Title: User Datagram Protocol
Content:  port number will likely be a well-known port number. ; Length ; This field specifies the length in bytes of the UDP header and UDP data. The minimum length is 8 bytes, 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. However the actual limit for the data length, which is imposed by the underlying IPv4 protocol, is 65,507 bytes (65,535 bytes − 8-byte UDP header − 20-byte IP header). ; Using IPv6 jumbograms it is possible to have UDP datagrams of size greater than 65,535 bytes. specifies that the length field is set to zero if 
Title: UDP-Lite
Content:  packet smaller than 12 octets is sent in such a setup, the checksum will cover the whole packet. On the receiving side a socket will by default drop all packets which are not covered completely (UDP emulation.) To permit for smaller coverage one can use: This will allow for packets where at minimum 12 octets of user data are checksummed. Any packet with a smaller coverage will be silently dropped as bad. If a packet has a coverage length of at least 20 octets (including header) and its checksum is correct, it will be delivered to application (whole or part of the payload can still 
Title: User Datagram Protocol
Content:  In computer networking, the User Datagram Protocol (UDP) is one of the core members of the Internet protocol suite. With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol (IP) network. Prior communications are not required in order to set up communication channels or data paths. UDP uses a simple connectionless communication model with a minimum of protocol mechanisms. UDP provides checksums for data integrity, and port numbers for addressing different functions at the source and destination of the datagram. It has no handshaking dialogues, and thus exposes the user's program to any unreliability of the underlying network; there is 
Title: Berkeley sockets
Content:  The User Datagram Protocol (UDP) is a connectionless protocol with no guarantee of delivery. UDP packets may arrive out of order, multiple times, or not at all. Because of this minimal design, UDP has considerably less overhead than TCP. Being connectionless means that there is no concept of a stream or permanent connection between two hosts. Such data are referred to as datagrams (Datagram Sockets). UDP address space, the space of UDP port numbers (in ISO terminology, the TSAPs), is completely disjoint from that of TCP ports.
Title: User Datagram Protocol
Content:  length of the UDP header plus UDP data is greater than 65,535. ; Checksum ; The checksum field may be used for error-checking of the header and data. This field is optional in IPv4, and mandatory in IPv6. The field carries all-zeros if unused. A UDP datagram consists of a datagram header and a data section. The UDP datagram header consists of 4 fields, each of which is 2 bytes (16 bits). The data section follows the header and is the payload data carried for the application. The use of the checksum and source port fields is optional in IPv4 (pink background in table). In IPv6 only the source port field is optional. 
Title: UDP-Lite
Content:  UDP-Lite is based on User Datagram Protocol (UDP), but unlike UDP, where either all or none of a packet is protected by a checksum, UDP-Lite allows for partial checksums that only covers part of a datagram (an arbitrary count of octets at the beginning of the packet), and will therefore deliver packets that have been partially corrupted. It is designed for multimedia protocols, such as Voice over IP (VoIP) or streamed video, in which receiving a packet with a damaged payload is better than receiving no packet at all. For conventional UDP and Transmission Control Protocol (TCP), a single bit in error will cause a "bad" 
Title: User Datagram Protocol
Content:  Applications can use datagram sockets to establish host-to-host communications. An application binds a socket to its endpoint of data transmission, which is a combination of an IP address and a port. In this way, UDP provides application multiplexing. A port is a software structure that is identified by the port number, a 16 bit integer value, allowing for port numbers between 0 and 65535. Port 0 is reserved, but is a permissible source port value if the sending process does not expect messages in response. The Internet Assigned Numbers Authority (IANA) has divided port numbers into three ranges. Port numbers 0 through 1023 are used for common, well-known services. On Unix-like operating systems, using one of these ports requires superuser operating permission. Port numbers 1024 through 49151 are the registered ports used for IANA-registered services. Ports 49152 through 65535 are dynamic ports that are not officially designated for any specific service, and may be used for any purpose. These may also be used as ephemeral ports, which software running on the host may use to dynamically create communications endpoints as needed.
Title: UDP-Lite
Content:  corrupted, because it could be not covered by checksum or because the checksum was correct incidentally, but the latter is very unlikely.) If the checksum is incorrect the packet will be dropped, because it is actually impossible to know if the error was inside the payload data or in the UDP-Lite header, so the packet could actually be destined for a different program. The smallest possible coverage is 8 octets. Headers need to be included in checksum. Packets with a smaller length of coverage will always be dropped independent of any settings (ignoring sniffers which are interested in all traffic) as not conforming to standard.
Title: IPv4
Content:  underlying network. ; Total Length ; This 16-bit field defines the entire packet size in bytes, including header and data. The minimum size is 20 bytes (header without data) and the maximum is 65,535 bytes. All hosts are required to be able to reassemble datagrams of size up to 576 bytes, but most modern hosts handle much larger packets. Links may impose further restrictions on the packet size, in which case datagrams must be fragmented. Fragmentation in IPv4 is performed in either the sending host or in routers. Reassembly is performed at the receiving host. ; Identification ; This field is an identification 
Title: UDP-based Data Transfer Protocol
Content:  UDP-based Data Transfer Protocol (UDT), is a high-performance data transfer protocol designed for transferring large volumetric datasets over high-speed wide area networks. Such settings are typically disadvantageous for the more common TCP protocol. Initial versions were developed and tested on very high-speed networks (1 Gbit/s, 10 Gbit/s, etc.); however, recent versions of the protocol have been updated to support the commodity Internet as well. For example, the protocol now supports rendezvous connection setup, which is a desirable feature for traversing NAT firewalls using UDP. UDT has an open source implementation which can be found on SourceForge. It is one of the most popular solutions for supporting high-speed data transfer and is part of many research projects and commercial products.

Question: what is the minimum size of a udp datagram?
Answer: 8 bytes.