POST [2001:db8::2:1]:61616/est/sen (CON)(1:0/1/256) 
                   {CSR (frag# 1)} -->

       <-- (ACK) (1:0/1/256) (2.31 Continue)
POST [2001:db8::2:1]:61616/est/sen (CON)(1:1/1/256) 
                   {CSR (frag# 2)} -->
       <-- (ACK) (1:1/1/256) (2.31 Continue)
                      .
                      .
                      .
POST [2001:db8::2:1]:61616/est/sen (CON)(1:N1/0/256)
                   {CSR(frag# N1+1)}-->
                      |
    ...........Immediate response  .........
                      |
  <-- (ACK) (1:N1/0/256)(2:0/1/256)(2.04 Changed)
                   {Cert resp (frag# 1)}
POST [2001:db8::2:1]:61616/est/sen (CON)(2:1/0/256)           -->
  <-- (ACK) (2:1/1/256)(2.04 Changed) 
                   {Cert resp (frag# 2)}
                      .
                      .
                      .
POST [2001:db8::2:321]:61616/est/sen (CON)(2:N2/0/256)          -->
  <-- (ACK) (2:N2/0/256) (2.04 Changed) 
                   {Cert resp (frag# N2+1)}