HTTP Alternative Services, Plan B

Internet-Draft	Alt-SvcB	October 2022
Thomson, et al.	Expires 27 April 2023	[Page]

1. Introduction

HTTP alternative services provide an HTTP server with a means to direct requests from clients to an alternative server instance. Clients that learn about an alternative service can establish a connection to the identified instance, which - if successfully established and authenticated - can be used for future requests. This design allows for nearly seamless service continuity in some conditions.¶

The use cases that might motivate a server to direct future requests to a different server instance vary.¶

Some deployments might seek to direct a client to a server instance that is better able to serve requests for that client. This might occur if DNS resolution of the server name produces the address of a server instance that is further from the client.¶
Servers that might seek to shed load, perhaps in anticipation of an imminent shutdown or maintenance action, might use an alternative service declaration to reduce either server load or the number of clients that might be affected.¶
Many deployments of HTTP/3 [HTTP/3] use the protocol identifiers in an alternative service declaration to make clients aware of support for the newer protocol.¶

These different use cases can sometimes create awkward trade-offs for deployments of alternative services.¶

1.1. Caching in Alternative Services

With RFC 7838 [ALT-SVC], setting the "ma" (or max-age) parameter for an alternative can be challenging for a server deployment. Setting too a large max-age can mean that clients use an alternative for longer than they should. Conversely, a short cache period for an advertisement for HTTP/3 can mean that earlier versions might be used more often than is optimal.¶

Alternative services can interact poorly with service configuration information that is published in the DNS. With the introduction of HTTPS records [SVCB], the availability of HTTP/3 can be advertised in the DNS, creating two independent sources of this information, with different approaches to caching.¶

Alternative services can also be highly dependent on networking conditions. RFC 7838 attempted to manage this by having clients be responsible for invalidating alternatives when changes in their network are detected, unless the alternative is explicitly marked as "persistent". In practice, detecting the necessary changes is difficult for many clients, so this requirement is not consistently implemented.¶

The alternative services mechanisms defined in RFC 7838 can produce suboptimal or even detrimental outcomes in some deployments. Consequently, this document obsoletes RFC 7838.¶

1.2. A New Alternative

This document describes a different approach to advertising alternative services. This approach uses the DNS as the singular source of information about service reachability. An alternative service advertisement only acts as a prompt for clients to seek updated information from the DNS.¶

To use this new design, a server advertises an alternative name using the "Alt-SvcB" field.¶

200 OK HTTP/1.1
date: Mon, 24 Oct 2022 02:58:31 GMT
alt-svcb: "instance31.example.com"
content-length: 0

Clients can then consult the DNS, making HTTPS queries [SVCB] starting with this name. The alternative name is used in place of the name of the authority and using HTTPS records is mandatory, but the process otherwise follows normal HTTPS record resolution and connection procedures. Section 2 defines how this name is used in detail.¶

Future connections for requests to resources on the same server use HTTPS record resolution to the name of the authority, but are reprioritized if a successful connection was previously made to an alternative service. Section 2.2 defines how this process works in more detail.¶

1.3. 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. Using Alternative Services

When a client learns about a new potential alternative from a server, they SHOULD attempt to use that alternative for future requests to that server. The client attempts to make a request for a resource on the same server using the alternative as follows:¶

The client makes a DNS query for HTTPS records for the alternative name, following the procedures in Section 3 of [SVCB]. Clients make this query as a "SVCB-reliant" SVCB client, treating a failure to obtain HTTPS records as a failed alternative. If this process fails to produce service parameters or IP addresses, abort this process. [[ISSUE: Maybe we can still be SVCB-optional here, but that places constraints on the choice of name.]]¶
The client establishes a connection using the service parameters and addresses learned from the DNS query. Important here is that the client uses a TLS server name indication [SNI] of the server name from the URL, not the alternative name. This allows the server to produce a certificate for that name, which the client can validate as applying to the URL it is resolving. If a connection cannot be established, abort this process.¶
The client validates that the server is authoritative for the resource. If the server is not authoritative, abort this process.¶
The client makes a query for the resource. If the server does not respond, responds with a 421 (Misdirected Request) (see Section 15.5.20 of [HTTP]), or a 5xx status code (see Section 15.6 of [HTTP]), abort this process.¶
Once a response is received, the connection to the alternative is complete. Any other connections can be closed and future requests directed to the alternative. The client SHOULD remember the alternative name and the service name (the TargetName from the HTTPS ServiceMode record that was used) that were used; see Section 2.1.¶

A client MUST NOT remember a service name for an alternative service until a request has been successfully completed with a 2xx or 3xx status code. A client MAY send additional requests using the newly established connection to the alternative service after it verifies that the server is authoritative. The alternative service is therefore active once the connection is established, but it will not be reused (Section 2.2) for future connections until a request completes successfully.¶

A client MAY continue sending other requests over any existing connection to the server until this process completes in order to minimize latency for those requests. A client MAY - when presented with an alternative name - proactively make a request for an arbitrary resource on the server, rather than waiting for the next time a request is needed. This might allow the connection to be available for future requests with less delay.¶

2.1. Retention of Alternatives

Clients SHOULD remember the successful use of an alternative service. Two pieces of information are retained:¶

the alternative name, which is the name provided by the server in the Alt-SvcB field or ALTSVCB frame, and¶
the service name, which is the TargetName from the ServiceMode HTTPS record that was used to successfully connect to the server.¶

These two names are saved for the server against the origin of the server [ORIGIN]. Clients MUST NOT reuse saved information for a server with a different hostname, port, or scheme.¶

The name given in an Alt-SvcB field or ALTSVCB frame is retained only so that the client can avoid initiating a connection to an alternative if it has already made an attempt. Any time that a server provides a different name in an Alt-SvcB field or ALTSVCB frame, any existing information MUST be discarded. A client MAY then initiate a DNS query and connection attempt to the identified alternative. A client can subsequently ignore repeated fields or frames.¶

A server might provide an Alt-SvcB field in all responses it sends. Only acting on a value once ensures that this repetition has no effect on clients. Though a server might repeat the field, clients MUST NOT consider an absent field as indicative of a retraction of a previous advertisement. An alternative name is only removed when explicitly replaced or when a remembered service name does not appear in the set of HTTPS query responses.¶

On the first attempt to use an alternative, a failed alternative SHOULD be remembered using an alternative name and a null or absent service name. This avoids making repeated attempts to use an alternative service that is not available if the server repeats the information in Alt-SvcB fields or ALTSVCB frames. A client MAY periodically attempt to retry a failed alternative if the information is repeated.¶

A server can explicitly request that a client remove any remembered service name by providing an alternative name of "invalid". The "invalid" domain name corresponds to a DNS name that will never successfully resolve (see Section 6.4 of [SUDN]), which guarantees that an attempt to use this name cannot succeed. Clients MAY recognize the name "invalid" as special and avoid any attempt to use this to discover an alternative service.¶

2.2. Reusing Alternatives

A client remembers the service name, or the TargetName from the ServiceMode HTTPS record that it successfully used to establish a connection to an alternative service. In subsequent connections to the same server, it makes HTTP queries for the server name. If this query returns a ServiceMode resource record (RR) that includes a TargetName that is identical to the remembered service name, the client SHOULD choose that over any alternatives, including those RRs that are marked "alt-only"; see Section 2.2.2.¶

A client does not make a query for the remembered alternative name. They make a query for the name of the server, using the QNAME derived from the URI of the target resource.¶

The RR that matches the remembered service name is selected, overriding any SvcPriority that might otherwise result in another ServiceMode record being chosen.¶

If a query for HTTPS records does not produce a ServiceMode record with a matching TargetName, any remembered information MUST be removed for that origin.¶

When reusing stored information, if a connection is unsuccessful for any reason (see Section 2), remembered information for that origin MUST be removed. Clients clear retained alternative service information on reuse to prevent stale information from affecting all future connection attempts. After removing remembered information, a client MAY make another attempt to connect using any other ServiceMode records that the DNS query produced.¶

2.2.1. Example of Reuse

A client that is fetching "https://example.com/" might originally perform a DNS query for "example.com" and receive in response:¶

example.com.  7200 IN HTTPS 1 . port=443
alt1.example. 7200 IN HTTPS 10 . port=8443
alt2.example. 7200 IN HTTPS 10 . port=8443
alt3.example. 7200 IN HTTPS 10 . port=8443

Under normal conditions, the SvcPriority of the "alt?.example." RRs would indicate that it is not preferred, so the "example.com" record would be used.¶

If the client received an alternative service advertisement from this server for "alt.example.net" it would then make a DNS query to that name. This might return a different set of records, as follows:¶

alt2.example. 7200 IN HTTPS 1 . port=8887
alt3.example. 7200 IN HTTPS 1 . port=8887

If the client selects "alt2.example." and successfully connects to that host, it remembers both the name given as an alternative name ("alt.example.net") and a service name (the TargetName from the ServiceMode HTTPS record, "alt2.example.").¶

In subsequent connections to "example.com", the client again queries the "example.com" name. Importantly, this is not any other name it might have learned. The resulting response - after following indirections through AliasMode, CNAME, or similar mechanisms - produces the same records as previously:¶

example.com.  7200 IN HTTPS 1 . port=443
alt1.example. 7200 IN HTTPS 10 . port=8443
alt2.example. 7200 IN HTTPS 10 . port=8443
alt3.example. 7200 IN HTTPS 10 . port=8443

The ServiceMode HTTPS record for "alt2.example." is used, even though this is a lower priority than other records. It is also used despite not using the same port number as previously.¶

2.2.2. Exclusive Alternative Services

ServiceMode HTTPS records can be marked as only being available for use as an alternative. This allows servers to use alternative services for specific server instances, without having clients connect to them without being first invited to do so.¶

This is achieved with a SvcParam with a key of "alt-only" (codepoint TBD). The value of this SvcParamKey MUST be empty. HTTPS ServiceMode records with this SvcParamKey MUST NOT be used unless the client is actively seeking an alternative, either as a result of actively looking up an alternative name or because the alternative has been remembered.¶

To prevent clients that do not support this specification from using these services, the "alt-only" SvcParamKey MUST be listed in the "mandatory" SvcParam.¶

In the following example, though "alt1.example" is listed at a higher priority than "example.com", clients will not use this service unless an alternative was provided by the server:¶

alt1.example. 7200 IN HTTPS 1 . port=443,alt-only,mandatory=alt-only
example.com.  7200 IN HTTPS 2 . port=443

[[ISSUE: Do we need this flag in addition to the priority override? Could one of the two be sufficient?]]¶

2.3. Port Numbers

The name that is provided in the Alt-SvcB field or ALTSVCB frame can be any valid DNS QNAME. This includes those with underscored labels [ATTRLEAF], including those that might be used to query for HTTPS records to a non-default port.¶

200 OK HTTP/1.1
date: Mon, 24 Oct 2022 02:58:31 GMT
alt-svcb: "_8443._https.example.com"
content-length: 0

This might be used to direct clients to connect to alternative ports. Note that the HTTPS records might direct clients to an entirely different port number than the name implies. Clients MUST NOT infer a port number from the provided name, instead treating this name no differently than any other.¶

2.4. Interaction with GOAWAY

Servers that advertise alternative services cannot expect clients to switch to the advertised alternative. Use of the alternative is entirely at the discretion of clients. If the client is unsuccessful in connecting to an alternative or does not attempt a connection, they could continue to use the existing connection for new requests.¶

A server that seeks to actively encourage clients to disconnect and seek service elsewhere needs to use graceful shutdown procedures of the HTTP version that is in use. HTTP/2 [HTTP/2] and HTTP/3 [HTTP/3] each provide a GOAWAY frame that can be used to initiate the graceful shutdown of a connection. Alternative services is not a substitute for these mechanisms.¶

2.5. Proxies

The procedures in this document apply to clients that connect to gateways or reverse proxies. However, clients that connect via a proxy, using HTTP CONNECT or similar methods, have a choice.¶

Clients that provide a proxy with the name of a service leave name resolution to the proxy. Such a client MUST ignore any alternative service advertisement it receives and MAY fallback to using legacy alternative services; see Section 2.6.¶

Clients that make HTTPS queries for any connection attempt via a proxy can use alternative services. Such a client MUST provide the proxy with the IP address of the server it wishes to contact, rather than providing a name.¶

2.6. Fallback to Alt-Svc

A client that successfully makes use of HTTPS records in resolving the name of an HTTP server MUST ignore any Alt-Svc fields or ALTSVC frames [ALT-SVC] that the server provides. This document deprecates the mechanisms defined in RFC 7838 [ALT-SVC].¶

Servers might provide Alt-Svc fields or ALTSVC frames [ALT-SVC] in order to support clients that cannot use HTTPS records.¶

2.7. No Authority

This design does not assume that information that a client learns about alternatives is authoritative in any way, either by virtue of being provided by an authoritative server. Instead, once a server is determined to be authorative (see Section 4.3 of [HTTP]), that server is treated as the authority on all aspects of service configuration. For protocol choice, [ALPN] and maybe [SNIP] extensions in the TLS handshake [TLS] determine what is used.¶

In contrast RFC 7838, sending alternative services over an HTTP connection ensures that the information is authoritative. Clients therefore might have been less concerned about attacks that compromise the integrity of alternative services when using RFC 7838.¶

Though integrity protection might appear to be valuable, it produced conflicts. For instance, information about the protocol is ostensibly authentic when provided in Alt-Svc fields or ALTSVC frames. However, protocol support is also authenticated when establishing a connection. This creates a potential conflict between two equally authoritative sources of the same information.¶

Conflicts also arise when alternative service information is retained as any retained state might disagree with what is currently deployed. This design avoids this contention by having the entire service resolution process occur almost entirely to the DNS.¶

An alternative service advertisement provides only a minimal nudge to perform a DNS query at the time it is made. On reconnection, remembered state only affects prioritization of active DNS records. Invalid configurations do not persist.¶

3. Protocol Elements

Multiple ways of encoding an alternative service name is defined. The Alt-SvcB field in Section 3.1 allows servers to indicate a preferred service in responses. The ALTSVCB frames in Section 3.2 allows a server to provide alternative names outside of the context of a query.¶

These approaches have different properties. Alt-SvcB fields are forwarded by intermediaries and so might reach clients through a gateway or reverse proxy. Clients that use a proxy without using CONNECT or similar tunnels, might also receive an alternative name using a field. In comparison, ALTSVCB frames each only apply to a single origin within the scope of a single connection.¶

3.1. Alt-SvcB Field

The "Alt-SvcB" response field is a List of String values (see Sections 3.1 and 3.3.3 of [STRUCTURED-FIELDS]). This response field MAY appear in a header or trailer section, though servers need to be aware that some clients might not process field values.¶

Each field value includes an alternative name. Each alternative name is encoded as an ASCII string, or a series of DNS A-labels, each separated by a single period character (".", U+2E). Each value MAY end with a period, though - for the purposes of the process in Section 2 - the string is treated as an absolute DNS QNAME whether or not a trailing period is present.¶

The applicable origin [ORIGIN] is derived from the origin of the Target Resource (see Section 7.1 of [HTTP]).¶

If multiple Alt-SvcB fields or field values are present in a response, the client MAY use any subset of the provided alternative names, including none, one, or all of the provided names.¶

Servers SHOULD NOT provide more than one name. The DNS provides ample opportunity to present clients with options, including the use of priority to help manage selection. A list is tolerated only to allow for the possibility that multiple field lines might be added to responses without proper coordination.¶

Clients MUST ignore unknown parameters that are provided with alternative names. This document does not define any parameters as the DNS is expected to provide supplementary information about services; a revision of this document would be required to enable the use of parameters.¶

3.2. ALTSVCB Frame

An ALTSVCB frame is defined for both HTTP/2 and HTTP/3. The frame provides an alternative name for an identified origin [ORIGIN].¶

In both protocols, the ALTSVCB frame uses the identifier TBD. The format for both protocols is the same; this is shown in Figure 1 using the notation from Section 3 of [QUIC].¶

ALTSVCB Frame {
  Origin Length (i),
  Origin (..),
  Alternative Name (..),
}

Figure 1: ALTSVCB Frame Format

The fields in the ALTSVCB frame are defined as follows:¶

Origin Length:: An integer, encoded as a QUIC variable-length integer (see Section 16 of [QUIC]) indicating the length of the Origin field, in bytes.¶
Origin:: The ASCII serialization of the affected origin; see Section 6.2 of [ORIGIN].¶
Alternative Name:: The remainder of the frame contains a single alternative name, encoded as an ASCII string; see the definition in Section 3.1 for more details on the encoding.¶

If a server sends multiple ALTSVCB frames for the same origin, clients MUST ignore any frames other than the most recent.¶

7. References

7.1. Normative References

[ALPN]: Friedl, S., Popov, A., Langley, A., and E. Stephan, "Transport Layer Security (TLS) Application-Layer Protocol Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301, July 2014, <https://www.rfc-editor.org/rfc/rfc7301>.
[ALT-SVC]: Nottingham, M., McManus, P., and J. Reschke, "HTTP Alternative Services", RFC 7838, DOI 10.17487/RFC7838, April 2016, <https://www.rfc-editor.org/rfc/rfc7838>.
[HTTP]: Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics", STD 97, RFC 9110, DOI 10.17487/RFC9110, June 2022, <https://www.rfc-editor.org/rfc/rfc9110>.
[ORIGIN]: Barth, A., "The Web Origin Concept", RFC 6454, DOI 10.17487/RFC6454, December 2011, <https://www.rfc-editor.org/rfc/rfc6454>.
[QUIC]: Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based Multiplexed and Secure Transport", RFC 9000, DOI 10.17487/RFC9000, May 2021, <https://www.rfc-editor.org/rfc/rfc9000>.
[RFC2119]: Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/rfc/rfc2119>.
[RFC5890]: Klensin, J., "Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework", RFC 5890, DOI 10.17487/RFC5890, August 2010, <https://www.rfc-editor.org/rfc/rfc5890>.
[RFC8174]: Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[SNI]: Eastlake 3rd, D., "Transport Layer Security (TLS) Extensions: Extension Definitions", RFC 6066, DOI 10.17487/RFC6066, January 2011, <https://www.rfc-editor.org/rfc/rfc6066>.
[STRUCTURED-FIELDS]: Nottingham, M. and P-H. Kamp, "Structured Field Values for HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021, <https://www.rfc-editor.org/rfc/rfc8941>.
[SVCB]: Schwartz, B. M., Bishop, M., and E. Nygren, "Service binding and parameter specification via the DNS (DNS SVCB and HTTPS RRs)", Work in Progress, Internet-Draft, draft-ietf-dnsop-svcb-https-11, 11 October 2022, <https://datatracker.ietf.org/doc/html/draft-ietf-dnsop-svcb-https-11>.
[TLS]: Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, <https://www.rfc-editor.org/rfc/rfc8446>.

7.2. Informative References

[ATTRLEAF]: Crocker, D., "Scoped Interpretation of DNS Resource Records through "Underscored" Naming of Attribute Leaves", BCP 222, RFC 8552, DOI 10.17487/RFC8552, March 2019, <https://www.rfc-editor.org/rfc/rfc8552>.
[HTTP/2]: Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113, DOI 10.17487/RFC9113, June 2022, <https://www.rfc-editor.org/rfc/rfc9113>.
[HTTP/3]: Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114, June 2022, <https://www.rfc-editor.org/rfc/rfc9114>.
[SNIP]: Thomson, M., "Secure Negotiation of Incompatible Protocols in TLS", Work in Progress, Internet-Draft, draft-ietf-tls-snip-02, 30 June 2022, <https://datatracker.ietf.org/doc/html/draft-ietf-tls-snip-02>.
[SUDN]: Cheshire, S. and M. Krochmal, "Special-Use Domain Names", RFC 6761, DOI 10.17487/RFC6761, February 2013, <https://www.rfc-editor.org/rfc/rfc6761>.

HTTP Alternative Services, Plan B

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