I just tested a ZT client connection behind a NAT:in gateway with its WAN link via LTE (with private APN)
The node connects OK (not in relay mode). The other nodes are all on a 100Mbit or better fiber with a NAT:ing Gateway.
When ping the mobile ZT node the latency is ap. 250-300ms with some 2 - 20s dropouts.
If I at the same time try to establish a ssh connection the ping session get a “very long” dropout and ssh client stalls. Some wireshark sniffing at station side reveals that some serious packet fragmentation is going on…
ZT interfaces gets a default MTU set to 2800 (where common support is normally 1500 to be safe)
If you’r node is on a fiber or any newer dsl link most of the infrastructure are likely to support MTU of 1500 AND “jumbo frames” up to 9000 (that imply that 2800 still is safe).
When you are on a Mobile Network (LTE or 3G) several parts in the infrastructure chain do NOT support “jumbo frames” and actively blocking it. This means 1500 including header is the theoretical maximal size. In reality there is also at least one VPN header overhead that you need to take off to get a usable MTU. Also in the “radio side” at the cells there are some bad behavior on larger packets (mostly on UDP ). My conclusions after some 15+ years of using IP networking on 2G -4G…
DON’T USE MTU LARGER THAN 1280, otherwise it will fragment as hell and all bad things that follow.
Now to the ZT and MTU settings:
You can NOT use ifconfig or IP command to set the node’s MTU in a persistent way, it will be overwritten by conf from ZT central, but it will work for some minutes or so…
I have NOT found any manual or instructions how to set a local MTU in local config or for a speciffic node at ZT central.
I have NOT found in ZT central UI any way to enter a “global” MTU setting.
I did find a solution using curl to post a “global” MTU setting (most likely related to address assignment)
but it also needs you to generate a API token… Not that user friendly !!
Still that would decrease the overall performance…
Anyone that successfully managed to modify MTU settings??
Best regards,
Anders