An apparatus comprising: a plurality of completer elements (45, 50, 55, 60, 65, 70) configured to process transactions: a requester element (10, 20, 30) configured to issue a sequence of ordered transactions; and an interconnect providing, for each completer element, a communication channel between that completer element and the requester element for transfer of signals between that completer element and the requester element; wherein: a given completer element that is processing a given transaction in the sequence is arranged to issue a response signal to the requester element over its associated communication channel characterized in that the response signal comprises an ordered channel indication to identify whether the associated communication channel has an ordered channel property, where the ordered channel property guarantees that processing of transactions issued by the requester element over that associated communication channel in a given order will be completed by the given completer element in the same given order; and the requester element is responsive to the ordered channel indication to control timing of issuance from the requester element of at least one signal relating to one or more transactions after the given transaction in the sequence. a requester element (10, 20, 30) configured to issue a sequence of ordered transactions; and an interconnect providing, for each completer element, a communication channel between that completer element and the requester element for transfer of signals between that completer element and the requester element; wherein: a given completer element that is processing a given transaction in the sequence is arranged to issue a response signal to the requester element over its associated communication channel characterized in that the response signal comprises an ordered channel indication to identify whether the associated communication channel has an ordered channel property, where the ordered channel property guarantees that processing of transactions issued by the requester element over that associated communication channel in a given order will be completed by the given completer element in the same given order; and the requester element is responsive to the ordered channel indication to control timing of issuance from the requester element of at least one signal relating to one or more transactions after the given transaction in the sequence. a given completer element that is processing a given transaction in the sequence is arranged to issue a response signal to the requester element over its associated communication channel characterized in that the response signal comprises an ordered channel indication to identify whether the associated communication channel has an ordered channel property, where the ordered channel property guarantees that processing of transactions issued by the requester element over that associated communication channel in a given order will be completed by the given completer element in the same given order; and the requester element is responsive to the ordered channel indication to control timing of issuance from the requester element of at least one signal relating to one or more transactions after the given transaction in the sequence.