One document matched: draft-ietf-httpbis-alt-svc-02.xml
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<rfc ipr="trust200902" category="std" docName="draft-ietf-httpbis-alt-svc-02">
<front>
<title abbrev="Alternative Services">HTTP Alternative Services</title>
<author initials="M." surname="Nottingham" fullname="Mark Nottingham">
<organization>Akamai</organization>
<address>
<email>mnot@mnot.net</email>
<uri>https://www.mnot.net/</uri>
</address>
</author>
<author initials="P." surname="McManus" fullname="Patrick McManus">
<organization>Mozilla</organization>
<address>
<email>mcmanus@ducksong.com</email>
<uri>https://mozillians.org/u/pmcmanus/</uri>
</address>
</author>
<author initials="J. F." surname="Reschke" fullname="Julian F. Reschke">
<organization abbrev="greenbytes">greenbytes GmbH</organization>
<address>
<email>julian.reschke@greenbytes.de</email>
<uri>https://greenbytes.de/tech/webdav/</uri>
</address>
</author>
<date year="2014" month="July" day="4"/>
<area>Applications</area>
<workgroup>HTTPbis Working Group</workgroup>
<keyword>HTTP</keyword>
<keyword>ALPN</keyword>
<keyword>Alternate Services</keyword>
<abstract>
<t>
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.</t>
</abstract>
<note title="Editorial Note (To be removed by RFC Editor)">
<t>
Discussion of this draft takes place on the HTTPBIS working group mailing list
(ietf-http-wg@w3.org), which is archived at <eref target="https://lists.w3.org/Archives/Public/ietf-http-wg/"/>.
</t>
<t>
Working Group information can be found at <eref target="https://tools.ietf.org/wg/httpbis/"/>
and <eref target="http://httpwg.github.io/"/>; source code and issues
list for tis draft can be found at
<eref target="https://github.com/httpwg/http-extensions"/>.
</t>
<t>
The changes in this draft are summarized in <xref target="change.log"/>.
</t>
</note>
</front>
<middle>
<section anchor="introduction" title="Introduction">
<t>
HTTP <xref target="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.
</t>
<t>
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.
</t>
<t>For example:
<list style="symbols">
<t>
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.
</t>
<t>
An origin server might wish to offer access to its resources using a new
protocol (such as HTTP/2, see <xref target="HTTP2"/>) or one using improved
security (such as Transport Layer Security (TLS), see <xref target="RFC5246"/>).
</t>
<t>
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 <xref target="RFC6066"/>) and those not
supporting it, for operational purposes.
</t>
</list>
</t>
<t>
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 <xref target="alternative"/>, along with specific mechanisms for advertising their
existence using HTTP header fields (<xref target="alt-svc"/>) or an HTTP/2
frame type (<xref target="frame"/>).
</t>
<t>
It also introduces a new status code in <xref target="status"/>, 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.
</t>
<section title="Notational Conventions" anchor="notational-conventions">
<t>
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 <xref target="RFC2119"/>.
</t>
<t>
This document uses the Augmented BNF defined in <xref target="RFC5234"/>
along with the "OWS", "delta-seconds", "parameter", "port", "quoted-string", "token",
and "uri-host" rules from <xref target="RFC7230"/>, and uses the "#rule"
extension defined in Section 7 of that document.
</t>
</section>
</section>
<section title="Alternative Services Concepts" anchor="alternative">
<t>
This specification defines a new concept in HTTP, the "alternative service".
When an origin (see <xref target="RFC6454"/>) has resources that are accessible through a
different protocol / host / port combination, it is said to have an alternative
service.
</t>
<t>
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 <xref target="RFC7230"/>, Section 9.1.
</t>
<figure>
<preamble>For example, an origin:</preamble>
<artwork type="example"><![CDATA[
("http", "www.example.com", "80")
]]></artwork>
</figure>
<figure>
<preamble>might declare that its resources are also accessible at the alternative service:</preamble>
<artwork type="example"><![CDATA[
("h2", "new.example.com", "81")
]]></artwork>
</figure>
<t>
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.
</t>
<t>
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.
</t>
<t>
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.
</t>
<t>
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.
</t>
<t>
Importantly, this includes its security context; in particular, when TLS
<xref target="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 (<xref target="RFC7230"/>, Section 5.4) is
still derived from the origin, not the alternative service (just as it would
if a CNAME were being used).
</t>
<t>
The changes MAY, however, be made visible in debugging tools, consoles, etc.
</t>
<t>
Formally, an alternative service is identified by the combination of:
<list style="symbols">
<t>An Application Layer Protocol Negotiation (ALPN) protocol, as per
<xref target="ALPN"/></t>
<t>A host, as per <xref target="RFC3986"/>, Section 3.2.2</t>
<t>A port, as per <xref target="RFC3986"/>, Section 3.2.3</t>
</list>
</t>
<t>
Additionally, each alternative service MUST have:
<list style="symbols">
<t>A freshness lifetime, expressed in seconds; see <xref target="caching"/></t>
</list>
</t>
<t>
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 (<xref target="alt-svc"/>) and an HTTP/2 frame type
(<xref target="frame"/>).
</t>
<section title="Host Authentication" anchor="host_auth">
<t>
Clients MUST NOT use alternative services with a host other than the origin's
without strong server authentication; this mitigates the attack described in
<xref target="host_security"/>. One way to achieve this is for the alternative to use TLS
with a certificate that is valid for that origin.
</t>
<t>
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.
</t>
<t>
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.
</t>
</section>
<section title="Alternative Service Caching" anchor="caching">
<t>
Mechanisms for discovering alternative services can associate a freshness
lifetime with them; for example, the Alt-Svc header field uses the "ma"
parameter.
</t>
<t>
Clients MAY choose to use an alternative service instead of the origin at any
time when it is considered fresh; see <xref target="switching"/> for specific
recommendations.
</t>
<t>
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.
</t>
<t>
To mitigate risks associated with caching compromised values (see
<xref target="host_security"/> 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.
</t>
</section>
<section title="Requiring Server Name Indication" anchor="requiring-server-name-indication">
<t>
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.
</t>
</section>
<section title="Using Alternative Services" anchor="switching">
<t>
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).
</t>
<t>
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.
</t>
<t>
When a client uses an alternate service, it MUST emit the Alt-Svc-Used
header field (<xref target="indicator"/>) on every request using that alternate
service.
</t>
<t>
The client does not need to block requests; the origin's connection 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.
</t>
<t>
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.
</t>
</section>
</section>
<section title="The Alt-Svc HTTP Header Field" anchor="alt-svc">
<t>
An HTTP(S) origin server can advertise the availability of alternative services
to clients by adding an Alt-Svc header field to responses.
</t>
<figure>
<artwork type="abnf2616"><![CDATA[
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
]]></artwork>
</figure>
<t>
ALPN protocol names are octet sequences with no
additional constraints on format.<!-- besides length?-->
Octets not allowed in tokens (<xref target="RFC7230"/>, Section 3.2.6)
MUST be percent-encoded as per Section 2.1 of <xref target="RFC3986"/>.
Consequently, the octet representing the percent character "%" (hex 25) MUST
be percent-encoded as well.
</t>
<t>
In order to have precisely one way to represent any ALPN protocol name, the following
additional constraints apply:
<list style="numbers">
<t>
Octets in the ALPN protocol MUST NOT be percent-encoded if they are valid
token characters except "%", and
</t>
<t>
When using percent-encoding, uppercase hex digits MUST be used.
</t>
</list>
</t>
<t>
With these constraints, recipients can apply simple string comparison to
match protocol identifiers.
</t>
<t>
The "alt-authority" component consists of an OPTIONAL uri-host
("host" in Section 3.2.2 of <xref target="RFC3986"/>), a colon (":"),
and a port number.
</t>
<figure>
<preamble>For example:</preamble>
<artwork type="example"><![CDATA[
Alt-Svc: http2=":8000"
]]></artwork>
<postamble>
This indicates the "http2" protocol on the same host using the
indicated port 8000.
</postamble>
</figure>
<figure>
<preamble>An example involving a change of host:</preamble>
<artwork type="example"><![CDATA[
Alt-Svc: http2="new.example.org:80"
]]></artwork>
<postamble>
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").
</postamble>
</figure>
<texttable style="all" align="left">
<preamble>Examples for protocol name escaping:</preamble>
<ttcol>ALPN protocol name</ttcol>
<ttcol>protocol-id</ttcol>
<ttcol>Note</ttcol>
<c>http2</c>
<c>http2</c>
<c>No escaping needed</c>
<c>w=x:y#z</c>
<c>w%3Dx%3Ay#z</c>
<c>"=" and ":" escaped</c>
<c>x%y</c>
<c>x%25y</c>
<c>"%" needs escaping</c>
</texttable>
<t>
Alt-Svc MAY occur in any HTTP response message, regardless of the status code.
</t>
<t>
Alt-Svc does not allow advertisement of alternative services on other hosts, to
protect against various header-based attacks.
</t>
<figure>
<preamble>It can, however, have multiple values:</preamble>
<artwork type="example"><![CDATA[
Alt-Svc: h2c=":8000", h2=":443"
]]></artwork>
</figure>
<t>
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.
</t>
<t>
To reduce the ability of servers to track individual clients over time (see <xref target="tracking"/>), an alternative service indication sent by a client SHOULD NOT include
any alternative service information other than the protocol, host and port.
</t>
<t>
When using HTTP/2 (<xref target="HTTP2"/>), clients SHOULD instead send an ALTSVC frame.
A single ALTSVC frame can be sent for a connection; a new frame is not needed for every
request.
</t>
<t>
Note that all field elements that allow "quoted-string" syntax MUST be processed
as per Section 3.2.6 of <xref target="RFC7230"/>.
</t>
<section title="Caching Alt-Svc Header Field Values" anchor="caching-alt-svc-header-field-values">
<t>
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;
</t>
<figure>
<artwork type="example"><![CDATA[
Alt-Svc: h2=":443"; ma=3600
]]></artwork>
</figure>
<t>
which indicates the number of seconds since the response was generated the
alternative service is considered fresh for.
</t>
<figure>
<artwork type="abnf2616"><![CDATA[
ma = delta-seconds
]]></artwork></figure>
<t>
See Section 4.2.3 of <xref target="RFC7234"/> for details of determining
response age.
</t>
<figure>
<preamble>For example, a response:</preamble>
<artwork type="message/http; msgtype="response""><![CDATA[
HTTP/1.1 200 OK
Content-Type: text/html
Cache-Control: 600
Age: 30
Alt-Svc: h2c=":8000"; ma=60
]]></artwork>
<postamble>
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.
</postamble>
</figure>
<t>
When an Alt-Svc response header field is received from an origin, its value
invalidates and replaces all cached alternative services for that origin.
</t>
<t>
See <xref target="caching"/> for general requirements on caching alternative services.
</t>
<t>
Note that the freshness lifetime for HTTP caching (here, 600 seconds) does not
affect caching of Alt-Svc values.
</t>
</section>
</section>
<section anchor="frame" title="The ALTSVC HTTP/2 Frame">
<t>
The ALTSVC HTTP/2 frame (<xref target="HTTP2"/>, Section 4)
advertises the availability of an alternative service to an HTTP/2 client.
</t>
<t>
The ALTSVC frame is a non-critical extension to HTTP/2. Endpoints that do
not support this frame can safely ignore it.
</t>
<t>
An ALTSVC frame on a client-initiated stream indicates that the conveyed
alternative service is associated with the origin of that stream.
</t>
<t>
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.
</t>
<t>
The ALTSVC frame type is 0xa (decimal 10).
</t>
<figure title="ALTSVC Frame Payload">
<artwork type="drawing"><![CDATA[
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? (*) ...
+---------------------------------------------------------------+
]]></artwork>
</figure>
<t>
The ALTSVC frame contains the following fields:
<list style="hanging">
<t hangText="Max-Age:">
An unsigned, 32-bit integer indicating the freshness lifetime of the alternative
service association, as per <xref target="caching"/>.
</t>
<t hangText="Port:">
An unsigned, 16-bit integer indicating the port that the alternative service is
available upon.
</t>
<t hangText="Proto-Len:">
An unsigned, 8-bit integer indicating the length, in octets, of the Protocol-ID field.
</t>
<t hangText="Protocol-ID:">
A sequence of bytes (length determined by <spanx style="verb">Proto-Len</spanx>)
containing the ALPN protocol identifier of the alternative service.
</t>
<t hangText="Host-Len:">
An unsigned, 8-bit integer indicating the length, in octets, of the Host header field.
</t>
<t hangText="Host:">
A sequence of characters (length determined by <spanx style="verb">Host-Len</spanx>)
containing an ASCII string indicating the host that the alternative service is
available upon.
</t>
<t hangText="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 (<xref target="RFC6454"/>, Section 6.2) that the alternate service is
applicable to.
</t>
</list>
</t>
<t>
The ALTSVC frame does not define any flags.
</t>
<t>
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.
</t>
<t>
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.
</t>
</section>
<section title="The Alt-Svc-Used HTTP Header Field" anchor="indicator">
<t>
The Alt-Svc-Used HTTP header field is used in requests to indicate that an
alternate service is in use.
</t>
<figure>
<artwork type="abnf2616"><![CDATA[
Alt-Svc-Used = ("1" / "0") *( OWS ";" OWS Alt-Svc-Used-Ext )
Alt-Svc-Used-Ext = token "=" ( token / quoted-string )
]]></artwork>
</figure>
<t>
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.
</t>
<t>
When using an alternative service, clients MUST include a Alt-Svc-Used
header field in all requests.
</t>
<figure>
<preamble>For example:</preamble>
<artwork type="message/http; msgtype="request""><![CDATA[
GET /thing HTTP/1.1
Host: origin.example.com
Alt-Svc-Used: 1
]]></artwork>
</figure>
<t>
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).
</t>
</section>
<section title="The 421 Not Authoritative HTTP Status Code" anchor="status">
<t>
The 421 (Not Authoritative) status code is defined in <xref target="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 <xref target="alternative"/>).
</t>
<t>
Clients receiving 421 (Not Authoritative) from an alternative service MUST
remove the corresponding entry from its alternative service cache (see <xref target="caching"/>) 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.
</t>
<t>
A 421 (Not Authoritative) response MAY carry an Alt-Svc header field.
</t>
</section>
<section title="IANA Considerations" anchor="iana-considerations">
<section title="Header Field Registrations">
<t>
HTTP header fields are registered within the "Message Headers" registry
maintained at
<eref target="https://www.iana.org/assignments/message-headers/"/>.
</t>
<t>
This document defines the following HTTP header fields, so their associated
registry entries shall be added according to the permanent registrations below
(see <xref target="BCP90"/>):
</t>
<texttable align="left" suppress-title="true" anchor="iana.header.registration.table">
<ttcol>Header Field Name</ttcol>
<ttcol>Protocol</ttcol>
<ttcol>Status</ttcol>
<ttcol>Reference</ttcol>
<c>Alt-Svc</c>
<c>http</c>
<c>standard</c>
<c>
<xref target="alt-svc"/>
</c>
<c>Alt-Svc-Used</c>
<c>http</c>
<c>standard</c>
<c>
<xref target="indicator"/>
</c>
</texttable>
<t>
The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
</t>
</section>
<section anchor="alt-svc-frame-registration" title="The ALTSVC HTTP/2 Frame Type">
<t>
This document registers the ALTSVC frame type in the HTTP/2 Frame Types
registry (<xref target="HTTP2"/>, Section 11.2).
<list style="empty">
<t>Frame Type: ALTSVC</t>
<t>Code: 0xa</t>
<t>Specification: <xref target="frame"/> of this document</t>
</list>
</t>
</section>
</section>
<section title="Internationalization Considerations" anchor="tnternationalization-considerations">
<t>
An internationalized domain name that appears in either the header field
(<xref target="alt-svc"/>) or the HTTP/2 frame (<xref target="frame"/>)
MUST be expressed using A-labels (<xref target="RFC5890"/>, Section 2.3.2.1).
</t>
</section>
<section title="Security Considerations" anchor="security-considerations">
<section title="Changing Ports" anchor="changing-ports">
<t>
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.
</t>
<t>
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.
</t>
<t>
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.
</t>
</section>
<section title="Changing Hosts" anchor="host_security">
<t>
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.
</t>
<t>
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).
</t>
<t>
This is the reason for the requirement in <xref target="host_auth"/> 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.
</t>
<t>
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.
</t>
<t>
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.
</t>
<t>
As a result, there is a requirement in <xref target="caching"/> to examine cached alternative
services when a network change is detected.
</t>
</section>
<section title="Changing Protocols" anchor="changing-protocols">
<t>
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.
</t>
<t>
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.
</t>
<t>
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.
</t>
</section>
<section title="Tracking Clients Using Alternative Services" anchor="tracking">
<t>
The <xref target="indicator">alternative service indicator</xref> 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.
</t>
<t>
Clients that do not wish to be tracked MAY choose to ignore alternative
service advertisements.
</t>
<t>
In a browser, any alternative service information MUST be removed when origin-specific data
is cleared (for instance, when cookies are cleared).
</t>
</section>
</section>
<section title="Acknowledgements" anchor="acknowledgements">
<t>
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.
</t>
<t>
The Alt-Svc header field was influenced by the design of the
Alternate-Protocol header field in SPDY.
</t>
</section>
</middle>
<back>
<references title="Normative References">
<reference anchor="RFC2119">
<front>
<title abbrev="RFC Key Words">Key words for use in RFCs to Indicate Requirement Levels</title>
<author initials="S." surname="Bradner" fullname="Scott Bradner">
<organization>Harvard University</organization>
</author>
<date year="1997" month="March"/>
</front>
<seriesInfo name="BCP" value="14"/>
<seriesInfo name="RFC" value="2119"/>
</reference>
<reference anchor="RFC3986">
<front>
<title abbrev="URI Generic Syntax">Uniform Resource Identifier (URI): Generic Syntax</title>
<author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
<organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
</author>
<author initials="R." surname="Fielding" fullname="Roy T. Fielding">
<organization abbrev="Day Software">Day Software</organization>
</author>
<author initials="L." surname="Masinter" fullname="Larry Masinter">
<organization abbrev="Adobe Systems">Adobe Systems Incorporated</organization>
</author>
<date year="2005" month="January"/>
</front>
<seriesInfo name="STD" value="66"/>
<seriesInfo name="RFC" value="3986"/>
</reference>
<reference anchor="RFC5234">
<front>
<title>Augmented BNF for Syntax Specifications: ABNF</title>
<author initials="D." surname="Crocker" fullname="D. Crocker"/>
<author initials="P." surname="Overell" fullname="P. Overell"/>
<date year="2008" month="January"/>
</front>
<seriesInfo name="STD" value="68"/>
<seriesInfo name="RFC" value="5234"/>
</reference>
<reference anchor="RFC5890">
<front>
<title>
Internationalized Domain Names for Applications (IDNA): Definitions and
Document Framework
</title>
<author initials="J." surname="Klensin" fullname="John Klensin"/>
<date year="2010" month="August"/>
</front>
<seriesInfo name="RFC" value="5890"/>
</reference>
<reference anchor="RFC6066">
<front>
<title>Transport Layer Security (TLS) Extensions: Extension Definitions</title>
<author initials="D." surname="Eastlake" fullname="D. Eastlake"/>
<date year="2011" month="January"/>
</front>
<seriesInfo name="RFC" value="6066"/>
</reference>
<reference anchor="RFC6454">
<front>
<title>The Web Origin Concept</title>
<author initials="A." surname="Barth" fullname="A. Barth"/>
<date year="2011" month="December"/>
</front>
<seriesInfo name="RFC" value="6454"/>
</reference>
<reference anchor="RFC7230">
<front>
<title>Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
<author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
<organization abbrev="Adobe">Adobe Systems Incorporated</organization>
<address><email>fielding@gbiv.com</email></address>
</author>
<author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
<organization abbrev="greenbytes">greenbytes GmbH</organization>
<address><email>julian.reschke@greenbytes.de</email></address>
</author>
<date month="June" year="2014"/>
</front>
<seriesInfo name="RFC" value="7230"/>
</reference>
<reference anchor="RFC7234">
<front>
<title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
<author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
<organization abbrev="Adobe">Adobe Systems Incorporated</organization>
<address><email>fielding@gbiv.com</email></address>
</author>
<author fullname="Mark Nottingham" initials="M." role="editor" surname="Nottingham">
<organization>Akamai</organization>
<address><email>mnot@mnot.net</email></address>
</author>
<author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
<organization abbrev="greenbytes">greenbytes GmbH</organization>
<address><email>julian.reschke@greenbytes.de</email></address>
</author>
<date month="June" year="2014"/>
</front>
<seriesInfo name="RFC" value="7234"/>
</reference>
<reference anchor="ALPN">
<front>
<title>Transport Layer Security (TLS) Application Layer Protocol Negotiation Extension</title>
<author initials="S." surname="Friedl" fullname="Stephan Friedl"/>
<author initials="A." surname="Popov" fullname="Andrey Popov"/>
<author initials="A." surname="Langley" fullname="Adam Langley"/>
<author initials="S." surname="Emile" fullname="Stephan Emile"/>
<date month="March" year="2014"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-tls-applayerprotoneg-05"/>
</reference>
<reference anchor="HTTP2">
<front>
<title>Hypertext Transfer Protocol version 2</title>
<author initials="M." surname="Belshe" fullname="Mike Belshe"/>
<author initials="R." surname="Peon" fullname="Roberto Peon"/>
<author initials="M." surname="Thomson" fullname="Martin Thomson" role="editor"/>
<date month="June" year="2014"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-http2-13"/>
</reference>
</references>
<references title="Informative References">
<reference anchor="BCP90">
<front>
<title>Registration Procedures for Message Header Fields</title>
<author initials="G." surname="Klyne" fullname="G. Klyne"/>
<author initials="M." surname="Nottingham" fullname="M. Nottingham"/>
<author initials="J." surname="Mogul" fullname="J. Mogul"/>
<date year="2004" month="September"/>
</front>
<seriesInfo name="BCP" value="90"/>
<seriesInfo name="RFC" value="3864"/>
</reference>
<reference anchor="RFC5246">
<front>
<title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
<author initials="T." surname="Dierks" fullname="T. Dierks"/>
<author initials="E." surname="Rescorla" fullname="E. Rescorla"/>
<date year="2008" month="August"/>
</front>
<seriesInfo name="RFC" value="5246"/>
</reference>
</references>
<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">
<section title="Since draft-nottingham-httpbis-alt-svc-05" anchor="changes.since.draft-nottingham-httpbis-alt-svc-05">
<t>
This is the first version after adoption of draft-nottingham-httpbis-alt-svc-05
as Working Group work item. It only contains editorial changes.
</t>
</section>
<section title="Since draft-ietf-httpbis-alt-svc-00" anchor="changes.since.draft-ietf-httpbis-alt-svc-00">
<t>
Selected 421 as proposed status code for "Not Authoritative".
</t>
<t>
Changed header field syntax to use percent-encoding of ALPN protocol names
(<eref target="https://github.com/http2/http2-spec/issues/446"/>).
</t>
</section>
<section title="Since draft-ietf-httpbis-alt-svc-01" anchor="changes.since.draft-ietf-httpbis-alt-svc-01">
<t>
Updated HTTP/1.1 references.
</t>
<t>
Renamed "Service" to "Alt-Svc-Used" and reduced information to a flag to address
fingerprinting concerns (<eref target="https://github.com/http2/http2-spec/issues/502"/>).
</t>
<t>
Note that ALTSVC frame is preferred to Alt-Svc header field (<eref target="https://github.com/http2/http2-spec/pull/503"/>).
</t>
<t>
Incorporate ALTSRV frame (<eref target="https://github.com/http2/http2-spec/pull/507"/>).
</t>
<t>
Moved definition of status code 421 to HTTP/2.
</t>
<t>
Partly resolved <eref target="https://github.com/httpwg/http-extensions/issues/5"/>.
</t>
</section>
</section>
</back>
</rfc>| PAFTECH AB 2003-2026 | 2026-04-24 14:50:08 |