General Networking/Lan/Wan/TCP window Size

Advertisement


Question
Hai,

I understand selection of window size should match with BDP (Bandwidth Delay Product), if window size is less, link will be under utilized and if  window size is more, link will have saw tooth effect….


Now my query is…..


If I have a VSAT link of 10Mbps with 600msec then required window size will be  750Kbytes….


Now link is having UDP strumming (say 5Mbps) then FTP traffic will it have saw tooth effect????

If udp traffic is changing randomly, how do I adjust TCP window sizes dynamically???

Regards

Answer
Well I'll HELP YOU ... but I'm not going to answer the question, since it's homework - Ok?  Next time - please provide YOUR ANSWERS and I'll guide them along.

>> I understand selection of window size should match with BDP (Bandwidth Delay Product), if window size is less, link will be under utilized and if  window size is more, link will have saw tooth effect….

Well not really.  It is a "contributing factor" perhaps but not usually the root cause.  The saw tooth pattern is really the result of TCP congestion detection, and then the speed ramping back up again.  It can also be caused by retransmissions / packet loss.

Here are some things worth reading
http://inst.eecs.berkeley.edu/~ee122/fa06/notes/18-Congestion-6up.pdf
http://www.hjp.at/doc/rfc/rfc2581.txt
http://www.ssfnet.org/Exchange/tcp/tcpTutorialNotes.html


>> If I have a VSAT link of 10Mbps with 600msec then required window size will be  750Kbytes….

Oh?  But the end stations have NO IDEA that the VSAT is in the middle.  They would need to be manually configured.  Better yet - instead of doing that, there are very specific technologies for Satellite links

http://www.vsat-systems.com/Education/Satellite-Internet-Explained/Performance/P
http://www.satellitetcp.com/chapters/SatTCP-chap13-satellitetcp.pdf
http://www.cs.ucla.edu/classes/winter02/cs216/l1/hw/0218-rhw3-dueFeb18/nash97.pd

>> Now link is having UDP strumming (say 5Mbps) then FTP traffic will it have saw tooth effect????

UDP strumming?  I have never heard of "udp strumming". Do you mean Streaming?

Anyway what you are saying is "hey there is a lot of UDP traffic clogging up the link and IF it causes congestion then the TCP traffic will be affected by Congestion Control.

So ... WILL 5 Mbps of UDP clog up that link ... or ... NOT?  Hmmmm ...  I'm not here to answer that FOR YOU.  So why don't you work up the numbers?  How fast will a bursty streamlined tcp application like FTP be able to go on a link with 600 ms of latency?  

Here are some links to help you ... but you'll still need to do the work, ok?  Oh ... and packet loss really really hurts stuff at high latency!

http://forums.anandtech.com/archive/index.php/t-1722863.html
http://bradhedlund.com/2008/12/19/how-to-calculate-tcp-throughput-for-long-dista
http://www.hpl.hp.com/personal/Praveen_Yalagandula/papers/SIGOPS09.pdf
http://www.speedguide.net/articles/the-tcp-window-latency-and-the-bandwidth-dela

>> If udp traffic is changing randomly, how do I adjust TCP window sizes dynamically???

End stations won't do this, since you have already manually set them.  So ... this question assumes that you are NOT manually setting them, but rather using a specialized device on each side to handle that stuff.

If you don't set tcp to use those manual settings then it will "adjust dynamically".  BUT - it will not do it WELL - not on high latency links like that.

Good luck!

General Networking/Lan/Wan

All Answers


Answers by Expert:


Ask Experts

Volunteer


Jeff K

Expertise

I'm a Network and Application Performance Specialist, and have worked for some of the best software companies in the world.

Experience

I have over 20 years in Information Technology & Networking.

Education/Credentials
Lots of hard work, study and real-world experience. I've had some formal training along the way but most of my knowledge is from working in the field, not the classroom.

©2016 About.com. All rights reserved.