Viewer: output format improvements, added new readme

The plugin now dependes on ipfixcol v2.1.0 (requires list iterators provided by libfds v0.2.0)

Author: Lukas955

src/plugins/output/viewer/README.rst

@@ -1,8 +1,19 @@
 Viewer (output plugin)
 =====================
 
-The plugin provides a way how to observe functionality of collector.
-Viewer module prints information about incoming IPFIX packets and data inside of them.
+The plugin converts IPFIX Messages into plain text and prints them on standard output.
+
+The main goal of the plugin is to show content of received IPFIX Messages in human readable form.
+Each IPFIX Message is broken down into IPFIX Sets and each IPFIX Set is further broken down into
+(Options) Template/Data records and so on. Fields of the Data records are formatted according
+to the expected data type, if their corresponding Information Element definitions are known to
+the collector. Therefore, the output can be also used to determine missing Information Element
+definitions.
+
+Biflow records and structured data types are also supported and appropriately formatted.
+Output is not supposed to be further parsed because its format can be changed in the future.
+However, if you are interested into processing Data Records, consider using other
+plugins such as JSON, UniRec, etc.
 
 Example configuration
 ---------------------
@@ -12,10 +23,74 @@ Example configuration
     <output>
         <name>Viewer output</name>
         <plugin>viewer</plugin>
-        <params>
-        </params>
+        <params/>
     </output>
 
 Parameters
 ----------
 
+Parameters are not supported by the plugin.
+
+Example output
+--------------
+
+Below you can see example output for anonymized IPFIX Messages. The message structure and
+available fields of Data Record vary from exporter to exporter.
+
+.. code-block::
+
+    IPFIX Message header:
+        Version:      10
+        Length:       176
+        Export time:  1455534348
+        Sequence no.: 0
+        ODID:         1
+
+    Set Header:
+        Set ID: 2 (Template Set)
+        Length: 160
+    Template Record (#1)
+        Template ID: 256
+        Field Count: 27
+        EN: 0  ID: 1    Size: 4  |  iana:octetDeltaCount
+        EN: 0  ID: 2    Size: 4  |  iana:packetDeltaCount
+        EN: 0  ID: 152  Size: 8  |  iana:flowStartMilliseconds
+        EN: 0  ID: 153  Size: 8  |  iana:flowEndMilliseconds
+        EN: 0  ID: 10   Size: 2  |  iana:ingressInterface
+        EN: 0  ID: 60   Size: 1  |  iana:ipVersion
+        EN: 0  ID: 8    Size: 4  |  iana:sourceIPv4Address
+        EN: 0  ID: 12   Size: 4  |  iana:destinationIPv4Address
+        EN: 0  ID: 5    Size: 1  |  iana:ipClassOfService
+        EN: 0  ID: 192  Size: 1  |  iana:ipTTL
+        EN: 0  ID: 4    Size: 1  |  iana:protocolIdentifier
+        EN: 0  ID: 7    Size: 2  |  iana:sourceTransportPort
+        EN: 0  ID: 11   Size: 2  |  iana:destinationTransportPort
+        ... <~ output shortened for clarity ~> ...
+
+    -------------------------------------------------------------------
+    IPFIX Message header:
+        Version:      10
+        Length:       216
+        Export time:  1455534348
+        Sequence no.: 0
+        ODID:         1
+
+    Set Header:
+        Set ID: 256 (Data Set)
+        Length: 200
+        Template ID: 256
+    Data Record (#1) [Length: 95]:
+        EN: 0  ID: 1    iana:octetDeltaCount          : 76 octets
+        EN: 0  ID: 2    iana:packetDeltaCount         : 1 packets
+        EN: 0  ID: 152  iana:flowStartMilliseconds    : 2016-02-15T11:05:48.150Z
+        EN: 0  ID: 153  iana:flowEndMilliseconds      : 2016-02-15T11:05:48.150Z
+        EN: 0  ID: 10   iana:ingressInterface         : 1
+        EN: 0  ID: 60   iana:ipVersion                : 4
+        EN: 0  ID: 8    iana:sourceIPv4Address        : 209.246.218.165
+        EN: 0  ID: 12   iana:destinationIPv4Address   : 93.113.168.59
+        EN: 0  ID: 5    iana:ipClassOfService         : 0
+        EN: 0  ID: 192  iana:ipTTL                    : 122 hops
+        EN: 0  ID: 4    iana:protocolIdentifier       : 17
+        EN: 0  ID: 7    iana:sourceTransportPort      : 62299
+        EN: 0  ID: 11   iana:destinationTransportPort : 53
+        ... <~ output shortened for clarity ~> ...