More UDP
Port number: Used for (de)multiplexing. Client ports are ephemeral (short-lived). Server ports are “well known”.
UDP checksum similar to IP header checksum, but includes a pseudo-header (to help check source/destination). Fig 11.3
UDP checksum optional, but RFC 1122/23 (host reqts) requires it to be enabled
Application message is simply encapsulated and sent to IP => can result in fragmentation. Newer systems use some path MTU discovery algorithms at the IP layer.