A method of handling mobility-related signaling in a communications system comprising a mobile node, a mobile router, and a peer node. The method comprises providing the mobile router with a delegation certificate that is cryptographically signed by or on behalf of the mobile node. At the mobile router, a mobility-related signaling exchange is initiated with the peer node on behalf of the mobile node, the mobile router providing to the peer node within this exchange, said delegation certificate or an identification of the certificate, and a sequence number associated with the certificate. At the peer node, the received sequence number is compared with a sequence number maintained by the peer node in respect of the delegation certificate, and the exchange authorized in dependence upon the result of the comparison.