User Datagram Protocol, UDP 指導老師：梁德昭 老師 學生：吳雅真 學號： 日期：

Introduction  RFC 768  Transport layer protocol  connectionless  unreliable (no acknowledgments)  transmit messages (called user datagrams)  does not reassemble incoming messages  no flow control  very limited error checking (checksum)

Why does the UDP exist?  simple  For a process which wants to send a small message and does not care much about reliability  minimum overhead

UDP Formate

UDP Header Formate The fields are as follows: Source port number Destination port number Length Checksum

UDP Header Formate  Source port number Port number range: from 0 to 65,535 It ’ s optional  Destination port number Port number range: from 0 to 65,535 Length defines the total length of the user datagram, header plus data the minimum length is 8 bytes

Well-known ports used with UDP PortProtocolDescription 7EchoEchoes a received datagram back to the sender 9DiscardDiscards any datagram that is received 11UsersActive Users 13DaytimeReturns the date and the time 17QuoteReturns a quote of the day 19ChargenReturns a string of characters 53NameserverDomain name services 67BootpsServer port to download bootstrap information 68BootpcClient port to download bootstrap information 69TFTPTrivial File Transfer Protocol 111RPCRemote Procedure Call 123NTPNetwork Time Protocol 161SNMPSimple Network Management Protocol 162SNMPSimple Network Management Protocol (trap)

Checksum  This field is used to detect errors over the entire user datagram (header + data)  It ’ s optional  It includes 3 sections: a pseudo-header the UDP header the data coming from the application layer

Checksum

Checksum example

Encapsulation and Decapsulation

Multiplexing and Demultiplexing