Tuesday, October 26, 2010

Breaking RFC compliance to improve monitoring

A colleague came to me today and had a troubling issue. He's using sipsak and nagios to monitor some SIP endpoints. Pretty standard so far, right? He noticed that when using UDP and checking on an endpoint that was completely offline sipsak would take over 30 seconds to finally return with an error. Meanwhile Nagios would block and wait for sipsak to return...

Without a simple command line option in sipsak that appeared to change this behavior, we had to enter the semi-complicated world of SIP timers. I feared that to change this behavior we'd have to do some things that might not necessarily be RFC compliant...

What's this? For once I'm actually suggesting you do something against the better advice of an RFC?

That's right, I am.

RFC3261 defines multiple timers and timeouts for messages and transactions. It says things like:

"If there is no final response for the original request in 64*T1 seconds"

"The UAC core considers the INVITE transaction completed 64*T1 seconds after the reception of the first 2xx response."

"The 2xx response is passed to the transport with an interval that starts at T1 seconds and doubles for each retransmission until it reaches T2 seconds"

Without even knowing what "T1" is you can start to see that it's a pretty important timing parameter and (more or less) serves as the father of all timeouts in SIP. Let's look at section 17 to find out what T1 is:

"The default value for T1 is 500 ms. T1 is an estimate of the RTT between the client and server transactions. Elements MAY (though it is NOT RECOMMENDED) use smaller values of T1 within closed, private networks that do not permit general Internet connection. T1 MAY be chosen larger, and this is RECOMMENDED if it is known in advance (such as on high latency access links) that the RTT is larger. Whatever the value of T1, the exponential backoffs on retransmissions described in this section MUST be used."

T1 is essentially a variable for RTT between two endpoints that serves as a multiplier for other timeouts. Unless we know better T1 should default to 500ms, which is quite high. Some implementations (such as Asterisk with the SIP peer qualify option) automatically send OPTIONS requests to endpoints in an effort to better determine RTT instead of using the RFC default of 500ms.

In reading through the sipsak source code it appeared to be RFC compliant for timing, using a default T1 value of 500ms and a transaction timeout value of 64*T1. This is why it was taking over 30 seconds (32 seconds to be exact) for sipsak to finally timeout and return the status code to nagios. This comes directly from the RFC:

"For any transport, the client transaction MUST start timer B with a value of 64*T1 seconds (Timer B controls transaction timeouts)."

This is all well and good but what happens when you don't have a way to dynamically determine T1 and you can't wait T1*64 (32s) for your results like my sipsak/nagios check earlier? Simple: you go renegade, throw out the RFC, and hack the sipsak source yourself!

So I had three options:

1) Change the default value of T1.
2) Change the value of T2 by changing the multiplier or setting a static timeout.
3) Some combination of both.

I decided to go with option #3 (RFC be damned). Why?

1) 500ms is crazy high for most of our endpoints. At a glance 100ms would be fine for ~90% of them. I'll pick 150ms.
2) I don't need that many retransmits. If the latency and/or packet loss is that bad I'm not going to wait (my RTP certainly isn't) and I just want to know about it that much quicker.

So I ended up with a quick easy patch to sipsak:

diff -urN sipsak-0.9.6.orig/sipsak.h sipsak-0.9.6/sipsak.h
--- sipsak-0.9.6.orig/sipsak.h 2006-01-28 16:11:50.000000000 -0500
+++ sipsak-0.9.6/sipsak.h 2010-10-26 18:38:45.000000000 -0400
@@ -102,11 +102,7 @@
# define FQDN_SIZE 100

-# define SIP_T1 500
+#define SIP_T1 150

#define SIP_T2 8*SIP_T1

diff -urN sipsak-0.9.6.orig/transport.c sipsak-0.9.6/transport.c
--- sipsak-0.9.6.orig/transport.c 2006-01-28 16:11:34.000000000 -0500
+++ sipsak-0.9.6/transport.c 2010-10-26 18:38:51.000000000 -0400
@@ -286,7 +286,7 @@
senddiff = deltaT(&(srt->starttime), &(srt->recvtime));
- if (senddiff > (float)64 * (float)SIP_T1) {
+ if (senddiff > inv_final) {
if (timing == 0) {
if (verbose>0)
printf("*** giving up, no final response after %.3f ms\n", senddiff);

This changes the value of T1 to 150ms (more reasonable for most networks) and allows you to specify the number of retransmits (and thus the total timeout) using -D on the sipsak command line:

kkmac:sipsak-0.9.6-build kris$ ./sipsak -p -s sip:ext_callqual@asterisk -D1 -v
** timeout after 150 ms**
*** giving up, no final response after 150.334 ms

kkmac:sipsak-0.9.6-build kris$ ./sipsak -p -s sip:ext_callqual@asterisk -D2 -v
** timeout after 150 ms**
** timeout after 300 ms**
*** giving up, no final response after 460.612 ms

kkmac:sipsak-0.9.6-build kris$ ./sipsak -p -s sip:ext_callqual@asterisk -D4 -v
** timeout after 150 ms**
** timeout after 300 ms**
** timeout after 600 ms**
*** giving up, no final response after 1071.137 ms

kkmac:sipsak-0.9.6-build kris$

Needless to say our monitoring situation is much improved.

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