Best MTU to avoid fragmentation of packets on send through

I’m trying to optimize my network to make most efficient use of my limited bandwidth. Part of that optimization is to ensure that my client send packets which do not amplify over the WAN links (in other words not fragmenting).

I’m on DSL connections (3 I’m trying to bond) my PPPoE interface MTU is 1492. I have SpeedFusion running which should take some overhead to send down the tunnel. My goal is to ensure that SpeedFusion never has to send a partial fragment down one side or the other as a result of a single packet being sent by the source. This should in-theory prevent the additional header send down the congested DSL lines for a packet fragment remainder (which on a 1500 MTU normal is only ever 80-100 bytes at best).

Can anyone tell me what the optimal MTU size to configure my computers and network for in order to prevent fragmented sends over PepVPN/SpeedFusion?

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So I found the following quote from the Best Practices guide on page 6:

SpeedFusion adds an additional 80 bytes of data to each packet sent over a SpeedFusion connection, no matter what size the original data packet is. This compares well to the 58 bytes of overhead required by IPsec, specially considering that SpeedFusion is not only providing advanced routing and load balancing but 256 bit AES encryption within the tunnel too.

So my theory would be the preferred client MTU size should be 1392 to avoid any packet fragmentation through the SpeedFusion tunnel. (1492 PPPoE MTU - 20 for IP header - 80 for SpeedFusion header == 1392 MTU)

This should ensure that every packet sent by a client on my network will traverse through the SpeedFusion tunnels without needing any additional fragmentation (and incurring the additional 100 bytes of header data to send for each of the fragment segments).

I would love if someone from PepLink (or the community at large) can confirm but I’ll do some preliminary testing and report back what I find.

Looks like I may have forgotten to add a header :slight_smile:

1492 (PPPoE MTU) - 20 for IP header - 8 for UDP header - 80 for Speed Fusion header = 1384 MTU to avoid SpeedFusion tunnel fragmentation on full packet send. I am still playing around with this.


I’d like an update to what you found if you don’t mind. I’m personally dealing with an MTU of 1430 on AT&T, 1500 on T-Mobile, and 1500 on the SpeedFusion hub in the Azure Data Center I’m hosting it in. What settings if any did you change to improve link conditions? I can only find the WAN MTU in the Fusion Hub and the individual WAN MTUs on my end. Did you reduce the one at the FusionHub or are you just playing with a client side (Windows) MTU to increase reliable throughput?

I seem to get the best response by setting all 3 WANs to 1430. There is still some reported packet loss reported by the FusionHub heading towards the site, but the site shows a clean connection.


Shortcut: The best MTU for AT&T cellular link is 1420. The best MTU for Verizon cellular link is also 1420.