Defining context identifier in header field compression

A method for defining a context identifier when compressing header fields of data packets, in which method, a context is defined for a compressor and decompressor of a data packet flow, the context controlling the operation of said compressor and decompressor. The context is identified by a context identifier attached to the data packet and the length of the context identifier is defined by data transfer between the compressor and decompressor in such a manner that the length of the context identifier is defined in the context identifier of the data packet being transmitted. The context identifier comprises a field of at least one bit for defining the length of the context identifier. The length of the context identifier can be defined in the context identifier of each data packet being transmitted.