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UDP - User Datagram Protocol

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Peter R. Egli
Peter R. Egli

Overview of UDP protocol. UDP (User Datagram Protocol) is a simple extension of the Internet Protocol services. It basically provides simple packet transport service without any quality of service functions. Unlike TCP, UDP is connection-less and packet-based. Application PDUs (application packets) sent over a UDP socket are delivered to the receiving host application as is without fragmentation. UDP is mostly used by applications with simple request-response communication patterns like DNS, DHCP, RADIUS, RIP or RPC. Since UDP does provide any error recovery such as retransmission of lost packets, the application protocols have to take care of these situations.

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© 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
© 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
© 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.
© 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.
© 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
© 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
© 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.).

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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.).