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title: The ORIGIN HTTP/2 Frame abbrev: ORIGIN Frames docname: draft-ietf-httpbis-origin-frame-latest date: 2017 category: std

ipr: trust200902 area: General workgroup: HTTP keyword: Internet-Draft

stand_alone: yes pi: [toc, tocindent, sortrefs, symrefs, strict, compact, comments, inline]

author:

ins: M. Nottingham
name: Mark Nottingham
organization: Akamai
email: mnot@mnot.net
uri: https://www.mnot.net/

normative: RFC2119:

informative:

--- abstract

This document specifies the ORIGIN frame for HTTP/2, to indicate what origins are available on a given connection.

--- note_Note_to_Readers

Discussion of this draft takes place on the HTTP working group mailing list (ietf-http-wg@w3.org), which is archived at https://lists.w3.org/Archives/Public/ietf-http-wg/.

Working Group information can be found at http://httpwg.github.io/; source code and issues list for this draft can be found at origin-frame .

--- middle

Introduction

HTTP/2 {{!RFC7540}} allows clients to coalesce different origins {{!RFC6454}} onto the same connection when certain conditions are met. However, in certain cases, a connection is is not usable for a coalesced origin, so the 421 (Misdirected Request) status code ({{?RFC7540}}, Section 9.1.2) was defined.

Using a status code in this manner allows clients to recover from misdirected requests, but at the penalty of adding latency. To address that, this specification defines a new HTTP/2 frame type, "ORIGIN", to allow servers to indicate what origins a connection is usable for.

Additionally, experience has shown that HTTP/2's requirement to establish server authority using both DNS and the server's certificate is onerous. This specification relaxes the requirement to check DNS when the ORIGIN frame is in use.

Notational Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in {{RFC2119}}.

The ORIGIN HTTP/2 Frame

The ORIGIN HTTP/2 frame ({{!RFC7540}}, Section 4) allows a server to indicate what origin(s) {{!RFC6454}} the server would like the client to consider as members of the Origin Set ({{set}}) for the connection it occurs within.

Syntax {#syntax}

The ORIGIN frame type is 0xb (decimal 11).

+-------------------------------+-------------------------------+
|         Origin-Len (16)       | ASCII-Origin? (*)           ...
+-------------------------------+-------------------------------+

The ORIGIN frame's payload contains the following fields, sets of which may be repeated within the frame to indicate multiple origins:

Origin-Len: : An unsigned, 16-bit integer indicating the length, in octets, of the ASCII-Origin field.

Origin: : An optional sequence of characters containing the ASCII serialization of an origin ({{!RFC6454}}, Section 6.2) that the sender believes this connection is or could be authoritative for.

The ORIGIN frame does not define any flags. However, future updates to this specification MAY define flags. See {{process}}.

Processing ORIGIN Frames {#process}

The ORIGIN frame is a non-critical extension to HTTP/2. Endpoints that do not support this frame can safely ignore it upon receipt.

When received by an implementing client, it is used to initialise and manipulate the Origin Set (see {{set}}), thereby changing how the client establishes authority for origin servers (see {{authority}}).

The origin frame MUST be sent on stream 0; an ORIGIN frame on any other stream is invalid and MUST be ignored.

Likewise, the ORIGIN frame is only valid on connections with the "h2" protocol identifier, or when specifically nominated by the protocol's definition; it MUST be ignored when received on a connection with the "h2c" protocol identifier.

This specification does not define any flags for the ORIGIN frame, but future updates might use them to change its semantics. The first four flags (0x1, 0x2, 0x4 and 0x8) are reserved for backwards-incompatible changes, and therefore when any of them are set, the ORIGIN frame containing them MUST be ignored by clients conforming to this specification. The remaining flags are reserved for backwards-compatible changes, and do not affect processing by clients conformant to this specification.

The ORIGIN frame is processed hop-by-hop. An intermediary MUST NOT forward ORIGIN frames. Clients configured to use a proxy MUST ignore any ORIGIN frames received from it.

Each ASCII-Origin field in the frame's payload MUST be parsed as an ASCII serialisation of an origin ({{!RFC6454}}, Section 6.2). If parsing fails, the field MUST be ignored.

Senders should note that, as per {{!RFC6454}} Section 4, the values in an ORIGIN header need to be case-normalised before serialisation.

Once parsed, the value MUST have:

  • a scheme of "https",
  • a host that is reflected in a subjectAltName of the connection's TLS certificate (using the wildcard rules defined in {{!RFC2818}}, Section 3.1), and
  • a port that reflects the connection's remote port on the client.

If any of these requirements are violated, the client MUST ignore the field (but still continue processing the frame).

See {{algo}} for an illustrative algorithm for processing ORIGIN frames.

The Origin Set {#set}

The set of origins (as per {{!RFC6454}}) that a given connection might be used for is known in this specification as the Origin Set.

By default, a connections's Origin Set is uninitialised. When an ORIGIN frame is first received and successfully processed by a client, the connection's Origin Set is defined to contain a single origin, composed from:

  • Scheme: "https"
  • Host: the value sent in Server Name Indication ({{!RFC6066}} Section 3), converted to lower case
  • Port: the local port of the connection on the server

The contents of that ORIGIN frame (and subsequent ones) allows the server to incrementally add new origins to the Origin Set, as described in {{process}}.

The Origin Set is also affected by the 421 (Misdirected Request) response status code, defined in {{!RFC7540}} Section 9.1.2. Upon receipt of a response with this status code, implementing clients MUST create the ASCII serialisation of the corresponding request's origin (as per {{!RFC6454}}, Section 6.2) and remove it from the connection's Origin Set, if present.

Note that the Origin Set does not affect existing streams on a connection in any way.

Authority, Push and Coalescing with ORIGIN {#authority}

{{!RFC7540}}, Section 10.1 uses both DNS and the presented TLS certificate to establish the origin server(s) that a connection is authoritative for, just as HTTP/1.1 does in {{?RFC7230}}. Furthermore, {{!RFC7540}} Section 9.1.1 explicitly allows a connection to be used for more than one origin server, if it is authoritative. This affects what requests can be sent on the connection, both in HEADERS frame by the client and as PUSH_PROMISE frames from the server.

Once an Origin Set has been initialised for a connection, clients that implement this specification change these behaviors in the following ways:

  • Clients MUST NOT consult DNS to establish authority for origins in the Origin Set. The TLS certificate MUST be used to do so, as described in {{!RFC7540}} Section 9.1.1.

  • Clients sending a new request SHOULD use an existing connection if the request's origin is in that connection's Origin Set, unless there are operational reasons for creating a new connection.

  • Clients MUST use the Origin Set to determine whether a received PUSH_PROMISE is authoritative, as described in {{!RFC7540}}, Section 8.2.2.

Note that this does not prevent clients from performing other certificate checks as required or allowed, either at connection time or when processing ORIGIN. See {{!RFC7540}} Section 9.1.1 for more information.

Because ORIGIN can change the set of origins a connection is used for over time, it is possible that a client might have more than one viable connection to an origin open at any time. When this occurs, clients SHOULD not emit new requests on any connection whose Origin Set is a subset of another connection's Origin Set, and SHOULD close it once all outstanding requests are satisified.

IANA Considerations

This specification adds an entry to the "HTTP/2 Frame Type" registry.

  • Frame Type: ORIGIN
  • Code: 0xb
  • Specification: [this document]

Security Considerations

Clients that blindly trust the ORIGIN frame's contents will be vulnerable to a large number of attacks. See {{authority}} for mitigations.

Relaxing the requirement to consult DNS when determining authority for an origin means that an attacker who possesses a valid certificate no longer needs to be on-path to redirect traffic to them; instead of modifying DNS, they need only convince the user to visit another Web site, in order to coalesce connections to the target onto their existing connection.

--- back

Non-Normative Processing Algorithm {#algo}

The following algorithm illustrates how a client could handle received ORIGIN frames:

  1. If the client is configured to use a proxy for the connection, ignore the frame and stop processing.
  2. If the connection is not identified with the "h2" protocol identifier or another protocol that has explicitly opted into this specification, ignore the frame and stop processing.
  3. If the frame occurs upon any stream except stream 0, ignore the frame and stop processing.
  4. If any of the flags 0x1, 0x2, 0x4 or 0x8 are set, ignore the frame and stop processing.
  5. If no previous ORIGIN frame on the connection has reached this step, initialise the Origin Set as per {{set}}.
  6. For each Origin field origin_raw in the frame payload:
    1. Parse origin_raw as an ASCII serialization of an origin ({{!RFC6454}}, Section 6.2) and let the result be parsed_origin. If parsing fails, skip to the next origin_raw.
    2. If the scheme of parsed_origin is not "https", skip to the next origin_raw.
    3. If the certificate presented by the server is not valid for the host of parsed_origin (see below), skip to the next origin_raw.
    4. If the port of parsed_origin does not match the connection's remote port, skip to the next origin_raw.
    5. Add parsed_origin to the Origin Set.

The certificate presented by the server is valid for a host if it passes the checks that the client would perform when forming a new TLS connection to the origin. This includes verifying that the host matches a dNSName value from the certificate subjectAltName field (using the wildcard rules defined in {{!RFC2818}}).

Operational Considerations for Servers {#server-ops}

The ORIGIN frame allows a server to indicate what for origins a given connection ought be used.

For example, it can be used to inform the client that the connection is to only be used for the SNI-based origin, by sending an empty ORIGIN frame. Or, a larger number of origins can be indicated by including a payload.

Generally, this information is most useful to send before sending any part of a response that might initiate a new connection; for example, Link headers {{?RFC5988}} in a response HEADERS, or links in the response body.

Therefore, the ORIGIN frame ought be sent as soon as possible on a connection, ideally before any HEADERS or PUSH_PROMISE frames.

However, if it's desirable to associate a large number of origins with a connection, doing so might introduce end-user perceived latency, due to their size. As a result, it might be necessary to select a "core" set of origins to send initially, expanding the set of origins the connection is used for with subsequent ORIGIN frames later (e.g., when the connection is idle).