我有这个疑问是因为 TCP segment 的首部里是有 source IP 和 dest. IP 的，那么为什么 UPD segment 的首部不需要这两个东西呢？
解释：https://networkengineering.stackexchange.com/a/6683
Because you're looking at the UDP Datagram, which is the payload in an IP Datagram. The IP Header has the src/dst IP addresses.
注：上述引用的文字里所谓 “UDP Datagram” 和我的博文标题里所说的 “UPD segment” 指的是同一个东西。我采用的术语和《自顶向下方法》这本书一致。
自顶向下方法（第7版）第191页最后一段：
To simplify terminology, we refer to the transport-layer packet as a segment. We mention, however, that the Internet literature (for example, the RFCs) also refers to the transport-layer packet for TCP as a segment but often uses the term datagram for the network-layer packet! For an introductory book on computer networking such as this, we believe that it is less confusing to refer to both TCP and UDP packets as segments, and reserve the term datagram for the network-layer packet.