HTTP Alternative Services
draft-ietf-httpbis-alt-svc-05

HTTPbis Working Group                                      M. Nottingham
Internet-Draft                                                    Akamai
Intended status: Standards Track                              P. McManus
Expires: June 4, 2015                                            Mozilla
                                                              J. Reschke
                                                              greenbytes
                                                        December 1, 2014

                       HTTP Alternative Services
                     draft-ietf-httpbis-alt-svc-05

Abstract

   This document specifies "alternative services" for HTTP, which allow
   an origin's resources to be authoritatively available at a separate
   network location, possibly accessed with a different protocol
   configuration.

Editorial Note (To be removed by RFC Editor)

   Discussion of this draft takes place on the HTTPBIS working group
   mailing list (ietf-http-wg@w3.org), which is archived at
   <https://lists.w3.org/Archives/Public/ietf-http-wg/>.

   Working Group information can be found at
   <https://tools.ietf.org/wg/httpbis/> and <http://httpwg.github.io/>;
   source code and issues list for this draft can be found at
   <https://github.com/httpwg/http-extensions>.

   The changes in this draft are summarized in Appendix A.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 4, 2015.

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Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Notational Conventions . . . . . . . . . . . . . . . . . .  4
   2.  Alternative Services Concepts  . . . . . . . . . . . . . . . .  5
     2.1.  Host Authentication  . . . . . . . . . . . . . . . . . . .  6
     2.2.  Alternative Service Caching  . . . . . . . . . . . . . . .  7
     2.3.  Requiring Server Name Indication . . . . . . . . . . . . .  7
     2.4.  Using Alternative Services . . . . . . . . . . . . . . . .  7
   3.  The Alt-Svc HTTP Header Field  . . . . . . . . . . . . . . . .  8
     3.1.  Caching Alt-Svc Header Field Values  . . . . . . . . . . . 10
   4.  The ALTSVC HTTP/2 Frame  . . . . . . . . . . . . . . . . . . . 10
   5.  The Alt-Used HTTP Header Field . . . . . . . . . . . . . . . . 11
   6.  The 421 Misdirected Request HTTP Status Code . . . . . . . . . 12
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
     7.1.  Header Field Registrations . . . . . . . . . . . . . . . . 12
     7.2.  The ALTSVC HTTP/2 Frame Type . . . . . . . . . . . . . . . 13
   8.  Internationalization Considerations  . . . . . . . . . . . . . 13
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
     9.1.  Changing Ports . . . . . . . . . . . . . . . . . . . . . . 13
     9.2.  Changing Hosts . . . . . . . . . . . . . . . . . . . . . . 14
     9.3.  Changing Protocols . . . . . . . . . . . . . . . . . . . . 14
     9.4.  Tracking Clients Using Alternative Services  . . . . . . . 15
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 15
     11.2. Informative References . . . . . . . . . . . . . . . . . . 16
   Appendix A.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 16
     A.1.  Since draft-nottingham-httpbis-alt-svc-05  . . . . . . . . 16
     A.2.  Since draft-ietf-httpbis-alt-svc-00  . . . . . . . . . . . 16
     A.3.  Since draft-ietf-httpbis-alt-svc-01  . . . . . . . . . . . 16
     A.4.  Since draft-ietf-httpbis-alt-svc-02  . . . . . . . . . . . 17
     A.5.  Since draft-ietf-httpbis-alt-svc-03  . . . . . . . . . . . 17
     A.6.  Since draft-ietf-httpbis-alt-svc-04  . . . . . . . . . . . 17

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1.  Introduction

   HTTP [RFC7230] conflates the identification of resources with their
   location.  In other words, "http://" (and "https://") URLs are used
   to both name and find things to interact with.

   In some cases, it is desirable to separate identification and
   location in HTTP; keeping the same identifier for a resource, but
   interacting with it at a different location on the network.

   For example:

   o  An origin server might wish to redirect a client to a different
      server when it needs to go down for maintenance, or it has found a
      server in a location that is more local to the client.

   o  An origin server might wish to offer access to its resources using
      a new protocol (such as HTTP/2, see [HTTP2]) or one using improved
      security (such as Transport Layer Security (TLS), see [RFC5246]).

   o  An origin server might wish to segment its clients into groups of
      capabilities, such as those supporting Server Name Indication
      (SNI, see Section 3 of [RFC6066]) and those not supporting it, for
      operational purposes.

   This specification defines a new concept in HTTP, "Alternative
   Services", that allows an origin server to nominate additional means
   of interacting with it on the network.  It defines a general
   framework for this in Section 2, along with specific mechanisms for
   advertising their existence using HTTP header fields (Section 3) or
   an HTTP/2 frame type (Section 4).

   It also introduces a new status code in Section 6, so that origin
   servers (or their nominated alternatives) can indicate that they are
   not authoritative for a given origin, in cases where the wrong
   location is used.

1.1.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   This document uses the Augmented BNF defined in [RFC5234] along with
   the "OWS", "delta-seconds", "parameter", "port", "quoted-string",
   "token", and "uri-host" rules from [RFC7230], and uses the "#rule"
   extension defined in Section 7 of that document.

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2.  Alternative Services Concepts

   This specification defines a new concept in HTTP, the "alternative
   service".  When an origin (see [RFC6454]) has resources that are
   accessible through a different protocol / host / port combination, it
   is said to have an alternative service available.

   An alternative service can be used to interact with the resources on
   an origin server at a separate location on the network, possibly
   using a different protocol configuration.  Alternative services are
   considered authoritative for an origin's resources, in the sense of
   [RFC7230], Section 9.1.

   For example, an origin:

   ("http", "www.example.com", "80")

   might declare that its resources are also accessible at the
   alternative service:

   ("h2", "new.example.com", "81")

   By their nature, alternative services are explicitly at the
   granularity of an origin; i.e., they cannot be selectively applied to
   resources within an origin.

   Alternative services do not replace or change the origin for any
   given resource; in general, they are not visible to the software
   "above" the access mechanism.  The alternative service is essentially
   alternative routing information that can also be used to reach the
   origin in the same way that DNS CNAME or SRV records define routing
   information at the name resolution level.  Each origin maps to a set
   of these routes -- the default route is derived from origin itself
   and the other routes are introduced based on alternative-protocol
   information.

   Furthermore, it is important to note that the first member of an
   alternative service tuple is different from the "scheme" component of
   an origin; it is more specific, identifying not only the major
   version of the protocol being used, but potentially communication
   options for that protocol.

   This means that clients using an alternative service can change the
   host, port and protocol that they are using to fetch resources, but
   these changes MUST NOT be propagated to the application that is using
   HTTP; from that standpoint, the URI being accessed and all
   information derived from it (scheme, host, port) are the same as
   before.

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   Importantly, this includes its security context; in particular, when
   TLS [RFC5246] is in use, the alternative service will need to present
   a certificate for the origin's host name, not that of the
   alternative.  Likewise, the Host header field ([RFC7230], Section
   5.4) is still derived from the origin, not the alternative service
   (just as it would if a CNAME were being used).

   The changes MAY, however, be made visible in debugging tools,
   consoles, etc.

   Formally, an alternative service is identified by the combination of:

   o  An Application Layer Protocol Negotiation (ALPN) protocol, as per
      [RFC7301]

   o  A host, as per [RFC3986], Section 3.2.2

   o  A port, as per [RFC3986], Section 3.2.3

   Additionally, each alternative service MUST have:

   o  A freshness lifetime, expressed in seconds; see Section 2.2

   There are many ways that a client could discover the alternative
   service(s) associated with an origin.  This document describes two
   such mechanisms: an HTTP header field (Section 3) and an HTTP/2 frame
   type (Section 4).

   The remainder of this section describes requirements that are common
   to alternative services, regardless of how they are discovered.

2.1.  Host Authentication

   Clients MUST NOT use alternative services with a host that is
   different than the origin's without strong server authentication;
   this mitigates the attack described in Section 9.2.  One way to
   achieve this is for the alternative to use TLS with a certificate
   that is valid for that origin.

   For example, if the origin's host is "www.example.com" and an
   alternative is offered on "other.example.com" with the "h2" protocol,
   and the certificate offered is valid for "www.example.com", the
   client can use the alternative.  However, if "other.example.com" is
   offered with the "h2c" protocol, the client cannot use it, because
   there is no mechanism in that protocol to establish strong server
   authentication.

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2.2.  Alternative Service Caching

   Mechanisms for discovering alternative services also associate a
   freshness lifetime with them; for example, the Alt-Svc header field
   uses the "ma" parameter.

   Clients MAY choose to use an alternative service instead of the
   origin at any time when it is considered fresh; see Section 2.4 for
   specific recommendations.

   Clients with existing connections to an alternative service do not
   need to stop using it 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.

   Clients ought to consider that changes in network configurations can
   result in suboptimal or compromised cached alternative services.

2.3.  Requiring Server Name Indication

   A client MUST only use a TLS-based alternative service if the client
   also supports TLS Server Name Indication (SNI).  This supports the
   conservation of IP addresses on the alternative service host.

   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
   field-value).

2.4.  Using Alternative Services

   By their nature, alternative services are OPTIONAL: clients do not
   need to use them.  However, it is advantageous for clients to behave
   in a predictable way when they are used by servers (e.g., for load
   balancing).

   Therefore, if a client becomes aware of an alternative service, the
   client SHOULD use that alternative service for all requests to the
   associated origin as soon as it is available, provided that the
   security properties of the alternative service protocol are
   desirable, as compared to the existing connection.

   If a client becomes aware of multiple alternative services, it MAY
   choose the most suitable according to its own criteria (again,
   keeping security properties in mind).  For example, an origin might
   advertise multiple alternative services to notify clients of support
   for multiple versions of HTTP; or, an alternative service might
   itself advertise an alternative.

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   When a client uses an alternate service, it MUST emit the Alt-Used
   header field (Section 5) on every request using that alternate
   service.

   The client does not need to block requests on any existing
   connection; it can be used until the alternative connection is
   established.  However, if the security properties of the existing
   connection are weak (e.g. cleartext HTTP/1.1) then it might make
   sense to block until the new connection is fully available in order
   to avoid information leakage.

   Furthermore, if the connection to the alternative service fails or is
   unresponsive, the client MAY fall back to using the origin or another
   alternative service.  Note, however, that this could be the basis of
   a downgrade attack, thus losing any enhanced security properties of
   the alternative service.

3.  The Alt-Svc HTTP Header Field

   An HTTP(S) origin server can advertise the availability of
   alternative services to clients by adding an Alt-Svc header field to
   responses.

   Alt-Svc       = 1#( alternative *( OWS ";" OWS parameter ) )
   alternative   = protocol-id "=" alt-authority
   protocol-id   = token ; percent-encoded ALPN protocol identifier
   alt-authority = quoted-string ; containing [ uri-host ] ":" port

   ALPN protocol names are octet sequences with no additional
   constraints on format.  Octets not allowed in tokens ([RFC7230],
   Section 3.2.6) MUST be percent-encoded as per Section 2.1 of
   [RFC3986].  Consequently, the octet representing the percent
   character "%" (hex 25) MUST be percent-encoded as well.

   In order to have precisely one way to represent any ALPN protocol
   name, the following additional constraints apply:

   1.  Octets in the ALPN protocol MUST NOT be percent-encoded if they
       are valid token characters except "%", and

   2.  When using percent-encoding, uppercase hex digits MUST be used.

   With these constraints, recipients can apply simple string comparison
   to match protocol identifiers.

   The "alt-authority" component consists of an OPTIONAL uri-host
   ("host" in Section 3.2.2 of [RFC3986]), a colon (":"), and a port
   number.

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   For example:

   Alt-Svc: h2=":8000"

   This indicates the "h2" protocol ([HTTP2]) on the same host using the
   indicated port 8000.

   An example involving a change of host:

   Alt-Svc: h2="new.example.org:80"

   This indicates the "h2" protocol on the host "new.example.org",
   running on port 80.  Note that the "quoted-string" syntax needs to be
   used because ":" is not an allowed character in "token".

   Examples for protocol name escaping:

   +--------------------+-------------+---------------------+
   | ALPN protocol name | protocol-id | Note                |
   +--------------------+-------------+---------------------+
   | h2                 | h2          | No escaping needed  |
   +--------------------+-------------+---------------------+
   | w=x:y#z            | w%3Dx%3Ay#z | "=" and ":" escaped |
   +--------------------+-------------+---------------------+
   | x%y                | x%25y       | "%" needs escaping  |
   +--------------------+-------------+---------------------+

   Alt-Svc MAY occur in any HTTP response message, regardless of the
   status code.

   The Alt-Svc field value can have multiple values:

   Alt-Svc: h2c=":8000", h2=":443"

   The value(s) advertised by Alt-Svc can be used by clients to open a
   new connection to one or more alternative services immediately, or
   simultaneously with subsequent requests on the same connection.

   When using HTTP/2 ([HTTP2]), servers SHOULD instead send an ALTSVC
   frame (Section 4).  A single ALTSVC frame can be sent for a
   connection; a new frame is not needed for every request.

   Note that all field elements that allow "quoted-string" syntax MUST
   be processed as per Section 3.2.6 of [RFC7230].

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3.1.  Caching Alt-Svc Header Field Values

   When an alternative service is advertised using Alt-Svc, it is
   considered fresh for 24 hours from generation of the message.  This
   can be modified with the 'ma' (max-age) parameter;

   Alt-Svc: h2=":443"; ma=3600

   which indicates the number of seconds since the response was
   generated the alternative service is considered fresh for.

   ma = delta-seconds

   See Section 4.2.3 of [RFC7234] for details of determining response
   age.

   For example, a response:

     HTTP/1.1 200 OK
     Content-Type: text/html
     Cache-Control: 600
     Age: 30
     Alt-Svc: h2c=":8000"; ma=60

   indicates that an alternative service is available and usable for the
   next 60 seconds.  However, the response has already been cached for
   30 seconds (as per the Age header field value), so therefore the
   alternative service is only fresh for the 30 seconds from when this
   response was received, minus estimated transit time.

   Note that the freshness lifetime for HTTP caching (here, 600 seconds)
   does not affect caching of Alt-Svc values.

   When an Alt-Svc response header field is received from an origin, its
   value invalidates and replaces all cached alternative services for
   that origin.

   See Section 2.2 for general requirements on caching alternative
   services.

4.  The ALTSVC HTTP/2 Frame

   The ALTSVC HTTP/2 frame ([HTTP2], Section 4) advertises the
   availability of an alternative service to an HTTP/2 client.

   The ALTSVC frame is a non-critical extension to HTTP/2.  Endpoints
   that do not support this frame can safely ignore it.

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   An ALTSVC frame from a server to a client on a client-initiated
   stream indicates that the conveyed alternative service is associated
   with the origin of that stream.

   An ALTSVC frame from a server to a client on stream 0 indicates that
   the conveyed alternative service is associated with the origin
   contained in the Origin field of the frame.  An association with an
   origin that the client does not consider authoritative for the
   current connection MUST be ignored.

   The ALTSVC frame type is 0xa (decimal 10).

   The payload of an ALTSVC frame is identical to the payload of the
   Alt-Svc field value defined in Section 3 (ABNF production "Alt-Svc").

   The ALTSVC frame does not define any flags.

   The ALTSVC frame is intended for receipt by clients; a server that
   receives an ALTSVC frame MUST treat it as a connection error of type
   PROTOCOL_ERROR.

   An ALTSVC frame on a client-initiated stream containing non-empty
   "Origin" information is invalid and MUST be ignored.  Likewise, an
   ALTSVC frame on stream 0 with empty (length 0) "Origin" information
   is invalid and MUST be ignored.

   The ALTSVC frame is processed hop-by-hop.  An intermediary MUST NOT
   forward ALTSVC frames, though it can use the information contained in
   ALTSVC frames in forming new ALTSVC frames to send to its own
   clients.

5.  The Alt-Used HTTP Header Field

   The Alt-Used header field is used in requests to indicate that an
   alternate service is in use.

   Alt-Used     = use-flag *( OWS ";" OWS ext-param )
   use-flag     = "1" / "0"
   ext-param    = token "=" ( token / quoted-string )

   Alt-Used is intended to allow alternate services to avoid sending
   alternative service indications where there is a risk of generating a
   loops.  It also allows a service to identify requests for accounting
   and load balancing purposes.

   When using an alternative service, clients MUST include a Alt-Used
   header field in all requests.

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   A flag value of "1" indicates that an alternate service was used,
   while "0" means it was not.

   For example:

     GET /thing HTTP/1.1
     Host: origin.example.com
     Alt-Used: 1

   The extension parameters (ext-param) are reserved for future use;
   specifications that want to define an extension will need to update
   this document (and ought to introduce an extension registry).

6.  The 421 Misdirected Request HTTP Status Code

   The 421 (Misdirected Request) status code is defined in Section 9.1.2
   of [HTTP2] to indicate that the current server instance is not
   authoritative for the requested resource.  This can be used to
   indicate that an alternative service is not authoritative; see
   Section 2).

   Clients receiving 421 (Misdirected Request) from an alternative
   service MUST remove the corresponding entry from its alternative
   service cache (see Section 2.2) for that origin.  Regardless of the
   idempotency of the request method, they MAY retry the request, either
   at another alternative server, or at the origin.

   A 421 (Misdirected Request) response MAY carry an Alt-Svc header
   field.

7.  IANA Considerations

7.1.  Header Field Registrations

   HTTP header fields are registered within the "Message Headers"
   registry maintained at
   <https://www.iana.org/assignments/message-headers/>.

   This document defines the following HTTP header fields, so their
   associated registry entries shall be added according to the permanent
   registrations below (see [BCP90]):

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   +-------------------+----------+----------+-----------+
   | Header Field Name | Protocol | Status   | Reference |
   +-------------------+----------+----------+-----------+
   | Alt-Svc           | http     | standard | Section 3 |
   | Alt-Used          | http     | standard | Section 5 |
   +-------------------+----------+----------+-----------+

   The change controller is: "IETF (iesg@ietf.org) - Internet
   Engineering Task Force".

7.2.  The ALTSVC HTTP/2 Frame Type

   This document registers the ALTSVC frame type in the HTTP/2 Frame
   Types registry ([HTTP2], Section 11.2).

      Frame Type: ALTSVC

      Code: 0xa

      Specification: Section 4 of this document

8.  Internationalization Considerations

   An internationalized domain name that appears in either the header
   field (Section 3) or the HTTP/2 frame (Section 4) MUST be expressed
   using A-labels ([RFC5890], Section 2.3.2.1).

9.  Security Considerations

9.1.  Changing Ports

   Using an alternative service implies accessing an origin's resources
   on an alternative port, at a minimum.  An attacker that can inject
   alternative services and listen at the advertised port is therefore
   able to hijack an origin.

   For example, an attacker that can add HTTP response header fields can
   redirect traffic to a different port on the same host using the Alt-
   Svc header field; if that port is under the attacker's control, they
   can thus masquerade as the HTTP server.

   This risk can be mitigated by restricting the ability to advertise
   alternative services, and restricting who can open a port for
   listening on that host.

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9.2.  Changing Hosts

   When the host is changed due to the use of an alternative service, it
   presents an opportunity for attackers to hijack communication to an
   origin.

   For example, if an attacker can convince a user agent to send all
   traffic for "innocent.example.org" to "evil.example.com" by
   successfully associating it as an alternative service, they can
   masquerade as that origin.  This can be done locally (see mitigations
   in Section 9.1) or remotely (e.g., by an intermediary as a man-in-
   the-middle attack).

   This is the reason for the requirement in Section 2.1 that any
   alternative service with a host different to the origin's be strongly
   authenticated with the origin's identity; i.e., presenting a
   certificate for the origin proves that the alternative service is
   authorized to serve traffic for the origin.

   However, this authorization is only as strong as the method used to
   authenticate the alternative service.  In particular, there are well-
   known exploits to make an attacker's certificate appear as
   legitimate.

   Alternative services could be used to persist such an attack; for
   example, an intermediary could man-in-the-middle TLS-protected
   communication to a target, and then direct all traffic to an
   alternative service with a large freshness lifetime, so that the user
   agent still directs traffic to the attacker even when not using the
   intermediary.

9.3.  Changing Protocols

   When the ALPN protocol is changed due to the use of an alternative
   service, the security properties of the new connection to the origin
   can be different from that of the "normal" connection to the origin,
   because the protocol identifier itself implies this.

   For example, if a "https://" URI had a protocol advertised that does
   not use some form of end-to-end encryption (most likely, TLS), it
   violates the expectations for security that the URI scheme implies.

   Therefore, clients cannot blindly use alternative services, but
   instead evaluate the option(s) presented to assure that security
   requirements and expectations (of specifications, implementations and
   end users) are met.

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9.4.  Tracking Clients Using Alternative Services

   Choosing an alternative service implies connecting to a new, server-
   supplied host name.  By using many different (potentially unique)
   host names, servers could conceivably track client requests.

   Clients concerned by the additional fingerprinting can choose to
   ignore alternative service advertisements.

   In a browser, any alternative service information MUST be removed
   when origin-specific data is cleared (for instance, when cookies are
   cleared).

10.  Acknowledgements

   Thanks to Adam Langley, Eliot Lear, Erik Nygren, Guy Podjarny, Paul
   Hoffman, Richard Barnes, Stephen Farrell, Stephen Ludin, and Will
   Chan for their feedback and suggestions.

   The Alt-Svc header field was influenced by the design of the
   Alternate-Protocol header field in SPDY.

11.  References

11.1.  Normative References

   [HTTP2]    Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol version 2", draft-ietf-httpbis-http2-16
              (work in progress), November 2014.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, August 2010.

   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
              Extension Definitions", RFC 6066, January 2011.

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,

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              December 2011.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, June 2014.

   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, June 2014.

   [RFC7301]  Friedl, S., Popov, A., Langley, A., and S. Emile,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, July 2014.

11.2.  Informative References

   [BCP90]    Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              September 2004.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

Appendix A.  Change Log (to be removed by RFC Editor before publication)

A.1.  Since draft-nottingham-httpbis-alt-svc-05

   This is the first version after adoption of
   draft-nottingham-httpbis-alt-svc-05 as Working Group work item.  It
   only contains editorial changes.

A.2.  Since draft-ietf-httpbis-alt-svc-00

   Selected 421 as proposed status code for "Not Authoritative".

   Changed header field syntax to use percent-encoding of ALPN protocol
   names (<https://github.com/http2/http2-spec/issues/446>).

A.3.  Since draft-ietf-httpbis-alt-svc-01

   Updated HTTP/1.1 references.

   Renamed "Service" to "Alt-Svc-Used" and reduced information to a flag
   to address fingerprinting concerns
   (<https://github.com/http2/http2-spec/issues/502>).

   Note that ALTSVC frame is preferred to Alt-Svc header field
   (<https://github.com/http2/http2-spec/pull/503>).

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   Incorporate ALTSRV frame
   (<https://github.com/http2/http2-spec/pull/507>).

   Moved definition of status code 421 to HTTP/2.

   Partly resolved <https://github.com/httpwg/http-extensions/issues/5>.

A.4.  Since draft-ietf-httpbis-alt-svc-02

   Updated ALPN reference.

   Resolved <https://github.com/httpwg/http-extensions/issues/2>.

A.5.  Since draft-ietf-httpbis-alt-svc-03

   Renamed "Alt-Svc-Used" to "Alt-Used"
   (<https://github.com/httpwg/http-extensions/issues/17>).

   Clarify ALTSVC Origin information requirements
   (<https://github.com/httpwg/http-extensions/issues/19>).

   Remove/tune language with respect to tracking risks (see
   <https://github.com/httpwg/http-extensions/issues/34>).

A.6.  Since draft-ietf-httpbis-alt-svc-04

   Mention tracking by alt-svc host name in Security Considerations
   (<https://github.com/httpwg/http-extensions/issues/36>).

   "421 (Not Authoritative)" -> "421 (Misdirected Request)".

   Allow the frame to carry multiple indicator and use the same payload
   formats for both
   (<https://github.com/httpwg/http-extensions/issues/37>).

Authors' Addresses

   Mark Nottingham
   Akamai

   EMail: mnot@mnot.net
   URI:   https://www.mnot.net/

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   Patrick McManus
   Mozilla

   EMail: mcmanus@ducksong.com
   URI:   https://mozillians.org/u/pmcmanus/

   Julian F. Reschke
   greenbytes GmbH

   EMail: julian.reschke@greenbytes.de
   URI:   https://greenbytes.de/tech/webdav/

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