PCHAR:  A TOOL FOR MEASURING NETWORK PATH CHARACTERISTICS
Bruce A. Mah
<bmah@acm.org>
$Id: FAQ,v 1.8 2001/06/13 17:39:48 bmah Exp $
---------------------------------------------------------

A compilation of questions, experiences, and suggestions from users of
pchar.

-----

Q1.  I compiled pchar using GCC on Solaris.  When I try to run pchar,
I get an error message that looks like this:

ld.so.1: ./pchar: fatal: libstdc++.so.2.10.0: open failed: No such file or directory
Killed

A1.  When trying to link shared libraries for processes running as
root, Solaris will only look for shared libraries in directories
explicitly named at compile-time or in a few hardcoded directories
such as /usr/lib.  The libraries provided with GCC are usually located
somewhere else.

To work around this problem, first determine where the library in
question resides (the example cited above is a C++ support library
from GCC, which installs by default into /usr/local/lib).  Then,
before configuring the build of pchar, set the LDFLAGS environment
variable, for example:

% setenv LDFLAGS "-R /usr/local/lib"
% ./configure

(Adjust appropriately for non-csh-like shells.)

Compile and install as normal.

-----

Q2.  pchar dies as soon as it detects a routing change.  How can I make
it not do this?

A2.  Pass the -c option to pchar, to get something similar to
pathchar's default behavior.

-----

Q3.  I see really high packet losses when tracing a path through
a Solaris machine.  Why?

A3.  Solaris and Cisco IOS implement rate control on some ICMP packets
they generate, typically limiting the rate to at most 2 packets per
second per destination.  This prevents ICMP generation from being used
in a possible (D)DOS attack.  Increase the gap between packets to
slightly over 0.5s, using the -g option.  This will make pchar
slightly slower for intermediate hops, but will speed up the last hop
significantly.

-----

Q4.  Sometimes I get negative values for round-trip-time and bandwidth
estimates.  What gives?

A4.  pchar relies on being able to infer the characteristics of
network links.  Essentially this involves sending packets into the
network and measuring their response times, and then analyzing the
results.  Any or all of the following conditions can cause negative
round-trip-times or bandwidths:

1.  Excessive packet loss along a link, leading to a lack of useful
data from which to estimate link parameters.

2.  Variations in the amount of time taken for routers to process
packets (for example, the forwarding codepath taking less time than
the ICMP packet generation codepath).

3.  Transient changes in network conditions for links that have been
previously measured.

pchar-1.3 and later will actually replace negative values here by
hyphens, to indicate that the estimates are most likely not valid.

-----

Q5.  Many of the measurement times recorded by my OSF/1
(a.k.a. Digital UNIX or Tru64) machine are zero, and I see the message
"Warning: No non-zero timestamps measured, bumping up to 0.0000001".
What can I do about this?

A5.  According to Jeffrey Mogul <mogul@pa.dec.com>, the default kernel
clock resolution on an OSF/1 machine is 1024Hz (which gives millisecond
resolution, rather than microsecond resolution, as is the case for
many other UNIX systems).  He writes that:

	...for Tru64, you [could] recommend enabling the MICRO_TIME
	kernel option, to get sufficient timing resolution.  To enable
	this option, add the following line to the kernel
	configuration file

	     options MICRO_TIME

	and rebuild the kernel.

-----



