21. Migration detention policies and procedures must not be discriminatory or make distinctions based on the legal conditions of the person.22 Detaining someone solely on the basis of a distinction such as race, colour, sex, language, religion, political or other opinion, national or social origin, economic position, birth, nationality or any other status will always be arbitrary. 22. The element of reasonableness requires that the detention be imposed in pursuance of a legitimate aim in each individual case. This must be prescribed by legislation that clearly defines and exhaustively lists the reasons that are legitimate aims justifying detention.23 Such reasons that would legitimize the detention include the necessity of identification of the person in an irregular situation or risk of absconding when their presence is necessary for further proceedings.24 23. The element of necessity requires that the detention be absolutely indispensable for achieving the intended purpose and that no other measure less onerous exists in the individual circumstances of the person who is in an irregular migration situation.25 24. The element of proportionality requires that a balance be struck between the gravity of the measure taken, which is the deprivation of liberty of a person in an irregular situation, including the effect of the detention on the physical and mental health of the individual, and the situation concerned.26 To ensure that the principle of proportionality is satisfied, alternatives to detention must always be considered.27 IV. Length of detention in the course of migration proceedings 25. A maximum detention period in the course of migration proceedings must be set by legislation, and such detention shall be permissible only for the shortest period of time. Excessive detention in the course of migration proceedings is arbitrary.28 Upon the expiry of the detention period set by law, the detained person must automatically be released.29 26. Indefinite detention of individuals in the course of migration proceedings cannot be justified and is arbitrary.30 27. There may be instances when the obstacle for identifying or removal of persons in an irregular situation from the territory is not attributable to them — including non-cooperation of the consular representation of the country of origin, the principle of non-refoulement31 or the unavailability of means of transportation — thus rendering expulsion impossible. In such cases, the detainee must be released to avoid potentially indefinite detention from occurring, which would be arbitrary.32 22 See Universal Declaration of Human Rights, arts. 2, 9, 10 and 11; and International Covenant on Civil and Political Rights, art. 9 (1). 23 See A/HRC/13/30, para. 59; and A/HRC/10/21, paras. 67 and 82. 24 See A/HRC/13/30, para. 59; and general comment No. 35, para. 18. 25 See A/HRC/7/4, para. 46. See also E/CN.4/1999/63/Add.3, para. 34; and E/CN.4/1999/63/Add.3, paras. 29 and 34. 26 See A/HRC/30/37, para. 111. 27 Ibid., para. 108. 28 See, for example, opinion No. 5/2009, 42/2017; E/CN.4/1999/63/Add.3, para. 35; and A/HRC/33/50/Add.1, paras. 49-50. 29 See A/HRC/13/30, para. 61. 30 See A/HRC/13/30, para. 63. See also opinions No. 42/2017 and No. 28/2017. 31 See the Convention against Torture and Other Cruel, Inhuman or Degrading Treatment or Punishment, art. 3; and the Convention relating to the Status of Refugees, art. 33. 32 See A/HRC/13/30, para. 63; A/HRC/7/4, para. 48; and A/HRC/10/21, para. 82. See also opinion No. 45/2006. 4