The Open System Interconnection (OSI) model defines a networking framework to implement protocols in seven layers:

  • Layer 7: The application layer
  • Layer 6: The presentation layer
  • Layer 5: The session layer
  • Layer 4: The transport layer
  • Layer 3: The network layer
  • Layer 2: The data-link layer
  • Layer 1: The physical layer

The OSI model doesn’t perform any functions in the networking process. It is a conceptual framework to better understand complex interactions that are happening.

Network firewalls are security devices that operate at levels one to three while load balancing happens between layers four to seven.

Load balancers have different capabilities, which include:

  • L4: directs traffic based on data from network and transport layer protocols, such as IP address and TCP port.
  • L7: adds content switching to load balancing. This allows routing decisions based on attributes like HTTP header, URL, SSL session ID and HTML form data.
  • GSLB: Global Server Load Balancing extends L4 and L7 capabilities to servers in different geographic locations.
  • Domain Name System (DNS) is also used in certain solutions and this topic is addressed when Azure Traffic Manager is used as an example of such an implementation.

As more enterprises seek to deploy cloud-native applications in public clouds, it is resulting in significant changes in the capability of load balancers.