FTP and SMTP – the rest of the application layer

While we have developed a deep understanding of HTTP due to its prominence in the daily lives of network programmers, we must also take the time to mention and briefly look at some of the other common application-layer protocols in use today. In this section, we'll look at the File Transfer Protocol (FTP) and SSH File Transfer Protocol (SFTP), which allow for remote file copy operations and filesystem navigation; and the Simple Mail Transfer Protocol (SMTP), which is used for email transmission over networks.

Interestingly, with each of these protocols operating on the application layer, it's not uncommon to see one protocol provide the functionality that has historically fallen under the domain of another protocol. For example, the data-agnostic nature of HTTP's plain-text message structure makes it trivially simple to use HTTP to transfer complete file data over an HTTP session. It's as simple as writing software on the server to transmit files through the message body of the response. For this reason, FTP, and to a lesser degree, SMTP, have fallen out of favor with network programmers in recent years, in favor of implementing their responsibilities in HTTP-aware software hosts. The protocols remain, however, and it will benefit us to consider what their flaws and advantages are.