TCP Proxy Load Balancing overview

Google Cloud TCP Proxy Load Balancing allows you to use a single IP address for all users worldwide. The TCP proxy load balancer automatically routes traffic to the backends that are closest to the user.

Global load balancing requires that you use the Premium Tier of Network Service Tiers, which is the default tier. Otherwise, load balancing is handled regionally.

TCP Proxy Load Balancing is intended for TCP traffic on specific well-known ports, such as port 25 for Simple Mail Transfer Protocol (SMTP). For more information, see Port specifications. For client traffic that is encrypted on these same ports, use SSL Proxy Load Balancing.

For information about how the Google Cloud load balancers differ from each other, see the following documents:

TCP Proxy Load Balancing supports both IPv4 and IPv6 addresses for client traffic. Client IPv6 requests are terminated at the load balancing layer, and then proxied over IPv4 to your backends.

When you use TCP Proxy Load Balancing, you can terminate client TCP sessions at the load balancing layer, and then forward the traffic to your backend instances by using TCP or SSL.

TCP Proxy Load Balancing example

By using TCP Proxy Load Balancing, traffic coming over a TCP connection is terminated at the load balancing layer, and then proxied to the closest available backend.

In this example, the connections for traffic from users in Iowa and Boston are terminated at the load balancing layer. These connections are labeled 1a and 2a. Separate connections are established from the load balancer to the selected backend instances. These connections are labeled 1b and 2b.

Cloud Load Balancing with TCP termination (click to enlarge)
Cloud Load Balancing with TCP termination (click to enlarge)

TCP Proxy Load Balancing can be configured as a global load balancing service. With this configuration, you can deploy your backends in multiple regions, and global load balancing automatically directs traffic to the region closest to the user. If a region is at capacity, the load balancer automatically directs new connections to another region with available capacity. Existing user connections remain in the current region.


Following are some benefits of using TCP Proxy Load Balancing:

  • Intelligent routing. The load balancer can route requests to backend locations where there is capacity. In contrast, an L3/L4 load balancer must route to regional backends without considering capacity. The use of smarter routing allows provisioning at N+1 or N+2 instead of x*N.

  • Security patching. If vulnerabilities arise in the TCP stack, Cloud Load Balancing applies patches at the load balancer automatically to keep your backends safe.

  • TCP Proxy Load Balancing support for the following ports: 25, 43, 110, 143, 195, 443, 465, 587, 700, 993, 995, 1883, and 5222.

Load balancer behavior in Network Service Tiers

TCP Proxy Load Balancing is a global service with Premium Tier. You can have only one backend service, and (in Premium Tier) it can have backends in multiple regions. Traffic is allocated to backends as follows:

  1. When a client sends a request, the load balancing service determines the approximate origin of the request from the source IP address.
  2. The load balancing service determines the locations of the backends owned by the backend service, their overall capacity, and their overall current usage.
  3. If the closest backend instances to the user have available capacity, the request is forwarded to that closest set of backends.
  4. Incoming requests to the given region are distributed evenly across all available backend instances in that region. However, at very small loads, the distribution might appear to be uneven.
  5. If there are no healthy backend instances with available capacity in a given region, the load balancer instead sends the request to the next closest region with available capacity.

With Standard Tier, TCP Proxy Load Balancing is a regional service. Its backends must all be located in the region used by the load balancer's external IP address and forwarding rule.


The following are components of TCP proxy load balancers.

Forwarding rules and addresses

Forwarding rules route traffic by IP address, port, and protocol to a load balancing configuration consisting of a target proxy and one or more backend services.

Each forwarding rule provides a single IP address that you can use in DNS records for your application. No DNS-based load balancing is required. You can either reserve a static IP address that you can use or let Cloud Load Balancing assign one for you. We recommend that you reserve a static IP address; otherwise, you must update your DNS record with the newly- assigned ephemeral IP address whenever you delete a forwarding rule and create a new one.

Target proxies

TCP Proxy Load Balancing terminates TCP connections from the client and creates new connections to the backends. By default, the original client IP address and port information is not preserved. You can preserve this information by using the PROXY protocol. The target proxies route incoming requests directly to backend services.

Backend services

Backend services direct incoming traffic to one or more attached backends. Each backend is composed of an instance group or network endpoint group and serving capacity metadata. Backend serving capacity can be based on CPU or requests per second (RPS).

Each backend service specifies the health checks to perform for the available backends.

To ensure minimal interruptions to your users, you can enable connection draining on backend services. Such interruptions might happen when a backend is terminated, removed manually, or removed by an autoscaler. To learn more about using connection draining to minimize service interruptions, see Enabling connection draining.

Protocol to the backends

When you configure a backend service for the TCP proxy load balancer, you set the protocol that the backend service uses to communicate with the backends. You can choose either SSL or TCP. The load balancer uses only the protocol that you specify, and does not attempt to negotiate a connection with the other protocol.

Firewall rules

Firewall rules allow traffic to reach your backends. You must configure firewall rules to allow traffic from both the load balancer and the health checker.

You can use a single firewall rule if the following is true:

  • The rule allows traffic on the port that your global forwarding rule uses.
  • Your health checker uses the same port.

If your health checker uses a different port, you must create a separate firewall rule for that port.

Firewall rules block and allow traffic at the backend level, not at the edges of the network. They cannot prevent traffic from reaching the load balancer itself.

Google Cloud uses a large range of IP addresses, which change over time. If you need to determine external IP addresses at a particular time, see the instructions in the Compute Engine FAQ.

Source IP addresses

The source IP addresses for packets, as seen by each backend virtual machine (VM) instance or container, is an IP address from these ranges:


The source IP address for actual load-balanced traffic is the same as the health checks probe IP ranges.

The source IP addresses for traffic, as seen by the backends, is not the Google Cloud external IP address of the load balancer. In other words, there are two HTTP, SSL, or TCP sessions:

  • Session 1, from original client to the load balancer (GFE):

    • Source IP address: the original client (or external IP address if the client is behind NAT).
    • Destination IP address: your load balancer's IP address.
  • Session 2, from the load balancer (GFE) to the backend VM or container:

    • Source IP address: an IP address in one of these ranges: or

      You cannot predict the actual source address.

    • Destination IP address: the internal IP address of the backend VM or container in the Virtual Private Cloud (VPC) network.

Balancing mode

When you add a backend to the backend service, you set a load balancing mode.

For TCP Proxy Load Balancing, the balancing mode can be connection or backend utilization.

If the load balancing mode is connection, the load is spread based on how many concurrent connections the backend can handle. You must also specify exactly one of the following parameters: maxConnections (except for regional managed instance groups), maxConnectionsPerInstance, or maxConnectionsPerEndpoint.

If the load balancing mode is backend utilization, the load is spread based on the utilization of instances in an instance group.

For information about comparing the load balancer types and the supported balancing modes, see Load balancing methods.

Session affinity

Session affinity sends all requests from the same client to the same backend, if the backend is healthy and has capacity.

TCP Proxy Load Balancing offers client IP affinity, which forwards all requests from the same client IP address to the same backend.

Open ports

The TCP proxy load balancers are reverse proxy load balancers. The load balancer terminates incoming connections, and then opens new connections from the load balancer to the backends. The reverse proxy functionality is provided by Google Front Ends (GFE).

GFEs have several open ports to support other Google Cloud load balancers and other Google services. If you run a security or port scan against the external IP address of your load balancer, additional ports appear to be open.

This does not affect TCP proxy load balancers. External forwarding rules, which are used in the definition of an SSL proxy load balancer, can only reference a specific set of ports. Traffic with a different TCP destination port is not forwarded to the load balancer's backend. You can verify that traffic to additional ports is not processed by attempting to open a TCP session to an unauthorized port. The GFE that handles your request closes the connection with a TCP reset (RST) packet.

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