UDP
UDP is a transport layer protocol which is only implemented at the hosts. It’s datagram oriented, unreliable and connectionless. It supports unicast and multicast and commonly be used for network control signaling services – Network management (SNMP), routing (RIP), DNS, TFTP.
However, UDP merely extends the host-to-host delivery service of IP datagram to an application-to-application service. The only thing that UDP adds is multiplexing and demultiplexing
UDP (and TCP) use port numbers to identify applications. There are 65535 UDP ports available per host. Dynamic/private, used by clients, randomly picked, >49151. Registered, used by ordinary user processes, 1024 – 49151. Well-known, used by serves, fixed, 1-1023.
Maximum Transmission Unit (MTU)
The frame size limit of the data link protocol specifies a limit on the size of the IP datagram that can be encapsulated by the protocol. MTUs vary for different data link protocol: Ethernet 1500, PPP 296, 802.2 1492.
IP datagram is fragmented into smaller units if the size of it exceeds the MTU. The smallest MTU of any data link is used as the Path MTU. IP routers split the datagram into several fragments. Fragmentation requires that the data potion of every fragment except the last be a multiple of 8-bytes. Fragmentation can be done at the sender and at intermediate routers and is only done at destination hosts.
If fragmentation is needed, but Don’t Fragmentation flag is set. The router drops the datagram and sends an ICMP error unreachable error message to the source. This can be used in Path MTU Discovery to find the smallest MTU along the path.