Internet DRAFT - draft-power-cdni-cache-control-metadata
draft-power-cdni-cache-control-metadata
Network Working Group W. Power
Internet-Draft G. Goldstein
Updates: 8006 (if approved) Lumen Technologies
Intended status: Standards Track 13 March 2023
Expires: 14 September 2023
CDNI Cache Control Metadata
draft-power-cdni-cache-control-metadata-00
Abstract
This specification adds to the basic Cache Control metadata defined
in RFC8006, providing Content Providers and uCDNs more fine-grained
control over dCDN caching. Use cases include overriding or adjusting
cache-control headers from the origin, bypassing caching altogether,
or altering cache keys with dynamically generated values.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. MI.CachePolicy . . . . . . . . . . . . . . . . . . . . . . . 3
3. MI.NegativeCachePolicy . . . . . . . . . . . . . . . . . . . 6
4. MI.StaleContentCachePolicy . . . . . . . . . . . . . . . . . 7
5. MI.CacheBypassPolicy . . . . . . . . . . . . . . . . . . . . 9
6. MI.ComputedCacheKey . . . . . . . . . . . . . . . . . . . . . 11
7. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 12
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
9.1. CDNI Payload Types . . . . . . . . . . . . . . . . . . . 12
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
11.1. Normative References . . . . . . . . . . . . . . . . . . 12
11.2. Informative References . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
In addition to the cache control parameters currently specified by
Cache object in [RFC8006], Content Providers and uCDNs often need
more fine-grained control over dCDN caching, including scenarios
where it is desirable to override or adjust cache-control headers
from the origin.
These capabilities are required for commercial CDN and open caching
use cases:
Positive Cache Control - Allows the uCDN to specify internal caching
policies for the dCDN and external caching policies advertised to
clients of the dCDN, overriding any cache control policy set in the
response from the uCDN.
1. Positive Cache Control - Allows the uCDN to specify internal
caching policies for the dCDN and external caching policies
advertised to clients of the dCDN, overriding any cache control
policy set in the response from the uCDN.
2. Negative Cache Control - Allows the specification of caching
policies based on error response codes received from the origin,
allowing for fine-grained control of the downstream caching of
error responses. For example, it may be desirable to cache error
responses at the dCDN for a short period of time to prevent an
overwhelmed origin service or uCDN from being flooded with
requests.
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3. Cache Bypass Control - Allows content providers to bypass CDN
caching when needed (typically for testing or performance
benchmarking purposes).
4. Stale Content Policies - Allows control over how the dCDN should
process requests for stale content. For example, this policy
allows the content provider to specify that stale content be
served from cache for a specified time period while refreshes
from the origin occur asynchronously.
5. Dynamically Constructed Cache Keys - It is typical in advanced
CDN configurations to generate cache keys that are dynamically
constructed via lightweight processing of various properties of
the HTTP request and/or response. As an example, an origin may
specify a cache key as a value returned in a specific HTTP
response header. The Metadata Expression Language is used to
allow for such advanced cache key construction.
2. MI.CachePolicy
CachePolicy is a new GenericMetadata object that allows for the uCDN
to specify internal caching policies for the dCDN, as well as
external caching policies advertised to clients of the dCDN
(overriding any cache control policy set in the response from the
uCDN).
Property: internal
- Description: Specifies the internal cache control policy to be
used by the dCDN.
- Type: Number in seconds encoded as string (e.g. 5 is a five
second cache ) and/or a list of Enumeration [as-is|no-cache|no-
store]
- Mandatory-to-Specify: No. The default is to use the cache
control policy specified in the response from the uCDN.
Property: external
- Description: Specifies the external cache control policy to be
used by clients of the dCDN.
- Type: Number in seconds encoded as string (e.g. 5 is a five
second cache ) and/or a list of Enumeration [as-is|no-cache|no-
store]
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- Mandatory-to-Specify: No. The default is to use the cache
control policy specified in the response from the uCDN.
Property: force-internal
- Description: If set to True, the metadata interface cache
policy defined in the MI.CachePolicy internal property value
will override any cache control policy set in the response from
the uCDN. If set to False, the MI.CachePolicy internal
property value is only used if there is no cache control policy
provided in the response from the uCDN.
- Type: Boolean
- Mandatory-to-Specify: No. The default is "False", which will
apply the MI.CachePolicy internal property value only if no
policy is provided in the response from the uCDN.
Property: force-external
- Description: If set to True, the metadata interface cache
policy defined in the MI.CachePolicy external property value
will override any cache control policy set in the response from
the uCDN. If set to False, the MI.CachePolicy external
property value is only used if there is no cache control policy
provided in the response from the uCDN.
- Type: Boolean
- Mandatory-to-Specify: No. The default is "False", which will
apply the MI.CachePolicy external property value only if no
policy is provided in the response from the uCDN.
Example 1: An MI.CachePolicy that sets the internal cache control
policy to five seconds. The external cache policy is set to 'no-
cache', and both policies are forced:
{
"generic-metadata-type": "MI.CachePolicy",
"generic-metadata-value": {
"internal": "5",
"external": "no-cache",
"force-internal": "true",
"force-external": "true"
}
}
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Example 2: An MI.CachePolicy that sets the internal cache control
policy to "as-is" (keep the policy set in the response from the
uCDN). The external cache policy is set to 'no-cache' and forced:
{
"generic-metadata-type": "MI.CachePolicy",
"generic-metadata-value": {
"internal": "as-is",
"external": "no-cache",
"force-external": "true"
}
}
Example 3: An MI.CachePolicy in the context of the processing stages
model that sets a caching policy only if the HTTP status code
received from the origin is a 200. In this example, the internal
cache control policy is set to five seconds. The external cache
policy is set to 'no-cache'. Internal and external force flags are
both set to ‘False’, indicating that the MI.CachePolicy only applies
if there is no cache policy in the response from the uCDN.
{
"generic-metadata-type": "MI.ProcessingStages",
"generic-metadata-value": {
"origin-response": [
{
"match": {
"expression": "resp.status == 200"
},
"stage-metadata": {
"generic-metadata": [
{
"generic-metadata-type": "MI.CachePolicy",
"generic-metadata-value": {
"internal": "5",
"external": "no-cache",
"force-internal": "false",
"force-external": "false"
}
}
]
}
}
]
}
}
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3. MI.NegativeCachePolicy
NegativeCachePolicy is a new GenericMetadata object that allows for
the specification of caching policies based on response codes
received from the origin.
Property: error-codes
- Description: Array of HTTP response status codes (See Sections
15.5 and 15.6 of [RFC9110]) , that if returned from the uCDN,
will be cached using the cache policy defined by the cache-
policy property.
- Type: Array of HTTP response status codes
- Values: Any HTTP status code from 100 to 599, or one of the
special values "4xx" or "5xx", where "xx" implies everything
from 00 to 99. Note that use of 4xx would specify that 416
responses are cached.
- Mandatory-to-Specify: No. The default is to revert to
[RFC8006] behavior. An empty or unspecified list may function
as a means to revoke a list inherited from an upper level
configuration
Property: cache-policy
- Description: MI.CachePolicy to apply to the HTTP response
status codes returned by the uCDN.
- Mandatory-to-Specify: Yes
Example: A MI.NegativeCachePolicy that applies to HTTP error codes:
"404", "503", "504" and sets the internal cache control policy to
five seconds and external to 'no-cache'.
{
"generic-metadata-type": "MI.NegativeCachePolicy",
"generic-metadata-value": {
"error-codes": [ "404", "503", "504" ],
"cache-policy": {
"internal": "5",
"external": "no-cache",
"force-internal": "true",
"force-external": "true"
}
}
}
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4. MI.StaleContentCachePolicy
MI.StaleContentCachePolicy is a new GenericMetadata object that
allows the uCDN to specify the policy to use by a dCDN when
responding with stale content. For example, this policy allows the
content provider to specify that stale content be served from cache
for a specified time period while refreshes from the origin occur
asynchronously.
Property: stale-while-revalidating
- Description: Instructs the dCDN to serve a stale version of a
resource while refreshing the resource with the uCDN. When set
to "True", the dCDN will return a previously cached version of
a resource while the resource is refreshed with the uCDN in the
background.
- Type: Boolean
- Mandatory-to-Specify: No. The default is False, which waits
for the uCDN to refresh a resource before responding to the
client.
Property: stale-if-error
- Description: Instructs the dCDN to serve a stale version of a
resource if any one of a specified set of HTTP status codes was
received when trying to refresh the resource with the uCDN. In
this case, the dCDN will return a previously cached version of
a resource instead of caching the error response. While this
capability is typically used for well-understood HTTP error
status codes, a list of any HTTP codes can be provided for
maximum flexibility.
- Type: Array of HTTP response status codes
- Values: Any HTTP status code from 100 to 599, or one of the
special values "4xx" or "5xx", where "xx" implies everything
from 00 to 99.
- Mandatory-to-Specify: No. The default is to not serve stale
content. An empty or unspecified list may function as a mean
to revoke a list inherited from an upper level configuration
Property: failed-refresh-ttl
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- Description: Instructs the dCDN to serve a stale version of a
resource for the number of seconds specified in failed-refresh-
ttl before trying to revalidate the resource with the uCDN.
Use of failed-refresh-ttl allows the load to be reduced on the
uCDN during times of system stress.
- Type: Integer
- Mandatory-to-Specify: No
Example 1: A MI.StaleContentCachePolicy where stale-while-
revalidating is true, instructing the dCDN to respond with a stale
cached version of the resource while it refreshes the resource with
the uCDN in the background:
{
"generic-metadata-type": "MI.StaleContentCachePolicy",
"generic-metadata-value": {
"stale-while-revalidating": true
}
}
Example 2: A MI.StaleContentCachePolicy where stale-if-error
instructs the dCDN to use the stale cached resource if it receives an
error of type 503 or 504 when trying to refresh the resource with the
uCDN.
failed-refresh-ttl instructs the dCDN to use a five second cache TTL
on the resource that receives an error when refreshing from the uCDN.
That is, after five seconds, the dCDN will attempt to refresh the
resource with the uCDN.
{
"generic-metadata-type": "MI.StaleContentCachePolicy",
"generic-metadata-value": {
"stale-if-error": [ "503", "504" ],
"failed-refresh-ttl": "5"
}
}
Example 3: A MI.StaleContentCachePolicy where stale-while-
revalidating is true, instructing the dCDN to respond with a stale
cached version of the resource while it refreshes the resource with
the uCDN in the background.
stale-if-error instructs the dCDN to use the stale cached resource if
it receives an error of type 404 or any 5xx status when trying to
refresh the resource with the uCDN.
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failed-refresh-ttl instructs the dCDN to use a five second cache TTL
on the resource that receives an error when refreshing from the uCDN.
That is, after five seconds, the dCDN will attempt to refresh the
resource with the uCDN.
{
"generic-metadata-type": "MI.StaleContentCachePolicy",
"generic-metadata-value": {
"stale-while-revalidating": "true",
"stale-if-error": [ "404", "5xx" ],
"failed-refresh-ttl": "5"
}
}
5. MI.CacheBypassPolicy
CacheBypassPolicy is a new GenericMetadata object that allows a
client request to be set as non-cacheable. It is expected that this
feature will be used to allow clients to bypass cache when testing
the uCDN fill path. Note: CacheBypassPolicy is typically used in
conjunction with a path match or match expression on a header value
or query parameter. Any content previously cached (by client
requests that do not set CacheBypassPolicy) is not evicted.
Property: bypass-cache
- Description: A Boolean value that can activate the feature for
a given client request. It is expected that this feature will
be used within ProcessingStages to allow a client request to be
marked to bypass cache.
- Type: Boolean
- Mandatory-to-Specify: No. The default is False.
Example 1: A MI.CacheBypassPolicy with the client HTTP header of:
CDN-BYPASS: “True”:
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{
"generic-metadata-type": "MI.ProcessingStages",
"generic-metadata-value": {
"client-request": [
{
"match": {
"expression": "req.h.cdn-bypass == 'true'"
},
"stage-metadata": {
"generic-metadata": [
{
"generic-metadata-type": "MI.CacheBypassPolicy",
"generic-metadata-value": {
"bypass-cache": "true"
}
}
]
}
}
]
}
}
Example 2: A MI.CacheBypassPolicy that applies to all requests where
the host header is bypass.example.com:
{
"generic-metadata-type": "MI.ProcessingStages",
"generic-metadata-value": {
"client-request": [
{
"match": {
"expression": "req.h.host == 'bypass.example.com'"
},
"stage-metadata": {
"generic-metadata": [
{
"generic-metadata-type": "MI.CacheBypassPolicy",
"generic-metadata-value": {
"bypass-cache": "true"
}
}
]
}
}
]
}
}
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6. MI.ComputedCacheKey
It is typical in advanced CDN configurations to generate cache keys
that are dynamically constructed via lightweight processing of
various properties of the HTTP request and/or response. As an
example, an origin may specify a cache key as a value returned in a
specific HTTP response header.
ComputedCacheKey is a new GenericMetadata object that allows for the
specification of a cache key using the metadata expression language.
Typical use cases would involve the construction of a cache key from
one or more elements of the HTTP request. In cases where both the
ComputedCacheKey and the Cache object are applied, the
ComputedCacheKey will take precedence.
Property: expression
- Description: The expression that specifies how the cache key
shall be constructed.
- Type: String. An expression using [CDNI-MEL] to dynamically
construct the cache key from elements of the HTTP request and/
or response.
- Mandatory-to-Specify: Yes
Example, using a custom request header as the cache key instead of
the URI path:
{
"generic-metadata-type": "MI.ComputedCacheKey",
"generic-metadata-value": {
"expression": "req.h.X-Cache-Key"
}
}
7. Conclusion
This specification has defined a new set of Cache Control Metadata
objects that meet the needs of Content Providers, CDNs, and Open
Caching Systems. As the standard matures and gains wider adoption,
it is expected that additions to this set of cache control policies
will be required.
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8. Security Considerations
The FCI and MI objects defined in the present document are
transferred via the interfaces defined in CDNI [RFC8006]. [RFC8006]
describes how to secure these interfaces, protecting the integrity,
confidentiality and ensuring the authenticity of the dCDN and uCDN.
The security provide by [RFC8006] should therefore address the above
security concerns.
9. IANA Considerations
9.1. CDNI Payload Types
TBD.
10. Acknowledgements
The authors would like to express their gratitude to the members of
the Streaming Video Technology Alliance [SVTA] Open Caching Working
Group for their guidance / contribution / reviews ...)
Particulary the following people contribute in one or other way to
the content of this draft:
Guillaume Bichot - Broadpeak
Pankaj Chaudhari - Disney Streaming Services
Yoav Gressel - Qwilt
Alfonso Siloniz - Telefonica
Ben Rosenblum - Vecima
11. References
11.1. Normative References
[RFC9110] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/info/rfc9110>.
[RFC8006] Niven-Jenkins, B., Murray, R., Caulfield, M., and K. Ma,
"Content Delivery Network Interconnection (CDNI)
Metadata", RFC 8006, DOI 10.17487/RFC8006, December 2016,
<https://www.rfc-editor.org/info/rfc8006>.
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11.2. Informative References
[SVTA] "Streaming Video Technology Alliance Home Page",
<https://www.svta.org>.
[CDNI-MEL] Goldstein, G., Ed., Chaudhari, P., Power, W., Gressel, Y.,
and A. Warshavsky, "SVTA Open Caching - Configuration
Interface Part 2a - Metadata Expression Language",
Version 2.0, 2 July 2021.
Authors' Addresses
Will Power
Lumen Technologies
United States of America
Email: wrpower@gmail.com
Glenn Goldstein
Lumen Technologies
United States of America
Email: glenng1215@gmail.com
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