We will consider RPC in greater detail below, in 16.5 Remote Procedure Call (RPC) for now, the point of UDP is that on a local LAN we can fall back on rather simple mechanisms for timeout and retransmission. The conceptual idea behind RPC is that one host invokes a procedure on another host the parameters and the return value are transported back and forth by UDP. In this setting it is common to use UDP as the transport basis for a Remote Procedure Call, or RPC, protocol. UDP is popular for “local” transport, confined to one LAN. Higher-level protocols that define ACK packets ( eg UDP-based RPC, below) typically include some form of flow control to prevent this. When the latter happens, it means that packets are arriving faster than the receiver can process them. UDP packets can be dropped due to queue overflows either at an intervening router or at the receiving host. If a NAT router rewrites an IP address or port, the UDP checksum must be updated. The UDP checksum covers the UDP header, the UDP data and also a “pseudo-IP header” that includes the source and destination IP addresses (and also a duplicate copy of the UDP-header length field). While it is seldom done today, the checksum can be disabled by setting the checksum field to the all-0-bits value, which never occurs as an actual ones-complement sum. UDP packets use the 16-bit Internet checksum ( 7.4 Error Detection) on the data. An old bit of Internet humor about UDP’s unreliability has it that if I send you a UDP joke, you might not get it.
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