About differntiating notions of datagram and packet see RFC 1812, 1547, 1661 etc. PDUs of the network layer are called (IP-)datagrams. As each technology of the data link layer defines restrictions for the length of its PDU, datagrams may need to be fragmented before incapsulation into a frame. So packet is a basic unit transmited to the data link layer fot incapsulation. It may be equal to a datagram, but may be not. The difference between ip-datagrams, ip-fragments and ip-packets is not structural, it is functional one. The all have the same messege structure. Full datagrams and fragments differs according to values of some header fields. Packets are identical either to datagrams (when they fit into underlieing frames) or to ip-fragments. But not every datagram can be a packet (if it does not fit into data link layer frame), and even not every fragment without repeated fragmentation can be a packet. A network PDU is a datagram, but a data link SDU is not an ip-datagram, it is an ip-packet.
RFC 1812:
Datagram
The unit transmitted between a pair of internet modules. Data,
called datagrams, from sources to destinations. The Internet
Protocol does not provide a reliable communication facility.
There are no acknowledgments either end-to-end or hop-by-hop.
There is no error no retransmissions. There is no flow control.
Fragment
An IP datagram that represents a portion of a higher layer's
packet that was too large to be sent in its entirety over the
output network.
Packet
A packet is the unit of data passed across the interface between
the Internet Layer and the Link Layer. It includes an IP header
and data. A packet may be a complete IP datagram or a fragment
of an IP datagram.
IP Datagram
An IP Datagram is the unit of end-to-end transmission in the
Internet Protocol. An IP Datagram consists of an IP header
followed by all of higher-layer data (such as TCP, UDP, ICMP,
and the like). An IP Datagram is an IP header followed by a
message.
An IP Datagram is a complete IP end-to-end transmission unit.
An IP Datagram is composed of one or more IP Fragments.
IP Fragment
An IP Fragment is a component of an IP Datagram. An IP Fragment
consists of an IP header followed by all or part of the higher-
layer of the original IP Datagram.
One or more IP Fragments comprises a single IP Datagram.
IP Packet
An IP Datagram or an IP Fragment.
PS sorry for my English
