UDP - User Datagram Protocol
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The document provides an overview of the User Datagram Protocol (UDP), outlining its characteristics, services, and comparisons with the Transmission Control Protocol (TCP). It emphasizes UDP's connectionless nature, lack of flow control, and minimal error checking through checksums, making it suitable for applications requiring short transactions without the overhead of connection establishment. Typical applications utilizing UDP include SNMP, DNS, and DHCP, while its limitations are highlighted in contrast to the reliability and features of TCP.
UDP - User Datagram Protocol
- 1. © Peter R. Egli 2015 1/7 Rev. 3.50 UDP - User Datagram Protocol indigoo.com INTRODUCTION TO USER DATAGRAM PROTOCOL, A SIMPLE PACKET TRANSPORT SERVICE IN THE INTERNET PROTOCOL SUITE PETER R. EGLI INDIGOO.COM USER DATAGRAM PROTOCOL UDP
- 2. © Peter R. Egli 2015 2/7 Rev. 3.50 UDP - User Datagram Protocol indigoo.com Contents 1. UDP (RFC768) characteristics 2. UDP Service 3. Typical UDP applications 4. UDP Checksum 5. UDP versus TCP
- 3. © Peter R. Egli 2015 3/7 Rev. 3.50 UDP - User Datagram Protocol indigoo.com 1. UDP (RFC768) characteristics • No connection establishment/teardown; data is just sent right away. • No flow control / congestion control, sender can overrun receiver‘s buffer: UDP is not suited for bulk data transfer. For data transfer with UDP a lock-step protocol is required (to be implemented by the application). • No error control; corrupted data is not retransmitted (even though UDP header has a checksum to detect errors and report these to the application). Packet 4 is dropped (buffer overrun). Host 1 Host 2 Packet 1 is sent to the application. Packet 4 is missing. The packet loss needs to be handled by the application (detection, retransmission). 1 Packet 1 is stored in the receive queue. 1 2 1 2 1 2 3 4 3 1 2 3 5 Packet 2 is stored in the receive queue. Packet 3 is stored in the receive queue. 2 3 5 1 Host 1 sends packets.
- 4. © Peter R. Egli 2015 4/7 Rev. 3.50 UDP - User Datagram Protocol indigoo.com 2. UDP service UDP is basically a simple extension of the IP datagram service. UDP adds multiplexing (on port number) to IP datagram service. Application writes are mapped 1:1 to UDP datagrams; UDP passes these 1:1 to the IP layer. UDP layer Appl. IP packets Application sends packets (APDU’s) over the socket interface Socket interface IP layer 2500 300 500 150 TPDU’s (UDP datagrams) 2500 300 500 150 300 500 15015001000 IP layer 300 2500 500 150 UDP layer 300 2500 500 150 Appl. Application reads packets (APDU’s) from the socket interface TPDU’s (UDP datagrams) 2500byte packet is too big. IP layer fragments the UDP datagram.
- 5. © Peter R. Egli 2015 5/7 Rev. 3.50 UDP - User Datagram Protocol indigoo.com 3. Typical UDP applications UDP is best suited for applications with short command-response type „transactions“ that do not justify the establishment / release prior to the data exchange. Host 1 Host 2 SNMP GET Request SNMP GET Response SNMP Host 1 Host 2 DNS Query DNS Response DNS Host 1 Host 2 Time request Time response SNTP Host 1 Host 2 DHCP Request DHCP Response DHCP/Bootp Host 1 Host 1 Host 2 RIP Route Update RIPHost 2 RADIUS Access Request RADIUS RADIUS Access Accept Host 1 Host 2 RPC Request RPC Reply RPC
- 6. © Peter R. Egli 2015 6/7 Rev. 3.50 UDP - User Datagram Protocol indigoo.com 4. UDP checksum UDP has a checksum too that provides minimal protection against transmission errors. The checksum is optional; if it is not used it shall be set to 0. Becaus the IP addresses are used in the UDP checksum calculation, UDP is tightly bound to the IP layer. Therefore UDP can only run on top of IP. Pseudo header The checksum is calculated over the pseudo header, UDP header and data (UDP payload). Source Port Destination Port Checksum Length Data UDP Length00000000 Protocol=17 UDP Length IP Destination Address IP Source Address UDP header
- 7. © Peter R. Egli 2015 7/7 Rev. 3.50 UDP - User Datagram Protocol indigoo.com 5. UDP versus TCP TCP Connection-oriented, point-to-point (unicast) Reliable end-to-end: No bit errors due to checksum. Packet ordering preserved. No duplicates. No packet loss. Stream-oriented (no message boundary preservation) Has flow control to maximise throughput Has congestion control to minimise packet loss Analogon: phone Examples of application protocols using TCP: HTTP, SMTP, FTP, TELNET UDP Connection-less, best-effort Not reliable (no retransmissions) Message boundary preservation No flow control No congestion control Analogon: mail (snail mail) Examples of application protocols using UDP: SNMP, DNS, TFTP, RTP, DHCP, SNTP N.B.: It is possible to run application protocols over both TCP and UDP. E.g. DNS is normally run on UDP, but for zone transfers (higher data volume) DNS uses TCP. Actually there is a shift towards using TCP instead UDP since TCP can better provide security (SSL/TLS, simpler filtering in firewalls etc.).