net/packet: rename from wgengine/packet.

Signed-off-by: David Anderson <danderson@tailscale.com>

net/packet/header.go

@@ -0,0 +1,48 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package packet
+
+import (
+	"errors"
+	"math"
+)
+
+const tcpHeaderLength = 20
+
+// maxPacketLength is the largest length that all headers support.
+// IPv4 headers using uint16 for this forces an upper bound of 64KB.
+const maxPacketLength = math.MaxUint16
+
+var (
+	errSmallBuffer = errors.New("buffer too small")
+	errLargePacket = errors.New("packet too large")
+)
+
+// Header is a packet header capable of marshaling itself into a byte buffer.
+type Header interface {
+	// Len returns the length of the header after marshaling.
+	Len() int
+	// Marshal serializes the header into buf in wire format.
+	// It clobbers the header region, which is the first h.Length() bytes of buf.
+	// It explicitly initializes every byte of the header region,
+	// so pre-zeroing it on reuse is not required. It does not allocate memory.
+	// It fails if and only if len(buf) < Length().
+	Marshal(buf []byte) error
+	// ToResponse transforms the header into one for a response packet.
+	// For instance, this swaps the source and destination IPs.
+	ToResponse()
+}
+
+// Generate generates a new packet with the given header and payload.
+// Unlike Header.Marshal, this does allocate memory.
+func Generate(h Header, payload []byte) []byte {
+	hlen := h.Len()
+	buf := make([]byte, hlen+len(payload))
+
+	copy(buf[hlen:], payload)
+	h.Marshal(buf)
+
+	return buf
+}