A method (30) performed in a device (4a, 4b) for compression of headers of Internet Protocol, IP, packets, is described. The method (30) comprises aggregating (31) a first IP packet (13; 25) and a reference IP packet (12; 24) into a single transmission unit (16); identifying (32) header information elements in the first IP packet (13; 25) to be redundant or non-redundant; indicating (33) the header information elements to be redundant or non-redundant by a bitmap; and compressing (34) the header of the first IP packet (13; 25) according to the bitmap. A corresponding method for decompressing is also provided, and devices, computer programs and computer program products.