The sendaddrv2 message signals support for receiving ADDRV2 messages (BIP155). It also implies that its sender can encode as ADDRV2 and would send ADDRV2 instead of ADDR to a peer that has signaled ADDRV2 support by sending SENDADDRV2.
Declared in <protocol.h>
inline constexpr char const* SENDADDRV2 = {"sendaddrv2"};