tailscale/net/packet/header.go

// 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 is returned when Marshal receives a buffer
	// too small to contain the header to marshal.
	errSmallBuffer = errors.New("buffer too small")
	// errLargePacket is returned when Marshal receives a payload
	// larger than the maximum representable size in header
	// fields.
	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 marshaled packet.
	Len() int
	// Marshal serializes the header into buf, which must be at
	// least Len() bytes long. Implementations of Marshal assume
	// that bytes after the first Len() are payload bytes for the
	// purpose of computing length and checksum fields. Marshal
	// implementations must not allocate memory.
	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. This function allocates memory, see Header.Marshal for an
// allocation-free option.
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
}