SIP Timers

http://www.cisco.com/c/en/us/td/docs/ios_xr_sw/iosxr_r3-4/sbc/configuration/guide/sbc_c34/sbc34str.pdf

https://community.acmepacket.com/t5/Interop-and-Standards/What-are-SIP-timers-and-how-are-they-used/td-p/135