Addressing Bill's comments

[furry13]

As Bill pointed out, we might want to explicitly define how to add the object in two cases:

• extension structure already exists;
• no structure, so new one needs to be added.

Adding two subsections to define the behaviour.

Open questions:

• is it crazy to suggest truncating the embedded invoking packet?
• does the proposed logic make sense (first adding the structure to ICMPv6, then translate. not "translate first, then add the structure"?)