We use a static record size of 4K. This gives a good balance of latency and throughput.
Example
http {
ssl_dyn_rec_enable on;
}By initialy sending small (1 TCP segment) sized records, we are able to avoid HoL blocking of the first byte. This means TTFB is sometime lower by a whole RTT.
By sending increasingly larger records later in the connection, when HoL is not a problem, we reduce the overhead of TLS record (29 bytes per record with GCM/CHACHA-POLY).
Start each connection with small records
(1369 byte default, change with ssl_dyn_rec_size_lo).
After a given number of records (40, change with ssl_dyn_rec_threshold)
start sending larger records (4229, ssl_dyn_rec_size_hi).
Eventually after the same number of records,
start sending the largest records (ssl_buffer_size).
In case the connection idles for a given amount of time
(1s, ssl_dyn_rec_timeout), the process repeats itself
(i.e. begin sending small records again).
- syntax:
dyn_rec_enable bool - default:
off - context:
http
- syntax:
dyn_rec_timeout number - default:
1000 - context:
http
We want the initial records to fit into one TCP segment so we don't get TCP HoL blocking due to TCP Slow Start.
A connection always starts with small records, but after a given amount of records sent, we make the records larger to reduce header overhead.
After a connection has idled for a given timeout, begin the process from the start. The actual parameters are configurable. If dyn_rec_timeout is 0, we assume dyn_rec is off.
- syntax:
dyn_rec_size_lo number - default:
1369 - context:
http
Default sizes for the dynamic record sizes are defined to fit maximal TLS + IPv6 overhead in a single TCP segment for lo and 3 segments for hi: 1369 = 1500 - 40 (IP) - 20 (TCP) - 10 (Time) - 61 (Max TLS overhead)
- syntax:
dyn_rec_size_hi number - default:
4229 - context:
http
4229 = (1500 - 40 - 20 - 10) * 3 - 61
- syntax:
dyn_rec_threshold number - default:
40 - context:
http