A first node (110; 210; 212; 310; 1000) for determining an indication of a maximum datagram size supported without fragmentation in communication between the first node (110; 210; 212; 310; 1000) and a second node (111; 211; 210; 220) in an Internet Protocol, IP, network (100; 201; 201-202; 300). The first node (110; 210; 212; 310; 1000): sends (301; 901), to the second node (111; 211; 210; 220), request messages in varying sized IP packets using an application layer protocol over UDP and which datagrams are configured not to be fragmented; receives (302; 902) response messages from the second node, each indicating a received request message that the response message is in response to, was received by the second node (111; 211; 210; 220); identifies (303; 903) a request message for which there is a change in receipt of response message compared to another request message of said varying sized IP packets; determines (304; 904), based on said identification, an indication of the maximum datagram size supported without fragmentation.