Distributed HTTP Origins: Solution Space Exploration

1.1. Conventions and Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

2.1. Content-Specific Hostnames

One solution, visible in multiple services, uses granular hostnames to identify the server or servers with the particular content in question, such as r2---sn-jpocxaa-j8bl.googlevideo.com. This hostname, with its own HTTP origin, controls a particular slice of the media available on YouTube.com. The YouTube service indicates to a player loading a video which origin contains or caches the requested content.¶

Note that there are several ways of providing these hostnames to clients, depending on the interaction model between the client and the server. For example:¶

• The server might generate HTML or JSON content in response to an initial request, providing absolute URIs for each dependent resource which indicate the specific host from which the resource can be retrieved¶
• The server might return a 3XX (Redirect) response to a client's query for a resource, directing the client to the resource at a different hostname¶
• An API might enable a client to query for the location of a resource before requesting it¶

One drawback of this approach is that the content belongs to a different origin than the primary origin of the page. While this is less of an issue in APIs or bulk data transfer, this limits the types of requests that can be made and the access to the data from scripts loaded by the primary origin without first making CORS preflight requests [CORS], which introduce additional latency.¶

This approach can also complicate certain protocol features which rely on previous contact with the server. The primary server typically cannot provide Alt-Svc entries for the secondary, though the targeting of the specific hostname may avoid the need for Alt-Svc. TLS session resumption and 0-RTT will typically not be usable, adding latency to the request.¶

2.2. Internal Load-Balancing

A second solution, which is generally not visible to the client, is to have all requests terminated by a front-end which does not cache or serve any content directly. Rather, this front-end is responsible for inspecting the request, identifying the server which can actually respond, and forwarding the request to that server.¶

This solution has its own challenges. While the data access and storage requirements can be distributed amongst back-end machines, throughput on the front-end load balancer becomes a bottleneck. For certain protocols, direct server return (DSR) avoids this bottleneck by sending response packets back to the client instead of sending them via the load balancer. However, DSR is challenging with reliable and encrypted protocols, and even moreso with multiplexed protocols like HTTP/2 or HTTP/3.¶

3.1. Out-of-Band Encoding

[OOB] describes an HTTP content coding that can be used to describe the location of a secondary resource that contains the payload. The origin returns an HTTP field set which describes the content, including a Content-Encoding header which indicates the content can be fetched from a different URL, typically hosted on a different origin server.¶

This approach is similar in spirit to Content-Specific Hostnames as described in Section 2.1, except that the resources continue to belong to a single origin regardless of which origin server actually delivers the bytes. Unlike Content-Specific Hostnames, however, a separate request must be made for each resource -- first to the origin server to receive the headers, then to the secondary server to retrieve the content of the response.¶

3.2. Alternative Services

[AltSvc] describes a way in which an origin server can delegate authority over the origin to another host which might be preferable in some way. However, this mechanism delegates the entire origin and cannot be subdivided.¶

A 421 response being used to work around this dramatically reduces efficiency, as the client has no insight into which paths the alternative might or might not support.¶

4.1. Scope-Restricted Alt-Svc Entries

When an alternative service is advertised by an origin, by default the indicated server is authoritative for all resources in the origin. The scope parameter can be used to adjust this scope.¶

The scope parameter contains the path portion of a URI; see Section 3.3 of [RFC3986]. The indicated alternative is authoritative only for resources where the path begins with the indicated prefix.¶

scope = DQUOTE path DQUOTE ; see [RFC3986], Section 3.3

For example:¶

Alt-Svc: h2=":443"; ma=3600; scope="/sn-jpocxaa-j8bl/",
         h2=":443"; ma=3600; scope="/sn-5ualdn7s"

A scope-restricted alternative SHOULD NOT be sent requests for resources unless the path portion of the URI is a prefix match with the indicated scope.¶

[AltSvc] indicates that parameters are optional to understand. Therefore, origin servers SHOULD NOT send an alternative service advertisement to a client which has not indicated support for this extension (Section 4.2). Alternatives MUST be prepared to receive requests for any resource in the origin. However, the alternative MAY respond with a 421 (Misdirected Request) to any request it is unable to serve.¶

4.2. Indicating Support for Alt-Svc Parameters

Certain origins might prefer to take different actions based on whether the client supports HTTP Alternative Services or not. For example, many clients are unable to implement the persist parameter defined in [AltSvc]. Servers that offer alternatives based on the client's current network connection might choose not to send Alt-Svc entries to such a client.¶

The client can optionally send an Accept-Alt-Svc request header field indicating which Alt-Svc parameters it is able to understand. The content of this field is an sf-list [RFC8941] of Alt-Svc parameter names. To reduce fingerprinting surface, the contents of the list SHOULD be sorted alphabetically.¶

For example:¶

Accept-Alt-Svc: host, ma, persist, scope

A server MAY publish alternative services containing parameters which are not understood by the client, since unknown parameters are ignored per [AltSvc].¶

While [AltSvc] enables an alternative to reside on a different host than the origin server, not all clients implement this behavior. This draft registers the "host" parameter for Alt-Svc to enable clients to indicate support for Alt-Svc entries which provide a different hostname from the origin. The "host" parameter MUST NOT be used in Alt-Svc field generation and MUST be ignored if present.¶

The presence of this header can be assumed to indicate support for Alt-Svc, even if empty.¶

4.3. Incremental Alt-Svc Advertisements

[AltSvc] says that when an Alt-Svc response header field is received from an origin, its value invalidates and replaces all cached alternative services for that origin.¶

In certain circumstances, a server might prefer not to publish the full list of alternatives, but instead incrementally add to them. For example, a server might provide scope-restricted alternatives as a client makes requests for resources in various scopes.¶

This draft defines the Additional-Alt-Svc header field. The parsing and semantics of this field are identical to that of Alt-Svc, with the following modifications:¶

• The value MUST NOT be "clear"¶
• The entries presented augment, rather than replace, any cached alternatives already known to the client.¶

4.4. The 3NN (Use Alternative) Status Code

This document defines a new status code directing that a client attempt to satisfy the request from an alternative.¶

A server MUST include an Alt-Svc or Additional-Alt-Svc header field in the response indicating which alternative(s) the client can use to satisfy the given request. A server MUST NOT send the 3NN status code in response to a request which did not contain the Accept-Alt-Svc header field.¶

Upon receipt of this status code, a client SHOULD choose an alternative service and retry the request with that alternative. If all configured alternatives are unsuccessful, or the client chooses not to use an alternative, the client MAY retry the request with the origin server, omitting the Accept-Alt-Svc header field.¶