Reserving the clear ALPN Protocol ID

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Table of Contents

1. Introduction

HTTP Alternative Services (Alt-Svc) [ALTSVC] are identified by a tuple of Application-Protocol Layer Negotiation (ALPN) [ALPN] protocol identifier, a host and a port. The wire format for Alt-Svc is defined in ABNF and encodes this tuple or the keyword "clear", which has a special meaning. This memo reserves the ALPN protocol identifier "clear" to reduce the chances of accidental aliasing with the "clear" keyword.¶

2. Aliasing and Avoidance

[ALTSVC] Section 3 defines the Alt-Svc header field using ABNF. It requires a custom parser, which introduces a possibility for custom implementation errors. The Alt-Svc header field value can either be the keyword "clear" - a special value that invalidates cached alternative services, or a list of alt-value, that includes an encoded ALPN protocol identifier [ALPN]. There is a chance that someone unwittingly defines the ALPN protocol identifier "clear" for genuine purposes but is unaware of the use of protocol identifiers in Alternative Services. This could trigger Alt-Svc parser errors that might lead to confusion between the keyword with the protocol identifier use. Since the "clear" keyword has special meaning, confusion might lead to detrimental effects.¶

To prevent unintended aliasing, this document registers the "clear" ALPN protocol identifier. It relates to no actual application-layer protocol, effectively reserving the code point and preventing any unintended aliasing.¶

3. Security Considerations

Broken Alt-Svc header field parsers might confuse a "clear" keyword with a "clear" ALPN protocol identifier. This could invalidate Alternative Service cache state but a conformant client should fall back safely as described in Section 2.4 of [ALTSVC].¶

4. IANA Considerations

Acknowledgments

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