Internet Engineering Task Force (IETF) M. Nottingham
Request for Comments: 8336
Category: Standards Track E. Nygren
ISSN: 2070-1721 Akamai Technologies
March 2018 The ORIGIN HTTP/2 Frame
This document specifies the ORIGIN frame for HTTP/2, to indicate what
origins are available on a given connection.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
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2. The ORIGIN HTTP/2 Frame
This document defines a new HTTP/2 frame type ([RFC7540], Section 4)
called ORIGIN, that allows a server to indicate what origin(s)
[RFC6454] the server would like the client to consider as members of
the Origin Set (Section 2.3) for the connection within which it
The ORIGIN frame type is 0xc (decimal 12) and contains zero or more
instances of the Origin-Entry field.
| Origin-Entry (*) ...
An Origin-Entry is a length-delimited string:
| Origin-Len (16) | ASCII-Origin? ...
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 asserts 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 Section 2.2.
2.2. Processing ORIGIN Frames
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 initialize and
manipulate the Origin Set (see Section 2.3), thereby changing how the
client establishes authority for origin servers (see Section 2.4).
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 to this specification (through IETF consensus)
might use them to change its semantics. The first four flags (0x1,
0x2, 0x4, and 0x8) are reserved for backwards-incompatible changes;
therefore, when any of them are set, the ORIGIN frame containing them
MUST be ignored by clients conforming to this specification, unless
the flag's semantics are understood. The remaining flags are
reserved for backwards-compatible changes and do not affect
processing by clients conformant to this specification.
The ORIGIN frame describes a property of the connection and therefore
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 serialization of an origin ([RFC6454], Section 6.2). If
parsing fails, the field MUST be ignored.
Note that the ORIGIN frame does not support wildcard names (e.g.,
"*.example.com") in Origin-Entry. As a result, sending ORIGIN when a
wildcard certificate is in use effectively disables any origins that
are not explicitly listed in the ORIGIN frame(s) (when the client
See Appendix A for an illustrative algorithm for processing ORIGIN
2.3. The Origin 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, the Origin Set for a connection is uninitialized. An
uninitialized Origin Set means that clients apply the coalescing
rules from Section 9.1.1 of [RFC7540].
When an ORIGIN frame is first received and successfully processed by
a client, the connection's Origin Set is defined to contain an
initial origin. The initial origin is composed from:
o Scheme: "https"
o Host: the value sent in Server Name Indication (SNI) ([RFC6066],
Section 3) converted to lower case; if SNI is not present, the
remote address of the connection (i.e., the server's IP address)
o Port: the remote port of the connection (i.e., the server's port)
The contents of that ORIGIN frame (and subsequent ones) allow the
server to incrementally add new origins to the Origin Set, as
described in Section 2.2.
The Origin Set is also affected by the 421 (Misdirected Request)
response status code, as defined in [RFC7540], Section 9.1.2. Upon
receipt of a response with this status code, implementing clients
MUST create the ASCII serialization of the corresponding request's
origin (as per [RFC6454], Section 6.2) and remove it from the
connection's Origin Set, if present.
Note: When sending an ORIGIN frame to a connection that is
initialized as an alternative service [RFC7838], the initial
Origin Set (Section 2.3) will contain an origin with the
appropriate scheme and hostname (since RFC 7838 specifies that the
origin's hostname be sent in SNI). However, it is possible that
the port will be different than that of the intended origin, since
the initial Origin Set is calculated using the actual port in use,
which can be different for the alternative service. In this case,
the intended origin needs to be sent in the ORIGIN frame
For example, a client making requests for "https://example.com" is
directed to an alternative service at ("h2", "x.example.net",
"8443"). If this alternative service sends an ORIGIN frame, the
initial origin will be "https://example.com:8443". The client
will not be able to use the alternative service to make requests
for "https://example.com" unless that origin is explicitly
included in the ORIGIN frame.
2.4. Authority, Push, and Coalescing with ORIGIN
Section 10.1 of [RFC7540] uses both DNS and the presented Transport
Layer Security (TLS) certificate to establish the origin server(s)
that a connection is authoritative for, just as HTTP/1.1 does in
Furthermore, Section 9.1.1 of [RFC7540] explicitly allows a
connection to be used for more than one origin server, if it is
authoritative. This affects what responses can be considered
authoritative, both for direct responses to requests and for server
push (see [RFC7540], Section 8.2.2). Indirectly, it also affects
what requests will be sent on a connection, since clients will
generally only send requests on connections that they believe to be
authoritative for the origin in question.
Once an Origin Set has been initialized for a connection, clients
that implement this specification use it to help determine what the
connection is authoritative for. Specifically, such clients MUST NOT
consider a connection to be authoritative for an origin not present
in the Origin Set, and they SHOULD use the connection for all
requests to origins in the Origin Set for which the connection is
authoritative, unless there are operational reasons for opening a new
Note that for a connection to be considered authoritative for a given
origin, the server is still required to authenticate with a
certificate that passes suitable checks; see Section 9.1.1 of
[RFC7540] for more information. This includes verifying that the
host matches a "dNSName" value from the certificate "subjectAltName"
field (using the rules defined in [RFC2818]; see also [RFC5280],
Additionally, clients MAY avoid consulting DNS to establish the
connection's authority for new requests to origins in the Origin Set;
however, those that do so face new risks, as explained in Section 4.
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 proper subset of another connection's Origin Set, and they
SHOULD close it once all outstanding requests are satisfied.
The Origin Set is unaffected by any alternative services [RFC7838]
advertisements made by the server. Advertising an alternative
service does not affect whether a server is authoritative.
3. IANA Considerations
This specification adds an entry to the "HTTP/2 Frame Type" registry.
o Frame Type: ORIGIN
o Code: 0xc
o Specification: RFC 8336
4. Security Considerations
Clients that blindly trust the ORIGIN frame's contents will be
vulnerable to a large number of attacks. See Section 2.4 for
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 website in order to coalesce connections to the target
onto their existing connection.
As a result, clients opting not to consult DNS ought to employ some
alternative means to establish a high degree of confidence that the
certificate is legitimate. For example, clients might skip
consulting DNS only if they receive proof of inclusion in a
Certificate Transparency log [RFC6962] or if they have a recent
Online Certificate Status Protocol (OCSP) response [RFC6960]
(possibly using the "status_request" TLS extension [RFC6066]) showing
that the certificate was not revoked.
The Origin Set's size is unbounded by this specification and thus
could be used by attackers to exhaust client resources. To mitigate
this risk, clients can monitor their state commitment and close the
connection if it is too high.
Appendix A. Non-Normative Processing Algorithm
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, initialize the Origin Set as per Section 2.3.
6. For each "Origin-Entry" in the frame payload:
1. Parse "ASCII-Origin" 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
2. Add "parsed_origin" to the Origin Set.
Appendix B. Operational Considerations for Servers
The ORIGIN frame allows a server to indicate for which origins a
given connection ought be used. The set of origins advertised using
this mechanism is under control of the server; servers are not
obligated to use it or to advertise all origins that they might be
able to answer a request for.
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" response header fields [RFC8288], or links in the response
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, and expand the set
of origins the connection is used for with subsequent ORIGIN frames
later (e.g., when the connection is idle).
That said, senders are encouraged to include as many origins as
practical within a single ORIGIN frame; clients need to make
decisions about creating connections on the fly, and if the Origin
Set is split across many frames, their behavior might be suboptimal.
Senders take note that, as per Section 4, Step 5, of [RFC6454], the
values in an ORIGIN header need to be case-normalized before
Finally, servers that host alternative services [RFC7838] will need
to explicitly advertise their origins when sending ORIGIN, because
the default contents of the Origin Set (as per Section 2.3) do not
contain any alternative services' origins, even if they have been
used previously on the connection.