Why does UDP have the field “UDP Length” twice in

Question

Why does UDP have the field "UDP Length" twice in its packet? Isn't it redundant? If it is required for some kind of error checking, please provide an example.

Answer 1

Your observation is correct. The length field is redundant because both the IP header and the UDP header has a length field. My only guess about the reason for this redundancy is, that it happened because UDP was designed at a time, where it was not yet clear what the IP protocol suite would look like.

All legitimate UDP packets should have a length field matching exactly what could be derived from the length field in the IP header. If you don't do that, you can't know for sure, what the receiver is going to do with the packet.

UDP packets with inconsistent length fields are seen in the wild on the Internet. I guess they are probing for buffer overflows, which might happen if one length field is used to allocate memory and the other length field is used when copying data to the allocated buffer.

In the newer UDP Lite protocol, the length field has been repurposed. The length field in the UDP Lite header does not indicate how much data there is in the packet, but rather how much of it has been covered by the checksum. The length of the data in a UDP Lite packet is always computed from the length field in the IP header. This is the only difference between the UDP and UDP Lite header formats.

Answer 2

I don't know where it appeared twice, only one "udp length" field in udp header.

Answer 3

From RFC 768:

Length is the length in octets of this user datagram including this header and the data. (This means the minimum value of the length is eight.)

The pseudo header conceptually prefixed to the UDP header contains the source address, the destination address, the protocol, and the UDP length. This information gives protection against misrouted datagrams. This checksum procedure is the same as is used in TCP.

              0      7 8     15 16    23 24    31
             +--------+--------+--------+--------+
             |          source address           |
             +--------+--------+--------+--------+
             |        destination address        |
             +--------+--------+--------+--------+
             |  zero  |protocol|   UDP length    |
             +--------+--------+--------+--------+

The REAL answer is that this is a "pseudo header" - that is, it is used for calculating the checksum, but not actually sent. at least that is what I conclude from What is the Significance of Pseudo Header used in UDP/TCP