Time to Live (TTL) is the “hop limit” or limit on the number of routing devices a message may pass through before expiring.
TTL enables RAP to operate on networks where there is a router between a Publisher and a Subscriber. TTL is represented as an 8-bit unsigned integer on each packet on a network. The TTL of a message is stored as part of the message and every time the message passes through a router, its TTL value is decreased by the amount of time the message spent at that routing station, with a minimum of one second. On modern hardware this is equivalent to the “hop limit”, or number of routers the packet may pass through. When the TTL on a packet reaches zero, the packet is discarded and an error datagram is returned to the sender.
TTL exists to prevent packets from becoming effectively immortal on the network – if a packet cannot reach its destination, its TTL will eventually expire, preventing it from clogging up the network forever.
For networks with routers between publisher and subscriber machines the TTL will need to be set for all publishers attempting to send data through the router.