draft-ietf-httpbis-rfc7838bis-00.txt   draft-ietf-httpbis-rfc7838bis-latest.txt 
HTTP Working Group M. Bishop, Ed. HTTP Working Group M. Bishop, Ed.
Internet-Draft Akamai Technologies Internet-Draft Akamai Technologies
Intended status: Standards Track M. Thomson, Ed. Intended status: Standards Track M. Thomson, Ed.
Expires: March 5, 2022 Mozilla Expires: March 20, 2022 Mozilla
September 01, 2021 September 16, 2021
HTTP Alternative Services HTTP Alternative Services
draft-ietf-httpbis-rfc7838bis-00 draft-ietf-httpbis-rfc7838bis-latest
Abstract Abstract
This document specifies "Alternative Services" for HTTP, which allow This document specifies "Alternative Services" for HTTP, which allow
an origin's resources to be authoritatively available at a separate an origin's resources to be authoritatively available at a separate
network location, possibly accessed with a different protocol network location, possibly accessed with a different protocol
configuration. configuration.
Note to Readers
_RFC EDITOR: please remove this section before publication_
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/ [1].
Working Group information can be found at https://httpwg.org/ [2];
source code and issues list for this draft can be found at
https://github.com/httpwg/http-extensions/labels/alt-svc [3].
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on March 5, 2022. This Internet-Draft will expire on March 20, 2022.
Copyright Notice Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Changes from RFC 7838 . . . . . . . . . . . . . . . . . . 3 1.1. Changes from RFC 7838 . . . . . . . . . . . . . . . . . . 3
1.2. Notational Conventions . . . . . . . . . . . . . . . . . 3 1.2. Notational Conventions . . . . . . . . . . . . . . . . . 4
2. Alternative Services Concepts . . . . . . . . . . . . . . . . 4 2. Alternative Services Concepts . . . . . . . . . . . . . . . . 4
2.1. Host Authentication . . . . . . . . . . . . . . . . . . . 5 2.1. Host Authentication . . . . . . . . . . . . . . . . . . . 6
2.2. Alternative Service Caching . . . . . . . . . . . . . . . 6 2.2. Alternative Service Caching . . . . . . . . . . . . . . . 6
2.3. Requiring Server Name Indication . . . . . . . . . . . . 6 2.3. Requiring Server Name Indication . . . . . . . . . . . . 7
2.4. Using Alternative Services . . . . . . . . . . . . . . . 7 2.4. Using Alternative Services . . . . . . . . . . . . . . . 7
3. The Alt-Svc HTTP Header Field . . . . . . . . . . . . . . . . 8 3. The Alt-Svc HTTP Header Field . . . . . . . . . . . . . . . . 8
3.1. Caching Alt-Svc Header Field Values . . . . . . . . . . . 10 3.1. Caching Alt-Svc Header Field Values . . . . . . . . . . . 10
4. The ALTSVC HTTP/2 Frame . . . . . . . . . . . . . . . . . . . 11 4. The ALTSVC Extension Frame . . . . . . . . . . . . . . . . . 11
5. The Alt-Used HTTP Header Field . . . . . . . . . . . . . . . 13 5. The Alt-Used HTTP Header Field . . . . . . . . . . . . . . . 13
6. The 421 (Misdirected Request) HTTP Status Code . . . . . . . 13 6. The 421 (Misdirected Request) HTTP Status Code . . . . . . . 13
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
7.1. Header Field Registrations . . . . . . . . . . . . . . . 13 7.1. Header Field Registrations . . . . . . . . . . . . . . . 14
7.2. The ALTSVC HTTP/2 Frame Type . . . . . . . . . . . . . . 14 7.2. The ALTSVC HTTP/2 Frame Type . . . . . . . . . . . . . . 14
7.3. Alt-Svc Parameter Registry . . . . . . . . . . . . . . . 14 7.3. The ALTSVC HTTP/3 Frame Type . . . . . . . . . . . . . . 14
7.3.1. Procedure . . . . . . . . . . . . . . . . . . . . . . 14 7.4. Alt-Svc Parameter Registry . . . . . . . . . . . . . . . 15
7.3.2. Registrations . . . . . . . . . . . . . . . . . . . . 15 7.4.1. Procedure . . . . . . . . . . . . . . . . . . . . . . 15
7.4.2. Registrations . . . . . . . . . . . . . . . . . . . . 15
8. Internationalization Considerations . . . . . . . . . . . . . 15 8. Internationalization Considerations . . . . . . . . . . . . . 15
9. Security Considerations . . . . . . . . . . . . . . . . . . . 15 9. Security Considerations . . . . . . . . . . . . . . . . . . . 15
9.1. Changing Ports . . . . . . . . . . . . . . . . . . . . . 15 9.1. Changing Ports . . . . . . . . . . . . . . . . . . . . . 15
9.2. Changing Hosts . . . . . . . . . . . . . . . . . . . . . 15 9.2. Changing Hosts . . . . . . . . . . . . . . . . . . . . . 16
9.3. Changing Protocols . . . . . . . . . . . . . . . . . . . 16 9.3. Changing Protocols . . . . . . . . . . . . . . . . . . . 17
9.4. Tracking Clients Using Alternative Services . . . . . . . 17 9.4. Tracking Clients Using Alternative Services . . . . . . . 17
9.5. Confusion regarding Request Scheme . . . . . . . . . . . 17 9.5. Confusion regarding Request Scheme . . . . . . . . . . . 17
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 18 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
10.1. Normative References . . . . . . . . . . . . . . . . . . 18 10.1. Normative References . . . . . . . . . . . . . . . . . . 18
10.2. Informative References . . . . . . . . . . . . . . . . . 19 10.2. Informative References . . . . . . . . . . . . . . . . . 19
10.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 20 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction 1. Introduction
HTTP [RFC7230] conflates the identification of resources with their HTTP [HTTP] conflates the identification of resources with their
location. In other words, "http://" and "https://" URIs are used to location. In other words, "http://" and "https://" URIs are used to
both name and find things to interact with. both name and find things to interact with.
In some cases, it is desirable to separate identification and In some cases, it is desirable to separate identification and
location in HTTP; keeping the same identifier for a resource, but location in HTTP; keeping the same identifier for a resource, but
interacting with it at a different location on the network. interacting with it at a different location on the network.
For example: For example:
o An origin server might wish to redirect a client to a different o An origin server might wish to redirect a client to a different
server when it is under load, or it has found a server in a server when it is under load, or it has found a server in a
location that is more local to the client. location that is more local to the client.
o An origin server might wish to offer access to its resources using o An origin server might wish to offer access to its resources using
a new protocol, such as HTTP/2 [RFC7540], or one using improved a new protocol, such as HTTP/3 [HTTP3], or one using improved
security, such as Transport Layer Security (TLS) [RFC5246]. security, such as Transport Layer Security (TLS) [RFC5246].
o An origin server might wish to segment its clients into groups of o An origin server might wish to segment its clients into groups of
capabilities, such as those supporting Server Name Indication capabilities, such as those supporting Server Name Indication
(SNI) (Section 3 of [RFC6066]), for operational purposes. (SNI) (Section 3 of [RFC6066]), for operational purposes.
This specification defines a new concept in HTTP, "Alternative This specification defines a new concept in HTTP, "Alternative
Services", that allows an origin server to nominate additional means Services", that allows an origin server to nominate additional means
of interacting with it on the network. It defines a general of interacting with it on the network. It defines a general
framework for this in Section 2, along with specific mechanisms for framework for this in Section 2, along with specific mechanisms for
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No substantive changes. No substantive changes.
1.2. Notational Conventions 1.2. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
This document uses the Augmented BNF defined in [RFC5234] and updated This document uses the Augmented BNF defined in [RFC5234] and updated
by [RFC7405] along with the "#rule" extension defined in Section 7 of by [RFC7405] along with the "#rule" extension defined in
[RFC7230]. The rules below are defined in [RFC5234], [RFC7230], and Section 5.6.1 of [HTTP]. The rules below are defined in [RFC5234],
[RFC7234]: [HTTP], and [Caching]:
OWS = <OWS, see [RFC7230], Section 3.2.3> OWS = <OWS, see [HTTP], Section 5.6.3>
delta-seconds = <delta-seconds; see [RFC7234], Section 1.2.1> delta-seconds = <delta-seconds; see [Caching], Section 1.2.2>
port = <port, see [RFC7230], Section 2.7> port = <port, see [HTTP], Section 4.1>
quoted-string = <quoted-string, see [RFC7230], Section 3.2.6> quoted-string = <quoted-string, see [HTTP], Section 5.6.4>
token = <token, see [RFC7230], Section 3.2.6> token = <token, see [HTTP], Section 5.6.2>
uri-host = <uri-host, see [RFC7230], Section 2.7> uri-host = <uri-host, see [HTTP], Section 4.1>
2. Alternative Services Concepts 2. Alternative Services Concepts
This specification defines a new concept in HTTP, the "Alternative This specification defines a new concept in HTTP, the "Alternative
Service". When an origin [RFC6454] has resources that are accessible Service". When an origin [RFC6454] has resources that are accessible
through a different protocol/host/port combination, it is said to through a different protocol/host/port combination, it is said to
have an alternative service available. have an alternative service available.
An alternative service can be used to interact with the resources on An alternative service can be used to interact with the resources on
an origin server at a separate location on the network, possibly an origin server at a separate location on the network, possibly
using a different protocol configuration. Alternative services are using a different protocol configuration. Alternative services are
considered authoritative for an origin's resources, in the sense of considered authoritative for an origin's resources, in the sense of
Section 9.1 of [RFC7230]. Section 4.3 of [HTTP].
For example, an origin: For example, an origin:
("http", "www.example.com", "80") ("http", "www.example.com", "80")
might declare that its resources are also accessible at the might declare that its resources are also accessible at the
alternative service: alternative service:
("h2", "new.example.com", "81") ("h2", "new.example.com", "81")
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This means that clients using an alternative service can change the This means that clients using an alternative service can change the
host, port, and protocol that they are using to fetch resources, but host, port, and protocol that they are using to fetch resources, but
these changes MUST NOT be propagated to the application that is using these changes MUST NOT be propagated to the application that is using
HTTP; from that standpoint, the URI being accessed and all HTTP; from that standpoint, the URI being accessed and all
information derived from it (scheme, host, and port) are the same as information derived from it (scheme, host, and port) are the same as
before. before.
Importantly, this includes its security context; in particular, when Importantly, this includes its security context; in particular, when
TLS [RFC5246] is used to authenticate, the alternative service will TLS [RFC5246] is used to authenticate, the alternative service will
need to present a certificate for the origin's host name, not that of need to present a certificate for the origin's host name, not that of
the alternative. Likewise, the Host header field ([RFC7230], the alternative. Likewise, the Host header field (Section 7.2 of
Section 5.4) is still derived from the origin, not the alternative [HTTP]) is still derived from the origin, not the alternative service
service (just as it would if a CNAME were being used). (just as it would if a CNAME were being used).
The changes MAY, however, be made visible in debugging tools, The changes MAY, however, be made visible in debugging tools,
consoles, etc. consoles, etc.
Formally, an alternative service is identified by the combination of: Formally, an alternative service is identified by the combination of:
o An Application Layer Protocol Negotiation (ALPN) protocol name, as o An Application-Layer Protocol Negotiation (ALPN) protocol name, as
per [RFC7301] per [RFC7301]
o A host, as per Section 3.2.2 of [RFC3986] o A host, as per Section 3.2.2 of [RFC3986]
o A port, as per Section 3.2.3 of [RFC3986] o A port, as per Section 3.2.3 of [RFC3986]
The ALPN protocol name is used to identify the application protocol The ALPN protocol name is used to identify the application protocol
or suite of protocols used by the alternative service. Note that for or suite of protocols used by the alternative service. Note that for
the purpose of this specification, an ALPN protocol name implicitly the purpose of this specification, an ALPN protocol name implicitly
includes TLS in the suite of protocols it identifies, unless includes TLS in the suite of protocols it identifies, unless
specified otherwise in its definition. In particular, the ALPN name specified otherwise in its definition. In particular, the ALPN name
"http/1.1", registered by Section 6 of [RFC7301], identifies HTTP/1.1 "http/1.1", registered by Section 6 of [RFC7301], identifies HTTP/1.1
over TLS. over TLS.
Additionally, each alternative service MUST have a freshness Additionally, each alternative service MUST have a freshness
lifetime, expressed in seconds (see Section 2.2). lifetime, expressed in seconds (see Section 2.2).
There are many ways that a client could discover the alternative There are many ways that a client could discover the alternative
service(s) associated with an origin. This document describes two service(s) associated with an origin. This document describes two
such mechanisms: the "Alt-Svc" HTTP header field (Section 3) and the such mechanisms: the "Alt-Svc" HTTP header field (Section 3) and the
"ALTSVC" HTTP/2 frame type (Section 4). "ALTSVC" frame type for HTTP/2 and HTTP/3 (Section 4).
The remainder of this section describes requirements that are common The remainder of this section describes requirements that are common
to alternative services, regardless of how they are discovered. to alternative services, regardless of how they are discovered.
2.1. Host Authentication 2.1. Host Authentication
Clients MUST have reasonable assurances that the alternative service Clients MUST have reasonable assurances that the alternative service
is under control of and valid for the whole origin. This mitigates is under control of and valid for the whole origin. This mitigates
the attack described in Section 9.2. the attack described in Section 9.2.
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The field value consists either of a list of values, each of which The field value consists either of a list of values, each of which
indicates one alternative service, or the keyword "clear". indicates one alternative service, or the keyword "clear".
A field value containing the special value "clear" indicates that the A field value containing the special value "clear" indicates that the
origin requests all alternatives for that origin to be invalidated origin requests all alternatives for that origin to be invalidated
(including those specified in the same response, in case of an (including those specified in the same response, in case of an
invalid reply containing both "clear" and alternative services). invalid reply containing both "clear" and alternative services).
ALPN protocol names are octet sequences with no additional ALPN protocol names are octet sequences with no additional
constraints on format. Octets not allowed in tokens ([RFC7230], constraints on format. Octets not allowed in tokens (Section 5.6.2
Section 3.2.6) MUST be percent-encoded as per Section 2.1 of of [HTTP]) MUST be percent-encoded as per Section 2.1 of [RFC3986].
[RFC3986]. Consequently, the octet representing the percent Consequently, the octet representing the percent character "%" (hex
character "%" (hex 25) MUST be percent-encoded as well. 25) MUST be percent-encoded as well.
In order to have precisely one way to represent any ALPN protocol In order to have precisely one way to represent any ALPN protocol
name, the following additional constraints apply: name, the following additional constraints apply:
1. Octets in the ALPN protocol name MUST NOT be percent-encoded if 1. Octets in the ALPN protocol name MUST NOT be percent-encoded if
they are valid token characters except "%", and they are valid token characters except "%", and
2. When using percent-encoding, uppercase hex digits MUST be used. 2. When using percent-encoding, uppercase hex digits MUST be used.
With these constraints, recipients can apply simple string comparison With these constraints, recipients can apply simple string comparison
to match protocol identifiers. to match protocol identifiers.
The "alt-authority" component consists of an OPTIONAL uri-host The "alt-authority" component consists of an OPTIONAL uri-host
("host" in Section 3.2.2 of [RFC3986]), a colon (":"), and a port ("host" in Section 3.2.2 of [RFC3986]), a colon (":"), and a port
number. number.
For example: For example:
Alt-Svc: h2=":8000" Alt-Svc: h2=":8000"
This indicates the "h2" protocol ([RFC7540]) on the same host using
the indicated port 8000. This indicates the "h2" protocol ([HTTP2]) on the same host using the
indicated port 8000.
An example involving a change of host: An example involving a change of host:
Alt-Svc: h2="new.example.org:80" Alt-Svc: h2="new.example.org:80"
This indicates the "h2" protocol on the host "new.example.org", This indicates the "h2" protocol on the host "new.example.org",
running on port 80. Note that the "quoted-string" syntax needs to be running on port 80. Note that the "quoted-string" syntax needs to be
used because ":" is not an allowed character in "token". used because ":" is not an allowed character in "token".
Examples for protocol name escaping: Examples for protocol name escaping:
skipping to change at page 9, line 29 skipping to change at page 9, line 46
+--------------------+-------------+---------------------+ +--------------------+-------------+---------------------+
| h2 | h2 | No escaping needed | | h2 | h2 | No escaping needed |
| | | | | | | |
| w=x:y#z | w%3Dx%3Ay#z | "=" and ":" escaped | | w=x:y#z | w%3Dx%3Ay#z | "=" and ":" escaped |
| | | | | | | |
| x%y | x%25y | "%" needs escaping | | x%y | x%25y | "%" needs escaping |
+--------------------+-------------+---------------------+ +--------------------+-------------+---------------------+
Alt-Svc MAY occur in any HTTP response message, regardless of the Alt-Svc MAY occur in any HTTP response message, regardless of the
status code. Note that recipients of Alt-Svc can ignore the header status code. Note that recipients of Alt-Svc can ignore the header
field (and are required to in some situations; see Sections 2.1 and field (and are required to in some situations; see Section 2.1 and
6). Section 6).
The Alt-Svc field value can have multiple values: The Alt-Svc field value can have multiple values:
Alt-Svc: h2="alt.example.com:8000", h2=":443" Alt-Svc: h2="alt.example.com:8000", h2=":443"
When multiple values are present, the order of the values reflects When multiple values are present, the order of the values reflects
the server's preference (with the first value being the most the server's preference (with the first value being the most
preferred alternative). preferred alternative).
The value(s) advertised by Alt-Svc can be used by clients to open a The value(s) advertised by Alt-Svc can be used by clients to open a
new connection to an alternative service. Subsequent requests can new connection to an alternative service. Subsequent requests can
start using this new connection immediately or can continue using the start using this new connection immediately or can continue using the
existing connection while the new connection is created. existing connection while the new connection is created.
When using HTTP/2 ([RFC7540]), servers SHOULD instead send an ALTSVC When using HTTP/2 ([HTTP2]), servers SHOULD instead send an ALTSVC
frame (Section 4). A single ALTSVC frame can be sent for a frame (Section 4). A single ALTSVC frame can be sent for a
connection; a new frame is not needed for every request. Note that, connection; a new frame is not needed for every request. Note that,
despite this recommendation, Alt-Svc header fields remain valid in despite this recommendation, Alt-Svc header fields remain valid in
responses delivered over HTTP/2. responses delivered over HTTP/2.
Each "alt-value" is followed by an OPTIONAL semicolon-separated list Each "alt-value" is followed by an OPTIONAL semicolon-separated list
of additional parameters, each such "parameter" comprising a name and of additional parameters, each such "parameter" comprising a name and
a value. a value.
This specification defines two parameters: "ma" and "persist", This specification defines two parameters: "ma" and "persist",
defined in Section 3.1. Unknown parameters MUST be ignored. That defined in Section 3.1. Unknown parameters MUST be ignored. That
is, the values (alt-value) they appear in MUST be processed as if the is, the values (alt-value) they appear in MUST be processed as if the
unknown parameter was not present. unknown parameter was not present.
New parameters can be defined in extension specifications (see New parameters can be defined in extension specifications (see
Section 7.3 for registration details). Section 7.4 for registration details).
Note that all field elements that allow "quoted-string" syntax MUST Note that all field elements that allow "quoted-string" syntax MUST
be processed as per Section 3.2.6 of [RFC7230]. be processed as per Section 5.6.4 of [HTTP].
3.1. Caching Alt-Svc Header Field Values 3.1. Caching Alt-Svc Header Field Values
When an alternative service is advertised using Alt-Svc, it is When an alternative service is advertised using Alt-Svc, it is
considered fresh for 24 hours from generation of the message. This considered fresh for 24 hours from generation of the message. This
can be modified with the "ma" (max-age) parameter. can be modified with the "ma" (max-age) parameter.
Syntax: Syntax:
ma = delta-seconds; see [RFC7234], Section 1.2.1 ma = delta-seconds; see [Caching], Section 1.2.2
The delta-seconds value indicates the number of seconds since the The delta-seconds value indicates the number of seconds since the
response was generated for which the alternative service is response was generated for which the alternative service is
considered fresh. considered fresh.
Alt-Svc: h2=":443"; ma=3600 Alt-Svc: h2=":443"; ma=3600
See Section 4.2.3 of [RFC7234] for details on determining the See Section 4.2.3 of [Caching] for details on determining the
response age. response age.
For example, a response: For example, a response:
HTTP/1.1 200 OK HTTP/1.1 200 OK
Content-Type: text/html Content-Type: text/html
Cache-Control: max-age=600 Cache-Control: max-age=600
Age: 30 Age: 30
Alt-Svc: h2=":8000"; ma=60 Alt-Svc: h2=":8000"; ma=60
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For example: For example:
Alt-Svc: h2=":443"; ma=2592000; persist=1 Alt-Svc: h2=":443"; ma=2592000; persist=1
This specification only defines a single value for "persist". This specification only defines a single value for "persist".
Clients MUST ignore "persist" parameters with values other than "1". Clients MUST ignore "persist" parameters with values other than "1".
See Section 2.2 for general requirements on caching alternative See Section 2.2 for general requirements on caching alternative
services. services.
4. The ALTSVC HTTP/2 Frame 4. The ALTSVC Extension Frame
The ALTSVC HTTP/2 frame (Section 4 of [RFC7540]) advertises the The ALTSVC frame advertises the availability of an alternative
availability of an alternative service to an HTTP/2 client. service to an HTTP/2 or HTTP/3 client.
The ALTSVC frame is a non-critical extension to HTTP/2. Endpoints The ALTSVC frame is a separate non-critical extension in each
that do not support this frame will ignore it (as per the protocol. Endpoints that do not support this frame will ignore it
extensibility rules defined in Section 4.1 of [RFC7540]). (as per the extensibility rules defined in Section 4.1 of [HTTP2] and
Section 4.1 of [HTTP3]).
An ALTSVC frame from a server to a client on a stream other than An ALTSVC frame from a server to a client on a request stream or a
stream 0 indicates that the conveyed alternative service is push stream indicates that the conveyed alternative service is
associated with the origin of that stream. associated with the origin of that stream.
An ALTSVC frame from a server to a client on stream 0 indicates that An ALTSVC frame from a server to a client on the control stream
(stream 0 in HTTP/2 or a stream of type 0 in HTTP/3) indicates that
the conveyed alternative service is associated with the origin the conveyed alternative service is associated with the origin
contained in the Origin field of the frame. An association with an contained in the Origin field of the frame. An association with an
origin that the client does not consider authoritative for the origin that the client does not consider authoritative for the
current connection MUST be ignored. current connection MUST be ignored.
The ALTSVC frame type is 0xa (decimal 10). The ALTSVC frame type is 0xa (decimal 10) in both protocols.
+-------------------------------+-------------------------------+ +-------------------------------+-------------------------------+
| Origin-Len (16) | Origin? (*) ... | Origin-Len (16) | Origin? (*) ...
+-------------------------------+-------------------------------+ +-------------------------------+-------------------------------+
| Alt-Svc-Field-Value (*) ... | Alt-Svc-Field-Value (*) ...
+---------------------------------------------------------------+ +---------------------------------------------------------------+
ALTSVC Frame Payload ALTSVC Frame Payload
The ALTSVC frame contains the following fields: The ALTSVC frame contains the following fields:
skipping to change at page 12, line 27 skipping to change at page 12, line 45
Origin: An OPTIONAL sequence of characters containing the ASCII Origin: An OPTIONAL sequence of characters containing the ASCII
serialization of an origin (Section 6.2 of [RFC6454]) to which the serialization of an origin (Section 6.2 of [RFC6454]) to which the
alternative service is applicable. alternative service is applicable.
Alt-Svc-Field-Value: A sequence of octets (length determined by Alt-Svc-Field-Value: A sequence of octets (length determined by
subtracting the length of all preceding fields from the frame subtracting the length of all preceding fields from the frame
length) containing a value identical to the Alt-Svc field value length) containing a value identical to the Alt-Svc field value
defined in Section 3 (ABNF production "Alt-Svc"). defined in Section 3 (ABNF production "Alt-Svc").
The ALTSVC frame does not define any flags. The ALTSVC frame does not define any flags in HTTP/2; there is no
generic flag field for HTTP/3 frames.
The ALTSVC frame is intended for receipt by clients. A device acting The ALTSVC frame is intended for receipt by clients. A device acting
as a server MUST ignore it. as a server MUST ignore it.
An ALTSVC frame on stream 0 with empty (length 0) "Origin" An ALTSVC frame on the control stream with empty (length 0) "Origin"
information is invalid and MUST be ignored. An ALTSVC frame on a information is invalid and MUST be ignored. An ALTSVC frame on a
stream other than stream 0 containing non-empty "Origin" information request or push stream containing non-empty "Origin" information is
is invalid and MUST be ignored. invalid and MUST be ignored.
The ALTSVC frame is processed hop-by-hop. An intermediary MUST NOT The ALTSVC frame is processed hop-by-hop. An intermediary MUST NOT
forward ALTSVC frames, though it can use the information contained in forward ALTSVC frames, though it can use the information contained in
ALTSVC frames in forming new ALTSVC frames to send to its own ALTSVC frames in forming new ALTSVC frames to send to its own
clients. clients.
Receiving an ALTSVC frame is semantically equivalent to receiving an Receiving an ALTSVC frame is semantically equivalent to receiving an
Alt-Svc header field. As a result, the ALTSVC frame causes Alt-Svc header field. As a result, the ALTSVC frame causes
alternative services for the corresponding origin to be replaced. alternative services for the corresponding origin to be replaced.
Note that it would be unwise to mix the use of Alt-Svc header fields Note that it would be unwise to mix the use of Alt-Svc header fields
with the use of ALTSVC frames, as the sequence of receipt might be with the use of ALTSVC frames, as the sequence of receipt might be
hard to predict. hard to predict.
5. The Alt-Used HTTP Header Field 5. The Alt-Used HTTP Header Field
The Alt-Used header field is used in requests to identify the The Alt-Used header field is used in requests to identify the
alternative service in use, just as the Host header field alternative service in use, just as the Host header field
(Section 5.4 of [RFC7230]) identifies the host and port of the (Section 7.2 of [HTTP]) identifies the host and port of the origin.
origin.
Alt-Used = uri-host [ ":" port ] Alt-Used = uri-host [ ":" port ]
Alt-Used is intended to allow alternative services to detect loops, Alt-Used is intended to allow alternative services to detect loops,
differentiate traffic for purposes of load balancing, and generally differentiate traffic for purposes of load balancing, and generally
to ensure that it is possible to identify the intended destination of to ensure that it is possible to identify the intended destination of
traffic, since introducing this information after a protocol is in traffic, since introducing this information after a protocol is in
use has proven to be problematic. use has proven to be problematic.
When using an alternative service, clients SHOULD include an Alt-Used When using an alternative service, clients SHOULD include an Alt-Used
header field in all requests. header field in all requests.
For example: For example:
GET /thing HTTP/1.1 GET /thing HTTP/1.1
Host: origin.example.com Host: origin.example.com
Alt-Used: alternate.example.net Alt-Used: alternate.example.net
6. The 421 (Misdirected Request) HTTP Status Code 6. The 421 (Misdirected Request) HTTP Status Code
The 421 (Misdirected Request) status code is defined in Section 9.1.2 The 421 (Misdirected Request) status code is defined in
of [RFC7540] to indicate that the current server instance is not Section 15.5.20 of [HTTP] to indicate that the current server
authoritative for the requested resource. This can be used to instance is not authoritative for the requested resource. This can
indicate that an alternative service is not authoritative; see be used to indicate that an alternative service is not authoritative;
Section 2). see Section 2).
Clients receiving 421 (Misdirected Request) from an alternative Clients receiving 421 (Misdirected Request) from an alternative
service MUST remove the corresponding entry from its alternative service MUST remove the corresponding entry from its alternative
service cache (see Section 2.2) for that origin. Regardless of the service cache (see Section 2.2) for that origin. Regardless of the
idempotency of the request method, they MAY retry the request, either idempotency of the request method, they MAY retry the request, either
at another alternative server, or at the origin. at another alternative server, or at the origin.
An Alt-Svc header field in a 421 (Misdirected Request) response MUST An Alt-Svc header field in a 421 (Misdirected Request) response MUST
be ignored. be ignored.
skipping to change at page 14, line 9 skipping to change at page 14, line 23
7.1. Header Field Registrations 7.1. Header Field Registrations
HTTP header fields are registered within the "Message Headers" HTTP header fields are registered within the "Message Headers"
registry maintained at <https://www.iana.org/assignments/message- registry maintained at <https://www.iana.org/assignments/message-
headers/>. headers/>.
This document defines the following HTTP header fields, so their This document defines the following HTTP header fields, so their
associated registry entries have been added according to the associated registry entries have been added according to the
permanent registrations below (see [BCP90]): permanent registrations below (see [BCP90]):
+-------------------+----------+----------+-----------+ +-------------------+----------+----------+------------+
| Header Field Name | Protocol | Status | Reference | | Header Field Name | Protocol | Status | Reference |
+-------------------+----------+----------+-----------+ +-------------------+----------+----------+------------+
| Alt-Svc | http | standard | Section 3 | | Alt-Svc | http | standard | Section 3 |
| | | | | | | | | |
| Alt-Used | http | standard | Section 5 | | Alt-Used | http | standard | Section 5 |
+-------------------+----------+----------+-----------+ +-------------------+----------+----------+------------+
The change controller is: "IETF (iesg@ietf.org) -- Internet The change controller is: "IETF (iesg@ietf.org) -- Internet
Engineering Task Force". Engineering Task Force".
7.2. The ALTSVC HTTP/2 Frame Type 7.2. The ALTSVC HTTP/2 Frame Type
This document registers the ALTSVC frame type in the "HTTP/2 Frame This document registers the ALTSVC frame type in the "HTTP/2 Frame
Type" registry (Section 11.2 of [RFC7540]). Type" registry (Section 11.2 of [HTTP2]).
Frame Type: ALTSVC Frame Type: ALTSVC
Code: 0xa Code: 0xa
Specification: Section 4 of this document Specification: Section 4 of this document
7.3. Alt-Svc Parameter Registry 7.3. The ALTSVC HTTP/3 Frame Type
This document registers the ALTSVC frame type in the "HTTP/3 Frame
Type" registry (Section 11.2.1 of [HTTP3]).
Frame Type: ALTSVC
Value: 0xa
Specification: Section 4 of this document
7.4. Alt-Svc Parameter Registry
The "Hypertext Transfer Protocol (HTTP) Alt-Svc Parameter Registry" The "Hypertext Transfer Protocol (HTTP) Alt-Svc Parameter Registry"
defines the name space for parameters. It has been created and will defines the name space for parameters. It has been created and will
be maintained at <http://www.iana.org/assignments/http-alt-svc- be maintained at <http://www.iana.org/assignments/http-alt-svc-
parameters>. parameters>.
7.3.1. Procedure 7.4.1. Procedure
A registration MUST include the following fields: A registration MUST include the following fields:
o Parameter Name o Parameter Name
o Pointer to specification text o Pointer to specification text
Values to be added to this name space require Expert Review (see Values to be added to this name space require Expert Review (see
Section 4.1 of [RFC5226]). Section 4.1 of [RFC5226]).
7.3.2. Registrations 7.4.2. Registrations
The "Hypertext Transfer Protocol (HTTP) Alt-Svc Parameter Registry" The "Hypertext Transfer Protocol (HTTP) Alt-Svc Parameter Registry"
has been populated with the registrations below: has been populated with the registrations below:
+-------------------+-------------+ +-------------------+--------------+
| Alt-Svc Parameter | Reference | | Alt-Svc Parameter | Reference |
+-------------------+-------------+ +-------------------+--------------+
| ma | Section 3.1 | | ma | Section 3.1 |
| | | | | |
| persist | Section 3.1 | | persist | Section 3.1 |
+-------------------+-------------+ +-------------------+--------------+
8. Internationalization Considerations 8. Internationalization Considerations
An internationalized domain name that appears in either the header An internationalized domain name that appears in either the header
field (Section 3) or the HTTP/2 frame (Section 4) MUST be expressed field (Section 3) or the HTTP/2 frame (Section 4) MUST be expressed
using A-labels (Section 2.3.2.1 of [RFC5890]). using A-labels (Section 2.3.2.1 of [RFC5890]).
9. Security Considerations 9. Security Considerations
9.1. Changing Ports 9.1. Changing Ports
skipping to change at page 17, line 44 skipping to change at page 18, line 16
connection to be migrated to a different protocol and port, these connection to be migrated to a different protocol and port, these
applications can become confused about the security properties of a applications can become confused about the security properties of a
given connection, sending information (for example, cookies and given connection, sending information (for example, cookies and
content) that is intended for a secure context (such as an "https://" content) that is intended for a secure context (such as an "https://"
URI) to a client that is not treating it as one. URI) to a client that is not treating it as one.
This risk can be mitigated in servers by using the URI scheme This risk can be mitigated in servers by using the URI scheme
explicitly carried by the protocol (such as ":scheme" in HTTP/2 or explicitly carried by the protocol (such as ":scheme" in HTTP/2 or
the "absolute form" of the request target in HTTP/1.1) as an the "absolute form" of the request target in HTTP/1.1) as an
indication of security context, instead of other connection indication of security context, instead of other connection
properties (Section 8.1.2.3 of [RFC7540] and Section 5.3.2 of properties (Section 8.3.1 of [HTTP2] and Section 3.2.2 of [HTTP11]).
[RFC7230]).
When the protocol does not explicitly carry the scheme (as is usually When the protocol does not explicitly carry the scheme (as is usually
the case for HTTP/1.1 over TLS), servers can mitigate this risk by the case for HTTP/1.1 over TLS), servers can mitigate this risk by
either assuming that all requests have an insecure context, or by either assuming that all requests have an insecure context, or by
refraining from advertising alternative services for insecure schemes refraining from advertising alternative services for insecure schemes
(for example, HTTP). (for example, HTTP).
10. References 10. References
10.1. Normative References 10.1. Normative References
[Caching] Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
Caching", draft-ietf-httpbis-cache-19 (work in progress),
September 2021.
[HTTP] Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
Semantics", draft-ietf-httpbis-semantics-19 (work in
progress), September 2021.
[HTTP11] Fielding, R. T., Nottingham, M., and J. Reschke,
"HTTP/1.1", draft-ietf-httpbis-messaging-19 (work in
progress), September 2021.
[HTTP2] Belshe, M., Peon, R., and M. Thomson, "Hypertext Transfer
Protocol Version 2 (HTTP/2)", draft-ietf-httpbis-http2-17
(work in progress), February 2015.
[HTTP3] Bishop, M., "Hypertext Transfer Protocol Version 3
(HTTP/3)", draft-ietf-quic-http-34 (work in progress),
February 2021.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000, DOI 10.17487/RFC2818, May 2000,
<https://www.rfc-editor.org/info/rfc2818>. <https://www.rfc-editor.org/info/rfc2818>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
skipping to change at page 18, line 47 skipping to change at page 19, line 38
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066, Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011, DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>. <https://www.rfc-editor.org/info/rfc6066>.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, [RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC6454, December 2011, DOI 10.17487/RFC6454, December 2011,
<https://www.rfc-editor.org/info/rfc6454>. <https://www.rfc-editor.org/info/rfc6454>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014,
<https://www.rfc-editor.org/info/rfc7234>.
[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan, [RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol "Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301, Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/info/rfc7301>. July 2014, <https://www.rfc-editor.org/info/rfc7301>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF", [RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014, RFC 7405, DOI 10.17487/RFC7405, December 2014,
<https://www.rfc-editor.org/info/rfc7405>. <https://www.rfc-editor.org/info/rfc7405>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.
10.2. Informative References 10.2. Informative References
[BCP90] Consisting of: [RFC3864], [BCP90] Consisting of: [RFC3864],
<https://www.rfc-editor.org/info/bcp90>. <https://www.rfc-editor.org/info/bcp90>.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864, Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004, DOI 10.17487/RFC3864, September 2004,
<https://www.rfc-editor.org/info/rfc3864>. <https://www.rfc-editor.org/info/rfc3864>.
skipping to change at page 20, line 5 skipping to change at page 20, line 27
<https://www.rfc-editor.org/info/rfc6265>. <https://www.rfc-editor.org/info/rfc6265>.
[RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning [RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
2015, <https://www.rfc-editor.org/info/rfc7469>. 2015, <https://www.rfc-editor.org/info/rfc7469>.
[RFC7838] Nottingham, M., McManus, P., and J. Reschke, "HTTP [RFC7838] Nottingham, M., McManus, P., and J. Reschke, "HTTP
Alternative Services", RFC 7838, DOI 10.17487/RFC7838, Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
April 2016, <https://www.rfc-editor.org/info/rfc7838>. April 2016, <https://www.rfc-editor.org/info/rfc7838>.
10.3. URIs
[1] https://lists.w3.org/Archives/Public/ietf-http-wg/
[2] https://httpwg.org/
[3] https://github.com/httpwg/http-extensions/labels/alt-svc
Acknowledgments Acknowledgments
The previous version of this document was authored by Mark The previous version of this document was authored by Mark
Nottingham, Patrick McManus, and Julian F. Reschke. [RFC7838] Nottingham, Patrick McManus, and Julian F. Reschke. [RFC7838]
contains a more extensive list of people who contributed to that contains a more extensive list of people who contributed to that
document. document.
Authors' Addresses Authors' Addresses
Mike Bishop (editor) Mike Bishop (editor)
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