单选题
A glue that holds the whole Internet together is the
network layer protocol, ______. Unlike most older network layer protocols, it
was designed from the beginning with internetworking in mind. Its job is to
provide a ______ way to transport datagrams 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 ______ layer takes data streams and breaks them up into
datagrams. 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 ______ 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 ______ part and a
variable length optional part.