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).

Support

ALPN is supported by these libraries:

• BSAFE Micro Edition Suite since version 5.0 ^[1]
• GnuTLS since version 3.2.0 released in May 2013 ^[2]
• MatrixSSL since version 3.7.1 released in December 2014 ^[3]
• Network Security Services since version 3.15.5 released in April 2014 ^[4]
• OpenSSL since version 1.0.2 released in January 2015 ^[5]
• LibreSSL since version 2.1.3 released in January 2015 ^[6]
• mbed TLS (previously PolarSSL) since version 1.3.6 released in April 2014 ^[7]
• s2n since its original public release in June 2015.
• wolfSSL (formerly CyaSSL) since version 3.7.0 released in October 2015 ^[8]
• Go (in the standard library crypto/tls package) since version 1.4 released in December 2014 ^[9]
• JSSE in Java since JDK 9 released in September 2017, ^[10] backported to JDK 8 released in April 2020 ^[11]
• Win32 SSPI since Windows 8.1 and Windows Server 2012 R2 were released October 18, 2013 ^[12]
• Rustls ^[13]

History

Next Protocol Negotiation

In January 2010, Google introduced IETF standard draft describing Next Protocol Negotiation TLS extension. ^[14] 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. ^[15]

TLS False Start was disabled in Google Chrome from version 20 (2012) onward except for websites with the earlier NPN extension. ^[16]

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:

HandshakeType:ClientHello(1)Length:141Version:TLS1.2(0x0303)Random:dd67b5943e5efd0740519f38071008b59efbd68ab3114587...SessionIDLength:0CipherSuitesLength:10CipherSuites(5suites)CompressionMethodsLength:1CompressionMethods(1method)ExtensionsLength:90[otherextensionsomitted]Extension:application_layer_protocol_negotiation(len=14)Type:application_layer_protocol_negotiation(16)Length:14ALPNExtensionLength:12ALPNProtocolALPNstringlength:2ALPNNextProtocol:h2ALPNstringlength:8ALPNNextProtocol: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:

HandshakeType:ServerHello(2)Length:94Version:TLS1.2(0x0303)Random:44e447964d7e8a7d3b404c4748423f02345241dcc9c7e332...SessionIDLength:32SessionID:7667476d1d698d0a90caa1d9a449be814b89a0b52f470e2d...CipherSuite:TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256(0xc02f)CompressionMethod:null(0)ExtensionsLength:22[otherextensionsomitted]Extension:application_layer_protocol_negotiation(len=5)Type:application_layer_protocol_negotiation(16)Length:5ALPNExtensionLength:3ALPNProtocolALPNstringlength:2ALPNNextProtocol:h2

References

1. "Dell BSAFE Micro Edition Suite 5.0 Release Advisory" . Retrieved 2022-10-18.
2. "gnutls 3.2.0". Archived from the original on 2016-01-31. Retrieved 2015-01-26.
3. "MatrixSSL - News". 2014-12-04. Archived from the original on 2015-02-14. Retrieved 2015-01-26.
4. "NSS 3.15.5 release notes". Mozilla Developer Network. Mozilla. Retrieved 2015-01-26.
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.
6. "LibreSSL 2.1.3 released". 2015-01-22. Retrieved 2015-01-26.
7. "Download overview - PolarSSL". 2014-04-11. Archived from the original on 2015-02-09. Retrieved 2015-01-26.
8. "wolfSSL Release Change Log". 2015-10-26. Retrieved 2015-09-11.
9. "Go 1.4 Release Notes". 2014-12-10. Retrieved 2017-11-28.
10. "JEP 244: TLS Application-Layer Protocol Negotiation Extension". 2017-08-07. Retrieved 2018-08-29.
11. "Release Note: TLS Application-Layer Protocol Negotiation Extension". 2020-04-30. Retrieved 2020-06-11.
12. "What's New in TLS/SSL (Schannel SSP)". 31 August 2016. Retrieved 2020-03-30.
13. "ClientConfig in rustls::client - Rust". docs.rs. Retrieved 2026-01-11.
14. Langley, A. (January 20, 2010). "Transport Layer Security (TLS) Next Protocol Negotiation Extension". IETF Datatracker.
15. Langley, Adam. "» NPN and ALPN" . Retrieved 2 April 2013.
16. Langley, Adam. "False Start's Failure (11 Apr 2012)" . Retrieved 25 September 2013.

External links

• The registry of ALPN protocol IDs is maintained by IANA as a TLS extension.
• draft-agl-tls-nextprotoneg-04 (NPN draft) (last updated: May 2012)
• RFC   7301 "Transport Layer Security (TLS) Application-Layer Protocol Negotiation Extension"