Reasons of Retransmission with TCP and what if received frame size is bigger than MTU configured on switch

[ashish7aditya]

Dear Experts,

I need your support to understand that what can be the possible reasons for packet re-transmission in TCP based applications?

also how the switch and router treats a frame if the size of the received frame is greater than the MTU size configured on the interface? Will it drop the frame or it will fragment the frame and will pass the frame?

I am sending a VLAN tagged Ethernet(802.3ac) frame towards a switch on whose receiving interface MTU =1500Bytes. So, all the frames for which payload is >1500Bytes i.e. Ethernet frame size will be 1542(7+1+6+6+4+2+1500+4+12)Bytes will be dropped or will be fragmented into smaller PDUs and will be passed?

Further I believe large round trip delay can also lead to packet re transmission? In this case how to check this and how to fix this ?

Best Regards!

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Interfaces receiving frames bigger than they are configured for should drop them.  Only too large packets, leaving a L3 interface, are fragmented.

Yes, excessive RTT can cause TCP re-transmission, although TCP generally tracks a RTT average so usually only highly variable RTTs would trip the timeout.  A packet trace would be perhaps the best way to examine what's happening at that level but you might see this issue too withing a ping test series.

[ashish7aditya]

Dear Joseph,

Many thanks for your support and feedback.

Further to it, if I make a ping test then what values of Average RTT should I consider alarming in this regard of packet re transmission ? I mean any numeric figure for RTT based on which I can suspect packet re-transmission ?

Best Regards!

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

If you see sequential ping times varying by more than 200 ms, that might be an indicator.

[ashish7aditya]

In the attached wire-shark snapshot,what do we mean by length values 1470 and 1394 (circled in red) ?