A glue that holds the whole Internet together is the network layer protocol, (71) .Unlike most older network layer protocols, it was designed from the beginning with internetworking in mind. Its job is to provide a (72) way to transport data grams from source to destination, without regard to whether these machines are on the same network or whether there are other networks in between them.

Communication in the Internet works as follows. The (73) layer takes data streams and breaks them up into data grams. Each datagram is transmitted through the Internet, possibly being fragmented into smaller units as it goes. When all the pieces finally get to the destination machine, they are reassembled by the (74) layer into the original datagram. This datagram is then handed to the transport layer, which inserts it into the receiving process’ input stream.

An IP datagram consists of a header part and a text part. The header has a (75) part and a variable length optional part.

73（）

A．data link

B．transport

C．network

D．application

在编制进度计划时充分考虑各阶段工作之间的合理搭接，是建设工程进度控制的( )要注意的问题之一。

A．系统

B．全过程控制

C．全方位控制

D．组织协调