Mastering curl and tcpdump
cURL is a command-line tool that supports a wide range of application-layer protocols even if it is mainly known for its HTTP support. Daniel Stenberg, the author of cURL created a 3.5 hours video with examples of the most important usages of cURL. If you prefer this information in textual format, Anton Zhiyanov summarized most of the video in a long blog post.
tcpdump is the standard tool to collect packet traces and dissect the protocols that are used in the captured packets. Daniel Miessler wrote an interesting article describing many of the common use cases for tcpdump.