discussion
Click on the red underlined text to get to the source
... packet processing. Appendix A describes how this specification
updates a number of existing specifications. Appendices B and C
expand on the discussion in Section 2.
...
... ICMP
Destination Unreachables, but that is the topic of Section 6.
For purposes of this discussion, it is useful to divide ICMP
messages into three classes:
...
... larger issue can be extremely complex [4], and is beyond the scope
of this memo. This discussion should therefore be considered only
an overview. Implementors should consult the Router ...