Using Early Data in HTTP

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TLS 1.3 [TLS13] introduces the concept of early data (also known as zero round-trip time (0-RTT) data). If the client has spoken to the same server recently, early data allows a client to send data to a server in the first round trip of a connection, without waiting for the TLS handshake to complete.

When used with HTTP [HTTP], early data allows clients to send requests immediately, thus avoiding the one or two round-trip delays needed for the TLS handshake. This is a significant performance enhancement; however, it has significant limitations.

The primary risk of using early data is that an attacker might capture and replay the request(s) it contains. TLS [TLS13] describes techniques that can be used to reduce the likelihood that an attacker can successfully replay a request, but these techniques can be difficult to deploy and still leave some possibility of a successful attack.

Note that this is different from automated or user-initiated retries; replays are initiated by an attacker without the awareness of the client.

To help mitigate the risk of replays in HTTP, this document gives an overview of techniques for controlling these risks in servers and defines requirements for clients when sending requests in early data.

The advice in this document also applies to use of 0-RTT in HTTP over QUIC [HQ].

Conceptually, early data is concatenated with other application data to form a single stream. This can mean that requests are entirely contained within early data or that only part of a request is early. In a multiplexed protocol, like HTTP/2 [RFC7540] or HTTP/QUIC [HQ], multiple requests might be partially delivered in early data.

The model that this document assumes is that once the TLS handshake completes, the early data received on that TLS connection is known to not be a replayed copy of that data. However, it is important to note that this does not mean that early data will not be or has not been replayed on another connection.

A server decides whether or not to offer a client the ability to send early data on future connections when sending the TLS session ticket.

TLS [TLS13] mandates the use of replay detection strategies that reduce the ability of an attacker to successfully replay early data. These anti-replay techniques reduce but don’t completely eliminate the chance of data being replayed and ensure a fixed upper limit to the number of replays.

1. The server can reject early data at the TLS layer. A server cannot selectively reject early data, so this results in all requests sent in early data being discarded.
2. The server can choose to delay processing of early data until after the TLS handshake completes. By deferring processing, it can ensure that only a successfully completed connection is used for the request(s) therein. This provides the server with some assurance that the early data was not replayed. If the server receives multiple requests in early data, it can determine whether to defer HTTP processing on a per-request basis.
3. The server can cause a client to retry individual requests and not use early data by responding with the 425 (Too Early) status code (Section 5.2) in cases where the risk of replay is judged too great.

All of these techniques are equally effective; a server can use the method that best suits it.

For a given request, the level of tolerance to replay risk is specific to the resource it operates upon (and therefore only known to the origin server). The primary risk associated with using early data is in the actions a server takes when processing a request; processing a duplicated request might result in duplicated effects and side effects. Appendix E.5 of [TLS13] also describes other effects produced by processing duplicated requests.

The request method’s safety ([RFC7231], Section 4.2.1) is one way to determine this. However, some resources produce side effects with safe methods, so this cannot be universally relied upon.

It is RECOMMENDED that origin servers allow resources to explicitly configure whether early data is appropriate in requests. Absent such explicit information, origin servers MUST either reject early data or implement the techniques described in this document for ensuring that requests are not processed prior to TLS handshake completion.

A request might be sent partially in early data with the remainder of the request being sent after the handshake completes. This does not necessarily affect handling of that request; what matters is when the server starts acting upon the contents of a request. Any time any server instance might initiate processing prior to completion of the handshake, all server instances need to account for the possibility of replay of early data and how that could affect that processing (see also Section 6.2).

A server can partially process requests that are incomplete. Parsing header fields -- without acting on the values -- and determining request routing is likely to be safe from side effects but other actions might not be.

Intermediary servers do not have sufficient information to decide whether early data can be processed, so Section 5.2 describes a way for the origin to signal to them that a particular request isn’t appropriate for early data. Intermediaries that accept early data MUST implement that mechanism.

Note that a server cannot choose to selectively reject early data at the TLS layer. TLS only permits a server to either accept all early data or none of it. Once a server has decided to accept early data, it MUST process all requests in early data, even if the server rejects the request by sending a 425 (Too Early) response.

A server can limit the amount of early data with the max_early_data_size field of the early_data TLS extension. This can be used to avoid committing an arbitrary amount of memory for requests that it might defer until the handshake completes.

A client that wishes to use early data commences by sending HTTP requests immediately after sending the TLS ClientHello.

By their nature, clients have control over whether a given request is sent in early data, thereby giving the client control over risk of replay. Absent other information, clients MAY send requests with safe HTTP methods ([RFC7231], Section 4.2.1) in early data when it is available and MUST NOT send unsafe methods (or methods whose safety is not known) in early data.

If the server rejects early data at the TLS layer, a client MUST start sending again as though the connection were new. This could entail using a different negotiated protocol [ALPN] than the one optimistically used for the early data. Any requests sent in early data will need to be sent again, unless the client decides to abandon those requests.

Automatic retry creates the potential for a replay attack. An attacker intercepts a connection that uses early data and copies the early data to another server instance. The second server instance accepts and processes the early data, even though it will not complete the TLS handshake. The attacker then allows the original connection to complete. Even if the early data is detected as a duplicate and rejected, the first server instance might allow the connection to complete. If the client then retries requests that were sent in early data, the request will be processed twice.

Replays are also possible if there are multiple server instances that will accept early data or if the same server accepts early data multiple times (though the latter would be in violation of requirements in Section 8 of [TLS13]).

Clients that use early data MUST retry requests upon receipt of a 425 (Too Early) status code; see Section 5.2.

An intermediary MUST NOT use early data when forwarding a request unless early data was used on a previous hop, or it knows that the request can be retried safely without consequences (typically, using out-of-band configuration). Absent better information, that means that an intermediary can only use early data if the request either arrived in early data or arrived with the Early-Data header field set to “1” (see Section 5.1).

Because HTTP requests can span multiple “hops”, it is necessary to explicitly communicate whether a request has been sent in early data on a previous hop. Likewise, it is necessary to have some means of explicitly triggering a retry when early data is not desired. Finally, it is necessary to know whether the client will actually perform such a retry.

• The Early-Data header field is included in requests that might have been forwarded by an intermediary prior to the completion of the TLS handshake with its client.
• The 425 (Too Early) status code is defined for a server to indicate that a request could not be processed due to the consequences of a possible replay attack.

They are designed to enable better coordination of the use of early data between the user agent and origin server, and also when a gateway (also “reverse proxy”, “Content Delivery Network”, or “surrogate”) is present.

Gateways typically don’t have specific information about whether a given request can be processed safely when it is sent in early data. In many cases, only the origin server has the necessary information to decide whether the risk of replay is acceptable. These extensions allow coordination between a gateway and its origin server.

Using early data exposes a client to the risk that their request is replayed. A retried or replayed request can produce different side effects on the server. In addition to those side effects, replays and retries might be used for traffic analysis to recover information about requests or the resources those requests target. In particular, a request that is replayed might result in a different response, which might be observable from the length of protected data even if the content remains confidential.

This document registers the Early-Data header field in the “Permanent Message Header Field Names” registry located at <https://www.iana.org/assignments/message-headers>.

Header field name:

Early-Data

Applicable protocol:

http

Status:

standard

Author/Change controller:

IETF

Specification document(s):

This document

Related information:

(empty)

This document registers the 425 (Too Early) status code in the “HTTP Status Codes” registry located at <https://www.iana.org/assignments/http-status-codes>.

Value:

425

Description:

Too Early

Reference:

This document

This document was not easy to produce. The following people made substantial contributions to the quality and completeness of the document: David Benjamin, Subodh Iyengar, Benjamin Kaduk, Ilari Liusavaara, Kazuho Oku, Eric Rescorla, Kyle Rose, and Victor Vasiliev.