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)}-->
  <-- (ACK) (1:N1/0/256) (5.03 Service Unavailable) (Max-Age)
                  |
                  |
  ... Client tries again after Max-Age with identical payload ...
                  |
                  |
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 when certificate is ready ...
                  |
  <-- (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::2:1]:61616/est/sen (CON)(2:N2/0/256)          -->
  <-- (ACK) (2:N2/0/256) (2.04 Changed) {Cert resp (frag# N2+1)}