h3x

Experimental HTTP/3 load generator using libh2o. As usual, unbuckled seatbelt. Use at your own risk.

Build

git submodule update --init
git -C deps/h2o apply "$(pwd)/patches/h2o-udp-gro-send-batch.patch"  # local h2o patches (patches/README.md)
cmake -S . -B build && cmake --build build --target h3x

Run the servers

cmake --build build --target h2o                       # once, builds the h2o server binary
build/deps/h2o/h2o -c bench/h2o.conf &                 # h2o (native)      https://127.0.0.1:14433/
docker start nginx-h3 haproxy-h3 caddy-h3 envoy-h3     # 14434 / 14435 / 14436 / 14437

Server configs live in bench/ (bind-mounted into the containers); every server runs 12 workers and serves the 1 KB bench/doc_root/index.html.

Run the load generator

./build/h3x -k --connections 512 -m 8 -d 10 https://127.0.0.1:14433/

Threads default to one worker per CPU the process may use; connections are spread across the threads; requests in flight = connections × -c.

Flag	Meaning (default)
-n <count>	total requests across all threads (100)
-d <seconds>	run for a duration instead; overrides -n
-t <num>	worker threads (all allowed CPUs)
--connections <n>	total connections, spread across threads (one per thread)
-m <num>	concurrent streams per connection (10; h2load calls this -m too)
--send-batch <n>	hold N freed slots before refilling, so requests pack into fewer datagrams (1 = off)
--socket-per-conn	one UDP socket per connection (unique 4-tuple, the h2load model); avoids servers that kill connections sharing a source port
--requests <file>	load request templates (method/path/headers/body) from a .http file; multiple requests become a round-robin mix (overrides -m/-H). See examples/, and --dump-requests to check parsing
--method <method>	request method (GET)
-H <name:value>	add a request header (repeatable)
-x <url>	connect to this host:port instead of the URL's (pin a backend / skip DNS)
-k	skip certificate verification
--key-exchange <name>	override the TLS key exchange (e.g. x25519)
--reconnect <N>	close each connection after N requests, so churn exercises 0-RTT resumption
--no-resumption	full handshake on every connection (disables the ticket cache)
-W <bytes>	HTTP/3 receive window, per stream
--max-udp-payload-size <bytes>	max_udp_payload_size transport parameter
--initial-udp-payload-size <bytes>	initial egress UDP payload size
--ack-frequency <0..1>	ACK frequency ratio
--disallow-delayed-ack	disable delayed ACKs
--no-ecn	disable ECN
--qpack-table <bytes>	QPACK encoder dynamic table capacity (4096)

Certificate verification uses the system CA bundle; override with H3X_CA_BUNDLE.