Oblivious Relay Feedback

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Table of Contents

1. Introduction

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 Resource via the RateLimit fields defined in [RATELIMIT]. This is useful when such servers identify traffic anomalies or unexpected request volumes. An Oblivious Relay Resource 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 Resource (rather than to the client).¶

How an Oblivious Relay Resource can use this information to avoid being throttled out or shape its request policy is outside the scope of this specification.¶

The proposed mechanism does not address on purpose the attack of an offending client attacking the server (e.g., the client is using an abnormal header that matches an attack pattern) because the application of the rate-limit can potentially allow the target to take advantage of the differential treatment applied by the relay to de-anonymize the client.¶

2. Terminology

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 Section 2.2 of [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].¶

3. Providing RateLimit Information to an Oblivious Proxy

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 Resource. 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 Resource MUST:¶

1. Identify the quota policy associated with the expiring limit.¶
2. Check whether the "ohttp-target" parameter is present and its syntax is correct.¶

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 Resource.¶

   RateLimit-Limit: 100
   RateLimit-Policy: 10;w=1, 100;w=60;ohttp-target
   RateLimit-Remaining: 8
   RateLimit-Reset: 15

4. The ohttp-target Quota Policy Parameter

5. The 'attack-severity' Quota Policy Parameter

The following quota policy parameter is defined for the RateLimit-Policy field defined in [RATELIMIT]:¶

attack-severity:

Is used by the Oblivious Gateway Resource or Target Resource to convey the severity of the attack. This parameter is OPTIONAL.¶

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 severity of the attack may be used as an operational alert to the Oblivious Relay Resource's security operation team and to automatically trigger the rate-limit mitigation action accordingly. Irrespective of the security level at which a rate-limit is triggered, the relay must follow the rules discussed in Section 4.1 to offer privacy to the clients it is serving. The severity of the attack can also be used to identify a "ramp-up" attack versus a "blast" attack.¶

6. Use of The ohttp-target Quota Policy Parameters: An Example

The example depicted in Figure 2 illustrates the use of the "ohttp-target" parameter. An oblivious target resource receives unexpected request volumes and uses the source IP address to identify that these were Encapsulated Requests decapsulated by an oblivious gateway resource. The Oblivious target resource adds the RateLimit fields along with the "ohttp-target" quota policy parameter to the HTTP response. The oblivious gateway resource proceeds as follows:¶

1. Copy the RateLimit fields from the original response.¶
2. Remove them from the original response before encapsulating it.¶
3. Generate a single 200 response containing the encapsulated response for the Oblivious Relay Resource along with the copied RateLimit fields.¶

+----+            +----------+       +----------+    +----------+
| C  |            | Relay    |       | Gateway  |    | Target   |
|    |            | Resource |       | Resource |    | Resource |
+-+--+            +----+-----+       +-----+----+    +-----+----+
  |                    |                   |               |
  | Encapsulated       |                   |               |
  +------------------->|                   |               |
  |  Request           |                   |               |
  |                    | Encapsulated      |               |
  |                    +------------------>|               |
  |                    |  Request          |               |
  |                    |                   | Request       | .----------------.
  |                    |                   +-------------->| | Identify       |
  |                    |                   |               +-+ unexpected     |
  |                    |                   |               | | request volumes|
  |                    |                   |  2xx response | '----------------'
  |                    |                   |<--------------+
  |                    |                   |               |
  |                    | 200 response with |               |
  |                    | RateLimit-Limit,  |               |
  |                    | RateLimit-Policy  |               |
  |                    | fields and the    |               |
  |                    | ohttp-target      |               |
  |                    | parameter         |               |
                       |<------------------+               |
.--------------------. | Encapsulated 2xx  |               |
| Process            | |    response       |               |
| ohttp-target       +-+                   |               |
| and rate-limit     |  |                  |               |
| requests from      |  |                  |               |
| clients            |  |                  |               |
'--------------------'  |                  |               |
  |                     |                  |               |
  |                     |                  |               |
  | Encapsulated 2xx    |                  |               |
  |<--------------------+                  |               |
  |     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;attack-severity="high";\
comment="Bandwidth Limit Exceeded"
  Content-Type: message/ohttp-res
  Content-Length: 38 <content is the encapsulated 400 response>
  ...encrypted content...

7. Ohttp-Outside-Encap Header

The "Ohttp-Outside-Encap" HTTP header field is defined in this specification (Section 9.2.1). Its purpose is to signal which HTTP headers will be removed by the Oblivious gateway. It is intended only for use in requests to an Oblivious target.¶

When an Oblivious gateway resource sends an HTTP request to an Oblivious target, 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 those header fields and values, and it then removes them 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 Resource 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. The syntax of this header field conforms to [RFC8941]. It is a List ([RFC8941], Section 3.1):¶

  Ohai-Outside-Encap =  sf-list

An example is illustrated below:¶

Ohttp-Outside-Encap: RateLimit-Limit,RateLimit-Remaining,RateLimit-Reset,RateLimit-Policy

8. Security Considerations

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.¶

9. IANA Considerations

+=================+=================+================+===============+
| 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  |
+-----------------+-----------------+----------------+---------------+

Authors' Addresses