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A UDP datagram is carried in a single IP packet and is hence limited to a maximum payload of 65,507 bytes for IPv4 and 65,527 bytes for IPv6. The transmission of large IP packets usually requires IP fragmentation.
1 Answer. The minimum length is 8 bytes because that is the length of the header. The field size sets a theoretical limit of 65,535 bytes (8 byte header + 65,527 bytes of data) for a UDP datagram.
Most often they are a few hundred bytes or less, and often may be around 100 bytes. That is, many messages are relatively short packets.
UDP datagrams have little to do with the MTU size you can make them as big as you like up to the 64K is maximum mentioned above. You can even send one of them in an entire packet as long as you are using jumbo frames with a size larger the large datagram.
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.
The maximum safe UDP payload is 508 bytes. This is a packet size of 576 (the "minimum maximum reassembly buffer size"), minus the maximum 60-byte IP header and the 8-byte UDP header.
Any UDP payload this size or smaller is guaranteed to be deliverable over IP (though not guaranteed to be delivered). Anything larger is allowed to be outright dropped by any router for any reason. Except on an IPv6-only route, where the maximum payload is 1,212 bytes. As others have mentioned, additional protocol headers could be added in some circumstances. A more conservative value of around 300-400 bytes may be preferred instead.
The maximum possible UDP payload is 67 KB, split into 45 IP packets, adding an additional 900 bytes of overhead (IPv4, MTU 1500, minimal 20-byte IP headers).
Any UDP packet may be fragmented. But this isn't too important, because losing a fragment has the same effect as losing an unfragmented packet: the entire packet is dropped. With UDP, this is going to happen either way.
IP packets include a fragment offset field, which indicates the byte offset of the UDP fragment in relation to its UDP packet. This field is 13-bit, allowing 8,192 values, which are in 8-byte units. So the range of such offsets an IP packet can refer to is 0...65,528 bytes. Being an offset, we add 1,480 for the last UDP fragment to get 67,008. Minus the UDP header in the first fragment gives us a nice, round 67 KB.
Sources: RFC 791, RFC 1122, RFC 2460
The theoretical limit (on Windows) for the maximum size of a UDP packet is 65507 bytes. This is documented here:
The correct maximum UDP message size is 65507, as determined by the following formula: 0xffff - (sizeof(IP Header) + sizeof(UDP Header)) = 65535-(20+8) = 65507
That being said, most protocols limit to a much smaller size - usually either 512 or occasionally 8192. You can often go higher than 548 safely if you are on a reliable network - but if you're broadcasting across the internet at large, the larger you go, the more likely you'll be to run into packet transmission problems and loss.
576 is the minimum maximum reassembly buffer size, i.e. each implementation must be able to reassemble packets of at least that size. See IETF RFC 1122 for details.
This article describes maximum transmission unit (MTU) http://en.wikipedia.org/wiki/Maximum_transmission_unit. It states that IP hosts must be able to process 576 bytes for an IP packet. However, it notes the minumum is 68. RFC 791: "Every internet module must be able to forward a datagram of 68 octets without further fragmentation. This is because an internet header may be up to 60 octets, and the minimum fragment is 8 octets."
Thus, safe packet size of 508 = 576 - 60 (IP header) - 8 (udp header) is reasonable.
As mentioned by user607811, fragmentation by other network layers must be reassembled. https://www.rfc-editor.org/rfc/rfc1122#page-56 3.3.2 Reassembly The IP layer MUST implement reassembly of IP datagrams. We designate the largest datagram size that can be reassembled by EMTU_R ("Effective MTU to receive"); this is sometimes called the "reassembly buffer size". EMTU_R MUST be greater than or equal to 576
IPv4 minimum reassembly buffer size is 576, IPv6 has it at 1500. Subtract header sizes from here. See UNIX Network Programming by W. Richard Stevens :)
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