Network Basics

L4 and L7 Load Balancer in Bullet Points

OSI and TCP/IP model

Load balancing can be performed at various layers in the Open Systems Interconnection (OSI) Reference Model.

L4 Load Balancer -

• Operate at OSI Model Layer 4 (Transport Layer)
• Traffic forwarded on basis of source and destination IP addresses and ports [part of header data in a packet]
• Does not matter what is content of the messages
• Transmission Control Protocol (TCP) operates at the transport layer (Layer 4).
• The source and destination IP of each packet is changed by the load balancer using NAT (Network address translation).
• No TLS termination

L7 Load Balancer -

• Application Load Balancers based on Layer 7 of OSI
• Operates at Layer 7 so can view actual content of each message
• Can route data based on URL, context root, cookie, Session etc
• Terminates the network traffic and reads the message within
• More CPU‑intensive then L4 LBs
• Most of the time called reverse proxy