D
DelJo63
The following is a description of PACKET FRAGMENTATION(PF) and how to stop it.
Bigger or max MTU(1500) is frequently counter productive.
Reducing your MTU at least to 1492 will stop fragmentation, but you can do even better.
Find the correct MTU for your ISP (regardless of DSL or Cable)
Get a command prompt and enter
eg sample: $ ping -f -l 1500 google.com
Pinging google.com [216.58.193.206] with 1500 bytes of data:
Packet needs to be fragmented but DF set.
So 1500 creates PF from my ISP to my laptop, so keep reducing $SIZE$ by 8 until you get
Reply from 216.58.193.206: bytes=64 (sent $SIZE$ ) time=14ms TTL=54
Why?
The maximum and default value of the MTU is 1500. This limits the packets sent and received to this value.
It also permits larger packets by fragmenting them into multiple units which then are reassembled upon receipt.
Fragmentation of packets IS a problem:
1) obviously slows both the send & receive.
2) the reassembly sometimes fails and the connection gets broken
3) and sometimes the router will also reboot.
Solution? Stop or inhibit fragmentation with an MTU < 1500 (value must be a multiple of 8)
Alternative approaches?
There are hacks available on the Internet which alter frame and window sizes (tcp internal values).
However, forcing more data on a link has a higher probability of failing than sending smaller size
because the error and retry occurs more quickly and with a better probability of success.
W A Y back in the dial-up days, guess what the MTU value was used?? 576, the smallest value possible.
Alternative values?
The value of 1492 nominated above solves the fragmentation problem, but may not be the best value for you
and your specific ISP connection. If you don't care, that's ok, just set 1492 as shown below and smile.
This works as most networks today implement MTU Discovery, where every node in
the path to you reports its unique MTU value and the server automatically selects the smallest reported
value for communications to you - - "auto mtu configuration" if you will.
Where?
The 'right' place to set the MTU is in your router. Typically in the WAN settings you have a choice
for the MTU - - set it here and every device connected to your router will get the benefit of avoiding
packet fragmentation.
The other advantage is for mobile devices. When the device connects to some other
ISP (say your favorite hotspot), your personal router is no longer in control and the mobile device
will correctly adapt to the hotspot settings.
The other location that can be use to set an MTU is on the device itself. Primarily, we're talking about
PCs, as personally I have no idea how or if you can tune a cellphone or tablet (sigh).
Any value manually set in the following manner will 'persistent' regardless of which ISP you connect to.
FIRST, from an admin loging, get a command prompt
To see your existing MTU
netsh interface ipv4 show subinterface "Local Area Connection"
netsh interface ipv4 show subinterface "Wireless Network Connection"
To set an MTU of 1492
netsh interface ipv4 set subinterface "Local Area Connection" mtu=1492 store=persistent
netsh interface ipv4 set subinterface "Wireless Network Connection" mtu=1492 store=persistent
The new value will become effective upon reboot.
Bigger or max MTU(1500) is frequently counter productive.
Reducing your MTU at least to 1492 will stop fragmentation, but you can do even better.
Find the correct MTU for your ISP (regardless of DSL or Cable)
Get a command prompt and enter
ping -f -l $SIZE$ google.com
where $SIZE is always a multiple of 8 and 1500 is the max.
where $SIZE is always a multiple of 8 and 1500 is the max.
eg sample: $ ping -f -l 1500 google.com
Pinging google.com [216.58.193.206] with 1500 bytes of data:
Packet needs to be fragmented but DF set.
So 1500 creates PF from my ISP to my laptop, so keep reducing $SIZE$ by 8 until you get
Reply from 216.58.193.206: bytes=64 (sent $SIZE$ ) time=14ms TTL=54
Why?
The maximum and default value of the MTU is 1500. This limits the packets sent and received to this value.
It also permits larger packets by fragmenting them into multiple units which then are reassembled upon receipt.
Fragmentation of packets IS a problem:
1) obviously slows both the send & receive.
2) the reassembly sometimes fails and the connection gets broken
3) and sometimes the router will also reboot.
Solution? Stop or inhibit fragmentation with an MTU < 1500 (value must be a multiple of 8)
Alternative approaches?
There are hacks available on the Internet which alter frame and window sizes (tcp internal values).
However, forcing more data on a link has a higher probability of failing than sending smaller size
because the error and retry occurs more quickly and with a better probability of success.
W A Y back in the dial-up days, guess what the MTU value was used?? 576, the smallest value possible.
Alternative values?
The value of 1492 nominated above solves the fragmentation problem, but may not be the best value for you
and your specific ISP connection. If you don't care, that's ok, just set 1492 as shown below and smile.
This works as most networks today implement MTU Discovery, where every node in
the path to you reports its unique MTU value and the server automatically selects the smallest reported
value for communications to you - - "auto mtu configuration" if you will.
Where?
The 'right' place to set the MTU is in your router. Typically in the WAN settings you have a choice
for the MTU - - set it here and every device connected to your router will get the benefit of avoiding
packet fragmentation.
The other advantage is for mobile devices. When the device connects to some other
ISP (say your favorite hotspot), your personal router is no longer in control and the mobile device
will correctly adapt to the hotspot settings.
The other location that can be use to set an MTU is on the device itself. Primarily, we're talking about
PCs, as personally I have no idea how or if you can tune a cellphone or tablet (sigh).
Any value manually set in the following manner will 'persistent' regardless of which ISP you connect to.
FIRST, from an admin loging, get a command prompt
To see your existing MTU
netsh interface ipv4 show subinterface "Local Area Connection"
netsh interface ipv4 show subinterface "Wireless Network Connection"
To set an MTU of 1492
netsh interface ipv4 set subinterface "Local Area Connection" mtu=1492 store=persistent
netsh interface ipv4 set subinterface "Wireless Network Connection" mtu=1492 store=persistent
The new value will become effective upon reboot.
