Set the QUIC InitialPacketSize to 1200 bytes#7886
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I did not see any mechanism to test this with the current Caddy test code, but I am open to suggestions for how to write tests for this. |
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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FYI @marten-seemann in case you have any opinions on this |
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quic-go defaults the
InitialPacketSizeto 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
InitialPacketSizeto 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
InitialPacketSizeis short-lived.No AI was used in making this PR.
Fixes: #7885