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// Copyright (c) Tailscale Inc & contributors
// SPDX-License-Identifier: BSD-3-Clause
package packet
import (
"errors"
"math"
)
const igmpHeaderLength = 8
const tcpHeaderLength = 20
const sctpHeaderLength = 12
// 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
}
// HeaderChecksummer is implemented by Header implementations that
// need to do a checksum over their payloads.
type HeaderChecksummer interface {
Header
// WriteCheck writes the correct checksum into buf, which should
// be be the already-marshalled header and payload.
WriteChecksum(buf []byte)
}
// 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)
if hc, ok := h.(HeaderChecksummer); ok {
hc.WriteChecksum(buf)
}
return buf
}
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