Nghttp2

nghttp2
Original author	Tatsuhiro Tsujikawa
Initial release	2013
Stable release	1.69.01  / 19 April 2026 (19 April 2026)
Written in	C
Platform	macOS, Windows, POSIX
Type	HTTP/2 implementation
License	MIT License
Website	nghttp2.org
Repository	github.com/nghttp2/nghttp2

nghttp2 is a C library. It is an implementation of HTTP/2.

nghttp2 was created by Tatsuhiro Tsujikawa as a derivative of spdylay, an implementation of SPDY, a communications protocol created by Google in 2009, in C.²³

Several well-known projects use nghttp2 to implement HTTP/2, including Apache and cURL.⁴⁵

nghttp2 will send a WINDOW_UPDATE frame upon consuming more than half of the flow control window. For instance, if the sender specifies the SETTINGS_INITIAL_WINDOW_SIZE [sic] as 65,535 octets in the SETTINGS frame, nghttp2 will send a WINDOW_UPDATE frame upon exceeding 32,768 octets. The initial window size may be changed using the -w and -W flags.⁶

nghttp2 offers multiple tools. nghttp is a command-line tool that uses nghttp2 to output HTTP/2 messages from a URL.⁷ nghttp's dependency-based priority is based on Firefox; when a connection is established, nghttp sends five PRIORITY frames.⁸ Other tools provided include nghttpd, an HTTP/2 server, nghttpx, an HTTP/2 proxy, h2load, an HTTP/2 load testing tool, and inflatehd and deflatehd, tools to decompress and compress using the HPACK header compression algorithm.⁹

nghttp3 is an implementation of HTTP/3 in C and authored by Tsujikawa. nghttp3 uses the QUIC network protocol designed by Jim Roskind at Google.¹⁰