A method and a first node (110) for providing an ACK/NACK-message of a HARQ process relating to a set of transport blocks received by the first node (110) are disclosed. Each transport block of the set of transport blocks comprises a set of code block groups. Each code block group comprises a set of code blocks. The first node (110) encodes (S060) bits for the ACK/NACK-message by use of a binary search to indicate one or more sets of code blocks to be re-transmitted. Each set of code blocks to be re-transmitted is indicated by a respective bit that is cleared. Said respective bit is included in the ACK/NACK-message in order according to the binary search. The first node (110) terminates (S070) the encoding (S060) of the bits for the ACK/NACK-message by use of the binary search when said each set of code blocks consists of one code block, or a number of bits, included in the ACK/NACK-message, has reached a predetermined number of bits. A corresponding method and a second node (120) are also disclosed.