frame

type Frame struct {
	// Bytes is the frame, ready to transmit.
	Bytes []byte

	// SrcPortOff and DstPortOff are where the UDP ports are, and SrcIPOff and
	// DstIPOff where the addresses are. Changing an address means fixing two
	// checksums; changing a port means fixing one, or leaving it zero.
	SrcIPOff, DstIPOff     int
	SrcPortOff, DstPortOff int

	// VLANTags is how many tags the frame carries, which is what decides how
	// much of it a device in L2 inline mode must be shown.
	VLANTags int

	// HeaderLen is everything before the UDP payload.
	HeaderLen int
}

type Spec struct {
	// SrcMAC and DstMAC are the Ethernet addresses. DstMAC must be the address
	// the far end will accept: with no switch learning involved, sending to a
	// made-up address usually means the frames are never seen again.
	SrcMAC, DstMAC net.HardwareAddr

	// VLAN is the tag to carry, or zero for untagged. Outer, if there are two.
	VLAN int

	// OuterVLAN, when set, makes the frame double tagged: OuterVLAN outside,
	// VLAN inside.
	OuterVLAN int

	// Priority is the priority code point in the tags.
	Priority int

	// SrcIP, DstIP, SrcPort and DstPort are the UDP flow.
	SrcIP, DstIP     netip.Addr
	SrcPort, DstPort uint16

	// Size is the whole frame length in bytes, not counting the frame check
	// sequence. 60 is the shortest legal frame and corresponds to what is
	// usually called a 64-byte packet.
	Size int
}