Abstract: To identify the encrypted data of data link layer,an independent identification scheme was proposed by mainly using the technique of frequency test within a block．As the identification rate of frequency test within a block is impressionable to the size of the block,a block size chosen scheme was provided based on the principle of variance．Furthermore,as the identification ability of frequency test within a block is also limited to short bit sequence,random sampling was introduced into its information extraction process to heighten the identification rate．Eventually,by taking the link layer bit sequence of a wiriness network as the identification object,the identification rate of the proposed schemes was verified．Experimental results demonstrate that the proposed schemes have high rates to identify encrypted data of data link layer and the correlative research paves the way for the further protocol identification research.

Key words: Data link layer,Encrypted data,Frequency test within a block,Variance principle,Random sampling