Internet-Draft Using HTTP/3 Stream Limits in HTTP/2 November 2023
Thomson & Pardue Expires 9 May 2024 [Page]
Intended Status:
Standards Track
M. Thomson
L. Pardue

Using HTTP/3 Stream Limits in HTTP/2


A variant mechanism for managing stream limits is described for HTTP/2. This scheme is based on that used in QUIC and is more robust against certain patterns of abuse.

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1. Introduction

HTTP/2 allows endpoints to declare the concurrency limit for streams created by their peer (SETTINGS_MAX_CONCURRENT_STREAMS; see Section 5.1.2 of [HTTP/2]). Initially, the stream concurrency limit is only bounded by the maximum number of streams that can be created, which is 230 for each endpoint, but the value can be changed at any time. Most endpoints set a smaller value in an attempt to protect the resources that are committed when processing a stream.

This limit is not effective in the case that streams are quickly cancelled. The creator of a stream can cancel it using the RST_STREAM frame. The creator of a stream could also cause its peer to send RST_STREAM by purposefully sending frames that violate HTTP/2 rules, unless errors also cause the peer to close the connection. Either of these methods has deterministic and immediate effect, which means that the stream no longer counts against the concurrent stream limit. This means that a malicious endpoint can create or cancel an unbounded number of streams as long as its peer does not set a limit of zero. For clarity, we'll refer to either of these methods as "cancelling".

If the creation of the stream results in the expenditure of resources, rapidly creating and cancelling streams can exhaust resources. For a server, clients that create and cancel many requests can effectively deny service; for a client, reserving and cancelling promised streams might have similar effect if a client does not disable server push using the SETTINGS_ENABLE_PUSH setting. This creates a denial of service exposure for which a remedy is not supported in the protocol.

However, as noted in Section 10.5 of [HTTP/2], many of the features in HTTP/2 that have some potential to create denial of service attacks also have constructive uses. Distinguishing constructive and destructive uses is often challenging. Some applications of HTTP might find that cancelling many requests is necessary; if cancellation is treated as abusive, it is possible that the treatment necessary to prevent attacks might unintentionally punish some clients.

The QUIC protocol [QUIC] on which HTTP/3 [HTTP/3] is built contains an alternative mechanism for limiting concurrency. The scheme in QUIC, as described in Section 4.6 of [QUIC], does not set a concurrent stream limit, but instead relies on a peer increasing the maximum allowed stream identifier. An endpoint cannot create new streams immediately after cancelling an open stream; their peer needs to send a message to make more streams available. In QUIC therefore, a malicious endpoint can only create and cancel a finite number of streams, after which their peer needs to provide consent to continue.

This document ports the QUIC stream concurrency limit mechanisms to HTTP/2. As use of this system is voluntary and therefore not necessarily guaranteed, this does not prevent abusive use of stream cancellation. However, deployment of this mechanism in popular implementations might allow endpoints to deploy more aggressive strategies for managing abuse in its absence.

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.


The MAX_STREAMS frame (type=0xTBD) is used to limit the streams that a recipient is permitted to create.

The format of the MAX_STREAMS frame is illustrated in Figure 1, using the notation defined in Section 1.3 of [QUIC].

  Length (24) = 0x04,
  Type (8) = 0x0TBD,

  Unused Flags (8),

  Reserved (1),
  Stream Identifier (31) = 0,

  Reserved (1),
  Maximum Stream Identifier (31),
Figure 1: MAX_STREAMS Frame Format

In addition to the common HTTP/2 frame header (see Section 4 of [HTTP/2]), the MAX_STREAMS contains a one-bit reserved field and a 31-bit Maxmimum Stream Identifier field.

The Reserved field MUST be set to zero when sending and ignored on receipt.

The Maximum Stream Identifier field contains the maximum value of stream identifier that a recipient of this frame can use when it creates or reserves a stream; see Section 5.1 of [HTTP/2] for how streams are created.

A MAX_STREAMS frame MUST be sent on stream 0. Receipt of a MAX_STREAMS frame on any other stream MUST be treated as a connection error of type PROTOCOL_ERROR; see Section 5.4.1 of [HTTP/2].

An endpoint MUST treat receipt of a MAX_STREAMS frame with a length other than 4 as a connection error of type FRAME_SIZE_ERROR; see Section 5.4.1 of [HTTP/2].

A client MUST treat receipt of an even-numbered Maximum Stream Identifier as a connection error of type PROTOCOL_ERROR; see Section 5.4.1 of [HTTP/2], with an exception of a value of 0, which can be used to indicate support for the feature without enabling the creation of streams (see Section 3.2). Similarly, a server MUST treat receipt of an odd-numbered Maximum Stream Identifier as a connection error.

3.1. Applying Stream Limits

An endpoint that receives a MAX_STREAMS frame MUST NOT create or reserve a stream with a number that exceeds the value of the Maximum Stream Identifier field from that frame, until it receives a MAX_STREAMS frame with a larger value.

An endpoint MUST treat the creation or reservation of a stream with a higher valued stream identifier than it included in a MAX_STREAMS frame as a connection error of type FLOW_CONTROL_ERROR; see Section 5.4.1 of [HTTP/2].

Endpoints can only increase the value that they include in a MAX_STREAMS frame. An endpoint MUST treat receipt of a Maximum Stream Identifier that is equal to or smaller than a value that it has previously received as a connection error of type PROTOCOL_ERROR; see Section 5.4.1 of [HTTP/2]. Note that a value of 0 can be sent one time by either client or server to indicate support for this feature without permitting the creation of streams; see Section 3.2.

An implementation can support a similar concurrency limit to that provided by SETTINGS_MAX_CONCURRENT_STREAMS. The initial value can be set to twice the value of SETTINGS_MAX_CONCURRENT_STREAMS, with a client adding 1 if the value is non-zero. The endpoint then increases the value of the Maximum Stream Identifier field as the streams its peer initiates are closed. An endpoint can avoid sending redundant MAX_STREAMS frames by processing all incoming data before increasing the maximum. This approach differs from SETTINGS_MAX_CONCURRENT_STREAMS in that a peer is not able to create and cancel streams arbitrarily, as new streams do not become available until after receiving a MAX_STREAMS frame.

3.2. Negotiating MAX_STREAMS Usage

This extension is not negotiated using the SETTINGS frame. Instead, the receipt of a MAX_STREAMS frame indicates support for the feature.

An endpoint that supports this extension MUST send a MAX_STREAMS frame after establishing a connection, after the HTTP/2 connection preface (Section 3.4 of [HTTP/2]). When a MAX_STREAMS frame is received, the endpoint MUST subsequently only create streams according to the rules in Section 3.1 and ignore any value of the SETTINGS_MAX_CONCURRENT_STREAMS setting.

4. Security Considerations

HTTP/2 considered the cancellation of a stream to reclaim the resources that might have been committed during its creation. The stream limits in HTTP/2 therefore did not provide a means to limit stream creation. Though cancellation is a potential source of abusive traffic, it was not explicitly mentioned (see Section 10.5 of [HTTP/2]). However, experience has shown that the creation of a stream has costs that cannot always be recovered when that stream is cancelled.

The use of the MAX_STREAMS frame provides endpoints with a means of limiting stream creation, using a method that has proven to be effective in QUIC. However, repeated use of the MAX_STREAMS frame is also a potential source of abuse, as described in Section 10.5 of [HTTP/2]. Though processing a MAX_STREAMS frame is likely to be trivial, receiving significantly more MAX_STREAMS frames than the number of streams that have been closed might indicate an attempt to waste effort.

5. IANA Considerations

This document registers a new entry in the "HTTP/2 Frame Type" registry, as documented in Section 11 of [HTTP/2]. This entry has the following values.

Frame Type:





Section 3

6. References

6.1. Normative References

Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113, DOI 10.17487/RFC9113, , <>.
Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based Multiplexed and Secure Transport", RFC 9000, DOI 10.17487/RFC9000, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.

6.2. Informative References

"HTTP/2 Rapid Reset CVE Record", , <>.
Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114, , <>.


This document is written in response to the problems discovered as part of [CVE-2023-44487], so maybe those behind the botnet responsible for that attack deserve some credit.

Authors' Addresses

Martin Thomson
Lucas Pardue