| Internet-Draft | Oblivious Relay Feedback | July 2022 |
| Reddy, et al. | Expires 11 January 2023 | [Page] |
To provide equitable service to clients, servers often rate-limit incoming requests, for example, based upon the source IP address. However, oblivious HTTP removes the ability for the server to distinguish amongst clients so the server can only rate-limit traffic from the oblivious relay. This harms all clients behind that oblivious relay.¶
This specification enables a server to convey rate-limit information to an oblivious relay, which can use it to apply rate-limit policies on clients. Cooperating oblivious relays can thus provide more equitable service to their distinguishable clients without impacting on all clients behind that oblivious relay.¶
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Oblivious HTTP [OHTTP] requires three parties to exchange HTTP messages: the client, the relay, and the target (formally, the Oblivious Gateway Resource and Oblivious Target Resource). Oblivious HTTP enables a client to send requests to a target in such a way that the target cannot tell whether two requests came from the same client, and the relay cannot see the contents of the requests.¶
Since clients are located behind a relay, a target cannot distinguish between well-behaving and malicious clients: an unexpected behavior from one or more clients can then impact on all the intermediated clients, as described in Section 8.2.1 of [OHTTP]. This can be problematic when the target implements rate limiting policies based on an information masked by the intermediary, such as the source IP address.¶
This document defines a mechanism that allows Oblivious gateway and target resource to provide rate-limit information to an Oblivious relay via the RateLimit fields defined in [RATELIMIT]. This is useful when such servers identify traffic anomalies or unexpected request volumes. The Oblivious relay can then use this information to apply rate-limit policies on clients.¶
While [RATELIMIT] provides enough information to generic clients to shape their request policy and avoid being throttled out, this specification allows an Oblivious gateway and target resource to indicate their RateLimit information is intended for the Oblivious relay (rather than to the client).¶
How an Oblivious relay can use this information to avoid being throttled out or shape its request policy is outside the scope of this specification.¶
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.¶
The terms "content", "receiver", "request", and "response" are to be interpreted as described in [HTTP].¶
The terms "Encapsulated request", "Encapsulated response", "Oblivious relay resource", "Oblivious gateway resource", "Oblivious target resource", and "Client" are to be interpreted as described in [OHTTP].¶
The collective term "Oblivious resource" indicates either an "Oblivious gateway resource" or an "Oblivious target resource".¶
The terms "quota policy", "service limit", "expiring limit", and "RateLimit fields" are to be interpreted as described in [RATELIMIT].¶
This document uses the Integer type from [STRUCTURED-FIELDS].¶
An Oblivious resource that uses RateLimit fields [RATELIMIT] to return service limit information MAY add the "ohttp-target" quota policy parameter defined in Section 4 to signal to the receiver that the associated quota policy is intended for an Oblivious relay. For example, when an Oblivious target identifies a high frequency or high volume anomalies in the HTTP requests it would include the "ohttp-target" parameter.¶
The term "Oblivious Relay Feedback" denotes both the mechanism described in this specification and the complete set of RateLimit fields together with the "ohttp-target" parameter.¶
To know whether the RateLimit fields provides Oblivious Relay Feedback (see Section 3.1), an Oblivious relay MUST:¶
In the example shown in Figure 1, the expiring limit value is "100", so the associated quota policy is the second one. This quota policy includes the "ohttp-target" parameter: this indicates that the RateLimit fields are intended for an Oblivious relay.¶
RateLimit-Limit: 100 RateLimit-Policy: 10;w=1, 100;w=60;ohttp-target=1 RateLimit-Remaining: 8 RateLimit-Reset: 15
The following quota policy parameter is defined for the RateLimit-Policy field [RATELIMIT]:¶
The "ohttp-target" parameter has the following syntax:¶
ohttp-target = sf-integer¶
Its value MUST be an Integer (Section 3.3.1 of [STRUCTURED-FIELDS]) and indicates whether the quota policy is applicable to all the clients that are serviced by the Oblivious relay or applicable only to a specific client. The "ohttp-target" parameter MUST have one of the following values:¶
Indicates that RateLimit fields are applicable only to the offending client. For example, this value is used if the client is attacking the server (e.g., the client is using an abnormal header that matches an attack pattern).¶
The Oblivious relay does not immediately act to rate-limit the traffic from the client but starts maintaining a count of responses to the client with "ohttp-target" parameter value set to "2" marked as "potential malicious requests" and responses without the parameter marked as "legitimate requests".¶
The Oblivious relay can rate-limit requests from the offending client for a certain duration only when the client has a high ratio of "potential malicious requests" to "legitimate requests". In other words, the Oblivious relay will rate-limit requests from a client if the target has seen an attack pattern in multiple requests from that same client. A malicious client sends malformed HTTP requests, whereas a benign client sends valid HTTP requests. The malformed HTTP requests are linkable whereas the valid HTTP requests are not linkable. Most importantly, the target will not be able to partition the anonymity set of legitimate clients.¶
Other values MUST cause the parameter to be ignored.¶
The "ohttp-target" parameter MUST NOT appear more than once in a quota policy. If the parameter is malformed or its value is invalid, it MUST be ignored, and the receiving Oblivious relay MUST NOT attempt to fix neither the parameter nor its value. That is, the RateLimit fields must not be considered as providing Oblivious Relay Feedback.¶
An Oblivious relay receiving RateLimit fields providing Oblivious Relay Feedback will do the following:¶
An Oblivious gateway resource receiving RateLimit fields providing Oblivious Relay Feedback MUST proceed as follows:¶
If the RateLimit fields along with the "ohttp-target" parameter are generated by the Oblivious gateway resource before removing the protection (including being unable to remove the encapsulation for any reason)(Section 6.2 of [OHTTP]), it will result in the RateLimit fields added in the response being sent without protection in response to a POST request from a client.¶
While this specification does not mandate specific traffic shaping actions for Oblivious proxies in addition to the ones indicated in [RATELIMIT], an Oblivious relay failing to reshape traffic from a specific client or from all the clients according to the received Oblivious Relay Feedback can experience different levels of service denial by the Oblivious gateway and target resources. There is no explicit mechanism for an Oblivious relay to indicate to the server that the rate-limit information was processed or was ignored.¶
The following quota policy parameter is defined for the RateLimit-Policy field defined in [RATELIMIT]:¶
attack-severity = sf-string¶
Note that sf-string is defined in Section 3.3.3 of [STRUCTURED-FIELDS].¶
The value of the "attack-severity" parameter is a String (Section 3.3.3 of [RFC8941]) that takes one of the values defined in [SEVERITY]. This parameter MUST NOT appear more than once in a quota policy. If the parameter is malformed or its value is invalid, the parameter MUST be ignored, and the relays MUST NOT attempt to fix neither the parameter nor the value.¶
The example depicted in Figure 2 illustrates the use of the "ohttp-target" parameter. An oblivious target resource receives a malformed request and uses the source IP address to identify that it was an encapsulated request decapsulated by an oblivious gateway resource. The Oblivious target resource generates a 400 response and adds the RateLimit fields along with the "ohttp-target" quota policy parameter. The oblivious gateway resource proceeds as follows:¶
+----+ +----------+ +----------+ +----------+
| C | | Relay | | Gateway | | Target |
| | | Resource | | Resource | | Resource |
+-+--+ +----+-----+ +-----+----+ +-----+----+
| | | |
| Encapsulated | | |
+------------------->| | |
| Request | | |
| | Encapsulated | |
| +------------------>| |
| | Request | |
| | | Request | .---------.
| | +-------------->| | Identify|
| | | +-+malformed|
| | | | | request |
| | | 400 response | '---------'
| | |<--------------+
| | | |
| | 200 response with | |
| | RateLimit-Limit, | |
| | RateLimit-Policy | |
| | fields and the | |
| | ohttp-target | |
| | parameter | |
|<------------------+ |
.--------------------. | Encapsulated 400 | |
| Process | | response | |
| ohttp-target +-+ | |
| and rate-limit | | | |
| requests from the | | | |
| offending client | | | |
'--------------------' | | |
| | |
| | | |
| Encapsulated 400 | | |
|<--------------------+ | |
| response | | |
| | | |
The response that is generated by the Oblivious gateway resource is depicted in Figure 3. This response includes an unregistered, informative "comment" quota policy parameter providing the rationale for the "attack- severity".¶
=============== NOTE: '\' line wrapping per RFC 8792 ================ HTTP/1.1 200 OK Date: Wed, 27 March 2022 04:45:07 GMT Cache-Control: private, no-store RateLimit-Limit: 10 RateLimit-Policy: 10;ohttp-target=2;attack-severity="high";\ comment="abnormal header matching a WAF rule" Content-Type: message/ohttp-res Content-Length: 38 <content is the encapsulated 400 response> ...encrypted content...
The "Ohttp-Outside-Encap" header is defined in this specification (Section 9.2.1). Its purpose is to signal which HTTP headers will be removed by the Oblivious gateway.¶
When an Oblivious gateway resource sends an HTTP request to an Oblivious taget, it adds the "Ohttp-Outside-Encap" header to indicate which headers will be removed from the response.¶
On receipt of an HTTP response from the Oblivious target resource, the Oblivious gateway resource copies the header fields signaled in the associated request and removes those headers from the HTTP response. The Oblivious gateway then encapsulates the HTTP response. The Oblivious gateway resource adds the copied header fields and values to the response containing the encapsulated response, so that the Oblivious relay can access and act on them.¶
The "Ohttp-Outside-Encap" header is useful in deployments where the Oblivious gateway resource and Oblivious target resource are managed by separate entities.¶
Figure 4 describes the syntax using Augmented Backus-Naur Form (ABNF) of the header field, using the grammar defined in [RFC5234] and the rules defined in Section 5 of [RFC9110]. The field values of the header field conform to the same rules.¶
Ohai-Outside-Encap = header-field *( OWS "|" OWS header-field) header-field = token
Optional white space (OWS) is used as defined in Section 5.6.3 of [RFC9110].¶
An example is illustrated below:¶
Ohttp-Outside-Encap: RateLimit-Limit|RateLimit-Remaining|RateLimit-Reset|RateLimit-Policy¶
The security considerations for the Oblivious HTTP protocol (Section 8 of [OHTTP]) as well as the ones for RateLimit fields (Section 6 of [RATELIMIT]) apply. The following sub-sections discuss security considerations specific to this specification.¶
While Oblivious HTTP relies upon an Oblivious relay to prevent leaking the client identity to the Oblivious resources, it might be the case that the Oblivious relay colludes with clients in attacking Oblivious resources. RateLimit fields might disclose operational capacity information useful to design denial of service attacks or to circumvent defensive measures put in place by the Oblivious resources (Section 6.2 of [RATELIMIT]). The Oblivious target and gateway resources SHOULD convey Oblivious Relay Feedback only to trusted Oblivious proxies.¶
Attacks against the Oblivious Gateway and Target Resources can be classified into three primary categories:¶
This specification requests IANA to add the following parameters to the "Hypertext Transfer Protocol (HTTP) RateLimit Parameters" registry defined in [RATELIMIT].¶
+=================+=================+================+===============+ | Field Name |Parameter Name |Description |Specification | +=================+=================+================+===============+ | RateLimit-Policy|ohttp-target |ohttp ratelimit |Section 3 of | | | | |this document | | RateLimit-Policy|attack-severity |ohttp ratelimit |Section 5 of | | | | |this document | +-----------------+-----------------+----------------+---------------+¶
This section describes a header field for registration in the Permanent Message Header Field Registry [RFC3864].¶
Thanks to Lucas Pardue, Rich Salz, Martin Thomson, Christopher A. Wood and Brandon Williams for the discussion and comments.¶