Load balancing Frequently Asked Questions
This page tries to answer frequently asked questions about Load Balancing, application optimization and acceleration.
What does layer 4 mean?
Layer 4 is related to fourth layer of the OSI model: transport level.
For example: TCP and UDP protocols are transport level.
What does layer 7 mean?
Layer 7 is related to seventh layer of the OSI model: application level.
For example: HTTP, FTP, SMTP, DNS protocols are application level.
What is layer 4 load-balancing?
A layer 4 load-balancer takes routing decision based on IPs and TCP or UDP ports.
It has a packet view of the traffic exchanged between the client and a server which means it takes decisions packet by packet.
The layer 4 connection is established between the client and the server.
It is really fast but can’t perform any action on the protocol above layer 4.
The fastest layer4 load-balancers uses an ASIC to take routing decision.
What is layer 7 load-balancing?
A layer 7 load-balancer takes routing decision based on IPs, TCP or UDP ports or any information it can get from the application protocol (mainly HTTP).
The layer 7 load-balancer acts as a proxy, which means it maintains two TCP connections: one with the client and one with the server.
The packets are re-assembled then the load-balancer can take a routing decision based on information it can find in the application requests or responses.
Even if this kind of processing seems slow, it is not that much: less than the millisecond.
What are the possible architectures with layer4 load balancers?
Mainly three architectures are doable, depending on your needs:
What are the possible architectures with layer7 load balancers?
Actually, only one: Proxy mode, but two main flavors of it are available: