Standards/Ethernet/UDP
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UDP payload
With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol (IP) network without requiring prior communications to set up special transmission channels or data paths. The protocol was designed by David P. Reed in 1980 and formally defined in RFC 768.
UDP uses a simple transmission model without implicit handshaking dialogues for providing reliability, ordering, or data integrity. Thus, UDP provides an unreliable service and datagrams may arrive out of order, appear duplicated, or go missing without notice. UDP assumes that error checking and correction is either not necessary or performed in the application, avoiding the overhead of such processing at the network interface level.
UDP provides application multiplexing via port numbers and integrity verification via checksum of the header and payload.
The following table shows the content of an UDP datagram
byte\bit offset | 0 – 15 | 16 – 31 | ||||||||||||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
0 | Source Port Number | Destination Port Number | ||||||||||||||||||||||||||||||
4 | Length | Checksum | ||||||||||||||||||||||||||||||
8 | Data |
The UDP header consists of 4 fields, each of which is 2 bytes.
Source port number
This field identifies the sender's port when meaningful and should be assumed to be the port to reply to if needed. If not used, then it should be zero. If the source host is the client, the port number is likely to be an ephemeral port number. If the source host is the server, the port number is likely to be a well-known port number.
Destination port number
This field identifies the receiver's port and is required. Similar to source port number, if the client is the destination host then the port number will likely be an ephemeral port number and if the destination host is the server then the port number will likely be a well-known port number.
Length
A field that specifies the length, in bytes, of the entire datagram: header and data. The minimum length is 8 bytes since that is the length of the header. The field size sets a theoretical limit of 65,535 bytes (8 byte header + 65,527 bytes of data) for an UDP datagram. The practical limit for the data length which is imposed by the underlying IPv4 protocol is 65,507 bytes (65,535 − 8 byte UDP header − 20 byte IP header).
Checksum
The checksum field is used for error-checking of the header and data. The checksum computation also takes in account a part of the IP header. If no checksum is generated by the transmitter, the field uses the value all-zeros. This field is not optional for IPv6.
The calculation method is the same as for the IP checksum (sum and bitwise inversion). If the checksum calculation results in the value zero (all 16 bits 0) it should be sent as its one's complement (all 1's).
For IPv4, the checksum calculation is done on the following data (pseudo-header, UDP header and data):
bits | 0 – 7 | 8 – 15 | 16 – 23 | 24 – 31 | ||||||||||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
0 | Source IP address | |||||||||||||||||||||||||||||||
32 | Destination IP address | |||||||||||||||||||||||||||||||
64 | Zeros | Protocol (11h) | UDP length | |||||||||||||||||||||||||||||
96 | Source UDP Port | Destination UDP Port | ||||||||||||||||||||||||||||||
128 | UDP length | Checksum (0000h) | ||||||||||||||||||||||||||||||
160 | UDP data |
Example
Sending hello world!<cr><lf> in an UDP frame from IP address / port 01234567 / 0017h to IP address / port 89ABCDEF / 1717h would give the following checksum calculation:
info | data | part. sum | total sum | checksum |
---|---|---|---|---|
Source IP address | 0123 | 9E4C | 348A | CB75 |
4567 | ||||
Destination IP address | 89AB | |||
CDEF | ||||
IP protocol (UDP) | 0011 | |||
UDP length | 0016 | |||
Source UDP port | 0017 | 1744 | ||
DestinationUDP port | 1717 | |||
UDP length | 0016 | |||
checksum | 0000 | |||
He | 4865 | 7EF9 | ||
ll | 6C6C | |||
o | 6F20 | |||
wo | 776F | |||
rl | 726C | |||
d! | 6421 | |||
<cr><lf> | 0D0A |
With this, the Ethernet frame will be:
Ethernet header | E0E1E2E3E4E5 F0F1F2F3F4F5 0800 |
---|---|
IP header | 4500 002A 0000 4000 1011 CC9E 0123 4567 89AB CDEF |
UDP header | 0017 1717 0016 CB75 |
Payload | 48 65 6C 6C 6F 20 77 6F 72 6C 64 21 0D 0A |
Padding | 00 00 00 00 |
Eth. CRC | 52 5D A4 CF |