Purpose of T1and T2 transaction timeout

[cihankomecoglu@xxxxxxxxx (Cihan Kömeçoglu)]

Hi

I am learning pjsip usage and also sip. I understood pjsip_t4_timeout and also td timeout value ,invite and non-invite.
 But i couldnt find any infirmation about purpose of pjsip_t1_timeout and pjsip_t2_timeout.
I will be appreciated if you explain this defines. Purpose of t1,t2 timeout value.

Thanks in advance
Sincerely

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