Early mentioning of the optimization properties.

draft-ietf-core-oscore-edhoc.md

@@ -81,6 +81,8 @@ This optimization is desirable, since the number of message exchanges can have a
 
 Without this optimization, it is not possible, not even in theory, to achieve the minimum number of round trips. This optimization makes it possible also in practice, since the message_3 of the EDHOC protocol can be made relatively small (see {{Section 1.2 of I-D.ietf-lake-edhoc}}), thus allowing additional OSCORE-protected CoAP data within target MTU sizes.
 
+The minimum number of two round trips can be achieved only if the default, forward message flow of EDHOC is used, i.e., when a CoAP client acts as EDHOC Initiator and a CoAP server acts as EDHOC Responder. The performance advantage of using this optimization can be lost when used in combination with Block-wise transfers {{RFC7959}} that rely on specific parameter values and block sizes.
+
 Furthermore, this document defines a number of parameters corresponding to different information elements of an EDHOC application profile (see {{web-linking}}). These can be specified as target attributes in the link to an EDHOC resource associated with that application profile, thus enabling an enhanced discovery of such a resource for CoAP clients.
 
 ## Terminology