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This section brings us to the top of the OSI stack to the application layer. Layer 7 is the most complex of the 7 layers and it's where the creative work gets done by way of applications. It's also the most intelligent layer of the seven. The are tons of protocols in the application layer. Among just a few are the browser protocols of http, https, and ftp. Protocols at this level are what drive the apps! Other layer 7 protocols include SMTP, SNMP, and POP3 and others too numerous to mention in this video. Functions such as non-repudiation via certificates, time services, directory services (e.g. ActiveDirectory) application proxies, and smart firewalls all exist at the application layer. Smart firewalls are discussed in comparison to packet filtering firewalls at layer 4. Content inspection is possible at layer 7 and this opens up data filtering and deep packet inspection using time-aware filters and user permission levels obtained from a directory service. It is noted that all this intelligence comes with both a financial as well as time cost. Kernel-level firewalls are quite expensive and not nearly as fast as layer 4 packet filtering firewalls.