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Applications HTTPbis HTTP ALPN Alternate Services 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. Discussion of this draft takes place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org), which is archived at . Working Group information can be found at and ; source code and issues list for this draft can be found at . The changes in this draft are summarized in .

HTTP 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: 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. An origin server might wish to offer access to its resources using a new protocol (such as HTTP/2, see ) or one using improved security (such as Transport Layer Security (TLS), see ). An origin server might wish to segment its clients into groups of capabilities, such as those supporting Server Name Indication (SNI, see ) 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 , along with specific mechanisms for advertising their existence using HTTP header fields () or an HTTP/2 frame type (). It also introduces a new status code in , 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.

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

This specification defines a new concept in HTTP, the "alternative service". When an origin (see ) 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 .

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. Importantly, this includes its security context; in particular, when TLS 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 () 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: An Application Layer Protocol Negotiation (ALPN) protocol, as per A host, as per A port, as per Additionally, each alternative service &MUST; have: A freshness lifetime, expressed in seconds; see 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 () and an HTTP/2 frame type (). The remainder of this section describes requirements that are common to alternative services, regardless of how they are discovered.

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 . 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.

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 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.

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).

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. When a client uses an alternate service, it &MUST; emit the Alt-Used header field () 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.

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 () &MUST; be percent-encoded as per . 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: Octets in the ALPN protocol &MUST-NOT; be percent-encoded if they are valid token characters except "%", and 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 ), a colon (":"), and a port number.

For example: Alt-Svc: h2=":8000" This indicates the "h2" protocol () 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 (), servers &SHOULD; instead send an ALTSVC frame (). A single ALTSVC frame can be sent for a connection; a new frame is not needed for every request. Note that all field elements that allow "quoted-string" syntax &MUST; be processed as per .

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 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 for general requirements on caching alternative services.

The ALTSVC HTTP/2 frame () advertises the availability of an alternative service to an HTTP/2 client. The ALTSVC frame is a non-critical extension to HTTP/2. Endpoints that do not support this frame can safely ignore it. An ALTSVC frame 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 (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.

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. 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).

The 421 (Misdirected Request) status code is defined in 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 ). Clients receiving 421 (Misdirected Request) from an alternative service &MUST; remove the corresponding entry from its alternative service cache (see ) 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.

HTTP header fields are registered within the "Message Headers" registry maintained at . This document defines the following HTTP header fields, so their associated registry entries shall be added according to the permanent registrations below (see ): Header Field Name Protocol Status Reference Alt-Svc http standard Alt-Used http standard The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".

This document registers the ALTSVC frame type in the HTTP/2 Frame Types registry (). Frame Type: ALTSVC Code: 0xa Specification: of this document

An internationalized domain name that appears in either the header field () or the HTTP/2 frame () &MUST; be expressed using A-labels ().

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.

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 ) or remotely (e.g., by an intermediary as a man-in-the-middle attack). This is the reason for the requirement in 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.

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.

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).

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.

Harvard University World Wide Web Consortium Day Software Adobe Systems Incorporated Adobe Systems Incorporated

fielding@gbiv.com

greenbytes GmbH

julian.reschke@greenbytes.de

Adobe Systems Incorporated

fielding@gbiv.com

Akamai

mnot@mnot.net

greenbytes GmbH

julian.reschke@greenbytes.de

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

Selected 421 as proposed status code for "Not Authoritative". Changed header field syntax to use percent-encoding of ALPN protocol names ().

Updated HTTP/1.1 references. Renamed "Service" to "Alt-Svc-Used" and reduced information to a flag to address fingerprinting concerns (). Note that ALTSVC frame is preferred to Alt-Svc header field (). Incorporate ALTSRV frame (). Moved definition of status code 421 to HTTP/2. Partly resolved .

Updated ALPN reference. Resolved .

Renamed "Alt-Svc-Used" to "Alt-Used" (). Clarify ALTSVC Origin information requirements (). Remove/tune language with respect to tracking risks (see ).

Mention tracking by alt-svc host name in Security Considerations (). "421 (Not Authoritative)" -> "421 (Misdirected Request)". Allow the frame to carry multiple indicator and use the same payload formats for both ().