RM and Intermediaries

Lets say S is the sender endpoint, R is the receiver endpoint, and i1, i2 
are intermediaries that pre-process the message.

S --> i1 --> i2 --> R

Do i1 and i2 have to understand RM ? or RM is just an end-to-end 
handshaking between S and R ?  If so, can S pick a different path in its 
message resend ?

Rgds, Ricky