title: RateLimit Header Fields for HTTP abbrev: docname: draft-ietf-httpapi-ratelimit-headers-latest category: std
ipr: trust200902 area: Applications and Real-Time workgroup: HTTPAPI keyword: Internet-Draft
stand_alone: yes pi: [toc, tocindent, sortrefs, symrefs, strict, compact, comments, inline, docmapping]
ins: R. Polli
name: Roberto Polli
org: Team Digitale, Italian Government
email: robipolli@gmail.com
country: Italy
- ins: A. Martinez name: Alejandro Martinez Ruiz org: Red Hat email: amr@redhat.com
normative: UNIX: title: The Single UNIX Specification, Version 2 - 6 Vol Set for UNIX 98 author: name: The Open Group ins: The Open Group date: 1997-02
informative: RFC6585:
--- abstract
This document defines the RateLimit-Limit, RateLimit-Remaining, RateLimit-Reset fields for HTTP, thus allowing servers to publish current request quotas and clients to shape their request policy and avoid being throttled out.
--- note_Note_to_Readers
RFC EDITOR: please remove this section before publication
Discussion of this draft takes place on the HTTP working group mailing list (httpapi@ietf.org), which is archived at https://lists.w3.org/Archives/Public/ietf-httpapi-wg/.
The source code and issues list for this draft can be found at https://github.com/ietf-wg-httpapi/ratelimit-headers.
--- middle
The widespreading of HTTP as a distributed computation protocol requires an explicit way of communicating service status and usage quotas.
This was partially addressed with the Retry-After
header field
defined in {{!SEMANTICS=I-D.ietf-httpbis-semantics}} to be returned in
429 Too Many Requests
or 503 Service Unavailable
responses.
Still, there is not a standard way to communicate service quotas so that the client can throttle its requests and prevent 4xx or 5xx responses.
Servers use quota mechanisms to avoid systems overload, to ensure an equitable distribution of computational resources or to enforce other policies - eg. monetization.
A basic quota mechanism limits the number of acceptable requests in a given time window, eg. 10 requests per second.
When quota is exceeded, servers usually do not serve the request
replying instead with a 4xx
HTTP status code (eg. 429 or 403)
or adopt more aggressive policies like dropping connections.
Quotas may be enforced on different basis (eg. per user, per IP, per geographic area, ..) and at different levels. For example, an user may be allowed to issue:
- 10 requests per second;
- limited to 60 request per minute;
- limited to 1000 request per hour.
Moreover system metrics, statistics and heuristics can be used to implement more complex policies, where the number of acceptable request and the time window are computed dynamically.
To help clients throttling their requests, servers may expose the counters used to evaluate quota policies via HTTP header fields.
Those response headers may be added by HTTP intermediaries such as API gateways and reverse proxies.
On the web we can find many different rate-limit headers, usually containing the number of allowed requests in a given time window, and when the window is reset.
The common choice is to return three headers containing:
- the maximum number of allowed requests in the time window;
- the number of remaining requests in the current window;
- the time remaining in the current window expressed in seconds or as a timestamp;
A major interoperability issue in throttling is the lack of standard headers, because:
- each implementation associates different semantics to the same header field names;
- header field names proliferates.
Client applications interfacing with different servers may thus need to process different headers, or the very same application interface that sits behind different reverse proxies may reply with different throttling headers.
This proposal defines syntax and semantics for the following fields:
RateLimit-Limit
: containing the requests quota in the time window;RateLimit-Remaining
: containing the remaining requests quota in the current window;RateLimit-Reset
: containing the time remaining in the current window, specified in seconds.
The behavior of RateLimit-Reset
is compatible with the delay-seconds
notation of Retry-After
.
The fields definition allows to describe complex policies, including the ones using multiple and variable time windows and dynamic quotas, or implementing concurrency limits.
The goals of this proposal are:
-
Standardizing the names and semantic of rate-limit headers;
-
Improve resiliency of HTTP infrastructures simplifying the enforcement and the adoption of rate-limit headers;
-
Simplify API documentation avoiding expliciting rate-limit fields semantic in documentation.
The goals do not include:
Authorization: : The rate-limit headers described here are not meant to support authorization or other kinds of access controls.
Throttling scope: : This specification does not cover the throttling scope, that may be the given resource-target, its parent path or the whole Origin (see Section 7 of {{!RFC6454}}).
Response status code: : The rate-limit headers may be returned in both Successful and non Successful responses. This specification does not cover whether non Successful responses count on quota usage.
Throttling policy: : This specification does not mandate a specific throttling policy. The values published in the headers, including the window size, can be statically or dynamically evaluated.
Service Level Agreement: : Conveyed quota hints do not imply any service guarantee. Server is free to throttle respectful clients under certain circumstances.
{::boilerplate bcp14}
This document uses the Augmented BNF defined in {{!RFC5234}} and updated by {{!RFC7405}} along with the "#rule" extension defined in Section 5.6.1 of {{SEMANTICS}}.
The term Origin is to be interpreted as described in Section 7 of {{!RFC6454}}.
The "delay-seconds" rule is defined in Section 10.2.4 of {{SEMANTICS}}.
Rate limit policies limit the number of acceptable requests in a given time window.
A time window is expressed in seconds, using the following syntax:
time-window = delay-seconds
Subsecond precision is not supported.
The request-quota is a value associated to the maximum number of requests
that the server is willing to accept
from one or more clients
on a given basis (originating IP, authenticated user, geographical, ..)
during a time-window
as defined in {{time-window}}.
The request-quota
is expressed in quota-units
and has the following syntax:
request-quota = quota-units
quota-units = 1*DIGIT
The request-quota
SHOULD match the maximum number of acceptable requests.
The request-quota
MAY differ from the total number of acceptable requests
when weight mechanisms, bursts, or other server policies are implemented.
If the request-quota
does not match the maximum number of acceptable requests
the relation with that SHOULD be communicated out-of-band.
Example: A server could
- count once requests like
/books/{id}
- count twice search requests like
/books?author=Camilleri
so that we have the following counters
GET /books/123 ; request-quota=4, remaining: 3, status=200
GET /books?author=Camilleri ; request-quota=4, remaining: 1, status=200
GET /books?author=Eco ; request-quota=4, remaining: 0, status=429
This specification allows describing a quota policy with the following syntax:
quota-policy = request-quota; "w" "=" time-window
*( OWS ";" OWS quota-comment)
quota-comment = token "=" (token / quoted-string)
quota-policy parameters like w
and
quota-comment tokens MUST NOT occur multiple times within the same quota-policy.
An example policy of 100 quota-units per minute.
100;w=60
Two examples of providing further details via custom parameters
in quota-comments
.
100;w=60;comment="fixed window"
12;w=1;burst=1000;policy="leaky bucket"
The following RateLimit
response fields are defined
The RateLimit-Limit
response field indicates
the request-quota
associated to the client
in the current time-window
.
If the client exceeds that limit, it MAY not be served.
The header value is
RateLimit-Limit = expiring-limit [, 1#quota-policy ]
expiring-limit = request-quota
The expiring-limit
value MUST be set to the request-quota
that is closer to reach its limit.
The quota-policy
is defined in {{quota-policy}}, and its values are informative.
RateLimit-Limit: 100
A time-window
associated to expiring-limit
can be communicated
via an optional quota-policy
value, like shown in the following example
RateLimit-Limit: 100, 100;w=10
If the expiring-limit
is not associated to a time-window
, the time-window
MUST either be:
- inferred by the value of
RateLimit-Reset
at the moment of the reset, or - communicated out-of-band (eg. in the documentation).
Policies using multiple quota limits MAY be returned using multiple
quota-policy
items, like shown in the following two examples:
RateLimit-Limit: 10, 10;w=1, 50;w=60, 1000;w=3600, 5000;w=86400
RateLimit-Limit: 10, 10;w=1;burst=1000, 1000;w=3600
This header MUST NOT occur multiple times and can be sent in a trailer section.
The RateLimit-Remaining
response field indicates the remaining quota-units
defined in {{request-quota}}
associated to the client.
The header value is
RateLimit-Remaining = quota-units
This header MUST NOT occur multiple times and can be sent in a trailer section.
Clients MUST NOT assume that a positive RateLimit-Remaining
value is
a guarantee of being served.
A low RateLimit-Remaining
value is like a yellow traffic-light: the red light
may arrive suddenly.
One example of RateLimit-Remaining
use is below.
RateLimit-Remaining: 50
The RateLimit-Reset
response field indicates either
- the number of seconds until the quota resets.
The header value is
RateLimit-Reset = delay-seconds
The delay-seconds format is used because:
- it does not rely on clock synchronization and is resilient to clock adjustment and clock skew between client and server (see Section 5.6.7 of {{SEMANTICS}});
- it mitigates the risk related to thundering herd when too many clients are serviced with the same timestamp.
This header MUST NOT occur multiple times and can be sent in a trailer section.
An example of RateLimit-Reset
use is below.
RateLimit-Reset: 50
The client MUST NOT assume that all its request-quota
will be restored
after the moment referenced by RateLimit-Reset
.
The server MAY arbitrarily alter the RateLimit-Reset
value between subsequent requests
eg. in case of resource saturation or to implement sliding window policies.
A server MAY use one or more RateLimit
response fields
defined in this document to communicate its quota policies.
The returned values refers to the metrics used to evaluate if the current request respects the quota policy and MAY not apply to subsequent requests.
Example: a successful response with the following fields
RateLimit-Limit: 10
RateLimit-Remaining: 1
RateLimit-Reset: 7
does not guarantee that the next request will be successful. Server metrics may be subject to other conditions like the one shown in the example from {{request-quota}}.
A server MAY return RateLimit
response fields independently
of the response status code. This includes throttled responses.
If a response contains both the Retry-After
and the RateLimit-Reset
fields,
the value of RateLimit-Reset
SHOULD reference the same point in time as
Retry-After
.
When using a policy involving more than one time-window
,
the server MUST reply with the RateLimit
headers related to the window
with the lower RateLimit-Remaining
values.
Under certain conditions, a server MAY artificially lower RateLimit
field values between subsequent requests,
eg. to respond to Denial of Service attacks or in case of resource saturation.
Servers usually establish whether the request is in-quota before creating a
response, so the RateLimit field values should be already available in that
moment.
Nonetheless servers MAY decide to send the RateLimit
fields
in a trailer section.
This section documents the considerations advised in Section 16.3.3 of {{SEMANTICS}}.
An intermediary that is not part of the originating service infrastructure and is not aware of the quota-policy semantic used by the Origin Server SHOULD NOT alter the RateLimit fields' values in such a way as to communicate a more permissive quota-policy; this includes removing the RateLimit fields.
An intermediary MAY alter the RateLimit fields in such a way as to communicate a more restrictive quota-policy when:
- it is aware of the quota-unit semantic used by the Origin Server;
- it implements this specification and enforces a quota-policy which is more restrictive than the one conveyed in the fields.
An intermediary SHOULD forward a request even when presuming that it might not be serviced; the service returning the RateLimit fields is the sole responsible of enforcing the communicated quota-policy, and it is always free to service incoming requests.
This specification does not mandate any behavior on intermediaries respect to retries, nor requires that intermediaries have any role in respecting quota-policies. For example, it is legitimate for a proxy to retransmit a request without notifying the client, and thus consuming quota-units.
As is the ordinary case for HTTP caching ({{?RFC7234}}), a response with
RateLimit fields might be cached and re-used for subsequent requests.
A cached RateLimit response, does not modify quota counters but could contain stale information.
Clients interested in determining the freshness of the RateLimit fields could rely on fields such as Date
and on the window
value of a quota-policy
.
A client MUST process the received RateLimit
headers.
A client MUST validate the values received in the RateLimit
headers before using them
and check if there are significant discrepancies
with the expected ones.
This includes a RateLimit-Reset
moment too far in the future or a request-quota
too high.
Malformed RateLimit
headers MAY be ignored.
A client SHOULD NOT exceed the quota-units
expressed in RateLimit-Remaining
before the time-window
expressed
in RateLimit-Reset
.
A client MAY still probe the server if the RateLimit-Reset
is considered too high.
The value of RateLimit-Reset
is generated at response time:
a client aware of a significant network latency MAY behave accordingly
and use other informations (eg. the Date
response header, or otherwise gathered metrics) to better
estimate the RateLimit-Reset
moment intended by the server.
The quota-policy
values and comments provided in RateLimit-Limit
are informative
and MAY be ignored.
If a response contains both the RateLimit-Reset
and Retry-After
fields,
the Retry-After
header field MUST take precedence and
the RateLimit-Reset
field MAY be ignored.
The client exhausted its request-quota for the next 50 seconds.
The time-window
is communicated out-of-band or inferred by the header values.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 100
Ratelimit-Remaining: 0
Ratelimit-Reset: 50
{"hello": "world"}
The server uses two custom headers,
namely acme-RateLimit-DayLimit
and acme-RateLimit-HourLimit
to expose the following policy:
- 5000 daily quota-units;
- 1000 hourly quota-units.
The client consumed 4900 quota-units in the first 14 hours.
Despite the next hourly limit of 1000 quota-units, the closest limit to reach is the daily one.
The server then exposes the RateLimit-*
headers to
inform the client that:
- it has only 100 quota-units left;
- the window will reset in 10 hours.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
acme-RateLimit-DayLimit: 5000
acme-RateLimit-HourLimit: 1000
RateLimit-Limit: 5000
RateLimit-Remaining: 100
RateLimit-Reset: 36000
{"hello": "world"}
Throttling headers may be used to limit concurrency, advertising limits that are lower than the usual ones in case of saturation, thus increasing availability.
The server adopted a basic policy of 100 quota-units
per minute,
and in case of resource exhaustion adapts the returned values
reducing both RateLimit-Limit
and RateLimit-Remaining
.
After 2 seconds the client consumed 40 quota-units
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 100
RateLimit-Remaining: 60
RateLimit-Reset: 58
{"elapsed": 2, "issued": 40}
At the subsequent request - due to resource exhaustion -
the server advertises only RateLimit-Remaining: 20
.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 100
RateLimit-Remaining: 20
RateLimit-Reset: 56
{"elapsed": 4, "issued": 41}
A client exhausted its quota and the server throttles the request
sending the Retry-After
response header field.
In this example, the values of Retry-After
and RateLimit-Reset
reference the same moment,
but this is not a requirement.
The 429 Too Many Requests
HTTP status code is just used as an example.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 429 Too Many Requests
Content-Type: application/json
Date: Mon, 05 Aug 2019 09:27:00 GMT
Retry-After: Mon, 05 Aug 2019 09:27:05 GMT
RateLimit-Reset: 5
RateLimit-Limit: 100
Ratelimit-Remaining: 0
{
"title": "Too Many Requests",
"status": 429,
"detail": "You have exceeded your quota"
}
The client has 99 quota-units
left for the next 50 seconds.
The time-window
is communicated by the w
parameter, so we know the throughput is 100 quota-units
per minute.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 100, 100;w=60
Ratelimit-Remaining: 99
Ratelimit-Reset: 50
{"hello": "world"}
The policy conveyed by RateLimit-Limit
states that
the server accepts 100 quota-units per minute.
To avoid resource exhaustion, the server artificially lowers the actual limits returned in the throttling headers.
The RateLimit-Remaining
then advertises
only 9 quota-units for the next 50 seconds to slow down the client.
Note that the server could have lowered even the other
values in RateLimit-Limit
: this specification
does not mandate any relation between the field values
contained in subsequent responses.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 10, 100;w=60
Ratelimit-Remaining: 9
Ratelimit-Reset: 50
{
"status": 200,
"detail": "Just slow down without waiting."
}
Continuing the previous example, let's say the client waits 10 seconds and
performs a new request which, due to resource exhaustion, the server rejects
and pushes back, advertising RateLimit-Remaining: 0
for the next 20 seconds.
The server advertises a smaller window with a lower limit to slow down the client for the rest of its original window after the 20 seconds elapse.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 429 Too Many Requests
Content-Type: application/json
RateLimit-Limit: 0, 15;w=20
Ratelimit-Remaining: 0
Ratelimit-Reset: 20
{
"status": 429,
"detail": "Wait 20 seconds, then slow down!"
}
Alternatively, given the same context where the previous example starts, we can convey the same information to the client via the Retry-After header, with the advantage that the server can now specify the policy's nominal limit and window that will apply after the reset, ie. assuming the resource exhaustion is likely to be gone by then, so the advertised policy does not need to be adjusted, yet we managed to stop requests for a while and slow down the rest of the current window.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 429 Too Many Requests
Content-Type: application/json
Retry-After: 20
RateLimit-Limit: 15, 100;w=60
Ratelimit-Remaining: 15
Ratelimit-Reset: 40
{
"status": 429,
"detail": "Wait 20 seconds, then slow down!"
}
Note that in this last response the client is expected to honor the
Retry-After
header and perform no requests for the specified amount of
time, whereas the previous example would not force the client to stop
requests before the reset time is elapsed, as it would still be free to
query again the server even if it is likely to have the request rejected.
The server does not expose RateLimit-Remaining
values, but
resets the limit counter every second.
It communicates to the client the limit of 10 quota-units per second
always returning the couple RateLimit-Limit
and RateLimit-Reset
.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 10
Ratelimit-Reset: 1
{"first": "request"}
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 Ok
Content-Type: application/json
RateLimit-Limit: 10
Ratelimit-Reset: 1
{"second": "request"}
This is a standardized way of describing the policy detailed in {{use-with-custom-headers}}:
- 5000 daily quota-units;
- 1000 hourly quota-units.
The client consumed 4900 quota-units in the first 14 hours.
Despite the next hourly limit of 1000 quota-units, the closest limit to reach is the daily one.
The server then exposes the RateLimit
headers to
inform the client that:
- it has only 100 quota-units left;
- the window will reset in 10 hours;
- the
expiring-limit
is 5000.
Request:
GET /items/123 HTTP/1.1
Host: api.example
Response:
HTTP/1.1 200 OK
Content-Type: application/json
RateLimit-Limit: 5000, 1000;w=3600, 5000;w=86400
RateLimit-Remaining: 100
RateLimit-Reset: 36000
{"hello": "world"}
This specification does not prevent clients to make over-quota requests.
Servers should always implement mechanisms to prevent resource exhaustion.
Servers should not disclose operational capacity informations that can be used to saturate its resources.
While this specification does not mandate whether non 2xx responses consume quota, if 401 and 403 responses count on quota a malicious client could probe the endpoint to get traffic informations of another user.
As intermediaries might retransmit requests and consume quota-units without prior knowledge of the User Agent, RateLimit headers might reveal the existence of an intermediary to the User Agent.
RateLimit-*
headers convey hints from the server
to the clients in order to avoid being throttled out.
Clients MUST NOT consider the quota-units
returned in RateLimit-Remaining
as a service level agreement.
In case of resource saturation, the server MAY artificially lower the returned values or not serve the request anyway.
Consider that request-quota
may not be restored after the moment referenced by RateLimit-Reset
,
and the RateLimit-Reset
value should not be considered fixed nor constant.
Subsequent requests may return an higher RateLimit-Reset
value to limit
concurrency or implement dynamic or adaptive throttling policies.
When returning RateLimit-Reset
you must be aware that many throttled
clients may come back at the very moment specified.
This is true for Retry-After
too.
For example, if the quota resets every day at 18:00:00
and your server returns the RateLimit-Reset
accordingly
Date: Tue, 15 Nov 1994 08:00:00 GMT
RateLimit-Reset: 36000
there's a high probability that all clients will show up at 18:00:00
.
This could be mitigated adding some jitter to the field-value.
RateLimit
fields may assume unexpected values by chance or purpose.
For example, an excessively high RateLimit-Remaining
value may be:
- used by a malicious intermediary to trigger a Denial of Service attack or consume client resources boosting its requests;
- passed by a misconfigured server;
or an high RateLimit-Reset
value could inhibit clients to contact
the server.
Clients MUST validate the received values to mitigate those risks.
This section registers the RateLimit-Limit
field in the
"Hypertext Transfer Protocol (HTTP) Field Name Registry" registry ({{SEMANTICS}}).
Field name: RateLimit-Limit
Status: permanent
Specification document(s): {{ratelimit-limit-header}} of this document
This section registers the RateLimit-Remaining
field in the
"Hypertext Transfer Protocol (HTTP) Field Name Registry" registry ({{SEMANTICS}}).
Field name: RateLimit-Remaining
Status: permanent
Specification document(s): {{ratelimit-remaining-header}} of this document
This section registers the RateLimit-Reset
field in the
"Hypertext Transfer Protocol (HTTP) Field Name Registry" registry ({{SEMANTICS}}).
Field name: RateLimit-Reset
Status: permanent
Specification document(s): {{ratelimit-reset-header}} of this document
--- back
RFC EDITOR PLEASE DELETE THIS SECTION.
Thanks to Willi Schoenborn, Alejandro Martinez Ruiz, Alessandro Ranellucci, Amos Jeffries, Martin Thomson, Erik Wilde and Mark Nottingham for being the initial contributors of these specifications. Kudos to the first community implementors: Aapo Talvensaari, Nathan Friedly and Sanyam Dogra.
RFC EDITOR PLEASE DELETE THIS SECTION.
Commonly used header field names are:
X-RateLimit-Limit
,X-RateLimit-Remaining
,X-RateLimit-Reset
;X-Rate-Limit-Limit
,X-Rate-Limit-Remaining
,X-Rate-Limit-Reset
.
There are variants too, where the window is specified in the header field name, eg:
x-ratelimit-limit-minute
,x-ratelimit-limit-hour
,x-ratelimit-limit-day
x-ratelimit-remaining-minute
,x-ratelimit-remaining-hour
,x-ratelimit-remaining-day
Here are some interoperability issues:
-
X-RateLimit-Remaining
references different values, depending on the implementation:- seconds remaining to the window expiration
- milliseconds remaining to the window expiration
- seconds since UTC, in UNIX Timestamp
- a datetime, either
IMF-fixdate
{{SEMANTICS}} or {{?RFC3339}}
-
different headers, with the same semantic, are used by different implementers:
- X-RateLimit-Limit and X-Rate-Limit-Limit
- X-RateLimit-Remaining and X-Rate-Limit-Remaining
- X-RateLimit-Reset and X-Rate-Limit-Reset
The semantic of RateLimit-Remaining depends on the windowing algorithm. A sliding window policy for example may result in having a ratelimit-remaining value related to the ratio between the current and the maximum throughput. Eg.
RateLimit-Limit: 12, 12;w=1
RateLimit-Remaining: 6 ; using 50% of throughput, that is 6 units/s
RateLimit-Reset: 1
If this is the case, the optimal solution is to achieve
RateLimit-Limit: 12, 12;w=1
RateLimit-Remaining: 1 ; using 100% of throughput, that is 12 units/s
RateLimit-Reset: 1
At this point you should stop increasing your request rate.
-
Why defining standard headers for throttling?
To simplify enforcement of throttling policies.
-
Can I use RateLimit-* in throttled responses (eg with status code 429)?
Yes, you can.
-
Are those specs tied to RFC 6585?
No. {{?RFC6585}} defines the
429
status code and we use it just as an example of a throttled request, that could instead use even 403 or whatever status code. -
Why don't pass the throttling scope as a parameter?
After a discussion on a similar thread we will probably add a new "RateLimit-Scope" header to this spec.
I'm open to suggestions: comment on this issue
-
Why using delay-seconds instead of a UNIX Timestamp? Why not using subsecond precision?
Using delay-seconds aligns with
Retry-After
, which is returned in similar contexts, eg on 429 responses.Timestamps require a clock synchronization protocol (see Section 5.6.7 of {{SEMANTICS}}). This may be problematic (eg. clock adjustment, clock skew, failure of hardcoded clock synchronization servers, IoT devices, ..). Moreover timestamps may not be monotonically increasing due to clock adjustment. See Another NTP client failure story
We did not use subsecond precision because:
- that is more subject to system clock correction like the one implemented via the adjtimex() Linux system call;
- response-time latency may not make it worth. A brief discussion on the subject is on the httpwg ml
- almost all rate-limit headers implementations do not use it.
-
Why not support multiple quota remaining?
While this might be of some value, my experience suggests that overly-complex quota implementations results in lower effectiveness of this policy. This spec allows the client to easily focusing on RateLimit-Remaining and RateLimit-Reset.
-
Shouldn't I limit concurrency instead of request rate?
You can use this specification to limit concurrency at the HTTP level (see {#use-for-limiting-concurrency}) and help clients to shape their requests avoiding being throttled out.
A problematic way to limit concurrency is connection dropping, especially when connections are multiplexed (eg. HTTP/2) because this results in unserviced client requests, which is something we want to avoid.
A semantic way to limit concurrency is to return 503 + Retry-After in case of resource saturation (eg. thrashing, connection queues too long, Service Level Objectives not meet, ..). Saturation conditions can be either dynamic or static: all this is out of the scope for the current document.
-
Do a positive value of
RateLimit-Remaining
imply any service guarantee for my future requests to be served?No. The returned values were used to decide whether to serve or not the current request and do not imply any guarantee that future requests will be successful.
Instead they help to understand when future requests will probably be throttled. A low value for
RateLimit-Remaining
should be interpreted as a yellow traffic-light for either the number of requests issued in thetime-window
or the request throughput. -
Is the quota-policy definition {{quota-policy}} too complex?
You can always return the simplest form of the 3 headers
RateLimit-Limit: 100
RateLimit-Remaining: 50
RateLimit-Reset: 60
The key runtime value is the first element of the list: expiring-limit
, the others quota-policy
are informative.
So for the following header:
RateLimit-Limit: 100, 100;w=60;burst=1000;comment="sliding window", 5000;w=3600;burst=0;comment="fixed window"
the key value is the one referencing the lowest limit: 100
- Can we use shorter names? Why don't put everything in one header?
The most common syntax we found on the web is X-RateLimit-*
and
when starting this I-D we opted for it
The basic form of those headers is easily parseable, even by implementors procesing responses using technologies like dynamic interpreter with limited syntax.
Using a single header complicates parsing and takes a significantly different approach from the existing ones: this can limit adoption.
-
Why don't mention connections?
Beware of the term "connection":   - it is just one possible saturation cause. Once you go that path  you will expose other infrastructural details (bandwidth, CPU, .. see {{sec-information-disclosure}})  and complicate client compliance;  - it is an infrastructural detail defined in terms of server and network  rather than the consumed service. This specification protects the services first, and then the infrastructures through client cooperation (see {{sec-throttling-does-not-prevent}}).   RateLimit headers enable sending on the same connection different limit values  on each response, depending on the policy scope (eg. per-user, per-custom-key, ..) 
-
Can intermediaries alter RateLimit fields?
Generally, they should not because it might result in unserviced requests. There are reasonable use cases for intermediaries mangling RateLimit fields though, e.g. when they enforce stricter quota-policies, or when they are an active component of the service. In those case we will consider them as part of the originating infrastructure.
{:numbered="false"}
RFC Editor: Please remove this section before publication.
{:numbered="false"}
- Use I-D.httpbis-semantics, which includes referencing
delay-seconds
instead ofdelta-seconds
. #5