Improve CONTRIBUTING (sample.pcap)

Improve "6) Add/update sample.pcap files" in CONTRIBUTING, clarifying the
options to use.

1 files changed, 5 insertions, 4 deletions

diff --git a/CONTRIBUTING b/CONTRIBUTING
index 186583ed..0f4a5d85 100644
--- a/CONTRIBUTING
+++ b/CONTRIBUTING
@@ -68,9 +68,10 @@ How to add new code and to update existing code
 
 6) Add/update sample.pcap files
    We use tests directory to do regression tests on the dissection of captured
-   packets, by running tcpdump against a savefile sample.pcap, created with -w
-   option and comparing the results with a text file sample.out giving the
-   expected results.
+   packets.  Those captured packets were saved running tcpdump with option "-w
+   sample.pcap".  Additional options like "-n" and "-t" are used to create
+   relevant and reproducible output.  The actual test compares the current text
+   output with the expected result (sample.out) saved from a previous version.
 
    Any new/updated fields in a dissector must be present in a sample.pcap file
    and the corresponding output file.
@@ -80,7 +81,7 @@ How to add new code and to update existing code
    test-name   sample.pcap   sample.out   tcpdump-options
 
    the sample.out file can be build by:
-   (cd tests && ../tcpdump -n -r sample.pcap tcpdump-options > sample.out)
+   (cd tests && ../tcpdump -n -t -r sample.pcap tcpdump-options > sample.out)
 
    It is often useful to have test outputs with different verbosity levels
    (none, -v, -vv, -vvv, etc.) depending on the code.