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Application-Layer Protocol Negotiation
Transport Layer Security extension for application layer protocol negotiation

Application-Layer Protocol Negotiation (ALPN) is a Transport Layer Security (TLS) extension that allows the application layer to negotiate which protocol should be performed over a secure connection in a manner that avoids additional round trips and which is independent of the application-layer protocols. It is used to establish HTTP/2 connections without additional round trips (client and server can communicate over two ports previously assigned to HTTPS with HTTP/1.1 and upgrade to use HTTP/2 or continue with HTTP/1.1 without closing the initial connection).

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Support

ALPN is supported by these libraries:

  • BSAFE Micro Edition Suite since version 5.01
  • GnuTLS since version 3.2.0 released in May 20132
  • MatrixSSL since version 3.7.1 released in December 20143
  • Network Security Services since version 3.15.5 released in April 20144
  • OpenSSL since version 1.0.2 released in January 20155
  • LibreSSL since version 2.1.3 released in January 20156
  • mbed TLS (previously PolarSSL) since version 1.3.6 released in April 20147
  • s2n since its original public release in June 2015.
  • wolfSSL (formerly CyaSSL) since version 3.7.0 released in October 20158
  • Go (in the standard library crypto/tls package) since version 1.4 released in December 20149
  • JSSE in Java since JDK 9 released in September 2017,10 backported to JDK 8 released in April 202011
  • Win32 SSPI since Windows 8.1 and Windows Server 2012 R2 were released October 18, 201312

History

Next Protocol Negotiation

In January 2010, Google introduced IETF standard draft describing Next Protocol Negotiation TLS extension.13 This extension was used to negotiate experimental SPDY connections between Google Chrome and some of Google's servers. As SPDY evolved, NPN was replaced with ALPN.

Application-Layer Protocol Negotiation

On July 11, 2014, ALPN was published as RFC 7301. ALPN replaces Next Protocol Negotiation (NPN) extension.14

TLS False Start was disabled in Google Chrome from version 20 (2012) onward except for websites with the earlier NPN extension.15

Example

ALPN is a TLS extension which is sent on the initial TLS handshake 'Client Hello', and it lists the protocols that the client (for example the web browser) supports:

Handshake Type: Client Hello (1) Length: 141 Version: TLS 1.2 (0x0303) Random: dd67b5943e5efd0740519f38071008b59efbd68ab3114587... Session ID Length: 0 Cipher Suites Length: 10 Cipher Suites (5 suites) Compression Methods Length: 1 Compression Methods (1 method) Extensions Length: 90 [other extensions omitted] Extension: application_layer_protocol_negotiation (len=14) Type: application_layer_protocol_negotiation (16) Length: 14 ALPN Extension Length: 12 ALPN Protocol ALPN string length: 2 ALPN Next Protocol: h2 ALPN string length: 8 ALPN Next Protocol: http/1.1

The resulting 'Server Hello' from the web server will also contain the ALPN extension, and it confirms which protocol will be used for the HTTP request:

Handshake Type: Server Hello (2) Length: 94 Version: TLS 1.2 (0x0303) Random: 44e447964d7e8a7d3b404c4748423f02345241dcc9c7e332... Session ID Length: 32 Session ID: 7667476d1d698d0a90caa1d9a449be814b89a0b52f470e2d... Cipher Suite: TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 (0xc02f) Compression Method: null (0) Extensions Length: 22 [other extensions omitted] Extension: application_layer_protocol_negotiation (len=5) Type: application_layer_protocol_negotiation (16) Length: 5 ALPN Extension Length: 3 ALPN Protocol ALPN string length: 2 ALPN Next Protocol: h2

References

  1. "Dell BSAFE Micro Edition Suite 5.0 Release Advisory". Retrieved 2022-10-18. https://www.dell.com/support/kbdoc/000204231/dell-bsafe-micro-edition-suite-5-0-release-advisory ↩

  2. "gnutls 3.2.0". Archived from the original on 2016-01-31. Retrieved 2015-01-26. https://web.archive.org/web/20160131230710/http://article.gmane.org/gmane.network.gnutls.general/3136 ↩

  3. "MatrixSSL - News". 2014-12-04. Archived from the original on 2015-02-14. Retrieved 2015-01-26. https://web.archive.org/web/20150214105056/http://www.matrixssl.org/news.html ↩

  4. "NSS 3.15.5 release notes". Mozilla Developer Network. Mozilla. Retrieved 2015-01-26. https://developer.mozilla.org/en-US/docs/Mozilla/Projects/NSS/NSS_3.15.5_release_notes ↩

  5. "OpenSSL 1.0.2 release notes". The OpenSSL Project. 2015-01-22. Archived from the original on 2014-09-04. Retrieved 2015-01-26. https://web.archive.org/web/20140904045720/http://www.openssl.org/news/openssl-1.0.2-notes.html ↩

  6. "LibreSSL 2.1.3 released". 2015-01-22. Retrieved 2015-01-26. https://marc.info/?l=openbsd-announce&m=142193407304782 ↩

  7. "Download overview - PolarSSL". 2014-04-11. Archived from the original on 2015-02-09. Retrieved 2015-01-26. https://web.archive.org/web/20150209195111/https://polarssl.org/tech-updates/releases/polarssl-1.3.6-released ↩

  8. "wolfSSL Release Change Log". 2015-10-26. Retrieved 2015-09-11. https://www.wolfssl.com/wolfSSL/Docs-wolfssl-changelog.html ↩

  9. "Go 1.4 Release Notes". 2014-12-10. Retrieved 2017-11-28. https://golang.org/doc/go1.4#minor_library_changes ↩

  10. "JEP 244: TLS Application-Layer Protocol Negotiation Extension". 2017-08-07. Retrieved 2018-08-29. https://bugs.openjdk.java.net/browse/JDK-8051498 ↩

  11. "Release Note: TLS Application-Layer Protocol Negotiation Extension". 2020-04-30. Retrieved 2020-06-11. https://bugs.openjdk.java.net/browse/JDK-8242894 ↩

  12. "What's New in TLS/SSL (Schannel SSP)". 31 August 2016. Retrieved 2020-03-30. https://docs.microsoft.com/en-us/previous-versions/windows/it-pro/windows-server-2012-R2-and-2012/hh831771(v=ws.11)?redirectedfrom=MSDN#whats-new-in-tlsssl-schannel-ssp-in-windows-server-2012-r2-and-windows-81 ↩

  13. Langley, A. (January 20, 2010). "Transport Layer Security (TLS) Next Protocol Negotiation Extension". IETF Datatracker. https://datatracker.ietf.org/doc/html/draft-agl-tls-nextprotoneg-00 ↩

  14. Langley, Adam. "» NPN and ALPN". Retrieved 2 April 2013. https://www.imperialviolet.org/2013/03/20/alpn.html ↩

  15. Langley, Adam. "False Start's Failure (11 Apr 2012)". Retrieved 25 September 2013. https://www.imperialviolet.org/2012/04/11/falsestart.html ↩