net/packet: implement Geneve header serialization (#15301)

Updates tailscale/corp#27100

Signed-off-by: Jordan Whited <jordan@tailscale.com>

net/packet/geneve.go

@@ -0,0 +1,104 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package packet
+
+import (
+	"encoding/binary"
+	"errors"
+	"io"
+)
+
+const (
+	// GeneveFixedHeaderLength is the length of the fixed size portion of the
+	// Geneve header, in bytes.
+	GeneveFixedHeaderLength = 8
+)
+
+const (
+	// GeneveProtocolDisco is the IEEE 802 Ethertype number used to represent
+	// the Tailscale Disco protocol in a Geneve header.
+	GeneveProtocolDisco uint16 = 0x7A11
+	// GeneveProtocolWireGuard is the IEEE 802 Ethertype number used to represent the
+	// WireGuard protocol in a Geneve header.
+	GeneveProtocolWireGuard uint16 = 0x7A12
+)
+
+// GeneveHeader represents the fixed size Geneve header from RFC8926.
+// TLVs/options are not implemented/supported.
+//
+//	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+//	|Ver|  Opt Len  |O|C|    Rsvd.  |          Protocol Type        |
+//	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+//	|        Virtual Network Identifier (VNI)       |    Reserved   |
+//	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+type GeneveHeader struct {
+	// Ver (2 bits): The current version number is 0. Packets received by a
+	// tunnel endpoint with an unknown version MUST be dropped. Transit devices
+	// interpreting Geneve packets with an unknown version number MUST treat
+	// them as UDP packets with an unknown payload.
+	Version uint8
+
+	// Protocol Type (16 bits): The type of protocol data unit appearing after
+	// the Geneve header. This follows the Ethertype [ETYPES] convention, with
+	// Ethernet itself being represented by the value 0x6558.
+	Protocol uint16
+
+	// Virtual Network Identifier (VNI) (24 bits): An identifier for a unique
+	// element of a virtual network. In many situations, this may represent an
+	// L2 segment; however, the control plane defines the forwarding semantics
+	// of decapsulated packets. The VNI MAY be used as part of ECMP forwarding
+	// decisions or MAY be used as a mechanism to distinguish between
+	// overlapping address spaces contained in the encapsulated packet when load
+	// balancing across CPUs.
+	VNI uint32
+
+	// O (1 bit): Control packet. This packet contains a control message.
+	// Control messages are sent between tunnel endpoints. Tunnel endpoints MUST
+	// NOT forward the payload, and transit devices MUST NOT attempt to
+	// interpret it. Since control messages are less frequent, it is RECOMMENDED
+	// that tunnel endpoints direct these packets to a high-priority control
+	// queue (for example, to direct the packet to a general purpose CPU from a
+	// forwarding Application-Specific Integrated Circuit (ASIC) or to separate
+	// out control traffic on a NIC). Transit devices MUST NOT alter forwarding
+	// behavior on the basis of this bit, such as ECMP link selection.
+	Control bool
+}
+
+// Encode encodes GeneveHeader into b. If len(b) < GeneveFixedHeaderLength an
+// io.ErrShortBuffer error is returned.
+func (h *GeneveHeader) Encode(b []byte) error {
+	if len(b) < GeneveFixedHeaderLength {
+		return io.ErrShortBuffer
+	}
+	if h.Version > 3 {
+		return errors.New("version must be <= 3")
+	}
+	b[0] = 0
+	b[1] = 0
+	b[0] |= h.Version << 6
+	if h.Control {
+		b[1] |= 0x80
+	}
+	binary.BigEndian.PutUint16(b[2:], h.Protocol)
+	if h.VNI > 1<<24-1 {
+		return errors.New("VNI must be <= 2^24-1")
+	}
+	binary.BigEndian.PutUint32(b[4:], h.VNI<<8)
+	return nil
+}
+
+// Decode decodes GeneveHeader from b. If len(b) < GeneveFixedHeaderLength an
+// io.ErrShortBuffer error is returned.
+func (h *GeneveHeader) Decode(b []byte) error {
+	if len(b) < GeneveFixedHeaderLength {
+		return io.ErrShortBuffer
+	}
+	h.Version = b[0] >> 6
+	if b[1]&0x80 != 0 {
+		h.Control = true
+	}
+	h.Protocol = binary.BigEndian.Uint16(b[2:])
+	h.VNI = binary.BigEndian.Uint32(b[4:]) >> 8
+	return nil
+}

net/packet/geneve_test.go

@@ -0,0 +1,32 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package packet
+
+import (
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestGeneveHeader(t *testing.T) {
+	in := GeneveHeader{
+		Version:  3,
+		Protocol: GeneveProtocolDisco,
+		VNI:      1<<24 - 1,
+		Control:  true,
+	}
+	b := make([]byte, GeneveFixedHeaderLength)
+	err := in.Encode(b)
+	if err != nil {
+		t.Fatal(err)
+	}
+	out := GeneveHeader{}
+	err = out.Decode(b)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if diff := cmp.Diff(out, in); diff != "" {
+		t.Fatalf("wrong results (-got +want)\n%s", diff)
+	}
+}