UDP packet and firewall

Question

I notice when I send udp packets from computer A to computer B, B can receive the upd packets correctly but if I send udp packets from computer B to computer A the udp packets are not received

A and B are not in the same network, and they are all with public IP, they are not behind NAT.

does this mean there is a firewall in between which prevents A from receiving udp packets from B? if A sends some udp packets to B before B sends udp packets to A, will the firewall remember this and then allow the udp packets from B to A ?

I know TCP is stateful and so firewall have a lot of measures to block some malicious TCP packets, like TCP SYN flooding, but how firewall block UDP packets? are there any good articles about this?

because A is my computer in a office. I want to build a system so that a UDP program on A can receive packets from outside. But it seems the firewall filters all the incoming udp packets. I'm wondering, if I use a commercial UDP-based program, like UDP-based video streaming tool or website, can I watch videos?

thanks

Solution

Possible reasons are (1) firewalls (2) wrong routing information (3) NAT somewhere on the path, even if you don't know about it.

Debug the network with traceroute and ping from both sides, see what is in between both computers, and see where the block is.

(As the question is 5 years old, it's unlikely to get more details...)