单项选择题

Each machine supporting TCP has a TCP transport entity, either a library procedure, a user process, or part of the kernel. In all case, it manages TCP streams and （） to the IP layer.A TCP （） accepts user data streams from local process, breaks them into pieces not exceeding 64KB, and sends each piece as a separate IP （） . When datagrams containing TCP data arrive at a machine, they are given to the TCP entity,which reconstructs the original byte streams. The IP layer gives no guarantee that datagrams will be delivered properly, so it is up to TCP to time out and （） them as need be. Datagrams do arrive may well do so in the wrong order, it is also up to TCP to （） them into messages in the proper sequence.

so it is up to TCP to time out and （） them as need be.