04/19/23 UDP 1

Internet Transport Services

• Reliable, in-order unicast delivery (TCP)– Congestion

– Flow control

– Connection setup

• Unreliable (“best-effort”), unordered unicast or multicast delivery: UDP

• Services not available: – Real-time

– Bandwidth guarantees

– Reliable multicast

04/19/23 UDP 2

UDP: User Datagram Protocol

• RFC 768

• “no frills,” “bare bones” Internet transport protocol

• “best effort” service, UDP segments may be:

– Lost

– Delivered out of order

• Connectionless:

– No handshaking between UDP sender, receiver

– Each UDP segment handled independently of others

Why is there a UDP?• No connection

establishment (which can add delay)

• Simple: no connection state at sender, receiver

• Small segment header• No congestion control:

UDP can blast away as fast as desired

04/19/23 UDP 3

UDP

• Often used for streaming multimedia apps

– Loss tolerant

– Rate sensitive

• Other UDP uses:– DNS

– SNMP

• Reliable transfer over UDP– Add reliability at application

layer

– Application-specific error recover!

source port # dest port #

32 bits

Applicationdata

(message)

UDP segment format

length checksumLength, in

bytes of UDPsegment,including

header

04/19/23 UDP 4

Header Fields

• Port numbers identify the sending and receiving processes (source port is optional)

• The length field identifies the length of the header and the data in bytes. The minimum value for this field is 8 bytes.

• Note the length is redundant, since the IP header contains the total length.

• 0 length data portions are okay!

04/19/23 UDP 5

UDP Checksum

• The UDP checksum covers the UDP header and UDP data.

• The checksum is optional, it is set to 0 if not used.• There are some differences in the way UDP

computes its checksum– if the UDP datagram consists of an odd umber of bytes

a 0 byte is added to the end (but it is not sent)– UDP appends a pseudo-header so that UDP can double

check that data has arrived at the correct destination– Note that the pseudo-header is never sent

04/19/23 UDP 6

UDP Pseudo-Header

16-bit source port 16-bit destination port

16-bit length 16-bit checksum

data (if any)

0 15 16 31

32-bit source IP address

32-bit destination IP address

zero 8-bit protocol 16-bit length

pad byte