Hello!
I want to send broadcast message to all peers in my local network. Message is a 32 bit integer. I can be sure, that message will not me fragmented, right? There will be two options: - peer will receive whole message at once - peer will not receive message at all
Going further, 4 bytes is maximum size of data, that can be sent in one UDP datagram? I use Ethernet based network in 99%.
Ethernet packets can be up to around 1500 bytes (and that's not counting jumbo frames). If you send broadcast messages with a payload of only 4 bytes, they shouldn't get fragmented at all. Fragmentation should only occur when the packet is larger than the Maximum Transmission Unit (so about 1500 bytes over Ethernet).
IPv4 specifies a minimum supported MTU of 576 bytes, including the IP header. Your 4 byte UDP payload will result in an IP packet far smaller than this, so you need not fear fragmentation.
Furthermore, your desired outcome - "peer will receive whole message at once or peer will not receive message at all" is always how UDP works, even in the presence of fragmentation. If a fragment doesn't arrive, your app won't recieve the packet at all.
The rules for UDP are "The packet may arrive out-of-order, duplicated, or not at all. If the packet does arrive, it will be the whole packet and error-free.". ("Error-free" is obviously only true within the modest limits of the IP checksum).