From 0299f3f790437d124d15f60489c774407325f82b Mon Sep 17 00:00:00 2001 From: Matt Caswell Date: Tue, 18 Jul 2017 14:54:23 +0100 Subject: [PATCH] Add some performance notes about early data In particular add information about the effect of Nagle's algorithm on early data. Fixes #3906 Reviewed-by: Ben Kaduk (Merged from https://github.com/openssl/openssl/pull/3955) --- doc/man3/SSL_read_early_data.pod | 24 ++++++++++++++++++++++++ 1 file changed, 24 insertions(+) diff --git a/doc/man3/SSL_read_early_data.pod b/doc/man3/SSL_read_early_data.pod index 38dffe5652..f0237faf40 100644 --- a/doc/man3/SSL_read_early_data.pod +++ b/doc/man3/SSL_read_early_data.pod @@ -168,6 +168,30 @@ In the event that the current maximum early data setting for the server is different to that originally specified in a session that a client is resuming with then the lower of the two values will apply. +=head1 NOTES + +The whole purpose of early data is to enable a client to start sending data to +the server before a full round trip of network traffic has occurred. Application +developers should ensure they consider optimisation of the underlying TCP socket +to obtain a performant solution. For example Nagle's algorithm is commonly used +by operating systems in an attempt to avoid lots of small TCP packets. In many +scenarios this is beneficial for performance, but it does not work well with the +early data solution as implemented in OpenSSL. In Nagle's algorithm the OS will +buffer outgoing TCP data if a TCP packet has already been sent which we have not +yet received an ACK for from the peer. The buffered data will only be +transmitted if enough data to fill an entire TCP packet is accumulated, or if +the ACK is received from the peer. The initial ClientHello will be sent as the +first TCP packet, causing the early application data from calls to +SSL_write_early_data() to be buffered by the OS and not sent until an ACK is +received for the ClientHello packet. This means the early data is not actually +sent until a complete round trip with the server has occurred which defeats the +objective of early data. + +In many operating systems the TCP_NODELAY socket option is available to disable +Nagle's algorithm. If an application opts to disable Nagle's algorithm +consideration should be given to turning it back on again after the handshake is +complete if appropriate. + =head1 RETURN VALUES SSL_write_early_data() returns 1 for success or 0 for failure. In the event of a -- 2.25.1