Segment 是TCP协议端到端传输的基本单元，协议内部会把大块数据分割成以报文段（segment）为单位的数据包进行管理，属于传输层协议
IP Datagram为ip协议端到端传输的基本单元
Packet是译为分组、数据包，是网络接口层和数据链路层传输的基本单元，分组既可以是一个IP数据报，也可以是IP数据报的一个片（fragment）
Frame是链路层的基本单元，包含链路层的头部信息（mac地址）和网络层的packet
数据封装的过程data=>segment=>packet=>frame,同样也是层层包含的关系，可以从Wireshark中查看详细的封包情况
Terminology
Segment
A segment is the unit of end-to-end transmission in the
TCP protocol. A segment consists of a TCP header followed
by application data. A segment is transmitted by
encapsulation inside an IP datagram.
IP Datagram
An IP datagram is the unit of end-to-end transmission in
the IP protocol. An IP datagram consists of an IP header
followed by transport layer data, i.e., of an IP header
followed by a message.
In the description of the internet layer (Section 3), the
unqualified term "datagram" should be understood to refer
to an IP datagram.
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.
Frame
A frame is the unit of transmission in a link layer
protocol, and consists of a link-layer header followed by
a packet.
以上摘自RFC1122-1.3.3 Terminology