quic-go defaults the `InitialPacketSize` to 1280 bytes. This is used for
the size of the payload plus UDP header. When quic-go creates its
initial handshake packet, it pads it to this size as an optimization
for the anti-amplification limit (which does not really apply to
Caddy's server connections).
Tailscale uses an MTU of 1280 (the minimum IPv6 MTU) because it is a
tunnel operating in unknown environments, possibly inside other
tunnels.
When wrapped in an IP header (either v4 or v6), the 1280 byte QUIC packet
constructed by quic-go exceeds the Tailscale MTU and gets dropped. This
makes it impossible to respond to an HTTP3 connection attempt over
Tailscale when using the default `InitialPacketSize`.
We explicitly set the `InitialPacketSize` to 1200 (its minimum) to support
HTTP3 connections over Tailscale and other low MTU connections.
Once the connection is established, quic-go can perform MTU discovery to
increase the packet size to the maximum supported by the connection, so
any throughput loss due to the lowered `InitialPacketSize` is
short-lived.
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