Added "Ensuring Serialization Length" section to the "Messages" chapter.

Author: arobenko

fields/optional.adoc

@@ -72,6 +72,7 @@ specified using **defaultMode** <<intro-properties, property>>.
 </schema>
 ----
 
+[[fields-optional-existence-conditions]]
 ==== Existence Condition ====
 Many protocols introduce optional fields, and the existence of such fields
 depend on the value of some other field. Classic example would be having 

intro/names.adoc

@@ -3,7 +3,7 @@
 Almost every element has a required **name** <<intro-properties, property>>. Provided
 value will be used to generate appropriate classes and/or relevant access functions.
 As the result, the chosen names must be only alphanumeric and
-`_' (underscore) characters, but also mustn't start with a number.
+'_' (underscore) characters, but also mustn't start with a number.
 The provided value is case sensitive. However, the code generator is allowed
 to change the case of to first letter of the provided value. It is up to the
 code generator to choose whether to use **camelCase** or **PascalCase** when

messages/messages.adoc

@@ -382,6 +382,31 @@ Sometimes an existing member field may be renamed and/or moved. It is possible t
 create alias names for the fields to keep the old client code being able to compile
 and work. Please refer to <<aliases-aliases, Aliases>> chapter for more details.
 
+[[messages-ensuring-serialization-length]]
+=== Ensuring Serialization Length ===
+Many protocol specifications indicate the expected message serialization length, especially
+when message has fixed length. The **CommsDSL** allows specifying expected minimal length
+using **validateMinLength** property with <<intro-numeric, unsigned>> value to help avoid some copy-paste or typo errors 
+at the time of schema parsing rather than debugging generated incorrect code.
+[source,xml]
+----
+<?xml version="1.0" encoding="UTF-8"?>
+<schema ...>
+    <message name="Msg1" id="1" validateMinLength="6">
+        <int name="F1" type="uint32" />
+        <int name="F2" type="int16" />
+        <optional name="F3" defaultMode="exist">
+            <int name="F3Field" type="uint8" />
+        </optional>
+    </message>
+</schema>
+----
+Note, that <<fields-optional, &lt;optional&gt; >> field is always expected to have 0 minimal length regardless
+of its <<fields-optional-existence-conditions, existence conditions>>.
+
+Also note, that the value of **validateMinLength** is the expected serialization length of 
+message fields __without__ the fields of the <<frames-frames, &lt;frame&gt; >>.
+
 [[messages-ensuring-overriding-code-injection]]
 === Ensuring Overriding Code Injection  ===
 Similar to <<fields-common-ensuring-overriding-code-injection, fields>> the default code