Internet Engineering Task Force (IETF) A. Gable
Request for Comments: 9773 Internet Security Research Group
Category: Standards Track May 2025
ISSN: 2070-1721
ACME Renewal Information (ARI) Extension
Abstract
This document specifies how an Automated Certificate Management
Environment (ACME) server may provide suggestions to ACME clients as
to when they should attempt to renew their certificates. This allows
servers to mitigate load spikes and ensures that clients do not make
false assumptions about appropriate certificate renewal periods.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9773.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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in the Revised BSD License.
Table of Contents
1. Introduction
2. Conventions and Definitions
3. Extensions to the Directory Object
4. Getting Renewal Information
4.1. The "renewalInfo" RenewalInfo Resource
4.2. RenewalInfo Objects
4.3. Schedule for Checking the RenewalInfo Resource
4.3.1. Server Choice of Retry-After
4.3.2. Client Handling of Retry-After
4.3.3. Error Handling
5. Extensions to the Order Object
6. Security Considerations
7. IANA Considerations
7.1. ACME Resource Type
7.2. ACME Renewal Info RenewalInfo Object Fields
7.3. ACME Order Object Fields
7.4. ACME Error Types
8. References
8.1. Normative References
8.2. Informative References
Appendix A. Example Certificate
Acknowledgments
Author's Address
1. Introduction
Most ACME [RFC8555] clients today choose when to attempt to renew a
certificate in one of three ways:
1. they may be configured to renew at a specific interval (e.g., via
cron),
2. they may parse the issued certificate to determine its expiration
date and renew a specific amount of time before then, or
3. they may parse the issued certificate and renew when some
percentage of its validity period has passed.
The first two create significant barriers against the issuing
Certification Authority (CA) changing certificate lifetimes. All
three ways may lead to load clustering for the issuing CA due to its
inability to schedule renewal requests.
Allowing issuing CAs to suggest a period in which clients should
renew their certificates enables dynamic time-based load balancing.
This allows a CA to better respond to exceptional circumstances. For
example:
* a CA could suggest that clients renew prior to a mass-revocation
event to mitigate the impact of the revocation, or
* a CA could suggest that clients renew earlier than they normally
would to reduce the size of an upcoming mass-renewal spike.
This document specifies the ACME Renewal Information (ARI) extension,
a mechanism by which ACME servers may provide suggested renewal
windows to ACME clients and by which ACME clients may inform ACME
servers that they have successfully renewed and replaced a
certificate.
2. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Throughout this document, the word "renewal" and its variants are
taken to encompass any combination of "Renewal", "Re-Key", and
"Modification" as defined in [RFC3647].
This document assumes that the certificates being issued by the ACME
server are in compliance with [RFC5280] and, in particular, contain
the Authority Key Identifier extension and the keyIdentifier field
within that extension.
3. Extensions to the Directory Object
An ACME server that wishes to provide renewal information MUST
include a new field, renewalInfo, "renewalInfo", in its directory object.
+=============+==============+
+=============+=====================+
| Field | URL in Value |
+=============+==============+
+=============+=====================+
| renewalInfo | Renewal info information |
+-------------+--------------+
+-------------+---------------------+
Table 1
HTTP/1.1 200 OK
Content-Type: application/json
{
"newNonce": "https://acme.example.com/new-nonce",
"newAccount": "https://acme.example.com/new-account",
"newOrder": "https://acme.example.com/new-order",
"newAuthz": "https://acme.example.com/new-authz",
"revokeCert": "https://acme.example.com/revoke-cert",
"keyChange": "https://acme.example.com/key-change",
"renewalInfo": "https://acme.example.com/renewal-info",
"meta": {
"termsOfService": "https://example.com/acme/terms",
"website": "https://example.com/acme/docs",
"caaIdentities": ["example.com"],
"externalAccountRequired": false
}
}
4. Getting Renewal Information
4.1. The "renewalInfo" RenewalInfo Resource
The "renewalInfo" RenewalInfo resource is a new resource type introduced to the
ACME protocol. This new resource allows clients to query the server
for suggestions on when they should renew certificates.
To request the suggested renewal information for a certificate, the
client sends an unauthenticated GET request to a path under the
server's renewalInfo URL.
The path component is a unique identifier for the certificate in
question. The unique identifier is constructed by concatenating the
base64url-encoding
base64url encoding [RFC4648] of the keyIdentifier field of the
certificate's Authority Key Identifier (AKI) [RFC5280] extension, the
period character ".", and the base64url-encoding base64url encoding of the DER-encoded
Serial Number field (without the tag and length bytes). All trailing
"=" characters MUST be stripped from both parts of the unique
identifier.
Thus, the full request URL is constructed as follows (split onto
multiple lines for readability), where the "||" operator indicates
string concatenation and the renewalInfo URL is taken from the
Directory object:
url = renewalInfo || '/' ||
base64url(AKI keyIdentifier) || '.' || base64url(Serial)
For example, to request renewal information for the end-entity
certificate given in Appendix A, the client would make the request as
follows:
1. The keyIdentifier field of the certificate's AKI extension has
the hexadecimal bytes
69:88:5B:6B:87:46:40:41:E1:B3:7B:84:7B:A0:AE:2C:DE:01:C8:D4 as
its ASN.1 Octet String value. The base64url encoding of those
bytes is aYhba4dGQEHhs3uEe6CuLN4ByNQ=.
2. The certificate's Serial Number field has the hexadecimal bytes
00:87:65:43:21 as its DER encoding (note the leading zero byte to
ensure the serial number remains positive despite the leading 1
bit in 0x87). The base64url encoding of those bytes is AIdlQyE=.
3. Stripping the trailing padding characters and concatenating with
the separator, the unique identifier is therefore
aYhba4dGQEHhs3uEe6CuLN4ByNQ.AIdlQyE, and the client makes the
request:
GET /renewal-info/aYhba4dGQEHhs3uEe6CuLN4ByNQ.AIdlQyE HTTP/1.1
Host: acme.example.com
Accept: application/json
4.2. RenewalInfo Objects
The structure of an ACME renewalInfo resource RenewalInfo object is as follows:
suggestedWindow (object, required):
A JSON object with two keys, "start" and "end", whose values are
timestamps, encoded in the format specified in [RFC3339], which
bound the window of time in which the CA recommends renewing the
certificate.
explanationURL (string, optional):
A URL pointing to a page that may explain why the suggested
renewal window has its current value. For example, it may be a
page explaining the CA's dynamic load-balancing strategy or a page
documenting which certificates are affected by a mass-revocation
event. Clients SHOULD provide this URL to their operator, if
present.
HTTP/1.1 200 OK
Content-Type: application/json
Retry-After: 21600
{
"suggestedWindow": {
"start": "2025-01-02T04:00:00Z",
"end": "2025-01-03T04:00:00Z"
},
"explanationURL": "https://acme.example.com/docs/ari"
}
Clients MUST attempt renewal at a time of their choosing based on the
suggested renewal window. The following algorithm is RECOMMENDED for
choosing a renewal time:
1. Query the Make a renewalInfo resource request to get a suggested renewal window.
2. Select a uniform random time within the suggested window.
3. If the selected time is in the past, attempt renewal immediately.
4. Otherwise, if the client can schedule itself to attempt renewal
at exactly the selected time, do so.
5. Otherwise, if the selected time is before the next time that the
client would wake up normally, attempt renewal immediately.
6. Otherwise, sleep until the time indicated by the Retry-After
header and return to Step 1.
In all cases, renewal attempts are subject to the client's existing
error backoff and retry intervals.
In particular, cron-based clients may find they need to increase
their run frequency to check ARI more frequently. Those clients will
need to store information about failures so that increasing their run
frequency doesn't lead to retrying failures without proper backoff.
Typical information stored should include: number of failures for a
given order (defined by the set of names on the order) and time of
the most recent failure.
A RenewalInfo object in which the end timestamp equals or precedes
the start timestamp is invalid. Servers MUST NOT serve such a
response, and clients MUST treat one as though they failed to receive
any response from the server (e.g., retry at an appropriate interval,
renew on a fallback schedule, etc.).
4.3. Schedule for Checking the RenewalInfo Resource
Clients SHOULD fetch a certificate's RenewalInfo immediately after
issuance.
During the lifetime of a certificate, the renewal information needs
to be fetched frequently enough that clients learn about changes in
the suggested window quickly, but without overwhelming the server.
This protocol uses the Retry-After header [RFC9110] to indicate to
clients how often to retry. Note that in other HTTP applications,
Retry-After often indicates the minimum time to wait before retrying
a request. In this protocol, it indicates the desired (i.e., both
requested minimum and maximum) amount of time to wait.
Clients MUST NOT check a certificate's RenewalInfo after the
certificate has expired. Clients MUST NOT check a certificate's
RenewalInfo after they consider the certificate to be replaced (for
instance, after a new certificate for the same identifiers has been
received and configured).
4.3.1. Server Choice of Retry-After
Servers set the Retry-After header based on their requirements on how
quickly to perform a revocation. For instance, a server that needs
to revoke certificates within 24 hours of notification of a problem
might choose to reserve twelve hours for investigation, six hours for
clients to fetch RenewalInfo, updated RenewalInfo objects, and six hours for
clients to perform a renewal. Setting a small value for Retry-After
means that clients can respond more quickly but also incurs more load
on the server. Servers should estimate their expected load based on
the number of clients, keeping in mind that third parties may also
monitor
RenewalInfo renewalInfo endpoints.
4.3.2. Client Handling of Retry-After
After an initial fetch of a certificate's RenewalInfo, clients MUST
fetch it again as soon as possible after the time indicated in the
Retry-After header (backoff on errors takes priority, though).
Clients MUST set reasonable limits on their checking interval. For
example, values under one minute could be treated as if they were one
minute, and values over one day could be treated as if they were one
day.
4.3.3. Error Handling
Temporary errors include, for instance:
* Connection timeout
* Request timeout
* 5xx HTTP errors
On receiving a temporary error, clients SHOULD do exponential backoff
with a capped number of tries. If all tries are exhausted, clients
MUST treat the request as a long-term error.
Examples of long-term errors include:
* Retry-After is invalid or not present
* RenewalInfo object is invalid
* DNS lookup failure
* Connection refused
* Non-5xx HTTP error
On receiving a long-term error, clients MUST perform make the next
RenewalInfo fetch
renewalInfo request as soon as possible after six hours have passed
(or some other locally configured default).
5. Extensions to the Order Object
In order to convey information regarding which certificate requests
represent renewals of previous certificates, a new field is added to
the Order object:
replaces (string, optional):
A string uniquely identifying a previously issued certificate that
this order is intended to replace. This unique identifier is
constructed in the same way as the path component for GET requests
described above.
Clients SHOULD include this field in New Order newOrder requests if there is a
clear predecessor certificate, as is the case for most certificate
renewals. Clients SHOULD NOT include this field if the ACME Server server
has not indicated that it supports this protocol by advertising the
renewalInfo resource in its Directory.
POST /new-order HTTP/1.1
Host: acme.example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://acme.example.com/acct/evOfKhNU60wg",
"nonce": "5XJ1L3lEkMG7tR6pA00clA",
"url": "https://acme.example.com/new-order"
}),
"payload": base64url({
"identifiers": [
{ "type": "dns", "value": "acme.example.com" }
],
"replaces": "aYhba4dGQEHhs3uEe6CuLN4ByNQ.AIdlQyE"
}),
"signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"
}
Servers SHOULD check that the identified certificate and the New
Order newOrder
request correspond to the same ACME Account, that they share at least
one identifier, and that the identified certificate has not already
been marked as replaced by a different Order that is not "invalid".
Correspondence checks beyond this (such as requiring exact identifier
matching) are left up to Server server policy. If any of these checks fail,
the Server server SHOULD reject the new-order newOrder request. If the Server server rejects
the request because the identified certificate has already been
marked as replaced, it MUST return an HTTP 409 (Conflict) with a
problem document of type "alreadyReplaced" (see Section 7.4).
If the Server server accepts a new-order newOrder request with a "replaces" field, it
MUST reflect that field in the response and in subsequent requests
for the corresponding Order object.
This replacement information may serve many purposes, including but
not limited to:
* granting New Order newOrder requests that arrive during the suggested
renewal window of their identified predecessor certificate higher
priority or allowing them to bypass rate limits, if the Server's server's
policy uses such;
* tracking the replacement of certificates that have been affected
by a compliance incident, so that they can be revoked immediately
after they are replaced; and
* tying together certificates issued under the same contract with an
entity identified by External Account Binding.
6. Security Considerations
The extensions to the ACME protocol described in this document build
upon the security considerations and threat model defined in
Section 10.1 of [RFC8555].
This document specifies that renewalInfo RenewalInfo resources are exposed and
accessed via unauthenticated GET requests, a departure from the
requirement in RFC 8555 that clients send POST-as-GET requests to
fetch resources from the server. This is because the information
contained in renewalInfo RenewalInfo resources is not considered confidential and
because allowing renewalInfo RenewalInfo resources to be easily cached is
advantageous to shed the load from clients that do not respect the
Retry-After header. As always, servers should take measures to
ensure that unauthenticated requests for renewal information cannot
result in denial-of-service attacks. These measures might include
ensuring that a cache does not include superfluous request headers or
query parameters in its cache key, instituting IP-based rate limits,
or other general best-practice measures.
Note that this protocol could exhibit undesired behavior in the
presence of significant clock skew between the ACME client and
server. For example, if a server places the suggested renewal window
wholly in the past to encourage a client to renew immediately, a
client with a sufficiently slow clock might nonetheless see the
window as being in the future. Similarly, a server that wishes to
schedule renewals very precisely may have difficulty doing so if some
clients have skewed clocks (or do no implement ARI at all). Server
operators should take this concern into account when setting
suggested renewal windows. However, many other protocols (including
TLS handshakes themselves) fall apart with sufficient clock skew, so
this is not unique to this protocol.
7. IANA Considerations
7.1. ACME Resource Type
IANA has added the following entry to the "ACME Resource Types"
registry within the "Automated Certificate Management Environment
(ACME) Protocol" registry group at <https://www.iana.org/assignments/
acme>:
+=============+=====================+===============+
+=============+====================+===============+
| Field Name | Resource Type | Reference |
+=============+=====================+===============+
+=============+====================+===============+
| renewalInfo | Renewal Info RenewalInfo object | This document |
+-------------+---------------------+---------------+
+-------------+--------------------+---------------+
Table 2
7.2. ACME Renewal Info RenewalInfo Object Fields
IANA has added the following new registry to the "Automated
Certificate Management Environment (ACME) Protocol" registry group at
<https://www.iana.org/assignments/acme>:
Registry Name:
ACME Renewal Info RenewalInfo Object Fields
Registration Procedure:
Specification Required (see [RFC8126]). The designated expert
should ensure that any new fields added to this registry carry
useful and unique information that does not better belong
elsewhere in the ACME protocol.
Template:
Field name: The string to be used as a field name in the JSON
object
Field type: The type of value to be provided, e.g., string,
boolean, array of string
Reference: Where this field is defined
Initial contents:
+=================+============+===============+
| Field Name | Field Type | Reference |
+=================+============+===============+
| suggestedWindow | object | This document |
+-----------------+------------+---------------+
| explanationURL | string | This document |
+-----------------+------------+---------------+
Table 3
7.3. ACME Order Object Fields
IANA has added the following entry to the "ACME Order Object Fields"
registry within the "Automated Certificate Management Environment
(ACME) Protocol" registry group at <https://www.iana.org/assignments/
acme>:
+============+============+==============+===============+
| Field Name | Field Type | Configurable | Reference |
+============+============+==============+===============+
| replaces | string | true | This document |
+------------+------------+--------------+---------------+
Table 4
7.4. ACME Error Types
IANA has added the following entry to the "ACME Error Types" registry
within the "Automated Certificate Management Environment (ACME)
Protocol" registry group at <https://www.iana.org/assignments/acme>:
+=================+==================================+===========+
| Type | Description | Reference |
+=================+==================================+===========+
| alreadyReplaced | The request specified a | This |
| | predecessor certificate that has | document |
| | already been marked as replaced | |
+-----------------+----------------------------------+-----------+
Table 5
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
<https://www.rfc-editor.org/info/rfc3339>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8555] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
Kasten, "Automatic Certificate Management Environment
(ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
<https://www.rfc-editor.org/info/rfc8555>.
[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>.
8.2. Informative References
[RFC3647] Chokhani, S., Ford, W., Sabett, R., Merrill, C., and S.
Wu, "Internet X.509 Public Key Infrastructure Certificate
Policy and Certification Practices Framework", RFC 3647,
DOI 10.17487/RFC3647, November 2003,
<https://www.rfc-editor.org/info/rfc3647>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
Appendix A. Example Certificate
-----BEGIN CERTIFICATE-----
MIIBQzCB66ADAgECAgUAh2VDITAKBggqhkjOPQQDAjAVMRMwEQYDVQQDEwpFeGFt
cGxlIENBMCIYDzAwMDEwMTAxMDAwMDAwWhgPMDAwMTAxMDEwMDAwMDBaMBYxFDAS
BgNVBAMTC2V4YW1wbGUuY29tMFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEeBZu
7cbpAYNXZLbbh8rNIzuOoqOOtmxA1v7cRm//AwyMwWxyHz4zfwmBhcSrf47NUAFf
qzLQ2PPQxdTXREYEnKMjMCEwHwYDVR0jBBgwFoAUaYhba4dGQEHhs3uEe6CuLN4B
yNQwCgYIKoZIzj0EAwIDRwAwRAIge09+S5TZAlw5tgtiVvuERV6cT4mfutXIlwTb
+FYN/8oCIClDsqBklhB9KAelFiYt9+6FDj3z4KGVelYM5MdsO3pK
-----END CERTIFICATE-----
Acknowledgments
My thanks to Roland Shoemaker and Jacob Hoffman-Andrews for coming up
with the initial idea of ARI and for helping me learn the IETF
process. Thanks also to Samantha Frank, Matt Holt, Ilari Liusvaara,
and Wouter Tinus for contributing client implementations, and to
Freddy Zhang for contributing an independent server implementation.
Finally, thanks to Rob Stradling, Andrew Ayer, and J.C. Jones for
providing meaningful feedback and suggestions that significantly
improved this specification.
Author's Address
A. Gable
Internet Security Research Group
Email: aaron@letsencrypt.org