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What is a Network Load Balancer?

Elastic Load Balancing automatically distributes your incoming traffic across multiple targets, such as EC2
instances, containers, and IP addresses, in one or more Availability Zones. It monitors the
health of its registered targets, and routes traffic only to the healthy targets. Elastic Load Balancing
scales your load balancer as your incoming traffic changes over time. It can automatically
scale to the vast majority of workloads.

Elastic Load Balancing supports the following load balancers: Application Load Balancers, Network Load Balancers, Gateway Load Balancers, and Classic Load Balancers.
You can select the type of load balancer that best suits your needs. This guide
discusses Network Load Balancers. For more information about the other load balancers, see the
User Guide for Application Load Balancers, the User Guide for Gateway Load Balancers, and the User Guide for Classic Load Balancers.

Network Load Balancer components

A load balancer serves as the single point of contact for
clients. The load balancer distributes incoming traffic across multiple targets, such as
Amazon EC2 instances. This increases the availability of your application. You add one or
more listeners to your load balancer.

A listener checks for connection requests from clients, using the
protocol and port that you configure, and forwards requests to a target group.

Each target group routes requests to one or more registered
targets, such as EC2 instances, using the TCP protocol and the port number that you
specify. You can register a target with multiple target groups. You can configure health
checks on a per target group basis. Health checks are performed on all targets
registered to a target group that is specified in a listener rule for your load
balancer.

For more information, see the following documentation:

Network Load Balancer overview

A Network Load Balancer functions at the fourth layer of the Open Systems Interconnection (OSI) model.
It can handle millions of requests per second. After the load balancer receives a
connection request, it selects a target from the target group for the default rule. It
attempts to open a TCP connection to the selected target on the port specified in the
listener configuration.

When you enable an Availability Zone for the load balancer, Elastic Load Balancing creates a load
balancer node in the Availability Zone. By default, each load balancer node distributes
traffic across the registered targets in its Availability Zone only. If you enable
cross-zone load balancing, each load balancer node distributes traffic across the
registered targets in all enabled Availability Zones. For more information, see Availability Zones.

To increase the fault tolerance of your applications, you can enable multiple
Availability Zones for your load balancer and ensure that each target group has at least
one target in each enabled Availability Zone. For example, if one or more target groups
does not have a healthy target in an Availability Zone, we remove the IP address for the
corresponding subnet from DNS, but the load balancer nodes in the other Availability
Zones are still available to route traffic. If a client doesn’t honor the time-to-live
(TTL) and sends requests to the IP address after it is removed from DNS, the requests
fail.

For TCP traffic, the load balancer selects a target using a flow hash algorithm based
on the protocol, source IP address, source port, destination IP address, destination
port, and TCP sequence number. The TCP connections from a client have different source
ports and sequence numbers, and can be routed to different targets. Each individual TCP
connection is routed to a single target for the life of the connection.

For UDP traffic, the load balancer selects a target using a flow hash algorithm based
on the protocol, source IP address, source port, destination IP address, and destination
port. A UDP flow has the same source and destination, so it is consistently routed to a
single target throughout its lifetime. Different UDP flows have different source IP
addresses and ports, so they can be routed to different targets.

Elastic Load Balancing creates a network interface for each Availability Zone you enable. Each load
balancer node in the Availability Zone uses this network interface to get a static IP
address. When you create an Internet-facing load balancer, you can optionally associate
one Elastic IP address per subnet.

When you create a target group, you specify its target type, which determines how you
register targets. For example, you can register instance IDs, IP addresses, or an Application Load Balancer.
The target type also affects whether the client IP addresses are preserved.
For more information, see Client IP preservation.

You can add and remove targets from your load balancer as your needs change, without
disrupting the overall flow of requests to your application. Elastic Load Balancing scales your load
balancer as traffic to your application changes over time. Elastic Load Balancing can scale to the vast
majority of workloads automatically.

You can configure health checks, which are used to monitor the health of the
registered targets so that the load balancer can send requests only to the healthy
targets.

For more information, see How Elastic Load Balancing works in
the Elastic Load Balancing User Guide.

Benefits of migrating from a
Classic Load Balancer

Using a Network Load Balancer instead of a Classic Load Balancer has the following benefits:

Ability to handle volatile workloads and scale to millions of requests per
second.
Support for static IP addresses for the load balancer. You can also assign one
Elastic IP address per subnet enabled for the load balancer.
Support for registering targets by IP address, including targets outside the
VPC for the load balancer.
Support for routing requests to multiple applications on a single EC2
instance. You can register each instance or IP address with the same target
group using multiple ports.
Support for containerized applications. Amazon Elastic Container Service (Amazon ECS) can select an unused
port when scheduling a task and register the task with a target group using this
port. This enables you to make efficient use of your clusters.
Support for monitoring the health of each service independently, as health
checks are defined at the target group level and many Amazon CloudWatch metrics are
reported at the target group level. Attaching a target group to an Auto Scaling group
enables you to scale each service dynamically based on demand.