code: func (b IPv4) EncodePartial(partialChecksum, totalLength uint16) {
  	b.SetTotalLength(totalLength)
  	checksum := Checksum(b[totalLen:totalLen+2], partialChecksum)
  	b.SetChecksum(^checksum)
  }
docs: // EncodePartial updates the total length and checksum fields of ipv4 header,
  // taking in the partial checksum, which is the checksum of the header without
  // the total length and checksum fields. It is useful in cases when similar
  // packets are produced.