Controlling an HTTP client's cache duration

When using a CDN, two levels of caching are available:

Caching at the Edge occurs when a CDN Edge server caches a piece of content from the origin.
Caching downstream from the Edge network of servers occurs when an user or HTTP client, such as a requesting browser, caches a piece of content from an Edge server.

The method that you choose to control how long content is cached at the requester, such as a browser, depends on the following factors:

Whether the Respect Header setting is ON or OFF (default).
Whether the origin server provides a max-age value in the Cache-Control header for a particular piece of content.

Regardless of how these factors change, your origin must provide a Cache-Control header for the intended content to the Edge, if you want Edge servers to send HTTP responses with the Cache-Control header for that content.

Essentially, Cache-Control headers sent from an Edge server downstream ask the requester to cache the associated content according to the caching directives or values that are specified by the Edge server.

Respect Header: Off

If your origin does provide a Cache-Control header with a max-age directive and value for a specific piece of content, then the cache duration for the specific piece of content that is cached on the Edge is still derived from your CDN's TTL settings. Additionally, the Edge server responds to the requester downstream with a Cache-Control max-age value equal to the lesser of:

The origin's Cache-Control max-age value.
The remaining time until the content goes stale on the Edge.

However, if your origin does not provide a Cache-Control header to the Edge server, then Edge servers will not provide a Cache-Control header to the requester. The Edge cache duration for your content is still derived from your CDN's TTL Settings.

Respect Header: On

If your origin does provide a Cache-Control header with max-age for a specific piece of content, the origin's Cache-Control max-age value becomes the cache duration for that specific piece of content cached on the Edge, overriding any applicable TTL settings for that piece of content. Additionally, the Edge responds to the requester with a Cache-Control max-age value equal to the remaining time until the content goes stale on the Edge server.

However, if your origin does not provide a Cache-Control header to the Edge server, then the Edge server will not provide a Cache-Control header to the requester. The Edge cache duration for your content is still derived from your CDN's TTL settings.

Summary

Table 1. Summary: HTTP client's cache duration
Respect Header	Origin provides Cache-Control	Specific Content's Cache Duration on Edge Server	Edge Server provides Cache-Control
On	Yes, origin specifies a `max-age`	Edge cache duration overridden with origin's `max-age` value	Yes, Edge also provides a `max-age` with a value that is the (overridden) time before the Edge needs to refresh the content from the origin
On	Yes, but origin does not specify a `max-age`	Edge cache duration based on the CDN's TTL configuration	Yes, Edge also provides a `max-age` with a value that is the time before the Edge needs to refresh the content from the origin
On	No	Edge cache duration based on the CDN's TTL configuration	No
Off	Yes, origin specifies a `max-age`	Edge cache duration base on the CDN's TTL configuration	Yes, Edge also provides a `max-age` value that is the lesser of origin's `max-age` value and the time before the Edge needs to refresh the content from the origin
Off	Yes, but origin does not specify a `max-age`	Edge cache duration based on the CDN's TTL configuration	Yes, Edge also provides a `max-age` that is the time before the Edge needs to refresh the content from the origin
Off	No	Edge cache duration based on the CDN's TTL configuration	No

More information

See cache-control as defined in section 14.9 of RFC 2616.