As noted before, the lifetime and hash are used to avoid excessive resources wastage. Actually, the lifetime metric is sufficient to know if a new negotiation procedure is needed or not. But, quantitatively, the Request_hash and Response_hash messages are less bulky than Request_SSP and Response_SSP messages: a hash size is about few bytes (for instance, SHA-1 hash size is 20 bytes) whereas the SSPD is planned to be stored in XML stream (for instance, an XML stream containing SSH, IPsec, and TLS mechanisms has a 1000 bytes size).