One of {HEAD ~ the layers} may be left unencrypted to allow {TAIL ~ free browsing} of a low quality service level 
left unencrypted to allow
One of { HEAD ~ the layers } may be left unencrypted to allow { TAIL ~ free browsing } of a low quality service level
OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH REL REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH