On Mon, 8 May 2023 14:10:07 -0400, "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote: > On Mon, May 08, 2023 at 09:25:48AM -0700, Stephen Hemminger wrote: > > On Mon, 8 May 2023 06:30:07 -0400 > > "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote: > > > > > > > > I don't know, in any scenario, when the hardware supports a large mtu, but we do > > > > > > not want the user to use it by default. > > > > > > > > > > When other devices on the same LAN have mtu set to 1500 and > > > > > won't accept bigger packets. > > > > > > > > So, that depends on pmtu/tcp-probe-mtu. > > > > > > > > If the os without pmtu/tcp-probe-mtu has a bigger mtu, then it's big packet > > > > will lost. > > > > > > > > Thanks. > > > > > > > > > > pmtu is designed for routing. LAN is supposed to be configured with > > > a consistent MTU. > > > > Virtio is often used with bridging or macvlan which can't support PMTU. > > PMTU only works when forwarding at layer 3 (ie routing) where there is > > a IP address to send the ICMP response. If doing L2 forwarding, the > > only thin the bridge can do is drop the packet. > > > > TCP cab recover but detecting an MTU blackhole requires retransmissions. > > Exactly. That's why we basically use the MTU advice supplied by device > by default - it's designed for use-cases of software devices where > the device might have more information about the MTU than the guest. > If hardware devices want e.g. a way to communicate support for > jumbo frames without communicating any information about the LAN, > a new feature will be needed. Let's think this question carefully. If necessary, we will try to introduce a new feature for virtio-net spec to support Jumbo Frame. Thanks. > > -- > MST > _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
