The maximum send length for UDP is the maximum size of a UDP datagram.

For UDP over IPv4, this is the maximum IPv4 packet size (65535) minus
the size of the IPv4 and UDP headers, arriving at 65507.

In theory for a system implementing IPv6 jumbogram support, there is
no maximum packet size for UDP.  In practice we've observed with
CentOS 5 a limitation of 65535 - 8, which is dictated by the size
field in the UDP header (it has a maximum value of 65535, but needs
to count both payload and header bytes, thus subtracting off the 8
bytes for the UDP header).

We take the most conservative approach and use the 65507 value
for UDP / IPv4.

This is (I believe) the last part of issue #212.