ELBs

ALB is layer 7 load balancer. It is context/application aware and capable of doing content based routing. It examines  the content of the request and forwards accordingly. It also supports AWS outpost.

NLB is layer 3/4 load balancer. It does not know the application it is load balancing. It just forwards traffic  based on  connection parameters ( source ip, source port, tcp sequence number ) . Each TCP connection can have different port and seq no even if from same client.  So request from same client can be forwarded to different targets.  Though traffic from single connection goes to same target for entire connection duration. Only difference for UDP traffic is , it does not have sequence number. So packets are forwarded based on source IP and source port. This kind of routing does not require lots of processing so it is very fast. Can process millions of requests per second. It also provides one  static & elastic IP ( elastic IP only if internet facing )  per zone.

NLB only preserves source IP if the target is registered using instance id. 

Because network load balancer is not application aware hence it can not assure that application is up while ALB can do so. Also it can not differentiate among two apps on same IP unless ports are different..but ALB can do so. 

Gateway Load Balancer is used for third party virtual appliances. 




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