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rfc:rfc7838

Internet Engineering Task Force (IETF) M. Nottingham Request for Comments: 7838 Akamai Category: Standards Track P. McManus ISSN: 2070-1721 Mozilla

                                                            J. Reschke
                                                            greenbytes
                                                            April 2016
                     HTTP Alternative Services

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.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7838.

Copyright Notice

 Copyright (c) 2016 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.

Nottingham, et al. Standards Track [Page 1] RFC 7838 HTTP Alternative Services April 2016

Table of Contents

 1. Introduction ....................................................2
    1.1. Notational Conventions .....................................3
 2. Alternative Services Concepts ...................................3
    2.1. Host Authentication ........................................5
    2.2. Alternative Service Caching ................................6
    2.3. Requiring Server Name Indication ...........................6
    2.4. Using Alternative Services .................................6
 3. The Alt-Svc HTTP Header Field ...................................8
    3.1. Caching Alt-Svc Header Field Values .......................10
 4. The ALTSVC HTTP/2 Frame ........................................11
 5. The Alt-Used HTTP Header Field .................................13
 6. The 421 (Misdirected Request) HTTP Status Code .................13
 7. IANA Considerations ............................................13
    7.1. Header Field Registrations ................................13
    7.2. The ALTSVC HTTP/2 Frame Type ..............................14
    7.3. Alt-Svc Parameter Registry ................................14
         7.3.1. Procedure ..........................................14
         7.3.2. Registrations ......................................15
 8. Internationalization Considerations ............................15
 9. Security Considerations ........................................15
    9.1. Changing Ports ............................................15
    9.2. Changing Hosts ............................................15
    9.3. Changing Protocols ........................................16
    9.4. Tracking Clients Using Alternative Services ...............17
    9.5. Confusion regarding Request Scheme ........................17
 10. References ....................................................18
    10.1. Normative References .....................................18
    10.2. Informative References ...................................19
 Acknowledgements ..................................................19
 Authors' Addresses ................................................20

1. Introduction

 HTTP [RFC7230] conflates the identification of resources with their
 location.  In other words, "http://" and "https://" URIs 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 is under load, or it has found a server in a
    location that is more local to the client.

Nottingham, et al. Standards Track [Page 2] RFC 7838 HTTP Alternative Services April 2016

 o  An origin server might wish to offer access to its resources using
    a new protocol, such as HTTP/2 [RFC7540], or one using improved
    security, such as Transport Layer Security (TLS) [RFC5246].
 o  An origin server might wish to segment its clients into groups of
    capabilities, such as those supporting Server Name Indication
    (SNI) (Section 3 of [RFC6066]), 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
 HTTP/2 frames (Section 4), plus a way to indicate that an alternative
 service was used (Section 5).
 It also endorses the status code 421 (Misdirected Request)
 (Section 6) that origin servers or their nominated alternatives can
 use to 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] and updated
 by [RFC7405] along with the "#rule" extension defined in Section 7 of
 [RFC7230].  The rules below are defined in [RFC5234], [RFC7230], and
 [RFC7234]:
 OWS           = <OWS, see [RFC7230], Section 3.2.3>
 delta-seconds = <delta-seconds; see [RFC7234], Section 1.2.1>
 port          = <port, see [RFC7230], Section 2.7>
 quoted-string = <quoted-string, see [RFC7230], Section 3.2.6>
 token         = <token, see [RFC7230], Section 3.2.6>
 uri-host      = <uri-host, see [RFC7230], Section 2.7>

2. Alternative Services Concepts

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

Nottingham, et al. Standards Track [Page 3] RFC 7838 HTTP Alternative Services April 2016

 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; 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 the origin
 itself and the other routes are introduced based on alternative-
 service 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 the communication
 options for that protocol as well.
 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, and port) are the same as
 before.
 Importantly, this includes its security context; in particular, when
 TLS [RFC5246] is used to authenticate, 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).

Nottingham, et al. Standards Track [Page 4] RFC 7838 HTTP Alternative Services April 2016

 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 name, as
    per [RFC7301]
 o  A host, as per [RFC3986], Section 3.2.2
 o  A port, as per [RFC3986], Section 3.2.3
 The ALPN protocol name is used to identify the application protocol
 or suite of protocols used by the alternative service.  Note that for
 the purpose of this specification, an ALPN protocol name implicitly
 includes TLS in the suite of protocols it identifies, unless
 specified otherwise in its definition.  In particular, the ALPN name
 "http/1.1", registered by Section 6 of [RFC7301], identifies HTTP/1.1
 over TLS.
 Additionally, each alternative service MUST have 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: the "Alt-Svc" HTTP header field (Section 3) and the
 "ALTSVC" 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 have reasonable assurances that the alternative service
 is under control of and valid for the whole origin.  This mitigates
 the attack described in Section 9.2.
 For the purposes of this document, "reasonable assurances" can be
 established through use of a TLS-based protocol with the certificate
 checks defined in [RFC2818].  Clients MAY impose additional criteria
 for establishing reasonable assurances.
 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 either is offered with
 the "h2c" protocol, the client cannot use it, because there is no

Nottingham, et al. Standards Track [Page 5] RFC 7838 HTTP Alternative Services April 2016

 mechanism (at the time of the publication of this specification) in
 that protocol to establish the relationship between the origin and
 the alternative.

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 can 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; the caching
 mechanism is intended for limiting how long an alternative service
 can be used for establishing new connections, not limiting the use of
 existing ones.
 Alternative services are fully authoritative for the origin in
 question, including the ability to clear or update cached alternative
 service entries, extend freshness lifetimes, and any other authority
 the origin server would have.
 When alternative services are used to send a client to the most
 optimal server, a change in network configuration can result in
 cached values becoming suboptimal.  Therefore, clients SHOULD remove
 from cache all alternative services that lack the "persist" flag with
 the value "1" when they detect such a change, when information about
 network state is available.

2.3. Requiring Server Name Indication

 A client MUST NOT use a TLS-based alternative service unless the
 client 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 alternative services are used by servers,
 to aid in purposes like load balancing.

Nottingham, et al. Standards Track [Page 6] RFC 7838 HTTP Alternative Services April 2016

 Therefore, if a client supporting this specification 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 the alternative service information is fresh
 (Section 2.2) and the security properties of the alternative service
 protocol are desirable, as compared to the existing connection.  A
 viable alternative service is then treated in every way as the
 origin; this includes the ability to advertise alternative services.
 If a client becomes aware of multiple alternative services, it
 chooses the most suitable according to its own criteria, keeping
 security properties in mind.  For example, an origin might advertise
 multiple alternative services to notify clients of support for
 multiple versions of HTTP.
 A client configured to use a proxy for a given request SHOULD NOT
 directly connect to an alternative service for this request, but
 instead route it through that proxy.
 When a client uses an alternative service for a request, it can
 indicate this to the server using the Alt-Used header field
 (Section 5).
 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 (for example, 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.  If the connection to the alternative
 service does not negotiate the expected protocol (for example, ALPN
 fails to negotiate h2, or an Upgrade request to h2c is not accepted),
 the connection to the alternative service MUST be considered to have
 failed.

Nottingham, et al. Standards Track [Page 7] RFC 7838 HTTP Alternative Services April 2016

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       = clear / 1#alt-value
 clear         = %s"clear"; "clear", case-sensitive
 alt-value     = alternative *( OWS ";" OWS parameter )
 alternative   = protocol-id "=" alt-authority
 protocol-id   = token ; percent-encoded ALPN protocol name
 alt-authority = quoted-string ; containing [ uri-host ] ":" port
 parameter     = token "=" ( token / quoted-string )
 The field value consists either of a list of values, each of which
 indicates one alternative service, or the keyword "clear".
 A field value containing the special value "clear" indicates that the
 origin requests all alternatives for that origin to be invalidated
 (including those specified in the same response, in case of an
 invalid reply containing both "clear" and alternative services).
 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 name 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.
 For example:
 Alt-Svc: h2=":8000"

Nottingham, et al. Standards Track [Page 8] RFC 7838 HTTP Alternative Services April 2016

 This indicates the "h2" protocol ([RFC7540]) 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.  Note that recipients of Alt-Svc can ignore the header
 field (and are required to in some situations; see Sections 2.1 and
 6).
 The Alt-Svc field value can have multiple values:
 Alt-Svc: h2="alt.example.com:8000", h2=":443"
 When multiple values are present, the order of the values reflects
 the server's preference (with the first value being the most
 preferred alternative).
 The value(s) advertised by Alt-Svc can be used by clients to open a
 new connection to an alternative service.  Subsequent requests can
 start using this new connection immediately or can continue using the
 existing connection while the new connection is created.
 When using HTTP/2 ([RFC7540]), 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,
 despite this recommendation, Alt-Svc header fields remain valid in
 responses delivered over HTTP/2.

Nottingham, et al. Standards Track [Page 9] RFC 7838 HTTP Alternative Services April 2016

 Each "alt-value" is followed by an OPTIONAL semicolon-separated list
 of additional parameters, each such "parameter" comprising a name and
 a value.
 This specification defines two parameters: "ma" and "persist",
 defined in Section 3.1.  Unknown parameters MUST be ignored.  That
 is, the values (alt-value) they appear in MUST be processed as if the
 unknown parameter was not present.
 New parameters can be defined in extension specifications (see
 Section 7.3 for registration details).
 Note that all field elements that allow "quoted-string" syntax MUST
 be processed as per Section 3.2.6 of [RFC7230].

3.1. Caching Alt-Svc Header Field Values

 When an alternative service is advertised using Alt-Svc, it is
 considered fresh for 24 hours from generation of the message.  This
 can be modified with the "ma" (max-age) parameter.
 Syntax:
 ma = delta-seconds; see [RFC7234], Section 1.2.1
 The delta-seconds value indicates the number of seconds since the
 response was generated for which the alternative service is
 considered fresh.
 Alt-Svc: h2=":443"; ma=3600
 See Section 4.2.3 of [RFC7234] for details on determining the
 response age.
 For example, a response:
   HTTP/1.1 200 OK
   Content-Type: text/html
   Cache-Control: max-age=600
   Age: 30
   Alt-Svc: h2=":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); therefore, the
 alternative service is only fresh for the 30 seconds from when this
 response was received, minus estimated transit time.

Nottingham, et al. Standards Track [Page 10] RFC 7838 HTTP Alternative Services April 2016

 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.
 By default, cached alternative services will be cleared when the
 client detects a network change.  Alternative services that are
 intended to be longer lived (such as those that are not specific to
 the client access network) can carry the "persist" parameter with a
 value "1" as a hint that the service is potentially useful beyond a
 network configuration change.
 Syntax:
 persist = "1"
 For example:
 Alt-Svc: h2=":443"; ma=2592000; persist=1
 This specification only defines a single value for "persist".
 Clients MUST ignore "persist" parameters with values other than "1".
 See Section 2.2 for general requirements on caching alternative
 services.

4. The ALTSVC HTTP/2 Frame

 The ALTSVC HTTP/2 frame ([RFC7540], 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 will ignore it (as per the
 extensibility rules defined in Section 4.1 of [RFC7540]).
 An ALTSVC frame from a server to a client on a stream other than
 stream 0 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).

Nottingham, et al. Standards Track [Page 11] RFC 7838 HTTP Alternative Services April 2016

  +-------------------------------+-------------------------------+
  |         Origin-Len (16)       | Origin? (*)                 ...
  +-------------------------------+-------------------------------+
  |                   Alt-Svc-Field-Value (*)                   ...
  +---------------------------------------------------------------+
                         ALTSVC Frame Payload
 The ALTSVC frame contains the following fields:
 Origin-Len:  An unsigned, 16-bit integer indicating the length, in
    octets, of the Origin field.
 Origin:  An OPTIONAL sequence of characters containing the ASCII
    serialization of an origin ([RFC6454], Section 6.2) to which the
    alternative service is applicable.
 Alt-Svc-Field-Value:  A sequence of octets (length determined by
    subtracting the length of all preceding fields from the frame
    length) containing a value identical to 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 device acting
 as a server MUST ignore it.
 An ALTSVC frame on stream 0 with empty (length 0) "Origin"
 information is invalid and MUST be ignored.  An ALTSVC frame on a
 stream other than stream 0 containing non-empty "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.
 Receiving an ALTSVC frame is semantically equivalent to receiving an
 Alt-Svc header field.  As a result, the ALTSVC frame causes
 alternative services for the corresponding origin to be replaced.
 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
 hard to predict.

Nottingham, et al. Standards Track [Page 12] RFC 7838 HTTP Alternative Services April 2016

5. The Alt-Used HTTP Header Field

 The Alt-Used header field is used in requests to identify the
 alternative service in use, just as the Host header field
 (Section 5.4 of [RFC7230]) identifies the host and port of the
 origin.
 Alt-Used     = uri-host [ ":" port ]
 Alt-Used is intended to allow alternative services to detect loops,
 differentiate traffic for purposes of load balancing, and generally
 to ensure that it is possible to identify the intended destination of
 traffic, since introducing this information after a protocol is in
 use has proven to be problematic.
 When using an alternative service, clients SHOULD include an Alt-Used
 header field in all requests.
 For example:
   GET /thing HTTP/1.1
   Host: origin.example.com
   Alt-Used: alternate.example.net

6. The 421 (Misdirected Request) HTTP Status Code

 The 421 (Misdirected Request) status code is defined in Section 9.1.2
 of [RFC7540] 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.
 An Alt-Svc header field in a 421 (Misdirected Request) response MUST
 be ignored.

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/>.

Nottingham, et al. Standards Track [Page 13] RFC 7838 HTTP Alternative Services April 2016

 This document defines the following HTTP header fields, so their
 associated registry entries have been added according to the
 permanent registrations below (see [BCP90]):
 +-------------------+----------+----------+------------+
 | 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
 Type" registry ([RFC7540], Section 11.2).
    Frame Type: ALTSVC
    Code: 0xa
    Specification: Section 4 of this document

7.3. Alt-Svc Parameter Registry

 The "Hypertext Transfer Protocol (HTTP) Alt-Svc Parameter Registry"
 defines the name space for parameters.  It has been created and will
 be maintained at <http://www.iana.org/assignments/http-alt-svc-
 parameters>.

7.3.1. Procedure

 A registration MUST include the following fields:
 o  Parameter Name
 o  Pointer to specification text
 Values to be added to this name space require Expert Review (see
 [RFC5226], Section 4.1).

Nottingham, et al. Standards Track [Page 14] RFC 7838 HTTP Alternative Services April 2016

7.3.2. Registrations

 The "Hypertext Transfer Protocol (HTTP) Alt-Svc Parameter Registry"
 has been populated with the registrations below:
 +-------------------+--------------+
 | Alt-Svc Parameter | Reference    |
 +-------------------+--------------+
 | ma                | Section 3.1  |
 | persist           | Section 3.1  |
 +-------------------+--------------+

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.  Therefore, an attacker that
 can inject alternative services and listen at the advertised port is
 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
 and also to accept requests on less-privileged ports.
 For example, an attacker that can add HTTP response header fields to
 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
 under the attacker's control, they can thus masquerade as the HTTP
 server.
 This risk is mitigated by the requirements in Section 2.1.
 On servers, this risk can also be reduced by restricting the ability
 to advertise alternative services, and restricting who can open a
 port for listening on that host.

9.2. Changing Hosts

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

Nottingham, et al. Standards Track [Page 15] RFC 7838 HTTP Alternative Services April 2016

 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 clients
 have reasonable assurances that the alternative service is under
 control of and valid for the whole origin; for example, presenting a
 certificate for the origin proves that the alternative service is
 authorized to serve traffic for the origin.
 Note that this assurance is only as strong as the method used to
 authenticate the alternative service.  In particular, when TLS
 authentication is used to do so, 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.
 Implementations MUST perform any certificate-pinning validation (such
 as [RFC7469]) on alternative services just as they would on direct
 connections to the origin.  Implementations might also choose to add
 other requirements around which certificates are acceptable for
 alternative services.

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 an "https://" URI has a protocol advertised that does
 not use some form of end-to-end encryption (most likely, TLS), this
 violates the expectations for security that the URI scheme implies.
 Therefore, clients cannot use alternative services blindly, but
 instead evaluate the option(s) presented to ensure that security
 requirements and expectations of specifications, implementations, and
 end users are met.

Nottingham, et al. Standards Track [Page 16] RFC 7838 HTTP Alternative Services April 2016

9.4. Tracking Clients Using Alternative Services

 Choosing an alternative service implies connecting to a new, server-
 supplied host name.  By using unique names, servers could conceivably
 track client requests.  Such tracking could follow users across
 multiple networks, when the "persist" flag is used.
 Clients that wish to prevent requests from being correlated can
 decide not to use alternative services for multiple requests that
 would not otherwise be allowed to be correlated.
 In a user agent, any alternative service information MUST be removed
 when origin-specific data is cleared (typically, when cookies
 [RFC6265] are cleared).

9.5. Confusion regarding Request Scheme

 Some server-side HTTP applications make assumptions about security
 based upon connection context; for example, equating being served
 upon port 443 with the use of an "https://" URI and the various
 security properties that implies.
 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
 example, a Web browser treats "https://" URIs differently than
 "http://" URIs in many ways, not just for purposes of protocol
 handling.
 Since one of the uses of Alternative Services is to allow a
 connection to be migrated to a different protocol and port, these
 applications can become confused about the security properties of a
 given connection, sending information (for example, cookies and
 content) that is intended for a secure context (such as an "https://"
 URI) to a client that is not treating it as one.
 This risk can be mitigated in servers by using the URI scheme
 explicitly carried by the protocol (such as ":scheme" in HTTP/2 or
 the "absolute form" of the request target in HTTP/1.1) as an
 indication of security context, instead of other connection
 properties ([RFC7540], Section 8.1.2.3 and [RFC7230], Section 5.3.2).
 When the protocol does not explicitly carry the scheme (as is usually
 the case for HTTP/1.1 over TLS), servers can mitigate this risk by
 either assuming that all requests have an insecure context, or by
 refraining from advertising alternative services for insecure schemes
 (for example, HTTP).

Nottingham, et al. Standards Track [Page 17] RFC 7838 HTTP Alternative Services April 2016

10. References

10.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
            DOI 10.17487/RFC2818, May 2000,
            <http://www.rfc-editor.org/info/rfc2818>.
 [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
            Resource Identifier (URI): Generic Syntax", STD 66,
            RFC 3986, DOI 10.17487/RFC3986, January 2005,
            <http://www.rfc-editor.org/info/rfc3986>.
 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            DOI 10.17487/RFC5226, May 2008,
            <http://www.rfc-editor.org/info/rfc5226>.
 [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", STD 68, RFC 5234,
            DOI 10.17487/RFC5234, January 2008,
            <http://www.rfc-editor.org/info/rfc5234>.
 [RFC5890]  Klensin, J., "Internationalized Domain Names for
            Applications (IDNA): Definitions and Document Framework",
            RFC 5890, DOI 10.17487/RFC5890, August 2010,
            <http://www.rfc-editor.org/info/rfc5890>.
 [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
            Extension Definitions", RFC 6066, DOI 10.17487/RFC6066,
            January 2011, <http://www.rfc-editor.org/info/rfc6066>.
 [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
            DOI 10.17487/RFC6454, December 2011,
            <http://www.rfc-editor.org/info/rfc6454>.
 [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
            Protocol (HTTP/1.1): Message Syntax and Routing",
            RFC 7230, DOI 10.17487/RFC7230, June 2014,
            <http://www.rfc-editor.org/info/rfc7230>.

Nottingham, et al. Standards Track [Page 18] RFC 7838 HTTP Alternative Services April 2016

 [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
            Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
            RFC 7234, DOI 10.17487/RFC7234, June 2014,
            <http://www.rfc-editor.org/info/rfc7234>.
 [RFC7301]  Friedl, S., Popov, A., Langley, A., and S. Emile,
            "Transport Layer Security (TLS) Application-Layer Protocol
            Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
            July 2014, <http://www.rfc-editor.org/info/rfc7301>.
 [RFC7405]  Kyzivat, P., "Case-Sensitive String Support in ABNF",
            RFC 7405, DOI 10.17487/RFC7405, December 2014,
            <http://www.rfc-editor.org/info/rfc7405>.
 [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
            Transfer Protocol version 2", RFC 7540,
            DOI 10.17487/RFC7540, May 2015,
            <http://www.rfc-editor.org/info/rfc7540>.

10.2. Informative References

 [BCP90]    Klyne, G., Nottingham, M., and J. Mogul, "Registration
            Procedures for Message Header Fields", BCP 90, RFC 3864,
            September 2004, <http://www.rfc-editor.org/info/bcp90>.
 [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.2", RFC 5246,
            DOI 10.17487/RFC5246, August 2008,
            <http://www.rfc-editor.org/info/rfc5246>.
 [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
            DOI 10.17487/RFC6265, April 2011,
            <http://www.rfc-editor.org/info/rfc6265>.
 [RFC7469]  Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
            Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
            2015, <http://www.rfc-editor.org/info/rfc7469>.

Acknowledgements

 Thanks to Adam Langley, Bence Beky, Chris Lonvick, Eliot Lear, Erik
 Nygren, Guy Podjarny, Herve Ruellan, Lucas Pardue, Martin Thomson,
 Matthew Kerwin, Mike Bishop, Paul Hoffman, Richard Barnes, Richard
 Bradbury, 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.

Nottingham, et al. Standards Track [Page 19] RFC 7838 HTTP Alternative Services April 2016

Authors' Addresses

 Mark Nottingham
 Akamai
 Email: mnot@mnot.net
 URI:   https://www.mnot.net/
 Patrick McManus
 Mozilla
 Email: mcmanus@ducksong.com
 URI:   https://mozillians.org/u/pmcmanus/
 Julian F. Reschke
 greenbytes GmbH
 Email: julian.reschke@greenbytes.de
 URI:   https://greenbytes.de/tech/webdav/

Nottingham, et al. Standards Track [Page 20]

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