HTTPbis Working Group M. Nottingham
Internet-Draft Akamai
Intended status: Standards Track P. McManus
Expires: January 5, 2015 Mozilla
J. Reschke
greenbytes
July 4, 2014
HTTP Alternative Services
draft-ietf-httpbis-alt-svc-02
Abstract
This document specifies "alternative services" for HTTP, which allow
an origin's resources to be authoritatively available at a separate
network location, possibly accessed with a different protocol
configuration.
Editorial Note (To be removed by RFC Editor)
Discussion of this draft takes place on the HTTPBIS working group
mailing list (ietf-http-wg@w3.org), which is archived at
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Working Group information can be found at
and ;
source code and issues list for tis draft can be found at
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The changes in this draft are summarized in Appendix A.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on January 5, 2015.
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Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . . 3
2. Alternative Services Concepts . . . . . . . . . . . . . . . . 4
2.1. Host Authentication . . . . . . . . . . . . . . . . . . . 5
2.2. Alternative Service Caching . . . . . . . . . . . . . . . 6
2.3. Requiring Server Name Indication . . . . . . . . . . . . . 6
2.4. Using Alternative Services . . . . . . . . . . . . . . . . 6
3. The Alt-Svc HTTP Header Field . . . . . . . . . . . . . . . . 7
3.1. Caching Alt-Svc Header Field Values . . . . . . . . . . . 9
4. The ALTSVC HTTP/2 Frame . . . . . . . . . . . . . . . . . . . 10
5. The Alt-Svc-Used HTTP Header Field . . . . . . . . . . . . . . 11
6. The 421 Not Authoritative HTTP Status Code . . . . . . . . . . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
7.1. Header Field Registrations . . . . . . . . . . . . . . . . 12
7.2. The ALTSVC HTTP/2 Frame Type . . . . . . . . . . . . . . . 13
8. Internationalization Considerations . . . . . . . . . . . . . 13
9. Security Considerations . . . . . . . . . . . . . . . . . . . 13
9.1. Changing Ports . . . . . . . . . . . . . . . . . . . . . . 13
9.2. Changing Hosts . . . . . . . . . . . . . . . . . . . . . . 13
9.3. Changing Protocols . . . . . . . . . . . . . . . . . . . . 14
9.4. Tracking Clients Using Alternative Services . . . . . . . 14
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
11.1. Normative References . . . . . . . . . . . . . . . . . . . 15
11.2. Informative References . . . . . . . . . . . . . . . . . . 16
Appendix A. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 16
A.1. Since draft-nottingham-httpbis-alt-svc-05 . . . . . . . . 16
A.2. Since draft-ietf-httpbis-alt-svc-00 . . . . . . . . . . . 16
A.3. Since draft-ietf-httpbis-alt-svc-01 . . . . . . . . . . . 16
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1. Introduction
HTTP [RFC7230] conflates the identification of resources with their
location. In other words, "http://" (and "https://") URLs are used
to both name and find things to interact with.
In some cases, it is desirable to separate these aspects; to be able
to keep the same identifier for a resource, but interact with it
using a different location on the network.
For example:
o An origin server might wish to redirect a client to an alternative
when it needs to go down for maintenance, or it has found an
alternative in a location that is more local to the client.
o An origin server might wish to offer access to its resources using
a new protocol (such as HTTP/2, see [HTTP2]) or one using improved
security (such as Transport Layer Security (TLS), see [RFC5246]).
o An origin server might wish to segment its clients into groups of
capabilities, such as those supporting Server Name Indication
(SNI, see Section 3 of [RFC6066]) and those not supporting it, for
operational purposes.
This specification defines a new concept in HTTP, "Alternative
Services", that allows a resource to nominate additional means of
interacting with it on the network. It defines a general framework
for this in Section 2, along with specific mechanisms for advertising
their existence using HTTP header fields (Section 3) or an HTTP/2
frame type (Section 4).
It also introduces a new status code in Section 6, so that origin
servers (or their nominated alternatives) can indicate that they are
not authoritative for a given origin, in cases where the wrong
location is used.
1.1. 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].
This document uses the Augmented BNF defined in [RFC5234] along with
the "OWS", "delta-seconds", "parameter", "port", "quoted-string",
"token", and "uri-host" rules from [RFC7230], and uses the "#rule"
extension defined in Appendix of that document.
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2. Alternative Services Concepts
This specification defines a new concept in HTTP, the ""alternative
service"". When an origin (see [RFC6454]) has resources that are
accessible through a different protocol / host / port combination, it
is said to have an alternative service.
An alternative service can be used to interact with the resources on
an origin server at a separate location on the network, possibly
using a different protocol configuration. Alternative services are
considered authoritative for an origin's resources, in the sense of
[RFC7230], Appendix .
For example, an origin:
("http", "www.example.com", "80")
might declare that its resources are also accessible at the
alternative service:
("h2", "new.example.com", "81")
By their nature, alternative services are explicitly at the
granularity of an origin; i.e., they cannot be selectively applied to
resources within an origin.
Alternative services do not replace or change the origin for any
given resource; in general, they are not visible to the software
"above" the access mechanism. The alternative service is essentially
alternative routing information that can also be used to reach the
origin in the same way that DNS CNAME or SRV records define routing
information at the name resolution level. Each origin maps to a set
of these routes -- the default route is derived from origin itself
and the other routes are introduced based on alternative-protocol
information.
Furthermore, it is important to note that the first member of an
alternative service tuple is different from the "scheme" component of
an origin; it is more specific, identifying not only the major
version of the protocol being used, but potentially communication
options for that protocol.
This means that clients using an alternative service will change the
host, port and protocol that they are using to fetch resources, but
these changes MUST NOT be propagated to the application that is using
HTTP; from that standpoint, the URI being accessed and all
information derived from it (scheme, host, port) are the same as
before.
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Importantly, this includes its security context; in particular, when
TLS [RFC5246] is in use, the alternative server will need to present
a certificate for the origin's host name, not that of the
alternative. Likewise, the Host header field ([RFC7230], Appendix )
is still derived from the origin, not the alternative service (just
as it would if a CNAME were being used).
The changes MAY, however, be made visible in debugging tools,
consoles, etc.
Formally, an alternative service is identified by the combination of:
o An Application Layer Protocol Negotiation (ALPN) protocol, as per
[ALPN]
o A host, as per [RFC3986], Section 3.2.2
o A port, as per [RFC3986], Section 3.2.3
Additionally, each alternative service MUST have:
o A freshness lifetime, expressed in seconds; see Section 2.2
There are many ways that a client could discover the alternative
service(s) associated with an origin. This document describes two
such mechanisms: an HTTP header field (Section 3) and an HTTP/2 frame
type (Section 4).
2.1. Host Authentication
Clients MUST NOT use alternative services with a host other than the
origin's without strong server authentication; this mitigates the
attack described in Section 9.2. One way to achieve this is for the
alternative to use TLS with a certificate that is valid for that
origin.
For example, if the origin's host is "www.example.com" and an
alternative is offered on "other.example.com" with the "h2" protocol,
and the certificate offered is valid for "www.example.com", the
client can use the alternative. However, if "other.example.com" is
offered with the "h2c" protocol, the client cannot use it, because
there is no mechanism in that protocol to establish strong server
authentication.
Furthermore, this means that the HTTP Host header field and the SNI
information provided in TLS by the client will be that of the origin,
not the alternative.
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2.2. Alternative Service Caching
Mechanisms for discovering alternative services can associate a
freshness lifetime with them; for example, the Alt-Svc header field
uses the "ma" parameter.
Clients MAY choose to use an alternative service instead of the
origin at any time when it is considered fresh; see Section 2.4 for
specific recommendations.
Clients with existing connections to alternative services are not
needed to fall back to the origin when its freshness lifetime ends;
i.e., the caching mechanism is intended for limiting how long an
alternative service can be used for establishing new requests, not
limiting the use of existing ones.
To mitigate risks associated with caching compromised values (see
Section 9.2 for details), user agents SHOULD examine cached
alternative services when they detect a change in network
configuration, and remove any that could be compromised (for example,
those whose association with the trust root is questionable). UAs
that do not have a means of detecting network changes SHOULD place an
upper bound on their lifetime.
2.3. Requiring Server Name Indication
A client MUST only use a TLS-based alternative service if the client
also supports TLS Server Name Indication (SNI). This supports the
conservation of IP addresses on the alternative service host.
2.4. Using Alternative Services
By their nature, alternative services are OPTIONAL: clients do not
need to use them. However, it is advantageous for clients to behave
in a predictable way when they are used by servers (e.g., for load
balancing).
Therefore, if a client becomes aware of an alternative service, the
client SHOULD use that alternative service for all requests to the
associated origin as soon as it is available, provided that the
security properties of the alternative service protocol are
desirable, as compared to the existing connection.
When a client uses an alternate service, it MUST emit the Alt-Svc-
Used header field (Section 5) on every request using that alternate
service.
The client does not need to block requests; the origin's connection
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can be used until the alternative connection is established.
However, if the security properties of the existing connection are
weak (e.g. cleartext HTTP/1.1) then it might make sense to block
until the new connection is fully available in order to avoid
information leakage.
Furthermore, if the connection to the alternative service fails or is
unresponsive, the client MAY fall back to using the origin. Note,
however, that this could be the basis of a downgrade attack, thus
losing any enhanced security properties of the alternative service.
3. The Alt-Svc HTTP Header Field
An HTTP(S) origin server can advertise the availability of
alternative services to clients by adding an Alt-Svc header field to
responses.
Alt-Svc = 1#( alternative *( OWS ";" OWS parameter ) )
alternative = protocol-id "=" alt-authority
protocol-id = token ; percent-encoded ALPN protocol identifier
alt-authority = token / quoted-string
; containing [ uri-host ] ":" port
ALPN protocol names are octet sequences with no additional
constraints on format. Octets not allowed in tokens ([RFC7230],
Appendix ) MUST be percent-encoded as per Section 2.1 of [RFC3986].
Consequently, the octet representing the percent character "%" (hex
25) MUST be percent-encoded as well.
In order to have precisely one way to represent any ALPN protocol
name, the following additional constraints apply:
1. Octets in the ALPN protocol MUST NOT be percent-encoded if they
are valid token characters except "%", and
2. When using percent-encoding, uppercase hex digits MUST be used.
With these constraints, recipients can apply simple string comparison
to match protocol identifiers.
The "alt-authority" component consists of an OPTIONAL uri-host
("host" in Section 3.2.2 of [RFC3986]), a colon (":"), and a port
number.
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For example:
Alt-Svc: http2=":8000"
This indicates the "http2" protocol on the same host using the
indicated port 8000.
An example involving a change of host:
Alt-Svc: http2="new.example.org:80"
This indicates the "http2" protocol on the host "new.example.org",
running on port 80. Note that the "quoted-string" syntax needs to be
used when a host is specified in addition to a port (":" is not an
allowed character in "token").
Examples for protocol name escaping:
+--------------------+-------------+---------------------+
| ALPN protocol name | protocol-id | Note |
+--------------------+-------------+---------------------+
| http2 | http2 | No escaping needed |
+--------------------+-------------+---------------------+
| w=x:y#z | w%3Dx%3Ay#z | "=" and ":" escaped |
+--------------------+-------------+---------------------+
| x%y | x%25y | "%" needs escaping |
+--------------------+-------------+---------------------+
Alt-Svc MAY occur in any HTTP response message, regardless of the
status code.
Alt-Svc does not allow advertisement of alternative services on other
hosts, to protect against various header-based attacks.
It can, however, have multiple values:
Alt-Svc: h2c=":8000", h2=":443"
The value(s) advertised by Alt-Svc can be used by clients to open a
new connection to one or more alternative services immediately, or
simultaneously with subsequent requests on the same connection.
To reduce the ability of servers to track individual clients over
time (see Section 9.4), an alternative service indication sent by a
client SHOULD NOT include any alternative service information other
than the protocol, host and port.
When using HTTP/2 ([HTTP2]), clients SHOULD instead send an ALTSVC
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frame. A single ALTSVC frame can be sent for a connection; a new
frame is not needed for every request.
Note that all field elements that allow "quoted-string" syntax MUST
be processed as per Appendix of [RFC7230].
3.1. Caching Alt-Svc Header Field Values
When an alternative service is advertised using Alt-Svc, it is
considered fresh for 24 hours from generation of the message. This
can be modified with the 'ma' (max-age) parameter;
Alt-Svc: h2=":443"; ma=3600
which indicates the number of seconds since the response was
generated the alternative service is considered fresh for.
ma = delta-seconds
See Section 4.2.3 of [RFC7234] for details of determining response
age.
For example, a response:
HTTP/1.1 200 OK
Content-Type: text/html
Cache-Control: 600
Age: 30
Alt-Svc: h2c=":8000"; ma=60
indicates that an alternative service is available and usable for the
next 60 seconds. However, the response has already been cached for
30 seconds (as per the Age header field value), so therefore the
alternative service is only fresh for the 30 seconds from when this
response was received, minus estimated transit time.
When an Alt-Svc response header field is received from an origin, its
value invalidates and replaces all cached alternative services for
that origin.
See Section 2.2 for general requirements on caching alternative
services.
Note that the freshness lifetime for HTTP caching (here, 600 seconds)
does not affect caching of Alt-Svc values.
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4. The ALTSVC HTTP/2 Frame
The ALTSVC HTTP/2 frame ([HTTP2], Section 4) advertises the
availability of an alternative service to an HTTP/2 client.
The ALTSVC frame is a non-critical extension to HTTP/2. Endpoints
that do not support this frame can safely ignore it.
An ALTSVC frame on a client-initiated stream indicates that the
conveyed alternative service is associated with the origin of that
stream.
An ALTSVC frame on stream 0 indicates that the conveyed alternative
service is associated with the origin contained in the Origin field
of the frame. An association with an origin that the client does not
consider authoritative for the current connection MUST be ignored.
The ALTSVC frame type is 0xa (decimal 10).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Max-Age (32) |
+-------------------------------+---------------+---------------+
| Port (16) | Proto-Len (8) |
+-------------------------------+---------------+---------------+
| Protocol-ID (*) |
+---------------+-----------------------------------------------+
| Host-Len (8) | Host (*) ...
+---------------+-----------------------------------------------+
| Origin? (*) ...
+---------------------------------------------------------------+
ALTSVC Frame Payload
The ALTSVC frame contains the following fields:
Max-Age: An unsigned, 32-bit integer indicating the freshness
lifetime of the alternative service association, as per
Section 2.2.
Port: An unsigned, 16-bit integer indicating the port that the
alternative service is available upon.
Proto-Len: An unsigned, 8-bit integer indicating the length, in
octets, of the Protocol-ID field.
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Protocol-ID: A sequence of bytes (length determined by "Proto-Len")
containing the ALPN protocol identifier of the alternative
service.
Host-Len: An unsigned, 8-bit integer indicating the length, in
octets, of the Host header field.
Host: A sequence of characters (length determined by "Host-Len")
containing an ASCII string indicating the host that the
alternative service is available upon.
Origin: An OPTIONAL sequence of characters (length determined by
subtracting the length of all preceding fields from the frame
length) containing the ASCII serialisation of an origin
([RFC6454], Section 6.2) that the alternate service is applicable
to.
The ALTSVC frame does not define any flags.
The ALTSVC frame is intended for receipt by clients; a server that
receives an ALTSVC frame MUST treat it as a connection error of type
PROTOCOL_ERROR.
The ALTSVC frame is processed hop-by-hop. An intermediary MUST NOT
forward ALTSVC frames, though it can use the information contained in
ALTSVC frames in forming new ALTSVC frames to send to its own
clients.
5. The Alt-Svc-Used HTTP Header Field
The Alt-Svc-Used HTTP header field is used in requests to indicate
that an alternate service is in use.
Alt-Svc-Used = ("1" / "0") *( OWS ";" OWS Alt-Svc-Used-Ext )
Alt-Svc-Used-Ext = token "=" ( token / quoted-string )
Alt-Svc-Used is intended to allow alternate services to avoid sending
alternative service indications where there is a risk of generating a
loops. It also allows a service to identify requests for accounting
and load balancing purposes.
When using an alternative service, clients MUST include a Alt-Svc-
Used header field in all requests.
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For example:
GET /thing HTTP/1.1
Host: origin.example.com
Alt-Svc-Used: 1
The extension parameters (Alt-Svc-Used-Ext) are reserved for future
use; specifications that want to define an extension will need to
update this document (and ought to introduce an extension registry).
6. The 421 Not Authoritative HTTP Status Code
The 421 (Not Authoritative) status code is defined in [HTTP2],
Section 9.1.2 to indicate that the current server instance is not
authoritative for the requested resource. This can be used to
indicate that an alternative service is not authoritative; see
Section 2).
Clients receiving 421 (Not Authoritative) from an alternative service
MUST remove the corresponding entry from its alternative service
cache (see Section 2.2) for that origin. Regardless of the
idempotency of the request method, they MAY retry the request, either
at another alternative server, or at the origin.
A 421 (Not Authoritative) response MAY carry an Alt-Svc header field.
7. IANA Considerations
7.1. Header Field Registrations
HTTP header fields are registered within the "Message Headers"
registry maintained at
.
This document defines the following HTTP header fields, so their
associated registry entries shall be added according to the permanent
registrations below (see [BCP90]):
+-------------------+----------+----------+-----------+
| Header Field Name | Protocol | Status | Reference |
+-------------------+----------+----------+-----------+
| Alt-Svc | http | standard | Section 3 |
| Alt-Svc-Used | http | standard | Section 5 |
+-------------------+----------+----------+-----------+
The change controller is: "IETF (iesg@ietf.org) - Internet
Engineering Task Force".
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7.2. The ALTSVC HTTP/2 Frame Type
This document registers the ALTSVC frame type in the HTTP/2 Frame
Types registry ([HTTP2], Section 11.2).
Frame Type: ALTSVC
Code: 0xa
Specification: Section 4 of this document
8. Internationalization Considerations
An internationalized domain name that appears in either the header
field (Section 3) or the HTTP/2 frame (Section 4) MUST be expressed
using A-labels ([RFC5890], Section 2.3.2.1).
9. Security Considerations
9.1. Changing Ports
Using an alternative service implies accessing an origin's resources
on an alternative port, at a minimum. An attacker that can inject
alternative services and listen at the advertised port is therefore
able to hijack an origin.
For example, an attacker that can add HTTP response header fields can
redirect traffic to a different port on the same host using the Alt-
Svc header field; if that port is under the attacker's control, they
can thus masquerade as the HTTP server.
This risk can be mitigated by restricting the ability to advertise
alternative services, and restricting who can open a port for
listening on that host.
9.2. Changing Hosts
When the host is changed due to the use of an alternative service, it
presents an opportunity for attackers to hijack communication to an
origin.
For example, if an attacker can convince a user agent to send all
traffic for "innocent.example.org" to "evil.example.com" by
successfully associating it as an alternative service, they can
masquerade as that origin. This can be done locally (see mitigations
above) or remotely (e.g., by an intermediary as a man-in-the-middle
attack).
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This is the reason for the requirement in Section 2.1 that any
alternative service with a host different to the origin's be strongly
authenticated with the origin's identity; i.e., presenting a
certificate for the origin proves that the alternative service is
authorized to serve traffic for the origin.
However, this authorization is only as strong as the method used to
authenticate the alternative service. In particular, there are well-
known exploits to make an attacker's certificate appear as
legitimate.
Alternative services could be used to persist such an attack; for
example, an intermediary could man-in-the-middle TLS-protected
communication to a target, and then direct all traffic to an
alternative service with a large freshness lifetime, so that the user
agent still directs traffic to the attacker even when not using the
intermediary.
As a result, there is a requirement in Section 2.2 to examine cached
alternative services when a network change is detected.
9.3. Changing Protocols
When the ALPN protocol is changed due to the use of an alternative
service, the security properties of the new connection to the origin
can be different from that of the "normal" connection to the origin,
because the protocol identifier itself implies this.
For example, if a "https://" URI had a protocol advertised that does
not use some form of end-to-end encryption (most likely, TLS), it
violates the expectations for security that the URI scheme implies.
Therefore, clients cannot blindly use alternative services, but
instead evaluate the option(s) presented to assure that security
requirements and expectations (of specifications, implementations and
end users) are met.
9.4. Tracking Clients Using Alternative Services
The alternative service indicator (Section 5) provided by clients
provides a server the means of correlating requests. If the
alternative service indicator includes a sufficiently unique
identifier, requests made to an alternative service can be correlated
with the original alternative service advertisement.
Clients that do not wish to be tracked MAY choose to ignore
alternative service advertisements.
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In a browser, any alternative service information MUST be removed
when origin-specific data is cleared (for instance, when cookies are
cleared).
10. Acknowledgements
Thanks to Eliot Lear, Stephen Farrell, Guy Podjarny, Stephen Ludin,
Erik Nygren, Paul Hoffman, Adam Langley, Will Chan and Richard Barnes
for their feedback and suggestions.
The Alt-Svc header field was influenced by the design of the
Alternate-Protocol header field in SPDY.
11. References
11.1. Normative References
[ALPN] Friedl, S., Popov, A., Langley, A., and S. Emile,
"Transport Layer Security (TLS) Application Layer Protocol
Negotiation Extension", draft-ietf-tls-applayerprotoneg-05
(work in progress), March 2014.
[HTTP2] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol version 2", draft-ietf-httpbis-http2-13
(work in progress), June 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
December 2011.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
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RFC 7230, June 2014.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, June 2014.
11.2. Informative References
[BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
Appendix A. Change Log (to be removed by RFC Editor before publication)
A.1. Since draft-nottingham-httpbis-alt-svc-05
This is the first version after adoption of
draft-nottingham-httpbis-alt-svc-05 as Working Group work item. It
only contains editorial changes.
A.2. Since draft-ietf-httpbis-alt-svc-00
Selected 421 as proposed status code for "Not Authoritative".
Changed header field syntax to use percent-encoding of ALPN protocol
names ().
A.3. Since draft-ietf-httpbis-alt-svc-01
Updated HTTP/1.1 references.
Renamed "Service" to "Alt-Svc-Used" and reduced information to a flag
to address fingerprinting concerns
().
Note that ALTSVC frame is preferred to Alt-Svc header field
().
Incorporate ALTSRV frame
().
Moved definition of status code 421 to HTTP/2.
Partly resolved .
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Internet-Draft Alternative Services July 2014
Authors' Addresses
Mark Nottingham
Akamai
EMail: mnot@mnot.net
URI: https://www.mnot.net/
Patrick McManus
Mozilla
EMail: mcmanus@ducksong.com
URI: https://mozillians.org/u/pmcmanus/
Julian F. Reschke
greenbytes GmbH
EMail: julian.reschke@greenbytes.de
URI: https://greenbytes.de/tech/webdav/
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