section 4.1, rewrite

[mcr]

Russ says:

Section 4.1, para 2: This paragraph is very confusing. I suggest that
you have separate paragraphs for:

1. The Join Proxy learns the IP address and port of the Registrar;
2. The Pledge discovers one or more Join Proxy and then selects one;
3. The Pledge sends a message to the Join Proxy; and
4. The Join Proxy forwards the message to the Registrar, keeping the
   needed context to send a response back the the Pledge.

[mcr]

Only in Figure 2 do we see an traffic that goes to the Pledge. The
above should probably be expanded to cover that too.