=pod

=head1 NAME

SSL_get_handshake_rtt
- get round trip time for SSL Handshake

=head1 SYNOPSIS

 #include <openssl/ssl.h>

 int SSL_get_handshake_rtt(const SSL *s, uint64_t *rtt);

=head1 DESCRIPTION

SSL_get_handshake_rtt() retrieves the round-trip time (RTT) for I<ssl>.

This metric is represented in microseconds (us) as a uint64_t data type.

=head1 NOTES

This metric is created by taking two timestamps during the handshake and
providing the difference between these two times.

When acting as the server, one timestamp is taken when the server is finished
writing to the client. This is during the ServerFinished in TLS 1.3 and
ServerHelloDone in TLS 1.2. The other timestamp is taken when the server is
done reading the client's response. This is after the client has responded
with ClientFinished.

When acting as the client, one timestamp is taken when the client is finished
writing the ClientHello and early data (if any). The other is taken when
client is done reading the server's response. This is after ServerFinished in
TLS 1.3 and after ServerHelloDone in TLS 1.2.

In addition to network propagation delay and network stack overhead, this
metric includes processing time on both endpoints, as this is based on TLS
protocol-level messages and the TLS protocol is not designed to measure
network timings. In some cases the processing time can be significant,
especially when the processing includes asymmetric cryptographic operations.

=head1 RETURN VALUES

Returns 1 if the TLS handshake RTT is successfully retrieved.
Returns 0 if the TLS handshake RTT cannot be determined yet.
Returns -1 if, while retrieving the TLS handshake RTT, an error occurs.

=head1 COPYRIGHT

Copyright 2023 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file LICENSE in the source distribution or at
L<https://www.openssl.org/source/license.html>.

=cut