In the CISSP books, SSL/TLS has been through the arguments and fell rest onto the transport layer 2 instead of the session. OK - Fine.

Port security 802.1X and EAP is a data link layer 2 authentication mechanism. Since EAP is a framework, it consists of EAP-TLS (among others like EAP-TTLS, PEAP and LEAP).

There maybe something silly I am missing here, but how can the book call EAP-TLS a layer 2 authentication mechanism when TLS doesn't even kick into the picture until layer 4?

  • 1
    I'm not sure I understand your question clearly. Could you perhaps elaborate a bit as to what "arguments" there have been over SSL/TLS, and your current understanding of SSL/TLS vs. EAP-TLS?
    – Iszi
    Commented May 2, 2011 at 12:18
  • @Iszi - Putting real life into OSI layers is not cool, but passing tests (CISSP) makes me do it. SSL has to go into 1 layer. My text book mentioned that some professionals said it should go into Layer 5 (session) while others said it should go into Layer 4 (transport). Layer 4 won. Any question on an exam that asks "Which layer of the OSI does SSL/TLS fall into" will have a correct answer (earning one a higher grade) of LAYER 4 (Transport).
    – Abdu
    Commented May 3, 2011 at 16:17
  • @Isza - Pornin answered my question: It doesn't matter what layer the text book calls it. A layer 2 authentication (802.1x) can use a (text book) layer 4 protocol (TLS in the EAP-TLS combo) without actually (physically) TRAVELLING UP THROUGH THE LAYERS to make the TLS handshakes happen. I was just thinking that if TLS is involved, the incoming streams of data remain insecure passing through Layer 2 and 3 (Data Link and Network) until it reaches Layer 4 (Transport) for the TLS handshakes of authentication, confidentiality and integrity.
    – Abdu
    Commented May 3, 2011 at 16:26
  • CONTINUED..... So I questioned the text book - why call 802.1x/EAP-TLS a layer 2 operation (which is secure because it secures the data stream as soon as the electrons are pulled off the wire) when TLS makes me think it is a layer 4 operation.
    – Abdu
    Commented May 3, 2011 at 16:26

2 Answers 2


The "layers" are from an old ISO abstract classification which has been applied to how things really run in the physical world, often with the rhetorical equivalent of a sledgehammer. You should not try to follow layers as gospel, it will often lead to confusion.

SSL/TLS is a protocol which can be applied over a bidirectional data tunnel, and which provides a bidirectional data tunnel, with authentication (server by client, or mutual), confidentiality (through encryption) and verified integrity. In terms of "layers", it goes right between layer 4 and layer 6 of the OSI model, which shows that the concept of "layers" fails to capture the actual position of SSL/TLS.

It so happens that the authentication part of SSL/TLS (the "handshake") consists in a series of messages, which are exchanged in an alternate sequence; see the TLS specification, page 36, for a diagram showing that sequence. In "normal" SSL/TLS, the messages are exchanged over whatever bidirectional data tunnel SSL/TLS is applied on, usually a TCP connection. However, the cryptographic properties of the handshake come from the message contents, not from how they are sent. At that point, consider EAP: this is a framework for authentication protocols, consisting in messages; EAP is meant to be used over any transport mechanism which can send messages ("layer 2" if you really want to think about it in layers). Hence EAP-TLS has been defined: this is an authentication protocol, consisting in a sequence of messages which are the ones specified for the SSL/TLS handshake -- but sent as so many messages over the transport mechanism used by EAP. EAP-TLS is not TLS, it does not provide a tunnel for data; it is a protocol which reuses parts of the handshake which occurs at the beginning of a TLS connection.



If you look at the 802.1x packet above it shows communication happening at layer-2.

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