The Open Systems Interconnection (OSI) model, developed by the International Standards Organization (ISO), divides network communication into seven layers. Layers 1-4 are considered the lower layers, and mostly concern themselves with moving data around. Layers 5-7, the upper layers, contain application-level data. Networks operate on one basic principle: "pass it on." Each layer takes care of a very specific job and then passes the data onto the next layer.
Layer 1 is called the physical layer, layer 2 is the data link layer, layer 3 is defined as the network layer, layer 4 is transport layer, layer 5 is referred to as session layer, layer 6 is the transport layer and layer 7 is application layer. OSI Model, Layer 7, supports application and end-user processes. This layer refers to the top communication layer, supporting applications and end-user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. Everything at this layer is application-specific. This layer provides application services for file transfers, e-mail, and other network software services. Telnet and FTP are applications that exist entirely at the application level.
Solutions with layer 7 device are able to gain additional insights about an end-user and an application request, and can also directly interact with the request—such as sending cookies or presenting a CAPTCHA challenge.
Layer 3 or Layer 4 network visibility solutions are limited to basic attributes, such as source and destination IPs, protocol types, and the number of active connections. These must be known in order to route network packets, but they offer no data about the packets’ actual payload. Whereas with layer 7 visibility, a user can gain insight into client type, request destination, a number of consecutive requests, etc.
Layer 7 visibility offers granular information to a security solution, which differentiates between legitimate users from malicious DDoS bots. In a load balancing context, Layer 7 visibility helps the user understand the exact load being transferred which is critical information for all traffic distribution decisions. It lets the system assess each server’s response time and then use this data as an indication of availability. The result is optimal load distribution, as opposed to hit or miss alternatives.
Layer 7 visibility is also useful for server health checks. With a layer 7 failover solution, a user is able to devise a more accurate health check process. For example, a user can set one up to monitor a specific URL that shows if the application’s database is up and running.