Load Balancer

A load balancer may act :

  - at the link level : this is called link load balancing, and it consists in
    choosing what network link to send a packet to;

  - at the network level : this is called network load balancing, and it
    consists in choosing what route a series of packets will follow;

  - at the server level : this is called server load balancing and it consists
    in deciding what server will process a connection or request.

Usually stateless, it can also be stateful (consider the session a packet belongs to and called layer4-LB or L4), may support DSR (direct server return, without passing through the LB again)