attempt to document some of the internal details

src/common/tag-list.h

@@ -28,6 +28,41 @@ namespace ns3 {
 
 struct TagListData;
 
+/**
+ * \brief keep track of the tags stored in a packet.
+ *
+ * This class is mostly private to the Packet implementation and users
+ * should never have to access it directly.
+ *
+ * \internal
+ * The implementation of this class is a bit tricky so, there are a couple
+ * of things to keep in mind here:
+ *
+ *   - it stores all tags in a single byte buffer: each tag is stored
+ *     as 4 32bit integers (TypeId, tag data size, start, end) followed 
+ *     by the tag data as generated by Tag::Serialize.
+ *
+ *   - the struct TagListData structure which contains the tag byte buffer
+ *     is shared and, thus, reference-counted. This data structure is unshared
+ *     as-needed to emulate COW semantics.
+ *
+ *   - each tag tags a unique set of bytes identified by the pair of offsets 
+ *     (start,end). These offsets are provided by Buffer::GetCurrentStartOffset
+ *     and Buffer::GetCurrentEndOffset which means that they are relative to 
+ *     the start of the 'virtual byte buffer' as explained in the documentation
+ *     for the ns3::Buffer class. Whenever the origin of the offset of the Buffer
+ *     instance associated to this TagList instance changes, the Buffer class
+ *     reports this to its container Packet class as a bool return value
+ *     in Buffer::AddAtStart and Buffer::AddAtEnd. In both cases, when this happens
+ *     the Packet class calls TagList::AddAtEnd and TagList::AddAtStart to update
+ *     the byte offsets of each tag in the TagList.
+ *
+ *   - whenever bytes are removed from the packet byte buffer, the TagList offsets 
+ *     are never updated because we rely on the fact that they will be updated in
+ *     either the next call to Packet::AddHeader or Packet::AddTrailer or when
+ *     the user iterates the tag list with Packet::GetTagIterator and 
+ *     TagIterator::Next.
+ */
 class TagList
 {
 public:
@@ -68,16 +103,57 @@ public:
   TagList &operator = (const TagList &o);
   ~TagList ();
 
+  /**
+   * \param tid the typeid of the tag added
+   * \param bufferSize the size of the tag when its serialization will 
+   *        be completed. Typically, the return value of Tag::GetSerializedSize
+   * \param start offset which uniquely identifies the first byte tagged by this tag.
+   * \param start offset which uniquely identifies the last byte tagged by this tag.
+   * \returns a buffer which can be used to write the tag data.     
+   *
+   * 
+   */
   TagBuffer Add (TypeId tid, uint32_t bufferSize, uint32_t start, uint32_t end);
 
+  /**
+   * \param o the other list of tags to aggregate.
+   *
+   * Aggregate the two lists of tags.
+   */
   void Add (const TagList &o);
 
+  /**
+   * \param i points to the item to remove from this list.
+   *
+   * Not implemented.
+   */
   void Remove (const Iterator &i);
   void RemoveAll (void);
 
+  /**
+   * \param offsetStart the offset which uniquely identifies the first data byte 
+   *        present in the byte buffer associated to this TagList.
+   * \param offsetEnd the offset which uniquely identifies the last data byte 
+   *        present in the byte buffer associated to this TagList.
+   * \returns an iterator
+   *
+   * The returned iterator will allow you to loop through the set of tags present
+   * in this list: the boundaries of each tag as reported by their start and
+   * end offsets will be included within the input offsetStart and offsetEnd.
+   */
   TagList::Iterator Begin (uint32_t offsetStart, uint32_t offsetEnd) const;
 
+  /**
+   * Adjust the offsets stored internally by the adjustment delta and
+   * make sure that all offsets are smaller than appendOffset which represents
+   * the location where new bytes have been added to the byte buffer.
+   */
   void AddAtEnd (int32_t adjustment, uint32_t appendOffset);
+  /**
+   * Adjust the offsets stored internally by the adjustment delta and
+   * make sure that all offsets are bigger than prependOffset which represents
+   * the location where new bytes have been added to the byte buffer.
+   */
   void AddAtStart (int32_t adjustment, uint32_t prependOffset);
 
 private: