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tailscale/net/packet/header.go

49 lines
1.5 KiB
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 = 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
}