What is the largest Safe UDP Packet Size on the Internet
I've read a number of articles about UDP packet sizes but have been unable to come to a conclusion on whats correct.
A number of services restrict the largest UDP packet to 512 bytes (like dns)
Given the minimum MTU on the internet is 576 , and the size of the IPv4 header is 20 bytes, and the UDP header 8 bytes. This leaves 548 bytes available for user data
Would I be able to use packets up to the size of 548 without packet fragmentation? Or is there something the creators of DNS knew about, and that why they restricted it to 512 bytes.
Could I even go higher than 548 bytes safely?
Answer
It is true that a typical IPv4 header is 20 bytes, and the UDP header is 8 bytes. However it is possible to include IP options which can increase the size of the IP header to as much as 60 bytes. In addition, sometimes it is necessary for intermediate nodes to encapsulate datagrams inside of another protocol such as IPsec (used for VPNs and the like) in order to route the packet to its destination. So if you do not know the MTU on your particular network path, it is best to leave a reasonable margin for other header information that you may not have anticipated. A 512-byte UDP payload is generally considered to do that, although even that does not leave quite enough space for a maximum size IP header.