Issue appears with UNICAST or NAKACK, example below uses NAKACK:

• Message M1 is received and placed into received_msgs. M1 has a FragHeader attached (FRAG2 was somewhere above NAKACK)
• M1 is retrieved from received_msgs and passed up
• FRAG2 removes the FragHeader
• A retransmission request is received for M1
• M1 is retrieved from received_msgs and passed down.
  Problem: the FragHeader is missing ! The receiver will not recognize M1 as a fragment and FRAG2 will not handle M1 !

SOLUTION 1: all protocols (maybe only the ones above UNICAST/NAKACK) should never remove headers. We could ensure this is the case by simply making removeHeader() a no-op
SOLUTION 2: investigate into who owns the message and who is allowed to modify it. Look into sealing techniques