Loadbalancing FAQ

Load balancing Frequently Asked Questions

This page tries to answer frequently asked questions about Load Balancing, application optimization and acceleration.


Summary

  • What does layer 4 mean?
  • What does layer 7 mean?
  • What is layer 4 load balancing?
  • What is layer 7 load balancing?
  • What are the possible architectures with layer4 load balancers?
  • What are the possible architectures with layer7 load balancers?


  • 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.

    Summary



    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.

    Summary



    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.

    Summary



    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.

    Summary



    What are the possible architectures with layer4 load balancers?


    Mainly three architectures are doable, depending on your needs:

    Summary



    What are the possible architectures with layer7 load balancers?


    Actually, only one: Proxy mode, but two main flavors of it are available:

    Summary