Performance Issues - PTR Records

Wed Nov 2 21:57:37 UTC 2011

I work for a regional ISP and very recently there has been an influx of
calls reporting "slowness" when accessing certain websites (i.e
google.com/voice/b) via HTTP.  After performing a tcpdump and analyzing the
session, I have been able to pinpoint the latency at the application
layer.  After the tcp session has been established, it takes up to 15-20
seconds before the application begins sending data.   The root of the
problem was that the PTR record for our customer(s) address does not
exist.  As soon as the record is created, latency from the application is
eliminated.  This is analogous to latency when accessing a server over SSH
when no PTR is available.

A seperate packet capture from another customer exhibiting similar
performance behavior showed many TCP retransmissions.  At first glance, I
assumed this was network related however this correlates with the
application not responding and inducing retransmissions at the TCP layer
(different symptoms, same problem).

Historically, there was no compelling reason to create PTR records for our
CPE however more and more applications seem to be dependent on it.
Although we will be assigning a record for each address, my question is why
is the application (specifically HTTP) dependent on a reverse record ?
What is the purpose?

Hope this is helpful as well

-- 
-Matt Chung