ACKs were received by the sender all indicating receipt of out-of-order segments, this indicates that several segments have already been removed from the flow of segments between the two devices. For efficiency reasons, then, the transmission rate can be increased more quickly than when congestion occurs in other ways. This improves performance compared to using the regular Congestion Avoidance algorithm after Fast Retransmit.

Relationship of Congestion Handling Mechanisms

In practice, these features are all related to each other. Slow Start and Congestion Avoidance are distinct algorithms but are implemented using a single mechanism, involving the definition of a Congestion Window that limits the size of transmissions and whose size is increased or decreased depending on congestion levels. Fast Retransmit and Fast Recovery are implemented as changes to the mechanism that implements Slow Start and Congestion Avoidance.

I realize that this is somewhat cryptic; congestion handling is a rather complex process. If you want to learn more, RFC 2001 contains the technical details, showing how each of the algorithms is implemented in each device. I have specifically omitted those details from this topic since I feel my summary descriptions are already complicated enough, and my goal was simply to help you get a feel for how congestion is handled in TCP in general terms.

Key Concept: TCP flow control is an essential part of regulating the traffic flow between TCP devices, but takes into account only how busy the two TCP endpoints are. It is also important to take into account the possibility of congestion of the networks over which any TCP session is established, which can lead to inefficiency through dropped segments. To deal with congestion and avoid contributing to it unnecessarily, modern TCP implementations include a set of congestion avoidance algorithms that alter the normal operation of the sliding window system to ensure more efficient overall operation.


 


Previous Topic/Section
TCP "Silly Window Syndrome" and Changes To the Sliding Window System For Avoiding Small-Window Problems
Previous Page
Pages in Current Topic/Section
12
3
Next Page
Summary Comparison of TCP/IP Transport Layer Protocols (UDP and TCP)
Next Topic/Section

If you find The TCP/IP Guide useful, please consider making a small Paypal donation to help the site, using one of the buttons below. You can also donate a custom amount using the far right button (not less than $1 please, or PayPal gets most/all of your money!) In lieu of a larger donation, you may wish to consider purchasing a download license of The TCP/IP Guide. Thanks for your support!
Donate $2
Donate $5
Donate $10
Donate $20
Donate $30
Donate: $



Home - Table Of Contents - Contact Us

The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005

© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.