Transport Layer Security (TLS)

Transport Layer Security (TLS) and its predecessor, Secure Sockets Layer (SSL), both frequently referred to as “SSL”, are cryptographic protocols that provide communications security over a computer network.

What layer is TLS/SSL?

위키피디아에 따르면 TLS의 정의는 ‘operating over some reliable transport layer’이므로 TCP/IP reference model에서는 application layer에 속하고 OSI model에서는 presentation layer에 속한다고 한다.

TLS and SSL are defined as ‘operating over some reliable transport layer’, which places them as application layer protocols in the TCP/IP reference model and as presentation layer protocols in the OSI model.

하지만 실제로 TLS가 전달하는 애플리케이션 데이터는 양방향 스트림 바이트들이고 애플리케이션들이 TLS를 transport protocal로 이용하기 때문에 transport layer에 해당한다고 한다.

However, what SSL/TLS conveys is “application data”, which is, in fact, a bidirectional stream of bytes. Applications that use SSL/TLS really use it as a transport protocol. They then use their own data representation and messages and semantics within that “application data”. Therefore, SSL/TLS cannot be, in the OSI model, beyond layer 4.

HTTPS가 이러한 방식으로 되어있다. HTTPS는 TLS에 의해 암호화된 커넥션으로 HTTP 통신을 한다.

HTTPS consists of communication over Hypertext Transfer Protocol (HTTP) within a connection encrypted by Transport Layer Security, or its predecessor, Secure Sockets Layer.

이처럼 TLS가 TCP/IP reference model 상에서 application layer와 transport layer 모두에 동시에 속하기 때문에 TLS는 이 모델에 맞지 않는다고 봐야한다. OSI model에서도 마찬가지다.

Thus, in the OSI model, SSL/TLS must be in layer 6 or 7, and, at the same time, in layer 4 or below. The conclusion is unescapable: the OSI model does not work with SSL/TLS. TLS is not in any layer.

References

https://en.wikipedia.org/wiki/Transport_Layer_Security http://security.stackexchange.com/questions/93333/what-layer-is-tls

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