External Application Load Balancer overview (original) (raw)

This document introduces the concepts that you need to understand how to configure an external Application Load Balancer.

An external Application Load Balancer is a proxy-based Layer 7 load balancer that enables you to run and scale your services behind a single external IP address. The external Application Load Balancer distributes HTTP and HTTPS traffic to backends hosted on a variety of Google Cloud platforms (such as Compute Engine, Google Kubernetes Engine (GKE), and Cloud Storage), as well as external backends connected over the internet or through hybrid connectivity. For details, see Application Load Balancer overview: Use cases.

Modes of operation

You can configure an external Application Load Balancer in the following modes:

• Global external Application Load Balancer. This is a global load balancer that is implemented as a managed service on Google Front Ends (GFEs). It uses the open-source Envoy proxyto support advanced traffic managementcapabilities such as traffic mirroring, weight-based traffic splitting, and request-based or response-based header transformations.
• Classic Application Load Balancer. This is the classic external Application Load Balancer that is global in Premium Tier but can be configured to be regional in Standard Tier. This load balancer is implemented on Google Front Ends (GFEs). GFEs are distributed globally and operate together using Google's global network and control plane.
• Regional external Application Load Balancer. This is a regional load balancer that is implemented as a managed service on the open-source Envoy proxy. It includesadvanced traffic managementcapabilities such as traffic mirroring, weight-based traffic splitting, and request-based or response-based header transformations.

Identify the mode

Console

1. In the Google Cloud console, go to the Load balancing page.
   Go to Load balancing
2. In the Load Balancers tab, the load balancer type, protocol, and region are displayed. If the region is blank, then the load balancer is global. The following table summarizes how to identify the mode of the load balancer.

gcloud

To determine the mode of a load balancer, run the following command:

gcloud compute forwarding-rules describe FORWARDING_RULE_NAME

In the command output, check the load balancing scheme, region, and network tier. The following table summarizes how to identify the mode of the load balancer.

Architecture

The following resources are required for an external Application Load Balancer deployment:

• For regional external Application Load Balancers only, a proxy-only subnet is used to send connections from the load balancer to the backends.
• An external forwarding rule specifies an external IP address, port, and target HTTP(S) proxy. Clients use the IP address and port to connect to the load balancer.
• A target HTTP(S) proxy receives a request from the client. The HTTP(S) proxy evaluates the request by using the URL map to make traffic routing decisions. The proxy can also authenticate communications by using SSL certificates.
  • For HTTPS load balancing, the target HTTPS proxy uses SSL certificates to prove its identity to clients. A target HTTPS proxy supports up to the documented number of SSL certificates.
• The HTTP(S) proxy uses a URL map to make a routing determination based on HTTP attributes (such as the request path, cookies, or headers). Based on the routing decision, the proxy forwards client requests to specific backend services or backend buckets. The URL map can specify additional actions, such as sending redirects to clients.
• A backend service distributes requests to healthybackends. The global external Application Load Balancers also support backend buckets. One or morebackends must be connected to the backend service or backend bucket.
• A health check periodically monitors the readiness of your backends. This reduces the risk that requests might be sent to backends that can't service the request.
• Firewall rules for your backends to accept health check probes. Regional external Application Load Balancers require an additional firewall rule to allow traffic from the proxy-only subnet to reach the backends.

Global

This diagram shows the components of a global external Application Load Balancer deployment. This architecture applies to both, the global external Application Load Balancer, and the classic Application Load Balancer in Premium Tier.

Global external Application Load Balancer components (click to enlarge).

Regional

This diagram shows the components of a regional external Application Load Balancer deployment.

Regional external Application Load Balancer components (click to enlarge).

Proxy-only subnet

Proxy-only subnets are only required for regional external Application Load Balancers.

The proxy-only subnet provides a set of IP addresses that Google uses to run Envoy proxies on your behalf. You must create one proxy-only subnet in each region of a VPC network where you use regional external Application Load Balancers. The --purpose flag for this proxy-only subnet is set toREGIONAL_MANAGED_PROXY. All regional Envoy-based load balancers in the same region and VPC network share a pool of Envoy proxies from the same proxy-only subnet. Further:

• Proxy-only subnets are only used for Envoy proxies, not your backends.
• Backend VMs or endpoints of all regional external Application Load Balancers in a region and VPC network receive connections from the proxy-only subnet.
• The IP address of the regional external Application Load Balancer is not located in the proxy-only subnet. The load balancer's IP address is defined by its external managed forwarding rule, which is described below.
• Proxy-only subnets can be configured with a stack type ofIPV4_IPV6 or IPV4_ONLY.

If you previously created a proxy-only subnet with--purpose=INTERNAL_HTTPS_LOAD_BALANCER, migrate the subnet's purpose toREGIONAL_MANAGED_PROXY before you can create other Envoy-based load balancers in the same region of the VPC network.

Forwarding rules and IP addresses

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

IP address specification. Each forwarding rule provides a single IP address that can be used in DNS records for your application. No DNS-based load balancing is required. You can either specify the IPv4 or IPv6 address to be used or let Cloud Load Balancing assign one for you.

Port specification. Each forwarding rule for an Application Load Balancer can reference a single port from 1-65535. To support multiple ports, you must configure multiple forwarding rules. You can configure multiple forwarding rules to use the same external IP address (VIP) and to reference the same target HTTP(S) proxy as long as the overall_combination_ of IP address, port, and protocol is unique for each forwarding rule. This way, you can use a single load balancer with a shared URL map as a proxy for multiple applications.

The type of forwarding rule, IP address, and load balancing scheme used by external Application Load Balancers depends on the mode of the load balancer and which Network Service Tier the load balancer is in.

For the complete list of protocols supported by external Application Load Balancer forwarding rules in each mode, see Load balancer features.

Forwarding rules and VPC networks

This section describes how forwarding rules used by external Application Load Balancers are associated with VPC networks.

Target proxies

Target proxies terminate HTTP(S) connections from clients. One or more forwarding rules direct traffic to the target proxy, and the target proxy consults the URL map to determine how to route traffic to backends.

Do not rely on the proxy to preserve the case of request or response header names. For example, a Server: Apache/1.0 response header might appear at the client as server: Apache/1.0.

The following table specifies the type of target proxy required by external Application Load Balancers.

In addition to headers added by the target proxy, the load balancer adjusts other HTTP headers in the following ways:

• For the global external Application Load Balancer, both request and response headers might be converted to lowercase.
  The only exception to this is when you use global internet NEG backends with HTTP/1.1. For details about how HTTP/1.1 headers are processed with global internet NEGs, see the Internet NEGs overview.
• For the classic Application Load Balancer, request and response headers are converted to lowercase except when you use HTTP/1.1. With HTTP/1.1, headers are proper-cased instead. The first letter of the header's key and every letter following a hyphen (-) is capitalized to preserve compatibility with HTTP/1.1 clients. For example, user-agent is changed to User-Agent, and content-encoding is changed to Content-Encoding.
• Some headers are coalesced. When there are multiple instances of the same header key (for example, Via), the load balancer combines their values into a single comma-separated list for a single header key. Only the headers whose values can be represented as a comma-separated list are coalesced. Other headers, such as Set-Cookie, are never coalesced.

When the load balancer makes the HTTP request, the load balancer preserves the Host header of the original request.

The load balancer appends two IP addresses to theX-Forwarded-For header, separated by a single comma, in the following order:

1. The IP address of the client that connects to the load balancer
2. The IP address of the load balancer's forwarding rule

If the incoming request does not include an X-Forwarded-For header, the resulting header is as follows:

X-Forwarded-For: <client-ip>,<load-balancer-ip>

If the incoming request already includes an X-Forwarded-For header, the load balancer appends its values to the existing header:

X-Forwarded-For: <existing-value>,<client-ip>,<load-balancer-ip>

Remove existing header values using a custom request header

It is possible to remove existing header values by using custom request headers on the backend service. The following example uses the --custom-request-header flag to recreate the X-Forwarded-For header by using the variablesclient_ip_address and server_ip_address. This configuration replaces the incoming X-Forwarded-For header with only the client and the load balancer IP address.

--custom-request-header=x-forwarded-for:{client_ip_address},{server_ip_address}

How backend reverse proxy software might modify the X-Forwarded-For header

If your load balancer's backends run HTTP reverse proxy software, the software might append one or both of the following IP addresses to the end of the X-Forwarded-For header:

1. The IP address of the GFE that connected to the backend. The GFE proxy ranges are listed in theFirewall rules document.
2. The IP address of the backend system itself.

As a result, an upstream system might see an X-Forwarded-For header structured as follows:

<existing-value>,<client-ip>,<load-balancer-ip>,<GFE-ip>,<backend-ip>

Cloud Trace support

Trace is not supported with Application Load Balancers. The global and classic Application Load Balancers add the X-Cloud-Trace-Context header if it is not present. The regional external Application Load Balancer does not add this header. If theX-Cloud-Trace-Context header is already present, it passes through the load balancers unmodified. However, no traces or spans are exported by the load balancer.

URL maps

URL maps define matching patterns for URL-based routing of requests to the appropriate backend services. The URL map allows you to divide your traffic by examining the URL components to send requests to different sets of backends. A default service is defined to handle any requests that don't match a specified host rule or path matching rule.

In some situations, such as the multi-region load balancing example, you might not define any URL rules and rely only on the default service.

URL maps support several advanced traffic management features such as header-based traffic steering, weight-based traffic splitting, and request mirroring. For more information, see the following:

• Traffic management overview for global external Application Load Balancer
• Traffic management overview for regional external Application Load Balancer

The following table specifies the type of URL map required by external Application Load Balancers in each mode.

SSL certificates

External Application Load Balancers using target HTTPS proxies require private keys and SSL certificates as part of the load balancer configuration.

• Google Cloud provides two configuration methods for assigning private keys and SSL certificates to target HTTPS proxies: Compute Engine SSL certificates and Certificate Manager. For a description of each configuration, see Certificate configuration methods in the SSL certificates overview.
• Google Cloud provides two certificate types: Self-managed and Google-managed. For a description of each type, see Certificate types in the SSL certificates overview.

The type of external Application Load Balancer (global, regional, or classic) determines which configuration methods and certificate types are supported. For details, seeCertificates and Google Cloud load balancers in the SSL certificates overview.

SSL policies

SSL policies specify the set of SSL features that Google Cloud load balancers use when negotiating SSL with clients.

By default, HTTPS Load Balancing uses a set of SSL features that provides good security and wide compatibility. Some applications require more control over which SSL versions and ciphers are used for their HTTPS or SSL connections. You can define an SSL policy to specify the set of SSL features that your load balancer uses when negotiating SSL with clients. In addition, you can apply that SSL policy to your target HTTPS proxy.

The following table specifies the SSL policy support for load balancers in each mode.

Backend services

A backend service provides configuration information to the load balancer so that it can direct requests to its backends—for example, Compute Engine instance groups or network endpoint groups (NEGs). For more information about backend services, see Backend services overview.

For an example showing how to set up a load balancer with a backend service and a Compute Engine backend, see Setting up an external Application Load Balancer with a Compute Engine backend.

Backend service scope

The following table indicates which backend service resource and scope is used by external Application Load Balancers:

Protocol to the backends

Backend services for Application Load Balancers must use one of the following protocols to send requests to backends:

• HTTP, which uses HTTP/1.1 and no TLS
• HTTPS, which uses HTTP/1.1 and TLS
• HTTP/2, which uses HTTP/2 and TLS (HTTP/2 without encryption isn't supported.)
• H2C, which uses HTTP/2 over TCP. TLS isn't required. H2C isn't supported for classic Application Load Balancers.

The load balancer only uses the backend service protocol that you specify to communicate with its backends. The load balancer doesn't fall back to a different protocol if it is unable to communicate with backends using the specified backend service protocol.

The backend service protocol doesn't need to match the protocol used by clients to communicate with the load balancer. For example, clients can send requests to the load balancer using HTTP/2, but the load balancer can communicate with backends using HTTP/1.1 (HTTP or HTTPS).

Backend buckets

Backend buckets direct incoming traffic to Cloud Storage buckets. For an example showing how to add a bucket to a external Application Load Balancer, see Setting up a load balancer with backend buckets. For general information about Cloud Storage, see What is Cloud Storage?

Backends

The following table specifies the backends and related features supported by external Application Load Balancers in each mode.

1Backends on a backend service must be the same type: all instance groups or all the same type of NEG. An exception to this rule is that bothGCE_VM_IP_PORT zonal NEGs and hybrid NEGs can be used on the same backend service to support ahybrid architecture.

2 IAP and Cloud CDN are incompatible with each other. They can't both be enabled on the same backend service.

3 Combinations of zonal unmanaged, zonal managed, and regional managed instance groups are supported on the same backend service. When using autoscaling for a managed instance group that's a backend for two or more backend services, configure the instance group's autoscaling policy to usemultiple signals.

4 Zonal NEGs must use GCE_VM_IP_PORT endpoints.

Backends and VPC networks

The restrictions on where backends can be located depend on the type of load balancer.

For global external Application Load Balancer backends, the following applies:

• Backend instances (for instance group backends) and backend endpoints (for NEG backends) can be located in any VPC network within the same organization. The different VPC networks don't need to be connected using VPC Network Peering because GFEs communicate directly with backends in their respective VPC networks.
• Cloud Storage buckets aren't associated with a VPC network. They can be located in any project within the same organization.
  Global external Application Load Balancers also support Shared VPC environments where you can share VPC networks and their associated resources across projects. However, for global external Application Load Balancers, you don't need to configure Shared VPC to be able to reference backend services or backend buckets from other projects in your organization.

For classic Application Load Balancer backends, the following applies:

• All backend instances from instance group backends and all backend endpoints from NEG backends must be located in the same project. However, an instance group backend or a NEG can use a different VPC network in that project. The different VPC networks don't need to be connected using VPC Network Peering because GFEs communicate directly with backends in their respective VPC networks.
• Cloud Storage buckets aren't associated with a VPC network. However, they must be located in the same project as the load balancer.

For regional external Application Load Balancer backends, the following applies:

• For instance groups, zonal NEGs, and hybrid connectivity NEGs, all backends must be located in the same project and region as the backend service. However, a load balancer can reference a backend that uses a different VPC network in the same project as the backend service. Connectivity between the load balancer's VPC network and the backend VPC network can be configured using either VPC Network Peering, Cloud VPN tunnels, Cloud Interconnect VLAN attachments, or a Network Connectivity Center framework.
  Backend network definition
  • For zonal NEGs and hybrid NEGs, you explicitly specify the VPC network when you create the NEG.
  • For managed instance groups, the VPC network is defined in the instance template.
  • For unmanaged instance groups, the instance group's VPC network is set to match the VPC network of the nic0 interface for the first VM added to the instance group.
    Backend network requirements
    Your backend's network must satisfy one of the following network requirements:
  • The backend's VPC network must exactly match the forwarding rule's VPC network.
  • The backend's VPC network must be connected to the forwarding rule's VPC network using VPC Network Peering. You must configure subnet route exchanges to allow communication between the proxy-only subnet in the forwarding rule's VPC network and the subnets used by the backend instances or endpoints.
    Backend IPv6 subnet requirements
    The IP version used for the frontend connection is independent of the backend connection. Since the proxy-only subnet isdual-stack (IPV4_IPV6), the proxy-only subnet can communicate with backends using either IPv4 or IPv6.
    If your backend instances are handling IPv6 traffic, the backend subnet can be configured with a stack type of IPV4_ONLY orIPV4_IPV6 (dual-stack). If the backend subnet's stack type includes IPv6, you must explicitly set the subnet's ipv6-access-type toEXTERNAL.
• Both the backend's VPC network and the forwarding rule's VPC network must be VPC spokesattached to the same NCC hub. Import and export filters must allow communication between the proxy-only subnet in the forwarding rule's VPC network and the subnets used by backend instances or endpoints.
• For all other backend types, all backends must be located in the same VPC network and region.
  Regional external Application Load Balancers also support Shared VPC environments where you can share VPC networks and their associated resources across projects. If you want the regional external Application Load Balancer's backend service and backends to be in a different project from the forwarding rule, you need to configure the load balancer in aShared VPC environment with cross-project service referencing.

Backends and network interfaces

If you use instance group backends, packets are always delivered to nic0. If you want to send packets to non-nic0 interfaces (either vNICs or Dynamic Network Interfaces), use NEG backends instead.

If you use zonal NEG backends, packets are sent to whatever network interface is represented by the endpoint in the NEG. The NEG endpoints must be in the same VPC network as the NEG's explicitly defined VPC network.

Health checks

Each backend service specifies a health check that periodically monitors the backends' readiness to receive a connection from the load balancer. This reduces the risk that requests might be sent to backends that can't service the request. Health checks don't check if the application itself is working.

For the health check probes, you must create an ingress allow firewall rule that allows health check probes to reach your backend instances. Typically, health check probes originate from Google's centralized health checking mechanism.

Regional external Application Load Balancers that use hybrid NEG backends are an exception to this rule because their health checks originate from the proxy-only subnet instead. For details, see the Hybrid NEGs overview.

Health check protocol

Although it is not required and not always possible, it is a best practice to use a health check whose protocol matches the protocol of the backend service. For example, an HTTP/2 health check most accurately tests HTTP/2 connectivity to backends. In contrast, regional external Application Load Balancers that use hybrid NEG backends don't support gRPC health checks. For the list of supported health check protocols, see Load balancing features.

The following table specifies the scope of health checks supported by external Application Load Balancers in each mode.

For more information about health checks, see the following:

• Health checks overview
• Creating health checks

Firewall rules

The load balancer requires the following firewall rules:

• For the global external Application Load Balancers, an ingress allow rule to permit traffic from Google Front Ends (GFEs) to reach your backends. For the regional external Application Load Balancer, an ingress allow rule to permit traffic from the proxy-only subnet to reach your backends.
• An ingress allow rule to permit traffic from the health check probes ranges. For more information about health check probes and why it's necessary to allow traffic from them, see Probe IP ranges and firewall rules.

Firewall rules are implemented at the VM instance level, not on GFE proxies. You cannot use Google Cloud firewall rules to prevent traffic from reaching the load balancer. For the global external Application Load Balancer and the classic Application Load Balancer, you can useGoogle Cloud Armor to achieve this.

The ports for these firewall rules must be configured as follows:

• Allow traffic to the destination port for each backend service's health check.
• For instance group backends: Determine the ports to be configured by the mapping between the backend service's named port and the port numbers associated with that named port on each instance group. The port numbers can vary among instance groups assigned to the same backend service.
• For GCE_VM_IP_PORT NEG backends: Allow traffic to the port numbers of the endpoints.

The following table summarizes the required source IP address ranges for the firewall rules:

GKE support

GKE uses external Application Load Balancers in the following ways:

• External Gateways created using the GKE Gateway controller can use any mode of an external Application Load Balancer. You control the load balancer's mode by choosing aGatewayClass. The GKE Gateway controller always uses GCE_VM_IP_PORT zonal NEG backends.
  When Cloud CDN is enabled for backends managed by GKE Gateway, caching is configured using the GKE GCPHTTPFilterfilters that are attached to HTTPRoute resources. For information about how to configure caching, see Configure Cloud CDN for Gateway.
• External Ingresses created using the GKE Ingress controller are always classic Application Load Balancers. The GKE Ingress controller prefers to use GCE_VM_IP_PORT zonal NEG backends, though instance group backends are also supported.
• You can use GCE_VM_IP_PORT zonal NEG created and managed by GKE Services as backends for any Application Load Balancer or Proxy Network Load Balancer. For more information, see Container-native load balancing through standalone zonal NEGs.

External Application Load Balancers support networks that use Shared VPC. Shared VPC lets organizations connect resources from multiple projects to a common VPC network so that they can communicate with each other securely and efficiently by using internal IP addresses from that network. If you're not already familiar with Shared VPC, read the Shared VPC overview.

There are many ways to configure an external Application Load Balancer within a Shared VPC network. Regardless of type of deployment, all the components of the load balancer must be in the same organization.

While you can create all the load balancing components and backends in the Shared VPC host project, this type of deployment doesn't separate network administration and service development responsibilities.

All load balancer components and backends in a service project

The following architecture diagram shows a standard Shared VPC deployment where all load balancer components and backends are in a service project. This deployment type is supported by all Application Load Balancers.

The load balancer components and backends must use the same VPC network.

Regional external Application Load Balancer on Shared VPC network (click to enlarge).

Serverless backends in a Shared VPC environment

For a load balancer that is using a serverless NEG backend, the backend Cloud Run or Cloud Run functions service must be in the same project as the serverless NEG.

Additionally, for the regional external Application Load Balancer that supports cross-project service referencing, the backend service, serverless NEG, and the Cloud Run service must always be in the same service project.

Cross-project service referencing

Cross-project service referencing is a deployment model where the load balancer's frontend and URL map are in one project and the load balancer's backend service and backends are in a different project.

Cross-project service referencing lets organizations configure one central load balancer and route traffic to hundreds of services distributed across multiple different projects. You can centrally manage all traffic routing rules and policies in one URL map. You can also associate the load balancer with a single set of hostnames and SSL certificates. You can therefore optimize the number of load balancers needed to deploy your application, and lower manageability, operational costs, and quota requirements.

By having different projects for each of your functional teams, you can also achieve separation of roles within your organization. Service owners can focus on building services in service projects, while network teams can provision and maintain load balancers in another project, and both can be connected by using cross-project service referencing.

Service owners can maintain autonomy over the exposure of their services and control which users can access their services by using the load balancer. This is achieved by a special IAM role called theCompute Load Balancer Services User role(roles/compute.loadBalancerServiceUser).

Cross-project service referencing support differs based on the type of load balancer:

• For global external Application Load Balancers: your load balancer's frontend and URL map can reference backend services or backend buckets from any project within the same organization. No VPC network restrictions apply. While you can use a Shared VPC environment to configure a cross-project deployment as shown in thisexample, this isn't a requirement.
• For regional external Application Load Balancers: you must create the load balancer in a Shared VPC environment. The load balancer's frontend and URL map must be in a host or service project, and the load balancer's backend services and backends can be distributed across host or service projects in the same Shared VPC environment.

To learn how to configure Shared VPC for a global external Application Load Balancer—with and without cross-project service referencing—see Set up a global external Application Load Balancer with Shared VPC.

To learn how to configure Shared VPC for a regional external Application Load Balancer—with and without cross-project service referencing—see Set up a regional external Application Load Balancer with Shared VPC.

Usage notes for cross-project service referencing

• Cross-project service referencing can be used with instance groups, serverless NEGs, and most other supported backend types. However, the following limitations apply:
  • With global external Application Load Balancers, you can't reference a cross-project backend service if the backend service has serverless NEG backends with App Engine.
  • With regional external Application Load Balancers, you can't reference a cross-project backend service if the backend service has regional internet NEG backends.
• Cross-project service referencing is not supported for the classic Application Load Balancer.
• Google Cloud doesn't differentiate between resources (for example, backend services) using the same name across multiple projects. Therefore, when you are using cross-project service referencing, we recommend that you use unique backend service names across projects within your organization.
• If you see an error such as "Cross-project references for this resource are not allowed", make sure that you have the permission to use the resource. An administrator of the project that owns the resource must grant you theCompute Load Balancer Services User role (roles/compute.loadBalancerServiceUser). This role can be granted at the project level or at the resource level. For an example, see Grant permissions to the Compute Load Balancer Admin to use the backend service or backend bucket.

Example 1: Load balancer frontend and backend in different service projects

Here is an example of a Shared VPC deployment where the load balancer's frontend and URL map are created in service project A and the URL map references a backend service in service project B.

In this case, Network Admins or Load Balancer Admins in service project A require access to backend services in service project B. Service project B admins grant the Compute Load Balancer Services User role (roles/compute.loadBalancerServiceUser) to Load Balancer Admins in service project A who want to reference the backend service in service project B.

Load balancer frontend and backend in different service projects (click to enlarge).

Example 2: Load balancer frontend in the host project and backends in service projects

Here is an example of a Shared VPC deployment where the load balancer's frontend and URL map are created in the host project and the backend services (and backends) are created in service projects.

In this case, Network Admins or Load Balancer Admins in the host project require access to backend services in the service project. Service project admins grant the Compute Load Balancer Services User role (roles/compute.loadBalancerServiceUser) to to Load Balancer Admins in the host project A who want to reference the backend service in the service project.

Load balancer frontend and URL map in host project (click to enlarge).

Example 3: Load balancer frontend and backends in different projects

Here is an example of a deployment where the global external Application Load Balancer's frontend and URL map are created in a different project from the load balancer's backend service and backends. This type of deployment doesn't use Shared VPC and is supported only for global external Application Load Balancers.

Load balancer frontend and backends in different projects (click to enlarge).

To learn more about this setup, seeSet up cross-project service referencing with a backend service and a backend bucket.

High availability and failover

High availability and failover in external Application Load Balancers can be configured at the load balancer level. This is handled by creating backup regional external Application Load Balancers in any region where you require backup.

The following table describes the failover behavior.

HTTP/2 support

HTTP/2 is a major revision of the HTTP/1 protocol. There are 2 modes of HTTP/2 support:

• HTTP/2 over TLS
• Cleartext HTTP/2 over TCP

HTTP/2 over TLS

HTTP/2 over TLS is supported for connections between clients and the external Application Load Balancer, and for connections between the load balancer and its backends.

The load balancer automatically negotiates HTTP/2 with clients as part of the TLS handshake by using the ALPN TLS extension. Even if a load balancer is configured to use HTTPS, modern clients default to HTTP/2. This is controlled_on the client_, not on the load balancer.

If a client doesn't support HTTP/2 and the load balancer is configured to use HTTP/2 between the load balancer and the backend instances, the load balancer might still negotiate an HTTPS connection or accept unsecured HTTP requests. Those HTTPS or HTTP requests are then transformed by the load balancer to proxy the requests over HTTP/2 to the backend instances.

To use HTTP/2 over TLS, you must enable TLS on your backends and set thebackend service protocol toHTTP2. For more information, see Encryption from the load balancer to the backends.

HTTP/2 max concurrent streams

The HTTP/2SETTINGS_MAX_CONCURRENT_STREAMSsetting describes the maximum number of streams that an endpoint accepts, initiated by the peer. The value advertised by an HTTP/2 client to a Google Cloud load balancer is effectively meaningless because the load balancer doesn't initiate streams to the client.

In cases where the load balancer uses HTTP/2 to communicate with a server that is running on a VM, the load balancer respects theSETTINGS_MAX_CONCURRENT_STREAMS value advertised by the server, up to a maximum value of 100. In the request direction (Google Cloud load balancer → gRPC server), the load balancer uses the initial SETTINGS frame from the gRPC server to determine how many streams per connection can be in use simultaneously. If the server advertises a value higher than 100, the load balancer uses 100 as the maximum number of concurrent streams. If a value of zero is advertised, the load balancer can't forward requests to the server, and this might result in errors.

HTTP/2 significantly improves upon HTTP/1.1 with features like multiplexing and HPACK header compression. HPACK uses a dynamic table that enhances header compression, making everything faster. To understand the impact of dynamic header table size changes in HTTP/2, how this feature can improve performance and how a specific bug in a various HTTP client libraries could cause issues in HPACK header compression, refer to the community article.

HTTP/2 limitations

• HTTP/2 between the load balancer and the instance can require significantly more TCP connections to the instance than HTTP or HTTPS. Connection pooling, an optimization that reduces the number of these connections with HTTP or HTTPS, isn't available with HTTP/2. As a result, you might see high backend latencies because backend connections are made more frequently.
• HTTP/2 between the load balancer and the backend doesn't support running the WebSocket Protocol over a single stream of an HTTP/2 connection (RFC 8441).
• HTTP/2 between the load balancer and the backend doesn't support server push.
• The gRPC error rate and request volume aren't visible in the Google Cloud API or the Google Cloud console. If the gRPC endpoint returns an error, the load balancer logs and the monitoring data report the200 OK HTTP status code.

HTTP/2 over cleartext TCP

HTTP/2 over cleartext TCP, represented by the string "h2c" per RFC 7540, lets you use HTTP/2 without TLS encryption. It is supported for the following connections:

• Client to load balancer: Supported automatically; no special configuration is required.
• Load balancer to its backends: Supported by setting thebackend service protocol to H2C.

H2C support is also available for load balancers created using the GKE Gateway controller and Cloud Service Mesh, but isn't supported for classic Application Load Balancers.

HTTP/3 support

HTTP/3 is a next-generation internet protocol. It is built on top of IETF QUIC, a protocol developed from the original Google QUIC protocol. HTTP/3 is supported between the external Application Load Balancer, Cloud CDN, and clients.

Specifically:

• IETF QUIC is a transport layer protocol that provides congestion control and reliability similar to TCP, uses TLS 1.3 for security, and improved performance.
• HTTP/3 is an application layer built on top of IETF QUIC, and it relies on QUIC to handle multiplexing, congestion control, loss detection, and retransmission.
• HTTP/3 allows faster client connection initiation, eliminates head-of-line blocking in multiplexed streams, and supports connection migration when a client's IP address changes.
• HTTP/3 is supported for connections between clients and the load balancer, not connections between the load balancer and its backends.
• HTTP/3 connections use the BBRcongestion control algorithm.

HTTP/3 on your load balancer can improve web page load times, reduce video rebuffering, and improve throughput on higher latency connections.

The following table specifies the HTTP/3 support for external Application Load Balancers in each mode.

How HTTP/3 is negotiated

When HTTP/3 is enabled, the load balancer advertises this support to clients, allowing clients that support HTTP/3 to attempt to establish HTTP/3 connections with the HTTPS load balancer.

• Properly implemented clients always fall back to HTTPS or HTTP/2 when they can't establish an HTTP/3 connection.
• Clients that support HTTP/3 use their cached prior knowledge of HTTP/3 support to save unnecessary round trips in the future.
• Because of this fallback, enabling or disabling HTTP/3 in the load balancer doesn't disrupt the load balancer's ability to connect to clients.

Support is advertised in theAlt-SvcHTTP response header. When HTTP/3 is enabled, responses from the load balancer include the following alt-svc header value:

alt-svc: h3=":443"; ma=2592000,h3-29=":443"; ma=2592000"

If HTTP/3 has been explicitly set to DISABLE, responses don't include analt-svc response header.

When you have HTTP/3 enabled on your HTTPS load balancer, some circumstances can cause your client to fall back to HTTPS or HTTP/2 instead of negotiating HTTP/3. These include the following:

• When a client supports versions of HTTP/3 that aren't compatible with the HTTP/3 versions supported by the HTTPS load balancer.
• When the load balancer detects that UDP traffic is blocked or rate-limited in a way that prevents HTTP/3 from working.
• The client doesn't support HTTP/3 at all, and thus doesn't attempt to negotiate an HTTP/3 connection.

When a connection falls back to HTTPS or HTTP/2, we don't count this as a failure of the load balancer.

Before you enable HTTP/3, ensure that the previously described behaviors are acceptable for your workloads.

Configure HTTP/3

Both NONE (the default) and ENABLE enable HTTP/3 support for your load balancer.

When HTTP/3 is enabled, the load balancer advertises it to clients, which allows clients that support it to negotiate an HTTP/3 version with the load balancer. Clients that don't support HTTP/3 don't negotiate an HTTP/3 connection. You don't need to explicitly disable HTTP/3 unless you have identified broken client implementations.

External Application Load Balancers provide three ways to configure HTTP/3 as shown in the following table.

To explicitly enable (or disable) HTTP/3, follow these steps.

Console: HTTPS

1. In the Google Cloud console, go to the Load balancing page.
   Go to Load balancing
2. Select the load balancer that you want to edit.
3. Click Frontend configuration.
4. Select the frontend IP address and port that you want to edit. To edit an HTTP/3 configuration, the protocol must be HTTPS.

Enable HTTP/3

1. Select the QUIC negotiation menu.
2. To explicitly enable HTTP/3 for this frontend, select Enabled.
3. If you have multiple frontend rules representing IPv4 and IPv6, make sure to enable HTTP/3 on each rule.

Disable HTTP/3

1. Select the QUIC negotiation menu.
2. To explicitly disable HTTP/3 for this frontend, select Disabled.
3. If you have multiple frontend rules representing IPv4 and IPv6, make sure to disable HTTP/3 for each rule.

gcloud: HTTPS

Before you run this command, you must create an SSL certificate resource for each certificate.

gcloud compute target-https-proxies create HTTPS_PROXY_NAME
--global
--quic-override=QUIC_SETTING

Replace QUIC_SETTING with one of the following:

• NONE (default): allows Google to control when HTTP/3 is advertised.
  When you select NONE, HTTP/3 is advertised to clients, but Google QUIC isn't advertised. In the Google Cloud console, this option is called Automatic (Default).
• ENABLE: advertises HTTP/3 to clients.
• DISABLE: doesn't advertise HTTP/3 to clients.

API: HTTPS

POST https://www.googleapis.com/v1/compute/projects/PROJECT_ID/global/targetHttpsProxies/TARGET_PROXY_NAME/setQuicOverride

{ "quicOverride": QUIC_SETTING }

Replace QUIC_SETTING with one of the following:

• NONE (default): Allows Google to control when HTTP/3 is advertised.
  When you select NONE, HTTP/3 is advertised to clients, but Google QUIC isn't advertised. In the Google Cloud console, this option is called Automatic (Default).
• ENABLE: Advertises HTTP/3 and Google QUIC to clients.
• DISABLE: Doesn't advertise HTTP/3 or Google QUIC to clients.

WebSocket support

Google Cloud HTTP(S)-based load balancers support the websocket protocol when you use HTTP or HTTPS as the protocol to the backend. The load balancer doesn't require any configuration to proxy websocket connections.

The websocket protocol provides a full-duplex communication channel between clients and the load balancer. For more information, see RFC 6455.

The websocket protocol works as follows:

1. The load balancer recognizes a websocket Upgrade request from an HTTP or HTTPS client. The request contains the Connection: Upgrade andUpgrade: websocket headers, followed by other relevant websocket related request headers.
2. Backend sends a websocket Upgrade response. The backend instance sends a101 switching protocol response with Connection: Upgrade andUpgrade: websocket headers and other other websocket related response headers.
3. The load balancer proxies bidirectional traffic for the duration of the current connection.

If the backend instance returns a status code 426 or 502, the load balancer closes the connection.

Websocket connection timeouts depend on the type of load balancer (global, regional, or classic). For details, see Backend service timeout.

Session affinity for websockets works the same as for any other request. For more information, see Session affinity.

gRPC support

gRPC is an open-source framework for remote procedure calls. It is based on the HTTP/2 standard. Use cases for gRPC include the following:

• Low-latency, highly scalable, distributed systems
• Developing mobile clients that communicate with a cloud server
• Designing new protocols that must be accurate, efficient, and language-independent
• Layered design to enable extension, authentication, and logging

To use gRPC with your Google Cloud applications, you must proxy requests end-to-end over HTTP/2. To do this, you create an Application Load Balancer with one of the following configurations:

• HTTP/2 over TLS between the client and the load balancer and H2C between the load balancer and the backend: you create an HTTPS load balancer (configured with a target HTTPS proxy and SSL certificate). Additionally, you configure the load balancer to use HTTP/2 for unencrypted connections between the load balancer and its backends by setting the backend service protocol to H2C.
• End-to-end encrypted traffic using HTTP/2 over TLS: you create an HTTPS load balancer (configured with a target HTTPS proxy and SSL certificate). The load balancer negotiates HTTP/2 with clients as part of the SSL handshake by using the ALPN TLS extension.
  Additionally, you must make sure that the backends can handle TLS traffic and configure the load balancer to use HTTP/2 for encrypted connections between the load balancer and its backends by setting the backend service protocol toHTTP2.
  The load balancer might still negotiate HTTPS with some clients or accept unsecured HTTP requests on a load balancer that is configured to use HTTP/2 between the load balancer and the backend instances. Those HTTP or HTTPS requests are transformed by the load balancer to proxy the requests over HTTP/2 to the backend instances.

If you want to configure an Application Load Balancer by using HTTP/2 with Google Kubernetes Engine Ingress or by using gRPC and HTTP/2 with Ingress, see HTTP/2 for load balancing with Ingress.

If you want to configure an external Application Load Balancer by using HTTP/2 with Cloud Run, see Use HTTP/2 behind a load balancer.

For information about troubleshooting problems with HTTP/2, see Troubleshooting issues with HTTP/2 to the backends.

For information about HTTP/2 limitations, see HTTP/2 limitations.

TLS support

By default, an HTTPS target proxy accepts only TLS 1.0, 1.1, 1.2, and 1.3 when terminating client SSL requests.

When the global external Application Load Balancer or the regional external Application Load Balancer use HTTPS as the backend service protocol, they can negotiate TLS 1.2 or 1.3 to the backend.

When the classic Application Load Balancer uses HTTPS as the backend service protocol, it can negotiate TLS 1.0, 1.1, 1.2, or 1.3 to the backend.

Mutual TLS support

Mutual TLS, or mTLS, is an industry standard protocol for mutual authentication between a client and a server. mTLS helps ensure that both the client and server authenticate each other by verifying that each holds a valid certificate issued by a trusted certificate authority (CA). Unlike standard TLS, where only the server is authenticated, mTLS requires both the client and server to present certificates, confirming the identities of both parties before communication is established.

All of the Application Load Balancers support mTLS. With mTLS, the load balancer requests that the client send a certificate to authenticate itself during the TLS handshake with the load balancer. You can configure aCertificate Manager trust store that the load balancer then uses to validate the client certificate's chain of trust.

For more information about mTLS, see Mutual TLS authentication.

TLS 1.3 early data support

TLS 1.3 early data is supported on the target HTTPS proxy of the following external Application Load Balancers for both HTTPS over TCP (HTTP/1.1, HTTP/2) and HTTP/3 over QUIC:

• Global external Application Load Balancers
• Classic Application Load Balancers

TLS 1.3 was defined in RFC 8446 and introduces the concept of early data, also known as_zero-round-trip time (0-RTT) data_, which can improve application performance for resumed connections by 30 to 50%.

With TLS 1.2, two round trips are required before data can be securely transmitted. TLS 1.3 reduces this to one round trip (1-RTT) for new connections, allowing clients to send application data immediately after the first server response. Additionally, TLS 1.3 introduces the concept of early data for resumed sessions, enabling clients to send application data with the initialClientHello, thereby reducing the effective round-tip time to zero (0-RTT). TLS 1.3 early data allows the backend server to begin processing client data before the handshake process with the client is complete, thereby reducing latency; however, care must be taken to mitigate replay risks.

Because early data is sent before the handshake is complete, an attacker can attempt to capture and replay requests. To mitigate this, the backend server must carefully control early data usage, limiting its use to idempotent requests. HTTP methods that are intended to be idempotent but which might trigger nonidempotent changes—such as a GET request modifying a database—must not accept early data. In such cases, the backend server must reject requests with the HTTP Early-Data: 1 header by returning an HTTP425 Too Early status code.

Requests with early data have the HTTP Early-Data header set to a value of1, which indicates to the backend server that the request has been conveyed in TLS early data. It also indicates that the client understands the HTTP 425 Too Early status code.

TLS early data (0-RTT) modes

You can configure TLS early data using one of the following modes on the target HTTPS proxy resource of the load balancer.

• STRICT. This enables TLS 1.3 early data for requests with safe HTTP methods (GET, HEAD, OPTIONS, TRACE), and HTTP requests that don't have query parameters. Requests that send early data containing nonidempotent HTTP methods (such as POST or PUT) or with query parameters are rejected with an HTTP 425 status code.
• PERMISSIVE. This enables TLS 1.3 early data for requests with safe HTTP methods (GET, HEAD, OPTIONS, TRACE). This mode doesn't deny requests that include query parameters. The application owner must ensure that early data is safe to use for each request path, particularly for endpoints where request replay might cause unintended side effects, such as logging or database updates triggered by GET requests.
• DISABLED. TLS 1.3 early data isn't advertised, and any (invalid) attempts to send early data are rejected. If your applications aren't equipped to handle early data requests safely, you must disable early data. TLS 1.3 early data is disabled by default.
• UNRESTRICTED (not recommended for most workloads). This enables TLS 1.3 early data for requests with any HTTP method including nonidempotent methods, such as POST. This mode doesn't enforce any other limitations. This mode can be valuable for gRPC use cases. However, we don't recommend this method unless you have evaluated your security posture and mitigated the risk of replay attacks using other mechanisms.

Configure TLS early data

To explicitly enable or disable TLS early data, do the following:

Console

1. In the Google Cloud console, go to the Load balancing page.
   Go to Load balancing
2. Select the load balancer for which you need to enable early data.
3. Click Edit.
4. Click Frontend configuration.
5. Select the frontend IP address and port that you want to edit. To enable TLS early data, the protocol must be HTTPS.
6. In the Early data (0-RTT) list, select a TLS early data mode.
7. Click Done.
8. To update the load balancer, click Update.

gcloud

1. Configure the TLS early data mode on the target HTTPS proxy of an Application Load Balancer.
   gcloud compute target-https-proxies update TARGET_HTTPS_PROXY \
   --tls-early-data=TLS_EARLY_DATA_MODE
   Replace the following:
   • TARGET_HTTPS_PROXY: the target HTTPS proxy of your load balancer
   • TLS_EARLY_DATA_MODE: STRICT, PERMISSIVE,DISABLED, or UNRESTRICTED

API

PATCH https://compute.googleapis.com/compute/v1/projects/{project}/global/targetHttpsProxies/TARGET_HTTPS_PROXY { "tlsEarlyData":"TLS_EARLY_DATA_MODE", "fingerprint": "FINGERPRINT" }

Replace the following:

• TARGET_HTTPS_PROXY: the target HTTPS proxy of your load balancer
• TLS_EARLY_DATA_MODE: STRICT, PERMISSIVE,DISABLED, or UNRESTRICTED
• FINGERPRINT: a Base64 encoded string. An up-to-date fingerprint must be provided in order to patch the target HTTPS proxy; otherwise, the request fails with an HTTP 412 Precondition Failedstatus code.

After you have configured TLS early data, you can issue requests from an HTTP client that supports TLS early data. You can observe lower latency for resumed requests.

If a non-RFC-compliant client sends a request with a nonidempotent method or with query parameters, the request is denied. You see an HTTP 425 Early status code in the load balancer logs and the following HTTP response:

HTTP/1.1 425 Too Early Content-Type: text/html; charset=UTF-8 Referrer-Policy: no-referrer Content-Length: 1558 Date: Thu, 03 Aug 2024 02:45:14 GMT Connection: close Error 425 (Too Early)

The request was sent to the server too early, please retry. That's all we know.

Google tag gateway for advertisers

Google tag gatewaylets website owners host and deploy Google tags through Google Cloud. These Google tags help you analyze traffic on your website. You can use a global external Application Load Balancer to route measurement traffic on your website through your domain for improved measurement data accuracy. This provides more reliable data for advertising campaign optimization.

Note that Google tag gateway is only supported on global external Application Load Balancers. If you use a classic Application Load Balancer,migrate your resources from classic to global external Application Load Balancer.

Configuration and management of the Google tag gateway are managed outside of the Google Cloud console. You must configure this feature using the Google Tag Manager interface.

• In the Google Cloud console, the Load balancer details page displays a read-only visual indicator showing that Google tag gateway is enabled.
• You can't edit or delete the Google tag gateway configuration from the Google Cloud console.

For complete instructions on how to set up the Google tag gateway for advertisers, seeSet up Google tag gateway for advertisers.

While configuration is managed with Google Ads, request processing and Google tag injections performed by the gateway are recorded in your global external Application Load Balancer's logging and monitoring. For more information, seeGlobal external Application Load Balancer logging and monitoring.

For information about troubleshooting issues with Google tag gateways, seeTroubleshoot Google tag gateway injection issues.

If you encounter issues during setup that the documentation can't resolve, contactGoogle Ads Support.

Limitations

• HTTPS load balancers don't send a close_notify closure alert when terminating SSL connections. That is, the load balancer closes the TCP connection instead of performing an SSL shutdown.
• HTTPS load balancers support only lowercase characters in domains in a common name (CN) attribute or a subject alternative name (SAN) attribute of the certificate. Certificates with uppercase characters in domains are returned only when set as the primary certificate in the target proxy.
• HTTPS load balancers don't use the Server Name Indication (SNI) extension when connecting to the backend, except for load balancers with Internet NEG backends. For more information, see Encryption from the load balancer to the backends.
• Google Cloud doesn't guarantee that an underlying TCP connection can remain open for the entirety of the value of the backend service timeout. Client systems must implement retry logic instead of relying on a TCP connection to be open for long periods of time.
• When you create a regional external Application Load Balancer in Premium Tier using the Google Cloud console, only regions supporting Standard Tier are available in the Google Cloud console. For access to other regions, use either the gcloud CLI or the API.
• The GFE proxies used by global and classic Application Load Balancers don't support_early_ 200 OK responses that are sent before the request's POST payload has been fully proxied to the backend. Sending an early 200 OK response causes the GFE to close the connection to the backend.
  Your backend must respond with 100 Continue responses after the request headers are received, and then wait until the request's POST payload has been fully proxied before responding with the final 200 OK response code.
• When using regional external Application Load Balancers with Cloud Run in a Shared VPC environment, standalone VPC networks in service projects can send traffic to any other Cloud Run services deployed in any other service projects within the same Shared VPC environment. This is a known issue.
• Cloud CDN lets you force an object or set of objects to be ignored by the cache by requesting a cache invalidation. When you're using a global external Application Load Balancer with Shared VPC cross-project service referencing, by default, service project admins won't have the required permissions to request cache invalidations. This is because cache invalidation is configured in the frontend project (that is, the project that has the forwarding rule, target proxy, and URL map of the load balancer). Thus, cache invalidations can only be issued by principals who have the IAM roles for configuring load balancer related resources in the frontend projects (for example, the Compute Network Admin role). Service admins, who control provisioning of the backend services in a separate project, need to work with the load balancer administrator of the frontend project to issue cache invalidation for their cross-project services.
• When using a classic Application Load Balancer, the load balancer automatically adds an HTTP Content-Length: 0 header to DELETE requests over HTTP/1.1 and HTTP/2, even if the original request did not specify a content length. This behavior might cause Cloud Armor security policies to block these requests if your policy is configured to filter or reject requests containing a content-length header. This issue is specific to a classic Application Load Balancer and does not occur with GET or HEAD requests.

What's next

• To learn how external Application Load Balancers handle connections, route traffic, and maintain session affinity, see Request distribution for external Application Load Balancers.
• To learn how to deploy a global external Application Load Balancer, see Setting up an external Application Load Balancer with a Compute Engine backend.
• To learn how to deploy a regional external Application Load Balancer, see Setting up a regional external Application Load Balancer with a Compute Engine backend.
• If you are an existing user of the classic Application Load Balancer, make sure that you review Migration overview when you plan a new deployment with the global external Application Load Balancer.
• To learn how to automate your external Application Load Balancer setup with Terraform, seeTerraform module examples for external Application Load Balancers.
• To learn how to configure advanced traffic management capabilities available with the global external Application Load Balancer, see Traffic management overview for global external Application Load Balancers.
• To learn how to configure advanced traffic management capabilities available with the regional external Application Load Balancer, see Traffic management overview for regional external Application Load Balancers.
• To learn about serving websites, see Serving websites.
• To find the locations for Google PoPs, seeGFE locations.
• To learn about capacity management, see Capacity management with load balancing tutorialand Application capacity optimizations with global load balancing.
• To learn how to use Certificate Manager to provision and manage SSL certificates, see the Certificate Manager overview.
• To insert custom logic into the load balancing data path, configureService Extensions plugins or callouts.
• For regional external Application Load Balancers only, you can try using Apigee Shadow API Discovery to find shadow APIs (also known as undocumented APIs) in your existing Google Cloud infrastructure. Make sure that you read the associatedlimitationsbefore you enable Shadow API Discovery.