If SSL_write_early_data() returned SSL_ERROR_WANT_WRITE, stop further reading
using a newly introduced c->ssl->write_blocked flag, as otherwise this would
result in SSL error "ssl3_write_bytes:bad length".  Eventually, normal reading
will be restored by read event posted from successful SSL_write_early_data().

While here, place "SSL_write_early_data: want write" debug on the path.

The directive allows to drop binding between a client and existing UDP stream
session after receiving a specified number of packets.  First packet from the
same client address and port will start a new session.  Old session continues
to exist and will terminate at moment defined by configuration: either after
receiving the expected number of responses, or after timeout, as specified by
the "proxy_responses" and/or "proxy_timeout" directives.

By default, proxy_requests is zero (disabled).

The session handler may result in session termination, thus a connection
pool (from which c->udp was allocated) may be destroyed.

With maximum version explicitly set, TLSv1.3 will not be unexpectedly
enabled if nginx compiled with OpenSSL 1.1.0 (without TLSv1.3 support)
will be run with OpenSSL 1.1.1 (with TLSv1.3 support).

The directives enable the use of the SO_KEEPALIVE option on
upstream connections.  By default, the value is left unchanged.

The "no suitable signature algorithm" errors are reported by OpenSSL 1.1.1
when using TLSv1.3 if there are no shared signature algorithms.  In
particular, this can happen if the client limits available signature
algorithms to something we don't have a certificate for, or to an empty
list.  For example, the following command:

    openssl s_client -connect 127.0.0.1:8443 -sigalgs rsa_pkcs1_sha1

will always result in the "no suitable signature algorithm" error
as the "rsa_pkcs1_sha1" algorithm refers solely to signatures which
appear in certificates and not defined for use in TLS 1.3 handshake
messages.

The SSL_R_NO_COMMON_SIGNATURE_ALGORITHMS error is what BoringSSL returns
in the same situation.

The "no suitable key share" errors are reported by OpenSSL 1.1.1 when
using TLSv1.3 if there are no shared groups (that is, elliptic curves).
In particular, it is easy enough to trigger by using only a single
curve in ssl_ecdh_curve:

    ssl_ecdh_curve secp384r1;

and using a different curve in the client:

    openssl s_client -connect 127.0.0.1:443 -curves prime256v1

On the client side it is seen as "sslv3 alert handshake failure",
"SSL alert number 40":

0:error:14094410:SSL routines:ssl3_read_bytes:sslv3 alert handshake failure:ssl/record/rec_layer_s3.c:1528:SSL alert number 40

It can be also triggered with default ssl_ecdh_curve by using a curve
which is not in the default list (X25519, prime256v1, X448, secp521r1,
secp384r1):

    openssl s_client -connect 127.0.0.1:8443 -curves brainpoolP512r1

Given that many clients hardcode prime256v1, these errors might become
a common problem with TLSv1.3 if ssl_ecdh_curve is redefined.  Previously
this resulted in not using ECDH with such clients, but with TLSv1.3 it
is no longer possible and will result in a handshake failure.

The SSL_R_NO_SHARED_GROUP error is what BoringSSL returns in the same
situation.

Seen at:

https://serverfault.com/questions/932102/nginx-ssl-handshake-error-no-suitable-key-share