【iOS之Payload、Trailer】
A packet generally contains three basic parts: a header that tells where the packet should be sent, a payload that contains the actual data, and a trailer that contains checksum information to ensure that the packet was received correctly. Some packet types include this checksum information as part of the header, and thus do not have a trailer. For example, shows the structure of an Ethernet packet.
一直没搞明白payload是个什么东西，现在搞明白了，理解为POST-DATA或者BODY-DATA即可。