An apparatus for communication with another apparatus (102-k) over a packet-switched network, wherein the apparatus comprises a transport protocol portion and a complementary transport protocol portion, wherein the transport protocol portion is arranged to cause the apparatus to carry out at least the following: transmit, to the other apparatus (102-k, 102-m, 102-n), user data in a sequence of data blocks having respective sequence numbers assigned thereto, receive, from the other apparatus (102-k, 102-m, 102-n), acknowledgements blocks acknowledging reception of at least some of said data blocks, retransmit, to the other apparatus (102-k, 102-m, 102-n), those ones of said data blocks that are not acknowledged by the other apparatus (102-k, 102-m, 102-n), and enter a congestion avoidance state in response to a predefined amount of said data blocks not being acknowledged by the other apparatus (102-k, 102-m, 102-n), wherein the complementary transport control protocol is arranged to selectively cause the apparatus to transmit, to the other apparatus (102-k, 102-m, 102-n), a control block instructing the other apparatus (102-k, 102-m, 102-n) to ignore any unreceived data blocks having assigned thereto a sequence number that is smaller than a second reference sequence number, characterized in that the complementary transport protocol portion is arranged to provide the transport protocol portion with information that indicates acknowledgement of all data blocks up to one having a first reference sequence number assigned thereto via modifying the acknowledgement blocks received from the other apparatus (102-k, 102-m, 102-n) before passing them to the transport protocol portion such that the respective acknowledgement blocks indicate reception of all data blocks up to one having the first reference sequence number assigned thereto. wherein the transport protocol portion is arranged to cause the apparatus to carry out at least the following: transmit, to the other apparatus (102-k, 102-m, 102-n), user data in a sequence of data blocks having respective sequence numbers assigned thereto, receive, from the other apparatus (102-k, 102-m, 102-n), acknowledgements blocks acknowledging reception of at least some of said data blocks, retransmit, to the other apparatus (102-k, 102-m, 102-n), those ones of said data blocks that are not acknowledged by the other apparatus (102-k, 102-m, 102-n), and enter a congestion avoidance state in response to a predefined amount of said data blocks not being acknowledged by the other apparatus (102-k, 102-m, 102-n), transmit, to the other apparatus (102-k, 102-m, 102-n), user data in a sequence of data blocks having respective sequence numbers assigned thereto, receive, from the other apparatus (102-k, 102-m, 102-n), acknowledgements blocks acknowledging reception of at least some of said data blocks, retransmit, to the other apparatus (102-k, 102-m, 102-n), those ones of said data blocks that are not acknowledged by the other apparatus (102-k, 102-m, 102-n), and enter a congestion avoidance state in response to a predefined amount of said data blocks not being acknowledged by the other apparatus (102-k, 102-m, 102-n), wherein the complementary transport control protocol is arranged to selectively cause the apparatus to transmit, to the other apparatus (102-k, 102-m, 102-n), a control block instructing the other apparatus (102-k, 102-m, 102-n) to ignore any unreceived data blocks having assigned thereto a sequence number that is smaller than a second reference sequence number, characterized in that the complementary transport protocol portion is arranged to provide the transport protocol portion with information that indicates acknowledgement of all data blocks up to one having a first reference sequence number assigned thereto via modifying the acknowledgement blocks received from the other apparatus (102-k, 102-m, 102-n) before passing them to the transport protocol portion such that the respective acknowledgement blocks indicate reception of all data blocks up to one having the first reference sequence number assigned thereto.