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Trying to Understand MTU !!!

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IAAI

Very Senior Member
Hi Guys ,

My connection goes like this : OTN > Cat 6 > Router (AC68U)
No ISP router in the middle :p

OTN Sat from ISP to 1492 MTU , PC sat to 1500 and IOS devices are sat for 1500

does this mean i m getting some delays (lag) because of the dropped packages ?

what would happen if i set to the router to the same value as the OTN/ISP prefered MTU 1492 ?
 
With Path MTU Discovery being enabled in almost every device, each client will determine the MTU dynamically, assuming that your firewall allows ICMP responses.
 
With Path MTU Discovery being enabled in almost every device, each client will determine the MTU dynamically, assuming that your firewall allows ICMP responses.

Correct. What it'll basically mean is that internally your devices will use 1500 bit packets, but anything that will pass through the internet will be reduced to 1492 bit packets before headed towards your router and out to the greater internet.
 
i think ethernet uses 1500 MTU by default and wifi uses above 2000. Every medium has its own MTU. Whenever a packet travelling between 2 mediums have a different MTU you will get more packets if it cant fit into a single packet. You can also use 1000 MTU if you want, usually every network MTU can be configured to any size but some mediums have a limit of a maximum packet size.

Even if you set a maximum MTU of 65535 bytes (software limit) it makes no difference if you keep sending all packets with 64 bytes of data each. The MTU is just the maximum size a packet can be before it is divided into smaller ones. MTUs dont have an effect of error rates as that is done in the overhead but smaller packets use more CPU as it has to process more packets. All network hardware performance is determined by how many packets it can handle. Increasing the MTU reduces network cpu usage but increases memory usage. If you set the MTU between LAN and WAN to the same than you use less processing overall since there are no packets that need to be fragmanted between LAN and WAN but if you have really fast network hardware than going for the highest MTU possible would be beneficial in CPU for your network clients when they communicate with each other.

You do get lag from lost packets but that would not be caused by fragmantation. Also some values stated are a bit different. If you want to set the same MTU than you want the data MTUs to be the same because every medium has different overhead sizes. Some devices say the physical MTU while some show the data MTU instead.

packet fragmantation only increases lag by an unnoticeable amount usually in microseconds in difference so having really fast network hardware helps.
 
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tried 1492 and got the same result as 1500 for ping and packet loss but the deference wasn't much in the jitter 0MS for 1500 and 1MS for 1492.
 
MSS Clamping exists to arbitrarily constrain TCP connections if you or your routes have known problems with Path MTU Discovery and/or ICMP responses.

Some sites say MSS Clamping is the devil since it breaks the end-to-end principle. Other sites say Path MTU Discovery is virtually broken and MSS Clamping saves the day.

I have no idea...


Unless you are well aware of how an undersized MTU/MSS can affect your connection, default to the maximum sane size.
 
Wifi MTU is 2302.

I believe it is actually one of the proposals for 802.11ax (ay? I am losing track of standards) to increase MTU to help with overall protocol efficiency. But I could be wrong about that.
 

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