While UDP gives no order guarantee, the USRP still sends them out in order. The uncertainty comes in cases where routing happens between the USRP and the host. Still, within a LAN you can expect with relative certainty, that packets will still arrive in order, as there is usually only one route from device to host.
1. The sequence of the packets is important. It would be rather bad if two bunches of samples in your IQ stream suddenly switched places.
2. The host PC network stack does no reordering. It can't, by definition of UDP, as there's nothing to reorder by.
3. AFAIK, the UHD also does no reordering. However, the packets arriving from the USRP __are__ numbered. If UHD detects a missing packet, it* prints a D (to signify a Dropped packet) to stdout, and emits a new rx_time tag for the next packet.
* Actually, I don't know whether that's UHD or gr-uhd that does this.
Hope it helped
Laur