What is a TTL?
Time to Live (TTL) is a mechanism that limits the lifespan or lifetime of data in a computer or network. In relation to SMS, this is what you set to limit the active lifetime of a pending message. So if a message has not been delivered to it's intended recipient before the expiry of it's TTL, it will fail.
The default setting for most TTL's is 72 hours from submission, but in some cases it may be beneficial to reduce this. More on this below.
Typically, mobile networks will have a retry plan built into their default TTL's. E.g. In the case of T-Mobile UK, if the message is not successful after 5 minutes, they automatically attempt redelivery. The same thing happens again after 1 hour, 6 hours, 12 hours, 24 hours and 48 hours, before finally failing on the 72 hour mark.
Some clients utilise a special retry plan in an effort to increase successful payments from their users. They set the expiry time (TTL) of their sent premium messages to 6 minutes, meaning that if they receive no "Delivered" delivery report after this time, they can consider the message to be failed and attempt to retry shortly afterwards (if they so wish).
Things to watch out for...
Some operators in some markets charge a fee or a "fine" for unsuccessful messages above a certain ratio (e.g. Vodafone UK, T-Mobile UK, Vodafone NL). If you intend to deploy your own retry scheme (whilst reducing default TTL's), you risk exposing yourself to these charges. Please be careful. If you're unsure, please ask us for help.
As a rule of thumb, this strategy should only be deployed in MT billed markets.