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16,580 byte packet

Posted on 2004-10-01
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Last Modified: 2013-12-07
Been watching traffic (file transfers between windows machines.)   I noticed this:
http:/mvpbaseball.cc/capture.jpg

Can 16,580 bytes really be sent in one packet?  I thought the MTU of ethernet was 1500
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Question by:dissolved
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Author Comment

by:dissolved
ID: 12209407
anyone?
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Expert Comment

by:VertigoRay
ID: 12214687
As for 1500 being the max packet size, I believe that Win2k (pre SP1) defaulted UDP at 2000.  Of course after some SP releases they changed it to 1465.  This was done to make sure Kerberos didn't have any issues, since that's the basis of 2k3's authentication/encryption/etc.  So basically, the max packet size is dependant on the protocol.

SMB packet sizes can get much larger than the typical TCP/IP packet size.  The global option of "max xmit" sets the maximum size of packets that Samba exchanges with a client.  In some cases, setting a smaller maximum packet size can increase performance, especially with Windows for Workgroups.  The default value for this option is 65535. You can override it as follows:

[global]
      max xmit = 4096

Since the percentage of each block required for overhead falls as the blocks get larger, max xmit is conventionally set as large as possible.  It defaults to the protocol's upper limit, which is 64 kilobytes.  The smallest value that doesn't cause significant slowdowns is 2048.  If it is set low enough, it will limit the largest packet size that Samba will be able to negotiate.  This can be used to simulate a small MTU if you need to test an unreliable network connection.  However, such a test should not be used in production for reducing the effective MTU.  Normally, one tries to set these socket options high.  The value of 16384 has been suggested as a good starting point: in a non-Samba test.  This starting point seems pretty close to your packet size.  Is this coincidence?
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Author Comment

by:dissolved
ID: 12216290
Are you speaking about MSS or MTU?  I"m confused. Say I  have an MSS of 1460 (which I do in thise case) and the MTU of ethernet is 1500bytes as we all know. ...So how is a 16,580 packet being sent?  First off, it *should* be sent as several segments instead of one.  Also:   Surely there are intermediate routers that are there whose interface has an MTU of 1500. If I send a 16580 packet via SMB protocol (which utilizes TCP), it's going to get fragmented.

However in my case this the packet never leaves my immediate network and the receiver receives the packet and indicated it wa indeed 16,580 bytes..  Any clarification appreciated.
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Accepted Solution

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VertigoRay earned 500 total points
ID: 12234063
I found the s ite where I read a lot of what I learned about this.  Here's a link for a topic that is closest to what you're asking (save me some key strocks if I just paste you the link):

http://www.aarnet.edu.au/engineering/networkdesign/mtu/size.html

I hope this helps!
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Author Comment

by:dissolved
ID: 12242810
great read thanks
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