On 2022-11-28 19:47, L A Walsh wrote:
I use a 9k mtu on my 10gb network @ home -- only exists between my desktop & my server where it can be worthwhile.
Bridged to that NW is a 1gb, 1.5k mtu network use for my home IOT (Internet of Things) This works for administrative traffic, for the most part, but in gaining some new 'things', some wish to talk to a media/file server on the higher BW net.
Someone referred me to possibly using tcp mss clamping, though they were using it in the context of a VPN tunnel which I'm not using.
I'm wondering if it is as simple as installing routes with mtu clamping so hosts wanting(needing) to goto a smaller mtu would take the appropriate network and talk with smaller packets. Seems there maybe something missing there since some traffic can be ip-level w/no tcp in play (ex: dns). Doesn't seem that would really work too well...
Are you talking about IPv4 or IPv6? Path MTU discovery is mandatory with IPv6, but IPv4 might still use fragmenting. Linux uses PMTUD for almost everything, IIRC, including UDP, but Windows uses PMTUD only for TCP. Regardless, IPv4 on either can fall back to fragmenting when PMTUD fails. With it, you should get an ICMP too big message coming back from whatever router is choking on that packet. However, I see you're bridging a network with a smaller MTU to your main network. That's a no no. Different size MTUs must be separated by a router, as a bridge (switch) has no way of dealing with an oversize packet. In fact, some switches can handle much larger packets than you're using.