draft-ietf-httpbis-alt-svc-14.txt   draft-ietf-httpbis-alt-svc-latest.txt 
HTTP Working Group M. Nottingham HTTP Working Group M. Nottingham
Internet-Draft Akamai Internet-Draft Akamai
Intended status: Standards Track P. McManus Intended status: Standards Track P. McManus
Expires: September 9, 2016 Mozilla Expires: April 16, 2026 Mozilla
J. Reschke J. Reschke
greenbytes greenbytes
March 8, 2016 October 13, 2025
HTTP Alternative Services HTTP Alternative Services
draft-ietf-httpbis-alt-svc-14 draft-ietf-httpbis-alt-svc-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.
Editorial Note (To be removed by RFC Editor) Editorial Note (To be removed by RFC Editor)
skipping to change at page 1, line 41 skipping to change at page 1, line 41
The changes in this draft are summarized in Appendix A. The changes in this draft are summarized in Appendix A.
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
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 9, 2016. This Internet-Draft will expire on April 16, 2026.
Copyright Notice Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . . 4 1.1. Notational Conventions . . . . . . . . . . . . . . . . . 4
2. Alternative Services Concepts . . . . . . . . . . . . . . . . 5 2. Alternative Services Concepts . . . . . . . . . . . . . . . . 4
2.1. Host Authentication . . . . . . . . . . . . . . . . . . . 7 2.1. Host Authentication . . . . . . . . . . . . . . . . . . . 6
2.2. Alternative Service Caching . . . . . . . . . . . . . . . 7 2.2. Alternative Service Caching . . . . . . . . . . . . . . . 6
2.3. Requiring Server Name Indication . . . . . . . . . . . . . 8 2.3. Requiring Server Name Indication . . . . . . . . . . . . 7
2.4. Using Alternative Services . . . . . . . . . . . . . . . . 8 2.4. Using Alternative Services . . . . . . . . . . . . . . . 7
3. The Alt-Svc HTTP Header Field . . . . . . . . . . . . . . . . 9 3. The Alt-Svc HTTP Header Field . . . . . . . . . . . . . . . . 8
3.1. Caching Alt-Svc Header Field Values . . . . . . . . . . . 11 3.1. Caching Alt-Svc Header Field Values . . . . . . . . . . . 11
4. The ALTSVC HTTP/2 Frame . . . . . . . . . . . . . . . . . . . 12 4. The ALTSVC HTTP/2 Frame . . . . . . . . . . . . . . . . . . . 12
5. The Alt-Used HTTP Header Field . . . . . . . . . . . . . . . . 14 5. The Alt-Used HTTP Header Field . . . . . . . . . . . . . . . 13
6. The 421 Misdirected Request HTTP Status Code . . . . . . . . . 14 6. The 421 (Misdirected Request) HTTP Status Code . . . . . . . 14
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
7.1. Header Field Registrations . . . . . . . . . . . . . . . . 15 7.1. Header Field Registrations . . . . . . . . . . . . . . . 14
7.2. The ALTSVC HTTP/2 Frame Type . . . . . . . . . . . . . . . 15 7.2. The ALTSVC HTTP/2 Frame Type . . . . . . . . . . . . . . 15
7.3. Alt-Svc Parameter Registry . . . . . . . . . . . . . . . . 15 7.3. Alt-Svc Parameter Registry . . . . . . . . . . . . . . . 15
7.3.1. Procedure . . . . . . . . . . . . . . . . . . . . . . 15 7.3.1. Procedure . . . . . . . . . . . . . . . . . . . . . . 15
7.3.2. Registrations . . . . . . . . . . . . . . . . . . . . 16 7.3.2. Registrations . . . . . . . . . . . . . . . . . . . . 15
8. Internationalization Considerations . . . . . . . . . . . . . 16 8. Internationalization Considerations . . . . . . . . . . . . . 15
9. Security Considerations . . . . . . . . . . . . . . . . . . . 16 9. Security Considerations . . . . . . . . . . . . . . . . . . . 16
9.1. Changing Ports . . . . . . . . . . . . . . . . . . . . . . 16 9.1. Changing Ports . . . . . . . . . . . . . . . . . . . . . 16
9.2. Changing Hosts . . . . . . . . . . . . . . . . . . . . . . 17 9.2. Changing Hosts . . . . . . . . . . . . . . . . . . . . . 16
9.3. Changing Protocols . . . . . . . . . . . . . . . . . . . . 17 9.3. Changing Protocols . . . . . . . . . . . . . . . . . . . 17
9.4. Tracking Clients Using Alternative Services . . . . . . . 18 9.4. Tracking Clients Using Alternative Services . . . . . . . 17
9.5. Confusion Regarding Request Scheme . . . . . . . . . . . . 18 9.5. Confusion regarding Request Scheme . . . . . . . . . . . 17
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
10.1. Normative References . . . . . . . . . . . . . . . . . . . 19 10.1. Normative References . . . . . . . . . . . . . . . . . . 18
10.2. Informative References . . . . . . . . . . . . . . . . . . 20 10.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. Change Log (to be removed by RFC Editor before Appendix A. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 20 publication) . . . . . . . . . . . . . . . . . . . . 21
A.1. Since draft-nottingham-httpbis-alt-svc-05 . . . . . . . . 20 A.1. Since draft-nottingham-httpbis-alt-svc-05 . . . . . . . . 21
A.2. Since draft-ietf-httpbis-alt-svc-00 . . . . . . . . . . . 21 A.2. Since draft-ietf-httpbis-alt-svc-00 . . . . . . . . . . . 21
A.3. Since draft-ietf-httpbis-alt-svc-01 . . . . . . . . . . . 21 A.3. Since draft-ietf-httpbis-alt-svc-01 . . . . . . . . . . . 21
A.4. Since draft-ietf-httpbis-alt-svc-02 . . . . . . . . . . . 21 A.4. Since draft-ietf-httpbis-alt-svc-02 . . . . . . . . . . . 21
A.5. Since draft-ietf-httpbis-alt-svc-03 . . . . . . . . . . . 21 A.5. Since draft-ietf-httpbis-alt-svc-03 . . . . . . . . . . . 21
A.6. Since draft-ietf-httpbis-alt-svc-04 . . . . . . . . . . . 21 A.6. Since draft-ietf-httpbis-alt-svc-04 . . . . . . . . . . . 22
A.7. Since draft-ietf-httpbis-alt-svc-05 . . . . . . . . . . . 22 A.7. Since draft-ietf-httpbis-alt-svc-05 . . . . . . . . . . . 22
A.8. Since draft-ietf-httpbis-alt-svc-06 . . . . . . . . . . . 22 A.8. Since draft-ietf-httpbis-alt-svc-06 . . . . . . . . . . . 22
A.9. Since draft-ietf-httpbis-alt-svc-07 . . . . . . . . . . . 22 A.9. Since draft-ietf-httpbis-alt-svc-07 . . . . . . . . . . . 22
A.10. Since draft-ietf-httpbis-alt-svc-08 . . . . . . . . . . . 23 A.10. Since draft-ietf-httpbis-alt-svc-08 . . . . . . . . . . . 23
A.11. Since draft-ietf-httpbis-alt-svc-09 . . . . . . . . . . . 24 A.11. Since draft-ietf-httpbis-alt-svc-09 . . . . . . . . . . . 24
A.12. Since draft-ietf-httpbis-alt-svc-10 . . . . . . . . . . . 24 A.12. Since draft-ietf-httpbis-alt-svc-10 . . . . . . . . . . . 24
A.13. Since draft-ietf-httpbis-alt-svc-11 . . . . . . . . . . . 24 A.13. Since draft-ietf-httpbis-alt-svc-11 . . . . . . . . . . . 24
A.14. Since draft-ietf-httpbis-alt-svc-12 . . . . . . . . . . . 24 A.14. Since draft-ietf-httpbis-alt-svc-12 . . . . . . . . . . . 24
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 24 A.15. Since draft-ietf-httpbis-alt-svc-13 . . . . . . . . . . . 25
A.16. Since draft-ietf-httpbis-alt-svc-14 . . . . . . . . . . . 25
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction 1. Introduction
HTTP [RFC7230] conflates the identification of resources with their HTTP [RFC7230] 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.
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delta-seconds = <delta-seconds; see [RFC7234], Section 1.2.1> delta-seconds = <delta-seconds; see [RFC7234], Section 1.2.1>
port = <port, see [RFC7230], Section 2.7> port = <port, see [RFC7230], Section 2.7>
quoted-string = <quoted-string, see [RFC7230], Section 3.2.6> quoted-string = <quoted-string, see [RFC7230], Section 3.2.6>
token = <token, see [RFC7230], Section 3.2.6> token = <token, see [RFC7230], Section 3.2.6>
uri-host = <uri-host, see [RFC7230], Section 2.7> uri-host = <uri-host, see [RFC7230], Section 2.7>
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 Service"". When an origin [RFC6454] has resources that are
accessible through a different protocol / host / port combination, it accessible through a different protocol/host/port combination, it is
is said to have an alternative service available. said to 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
[RFC7230], Section 9.1. [RFC7230], Section 9.1.
For example, an origin: For example, an origin:
("http", "www.example.com", "80") ("http", "www.example.com", "80")
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[RFC7230], Section 9.1. [RFC7230], Section 9.1.
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")
By their nature, alternative services are explicitly at the By their nature, alternative services are explicitly at the
granularity of an origin; they cannot be selectively applied to granularity of an origin; they cannot be selectively applied to
resources within an origin. resources within an origin.
Alternative services do not replace or change the origin for any Alternative services do not replace or change the origin for any
given resource; in general, they are not visible to the software given resource; in general, they are not visible to the software
"above" the access mechanism. The alternative service is essentially "above" the access mechanism. The alternative service is essentially
alternative routing information that can also be used to reach the alternative routing information that can also be used to reach the
origin in the same way that DNS CNAME or SRV records define routing 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 information at the name resolution level. Each origin maps to a set
of these routes -- the default route is derived from the origin of these routes -- the default route is derived from the origin
itself and the other routes are introduced based on alternative- itself and the other routes are introduced based on alternative-
service information. service information.
Furthermore, it is important to note that the first member of an Furthermore, it is important to note that the first member of an
alternative service tuple is different from the "scheme" component of alternative service tuple is different from the "scheme" component of
an origin; it is more specific, identifying not only the major an origin; it is more specific, identifying not only the major
version of the protocol being used, but potentially communication version of the protocol being used, but potentially the communication
options for that protocol. options for that protocol as well.
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, 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], Section the alternative. Likewise, the Host header field ([RFC7230],
5.4) is still derived from the origin, not the alternative service Section 5.4) is still derived from the origin, not the alternative
(just as it would if a CNAME were being used). service (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 [RFC3986], Section 3.2.2 o A host, as per [RFC3986], Section 3.2.2
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o A port, as per [RFC3986], Section 3.2.3 o A port, as per [RFC3986], Section 3.2.3
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: Additionally, each alternative service MUST have a freshness
lifetime, expressed in seconds (see Section 2.2).
o A freshness 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" HTTP/2 frame type (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
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Note that the SNI information provided in TLS by the client will be Note that the SNI information provided in TLS by the client will be
that of the origin, not the alternative (as will the Host HTTP header that of the origin, not the alternative (as will the Host HTTP header
field value). field value).
2.4. Using Alternative Services 2.4. Using Alternative Services
By their nature, alternative services are OPTIONAL: clients do not By their nature, alternative services are OPTIONAL: clients do not
need to use them. However, it is advantageous for clients to behave need to use them. However, it is advantageous for clients to behave
in a predictable way when alternative services are used by servers, in a predictable way when alternative services are used by servers,
to aid purposes like load balancing. to aid in purposes like load balancing.
Therefore, if a client supporting this specification becomes aware of Therefore, if a client supporting this specification becomes aware of
an alternative service, the client SHOULD use that alternative an alternative service, the client SHOULD use that alternative
service for all requests to the associated origin as soon as it is service for all requests to the associated origin as soon as it is
available, provided the alternative service information is fresh available, provided the alternative service information is fresh
(Section 2.2) and the security properties of the alternative service (Section 2.2) and the security properties of the alternative service
protocol are desirable, as compared to the existing connection. A protocol are desirable, as compared to the existing connection. A
viable alternative service is then treated in every way as the viable alternative service is then treated in every way as the
origin; this includes the ability to advertise alternative services. origin; this includes the ability to advertise alternative services.
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directly connect to an alternative service for this request, but directly connect to an alternative service for this request, but
instead route it through that proxy. instead route it through that proxy.
When a client uses an alternative service for a request, it can When a client uses an alternative service for a request, it can
indicate this to the server using the Alt-Used header field indicate this to the server using the Alt-Used header field
(Section 5). (Section 5).
The client does not need to block requests on any existing The client does not need to block requests on any existing
connection; it can be used until the alternative connection is connection; it can be used until the alternative connection is
established. However, if the security properties of the existing established. However, if the security properties of the existing
connection are weak (for example, cleartext HTTP/1.1) then it might connection are weak (for example, cleartext HTTP/1.1), then it might
make sense to block until the new connection is fully available in make sense to block until the new connection is fully available in
order to avoid information leakage. order to avoid information leakage.
Furthermore, if the connection to the alternative service fails or is Furthermore, if the connection to the alternative service fails or is
unresponsive, the client MAY fall back to using the origin or another unresponsive, the client MAY fall back to using the origin or another
alternative service. Note, however, that this could be the basis of alternative service. Note, however, that this could be the basis of
a downgrade attack, thus losing any enhanced security properties of a downgrade attack, thus losing any enhanced security properties of
the alternative service. If the connection to the alternative the alternative service. If the connection to the alternative
service does not negotiate the expected protocol (for example, ALPN service does not negotiate the expected protocol (for example, ALPN
fails to negotiate h2, or an Upgrade request to h2c is not accepted), fails to negotiate h2, or an Upgrade request to h2c is not accepted),
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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 start using this new connection immediately or can continue using the
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 ([RFC7540]), 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.
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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 7.3 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 3.2.6 of [RFC7230].
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 [RFC7234], Section 1.2.1
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 the alternative service is considered fresh response was generated for which the alternative service is
for. considered fresh.
Alt-Svc: h2=":443"; ma=3600 Alt-Svc: h2=":443"; ma=3600
See Section 4.2.3 of [RFC7234] for details of determining response See Section 4.2.3 of [RFC7234] for details on determining the
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
indicates that an alternative service is available and usable for the indicates that an alternative service is available and usable for the
next 60 seconds. However, the response has already been cached for next 60 seconds. However, the response has already been cached for
30 seconds (as per the Age header field value), so therefore the 30 seconds (as per the Age header field value); therefore, the
alternative service is only fresh for the 30 seconds from when this alternative service is only fresh for the 30 seconds from when this
response was received, minus estimated transit time. response was received, minus estimated transit time.
Note that the freshness lifetime for HTTP caching (here, 600 seconds) Note that the freshness lifetime for HTTP caching (here, 600 seconds)
does not affect caching of Alt-Svc values. does not affect caching of Alt-Svc values.
When an Alt-Svc response header field is received from an origin, its When an Alt-Svc response header field is received from an origin, its
value invalidates and replaces all cached alternative services for value invalidates and replaces all cached alternative services for
that origin. that origin.
By default, cached alternative services will be cleared when the By default, cached alternative services will be cleared when the
client detects a network change. Alternative services that are client detects a network change. Alternative services that are
intended to be longer-lived (such as those that are not specific to intended to be longer lived (such as those that are not specific to
the client access network) can carry the "persist" parameter with a the client access network) can carry the "persist" parameter with a
value "1" as a hint that the service is potentially useful beyond a value "1" as a hint that the service is potentially useful beyond a
network configuration change. network configuration change.
Syntax: Syntax:
persist = "1" persist = "1"
For example: For example:
skipping to change at page 13, line 33 skipping to change at page 13, line 9
+---------------------------------------------------------------+ +---------------------------------------------------------------+
ALTSVC Frame Payload ALTSVC Frame Payload
The ALTSVC frame contains the following fields: The ALTSVC frame contains the following fields:
Origin-Len: An unsigned, 16-bit integer indicating the length, in Origin-Len: An unsigned, 16-bit integer indicating the length, in
octets, of the Origin field. octets, of the Origin field.
Origin: An OPTIONAL sequence of characters containing the ASCII Origin: An OPTIONAL sequence of characters containing the ASCII
serialization of an origin ([RFC6454], Section 6.2) that the serialization of an origin ([RFC6454], Section 6.2) to which the
alternative service is applicable to. 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.
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.
skipping to change at page 14, line 17 skipping to change at page 13, line 41
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 indicate the The Alt-Used header field is used in requests to identify the
identity of the alternative service in use, just as the Host header alternative service in use, just as the Host header field
field (Section 5.4 of [RFC7230]) identifies the host and port of the (Section 5.4 of [RFC7230]) 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 Section 9.1.2
of [RFC7540] to indicate that the current server instance is not of [RFC7540] to indicate that the current server instance is not
authoritative for the requested resource. This can be used to authoritative for the requested resource. This can be used to
indicate that an alternative service is not authoritative; see indicate that an alternative service is not authoritative; see
Section 2). 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
skipping to change at page 15, line 13 skipping to change at page 14, line 36
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.
7. IANA Considerations 7. IANA Considerations
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 registry maintained at <https://www.iana.org/assignments/message-
<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 shall be added according to the permanent associated registry entries have been added according to the
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
Types registry ([RFC7540], Section 11.2). Type" registry ([RFC7540], Section 11.2).
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. Alt-Svc Parameter Registry
The HTTP Alt-Svc Parameter Registry defines the name space for The "Hypertext Transfer Protocol (HTTP) Alt-Svc Parameter Registry"
parameters. It will be created and maintained at (the suggested URI) defines the name space for parameters. It has been created and will
<http://www.iana.org/assignments/http-alt-svc-parameters>. be maintained at <http://www.iana.org/assignments/http-alt-svc-
parameters>.
7.3.1. Procedure 7.3.1. 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
[RFC5226], Section 4.1). [RFC5226], Section 4.1).
7.3.2. Registrations 7.3.2. Registrations
The HTTP Alt-Svc Parameter Registry is to be populated with the The "Hypertext Transfer Protocol (HTTP) Alt-Svc Parameter Registry"
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 ([RFC5890], Section 2.3.2.1). using A-labels ([RFC5890], Section 2.3.2.1).
9. Security Considerations 9. Security Considerations
9.1. Changing Ports 9.1. Changing Ports
Using an alternative service implies accessing an origin's resources Using an alternative service implies accessing an origin's resources
on an alternative port, at a minimum. An attacker that can inject on an alternative port, at a minimum. Therefore, an attacker that
alternative services and listen at the advertised port is therefore can inject alternative services and listen at the advertised port is
able to hijack an origin. On certain servers, it is normal for users able to hijack an origin. On certain servers, it is normal for users
to be able to control some personal pages available on a shared port, to be able to control some personal pages available on a shared port
and also to accept to requests on less-privileged ports. and also to accept requests on less-privileged ports.
For example, an attacker that can add HTTP response header fields to For example, an attacker that can add HTTP response header fields to
some pages can redirect traffic for an entire origin to a different some pages can redirect traffic for an entire origin to a different
port on the same host using the Alt-Svc header field; if that port is 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 under the attacker's control, they can thus masquerade as the HTTP
server. server.
This risk is mitigated by the requirements in Section 2.1. This risk is mitigated by the requirements in Section 2.1.
On servers, this risk can also be reduced by restricting the ability On servers, this risk can also be reduced by restricting the ability
to advertise alternative services, and restricting who can open a to advertise alternative services, and restricting who can open a
port for listening on that host. port for listening on that host.
9.2. Changing Hosts 9.2. Changing Hosts
When the host is changed due to the use of an alternative service, it When the host is changed due to the use of an alternative service,
presents an opportunity for attackers to hijack communication to an this presents an opportunity for attackers to hijack communication to
origin. an origin.
For example, if an attacker can convince a user agent to send all For example, if an attacker can convince a user agent to send all
traffic for "innocent.example.org" to "evil.example.com" by traffic for "innocent.example.org" to "evil.example.com" by
successfully associating it as an alternative service, they can successfully associating it as an alternative service, they can
masquerade as that origin. This can be done locally (see mitigations masquerade as that origin. This can be done locally (see mitigations
in Section 9.1) or remotely (e.g., by an intermediary as a man-in- in Section 9.1) or remotely (e.g., by an intermediary as a man-in-
the-middle attack). the-middle attack).
This is the reason for the requirement in Section 2.1 that clients This is the reason for the requirement in Section 2.1 that clients
have reasonable assurances that the alternative service is under have reasonable assurances that the alternative service is under
skipping to change at page 17, line 31 skipping to change at page 17, line 7
certificate for the origin proves that the alternative service is certificate for the origin proves that the alternative service is
authorized to serve traffic for the origin. authorized to serve traffic for the origin.
Note that this assurance is only as strong as the method used to Note that this assurance is only as strong as the method used to
authenticate the alternative service. In particular, when TLS authenticate the alternative service. In particular, when TLS
authentication is used to do so, there are well-known exploits to authentication is used to do so, there are well-known exploits to
make an attacker's certificate appear as legitimate. make an attacker's certificate appear as legitimate.
Alternative services could be used to persist such an attack. For Alternative services could be used to persist such an attack. For
example, an intermediary could man-in-the-middle TLS-protected example, an intermediary could man-in-the-middle TLS-protected
communication to a target, and then direct all traffic to an communication to a target and then direct all traffic to an
alternative service with a large freshness lifetime, so that the user alternative service with a large freshness lifetime so that the user
agent still directs traffic to the attacker even when not using the agent still directs traffic to the attacker even when not using the
intermediary. intermediary.
Implementations MUST perform any certificate-pinning validation (such Implementations MUST perform any certificate-pinning validation (such
as [RFC7469]) on alternative services just as they would on direct as [RFC7469]) on alternative services just as they would on direct
connections to the origin. Implementations might also choose to add connections to the origin. Implementations might also choose to add
other requirements around which certificates are acceptable for other requirements around which certificates are acceptable for
alternative services. alternative services.
9.3. Changing Protocols 9.3. Changing Protocols
When the ALPN protocol is changed due to the use of an alternative When the ALPN protocol is changed due to the use of an alternative
service, the security properties of the new connection to the origin service, the security properties of the new connection to the origin
can be different from that of the "normal" connection to the origin, can be different from that of the "normal" connection to the origin,
because the protocol identifier itself implies this. because the protocol identifier itself implies this.
For example, if an "https://" URI has a protocol advertised that does For example, if an "https://" URI has a protocol advertised that does
not use some form of end-to-end encryption (most likely, TLS), it not use some form of end-to-end encryption (most likely, TLS), this
violates the expectations for security that the URI scheme implies. violates the expectations for security that the URI scheme implies.
Therefore, clients cannot blindly use alternative services, but Therefore, clients cannot use alternative services blindly, but
instead evaluate the option(s) presented to assure that security instead evaluate the option(s) presented to ensure that security
requirements and expectations of specifications, implementations and requirements and expectations of specifications, implementations, and
end users are met. end users are met.
9.4. Tracking Clients Using Alternative Services 9.4. Tracking Clients Using Alternative Services
Choosing an alternative service implies connecting to a new, server- Choosing an alternative service implies connecting to a new, server-
supplied host name. By using unique names, servers could conceivably supplied host name. By using unique names, servers could conceivably
track client requests. Such tracking could follow users across track client requests. Such tracking could follow users across
multiple networks, when the "persist" flag is used. multiple networks, when the "persist" flag is used.
Clients that wish to prevent requests from being correlated can Clients that wish to prevent requests from being correlated can
decide not to use alternative services for multiple requests that decide not to use alternative services for multiple requests that
would not otherwise be allowed to be correlated. would not otherwise be allowed to be correlated.
In a user agent, any alternative service information MUST be removed In a user agent, any alternative service information MUST be removed
when origin-specific data is cleared (typically, when cookies when origin-specific data is cleared (typically, when cookies
[RFC6265] are cleared). [RFC6265] are cleared).
9.5. Confusion Regarding Request Scheme 9.5. Confusion regarding Request Scheme
Some server-side HTTP applications make assumptions about security Some server-side HTTP applications make assumptions about security
based upon connection context; for example, equating being served based upon connection context; for example, equating being served
upon port 443 with the use of an "https://" URI and the various upon port 443 with the use of an "https://" URI and the various
security properties that implies. security properties that implies.
This affects not only the security properties of the connection This affects not only the security properties of the connection
itself, but also the state of the client at the other end of it; for itself, but also the state of the client at the other end of it; for
example, a Web browser treats "https://" URIs differently than example, a Web browser treats "https://" URIs differently than
"http://" URIs in many ways, not just for purposes of protocol "http://" URIs in many ways, not just for purposes of protocol
skipping to change at page 19, line 12 skipping to change at page 18, line 37
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
[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, DOI 10.17487/ Requirement Levels", BCP 14, RFC 2119,
RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, DOI 10.17487/ [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
RFC2818, May 2000, DOI 10.17487/RFC2818, May 2000,
<http://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
Resource Identifier (URI): Generic Syntax", STD 66, Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005, RFC 3986, DOI 10.17487/RFC3986, January 2005,
<http://www.rfc-editor.org/info/rfc3986>. <https://www.rfc-editor.org/info/rfc3986>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226, IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008, DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>. <https://www.rfc-editor.org/info/rfc5226>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/ Specifications: ABNF", STD 68, RFC 5234,
RFC5234, January 2008, DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>. <https://www.rfc-editor.org/info/rfc5234>.
[RFC5890] Klensin, J., "Internationalized Domain Names for [RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework", Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010, RFC 5890, DOI 10.17487/RFC5890, August 2010,
<http://www.rfc-editor.org/info/rfc5890>. <https://www.rfc-editor.org/info/rfc5890>.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions: [RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, DOI 10.17487/RFC6066, Extension Definitions", RFC 6066, DOI 10.17487/RFC6066,
January 2011, <http://www.rfc-editor.org/info/rfc6066>. January 2011, <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,
<http://www.rfc-editor.org/info/rfc6454>. <https://www.rfc-editor.org/info/rfc6454>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing", Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014, RFC 7230, DOI 10.17487/RFC7230, June 2014,
<http://www.rfc-editor.org/info/rfc7230>. <https://www.rfc-editor.org/info/rfc7230>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching", Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014, RFC 7234, DOI 10.17487/RFC7234, June 2014,
<http://www.rfc-editor.org/info/rfc7234>. <https://www.rfc-editor.org/info/rfc7234>.
[RFC7301] Friedl, S., Popov, A., Langley, A., and S. Emile, [RFC7301] Friedl, S., Popov, A., Langley, A., and S. Emile,
"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, <http://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,
<http://www.rfc-editor.org/info/rfc7405>. <https://www.rfc-editor.org/info/rfc7405>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext [RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol version 2", RFC 7540, DOI 10.17487/ Transfer Protocol version 2", RFC 7540,
RFC7540, May 2015, DOI 10.17487/RFC7540, May 2015,
<http://www.rfc-editor.org/info/rfc7540>. <https://www.rfc-editor.org/info/rfc7540>.
10.2. Informative References 10.2. Informative References
[BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration [BCP90] 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,
September 2004, <http://www.rfc-editor.org/info/bcp90>. September 2004, <https://www.rfc-editor.org/info/bcp90>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/ (TLS) Protocol Version 1.2", RFC 5246,
RFC5246, August 2008, DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>. <https://www.rfc-editor.org/info/rfc5246>.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011, DOI 10.17487/RFC6265, April 2011,
<http://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, Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
April 2015, <http://www.rfc-editor.org/info/rfc7469>. 2015, <https://www.rfc-editor.org/info/rfc7469>.
Appendix A. Change Log (to be removed by RFC Editor before publication) Appendix A. Change Log (to be removed by RFC Editor before publication)
A.1. Since draft-nottingham-httpbis-alt-svc-05 A.1. Since draft-nottingham-httpbis-alt-svc-05
This is the first version after adoption of This is the first version after adoption of draft-nottingham-httpbis-
draft-nottingham-httpbis-alt-svc-05 as Working Group work item. It alt-svc-05 as Working Group work item. It only contains editorial
only contains editorial changes. changes.
A.2. Since draft-ietf-httpbis-alt-svc-00 A.2. Since draft-ietf-httpbis-alt-svc-00
Selected 421 as proposed status code for "Not Authoritative". Selected 421 as proposed status code for "Not Authoritative".
Changed header field syntax to use percent-encoding of ALPN protocol Changed header field syntax to use percent-encoding of ALPN protocol
names (<https://github.com/http2/http2-spec/issues/446>). names (<https://github.com/http2/http2-spec/issues/446>).
A.3. Since draft-ietf-httpbis-alt-svc-01 A.3. Since draft-ietf-httpbis-alt-svc-01
Updated HTTP/1.1 references. Updated HTTP/1.1 references.
Renamed "Service" to "Alt-Svc-Used" and reduced information to a flag Renamed "Service" to "Alt-Svc-Used" and reduced information to a flag
to address fingerprinting concerns to address fingerprinting concerns (<https://github.com/http2/http2-
(<https://github.com/http2/http2-spec/issues/502>). spec/issues/502>).
Note that ALTSVC frame is preferred to Alt-Svc header field Note that ALTSVC frame is preferred to Alt-Svc header field
(<https://github.com/http2/http2-spec/pull/503>). (<https://github.com/http2/http2-spec/pull/503>).
Incorporate ALTSRV frame Incorporate ALTSRV frame (<https://github.com/http2/http2-spec/
(<https://github.com/http2/http2-spec/pull/507>). pull/507>).
Moved definition of status code 421 to HTTP/2. Moved definition of status code 421 to HTTP/2.
Partly resolved <https://github.com/httpwg/http-extensions/issues/5>. Partly resolved <https://github.com/httpwg/http-extensions/issues/5>.
A.4. Since draft-ietf-httpbis-alt-svc-02 A.4. Since draft-ietf-httpbis-alt-svc-02
Updated ALPN reference. Updated ALPN reference.
Resolved <https://github.com/httpwg/http-extensions/issues/2>. Resolved <https://github.com/httpwg/http-extensions/issues/2>.
A.5. Since draft-ietf-httpbis-alt-svc-03 A.5. Since draft-ietf-httpbis-alt-svc-03
Renamed "Alt-Svc-Used" to "Alt-Used" Renamed "Alt-Svc-Used" to "Alt-Used" (<https://github.com/httpwg/
(<https://github.com/httpwg/http-extensions/issues/17>). http-extensions/issues/17>).
Clarify ALTSVC Origin information requirements Clarify ALTSVC Origin information requirements
(<https://github.com/httpwg/http-extensions/issues/19>). (<https://github.com/httpwg/http-extensions/issues/19>).
Remove/tune language with respect to tracking risks (see Remove/tune language with respect to tracking risks (see
<https://github.com/httpwg/http-extensions/issues/34>). <https://github.com/httpwg/http-extensions/issues/34>).
A.6. Since draft-ietf-httpbis-alt-svc-04 A.6. Since draft-ietf-httpbis-alt-svc-04
Mention tracking by alt-svc host name in Security Considerations Mention tracking by alt-svc host name in Security Considerations
(<https://github.com/httpwg/http-extensions/issues/36>). (<https://github.com/httpwg/http-extensions/issues/36>).
"421 (Not Authoritative)" -> "421 (Misdirected Request)". "421 (Not Authoritative)" -> "421 (Misdirected Request)".
Allow the frame to carry multiple indicator and use the same payload Allow the frame to carry multiple indicator and use the same payload
formats for both formats for both (<https://github.com/httpwg/http-extensions/
(<https://github.com/httpwg/http-extensions/issues/37>). issues/37>).
A.7. Since draft-ietf-httpbis-alt-svc-05 A.7. Since draft-ietf-httpbis-alt-svc-05
Go back to specifying the origin in Alt-Used, but make it a "SHOULD" Go back to specifying the origin in Alt-Used, but make it a "SHOULD"
(<https://github.com/httpwg/http-extensions/issues/34>). (<https://github.com/httpwg/http-extensions/issues/34>).
Restore Origin field in ALT-SVC frame Restore Origin field in ALT-SVC frame (<https://github.com/httpwg/
(<https://github.com/httpwg/http-extensions/issues/38>). http-extensions/issues/38>).
A.8. Since draft-ietf-httpbis-alt-svc-06 A.8. Since draft-ietf-httpbis-alt-svc-06
Disallow use of alternative services when the protocol might not Disallow use of alternative services when the protocol might not
carry the scheme carry the scheme (<https://github.com/httpwg/http-extensions/
(<https://github.com/httpwg/http-extensions/issues/12>). issues/12>).
Align opp-sec and alt-svc Align opp-sec and alt-svc (<https://github.com/httpwg/http-
(<https://github.com/httpwg/http-extensions/issues/33>). extensions/issues/33>).
alt svc frame on pushed (even and non-0) frame alt svc frame on pushed (even and non-0) frame
(<https://github.com/httpwg/http-extensions/issues/44>). (<https://github.com/httpwg/http-extensions/issues/44>).
"browser" -> "user agent" "browser" -> "user agent" (<https://github.com/httpwg/http-
(<https://github.com/httpwg/http-extensions/pull/61>). extensions/pull/61>).
ABNF for "parameter" ABNF for "parameter" (<https://github.com/httpwg/http-extensions/
(<https://github.com/httpwg/http-extensions/issues/65>). issues/65>).
Updated HTTP/2 reference. Updated HTTP/2 reference.
A.9. Since draft-ietf-httpbis-alt-svc-07 A.9. Since draft-ietf-httpbis-alt-svc-07
Alt-Svc alternative cache invalidation Alt-Svc alternative cache invalidation (<https://github.com/httpwg/
(<https://github.com/httpwg/http-extensions/issues/16>). http-extensions/issues/16>).
Unexpected Alt-Svc frames Unexpected Alt-Svc frames (<https://github.com/httpwg/http-
(<https://github.com/httpwg/http-extensions/issues/18>). extensions/issues/18>).
Associating Alt-Svc header with an origin Associating Alt-Svc header with an origin
(<https://github.com/httpwg/http-extensions/issues/21>). (<https://github.com/httpwg/http-extensions/issues/21>).
ALPN identifiers in Alt-Svc ALPN identifiers in Alt-Svc (<https://github.com/httpwg/http-
(<https://github.com/httpwg/http-extensions/issues/43>). extensions/issues/43>).
Number of alternate services used Number of alternate services used (<https://github.com/httpwg/http-
(<https://github.com/httpwg/http-extensions/issues/58>). extensions/issues/58>).
Proxy and .pac interaction Proxy and .pac interaction (<https://github.com/httpwg/http-
(<https://github.com/httpwg/http-extensions/issues/62>). extensions/issues/62>).
Need to define extensibility for alt-svc parameters Need to define extensibility for alt-svc parameters
(<https://github.com/httpwg/http-extensions/issues/69>). (<https://github.com/httpwg/http-extensions/issues/69>).
Persistence of alternates across network changes Persistence of alternates across network changes
(<https://github.com/httpwg/http-extensions/issues/71>). (<https://github.com/httpwg/http-extensions/issues/71>).
Alt-Svc header with 421 status Alt-Svc header with 421 status (<https://github.com/httpwg/http-
(<https://github.com/httpwg/http-extensions/issues/75>). extensions/issues/75>).
Incorporate several editorial improvements suggested by Mike Bishop Incorporate several editorial improvements suggested by Mike Bishop
(<https://github.com/httpwg/http-extensions/pull/77>, (<https://github.com/httpwg/http-extensions/pull/77>,
<https://github.com/httpwg/http-extensions/pull/78>). <https://github.com/httpwg/http-extensions/pull/78>).
Alt-Svc response header field in HTTP/2 frame Alt-Svc response header field in HTTP/2 frame
(<https://github.com/httpwg/http-extensions/issues/87>). (<https://github.com/httpwg/http-extensions/issues/87>).
A.10. Since draft-ietf-httpbis-alt-svc-08 A.10. Since draft-ietf-httpbis-alt-svc-08
Remove left over text about ext-params, applying to an earlier Remove left over text about ext-params, applying to an earlier
version of Alt-Used (see version of Alt-Used (see <https://github.com/httpwg/http-extensions/
<https://github.com/httpwg/http-extensions/issues/34>). issues/34>).
Conflicts between Alt-Svc and ALPN Conflicts between Alt-Svc and ALPN (<https://github.com/httpwg/http-
(<https://github.com/httpwg/http-extensions/issues/72>). extensions/issues/72>).
Elevation of privilege Elevation of privilege (<https://github.com/httpwg/http-extensions/
(<https://github.com/httpwg/http-extensions/issues/73>). issues/73>).
Alternates of alternates Alternates of alternates (<https://github.com/httpwg/http-extensions/
(<https://github.com/httpwg/http-extensions/issues/74>). issues/74>).
Alt-Svc and Cert Pinning Alt-Svc and Cert Pinning (<https://github.com/httpwg/http-extensions/
(<https://github.com/httpwg/http-extensions/issues/76>). issues/76>).
Using alt-svc on localhost (no change to spec, see Using alt-svc on localhost (no change to spec, see
<https://github.com/httpwg/http-extensions/issues/89>). <https://github.com/httpwg/http-extensions/issues/89>).
IANA procedure for alt-svc parameters IANA procedure for alt-svc parameters (<https://github.com/httpwg/
(<https://github.com/httpwg/http-extensions/issues/96>). http-extensions/issues/96>).
Alt-svc from https (1.1) to https (1.1) Alt-svc from https (1.1) to https (1.1) (<https://github.com/httpwg/
(<https://github.com/httpwg/http-extensions/issues/91>). http-extensions/issues/91>).
Alt-svc vs the ability to convey the scheme inside the protocol Alt-svc vs the ability to convey the scheme inside the protocol
(<https://github.com/httpwg/http-extensions/issues/92>). (<https://github.com/httpwg/http-extensions/issues/92>).
Reconciling MAY/can vs. SHOULD Reconciling MAY/can vs. SHOULD (<https://github.com/httpwg/http-
(<https://github.com/httpwg/http-extensions/issues/101>). extensions/issues/101>).
Typo in alt-svc caching example Typo in alt-svc caching example (<https://github.com/httpwg/http-
(<https://github.com/httpwg/http-extensions/issues/117>). extensions/issues/117>).
A.11. Since draft-ietf-httpbis-alt-svc-09 A.11. Since draft-ietf-httpbis-alt-svc-09
Editorial improvements Editorial improvements (<https://github.com/httpwg/http-extensions/
(<https://github.com/httpwg/http-extensions/issues/118>, issues/118>, <https://github.com/httpwg/http-extensions/issues/119>,
<https://github.com/httpwg/http-extensions/issues/119>,
<https://github.com/httpwg/http-extensions/issues/120>, <https://github.com/httpwg/http-extensions/issues/120>,
<https://github.com/httpwg/http-extensions/issues/121>, <https://github.com/httpwg/http-extensions/issues/121>,
<https://github.com/httpwg/http-extensions/issues/122>, <https://github.com/httpwg/http-extensions/issues/122>,
<https://github.com/httpwg/http-extensions/issues/123>, <https://github.com/httpwg/http-extensions/issues/123>,
<https://github.com/httpwg/http-extensions/issues/125>, <https://github.com/httpwg/http-extensions/issues/125>,
<https://github.com/httpwg/http-extensions/issues/126>). <https://github.com/httpwg/http-extensions/issues/126>).
A.12. Since draft-ietf-httpbis-alt-svc-10 A.12. Since draft-ietf-httpbis-alt-svc-10
Editorial improvements Editorial improvements (<https://github.com/httpwg/http-extensions/
(<https://github.com/httpwg/http-extensions/issues/130>). issues/130>).
Use RFC 7405 ABNF extension Use RFC 7405 ABNF extension (<https://github.com/httpwg/http-
(<https://github.com/httpwg/http-extensions/issues/131>). extensions/issues/131>).
A.13. Since draft-ietf-httpbis-alt-svc-11 A.13. Since draft-ietf-httpbis-alt-svc-11
Security considerations wrt system ports Security considerations wrt system ports (<https://github.com/httpwg/
(<https://github.com/httpwg/http-extensions/issues/139>). http-extensions/issues/139>).
A.14. Since draft-ietf-httpbis-alt-svc-12 A.14. Since draft-ietf-httpbis-alt-svc-12
Editorial changes triggered by <https://lists.w3.org/Archives/Public/ Editorial changes triggered by <https://lists.w3.org/Archives/Public/
ietf-http-wg/2016JanMar/0243.html>. ietf-http-wg/2016JanMar/0243.html>.
Reasonable Assurances and H2C Reasonable Assurances and H2C (<https://github.com/httpwg/http-
(<https://github.com/httpwg/http-extensions/issues/148>). extensions/issues/148>).
Appendix B. Acknowledgements A.15. Since draft-ietf-httpbis-alt-svc-13
Editorial improvements. Also relaxed the requirements for
"reasonable assurances" (see <https://github.com/httpwg/http-
extensions/issues/148>).
A.16. Since draft-ietf-httpbis-alt-svc-14
Editorial changes made during RFC Editor's AUTH48 stage
(<https://github.com/httpwg/http-extensions/issues/165>).
Acknowledgements
Thanks to Adam Langley, Bence Beky, Chris Lonvick, Eliot Lear, Erik Thanks to Adam Langley, Bence Beky, Chris Lonvick, Eliot Lear, Erik
Nygren, Guy Podjarny, Herve Ruellan, Lucas Pardue, Martin Thomson, Nygren, Guy Podjarny, Herve Ruellan, Lucas Pardue, Martin Thomson,
Matthew Kerwin, Mike Bishop, Paul Hoffman, Richard Barnes, Richard Matthew Kerwin, Mike Bishop, Paul Hoffman, Richard Barnes, Richard
Bradbury, Stephen Farrell, Stephen Ludin, and Will Chan for their Bradbury, Stephen Farrell, Stephen Ludin, and Will Chan for their
feedback and suggestions. feedback and suggestions.
The Alt-Svc header field was influenced by the design of the The Alt-Svc header field was influenced by the design of the
Alternate-Protocol header field in SPDY. Alternate-Protocol header field in SPDY.
 End of changes. 91 change blocks. 
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