A method of defining header field compression for a data packet connection and a header field compression system, in which a context for controlling the operation of a compressor and decompressor is defined as one parameter of the connection. A length is defined for a context identifier used in identifying data packet connections for data transmission between the compressor and decompressor, said length defining the maximum number of data packet connections transmitted on one connection. Each data packet connection is identified by its own context identifier. The parameters of the connection are defined in such a manner that at least the number of header fields of data packet connections allowed by the defined context identifier length can be compressed despite the fact that the number of data packet connections allowed by said context identifier length is exceeded.