RRSet size limitation lower than predicted by RDLENGTH field size

[Tom Byrnes]

We're pushing the limits of RRSet sizes for A records in the responses to
queries for our lists, but we're finding that the practical limit is much
lower than that predicted in the binary message format specs.
 
The octets in the RDLENGTH param (16 bit unsigned = 65535) should allow
16384 A records in a single RRSET using TCP, but the behavior we are
observing in BIND is a limitation of 4096 A records.
 
We're using Bind 9.4.1-P1 on Gentoo.
 
Any ideas what's causing this, or how to fix it?
 
Thanks in advance.
 
Tom Byrnes
CTO
ThreatSTOP