UDP Header

Posted on October 6, 2009. Filed under: CCNA, ICND1 break down |

The following is UPD segment structue.

bits 0 – 15 16 – 31
0 Source Port Destination Port
32 Length Checksum
64  Data 

The UDP header consists of only 4 fields.

  • Source port — This field identifies the sending port when meaningful and should be assumed to be the port to reply to if needed. If not used, then it should be zero.
  • Destination port — This field identifies the destination port and is required.
  • Length  — A 16-bit field that specifies the length in bytes of the entire datagram: header and data. The minimum length is 8 bytes since that’s the length of the header. The field size sets a theoretical limit of 65,535 bytes (8 byte header + 65527 bytes of data) for a UDP datagram. The practical limit for the data length which is imposed by the underlying IPv4 protocol is 65,507 bytes.
  • Checksum  — The 16-bit checksum field is used for error-checking of the header and data. The algorithm for computing the checksum is different for transport over IPv4 and IPv6.

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