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NPN: remove support for and use of

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Next Protocol Negotiation is a TLS extension that was created and used
for agreeing to use the SPDY protocol (the precursor to HTTP/2) for
HTTPS. In the early days of HTTP/2, before the spec was finalized and
shipped, the protocol could be enabled using this extension with some
servers.

curl supports the NPN extension with some TLS backends since then, with
a command line option `--npn` and in libcurl with
`CURLOPT_SSL_ENABLE_NPN`.

HTTP/2 proper is made to use the ALPN (Application-Layer Protocol
Negotiation) extension and the NPN extension has no purposes
anymore. The HTTP/2 spec was published in May 2015.

Today, use of NPN in the wild should be extremely rare and most likely
totally extinct. Chrome removed NPN support in Chrome 51, shipped in
June 2016. Removed in Firefox 53, April 2017.

Closes #9307
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Daniel Stenberg 2022-09-01 09:23:22 +02:00
parent e08c82f046
commit 472f1cbe7e
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29
docs/HTTP2.md
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@ -35,35 +35,16 @@ Over an https:// URL
--------------------
If `CURLOPT_HTTP_VERSION` is set to `CURL_HTTP_VERSION_2_0`, libcurl will use
ALPN (or NPN) to negotiate which protocol to continue with. Possibly introduce
an option that will cause libcurl to fail if not possible to use HTTP/2.
ALPN to negotiate which protocol to continue with. Possibly introduce an
option that will cause libcurl to fail if not possible to use HTTP/2.
`CURL_HTTP_VERSION_2TLS` was added in 7.47.0 as a way to ask libcurl to prefer
HTTP/2 for HTTPS but stick to 1.1 by default for plain old HTTP connections.
ALPN is the TLS extension that HTTP/2 is expected to use. The NPN extension is
for a similar purpose, was made prior to ALPN and is used for SPDY so early
HTTP/2 servers are implemented using NPN before ALPN support is widespread.
ALPN is the TLS extension that HTTP/2 is expected to use.
`CURLOPT_SSL_ENABLE_ALPN` and `CURLOPT_SSL_ENABLE_NPN` are offered to allow
applications to explicitly disable ALPN or NPN.
SSL libs
--------
The challenge is the ALPN and NPN support and all our different SSL
backends. You may need a fairly updated SSL library version for it to provide
the necessary TLS features. Right now we support:
- OpenSSL: ALPN and NPN
- libressl: ALPN and NPN
- BoringSSL: ALPN and NPN
- NSS: ALPN and NPN
- GnuTLS: ALPN
- mbedTLS: ALPN
- Schannel: ALPN
- wolfSSL: ALPN
- Secure Transport: ALPN
`CURLOPT_SSL_ENABLE_ALPN` is offered to allow applications to explicitly
disable ALPN.
Multiplexing
------------